Malton, Norton and Old Malton Flood Study Final Report North Yorkshire County Council Malton, Norton and Old Malton Flood Study Final Report
Total Page:16
File Type:pdf, Size:1020Kb
North Yorkshire County Council CONTRACT REFERENCE – 9P7C-JXSERY | October 2015 Malton, Norton and Old Malton Flood Study Final Report North Yorkshire County Council Malton, Norton and Old Malton Flood Study Final Report Revision | October 2015 This report takes into account the particular instructions and requirements of our client. It is not intended for and should not be relied upon by any third party and no responsibility is undertaken to any third party. Job number 239474 Ove Arup & Partners Ltd Admiral House Rose Wharf 78 East Street Leeds LS9 8EE United Kingdom www.arup.com Document Verification Job title Malton, Norton and Old Malton Flood Study Job number 239474 Document title Final Report File reference Document ref Revision Date Filename Malton-Norton Flood Study Report260615.docx Draft 1 26 Jun Description First issue for information/update purposes only 2015 Prepared by Checked by Approved by Various Nicole Rabier/ Name Will McBain Luke Ballantyne Signature Issue 3 Jul Filename Malton-Norton Flood Study Report030715.docx 2015 Description Issue to NYCC for comment Prepared by Checked by Approved by Luke Ballantyne, Name Various Nicole Rabier, Will McBain Rory Canavan Signature Revision Sep Filename Malton-Norton Flood Study Report250915.docx 2015 Description Revision following comments from NYCC and Multi-Agency Flood Group. Prepared by Checked by Approved by Luke Ballantyne Will McBain Name Various Signature Final Oct Filename Malton-Norton Flood Study Report291015.docx 2015 Description Minor edits requested by Kathy Stevenson Prepared by Checked by Approved by Name Various Nicole Rabier Will McBain Signature Issue Document Verification with Document | Revision | September 2015 \\GLOBAL\EUROPE\LEEDS\JOBS\230000\239474-00\0 ARUP\0-12 WATER\0-12-08 REPORTS\REPORT\REVISION A\MALTON-NORTON FLOOD STUDY REPORT250915.DOCX North Yorkshire County Council Malton, Norton and Old Malton Flood Study Final Report Contents Page Executive Summary iii 1 Introduction 1 1.1 Purpose of this report 1 1.2 Study Objectives 1 2 Background 2 2.1 History and wider context 2 2.2 Current approach to flood risk management 4 3 Problem definition and objectives 5 3.1 Outline of the problem 5 3.2 Consequences of walking away 5 3.3 Strategic issues 7 3.4 Key constraints 7 3.5 Project Objectives 8 4 Long list of options 9 5 Option shortlisting 14 5.1 Shortlisting process 14 5.2 Malton 14 5.3 Norton 15 5.4 Old Malton 17 6 Initial appraisal 19 6.1 Social and environmental impacts 19 6.2 Economic impacts, cost and benefits 20 7 Conclusions and recommendations 24 7.1 Conclusions 24 7.2 Next steps 24 Figures Figure 1 Malton Key Plan Figure 2 Norton Key Plan Figure 3 Old Malton Key Plan | Revision | September 2015 i \\GLOBAL\EUROPE\LEEDS\JOBS\230000\239474-00\0 ARUP\0-12 WATER\0-12-08 REPORTS\REPORT\REVISION A\MALTON-NORTON FLOOD STUDY REPORT250915.DOCX North Yorkshire County Council Malton, Norton and Old Malton Flood Study Final Report Appendices Appendix A Flood History – record of past events and flood mechanisms in Malton, Old Malton and Norton Appendix B Past Studies – summary of previous studies and models in the area Appendix C Hydrology – record of calculations for model hydrology Appendix D Hydrogeology – desk study and conceptual groundwater model Appendix E Joint probability – analysis of coincidence of high levels on Derwent and high flows in drainage systems Appendix F Hydraulic modelling – record of model build and resulting water levels under different options Appendix G Economics – coarse cost benefit analysis of options Appendix H Environmental – desk study of environmental, cultural and heritage features to inform options assessment Appendix I Shortlisting Process – including descriptions of the shortlisted options Appendix J Flood Maps – flood extents for do nothing and do something scenarios | Revision | September 2015 ii \\GLOBAL\EUROPE\LEEDS\JOBS\230000\239474-00\0 ARUP\0-12 WATER\0-12-08 REPORTS\REPORT\REVISION A\MALTON-NORTON FLOOD STUDY REPORT250915.DOCX North Yorkshire County Council Malton, Norton and Old Malton Flood Study Final Report Executive Summary Arup have been appointed by North Yorkshire County Council to identify an initial business case for options to prevent flood risk to the towns of Malton, Norton and Old Malton. This report summarises that work, identifying a range of potential options and their relative economic and technical merits. Options presented in this report do not represent a final decision to be implemented; rather the report identifies the likely front running options, as well as the work required to bring these to an implementation phase. Significant flooding occurred in Malton, Norton and Old Malton in November 2012. The areas most significantly affected were as follows: • Castlegate, Sheepfoot Hill and Railway Street, Malton; • Welham Road, Church Street and St Nicholas Street, Norton; • Old Malton Road and Town Street, Old Malton. The combination of existing defences and operational response