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APRIL 2009

Compiled for: Compiled by: Theewaterskloof Municipality Institute for Water & Environmental Engineering P O Box 24 Department of Civil Engineering Caledon University of Stellenbosch

7230 Private Bag X1, Matieland, 7602 SOUTH AFRICA

Greyton River Management Plan

Floods, flow patterns, river stability and mitigation measures

Hydraulic Model Study FINAL APRIL 2009

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EXECUTIVE SUMMARY

Greyton has experienced severe flooding with damage to properties during recent years with the last flood occurring in November 2008. IWEE was appointed during 2008 to carry out a detailed investigation of the flood hydrology and the river hydraulics of the main rivers: Plattekloof, Noupoort, Gobos and Scholtz Rivers at the town. The key aims of the study were to identify the 1:100 year floodline for the current development, also considering the fluvial morphology of the Gobos River, and to identify possible mitigation measures where properties are at risk of flooding. The study was carried out by building a physical hydraulic model of the rivers at the Hydraulics Laboratory of the University of Stellenbosch. The key findings of the study are as follows:

The flood hydrology was determined and analysis of the data of an automatic rain gauge located in Greyton indicated that the 2007 and November 2008 storms were average storm events which can be expected on a regular base

The main areas where properties will be inundated during the 1:100 year flood are: o Right bank of Plattekloof River where the floodplain falls away from the river o The Gobos River near the southern end of the town has properties on both

sides of the river which could be affected o The Scholtz River is the most critical area with wide floodplain flow even

during small floods. The main channel is very small and surrounded by houses on existing properties. Hydraulics structures also constricted the flow in the past

Hydraulic structures at risk to flood damage are: o The Gobos Road bridge on the Greyton-Riviersonderend Road where the

approach roads have scoured during the 2008 flood o The pipe culvert on the Scholtz River where the pipes were damaged and the

approach road washed away

Movable bed tests on the Gobos River indicated that the lower Gobos at the Southern end of the town could migrate further to the west over time. The situation has to monitored in the field by annual river bank surveys. In this reach the right bank has been protected with gabions in the past. For future protection riprap is proposed. One new house on the right bank next to the river needs immediate erosion and flood protection at this stage.

Flood mitigation measures are required as follows: o A flood levee on the right floodplain of the Plattekloof River o The pedestrian bridge and its fixed bed and fixed abutments on the Plattekloof

River have to be removed o The culvert on the Plattekloof River near the confluence with the Gobos River

has to be removed and the upstream gabion protected low water crossing should be maintained

o The flow through the Gobos River Road bridge could be streamlined by using spur dykes protected with riprap or the road should be protected against erosion

o A levee with riprap protection should be constructed on the right bank of the Gobos River at the southern end of the town where a new house was constructed on the river bank recently.

o On the Scholtz River the existing canal and hydraulic structures are too small. A canal could be constructed along the road where the river currently flows.




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The canal could be concrete lined, or lined with riprap (dumped rock/boulders), Reno mattresses, or grouted stone pitching. The bed slope of the canal would be steep at 1:50 and the flow would be supercritical. Different flows and canal options were investigated. The full 1:100 year flood could be conveyed by the canal, or if a flood attenuation dam is constructed upstream of the canal, the canal discharge could be decreased resulting in a smaller canal.

Possible flood attenuation dams were evaluated with and without excavation of the reservoir area, and a fully open or 50% blocked outlet was considered. In addition a double peaked flood was considered, without outlet blockage, to evaluate the safety of the canal design. The attenuation dam could be an earth embankment with concrete bottom outlet and concrete uncontrolled spillway. Stilling basins are required at the dam and outlet. The canal should be lined from the dam downstream to the end of the development at the southern end of the town. The canal-flood attenuation dam system could be sized to maximize the use of the existing main road culvert at about 16 m3/s, or else the culvert should be widened (without a pier in the flow). All other culverts should be removed and now structures should be constructed in the proposed canal. At the downstream end of the canal and where the canal bends at the end of the road concrete stilling basins are required. Possible reduction of the canal slope by steps was considered, but this creates a deep canal and the flow becomes unstable for some flow conditions when 0.8<Fr<1.2. It is proposed that a riprap lined canal, possibly with a flood attenuation dam is implemented. The riprap could consist of river boulders which would make it look natural and some vegetation could be allowed to establish over time in the finer deposited sediment in the riprap.

The 1:100 year floodlines determined in this study should be used to prevent further development on land within the floodlines indicated. On existing properties which fall within the floodlines, further or any future development should be controlled until the recommendations of this report has been implemented to ensure the safety of people and property. On the Gobos River until the bank erosion protection is in place, the widest of the 1:100 year floodline or the simulated possible lateral erosion should be taken as the floodline. Refer to Figure 1.




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Figure 1 High risk of flooding of properties in Greyton (red) considering the 1:100 year flood and possible bank erosion where further development should be controlled

until proposed mitigation measures have been implemented




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TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY ......................................................................................................................................... II TABLE OF CONTENTS ............................................................................................................................................ V LIST OF FIGURES ................................................................................................................................................. VII

1. INTRODUCTION .................................................................................................................................. 1

2. AIMS OF THE STUDY ........................................................................................................................... 1

3. RECENT FLOODING EXPERIENCED IN GREYTON................................................................................... 1

4. FLOOD HYDROLOGY .......................................................................................................................... 10

4.1 INTRODUCTION ....................................................................................................................................... 10 4.2 RESULTS .................................................................................................................................................. 11 4.3 ANALYSIS OF THE STORM EVENT 11 NOVEMBER 2008. ...................................................................................... 11

5. DESIGN OF THE PHYSICAL MODEL OF THE RIVERS............................................................................. 15

6. MODEL CALIBRATION ....................................................................................................................... 17

7. MODEL SIMULATED FLOOD LEVELS ................................................................................................... 19

7.1 GOBOS RIVER .......................................................................................................................................... 19 7.2 SCHOLTZ RIVER ........................................................................................................................................ 22 7.3 PLATTEKLOOF AND NOUPOORT RIVERS ......................................................................................................... 27

8 GOBOS RIVER MORPHOLOGICAL STABILITY ...................................................................................... 32

8.1 GENERAL .................................................................................................................................................... 32 8.2 HISTORICAL FLOW PATTERNS BASED ON AERIAL PHOTOS ....................................................................................... 32 8.3 SIMULATED GOBOS RIVER MIGRATION PATTERNS BASED ON PHYSICAL MODEL .......................................................... 34

9. MITIGATION MEASURES TO LIMIT FLOODING .................................................................................. 36

9.1 GOBOS RIVER .............................................................................................................................................. 36

9.1.1 General ............................................................................................................................................ 36

9.1.2 Gobos River bridge .......................................................................................................................... 36

9.1.3 Existing properties to the east of the current main channel ........................................................... 38

9.1.4 Existing properties at the right bank at the southern end of Greyton............................................. 38

9.2 PLATTEKLOOF AND NOUPOORT RIVERS............................................................................................................. 39 9.3 SCHOLTZ RIVER ............................................................................................................................................ 41

9.3.1 Possible mitigation measures .......................................................................................................... 41




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9.3.2 Canal lining options ......................................................................................................................... 41

9.3.3 Flood attenuation dam .................................................................................................................... 52

10. MITIGATION MEASURES TO LIMIT POSSIBLE LATERAL EROSION ON THE GOBOS RIVER ................... 61

11. PROPERTIES WHERE FURTHER DEVELOPMENT SHOULD BE CONTROLLED DUE TO RISK OF FLOODING

......................................................................................................................................................... 63

12. CONCLUSIONS AND RECOMMENDATIONS ........................................................................................ 64

APPENDIX A FLOOD HYDROLOGY CALCULATIONS

APPENDIX B RIVER SURVEY DATA AND CROSS-SECTION ONS ON CD

APPENDIX C FLOODLINE DRAWING & MITIGATION MEASURES

APPENDIX D DVD WITH LABORATORY HYDRAULIC MODEL TESTS




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LIST OF FIGURES

Figure 1 High risk of flooding of properties in Greyton (red) considering the 1:100 year flood and possible bank erosion where further development should be controlled until proposed mitigation measures have been implemented ............................... iv

Figure 3-1 Greyton Town layout ........................................................................................ 2 Figure 3-2 Main rivers and hydraulic structures at Greyton ................................................ 3 Figure 3-3 Upper Scholtz River culvert damaged during 2008 flood .................................. 4 Figure 3-4a Excavated river canal after the 2007 flood on the Scholtz River ................... 5 Figure 3-4b Scoured channel after the 2008 flood on the Scholtz River ........................... 5 Figure 3-5 Overflowing of culverts during November 2008 flood due to discharge capacity

limitations and partial blockage ...................................................................... 6 Figure 3-6 Flooding of new development on erf 1545 during November 2008 ................... 6 Figure 3-7 Flood flow through Erf 1545 towards the Gobos River during November 2008 . 7 Figure 3-8a New house at southern end of town on Gobos River during July 2008 ......... 7 Figure 3-8b New house at southern end of town on Gobos River following November 2008

flood with scour of the bank visible ................................................................. 8 Figure 3-9 Plattekloof culvert near confluence with Gobos River with approach roads

washed away two years ago. The culvert should be removed. ......................... 8 Figure 3-10 Gobos Road bridge left approach road scour after the November 2008 flood 9 Figure 4-1 Recorded rainfall at 30 minute time intervals ...................................................12 Figure 4-2 Cumulative rainfall of the 2008 storm ..............................................................13 Figure 4-3 Observed rainfall at 3 h time intervals .............................................................14 Figure 5-1 Greyton model layout ......................................................................................15 Figure 5-2 Photograph of the Greyton physical model ......................................................16 Figure 6-1 Mathematical model bathymetry of the Gobos River .......................................17 Figure 6-2 Mathematical model simulated flow depths on Gobos River ............................18 Figure 6-3 Mathematical model simulated flow velocity on Gobos River ...........................19 Figure 7.1-1 Floodlines along the Gobos River as simulated in the physical model .........20 Figure 7.1-2 Longitudinal profile of the flood levels along the Gobos River ......................20 Figure 7.1-3 Longitudinal profile of the flood levels along the Gobos River ......................21 Figure 7.1-4 Simulated flow patterns during the 1:100 year flood on the Gobos River .....21 Figure 7.1-5 Simulated flow patterns during the 1:100 year flood at the Gobos River Road

