Newark Sewerage Strategy Overview Flood Risk Assessment Severn Trent Water January 2015 Newark Sewerage Strategy Overview Flood Risk Assessment Notice This document and its contents have been prepared and are intended solely for Severn Trent Water’s information and use in relation to the Newark Sewerage Strategy, Overview Flood Risk Assessment. Atkins Ltd assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents. This document has 28 pages including the cover. Document history Job number: 5117326 Document ref: 71_DG_002 Revision Purpose description Originated Checked Reviewed Authorised Date Rev 1.0 Draft for Client Review L. Hillcoat J. Lamont T. Rouse J. Lamont Nov 2013 H. Stockham Rev 2.0 Revision following Client L. Hillcoat - T. Rouse T. Rouse Dec 2013 Review Rev 3.0 Revision following L. Hillcoat C. Nunns T. Rouse T. Rouse Nov 2014 Environment Agency comments Rev 4.0 Final for issue L. Hillcoat C. Nunns T. Rouse T. Rouse Jan 2015 Atkins Overview Flood Risk Assessment | Version 4.0 | January 2015 | 5117326 Newark Sewerage Strategy Overview Flood Risk Assessment Table of contents Chapter Pages Glossary i Executive Summary ii 1. Introduction 1 1.1. Background 1 1.2. Scheme Proposals 1 1.3. Scope of this Overview FRA 4 2. Assessment of Flood Risk 6 2.1. Historical Incidents of Flooding 6 2.2. Fluvial Flooding 6 2.3. Surface Water Flood Risk 12 2.4. Groundwater Flood Risk 13 2.5. Flood Risk from the Sewerage Network 14 2.6. Other Sources of Flood Risk 14 2.7. Climate Change Implications 14 3. Conclusions and Recommendations 16 3.1. Conclusions 16 3.2. Recommendations 16 4. Bibliography 17 Appendices 18 Appendix A. Scheme Proposal 19 Appendix B. Environment Agency Flood Risk Data 20 Appendix C. Hydraulic Modelling Technical Note 21 Appendix D. Initial compensation assessment 22 Tables Table 1-1 Number of properties and existing risk of sewer flooding to properties 3 Table 2-1 Change in discharge rates for the 1% AEP event 8 Table 2-2 Hydraulic Modelling Results 9 Table 2-3 Peak levels for the design event combination analysis 11 Table 2-4 Impact of climate change, taken from Table 5 in the NPPF 15 Figures Figure 1-1 Scheme Overview Location Plan in Newark-on-Trent 2 Figure 1-2 Location of property at risk from flooding 4 Figure 2-1 Flood Zone Extent 7 Figure 2-2 Key locations for hydraulic model results 10 Atkins Overview Flood Risk Assessment | Version 4.0 | January 2015 | 5117326 Newark Sewerage Strategy Overview Flood Risk Assessment Glossary Term Definition AEP Annual Exceedance Probability FRA Flood Risk Assessment PFRA Preliminary Flood Risk Assessment SFRA Strategic Flood Risk Assessment SuDs Sustainable Drainage Systems WCS Water Cycle Study Annual Exceedance Probability (AEP) The severities of the events discussed in this report are defined mainly as Annual Exceedance Probabilities (AEP). The AEP is the probability that there will be an event exceeding a particular severity in any one year. Annual Exceedance Probability Probability (chance in any given year) 50% 1 in 2 10% 1 in 10 4% 1 in 25 3.3% 1 in 30 2% 1 in 50 1.33% 1 in 75 1% 1 in 100 0.5% 1 in 200 0.4% 1 in 250 0.1% 1 in 1000 Atkins Overview Flood Risk Assessment | Version 4.0 | January 2015 | 5117326 i Newark Sewerage Strategy Overview Flood Risk Assessment Executive Summary Severn Trent Water has highlighted the necessity to improve the existing sewer system within Newark-on- Trent owing to the current flooding problems, the aging system and to a lesser extent development growth within this area. The proposed improvements include increasing the capacity of the existing system and relocating or closing combined sewer overflow (CSO) outfalls. Information provided by Severn Trent Water predicts that a total of 118 properties would significantly benefit, in relation to sewer flood risk, as a result of the proposed scheme. This includes a reduction in internal property flooding to 50 properties. Severn Trent Water has commissioned Atkins to complete this Overview Flood Risk Assessment (FRA) for these proposed improvements. The aims of this Overview FRA are to provide reassurance that the overall scheme will not have negative impact on flood risk from all sources, and to inform the Level 3 FRAs which are necessary for the two elements requiring planning permission. Whilst this Overview FRA has been completed utilising current FRA guidance, it does slightly deviate from standard FRAs owing to the general scheme falling within Severn Trent Water’s permissive power which do not require planning permission. The proposed changes to CSO outfalls within the scheme have the potential to impact fluvial flood risk along the River Trent. However hydraulic modelling completed for this Overview FRA indicates the proposed rearrangement of CSO outfalls will cause minor localised changes in peak flood levels although these changes will have negligible impact