Horton Landfill, Small Dole, West Sussex. Infill of geological SSSI. Surface Water Management Assessment. Current Approved Design

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1 Horton Landfill, Small Dole, West Sussex Infill of geological SSSI Surface Water Management Assessment Current Approved Design Planning Permission DC/814/07 (UB) Condition 21 (Surface Water Management) was discharged on 18 January 2010 (ref: Danny Tussler (DT), West Sussex County Council letter of the same date), following design and submission of a surface water management scheme by SLR on behalf of Viridor. The scheme related to the approved landfill restoration profile at that time. The approved surface water management design in the vicinity of the SSSI infill comprises a perimeter ditch with outflow controlled conditions by a piped discharge to the fishing lake located to the west of the Horton Site. The attenuated water in the fishing lake then discharges to the River Adur, during overflow conditions only. The approved SLR Horton Landfill perimeter drainage ditch design is detailed in letters to DT with enclosed drawings and calculations. The SLR letters, drawings and calculations are appended to this statement. The ditch proposed in the vicinity of the SSSI infill area is referred to as the North ditch. The catchment area for the North ditch is referred to as Area C (see appended SLR drawings and calculations). The North ditch was sized to ensure it could accommodate the peak design rainfall event, without overtopping. The anticipate peak flow was calculated using the Rational Method and using rainfall from the Flood Estimation Handbook (FEH) CD-ROM (Version 2). The peak design rainfall event used was the 15 minute rainfall event with 100 year return period, plus 30% allowance for climate change. The North ditch channel dimensions were determined using Mannings Equation to ensure that the channel had sufficient capacity to accommodate the peak design flow. The North ditch design took into account the site-wide typical restoration profile and ditch section specific slope around the perimeter of the site. The North ditch design assumed that all surface water runoff from catchment Area C would enter the North ditch via overland flow, with a runoff factor of 0.7 applied to take into account the average slope of the approved restoration profile and ground type (capped landfill, vegetated). The approved design did not take into account that some surface water runoff in catchment Area A would collect in the base of the SSSI bowl and be pumped out, rather than entering the North ditch directly. The pumped rate is likely to be lower than the direct peak runoff rate for the same area. Therefore it is considered that a worst case scenario was designed by SLR. URS Infrastructure and Environment UK Ltd Horton Landfill September 2013

2 Proposed Design It is proposed that, once the SSSI infill has been carried out, the existing approved North ditch alignment be amended to cut across the SSSI infill area, rather than following the Horton Site perimeter to the access in the east corner of the site (see figure SS.002), prior to joining with the existing perimeter ditch profile. This alignment best suits the infill design and site drainage requirements. A review of the SLR drainage design has been undertaken to determine that the proposed drainage design for the SSSI infill area and downstream drainage system remains adequate. The proposed SSSI infill and change in ditch alignment has not changed the design catchment Area C for the North ditch. The restoration profile of the infill is in keeping with, and no steeper than the existing landfill restoration profile considered in the approved surface water ditch design. The infill area will not be subject to the impermeable capping that the landfill area has received. Therefore the infill of the SSSI area is likely to result in a slightly reduced peak runoff rate compared to the situation considered in the SLR approved surface water ditch design. The gradient of the proposed section of ditch is 1:110, in keeping with the approved North ditch design gradient. Therefore, the approved North ditch channel dimensions are considered suitable for the proposed ditch channel dimensions, as flows into the ditch will be the same or less than the approved design and the ditch capacity will be the same. Overall in catchment Area C, the peak runoff is likely to be marginally reduced and ditch flows will not be increased by the SSSI infill and North ditch realignment. Therefore the downstream drainage system remains adequate based on the SLR design. URS Infrastructure and Environment UK Ltd Horton Landfill September 2013

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4 01 September 2009 Danny Trussler West Sussex County Council County Development Tower Street Chichester West Sussex PO19 1RH Our Ref: Your Ref: _ _L_DT_Condition21 Dear Mr Trussler, RE: HORTON LANDFILL, SMALL DOLE, WEST SUSSEX. PURSUANT TO CONDITION 21, PLANNING PERMISSION DC/814/07 (UB) Further to your recent correspondence with Viridor, pursuant to condition 21, please find attached perimeter drainage ditch calculations for Horton Landfill. Surface Water Drains - Sizing Calculations It is proposed that surface water runoff from the restored Horton Landfill shall be intercepted by open perimeter drains and routed to the River Adur to the immediate west of the site. It is acknowledged that the perimeter drains must be appropriately sized to ensure they can accommodate the design rainfall event without overtopping. Therefore, sizing calculations have been undertaken to determine appropriate dimensions for the perimeter drains. A twostage approach has been taken as follows: Stage One: Calculation of peak flow within perimeter drains during design rainfall event; and Stage Two: Determine appropriate channel dimensions to accommodate the peak flow calculated in Stage One. Peak Flow Calculations Two perimeter drains are proposed, with alignment based on the pre-settlement restoration contours. A primary northern drain collects runoff from the majority of the restored site and drains from the south-east corner of the site around the eastern, northern and western site boundaries prior to discharge to the fishing lake. A second, shorter drain collects runoff from a small southern catchment area and drains in a westerly direction along the southern site boundary. The northern perimeter drain has been subdivided into three sections based on topographic gradient as indicated on the enclosed plan. The corresponding catchment areas are referred to as West, Northwest and North and appropriate channel dimensions shall be determined for each.

