Amendment 2 31 March 2017 Annexure S Amended

AMENDMENT 2

31 MARCH 2017

ANNEXURE S

AMENDED STORMWATER MANAGEMENT PLAN

CONRADIE BLMEP

CONRADIE BLMEP STORMWATER MANAGEMENT PLAN

HHO Africa Infrastructure Engineers 7293-700-8001 Cape Town March 2017-Rev A

Form QS31-SF8 Rev 3 Page 1 of 1

STORMWATER MANAGEMENT PLAN

PROJECT NO: 7293 REPORT NO: REP-HHO-700-8001-A

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TABLE OF CONTENTS

Section No Description Page No

1.0 INTRODUCTION 1

1.1. BACKGROUND 1 1.2. PREVIOUS FLOODING STUDIES 1 1.3. CITY OF CAPE TOWN POLICY OBJECTIVES 2 1.4. TERMS OF REFERENCE 3 1.5. APPROVAL PROCESS 4

2.0 METHODOLOGY 5

3.0 REVIEW OF ELSIESKRAAL CANAL MODELLING 6

4.0 PROPOSED STORMWATER INFRASTRUCTURE 7

4.1 DETENTION PONDS 7 4.2 SWALES 8 4.3 INTERNAL ROADS & SITE LEVELS 8 4.4 ELSIESKRAAL CANAL 9

5.0 HYDROLOGY AND HYDRAULIC MODELLING 10

5.1 WATER QUALITY 11 5.2 QUANTITY AND RATE OF RUNOFF 12 5.3 HIGH HAZARD ZONES AND FLOOD LINES 19

6.0 BULK EARTHWORKS & COST ESTIMATE 22

6.1 BULK EARTHWORKS 22 6.2 COST ESTIMATE 22

7.0 CONCLUSION 23

8.0 RECOMMENDATIONS 24

9.0 REFERENCES 25

APPENDICES

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LIST OF FIGURES (TO BE UPDATED)

Figure No Description Page No

FIGURE 3.1 CONRADIE BLMEP INFLOW 3

FIGURE 3.2 CONRADIE BLMEP OUTFLOW 3

FIGURE 4.1 RATANGA ROAD OVERFLOW AT KINETIC WAY EXTENSION 5

FIGURE 4.2 ANTICIPATED WATER DEPTH AT RATANGA ROAD 5

FIGURE 4.3 FULL DEVELOPMENT: SYSTEM INFLOW 7

FIGURE 4.4 FULL DEVELOPMENT: SYSTEM OUTFLOW 7

FIGURE 4.5 FULL DEVELOPMENT: SYSTEM STORAGE 8

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LIST OF TABLES (TO BE UPDATED)

Table No Description Page No

TABLE 3.1 FINDINGS FROM 2011 SUDS STUDY 2

TABLE 3.2 CONRADIE BLMEP FLOWS FROM 2011 SUDS MODEL 2

TABLE 4.1 CONRADIE BLMEP FULL DEVELOPMENT COMPARISON WITH 2011 SUDS STUDY 6

TABLE 5.1 PROPOSED WATER BODIES 9

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EXECUTIVE SUMMARY

The Elsieskraal canal represents a significant public safety hazard, which needs to be managed by the City of Cape Town. It has been demonstrated that there is no benefit in realigning the canal and that filling the site does not detrimentally affect the flood regime, the floodplain or adjacent properties.

Although based on limited information and a basic urban design, this report has demonstrated that the Stormwater Management System (SWMS) proposed for the Conradie BLMEP can achieve the parameters for a Sustainable Urban Drainage System (SUDS) as defined by the City of Cape Town. The future Conradie BLMEP developer would need to demonstrate to a greater level of detail, based on preliminary design, how the City’s policy requirements can be achieved.

Key Findings

• Although the site is currently designated a High Hazard Zone (HHZ), the 100 year Recurrence Interval (RI) flood can be contained within the Elsieskraal canal reserve and the risk of flooding on the site can almost entirely be mitigated by the implementation of the SWMP. • The proposed SWMS can effectively reduce flood peaks both for relatively minor nuisance floods and major, extreme floods. Preliminary results indicate that the requirements of the City’s policies can be achieved. • The proposed SWMS form effective sediment and litter traps that are simple to maintain and operate. The City of Cape Town’s water quality objectives are achievable.

