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GGRREEYYTTOONN RRIIVVEERR MMAANNAAGGEEMMEENNTT PPLLAANN
FFllooooddss,, ffllooww ppaatttteerrnnss,, rriivveerr ssttaabbiilliittyy aanndd
mmiittiiggaattiioonn mmeeaassuurreess
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
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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
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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
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(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
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Figure 6-2 Mathematical model simulated flow depths on Gobos River
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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.
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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
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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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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.
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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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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
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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
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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
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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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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
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Figure 8-1 Historical Gobos River flow patterns based on aerial photos
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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.
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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
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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)
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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
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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
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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.
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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
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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
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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
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Figure 9.3-8 Plan layout of proposed canal route
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Figure 9.3-9 Typical canal cross-sections
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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
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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
Figure 9.3-11 Longitudinal profiles of the proposed canals at 25 m3/s
Canal design discharge - 25 m3/s
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Figure 9.3-12 Longitudinal profiles of the proposed canals at 44 m3/s
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
Canal design discharge - 44 m3/s
200
205
210
215
220
225
230
235
0 200 400 600 800 1000 1200Chainage (m)
Ele
vati
on
(m
asl)
Bed elevation Riprap canal-water level Concrete canal-water level
Culvert
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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.
0
5
10
15
20
25
30
35
40
45
50
0 1000 2000 3000 4000 5000 6000 7000 8000
Duration (sec)
Dis
ch
arg
e (
m3/s
)
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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
0
5
10
15
20
25
30
35
40
45
50
0 5000 10000 15000 20000 25000
Duration (sec)
Dis
ch
arg
e (
m3/s
)
Double peaked Inflow hydrograph No blockage outflow
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
0
5
10
15
20
25
30
35
40
45
50
0 5000 10000 15000 20000 25000
Duration (sec)
Dis
ch
arg
e (
m3/s
)
Double peaked Inflow hydrograph No blockage outflow
50% blockage outflow No blockage & double peak hydrograph-outflow
Inflow and outflow hydrographs for 25 m3/s outflow flood peak-with excavation
0
5
10
15
20
25
30
35
40
45
50
0 5000 10000 15000 20000 25000 30000
Duration (sec)
Dis
ch
arg
e (
m3/s
)
Double peaked Inflow hydrograph No blockage outflow
50% blockage outflow No blockage & double peak hydrograph-outflow
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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.
Inflow and outflow hydrographs for 16 m3/s outflow flood peak-with excavation
0
5
10
15
20
25
30
35
40
45
50
0 5000 10000 15000 20000 25000 30000
Duration (sec)
Dis
ch
arg
e (
m3/s
)
Double peaked Inflow hydrograph No blockage outflow
50% blockage outflow No blockage & double peak hydrograph-outflow
Inflow and outflow hydrographs for 5 m3/s outflow flood peak-with excavation
0
5
10
15
20
25
30
35
40
45
50
0 5000 10000 15000 20000 25000 30000
Duration (sec)
Dis
ch
arg
e (
m3/s
)
Double peaked Inflow hydrograph No blockage outflow
50% blockage outflow No blockage & double peak hydrograph-outflow
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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
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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
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Figure 10-2 Proposed flood levee at southern end of Greyton as observed in the
laboratory
Levee
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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
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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.
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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.
