12 FLOOD RISK ASSESSMENT Introduction

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1 12 FLOOD RISK ASSESSMENT 12.1 Introduction A full Flood Risk Assessment (FRA) has been prepared by Wardell Armstrong on behalf of FCC Environment in relation to the proposed WTS at London Road East and is included as Appendix 12.1 to this ES. Set out below is summary of the report Methodology The assessment was carried out in accordance with the guidance set out in the National Planning Policy Framework (NPPF) The methodology for the FRA has comprised a desktop study, supplemented by liaison with the Environment Agency, Chiltern District Council, Buckinghamshire County Council and Thames Water. Full details of the methodology is included in section 1.2 of the FRA (Appendix 12.1) 12.3 Site and Catchment Characteristics The site boundary, approximately 2.9 hectares in size, comprises in the majority of an irregularly shaped field of bramble thicket, tall ruderal vegetation, scrub and regenerating grassland with a broad margin of planted and semi-natural deciduous woodland The topography of the site generally falls in a south-western direction from circa 89.0mAOD, along the north-eastern boundary adjacent to London Road East, to circa 80.0mAOD at the south-eastern and southern boundaries adjacent to the River Misbourne. The A413 (London Road East) sits at a higher elevation of circa 92mAOD in relation to the development site. In addition, central areas of the site are fairly constant in levels of circa 86.0mAOD, however, fall quite steeply towards the north-western part of the site at it s interface with the existing HWRC access road, down to circa 81.6mAOD, indicating a level difference in the order of circa 4.4m. A site topographical survey is included on Drawing Number ST Existing Site Layout Local Watercourses and Surface Water Features The River Misbourne, an Environment Agency (EA) designated Main River, flows down the Misbourne Valley in a west to east direction through Amersham Old Town, before flowing in a south-easterly direction immediately adjacent to the southern boundary of the site. The River Misbourne continues its route through

2 South Buckinghamshire before arriving at its confluence with the River Colne, approximately 13km downstream from the site, at the border with the London Borough of Hillingdon The River Misbourne is described as a Chalk stream with a porous bed and is traditionally low during dry weather conditions, largely due to the Chalk geology. As a consequence, the river needs a high groundwater table in order to maintain its flows. It is understood that parts of the River Misbourne can regularly run dry for several seasons or even years and furthermore, historically the River Misbourne suffered from reduced flows for a number of years due to extensive abstraction from aquifers that maintained its base flow. Environment Agency remedial measures along the Misbourne Valley have, however, restored flows to the River Flood Risk Setting The Environment Agency s (EA) online Flood Map for Planning shows the majority of the site to be located within Flood Zone 1 (ie in an area defined as having a low annual probability of flooding from rivers (<0.1%)), although, an extent of Flood Zone 2 (ie in an area defined as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% 0.1%) and Flood Zone 3 (ie in an area defined as having a 1 in 100 or greater annual probability of river flooding (>1%) is also shown to encroach the north-western (existing HWRC access road) and southern boundaries of the site immediately adjacent to the River Misbourne. Further information is documented in Section 2.3 of the FRA (Appendix 12.1 to this ES) Existing Drainage Systems Thames Water s public sewer records are provided as Appendix 3.2 to the FRA (Appendix 12.1 of this ES) The existing 300mm diameter public combined water sewer that bisects the development site will be diverted in consultation with Thames Water to enable the construction of the proposed WTS building. Diversion proposals will be confirmed during detailed design stages Drainage Proposals As described in detail in the FRA, an existing 300mm diameter public combined water sewer that bisects the site will require diversion to accommodate the proposed WTS building. The proposed diversion will be confirmed in consultation