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GLOSSARY

BLMEP Better Living Model Exemplar Project DWS Department of Water and Sanitation GA General Authorisation HEC-RAS

HHZ High Hazard Zone HOA Home Owners’ Association NGL Natural Ground Level POS Public Open Space PCSWMM RFP Request for Proposal RI Recurrence Interval SCS Soil Conservation Survey SS Suspended Solids SUDS Sustainable Urban Drainage System SWMM US EPA Stormwater Management Model SWMP Stormwater Management Plan SWMS Stormwater Management System TA Technical Advisor TP Total Phosphorus VAT Value Added Tax WCG Western Cape Provincial Government

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1.0 INTRODUCTION

1.1. BACKGROUND

The proposed Conradie Better Living Model Exemplar Project (BLMEP), which is being undertaken by the Western Cape Provincial Government (WCG), requires the existing Elsieskraal canal to be deviated for the following reasons:

• The entire Conradie site is a flooding High Hazard Zone (HHZ) and consequently cannot be developed for housing unless the HHZ can be contained in a controlled manner without affecting downstream users.

• The urban development framework, on which the BLMEP is based, requires the canal to be realigned. This would create development areas on either side of the canal, whilst providing a recreational area and public open space.

1.2. PREVIOUS FLOODING STUDIES

The following reports related to the BLMEP have been compiled to date. These are attached in the Appendices, but are summarised below.

1.2.1 Stormwater Concept Design, Management and Riverine Development Plan (WSP, August 2016)

This BLMEP report presents a construction cost estimate and concept design for the realigned Elsieskraal canal that addresses the following City of Cape Town (City) policies:

• Management of Urban Stormwater Impacts Policy • Floodplain and River Corridor Management Policy

Based on a 2012 report by SRK entitled, “Salt River High Level Stormwater Master Plan,” the report does not take into account surface water inflows from Thornton and states that the piped inflows from that suburb are insignificant. WSP also alludes to current work being undertaken by the City of Cape Town to assess the upstream capacity of the Elsieskraal canal, which may have a positive impact on this project.

The WSP report contains a number of inconsistencies and ambiguities, some of which are addressed in Aurecon’s March 2017 report, referred to below.

1.2.2 Elsieskraal River- Flood Mitigation Study (Aurecon, October 2016)

This report was commissioned by the City of Cape Town, who had appointed Aurecon to study the Elsieskraal River. It considered only the existing Elsieskraal canal and not the proposed realignment. The study found that the flood levels adopted by WSP, which had been determined SRK in 2012, are based on energy levels as opposed to actual water surface levels. Although in keeping with the City’s policy, the results are overly conservative.

The study also assessed the impact of local minor and major stormwater systems on the existing canal. Importantly, it was based on 2D modelling, which provides the following advantages over the 1D modelling previously used:

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• 2D modelling takes topography into account, which means that overland flow paths and overtopping can be assessed.

• It is based on water surface elevations and not energy levels. As such, more realistic answers are obtained, although not strictly in keeping with the City’s policy.

This study reviewed and compared the SRK 2012 PCSWMM (1D) and HEC-RAS models and found several inconsistencies and inaccuracies. These were corrected to better represent actual conditions. Subsequently, a number of PCSWMM 2D models were created, which indicated that flooding on the existing Conradie Hospital site is less likely and less severe than previously thought.

1.2.3 Modelling of the Conradie Hospital’s Stormwater Concept Design, Management and Riverine Development Plan (Aurecon, March 2017)

Subsequent to the above study a meeting was held between WSP, WCG, HHO and Aurecon to obtain clarity on the WSP concept design and it’s supporting calculations and PCSWMM model. It became clear that further development of the concept was required and Aurecon was asked by the City to undertake this work through their current appointment. Aurecon was to assess the WSP concept design to determine whether it would perform appropriately. Furthermore, any areas of possible concern were to be identified.

The report concluded that:

• Aurecon was unable to accurately model the concept design as intended by WSP due to the paucity of information.

• Further concept design development work is required before it can be definitively stated that the concept will work.

• The width of the intended realigned canal reserve appears adequate to accommodate the 100 year flood, provided that key determinants are adequately addressed in the preliminary design stage.

1.3. CITY OF CAPE TOWN POLICY OBJECTIVES

The storm water design objectives can be categorised in two sections: • External flows: Accommodate the Elsieskraal canal, which passes through the site, and containing the HHZ such that the site can be developed. • Internal flows: Control the quantity and rate of runoff emanating from the site itself and improve the quality if runoff.

It is important to note that the design objective is not to attenuate or treat the incoming Elsieskraal canal flows. However, all runoff originating on site must be fully treated and attenuated as required by the City’s policies. The objectives of these may be may be summarised as follows:

• City of Cape Town’s Management of Urban Stormwater Impacts Policy objectives: − Improve quality of runoff − Control quantity and rate of runoff − Encourage ground water recharge

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• City of Cape Town’s Floodplain and River Corridor Management Policy objectives: − Buildings catering for residential or general business should have floors above the 100 year flood level and non-habitable basements flood proofed against the 50 year flood. − Educational facilities, public halls and places of worship should be situated above the 100 year flood level. − Public open spaces and sports fields could be situated lower than the 20 year flood level (but not lower than the 2 year flood level) provided that any clubhouse (or similar) building structures have floors above the 50 year flood level. − High Hazard Zones (HHZ) should be identified in terms of the 100 year flood and public safety ensured in such areas. Figure 1.1 illustrates the water depth and velocity parameters associated with flood zones.

