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Eastern CFRAM Study HA09 Hydraulics Report - DRAFT FINAL

IBE0600Rp0027 Rev F03

Eastern CFRAM

Study

HA09 Hydraulics Report

Santry Model

DOCUMENT CONTROL SHEET

Client OPW

Project Title Eastern CFRAM Study

Document Title IBE0600Rp0027_HA09 Hydraulics Report

Model Name HA09_Sant1

Rev. Status Author Reviewed By Approved By Office of Origin Issue Date

D01 Draft M.Houston S. Cullen G. Glasgow Belfast 25/02/2014

D02 Draft M.Houston S. Cullen G. Glasgow Belfast 02/07/2014

F01 Draft A. Sloan S. Patterson G. Glasgow Belfast 25/02/2015

F02 Draft A. Sloan S. Patterson G. Glasgow Belfast 13/08/2015

F03 Draft Final A. Sloan S. Patterson G. Glasgow Belfast 05/08/2016

Eastern CFRAM Study HA09 Hydraulics Report - DRAFT FINAL

IBE0600Rp0027 Rev F03

Table of Reference Reports

Report Issue Date Report Reference Relevant Section

Eastern CFRAM Study Flood Risk Review

December 2011

IBE0600Rp0001_Flood Risk Review_F02

3.2.1

Eastern CFRAM Study Inception Report UoM09

August 2012 IBE0600Rp0008_HA09 Inception Report_F02

4.3.2

Eastern CFRAM Study Hydrology Report UoM09

September 2013

IBE0600Rp0016_HA09_Hydrology Report_F01

7.2

Eastern CFRAM Study HA09 Liffey Survey Contract Report

November 2012

2001s4884- SC2 Survey Report v1 Various

Eastern CFRAM Study HA09 Hydraulics Report – DRAFT FINAL

IBE0600Rp00027 4.10 - 1 Rev F03

4 HYDRAULIC MODEL DETAILS

4.10 SANTRY MODEL

4.10.1 General Hydraulic Model Information

(1) Introduction:

The Eastern CFRAM Flood Risk Review (IBE0600Rp0001_Flood Risk Review) highlighted Santry as an

AFA and the Santry River itself as a HPW for fluvial flood risk under the ECFRAM Study based on a

review of historic flooding and the extents of flood risk determined during the Preliminary Flood Risk

Assessment.

The Santry watercourse is within HA09, but is independent from the main Liffey system and flows directly

into Dublin Bay at Raheny, to the west of North Bull Island.

The watercourse originates from the southern edge of Dublin Airport and drainage drawings supplied by

Dublin Airport Authority indicate that the drainage system for the western portion of the southern runway

and some car park hardstanding both discharge to the headwaters of the Santry through conventional

piped drainage systems.

The Santry watercourse has a narrow catchment and has no significant tributaries. The total catchment

area of the model at the downstream extents (at Raheny) is 13.96km². The catchment is heavily urbanised

(62%) and includes an attenuating structure and online storage pond at Santry Demesne to the south of

Northwood Avenue. Dublin City Council has also recently installed a flow control structure at the

Harmonstown Road bridge.

The model can be considered to represent a gauged catchment with the Cadburys hydrometric gauging

station (09102 – EPA) located approximately two thirds of the length of the river from Dublin Airport. The

gauging station was not given a classification under FSU and the rating curve does not extend up to the

range of the observed corresponding water level at Qmed. As such the gauging station cannot be

considered to have a high degree of certainty at flood flows. The observed Qmed of 3.37m3/s is derived from

only 11 complete hydrological years of data which again does not afford high statistical certainty.

A rainfall run-off model has been developed of the contributing catchment to the Cadburys gauging station

in order to simulate a longer AMAX series and increase confidence in the Cadburys gauged Qmed. The

model was calibrated against the existing continuous flow record of the gauging station from 2001 to 2011

and utilises rainfall data from the hourly gauge at Dublin Airport as input data. The hourly rainfall gauge

was used at Dublin Airport due to its location at the edge of the Santry catchment which is small and

therefore has little spatial variation in meteorological conditions which would be captured by the rainfall

radar also located at Dublin Airport. The rainfall run-off (NAM) model was calibrated to the period for which

there is corresponding flow data and rainfall gauge data (2001 – 2010) and good calibration was achieved.

Hourly rainfall information is available for the period of 1941 – 2010 and was input to the rainfall run-off


IBE0600Rp00027 4.10 - 2 Rev F03

model. Analysis of the simulated flow trace shows that the simulated Qmed from this period is 3.25m3/s.

This compares to values derived from catchment descriptor based methods at the gauge of 4.55m3/s and

2.53m3/s using IH124 and FSU methods respectively. Due to the catchment sizes the IH124 values have

been retained and adjusted downwards based on the relationship between the NAM simulated and IH124

derived index flood flow values at the Cadburys gauging station.

The entire length of the Santry model is a HPW and was modelled as 1D-2D using the Infoworks ICM

flexible mesh software.

A number of catchments in the Greater Dublin area have legacy drainage network models constructed in

InfoWorks CS. Survey data gathered as part of the CFRAM study has been augmented with culvert and

manhole information from these GDSDS models to allow a more accurate line and gradient of pipe

networks to be represented in the model. Comprehensive data collection of existing sewer network

records and survey of culverted reaches was undertaken for the GDSDS in order to capture detail in

complex drainage networks i.e. changes to internal diameter and gradient. Therefore OPW and RPS

selected ICM use to allow better representation of culverted river networks and enable better utilisation of,

and future integration with, the existing sewer network models. ICM also provides a very stable 2D

modelling regime for coastal inundation modelling, therefore ICM models (driven by a MIKE21 coastal

model) were used for Dublin Bay coastal AFAs, including Raheny AFA, to provide a consistent approach

throughout the Greater Dublin area, facilitating integration with existing models.

