STORMWATER MANAGEMENT PLAN: ERAND GARDENS X70

Transcription

1 STORMWATER MANAGEMENT PLAN: ERAND GARDENS X70 Client: Central Developments 09 April 2018 _ Rev00 Report no: SWMP/410/00

2 SWMP FOR ERAND GARDENS X70 TABLE OF CONTENTS 1 INTRODUCTION GENERAL INFORMATION PROPERTY DESCRIPTION AND CURRENT STORMWATER APPROVALS STORMWATER MANAGEMENT PLAN METHODOLOGY EXISTING STORMWATER EXISTING STORMWATER DRAINAGE SYSTEMS AND SURROUNDING INFRASTRUCTURE SYSTEMS STORMWATER RUN-OFF COEFFICIENT STORMWATER RUN-OFF STATUS OF EXISTING STORMWATER DRAINAGE SYSTEMS AND INFRASTRUCTURE POST DEVELOPED STORMWATER EXTERNAL STORMWATER DRAINAGE SYSTEMS STORMWATER RUN-OFF COEFFICIENT STORMWATER RUN-OFF CONCLUSION REGIONAL STORMWATER CONTROLS STORMWATER ATTENUATION STRUCTURES CALCULATIONS Attenuation Parameters Overflow Structures PROPOSED ATTENUATION STRUCTURE LAYOUT MAINTENANCE REQUIREMENTS, PHILOSOPHY AND IMPLEMENTATION RELEVANT PERMISSIONS (SERVITUDES, ETC.)... 9 ANNEXURES ANNEXURE A Locality Map ANNEXURE B General Plan ANNEXURE C Copy of part of Outlined Scheme Report (LSC Brunette & Associates) ANNEXURE D Existing Stormwater Layout (Drg No. 410-SWMP-002_RevA) ANNEXURE E Stormwater Hydrology Layout (Drg No. 410-SWMP-001_RevA) ANNEXURE F Post Developed Stormwater Layout (Drg No. 410-SWMP-003_RevA) ANNEXURE G Detail Stormwater Attenuation Calculations ANNEXURE H Stormwater Attenuation Structures Details (DRG. 410-SWMP-004_RevA) ANNEXURE I Stormwater Maintenance Plan Example

3 SWMP FOR ERAND GARDENS X70 1 Introduction Civil Engineering Developments was appointed by Central Developments to compile a Stormwater Management Plan (SWMP) for Erand Gardens X70. The purpose of this report is to provide clarification of the proposed Stormwater Management System for Erf 765 Erand Gardens X70. The report will serve as a supplementary report for the Outlined Scheme Report for Erand Gardens X70 compiled in July General Information 2.1 Property Description and current stormwater approvals The Erand Gardens X70 Township is located in Midrand, with George Road and Fourteenth Road forming the northern and eastern boundaries to the township. Refer to Locality Plan in Annexure A. The Erand Gardens X70 Township was established in 2007 and consist out of two erven; Erf 765 zoned Residential 3 with a density of 110 units per Ha ( Ha in size) and Erf 764 zoned as a park Ha in size. Refer to General Plan in Annexure B. As part of the township establishment process an Outlined Scheme Report was compiled by LSC Brunette & Associates in conjunction with John Bennett & Associates in July 2005 for the now known Erand Gardens X70 township with its surrounding farm portions, measuring in total extend of about 13.8 Ha. Refer to copy of part of the OSR obtained in Annexure C. The report indicated that stormwater attenuation facilities will be provided to reduce the peak stormwater runoff into the natural watercourse. Two attenuation dams have been constructed on Holdings 194 (Erand Agricultural Holdings X1) as indicated in the google earth image below. Image 1 Existing Attenuation dams 1 P a g e

4 SWMP FOR ERAND GARDENS X70 Only the northern portion of Erf 765 Erand Gardens has been developed till date and the stormwater runoff from this development drains through an internal road and stormwater network, discharging into various natural opens canals along the western boundary which drains through Portion 620 of the farm Randjesfontein towards the existing attenuation dams build on Holding 194. No detail information is available to verify if the existing attenuation structures had been designed to accommodate the post developed stormwater runoff from the southern portion of Erf 765 as well. The natural stormwater runoff pattern over the southern undeveloped portion of Erf 765 also transformed due to post 2007 upstream developments as indicated in the Wetland hydropedology identification and management report compiled by Terrasoil dated March Image 2 Upstream stormwater runoff As indicated in Image 1, the upstream stormwater runoff generated from the VodaWorld development is discharged into a sub-standard open stromwater trench draining toward the natural watercourse over Holding 192. Over the years this open trench frequently overflowed unto Holding 192 draining through Erf 765 (Erand Gardens X70) towards the natural watercourse. This created an artificial wetland area over a large portion of Holdings 192 and a small extend of the southern portion of Erf 765. Once the partly functional stormwater trench along the southern boundary of holding 192 had been upgraded with a formal pipe system up to the natural watercourse by JRA, the artificial wetland would disappear. The purpose of this SWMP is to address these upstream stormwater discharges over the property and to provide onsite attenuation for the southern portion of Erf 765 due to insufficient design information available for the existing attenuation dams. 2 P a g e