FIGURE 1.1: FLOOD HAZARD ZONES

1.4. TERMS OF REFERENCE

HHO Africa was appointed as Technical Advisor (TA) to the WCG on 18 October 2016. Whilst the appointment relates primarily to the Request for Proposal (RFP) process, it soon became apparent that further work was required on the concept stormwater design. Following a collaborative process with the authorities and other consulting engineers, HHO were instructed by WCG to provide a new Stormwater Management Plan (SWMP) report containing unambiguous stormwater and drainage requirements. This report would be appended to the rezoning application and forwarded to bidders. Furthermore, the cost estimate and phasing plan had to be verified, or alternatives provided.

In March 217 it became clear that the concept to realign the canal did not achieve its intended objective of flood peak attenuation. Furthermore, from an urban design perspective, it was unlikely that the previously proposed realignment, once geometrically corrected, would permit much development between Thornton and itself. There were also unanswered questions about the size, depth, etc. of the proposed detention pond and sports field and the amount of additional bulk earthworks required to prevent flooding. The earthworks levels (as adjusted to suit the models)

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needed to be assessed against the surrounding rounds, internal heritage areas and adjacent properties. The following significant amendments were proposed:

1) To keep the existing canal in its current position.

2) To create an adequate floodplain / public amenity adjacent the canal.

3) To revise the proposed site levels to: a. Ideally keep the development above the 100 year flood level b. Respect heritage buildings, adjacent roads and properties.

4) To separate the canal hydrology and flow from the development itself. An Important implication would be that the development itself would satisfy the City’s policies, but that the canal flows would not be integrated with the development as in the previous concept.

1.5. APPROVAL PROCESS

The proposal to realign the Elsieskraal Canal has been approved by the National Department of Water and Sanitation (DWS), who have granted General Authorisation (GA) based on the WSP concept design. This letter is attached as Annexure A.

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2.0 METHODOLOGY

The WCG commissioned this study in response to the concept design report issued by WSP and the hydraulic modelling undertaken on behalf of the City by Aurecon. As this report relies on the work done by others, the following methodology was followed:

1) The previous concept design and report by WSP was assessed. 2) The Aurecon reports on their 1D and 2D PCSWMM and HEC-RAS hydraulic modelling were assessed. 3) Meetings were held between the City of Cape Town, WCG, Aurecon and WSP. 4) Once the decision was made not to realign the Elsieskraal canal, a new high level concept design was required. This was achieved as follows: a. A topographic survey of the Elsieskraal canal was provided by Aurecon. b. Cadastral information and base information was provided by ArG Architects. c. New site levels were calculated in accordance with the 100 year RI flood water levels calculated by Aurecon. d. A berm was proposed along the Elsieskraal canal in accordance with the 100 year RI flood energy levels calculated by Aurecon. e. Bulk stormwater infrastructure was designed to satisfy the City’s policies. f. A simplistic and conservative 1D PCSWMM model was created for the development. g. Policy objectives were assessed using the model. 5) Bulk earthwork costs were estimated. 6) A draft SWMP report was written.

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3.0 REVIEW OF ELSIESKRAAL CANAL MODELLING

Aurecon undertook two modelling studies of the Elsieskraal canal on behalf of the City of Cape Town. The former studied the existing canal alignment, whereas the latter study considered the concept proposed by WSP. The following salient findings affect this SWMP, which proposes to keep the existing canal alignment: • Flood levels are based on energy levels, which take flow velocity into account. Whilst applicable to open channels and rivers, this approach renders unrealistic flood levels in very wide floodplains, such as the current Conradie site.

• The previous models, which defined the current floodplain, were overly conservative. The potential for flooding on site due to the canal is less likely and less severe than previously indicated.

• Flow velocities in the 100 year RI flood event, are very high in the Elsieskraal canal. Velocities of 4 to 5 m/s can be expected. This implies that energy (flood) levels are between 0.8m and 1.3m higher than water surface levels.

• Actual canal water surface levels during the 100 year RI flood event are likely to peak at approximately 12.0m adjacent the lower end of the site, upstream of the railway bridge. At the upper end of the site, the water level would be about 13.0m. Water depth in the canal would be roughly 3.0m.

• The railway culvert has adequate capacity to convey the 100 year RI flood peak of 115 m3/s.