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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
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Secondary drainage region number
Tertiary drainage region number
Quaternary drainage region number
Catchment description
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)
Actual velocity (m/s) Max 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
Total 100 Total 0.190 0 Total 0.000
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 0.194 0 Total 0.000
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
TIME OF CONCENTRATION (Tc)
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
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B
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
CATCHMENT DATA & GENERAL INFORMATION
Stream
OK
Overland flow (L) (km)
Noupoort/Platkloof
5. FLOW PATHS: NATURAL
4. CATCHMENT CLASSIFICATION
2. AREA DISTRIBUTION FACTORS
November 12, 2008
STREET FLOW
Checked
Date
MAP (mm)
623
790
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
100
0
1. LOCATION
Greyton
Noupoort/Platkloof
9.5
5.825
3. PRECIPITATION DATA
0.10045
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
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Secondary drainage region number
Tertiary drainage region number
Quaternary drainage region number
Catchment description
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)
Actual velocity (m/s) Max velocity (m/s)
Surface slope % Factor Cs % Factor C2
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
Total 100 Total 0.210 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
Total 100 Total 0.190 0 Total 0.000
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 0.194 0 Total 0.000
Total 100 Total C1 0.594 0 Total C2 0.000
Correction factor (t) for defined water course: 1.511
0.000 hours hours 0.000 hours 0.624 hours
Return period (T) 2 5 10 20 50 100
Run-off coefficient (C1) 0.594 0.594 0.594 0.594 0.594 0.594
Adjusted run-off coefficient (C1D) 0.594 0.594 0.594 0.594 0.594 0.594
Adjustment factor (FT) 0.750 0.800 0.850 0.900 0.950 1.000
Adjusted run-off coefficient (C1T) 0.445 0.475 0.505 0.534 0.564 0.594
Weighted runoff coefficient (CT) 0.445 0.475 0.505 0.534 0.564 0.594
Return period (T) 2 5 10 20 50 100
Point precipitation (mm), PT (Alexander)
Point precipitation (mm), PT (Smithers & Schulze) 13.091 19.237 23.984 29.231 36.975 44.366
Point intensity (mm/h), PiT 20.977 30.825 38.431 46.838 59.247 71.089
Area reduction factor (%) 98.706 98.706 98.706 98.706 98.706 98.706
Area reduction factor (%) (Smithers & Schulze) 100.000 100.000 100.000 100.000 100.000 100.000
Average intensity (mm/h), IT 20.977 30.825 38.431 46.838 59.247 71.089
Peak flow (m3/s), QT 25 39 51 66 88 111
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
0.713
Overland flow
Sandy, flat (<2%)
9.500
PHYSICAL CHARACTERISTICS
790
AREA DISTRIBUTION FACTORSCoastal/winter
Rural areas (a)
Dolomite area (D)
Total
ARTIFICIAL FLOW
0.624
Length of flow path (km)
Street flow
m/m0.10045
Heavy soil, steep (>7%)
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
TIME OF CONCENTRATION (Tc)
NOTES
TIME OF CONCENTRATION (Tc)
Artificial flow/ streets
Light industry
Average industry
Heavy industry
Total
Business
City centre
Suburban
0.594
Total
Noupoort/Platkloof
Greyton Main watercourse/ river
Designed
Checked
Noupoort/Platkloof
Date
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C
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) (%)
Overland flow: Surface description
Longest watercourse (L) (km)
Dolomite area (D) (%) 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
USER INPUT: MAP (mm) (Optional) 790
34
17.910
3. PRECIPITATION DATA
0.02931
100
0
2. AREA DISTRIBUTION FACTORS
1. LOCATION
Greyton
JA du Plessis
November 7, 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 7, 2008
