Vital Earth Flood Risk and Drainage Statement Final December 2011 Prepared for Vital Earth Ltd
Revision Schedule Flood Risk and Drainage Statement December 2011 Rev Date Details Prepared by Reviewed by Approved by 01 December 2011 Final Jo Somerton Senior Flood Risk Specialist Sarah Mason Principal Hydrologist Jim MacQueen Principal Consultant URS Scott Wilson WestOne Wellington Street Leeds LS1 1BA Tel 0113 204 5000 Fax 0113 204 5001 www.urs-scottwilson.com
Limitations URS Scott Wilson Ltd ( URS Scott Wilson ) has prepared this Report for the use of Vital Earth Limited ( Client ) in accordance with the Agreement under which our services were performed.. No other warranty, expressed or implied, is made as to the professional advice included in this Report or any other services provided by URS Scott Wilson. The conclusions and recommendations contained in this Report are based upon information provided by others and upon the assumption that all relevant information has been provided by those parties from whom it has been requested and that such information is accurate. Information obtained by URS Scott Wilson has not been independently verified by URS Scott Wilson, unless otherwise stated in the Report. The methodology adopted and the sources of information used by URS Scott Wilson in providing its services are outlined in this Report. The work described in this Report was undertaken between May 2011 and December 2011 and is based on the conditions encountered and the information available during the said period of time. The scope of this Report and the services are accordingly factually limited by these circumstances. Where assessments of works or costs identified in this Report are made, such assessments are based upon the information available at the time and where appropriate are subject to further investigations or information which may become available. URS Scott Wilson disclaim any undertaking or obligation to advise any person of any change in any matter affecting the Report, which may come or be brought to URS Scott Wilson s attention after the date of the Report. Certain statements made in the Report that are not historical facts may constitute estimates, projections or other forward-looking statements and even though they are based on reasonable assumptions as of the date of the Report, such forward-looking statements by their nature involve risks and uncertainties that could cause actual results to differ materially from the results predicted. URS Scott Wilson specifically does not guarantee or warrant any estimate or projections contained in this Report. Copyright This Report is the copyright of URS Scott Wilson Ltd. Any unauthorised reproduction or usage by any person other than the addressee is strictly prohibited.
Table of Contents 1 Introduction... 1 The Site... 1 2 Flood Risk Statement... 3 Identification of Potential Sources of Flood Risk... 3 Fluvial Flooding... 3 Tidal Flooding... 4 Groundwater Flooding... 4 Surface Water Run-off and Overland Flow... 4 Sewerage Infrastructure... 5 Climate Change... 5 3 Surface Water Drainage... 7 Existing Surface Water Drainage System... 7 4 Foul Water Drainage... 8 5 Conclusion... 9 Figure 1 Location Plan
1 Introduction Background 1.1.1 This report assesses flood risk and drainage issues at the Vital Earth composting facility at Ashbourne, Derbyshire. 1.1.2 Vital Earth was granted planning permission in 2005 to construct and run the composting facility at Ashbourne at a capacity of 60,000 tonnes of biodegradable waste per annum. Retrospective planning consent is now being sought to increase the amount of biodegradable waste processed at the site from 60,000 to 85,000 tonnes per annum. 1.1.3 To accommodate the 85,000 tonnes per annum land to the south of the composting facility has been leased to provide adequate compost storage space, including bagged and loose finished compost, increasing the total site area to approximately 4.9 ha. 1.1.4 In June 2011, the Council granted permission for the change of use of land to the south of the site to a compost dispatch storage area. The permission granted the stripping of topsoil from the leased land, the laying of permeable hardcore, the erection of three new bunkers (with associated tank and a below ground sump), new access points to the leased land and new fencing. 1.1.5 The compost dispatch storage area comprises permeable surfacing (hardcore) that will continue to allow infiltration of rainwater into the ground. Flood risk and drainage issues relating to the compost dispatch storage area have been considered as part of the previous planning application and therefore reference to the compost dispatch storage area is for context only. 1.1.6 This report concentrates on the composting facility operating at the increased capacity of 85,000 tonnes per annum. The Site 1.1.7 The composting facility is located off Blenheim Road on the Ashbourne Airfield Industrial Estate, to the south-east of Ashbourne in Derbyshire. The location of the site can be seen in Figure 1. The National Grid Reference (NGR) for the centre of the site is SK 198 453. The existing site occupies an area of approximately 2.85 hectares. 1.1.8 The existing site comprises the Vital Earth and is 100% hard standing (impermeable). Existing development on the site comprises a weighbridge, reception building, hard standing areas for compost vessel storage, a changeover bay, control centre, maturation shed, site laboratory and a product blending and packaging area. 1.1.9 The leased land to the south of the site has an area of approximately 2.17 ha and comprises agricultural or Greenfield land. 1.1.10 Access to the composting facility is from Blenheim Lane and will remain as the current situation. 1.1.11 The Airfield Industrial Estate bounds the site to the north-west and the remaining disused airfield, currently used for agriculture, bounds the site to the north and east. 1