Bridge............................................................................................................22 Figure 7.2-1 Floodlines along the Scholtz River as simulated in the physical model ........23 Figure 7.2-2 Pipe culvert structure during the 1:100 year flood in the laboratory ..............24 Figure 7.2-3 Pipe culvert following the 2008 flood ............................................................24 Figure 7.2-4 Scholtz River culvert on the main road viewed from upstream .....................25 Figure 7.2-5 Longitudinal profile of the Scholtz River showing the river bed level and 1:100

year flood level ..............................................................................................25 Figure 7.2-6 Model simulated 1:100 year flood on the Scholtz River ................................26 Figure 7.3-1 Floodlines along the Plattekloof River as simulated in the physical model ...27 Figure 7.3-2 Plattekloof 1:100 year flood as simulated in the model ................................28 Figure 7.3-2 Pedestrian bridge on Plattekloof River viewed from upstream .....................29 Figure 7.3-3 Pedestrian bridge on Plattekloof River with fixed bed level which has caused

local scour downstream .................................................................................29 Figure 7.3-4 Right bank stone pitching bank protection on the Plattekloof River upstream of

the pedestrian bridge .....................................................................................30 Figure 7.3-5 Bed profile along the Plattekloof River near the pedestrian bridge ...............30 Figure 7.3-6 Simulated 1:100 year flood flow at the pedestrian bridge .............................31 Figure 8-1 Historical Gobos River flow patterns based on aerial photos ...........................33




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Figure 8.3-1a Simulated migrated right bank of upper Gobos River ...................................34 Figure 8.3-1b Simulated migrated right bank of lower Gobos River ...................................35 Figure 8.3-2 River bed scour at end of tests showing the different flow patterns of the

Gobos River ..................................................................................................35 Figure 9.1-1 Temporary rehabilitation of the road after the 2008 flood (viewed from

upstream) ......................................................................................................36 Figure 9.1-2 Proposed spur dykes as tested in the physical model at the Greyton-Riverdale

Road bridge...................................................................................................37 Figure 9.1-3 Proposed levees as tested in the physical model at the properties near

southern Greyton on the Gobos River ...........................................................38 Figure 9.2-1 Flood levee location .....................................................................................39 Figure 9.2-2 Flood levee location downstream of Noupoort River ....................................40 Figure 9.2-3 Proposed location of the levee along the Plattekloof River...........................41 Figure 9.3-1 Concrete lined canal ....................................................................................42 Figure 9.3-2 Riprap bank protection on the Franschhoek River .......................................43 Figure 9.3-3 Gabion boxes bank protection on the Scholtz River upstream of the main road

culvert ...........................................................................................................44 Figure 9.3-4 Typical Armorflex lining ................................................................................45 Figure 9.3-5 Big Lotus River, Cape Town, with Armorflex lining ......................................45 Figure 9.3-6 Grouted stone pitching river bank with riprap at river bed ............................46 Figure 9.3-7 Stepped canal to dissipate the energy .........................................................46 Figure 9.3-8 Plan layout of proposed canal route .............................................................48 Figure 9.3-9 Typical canal cross-sections ........................................................................49 Figure 9.3-10 Longitudinal profiles of the proposed canals at 16 m3/s ...............................51 Figure 9.3-11 Longitudinal profiles of the proposed canals at 25 m3/s ...............................51 Figure 9.3-12 Longitudinal profiles of the proposed canals at 44 m3/s ...............................52 Figure 9.3-13 Flood attenuation dam site ...........................................................................53 Figure 9.3-14 Flood hydrograph of the 1:100 year flood ....................................................54 Figure 9.3-15 Inflow and outflow hydrographs for 16 m3/s outflow flood peaks-no excavation

of reservoir ....................................................................................................55 Figure 9.3-16a Inflow and outflow hydrographs for 25 m3/s outflow flood peaks-no excavation

.................................................................................................................56 Figure 9.3-16b Inflow and outflow hydrographs for 25 m3/s outflow flood peaks-with

excavation .....................................................................................................56 Figure 9.3-17 Inflow and outflow hydrographs for 16 m3/s outflow flood peaks-with

excavation .....................................................................................................57 Figure 9.3-18 Inflow and outflow hydrographs for 5 m3/s outflow flood peaks-with excavation

of the reservoir basin .....................................................................................57 Figure 9.3-19 Plan layout of outlet works and spillway .......................................................58 Figure 9.3-20 Plan layout of outlet works and spillway .......................................................59 Figure 9.3-21 Cross-section at outlet works and stilling basin ............................................59 Figure 9.3-22 Flood attenuation dam inundation for 7m high dam wall to FSL and no

reservoir excavation ......................................................................................60 Figure 9.3-23 Flood attenuation Dam inundation for 7m high dam wall to FSL and with

reservoir excavation ......................................................................................60 Figure 10-1 Existing gabion boxes on right bank of the Gobos River near southern end of

town ..............................................................................................................61 Figure 10-2 Proposed flood levee at southern end of Greyton as observed in the

laboratory ......................................................................................................62 Figure 11-1 High risk of flooding of properties in Greyton (red) considering the 1:100 year

flood and possible bank erosion where further development should be controlled until proposed mitigation measures have been implemented ........63




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LIST OF TABLES

Table 4.1 KV3 flood peaks ..............................................................................................10 Table 4.2 Flood Peaks at Gobos upstream of Greyton ....................................................11 Table 4.3 Summary of results .........................................................................................11 Table 9.3-1 Proposed canal typical hydraulic characteristics at a bed slope of 1:50 .......50 Table 9.3-2 Flood attenuation dam characteristics .........................................................55




1

1. INTRODUCTION

The town of Greyton has experienced severe flood damage during recent years. The Theewaterskloof Municipality appointed the Institute for Water and Environmental Engineering (IWEE) of the Department of Civil Engineering, University of Stellenbosch during 2008 to investigate the floods and associated flow patterns through the town and to propose flood mitigation measures to limit future flooding.

2. AIMS OF THE STUDY The aims of the study were as follows: a) Review of the flood hydrology carried out previously for the 2001 floodline report and to

investigate the severity and frequency of recent storm events b) Investigation of flooded areas in the model during the 1:100 year flood c) Review of the Gobos River fluvial morphology stability d) Investigation of suitable mitigation measures to limit possible flooding of properties

3. RECENT FLOODING EXPERIENCED IN GREYTON In recent years there has been a significant growth in property development in Greyton, both in the existing town and with extensions of the town to the south (refer to Figure 3-1) Many of the old town properties are located on the floodplains of the rivers running through the town. Figure 3-2 shows the main rivers at Greyton, with the Gobos the largest river with a shallow and wide river coarse. Upstream of the town the Gobos is joined by the Plattekloof River. The Noupoort River also joins the Plattekloof River before the latter joins the Gobos River. Near the Southern end of the town the Gobos River is joined by the Boesmanskloof River, also known as the Scholz River. At the confluence of the Gobos and Plattekloof Rivers the contribution of the Gobos River to the 1:100 year flood peak is about 63 %, while at the confluence of the Scholz River with the Gobos, the contribution of the Scholz River is about 17 %.




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Figure 3-1 Greyton Town layout




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Figure 3-2 Main rivers and hydraulic structures at Greyton

Gobos River

Plattekloof River

Scholtz River

Road bridge Greyton to Riverdale

Main Road culvert

Pipe culvert

Pedestrian bridge

Low water drift

Damaged box culvert

NoupoortRiver




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The most recent floods in Greyton occurred in November 2008. As during previous floods, most of the damage occurred along the Scholz River. The main flood damage experienced recently in Greyton are as follows (for locations refer to Figure 3-2):

Upper Scholz River Road pipe culvert damaged and road washed away on left bank (Figure 3-3)

Flood damage to properties along the Scholz River, especially downstream of the main road culvert. During the 2008 flood two culverts that caused damming along the Scholz River were removed by the Municipality. The existing river is only a few meters wide and floodplain flow occurs regularly. The newly constructed retirement village has also been severely affected by the floods (Figures 3-4 to 3-7).

On the Gobos River at the southern end of the town a new development and nearly completed house was inundated on the river bank with the river bank also moving very close to the house (Figure 3-8).

The road approaches to a culvert on the Plattekloof River near the confluence with the Gobos River were washed away two years ago (Figure 3-9). Further upstream on the same Plattekloof River a low water crossing also experienced damage to gabion erosion protection measures and a pipeline crossing the river.

