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South Western CFRAM Study

Final Hydrology Report, Unit of Management 18

June 2016

The Office of Public Works

296235 IWE CCW R014 D

C:\Users\pig44561\AppData\Roaming\OpenText\OTEdit\EC_EUNAPiMS\c1500321418\296235-IWE-CCW-R014-D-

Hydrology_Report_UoM18.docx June 2016


Final Hydrology Report,Unit of Management 18


Final Hydrology Report, Unit of Management 18

June 2016

The Office of Public Works

Mott MacDonald, 5 Eastgate Avenue, Eastgate, Little Island, Cork, Ireland

T +353 (0)21 4809 800 F +353 (0)21 4809 801 W www.mottmac.com

Jonathan Swift Street, Trim Co. Meath

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South Western CFRAM Study Final Hydrology Report,Unit of Management 18

296235/IWE/CCW/R014/D June 2016 C:\Users\pig44561\AppData\Roaming\OpenText\OTEdit\EC_EUNAPiMS\c1500321418\296235-IWE-CCW-R014-D-Hydrology_Report_UoM18.docx

Revision Date Originator Checker Approver Description Standard

A August 2013 M Piggott C Jones S Pipe

R Gamble R Gamble Draft

B February 2014 M Piggott R Gamble R Gamble Draft Final

C October 2015 M Piggott B O’Connor B O’Connor Draft Final Minor amendments

D June 2016 M Piggott B O’Connor B O’Connor Revised to Final Status

Issue and revision record

This document is issued for the party which commissioned it and for specific purposes connected with the above-captioned project only. It should not be relied upon by any other party or used for any other purpose.

We accept no responsibility for the consequences of this document being relied upon by any other party, or being used for any other purpose, or containing any error or omission which is due to an error or omission in data supplied to us by other parties.

This document contains confidential information and proprietary intellectual property. It should not be shown to other parties without consent from us and from the party which commissioned it..



Chapter Title Page

Executive Summary i

1 Introduction 1

1.1 Context of the CFRAM Study __________________________________________________________ 1 1.2 SW CFRAM Study Process ___________________________________________________________ 1 1.3 Report Structure ____________________________________________________________________ 2 1.4 Flood Probabilities __________________________________________________________________ 4

2 Description of the Study Area 5

2.1 Extent ____________________________________________________________________________ 5 2.2 Characteristics of Rivers ______________________________________________________________ 7 2.3 Coastal Features ___________________________________________________________________ 8 2.4 Topography _______________________________________________________________________ 9 2.5 Rainfall ___________________________________________________________________________ 9 2.6 Geology _________________________________________________________________________ 13 2.7 Land Use ________________________________________________________________________ 13

3 Data Collection and Review 16

3.1 Data Register _____________________________________________________________________ 16 3.2 River Gauge Data __________________________________________________________________ 16 3.3 Rainfall Data ______________________________________________________________________ 22 3.4 Coastal Data ______________________________________________________________________ 24

4 Historical Flood Review 26

4.1 Historical Flood Events ______________________________________________________________ 26 4.2 Historical Flood Mechanisms _________________________________________________________ 31 4.3 Historical Flood Frequency Estimates __________________________________________________ 32

5 Rating Reviews 35

5.1 Gauge Review Selection ____________________________________________________________ 35 5.2 River Bride at Mogeely (Gauge 18001) High Flows Rating Review ____________________________ 35 5.3 River Blackwater at Ballyduff (Gauge 18002) High Flows Rating Review _______________________ 41 5.4 River Allow at Riverview (Gauge 18009) High Flows Rating Review ___________________________ 48 5.5 River Dalua at Allen’s Bridge (Gauge 18010) High Flows Rating Review _______________________ 54 5.6 River Blackwater Rating Checks ______________________________________________________ 59

6 Design Flows 68

6.1 Overview ________________________________________________________________________ 68 6.2 Definition of Sub-Catchments _________________________________________________________ 68 6.3 Flood Frequency Analysis ___________________________________________________________ 72 6.4 Hydrograph Generation _____________________________________________________________ 81

Contents



6.5 Coastal Conditions _________________________________________________________________ 83

7 Hydrological Calibration, Sensitivity Testing and Uncertainty 89

7.1 Calibration Events _________________________________________________________________ 89 7.2 Uncertainty and Sensitivity Testing____________________________________________________ 101

8 Summary of Design Flows 103

9 Considerations for Hydrological and Hydraulic Model Integration 107

9.1 Inflows _________________________________________________________________________ 107 9.2 Downstream Conditions ____________________________________________________________ 108

10 Hydrogeomorphology 110

10.1 Approach _______________________________________________________________________ 110 10.2 Assessment _____________________________________________________________________ 110 10.3 Impact on Flood Risk ______________________________________________________________ 116

11 Joint Probability 117

11.1 Overview _______________________________________________________________________ 117 11.2 Fluvial-Fluvial Dependence _________________________________________________________ 117 11.3 Fluvial-Coastal Dependence ________________________________________________________ 119

12 Future Scenarios 121

12.1 Potential Climate Changes __________________________________________________________ 121 12.2 Potential Catchment Changes _______________________________________________________ 121 12.3 Design Future Scenario Conditions ___________________________________________________ 124

13 Conclusions, Key Findings and Recommendations 125

13.1 Conclusions and Key Findings _______________________________________________________ 125 13.2 Recommendations ________________________________________________________________ 127

Glossary 129


i


The Office of Public Works (OPW) is undertaking six catchment-based flood risk assessment and

management (CFRAM) studies to identify and map areas across Ireland which are at existing and potential

future risk of flooding. Mott MacDonald Ireland Ltd. has been appointed by the OPW to assess flood risk

and develop flood risk management options in the South Western River Basin District. This hydrology

report is one of a series of reports being produced as part of the South Western Catchment Flood Risk

Assessment and Management Study (SW CFRAM Study). This report details the assessment of the

hydrological conditions across Unit of Management 18 (the Munster Blackwater catchment) which will form

the inputs into the subsequent hydraulic modelling and mapping of the key areas at risk.

A review and analysis of historical flood events, hydrometric data and hydrogeomorphological processes

has highlighted flooding issues to urban areas including Freemount, Kanturk, Mallow, Fermoy, Ballyduff,

Youghal, Rathcormac Tallow and Aglish. The Flood Studies Update methodologies have been used to

determine the design peak flows and characteristic flood hydrographs for eight specified flood probabilities

across the sub-catchments. Corresponding coastal conditions have been developed for Youghal.

Calibration events were identified across the catchment where there was sufficient historical flood data.

Potential future catchment changes relevant to the Blackwater catchment have been assessed including

changes in urban development, land use and hydrology related to global climate change. Two future

scenarios have been developed from this analysis, a Mid Range Future Scenario and High End Future

Scenario, which have been used to develop potential future flows and extreme sea levels.

The resultant design flood hydrographs and coastal conditions will form the inflows for the hydraulic

models. The knowledge of the hydrological processes and the historical flooding issues in the Blackwater

catchment established in this report will support the development of sustainable and appropriate flood risk

management options in those areas at greatest flood risk.

Executive Summary



1

1.1 Context of the CFRAM Study

Flooding is a natural process that occurs throughout Ireland as a result of extreme rainfall, river flows,

storm surges, waves, and high groundwater. Flooding can become an issue where the flood waters

interact with people, property, farmland and protected habitats.

Flood risk in Ireland has historically been addressed through the use of structural or engineered solutions

(arterial drainage schemes and / or flood relief schemes). In line with internationally changing perspectives,

the Government adopted a new policy in 2004 that shifted the emphasis in addressing flood risk towards:

A catchment-based context for managing risk;

More pro-active flood hazard and risk assessment and management, with a view to avoiding or

minimising future increases in risk, such as that which might arise from development in floodplains;

Increased use of non-structural and flood impact mitigation measures.

A further influence on the management of flood risk in Ireland is the 'Floods' Directive [2007/60/EC]. The

aim of this Directive is to reduce the adverse consequences of flooding on human health, the environment,

cultural heritage and economic activity.

The Office of Public Works (OPW) is the lead agency in implementing flood management policy in Ireland.

The OPW have commissioned a number of Catchment Flood Risk Assessment and Management Studies

in order to assess and develop Flood Risk Management Plans (FRMPs) to manage the existing flood risk

and also the potential for significant increases in this risk due to climate change, ongoing development and

other pressures that may arise in the future.

Mott MacDonald Ireland Ltd. has been appointed by the OPW to undertake the Catchment-Based Flood

Risk Assessment and Management Study (CFRAM Study) for the South Western River Basin District,

henceforth referred to as the SW CFRAM Study. Under the project, Mott MacDonald will produce FRMPs

which will set out recommendations for the management of existing flood risk in the Study Area, and also

assess the potential for significant increases in this risk due to climate change, ongoing development and

other pressures that may arise in the future.

1.2 SW CFRAM Study Process

The overarching aims of the SW CFRAM Study are as follows:

Identify and map the existing and potential future flood hazard;

Assess and map the existing and potential future flood risk; and,

Identify viable structural and non-structural options and measures for the effective and sustainable

management of flood risk in the South Western River Basin District.

1 Introduction



2

In order to achieve the overarching aims, the study is being undertaken in the following stages:

Data collection;

Hydrological analysis;

Hydraulic analysis;

Development of flood maps;

Strategic Environmental Assessment and a Habitats Directive Appropriate Assessment;

Flood risk assessment of people, economy and environment;

Development and assessment of flood risk mitigation options; and,

Development of the Flood Risk Management Plan (FRMP).

The resultant FRMP will set out recommendations for the management of existing flood risk and the

potential for significant increases in this risk due to climate change, ongoing development and other

pressures that may arise in the future.

The South Western River Basin District is split into five Units of Management (UoM). These Units follow

watershed catchment boundaries and do not relate to political boundaries. The Units are as follows;

The Blackwater catchment (UoM18)

The Lee / Cork Harbour Catchment (UoM19)

The Bandon / Skibbereen Catchment (UoM20)

The Dunmanus / Bantry / Kenmare Bay Catchment (UoM21)

The Laune / Maine / Dingle Bay Catchment (UoM22)

1.3 Report Structure

This report aims to assess the hydrological conditions across the Munster Blackwater catchment and

derive design peak flows, levels and hydrographs to be used in subsequent hydraulic modelling and

mapping of key areas at risk.

Table 1.1 outlines the report structure and scope of work with a description of the key contents.



3

Table 1.1: Report Structure

Chapter Key Contents of Chapter

1. Introduction Context of the Study The SW CFRAM process and aims Scope of Work Flood Probabilities

2. Description of Study Area Description of study area Description of hydrological characteristics of study area

3. Data Collection and Review Overview of data used in the hydrological analysis Review and quality assessment of river level and flow data Review and quality assessment of rainfall data

Review and quality assessment of coastal data

4. Historical Flood Review Review of historical flood events Review of significant sources, pathways and receptors of

flooding

Estimation of flood probability for key historical events

5. Rating Reviews Analysis of spot gaugings at review gauge locations Hydraulic modelling used to extend rating curve Modelled rating curve extension

Application of revised rating curve

6. Design Flows Definition of sub-catchments Derivation of the index flood, design peak flows and flow

hydrographs Derivation of extreme sea levels and tidal curves

7. Hydrological Calibration, Sensitivity and Uncertainty

Review of historical data and selection of calibration events Derivation of calibration conditions Hydrological sensitivity and uncertainty in design hydrology

8. Summary of Design Flows Principal outputs and findings of design hydrology Preliminary design flows and hydrographs for hydraulic

modelling

9. Consideration for Hydrological and Hydraulic Model Integration

Full methodological approach to integrate hydrological outputs and hydraulic models

10. Hydrogeomorphology Assessment of existing hydrogeomorphological processes Consideration of flood risk impacts

11. Joint Probability Analysis Joint probability of fluvial events Joint probability of coastal events

12. Future Scenarios Potential impacts of climate change to rainfall, river flows, sea level and land movement

Potential catchment changes to land use and urbanisation Derivation of hydrology under future scenarios

13. Conclusions, Key Findings and Recommendations

Conclusions and key findings from the hydrological analysis and assessment

Summary of Design Existing and Future Hydrology Recommendations for hydraulic modelling and the FRMP Recommendations for future improvements in the

hydrological analysis



4

1.4 Flood Probabilities

The SW CFRAM Study refers to flood probabilities in terms of annual exceedance probability in preference

to the use of “return periods” as used in previous reports. The probability or chance of a flood event

occurring in any given year can be a useful tool to better understand the rarity of specific magnitude events

for flood risk management. Due to popular descriptors of floods involving terms like the “1 in 100 year

flood” there can be a public misunderstanding that a location will be safe from a repeat event of the same

magnitude, extent and volume for the duration of the term (100 years in the above example). In reality,

flood events of a similar or greater magnitude can occur again at any time.

Annual Exceedance Probability, henceforth referred to as AEP, is a term used throughout this report and

the wider CFRAM studies to refer to the rarity of a flood event. The probability of a flood relates to the

likelihood of an event of that size or larger occurring within any one year period. For example, a one in

hundred year flood has a one chance in a hundred of occurring in any given year; 1:100 odds of occurring

in any given year; or a 1% likelihood of occurring. This is described as a 1% annual exceedance probability

(AEP) flood event.

Table 1.2 converts the ‘return periods’ to %AEP for key flood events as a reference to previous studies.

Table 1.2: Flood Probabilities

% Annual Exceedance Probability (%AEP)

Odds of a Flood Event in Any Given Year

Chance of a Flood Event in Any Given Year or

Previous ‘Return Period’

50% 1:2 1 in 2

20% 1:5 1 in 5

10% 1:10 1 in 10

5% 1:20 1 in 20

2% 1:50 1 in 50

1% 1:100 1 in 100

0.5% 1:200 1 in 200

0.1% 1:1000 1 in 1000

The hydrological analysis uses a number of other acronyms and technical terminology which are defined in

the glossary of this report.



5

2.1 Extent

The South Western River Basin District covers an area of approximately 11,160 km2. The Study Area

includes most of County Cork, large parts of counties Kerry and Waterford along with small parts of the

counties of Tipperary and Limerick. The Study Area contains over 1,800 km of coastline along the Atlantic

Ocean and the Celtic Sea. There are five Units of Management within the South Western River Basin

District, which are listed below:

The River Blackwater catchment (UoM18)

The Lee / Cork Harbour Catchment (UoM19)

The Bandon / Skibbereen Catchment (UoM20)

The Dunmanus / Bantry / Kenmare Bay Catchment (UoM21)

The Laune / Maine / Dingle Bay Catchment (UoM22)

This report covers the Munster Blackwater in Unit of Management 18. It includes the Munster Blackwater

(henceforth referred to as Blackwater) downstream of Banteer to its outfall at Youghal, the River Allow,

River Bride and a number of smaller tributaries (Map 2.1). Unit of Management 18 contains nine Areas for

Further Assessment (AFAs) and over 238 km of high and medium priority watercourse associated with

these AFAs (Table 2.1).

Table 2.1: Areas for Further Assessment

Name Unique

ID Fluvial

Flood Risk Coastal

Flood Risk County Easting Northing

Contributing Catchment Area (km2)

Aglish 180247 Yes No Waterford 212250 91500 2.4

Ballyduff 180248 Yes No Waterford 196500 99500 2333.7

Fermoy 180252 Yes No Cork 182750 99500 1753.7

Freemount 180253 Yes No Cork 139500 114250 4.3

Kanturk 180254 Yes No Cork 138250 102750 307.5

Mallow 180262 Yes No Cork 155250 98500 1207.6

Rathcormac 180265 Yes No Cork 181750 91000 21.6

Tallow 180266 Yes No Waterford 199750 93750 18.1

Youghal 180267 Yes Yes Cork 210250 78750 > 3000

2 Description of the Study Area

6 296235/IWE/CCW/R014/D June 2016 C:\Users\pig44561\AppData\Roaming\OpenText\OTEdit\EC_EUNAPiMS\c1500321418\296235-IWE-CCW-R014-D-Hydrology_Report_UoM18.docx


Map 2.1: Unit of Management 18 Study Area

Allow Catchment

Blackwater Catchment

Bride Catchment



7

2.2 Characteristics of Rivers

The Study considers 80km of High Priority Watercourse (HPW) in AFAs and 158km of Medium Priority

Watercourse (MPW) (Map 2.1). All grid references are to Irish National Grid (ING) and levels are to

Ordnance Datum Malin Head (mODM).

Allow Sub-Catchment, including Freemount and Kanturk AFAs

The Study considers the River Allow from Freemount to its confluence with the River Blackwater near

Banteer. The River Allow/Glashawee River rises near the Mullaghereirk Mountains (126480, 118570) and

flows in a south-easterly direction towards Freemount where it is joined by the Freemount Stream at Allow

Bridge. The River Allow then flows southwards towards Kanturk where it is joined by the similarly sized

River Dalua immediately downstream of Greenane Bridge in the town. The Allow continues to flow

southwards where it is joined by minor tributaries such as the Brogeen Stream before it flows through

Riverview gauge to its confluence with the Blackwater at Leaders Bridge (138500, 098760). The major

tributary of the River Dalua has been considered from Riverview Gauge (133745, 104485) to its confluence

with the River Allow downstream of Greenane Bridge, Kanturk (138255, 103065).

Smaller tributaries such as the Greenane Stream in Kanturk AFA and Kilknockane Stream along the Allow

MPW have not been considered separately in the hydrology as their contributing area is less than 1km2

and/or their contributing flow is less than 10% to the downstream reach. The recurring flooding at

Greenane was attributed to the urban drainage system which is not considered under the CFRAM brief.

Blackwater Sub-Catchment, including Mallow, Fermoy, Ballyduff and Youghal AFAs

The River Blackwater rises near Glenatripple (112420, 109040) flowing southwards to Rathmore, before

flowing eastwards to Banteer where it is joined by the River Allow from the north (138500, 098750). This

upstream reach of the Blackwater will not be modelled or mapped as part of this study because there were

no AFAs identified upstream. However, the flow contribution from the upper Blackwater is considered.

The River Blackwater continues eastwards where it is joined by the Glen River from the south and Awbeg

Minor from the north before flowing into Mallow. Within Mallow, there is a flood defence scheme that

comprises of a number of walls, embankments, penstocks and the extension of the culvert through Tipp

O’Neill Park. There are a number of smaller urbanised tributaries that join with the River Blackwater in

Mallow. The most significant of these are the Clyda River which joins from the south upstream of

Quartertown and Spa Glen which joins downstream of Mallow Town Bridge.

The River Blackwater continues to flow east downstream of Mallow towards the gauge at Killavullen some

10km downstream before being joined by the Awbeg Major from the north downstream of Castletownroche

(169340, 099930) and flowing eastwards into Fermoy. A series of flood embankments and demountable

defences form the Fermoy flood defence scheme to mitigate flood risk from the high water levels in the

River Blackwater during floods. The left bank (north) was completed in 2008. The walls and embankments



8

along the right bank (south) are under construction at the time of this study. The River Blackwater is then

joined by the River Funshion and Araglin River from the north, 2km and 2.7km downstream of Fermoy.

The gradient of the River Blackwater continues to reduce as it flows eastwards through Ballyduff to

Lismore Weir (202250, 099080). Downstream of Lismore, the River Blackwater is considered fully tidal.

The river channel continues eastwards for another 6km before turning to flow southwards at the confluence

with the Glenshealan River. The River Blackwater flows southwards through Villerstown Gap before being

joined by the River Bride from the west and River Licky from the east. The River Blackwater then continues

to flow southwards to outfall into the Celtic Sea at Youghal (211500, 076250).

Bride Sub-Catchment including Rathcormac and Tallow AFAs

The River Bride is the second largest river within UoM18. The Bride rises from the Nagles Mountains

(164950,094090) and flows eastwards under the N8 road to Rathcormac, where it is joined by the

Shanowen River from Rathcormac ( 182060,091110) and the River Flesk from the south (182450,

091040). The River Bride continues to meander eastwards where it is joined by a number of smaller

tributaries before reaching Mogeely gauge (195640, 094130). Downstream of Mogeely, the Bride

continues east towards Tallow Bridge ( 199910,094330) where it becomes fully tidal, before joining the

River Blackwater some 12km downstream (209020, 091110). The much steeper and smaller Glenaboy

River flows from the south through Tallow before joining the River Bride upstream of Tallow Bridge.

Aglish AFA

The town of Aglish is situated in a minor sub-catchment of the lower Blackwater on the Ballynaparka River.

The small, steep Ballynaparka River rises 2km upstream of the town (213880,09090) flowing north-west

along the main street through Aglish before joining with the tributary immediately downstream of

Ballynaparka Bridge (212110,091508). Downstream of the confluence, the river flows west through Bleach

to join the tidal Goish River (210160,091750) and the River Blackwater 1km further downstream (209654,

092027).

2.3 Coastal Features

The River Blackwater can be considered tidal as far as Lismore, some 33km inland and the River Bride

can be consider tidal as far as Tallow Bridge, 30km inland. However, the relatively narrow channel and

floodplain between the steep valley sides limits the presence of in-channel bars, tidal loop channels and

wide estuarine flats until Youghal. Ferry Point, opposite Youghal (211110, 078060) constrains the incoming

tide, and protects Newtown and the eastern bank from extreme wave action. However, the western bank

from Youghal Mudlands (210140,080020) to Claycastle in Youghal (209115,75050) is vulnerable to wave

action and storm surges as identified by the Ireland Coastal Water level and Wave Study 2013 (ICWWS).

There is 1.7km of open coastline frontage at Claycastle, Youghal which goes across the fluvial watershed

between UoM18 and UoM19. This coastline has been considered as part of UoM18 since any coastal

flooding arising from this reach would affect the Youghal AFA.



9

2.4 Topography

Map 2.2 displays the variation in elevation and topography of UoM18. The River Blackwater catchment

ranges from less than 5mODM at Youghal Mudlands to 440mODM at the source of the Blackwater.

Elevations can reach over 900mODM at Galtymore Mountain the headwaters of the Funshion. The areas

of high relief and steepest slopes are associated with the more resistant geology in the west and north as

described in Section 2.6 below. The River Blackwater and lower reaches of the Awbeg (Major) and

Funshion have much shallower gradients ranging from 1 in 840 at the confluence with the Allow to 1 in

3000 downstream of Lismore. This very low gradient for the lower 30km of the Blackwater results in

significant attenuation of flood discharges. The floodplain is typically 1km wide upstream of Cappoquin

bounded by the more resistant valley sides. The land to the east of Cappoquin is low-lying and forms a

small area of floodplain when the Blackwater overtops its banks. Downstream of Cappoquin, the tidal

floodplain is constrained by the more resistant geology on either side of the river.

The River Bride catchment ranges from 2mODM up to 400mODM in its headwaters with the bed slope

typically ranging from 1 in 600 in the upper reaches to 1 in 1700 in the lower tidal reaches. The southern

tributaries to the Bride tend to be steeper and have higher relief than those that drain areas to the north of

the Bride. The floodplain is relatively wide, ranging from 2 to 3km along its length.

The River Allow catchment typically has higher relief as it forms the Blackwater headwaters. Elevations

range from 70mODM at its outfall to 400mODM at the source (Mullaghareirk). The River Allow has a

typical gradient of 1 in 320 whilst the River Dalua tributary has a steeper gradient of 1 in 260. The steeper

gradient of the Dalua results in a slightly faster response to rainfall than the Allow.