• The current flooding of the Conradie site, as previously defined, offers negligible flood peak attenuation. This implies that site levels could be raised without detriment to adjoining properties.

• It was recommended that the site be infilled to at least the level of the existing levee.

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4.0 PROPOSED STORMWATER INFRASTRUCTURE

The SWMP entails the following design objectives:

Talk about policies?

• To keep the existing canal in its current position. • To create an adequate floodplain / public amenity adjacent the canal. • To revise the proposed site levels to: - Ideally keep the development above the 100 year flood level - Respect heritage buildings, adjacent roads and properties. • To separate the canal hydrology and flow from the development itself. An Important implication would be that the development itself would satisfy the City’s policies, but that the canal flows would not be integrated with the development as in the previous concept.

HHO Africa has assessed the stormwater implications of the full development scenario in accordance with the latest Urban Design Framework dated March 2017. The SWMP is included in Annexure B. The following infrastructure is proposed:

• Detention ponds with overflow weirs and discharge pipes • Overland channels and swales • Underground stormwater pipes and culverts • Roadways, which act as overland stormwater conduits for major storms • Public open space enhancements along the Elsieskraal canal

4.1 DETENTION PONDS

Two detention ponds are proposed. These are situated adjacent the proposed 25m Elsieskraal canal Public Open Space (POS) or buffer zone and each collects roughly half of the site runoff. The ponds’ main function is to attenuate flood peaks and to trap litter and silt.

Although flap gates to the outlet pipes are proposed to prevent ingress of flood water from Elsieskraal canal, the system is designed to function without these. All habitable buildings are situated above the 100 year RI flood water level. Apart from the overflow pipes, weirs are provided to the 100 year RI flood level (water level) of the Elsieskraal canal.

For the purposes of this assessment, the following detention pond design has been assumed (see Table 4.1). Both detention ponds would be identical in size and operation. Figure 4.1 illustrates the concept.

TABLE 4.1: DETENTION POND DESIGN

ITEM DESCRIPTION Bottom area 1000m2 Side slopes 1V:2H Pond floor level 0.5m below swale entry level Outflow pipe DN375 Outflow pipe invert level 0.5m above pond floor Weir level 2.1m above pond floor Weir length 1m Top of berm level 3.0m above pond floor

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Insert 4.1

FIGURE 4.1: TYPICAL DETENTION POND DETAILS

4.2 SWALES

All surface water runoff from the Conradie BLMEP is conveyed to the proposed overland channels/ swales, which collect, convey and treat surface water runoff. Two swales are proposed, which traverse the site roughly perpendicular to the Elsieskraal canal. These each discharge into a detention pond, which in turn discharge into the Elsieskraal canal. Figure 4.2 illustrates the typical swale cross section that is proposed. For the purposes of the hydraulic model, a slope of 1:400 (0.25%) has been assumed. The two swales have considerable length, measuring 390m and 335m long respectively.

Insert 4.2

FIGURE 4.2 TYPICAL SWALE CROSS SECTION

4.3 INTERNAL ROADS & SITE LEVELS

All internal roads are designed as stormwater carriers in the event of major storms. All roads and underground pipes discharge into one of two overland channels that traverse the development. In instances where direct discharge into the channels or detention ponds is not possible, underground conduits are provided to relieve trapped lows. These can accommodate storm events of up to 100 year Recurrence Interval (RI).

All buildings are all elevated above the 100y RI flood (energy) level as required by the City’s Floodplain and River Corridor Management Policy:

• All residential floors are elevated above the 100 year RI flood (energy) level.

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• Parking areas underneath certain buildings are below the 100 year RI flood (energy) level, but above the 100 year RI flood (water) level. This means that the real risk of flooding is greatly reduced.

• There is a significant difference in elevation between the water and energy (flood) level due to the very high water velocities in the Elsieskraal canal. Typically, the theoretical flood level is expected to be between 0.8m and 1.3m above the water surface level, due to the velocity of about 5m/s in the 100 year RI flood event.

4.4 ELSIESKRAAL CANAL

Although the Elsieskraal canal is not realigned or altered in any way, the following improvements are proposed. Figure 4.3 illustrates a typical cross section.

• A 25m wide POS/ buffer zone, which abuts the canal, is proposed. This zone would be incorporated into the Elsieskraal canal floodplain. The responsibility for maintenance, public safety, etc. would resort with the Homeowners Association. The space inside the POS/ buffer zone would be configured as follows: - The level floodplain on the development side is widened to approximately 8m in order to create a better public amenity, similar to the downstream canal in Pinelands. - A further 9m is required to provide landscaped, gently sloped berm(s), which would raise ground levels to the 100 year RI flood level (energy level), thus containing the Elsieskraal canal flood waters. - Another 6m is provided for a pedestrian walkway and trees, beyond which another 2m provides space to adjust bulk earthwork levels to suit the design.

