1 First Chapter

Cornwall Region Truro Reservoir Inundation Modelling SW877 Draft October 2008 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008 Issue Box Issue Date Version Status Revisions Originated Checked Approved By By By 07/10/08 1 Draft - DB KP GB Capita Symonds Ltd Quays Office Park Conference Avenue Portishead Bristol BS20 7LZ Tel: 01275 840 840 Fax: 01275 840 830 Project Manager: George Baker Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008 CONTENTS 1 INTRODUCTION.....................................................................1 1.1 PROJECT BACKGROUND............................................................................1 1.2 STUDY OBJECTIVES ..................................................................................2 1.3 REPORT STRUCTURE ................................................................................2 2 METHODOLOGY....................................................................3 2.1 TRURO PARENT MODEL ............................................................................3 2.2 RESERVOIR INUNDATION MODEL ................................................................4 2.3 TIDAL BOUNDARY .....................................................................................5 2.4 BREACH APPROACH..................................................................................5 2.5 HYDROLOGY.............................................................................................5 2.6 RESTART FILES.........................................................................................6 3 DISCUSSION OF RESULTS .....................................................7 3.1 INTRODUCTION..........................................................................................7 3.2 NEW MILLS ..............................................................................................8 3.3 IDLESS .....................................................................................................9 3.4 MODEL OPERATION MANUAL UPDATE........................................................10 4 CONCLUSIONS AND RECOMMENDATIONS.............................11 4.1 INTRODUCTION........................................................................................11 4.2 IMPLICATIONS OF MODEL RESULTS ...........................................................11 4.3 TRURO PARENT MODEL DEVELOPMENT ....................................................12 4.4 RECOMMENDATIONS FOR FURTHER WORK.................................................13 4.5 REVIEW OF OBJECTIVES AND CONCLUSIONS..............................................13 APPENDIX A .........................................................................15 APPENDIX B .........................................................................16 i Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008 1 Introduction 1.1 Project Background The Pitt Review1 highlights the requirements for greater openness about flood risk, particularly in relation to dam and reservoir safety. The government is working towards introducing reservoir plans for Large Raised Reservoirs (Reservoirs with a capacity greater than 25,000m3) to be provided by reservoir undertakers. The Pitt Review identifies that there are three elements to a flood plan, and the requirements and methodology for these are currently under development by Defra: • On-site plan – To specify the response to a potential breach of a reservoir to reduce the risk or extent of any failure; • Inundation map – A map showing areas that would be inundated by a reservoir breach; and • Communications plan – A plan of how the reservoir undertaker and local emergency services should communicate with each other. The ‘Truro Reservoir Inundation Modelling’ study (SW877) has been commissioned by the Cornwall Area of the Environment Agency. The scope of the commission was to model a rapid and total breach of the reservoirs at New Mills and Idless using the Truro Parent Model, and map the areas of inundation. This work follows on from recommendations in the Pitt Review, but proceeds guidance from Defra on the methodology for inundation mapping, and therefore a full description of the method used will be provided within the report. The Truro Parent Model was developed for the Cornwall area Environment Agency as part of the Strategic Flood Risk Management (SFRM) commission for Truro2. The purpose of the Parent Model is to realise efficiencies for the Environment Agency across their range of functions as the model can be used and developed to inform further studies. The Parent Model was adopted by the Environment Agency for Truro as a ‘live’ model that will require routine review and updating. The scoping report for Truro, Par and Plympton3 set out a more strategic approach to hydraulic modelling of areas at significant risk of flooding. The model management strategy for Truro aims to deliver the following improvements and benefits: • Better management and understanding of model datasets and outputs; • Improved distribution, knowledge and access to models, modelling programmes and their outputs; • The adoption of a phased risk-based approach to delivering model deliverables; • Development of models as ‘live’ datasets, which require routine review and updating; • Better co-ordination on model construction and reuse within the different FRM functions; • Improved technical knowledge of Agency staff; 1 The Pitt Review – Learning lessons from the 2007 floods, June 2008. 2 Truro SFRM Study – Final (SW859), August 2008, Capita Symonds. 3 SFRM Scoping Report for Truro, Par and Plympton – Final, March 2008, Capita Symonds. 1 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008 • Better supply of best available information to third parties; and • Improved auditability, repeatability and accountability of models and their outputs. This present commission is one of three studies undertaken using the Truro Parent model. The parallel commissions are as follows: • Surface water modelling (SW873) – Prepare a broad scale model of surface water flooding in the urban area of Truro using the Parent Model for a range of scenarios; and • Blockage assessment (SW876) – Investigate the impact of asset blockage or failure on flood risk by modelling a range of blockage scenarios and mapping outputs of the model results. 1.2 Study Objectives The aim of this study is to undertake reservoir inundation modelling for Truro using the Parent Model. The key objectives of this commission are listed below: • Use the Parent Model for Truro to model a rapid and total failure of New Mills and Idless Reservoirs; • Clearly describe the modelling methodology adopted for reservoir inundation, as this project precedes any guidance being issued; • Produce a full set of mapped outputs suitable for development control and emergency planning; and • Update the Model Operation Manual for the Truro Parent Model. 1.3 Report Structure This report describes the modelling methodology and the inundation mapping produced for the representation of failure at New Mills and Idless reservoirs. The report structure is summarised below: • Chapter 1: Introduction – This chapter, outlines the background to the commission, in particular the Parent Model approach adopted for Truro, and the aims and objectives of the study are stated; • Chapter 2: Methodology – The approach taken to adapt the Truro Parent Model for the purpose of reservoir inundation modelling is described. The approach to filling the reservoirs and then creating a rapid and total breach is fully detailed; • Chapter 3: Discussion of results – The results of the reservoir inundation are described and discussed, with a focus on flood depth, velocity and hazard. The propagation of the flood wave from the breach is described; and • Chapter 4: Conclusions and recommendations – The findings of the commission are reviewed against the objectives set out for the study. The major conclusions are summarised and the recommendations for future work are outlined. 2 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008 2 Methodology 2.1 Truro Parent Model The Truro Parent Model covers an area that extends from upstream of the reservoirs at New Mills on the River Kenwyn, and Idless on the River Allen. The confluence of these two rivers is downstream of the City where the river is then called the Truro River as it passes through the tidal basin and into the estuary. The downstream extent of the model is to the downstream side of the tidal gates. The Parent Model for Truro has been developed as a 1D/2D ISIS-TUFLOW model. The 1D model elements define within bank flows and out of bank flow is modelled in the 2D domain. The 2D domain has a model grid size of 5m. The tide gate is modelled in ISIS as this allows logical control rules to be applied. The ISIS model elements extend from the downstream boundary to just upstream of the tide gate, where there is a dynamic link to the TUFLOW 2D model domain. The Truro Parent Model has been run for a range of design events both defended and undefended as part of the original SFRM commission. In the SFRM commission, FEH hydrological methods have been used to derive model inflows and a MHWS is applied at the tide boundary. The locations of these inflows and boundaries are summarised in Table 2.1. A full and detailed description of the Truro Parent Model can be found in the SFRM report4 and the Model Operation Manual. Table 2.1 Inflow and boundary locations Node Grid Reference Watercourse Location

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