2.5 Rainfall

Map 2.3 shows the variation in Standard Average Annual Rainfall across UoM18. Rainfall tends to be

greater in the west and decreases towards the east. This corresponds with the dominant wind direction in

the South West where storms tend to track west to east. Areas of high relief east of Mitchelstown also

experience higher SAAR. Map 2.4 shows the distribution of rainfall during the extreme event on the 19th

November 2009. The higher relief in the west combined with the dominant storm track leads to greater

rainfall in the headwaters of the Blackwater, Bride and over the higher relief at Cappoquin. The northern

catchments of the Awbeg Major and Minor both have lower rainfall totals. Their lower topography and

karstic geology leads to much lower flows than would be expected for a catchment of their size.

Given the large size of the Blackwater catchment (> 3000km2), rainfall and high flows in the upper

catchments is unlikely to occur at the same time as rainfall and high flows in the lower catchments as the

storm tracks across the catchment. A single storm tracking from west to east can exacerbate flooding as all

the flows from the upper catchment converge with the peak flow from tributaries in the lower catchment ,

raising water levels. However, gauge records indicate that the Bride tends to peak before the Blackwater at

Ballyduff.



Map 2.2: Topography



Map 2.3: Standard Average Annual Rainfall



Map 2.4: Rainfall Distribution Across UoM18 on 19th

November 2009 Event



13

2.6 Geology

Map 2.5 provides the underlying geology of UoM18. The geology can be summarised into relatively

resistant Devonian sandstone and volcanic geology and the less resistant Dinantian Limestones that run

west to east in bands. The resistant geology forms the major watershed boundaries between the

Blackwater and Bride catchments. The small tributaries underlain by the sandstone geology are more

prone to ‘flashy’ or a faster response to rainfall compared with the more permeable catchment in the north

of the Blackwater sub-catchment.

Conversely, the less resistant limestones form the river valleys along the Blackwater and Bride. The

majority of the River Funshion, River Awbeg (major), the River Bride and parts of the Blackwater at Mallow

to Killavullen are underlain by highly permeable karst geology which forms regionally important aquifers.

The permeable nature of these reaches is likely to mitigate flood peaks when unsaturated, but could

exacerbate and prolong flooding when the groundwater system is saturated.

2.7 Land Use

The Blackwater, Allow and Bride catchments are predominately rural in regards to land use, with the major

urbanised areas located around Mallow, Fermoy, Kanturk and Youghal. The smaller catchments of

Gooldshill Stream, Bearforest Stream and Hospital Stream in Mallow are more urban with between 9% and

19% covered by impermeable surfaces such as paving and tarmac. These surfaces increase surface water

runoff which reduces the time to peak giving a flashy response to any rainfall.

The rural land use comprises of largely pasture, with mixed agriculture interspersed with smaller wooded

areas on the valley sides. The significant wooded areas tend to be located on higher grounds near the

Araglin and Knockanore areas in the Lower Blackwater catchment. It was observed that polythene

coverings and polytunnels are used throughout the Allow, Blackwater and Bride catchments during

spring/summer which could significantly increase runoff. However, site visits indicate that this practice was

only applied to the minority of fields and was of short duration during the growing season. Therefore, the

impact of increased runoff due to polythene coverings was not considered in the design hydrology but may

be of local significance for specific events.

The headwaters of the Funshion, Araglin and Bride are dominated by the presence of peat bog and

moorland on the Knockmealdown and Nagles Mountains which could attenuate runoff reaching the river

channels. However, the impact of the peat bog and moorland reduced to less than 5% by the time these

rivers reach the study area on the River Blackwater and Bride respectively.

European Union agro-forestry policies since 1973 have led towards more intensive agriculture and

commercial forestry with associated land drainage. In particular, the removal of field boundary ditches as

natural drainage barriers and the raised embankments for the M8 motorway which have altered flow paths.

There is some evidence to support increased runoff and flows at the long-term gauge stations within the

Allow and Bride catchments. However, similar increases in flows have been observed in other catchments

in Ireland which have not experienced agricultural intensification. Therefore, climatic changes as well as

land use changes are involved in the change in flood response. Chapter 12 of this report assesses the

potential future changes in the catchment.



Map 2.5: Geology



15

The major urban areas are located at Mallow (population 11605), Youghal (population 8200) Fermoy

(population 7000) and Kanturk (population 1900). Of these Mallow is the most densely populated and

includes industrial areas along the River Blackwater valley upstream of the town. The increased presence

of tarmac and other impermeable materials increase the runoff and flashy response of the smaller urban

catchments. However, these urban areas form a relatively small proportion (<1%) of the larger Allow,

Blackwater and Bride catchment. Thus the urban land use is unlikely to significantly affect flows. The

remaining smaller settlements tend be located at the edge of the floodplains of the major rivers such as

Freemount and Aglish, or at crossing points such as Ballyduff and Lismore.



16

3.1 Data Register

A range of different data sources has been used to undertake the hydrological data analysis for UoM18

(Table 3.1). The use of local hydrometric data can greatly improve and validate flood flows for historic

events and design flood events and has been reviewed in the following sections of this Chapter.

Table 3.1: Summary of Available Data

Type Details Owner Period of Available Data

River Flows 15 minute interval data series at 12 gauges with flow converted from water

level

The OPW

EPA (operated by Cork and Waterford County Council)

Various up to 2012

River Levels 15 minute interval data series at 21 gauges

The OPW

EPA (operated by Kerry County Council)

Various up to 2012

Rainfall Gauges Daily rainfall values at 53 gauges

Hourly rainfall at 32 gauges in the Mallow Flood Forecasting System

Hourly rainfall series at Valentia Observatory

Met Eireann

The OPW

Various up to 2012

Extreme Sea Level

Irish Costal Protection Strategy Study Total tide +surge design levels in

Youghal Bay.

The OPW Calculated for 2012

Wave Conditions Water levels, wave heights and wave periods at Youghal Harbour.

The OPW

Calculated for 2013

Sea level Ballycotton Tidal Gauge The OPW tidal network 2007 - 2012

A full register of hydrometric data used in this study can be found in Appendix A.

3.2 River Gauge Data

Map 3.1 shows the locations of river gauges in UoM18 with available water level and flow data. Chapter 6

discusses the analysis of those gauges selected for the modelled HPW and MPW reaches. The existing

hydrometric data from the wider area has been assessed for the following common issues:

Anomalous spikes or dips in water level and/or flow from the continuous data records;

Capping of water level and/or flow;

Trends in water level or flow over time that might be caused by systematic error of gauging equipment

or erosion/sedimentation;

Sudden shifts in level of the gauging datum;

Comparison of AMAX flows and levels from digital gauged data with manually extracted AMAX series;

Anomalous AMAX flood peaks in the AMAX series at each gauge;

Consistency of concurrent high flows downstream for AMAX events;

Length of data record to enable hydrological analysis; and,

Any significant data gaps.

Dromcummer and other gauges with shorter records have been adjusted for the missing periods based

on longer term gauges within the catchment using a pooled approach to extend records.

3 Data Collection and Review



Map 3.1: Available Hydrometric Data



18

Stations 18050, 18048, 18016

Long term flow and level records are available at Duncannon, Duarrigle and Dromcummer on the River

Blackwater. Of these only Dromcummer gauge is located within the modelled reach of the Blackwater. The

Dromcummer gauge has 22 years of reliable flow data up to 2004 available for this study. This can still be

used for statistically analysis but any long term estimates of flows should be adjusted to account for the

wetter years since 2002 based on neighbouring gauges with longer records. Duncannon and Duarrigle

should be used with caution as they are underlain by permeable geology which may make them unsuitable

for use as pivotal sites in impermeable areas.

Stations 18006, 18055, 18003, 18106 and 18107

There are long records available on the Blackwater between Mallow and Fermoy at the Comhlucht Siúcre

Eireann Teoranta (CSET) Mallow gauge (18006) and Killavullen gauge (18003). Approximately 10 years’

of flow data is available at Mallow Rail Bridge (18055) and 10 years’ of level data at Fermoy Bridge gauges

(18106 and 18107). These gauges has a complete and consistent record with the exception of the CSET

Mallow gauge during the 2009 floods where the original record indicated the flood peak occurring several

days after the peak at neighbouring upstream and downstream gauges. The level record has been

reviewed by EPA and corrected during this study such that the peak occurs on 19th November 2009 in line

with neighbouring gauges.

The initial hydrometric review of the flow series along the Blackwater indicated that the gauges in Mallow

consistently experience higher peak flows than Killavullen despite additional inflows from tributaries and a

30% increase in contributing area. Therefore a check of the high flow rating curves was undertaken (see

Chapter 5 of this report). The high flows ratings were subsequently updated at CSET Mallow, Killavullen

and a full rating curve derived for the downstream Fermoy Bridge gauge.

The progression and any attenuation of the flood flows were then reviewed using the 1D-2D hydraulic

model of Mallow as a routing model for a range of in-bank to out-of bank events. Figure 3.1 compares peak

flow at Mallow and Killavullen. Typically flow increases with increasing contributing area. Therefore the

flows at Killavullen should be above the 1:1 ratio dashed line. However, the modelled peak flows and

concurrent spot gaugings are at or below this 1:1 ratio between bankfull and QMED. This apparent loss

can be explained by greater storage on the floodplain as water spills out-of-bank but does not return to the

channel, thus is not recorded at the downstream gauge. Furthermore, karstic caves and depressions on

the floodplain may cause additional losses to groundwater when the ground is not saturated but were not

considered in this surface water analysis. However, above QMED the floodplain flow is fully connected to

the river channel and flow increases at Killavullen again. Figure 3.2 demonstrates these mechanisms from

the preliminary hydraulic model results.



19

Hence, we can conclude that all the flow gauges between Mallow and Fermoy can be relied upon for

statistical analysis of extreme flood events above QMED flows as required for the CFRAM Study.

However, recorded peak flows at Killavullen between bankfull and QMED should be carefully considered in

combination with the floodplain conditions between Mallow and Killavullen before use.

Figure 3.1: Comparison of Flood Peaks between Mallow and Killavullen Gauges

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600

Flo

w a

t K

illav

ulle

n 1

80

03

(m

3/s

)

Flow at Mallow Rail Bridge 18055 (m3/s)

Spot Gaugings 1:1 Ratio Modelled Flood Peaks Bankfull d/s of Mallow QMED



20

Figure 3.2: Floodplain Snapshots of the Blackwater Mallow-Killavullen Reach

A: Below Bankfull Conditions < 130m3/s

B: Partial or Disconnected Floodplain Flooding 130-300 m3/s

C; Fully Connected Floodplain Flooding > 300m3/s



21

Station 18002

Ballyduff on the Blackwater has a long record of 15 minute level data over 56 years with no significant data

gaps or datum shifts in the data set provided. These levels have been converted to flow using rating curves

over the years based on in-bank spot gaugings. These in-bank rating curves have been extrapolated to

estimate extreme out-of-bank flows which may underestimate floodplain and any by-passing flow over the

road at this site. Therefore, the rating curves will be reviewed at Ballyduff as well as, as number of other

gauges identified with potential by-passing flow in Chapter 5. The resultant flows at these gauges will be

used to inform the calibration of the hydraulic models in the relevant AFAs (Ballyduff, Kanturk, Mallow and

Fermoy).

Stations 18010, 18009, 18005, 18004 and 18001

Long terms records over 10 years are also available on the Blackwater tributaries including: Allen's Bridge

on the Dalua; Riverview on the Allow; Downing Bridge on the Funshion; Ballynomona on the Awbeg

(Major); and, Mogeely on the Bride respectively. The majority of the flow records have been edited by

OPW but are consistent with level records and are suitable for use following the rating review for the

Allen's Bridge and Riverview gauges.

Stations 18110, 18111 18019, 18109, 18056, 18105 and 18117

Level records were available for Kilbrin Road on the Allow, Church Road on the Dalua, Murphy's Bridge on

the Glen; Lombardstown, Mallow Town Bridge, Castlelands and Fermoy Mill on the Blackwater

respectively. The preliminary data review has highlighted several missing periods around 2006 and 2007

for Kilbrin Road and Murphy’s Bridge. The level data on the Blackwater gauges listed was of good quality

but typically less than 10 years in length and limited or no spot gaugings to develop a rating curve to

estimate flows. Therefore the level-only gauges across the catchment will be used to inform the calibration

of the hydraulic models local to these gauges but not in the hydrological analysis. Appendix A contains a

full list of the selected gauges and plots data quality.



22

3.3 Rainfall Data

Available meteorological data from rain gauges and synoptic stations in and near to the catchment are

shown in Map 3.2.

Spatial distribution of intensity loggers and respective storage gauges (event based);

Identification of gaps or erroneous data which have been cross-referenced with the Met Eireann climate

stations to assess if significant events have been omitted;

Identification of shifts in rainfall records using temporal and cumulative plots; and,

Analysis of cumulative rainfall for key historic events.

The 32 rainfall gauges operated by OPW as part of the Mallow Flood Forecasting Scheme provide good

coverage across the Blackwater catchment to Lismore and Bride catchment to Tallow. The lower reaches

from Lismore to Youghal are not represented in this rainfall gauge network. However, the flood risk in the

lower reaches is dominated by coastal sources rather than fluvial or pluvial so rainfall data can be

interpolated from nearby sites. The data record available for these gauges is relatively short (< 3 years)

making them unsuitable for long term statistical analysis and rainfall-runoff routing of design events.

However, the rainfall gauge data provides details for recent calibration events since 2007, such as the

November 2009 event.

Based on the available data from Met Éireann and OPW, there are 33 hourly rainfall gauges within UoM18.

Additional detailed hourly rainfall data at Cork Airport (3904) and Roches Point (1004) will be used to

supplement and validate the rainfall data in UoM18, in conjunction with the daily rainfall gauges.

The preliminary meteorological analysis found a number of gaps in the data records at Fermoy,

Freemount, Kanturk, Mallow and Youghal gauges (3606, 5806, 1406, 6606 and 4106) particularly during

summer months. However, it is not expected that this will impact the hydrological analysis significantly as

most flood events occur in the winter months (October to March). Appendix A provides a summary of the

key rainfall gauges in the catchment.

Radar analysis is not necessarily appropriate because the accuracy of radar will be limited by the rain-

shadow effect in the mountainous areas and the distance from the Shannon radar station. It was agreed

with OPW that the daily storage gauges and river gauges within the catchments would be representative of

conditions on the ground. Therefore, radar data has not been considered further in the hydrological

analysis.



Map 3.2: Available Meteorological Data



24

3.4 Coastal Data

The locations of tidal gauges, extreme water sea level points and extreme wave condition points with

available coastal data in and near to the catchment are shown in Map 3.3.

Sea level data is also available at Ballycotton gauge since 2007. The data record was checked for

erroneous or poor quality data such as shifts in the datum, anomalous spikes and capping. There was

minor variation in the peak tide level and low tide levels, probably as a result of the gauging equipment and

variable atmospheric influences. The oscillation was within a 0.1 m tolerance and deemed suitable for

analysis. Therefore the Ballycotton gauge was deemed suitable to inform the total tide plus surge levels for

historic events since 2007. The Admiralty predicted tide level will be used to inform the astronomic tide

level for events prior to 2007.

The Irish Coastal Protection Strategy Study (ICPSS) data at point S_31 has been approved by OPW for

use directly as the coastal boundary to the Lower Blackwater and Youghal models. The extreme sea levels

will be used to define the magnitude of the tidal events along the coast for all AFAs. The hydraulic model of

the lower Blackwater will account for tidal influence when modelling fluvial events.

The Irish Coastal Water Level and Wave Study (ICWWS) also provide extreme wave heights, wave

periods and mean wave direction for those areas highlighted red in Map 3.3. The ICWWS data for Youghal

Harbour has been available for this report and covers the open coastline from Claycastle to Youghal

Mudlands.



Map 3.3: Available Coastal Data



26

4.1 Historical Flood Events

Table 4.1 summarises the source, extent and impact of flooding for the historic events identified where

sufficient evidence was available. Historic flood events in UoM22 were identified from the floods database

(www.floods.ie), previous reports, and interviews with Local Authority personnel and residents during the

Flood Risk Review. There were limited details available for historic flood events as detailed records of

impacts for events more than 20 years ago were scarce.

Flood Event of 17th of October 2012 affecting Youghal

The most recent flooding to take place in UoM18 was an extreme storm surge along the south west coast

of Ireland which inundated parts of Youghal town centre including areas along the quayside and adjacent

streets. An elevated water level of 2.6mODM due to high tide and 0.82m storm surge (as recorded at

Ballycotton gauge) spilled over the quayside walls and flowed down the roads. Approximately 30 to 40

properties would have been flooded if the property owners had not deployed individual flood gates or

sandbags1. Interviews with the town council indicated that flooding also occurs on Graffan Street, Market

Street and Catherine Street every one to two years.

Flood Event of 4th of August 2012 affecting Rathcormac

Flash-flooding in Rathcormac was caused by intense rainfall on 24th August 2012 between 2:00 am and

4:00 am. Intense rainfall fell, exceeding the capacity of the Shanowen River and a tributary through the

town which overtopped its banks at the junction to the north west of the School and flowed down the

surrounding roads. Subsequent investigations by Cork County Council found a major blockage on the

buried culvert between the Garda station and the graveyard outfall. This was subsequently removed.

However, the local engineer indicated that there is still a capacity issue with the Shanowen Stream that

causes the banks to overtop at a number of locations affecting the north of the village.

Flood Event 19th November 2009 affecting Kanturk, Mallow, Fermoy and Ballyduff

Widespread flooding occurred across the River Blackwater and River Bride catchments in the November

2009 event as a result of prolonged rainfall on already saturated catchments. This was the first significant

flood since the completion of phase 1 of the Mallow flood defence works. The Town Park and Mallow

Racecourse areas were flooded which closed the major road N72 for several days. A total of 8 properties

(7 residential and 1 commercial) were affected by flooding2. At Fermoy, flooding affected both banks of the

river, flooding a total of 22 residential properties, 16 commercial premises along Ashe Quay and O’Neill-

Crowley Quay.3 Access to the Hospital was also affected by flooding. There was also flooding of the N72 at

Mallow and Killavullen. Figure 4.1 shows the progression of the 2009 flood along the catchment which

generally increases down the catchment.

1 Mott MacDonald ( 2012) Flood Event Data Collection Youghal 17.10.12 on behalf of the OPW

2 Arup (2003) Munster Blackwater (Mallow) Drainage Scheme, Hydrology and Hydraulic Modelling Report.

3 Jacobs Babtie (2003) Munster Blackwater River (Fermoy) Drainage Scheme, Hydrology Report.

4 Historical Flood Review



Table 4.1: Key Historical Flood Events

Date Flooding Mechanisms Areas Affected Properties Flooded Reported Duration of Flooding (Hours)

02/11/1980 Fluvial flooding along the Blackwater overtopping river banks

Mallow: Navigation Road, Bridge Street, Ward Terrace and Town Park

Not reported, Estimated to be over 100

48 hours


Mallow: Bridge Street and Town Park Over 70 properties flooded 32 hours

22/10/1988 No flooding reported but high flows recorded at gauge


26/08/1997 Flash flooding along the Freemount Stream combined with blockage at key culverts.

Freemount: R578 and Main Street

Estimated 20 properties flooded < 2 hours



06/11/2000 Fluvial flooding on an already saturated catchment

Mallow: Racecourse, Bridge Street and Town Park

Over 70 properties flooded 48 hours

27/10/2004 Tidal and fluvial flooding. The high tide overtopped quay walls in Youghal and high

river flow from the heavy rain flooded the Allow and Blackwater.

Youghal: Quayside, The Mall, Market Place and Catherine Street

Kanturk: Brogeen

40 residential properties

None reported

14 hours


Mallow: Bridge Street, Park Street and Meadowlands

Fermoy

Over 70 properties flooded

30/01/2009 Fluvial event due to intense rainfall overtopping banks by the National Primary School

Rathcormac: Main Street 1 residential property and 1 commercial

< 2 hours

19/11/2009 Fluvial event exacerbated by saturated catchment conditions led to prolong flooding

across the catchment.

Limited properties were affected due to the completion of the Mallow and Fermoy schemes

Mallow: Town Park area

Fermoy: Town Bridge and right bank

Killavullen: Fields flooded

Ballyduff: fields flooded

7 residential , 1 commercial

22 residential 16 commercial

None reported

None reported

17 hours +

04/08/2012 Fluvial event due to intense rainfall overtopping banks by the National Primary School

Rathcormac: Main Street 1 residential property and 1 commercial

< 2 hours

17/10/2012 Tidal flooding from storm surge overtopped quay walls.

Youghal: Quayside, The Mall, Market Place and Catherine Street

1 commercial, no residential because individual flood protection

measures in place (up to 40 if protection not in place)

14 hours



Figure 4.1: Progression of the November 2009 Flood

0

100

200

300

400

500

600

700

18/11/2009

19/11/2009

20/11/2009

21/11/2009

22/11/2009

23/11/2009

24/11/2009

25/11/2009Fl

ow (m

3/s)

18006 CSET MALLOW FLOW Revised Rating 18055 MALLOW RAILWAY BR FLOW OPW Rating

18003 KILLAVULLEN FLOW Revised Rating 18107 FERMOY DS FLOW MM Applied Rating

18002 BALLYDUFF FLOW Revised Rating



29

Flood Event of 10th January 2008

The Mallow, Fermoy and Ballyduff gauges record a high flow on the 10th January 2008 along the

Blackwater. Aerial photographs indicated flooding across Mallow Town Park to Lidl and flooding along

Meadowlands prior to the completion of the Mallow Drainage scheme. The flood risk to these areas has

since been reduced due to the installation of the embankments and flood walls as part of the scheme.

Flood Event of 27th October 2004 affecting Youghal

Youghal was flooded on 27th October 2004 by a combination of extreme high tides and an extreme storm

surge (1.5m over the predicted astronomical high tide level). Extreme waves damaged and overtopped the

sea defences at Youghal, which were known to be in a poor condition at the time. The beach at Front

Strand and Claycastle to the rear flooded with much of Youghal’s main harbour area inundated with flood

waters, flooding Catherine Street, Market Square and reaching up to North Main Street. Water levels were

at their highest at Barry’s Lane and Youghal Fire Station. Property flooding was reported, but the final

damage estimates are unknown.

The heavy rainfall also resulted in high flows along the Blackwater and Allow. However no property

flooding was reported in Mallow or Fermoy. The Kanturk area was affected when flooding occurred

downstream of the main town, due to the overtopping of the Brogeen River.

Flooding of 6th November 2000 affecting Dromcummer, Mallow, Fermoy and Ballyduff

Two large flood events occurred on the 5th and 6

th November 2000. The second event caused flooded as

the catchment was already saturated and river levels were high from the day before. Water depths rose to

2.8m early on the 6th of November in Mallow Town Park. Significant flooding was also reported at the Race

Course, Bridge Street and along Park Road. Extensive areas of Blackwater valley were flooded from

Dromcummer to Fermoy and Ballyduff as shown in the aerial videos taken just after the flood event. The

video extents have been visually inspected and used to verify with the hydraulic model.

Flood Event of 26th August 1997 affecting Freemount

Freemount was flooded due to intense rainfall falling over a relatively short period. Between the hours of

6:00PM and 11:00PM over 90mm of rain fell, exceeding the capacity of a tributary to the River Allow,

known locally as ‘Freemount Stream’. A considerable amount of debris was moved by the high flows which

blocked the four main culverts towards the east of the village, resulting in a flood depth of up to 1 m at the

right bank. Excess flood waters flowed down Main Street and an estimated IR£210,000 worth of damage to

private property (houses, cars, gardens) and a further IR£15,000 cost of cleanup (Cork County Council,

1997).4

4 Cork County Council (1997) Freemount Flood Report [online] www.floodmaps.ie



30

Flood Event of 22nd

of October 1988 affecting Mallow and Fermoy

The floods that hit Fermoy and Mallow during October 1988 were due to heavy rainfall after a period of

consistently high rainfall events in the weeks preceding. With over 40mm of rain falling over the 21st

October, Mallow Town Park alongside the River Blackwater was flooded by 11:30 AM and Bridge Street

began to flood by 5:45 PM. Later in the day, flooding began at Ballyhadeen (07:15 PM) and Broom Lane

(07:30 PM). The maximum flood depth occurred at Bridge Street, with a depth of 1.6 m. The flood water

receded by 06:00 PM the next day. At Fermoy, the left bank at Fermoy Bridge to Brian Boru Square,

Frances Street and Rathealy Road flooded. Flooding also occurred to the south at Ashe Quay and O’Neil

Crowley Quay. The flood is ranked the 3rd

largest on record according to the available data at both Mallow

and Fermoy.