STREET FLOW
Checked
Date
MAP (mm)
623
CATCHMENT DATA & GENERAL INFORMATION
Bo Gobos
Overland flow (L) (km)
Bo-GOBOS
5. FLOW PATHS: NATURAL
Lakes () (%)
4. CATCHMENT CLASSIFICATION
SINGLE WEATHER-/ PRECIPITATION STATION
MULTIPLE WEATHER-/ PRECIPITATION STATIONS
INLAND-/ SUMMER PRECIPITATION
COASTAL-/ WINTER PRECIPITATION
CATCHMENT: FLAT & PERMEABLE
CATCHMENT: STEEP & IMPERMEABLE
Poor grass cover on highly erodable soil
YES NO
Average grass cover
Greyton River Management Plan Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
71
Secondary drainage region number
Tertiary drainage region number
Quaternary drainage region number
Catchment description
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)
Actual velocity (m/s) Max 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%) 8.6 0.160 0.014 0.170
Steep areas (>30%) 91.4 0.260 0.238 0.350
Total 100 Total 0.251 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
Total 100 Total 0.190 0 Total 0.000
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 0.194 0 Total 0.000
Total 100 Total C1 0.635 0 Total C2 0.000
Correction factor (t) for defined water course: 1.234
0.000 hours hours 0.000 hours 2.381 hours
Return period (T) 2 5 10 20 50 100
Run-off coefficient (C1) 0.635 0.635 0.635 0.635 0.635 0.635
Adjusted run-off coefficient (C1D) 0.635 0.635 0.635 0.635 0.635 0.635
Adjustment factor (FT) 0.750 0.800 0.850 0.900 0.950 1.000
Adjusted run-off coefficient (C1T) 0.477 0.508 0.540 0.572 0.604 0.635
Weighted runoff coefficient (CT) 0.477 0.508 0.540 0.572 0.604 0.635
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
Point intensity (mm/h), PiT 11.382 16.169 19.739 23.519 28.979 33.557
Area reduction factor (%) 100.000 100.000 100.000 100.000 100.000 100.000
Average intensity (mm/h), IT 11.382 16.169 19.739 23.519 28.979 33.557
Peak flow (m3/s), QT 51 78 101 127 165 201
Bo-GOBOS
Greyton Main watercourse/ river
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
TIME OF CONCENTRATION (Tc)
NOTES
TIME OF CONCENTRATION (Tc)
Artificial flow/ streets
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
Length of flow path (km)
Street flow
m/m0.02931
Heavy soil, steep (>7%)
Lawns
Sandy, flat (<2%)
34.000
PHYSICAL CHARACTERISTICS
790
AREA DISTRIBUTION FACTORSCoastal/winter
Rural areas (a)
Dolomite area (D)
PRECIPITATION
Streets
Maximum flood
Total Tc
Total
0.762
Overland flow
Heavy soil, flat (<2%)
Average channel slope (Sav)
Defined water course
Residential areas
Houses
URBAN
Flats
Total
Sandy, steep (>7%)
Industry
Greyton River Management Plan Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
72
C’
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
20.160
3. PRECIPITATION DATA
0.02430
100
0
1. LOCATION
Greyton
All excl Scholtz
46.4
JA du Plessis
November 8, 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 8, 2008
STREET FLOW
Checked
Date
MAP (mm)
623
790
CATCHMENT DATA & GENERAL INFORMATION
Stream
OK
Overland flow (L) (km)
Gobos Plus
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
Greyton River Management Plan Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
73
Secondary drainage region number
Tertiary drainage region number
Quaternary drainage region number
Catchment description
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)
Actual velocity (m/s) Max velocity (m/s)
Surface slope % Factor Cs % Factor C2
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
Total 100 Total 0.238 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
Total 100 Total 0.190 0 Total 0.000
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 0.194 0 Total 0.000
Total 100 Total C1 0.622 0 Total C2 0.000
Correction factor (t) for defined water course: 1.167
0.000 hours hours 0.000 hours 2.803 hours
Return period (T) 2 5 10 20 50 100
Run-off coefficient (C1) 0.622 0.622 0.622 0.622 0.622 0.622