Gobos Road bridge on the Greyton to Riviersonderend Road approach roads experienced severe scour (Figure 3-10)

Flooding has been experienced on the Plattekloof River floodplain in the past.

Figure 3-3 Upper Scholtz River culvert damaged during 2008 flood




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Figure 3-4a Excavated river canal after the 2007 flood on the Scholtz River

Figure 3-4b Scoured channel after the 2008 flood on the Scholtz River




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Figure 3-5 Overflowing of culverts during November 2008 flood due to discharge

capacity limitations and partial blockage

Figure 3-6 Flooding of new development on erf 1545 during November 2008




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Figure 3-7 Flood flow through Erf 1545 towards the Gobos River during November

2008

Figure 3-8a New house at southern end of town on Gobos River during July 2008




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Figure 3-8b New house at southern end of town on Gobos River following November

2008 flood with scour of the bank visible

Figure 3-9 Plattekloof culvert near confluence with Gobos River with approach

roads washed away two years ago. The culvert should be removed.




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Figure 3-10 Gobos Road bridge left approach road scour after the November 2008

flood




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4. FLOOD HYDROLOGY

4.1 INTRODUCTION

KV3 Engineers carried out the flood hydrology for the 2001 floodline study and were appointed by the Institute for Water and Environmental Engineering (IWEE) at the University of Stellenbosch during this study to do an assessment of the expected floods in the Greyton area as part of the Greyton River Management Study. Their conclusions are provided in Report 23826KP0, dated 11 September 2008, and are enclosed in Appendix A. Five areas are of specific interest to the IWEE. These areas are shown on the map in Appendix A and are: :

A. Flood peak of the Plattekloof stream upstream of Greyton, B. Flood peak of the combined Plattekloof and Noupoort catchment, at the confluence

with the Gobos, C. Flood peak of the Gobos upstream of Greyton, D. The flood peaks of the Scholtz River and E. The expected peaks just downstream of the confluence of the Scholtz and Gobos

Rivers. The findings of the KV3 report are summarised in Table 4.1 below for some of the catchments.

Table 4.1 KV3 flood peaks

Location Q50

(m3/s) Q100

(m3/s)

Gobos, upstream of Greyton 152 195

Scholtz River 43 55

Gobos & Scholtz (Downstream of Greyton)

199 255

Noupoort at Gobos confluence 105 136

Platkloof at Noupoort confluence 34 45

The recommended results by KV3 were all based on the Rational method and on the MAP as measured at Greyton, station number 0007183W. Although small differences were found in catchment characteristics as calculated by KV3 in comparison with those calculated by the IWEE in a review, it was decided to use the characteristics as calculated by IWEE. The rainfall was however not only obtained from a single station as done by KV3, but regional data was used instead, as suggested by Smithers and Schulze (WRC K5/1060). The Gobos upstream of Greyton was use as a comparative catchment and the results obtained using the following catchment characteristics are shown in Table 4.2. Catchment area : 34 km2 MAP : 790 mm (Based on WRC K5/1060) L : 17.91 km Lc : 7.9 km Avg watercourse slope : 0.02931 m/m (Taylor-Swartz method)




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Table 4.2 Flood Peaks at Gobos upstream of Greyton

Method Q50

(m3/s) Q100

(m3/s)

Rational (KV3) 152 195

Rational 165 201

Alternative Rational 139 169

SDF 153 188

Unit Hydrograph 112 140

The Alternative Rational method only differs from the Rational method in the way the point precipitation is calculated. Since the work done as suggested in WRC K5/1060 is based on regional data with the most up to date analysis, it was decided not to use the Alternative Rational method. While the SDF provided comparable results with those calculated by KV3, it was once again decided to give preference to the most recent regional analysed rainfall data as an input parameter as opposed to a single reference rainfall station used in the SDF. It is however comforting to note the similarity in the final results. The Unit Hydrograph method provides significant lower values, which is due to the small catchment areas investigated. These areas fall outside the recommended scope of the method and the method was therefore not used any further.

4.2 RESULTS

The Rational method was applied to calculate the required flood peaks at the five sites of interest and the results are summarised in Table 4.3. The calculation tables and the

positions of the catchments are shown in Appendix A. In all the cases the regional storm

rainfall was used as suggested in WRC K5/1060. Table 4.3 Summary of results

Area Tc (h) Flood peaks in m3/s

1:20 1:50 1:100 1:200 RMF

A- Plattekloof stream 0.41 29 35 46 56

B- Plattekloof & Noupoort 0.62 66 88 111 154 161

C- Gobos, upstream of Greyton

2.38 127 165 201 278 321

C’- Gobos, Noupoort and Plattekloof

2.8 155 202 246 339 430

D- Scholtz River 0.7 26 34 44 60 171

E- Downstream of Gobos and Scholtz

2.8 168 219 267 369 444

4.3 ANALYSIS OF THE STORM EVENT 11 NOVEMBER 2008.

For comparative reasons, the actual rainfall data at a rainfall station situated in Greyton and owned by Mr Derek Crabtree, was obtained and analysed. The rainfall data was logged over a period of approximately 2 years and cover the period October 2006 to December 2008.




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Rainfall was logged using a tipping bucket gauge, set to record at varying time intervals. The worst storm recorded at this station during the recording period occurred in November 2008, with the actual storm rainfall event starting at 4h00 on 11 November 2008. Figure 4-1. provides the actual rainfall per 30 min time interval, while Figure 4-2 provides the cumulative rainfall during the storm period. Figure 4-2 clearly shows an evenly distributed storm over the analysis period.

Figure 4-1 Recorded rainfall at 30 minute time intervals

A detail analysis of this storm shows that the most severe 24h rainfall occurs between 9h00 on 11 November 2008 and 9h00 on 12 November 2008, when a rainfall of 156.4 mm for the 24hour period was recorded. Comparison with the long term data available from station 0007062W, also situated in Greyton (Municipality), indicate that this rainfall represents a 24h storm event with a probability of occurrence varying between 1 in 100 (145 mm) and 1 in 200 (170 mm) years.

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Figure 4-2 Cumulative rainfall of the 2008 storm

Analysis of the specific storm event for a short period to correspond with a storm duration of 3h (Tc = 2.8h) which will provide the highest flood peak of the full catchment of the Gobos downstream of the town, however provides different results as shown in Figure 4-3. According to this analysis the specific storm event only had 2 distinct 3h storm peaks over the 3 day period with storm peaks in excess of the expected 1 in 2 year storm event. Although a significant amount of rainfall did occur, the flood producing storm events can be considered as rather average for the area. A 3 hour storm with similar magnitude also occurred on 21 November 2007. These 2 storms were the only events of significance during the record period and both occurred during the summer months and not during the winter months as could be expected.

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Figure 4-3 Observed rainfall at 3 h time intervals

Although it is extremely difficult to compare the results from a long 24h rainfall data range with the results obtained from a very short data set, reflecting actual measured short rainfall events, it can be concluded that the storms experienced in November 2007 and again in November 2008, were average storm events which can be expected on a regular base.

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5. DESIGN OF THE PHYSICAL MODEL OF THE RIVERS A physical model of the rivers at Greyton was designed and constructed during this study at the Hydraulics Laboratory of the University of Stellenbosch. Due to space limitations a horizontal scale of 1:125 was selected and a vertical scale of 1:40. The vertical model distortion was required to maintain rough turbulent flow conditions in the laboratory. 1 m horizontal distance in the model therefore represents 125 m in the field, and vertically 1 m in the model represents 40 m in the prototype. The layout of the model is shown in Figures 5-1 and 5-2.

Figure 5-1 Greyton model layout

Model boundary




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Figure 5-2 Photograph of the Greyton physical model

The model flow was scaled based on Froude similarity. The flow in the model is 31623 times smaller than in the prototype. For a flood of 250 m3/s on the Gobos River the model discharge was 7.9 l/s. The flow velocity observed in the model is 6.325 times smaller than in the prototype. The bridge on the Gobos River was included in the model. On the Plattekloof River only the small private pedestrian bridge was included, since the one further downstream near the Gobos River confluence road was washed away and it is clear that the type of structure obstructs the flow and it would therefore be a recommendation to remove the bridge. On the Scholtz River two culvert structures were included in the model at the main street and further upstream (refer to Figure 3-2). Other culverts were removed during the recent flood and were not considered again in the model. The tailwater level of the model was determined by calculating the 1:100 year flood level in the Sonderend River, and calculation of the backwater effect at the downstream end of the model. The 1:100 flood in the Sonderend River was determined as 1678 m3/s for a catchment area of 930 km2. The method of TR137 was used to determine the RMF and a scaled 1:100 flood peak was then determined. The 1:100 year flood tailwater level at the downstream end of the model is 207.5 m, while the river bed level at this location is 205.5 m. The Gobos River was constructed 100 mm lower in the model (4 m in the prototype) than the other rivers so that a movable bed could be added following fixed bed tests of the river. Boulders were sampled in the field to obtain the sediment grading analysis. The grading calculations are shown Appendix A. The median sediment diameter was found to be 79 mm in the field. If this is scaled down for the movable bed in the model, a diameter of 6 mm is obtained based on the same sediment density and on incipient motion conditions. The boulders are present in the main channels of the Gobos River and limit deep scour, but is conducive to lateral erosion of the river. It was therefore decided to place a 2 m layer of boulders and on top a layer of fine sand in the model to give a total bed thickness of 4 m

Gobos River

Scholtz

Plattekloof




17

(prototype) (0.1 m in laboratory). The fine sand had a median diameter of 0.15 mm in the model. The model was constructed based on a new survey of cross-sections in the field. The survey data and cross-sections are attached on CD in Appendix B.