(2) Model Reference: HA09_SANT1

(3) AFAs included in the model: Santry (flows through Raheny AFA where coastal flood risk is

addressed by a separate model)

(4) Primary Watercourses / Water Bodies (including local names):

SANTRY

(5) Software Type (and version):

(a) 1D Domain:

Infoworks ICM

(b) 2D Domain:

Infoworks ICM v5 Flexible Mesh

(c) Other model elements:

N/A

4.10.2 Hydraulic Model Schematisation

(1) Map of Model Extents:

Figure 4.10.1 illustrates the extent of the modelled catchment, river centre lines, HEP locations and AFA

extents as applicable. The Santry modelled watercourse lies within the Dublin AFA and includes 1

Upstream Limit HEP, 1 Downstream Limit HEP, 1 Intermediate HEP and 1 Gauging Station HEP. There

are no Tributary HEPs along this watercourse.


IBE0600Rp00027 4.10 - 3 Rev F03

Figure 4.10.1: Santry HPW/AFA Modelled Catchment and HEPs

(2) x-y Coordinates of River (Upstream extent):

River Name x y

SANTRY 313968 242314

(3) Total Modelled Watercourse Length: 11.1 km

(4) 1D Domain only Watercourse Length: N/A (5) 1D-2D Domain

Watercourse Length:

11.1 km

(6) 2D Domain Mesh Type / Resolution / Area: Flexible Mesh / 1-25 m2 / 13.7 km2

(7) 2D Domain Model Extent:

Figure 42 shows the modelled extents and the general topography of the catchment within the 2D model

domain. The river centre-line is shown in blue, changes in the vertical elevation of this map are outlined

by the legend, all levels have been set to OD Malin (metres).

The ground elevation (based on LiDAR data used to generate a 2D flexible mesh) is shown to provide an

overview of the modelled area topography.


IBE0600Rp00027 4.10 - 4 Rev F03

Figure 4.10.2: 2D Domain Model Extent


IBE0600Rp00027 4.10 - 5 Rev F03

Figure 4.10.3 is an overview of the model schematisation. Figure 4.10.4-4.10.6 show detailed views.

The overview diagram covers the model extents, showing the cross-section locations, AFA boundary and

river centre line. It also shows the area covered by the 2D model domain.

The detailed areas are provided where there is the most significant risk of flooding. These diagrams

include the surveyed cross-section locations, AFA boundary and river centre. They also show the location

of the critical structures as discussed in Section 4.10.3, along with the location and extent of the links

between the 1D and 2D models.

Figure 4.10.3: Overview of Model Schematisation


IBE0600Rp00027 4.10 - 6 Rev F03

Figure 4.10.4: Model Schematisation in the vicinity of Harmonstown Road Flow Control Structure


IBE0600Rp00027 4.10 - 7 Rev F03

Figure 4.10.5: Model Schematisation in the vicinity of Cadbury's Gauging Station

Figure 4.10.6: Model Schematisation in the vicinity of Santry Attenuation Ponds

(8) Survey Information

(a) Survey Folder Structure:

First Level Folder Second Level Folder Third Level Folder

Murphy_E09_M01_WP2_120622_Santry_A

Where:

Murphy – Surveyor Name

E09 – Eastern CFRAM Study Area,

Hydrometric Area 09

M01 – Model Number 1

WP2 – Work Package 2

120622 – Date Issued (22nd June 2012)

Santry_A – River Reference

GIS and Floodplain

Photos

Structure Register

Surveyed Cross Section Lines

Photos

Videos

Ascii

Drawings and PDFs

(b) Survey Folder References:

SANTRY Murphy_E09_M01_WP1_120504_SANT_Santry Gauge

Murphy_E09_M01_WP2_120622_Santry_A

Murphy_E09_M01_WP2_120622_Santry_B

2m resolution LiDAR of the entire modelled area was used to generate the 2D domain computational


IBE0600Rp00027 4.10 - 8 Rev F03

flexible mesh. The vertical accuracy of the LiDAR is quoted as 0.2m RMSE which was considered

satisfactory for model application. A comparison of topographical survey levels along James Larkin Road

and the LiDAR data for the same area was undertaken. The comparison indicated a good agreement

between the datasets with level differences of between 0mm and 120mm found at the locations

investigated.

The LiDAR data was augmented during the meshing process with the OSI building layer dataset to

integrate the building footprints into the model. The buildings are represented in the computational mesh

as porous polygons with a porosity value set to zero, allowing no flow to pass through them. No other

amendments were made to the computational mesh.

(9) Survey Issues: An infill survey of the flow control structure at 09SANT00190D and additional crest levels of the bridge

parapet wall were requested to ensure accurate representation in the model.

4.10.3 Hydraulic Model Construction

(1) 1D Structures in the 1D domain: See Appendix A.1 for details.

Number of Bridges and Culverts: 43

Number of Weirs: 12

Critical Structures

Figure 4.10.7: Culvert on Santry River with Trash Screen Fitted (09SANT01056I)

Figure 4.10.7 shows one of a number of culverts along the Santry River which has a trash screen


IBE0600Rp00027 4.10 - 9 Rev F03

installed. Parts of this screen appear to retain debris which vegetation growth can anchor in place,

increasing the risk of blockage and consequently flooding.

Figure 4.10.8: Sluice Gate at Santry Desmene (09SANT00713I)

Figure 4.10.8 shows a sluice gate located in Santry Demense which can control the flow allowed to

continue downstream, this gate has been modelled as fully open to enable as much flow as possible to

move downstream to the more densely populated reaches of the Santry River. If the sluice gate is closed

more storage will be available immediately upstream, however the sluice structure will be overtopped

before flooding of properties will occur.