Flood Event of 6th August 1986 affecting Kanturk

On this date, flash flooding occurred throughout County Cork, Kanturk suffered a large flood in which the

River Allow and the River Dalua overtopped the river banks, causing flooding to properties around Market

Square. Mallow town was also affected with the areas around Mallow Town Park flooding.

Flood Event of the 2nd

of November 1980 affecting Kanturk, Mallow and Fermoy

The 1980 flood is the largest flood recorded at Mallow. At 11:30 AM the water level rapidly rose to flood

Mallow town centre at around 12:00 Noon. Peak level occurred at 03:00PM and at this point it was not

possible to pass Mallow Town Bridge. Bridge Street was flooded with water up to 2.5m deep and the

flooding had subsided within 24 hours (ARUP, 2002).5 Fermoy was also flooded along the north and south

banks at Ashe and O’Neill Quays, although more detailed information on the properties flooded was not

available.

Further flooding at Kanturk was caused by the overtopping of the River Allow and Dalua, a short distance

to the south of the confluence, and the western areas of town flooded to a maximum depth of 2m. An

estimated IR£370,000 of damage was caused with 178 houses affected. R579 Strand Street was also

flooded.

Other Recurring Events

The floodmaps.ie website also detailed a number of other recurring flood events without specific dates:

Kanturk – Recurring flooding from the Dalua at Town Park and the R578 twice a year.

Kanturk – Recurring flooding from the Allow at Strand Street.

Tallow – Recurring flooding from the River Bride due to a combination of high tides and heavy rainfall.

5 ARUP (2002) River Blackwater (Mallow) Drainage Scheme [online] www.floodmaps.ie



31

4.2 Historical Flood Mechanisms

Following the review of the historic reports and other data, the key flood mechanisms identified in UoM18

include:

Fluvial or river flooding: Fluvial flooding can occur when the capacity of the river channel is exceeded

due to excess flow from heavy rainfall or releases from reservoirs upstream. Flood waters typically

overtop river banks at low sections or where water is constricted by bridges or culverts forcing water

levels to rise upstream and flood surrounding areas. Most of the flooding reported in UoM18 is

attributed to fluvial flooding mechanisms.

Pluvial or surface water flooding: Pluvial flooding can occur when overland flow from intense rainfall

or prolonged heavy rainfall is unable to enter the urban drainage network or river channel either

because they are already full or there is a blockage. Pluvial flooding is exacerbated by the increase of

impermeable areas (such as concrete or tarmac) associated with urbanisation which increases the

amount of overland flow. The most recent flooding in Rathcormac was partly attributed to pluvial

flooding. It should be noted that the study of pluvial flooding is not included in the scope of the CFRAM

Study.

Coastal or tidal flooding: Extreme sea levels, waves and storm surges overtop coastal defences and

river banks in tidally influenced reaches, particularly when combined with high river flows for tidal rivers.

The October 2004 event in Youghal was attributed to wave overtopping and the tide-locking of the

urban tributaries. According to anecdotal evidence Tallow is also at risk from tidal flooding when

combined with high flows on the River Bride.

In addition to the mechanisms listed above, flooding in Ireland can also occur from groundwater.

Groundwater flooding can occur when water levels rise above the ground to flood low-lying fields and

property basements, typically when the catchment is saturated. The onset of flooding is very slow and

therefore hazard to people is limited. The River Funshion and parts of the Blackwater between

Dromcummer and Ballyduff are susceptible to this form of flooding as they are underlain by highly

permeable karstic systems. However, there are no records of groundwater flooding. Hence, groundwater

flooding has been discounted from further analysis. It should be noted that the study of groundwater

flooding is not included in the scope of the CFRAM Study.

Based on the historical flood evidence, the key mechanisms for each of the AFAs are as follows:

Aglish: Flooding occurs at the Ballynaparka Bridge on the Ballnaparka Stream at Aglish, affecting

several nearby properties. The local engineers indicated that such flooding occurs annually although no

specific dates were given. Anecdotal evidence also suggests flooding from the Goish River to

Ballycullane when the River Blackwater is in flood. The key mechanisms will be verified during the

hydraulic modelling analysis.

Ballyduff: Flooding is caused by the overtopping of the River Blackwater flooding and inundates the

surrounding fields. However, it was reported to be contained within the masonry wall on the left bank in

the November 2000 event.

Fermoy: Flooding is caused the overtopping of the River Blackwater along the left bank at Thomas

Street and along right bank, flooding roads near the hospital. Frequent flooding along both the right

and left banks has occurred over recent decades. This has resulted in the development of flood



32

embankments along the left bank and flood walls along the right bank. Previous studies have shown

that the weir and town bridge do not significantly affect flood levels.

Freemount: Flooding is caused by the overtopping of the River Allow and the tributary known as the

River Keen that runs through Freemount affecting low-lying properties. Under-capacity culverts under

Main Street used to result in water flowing down the road and inundating properties. The road culvert

has subsequently been improved and no flooding has been reported since.

Kanturk: Flooding is caused by the overtopping of the rivers particularly at Dalua Footbridge when

flooding on the River Dalua interacts with high flows on the River Allow flooding the town park area.

Mallow: Recurring flooding is caused by the overtopping of the River Blackwater and Spa Glen. The

constriction of flow at the bridges combined with the inflows from Spa Glen causes flood levels to

increase and flood Bridge Street and the park area on the left bank. Rapid runoff and under capacity of

the urban drainage systems can also cause flooding on the various urban tributaries that flow through

Mallow. The frequent flooding of Mallow led to the development of flood defence walls, embankments

and pumping stations at Bridge Street to protect vulnerable properties.

Rathcormac: Flooding occurs when the Shanowen River overtops the river banks at culverted and

bridged sections flooding Main Street. The more recent flood events have been caused by intense

rainfall events and under-capacity urban drainage network.

Tallow: Overtopping of the River Bride at Tallow Bridge is caused by a combination of high tide and

high flows in the River Bride and the Glenaboy River that flows through the town. The hydraulic model

will extend along the Bride and lower Blackwater to fully consider the interaction between the high river

flows and high tidal conditions at Youghal.

Youghal: Primarily coastal flooding from extreme storm surges causes flooding in the Town although

the Claycastle area is at risk from wave overtopping as well. High levels in the Blackwater Estuary can

also prevent discharge from the smaller tributaries causing flooding as water “backs up” behind the tidal

sluices at Youghal Mudlands.

4.3 Historical Flood Frequency Estimates

An estimate has been made of the frequency for the historical flood events where there were recorded

river flows for the AFAs in UoM18. The number of river flow gauges provides sufficient coverage for

estimate of historical floods for the AFAs .The recorded peak flow at the various gauges was compared to

their annual maximum series, and the relative frequency derived using the Gringorten formula:

Where i is the relative rank in the annual maximum flow series (AMAX) and n is the number of values in

the AMAX series. The Gringorten plotting position is the most appropriate plotting formula when

considering the EV1 and GLO distributions. The Gringorten estimate was then reviewed against the

design flows detailed in Chapter 6 of this report to establish the final %AEP estimate (Table 4.2).

12.0

44.0

n

iFi



Table 4.2: Estimation of Flood Frequency for Historical Flood Events with Records of Flooding

AFA/MPW

Nearest Gauging Station Historical Flood Event

Station No. Location Date Peak Flow (m3/s) Rank AEP (%) Comments

Freemount/Allow 18009 Riverview factor to Freemount*

26/08/1997 < 23 1 >50% Localised event

Rainfall indicates a 2%AEP storm event

Rainfall Estimate 41 2%

Kanturk/Allow 18009 Riverview* 06/08/1986 284 2 2%

21/10/1988 187 5 20%

30/12/1998 167 7 20-50%

30/11/2000 165 8 20-50%

01/08/2009 313 1 1.3%

19/11/2009 160 10 20-50%

Mallow/Blackwater 18006 CSET Mallow* 02/11/1980 695 1 0.5-1%

06/08/1986 520 2 5%

21/10/1988 517 3 5%

30/12/1998 444 5 10-20%

06/11/2000 421 9 20%

10/01/2008 436 7 10-20%

19/11/2009 443 6 10-20%

Fermoy/Blackwater 18003 Killavullen factored to Fermoy Bridge*

02/11/1980 946 1 0.5-0.1%

06/08/1986 537 5 10-20%

22/10/1988 644 2 5%

30/12/1998 555 4 10-20%

18107 Fermoy Bridge* 10/01/2008 510 6 20%

19/11/2009 570 3 10%

Ballyduff/ Blackwater 18002 Ballyduff* 03/11/1980 700 6 5-10%

22/10/1988 814 1 2-5%

30/12/1998 571 13 10-20%

06/11/2000 669 8 10%

20/11/2009 734 2 5%



AFA/MPW

Nearest Gauging Station Historical Flood Event

Station No. Location Date Peak Flow (m3/s) Rank AEP (%) Comments

Youghal/Blackwater 19068 Ballycotton 27/10/2004 2.80mODM 1 0.1 Coastal

17/10/2012 2.60mODM 2 1 Coastal

Rathcormac/Shanowen 18001 Mogeely factored to Shanowen Stream in Rathcormac*

30/01/2009 5 2 10%

19/11/2009 6 1 5%

04/08/2012 No Data 1 - Localised rainfall event of similar magnitude to 19/11/2009

Aglish/ Ballynaparka‡ N/A N/A N/A No Data - -

‡ No flood events found on internet, literature and official sources.

* Riverview, CSET Mallow, Killavullen, Ballyduff and Mogeely use the revised rating curves from Chapter 5

N.B. The ranking of events is relative to the record length. However the estimate of %AEP has been adjusted to consider longer records for those gauges with short periods of data such as Fermoy.



35

5.1 Gauge Review Selection

Extreme flood flows can be estimated from recorded water levels at gauging stations where a stage-

discharge relationship is known. Historically, rating curves have been derived from in-bank gaugings and

extrapolated to estimate extreme flood flows. If the gauge is by-passed, the use of an in-bank rating curve

may significantly underestimate flood flows. However, it is not always safe or practical to observe level and

flow during out-of-bank conditions. Therefore, hydraulic modelling has been used in the CFRAM study to

simulate out-of-bank conditions and extend the rating curve for high flows.

A number of key gauges were identified as having significant bypass flow or only low-flow gaugings in

UoM18. A high flows rating review was undertaken at the sites listed in Table 5.1 to improve the estimates

of out-of-bank flows and to update the AMAX series for subsequent use in the derivation of the design

flows.

Table 5.1: Gauges Requiring Rating Reviews

Gauge Name Gauge Number Watercourse Nearest AFA

Mogeely 18001 River Bride Tallow

Ballyduff 18002 River Blackwater Ballyduff

Riverview 18009 River Blackwater Kanturk

Allen’s Bridge 18010 River Dalua Kanturk

The previous flood relief schemes at Mallow and Fermoy had identified atypical progression of flows

between the two towns where flows were recorded as decreasing downstream. Therefore, the high flows

rating curves at four additional gauging stations were also checked to assess the relatively of peak flows in

the AMAX series. These included:

18006 CSET Mallow Gauge on the River Blackwater

18055 Mallow Railway Bridge Gauge on the River Blackwater

18003 Killavullen Gauge on the River Blackwater

18107 Fermoy Bridge Gauge(s) on the River Blackwater

5.2 River Bride at Mogeely (Gauge 18001) High Flows Rating Review

Gauge Description

The gauge at Mogeely is located on the River Bride immediately upstream of Mogeely Bridge on the right-

bank. Water levels are recorded via a stilling well and converted to flow using the existing OPW rating

equation (valid since 01 April 2007). In-bank levels and flows are principally controlled by the bridge

structure and constriction of the channel 15 m downstream of the bridge. Out-of bank flows are constrained

by the valley sides and the road until water levels rise and overtop the road through the gaps, bypassing

the gauge (Figure 5.1). Although the wall along the road has gaps and will therefore not fully prevent flow

over the road, they are taken account of in the model by incorporating a lower spill coefficient to represent

the obstruction they provide.

5 Rating Reviews



36

The OPW has provided 76 spot gaugings of level and flow at the gauge between 1965 and 2009. 44 of

these have been measured to the current gauge datum, and only 5 have been measured since the

application of the latest rating curves. The spot gaugings to the previous gauge datum have not been

considered in this review as the corresponding cross-section elevations were not available. The highest

observed level is 9.7mODM which is below bankfull and approximately 50% of QMED. This warrants a

review of the high flow rating. Figure 5.2, 5.3 and 5.4 analyse the relevant spot gaugings for hysteresis,

seasonality and periodicity. The spot gauging at 16 m3/s and 0.9m was identified as an outlier from the

other spot gaugings although there were no comments in the spot gaugings logs on the reliability of this

measurement. All the other spot gaugings since 1983 are relevant for the current datum of 8.43 mODM

and were used to calibrate the rating curve.

Figure 5.1: Flood Flow Paths During High Flow Conditions at Mogeely Gauge

2.0m Flood Depth

1.0m

0.5m

0.2m

0.0m

Gauge



Figure 5.2: Spot Gauging Hysterisis At Mogeely Figure 5.3: Spot Gauging Seasonality At Mogeely

Figure 5.4: Spot Gauging Over Time Periods At Mogeely

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40 50

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)Rising Falling

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40 50

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)Summer Winter

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40 50

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)1983-1989 1989-1998 1998-1999 1999-2007 2007-2009

Outlier



38

A 1D-2D hydraulic model was developed of the River Allow extending over 1km upstream to consider all

incoming flows and 3km downstream so that the downstream boundary assumptions did not affect the

gauge. Please refer to the map provided in Appendix B showing the model extent. A digital terrain model

of the floodplain was developed using the national SAR DTM adjusted to match the recent river channel

survey in this area. Appendix B provides a geoschematic of the model extent, details of the model

development and assumptions made.

Revised High Flow Rating

The hydraulic parameters, such as the Manning’s ‘n’ values and downstream boundary, were adjusted until

the model matched the spot gaugings thus forming the “design scenario”. The following variations from the

design hydraulic parameters were then used to assess the sensitivity of the rating curve at the gauge:

Increased Manning’s ‘n’ to upper recommended limit

Reduced Manning’s ‘n’ to lower recommended limit

Raised downstream boundary stage-discharge relationship

The model results were converted to relative stage based on the surveyed gauge datum and compared

with the spot gaugings (Figure 5.5). The modelled stage-discharge matched well with the spot gaugings up

to 0.8m stage and was within the scatter of the higher spot gaugings. Therefore, the model was deemed

suitable to estimate out-of-banks flows subject to refinement in the future with high flows gaugings.

Figure 5.5: Revised Rating Curve at Mogeely

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 50 100 150 200

Sta

ge

(m

)

Flow (m3/s)

Spot Gaugings Maximum Recorded Stage Bankfull Level

Existing Rating Design Scenario Increased Manning's N

Raised Downstream Boundary Reduced Manning's N



39

Based on the spot gaugings and model results, it is recommended that the original rating curve be used for

low flows up to 1.84m but the modelled stage-discharge is used to revise the rating curve from 1.84m to

3.27m stage. The modelled stage-discharge was spilt up into 2 segments to represent the following

changes in gradient for out-of-bank flow:

1.844m to 2.985m – out-of-bank flow;

2.986m to 3.271m – significant bypass flow on right bank.

Regression analysis was then carried out for each section to derive the rating curve equation with the best

fit where the correlation coefficients (R2) for all the segments were all greater than 0.999. The resultant

regression curves were then interpolated to find the upper transition stage and rating curve parameters

derived. The revised high flow rating is presented in Table 5.2 as the power law format Q=C(h-e)β where;

Q is discharge;

h is the gauge height of the water surface;

e is the gauge height of zero flow for a control of regular shape, or of effective zero flow control for a

control of irregular shape;

C (constant) is the discharge when the head (h-e) equals 1.0;

β (constant) is the slope of the rating curve when plotted on a log scale (ratio of the horizontal distance

to the vertical distance).

Table 5.2: Mogeely 18001 Recommended Revised Rating Curve Parameters

Segment Lower Limit

(m stage)

Upper Limit

(m stage) C e β

1 (Original Rating 7.1) 0.000 0.562 70.000 0.100 2.360

2 (Original Rating 7.2) 0.563 1.756 17.500 -0.200 1.600

3 1.757 2.985 15.361 0.019 2.180

4 2.986 3.271 35.820 0.890 2.060

Bold denotes recommended changes to the existing rating

The resultant rating curve is provided in Table 5.3. Further details on the modelling decisions and rating

development can be found in Appendix B.



40

Table 5.3: Mogeely 18001 Recommended Revised Rating Curve

Stage (m above Gauge Datum) Flow (m3/s)

0.15 0.06

0.30 1.57

0.40 4.08

0.50 8.05

0.60 12.25

0.70 14.79

0.80 17.50

0.90 20.38

1.00 23.43

1.10 26.63

1.20 29.98

1.30 33.48

1.40 37.12

1.50 40.90

1.60 44.82

1.70 48.87

1.80 54.06

1.90 60.90

2.00 68.18

2.10 75.90

2.20 84.08

2.30 92.71

2.40 101.80

2.50 111.35

2.60 121.37

2.70 131.86

2.80 142.82

2.90 154.25

3.00 166.78

3.10 183.47

3.20 200.99

3.27 213.73

Note: Revised rating curve shown for modelled range 0.15m to 3.27m only

Figure 5.6 displays the suggested updated AMAX series based on the revised rating curve above.



41

Figure 5.6: Mogeely 18001 Recommended Updated AMAX Series

5.3 River Blackwater at Ballyduff (Gauge 18002) High Flows Rating Review

Gauge Description

The gauge at Ballyduff is located on the River Blackwater immediately downstream of Ballyduff Bridge on

the left-bank. Water levels are recorded via a stilling well and converted to flow using the existing OPW

rating equation (valid since 01 March 1999). The Blackwater at Ballyduff has a very flat water level profile

and levels are controlled by local bed levels at low flows and Lismore Weir, 8 km downstream, during flood

conditions. Interviews with the local County Waterford engineers indicated that the site could also become

tide locked and flooding is likely with high tide combined with high flows from Killavullen. Out-of bank flows

are constrained within the narrow valley and the raised road to the north of the Bridge acts as a barrier to

flows above 11mODM. The flat gradient along this reach means that any flooding downstream towards

Lismore limits the volume available at Ballyduff and can further constrain flows.

The OPW has provided 121 spot gaugings of level and flow at the gauge between 1948 and 2010.

However, only 25 of these have been measured to the current gauge datum and during the period of the

latest rating equation. The highest observed flow of 303m3/s is approximately at bankfull. However, the

maximum recorded level in 2009 is 0.9m above this, warranting a review of the high flow rating. Figure

5.7, 5.6 and 5.9 analyse the relevant spot gaugings for hysteresis, seasonality and periodicity.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

120.0

130.0

140.0

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

2001

2003

2005

2007

2009

AM

AX

Flo

w (m

3/s)

Year

Original Updated

Previous QMED (76.1 m3/s)

Updated QMED (84.9 m3/s)



Figure 5.7: Spot Gauging Hysterisis At Ballyduff Figure 5.8: Spot Gauging Seasonality At Ballyduff

Figure 5.9: Spot Gauging Over Time Periods at Ballyduff

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100 150 200 250 300 350

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)Faling Rising

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100 150 200 250 300 350

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)Summer Winter

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100 150 200 250 300 350

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)1971-1994 1948-1971 1994-1999 1999 - present



43

There was no hysteresis observed in-bank, although reports by local engineers indicated that out-of-bank

flows were dependent on downstream conditions. There were no obvious seasonality trends in the spot

gaugings. The latest spot gaugings have a slightly higher stage due to the changes in gauge datum and

the reprofiling of the nearby channel sections over time. There is only one high flows spot gauging near

bankfull. However, the OPW hydrometrics team confirm this observation was reliable and no issues were

recorded at the time of the observation. Therefore, the highest spot gauging of 303 m3/s has been used to

calibrate the model.

A 1D-2D hydraulic model was developed of the River Blackwater at Ballyduff extending over 2.5km

upstream to consider all incoming flows and 8km downstream to Lismore Weir which is the controlling

hydraulic structure during high flows. A digital terrain model of the floodplain was developed using latest

LiDAR and the recent river channel survey was used to develop the in-bank model. The watercourses

crossing the floodplain have been represented using 2D breaklines based on survey. Dense vegetation at

field boundaries have been explicitly represented on the floodplain based on OSi mapping and site

observations. Appendix B provides a geoschematic of the model extent, details of the model development

and assumptions made.






Decreased Manning’s ‘n’ to lower recommended limit

Increased downstream boundary stage-discharge relationship (i.e. greater backwater)

The increase in the downstream boundary provides an assessment of any tidal locking at Lismore as

anecdotally reported by the County Engineer.


with the spot gaugings (Figure 5.10). The Manning’s ‘n values were calibrated so that modelled stage-

discharge matched well with the spot gaugings up to bankfull. The model predicted out-of-bank flow above

3.1m stage (10.38mODM) and bypass flow over the road above 3.5m (10.78mODM) stage which

significantly increases the high flow estimates compared to the existing rating.

The decreased Manning’s ‘n’ test significantly increased flow both in-bank and out-of-bank. Therefore the

rating is relatively sensitive to the Manning’s ‘n’ assumed. The increased downstream boundary test did

not differ significantly to the design scenario on the rising limb. However, the raised downstream boundary

caused greater backwater on the falling limb.



Figure 5.10: Revised Rating Curve at Ballyduff

8

8.5

9

9.5

10

10.5

11

11.5

12

0 100 200 300 400 500 600 700

Lev

el

(mO

DM

)

Flow (m3/s)

Design Scenario Decreased Manning's 'n' Increased Manning's 'n'

Increased Downstream Boundary Spot Gaugings Bankfull

Existing Rating

Gauge

11.5mODN Water Level at Gauge





45


low flows up to bankfull, but the modelled stage-discharge is used to revise the rating curve for out-of-bank

flows. The modelled stage-discharge was spilt up into 2 segments:

0.590 to 3.095m – in-bank flow;

3.096m to 6.010m – out-of-bank flow with backwater effect on floodplain.





Q is discharge;







Table 5.4: Ballyduff 18002 Recommended Revised Rating Curve Parameters

Segment

Lower Limit

(m stage) Upper Limit (m

stage) C e β

1 (Existing Rating) 0.590 3.095 40.700 -0.473 1.560

2 3.096 6.010 64.802 1.100 2.199



Table 5.5: Ballyduff 18002 Recommended Revised Rating Curve


0.59 44.84

0.70 52.20

0.80 59.31

0.90 66.74

1.00 74.47

1.10 82.51

1.20 90.83

1.30 99.44

1.40 108.33

1.50 117.49

1.60 126.91

1.70 136.59



46


1.80 146.52

1.90 156.70

2.00 167.12

2.10 177.78

2.20 188.67

2.30 199.80

2.40 211.15

2.50 222.73

2.60 234.53

2.70 246.54

2.80 258.77

2.90 271.21

3.00 283.85

3.10 297.74

3.20 331.48

3.30 367.20

3.40 404.93

3.50 444.67

3.60 486.46

3.70 530.29

3.80 576.20

3.90 624.20

4.00 674.30

4.10 726.51

4.20 780.86

4.30 837.35

4.40 896.00

4.50 956.82

4.60 1019.82

4.70 1085.03

4.80 1152.44

4.90 1222.08

5.00 1293.95

5.10 1368.07

5.20 1444.44

5.30 1523.08

5.40 1604.00

5.50 1687.22

5.60 1772.73



47


5.70 1860.55

5.80 1950.70

5.90 2043.18

6.00 2137.99


Figure 5.11 displays the suggested updated AMAX series based on the revised rating curve above.