5 SWMP FOR ERAND GARDENS X Stormwater Management Plan Methodology The stormwater management system will be designed in accordance with the requirements of the City of Johannesburg and the Environmental Management Plan with the management of drainage on the basis of Sustainable Urban Drainage Systems (SuDS) and Water Sensitive Urban Design system approaches. With the SuDS system stormwater is managed through a series of unit processes, collectively known as a treatment train. This treatment train consist out of three key stages, each having slightly different combinations of SuDS options to control the stormwater: 1) Source Controls manage stormwater as close to its source as possible (green roofs, rainwater harvesting, permeable pavements and infiltration basins) 2) Local Controls manage stormwater runoff in the local area, typically within the road reserves (bio-retention areas, filter strips, infiltration trenches and swales) 3) Regional Controls manage the combined stormwater runoff from several developments (constructed wetlands, detention ponds, and retention ponds (attenuation structures)) This SWMP addresses the stormwater requirements for the southern portion of Erf 765 Erand Gardens X70. The proposed Regional Control is to provide an attenuation structure for each of the regional drainage areas according to the JRA guidelines. Stormwater attenuation will be designed according to Joburg Roads Stormwater Management Policy s (JRA, 2005); with the following policy requirements: All developments on land exceeding 8500m 2 (0.85ha) are subject to stormwater attenuation on site The preferred means of attenuation is on surface The runoff associated with the development is to be attenuated such that the predevelopment flows for the 1:5 as well as the 1:25 year storm events are not exceeded. The attenuation structure must be able to withstand the 1:50-year storm event. All attenuation structures to be designed according to the environmental acceptable standards. The focus of this SWMP is to provide the post development drainage routes and detail information regarding the regional controls proposed for the Erand Gardens X70 township. This will be achieved through an understanding of the existing stormwater system, followed by the incorporation of the proposed post development impact, which will lead to the implementation of regional stormwater controls to mitigate the post development impact. Detail regarding the Source control and Local control structures would be provided with the site development plan (SDP) for the proposed development. 3 P a g e

6 SWMP FOR ERAND GARDENS X70 3 Existing Stormwater 3.1 Existing Stormwater Drainage Systems and surrounding infrastructure systems The natural topology of the study area (Erand Gardens Extensions and Erand Agricultural Holdings) drains in a north western direction towards the natural watercourse. Refer to Existing Stormwater Layout Drawing in Annexure D. The natural watercourse, Fourteenth Road and the N1 Highway are part of the mayor stormwater system draining the mayor storm events. The minor storm events are drained via the kerb inlet and stormwater pipe network installed inside Fourteenth Road. The stormwater pipe system regularly discharge on the lower laying properties on the western side of Fourteenth Road. From here the stormwater drains via informal open channels / trenches towards the natural watercourse. As mentioned in paragraph 2.1, the stormwater runoff from the Voda World development is discharged onto Erand Argricultural Holdings X1 Holdings 192 along the southern boundary in an open trench towards the natural watercourse. Minor storm event runoff from the higher laying Erand Argricultural Holdings X1 Holdings 356 and erven 762 & 763 Erand Gardens X47 drains towards the kerb inlet and pipe system inside Fourteenth Road. This pipe system discharges unto Erf 765 Erand Gardens X70 and formed a stormwater trench over the years as it drains towards the natural watercourse. Only the northern portion of Erf 765 Erand Gardens X70 has been developed till date and the stormwater runoff from this development drains through an internal road and stormwater network, discharging into various natural opens canals along the western boundary which drains through Portion 620 of the farm Randjesfontein towards the existing attenuation dams build on Holding 194. The southern portion of Erf 765 is undeveloped and drains naturally in a north western direction towards the natural watercourse. 3.2 Stormwater Run-off Coefficient The development site is currently covered with grass and natural vegetation. The pre-developed run-off coefficients for all developments are given in the table below, as derived from the values given in the SANRAL Drainage Manual (Rational Method): Pre Developed Run-Off Factor Building Paving NGL Coverage % Runoff Factor C-Value Total Factor Stormwater Run-off The pre-developed stormwater run-off for each area were determined with the rational method as set out in the SANRAL Drainage Manual, as given in the table below (Refer to Stormwater Hydrology layout drawing in Annexure E): Pre-Developed Stormwater Runoff CATCHMENT 1 CATCHMENT 1 Design Method Rational method Rational method Catchment Area (m²) Runoff coefficient :5 year flood (m³/s) :25 year flood (m³/s) :50 year flood (m³/s) P a g e