6. MODEL CALIBRATION Model calibration of the fixed bed Gobos River was required to simulate the correct water levels. A two dimensional mathematical model was used to simulate the 1:100 year flood peak water levels, assuming conservatively high hydraulics roughness values of Manning n = 0.045 and 0.060 in the main channel and floodplains respectively. The model bathymetry is shown in Figure 6-1 and the simulated water depths and flow velocities in Figures 6-2 and 6-3. Roughness elements were added in the physical model to obtain these flow depths in the main channel of the river.

Figure 6-1 Mathematical model bathymetry of the Gobos River




18

Figure 6-2 Mathematical model simulated flow depths on Gobos River




19

Figure 6-3 Mathematical model simulated flow velocity on Gobos River

Houses were also added in the physical model next to the main rivers since they create secondary energy losses. The location and dimensions of the houses were obtained from 2001 aerial photography and satellite images for the new developments.

7. MODEL SIMULATED FLOOD LEVELS The physical model simulated 1:50 and 1:100 flood lines are indicated in the figures below and in the drawing in Appendix C. The key findings are:

7.1 GOBOS RIVER

The main flooding problems are near the downstream end of the town, where houses are located on the left and right banks of the main channel. Flood levees could possibly solve the problem (Figures 7.1-1 and 7.1-2). The bridge across the Gobos River (road to Riviersonderend) scoured during the November 2008 flood at its left (eastern) approach road. The freeboard at the bridge is however sufficient during the 1:100 year flood.




20

The flood flow is also across the road to Riviersonderend to the east of Greyton and the bridge, which also helps to lower the flood levels at Greyton. A longitudinal profile of the river bed level and the 1:100 year flood level is shown in Figure 7.1-3 .

Figure 7.1-1 Floodlines along the Gobos River as simulated in the physical model

Figure 7.1-2 Longitudinal profile of the flood levels along the Gobos River

Model boundary




21

Figure 7.1-3 Longitudinal profile of the flood levels along the Gobos River

Model simulated flows during the 1:100 year flood are shown in Figures 7.1-4 on the Gobos River and Figure 7.1-5 shows the flow pattern at the road bridge on the Greyton-Riviersonderend Road.

Figure 7.1-4 Simulated flow patterns during the 1:100 year flood on the Gobos River

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1:100 year flood level Gobos bed level

Bridge




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Figure 7.1-5 Simulated flow patterns during the 1:100 year flood at the Gobos River

Road Bridge

7.2 SCHOLTZ RIVER

Figure 7.2-1 shows the floodlines for the river as simulated in the physical model. It was clear in the model tests that at sharp river bends the river climbs out onto the banks and floodplains. Due to the steep slope (about 1:50) of the stream, the flow velocities are high and super critical which could make evacuation of the properties extremely dangerous. The existing culverts are too small and during the 1:100 year flood the river channel discharge capacity is exceeded, resulting in wide floodplain flow through the existing houses. It is estimated that the river channel has a discharge capacity of only 5 m3/s (compared to the 1:100 year peak discharge of 44 m3/s). Therefore smaller more regular floods such as a flood that would occur once in 10 years also have extensive flooding risk. Only two culverts remained along the river after other smaller culverts were removed during the 2008 flood to limit damming caused by blockage of the culverts. The culvert at location 1 (Figure 7.2-1) has 3 x 0.9 m diameter pipes and the maximum discharge capacity is estimated at 5 m3/s. Therefore most of the flow would flow across the road and damage to the approach roads could be expected as was experienced during especially the 2008 flood. See Figures 7.2-2 and 7.2-3. The main road culvert (location 2 on Figure 7.2-1) is 3.5 m wide and 2.1 m high, with a discharge capacity of 16 m3/s. The culvert can therefore currently only handle a 1:10 to 1:15 year flood (Figure 7.2-4). A longitudinal profile of the river with the simulated 1:100 year flood levels is indicated in Figure 7.2-5.




23

Of all the flood affected areas in Greyton, the Scholtz River is the highest risk river based on possible loss of lives and damage to property. The river main channel is currently quite small and house are located very close to the river. The new housing development at the southern end of the town is now also located right in the middle of the flood flow. Although the water flow depth on the floodplains would be relatively shallow, the floodplain is hydraulically steep (about 1:50 slope), which would make it very difficult for people to evacuate during a flood. The time of concentration of the flood is also very short, only 0.7 h, so there would be no warning time to evacuate the floodplain.

Figure 7.2-1 Floodlines along the Scholtz River as simulated in the physical model

Location 1

Location 2

River

Model boundary




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Figure 7.2-2 Pipe culvert structure during the 1:100 year flood in the laboratory

Figure 7.2-3 Pipe culvert following the 2008 flood




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Figure 7.2-4 Scholtz River culvert on the main road viewed from upstream

Figure 7.2-5 Longitudinal profile of the Scholtz River showing the river bed level and

1:100 year flood level

A photograph of the 1:100 year flood on the Scholtz River is shown in Figure 7.2-6.

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Main road box culvert




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Figure 7.2-6 Model simulated 1:100 year flood on the Scholtz River




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7.3 PLATTEKLOOF AND NOUPOORT RIVERS

Figure 7.3-1 shows the physical model simulated floodlines. The topography of the right bank floodplain of the Plattekloof River drops to the right away from the main channel and therefore flood water will flow through the existing properties during a major flood. A photograph of the 1:100 year flood as simulated is shown in Figure 7.3-2.

Figure 7.3-1 Floodlines along the Plattekloof River as simulated in the physical

model




28

Figure 7.3-2 Plattekloof 1:100 year flood as simulated in the model

The pedestrian bridge and its abutments across the river has to be removed since its abutments and pier (with debris) constricts the flow, causing acceleration downstream, which




29

has scoured the bed already 1 m deep (based on the 2008 survey). The fixed bed at the bridge should also be removed. The bridge is shown in Figures 7.3-2 to 7.3-4. A longitudinal profile of the river bed is shown in Figure 7.3-5, and the 1:100 year flood flow pattern is shown in Figure 7.3-6.

Figure 7.3-2 Pedestrian bridge on Plattekloof River viewed from upstream

Figure 7.3-3 Pedestrian bridge on Plattekloof River with fixed bed level which has

caused local scour downstream




30

The existing grouted stone pitching upstream of the pedestrian bridge on the Plattekloof River is shown in Figure 7.3-4.

Figure 7.3-4 Right bank stone pitching bank protection on the Plattekloof River

upstream of the pedestrian bridge

Figure 7.3-5 Bed profile along the Plattekloof River near the pedestrian bridge

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Noupoort River

Pedestrian bridge




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Figure 7.3-6 Simulated 1:100 year flood flow at the pedestrian bridge




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8 GOBOS RIVER MORPHOLOGICAL STABILITY

8.1 GENERAL

Before considering detailed mitigation measures it is important to evaluate the river bed stability. This was done by evaluating historical river bed locations from historical aerial photography, and by evaluating movable bed flow patterns in the physical model.

8.2 HISTORICAL FLOW PATTERNS BASED ON AERIAL PHOTOS

The river banks of the aerial photos are plotted in Figure 8-1. The Gobos River acts like a braided river which typically happens when the sediment is coarse and the river bed steep. Such a river could easily migrate sideways during a flood and cause flood problems. From the historical patterns the river reach south of the Greyton-Riviersonderend Road seems to be critical where houses were constructed near the river




33

Figure 8-1 Historical Gobos River flow patterns based on aerial photos




34

8.3 SIMULATED GOBOS RIVER MIGRATION PATTERNS BASED ON PHYSICAL MODEL

The simulation of river migration patterns were carried out as follows: A low steady flow of 9 m3/s was allowed to flow in the Gobos River until a new equilibrium was reached in the bed morphology. This equilibrium right bank location (western bank near Greyton) was then surveyed in the model. A larger 1:2 year flood of 51 m3/s was then switched on in the model and allowed to run at a steady discharge until a new equilibrium was reached and again the river bank location was surveyed. The 1:2 year flood is often seen as the dominant flood that determines the long-term stability of a river. Following the low flow and 1:2 year flood simulations, an envelope line (nearest to town) of the surveyed right bank was determined and is plotted in Figure 8.3-1. Where this line is falling on existing property care should be taken to limit lateral erosion of the river in those locations. It should be noted that the effect of riparian vegetation to limit lateral erosion was not considered, since in many locations in the town the natural vegetation on the river banks is now limited. Vegetation on the banks could however affect the flow patterns and behaviour of the river and the case simulated in the laboratory could be seen as a conservative approach.

Figure 8.3-1a Simulated migrated right bank of upper Gobos River

Based on the simulation results, it is especially the southern part of Greyton where river migration could be experienced. This was to some extent confirmed during the 2008 flood when the river actually moved several meters to the west.