Figure 5.11: 18002 Ballyduff Updated AMAX Series

0

100

200

300

400

500

600

700

800

900

19

55

19

57

19

59

19

61

19

63

19

65

19

67

19

69

19

71

19

73

19

75

19

77

19

79

19

81

19

83

19

85

19

87

19

89

19

91

19

93

19

95

19

97

19

99

20

01

20

03

20

05

20

07

20

09

AM

AX

Flo

w (

m3

/s)

Original Revised Rating



48

5.4 River Allow at Riverview (Gauge 18009) High Flows Rating Review

Gauge Description

The gauge at Riverview is located 180m upstream of the ford on the River Allow on the right-bank. The

Water levels are recorded via a stilling well and converted to flow using the existing EPA rating equation

C6 (valid since 12 April 2010). In-bank levels and flows are principally controlled by the ford acting as a

weir downstream (see Appendix B for photographs). Out-of bank flows are constrained by the valley sides

and slope. The bridge downstream and River Blackwater downstream do not cause backwater at this site.

The EPA and Cork County Council have provided spot gaugings of level and flow at the gauge between

1977 and 2012. Over 50 measurements have been observed to the current gauge datum since 2000

permitting good calibration of the rating curve within the observed range. The highest observed flow of

39.3 m3/s is less than 50% of QMED, warranting a review of the high flows rating. Figures 5.12, 5.13 and

5.14 analyse the relevant spot gaugings for hysteresis, seasonality and periodicity.



Figure 5.12: Key Spot Gauging Hysterisis At Riverview Figure 5.13: Key Spot Gauging Seasonality At Riverview

Figure 5.14: Spot Gauging Over Time Periods At Riverview

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)

Rising Falling

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)

Summer Winter

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)

2001-present 1994-2001 1986-1994 1977-1986



50

There was no hysteresis observed in the spot gaugings and no obvious indications of differences in the

stage-discharge relationship between summer and winter months. The latest spot gaugings have a higher

stage due to the changes in gauge datum over time. No information has been provided by the EPA on

datum changes. Site visit observations would suggest this datum change is caused by recalibration of

gauging equipment and minor silt build up behind weir/ford downstream of Riverview gauge. Only those

spot gaugings measured since 2001 were used to calibrate the model as these were consistent with the

current gauge datum and cross-section.

A 1D hydraulic model was developed of the River Allow from the confluence of the Allow and Brogeen to

the confluence of the Allow with the Blackwater at Leaders Bridge. This allows the model to account for all

incoming flows from upstream and any backwater influence downstream. A 1D approach was deemed

appropriate because the floodplain was at or above the bank levels so the velocities across the valley

section can be assumed to be similar. The small ditch on the left bank at 138534, 100845 can be assumed

to become flooded as water level rises because water can enter this feature further upstream. A digital

terrain model of the floodplain was developed using latest LiDAR, and the recent river channel survey was

used to develop the in-bank model. Appendix B provides a geoschematic of the model extent, details of the

model development and assumptions made.







Raised downstream boundary stage-discharge relationship


with the spot gaugings (Figure 5.15). The modelled stage-discharge was within 0.02m of the higher spot

gaugings up to 73.1mODM or 1.32m stage (the highest gauged flow).



51

Figure 5.15: Revised Rating Curve at Riverview

Flow paths estimated from 1D model of

floodplain and DTM


low flows up to 1.91m but the modelled stage-discharge is used to revise the rating curve from 1.91m to

3.73m stage. The modelled stage-discharge was spilt up into 3 segments to represent the following

changes in gradient:

1.991 to 2.812m – High flows in-bank

2.813 to 3.728m – Bypass flow across the floodplain.


fit, where the correlation coefficients (R2) for all the segments were all greater than 0.999. The resultant



Q is discharge;









52

Table 5.6: Riverview 18009 Recommended Revised Rating Curve Parameters

Segment

Lower Limit

(m stage) Upper Limit (m

stage) C e β

1 (Existing Rating) 0.158 1.910 24.3027 0.000 1.926

2 1.911 2.812 20.844 0.000 2.221

3 2.813 3.728 17.702 0.500 2.832



Table 5.7: Riverview 18009 Recommended Revised Rating Curve


0.16 0.70

0.30 2.39

0.40 4.16

0.50 6.40

0.60 9.09

0.70 12.23

0.80 15.81

0.90 19.84

1.00 24.30

1.10 29.20

1.20 34.52

1.30 40.28

1.40 46.45

1.50 53.06

1.60 60.08

1.70 67.51

1.80 75.37

1.90 83.64

2.00 92.32

2.10 101.42

2.20 110.92

2.30 120.83

2.40 131.15

2.50 141.88

2.60 156.07

2.70 175.97

2.80 197.45

2.90 220.58

3.00 245.41



53


3.10 272.00

3.20 300.41

3.30 330.68

3.40 362.88

3.50 397.07

3.60 433.29

3.70 471.61

3.72 480.72


Figure 5.16 displays the revised rating AMAX flows and suggested updated AMAX series for statistical

analysis.

Figure 5.16: Riverview 18009 Updated AMAX Series

0

50

100

150

200

250

300

350

19

82

19

83

19

84

19

85

19

86

19

87

19

88

19

89

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

20

09

20

10

20

11

20

12

AM

AX

Flo

w (

m3/s

)

Previous AMAX (EPA rating C6) Revised AMAX Series



54

5.5 River Dalua at Allen’s Bridge (Gauge 18010) High Flows Rating Review

Gauge Description

The gauge at Allen’s Bridge is located on the River Dalua, 12m upstream of the Bridge on the right-bank.

A smaller tributary, the Rampart Stream, joins the River Dalua 900m downstream of the bridge. However,

any backwater at this confluence is unlikely to affect the gauge as the bed level and water levels are

significantly below the gauge channel section. Moreover, no interaction was observed between the two

channels upstream of the bridge due to higher ground levels on the floodplain separating the two

catchments. Water levels are recorded via a stilling well and converted to flow using the existing EPA

rating equation (valid since 13 September 1997). In-bank levels and flows are principally controlled by the

weir downstream of the bridge, with the bridge openings influencing only the most extreme flows. Flows

are liable to by-pass the bridge before water levels reach the soffit, as the flood waters are able to flow

over the road on the left bank below soffit level.

The OPW has provided 127 spot gaugings of level and flow at the gauge between 1977 and 2009.

However, only 21 of these have been measured to the current gauge datum since 2001. The highest

measured flow of 14 m3/s is below bankfull and the bridge soffit and approximately 30% of QMED. This

warrants a review of the high flow rating. Figures 5.17, 5.18 and 5.19 analyse the relevant spot gaugings

for hysteresis, seasonality and periodicity.

The highest spot gaugings tended to be observed on the rising limb rather than the falling limb of an event

but the spot gaugings did not suggest any hysteresis effect in-bank. There was no discernible difference

between spot gaugings observed in the summer and winter months for in-bank flows. The gauge datum

and channel section has changes less than 0.01m over the past few decades. Therefore, the stage-

discharge relationship from all spot gaugings is fairly consistent over the entire period.

A 1D hydraulic model was developed of the River Dalua from the opening of the floodplain 800m upstream

of the gauge and was extended 950m downstream of the bridge to ensure the model downstream

boundary does not affect the gauge. A 1D approach was deemed appropriate because the floodplain was

at or above the bank levels so the velocities across the valley section can be assumed to be similar. A

digital terrain model of the floodplain was developed using the national SAR DTM. This SAR data was

adjusted to meet the recent river channel survey and then was used to develop extended sections for the

floodplain. Appendix B provides a geoschematic of the model extent, details of the model development and

assumptions made.



Figure 5.17: Spot Gauging Hysteresis At Allen’s Bridge Figure 5.18: Spot Gauging Seasonality At Allen’s Bridge

Figure 5.19: Spot Gauging Over Time Periods At Allen’s Bridge

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10 12 14 16

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)

Rising Falling

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10 12 14 16

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)

Summer Winter

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10 12 14 16

Stag

e (m

abo

ve G

auge

Dat

um)

Flow (m3/s)

1977-1982 1982-1994 1994-1997 1997-2001 2001- present



56




design hydraulic parameters were then used to assess the sensitivity of the rating curve at the gauge

(Figure 5.20):



Raised spill coefficient over the road

Reduced spill coefficient over the road

Raised Weir coefficient

Raised downstream boundary gradient

Figure 5.20: Revised Rating Curve at Allen’s Bridge

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 50 100 150 200 250 300 350

Stag

e (

m a

bo

ve G

auge

Dat

um

)

Flow (m3/s)

Design Scenario Spot GaugingsRaised Downstream Boundary Decreased Manning's NIncreased Weir Coefficient Decreased Spill Coefficient Over RoadIncreased Spill Coefficient Over Road Existing RatingIncreased Manning's N Bankfull



57

The existing rating curve has been derived from the spot gaugings measured at low flows when there is

negligible backwater influences from the weir and bridge structure. The hydraulic modelling results fit well

with the low flow spot gaugings, but there is significant backwater above 1m stage as the constriction of

the bridge and weir becomes more significant. The modelled backwater effect was verified during the

survey and results in lower flows than the previous EPA rating which does not consider backwater. Above

2.75m stage, the gauge is bypassed with flood waters spilling over the road increasing flow significantly

compared to the increase in water level. Figure 5.20 shows limited difference between the design scenario

and the Manning’s ‘n ’and raised downstream boundary sensitivity test which have less than 1m3/s

difference. The spill coefficient over the road sensitivity tests differ from the design above 2.7m stage by

less 15m3/s. However, the model was sensitive to the weir coefficient assumed under the bridge but the

raised weir coefficient did not match the spot gaugings at the low levels.

Based on the spot gaugings and design scenario model results, it is recommended that the original rating

equations be used for low flows up to 0.8m but the modelled stage-discharge is used to revise the rating

curve from 0.81m to 3.55m stage. The modelled stage-discharge was spilt up into segments to represent

the following changes in gradient:

0.806 to 2.754m – Flow influenced by backwater from the bridge and weir structure.

2.755 to 3.553m – Bypass flow spilling over the road.





Q is discharge;







Table 5.8: Allen’s Bridge 18010 Recommended Revised Rating Curve Parameters

Segment Lower Limit (m

stage) Upper Limit (m

stage) C e β

1 (Existing equation 1) 0.21 0.373 106.017 0.000 4.111

2 (Existing equation 2) 0.374 0.805 22.586 0.000 2.544

3 0.806 2.754 19.369 0.000 1.835

4 2.755 3.553 18.245 0.851 2.983





58

Table 5.9: Allen’s Bridge 18010 Recommended Revised Rating Curve


0.21 0.17

0.30 0.75

0.40 2.19

0.50 3.87

0.60 6.16

0.70 9.11

0.80 12.80

0.90 15.96

1.00 19.37

1.10 23.07

1.20 27.07

1.30 31.35

1.40 35.92

1.50 40.77

1.60 45.89

1.70 51.29

1.80 56.97

1.90 62.91

2.00 69.12

2.10 75.60

2.20 82.33

2.30 89.33

2.40 96.59

2.50 104.11

2.60 111.88

2.70 119.90

2.80 133.51

2.90 155.00

3.00 178.67

3.10 204.63

3.20 232.98

3.30 263.83

3.40 297.27

3.50 333.43

3.54 349.44




59

Figure 5.21 displays the updated AMAX flows which have reduced by up to 50% due to the backwater

effect from the bridge downstream.

Figure 5.21: Allen’s Bridge 18010 Updated AMAX Series

5.6 River Blackwater Rating Checks

5.6.1 CSET Mallow 18006

The CSET Mallow gauge is situated by the old sugar factory on the right hand side of the floodplain at the

race course. The previous Mallow Drainage Scheme identified underestimation of high flows above

bankfull. The 1D-2D hydraulic model developed for the flood mapping of Mallow AFA supports this

underestimation. Figure 5.22 compares the existing rating with the Appendix B Mallow Drainage Scheme

rating and the CFRAM study results. Screenshots of the model results and aerial footage of the 2009 event

show flows exiting upstream of the gauge, flowing across the racecourse and re-entering at the

downstream meander. Therefore, the Appendix B Mallow Drainage Scheme rating curve was used to

update the AMAX series and flow series at CSET Mallow (Table 5.10). The resultant AMAX series is

shown in Figure 5.23.

0

20

40

60

80

100

120

140

160

19

81

19

82

19

83

19

84

19

85

19

86

19

87

19

88

19

89

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

20

09

AM

AX

Flo

w (

m3/s

)

Previous AMAX (EPA Rating C4) Updated AMAX (Revised rating)



Figure 5.22: Check of CSET Mallow High Flows Rating



61

Table 5.10: CSET Mallow 18006 Applied Rating Curve Parameters



stage) C e β

1 (Existing equation) 0.30 4.508 24.446 0.000 1.686

2 (Mallow Drainage Scheme/CFRAM Study) 4.509 6.000 38.430 2.070 2.341

Figure 5.23: Updated AMAX Series for CSET Mallow 18006

0

100

200

300

400

500

600

700

800

19

78

19

79

19

80

19

81

19

82

19

83

19

84

19

85

19

86

19

87

19

88

19

89

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

20

09

20

10

20

11

20

12

Flo

w (

m3

/s)

Updated AMAX Series Existing AMAX Series (EPA)



62

5.6.2 Mallow Railway Bridge 18055

The Mallow Railway Bridge gauge is located on the right bank upstream of the Railway Bridge. The

majority of flows are gauged as the railway embankment prevents significant bypassing of the gauge.

Figure 5.24 compares the CFRAM model results with the spot gaugings and existing rating curve. Although

there some scatter in spot gaugings around 47mODM (road level on the right bank), the model results

agree well with the existing rating. Therefore the AMAX series has not been revised at this gauge.

Figure 5.24: Check of Mallow Rail Bridge High Flows Rating

5.6.3 Killavullen 18003

The Killavullen gauge is situated on the upstream face of Killavullen Bridge. The gauge is bypassed when

water overtops the raised road embankment on the left bank. The previous Fermoy Drainage Scheme

(2006) identified underestimation of high flows above bankfull based on section data. The 1D-2D hydraulic

model of Mallow AFA was extended to Killavullen using the latest detailed LIDAR to fully consider

floodplain flows. Figure 5.25 compares the existing rating with the Fermoy Drainage Scheme 2006 rating

and the CFRAM study results. Screenshots of the model results and aerial footage of the 2009 event show

considerable bypass flow on the left bank.

43

43.5

44

44.5

45

45.5

46

46.5

47

47.5

48

0 100 200 300 400 500 600

Leve

l (m

OD

M)

Flow (m3/s)

Spot gaugings Nov 2009 Event Peak Water Level Model Results (1D-2D) Existing Rating



Figure 5.25: Check of Killavullen High Flows Rating



64

The CFRAM model results agree well with the spot gaugings once the unreliable 2008 spot gauging has

been discounted due to issues whilst recording the high flow. The model results support the previous

Fermoy Drainage Scheme high flows above 38mODM. Therefore the Fermoy Drainage Scheme 2006 high

flows rating (Table 5.11) was used to update the AMAX series (Figure 5.26).

Table 5.11: CSET Mallow 18006 Applied Rating Curve Parameters



stage) C e β

1 (Existing equation) 1.381 4.069 36.2 0.54 1.528

2 (Fermoy Drainage Scheme/CFRAM Study) 4.070 - 3.617 0.0036 3.016

Figure 5.26: Updated AMAX Series for Killavullen 18003

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1955 1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011

Existing AMAX Updated AMAX



65

5.6.4 Fermoy Bridge 18106/18107

The 18106 and 18107 gauges at Fermoy Bridge are situated on the right bank upstream of the skew weir

and on the left bank downstream of the skew weir and bridge respectively (Figure 5.27a). The existing

Fermoy Drainage Scheme SOBEK hydraulic model has been converted to a full 1D-2D ISIS-TUFLOW

model to fully consider any floodplain flow for flood mapping. The combination of the skew weir and

arched bridge leads to complex interaction of flows which are simplified in the 1D approach. There is some

uncertainty in the threshold at which the weir drowns out both in the modelling and in the limited spot

gaugings available. Therefore, the analysis has been undertaken at the downstream gauge for the

hydrological analysis.

Figure 5.27 compares the CFRAM study model results with the OPW’s draft rating curve derived from spot

gaugings only. Screenshots of the 2009 event shows no bypassing on the right bank and only limited

bypass flow on the left bank. Based on the spot gaugings and model results, it is recommended that the

modelled stage-discharge was spilt up into segments to represent the following changes in gradient:

0.150m to 4.193m – In-bank flow at the downstream gauge

4.194m to 6.150m – Bypass flow spilling over the road on left bank.




derived. The rating equations applied for the CFRAM Study are presented in Table 5.12.

Table 5.12: Fermoy Bridge 18107 Applied Rating Curve Parameters



stage) C e β

1 0.150 4.193 45.391 -0.300 1.637

2 4.194 6.150 17.812 -0.461 2.208

Figure 5.28 displays the calculated AMAX flows based on the rating equations above.



Figure 5.27: Fermoy Bridge Rating Curve

A: Model Configuration B: Rating Curve



67

Figure 5.28: Fermoy Bridge 18107 Calculated AMAX Series

0

100

200

300

400

500

600

700

2001 2002 2003 2004 2005 2006 2007 2008 2009

Flo

w (

m3/s

)

AMAX Year



68

6.1 Overview

The hydrological approach draws on the data review described in Chapters 3, 4 and 5 of this report and

the latest Flood Studies Update (FSU) guidance. The hydrological analysis to derive design fluvial

hydrographs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1% AEP has been undertaken as follows:

Define the sub-catchments and locations at which to calculate design flows (Section 6.2);

Estimate the index flood flow for the 50% AEP flood (Section 6.3);

Estimate the flood growth curve to derive more extreme flood events (Section 6.3); and

Estimate the typical flood hydrograph shape (Section 6.4).

The hydrological analysis to derive design coastal conditions for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5%

and 0.1% AEP has been undertaken as follows:

Transformation of total tide plus surge levels along the coast to the model downstream extent (Section

6.5.1);

Estimate the typical tide plus surge profile (Section 6.5.1);

Estimate wave overtopping discharges at vulnerable locations (Section 6.5.2).

6.2 Definition of Sub-Catchments

6.2.1 Hydrological Estimation Points

Hydrological estimation points (HEPs) have been chosen at key locations in the River Blackwater

catchment to form the hydraulic model inflows, intermediate target flows for the model to achieve, and

downstream conditions for the model.

The HEPs were identified through a GIS analysis based on the following principles from Section 6.5.3 of

the Generic CFRAM Specification:

A central location within the AFA;

Flow gauging stations used in the hydrological analysis;

Upstream and downstream limits of each hydraulic model reach;

Major confluences which contribute significant flow to the modelled reach; and,

Locations where the physical catchment descriptors (PCD) significantly change from the upstream

catchment i.e. catchment centroid more than 25km away, ±0.15 change in BFI and ±0.07 change in

FARL.

Table 6.1 summarises the selected HEPs prior to hydraulic modelling. Individual maps and catchment

descriptors for each AFA and MPW reach are given in Appendix C.

6 Design Flows



69

Table 6.1: Selected HEPs

HEP Type Number in UoM18

Gauged 9

Model Inflow 48

Downstream target/inflow 6

Target 98

Downstream tidal 1

TOTAL 155

6.2.2 Sub-Catchment Delineation

The River Blackwater catchments were conceptualised into sub-catchments based on the latest Flood

Studies Update (FSU) database (supplied 2011). Map 6.1 displays the three key sub-catchments.

GIS spatial analysis was undertaken on the national digital elevation model to determine slope aspect and

subsequently identify the watersheds for each catchment. The output from this GIS analysis was compared

with the automated FSU catchment boundaries and verified against manual interpretation from Ordnance

Survey mapping at 1:50,000 scale, previous hydrological reports, and observations from site visits. The

other physical catchment descriptors were also reviewed including; average slope (S1085); average

rainfall (SAAR); runoff indicators (SPR); permeability indicators (BFI); and attenuation (FARL). Information

from the Geological Survey of Ireland (GSI) was also used to assess the impact of underlying geology and

aquifers on permeability and groundwater dominance, as well as to inform those catchments influenced by

karstic systems.

Overall, the automated FSU catchment boundaries were found to match the Ordnance Survey mapping

well in areas of steep relief. However, where the terrain is flatter and the watershed less distinct, there

were some discrepancies between the FSU catchments, those derived from OSI mapping and the more

detailed 5m resolution national DTM (see Map 6.2). Therefore, the boundaries were modified and the

revisions adopted. However, these modifications were minor, were less than 1km2 in area and did not

significantly change the parameters for the HPW and MPWs reaches assessed as part of this CFRAM

study, nor the area draining into the neighbouring river basin district.



Map 6.1: Sub-Catchments

Allow Catchment

Blackwater Catchment

Bride Catchment



71

Map 6.2: Example Catchment Boundary Modification, Sruhaneballiv Stream

The other physical catchment descriptors were also reviewed including; average slope (S1805); average

rainfall (SAAR); runoff indicators (SPR); permeability indicators (BFI); and attenuation (FARL). Information

from the Geological Survey of Ireland (GSI) was also used to assess the impact of underlying geology and

aquifers on permeability and groundwater dominance, as well as inform those catchments influenced by

karstic systems.

Analysis of the catchment parameters for UoM18 indicates that:

The upper catchments of the Upper Blackwater, River Allow, River Dalua all have low BFI indicating

lower permeability and a faster hydrograph response to rainfall in the North West of UoM18.

Catchments to the north of the River Blackwater have a higher BFI value indicating much higher

permeability and a slower hydrograph response to rainfall.

The River Awbeg Minor, Awbeg Major and Funshion to the north of Mallow and Fermoy are underlain

by karst, and these rivers are spring fed in their upper reaches indicating groundwater dominance for

low flows.

The highest standard average rainfall is in the west and north east of the Blackwater catchment but the

Awbeg catchment has the lowest rainfall as it partly falls in a rainshadow effect from the western

mountainous areas.

All the modifications made to the original FSU database are provided in Appendix C.



72

6.3 Flood Frequency Analysis

6.3.1 Methodology

Flood frequency analysis was undertaken at gauged and ungauged sites to derive the design fluvial

hydrographs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1% AEP events as boundary conditions for

the hydraulic modelling.

Gauged Sites

The index flood flow was derived from the median value of the Annual Maximum Flood Series (AMAX) at

gauged sites within or linked to the AFAs, and compared with the FSU 7-variable QMED rural estimate

(FSU WP2.3). Previous research by the FSR indicated that the index flood is proportional to AREA0.77

.

This relationship was applied as a check to identify atypical QMED flows for catchment size.

The Extreme Values (EV1), logistic (LO), generalised logistic (GLO) and generalised extreme value (GEV)

distributions were then fitted to the AMAX series to establish the most appropriate flood growth curve for

%AEP up to twice the record length at the gauged location (FSU WP2.2). For rarer, more extreme events,

hydrologically similar gauge sites were selected to form a pooling group based on the Euclidian distance

measure (dij) between catchment characteristics at the gauged site. Descriptors considered include AREA,

SAAR, BFISOILS, the ratio of the highest gauged flow to QMED, the presence of underlying karstic

features and any issues highlighted by the OPW hydrometric team.