IBE0600Rp00027 4.10 - 10 Rev F03

Figure 4.10.9: Attenuation Pond on Santry River (09SANT00678)

Figure 4.10.10: Attenuation Pond Outlet on Santry River (09SANT00667)

Figure 4. and Figure 4. show an attenuation Pond and pond outlet which provide online storage to reduce

peak flow passing downstream. The sluice gate at 09SANT00713l (Figure 4.) can regulate the flow

entering this pond. The outlet has also been reported to block in the past. The attenuation pond was

modelled using river cross sections as the outflow is controlled by the outlet structure in Figure 4.10.9.

This structure will determine the attenuation afforded by the pond.

Figure 4.10.11: Flow Control Structure at Harmonstown Road Bridge (09SANT00190)


IBE0600Rp00027 4.10 - 11 Rev F03

Figure 4.10.11 shows a flow control structure recently installed by Dublin City Council which reduces the

capacity of the Harmonstown Road bridge, controlling the peak flow allowed to pass downstream by

creating an online storage area during periods of high flow. This structure was represented in the model

using two orifice units, one for each of the openings. The parapet walls were represented in the 2D

domain using base linear structures. Further discussion of this structure is contained in Section 4.10.3(2).

(2) 1D Structures in the 2D domain:

As stated in Section 4.10.3(1) the parapet walls of the Harmonstown Road Bridge were represented by

base linear structures in the 2D domain. The use of a base linear structure enabled the correct storage

potential of the river valley upstream of the bridge to be modelled while allowing the potential flow path

around the wall to be represented.

(3) 2D Model structures: N/A

The 2D flexible mesh is generated using the Shewchuk Triangle meshing functionality which produces

Delaunay triangulations. Mesh triangles are aggregated into mesh elements if the triangle size is smaller

than the minimum element size defined in the mesh properties. The maximum triangle area was set at

25m2 with a minimum element area of 1m2 visual inspection of the mesh confirmed that topographical

features were being suitably defined in the 2D mesh.

Terrain sensitive meshing was used to automatically increase the mesh resolution throughout the 2D

domain in areas where topographical variation would required increased definition.

(4) Defences:

A flow control, see Figure 4.10.12, was installed by Dublin City Council at the Harmonstown Road Bridge,

model section ID 09SANT00190. This structure is aimed at limiting the flow which can pass downstream

during flood flow conditions, creating a flood storage area in the sports grounds immediately upstream of

the bridge.

The online ponds at Santry Demesne provide some attenuation of flow on the Santry River. The flow is

limited by the capacity of the outlet culverts under the Swords Road.

There are no raised defences on the Santry River.

(5) Model Boundaries - Inflows:

Full details of the flow estimates are provided in the Hydrology Report (IBE0600Rp0016_HA09 Hydrology

Report_F02 - Section 4.1 and Appendix D). The boundary conditions implemented in the model are

shown in Figure 4.10.12:

Inflow hydrographs were applied as a point inflow at the upstream end of the model and as lateral inflows

to each river reach and conduit along the length of the model. The upstream inflow and total lateral inflow

hydrographs are included in the above referenced hydrology report. The upstream inflow hydrograph was

applied at node 09SANT01072 which is the most upstream location in the modelled section of the Santry

River, Figure 4.10.12 illustrates the 1% AEP inflow hydrograph at 09SANT01072. The lateral inflows have

been calculated for three distinct sections of the watercourse. The total lateral inflow hydrograph has then


IBE0600Rp00027 4.10 - 12 Rev F03

been applied on a pro rata basis to each model reach (river reach or conduit) based on reach length. A

total of 102 hydrographs have been applied along the modelled length of watercourse.

Figure 4.10.12: 1% AEP Upstream Point Inflow Hydrograph (09SANT01072)

(6) Model Boundaries –

Downstream Conditions:

Coastal Water Level Boundary

Outputs from the Irish Coastal Protection Strategy Study (ICPSS) have resulted in extreme tidal and storm

surge water levels being made available around the Irish Coast for a range of Annual Exceedance

Probabilities (AEPs). The locations of the relevant AFAs and ICPSS nodes for Dublin Bay are shown in

Figure 4.10.13. The associated AEP water levels for each of the nodes are contained in Table 4.10.1.

following the location diagram. Node NE19 was used to inform the peak levels for the time varying coastal

water level boundaries included in the inundation model for the Santry River as it provided slightly more

conservative values than NE21 over the CFRAM Study design event range and is in reasonable

agreement with observed or estimated levels from a previous Dublin City Council study at this location.

For additional detail on the determination of the water levels generated during the ICPSS project reference

should be made to the ICPSS technical report (Irish Coastal Protection Strategy Study Phase 3 - North

East Coastal, Work Packages 2, 3 & 4A - Technical Report).

The flood extents currently available for this portion of the AFA are comprised of inundation due to fluvial

flood flows and time varying coastal water levels, wave overtopping assessment is not required for this

watercourse.


IBE0600Rp00027 4.10 - 13 Rev F03

Figure 4.10.13: ICPSS Node Locations (IBE0600Rp0016_HA09 Hydrology Report_F02)

Table 4.10.1: ICPSS Level in Close Proximity to HA09 AFAs / HPWs (IBE0600Rp0016_HA09 Hydrology Report_F02)

AEP (%) Elevation (m) to OD Malin for a Range of AEP

NE_17 NE_19 NE_20 NE_21 NE_22 NE_23

50 2.52 2.44 2.42 2.46 2.46 2.43

20 2.65 2.57 2.54 2.58 2.58 2.55

10 2.75 2.67 2.63 2.67 2.67 2.64

5 2.85 2.77 2.73 2.76 2.76 2.74

2 2.98 2.90 2.85 2.88 2.88 2.86

1 3.08 3.00 2.94 2.97 2.97 2.95

0.5 3.18 3.11 3.04 3.06 3.07 3.04

0.1 3.41 3.34 3.26 3.27 3.28 3.25

Representative tidal profiles for Santry were extracted from a tidal model of the Irish Sea and Dublin Bay


IBE0600Rp00027 4.10 - 14 Rev F03

for a 70 hour period. The tidal model of Dublin Bay is part of a wider Irish Sea tidal model. As such the

model is driven by global tidal predictions which are propagated into Dublin Bay; the model has been

calibrated to all relevant tide gauge data, including the Dublin Port gauge at Alexandra Basin.

