86 Chalkshire Road, Butlers Cross, Ellesborough Response to Lead Local Flood Authority’S Comments
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86 Chalkshire Road, Butlers Cross, Ellesborough Response to Lead Local Flood Authority’s Comments 1.0 Introduction 1.1 This note has been prepared by Glanville Consultants on behalf of St John Homes (Thames Valley) in response to Buckinghamshire Council’s comments, in its capacity as Lead Local Flood Authority, received in relation to the Drainage Statement produced by Glanville (ref. 003_8210376_AQ_Drainage_Statement_(Issue_3)) submitted to discharge Condition 16 attached to planning permission ref. 20/07667/FUL, which requires the approval of a surface water drainage scheme for the proposed residential development at 86 Chalkshire Road, Butlers Cross, Ellesborough, Buckinghamshire, HP17 0TJ. 1.2 The comments were provided by Amy Fowler, the Sustainable Drainage Officer at Buckinghamshire Council, dated 2 July 2021. A copy of the consultation response can be found at Appendix A. 1.3 The following sections of this note deal with each of the comments made by Buckinghamshire Council in relation to surface water drainage. The key issues are summarised as follows: • Details of the winter ground water monitoring; • Floatation calculations for all sub-surface SuDS/drainage components; • Updated calculations including: o infiltration co-efficient base and side for the cellular storage to be set to 0.0mm/hr. o an urban creep allowance of 10%. • Updated drainage layout; • Construction drawings for all SuDS and drainage components; and • Updated maintenance plan for all SuDS and drainage components, including: o maintenance tasks and frequencies. o details of maintenance responsibility. o details of how the full drainage system will be accessed for maintenance. 2.0 Comments & Responses 2.1 Each of Buckinghamshire Council’s specific comments in their consultation response are detailed in bold italic text and a response provided beneath. Groundwater Monitoring Details Section 2.11 of the Drainage Statement outlines that groundwater monitoring has been undertaken on site and that groundwater was encountered at a minimum depth of 0.90m below ground level (bgl). The LLFA are not however in receipt of this groundwater monitoring data/report and it is requested that this information is provided in support of the application. Ref: 005_8210376_AQ_Drainage_Technical_Note 1 Issue 2: 23 July 2021 2.2 Ground Investigation Services (Southern) Ltd carried out groundwater monitoring in January 2020 and March 2021 and the results confirm maximum groundwater levels of 0.96m (January 2020) and 1.62m (March 2021) below ground level. Monitoring data supplied by Ground Investigation Services (Southern) Ltd is provided in Appendix B. Floatation Calculations Whilst site-specific groundwater levels have been identified, the applicant has not provided floatation calculations for all sub-surface assets. Floatation calculations must be provided for all sub-surface components. These should be based upon the worst case scenario of groundwater assumed to be at surface level or based on the shallowest groundwater levels encountered on site (0.90m bgl) over the winter period. This information is required to ensure the functionality of the system is maintained during a high groundwater event. 2.3 Due to the high groundwater levels encountered, all storage features will be lined with an impermeable membrane to prevent groundwater ingress. Floatation calculations have therefore been undertaken for the attenuation crates. These are included in Appendix C and demonstrate that the attenuation crate structure proposed has a factor of safety against floatation greater than 1.2, and therefore will not become buoyant in the event of high groundwater. These calculations have been undertaken using the highest groundwater level encountered during the groundwater investigations (0.96m bgl). It should be noted that floatation calculations have been undertaken based on the amended drainage strategy as described in the following sections. Calculations Calculations have been provided to demonstrate that the system has been designed to accommodate all storm events up to and including the 1 in 100 year storm event +40% climate change allowance without flooding. The LLFA would however like to query why the cellular storage has been assigned infiltration rate values for the infiltration ‘co-efficient base’ and ‘infiltration co-efficient side’. This is an attenuation component and should be designed to show no infiltration. The calculations must be updated to reflect an attenuation tank rather than a soakaway. The applicant should be aware that the calculations must also be updated to be inclusive of an urban creep allowance of 10% to take account of any future increases in impermeable areas within the site. This includes areas such as patios, conservatories and small extensions (Section 24.7.2, CIRIA SuDS Manual, 2015). Please note that following any changes to be required storage