It was not always possible to find sufficient pooling sites of a similar size, BFI and SAAR, and the selection

criteria had to be relaxed in order to achieve the target record length of 500 years ( 5 times the target

1%AEP). The selection of the pooling group was a balance between selecting hydraulically similar sites,

maintaining homogeneity across the group and achieving the required record length. The pooled L-

Moment average for each pooling group was then compared with the various distributions to guide the

selection of the most appropriate flood growth curve.

Ungauged Sites

At ungauged locations, the QMEDrural values were estimated using the 7 variable equation (FSU WP 2.3)

based on gauged data from 190 sites across Ireland:

408.0

185.0341.0217.2306.1922.0937.05

)21(

108510237.1

ARTDRAIN

SDRAINDFARLSAARBFISOILSAREAQMEDrural



73

Where:

AREA is the total contributing area of the catchment

BFISOILS is an index of permeability

SAAR is the Standard Annual Average Rainfall between 1961 and 1990

FARL is an index of floodplain attenuation

S1085 is the typical slope between 10% and 85% along the river reach

ARTDRAIN2 is a proportion of the catchment which is artificially drained.

Pivotal gauged sites were then used to adjust the QMEDrural as recommended by FSU WP 2.3. The pivotal

gauged sites were selected from hydrologically similar gauges across Ireland with a preference for

geographically close locations to better represent rainfall characteristics in the South West area.

Hydrological similarity was guided by the similarity of physical catchment descriptors based on FSU

hydrological guidelines:

Area of pivotal site within a factor of 5 of the target ungauged HEP;

BFI soils index within 0.18 of the target ungauged HEP;

SAAR within a factor of 1.25 of the target ungauged HEP;

FARL within 0.05 of the target ungauged HEP.

Grade A gauges were assumed to be of reliable quality unless otherwise stated by the FSU report. Grade

B gauges were further assessed for the presence of lakes/reservoirs, significant karstified features and

FSU quality of the gauge, to ensure the gauge was suitable to inform the adjustment of QMED at the

ungauged target HEP.

The pooled analysis was used to derive appropriate flood growth curves for all ungauged sites. The

pooling group AMAX data was collated to create a combined record length of 500 years, which is in

accordance with the 5T rule of five times the record length of the target design event, i.e. the 1 in 100 year

or 1%AEP event. The criteria were lowered for selection of pooling group sites in the smaller tributary

catchments of Rathcormac, Fermoy, Aglish and Ballyduff in order to achieve a balance between finding

hydrologically similar sites and achieving the 500 years pooled record length from the target 1%AEP.

The pooling group was reviewed for gauges influenced by karstic geology based on the Geological Survey

of Ireland data and compared with the BFIsoils parameter. Sites influenced by karst were not necessarily

rejected as HEPs on the Blackwater, Awbeg, Funshion and Araglin are also karstic. However, gauges

19001, 19031, 21004 and 22009 were also rejected from pooling analysis due to the OPW’s hydrometric

team’s concerns with the estimation of high flows at these sites. The pooled L-Moment average for each

pooling group was used to identify discordant sites and select the most appropriate statistical distribution.



74

Alternative approaches considered for ungauged and smaller catchments

It should be noted that the FSU 7 variable equation was not developed for catchments less than 5km2 in

size due to the lack of reliable gauge records for such small catchments in Ireland. Alternative methods,

including the rational method, were found to better represent small catchments on average but tended to

over predict peak flows for small lowland catchments (Institute of Hydrology 1978).The modified rational

method (1981) is also not suitable to estimate greenfield runoff as it was developed specifically for sewer

design. The consensus from an exhaustive literature review was that it was not possible to verify the most

appropriate methodology without gauged records.

6.3.2 Estimation of the Index Flood

The index flood flow was derived from the median value of the Annual Maximum Flood Series (AMAX) as

provided by OPW at gauged locations (Table 6.2). The revised AMAX series at Mogeely, Ballyduff,

Fermoy, Killavullen, CSET Mallow, Riverview and Allen’s Bridge were used to derive the revised QMED

flow.

Table 6.2: Gauged QMED Values

Gauge ID Name Watercourse

AMAX Series Length (Years)

QMED Since 2000

(Wet Period)

QMED for Full Record

Adjustment Factor to

Full Record Selected

QMED

18001 Mogeely Bride 41 96.8 84.9 0.88 84

18002 Ballyduff Blackwater 57 461.0 404.8 0.88 404.8

18107 Fermoy Blackwater 11 397.5 N/A 0.93Killavullen 369.6

18003 Killavullen Blackwater 28 362.2 337.4 0.93 337.4

18055 Mallow Rail Bridge

Blackwater 11 355.7 N/A 0.93Killavullen 330.7

18006 CSET Mallow Blackwater 35 305.2 302.2 0.99 302.2

18048 Dromcummer Blackwater 24 N/A 220.0up to 2002 1.18Riverview

Factor Reversed 259.6

18009 Riverview Allow 30 136.5 115.6 0.84 115.6

18010 Allen’s Bridge Dalua 27 46.7 46.9 1.01

46.9

The AMAX series at Fermoy and Mallow Rail Bridge gauges were only available since 2000. This

represents a wetter period in the flow and rainfall records at the longer term gauges, i.e. CSET Mallow,

Killavullen and Ballyduff. Therefore, the QMED at Fermoy and Mallow Rail Bridge were adjusted based on

the difference recorded at the longer term gauges (Table 6.2). Conversely the AMAX series for

Dromcummer was not available since 2002 thus underestimating QMED as it misses the wetter years and

significant 2009 event. Therefore, Dromcummer was adjusted to account for the wetter period based on

the reversed factor at the nearby Riverview gauge.



75

For ungauged sites, the QMED was calculated using the FSU 7 variable approach and adjusted using

pivotal sites. The aforementioned gauges within the catchment were typically used as pivotal sites in the

Blackwater catchment as they were hydrologically similar. Different pivotal sites were used to adjust QMED

for the smaller ungauged tributaries in Mallow and Fermoy based on more hydrologically similar pivotal

sites and the previous Mallow Drainage Scheme flows.

Figure 6.1 provides a summary of the progression of QMED through the Blackwater catchment. The details

of the selected pivotal sites, QMED estimate and schematics for all HEPs are provided in Appendix D

along with the 95th percentile upper limit. The upper confidence limit of QMED (95

th percentile) has been

calculated for each HEP based on the factorial standard error of 1.37 (see WP 2.3).

QMED was checked to ensure the index flow value increased downstream with contributing area. The

increase in QMED flow between Mallow and Fermoy is relatively small compared to the increase in area.

This is the result of floodplain attenuation as water spill out-of-bank for flows around QMED. The recorded

QMED values at gauges were indexed to A0.77

/10 and factors were typically found to be between 10 and

14 at gauges across UoM18 and between 8 and 22 across the South West region. The ground-water

dominated tributaries of the Awbeg, Funshion and Araglin tended to have lower ratios due to the karstic

influence. The smaller tributaries in all the AFAs had lower ratios because the trends at the gauges with

much larger catchments do not necessarily reflect the hydrology in the smaller flashy catchments.

However, the FSU estimate was used to maintain consistency of approach across the study and

progression of flows along the catchment.



Figure 6.1: Summary Schematic of QMED for the Blackwater Catchment



77

6.3.3 Derivation of Flood Growth Curves

Flood frequency analysis was undertaken on the revised AMAX series at the gauges along the modelled

reach and used to derive appropriate flood growth curves as set out in the FSU methodology (Section

6.3.1). Pooling groups were derived and used to extend beyond the twice the record length for ungauged

catchments. The full pooling group details used to derive the flood growth curves are provided in

Appendix D.

Table 6.3 summarises the key flood growth curves selected at gauges along the modelled reach. The

single site EV1, LO and LN3 distributions were found to best fit the gauged data within the gauged record

and the pooled GLO distribution was typically found to be the best fit to extend the flood growth curve to

the 0.5%AEP and 0.1%AEP events.

Table 6.3: Selected Flood Growth Curves at Gauges in UoM18

Selected Flood Growth Curve

Flood Growth Factor for %AEP

ID Name 50% 20% 10% 5% 2% 1% 0.5% 0.1%

18001 Mogeely LO Single/GLO Pooled 1.00 1.22 1.35 1.46 1.61 2.05 2.30 3.03

18002 Ballyduff EV1 Single 1.00 1.35 1.58 1.79 2.08 2.29 2.50 3.00

18107 Fermoy EV1 Single 1.00 1.35 1.58 1.8 2.09 2.31 2.52 3.02

18003 Killavullen EV1 Single 1.00 1.35 1.58 1.80 2.09 2.31 2.52 3.01

18055 Mallow Rail Bridge

LN3 Single Site/Mallow Drainage Scheme 1.00 1.37 1.56 1.75 2.02 2.23 2.44 3.00

18006 CSET Mallow 1.00 1.37 1.56 1.75 2.02 2.23 2.44 3.00

18048 Dromcummer Mallow Drainage Scheme 1.00 1.37 1.56 1.75 2.02 2.23 2.44 3.00

18009 Riverview EV1 Single / GLO Pooled 1.00 1.32 1.54 1.76 2.05 2.28 2.87 3.15

18010 Allen’s Bridge EV1 Single / GLO Pooled 1.00 1.28 1.47 1.66 1.92 2.08 2.34 3.08

There have been a number of recent flood defence schemes in the Blackwater catchment. Therefore, the

design flows at CSET Mallow and Killavullen gauges were derived from the FSU statistical analysis

(Section 6.3.1) and compared with the recent flood defence scheme flows at Mallow and Fermoy to ensure

consistency between the different studies.

Figure 6.2 provides the resultant flood growth curves at CSET Mallow. Figure 6.3 provides the

corresponding L Moment plot of the pooled average. The FSU LN3 single site flood growth curve and

Mallow Drainage Scheme flood growth curve are similar and provide a good fit to the gauged data up to

the 1%AEP. The Mallow Drainage Scheme flow growth curve was found to be appropriate because it was

more consistent with the flood curves at Killavullen, Fermoy and Ballyduff. The Mallow Drainage flood

growth curve was applied as the design flood curve along the Blackwater for the hydrologically similar

reach between Dromcummer and Mallow Rail Bridge to ensure consistency of design flows along the

catchment.



78

Figure 6.4 provides the resultant flood growth curves at Killavullen and Figure 6.5 provides the

corresponding L Moment plot. The EV1 single site flood growth curve was found to best fit the revised

gauge data. The record length at Killavullen is relatively long giving confidence in the estimate for more

extreme %AEP events. The Fermoy Drainage Scheme flood growth curve was very similar to the FSU

single sites analysis only diverging for the most extreme %AEP. The single site EV1 flood growth curve

was applied to the hydrologically similar reach from Killavullen to Fermoy to ensure consistency with the

EV1 flood growth curves selected at Fermoy and Ballyduff.

Table 6.4 summarises the changes in design flows between the CFRAM Study and previous drainage

schemes in Mallow in Fermoy accounting for changes in QMED and flood growth curves.

Table 6.4: Comparison of FSU Design Flows with Scheme Flows in Mallow and Fermoy

%AEP Peak Flows (m3/s)

CSET Mallow

Mallow Drainage Scheme Hydrology Table A11(2003)

CSET Mallow

FSU single site estimate

Killavullen

Fermoy Drainage Scheme Revised Hydrology (2006)

Killavullen

FSU single site estimate

50% 302 302 285 337

20% 413 376 399 420

10% 471 435 480 478

5% 529 500 562 538

2% 609 596 679 627

1% 674 676 771 701

0.5% 736 765 872 784

0.1% 904 1005 1131 1015

Selected Selected

Italics denote interpolated or extrapolated flow not modelled in the previous study.



79

Figure 6.2: Flood Growth Curves at CSET Mallow Gauge (18006)

Figure 6.3: L-Moment Plot for CSET Mallow Gauge (18006)

50% 20% 10% 5% 2% 1% 0.5% 0.1%1

1.5

2

2.5

3

3.5

Flo

od

Gro

wth

Fa

cto

r

%AEP

EV1

LO

LN2

GEV

GLO

LN3

Mallow Scheme FGC

Single Site

AMAX

0.0

0.1

0.1

0.2

0.2

0.3

0.3

0.4

0.4

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

L-K

urt

osi

s

L-Skewness

Series1 Pooled L-Moments LO LN2 EV1 GEV GLO LN3 polynomial Fitted Trendline



80

Figure 6.4: Flood Growth Curves at Killavullen Gauge (18003)

Figure 6.5: L-Moment Plot for Killavullen Gauge (18003)

50% 20% 10% 5% 2% 1% 0.5% 0.1%1

1.5

2

2.5

3

3.5

4

Flo

od

Gro

wth

Fa

cto

r

%AEP

EV1

LO

LN2

GEV

GLO

LN3

Single

Fermoy Scheme FGC

AMAX

0.0

0.1

0.1

0.2

0.2

0.3

0.3

0.4

0.4

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

L-Ku

rtosis

L-Skewness

Series1 Pooled L-Moments LO LN2 EV1 GEV GLO LN3 polynomial Fitted Trendline



81

6.4 Hydrograph Generation

Flood extent, depth, velocity and hazard are governed by the shape and duration of a flood flow

hydrograph as well as the magnitude of the peak flow. Therefore, design inflow hydrographs were derived

at each HEP as follows.

At gauged locations, the hydrograph width analysis approach was used to derive the median flood

hydrograph as the characteristic flood hydrograph for subsequent use in the hydraulic modelling and

development of flood risk management options. The 15 minute flow data was extracted for each of the

AMAX events at each fluvial gauge, standardised by the peak flow, and the width exceedance for each

event derived at specified percentiles of the peak flow. The median of the width exceedance was then

used to compile the design flood hydrograph.

Figure 6.6 shows the progression of the standardised design flood hydrograph shape between CSET

Mallow and Ballyduff on the Blackwater. The flood hydrograph duration increases from 46 hours (< 2 days)

at Mallow to over 100 hours (> 4 days) at Ballyduff. The rising and falling limb also become more

prolonged and less flashy as the flood progresses down the catchment and the flood flows are attenuated

on the floodplain.

The FSU hydrograph pivotal site that best matched the gauged hydrographs was then used to derive the

design flows. Hydrograph pivotal site 25001, 15003 and 16001 were used for Mallow, Killavullen and

Ballyduff respectively as presented in Appendix D.

Figure 6.6: Progression of the Median Flood Hydrograph in the River Blackwater Catchment

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

-50 -40 -30 -20 -10 0 10 20 30 40 50

% o

f Pea

k Fl

ow

Time to Peak Flow (Hours)

18002 Ballyduff Median 18003 Killavullen Median 18006 CSET Mallow Median



82

For the ungauged HEPs, the regression-based UPO-ERR-gamma curve was calculated from the physical

catchment descriptors as detailed in 6.3.1.

The three components of the hydrograph are:

Gamma Curve (Rising Limb) - n

𝑦 = (x + Tr

Tr

)n−1

[𝐸𝑥𝑝 (−x(n − 1)

Tr

)]

Inflection Point (Starting point of Recession Limb) - Tr

𝑥𝑜 =Tr

√𝑛 − 1 𝑦

𝑜= (

𝑥𝑜 + Tr

Tr

)𝑛−1

𝐸𝑥𝑝 (𝑥𝑜(𝑛−1)

𝑇𝑟

)

Exponential Decay Curve (Recession Limb) - C

𝑦 = 𝑦𝑜 𝐸𝑥𝑝 (–𝑥 − 𝑥𝑜

𝐶)

The n, Tr and C parameters were estimated from the physical catchment descriptors for the study area and

were used to derive an initial estimate of the flow hydrograph. The Tr and C values were subsequently

adjusted based on hydrograph pivotal sites from the FSU database. Hydrologically similar sites were

selected based on slope, attenuation and permeability and compared to the target sites catchment area,

SAAR and critical duration to ensure similar responses to rainfall. In some cases, the hydrograph pivotal

site differs to the pivotal site used to adjust QMED because not all gauges were available in the

hydrograph. The recession limb was adjusted where the UPO-ERR estimate was excessive in relation to

catchment area and the FSSR16 time to peak estimate.

Pivotal sites 15003, 35002, 25001, 15006 and 16001 were typically used for the Dalua, Allow, Blackwater

Mallow, Blackwater Killavullen and Blackwater Ballyduff reaches respectively based on the best match to

the gauged median hydrographs. The recession parameter “C” was manually adjusted to 10 for the River

Allow to better match the gauged hydrographs. The C parameter was also adjusted from 5 to1.6 for the

tributaries in Youghal to better match the time to peak suggested by FSSR16.The smaller tributaries, such

as the Knockawillin Stream, tended to better match pivotal site 36021 although more permeable larger

catchments, such as the Funshion, better matched pivotal sites 15005 or 16005.



83

The details of the selected pivotal sites and typical design flood hydrographs for each reach are provided in

Appendix D.

6.5 Coastal Conditions

6.5.1 Total Tide plus Surge Levels

Extreme sea levels around the Irish coastline incorporate both the astronomic tide (caused by planetary

forcing) and storm surge elements (caused by atmospheric pressure), henceforth referred to as “total tide

plus surge levels”. The flood frequency analysis for extreme sea levels has already been undertaken as

part of ICPSS (2012) for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1% AEP events.

Total tide plus surge levels have been derived at Youghal as the tidal outfall of the River Blackwater. There

is no other AFA or MPW affected by coastal conditions in UoM18. In the absence of gauged data, the

CFRAM Study has assumed the same total tide plus surge levels at the tidal outfall of the Blackwater

(211780, 076350) as provided at ICPSS point S31. The resultant total tide plus surge level are

summarised in Chapter 8. The hydraulic model of the Lower Blackwater will be used to transform the tide

plus surge inland.

6.5.2 Design Tidal Curve

The shape of the astronomic curve defines the duration of the rising (flood) and falling (ebb) tide. In deep

water the astronomic curve can be assumed to be largely symmetrical depending on the relative phasing

of the various harmonic components. However, the shoaling of the tide in shallow estuarine areas can

modify the shape.

The admiralty tide tables6 were used to inform time differences in mean high water and low water between

the primary port (Cobh) and the local prediction points at Youghal to modify the astronomic tidal curve.

Storm surges caused by Atlantic storms can often cause elevated sea levels over several diurnal tidal

cycles. Surge residuals were calculated from the tidal gauge data along the south west coast for the most

extreme events (Figure 6.2). It is apparent that larger events tend to have a shorter duration than the

smaller event. The 48 hour duration has been assumed as a credible duration for an extreme surge event

and a symmetrical surge profile assumed in the absence of detailed gauge data at Youghal Harbour itself.

6 United Kingdom Hydrographic Office (2013) Admiralty Tidal Tables Volume 1, 2013.



84

Figure 6.7: Typical Surge Duration in South West Ireland

The design surge profile was then standardised by the peak surge residual and scaled on top of the

astronomic curve to achieve the design extreme sea levels (Figure 6.3). It was assumed that the peak of

the surge and the peak of the spring astronomical high tide coincide. This provided a conservative estimate

of the combined tidal curve. It is recognised that the peak of the astronomic tide does not necessarily

correspond with the peak surge as they are governed by different mechanisms. However, without long

term tidal and surge residual data along the South West coast it is not possible to assess the joint

probability between these two elements

Figure 6.6 displays the combined tidal curves for the design 50%AEP event at Youghal.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120 140 160

Surg

e R

esi

du

al (

m)

Duration above Predicted Tide (Hours)

Clonakilty Temporary Gauge Ballycotton Tidal Gauge



85

Figure 6.8: Example Tide Plus Surge Curve Generation at Youghal

6.5.3 Wave Overtopping

The ICWWS identified three sections of coastline in Youghal that were potentially vulnerable to wave

overtopping from Claycastle to Youghal mudlands (Map 6.3). Reach B and C were further split as part of

this study to account for changes in defence type and orientation to wave attack. There are no other

reaches of coastline in UoM18.

The source-pathway-receptor model can be readily applied to wave overtopping:

Source – wave overtopping volumes based on wave run-up spilling over the coastal frontage

Pathway – flow path of the wave overtopping discharge from the coastal defence to the receptors

considering topography behind the defence.

Receptors – roads, properties, environmental designations etc. affected by the wave overtopping and

their relative location to the wave overtopping.

A screening process was undertaken for the vulnerable reaches and three approaches to assessing wave

overtopping were developed for the CFRAM study:

Wave overtopping unit discharges – the calculation of unit discharge is sufficient to inform flood risk

where wave overtopping volume is insufficient to flow down the backslope of coastal defences or the

water would immediately drain back to the sea due to high relief inland.

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 6 12 18 24 30 36 42 48

Wate

r Leve

l (m

OD

M)

Time (Hours)

Surge Profile scaled Astronomic curve Design Combined Tidal Curve 50% AEP



86

Mapping of wave overtopping volumes – the mapping of total wave overtopping volumes is required

where wave overtopping discharges are able to flow down the backslope of coastal defences to affect

receptors, often in locations where the defences are above the coastal floodplain.

No consideration of wave overtopping – wave overtopping calculations are not required where still

water overtopping (Mechanism1) dominates as the additional volume from wave overtopping can be

considered negligible in comparison with the volume of the incoming tide.

Table 6.4 outlines the approach for each section based on the criteria above.

Table 6.5: Wave Overtopping Approach

Reach Source Pathway Receptors Approach

A Wave overtopping of grassed embankment

Flows away from grass embankment to low-lying areas inland.

Agricultural areas inland.

Mapping of wave overtopping volume for relevant scenarios

B1 Still water overtopping of a vertical concrete wall level with the quayside

Flows across quay and down roads towards Catherine Street.

Road and a few properties adjacent.

Wave volume is negligible so reach not considered in wave overtopping scenarios.

B2 Wave overtopping of a vertical concrete wall above the quayside

Flows across quay and down roads towards Market Square.

Road and a few properties adjacent.


C1 Wave overtopping of a vertical concrete wall above the quayside – similar to reach B2

Flows across down roads and round properties towards Strand Street.

Road and properties adjacent.


C2 Wave overtopping of vertical wall at top of riprap and shingle slope

Flows away from embankment to low-lying areas inland.

Roads, properties and caravans adjacent to Front Strand as well as agricultural areas inland.


The wave overtopping discharges were calculated for the Youghal sections above using empirical

equations of wave run up for simple slopes, composite slopes with walls and vertical walls and general

hydraulic principles to fully account for the transition from the valid limit of the empirical equations

(mechanism 2) to full still water overtopping (mechanism 1).

The six different combinations of total tide plus surge levels and wave heights from the ICWWS were

assessed to find the critical scenario for wave overtopping for each AEP. It should be noted that the

ICWWS uses the total tide plus surge levels from point ICPSS point S35 further along the coast for

computation efficiency. These levels have been combined with different extreme wave heights to represent

the target joint probability at Youghal.

Table 6.5 summarises the critical discharges for the target %AEP events. Reach C2 along Front Strand

was found to be most at risk from wave overtopping. The quayside (reach B10 was at risk from still water

overtopping in all scenarios therefore, further consideration of wave overtopping is not required. Full details

of the analysis for all scenarios can be found in Appendix D.