35

Figure 8.3-1b Simulated migrated right bank of lower Gobos River

Figure 8.3-2 River bed scour at end of tests showing the different flow patterns of

the Gobos River




36

9. MITIGATION MEASURES TO LIMIT FLOODING

9.1 GOBOS RIVER

9.1.1 General

On the Gobos River the work required is mainly at the Greyton-Riviersonderend bridge, and further downstream where houses were constructed on the floodplains.

9.1.2 Gobos River bridge

The Gobos River bridge needs erosion protection upstream to protect the approach roads against erosion and to minimize energy losses through the bridge. Rehabilitation work was also carried out after the 2008 flood (Figure 9.1-1), but spur dykes are proposed to streamline the flow patterns. The layout of the spur dykes as tested in the physical model is as shown in Figure 9.1-2. The spur dykes should be protected with riprap. The dyke orientation should be in line with the bridge abutments, and on the downstream left bank the dyke should be extended to control the flow direction in order to help protect the downstream houses on the left bank of the river. As alternative to the spur dykes the road embankment should be protected against erosion by riprap.

Figure 9.1-1 Temporary rehabilitation of the road after the 2008 flood (viewed from

upstream)




37

Figure 9.1-2 Proposed spur dykes as tested in the physical model at the Greyton-

Riverdale Road bridge

Spur dyke




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9.1.3 Existing properties to the east of the current main channel

Some houses are located on the floodplain to the east of the current main channel of the river. During a major flood these properties will be under water and will be cut off from the rest of the town. A possible solution is a levee around these properties which is open on the downstream side. Figure 9.1-3 shows a laboratory layout of the proposed levee. The levee should be protected with riprap.

Figure 9.1-3 Proposed levees as tested in the physical model at the properties near southern Greyton on the Gobos River

9.1.4 Existing properties at the right bank at the southern end of Greyton

Existing houses and properties will be flooded during a major flood on the right bank near the Southern end of Greyton. It is proposed that a riprap protected levee is designed around the most critical house on the bank (Figure 9.1-3), and that the rest of the properties upstream are monitored by annual river bank surveys.

Levee protected with riprap

House on bank with proposed levee

Possible upstream extension of levee




39

9.2 PLATTEKLOOF AND NOUPOORT RIVERS

There is sufficient space on the right floodplain to construct a riprap protected flood levee in the upper reach and further downstream of the Noupoort River, grouted stone pitching of the right river bank could be used where space is limited if needed. The location of the proposed levee is shown in Figures 9.2-1 to 9.2-3.

Figure 9.2-1 Flood levee location

Proposed Flood levee




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Figure 9.2-2 Flood levee location downstream of Noupoort River

Proposed low levee

Proposed flood levee




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Figure 9.2-3 Proposed location of the levee along the Plattekloof River

9.3 SCHOLTZ RIVER

9.3.1 Possible mitigation measures

The current river channel and culverts are too small. To handle the 1:100 year flood the following is proposed:

A solution could be the construction of a lined canal with suitable energy dissipation structures. Sufficient freeboard has to be provided in the canal and at the bridge crossings in order not to cause any obstructions and damming to the flow.

An alternative could be a flood attenuation dam with a smaller lined canal downstream.

9.3.2 Canal lining options

The canal downstream has to be lined due to the steep bed slope which leads to supercritical flow with high flow velocities. Possible linings that were considered are:

Concrete

Riprap (dumped rock or boulders)

Reno mattresses (wire cages filled with stones)

Armorflex (concrete blocks)

Grouted stone pitching The benefit of a concrete canal is that a relatively small canal cross-section can be used due to the smooth walls of the canal and this leads to high flow velocities. The canal could be seen as a large stormwater canal, but is hydraulically very effective and could be the most




42

economical solution. Special care should be taken in the design of the concrete joints due to the high flow velocities. Figure 9.3-1 shows a typical concrete canal.

Figure 9.3-1 Concrete lined canal

Riprap as solution uses dumped rock. The rock diameter is calculated based on the hydraulic conditions. The riprap layer is placed on a natural filter layer (sand, gravel and cobbles) or geotextile. Over time finer sediment from the river washes in between the rocks and vegetation establishes between them. The end result is a river channel that looks quite natural. An example of riprap bank protection is shown in Figure 9.3-2. In the case of the Scholtz River river boulders could probably also be used to protect the bed and banks of the canal.




43

Figure 9.3-2 Riprap bank protection on the Franschhoek River

Reno mattresses could be used to line the canal. The drawbacks of these mattresses are however that they could be damaged by bed load sediment transport and due to vandalism. Sometimes the Reno mattresses are used with gabion boxes. An example of gabion boxes as bank protection on the Scholtz River is shown in Figure 9.3-3. The boxes are however prone to toe scour and should in general rather be replaced by Reno mattresses with a trapezoidal canal layout. With coarse bed load, Reno mattresses or gabion boxes are however not recommended.

400 mm riprap on banks




44

Figure 9.3-3 Gabion boxes bank protection on the Scholtz River upstream of the

main road culvert Armorflex could be used as shown in Figure 9.3-4, but in steep rivers its unit weight is often too light. When vegetation starts to grow in the openings in the blocks, the canal could look quite natural (Figure 9.3-5), but the discharge capacity is reduced due to the vegetation. Grouted stone pitching is found at many rivers in the Western Cape and also on the Plattekloof River in Greyton. Smaller stones could be used than with riprap. Figure 9.3-6 shows grouted stone pitching at the river banks with riprap at the bed of the river. In the case of the Scholtz River a more trapezoidal canal shape could be used with grouted stone pitching. Grouted stone pitching is smoother than riprap or Reno mattresses with higher flow velocities. Special care should therefore be taken in the design for the canal to deal with these conditions.




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Figure 9.3-4 Typical Armorflex lining

Figure 9.3-5 Big Lotus River, Cape Town, with Armorflex lining




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Figure 9.3-6 Grouted stone pitching river bank with riprap at river bed

Stepped canals could also be used to reduce the bed slope and to lower the flow velocity in a canal. The drop structures at the steps should dissipate the energy and are sometimes quite large. The stepped layout also causes relatively deep canal sections, and a rail at the top of the canal could be required. An example of a small stepped canal is shown in Figure 9.3-7. In the case of the Scholz River the canal between the steps should also be protected against erosion.

Figure 9.3-7 Stepped canal to dissipate the energy




47

For the Scholtz River the following scenarios were considered: a) A canal designed for 16 m3/s and a flood attenuation dam upstream. The discharge capacity of the main road culvert could be increased to 16 m3/s. b) A larger canal designed for 25 m3/s with a flood attenuation dam upstream c) A large canal without a flood attenuation dam, designed for 44 m3/s, the 1:100 year flood. Figure 9.3-8 shows the proposed plan layout of the canal. Sharp bends were given longer radii and freeboard for bends has to be considered in the detail design. The canal should end at the Gobos River at a stilling basin to dissipate the energy. The canal has to be designed to fit in along the existing road and should start at the existing pipe culverts (to be removed) or at the flood attenuation dam upstream of the pipe culverts. The available width between properties to fit in the canal is 22 m. If a road width of 6 m is selected plus 1 m each side of the road is left open for other services, then the available space is 22-8 = 14 m for a canal. In the case of a concrete canal the width could be less than 14 m, but with a riprap canal one wants to reduce the hydraulic radius as much as possible to minimize the required rock diameter and cost. Typical canal cross-sections are shown in Figures 9.3-9. The hydraulic characteristics of the canals are shown in Table 9.3-1. The hydraulic roughness Manning n values that were assumed are:

Concrete canal: n = 0.016 (aged)

Riprap and Reno mattress: n = 0.032 (depends on rock size)

Grouted stone pitching: n = 0.025 (depends on stone size)

Armorflex: could not be used




48

Figure 9.3-8 Plan layout of proposed canal route




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Figure 9.3-9 Typical canal cross-sections




50

Table 9.3-1 Proposed canal typical hydraulic characteristics at a bed slope of 1:50

Canal type Design discharge

(m3/s)

Bottom width (m)

Top width (m)

Bank slope (1:__)

Flow depth (m)

Flow velocity (m/s)

Freeboard (m)

Froude number

Shallow Concrete

16 4 8.9 3.5 0.47 6.48 0.3 3.560

25 4 9.6 3.5 0.57 7.83 0.2 3.920

44 4.5 12.2 3.5 0.68 8.74 0.4 3.860

Hydraulically effective concrete

16 2 4 1.0 0.643 8.587 0.2 3.759

25 3 5.2 1.0 0.704 9.275 0.3 3.789

44 3 6 1.0 0.966 11.249 0.2 4.033

Riprap & Reno

mattress

16 4 11.7 3.5 0.7 3.191 0.2 1.406

25 4 13.8 3.5 0.92 3.72 0.1 1.474

44 4.5 17..1 3.5 1.18 4.34 0.2 1.530

Grouted stone

pitching

16 6 10 2.5 0.612 3.876 0.2 1.811

25 6 11.5 2.5 0.796 4.514 0.2 1.896

44 7 13.5 2.5 0.984 5.204 0.3 1.971

Armorflex

16 Too light to

use Too light to

use Too light to

use Too light to

use Too light to

use Too light to

use Too light to

use 25

44

Greyton River Management Plan Floods, flow patterns, river stability and mitigation measures


51

Longitudinal profiles of the canal (concrete and riprap) are shown in Figure 9.3-10 to 9.3-12.