Insert 4.3

FIGURE 4.3 TYPICAL CROSS SECTION SHOWING PROPOSED 25M WIDE POS/ BUFFER ZONE AT ELSIESKRAAL CANAL

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5.0 HYDROLOGY AND HYDRAULIC MODELLING

The software package Design rainfall of South Africa by Smithers and Schulze was used to determine the design rainfall for various storm events, as shown in Table 5.1 below. Although the design rainfall data did not contain data for storms smaller than 2-year RI, rainfall for the 1-year and ½ year RI storm events was calculated using logarithmic extrapolation. SCS Type I storms were assumed for all scenarios, as this distribution has been demonstrated as being realistic for South African coastal regions with winter rainfall.

TABLE 5.1: DESIGN RAINFALL (mm)

RECURRENCE INTERVAL (YEARS) DURATION 2.0 5.0 10.0 20.0 50.0 100.0 200.0 5 m 4.5 6.0 7.2 8.3 10.0 11.3 12.7 10 m 6.5 8.7 10.4 12.1 14.4 16.4 18.4 15 m 8.1 10.9 12.9 15.0 17.9 20.3 22.9 30 m 10.8 14.6 17.3 20.1 24.0 27.2 30.7 45 m 12.9 17.3 20.5 23.8 28.5 32.3 36.4 1 h 14.5 19.5 23.1 26.8 32.1 36.4 41.1 1.5 h 17.2 23.1 27.4 31.8 38.1 43.2 48.7 2 h 19.4 26.1 30.9 35.9 43.0 48.8 54.9 4 h 24.2 32.4 38.4 44.7 53.5 60.6 68.3 6 h 27.4 36.8 43.6 50.7 60.7 68.9 77.6 8 h 30.0 40.3 47.8 55.6 66.5 75.4 85.0 10 h 32.2 43.2 51.3 59.6 71.3 80.9 91.2 12 h 34.1 45.8 54.3 63.1 75.5 85.7 96.5 16 h 37.4 50.1 59.4 69.1 82.7 93.8 105.7 20 h 40.1 53.8 63.8 74.1 88.7 100.6 113.4 24 h 42.5 56.9 67.5 78.5 94.0 106.6 120.1

HHO Africa calculated the impact of the proposed development by using the Rational Method and modelling the proposed major stormwater infrastructure using PCSWMM. Calculations were done to verify compliance with the City of Cape Town’s 2009 Management of Urban Stormwater Impacts Policy, bearing in mind the approximate flood stage in the adjacent Elsieskraal canal.

Given the lack of historic data for the site, certain unverifiable assumptions have had to be made regarding the pre-development scenario. Although required for the SUDS evaluation, the veracity of this scenario cannot be ascertained. Other unknown information included geological or geotechnical information, a topographic survey of the site and hydrographs or flood models for the Elsieskraal canal catchment. Assumptions had to be made based on observations and local knowledge. For the purposes of this evaluation, the following parameters were assumed:

• Post-development runoff coefficient, Cpost: 0.7

• Pre-development runoff coefficient, Cpre: 0.2 • Approximate Natural Ground Level (NGL): 12.0m • Assumed flood levels in the Elsieskraal canal are stated in the relevant sections that follow.

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5.1 WATER QUALITY

The City’s Management of Urban Stormwater Impacts Policy requires on-site reduction of post- development annual stormwater pollutant load of 80% of Suspended Solids (SS) and 45% of Total Phosphorus (TP) based on the 0.5 year RI, 24 hour storm. Figure 5.1 illustrates that outflow into the Elsieskraal canal would commence some 19 hours after the start of the storm event, or 7 hours after the peak. This should allow sufficient time for pollutant removal, as detailed in Table 5.2 below. It is assumed that the peak flood level in the Elsieskraal canal during this storm event is 11.4m.

FIGURE 5.1: FLOOD PEAK ATTENUATION AT A SINGLE DETENTION POND: ½ YEAR RI, 24 HOUR STORM

It is proposed that the City’s water quality targets may be achieved as follows:

The very flat longitudinal swale gradient of 0.25% (1:400) would result in a flow velocity of 0.5m/s for the design storm. As this velocity is not sufficient to keep SS in suspension, SS would settle out along the swale. Any SS that is conveyed the full length of the swale would be discharged into the detention pond, which features a permanent wet well with elevated outlet pipe. Any remaining SS would thus settle on the pond floor. Regular maintenance would be required to clean the ponds and swales. This mechanism is expected to reduce suspended solids by at least 80% as required by the Policy.