volume of the attenuation tank, all relevant documents/drawings must be updated accordingly. The updated calculations must demonstrate that the proposed drainage system can contain up to the 1 in 30 storm event without flooding. Any onsite flooding between the 1 in 30 and the 1 in 100 plus 40% climate change storm event should be safely contained on site. These calculations must include details of critical storm durations and demonstrate how the proposed system as a whole will function during different storm events. If any flooding occurs for the 1 in 100 year plus 40% climate change event, then we require details of where this flooding will occur and the volume of the flooding. Ref: 005_8210376_AQ_Drainage_Technical_Note 2 Issue 2: 23 July 2021 2.4 Section 3 of the previous Drainage Statement indicates that the proposed development site will involve a total impermeable area of 441m2. An additional 17m2 of impermeable area has been included in the calculations associated with the proposed timber bike sheds and paving flags established between the dwellings. In addition, a 10% allowance for urban creep has been included in response to Buckinghamshire Council’s comments, resulting in a total impermeable area of 504m2. 2.5 The geo-cellular storage tank will be used to attenuate surface water flows with suitable outflow control. For the purposes of modelling, Polypipe Permavoid system PVPP150 (708mm x 354mm x 150mm deep) with 95% void ratio has been used to size the storage tank. 2.6 As shown in paragraph 2.5 of the previous Drainage Statement, the closest watercourse to the site is a ditch located approximately 200m to the east of the site. Attenuated outflows from the storage tank will be intercepted and conveyed through an underground pipe towards the ditch. The applicant has rights to drain across the land which lies between the application site and the ditch, so this outfall route is deliverable. 2.7 Attenuated outflow rates from the geo-cellular storage tank would be restricted to the minimum value of 2l/s for the total impermeable area of the site, as well as all rainfall events up to 1 in 100 year with an appropriate allowance for climate change, in accordance with Buckinghamshire Council’s requirements. 2.8 The base of the storage tank has been established 1.15m below ground level. The geo- cellular storage tank has been designed to be 0.75m deep, i.e. 5 No. Permavoid (PVPP150) units. Infiltration coefficients for the base and side of the crates have been removed. 2.9 The hydraulic calculations included in Appendix D demonstrate that the geo-cellular storage tank with base area of 27.0m2 (i.e. 25.5m long x 1.06m wide) and 0.75m deep will be able to accommodate surface water run-off from all rainfall events up to and including the 1 in 100 year event, including a 40% increase in rainfall intensity as allowance for the potential effects of climate change, without flooding from surface water. As confirmed in paragraph 2.4 of this report, a 10% addition for urban creep has been included within the proposed impermeable areas, making a total of 0.051ha. The attenuation storage was shown to reach a maximum flood depth of 0.734m and a maximum volume of approximately 18.8m3 associated with the critical storm duration of 60 min Winter storm. 2.10 The design utilises a Hydro-Brake vortex flow control to attenuate surface water run-off from the proposed residential development. To achieve the required outflow rates, as well as reduced the blockage risk, the unit selected has a 70mm orifice size (MD-SHE-0070- 2000-0800-2000), but as it is located within a chamber with a sump, this dimension is not likely to give rise to any undue maintenance burden. 2.11 MicroDrainage shows a maximum outflow rate of 2.0l/s for the 1:100 year+40%CC rainfall event. Therefore, run-off from the site has been limited to the target run-off rate of 2.0l/s for all return periods up to the 100 year design rainstorm plus 40% climate change. Ref: 005_8210376_AQ_Drainage_Technical_Note 3 Issue 2: 23 July 2021 2.12 Additional storage will be provided within the permeable pavement sub-base layer, but the attenuation storage potential of this sub-base layer has not been quantified or included in the hydraulic calculations. As such, a conservative approach has been followed when preparing the drainage strategy and sizing the storage devices. 2.13 The calculations demonstrate that the development of the site will not increase flow rates off-site, and therefore the risk of flooding from surface water will not increase as a consequence of the development proposals. The introduction of a positive drainage system will result in betterment with improvements to the surface water flood risk situation within the site and in the surrounding area, by directing flows more effectively to a downstream watercourse at restricted rates. Drainage Layout It is not clear why a 5m buffer distance has been provided between the building foundations and the attenuation tank; the applicant should provide clarification regarding this.