Map 6.3: Wave Overtopping Sections at Youghal AFA



88

Table 6.6: Critical Wave Overtopping Unit Discharges for Key %AEP

Critical Unit Discharge (l/s/m)

Reach Defence Type Effective Crest Level (mODM)

10%AEP 0.5%AEP 0.1%AEP

A Grassed Embankment

3.93 0.00 0.04 0.29

B1

Concrete vertical walls with no or bypassed walls above quay level

2.59 Still water

overtopping Still water overtopping

Still water overtopping

B2 Concrete vertical walls with walls above quay level

5.16 0.00 0.00 0.00

C1 Concrete vertical walls with walls above quay level

5.23 0.00 0.00 0.00

C2 Shingle and riprap leading to wall

4.73 62.59 185.09 278.76



89

7.1 Calibration Events

7.1.1 Selection of Events

During the Flood Risk Review, historical flood evidence was collated for those events listed in Chapter 4.

Information was gathered from post-flood surveys, aerial footage and anecdotal evidence from local

residents. Table 7.1 scores each of these events based on a number of criteria related to the location,

hydrology and data availability on a scale of 0 to 3 where:

0 is not available

1 is poor or unlikely

2 is fair or possible

3 is good or likely

These scores are then combined to create an indicative calibration confidence score for the available

historical flood evidence in accordance with Guidance Note 237. The following events have been

considered for the calibration of the entire Allow and Blackwater sub-catchments based on the indicative

calibration score:

30th December 1998 – extreme fluvial event along the Blackwater and Allow;

5th/6

th December 2000 – extreme fluvial event along the Blackwater and Allow;

19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow.

The following events have been considered for the calibration of the Bride sub-catchments based on the

indicative calibration score:

30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride;

19th November 2009 – extreme fluvial event along the Bride.

Youghal will also be calibrated for the 17th October 2012 coastal event to calibrate coastal flood

mechanisms in the town.

There was no recorded flood history at Aglish therefore the verification of model parameters will rely on

sensitivity tests. There was insufficient evidence to support full calibration at Freemount as the hydraulic

structures under Main Street have changed since the flooding in 1997 and the current culverts do not

represent the historic conditions. Therefore the modelled outline will be verified taking account of the

historical flood frequency from recurring flooding reports, and sensitivity analysis on the key hydraulic

parameters used in accordance with GN23.

There were insufficient gauge levels, wrack marks or photographs to undertake a full calibration in Tallow.

However, key flow paths will be verified based on the existing flood reports and flood mechanisms

identified by the local engineer in Chapter 4.

7Jacobs, (January 2013) Guidance Note 23 Model Calibration. Version 1.

7 Hydrological Calibration, Sensitivity Testing and Uncertainty



Table 7.1: Selection of Calibration Events

Event AFA/ Watercourse Lik

ely

Ac

cu

racy

of

Flo

w

Es

tim

ate

1

Lik

ely

Ac

cu

racy

of

Ga

ug

ed

L

eve

l

Es

tim

ate

Kn

ow

n

Hy

dra

ulic

Co

nd

itio

ns

2

Lik

ely

Ac

cu

racy

o

f S

po

t

Le

ve

ls3

Re

lia

ble

Flo

od

His

tory

4

Indicative Calibration

Score Calibration Approach

02/11/1980 Mallow/Blackwater

Kanturk/ Allow & Dalua

2 2 1 0 2 7

Significant catchment changes since event makes calibration difficult. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.


Kanturk/ Allow & Dalua

2 2 1 0 2 7



2 2 1 0 2 7


26/08/1997 Freemount/Keen (Freemount Stream)

1 0 1 0 2 4

Culvert changed significantly after this event. Previous culvert dimensions not available. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.


Fermoy /Blackwater 3 3 2 2 3 13

Entire catchment calibration available. Significant catchment changes since event makes calibration difficult. Calibrate main channel to large event data considering spot levels are accurate to within +/- 0.25m. Smaller tributaries should take note of uncertainties due to blockage.


Fermoy

3 3 3 1 3 12

Significant catchment changes since event makes calibration difficult. Extensive outline and photo information but no spot levels. Calibrate main channel to large event data considering that spot levels are derived from extent. Smaller tributaries in Mallow should take note of uncertainties due to blockage.

27/10/2004 Youghal/Coastal

Kanturk/Brogeen N/A for coastal

2 at Kanturk

0 1 1 3 7

Spot levels inferred from flood outline from Council. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.


Fermoy /Blackwater 1 1 1 2 3 8 Gauge data is incomplete for the full event at a number of sites between Mallow and Fermoy and the event is of a similar magnitude as the other selected events.



Event AFA/ Watercourse Lik

ely

Ac

cu

racy

of

Flo

w

Es

tim

ate

1

Lik

ely

Ac

cu

racy

of

Ga

ug

ed

L

eve

l

Es

tim

ate

Kn

ow

n

Hy

dra

ulic

Co

nd

itio

ns

2

Lik

ely

Ac

cu

racy

o

f S

po

t

Le

ve

ls3

Re

lia

ble

Flo

od

His

tory

4

Indicative Calibration

Score Calibration Approach

30/01/2009 Rathcormac/Dromore Stream and School

Stream 2 1 2 1 2 10

Spot levels inferred from flood extent from Council. Calibrate main channel and Rathcormac tributaries to event data and compare with 2012 event spot levels.


Fermoy/Blackwater

Ballyduff/Blackwater

3 3 2 1 3 12

Entire catchment calibration available. Spot level provided but seems to be estimated from flood outlines. Calibrate main channel to large event data. Smaller tributaries in Mallow should take note of uncertainties due to blockage.

04/08/2012 Rathcormac/Dromore Stream

0 0 2 3 3 8

Spot levels and outline surveyed after event but gauge data not available. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.

17/10/2012 Youghal/coastal N/A for coastal

2 3 3 3 11 Level transferred from Ballycotton and Council observations. Calibrate main channel and coastal flood risk to large event data.

Note 1: 3 = gauged flows are available in the catchment, 2 = gauged flows used from pivotal gauges nearby, 1 = rainfall data used to estimate flows and 0= no flow estimate available

Note 2: Hydraulic conditions relate to controls on water levels during a flood e.g. level of blockage, wall collapse etc.

Note 3 Levels during a known flood event NOT at a gauged location that represents a true flood level rather than a localised issue.

Note 4 Any information that includes date/time, precise location and mechanism of flooding.



92

7.1.2 Calibration Hydrology Approach

The prevalence of reliable river gauge data in AFAs means the river gauge data has been used for the

calibration events rather than detailed analysis of the rainfall data for calibration events prior to 2007.

Therefore, the inflows for ungauged tributaries have been transferred from these gauges based on the

relative flood frequency of each event. The phasing of the inflows from the ungauged HEPs will be

adjusted to ensure flood levels are met at the gauged locations.

More detailed rainfall data was available for the 2009 event. However, the prevalence of permeable

catchments across UoM18 made the FSSR16 approach unsuitable for many of the tributaries. The river

flow gauges were deemed to provide a better estimate of catchment hydrology as they account for any

permeable influences within the flow data.

30th December 1998

Figure 7.1 presents the gauged hydrographs throughout the Blackwater sub-catchment for the flood event

on 30th December 1998.

Figure 7.1: Progression of the December 1998 Flood Event

0

100

200

300

400

500

600

700

29

/12

/19

98

00

:00

29

/12

/19

98

12

:00

30

/12

/19

98

00

:00

30

/12

/19

98

12

:00

31

/12

/19

98

00

:00

31

/12

/19

98

12

:00

01

/01

/19

99

00

:00

01

/01

/19

99

12

:00

02

/01

/19

99

00

:00

Flo

w (

m3 /

s)

18003 KILLAVULLEN FLOW Revised Rating 18002 BALLYDUFF FLOW Revised Rating 18006 CSET MALLOW FLOW Revised Rating

18010 ALLENS BRIDGE FLOW Revised Rating 18009 RIVERVIEW FLOW Revised Rating



93

Table 7.2 details the relative flood frequency applied to each model reach for 1998 calibration event. The

Ballycotton tidal gauge was not in operation for this event. Therefore, the tidal conditions have been

derived from the Admiralty tide table for 1998. The resultant total tide plus surge curve is shown in Figure

7.2 and will be applied to the lower Blackwater model at Youghal.

Table 7.2: Calibration Inflows for 30th

December 1998

Sub-catchment Reach Gauge %AEP

Blackwater Blackwater from Allow to Mallow and tributaries

CSET Mallow

(Dromcummer inactive)

15

Blackwater and tributaries through Mallow

CSET Mallow 15

Blackwater and tributaries from Killavullen to Fermoy

Killavullen 10

Awbeg, Funshion and Araglin tributaries

Downing Bridge 15

Blackwater downstream of Araglin to Ballyduff and Lismore

Ballyduff 15

Blackwater downstream of Lismore and tributaries

Ballyduff 15

Blackwater Outfall, Youghal Admiralty Tide Table MHWS

Allow Allow inflow Riverview 10

Figure 7.2: Total Tide Plus Surge at Youghal 30th

December 1998

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 6 12 18 24 30 36 42 48

Wat

er Le

vel (

mO

DM)

Time (Hours)

MHWS Curve



94

5th – 6

th November 2000

Figure 7.3 presents the gauged hydrographs along the Blackwater sub-catchment for the flood event that

occurred from 5th to 6

th November 2000.

Figure 7.3: Progression of the November 2000 Flood Event

Table 7.3 details the relative flood frequency applied to each model reach for the 2000 calibration event.

The Ballycotton tidal gauge was not in operation for this event. Therefore, the tidal conditions have been

derived from the Admiralty tide table for 2000. The resultant total tide plus surge curve is shown in Figure

7.4 and will be applied to the lower Blackwater model at Youghal.

0

100

200

300

400

500

600

05

/11

/20

00

00

:00

05

/11

/20

00

12

:00

06

/11

/20

00

00

:00

06

/11

/20

00

12

:00

07

/11

/20

00

00

:00

07

/11

/20

00

12

:00

08

/11

/20

00

00

:00

08

/11

/20

00

12

:00

09

/11

/20

00

00

:00

Flo

w (

m3/s

)

18006 CSET MALLOW FLOW Revised Rating 18003 KILLAVULLEN FLOW Revised Rating 18002 BALLYDUFF FLOW Revised Rating

18001 MOGEELY FLOW Revised Rating 18010 ALLENS BRIDGE FLOW Revised Rating 18009 RIVERVIEW FLOW Revised Rating



95

Table 7.3: Calibration Inflows for 5th-6

th November 2000



CSET Mallow


20


CSET Mallow 20


Killavullen 20


Downing Bridge 1


Ballyduff 20


Ballyduff 20

Blackwater Outfall, Youghal Admiralty Tide Table Less than MHWS. MHWS assumed.

Allow Allow inflow Riverview 7

Figure 7.4: Total Tide Plus Surge at Youghal 5th – 6

th November 2000

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 6 12 18 24 30 36 42 48

Wat

er

Leve

l (m

OD

M)

Time (Hours)

MHWS Curve



96

30th January 2009

The 30th January 2009 event was similar in magnitude and duration to the more recent 24

th August 2012

event. Therefore the modelled flood outline, mechanisms and levels will be compared with the spot levels

from the 24th August 2012 to verify the model. Figure 7.5 presents the gauged hydrograph at Mogeely for

the flood event that occurred on 30th January 2009.

Figure 7.5: Recorded January 2009 Flood Event at Mogeely

Table 7.4 details the relative flood frequency applied to the Bride reach for the January 2009 catchment

calibration event. The Ballycotton tidal gauge has been used to inform tidal conditions at the Blackwater

outfall (Figure 7.6). The total tide plus surge curve will then be transferred to the Bride downstream based

on the modelled water level profile for the lower Blackwater.

0

20

40

60

80

100

120

140

29

/01

/20

09

12

:00

30

/01

/20

09

00

:00

30

/01

/20

09

12

:00

31

/01

/20

09

00

:00

31

/01

/20

09

12

:00

01

/02

/20

09

00

:00

01

/02

/20

09

12

:00

02

/02

/20

09

00

:00

02

/02

/20

09

12

:00

03

/02

/20

09

00

:00

Flo

w (

m3

/s)



97


January 2009


Bride Bride, Rathcormac and Tallow tributaries

Mogeely 10

Blackwater Outfall, Youghal Ballycotton >50

(1.935mODM)


January 2009 – 2nd

February 2009

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

-24 -18 -12 -6 0 6 12 18 24 30 36 42 48 54 60 66 72

Tota

l Tid

e P

lus

Surg

e L

eve

l (m

OD

M)

Time from Peak Flow at Mogeely(Hours from 30/01/2009 16:00)

Ballycotton Tide + Surge Level



98

19th

– 21st November 2009

Figure 7.7 presents the gauged hydrographs throughout the Allow, Bride and Blackwater sub-catchments

for the flood event that occurred from 19th to 21

st November 2009.

Figure 7.7: Progression of the November 2009 Flood Event

Table 7.5 details the relative flood frequency applied to each model reach for the November 2009

catchment calibration event. The Ballycotton tidal gauge has been used to inform the tidal conditions at the

Blackwater outfall. The resultant total tide plus surge curve is shown in Figure 7.8 and will be applied to the

lower Blackwater model at Youghal.

0

100

200

300

400

500

600

700

19

/11

/20

09

19

/11

/20

09

20

/11

/20

09

20

/11

/20

09

21

/11

/20

09

21

/11

/20

09

Flo

w (

m3

/s)

18006 CSET MALLOW FLOW Revised Rating 18055 MALLOW RAILWAY BR FLOW OPW Rating

18003 KILLAVULLEN FLOW Revised Rating 18107 FERMOY DS FLOW MM Applied Rating

18002 BALLYDUFF FLOW Revised Rating



99


- 21st November 2009


Allow Freemount and Allow upstream of Dalua confluence

Riverview 30

Dalua and Brogeen Allen’s Bridge 20

Allow downstream of Dalua confluence

Riverview 30

Bride Bride, Rathcormac and Tallow tributaries

Mogeely 20


CSET Mallow


10


CSET Mallow 10


Killavullen 5


Downing Bridge 2


Ballyduff 10


Ballyduff 10

Blackwater Outfall, Youghal Ballycotton >50

( 1.909mODM)



100


-21st November 2009

17th October 2012

The October 2012 event was caused by extreme coastal conditions at Youghal. Figure 7.9 displays the

recorded total tide plus surge hydrograph at Ballycotton gauge in Youghal Bay. This will be applied directly

to the Youghal model.

Table 7.6 details the relative flood frequency of this coastal event for the Lower Blackwater reach. River

flows were observed to be within-bank. Therefore flow less than the 50%AEP design flow will be applied to

the minor tributaries through Youghal.

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

-54 -48 -42 -36 -30 -24 -18 -12 -6 0 6 12 18 24 30 36

Tota

l Tid

e P

lus

Surg

e L

eve

l (m

OD

M)

Time from Peak Flow at Mallow (Hours from 20/11/2009 03:00)

Ballycotton Tide + Surge Level



101


October 2012


October 2012


Blackwater Youghal Tributaries Ballyduff >50%

Blackwater Outfall, Youghal Ballycotton

Council observations

0.93%

7.2 Uncertainty and Sensitivity Testing

The SW CFRAM study requires an understanding of sensitivity in hydrological and hydraulic parameters in

order to inform the uncertainty analysis in the flood mapping process. The key areas of uncertainty in the

hydrological analysis of UoM18 are:

Uncertainty in the QMEDrural regression equation;

Uncertainty in the pooling group and statistical distribution used to estimate the flood growth curve;

Uncertainty in the transformation of water levels to Youghal.

All sensitivity analysis has been assessed at the 1%AEP as this is the target fluvial AEP for the CFRAM

study and the AEP event used in planning decisions and in agreement with Guidance Note 26. Uncertainty

in flow and level for more frequent events are considered within the error bounds for the 1%AEP.

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

-36 -30 -24 -18 -12 -6 0 6 12 18 24 30 36 42 48 Surg

e R

esi

du

al (

m)

Tota

l Tid

e +

Su

rge

Lev

el (

mO

DM

)

Hours to Peak Water level @ 17/12/2012 18:15

Ballycotton Total Tide + Surge Level surge residual



102

Sensitivity in Flows

The FSU WP 2.3 states a factorial standard error (FSE) of 1.37 in the QMED rural regression equation

based on the 190 gauges across Ireland used to derive the equation coefficients. Approximate 95% upper

confidence limits for QMED were then calculated as follows:

95% 𝑐𝑜𝑛𝑓𝑖𝑑𝑒𝑛𝑐𝑒 𝑙𝑖𝑚𝑖𝑡 = 𝑄𝑀𝐸𝐷 ∗ 𝐹𝑆𝐸2

The uncertainty in the flood growth curves and pooling groups selected for a sample of 85 gaugings

stations across Ireland was investigated as part of the FSU WP 2.2. The percentage standard error in

design peak flow varied from 4.0 to 9.0 at the target fluvial 1%AEP.

The upper confidence limits from each source of peak flow uncertainty were combined to estimate overall

uncertainty in design peak flow at the target 1%AEP for ungauged HEPs. This resultant upper limit of the

1%AEP flow was typically within 10% to 30% of the design 1%AEP peak flow (see Appendix D). Therefore,

it was deemed that a sensitivity test of a 30% increase in peak flow at the target 1%AEP should be

considered in the subsequent hydraulic modelling of all HEPs in UoM18.

Sensitivity in Total Tide plus Surge Level

The Total Tide plus Surge Levels have been extracted from the RPS coastal model at offshore points

along the coast based on Extreme Value Analysis. There is some uncertainty in the transformation of the

total tide plus surge level to the near shore at Youghal as the frictional effects of the near shore bathymetry

has not necessarily been modelled. There is also inherent uncertainty in the derivation of the extreme

values for the rare %AEP events.

It was not possible to quantify the uncertainty at Youghal without long term tidal gauges records. The flood

history of coastal events at Youghal indicates two 1%AEP events in the past decade suggesting that the

ICPSS levels are underestimating the frequency of extreme events. The nearby Ballycotton tidal gauge

records are less than 5 years in length making an assessment of extreme total tide plus surge levels

unsuitable. Therefore, the GN 22 guidance was applied to consider a 0.5 m increase in water levels for

the design events which is broadly equivalent to the mid-range future scenario.



103

The design flows from this hydrology report inform the inflows to the hydraulic model to assess flood risk

from the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP fluvial and tidal flood events. The key

hydrological findings for design flows in UoM18 are as follows:

Historic flood events

Major flood events were identified in UoM18 since 1980 from extreme river flows along the

Blackwater, Bride and Allow.

Extreme storm surges at Youghal were also identified as a source of coastal flood risk with major

events in 2004 and 2012.

The largest gauged events were on 19th November 2009 and 2nd November 1980 along the

Blackwater.

There have been a number of catchment changes since the 1980s and more recently the Mallow

and Fermoy Schemes have been completed changing the impact of flooding in these AFAs.

There was no flood history identified in Aglish.

The calibration in Blackwater and Allow sub-catchments will be based on the following events where

there is sufficient information:

30th December 1998 – extreme fluvial event along the Blackwater and Allow

5th/6

th December 2000 – extreme fluvial event along the Blackwater and Allow

19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow

The calibration in Bride sub-catchment will be based on the following events where there is sufficient

information:

30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride

19th November 2009 – extreme fluvial event along the Bride

Rating Reviews

The high flows rating equations were revised for four gauges within UoM18 at Allen’s Bridge,

Riverview, Ballyduff and Mogeely.

High flows rating equations were checks for a further four gauges at CSET Mallow, Mallow Rail

Bridge, Killavullen and Fermoy.

The revised rating curves increased out-of-bank flow estimates for all gauges except Allen’s Bridge,

where flows were reduced to account for the backwater effect from the bridge and weir downstream.

The revised high flows rating equations were used to update the AMAX series at these gauged

HEPs and update the QMEDrural adjustment factor for hydrologically similar ungauged HEPS

Design flood flows

Peak flood flows were derived along the Allow, Dalua, Blackwater, Bride and Ballynaparka Stream

and various tributaries within the AFAs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP

events using the recommended FSU methodology outlined in Work Package 2.2 and 2.3.

The design flood hydrograph at gauged HEPs was derived from the median flood width exceedance

above 50% of the peak flow.

The design flood hydrograph for ungauged HEPs was based on the hydrograph pivotal site fitted to

the observed median hydrograph at the gauges within the catchment.

Design coastal conditions

The design extreme sea levels were extracted from the ICPSS for the 50%, 20%, 10%, 5%, 2%, 1%,

0.5% and 0.1%AEP tidal events.

ICPSS point S_31 was used to derive the total tide plus surge levels at Youghal.

8 Summary of Design Flows



104

The astronomic curve and surge profile were derived from the admiralty predicted astronomic tide

and typical duration of surge events in the South West.

The final design tidal curve was derived from the combined astronomic tide and design surge profile

scaled to meet the design extreme sea levels.

The ICWWS water level and wind conditions were used to derive wave overtopping at Youghal.

Youghal quayside was found to be at risk from still water overtopping (mechanism 1) in all

scenarios. The wave overtopping volume would be negligible in comparison to the still water

overtopping volumes. Therefore, wave overtopping has not been considered further for this reach.

Youghal at Claycastle was found to be at risk from wave overtopping in the design scenario and the

wave overtopping volume will be mapped in the subsequent hydraulic modelling and mapping stage.

Uncertainty and Sensitivity

The uncertainty of the 1%AEP target peak flow was estimated to range up to +30% in UoM18

ungauged HEPs which will inform the sensitivity tests in the hydraulic modelling.

The total tide plus surge levels could underestimate coastal risk based on the historical flood

frequency of storm surges at Youghal. A sensitivity test which raises the total tide plus surge level by

0.5m has been proposed in accordance with GN22

Tables 8.1 and 8.2 provide the design peak flows and total tide plus surge levels at key locations

respectively. These flows and levels are subject to change following the subsequent integration into the

hydraulic model and calibration processes.