A normalised 48 hour surge profile was scaled based on the difference between the peak water level

extracted from the tidal profile and the target extreme water level from the table above. The scaled surge

profile was then appended to the tidal profile, with coincident peaks, to achieve a representative combined

tidal and storm surge profile for the required AEP events, this calculation is detailed below.

ICPSS Extreme Water Level - Peak Astronomical Tide Level = Peak Surge Value

Peak Surge Value x Normalised 48Hr Representative Surge Profile = Scaled Surge Profile

Astronomical Tide Profile + Scaled Surge Profile = Extreme Coastal Water Level Profile

It should be noted that the peak water level values quoted in Table 4.10.1 included an allowance for the

affect of seiching in Dublin Bay. Figure 4.10.14 illustrates the tidal profile, storm surge profile and resultant

combined water level profile for the coastal boundary.

The appropriate water level profile was applied to the downstream node of the Santry model

(09SANT00000J) as a level boundary. The application of the oscillating water level to these boundaries

allows the model to simulate the ingress and egress of the coastal water levels.

The HA09 hydrology report concluded that correlation between total water levels and fluvial flood flow on

the Santry River can be considered to be negligible and it is proposed to follow a simplified conservative

approach whereby the 50% AEP design event is maintained for one mechanism while the whole range of

probabilities for the other mechanism are tested and vice versa, subject to sensitivity testing against

average winter conditions to ensure the approach does not yield results which could lead to unrealistic

flood extents or over design of measures.


IBE0600Rp00027 4.10 - 15 Rev F03

Figure 4.10.24: Extreme Water Level Profile for Santry Downstream Boundary

(7) Model Roughness:

(a) In-Bank (1D Domain) Minimum 'n' value: 0.020 Maximum 'n' value: 0.070

(b) HPW Out-of-Bank (1D) Minimum 'n' value: 0.030 Maximum 'n' value: 0.100

(c) MPW/HPW Out-of-Bank

(2D)

Minimum 'n' value: 0.013 Maximum 'n' value: 0.045

‐3

‐2

‐1

0

1

2

3

4

0 20 40 60 80 100

Water Elevation (mAOD)

Time (hrs)

Tide and Surge Profiles

Astronomical Tide

Normalised 48hrRepresentative Surge Profile

0.5 % Scaled Surge

0.5% AEP Water Level Profile


IBE0600Rp00027 4.10 - 16 Rev F03

Figure 4.10.35: Map of Roughness (Manning’s n) across 2D Domain

Figure 4.10.310.15 illustrates the roughness values applied within the 2D domain of the Santry

computational model. Roughness in the 2D domain was applied based on land type areas defined in the

Corine Land Cover Map with representative roughness values associated with each of the land cover

classes in the dataset.


IBE0600Rp00027 4.10 - 17 Rev F03

(d) Examples of In-Bank Roughness Coefficients (ref Section 2.5)

Figure 4.10.16: Section 09SANT01068

'n' = 0.050

Some in-channel weeds, lightly vegetated banks

with obstructions


'n' = 0.070

Heavily vegetated channel


‘n’ = 0.040

Straight, maintained


‘n’ = 0.050

Vegetated, unmaintained


'n' = 0.020

Straight concrete channel


'n' = 0.070

Attenuation Pond (deep slow moving water)


IBE0600Rp00027 4.10 - 18 Rev F03

4.10.4 Sensitivity Analysis

To be completed.

4.10.5 Hydraulic Model Calibration and Verification

(1) Key Historical Floods (from IBE0600Rp0008_HA09 Inception Report_F02 unless otherwise

specified):

(a) Oct 2011 The floods that hit Dublin, Wicklow and parts of Kildare in October 2011 resulted from

extremely heavy rainfall. The floods, which resulted in the deaths of two people,

affected mainly the city, south Dublin and Wicklow.

The estimated peak flow recorded at the Cadburys gauging station, provided by the

EPA, was 7.2m3/s. A flow of this magnitude equates to approximately a 5% AEP

event based on the CFRAM hydrological flow estimation

The Santry River Flooding at Raheny Assessment and Solutions Report,

MDW0484Rp0002, was commissioned by DCC to determine possible flood control

solutions in the Santry catchment. This study carried out flow estimation using

rainfall data from the Grange Tank rain gauge and the GDSDS model of the Santry

catchment. This study estimated that the peak flow at section ID 09SANT00257D

was 10.11m3/s during this event, this flow equates to approximately a 2% AEP event

at this location, based on the CFRAM hydrological estimation.

Santry Villas and Santry Avenue were impassable due to large amounts of rainfall

and there was also flooding on the M50 at Santry. No details were found on any

damage caused. Santry Villas and Santry Avenue are both remote from the Santry

River, the source of the reported flooding is therefore likely to have been pluvial.

Significant flooding was experienced in the area adjacent to Howth Road, Main Street

and Watermill Road in Raheny. Figure 4.10.410.22 provides a comparison of the

recorded extent of flooding and the modelled 2% AEP extents of flooding in this area.

Post flood investigations indicated that the flooding at this location may have been

exacerbated by a fallen tree in the Santry River between Howth Road and Main

Street. The modelled extents in Figure 4.10.22 have been produced with a 30%

channel blockage in place. The comparison indicates that the model can replicate

historic flood events with a good degree of accuracy.