ensured that the level of property flooding that occurred was relatively low – only 20 properties suffered internal flooding. However, the impact upon the community was significant. This report was commissioned by North Yorkshire Council (NYCC) and sets out an initial business case for investment in additional measures to reduce local flood risks in the above three locations. Flood risk in Malton, Norton and Old Malton is currently managed through operation and maintenance of: the River Derwent flood defences (including associated drainage outfall non-return valves, flood gates and land drainage pumping stations), highway/land drains and the combined sewer network, including sewerage pumping stations. Flood warning, emergency preparedness, planning and response measures are used to manage the residual risk (which is primarily that associated with surface water flooding). Should all these activities cease, the Net Present Value (NPV) cost of the flood damages that would occur over the next 100 years is estimated to be just under £30m if current flood management activities all ceased. The primary cause of the flooding problems experienced in 2012 is ‘flood- locking’, whereby gravity drainage systems cannot discharge to the river as a result of high river levels. Surface water flooding generally coincides with flows in the River Derwent that exceed 80m3/s, which corresponds broadly with the threshold at which gravity drainage becomes impeded. There have been seven occasions when a flow of greater than 80m3/s has occurred in the River Derwent, Malton since 2003, when the Main River flood defences were constructed. In 2012 this flow was exceeded for ten days and this required a major operation to over-pump the flood defences using temporary pumps. The related problems this causes can be summarised as follows: Whilst local surface water flooding may not affect as many properties as would flood from the River Derwent, sewer flooding from the overloaded combined sewer network makes it particularly unpleasant for the residents and businesses affected; Flood warnings in Malton are based on river levels and hence flood warning response to surface water and groundwater is reactive. Knowing when and | Revision | September 2015 iii \\GLOBAL\EUROPE\LEEDS\JOBS\230000\239474-00\0 ARUP\0-12 WATER\0-12-08 REPORTS\REPORT\REVISION A\MALTON-NORTON FLOOD STUDY REPORT250915.DOCX North Yorkshire County Council Malton, Norton and Old Malton Flood Study Final Report where temporary surface water pumps need to be deployed has to rely on anecdotal and eyewitness accounts; The residual risk of surface water flooding in Malton, Norton and Old Malton is potentially too high for the emergency response procedures developed by the Multi-Agency group to fully make sense as a long-term solution, if an economically viable investment now could save costs in the longer term; Relying on temporary pumping in emergencies is not an ideal arrangement because: o the pumps are not absolutely guaranteed to be available when required; o there are no formal ‘well’ points connected into the drainage systems in which to deploy them; o the arrangements still result in disruption to local residents and the local transport network. High groundwater levels cause infiltration into the combined sewer network, which reduces the network capacity for wastewater and stormwater, especially during flood-locked periods. In assessing potential options to reduce flood risk to businesses and communities in Malton, Norton and Old Malton, the appraisal has been guided by the following overriding objectives: To reduce flood risk in a manner which represents best value for money in the short, medium and long term; To adopt solutions that are socially and environmentally acceptable to local people and statutory authorities, which respect the heritage setting and avoid disruption to local residents and businesses where possible. Engineering judgement and experience, coarse economic baseline assessment and an understanding of the catchment and flood mechanisms from data and models were used to assess viable measures with regard to their technical and economic viability, their social/environmental impacts and their level of resilience. The shortlisted options for each of the three sites can be described generically as follows: Option 1: Under this purely theoretical scenario, all spending on activities and infrastructure to reduce flood risk would cease. This allows the benefits afforded by existing spending to be identified and is the baseline required for appraising schemes in line with national guidance; Option 2: Maintain existing levels of support; Option 3: Improve local flood warning procedures, construct permanent pumping chambers, in which to deploy the temporary pumps, and reduce the residual risk with property level protection measures. In Norton this option also involves optimising the operation of the existing Mill Beck pumping station and formalising a pipe-crossing of the railway line through which water can be pumped when necessary; Option 4: As above, but with wider rationalisation of the drainage systems. In Old Malton this would involve diverting Riggs Road drain.