Appendices 11 to 14
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Appendix 11: Stormwater Management Plan STORMWATER MANAGEMENT PLAN FOR THE DEVELOPMENT OF PORTION 4 OF THE FARM GWAYANG No 208 Submission date: 16 July 2018 Prepared by: Infrastructure Consulting Engineers Contact person: De Bruyn Joubert T: +27 12 349 2022 F: +27 12 349 2026 E: [email protected] Table of Contents 1. Introduction ................................................................................................................................. 1 1.1. Background to Report ...................................................................................................... 1 1.2. Location of Development ................................................................................................. 1 1.3. Objective of Report ............................................................................................................ 2 1.4. Development planning ...................................................................................................... 2 1.5. Current upstream drainage system .............................................................................. 3 1.6. Downstream receiving water body ................................................................................ 7 1.7. Overview of proposed stormwater drainage system ............................................... 7 2. Underground storage of hazardous substances ............................................................. 10 2.1. General .................................................................................................................................... 10 2.2. Fuel tanks ............................................................................................................................... 11 2.3. Fuel delivery .......................................................................................................................... 12 2.4. Fuel Conveyance .................................................................................................................. 12 2.5. Fuel pump system ............................................................................................................... 13 3. Drainage of surfaces with a high risk of hydrocarbon contamination ...................... 14 4. Design and Maintenance ........................................................................................................ 17 1. Introduction 1.1. Background to Report 8 Mile Investments is planning for the rezoning and subdividing of Portion 4 of the Farm Gwayang no 208, situated opposite the main entrance road to George Airport in the Western Cape. The current zoning of the property is “Agriculture Zone I”. The property is approximately 11.0433ha in extent. The land use application to be lodged to George Municipality will be for: i The subdivision of Portion 4 of the Farm Gwayang No 208 into 14 Portions (7 x Industrial Zone I portions; 1 x Business Zone VI portion; 1 x Transport Zone II portion; 4 x Open Space Zone II portions & 1 x Agriculture Zone I portion (the Remainder)) in terms of Section 15(2)(d) of the Land-Use Planning By-Law for George Municipality, 2015; ii The rezoning of Portion 4 of the Farm Gwayang No 208 from “Agriculture Zone I” to “Subdivisional Area” in terms of Section 15(2)(a) from the Land-Use Planning By-Law for George Municipality, 2015. Infrastructure Consulting Engineers (ICE) were appointed to do provisional planning of roads and services for the proposed township, which includes the necessary infrastructure to collect and control stormwater runoff. The Stormwater Management Plan was compiled to guide the future development of erven and the drainage system inside proposed road reserves. The purpose of the plan is to mitigate the impact of the post-development stormwater drainage system on the current drainage system downstream of the development. This report also addresses mitigating measures related to the potential impact of the storage of hazardous substances in underground fuel tanks on proposed Erf 1. Erf 1 is earmarked for the development of a fuel service station. 1.2. Location of Development The property is situated to the east of the R404 at the intersection with the George Airport main access road. The site locality plan is shown in Figure 1 below. 1 Figure 1: Site Locality Plan 1.3. Objective of Report The proposed development of Portion 4 will impact on the drainage response of the sub-catchment. This is caused by several characteristics that are inherently part of urbanisation. These are typically: • Reduction in permeability of the catchment area; • Reduction in the response time of the catchment. This in turn impacts on the appropriate storm duration and the storm intensity; • Reduction in rainfall intensity impacts on the groundwater table; • Possible deterioration of quality of runoff. The reduction in permeability and the increase in rainfall intensity (associated with the reduced catchment response time), increase the runoff into the receiving water bodies. This consequence potentially destabilises the downstream natural water courses. It furthermore impacts on the risk of flooding of downstream properties. It is therefore important to mitigate the mentioned potential negative impacts on the receiving water systems. 1.4. Development planning The property is currently undeveloped and is used for agricultural activities. The property is 11.0433 ha in extent. The proposed land-uses within the township will consist of Industrial Zone 1 and Business Zone VI. Industrial zoning limits coverage to 75 % of the erf area. Building lines furthermore limit the extent of development. Parking requirements of 2 per 100 m² of building, plus vehicle on-site circulation 2 areas imply that the maximum coverage will in general be limited to 50%. For the purpose of the mitigating measures it is however assumed that the maximum coverage will be implemented. 1.5. Current upstream drainage system In order to develop a stormwater masterplan for the proposed development it is imperative to consider the impact of runoff from higher laying areas on the receiving area of Portion 4. Due to the topography of the terrain, stormwater drains in general from the north to the south. Portion 4 therefore receives runoff from Portions 130 and 139 of the Farm Gwayang no 208 located to the north. See Stormwater Layout Plan attached in Annexure A. Figure 2 below is an extract from the Gwayang Local Spatial Development Framework. The development of Portion 4 is guided by this document. The area between the northern boundary of Portion 4 and the R102 is proposed to be part of the Airport Support Zone (coloured dark blue). Figure 2: Gwayang Local Spatial Development Framework The existing road system in the area has a significant impact on the drainage of the higher laying land to the north of Portion 4. Provincial road R102 runs from west to east towards the north of Portions 130 and 139. The road has a well-defined drainage system. The drainage system consists of open drains on the northern side of the road as well as along the southern side of the road. (See photo below). 3 Figure 3: R102 Open Drainage The profile of the road shows a high point (crest) at a position approximately 200 m to the east of the R102/R404 intersection. The drainage channel on the northern side of the road drains from the high point to a culvert passing underneath R102 directly north of a farm dam located on Portion 139. Figure 4: Culvert Underneath R102 4 It furthermore drains to the west from the high point to the intersection of R102/R404 where it passes through a culvert to the west of the R404. To the west of the mentioned intersection runoff drains along a concrete lined open drain running along the northern edge of the R102. See photo below. Figure 5: Concrete Open Drain Along R102 The drainage system of the R102 therefore routes runoff from the north of the road to the natural drainage channel leading into the dam as well as the drainage system of the R102 to the west of the R102/R404 intersection. The existing drainage system is indicated on the Stormwater Layout Plan attached in Annexure A. The proposed development on Portion 4 receives runoff from the north via the well-defined water course passing through the site along the eastern part of the site. This natural drainage channel receives runoff from the above-mentioned farm dam located to the north of the proposed development. The farm dam can be seen on the drawing attached in Annexure A. The drainage system of the R404 also has a significant impact on the drainage system leading to Portion 4. The photo below shows the well-defined drainage channel along the eastern edge of R404. This channel feeds into a dam in the south western corner of Portion 4. 5 Figure 6: Drainage Channel Along R404 The contours on the Subdivision Plan in Annexure B shows a valley extending from the dam in the south western corner towards the north. This valley forms a natural water course extending to the northern boundary of Portion 4. Beyond the northern boundary the water course runs parallel to the northern boundary as indicated on the Stormwater Layout Plan in Annexure A. The photo below shows how the watercourse runs parallel to the northern boundary inside Portion 130. Figure 7: Watercourse on Portion 130 6 There are therefore two distinct natural water courses present on Portion 4. These are the valley downstream of the farm dam on Portion 139 and the valley draining into the dam on the south western corner of Portion 4. 1.6. Downstream receiving water body The two natural