IBE0600Rp00027 4.10 - 19 Rev F03

Figure 4.10.42: Recorded extent of Oct. 2011 flooding in Raheny

This event was used for model validation in the Raheny area.

(b) July 2009 Flooding occurred on 2 July after several hours of heavy rainfall in the Dublin area

from midnight to 9.00am. 38.2mm of rain fell at Dublin Airport over 9 hours, with

26.5mm falling in one of those hours. This represents an approximately 3% AEP 1

hour duration rainfall event.

The flooding was reported less severe in the Santry area with the main flooding

problem being surface water on roads in the vicinity. The estimated peak discharge of

the River Santry was estimated to be 8.0m3/s at Cadburys Hydrometric Station which

is approximately a 5% AEP - 2% AEP event.

There was no further information on the extent of the flooding or details of any

damage caused by the July 2009 flood event, as such it could not be used for

calibration or verification of the model.

(c) Aug 2008 On the 9th August 2008 Dublin Airport recorded its highest August daily rainfall total

to date of 76.2mm which at that time had only once been exceeded before, in June

1993, it has since been exceeded again in October 2011 making it the third highest

24hour rainfall total on record.

The maximum 1hr rainfall recorded on the 9th August 2008 was 35.9mm which is

estimated to be a 0.8% AEP event. The daily total of 76.2mm fell over an 18 hour


IBE0600Rp00027 4.10 - 20 Rev F03

period which corresponds to a 1.5% AEP event.

No incidents of flooding are recorded on the FloodMaps.ie website, however

additional anecdotal evidence suggests that 10 properties were flooded in Raheny

Village. The location of these properties is unknown however it is likely they are

situated in the vicinity of Main Street. No flow records are available from the 9th

August through to the 18th August at the Cadburys gauging station which may

suggest that it was damaged during this event. No adjustments were made to the

model based on this event.

(d) Oct 2004 Flooding occurred in Dublin in October 2004 as a result of high tides and strong

easterly winds. The peak flow recorded during this event at Cadbury's gauging station

was 3.32m3/s. This is estimated to be approximately a Qmed event, there are no

reported incidents of property flooding during this event. No adjustments were made

to the model based on this event.

(e) Nov 2002 Widespread flooding occurred in mid November 2002 as a result of heavy and

prolonged rainfall. The total rainfall depth measured at Dublin Airport during this

event was 87mm, while 72mm of rainfall was recorded at Casement. The rainfall

event was estimated to be a 2% AEP event at Dublin Airport, the peak flow recorded

at Cadbury's gauging station during this event was 5.8m3/s which is estimated to be a

10% AEP event.

Flooding was severe in some parts as catchments were already somewhat saturated

following high levels of rainfall in October and early November.

In Santry, high water levels in the River Santry contributed to the flooding which

occurred at Santry Close, on the Old Airport Road.

Figure 4.10.5: Modelled 2% AEP flood extents for Santry Close

Santry Close


IBE0600Rp00027 4.10 - 21 Rev F03

The model output shown in Figure 4.10.5 is in agreement with the above stated

flooding at Santry Close. This event supported model validation in the Santry area.

(f) Jan 1965 A flood event occurred in Santry in January 1965 as a result of heavy rainfall. This

caused the Wad River to back up due to a blocked trash screen. However, no further

details or additional information could be gathered from http://www.floodmaps.ie or

other sources, so the event was not suitable for use in model calibration or validation.

(2) Post Public Consultation Updates:

All recorded comments were investigated following informal public consultation and formal S.I. public

consultation periods in 2015, however no model updates were required for Final issue.

(3) Standard of Protection of Existing Formal Defences:

None

(4) Gauging Stations:

There is one gauging station located on the Santry River, near Coolock, Dublin (Station No. 09102,

Cadbury's). It is approximately 3.3km upstream of its discharge to point to Dublin Bay at Raheny. The

gauge is located 290m upstream of the Malahide Road and has a weir type control structure (Figure

4.10.6). The open channel section at the gauge is an approximately 2.5m wide weir with an 8m channel

top width and a minimum bed level of 28.803m OD Malin. The bank levels are 32.92m OD Malin (left

bank) and 32.37m OD Malin (right bank). The current ordnance level of the gauge zero is 29.232m OD

Malin (as stated on the HydroNet website).

The gauge is operated by the Dublin City Council, with continuous 15 minute water level and derived flow

records available from 2001 to present.


IBE0600Rp00027 4.10 - 22 Rev F03

Figure 4.10.6: Photo of Gauge Location

Figure 4.10.7: Cadbury's gauging station model cross section

A rating review was undertaken in order to improve flood flow estimates at the gauging station and in order

to support calibration of the model. The rating review carried out at this station found that the model

calibration to the spot gaugings and to the existing rating was good with a maximum difference of 50mm

between modelled Q-h and spot gaugings. However the highest spot gaugings is 0.43m above the staff

gauge zero and the reliable limit of the existing rating equation is given as 0.66m.

The best fit rating curve was achieved using a Manning's n roughness co-efficient for the in-channel flows

of 0.035. This is consistent with a clean, straight channel which best describes the Santry at this location.

Initially an in-channel weir coefficient of 1.8 was applied to the control structure, this was increased to 1.85

29.0

30.0

31.0

32.0

33.0

34.0

Elev

atio

n (m

AD)

0.0 5.0 10.0 15.0Offset (m)


IBE0600Rp00027 4.10 - 23 Rev F03

during calibration runs to provide a better fit with low flow spot gaugings. An irregular weir with a

discharge coefficient of 0.8 was used for the high flow channel. The high flow channel at the gauging

station has a number of irregularities which would reduce the efficiency of the channel to convey water.

