Amendment 2 31 March 2017 Annexure S Amended
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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 DOCUMENT VERIFICATION Rev Date Prepared by Checked by Approved by Description Status NAME NAME NAME Not A March 2017 F de Villiers M Woodward C Avenant First Issue (Draft) Issued SIGNATURE SIGNATURE SIGNATURE Rev Date Prepared by Checked by Approved by Description Status NAME NAME NAME SIGNATURE SIGNATURE SIGNATURE Rev Date Prepared by Checked by Approved by Description Status NAME NAME NAME SIGNATURE SIGNATURE SIGNATURE Rev Date Prepared by Checked by Approved by Description Status NAME NAME NAME SIGNATURE SIGNATURE SIGNATURE i 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 Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO AFrica ii 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 Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO AFrica iii 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 Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO AFrica iv 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. Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO AFrica v 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 Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO AFrica 1 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: Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO AFrica 2 • 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 Conradie BLMEP: Stormwater Management Plan March 2017-Rev A Western Cape Government HHO AFrica 3 • 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.