1.1.12 The site is bound by Blenheim Road to the south and west beyond which there is light industrial development and four residential dwellings located along the top of Snipesmoor Lane. The nearest dwelling is approximately 100 m from the south west site boundary. 1.1.13 In the wider vicinity the site has a rural aspect to the north, east and south whilst the town of Ashbourne is located to the west, beyond the Airfield Industrial Estate. 1.1.14 As a previous airfield, the site is relatively flat with levels ranging from approximately 181 m AOD to 179.5 m AOD. 2
2 Flood Risk Statement Identification of Potential Sources of Flood Risk 2.1.1 Policy guidance for development and flood risk, Planning Policy Statement 25 (PPS 25): Development and Flood Risk, was published in March 2010. 2.1.2 This Flood Risk and Drainage Statement considers the composting facility with regard to PPS 25. The findings are based on the Environment Agency s Flood Zones (indicating fluvial flood risk) and the Derbyshire Dales Strategic Flood Risk Assessment combined with historical flood records. 2.1.3 No correspondence has been sought with regard to flood risk from the local drainage authority or water authority at this time. Fluvial Flooding 2.1.4 There are no Environment Agency main rivers within the site boundary or in the vicinity of the proposed scheme site. 2.1.5 The Environment Agency has stated that the composting facility is located within the upper catchment of the Hilton Brook. The Hilton Brook rises within the Derbyshire Dales district, where it is referred to as the Brailsford Brook. Brailsford Brook (Hilton Brook) is located approximately 4.3 km east of the compost facility. 2.1.6 The nearest surface watercourse to the composting facility is known as Shirley Brook, a tributary of Brailsford Brook. The watercourse is classed as an Ordinary Watercourse and is located approximately 543 m to the north-east of the site. The watercourse rises within the disused airfield and generally flows from north-west to south-east away from the airfield and associated industrial areas. 2.1.7 Snipesmoor Brook is a culverted watercourse that flows adjacent to the west site boundary. The watercourse flows through a 450 mm diameter culvert that passes beneath Blenheim Road. The culverted watercourse receives surface water drainage from the composting facility. There is no open section of the watercourse, both upstream and downstream, in the wider vicinity of the composting facility therefore the watercourse is considered to be a surface water drainage feature. 2.1.8 The site is located within Flood Zone 1 on the Environment Agency Flood Zone Maps. Flood Zone 1 is the low probability risk zone as defined in Table D1 of PPS 25. These are areas where there is a less than 1 in 1000 (0.1%) chance of flooding from rivers in any one year. 2.1.9 The Derbyshire Dales Strategic Flood Risk Assessment confirms that the site is located outside of the floodplain of the Shirley Brook. 2.1.10 It is considered that the risk of flooding to the site from fluvial sources is low. 3
Tidal Flooding 2.1.11 The proposed scheme site is not considered to be at risk from tidal flooding due to its inland location. Groundwater Flooding 2.1.12 The Environment Agency classifies the underlying bedrock geology as a Secondary B Aquifer. These are layers of rock or drift deposits that have permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. 2.1.13 The composting facility is not located within a Groundwater Source Protection Zone (SPZ). A total catchment SPZ is located approximately 1 km to the north-west of the development. 2.1.14 The SFRA reports that there are no known problems with flooding from groundwater in the area. 2.1.15 The Trent Catchment Flood Management Plan (CFMP) states that groundwater flooding is thought not to be significant in the Derbyshire Dales area. 2.1.16 Based on the evidence received to data it is assessed there is a low risk of groundwater flooding at the proposed scheme site. Surface Water Run-off and Overland Flow 2.1.17 Following prolonged rainfall or short intense storms the ground can become saturated causing water to quickly run off and potentially contribute to localised flooding conditions. As noted in PPS 25, local topography and the design of a development can have a strong influence on the depth and direction of runoff. 2.1.18 The CFMP states that surface water flooding is most associated with the steeper upland areas such as Ashbourne however there are no historic records of surface water flooding for the composting facility area within the Airfield Business Park. 2.1.19 Ground levels across the site fall from west to east, therefore any overland flow would be routed towards the composting facility from other existing developments within the Airfield Business Park. As the wider Business Park comprises development and roads with associated drainage, there is little possibility of overland flow entering the site from adjacent land. However, if highway drains were to become blocked, overland flow could potentially enter the site and follow natural topography ponding in low spots within the site boundary. 2.1.20 The existing operational site is currently 100% hard standing; therefore there will be no increase in surface water from the composting facility site as a result of the retrospective proposals. 2.1.21 The existing site drainage comprises attenuation tanks for surface water storage and the flow of surface water from the site to the Snipesmoor Brook and is restricted by a hydro-slide flow control device. There is also a French drain around the composting facility boundary and an additional infiltration ditch along the west site boundary. 4