It is the designed intention that the required reduction in TP would be achieved through nutrient absorption by approved plant species along the swale and in the detention pond. As the swales are unlined, significant groundwater infiltration would be expected, which would also capture some TP. Based on established best practice it is expected that TP would be reduced by 45%.

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TABLE 5.2: SUDS WATER QUALITY OBJECTIVES

SUDS OBJECTIVE: ON-SITE REDUCTION OF POST-DEVELOPMENT POLLUTANT LOAD SUDS TARGET: 80% SS & 45% TP REDUCTION OF THE 0.5-YEAR RI, 24H STORM PARAMETER TARGET ACHIEVED RESULT Runoff Volume - 1,900 m3 - Outflow Volume - 980 m3 - Swale Gradient - 0.25% - Swale Flow Velocity < 0.9 m/s 0.5 m/s OK Pond Wet Well Volume - 1,150 m3 - Sufficient Time for SS to settle (SS & SS & TP 7 h OK TP)

5.2 QUANTITY AND RATE OF RUNOFF

In order to assess the proposed Conradie BLMEP SWMS for quantity and rate of runoff, the objectives below were measured against the following storm events:

TABLE 5.3: SUDS QUANTITY & RATE OF RUNOFF OBJECTIVES

To protect the stability of downstream 24 hour extended detention of the 1-year channels recurrence interval, 24 hour storm event.

To protect downstream properties from fairly Up to the 10-year recurrence interval peak frequent nuisance floods flow reduced to pre-development level.

To protect floodplain developments and Up to the 50-year recurrence interval peak floodplains from adverse impacts of extreme flow reduced to existing development levels. floods Evaluate the effects of the 100-year recurrence interval storm event on the stormwater management system, adjacent property, and downstream facilities and property. Manage the impacts through detention controls and/or floodplain management.

The modelled scenarios are presented in greater detail below.

5.2.1 Stability of Downstream Channels

This requirement of the Policy was evaluated using the 1-year recurrence interval, 24 hour storm event. The inflow hydrograph of the SCS Type 1 storm peaks after 12 hours from the start of the simulation and rainfall ceases after 24 hours. Figure 5.2 demonstrates that the total peak inflow is roughly 0.5 m3/s. This flow would apply equally to either detention pond. The total runoff from the development is conservatively calculated as being 0.09 m3/s. It is assumed that the peak Elsieskraal canal water level during this storm event would be approximately 11.5m.

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Figure 5.3 shows that the outflow hydrograph peak is roughly 0.09 m3/s. This is approximately 18% of the inflow peak, illustrating significant attenuation. The outflow hydrograph in Figure 5.3 and Table 5.4 illustrate that the design storm is attenuated for more than 24 hours, thus achieving the Policy requirement.

FIGURE 5.2: FLOOD PEAK ATTENUATION AT A SINGLE DETENTION POND: 1 YEAR RI, 24 HOUR STORM

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FIGURE 5.3: EXTENDED DETENTION: 1 YEAR RI, 24 HOUR STORM

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TABLE 5.4: STABILITY OF DOWNSTREAM CHANNELS

SUDS OBJECTIVE: TO PROTECT THE STABILITY OF DOWNSTREAM CHANNELS SUDS TARGET: 24H EXTENDED DETENTION OF THE 1-YEAR RI, 24H STORM PARAMETER TARGET ACHIEVED RESULT Runoff Peak Flow - 1.0 m3/s - Peak Outflow - 0.2 m3/s - Detention Period 24 h >24 h OK

5.2.2 Fairly Frequent Nuisance Floods

To protect downstream properties from fairly frequent nuisance floods, the Policy requires that runoff be reduced to that which was likely before any human intervention or infrastructure development took place. It requires up to the 10-year recurrence interval peak flow to be reduced to pre-development level.

Before the Conradie Hospital site was first developed, it is assumed that the area was characterised by flat topography, sandy soils and Cape fynbos vegetation. Figure 5.4 illustrates a suggested pre- development runoff hydrograph for such assumptions, based on an SCS Type I design storm.

FIGURE 5.4: SUGGESTED PRE-DEVELOPMENT RUNOFF HYDROGRAPH

The simple and conservative PCSWMM model of half the proposed development, which illustrates the functioning of a single swale and detention pond combination, reveals that the post- development peak runoff of approximately 1.1 m3/s is reduced to a peak outflow of about 0.3 m3/s. The post-development hydrograph is shown in Figure 5.5. These flows may be doubled to obtain the

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effect of the full development. It is assumed that the peak Elsieskraal canal water level during this storm event would be approximately 11.7m.