The discharge coefficient of 0.8 was used to take account of these channel irregularities, however as no

high flow spot gaugings are available this coefficient could not be calibrated. The modelled Q-h closely

matches the existing rating at Qmed, at which point the existing rating is extrapolated, and therefore there is

high confidence in the existing Qmed of 3.3m3/s following the rating review. The modelled Q-h, existing

rating and spot gaugings are shown in Figure 4.10.8.

Figure 4.10.8: Comparison of Modelled Rating Curves with Existing Rating and Spot Gaugings

(5) Summary of Calibration

It should be noted that this AFA/HPW within the Dublin AFA has limited quantitative hydrometric data from

these events that can be used to undertake detailed model calibration. The historical review outputs were

used to the degree possible to validate the model based on reported information such as flood extents,

recorded flood levels in urban areas, or aerial imagery.

In addition the model was calibrated to the Cadbury’s gauging station on the Santry river. This calibration

yielded good correlation between the recorded spot gaugings and the model output for low flows. See

Section 4.10.5(4) for additional gauging station and calibration details.

Based on the comparison of the modelled flood extents with the recorded historic information the model

provides a good representation of flooding in the Santry AFA and associated HPWs. Specific flooding

mechanisms in locations where information is available to verify the source are being replicated with

satisfactory accuracy.

Appendix A2 shows the flow comparison of the model which produces satisfactory agreement between

model and check flows.


IBE0600Rp00027 4.10 - 24 Rev F03

The mass balance assessment of the model is also within acceptable bounds with a Mass Error Balance

of 0.0217%.

As a result of the draft mapping review workshops, Local Authorities provided information on past flood

events and catchment features that has contributed further to the model verification. A summary of the

comments provided by the Progress Group Member is given below:

A new flood alleviation measure was recently installed at the Harmonstown Road Bridge

Culvert blockage risk at 09SANT00667, the outlet from the attenuation ponds

(6) Other Information:

No other information

4.10.6 Hydraulic Model Assumptions, Limitations and Handover Notes

(1) Hydraulic Model Assumptions:

(a) Please refer to Chapter 3.4 and 3.5 for general assumptions using the Infoworks ICM modelling

software

(b) In channel roughness values have been selected based on normal bounds values which have been

reviewed during the calibration and verification process.

(c) All culverts and bridges have been assumed to run clean during design events.

(d) No specific afflux information is available for calibration of headloss across bridges, as such all bridge

coefficients have been left at default values.

(2) Hydraulic Model Limitations and Parameters:

(a) Road and street networks have been defined by the inclusion of building polygons, but have not been

specifically embedded in the 2D mesh.

(b) Only fluvial flooding from the main channel of the watercourse has been considered. Due to the highly

urbanised nature of the majority of this catchment it is likely that the flooding which occurs at the periphery

of the river corridor during low frequency events is associated with the minor drainage network, as

evidenced by the October 2011 event when flooding was reported at locations remote from the main river

channel at Santry Villas and Santry Avenue and also reported surface water flooding during the July 2009

event.. As such the model output may not be fully representative of recorded flooding within the Santry

catchment.

(c) A mesh resolution of 1m2 to 25m2 has been applied with terrain sensitive meshing.

(d) Default values for the network simulation parameters have been retained.

Hydraulic Model Parameters:

1D Domain

Timestep (seconds) 1

Min / Max Space Step 0.5m / 100m


IBE0600Rp00027 4.10 - 25 Rev F03

Max Timestep Halvings 10

Max Iterations 10

2D Domain

Timestep (seconds) Dynamic

Timestep Stability Control 0.95

Maximum Velocity 10m/s

Theta 0.9

Inundation Mapping depth threshold 0.01m

(3) Design Event Runs & Hydraulic Model Handover Notes:

(a) A model timestep of 1 second has been applied to the model.

(b) No drainage networks have been included in the model, as such flows have been introduced directly to

the 1D domain as point or lateral inflows as determined in the hydrological analysis.

(c) Some minor flooding is predicted in Silloge Park Golf Club in all modelled return periods. The flooding

is predicted to spill onto the M50 during the 0.1% AEP at the Ballymun Junction as shown in Figure

4.10.27. The depth of flooding on the M50 is predicted to reach a depth of 2m in the short stretch where

the flood water is ponding. No properties are predicted to be affected in this area.

Figure 4.10.27: Ponded flood water on the M50

M50

Silloge Park

Golf Club


IBE0600Rp00027 4.10 - 26 Rev F03

(d) Flooding is predicted downstream of the attenuation ponds at Santry Demesne. Flood water crosses

the Swords Road causing flooding in Santry Close as shown in Figure 4.10.28. Flooding in this area is

predicted during the 10% AEP, 1% AEP and 0.1% AEP events. Flooding during the 10% AEP event is low

depth with ponding occurring on the roadway to a depth of 300mm in Santry Close. More substantial

flooding is predicted in the 1% and 0.1% AEP events with ponding up to a depth of 800mm during the

0.1% AEP event. Approximately 20 properties are predicted to be affected during the 1% and 0.1% AEP

events.

Figure 4.10.28: Predicted flooding on the Swords Road and Santry Close

(e) As illustrated in Figure 4.10.29 flood water is predicted to bypass the Harmonstown Road Flow control

structure during the 10% AEP event. The flood water is able to reach the Harmonstown Road via an area

of lower ground immediately upstream of the road. The parapet wall of the Harmonstown Road Bridge

extends 60m from the bridge, however flood water is able to circumvent this wall, flowing across the road

and back into the river on the downstream side of the bridge. Some minor additional flooding from the

storage area created by the flow control structure is predicted during the 0.1% AEP event. Flood water

inundates a small stretch of Lein Park ponding to a depth of approximately 500mm. No properties are

predicted to be affected in this area.