Figure 9.3-10 Longitudinal profiles of the proposed canals at 16 m3/s


Canal design discharge - 25 m3/s

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9.3.3 Flood attenuation dam

The location of a possible flood attenuation dam is indicated in Figure 9.3-13. The conceptual design considered the following aspects:

The dam site was selected to also trap the tributary from the north east at this location.

The dam should have an uncontrolled culvert type bottom outlet

The dam should have an uncontrolled auxiliary spillway in case of partial blockage of the bottom outlet. 50 % blockage was allowed.

The dam outlet works should be designed for the 1:100 year hydrograph (Figure 9.3-14), but should also be able to handle a double peaked hydrograph

The volume of the 1:100 year inflow hydrograph is 166320 m3

The dam should be designed to fill and empty rapidly

Suitable energy dissipation structures should be designed downstream of the dam

The existing pipe culvert downstream of the attenuation dam should be removed

Only the main road culvert should be kept in place, possibly with streamlining of the flow through the culvert.

Excavation of the reservoir to improve attenuation was considered


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Figure 9.3-13 Flood attenuation dam site



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Figure 9.3-14 Flood hydrograph of the 1:100 year flood

The characteristics of the flood attenuation dam should be as indicated in Table 9.3-2 for the different possible canal designs. The flood routing through the flood attenuation dam was carried out with level pool routing. Attenuation dam scenarios with and without excavation of the reservoir were investigated. To achieve a maximum outflow at the dam of say 25 m3/s, without an excavated reservoir to create a larger storage capacity, the maximum damming would be 7.29 m above the river bed level (this includes a consideration of 50% blockage of the bottom outlet). This is with bottom outlet dimensions of 2.2 m x 1.5 m. If a double peaked inflow hydrograph is considered, the maximum outflow would be slightly less than 25 m3/s and the canal would not be damaged. An alternative scenario that could be considered is a large flood attenuation dam with a small outlet that attenuates most of the flood. For this scenario the existing river canal downstream could possibly be unlined.

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Table 9.3-2 Flood attenuation dam characteristics

Excavation of reservoir volume (m3)

0 0 141933 141933 141933

Peak outflow no blockage (m3/s) 25 16 25 16 5*

Peak outflow 50% blockage (m3/s) 19.3 21.0 15.2 8.8 2.4

Maximum water level in reservoir above original river bed level (masl)

236.79 237.03 232.29 233 233.36

Water depth measured above original river bed level(m)

7.29 7.53 2.79 3.5 3.86

Peak outflow with no blockage & a double peaked inflow hydrograph (m3/s)**

24.72 15.3 29.76 26.58 35.44

Culvert opening width (m) 2.2 1.5 5 2.5 1.2

Culvert opening height (m) 1.5 1.2 2 1.5 0.6

Full supply level (masl) 236.5 237.5 233 233 236.5

Note: * existing canal capacity ** to be critically evaluated in detail design The double peaked hydrograph outflow was not used as design discharge, but rather as safety evaluation discharge for the canal design to check on the available freeboard. The outflow hydrographs for the scenarios in Table 9.3-2 are indicated in Figure 9.3-15 to 9.3-18.

Figure 9.3-15 Inflow and outflow hydrographs for 16 m3/s outflow flood peaks-no

excavation of reservoir

Inflow and outflow hydrographs for 16 m3/s outflow flood peak-no excavation

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50% blockage outflow No blockage & double peak hydrograph-outflow



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Figure 9.3-16a Inflow and outflow hydrographs for 25 m3/s outflow flood

peaks-no excavation

Figure 9.3-16b Inflow and outflow hydrographs for 25 m3/s outflow flood

peaks-with excavation

Inflow and outflow hydrographs for 25 m3/s outflow flood peak-no excavation

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Figure 9.3-17 Inflow and outflow hydrographs for 16 m3/s outflow flood peaks-with

excavation

Figure 9.3-18 Inflow and outflow hydrographs for 5 m3/s outflow flood peaks-with

excavation of the reservoir basin The excavated flood attenuation dam helps to lower the damming depth considerably. The volume of excavation needs to be optimized in the detail design.


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In the detail design of the dam the following should also be considered: The dam should be designed using the SANCOLD interim guidelines on Freeboard for Dams (1990). The Recommended design discharge based on the dam height and hazard classification would probably be the 1:100 year flood and freeboard to allow for wind generated waves etc should be added. In addition the Safety Evaluation Discharge (SED) should be evaluated (without freeboard components) based on the SANCOLD guideline Safety in relation to floods (1991). For a high hazard rating and a dam between 5 and 12 m high, the Regional Maximum Flood (RMF) has to be considered for the SED. In this case the SED should not be allowed to dam higher than the Non Overspill Crest (NOC) of the dam. The school on the right bank should be considered when designing the dam. The flood attenuation dam is normally empty and when it is filling up care should be taken that lateral flow from the tributary does not erode the embankment dam toe. Riprap should be used for erosion protection at the toe. The plan layout of the outlet works and a cross-section are indicated in Figures 9.3-19 and 9.3-20. It is important to prevent trees to from blocking the bottom outlet. This could be achieved by designing a trashrack inlet system. A stilling basin is required at the outlet.

Figure 9.3-19 Plan layout of outlet works and spillway

Ogee



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Figure 9.3-20 Plan layout of outlet works and spillway

Figure 9.3-21 shows a longitudinal profile of the bottom outlet. Figures 9.3-22 and 9.3-23 show the possible flooded areas during floods upstream of the dam.

Figure 9.3-21 Cross-section at outlet works and stilling basin

Ogee spillway



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Figure 9.3-22 Flood attenuation dam inundation for 7m high dam wall to FSL and

no reservoir excavation

Figure 9.3-23 Flood attenuation Dam inundation for 7m high dam wall to

FSL and with reservoir excavation



61

10. MITIGATION MEASURES TO LIMIT POSSIBLE LATERAL EROSION ON THE GOBOS RIVER Along the Gobos River near the southern side of Greyton, right bank erosion has been experienced in the past and gabions boxes were used to limit further bank erosion (Figure 10-1). The physical model tests indicated that further erosion is possible as shown in Figure 8.3-1. It is proposed that the possible further bank erosion is monitored by annual surveys of the bank line after winter. Work is however required at the new development at the southern end of the town where one house has been constructed at the edge of the river and where the 2008 flood caused lateral erosion. At this location a riprap levee is proposed as indicated in Figure 10-2.

Figure 10-1 Existing gabion boxes on right bank of the Gobos River near southern end of town

Gabion boxes as bank protection



62

Figure 10-2 Proposed flood levee at southern end of Greyton as observed in the

laboratory

Levee



63

11. PROPERTIES WHERE FURTHER DEVELOPMENT SHOULD

BE CONTROLLED DUE TO RISK OF FLOODING

The 1:100 year floodlines determined in this study should be used to prevent further development on land within the floodlines indicated. On existing properties which fall within the floodlines, further or any future development should be controlled until the recommendations of this report has been implemented to ensure the safety of people and property. On the Gobos River until the bank erosion protection is in place, the widest of the 1:100 year floodline or the simulated possible lateral erosion should be taken as the floodline. Refer to Figure 11-1.

Figure 11-1 High risk of flooding of properties in Greyton (red) considering the 1:100 year flood and possible bank erosion where further development should be

controlled until proposed mitigation measures have been implemented



64

12. CONCLUSIONS AND RECOMMENDATIONS

Greyton has experienced severe flooding with damage to properties during recent years with the last flood occurring in November 2008. IWEE was appointed during 2008 to carry out a detailed investigation of the flood hydrology and the river hydraulics of the main rivers: Plattekloof, Noupoort, Gobos and Scholtz Rivers at the town. The study was carried out by building a physical hydraulic model of the rivers at the Hydraulics Laboratory of the University of Stellenbosch. The key findings of the study are as follows:

The flood magnitudes of recurrence interval floods were found similar to the 2001 floodline study carried out by KweziV3. Analysis of the data of an automatic rain gauge located in Greyton indicated that the November 2008 storm precipitation was an event to could occur frequently.

The main areas where properties will be inundated during the 1:100 year flood are:

o Right bank of Plattekloof River where the floodplain falls away from the river

o The Gobos River near the southern end of the town has properties on both sides of the river which could be affected

o The Scholtz River is the most critical area with wide floodplain flow even during small floods. The main channel is very small and surrounded by houses on existing properties. Hydraulics structures also constricted the flow in the past

Hydraulic structures at risk to flood damage are: o The Gobos Road bridge on the Greyton-Riviersonderend Road where the

approach roads have scoured during the 2008 flood o The pipe culvert on the Scholtz River where the pipes were damaged and

the approach road washed away

Movable bed tests on the Gobos River indicated that the lower Gobos at the Southern end of the town could migrate further to the west over time. The situation has to monitored in the field by annual river bank surveys. In this reach the right bank has been protected with gabions in the past. For future protection riprap is proposed. One new house on the right bank next to the river needs immediate erosion and flood protection at this stage.

Flood mitigation measures are required as follows: o A flood levee on the right floodplain of the Plattekloof River o The pedestrian bridge and its fixed bed and fixed abutments on the

Plattekloof River have to be removed o The culvert on the Plattekloof River near the confluence with the Gobos

River has to be removed and the upstream gabion protected low water crossing should be maintained

o The flow through the Gobos River Road bridge could be streamlined by using spur dykes protected with riprap or the road should be protected against erosion

o A levee with riprap protection should be constructed on the right bank of the Gobos River at the southern end of the town where a new house was constructed on the river bank recently.