2.1.22 It is assumed that the existing surface water drainage system includes an allowance for the impacts of climate change. 2.1.23 Considering surrounding topography, land use and the existing surface water drainage system including mitigation the risk of flooding from surface water run-off and overland flow is considered to be low. Residual risk still remains, however only for extreme events. Sewerage Infrastructure 2.1.24 Flooding from sewers can occur when the artificial drainage system is overwhelmed, hydraulically, becomes blocked or suffers structural failure or pump failure. Blockage and structural failure incidents tend to be isolated and unpredictable. 2.1.25 Severn Trent Water is responsible for the management of the urban drainage system throughout Ashbourne including surface water and foul sewerage. Severn Trent Water has procedures in place to respond to and rectify such incidents, which are also recorded on databases to inform maintenance and improvement plans. 2.1.26 Surface water runoff from the composting facility discharges to the Snipesmoor Brook which flows in culverted section adjacent to the western boundary of the composting facility. The flow of surface water to the Brook is restricted by a flow device, part of the existing Sustainable urban Drainage System (SuDS) at the site. 2.1.27 The proposed increased tonnage of organic waste will not increase the rate or volume of surface water run-off leaving the site, therefore the capacity of the culverted Snipesmoor Brook will not be affected. 2.1.28 Any flows resulting from surcharging of the drainage system during extreme rainfall events would be short term, relatively shallow in depth and would pass through the site following the natural topography. 2.1.29 Therefore, it is considered that the risk from surcharged sewer flooding is low. Residual risk still remains, however only for extreme events and incidents of sewer blockage etc. Climate Change 2.1.30 Climate change is expected to have a major influence on future flood risk. The expectations are that winter floods will happen more often and in urban areas flooding from thunderstorms will be more regular and more severe. 2.1.31 The Environment Agency Flood Map does not currently take account of future climate change. Annex A of PPS 25 advises that the effects of climate change at a regional level are likely to mean an increase in peak river flows and rainfall intensities. 2.1.32 Table B.2 (Annex A) of PPS 25 suggests sensitivity ranges in peak flows and rainfall intensities, and indicates a 20% increase in both river flows and rainfall intensities could occur over the period 2025-2085. 2.1.33 The predicted increase in rainfall intensity will result in an increase in surface water run-off at the site and, as a consequence, flood risk both too and from the proposed scheme site will also increase. 5
2.1.34 It is considered that with the current surface water drainage system (see Section 3), (which it is assumed includes an allowance for climate change and restricts discharge from the composting facility), that any increase in flood risk is manageable. 6
3 Surface Water Drainage 3.1.1 There are no open surface water features within the site boundary or directly adjacent to the site. Existing Surface Water Drainage System 3.1.2 Water draining from the roof areas and clean external yard areas is collected in a separate surface water drainage system within the site boundary. 3.1.3 Surface water attenuation storage (a total of 664 m 3 ) is provided within the site boundary in the form of three below ground poly-storm storage tanks located to the south-east and south-west of the site. The rate at which surface water leaves the site is controlled by a hydro-slide flow control device located in a flow control chamber to the north of the access road. 3.1.4 From the flow control chamber the system then connects to the existing surface water drainage system (900 mm diameter pipe) via a manhole located in the access road. This discharges into the culverted section of Snipesmoor Brook, adjacent to the western boundary of the composting facility, via a connection beneath Blenheim Road. 3.1.5 A French drain also surrounds the site and there is an infiltration trench along the length of the west site boundary. 7
4 Foul Water Drainage 4.1.1 Composting vessels and site plant entering the reception building are cleaned using power washing equipment prior to leaving for other areas of the site. All wash water from washing down of operational areas is collected in the foul water drainage system. 4.1.2 The changeover bay has an impermeable concrete base with 3 m high walls. The changeover bay is cleaned prior to re-use. Runoff from the bay is directed to the foul water drainage system. 4.1.3 To the north of the reception building are two large storage units for dirty water. These have a capacity of 80,000 litres each and collect dirty water from across the site. 4.1.4 The foul water drainage system used on site discharges to the foul sewer network in Blenheim Road. The separate foul water system collects water from the waste storage area, cleaning area, maturation building, staff facilities, and changeover bay. 8
5 Conclusion 5.1.1 The site is located in Flood Zone 1 and the fluvial flood risk to the site is generally considered to be low. Due to the inland location of the composting facility tidal flooding is not considered. 5.1.2 Current available information suggests the risk of localised surface water flooding and groundwater flooding is considered to be low. Due to the positively drained urban nature of the composting facility and its surroundings and the existing SuDs system draining the site, there is considered to be a low risk of flooding from sewers. 5.1.3 The drainage capacity of the site will not change as a result of the composting facility operating with the increased tonnage of waste and surface water run off rates for the site will remain the same as the currently operating facility. 5.1.4 The proposed scheme will not result in an increase in flood risk to the site or third party land in the wider vicinity of the site. 9
Figure 1 Location Plan Figure 1
High Trees Orchard House Coxbridge The Bungalow LB N KEY Site location Proposed scheme site boundary Existing site area Contains Ordnance Survey data Crown copyright and database right 2011 Tank Crown Copyright 2011. All rights reserved. Licence number 100020449 Drawing Title Vital Earth Ltd Site Location Plan Scale at A4 : 1:50,000 & 1:5,000 Drw Chk FIGURE 1 GB App JM Rev HP Date 29.09.11 Date www.urs-scottwilson.com