Table 8.1: UoM18 Design Peak Flood Flows at Key Locations

HEP ID Gauge/ Ungauged Location 50%AEP (m3/s) 20%AEP 10%AEP 5%AEP 2%AEP 1%AEP 0.5%AEP 0.1%AEP

Freemount AFA

18_2681_3 Knockeen/Freemount Stream 1.9 2.4 2.8 3.1 3.7 4.1 4.7 6.2

18_2734_2 River Allow downstream of Freemount Bridge 23 28 32 35 41 46 52 68

Allow MPW

18_2672_1 Allow d/s of Glannycumman confluence 27 33 37 41 48 54 60 79

18_546_1 Allow d/s of Knockawillin confluence 47 62 72 82 96 107 118 148

18_548_1 Allow d/s of Ballynoe confluence (Kanturk upstream) 48 64 75 85 99 110 122 152

Kanturk AFA

18_548_5 Kilbrin Road Level Gauge 18110 49 64 75 86 100 111 123 154

18_552_3 Riverview (18009) 116 153 178 203 237 264 292 364

18_1762_9 Allen's Bridge Gauge 18010 47 60 69 78 90 97 110 144

18_1756_3 Church Street 18111 65 83 95 107 125 135 151 200

18_2121_10 Brogeen d/s 17 21 23 26 30 34 37 49

Blackwater Reach 1 MPW

18_394_3 Allow at Leaders Bridge 119 157 183 209 244 271 341 374

18_393_4 Blackwater upstream of Allow confluence 137 188 214 241 277 306 334 411

18_802_3 DROMCUMMER 18048 (downstream of Glen) 260 355 405 455 523 579 633 778

18_2747_1 Blackwater downstream of Awbeg 273 373 425 478 550 608 664 817

18_2382_2 CSET MALLOW 18006 302 413 471 530 609 674 736 905

Mallow AFA

18_1638_2 Blackwater (Mallow Railway Bridge) 331 452 516 580 667 738 806 991

18_1632_3 Blackwater Town Bridge (upstream of Bearforest confluence) 334 454 522 586 674 746 815 1002

18_1630_1 Blackwater d/s of Mallow 337 455 526 591 680 752 821 1010

18_2541_9 Clyda d/s at confluence 33 48 55 64 75 83 91 111

18_1104_5 Gooldshill d/s at confluence 1.4 1.9 2.2 2.6 3.1 3.6 4.1 5.7

18_2474_4 Hospital Stream d/s at confluence 4.9 6.4 7.5 8.6 10.3 11.8 13.4 18.1

18_1631_3 Bearforest Stream d/s at confluence 1.8 2.6 3.1 3.6 4.4 5.0 5.6 7.1

18_2594_13 Spa Glen d/s at confluence 5.9 8.6 10.3 12.0 14.5 16.4 18.3 23.5


18_1616_5 Killavullen Gauge 18003 337 456 533 607 705 779 850 1016

18_352_1 Blackwater d/s of Awbeg confluence 359 484 567 647 750 828 905 1083

18_351_2 Blackwater Castlehyde 365 492 577 658 763 842 920 1102

Fermoy AFA

18_1158_5 Fermoy Bridger Downstream 18107 370 499 585 667 773 853 933 1117

18_1158_8 Blackwater Fermoy d/s ( M8 bridge) 371 500 586 669 776 856 935 1120


18_2286_1 Blackwater d/s of Funshion 389 524 613 698 809 892 974 1166

18_2462_1 Blackwater d/s of Araglin 400 538 630 718 832 917 1002 1199

Ballyduff AFA

18_2297_6 Ballyduff Gauge 18002 405 545 638 726 842 928 1014 1213


18_2307_2 Blackwater at Lismore 416 560 655 747 865 954 1042 1247

18_2822_7+ Blackwater Youghal 535 720 842 960 1112 1226 1339 1602

Youghal AFA

18_2824_5 Tourig at confluence with Blackwater 12.1 15.4 17.7 20.2 23.9 27.1 30.7 41.0

18_967_9 Kilnatoora at confluence with Blackwater 1.5 1.8 2.1 2.4 2.8 3.1 3.5 4.7



HEP ID Gauge/ Ungauged Location 50%AEP (m3/s) 20%AEP 10%AEP 5%AEP 2%AEP 1%AEP 0.5%AEP 0.1%AEP

Rathcormac AFA

18_1964_4 Shanowen d/s at confluence with Bride 3.7 4.6 5.2 5.8 6.8 7.7 8.6 11.4

18_1605_12 Bride u/s at M8 29 35 38 42 46 58 66 86

18_1600_1 Bride d/s of Shanowen confluence 38 47 51 56 62 78 88 116

Bride Reach 1 MPW

18_344_1 Bride downstream at Flesk 55 67 74 80 88 112 126 166

18_350_1 Bride downstream at Douglas 74 90 100 108 119 152 170 224

Mogeely Gauge

18_341_3 Mogeely Gauge 18001 85 103 114 124 137 174 196 257

Bride Reach 2 MPW

18_343_1 Bride bear Limekilnclose 87 106 117 128 141 179 201 264

18_2778_1 Bride at tidal limit (near Tallow Bridge) 91 111 123 134 147 187 210 276

18_2798_3+ Bride downstream 109 133 146 159 176 223 251 330

Tallow AFA

18_2186_4 Carrigroe d/s at Glenaboy confluence 0.7 0.9 1.1 1.2 1.5 1.6 1.9 2.5

18_962_6 Glenaboy u/s of Carrigroe confluence 4.9 6.2 7.1 8.1 9.5 10.7 12.1 16.1

18_910_5 Glenaboy d/s at Bride confluence 5.8 7.3 8.4 9.5 11.2 12.6 14.3 19.0

Aglish AFA

18_2805_2 Ballynaparka (lower) Downstream 4.0 5.0 5.8 6.5 7.7 8.7 9.8 13.1

18_2808_2 Goish River (MPW) Downstream 8.6 10.6 12.1 13.6 15.9 17.9 20.2 26.7

Table 8.2: UoM18 Design Total Tide Plus Surge Levels

Location Location 50%AEP (mODM) 20%AEP 10%AEP 5%AEP 2%AEP 1%AEP 0.5%AEP 0.1%AEP

Youghal ICPSS Point S31 2.19 2.28 2.36 2.42 2.52 2.58 2.65 2.81



107

9.1 Inflows

Design hydrographs have been derived at HEPs to represent the hydrological processes across the

Blackwater catchment as discussed in Chapter 6 of this report. The HEPs will be integrated with the

subsequent hydraulic models as follows:

Point inflows at the upstream model extents;

Point inflows at key tributary inflows;

Lateral inflows representing the inflow from the intervening areas between target HEPs.

The point inflows representing the upstream model extents and tributary inflow will be integrated with the

relevant cross-section in the hydraulic model accounting for a significant displacement from the HEP

calculated location. The lateral inflows will be integrated with the relevant cross-sections at locations which

fit the following criteria:

Natural inflows from minor watercourses which are not considered explicitly within the hydrology;

Overland flow paths identified from surveyed low points in the river bank and site walkover.

The lateral inflows will be calculated from the difference between the design flow hydrographs from the

upstream and downstream HEPs for a reach and scaled accordingly to meet the design peak flows on the

main reach. The resultant hydrograph will be distributed evenly across those locations where the

contributing area increases linearly downstream or area weighted where the contributing area increases

disproportionally downstream. Table 9.1 outlines the total number of inflows based on the criteria above for

each model. These will be further refined and discussed in the hydraulics report.

Table 9.1: Model Inflows

Model Number of Inflows

Freemount 3

Allow 6

Kanturk 5

Blackwater Reach 1 8

Mallow 8


Fermoy 3


Ballyduff 3


Youghal 3

Rathcormac 6

Bride Reach 1 5

Mogeely 1

Bride Reach 2 4

Tallow 4

Aglish 4

9 Considerations for Hydrological and Hydraulic Model Integration



108

In order to enhance the modelling outputs and ensure hydrological continuity along the larger catchments,

the hydraulic models will be calibrated to the design peak flows derived at the target HEPs. The hydraulic

parameters will be adjusted and hydrological inflows scaled such that the hydraulic model maintains the

design peak flows along the reach. However, it should be noted that the design fluvial flows do not

consider the following hydraulic processes:

Backwater effect at confluences;

Exchange of flows between tributaries at confluences; and,

Significant modification to the hydrograph shape due to floodplain attenuation.

Therefore, it is not appropriate to calibrate the hydraulic model to HEPs upstream of confluences where

there are significant out-of-bank flows.

In UoM18, the median width hydrographs have been derived at the gauged locations to establish the

design storm duration at target HEPs across each catchment. The duration of the tributary inflows are

based on the FSR time peak equation (function of SAAR, S1085 and MSL) but will be iteratively refined to

achieve the flow at the gauges as part of the hydraulic modelling. The intermediate inflows account for the

difference in duration between the target HEPs within the same hydrological catchment. Table 9.2 outlines

preliminary design storm durations for UoM18.

Table 9.2: Preliminary Design Storm Duration

AFA/MPW Gauge Design Duration (Hours)

Freemount and Allow Tributaries FSR estimate 8

Kanturk/Allow 18009 20

18010 17

Blackwater Dromcummer to Mallow 18048 27

Mallow/ Blackwater 18006 30

Mallow Tributaries FSR estimate 6

Killavullen 18003 38

Fermoy/Blackwater 18107 40

Fermoy Tributaries FSR estimate 5

Ballyduff/Blackwater 18002 59

Ballyduff Tributaries FSR estimate 7

Youghal Tributaries FSR estimate 11

Bride 18001 46

Rathcormac and Tallow Tributaries FSR estimate 11

9.2 Downstream Conditions

The downstream conditions will be defined for each model as outlined in Table 9.3 to fully account for the

relevant fluvial and tidal backwater effects as appropriate. An iterative approach will be used to phase the

design tidal curves so that the peak tide coincides with the peak flow as a conservative estimate of flood

risk.



109

Table 9.3: Downstream Boundary Conditions

Model Downstream Condition

Freemount Stage-discharge relationship based on the downstream water slope in the Allow model.

Allow Stage-discharge relationship based on the downstream water slope in the Kanturk model.

Kanturk Stage-discharge relationship based on the downstream water slope in the Blackwater Reach 1 model.

Blackwater Reach 1 Stage-discharge relationship based on the downstream water slope in the Mallow model.

Mallow Stage-discharge relationship based on the downstream water slope in the Blackwater Reach 2 model.

Blackwater Reach 2 Stage-discharge relationship based on the downstream water slope in the Fermoy model.

Fermoy Stage-discharge relationship based on the downstream water slope in the Blackwater Reach 3 model.

Blackwater Reach 3 Stage-discharge relationship based on the downstream water slope in the Ballyduff model.

Ballyduff Stage-discharge relationship based on the downstream water slope or water level over time in the Blackwater Reach 4 model depending on the relative tidal influence.

Blackwater Reach 4 Full tidal boundary using the results from the design tidal curves set out in Chapter 6

Youghal Full tidal boundary using the results from the design tidal curves set out in Chapter 6

Rathcormac Stage-discharge relationship based on the downstream water slope in the Bride Reach 1 model.

Bride Reach 1 Stage-discharge relationship based on the downstream water slope in the Mogeely model.

Mogeely Stage-discharge relationship based on the downstream water slope in the Bride Reach 2 model.

Bride Reach 2 Stage-discharge relationship based on the downstream water slope or water level over time in the Blackwater Reach 4 model depending on the relative tidal influence.

Tallow Stage-discharge relationship based on the downstream water slope or water level over time in the Bride Reach 2 model depending on the relative tidal influence.

Aglish Stage-discharge relationship based on the downstream water slope or water level over time in the Blackwater Reach 4 model depending on the relative tidal influence.



110

10.1 Approach

The hydrogeomorphological processes ongoing in the river channels can have a significant impact on flood

flows and the resultant flood risk. The assessment of hydrogeomorphological features focuses on whether

the processes appear to be in equilibrium and whether there are any processes taking place at present

which are likely to affect the flood risk indicators. This may include:

Recent interventions to the channel/hydrology to control flood risk which have accelerated erosion or

deposition;

The use of inappropriate bank protection which may transfer erosion downstream; or

Straightening or reprofiling the channel which may cause the watercourse to attempt to revert back to a

more natural state.

This has included an assessment of:

Typical land use, soils and geology as provided in Chapter 2;

Channel gradient based on the river channel survey;

Bank and bed material and condition based on site visits, aerial photographs and survey photographs;

Channel planform based on Ordnance Survey maps and aerial photography; and

The presence of structures (bridges, weirs, culverts) /channel modifications (e.g. straightening, bank

protection, bank reprofiling).

The survey data and photographs are provided in a separate survey report. Key photographs have been

included in this report to inform the analysis.

10.2 Assessment

The HPW and MPW were split into broad reaches of similar hydrogeomorphological characteristics based

on the approach above, and an assessment made on the current erosion and deposition features (Map

10.1).

10 Hydrogeomorphology



Map 10.1: Hydrogeomorphological Reaches



112

River Allow Catchment

The River Allow and the River Dalua have transitory meandering planforms which are actively migrating

across the valley floodplain as evidenced by the changes over time in the river profiles from the pre-1900

boundary lines and aerial photography. The rate of erosion is greatest on the outside of the various

tortuous meander bends where in-channel velocities and stream power are high, undermining the non-

cohesive alluvial soils. Erosion is further exacerbated by trampling of cattle along open river banks in some

reaches. The Allow and Dalua’s meandering planforms are a natural response to the following factors:

The natural tendency of the river to adjust its channel gradient (bed slope) and planform to transport

the sediment load;

The trend of land drainage due to European Union agro-forestry policies potentially increasing

sediment load and runoff; and,

The increase in flows due to a climatic “wetter” period since 2000 as evident in the Riverview gauge

record.

There were a number of localised informal bank protection measures

observed during site visits along the Allow and Dalua limiting the sources

of sediment. However, it is likely that banks will continue to naturally

erode due to the reasons stated above. Riffles and pools have formed

within Kanturk as the rivers naturally adjust to the straightened planforms

between the walls, embankments and changes in velocities through the

various bridges and weirs.

There are a number of cut-off remnants of previous meander bends

(known as oxbow lakes where flooded) along the Dalua and to a lesser

extent on the Allow. These are likely to provide additional storage below

the general floodplain level but may also form flood flow routes during

flood. Woody debris was observed in channel throughout the more rural

reaches of the Allow and Dalua due to the presence of heavily vegetated

banks and wooded areas upstream. Such debris may raise upstream

water levels locally, increasing flood risk. However, these temporary

debris blockages would be quickly bypassed during a flood and may

become mobilised presenting a blockage risk to structures downstream.

River Blackwater Catchment Upstream of Mallow

The River Blackwater downstream of the Allow confluence has low sinuosity, gently meandering across the

valley floodplain. Bank erosion was observed on the outside of some meander bends with associated

gravel bars deposited on the inside of bends. Bank erosion and slumping was particularly evident where

cattle have unrestricted access to the river. However, the rate of erosion is less than observed on the

Allow because the Blackwater is less sinuous and therefore the near-bank velocities are not as severe.

Site and survey observations indicated there was some woody debris in the rivers but this was not likely to

block bridges given the relative size of the bridge openings.

Photo 10.1: Bank Erosion on the

Allow



113

The Blackwater has been constrained by raised embankments and walls

through Mallow AFA. Scour protection and bank protection was

observed at a number of bridge structures indicating active erosion

upstream as the velocities change through these structures. Recent

reprofiling works downstream of Mallow Town Bridge associated with the

flood defence scheme have developed a two-stage channel, altering the

in-channel capacity to store flood waters for more extreme events.

Gravel bars tend to be deposited in areas of shallow flow near the

Mallow Bridge and weir where the channel has been over-widened in

the past to support the bridge structure.

The River Clyda, Spa Glen, Hospital, Bearforest and Gooldshill tributaries

are all steeper smaller catchments all with woody debris present in their

upper reaches. This woody debris combined with urban debris in the

urbanised reaches can modify water levels and flow paths locally but are

likely to be washed away during floods. However, this debris presents a

blockage risk to structures downstream which is managed by a number of

trash screens at key structures. Siltation was observed on the approach

to and under a number of bridge structures where the channel has been

over-widened, such as Lower Beecher Street. However, the rate of

deposition was not deemed to be significant in comparison to the

capacity of the structures. Sediment load and rate of deposition in the

flood storage areas is monitored as part of the scheme maintenance.

Only the lower reaches of the River Clyda were observed to be actively eroding banks where cattle had

unrestricted access to the riverside. The excess sediment is deposited in channel or in bars at the

confluence with the Blackwater and is not deemed to be out of balance.

Photo 10.2: Reprofiling at Mallow

Photo 10.3: Spa Glen Trash

Screen



114

River Blackwater Catchment Downstream of Mallow

Between Mallow and Fermoy, the Blackwater develops a distinct riffle-

pool sequence in response to the constrained floodplain between the

more resistant valley sides. For example upstream of Ballyhooly, the river

meets the resistant geology forming riverside cliffs. There are a number

of in-channel gravel bars at riffle features, but these are unlikely to

present additional flood risk as they are likely to be bypassed on the

floodplain and the sediment will become mobile during extreme floods.

The larger in-channel islands are vegetated, such as downstream of

Killavullen gauge, and will have a greater impact on low flows. However,

these islands are still likely to modify flow paths during floods. The River

Awbeg (Major) joins the Blackwater at an acute angle causing flow

inefficiencies and deposition of sediment sands and gravels. The

watercourse has been artificially straightened and is trapezoidal in cross

section through Fermoy as part of the flood defence scheme. The banks

generally consist of a variety of engineered walls or embankments. Such

heavily engineered walls reduce sources of sediment within this reach. Scour of the bed was observed

upstream of the Fermoy labyrinth weir with the change in velocities and flow paths over this structure.

Downstream the river has been over-widened historically leading to deposition of gravel bars in the shallow

channel downstream of the bridge.

Downstream of Fermoy, the Blackwater is joined by the Funshion and Araglin and widens by 10m with the

additional flows. The river banks in this reach were less vegetated than upstream, reducing the presence

of woody debris. No active bank erosion was observed in the lower Blackwater. However, the bank profile

was evidently maintained, and access for cattle restricted, limiting the sources of sediment for this reach.

Lismore Weir and the associated in-channel islands control water level and velocities upstream for in-bank

flows. Downstream of Lismore weir, the Blackwater becomes tidal and silts and fine sediments are

naturally deposited between low and high tide during periods of slack water (low velocities). Despite being

tidal, there are no wide tidal flats or floodplain until Youghal as the river is constrained by the resistant

geology of the valley sides. Downstream of the N25 Bridge at Youghal, the Blackwater widens to over

800m resulting in wider intertidal flats on the left bank. Ferry Point constrains the tide further, trapping

sediment on the left bank and protecting this reach from extreme coastal conditions and associated

erosion.

Photo 10.4: Fermoy Bridge



115

River Bride Catchment

The Rive Bride downstream of the N8 is typically meandering in planform

with typical sediment size ranging from gravels to small boulders at

Rathcormac, to silts and fines in the tidal reaches downstream of Tallow

Bridge. Woody debris was observed in the upstream reaches originated

from the vegetated banks. The accumulation of woody debris in the

upstream reach modifies local flow paths and scour but is unlikely to

impact on flood risk as these features are washed away during flood.

However, the woody debris may present a blockage risk to the smaller

bridges at Ballinterry Cross Roads and along the smaller tributaries

through Rathcormac. There are well developed pool and riffles

sequences along the Bride as the bed slope adjusts to the changes in

velocities around the meander bends. The river channel has been over-

widened at Mogeely Bridge causing deposition under the bridge which

may change the gauge datum overtime. Bank protection at bridge

structures near Conna limit the sources of sediment in these reaches. Downstream of Tallow Bridge, the

watercourse becomes more sinuous, widens and has tidal flows. Silts and fines are naturally deposited

between low and high tide during periods of slack water (low velocities). However, bank erosion was

observed where cattle had unrestricted access to the river from the adjoining pasture lands. In the lower

reaches, the river banks are raised above the floodplain such that any out-of –bank flow will be unable to

return to the river, thus increasing deposition on the floodplain during times of flood. The River Bride joins

the Blackwater at a more acute angle with deposition of silts and fines observed on the upstream/left bank

of the confluence where velocities between the two rivers are lowest.

Other Small Catchments

The Ballynaparka Stream flows through Aglish town and is relatively steep. The river channel is constricted

by a number of masonry walls and maintained embankments which limit bank erosion and sources of

sediment. However, deposition of sediment from upstream was observed where flows were constricted

through the various bridges and culverts as the velocities change through these structures. The channel is

heavily vegetated during the summer months, which further encourages deposition. Over time, the

deposition may limit the capacity of structures and the channel to convey flows. Further downstream the

Ballynaparka Stream is joined by a tributary from the east at Ballynaparka Bridge. This tributary is actively

eroding its banks in the upper reaches where cattle have unrestricted access to the river. The river channel

becomes embanked where it joins with the Goish River and develops a meandering planform constrained

by the flood defences. The River Goish has a sinuous meander planform until it is embanked at the tidal

outfall. Some bank erosion was observed further upstream. However, the historic mapping does not

indicate active meander migration since 1900. The excess sediment from the bank erosion upstream on

the Goish and Ballynaparka tributary is deposited in the flatter tidal reaches during periods of slack water

when velocities are low. Over time this deposition may reduce the tidal channel capacity as it is embanked

above the floodplain.

Photo 10.5: Lower Bridge Bank

Erosion



116

10.3 Impact on Flood Risk

In summary, the River Allow and Dalua were observed to have the faster rate of erosion and

geomorphological change due to land use and climatic changes in the catchment over the previous

decades. This effect is largely located in rural agricultural reaches and is unlikely to significantly increase

risk of flooding downstream as the sediment load is unlikely to reduce capacity at structures in Kanturk

during floods. However, flood risk management options should consider the rate of erosion, bank stability

and sediment load if applied in these more rural reaches upstream of Kanturk.

Woody and urban debris was deemed to be a significant issue on the Mallow tributaries as this could

reduce the capacity and/or block the smaller bridges and culverts through the town. Trash screens have

been installed as part of the flood defence scheme at key structures to limit the blockage risk during floods.

However, the level and frequency of screen maintenance should be considered for the operation of the

flood relief scheme going forward.

The Lower Blackwater, River Bride and Ballynaparka Stream were all assessed to be largely in equilibrium

with no significant change in channel shape or planform over time. However, deposition in the tidal

embanked reaches of the River Bride and Ballynaparka Stream may reduce the channel capacity over time

thus increasing flood risk to the adjacent low-lying areas.



117

11.1 Overview

The design flows on each river reach and total tide plus surge levels provided in Chapter 8 have been

derived independently of each other. In reality, there can be dependency between sources of flooding

which can be described by the joint probability to achieve a target %AEP event. The CFRAM study

considers the following joint probabilities:

Fluvial-fluvial – Where a range of combinations of flow on a main river combines with flow on a tributary

to generate a specific %AEP flood downstream.

Fluvial-coastal – Where an approaching depression generates a storm surge which combines with a

river flood to generate a specific %AEP at the coast.

The joint probability between total tide plus surge levels and extreme waves has been considered

separately under the ICWWS study. The resultant combinations have been assessed in Chapter 6 to

establish the critical scenario for wave overtopping for each target %AEP. Therefore, this will not be re-

examined in the following sections.

11.2 Fluvial-Fluvial Dependence

The joint probability between fluvial flows on the main watercourse and its tributaries was guided by the

methodology set out in Flood Studies Update Work Package 3.4. The FSU methodology assessed the

dependence between fluvial inflows based on the distance between catchment centroids; the ratio in

catchment area; and, the difference in FARL, a measure of flood attenuation due to reservoirs and lakes.

Table 11.1 sets out the different combinations in UoM18 for tributary inflows to achieve the target %AEP

on the main watercourse.

In UoM18, the joint probability of tributaries was found to be largely dictated by the size of the incoming

catchment relative to the main watercourse. The joint probability %AEP on the smaller tributary inflows

tended to be the more frequent smaller events in order to achieve the target flow on the main watercourse.

The flows on the smaller tributaries upstream of Fermoy and tributaries on the Bride were more correlated

with high flows on the main rivers because the same storm produced the high flows. However, smaller

tributaries downstream of Fermoy were less correlated with flows on the main river because the catchment

is remote from the Blackwater centroid and the same storm is less likely to produce high flows on both

catchments.

The exception was the River Dalua-River Allow and River Allow –River Blackwater combinations. These

have similar probabilities to the main river as the tributaries contribute approximately half of the flow to the

downstream reach.

11 Joint Probability



Table 11.1: Key Joint Probabilities of Inflows

Target %AEP at downstream HEP on main

watercourse

50%

20% 10% 5 %

2 %

1% 0.5%

0.1%

Reach inflow WP 3.4 Table 13.1 Scenario Associated %AEP of Tributary Inflow

River Bride-Blackwater

Allow tributaries (including Glannycumman, Knockcloon, Garragort, Knockawillin, Ballynoe and Brogeen)

Blackwater reach 1 tributaries (including Glen River and Awbeg Minor)

Mallow tributaries ( including Clyda, Hospital, Spa Glen, Gooldshill and Bearforest)

Blackwater reach 2 tributaries (including Awbeg Major)

Fermoy tributaries (including Strawhill and Glenabo)

Bride tributaries (including Shanowen,Flesk,Shanowennadrimina and Douglas)

Catchment centroid within 25km

Significantly smaller catchment (Ratio of area greater than 2.7)

Difference in FARL less than 0.07

71 46 35 23 10 6.1 3.8 1.2

Blackwater reach 3 tributaries (including Funshion and Araglin)

Blackwater reach 4 tributaries (including Glenshelan, Goish, Finnisk and Licky River)

Youghal tributaries (including Tourig and Kilnatoora)

Catchment centroid beyond 25km from target

Significantly smaller catchment (Ratio of area greater than 2.7)


93 79 65 51 34 25 17 7.5

Dalua-Allow

Blackwater-Allow

Catchment centroid within 25km

Similar sized catchment (Ratio of area within 2.7)


57 30 17 9.4 4.3 2.3 1.2 0.3

%AEP values to 2 significant figures



119

11.3 Fluvial-Coastal Dependence

It is not possible to statistically assess the joint probability between fluvial and tidal events along the South

West coast as there is limited concurrent river flow and with relatively short 3 year record of tidal gauge

data at Ballycotton gauge (ID 19068). A short record of less than 3 years is not deemed suitable to

undertake long term frequency and dependency analysis at this location. Therefore, the DEFRA

FD2308_TR1 desk-based assessment was used to estimate the fluvial-tidal joint probability combinations

in accordance with Guidance Note 20, Joint Probability Analysis ( RPS November 2012).