65

o On the Scholtz River the existing canal and hydraulic structures are too small. A canal could be constructed along the road where the river currently flows. The canal could be concrete lined, or lined with riprap (dumped rock/boulders), Reno mattresses, or grouted stone pitching. The bed slope of the canal would be steep at 1:50 and the flow would be supercritical. Different flows and canal options were investigated. The full 1:100 year flood could be conveyed by the canal, or if a flood attenuation dam is constructed upstream of the canal, the canal discharge could be decreased resulting in a smaller canal.

Possible flood attenuation dams were evaluated with and without excavation of the reservoir area, and a fully open or 50% blocked outlet was considered. In addition a double peaked flood was considered, without outlet blockage, to evaluate the safety of the canal design. The attenuation dam could be an earth embankment with concrete bottom outlet and concrete uncontrolled spillway. Stilling basins are required at the dam and outlet. The canal should be lined from the dam downstream to the end of the development at the southern end of the town. The canal-flood attenuation dam system could be sized to maximize the use of the existing main road culvert at about 16 m3/s, or else the culvert should be widened (without a pier in the flow). All other culverts should be removed and no structures should be constructed in the proposed canal. At the downstream end of the canal and where the canal bends at the end of the road concrete stilling basins are required. Possible reduction of the canal slope by steps was considered, but this creates a deep canal and the flow becomes unstable for some flow conditions when 0.8<Fr<1.2. It is proposed that a riprap lined canal, possibly with a flood attenuation dam is implemented. The riprap could consist of river boulders which would make it look natural and some vegetation could be allowed to establish over time in the finer deposited sediment in the riprap.

The 1:100 year floodlines determined in this study should be used to prevent further development on land within the floodlines indicated. On existing properties which fall within the floodlines, further or any future development should be controlled until the recommendations of this report has been implemented to ensure the safety of people and property. On the Gobos River until the bank erosion protection is in place, the widest of the 1:100 year floodline or the simulated possible lateral erosion should be taken as the floodline. Refer to Figure 11-1.



66

APPENDIX A: FLOOD HYDROLOGY CALCULATIONS

i) Review by IWEE

A

Secondary drainage region number

Tertiary drainage region number

Quaternary drainage region number

Catchment description

Size of catchment (A) (km²)

Rural areas (a) (%)

Urban areas (b) (%)

Lakes () (%) Overland flow: Surface description

Dolomite area (D) (%) Longest watercourse (L) (km)

Check: Area-distribution total (%) Average slope: Watercourse (m/m)

Length of flow path (km) Canal length (km)

Slope (m/m) Actual velocity (m/s)

Manning's n-value

Actual velocity (m/s) Max velocity (m/s)

Designed

Date

5.000

3. PRECIPITATION DATA

0.21867

100

0

1. LOCATION

Greyton

Platkloof

3.1

JA du Plessis

November 12, 2008

Main watercourse/ river

7. DESIGNER'S & SUPERVISOR'S DETAILS

CANAL FLOW

6. FLOW PATHS: ARTIFICIAL

Height difference: Overland flow (H) (m)

JA du Plessis

November 12, 2008

STREET FLOW

Checked

Date

MAP (mm)

623

790

CATCHMENT DATA & GENERAL INFORMATION

Stream

OK

Overland flow (L) (km)

Platkloof

5. FLOW PATHS: NATURAL

4. CATCHMENT CLASSIFICATION

2. AREA DISTRIBUTION FACTORS

INLAND-/ SUMMER PRECIPITATION

COASTAL-/ WINTER PRECIPITATION

CATCHMENT: FLAT & PERMEABLE

CATCHMENT: STEEP & IMPERMEABLE

Poor grass cover on highly erodable soil

YES NO

Average grass cover

SINGLE WEATHER-/ PRECIPITATION STATION

MULTIPLE WEATHER-/ PRECIPITATION STATIONS

1' X 1' GRID DESIGN PRECIPITATION DEPTHS



67





Precipitation region

MAP (mm) mm 100 %

Size of catchment (A) km² 0 %

Distance of overland flow (L) km 0 %

Height difference (H) m 0 %

Overland: Slope (S) m/m

Overland: r-Value

Longest watercourse (L) km Canal length (km)

Actual velocity (m/s)


Surface slope % Factor Cs % Factor C2

Vleis and pans (0-3%) 0.030 0.000 0.100

Flat areas (3- 10%) 0.080 0.000 0.200

Hilly (10-30%) 20 0.160 0.032 0.170

Steep areas (>30%) 80 0.260 0.208 0.350

Total 100 Total 0.240 0 Total 0.000

Soil class/ permeability % Factor Cp % Factor C2

Very permeable (A) 0.040 0.500

Very permeable (A/B) 0.060 0.700

Permeable (B) 0.080 0 Total 0.000

Permeable (B/C) 0.120 % Factor C2

Semi-perrneable (C) 70 0.160 0.112 0.800

Semi-perrneable (C/D) 0.210 0.850

Impermeable (D) 30 0.260 0.078 0.900


Land-use/ vegetation % Factor Cv % Factor C2

Thick bush and plantation 10 0.040 0.004 0.950

Light bush and farm lands 20 0.110 0.022 0.700

Grass lands 40 0.210 0.084 0.950

Cultivated land, contoured 0.110 0.000 1.000

Cultivated land 0.210 0.000

No vegetation 30 0.280 0.084


Total 100 Total C1 0.624 0 Total C2 0.000

Correction factor (t) for defined water course: 1.754

0.000 hours hours 0.000 hours 0.411 hours

Return period (T) 2 5 10 20 50 100

Run-off coefficient (C1) 0.624 0.624 0.624 0.624 0.624 0.624

Adjusted run-off coefficient (C1D) 0.624 0.624 0.624 0.624 0.624 0.624

Adjustment factor (FT) 0.750 0.800 0.850 0.900 0.950 1.000

Adjusted run-off coefficient (C1T) 0.468 0.499 0.530 0.562 0.593 0.624

Weighted runoff coefficient (CT) 0.468 0.499 0.530 0.562 0.593 0.624

Return period (T) 2 5 10 20 50 100

Point precipitation (mm), PT (Alexander)

Point precipitation (mm), PT (Smithers & Schulze) 13.338 17.905 21.050 24.260 28.498 34.925

Point intensity (mm/h), PiT 32.433 43.539 51.187 58.991 69.297 84.926

Area reduction factor (%) 102.755 102.755 102.755 102.755 102.755 102.755

Area reduction factor (%) (Smithers & Schulze) 100.000 100.000 100.000 100.000 100.000 100.000

Average intensity (mm/h), IT 32.433 43.539 51.187 58.991 69.297 84.926

Peak flow (m3/s), QT 13.1 18.7 23.4 28.5 35.4 45.6

Heavy soil, flat (<2%)

Average channel slope (Sav)

Defined water course

Residential areas

Houses

URBAN

Flats

Total

Sandy, steep (>7%)

Industry

PRECIPITATION

Streets

Maximum flood

Total Tc

Total

0.749

Overland flow

Sandy, flat (<2%)

3.100

PHYSICAL CHARACTERISTICS

790

AREA DISTRIBUTION FACTORSCoastal/winter

Rural areas (a)

Dolomite area (D)

Total

ARTIFICIAL FLOW

0.411

Length of flow path (km)

Street flow

m/m0.21867

Heavy soil, steep (>7%)

Lawns

0.749

RURAL

Slope (m/m)

1.200

0.624

5.000

Canal flow

RUNOFF COEFFICIENTS

55.6

86.279

200

86.279

102.755

35.482

100.000

RATIONAL METHOD

Stream

JA du Plessis

JA du Plessis

November 12, 2008

Urban areas (b)

Lakes ()

200

TIME OF CONCENTRATION (Tc)

NOTES


Artificial flow/ streets

Light industry

Average industry

Heavy industry

Total

Business

City centre

Suburban

0.624

Total

Platkloof

Greyton Main watercourse/ river

Designed

Checked

Platkloof

Date



68

B






Rural areas (a) (%)

Urban areas (b) (%)






Manning's n-value


Designed

Date


Stream

OK


Noupoort/Platkloof




November 12, 2008

STREET FLOW

Checked

Date

MAP (mm)

623

790

JA du Plessis

November 12, 2008



CANAL FLOW



JA du Plessis

100

0

1. LOCATION

Greyton

Noupoort/Platkloof

9.5

5.825


0.10045






YES NO

Average grass cover






69






MAP (mm) mm 100 %





Overland: r-Value





Vleis and pans (0-3%) 2 0.030 0.001 0.100

Flat areas (3- 10%) 22 0.080 0.018 0.200

Hilly (10-30%) 6 0.160 0.010 0.170

Steep areas (>30%) 70 0.260 0.182 0.350









Impermeable (D) 30 0.260 0.078 0.900





Grass lands 40 0.210 0.084 0.950








Return period (T) 2 5 10 20 50 100






Return period (T) 2 5 10 20 50 100







Peak flow (m3/s), QT 25 39 51 66 88 111




Residential areas

Houses

URBAN

Flats

Total

Sandy, steep (>7%)

Industry

PRECIPITATION

Streets

Maximum flood

Total Tc

Total

0.713

Overland flow

Sandy, flat (<2%)

