10.0 FLOOD RISK ASSESSMENT AND SURFACE WATER DRAINAGE STRATEGY This chapter of the Environmental Statement is based on a Flood Risk

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1 10.0 FLOOD RISK ASSESSMENT AND SURFACE WATER DRAINAGE STRATEGY Introduction 10.1 This chapter of the is based on a Flood Risk Assessment (FRA) incorporating a Surface Water Drainage Strategy and Flood Risk Management Plan and Access Strategy (Access Strategy) produced by Odyssey Consulting Engineers to support the planning application. The purpose of the FRA is to: Provide a preliminary Surface Water Drainage Strategy identifying strategic proposals for the surface water drainage that will ensure current Government guidance is complied with, such as Planning Policy Statement 25 (PPS25) and Sustainable Drainage Systems guidance (SUDS); Undertake a FRA to confirm the sites suitability for development; Assess surface water run-off from the existing land use and compare this with the proposed development run-off; Review existing surface water systems; Review the impact (if any) of the proposed access location and construction makeup to flood risk both downstream and upstream of the site The Flood Risk Management Plan and Access Strategy demonstrates the Flood Risk Management procedures that will be in place (to be secured by condition) and concludes that there would be no risk to users of the proposed access road. Site Description 10.3 Chapter 2 of the FRA provides a site description. The development area is sited within arable land and has a level difference of 4 meters from east to west towards the River Soar. This represents a gradient of 1: The site is shown on the 1:50,000 British Geological Survey map no.169 Coventry. Data for the site and surrounding area suggests the underlying sub-strata is predominantly Till or Boulder Clay with some

2 Mercia Mudstone and Alluvium deposits associated with steams, rivers and other watercourses. Planning Policy 10.5 Planning Policy Statement 25 (PPS25) Development and Flood Risk requires a FRA to be undertaken on all development sites over 1 hectare in size whether in floodplain or not. In terms of fluvial flooding, the guidance categorises flood zones in three levels of risk as follows: Figure 18 - Table 4.1 of FRA Flood Zone 1. Little or no risk 2. Low to medium risk 3. High risk Annual Probability of Flooding <0.1% % >1.0% 10.6 Pursuant to PPS25 Annex D the proposed development of the AD plant will fall within the classification of Less Vulnerable and is considered appropriate in areas categorised by Flood Zones 1 or 2 and 3a This section of the document considers the requirements of the guidance and demonstrates that the proposed development lies within the acceptable flood risk parameters. Site Specific Information 10.8 Environment Agency (the Agency) Section 105 flood mapping and detailed flood modelling provided by the Agency illustrates that the proposed site is located within Flood Zone 1 with a small section of the undeveloped site located on the fringe of a section of Flood Zone 2. This is further confirmed by correspondence with the Agency, shown within Appendix C of the FRA The detailed flood mapping indicates that the proposed main access and egress is located mainly within Flood Zone 2 but with small sections

3 (as shown blue on drawing number /006 and comprising less than 40% of the access road) within Flood Zone The site is indicated on the British Geological Survey map no.169 Coventry as predominantly Till or Boulder Clay with some Mercia Mudstone and Alluvium deposits associated with streams, rivers and other watercourses. The geology of the site has been re-affirmed by a site specific intrusive investigation into the underlying sub-strata The three dimensional level survey completed has been used to establish terrain contour mapping for use in completing the FRA. Drawing /002 affixed within Appendix B of the FRA shows the existing site levels. It can be seen from the site topographic survey that the site falls at a steady gradient from a level of approximately 79.98m at the east corner to a level of 75.01m at the west corner. Flood Levels Modelled flood levels provided by the Agency for the upstream and downstream face of Stoney Bridge are as follows: Figure 19 - Table 4.1 of Access Strategy Environment Agency Modelled Flood Levels Return Period Description Node Label Upstream face Stoney Bridge Downstream face Stoney Bridge The above modelled flood levels within Table 4.1 do not include an allowance for climate change, therefore based on the Agency s recommendation 300mm should be added to these levels.