3 with Thames Water following the gaining of planning consent and during detailed design stages. Drawing Number ST Proposed Drainage Strategy and Surface Water Management Plan shows one option for diversion of this sewer Currently the site has no surface water management, with surface water infiltrating the ground or flowing overland offsite to the River Misbourne at un-attenuated rates The site will require construction of both foul water and surface water drains. An indicative drainage strategy is included on Drawing Number ST (refer to Appendix 12.1) The total development site boundary, 2.9ha in size, will comprise of 71% (1.91ha) of existing permeable and proposed landscaping that will have no positive drainage systems, 0.21ha of existing impermeable area (existing HWRC access road that is discounted from this site percentage make up) and 29% (0.78 ha) of proposed impermeable surfaces The existing access road serving the HWRC, which is included within the application site boundary, consists of 0.21ha of existing impermeable area. Drainage for the existing access road is understood to discharge to the Thames Water public combined water sewer, located within the access road, via an oil interceptor, and it is proposed that the surface water drainage system will remain to function as the current situation with no requirement for restriction of surface water flows The majority of surface water generated from proposed impermeable areas of the site will be discharged to the River Misbourne (subject to agreement with the Environment Agency), at Greenfield rates. A portion of surface water generated from the proposed WTS internal access road is proposed to discharge to the existing Thames Water public sewer located within the existing HWRC access road (subject to agreement with Thames Water), at Greenfield rates. Attenuation systems and flow control devices will be designed in detail to ensure that the site discharge does not exceed the Greenfield runoff rate including an allowance for climate change The proposed WTS internal access road, which will be constructed off the existing HWRC access road, will need to rise steeply to accommodate a circa 4.4m difference in levels between the site development platforms and the interface with the existing HWRC access road. In terms of surface water drainage, it is considered unsustainable to provide an infinitely pumped surface water solution for capturing

4 surface water flows from this catchment area. It is proposed, therefore, that due to the nature of the site levels, that surface water collected over an impermeable surface area of circa 0.1ha, be discharged to the Thames Water public combined water sewer (subject to agreement with Thames Water), in addition to the surface water originating from the existing HWRC access road, via an oil interceptor, prior to discharge to the sewer. Thames Water has indicated the potential to accommodate a surface water discharge limited to 5 litres/second (refer to Appendix 3.1 of the FRA, Appendix 12.1) Foul water flows will be discharged to the Thames Water public combined water sewer system, subject to formal agreement with Thames Water. Thames Water has indicated that foul water flows can be accommodated within the sewer network (refer to Appendix 3.1 of the FRA, Appendix 12.1) In addition to both foul and surface water drainage, there may be a requirement for provision of trade effluent drainage (subject to agreement with Thames Water). In this circumstance, it is anticipated that any such infrequent flows will be only be produced during activities such as strategic maintenance of the internal WTS building, which may include periodic wash down of the internal building areas. It is proposed that any generated trade effluent flows will be discharged at a Greenfield rates Flood Risk Assessment Fluvial (River) Flooding The proposed development site including the majority of the WTS access road, operational areas and the WTS building will be at situated at levels of circa 4m above Flood Zone 2 and Flood Zone 3 areas and will be located entirely within Flood Zone 1. No proposed buildings will be located within areas noted as being in Flood Zone 3. In addition, any proposed security fencing located within floodplain areas will be designed to ensure that the current movement of floodwaters is retained and, therefore, there will be no compromise of the effective floodplain and no effect to flood water flows It is considered, therefore, that the risk of flooding from fluvial sources to the site will be low.

5 Surface Water It is considered that due to the higher elevated topography of the site and the proposed installation of drainage systems that will manage surface water runoff, the risk of flooding from this source to personnel at the site and adjacent areas will be low. The provision of an emergency egress route from the site will ensure the safety of personnel if surface water were to pond and flood the existing HWRC access road (refer to Section 4.4 of the FRA, Appendix 12.1). Groundwater The operational development site will be situated at levels of circa 4m above areas identified as being susceptible to groundwater flooding and, therefore, it is considered that the risk of flooding to the site from groundwater is low. If, however, groundwater flooding should occur, it would be contained within the existing HWRC access road and would eventually be routed towards the River Misbourne and would not cause a risk to personnel at the site due to the provision of emergency egress routes (see later sections of this report). Sewer Flooding The public sewers within the site boundary could potentially act as a source of flooding. It is considered, however, that due to relative levels floodwater would be routed overland in a southerly direction towards the River Misbourne and would not have a significant impact on the development. The risk of flooding from this source is, therefore, considered, to be low. Flood Risk posed by the Development Since the majority of the site is located within Flood Zone 1 and there will be no proposed development that could impede flows within areas of Flood Zone 2 and Flood Zone 3, there will be no effect on flood flow routes or floodplain storage (ie floodwater will not be displaced from the site). Surface Water Runoff Existing vegetation covers up to 93% of the site area. For the purposes of assessing the existing runoff rates from the site, the site will be considered as a Greenfield site, with Greenfield runoff rates. It is estimated that under the development