5 Viridor 2 Ref: Pursuant to Condition 21, Horton Landfill 01 September 2009 The anticipated peak flow during the design rainfall event has been calculated using the Rational Method and rainfall data from the Flood Estimation Handbook (FEH) CD-ROM (Version 2). The Rational Method states that the peak flow (Q) is related to catchment area (A), runoff coefficient (K) and rainfall intensity (I) via the following equation: Q = 2.78 AKI Due to the relatively small catchment area of the site and steep restoration profile it is considered that the time of concentration (i.e. time that peak runoff rate occurs) would be during a 15 minute rainfall event. The rainfall intensity during a 100-year, 15-minute event for the site catchment are reported from the FEH CD-ROM as 125.4mm/hour. The rainfall intensity with allowance for long-term climate change can therefore be calculated as 163mm/hour (100-year plus 30% allowance for climate change). The input parameters for the Rational Method can therefore be derived as follows: A (catchment area) = West: 3.9 hectares Northwest: 14.3 hectares North: 8.3 hectares South: 6.7 hectares (measured in AutoCAD) K (runoff coefficient) = 0.70 (based on restoration profile) I (rainfall intensity during 100-year + climate change event) = 163mm/hour The peak flow during a 100-year + climate change, 15 minute duration rainfall event is therefore calculated via the Rational Method as: Channel Dimensions West: 1.237m 3 /s Northwest: 4.536m 3 /s North: 2.633m 3 /s South: 2.125m 3 /s The channel capacity design peak flow of the existing open perimeter drains have been determined using Manning s Equation to ensure that the channel has adequate capacity to accommodate the rainfall event, (see Drawing SWD1). Calculations are enclosed (Appendix A) detailing the input parameters and existing channel dimensions. A Manning s roughness coefficient (Manning s n ) of 0.02 has been assumed. The longitudinal gradient of the channel has been determined based on the contours and ditch alignment. The calculations have identified a number of ditch sections within each catchment area that require re-profiling (see Drawing SWD2). The required new channel capacities and ditch dimensions are summarised overleaf, calculations are included in Appendix B _ _L_DT_Condition21.doc SLR

6 Viridor 3 Ref: Pursuant to Condition 21, Horton Landfill 01 September 2009 Catchment West (ch25-75) West (ch ) North West (ch25-125) North (ch70-145) South Longitudinal Gradient Channel Depth (m) Channel base width (m) Channel capacity (m 3 /s) Peak flow from Rational Method (m 3 /s) Closure The review has identified a number of locations where the perimeter collection ditches require re-profiling to attain the peak flow capacities. The peak flow capacities assume that the site is fully restored. It is therefore recommended that the re-profiling is undertaken before the completion of restoration at the site. We trust you will find everything in order to fully discharge this condition, should you have any further queries, do not hesitate to contact the undersigned. Yours sincerely SLR Consulting Limited [By ] Lee Miller Principal cc Ian Crofts Viridor Adrian Frost Viridor Enc. SWD1 Existing Perimeter Ditch Sections SWD2 Ditch Requirements following Restoration Appendix A Peak Flow Calculations for existing ditch sections Appendix B Peak Flow Calculations for re-profiled ditch sections _ _L_DT_Condition21.doc SLR

7 18 November 2009 Danny Trussler West Sussex County Council County Development Tower Street Chichester West Sussex PO19 1RH Our Ref: Your Ref: _ _DT_SW Dear Mr Trussler, RE: HORTON LANDFILL, SMALL DOLE, WEST SUSSEX. PURSUANT TO CONDITION 21, PLANNING PERMISSION DC/814/07 (UB) Further to your recent correspondence with Viridor regarding the surface water management at the site, please find the following clarifications to the queries raised. The outflows from the site are as the previously approved scheme. The primary northern drain collects runoff from the majority of the restored site and drains around the eastern, northern and western site boundaries prior to discharge to the fishing lake. The outflow from the northern ditch is controlled by an underground piped discharge to the fishing lake. The attenuated surface water in the fishing lake then discharges to the River Adur, during overflow conditions only. The second, shorter drain collects runoff from a smaller southern catchment area and drains in a westerly direction along the southern site boundary, this in turn drains to the meandering open brook pastures and onward to the River Adur. To conclude, all discharges are as the previously approved scheme, and at the locations approved within the site s Environmental Permit. The surface water discharges are also frequently monitored for quality as required under the permitting regime. We trust you will find everything in order to fully discharge this condition, should you have any further queries, do not hesitate to contact the undersigned. Yours sincerely SLR Consulting Limited [By ] Lee Miller Principal cc Adrian Frost Viridor

8 Michael Elkington Divisional Manager, County Development Please respond to Danny Trussler Tel: (01243) Fax: (01243) Infrastructure Business Unit The Grange Tower Street Chichester West Sussex PO19 1RG Switchboard: Adrian Frost, Project Manager Viridor (S East), 42, Kings Hill Avenue, Kings Hill, West Malling, Kent. ME19 4AJ 18 th January 2010 Our reference: DT/DC//814/07 (UB) Dear Adrian, Re: Horton Landfill, Small Dole, West Sussex. Planning Permission DC/814/07 (UB) Condition 21 (Surface Water Management). I refer to your of the 20 th November 2009 and your consultant s letter of the 18 th November 2009 which provided clarifications to some queries from the Environment Agency, concerning the above. I have now heard from the Environment Agency and they have no objection to this Authority discharging condition 21 on the basis of the further information received. Therefore, condition 21 of planning permission DC/814/07(UB) is formally discharged by this Authority. Thank you for your cooperation in this matter. Yours sincerely Danny Trussler Team Leader Planning Enforcement County Development Division Infrastructure Business Unit West Sussex County Council