4 Figure 20 - Table 4.2 of Access Strategy Environment Agency Modelled Flood Levels, Including 300mm for Climate Change Return Period Description Node Label Upstream face Stoney Bridge Downstream face Stoney Bridge Based on the above, a review was undertaken based on the modelled flood levels and existing ground levels. OCE drawing within the Access Strategy illustrates a long section of the proposed road (at existing ground levels) plotted against the EA modelled flood levels, indicating the predicted depth of flood waters during the 1 in 100 year flood event plus 300mm for climate change. Existing Surface Water Drainage Scheme Based on local records, there are no existing surface water sewers in the vicinity of the site A walkover survey has confirmed that there is an existing overland flow path for surface water which does not infiltrate directly into the substrata. The overland flow path follows the existing ground profile of the site and flows east to west towards the River Soar. Drawing /004 within Appendix B of the FRA (contained within Appendix 5 of this statement) illustrates this overland flow path. Proposed Site Access As previously discussed, the proposed access for the development will be mainly within Flood Zone 2 but partly within Flood Zone 3 (less than 40% at the access road).

5 10.18 Initial discussions with the Agency raised an objection on flood risk grounds due to the then proposed access road being raised above existing levels and therefore having the potential of increasing flood risk to the up and downstream catchments Further correspondence provided by the Agency based on a revised access, shown within Appendix C of the Access Strategy, has lifted their previous objection of the proposed access road. This access will be provided at existing levels to ensure the floodplain is not impacted Within this correspondence the Agency have indicated that there is a potential that the access road will be subject to flooding anywhere from 0 days in a dry year to days of the year depending upon rainfall and local hydraulics. These durations are not based on site specific information but for the local area as a whole Following previous correspondence with the Agency it has been agreed that for this area vehicles can be safely operational during times of flood up to a flood depth of 300mm while floods in excess of this may have a risk to life if the access remains operational From reviewing the Agency s Upper Sour ISIS model, for a comparison of flood depths against duration, the access road will be under flood conditions for storms in excess of a 1 in 50 year return period, and only then if 300mm has been included for climate change. The duration that has been indicated from this review demonstrates that the access road will be subject to an unacceptable depth of flooding for an approximate duration of around 18 hours during the 1 in 50 year storm event During these 18 hours the main access gate will be locked and no deliveries made, a secondary access will be utilised for emergency use and exit from the site. This emergency access is an existing track that traverses the farmland from the junction of the B4114 / Fosse Way past the development on the northern side through Sutton Lodge Farm and

6 joining onto Frolesworth Road to the east of the development. Figure 6 within Appendix A of the FRA highlights this route. Preliminary Surface Water Drainage Strategy Current Planning Policy (PPS25) and Environment Agency guidance requires developments to employ Sustainable Drainage measures (SUDS) wherever feasible. Careful design of SUDS features can ensure that the site storm water drainage closely reflects the natural hydrology and hydrogeology of the site. Such systems can also improve the quality of water discharged from development prior to reaching the receiving watercourse Source control is a key element of SUDS and is now identified in Part H of the Building Regulations 2002 as taking preference over conventional drainage systems. Part H now states: Rainwater from a system provided pursuant to sub-paragraphs (1) or (2) shall discharge to one of the following, listed in order of priority: (a) an adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable, (b) a watercourse; or where that is not reasonably practicable, (c) a sewer Source control systems treat water close to the point of collection, in features such as soakaways, porous pavements, infiltration trenches, swales and basins. The use of these can have the benefit of discharging surface water back to ground rather than just temporarily attenuating peak flows before discharging it to the receiving watercourse As source control measures generally rely upon the infiltration of surface water to ground, it is a prerequisite that the ground conditions are appropriate for such. Published geology for the local area suggests the subsoil will have poor permeability, hence the use of groundwater recharge systems is likely to be low.