on flood extents and risk to vulnerable property. There are minor localised impacts of 20mm during the 5% Annual Exceedance Probability (AEP) event and 10mm during the 1% AEP event to green open spaces and other such land uses not considered vulnerable. The proposed scheme involves development within Flood Zone 3 at Crankley Point. It was requested by the Environment Agency that an initial compensation assessment was completed for these works to inform this overview FRA owing to the limited area available for compensation. This initial compensation assessment has identified that sufficient compensation could be provided in the western area of the site, based on topography alone. Other potential constraints would need to be considered during the scheme design. There are currently surface water and sewer flood risk issues in Newark-on-Trent. However the proposed scheme is for works to increase the capacity of the existing system and alleviate the existing flood risk. Although there is a low to moderate risk of groundwater flooding the scheme, it is anticipated that the works will have a negligible impact on existing groundwater flood risk within the study area. There are no other known sources of flood risk to the study area, or that would be increased as a result of the proposed scheme. Climate change is predicted to cause an increase in flood risk to Newark-on-Trent in the future, particularly in relation to increased river flow and rainfall intensity. The increased fluvial flood risk as a result of climate change has been quantified through the hydraulic modelling completed for this Overview FRA. The proposed scheme will increase the capacity of the existing sewer system and hence will provide benefits in relation to surface water flooding in the future. Areas at an increased fluvial flood risk as a result of climate change will not be put at additional risk as result of the proposed scheme, as demonstrated through the hydraulic modelling completed for this Overview FRA. In summary, owing to the driver of the proposed scheme being to alleviate flooding from the existing surface water and sewer network, this Overview FRA concludes that the proposed scheme will provide a benefit in relation to flood risk from these sources within Newark-on-Trent. In addition, the hydraulic modelling completed for this assessment concludes the proposed scheme, in relation to the rearrangement of CSO outfalls, will not have an adverse impact on fluvial flood risk. Although there is potential that sufficient floodplain compensation could be provided for the works at Crankley Point, wider constraints need to be considered. The potential increase in risk as a result of the two elements of the scheme requiring planning permission (specifically in relation to development within the floodplain) will be further assessed through the completion of Level 3 FRAs. Atkins Overview Flood Risk Assessment | Version 4.0 | January 2015 | 5117326 ii Newark Sewerage Strategy Overview Flood Risk Assessment 1. Introduction 1.1. Background Severn Trent Water have identified that there is problem with sewer flooding within Newark-on-Trent as the existing combined sewers do not have sufficient capacity during high intensity rainfall events. The insufficient capacity causes flooding within Newark-on-Trent and the excess surface water runoff discharges into the River Trent. Newark-on-Trent is a growing town, with a high density of properties. Severn Trent Water have highlighted that the sewerage from properties needs to be improved due to the aging sewer system, the existing insufficient capacity and to support the town’s growth. Severn Trent Water is proposing a sewer replacement scheme to provide additional capacity and alleviate the existing flooding in Newark-on-Trent. Atkins has been commissioned to assess the flood risk implications of the scheme. The first element of this process is this Overview Flood Risk Assessment (FRA), the purpose of which is to assess the overall impact of the proposed scheme and in particular relevance to the changes in discharge rates and locations to the River Trent. This will then be followed by more detailed Level 3 FRAs for the two proposed elements of the scheme that require planning permission. Further details of the aims and scope of this assessment is provided in Section 1.3. 1.2. Scheme Proposals The purpose of the scheme is to alleviate flooding to over 130 locations. It is proposed that this will be achieved through upsizing the capacity of the sewers in Newark-on-Trent. In addition to this, the main storm relief outfall will become a single point discharge within the boundary of the existing sewage treatment works, with a smaller discharge further upstream. The outcome of which will be a net reduction in the peak discharge rate to the River Trent, through Newark-on-Trent, during periods of heavy rainfall.