Adjusted run-off coefficient (C1D) 0.622 0.622 0.622 0.622 0.622 0.622
Adjustment factor (FT) 0.750 0.800 0.850 0.900 0.950 1.000
Adjusted run-off coefficient (C1T) 0.467 0.498 0.529 0.560 0.591 0.622
Weighted runoff coefficient (CT) 0.467 0.498 0.529 0.560 0.591 0.622
Return period (T) 2 5 10 20 50 100
Point precipitation (mm), PT (Alexander)
Point precipitation (mm), PT (Smithers & Schulze) 29.116 41.323 50.428 60.113 74.161 85.828
Point intensity (mm/h), PiT 10.386 14.740 17.988 21.443 26.454 30.616
Area reduction factor (%) 96.409 96.409 96.409 96.409 96.409 96.409
Area reduction factor (%) (Smithers & Schulze) 100.000 100.000 100.000 100.000 100.000 100.000
Average intensity (mm/h), IT 10.386 14.740 17.988 21.443 26.454 30.616
Peak flow (m3/s), QT 62 95 123 155 202 246
Gobos Plus
Greyton Main watercourse/ river
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
TIME OF CONCENTRATION (Tc)
NOTES
TIME OF CONCENTRATION (Tc)
Artificial flow/ streets
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
Length of flow path (km)
Street flow
m/m0.02430
Heavy soil, steep (>7%)
Lawns
Sandy, flat (<2%)
46.400
PHYSICAL CHARACTERISTICS
790
AREA DISTRIBUTION FACTORSCoastal/winter
Rural areas (a)
Dolomite area (D)
PRECIPITATION
Streets
Maximum flood
Total Tc
Total
0.747
Overland flow
Heavy soil, flat (<2%)
Average channel slope (Sav)
Defined water course
Residential areas
Houses
URBAN
Flats
Total
Sandy, steep (>7%)
Industry
Greyton River Management Plan Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
74
D
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
6.260
3. PRECIPITATION DATA
0.08488
100
0
1. LOCATION
Greyton
Scholtz River
4.1
JA du Plessis
November 8, 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 8, 2008
STREET FLOW
Checked
Date
MAP (mm)
623
790
CATCHMENT DATA & GENERAL INFORMATION
Stream
OK
Overland flow (L) (km)
Scholtz River
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
Greyton River Management Plan Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
75
Secondary drainage region number
Tertiary drainage region number
Quaternary drainage region number
Catchment description
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)
Actual velocity (m/s) Max velocity (m/s)
Surface slope % Factor Cs % Factor C2
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
Total 100 Total 0.184 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
Total 100 Total 0.190 0 Total 0.000
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 0.194 0 Total 0.000
Total 100 Total C1 0.568 0 Total C2 0.000
Correction factor (t) for defined water course: 1.694
0.000 hours hours 0.000 hours 0.704 hours
Return period (T) 2 5 10 20 50 100
Run-off coefficient (C1) 0.568 0.568 0.568 0.568 0.568 0.568
Adjusted run-off coefficient (C1D) 0.568 0.568 0.568 0.568 0.568 0.568
Adjustment factor (FT) 0.750 0.800 0.850 0.900 0.950 1.000
Adjusted run-off coefficient (C1T) 0.426 0.454 0.483 0.511 0.540 0.568
Weighted runoff coefficient (CT) 0.426 0.454 0.483 0.511 0.540 0.568
Return period (T) 2 5 10 20 50 100
Point precipitation (mm), PT (Alexander)
Point precipitation (mm), PT (Smithers & Schulze) 13.857 20.354 25.388 30.922 39.145 47.365
Point intensity (mm/h), PiT 19.687 28.918 36.069 43.932 55.615 67.293
Area reduction factor (%) 103.406 103.406 103.406 103.406 103.406 103.406
Area reduction factor (%) (Smithers & Schulze) 100.000 100.000 100.000 100.000 100.000 100.000
Average intensity (mm/h), IT 19.687 28.918 36.069 43.932 55.615 67.293
Peak flow (m3/s), QT 9.6 15.0 19.8 25.6 34.2 43.5
Scholtz River
Greyton Main watercourse/ 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
TIME OF CONCENTRATION (Tc)
NOTES
TIME OF CONCENTRATION (Tc)
Artificial flow/ streets
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
Length of flow path (km)
Street flow