It was assumed that Youghal Harbour was similar to estuaries along the south coast of England in terms of

orientation to the dominant storm track as described in DEFRA FD2308_TR1. Based on the FD2308

research, the dependence of river flow and storm surge in these estuaries tended to be “well” to “strongly”

correlated. The strongly correlated CF (ratio of the actual frequency of occurrence of a particular joint

exceedence event to its probability of occurrence if the two variables were independent) was applied to

Youghal Harbour as a conservative estimate in the absence of detailed concurrent gauge data. Figure 11.1

outlines the resultant joint probabilities.

Figure 11.1: Joint Probability Curves of Tidal and Fluvial Events for Strongly Correlated Estuaries

0.1

1

10

100

1000

0 1 10 100 1000

Tidal Surge (Return Period)

Riv

er

Flo

w (

Re

turn

Pe

rio

d)

2

5

10

20

50

100

200

1000



120

Fluvial-coastal dependence between river gauges on the Lower Lee and tidal gauges in Cork Harbour

were also assessed as part of the Lee CFRAM pilot study. This analysis concluded there was some

correlation between high flows and higher storm surges as the storm events that caused the surge also

caused high rainfall in the Lower Lee catchment. Extensive sensitivity analysis was undertaken on the

0.5% AEP event as part of the pilot study and found the two main critical scenarios to be as follows:

Target flow and the MHWS tide; and

50%AEP Flow and the target Total tide plus surge level.

Based on this analysis, the design scenarios will combine the design fluvial %AEP with the appropriate

tidal surge to achieve the target fluvial dominated %AEP and vice versa for the coastal-dominated

scenario. For example, the 0.5% tidal event combined with the >50% fluvial event will produce the design

0.5% extent. This approach ensures easy interpretation of the maximum fluvial dominant flood and

maximum coastal dominant flood for the design scenario.



121

12.1 Potential Climate Changes

The range of potential impacts of climate change varies as there are significant uncertainties associated

with global climate predictions and local hydrological variation for periods more than 20 years in the future.

Therefore, two scenarios have been assessed to quantify the sensitivity of flood risk to potential climate

change namely, the Mid-Range future scenario (MRFS) and the High-Range future scenario (HRFS) as

detailed in Table 12.1.

Table 12.1: Allowance for Climate Change in Catchment Parameters

Catchment Parameter MRFS HRFS

Extreme Rainfall Depth +20% +30%

Flood Flows +20% +30%

Mean Sea Level Rise +0.5m +1.0m

Land Movement -0.5mm/year

i.e. +0.05m relative sea level rise over 100 years

-0.5mm/year

i.e. +0.05m relative sea level rise over 100 years

Source: Reproduced from Appendix F of National Flood Risk Assessment and Management Programme, Catchment-Based Flood

Risk Assessment and Management (CFRAM) Studies, Stage I Tender Documents: Project Brief.

The land movements quoted in Table 6.1 refer to postglacial readjustment of the underlying tectonic plate

since the last glacial period in Southern Ireland. This readjustment is not a climatic change but it does alter

the effective rate of sea level rise predicted with climate change.

It is important to note that the increase in sea level and flood flows applies to the entire tidal curve and

flood hydrograph, not just the peak.

12.2 Potential Catchment Changes

12.2.1 Urban Development

The way in which the land is used can significantly impact the flow routes across the catchment, how much

rainfall is stored, how much infiltrates into the ground, and how much evaporates. Future urban

development is likely to influence hydrology and flood risk in the following ways:

Increase the surface runoff from the catchment by increasing the area covered by impermeable

surfaces on previously undeveloped (“Greenfield”) sites;

Increase the proportion of surface runoff draining to urban drainage networks; and,

Increase the proportion of the population, properties and infrastructure within areas of flood risk.

All of these changes cause more water to reach the river channels quicker and affect more people,

property and environments.

12 Future Scenarios



122

The greatest concentration of urban development and urban change is located at Mallow at Youghal.

However, there has been significant growth in smaller towns such as Tallow and Kilworth over the past

decade. Furthermore, the regional plans identify the Greater cork area and Mallow as a “hub” of growth

over the next 20 years.

Table 12.2 outlines the urban growth in housing units according to the South West Regional Authority

(SWRA) Planning Guidelines and linear extrapolation to estimate urban growth for the MRFS and HEFS.

The SWRA data is based on a 2010 baseline and accounts for the economic downturn in forecasts beyond

2010. The MRFS growth rate has been estimated on the projected increase in housing units between 2016

and 2022 accounting for the economic downturn. The HEFS growth rate has been estimated on the

average projected increase from the entire regional plan with a lesser impact from the economic downturn.

Table 12.2: Future Urban Growth

SWRA Plan Area

Housing Units Required MRFS% Growth

HEFS % Growth

2006 2010 2016 2022

Cork Gateway 111,581 127,749 153,000 182,044 3.16% 3.54%

Mallow Hub 4,191 5,341 7,500 10,498 6.66% 8.05%

Ring towns and Rural areas

42,951 46,472 50,317 54,160 1.27% 1.38%

Greater Cork area

154,532 174,221 203,317 236,203 2.70% 2.96%

Tralee Killarney Hub area

15,284 17,099 20,318 23,573 2.67% 3.16%

Kerry linked hub

29,565 33,541 39,855 46,239 2.67% 3.15%

Northern Area 33,497 37,993 43,885 46,186 0.87% 1.80%

Western area 36,606 41,745 47,989 50,729 0.95% 1.79%

Source: South West Regional Plan. BOLD text signifies relevant areas to the UoM.

In agreement with OPW, the forecast growth in housing units was assumed to be on previously

undeveloped land as a conservative estimate of urbanisation. The MRFS and HEFS do not account for any

beneficial impacts of Sustainable Drainage Systems in the future.

12.2.2 Land Use Change

The majority of the Blackwater catchment is currently rural and dedicated to agricultural or pastoral use.

The type of crops that are grown, the way the land is prepared and changes in land drainage practice all

affect how quickly rainfall reaches the watercourses. Land management practices also affect the amount of

silt that gets washed from the fields into the rivers during rainfall events. Given that these processes can

influence flood risk, both in a positive and negative way, we need to consider how land use and land

management may change in the future.



123

There are many uncertainties surrounding the future of agriculture within the catchment. Land use will

depend upon society’s aspirations and needs, and will be driven by policies being implemented by both the

Irish government and the EU. The pressures and drivers that will affect how land is used in UoM22 include:

change to agricultural policy and land management subsidies in the EU;

opening of world markets making agriculture and pastoral activity less economically viable;

growth in world population increasing demand for food production;

change in typical annual temperatures with climate change resulting in changes in crop types grown;

diversification to other land uses, particularly for tourist related attractions;

drive to enhance and restore environmental habitats and landscapes;

drive to reduce carbon dioxide emissions through the use of carbon sinks and biofuels; and,

increasing energy prices could lead to increased biofuel use or make importing of produce

uneconomic.

All of these changes can either lead to intensification of activities and associated increased land drainage

and runoff or reduction in activities with associated increased infiltration and reduced runoff. There is very

limited information on most of these land cover changes as they are often driven by economic factors

which are rarely predicted more than 5 years into the future.

Deforestation to increase productivity of agricultural land can be a significant impact on rural land use in

Europe under the EU Common Agricultural Policies. Forested areas intercept rainfall, increase storage and

infiltration and slow surface water runoff into the river channels. The removal of natural forests can

encourage greater runoff. There is only limited evidence to suggest the extent of forest cover is a

significant controlling parameter on the regression equations used to estimate peak flood flows8. However,

the OPW guidelines identify commercial afforestation to increase productivity as the significant pressure on

rural land use in Ireland. Increased irrigation and drainage for the commercial forests can route more water

to the rivers thus reducing the time to peak. The OPW future scenarios guidelines recommend that

changes in forest cover can be reflected in a reduced time to peak due to these associated drainage

works.

Between 11% and 14% of the Allow, Blackwater and Bride sub-catchments are covered by forest as

defined by the Floods Studies Update. Therefore, any decrease in forested area is unlikely to impact future

flood hydrographs as forest covers such a small proportion of the catchment at present. Forest cover

increases to 31% on the Glen River catchment and to 38% on the Araglin catchment which flow into the

River Blackwater. The projected decrease in forest cover could reduce the time to peak by 17% and 33%

for the MRFS and HEFS respectively. The increased flashy response in these catchments would change

flood risk locally but would be unlikely to change the flood response of the larger rivers given their relatively

small catchment area.

8 Institute of Hydrology (1991). Plynlimon research: The first two decades. Report No. 109, Institute of Hydrology.



124

12.3 Design Future Scenario Conditions

The present day design hydrology (derived in Chapter 5 of this report) was modified to consider the

relevant catchment and climate changes discussed in the previous sections. Table 12.3 summarises the

final Mid-Range and High-End Future Scenarios.

Table 12.3: Allowance for Future Condition in Catchment Parameters

Catchment Parameter MRFS HEFS

Flood Flows +20% +30%

Mean Sea Level Rise +0.5m +1.0m

Land Movement -0.5mm/year

i.e. -0.05m over 100 years

-0.5mm/year

i.e. -0.05m over 100 years

Urbanisation 0.87 to 6.66%/year 1.8 to 8.05%/ year

Forestation -1/6 Tp -1/3Tp

+ 10% PR

The design hydrology under future conditions has been adjusted for the predicted decrease in forest cover

in the relevant Glen and Araglin catchments only. The resultant future peak flood flows and future extreme

sea levels based on the Mid-Range and High End Future Scenarios are provided in Appendix E.

The predicted increase in river flows and sea level rise attributed to predicted climate change is the most

significant factor that influences design peak flows and levels in UoM18. Urbanisation has a relatively small

impact on design peak flows as the catchment remains predominately rural in both the MRFS and HEFS.

The degree to which the increased river flows and sea levels change flood risk to the AFAs will be

assessed as part of the subsequent hydraulic modelling and mapping. The relative increase in flows and

period of any tide-locking associated with the impacts of climate change should be considered in the sizing

of any floodplain storage options and frequency of maintenance activities.



125

13.1 Conclusions and Key Findings

The design flows from this hydrology report inform the inflows to the hydraulic model to assess flood risk

from the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP fluvial and tidal flood events. The key

hydrological findings in UoM18 are as follows:

Historic flood events

Major flood events were identified in UoM18 since 1980 from extreme river flows along the

Blackwater, Bride and Allow.

Extreme storm surges at Youghal were also identified as a source of coastal flood risk with major

events in 2004 and 2012.

The largest gauged fluvial events were on 19th November 2009 and 2nd November 1980 along the

Blackwater.

The calibration in Blackwater and Allow sub-catchments will be based on the following events where

there is sufficient information:

30th December 1998 – extreme fluvial event along the Blackwater and Allow

5th/6


19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow

The calibration in the Bride sub-catchment will be based on the following events where there is sufficient

information:

30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride

19th November 2009 – extreme fluvial event along the Bride

The Freemount and Tallow AFA model results will be verified against the historical flood frequency

(where available) and sensitivity analysis on the key hydraulic parameters as there is insufficient

evidence to undertake full calibration on the floodplain.

The Aglish model results will be checked based on sensitivity testing of hydraulic parameters.

Rating Reviews

The high flows rating equations were revised for four gauges within UoM18 at Allen’s Bridge,

Riverview, Ballyduff and Mogeely.

High flows rating equations were checks for a further four gauges at CSET Mallow, Mallow Rail

Bridge, Killavullen and Fermoy.

The revised rating curves increased out-of-bank flow estimates for all gauges except Allen’s Bridge,

where flows were reduced to account for the backwater effect from the bridge and weir downstream.

The revised high flows rating equations were used to update the AMAX series at these gauged

HEPs and update the QMEDrural adjustment factor for hydrologically similar ungauged HEPS

13 Conclusions, Key Findings and Recommendations



126

Design flood flows

Peak flood flows were derived along the Allow, Dalua, Blackwater, Bride and Ballynaparka Stream

and various tributaries within the AFAs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP

events using the recommended FSU methodology outlined in Work Package 2.2 and 2.3.

The design flood hydrograph at gauged HEPs was derived using the median flood width

exceedance above 50% of the peak flow.

The design flood hydrograph for ungauged HEPs was based on the hydrograph pivotal site and

informed by the FSR calculated duration but will be iteratively refined to achieve the flow at the

gauges as part of the hydraulic modelling.

The design flood hydrographs will be applied to the hydraulic models as inflows to the upstream of

each river reach, tributary inflows and intermediate inflows for the catchment in between.

The outflow for the upstream models will form the inflow to the downstream models iteratively down

the Blackwater and Bride catchments except at Mallow where the CSET Mallow gauge will form the

inflow.

The joint probability between tributary inflows and the main watercourse was informed by FSU

WP3.4. The joint probability was found to be largely dictated by the size of the incoming catchment

in UoM18.

The exception was the River Dalua and River Allow as these tributaries join the River Allow and

River Blackwater respectively. These tributaries are of a similar size to the downstream reach and

therefore have broadly equal probabilities as they contribute approximately half of the flow to the

downstream reach.

Design coastal conditions

The design extreme sea levels were extracted from the ICPSS for the 50%, 20%, 10%, 5%, 2%, 1%,

0.5% and 0.1%AEP tidal events.

ICPSS point S_31 was used to derive the total tide plus surge levels at Youghal.

The astronomic curve and surge profile were derived from the admiralty predicted astronomic tide

and typical duration of surge events in the South West.

The final design tidal curve was derived from the combined astronomic tide and design surge profile

scaled to meet the design extreme sea levels.

The ICWWS water level and wind conditions were used to derive wave overtopping at Youghal.

Youghal quayside was found to be at risk from still water overtopping (mechanism 1) in all

scenarios. The wave overtopping volume would be negligible in comparison to the still water

overtopping volumes. Therefore, wave overtopping has not been considered further for this reach.

Youghal at Claycastle was found to be at risk from wave overtopping in the design scenario and the

wave overtopping volume will be mapped in the subsequent hydraulic modelling and mapping stage.

Storm surge events in Youghal Harbour were assessed to be strongly correlated to rainfall-river

flood events due to their location on the west coast and orientation to incoming storms.

Joint probability between the storm surge and river flood was calculated using the DEFRA FD2308

desk-based approach as per GN 22.



127

Uncertainty and Sensitivity

The uncertainty of the 1%AEP target peak flow was estimated to range up to +30% in UoM18

ungauged HEPs which will inform the sensitivity tests in the hydraulic modelling.

The total tide plus surge levels could underestimate coastal risk based on the historical flood

frequency of storm surges at Youghal. A sensitivity test which raises the total tide plus surge level by

0.5m has been proposed in accordance with GN22

Hydrogeomorphology

The current erosion and deposition processes were assessed for all AFAs and intervening MPWs.

The fastest rate of erosion was observed in the upper Allow associated with natural meander

migration due to climate and catchment changes since the 1960s.

The largest deposition features were associated with the tidal reaches of the Blackwater, Bride and

Goish where siltation reduces the channel capacity. The deposition is likely to be washed away

during floods. These reaches are thus deemed to be in equilibrium.

Localised depositions was observed along the Ballynaparka Stream in Aglish and the Kilnatoora in

Youghal where the sediment load from the steep upstream reaches was greater than typical flows

creation siltation at structures.

Structures on the tributaries in Mallow were found to be at risk from blockage from urban debris and

the wooded section upstream. This is currently being managed by trash screens at key structures.

Future conditions

Two future scenarios were developed to assess potential future changes namely, the Mid-Range

future scenario (MRFS) and the High-End future scenario (HEFS).

River flows were predicted to increase by 20% and 30% due to climatic changes under MRFS and

HEFS respectively.

Sea levels were predicted to rise by 0.55m and 1.05m for the MRFS and HEFS respectively,

including allowance for 0.5mm/year post-glacial rebound land movements.

Urban extent was predicted to increase between 1% and 6% per year, and 2% and 8% year for the

MRFS and HEFS respectively, based on the forecasted rates in the South West Regional Authority

planning guidelines.

Time to peak was predicted to reduce by 17% and 33% for the MRFS and HEFS respectively along

the Glen and Araglin tributaries due to change in forest cover in their upper reaches.

The design peak flood flows and total tide plus surge levels were adjusted to represent the climatic

and catchment changes above for the MRFS and HEFS future scenarios accordingly.

13.2 Recommendations

The following recommendations can be drawn from the key findings above for the subsequent hydraulic

modelling, flood risk assessment, preliminary option development and FRMP:

The design peak flows and design total tidal levels presented in Table 8.1 and 8.2 should be used to

inform the subsequent hydraulic modelling in UoM18.

Inflows for intervening catchments should be distributed across minor watercourses and overland flow

paths identified from the survey based on the proportional increase in contributing area.



128

The joint probability approach and analysis in Chapter 11 should be used to inform the combinations of

inflows and coastal conditions for the model boundaries.

The relevant hydraulic models should be calibrated as far as possible to these historic flood events:

– 30th December 1998 – extreme fluvial event along the Blackwater and Allow

– 5th/6


– 19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow

– 30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride

– 19th November 2009 – extreme fluvial event along the Bride

– 17th October 2012 – extreme coastal event at Youghal

The remaining models which do not have sufficient historic information or gauge information should

use reasonable hydraulic parameters and the modelled flood outline should be compared with the

relative historical flood frequency.

The following sensitivity tests should be considered to assess the impact of hydrological assumptions

on flood extent and levels in the subsequent hydraulic modelling:

– Peak flow

– Downstream tide plus surge levels

The mapping of wave overtopping volumes should be considered for Claycastle in Youghal (Reach C2)

where wave overtopping discharges were found to be significant and the floodplain was located below

the defence.

Extreme wave scenarios have not been considered along Youghal quayside because the quayside

was flooded via mechanism 1, still water overtopping. Wave overtopping is negligible in comparison to

mechanism 1.

The following recommendations can be drawn from the hydrological analysis for future analysis in the

catchment:

Continued effort to obtain concurrent spot gaugings (where safe) for bankfull and out-of-bank

conditions at Allen’s Bridge, Riverview, CSET Mallow, Mallow Rail Bridge, Killavullen and Ballyduff.

Spot gaugings at Fermoy Mill Gauge 18117 to verify flow estimates from the Fermoy Bridge gauges.

The Fermoy Mill gauge is located away from the complex weir-bridge structure thus avoids any

uncertainties associated assumptions taken for these structures during the rating development.

Concurrent monitoring of sea levels at Ballycotton and Youghal Harbour quayside during storm surge

events would help verify the transformation of extreme coastal events.

The %AEP estimates for total tide plus surge levels should be reviewed periodically at Ballycotton as a

longer period of data becomes available.

The rate of meander migration (erosion) in the River Allow and Dalua should be taken into

consideration in the design of any flood mitigation options and sediment yields. It should be noted that

the meander migration is part of the rivers natural hydrogeomorphological response to climatic

changes in the past few decades.

The level and frequency of screen maintenance in Mallow should be considered for the operation of the

flood relief scheme going forward.



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AEP Annual Exceedance Probability; this represents the probability of an event being exceeded in any one year and is an alternative method of defining flood probability to ‘return periods’. The 10%, 1% and 0.1% AEP events are equivalent to 10-year, 100-year and 1000-year return period events respectively.

AFA Area for Further Assessment – Areas where, based on the Preliminary Flood Risk Assessment and the CFRAM Study Flood Risk Review, the risks associated with flooding are potentially significant, and where further, more detailed assessment is required to determine the degree of flood risk, and develop measures to manage and reduce the flood risk.

AMAX Annual Maximum Flood

BFISOILS Baseflow index from Irish Geological Soils dataset. Often used as a permeability indicator.

CFRAM Catchment Flood Risk Assessment and Management – The ‘CFRAM’ Studies will develop more detailed flood mapping and measures to manage and reduce the flood risk for the AFAs.

DAD Defence Asset Database

DAS Defence Asset Survey

EU European Union

EPA Environmental Protection Agency

FARL Index of flood attenuation due to reservoirs and lakes

FRMP Flood Risk Management Plan. This is the final output of the CFRAM study. It will contain measures to mitigate flood risk in the AFAs.

FRR Flood Risk Review – an appraisal of the output from the PFRA involving on site verification of the predictive flood extent mapping, the receptors and historic information.

FSU (WP) Flood Studies Update (Work Package) (2008 to 2011)

FSR Flood Studies Report (HR Wallingford, 1975)

GIS Geographical Information Systems

HA Hydrometric Area. Ireland is divided up into 40 Hydrometric Areas.

HEFS High-End Future Scenario to assess climate and catchment changes over the next 100 years assuming high emission predictions from the International Panel on Climate Change.

HEP Hydrological Estimation Point

HPW High Priority Watercourse. A watercourse within an AFA.

ICPSS Irish Coastal Protection Strategy Study (2012)

ICWWS Irish Coastal Water Level and Wave Study (2013)

ING Irish National Grid system, Ordnance Survey of Ireland

Glossary



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MPW Medium Priority Watercourse. A watercourse between AFAs, and between an AFA and the sea.

MRFS Mid-Range Future Scenario to assess climate and catchment changes over the next 100 years assuming medium emission predictions from the International Panel on Climate Change.

ODM Ordnance Datum Malin.

The current geodetic datum of Irish National Grid which references the mean sea level at Malin Head between 1960 and 1969.

OPW Office of Public Works, Ireland

OSi Ordnance Survey Ireland

PFRA Preliminary Flood Risk Assessment – A national screening exercise, based on available and readily-derivable information, to identify areas where there may be a significant risk associated with flooding.

QMED Median annual flood used as the index flood in the Flood Studies Update. The QMED flood has an approximate 50%AEP.

QMEDamax QMED derived from the annual maximum series at a gauged location

QMEDrural QMED derived from physical catchment descriptors according to the Flood Studies Update methodology.

QMEDadj QMED adjusted by the ratio of QMEDamax:QMEDrural at a hydrologically similar Pivotal site.

QMEDurban QMED adjusted to account for the impacts of urban areas according to the Flood Studies Update methodology.

S1085 Typical slope of the river reach between 10%ile and 85%ile along its length.

SAAR Standard average annual rainfall 1961 to 1990

SEA Strategic Environmental Assessment. A high level assessment of the potential of the FRMPs to have an impact on the Environment within a UoM.

SW CFRAM South Western Catchment Flood Risk Assessment and Management study

Total tide plus surge level Total tidal level formed of the astronomic tide and storm surge elements.

UoM Unit of Management. The divisions into which the RBD is split in order to study flood risk. In this case a HA.

WFD Water Framework Directive. A European Directive for the protection of water bodies that aims to, prevent further deterioration of our waters, to enhance the quality of our waters, to promote sustainable water use, and to reduce chemical pollution of our waters.

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