6 proposals, the impermeable area of the site will increase to approximately 34% and therefore, the overall rate and volume of surface water runoff will increase In addition, climate change can be expected to cause an increase in rainfall intensity, and hence runoff rates and volumes, over the lifetime of the development. It will be necessary, therefore, to provide mitigation measures to ensure that flood risk is not increased to areas downstream of the site as a result of the proposed development Further detail is provided in Table 3 of the FRA (Appendix 12.1): Catchment Area and Proposed Discharge Locations. Flood Risk Mitigation Measures To mitigate the potential increase in flood risk to areas downstream of the site, surface water runoff from the proposed development will be restricted to predevelopment (Greenfield) rates for all storm events, up to and including the 1 in 100 year event (including an allowance for climate change) The estimated pre-development Greenfield surface water runoff rate (QBAR) is 1.2 litres/second (see Appendix 12.1 FRA (Appendix 6)) It is proposed to use a vortex flow control device for limiting the discharge of surface water flows from the site. Surface water from the development will, therefore, be discharged at a maximum rate of 5 litres/second to the River Misbourne and Thames Water public sewer system, subject to agreement with the Environment Agency and Thames Water. Any flows in excess of this will be attenuated on site for all storm events up to and including the 1 in 100 year event, including a 20% allowance for climate change The estimated attenuation volume for the 1 in 100 year event, including a 20% allowance for climate change, is 420m 3 for a discharge to the River Misbourne and 26m 3 for a discharge to the Thames Water public sewer (see Appendix 7 of the FRA, Appendix 12.1 to this ES) It is proposed that the required attenuation volume is distributed throughout the site in various SuDS features, which will provide water quality treatment and enhance amenity and biodiversity, as well as provide control of water quantity. As

7 far as possible, surface water runoff will be managed at source, rather than conveying it through the site to a single attenuation feature Further detail is provided on Drawing No. ST Proposed Drainage Strategy and Surface Water Management Plan, included as part of the FRA (Appendix 12.1) The proposed scheme is provided in Section 4.3 of the FRA (Appendix 12.1) including details on the following: Pond/Wetland Attenuation Below Ground Stone Blanket Layer Filter Drain/Filter Strip Below Ground Circular Pipe Attenuation Rainwater Harvesting Residual Risk On-site attenuation will be provided for storm events up to and including the 1 in 100 year event, including an appropriate allowance for climate change. For storm events with an annual probability of occurrence greater than this, the on-site attenuation may be exceeded. It is considered, however, that the probability of this is extremely low and, therefore, the residual risk is negligible Conclusions The site is located within Flood Zones 3, 2 and 1 in accordance with the Environment Agency Flood Risk Mapping. The site, however, lies mostly outside of Flood Zone 3 and in the majority is located within Flood Zone 1. Proposed development will only be in Flood Zone The development type, a Waste Transfer Station (WTS) for the bulking of nonrecyclable waste such as residual/black bin bag waste, is classified as More Vulnerable development, as described in Table 2: Flood Risk Vulnerability Classification, Flood Zone and Flood Risk Tables of the Planning Practice Guidance. Table 3: Flood risk vulnerability and flood zone compatibility, Flood Zone and Flood Risk Tables of the Planning Practice Guidance, shows that More Vulnerable

8 development located within Flood Zone 3 should only be permitted following successful application of the Sequential and Exception Tests It is considered that the Sequential Test as defined in the NPPF has been passed due to the allocation of the site within the Buckinghamshire Minerals and Waste Core Strategy Development Plan Document - Minerals and Waste Local Development Framework that was Adopted November It is expected that the information presented in the Flood Risk Assessment demonstrates that the development site will be safe to satisfy the second part of the Exception Test The site is considered not to be at risk of flooding from the sea. The risk of flooding from fluvial, groundwater, sewers, artificial sources, and surface water is considered to be low Flood risk management measures will be put in place to ensure that the risk of flooding to areas downstream of the site is not increased as a result of the development. This will include a restriction on surface water discharge rates to mimic Greenfield performance for a range of storm events Surface water runoff will be managed in a sustainable manner. On-site attenuation will be provided to accommodate flows up to and including the 1 in 100 year storm event, including an appropriate allowance for climate change and will be regulated by a vortex flow control device and Sustainable Drainage Systems (SuDS) features such as a pond/wetland, below ground stone blanket layer, filter drain/filter strips, circular attenuation pipe and rainwater harvesting. In addition to minimising flood risk, the proposed SuDS features will help to improve water quality by reducing pollutants, reduce the demand for potable water through inclusion of rainwater harvesting and improve wildlife habitats Safe access and egress will be provided to and from the site during any event that exceeds the design storm event The full FRA is provided as Appendix 12.1 to this ES. Overall, it is considered that the site is suitable for the type of development proposed with the above measures in place.