m/m0.08488
Heavy soil, steep (>7%)
Lawns
Sandy, flat (<2%)
4.100
PHYSICAL CHARACTERISTICS
790
AREA DISTRIBUTION FACTORSCoastal/winter
Rural areas (a)
Dolomite area (D)
PRECIPITATION
Streets
Maximum flood
Total Tc
Total
0.681
Overland flow
Heavy soil, flat (<2%)
Average channel slope (Sav)
Defined water course
Residential areas
Houses
URBAN
Flats
Total
Sandy, steep (>7%)
Industry
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
76
E
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
CATCHMENT DATA & GENERAL INFORMATION
Stream
OK
Overland flow (L) (km)
Gobos / Scholtz
5. FLOW PATHS: NATURAL
4. CATCHMENT CLASSIFICATION
2. AREA DISTRIBUTION FACTORS
November 8, 2008
STREET FLOW
Checked
Date
MAP (mm)
623
790
JA du Plessis
November 8, 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
100
0
1. LOCATION
Greyton
Full Catchment
50.44
20.160
3. PRECIPITATION DATA
0.02430
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
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
77
Secondary drainage region number
Tertiary drainage region number
Quaternary drainage region number
Catchment description
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)
Actual velocity (m/s) Max velocity (m/s)
Surface slope % Factor Cs % Factor C2
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
Total 100 Total 0.238 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
Total 100 Total 0.190 0 Total 0.000
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 0.194 0 Total 0.000
Total 100 Total C1 0.622 0 Total C2 0.000
Correction factor (t) for defined water course: 1.149
0.000 hours hours 0.000 hours 2.803 hours
Return period (T) 2 5 10 20 50 100
Run-off coefficient (C1) 0.622 0.622 0.622 0.622 0.622 0.622
Adjusted run-off coefficient (C1D) 0.622 0.622 0.622 0.622 0.622 0.622
Adjustment factor (FT) 0.750 0.800 0.850 0.900 0.950 1.000
Adjusted run-off coefficient (C1T) 0.467 0.498 0.529 0.560 0.591 0.622
Weighted runoff coefficient (CT) 0.467 0.498 0.529 0.560 0.591 0.622
Return period (T) 2 5 10 20 50 100
Point precipitation (mm), PT (Alexander)
Point precipitation (mm), PT (Smithers & Schulze) 29.116 41.323 50.428 60.113 74.161 85.828
Point intensity (mm/h), PiT 10.386 14.740 17.988 21.443 26.454 30.616
Area reduction factor (%) 95.956 95.956 95.956 95.956 95.956 95.956
Area reduction factor (%) (Smithers & Schulze) 100.000 100.000 100.000 100.000 100.000 100.000
Average intensity (mm/h), IT 10.386 14.740 17.988 21.443 26.454 30.616
Peak flow (m3/s), QT 68 103 133 168 219 267
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
0.747
Overland flow
Sandy, flat (<2%)
50.440
PHYSICAL CHARACTERISTICS
790
AREA DISTRIBUTION FACTORSCoastal/winter
Rural areas (a)
Dolomite area (D)
Total
ARTIFICIAL FLOW
2.803
Length of flow path (km)
Street flow
m/m0.02430
Heavy soil, steep (>7%)
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
TIME OF CONCENTRATION (Tc)
NOTES
TIME OF CONCENTRATION (Tc)
Artificial flow/ streets
Light industry
Average industry
Heavy industry
Total
Business
City centre
Suburban
0.622
Total
Gobos / Scholtz
Greyton Main watercourse/ river
Designed
Checked
Full Catchment
Date
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
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
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
79
ii) KweziV3 Flood Hydrology Report (2008)
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
80
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
81
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
82
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
83
APPENDIX B
RIVER SURVEY DATA AND CROSS-SECTIONS
(On cd)
Greyton River Management Plan
Floods, flow patterns, river stability and mitigation measures
Hydraulic Model Study FINAL APRIL 2009
84
APPENDIX C
DRAWINGS: FLOODLINES, RIVER MIGRATION & MITIGATION
MEASURES
(On cd)