Swords Road

Santry Close


IBE0600Rp00027 4.10 - 27 Rev F03

Figure 4.10.29: Predicted flooding in the vicinity of the Harmonstown Road Bridge and Lein Park

(f) Extensive flooding is predicted downstream of the railway embankment during the 1% and 0.1% AEP

events as shown in Figure 4.10.30. Flooding of the low ground and building to the rear of Raheny

Shopping Centre are predicted to be inundated to a depth of approximately 300mm during the 1% AEP

event and 1.7m during the 0.1% AEP event. Several properties are predicted to be at risk of flooding in

the area between Howth Road and Main Street during the 1% and 0.1% AEP events. Manor House

School downstream of Main Street is also predicted to be affected during these events. Properties in

Water Mill Lawn are also predicted to be at risk during the 0.1% AEP event with depths up to 250mm.

Apartment blocks in the located near the junction of Watermill Road and The Village are shown to be at

risk during the 0.1% AEP event. The watercourse is culverted along this reach however flooding from the

open watercourse upstream is predicted to flow through this area and back into the river on the

downstream side of The Village. A number of manholes are located along this culverted reach, however

the available information indicates that they are sealed and as such no flooding is predicted from them.

Extensive flooding of the James Larkin Road is predicted during the 1% and 0.1% AEP events in the

Raheny AFA, however no properties are indicated to be at risk.

Harmonstown

Road

Lein Park


IBE0600Rp00027 4.10 - 28 Rev F03

Figure 4.10.30: Predicted flood hazard between the railway line and the coast

(4) Hydraulic Model Deliverables:

Model deliverables are supplied in an accompanying InfoWorks ICM transportable database containing all

model files as required by the brief and the relevant network and event files.

Please see Appendix A.4 for a list of all GIS files provided with this report.

(5) Quality Assurance:

Model Constructed by:

Model Reviewed by:

Model Approved by:

Scott Cullen/Andrew Sloan

Andrew Jackson

Grace Glasgow

Railway

Embankment

Howth

Road Main

Street

Manor

House

School

Watermill

Lawn

Watermill

Road

The

Village

Eastern CFRAM Study

IBE0600Rp00027 4.9-29 Rev F03

APPENDIX A.1

STRUCTURE DETAILS

SANTRY BRIDGE OPENING UNITS

ID Length (m) Shape ID

Width (m)

Height (m)

Springing height (m)

Roughness Manning's n

09SANT00938 2.1 RECTANGULAR 4.69 1.22 NA 0.04


09SANT00749 1.2 ARCH 1.33 1.34 1.02 0.045

09SANT00740 (1) 9 RECTANGULAR 6.9 5.18 NA 0.045



09SANT00700 4.6 ARCH 3.2 2.63 1.91 0.04


09SANT00546 3.2 RECTANGULAR 6.73 2 NA 0.045

09SANT00417 2.4 ARCH 3 1.88 1 0.04

09SANT00398 (1) 4.2 ARCH 1.65 2.06 1.52 0.04

09SANT00398 (2) 4.2 ARCH 1.69 2.07 1.57 0.04

09SANT00385 (1) 3.9 ARCH 1.86 2.36 1.82 0.04

09SANT00385 (2) 3.9 ARCH 1.9 2.36 1.8 0.04

09SANT00370 1.8 ARCH 3 2.08 0.58 0.04

09SANT00356 6.5 ARCH 2.97 1.674 0.884 0.04

09SANT00309 3 RECTANGULAR 9 1.7 NA 0.04




Eastern CFRAM Study

IBE0600Rp00027 4.9-30 Rev F03

SANTRY CULVERT UNITS

US Xsec ID Length (m) Shape ID

Width (mm)

Height (mm)

Roughness type

Roughness Value

09SANT01068I 5.9 CIRCULAR 300 300 CW 1.5



09SANT00981ID 5.5 RECTANGULAR 1630 1300 CW 15

09SANT00935I 71 CIRCULAR 600 600 CW 1.5

09SANT00923D (1) 2.9 RECTANGULAR 2190 1200 CW 1.5

09SANT00923D (2) 2.9 RECTANGULAR 2090 1200 CW 1.5

09SANT00921I (1) 13.2 CIRCULAR 600 600 CW 1.5

09SANT00921I (2) 13.2 CIRCULAR 600 600 CW 1.5

09SANT00915D 2.3 RECTANGULAR 1300 300 n 0.02



09SANT00755 2.5 RECTANGULAR 3070 1310 CW 1.5

09SANT00713I 14.7 RECTANGULAR 1950 1640 CW 1.5

06SANT00667 (1) 53.6 CIRCULAR 600 600 CW 1.5

06SANT00667 (2) 53.6 CIRCULAR 600 600 CW 1.5

06SANT00667 (3) 53.6 CIRCULAR 600 600 CW 1.5

06SANT00667 (4) 53.6 CIRCULAR 600 600 CW 1.5

09SANT00616 (1) 44.8 RECTANGULAR 2710 950 CW 1.5


09SANT00493 60.5 RECTANGULAR 3400 1750 N 0.04

09SANT00472 63.1 CIRCULAR 1500 1500 CW 1.5

09SANT00421 32.8 ARCH 3000 2000 N 0.04

09SANT00363 25.1 RECTANGULAR 2920 1945 CW 1.5

09SANT00307I (1) 26.9 ARCH 1750 1520 CW 1.5

09SANT00307I (2) 26.9 ARCH 2130 1730 CW 1.5

09SANT00307I (3) 26.9 ARCH 1750 1650 CW 1.5

09SANT00282 (1) 48 RECTANGULAR 2370 1550 CW 1.5

09SANT00282 (2) 48 RECTANGULAR 2340 1550 CW 1.5

09SANT00141 11.3 RECTANGULAR 3230 4220 N 0.04

09SANT00123 25.2 ARCH 3140 1600 CW 0.04

09SANT00107I (1) 33.5 RECTANGULAR 2340 1640 N 0.04

09SANT00107I (2) 33.5 RECTANGULAR 2400 1640 N 0.04

09SANT00044 82 RECTANGULAR 3500 1880 CW 1.5

SO223717X1 70 RECTANGULAR 3500 1880 CW 1.5

SO22371714 48 RECTANGULAR 3500 1880 CW 1.5

SP22371607 10 RECTANGULAR 3500 1800 CW 1.5



Eastern CFRAM Study

IBE0600Rp00027 4.9-31 Rev F03

SANTRY WEIR UNITS

ID Type Crest (m AD)