FIGURE 5.5: FLOOD PEAK ATTENUATION AT A SINGLE DETENTION POND: 10 YEAR RI, 24 HOUR STORM

As can be seen in Table 5.5, the assumed pre-development peak runoff of 0.4 m3/s may not be quite matched by the post-development peak outflow of 0.5m3/s. However, as the post-development model is conservative, the actual outflow may match the objective. We can therefore conclude that the intention of this particular Policy objective is satisfied.

TABLE 5.5: FAIRLY FREQUENT NUISANCE FLOODS

SUDS OBJECTIVE: TO PROTECT DOWNSTREAM PROPERTIES SUDS TARGET: 10-YEAR PEAK FLOW REDUCED TO PRE-DEVELOPMENT LEVEL PARAMETER TARGET ACHIEVED RESULT Runoff Peak Flow - 2.0 m3/s - Peak Outflow 0.4 m3/s 0.5 m3/s OK

5.2.3 Extreme Floods

To protect floodplain developments and floodplains from adverse impacts of extreme floods, the Policy requires that the effects of severe storm events (100-year recurrence interval) be evaluated in the context of its impact on the stormwater management system, adjacent property and downstream infrastructure. Also, the peak flow from a 50-year recurrence interval storm for the fully developed scenario needs to be restricted to the current peak flow. The modelling results are presented below.

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50-Year RI Storm Event

The Policy requires up to the 50-year RI peak flow to be reduced to existing development levels. Currently the site consists of demolished buildings and derelict roads and gardens. Figure 5.6 illustrates a suggested current-development runoff hydrograph, based on an SCS Type I design storm.

FIGURE 5.6: CURRENT DEVELOPMENT RUNOFF HYDROGRAPH

As before, the simplified PCSWMM model, which illustrates the functioning of a single swale and detention pond combination for half the proposed development, reveals that the post-development peak runoff of approximately 1.5 m3/s is reduced to a peak outflow of about 0.5 m3/s, of which roughly 40% would overtop the weir. The hydrograph in Figure 5.7 provides illustration. These flows may be doubled to obtain the effect of the full development. It is assumed that the peak Elsieskraal canal water level during this storm event would be approximately 11.9m. Table 5.6 provides a summary of this scenario.

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FIGURE 5.7: FLOOD PEAK ATTENUATION AT A SINGLE DETENTION POND: 50 YEAR RI, 24 HOUR STORM

TABLE 5.6: REDUCTION OF EXTREME FLOOD PEAKS

SUDS OBJECTIVE: TO PROTECT FLOODPLAIN DEVELOPMENTS & FLOODPLAINS SUDS TARGET: 50-YEAR PEAK FLOW REDUCED TO CURRENT DEVELOPMENT LEVEL PARAMETER TARGET ACHIEVED RESULT Runoff Peak Flow - 3.0 m3/s - Peak Outflow 1.3 m3/s 1.0 m3/s OK

100-Year RI Storm Event

The Policy requires evaluation of the effects of the 100-year RI storm event on the stormwater management system, adjacent property, and downstream facilities and property. Its impacts must be managed through detention controls and/or floodplain management.

The PCSWMM model, which illustrates the functioning of a single swale and detention pond combination for half the proposed development, reveals that the post-development peak runoff of approximately 1.6 m3/s is reduced to a peak outflow of about 0.8 m3/s, of which roughly half would overtop the weir. Figure 5.8 illustrates inflows and outflows. These flows may be doubled to obtain the effect of the full development as revealed in Table 5.7. It is assumed that the peak Elsieskraal canal water level during this storm event would be approximately 12.0m.

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FIGURE 5.8: FLOOD PEAK ATTENUATION AT A SINGLE DETENTION POND: 100 YEAR RI, 24 HOUR STORM

TABLE 5.7: EVALUATION OF EXTREME FLOODS

SUDS OBJECTIVE: TO PROTECT FLOODPLAIN DEVELOPMENTS & FLOODPLAINS SUDS TARGET: EVALUATE EFFECTS OF THE 100-YEAR RI STORM EVENT ON SWMS, ADJACENT PROPERTY, DOWNSTREAM FACILITIES & PROPERTY PARAMETER TARGET ACHIEVED RESULT Runoff Peak Flow - 3.2 m3/s - Peak Outflow - 1.6 m3/s OK

This significant attenuation of the 100 year RI flood peak demonstrates that the Conradie BLMEP SWMS more than adequately buffers downstream properties against flood damage.

5.3 HIGH HAZARD ZONES AND FLOOD LINES

The entire Elsieskraal canal is a safety hazard when in flood. In terms of the definition of HHZ, which is based on water depth and velocity, it would be impossible for this canal to not be classified such. The canal passes through the site and this represents danger to the public, which needs to be managed.