7 SWMP FOR ERAND GARDENS X Status of existing Stormwater drainage systems and infrastructure As mentioned in paragraph 3.1, the existing kerb inlet and pipe network along Fourteenth Road is incomplete as the stormwater is discharged onto the lower laying properties and not formally channelled or piped towards the natural watercourse as required by JRA. 4 Post Developed Stormwater 4.1 External Stormwater drainage systems It is recommended that the minor stormwater system from Catchment 2 which currently discharge via a 675mm diameter stormwater pipe outlet freely onto the southern portion of Erf 765, be extended with a municipal 675mm diameter concrete stormwater pipe up to the floodline. This pipe need to be installed inside a newly registered 3m wide municipal servitude and the construction cost would by for the Johannesburg Road Agency s account. A temporary stormwater soil berm would also be constructed along the southern boundary of Erf 765 to prevent that stormwater generated from the upstream VodaWorld development traverse across Erf 765 due to the lack of a proper municipal stormwater system along the southern boundary of Holding Stormwater Run-off Coefficient With reference to the new zoning rights, hard surface areas will increase with 80% coverage, thus decreasing the amount of impermeable surfaces. The post-developed run-off coefficients for all developments are given in the table below, as derived from the values given in the SANRAL Drainage Manual (Rational Method): Post Development Run-Off Factor Building Paving NGL Coverage % Runoff Factor C-Value Total Factor Stormwater Run-off The post-developed stormwater run-off for each area were determined with the rational method as set out in the SANRAL Drainage Manual, as given in the table below (Refer to Stormwater Hydrology layout drawing in Annexure E): Post-Developed Stormwater Runoff CATCHMENT 2 CATCHMENT 2 Design Method Rational method Rational method Catchment Area (m²) Runoff coefficient :5 year flood (m³/s) :25 year flood (m³/s) :50 year flood (m³/s) P a g e

8 SWMP FOR ERAND GARDENS X Conclusion From the pre- and post-development stormwater run-off calculations, it is concluded that an attenuation structure is required to ensure pre-development run-off discharge towards the natural watercourse for the southern portion of Erf 675. This report only focusses on the stormwater management for the southern portion of Erf 765 (Catchment 1) and the runoff calculations for Catchment 2 were only used to size the municipal stormwater pipe required through Erf 765. The following table summarises the required attenuation for the southern portion of Erf 765 (Catchment 1) as basis for the Attenuation Structure design: Increase in Stormwater Runoff Peaks CATCHMENT 1 1:5 year flood (m³/s) :25 year flood (m³/s) Refer to Annexure F for the Post Developed Stormwater Layout. 6 P a g e

9 SWMP FOR ERAND GARDENS X70 5 Regional Stormwater Controls 5.1 Stormwater Attenuation Structures Calculations As per paragraph 4.4 the selected regional control structure for the southern portion of Erf 765 is to provide a stormwater attenuation structure Attenuation Parameters Refer to Annexure G for the detail stormwater attenuation calculations sheets, summarised in the table below: 1:5 and 1:25yr flood: Attenuation Parameters Parameter: ERF SOUTH Outlet Size: 1:5yr 450mm 1:25yr 525mm Storage dam volume (m³) 852 Maximum water level: 1:5yr 0.711m 1:25yr 1.216m Peak Inflows: 1:5yr 819 l/s 1:25yr l/s Peak Outflows: 1:5yr 295 l/s (307 l/s pre) 1:25yr 562 l/s (607 l/s pre) Attenuation 1:5yr 524 l/s 1:25yr l/s Overflow Structures To accommodate the 1:50 year flood as per JRA standards the attenuation dam mentioned in paragraph 5.2 requires an overflow structure with the following parameters: 1:50yr flood: Dam Overflow Structure Parameter: ERF SOUTH Design flood: 1:50yr (m³/s) = Flow formula Discharge coefficient Water overflow height (H) Width of Weir (L) Overflow capacity (m3/s): Proposed Attenuation Structure Layout The proposed attenuation structure will be constructed according to the attenuation parameters given in paragraph A chimney structure will be provided when the orifice size for the 1:5 year and 1:25 year flood occurrence differs from each other. The chimney structure will discharge the 1:5 year flood occurrence through an orifice at the bottom of the chimney, where the opening at the top of the chimney structure and the outlet pipes will have sufficient capacity to discharge the 1:25 year flood occurrence. The attenuation structures will have an overflow structure built according to the parameters given in paragraph Refer to Annexure H for the stormwater attenuation structure details. 7 P a g e