9.500


790


Rural areas (a)

Dolomite area (D)

Total

ARTIFICIAL FLOW

0.624


Street flow

m/m0.10045


Lawns

0.713

RURAL

Slope (m/m)

1.200

0.594

5.825

Canal flow

RUNOFF COEFFICIENTS

154

82.145

200

82.145

98.706

51.266

100.000

RATIONAL METHOD

Stream

JA du Plessis

JA du Plessis

November 12, 2008

Urban areas (b)

Lakes ()

200


NOTES



Light industry

Average industry

Heavy industry

Total

Business

City centre

Suburban

0.594

Total

Noupoort/Platkloof


Designed

Checked

Noupoort/Platkloof

Date



70

C






Rural areas (a) (%)

Urban areas (b) (%)

Overland flow: Surface description

Longest watercourse (L) (km)

Dolomite area (D) (%) Average slope: Watercourse (m/m)



Manning's n-value


Designed

Date

USER INPUT: MAP (mm) (Optional) 790

34

17.910


0.02931

100

0


1. LOCATION

Greyton

JA du Plessis

November 7, 2008



CANAL FLOW



JA du Plessis

November 7, 2008

STREET FLOW

Checked

Date

MAP (mm)

623


Bo Gobos


Bo-GOBOS


Lakes () (%)









YES NO

Average grass cover



71






MAP (mm) mm 100 %





Overland: r-Value





Vleis and pans (0-3%) 0.030 0.000 0.100

Flat areas (3- 10%) 0.080 0.000 0.200

Hilly (10-30%) 8.6 0.160 0.014 0.170

Steep areas (>30%) 91.4 0.260 0.238 0.350









Impermeable (D) 30 0.260 0.078 0.900





Grass lands 40 0.210 0.084 0.950








Return period (T) 2 5 10 20 50 100






Return period (T) 2 5 10 20 50 100

Point precipitation (mm), PT (Alexander) 27.100 38.500 47.000 56.000 69.000 79.900




Peak flow (m3/s), QT 51 78 101 127 165 201

Bo-GOBOS


Designed

Checked

Date

Business

City centre

Suburban

0.635

Total

Light industry

Average industry

Heavy industry

Total

November 7, 2008

Urban areas (b)

Lakes ()

200


NOTES



RATIONAL METHOD

Bo Gobos

JA du Plessis

JA du Plessis

278

38.596

200

38.596

100.000

91.900

0.762

RURAL

Slope (m/m)

1.200

0.635

17.910

Canal flow

RUNOFF COEFFICIENTS

Total

ARTIFICIAL FLOW

2.381


Street flow

m/m0.02931


Lawns

Sandy, flat (<2%)

34.000


790


Rural areas (a)

Dolomite area (D)

PRECIPITATION

Streets

Maximum flood

Total Tc

Total

0.762

Overland flow




Residential areas

Houses

URBAN

Flats

Total

Sandy, steep (>7%)

Industry



72

C’






Rural areas (a) (%)

Urban areas (b) (%)






Manning's n-value


Designed

Date

20.160


0.02430

100

0

1. LOCATION

Greyton

All excl Scholtz

46.4

JA du Plessis

November 8, 2008



CANAL FLOW



JA du Plessis

November 8, 2008

STREET FLOW

Checked

Date

MAP (mm)

623

790


Stream

OK


Gobos Plus









YES NO

Average grass cover






73






MAP (mm) mm 100 %





Overland: r-Value





Vleis and pans (0-3%) 2 0.030 0.001 0.100

Flat areas (3- 10%) 4 0.080 0.003 0.200

Hilly (10-30%) 10 0.160 0.016 0.170

Steep areas (>30%) 84 0.260 0.218 0.350









Impermeable (D) 30 0.260 0.078 0.900





Grass lands 40 0.210 0.084 0.950








Return period (T) 2 5 10 20 50 100






Return period (T) 2 5 10 20 50 100







Peak flow (m3/s), QT 62 95 123 155 202 246

Gobos Plus


Designed

Checked

All excl Scholtz

Date

Business

City centre

Suburban

0.622

Total

Light industry

Average industry

Heavy industry

Total

November 8, 2008

Urban areas (b)

Lakes ()

200


NOTES



RATIONAL METHOD

Stream

JA du Plessis

JA du Plessis

339

35.227

200

35.227

96.409

98.755

100.000

0.747

RURAL

Slope (m/m)

1.200

0.622

20.160

Canal flow

RUNOFF COEFFICIENTS

Total

ARTIFICIAL FLOW

2.803


Street flow

m/m0.02430


Lawns

Sandy, flat (<2%)

46.400


790


Rural areas (a)

Dolomite area (D)

PRECIPITATION

Streets

Maximum flood

Total Tc

Total

0.747

Overland flow




Residential areas

Houses

URBAN

Flats

Total

Sandy, steep (>7%)

Industry



74

D






Rural areas (a) (%)

Urban areas (b) (%)






Manning's n-value


Designed

Date

6.260


0.08488

100

0

1. LOCATION

Greyton

Scholtz River

4.1

JA du Plessis

November 8, 2008



CANAL FLOW



JA du Plessis

November 8, 2008

STREET FLOW

Checked

Date

MAP (mm)

623

790


Stream

OK


Scholtz River









YES NO

Average grass cover






75






MAP (mm) mm 100 %





Overland: r-Value





Vleis and pans (0-3%) 5.7 0.030 0.002 0.100

Flat areas (3- 10%) 15.1 0.080 0.012 0.200

Hilly (10-30%) 35.8 0.160 0.057 0.170

Steep areas (>30%) 43.4 0.260 0.113 0.350









Impermeable (D) 30 0.260 0.078 0.900





Grass lands 40 0.210 0.084 0.950








Return period (T) 2 5 10 20 50 100






Return period (T) 2 5 10 20 50 100







Peak flow (m3/s), QT 9.6 15.0 19.8 25.6 34.2 43.5

Scholtz River


Designed

Checked

Scholtz River

Date

Business

City centre

Suburban

0.568

Total

Light industry

Average industry

Heavy industry

Total

November 8, 2008

Urban areas (b)

Lakes ()

200


NOTES



RATIONAL METHOD

Stream

JA du Plessis

JA du Plessis

59.8

77.096

200

77.096

103.406

54.265

100.000

0.681

RURAL

Slope (m/m)

1.200

0.568

6.260

Canal flow

RUNOFF COEFFICIENTS

Total

ARTIFICIAL FLOW

0.704


Street flow

m/m0.08488


Lawns

Sandy, flat (<2%)

4.100


790


Rural areas (a)

Dolomite area (D)

PRECIPITATION

Streets

Maximum flood

Total Tc

Total

0.681

Overland flow




Residential areas

Houses

URBAN

Flats

Total

Sandy, steep (>7%)

Industry




76

E






Rural areas (a) (%)

Urban areas (b) (%)






Manning's n-value


Designed

Date


Stream

OK


Gobos / Scholtz




November 8, 2008

STREET FLOW

Checked

Date

MAP (mm)

623

790

JA du Plessis

November 8, 2008



CANAL FLOW



JA du Plessis

100

0

1. LOCATION

Greyton

Full Catchment

50.44

20.160


0.02430






YES NO

Average grass cover







77






MAP (mm) mm 100 %





Overland: r-Value





Vleis and pans (0-3%) 2 0.030 0.001 0.100

Flat areas (3- 10%) 4 0.080 0.003 0.200

Hilly (10-30%) 10 0.160 0.016 0.170

Steep areas (>30%) 84 0.260 0.218 0.350









Impermeable (D) 30 0.260 0.078 0.900





Grass lands 40 0.210 0.084 0.950








Return period (T) 2 5 10 20 50 100






Return period (T) 2 5 10 20 50 100







Peak flow (m3/s), QT 68 103 133 168 219 267




Residential areas

Houses

URBAN

Flats

Total

Sandy, steep (>7%)

Industry

PRECIPITATION

Streets

Maximum flood

Total Tc

Total

0.747

Overland flow

Sandy, flat (<2%)

50.440


790


Rural areas (a)

Dolomite area (D)

Total

ARTIFICIAL FLOW

2.803


Street flow

m/m0.02430


Lawns

0.747

RURAL

Slope (m/m)

1.200

0.622

20.160

Canal flow

RUNOFF COEFFICIENTS

369

35.227

200

35.227

95.956

98.755

100.000

RATIONAL METHOD

Stream

JA du Plessis

JA du Plessis

November 8, 2008

Urban areas (b)

Lakes ()

200


NOTES



Light industry

Average industry

Heavy industry

Total

Business

City centre

Suburban

0.622

Total

Gobos / Scholtz


Designed

Checked

Full Catchment

Date




78

Figure A1 Catchments used in flood hydrology calculations

C - Gobos

B- Platkloof & Noupoort

A- Platkloof

D- Scholtz

C`- Full catchment excluding D

E - Full Catchment

Different catchments

used.

GREYTON




79

ii) KweziV3 Flood Hydrology Report (2008)




80




81




82




83

APPENDIX B

RIVER SURVEY DATA AND CROSS-SECTIONS

(On cd)




84

APPENDIX C

DRAWINGS: FLOODLINES, RIVER MIGRATION & MITIGATION

MEASURES

(On cd)




85

APPENDIX D

DVD WITH PHYSICAL MODEL TESTS

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