7 10.28 Next in the search sequence, defined by Part H, is discharge to a watercourse. The site has only one designated nearby watercourse approximately 130m to the west. It is clear that an existing overland flow path is available for the site to drain into the existing watercourse Last in the search sequence is discharge to a sewer. There are no known sewers within the vicinity of the proposed development. Further to this, there is an existing watercourse in close proximity to the development and, for sustainability, discharge to a sewer has not been considered further The search demonstrates that the potential to recharge the ground via the proposed attenuation pond and the existing watercourse is the most appropriate outfall via overland flow for extreme flood events Accordingly, initial drainage proposals have been developed employing a piped on-site network that discharges into the attenuation pond before discharging into the River Soar not exceeding greenfield run-off rates. Appraisal In establishing the permitted discharge from the development areas, an appraisal has been completed in accordance with IH 124. A copy of the existing calculations are contained in Appendix C of the FRA. Taking into account the indicative catchment area, the permitted discharges from the site will be as follows: Figure 21 - Table 4.4 of the FRA Storm Event 1 in 1 year 1 in 30 year 1 in 100 year Flow l/s

8 10.33 Preliminary proposals have been developed to demonstrate the proposed surface water management scheme. The proposals are indicated on drawing /005 affixed within Appendix B of the FRA, with post development calculations provided within Appendix D of the FRA. The calculations within Appendix D were calculated using WINDES (industry standard design package) to the design criteria within Table 4.3 (figure 20) The outline drainage scheme provides surface water drainage in the form of an attenuation pond before discharging into the River Soar via outflow pipe and headwall. Extreme storm events will utilise the existing overland flow path. Discharge rates will be limited to the existing greenfield run-off rates as detailed above. All surface water storage is catered for within the proposed pond The surface water network has been sized to accommodate storage for the 1 in 1, 1 in 30 and the 1 in 100 year plus 20% climate change whilst still maintaining greenfield discharge rates On the rare occasions when storm events exceed 1 in 30 years, existing overland flow path to the watercourse to the north west will be maintained. This is indicated schematically on drawing /005 shown within Appendix B of the FRA. Conclusions The proposed AD buildings are indicated on the Agency s Section 105 flood mapping as within Flood Zone 1. PPS25 states that Flood Zone 1 is appropriate for all uses of land. Agency. This has been confirmed by the The proposed access road is located mostly within land designated as Flood Zone 2 but with small sections (<40%) within Flood Zone 3. The Agency has lifted their previous objection of the proposed access road located within this zone.

9 10.39 Review of the Agency s Upper Sour ISIS model has indicated that the road will be subject to unacceptable depths of flooding for an approximate duration of around 18 hours during the 1 in 50 year storm event During these 18 hours the main access will be gated with no deliveries being made. During this closure there is a secondary access that will be utilised for emergency use. The emergency access is an existing lane that traverses the farmland from the junction of the B4114 / Fosse Way past the development on the northern side through Sutton Lodge Farm and joining onto Frolesworth Road to the east of the development. Figure 6 within Appendix A of the FRA highlights this route The risk of pollutants entering the groundwater is extremely small due to the employment of source control measures The preliminary storm water drainage scheme proposed has demonstrated that SUDS techniques can and will be utilised on this site in the form of an attenuation pond The proposal will limit the surface water discharge to replicate greenfield run-off and therefore will not increase flood risk. greenfield run-off is shown in Table 4.4. Existing The Flood Risk Management Plan and Access Strategy demonstrates: There is no significant flood risk to users of the main access road. Any risk would be fully addressed and mitigated by the detailed strategy which would be secured by appropriate planning conditions The closure of the site for up to 14 days would not create a significant impact on the facility. Closure of up to 30 days would have an economic impact on the plant but would not be long enough for the biological digestion process to be completely stopped. Based on the flood depth / duration hydrograph, the proposed access road will have

10 an average closed duration of around 18 hours during the 1 in 50 year storm event. This duration will have no effect on the operation of the AD facility The findings of the FRA demonstrate that the proposed development will fully comply with the requirements of PPS25. The proposed plant will be located within Flood Zone 1 and a safe means of access to serve the development will be provided in all circumstances. There are no flood risk or other hydrological constraints or impacts which indicate planning permission should be refused.