If implemented, the SWMP would isolate the Elsieskraal canal from the development, thereby containing the HHZ and preventing flooding of the site. Annexure C details the 100 year RI flood lines and demonstrates that the HHZ inside the development can be contained to the detention ponds. This is shown in Figure 5.9 below.

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Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO Africa 21

[GET THIS FROM THE ANNEXURE C PDF)

Figure 5.9: Flood Lines and HHZ

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6.0 BULK EARTHWORKS & COST ESTIMATE

6.1 BULK EARTHWORKS

The Conradie BLMEP requires significant bulk earthworks to effect the SWMP. In the absence of a topographic survey, the City’s LIDAR data was used to estimate the Natural Ground Level on site. Generally speaking, the NGL has been assumed to be roughly 12.0m. Platform levels range between 12.5m and 14.0m, with the berm adjacent the Elsieskraal canal ranging in height from approximately 13.0m to 14.0m. Preliminary bulk earthworks quantities are presented in Table 6.1.

TABLE 6.1 BULK EARTHWORKS

ITEM UNIT QUANTITY Topsoil Strip m3 22 000 Cut to Fill m3 18 500 Imported Fill m3 166 000

6.2 COST ESTIMATE

Whilst it is not the objective of this SWMP to provide an accurate or detailed cost estimate, the following construction costs may be estimated for the following major system elements: • Bulk earthworks • Detention ponds • Swales, including culverts at road crossings

Table 6.2 summarises the anticipated construction cost. The following costs are not included in this estimate: • Planting to swales and detention ponds • Internal roads • Catchpits and underground stormwater pipes, which construe the minor system • Landscaping, footways, lighting, etc. • Fencing, or any other public safety devices • Professional fees, escalation and VAT

TABLE 6.2 CONSTRUCTION COST ESTIMATE

SWMP CONSTRUCTION COST ESTIMATE ITEM DESCRIPTION UNIT QUANTITY AMOUNT 1.0 Bulk Earthworks 3 Topsoil cut to spoil m 22000 R 1 270 000.00 3 Cut to fill m 18500 R 3 966 000.00 3 Imported fill m 166000 R 54 736 000.00 3 Overhaul in excess of freehaul m .km 382800 R 3 156 000.00 2.0 Detention pond No. 2 R 623 000.00 3.0 Swales, including culverts m 715 R 5 846 000.00

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Total (Excl VAT) R 69 597 000.00 7.0 CONCLUSION

Although based on limited information and a basic urban design, this report has demonstrated that the SWMS proposed for the Conradie BLMEP can achieve the parameters for a Sustainable Urban Drainage System (SUDS) as defined by the City of Cape Town. The future Conradie BLMEP developer would need to demonstrate to a greater level of detail, based on preliminary design, how the City’s policy requirements can be achieved.

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8.0 RECOMMENDATIONS

• Regular maintenance and annual clearing of the swales and detention ponds would be required to remove silt and sediment. Other regular maintenance tasks would include the removal of litter from the POS and water bodies and possible repair of the flap gates to detention pond outlet pipes.

• A formal agreement between City and future developer should be drafted to address common issues such as maintenance, access and public safety.

• Safety measures, such as signage and safety equipment should be implemented. Given the severity of the public hazard at the Elsieskraal canal, a flood warning alert system should be considered.

• Preliminary design should be completed before any proposals contained in this SWMP are implemented or acted upon. This should include detailed 2D modelling.

• Flood water levels must be determined in the Esieskraal canal for the full range of design storms (½, 1, 2, 10, 50 and 100 year RI storm events).

• Any pedestrian or vehicular bridges over the Elsieskraal canal would require backwater calculations to determine the effect on flood levels.

• Stormwater runoff from Thornton and Viking Park needs to be better understood and modelled.

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9.0 REFERENCES

1. Management of Urban Stormwater Impacts Policy, Catchment, Stormwater and River Management Branch, Roads & Stormwater Department, City of Cape Town, Cape Town May 2009

2. Floodplain and River Corridor Management Policy, City of Cape Town

3. Stormwater Concept Design, Management and Riverine Development Plan Prepared by WSP, August 2016

4. Elsieskraal River- Flood Mitigation Study, Prepared by Aurecon, October 2016

5. Modelling of the Conradie Hospital’s Stormwater Concept Design, Management and Riverine Development Plan, Prepared by Aurecon, March 2017

6. WRC Report No. 1060/1/03 entitled "Design Rainfall and Flood Estimation in South Africa" Prepared by JC Smithers and RE Schulze Version 3, July 2012

7. Minimum Standards for Civil Engineering Services in Townships (Version 1), City of Cape Town July 2013

Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO Africa

APPENDICES

A: Approval B: SWMP C: Flood Lines and HHZ

Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO Africa