Width (m)

Length (m) Discharge coefficient

09SANT00740 BROAD CRESTED 49.32 3 1.5 na

09SANT00446 IRREGULAR 35.293 3 NA 1.7

09SANT00387W BROAD CRESTED 32.94 4.5 0.3

Na

09SANT00336 IRREGULAR 29.234 3 NA 1.85

09SANT00257 BROAD CRESTED 23.63 4.5 0.2

NA


NA


NA


NA


NA


NA


NA


NA


IBE0600Rp00027 4.9-32 Rev F03

APPENDIX A.2

FLOW COMPARISON

IBE0600 EAST CFRAM STUDY RPS

PEAK WATER FLOWS

AFA Name Santry River HPW

Model Code HA09_SANT1

Status DRAFT

Date extracted from model 18/02/2015

Peak Water Flows

River Name & Chainage AEP Check Flow (m3/s) Model Flow (m3/s) Diff (%)

Santry River 5070 50% 2.48 2.44 -1.6

09_1507_6_RPS 10% 4.46 4.15 -7

1% 8.25 7.14 -13.5

0.1% 14.71 10.87 -26.1

Santry River 3360 50% 3.25 3.15 -3

09012_RPS 10% 5.83 5.43 -6.8

1% 10.78 9.54 -11.5

0.1% 19.23 15.22 -21

Santry River 00001 50% 5.24 5.03 -4

09_1507_17_RPS 10% 9.43 8.82 -6.5

1% 17.42 15.42 -11.5

0.1% 31.06 24.56 -20.9

The table above provides details of the hydrological flow estimation, the modelled flow and the

percentage difference at the check HEPs along the Santry River.

The table indicates a good correlation between the hydrological and modelled flows for the 50% and

10% AEP events, with the maximum difference being 7% during the 10% event.

Deviation of the modelled flows from the hydrological estimates is evident in the 1% AEP and 0.1%

AEP events which is to be expected in a river such as the Santry. The Santry River has a number of

attenuation structures and ponds which will affect the propagation of peak flows along the river

system.

Figure 4.10.31 illustrates the affect the attenuation pond at Santry Demesne has on the flow

hydrograph during the 1% AEP event. The blue trace defines the inflow hydrograph to the pond while

the green trace defines the outflow flow hydrograph through the outlet structure adjacent to the


IBE0600Rp00027 4.9-33 Rev F03

Swords Road. Flattening of the outflow hydrograph is caused by the pond level reaching the point

when flood water begins to spill across the Swords Road. As this flow path will not be as efficient as

through the channel and conduit network the hydrograph will become attenuated.

Figure 4.10.31 Santry Demesne pond inflow and outflow hydrographs

The flow control structure at the Harmonstown Road also has an effect on the in-channel flows, as

indicated by the hydrographs shown in Figure 4.10.32. However as discussed in Section 4.10.6 (3e)

flow is able to bypass the structure once the available storage is depleted.


IBE0600Rp00027 4.9-34 Rev F03

Figure 4.10.32 Harmonstown Road Bridge in-channel hydrographs

There are also some locations where flood flow leaves the system ponding in remote locations and not

returning to the river channel. This is particularly evident during the 0.1% AEP event at the M50

crossing which does not have sufficient capacity to carry the 0.1% AEP flow. Flood water ponds

upstream of the culvert, eventually spilling onto the M50 (see Figure 4.10.27).


IBE0600Rp00027 4.9-35 Rev F03

APPENDIX A.3

LONG SECTION - DOWNSTREAM OF CLONSHAUGH ROAD

Hormonstown Road Bridge


IBE0600Rp00027 4.9-36 Rev F03

LONG SECTION - UPSTREAM OF CLONSHAUGH ROAD

Santry Attenuation Pond


IBE0600Rp00027 4.9-37 Rev F03

Appendix A.4 Deliverables


IBE0600Rp00027 4.9-38 Rev F03

Fluvial Model Files - ICM Transportable Database containing all relevant model files linked by simulation file for each required run -

HA09_SANT1_F01.ICMT

GIS Deliverables - Hazard

Flood Extent Files (Shapefiles) Flood Depth Files (Raster) Water Level and Flows (Shapefiles) Fluvial Fluvial Fluvial E38EXFCD100F0 E38DPFCD100F0 E38NFCDF0 E38EXFCD010F0 E38DPFCD010F0 E38EXFCD001F0 Coastal E38EXCCD100F0 E38EXCCD005F0 E38EXCCD001F0

E38DPFCD001F0 Coastal E38DPCCD100F0 E38DPCCD005F0 E38DPCCD001F0

Flood Zone Files (Shapefiles) Flood Velocity Files (Raster) Flood Defence Files (Shapefiles) E38ZNA_FCDF0 E38VLFCD100F0 N/A E38ZNB_FCDF0 E38VLFCD010F0

E38VLFCD001F0 Coastal E38VLCCD100F0 E38VLCCD005F0 E38VLCCD001F0


IBE0600Rp00027 4.9-39 Rev F03

GIS Deliverables - Risk

Specific Risk - Inhabitants (Raster) General Risk - Economic (Shapefiles) General Risk-Environmental (Shapefiles) Fluvial One UoM Map E38RIFCD100F0 E38RIFCD010F0 E38RIFCD001F0 Coastal E38RICCD001

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