10 SWMP FOR ERAND GARDENS X70 As shown in the figure below the parameters as given in paragraphs & will be incorporated into the design of the structure for Erf 765 as follows: The dam depth is the sum of the 1:25 year flood occurrence maximum water level and the1:50 year water overflow height with freeboard. The height of the chimney will be equal to the 1:5 year occurrence maximum water level. The Inlet size (orifice) is equal to the pipe (orifice) size needed to attenuate the 1:5 year flood occurrence. The outlet size is equal to the pipe (orifice) size needed to attenuate the 1:25 year flood occurrence. The opening at the top of the chimney will have the same discharge capacity as the outlet pipe. The weir depth and width is determined by the 1:50 year water overflow height plus freeboard. 8 P a g e

11 SWMP FOR ERAND GARDENS X Maintenance requirements, philosophy and implementation The minimum maintenance requirements, philosophy and implementation are described in the example Stormwater Maintenance Plan and attached as Annexure I. 5.4 Relevant permissions (servitudes, etc.) A new 3m wide stormwater servitude would be registered through Erf 765 for the 675mm diameter JRA stormwater pipe required as mentioned in paragraph 4.1. Should more details be required regarding this SWMP, please do not hesitate to contact the undersigned.. Eben Beetge PrEng Civil Engineering Developments cc 09 April P a g e

12 Annexure A:

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14 Annexure B:

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29 Annexure D

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31 Annexure E

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41 ERAND GARDENS X70 STORMWATER MAINTENANCE PLAN (example) This schedule should be read in conjunction with report entitled STORMWATER MANAGEMENT REPORT: ERAND GARDENS X70, dated 02 April BACKGROUND AND SUMMARY The Erand Gardens X70 township consists of one Residential 3 erf. This erf has an attenuation structure to accommodate its stormwater flow. MAINTENANCE PRINCIPLES The inlet structures, overflow channels and energy dissipating structures will be filled with large debris, litter, leaves, etc. To ensure the optimal functionality of these structures, the debris, litter, leaves, etc. needs to be removed daily during the rainy season (October March) and Monthly during the dry season (April September). Dead vegetation may cause erosion to the wall of the attenuation structure itself, this need to be checked daily during the rainy season (October March) and Monthly during the dry season (April September). Should there be any dead vegetation, it should be removed and new vegetation should be planted. DECLARATION AND ACCEPTANCE I,.., i n my capacity as. of the subject property, herewith accept the contents of the Stormwater Impact Assessment report #SWMP/410/00 as compiled by Civil Engineering Developments. I hereby also accept responsibility to implement the scheme to the required specifications, and to maintain the system as specified by the Engineer, and as described in the Maintenance Plan attached to the relevant report. I further undertake to inform the Engineer of any changes to the scheme, and to inform any future owner or managing agent of this document, and the responsibilities contained therein. SIGNED IN FULL DATE WITNESS 1 WITNESS 2

42 MAINTENANCE ACTIVITY* DESCRIPTION RECURRENCE 1. Pipes and inlets (refer to detail 1 per drawing #AA-BB-01 of report) 1.1. Inlet Chimney/Orifice Remove all blocking debris, leaves or litter from Inlet Chimney/Orifice Daily between October and March Monthly between April and September 1.2 Attenuation Structure Make sure the structural integrity of the wall is in place by reporting any dead vegetation and possible erosion which took place. Remove any dead vegetation and plant new vegetation. 1.3 Overflow Channel Keep channel clean from any debris, leaved or litter. Ensure gabion boxes are tied together and filled with stones and rocks. 1.4 Energy Dissipating Structure Keep gabion boxes and field inlet clean from any debris, leaved or litter. Ensure gabion boxes are tied together and filled with stones and rocks 2. Swale N/A Daily between October and March Monthly between April and September Daily between October and March Monthly between April and September Daily between October and March Monthly between April and September SIGNED IN FULL DATE