Cornwall Region

Truro Reservoir Inundation Modelling

SW877

Draft

October 2008

Environment Agency 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 ...... 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 , and Idless on the . The confluence of these two rivers is downstream of the City where the river is then called the 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

KE01 SW 79877 46193 River Kenwyn Upstream of New Mills FAS

Upstream of the Truro gauging KE02 SW 81944 45037 River Kenwyn station on the Kenwyn

KE03 SW 82776 44691 River Kenwyn Downstream of Lemon Quay

Confluence of the River HI01 SW 81284 45336 River Kenwyn Kenwyn and Highertown Stream

AL01 SW 81639 48195 River Allen Upstream of Idless FAS

AL02 SW82497 45208 River Allen Downstream of the Viaduct

AL03 SW 82799 44871 River Allen St Clement Street, car park

TR01 SW 82875 44843 River Allen Upstream of A390 road bridge

River Truro downstream of tide TG01 SW 83493 43216 River Truro gate

4 Truro SFRM Study – Final (SW859), August 2008, Capita Symonds.

3 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

2.2 Reservoir Inundation Model In the reservoir inundation model the dams at New Mills and Idless are represented in the same way in the Truro Parent Model (Figure 2.1). Flow upstream of the dams is represented in the 2D model domain. Flow through the dams is represented by a series of culverts, weirs and open channel sections. The dams are represented in the 2D domain using the LIDAR data. To ensure that the dam crests are raised to the correct level, z-lines are applied across the crests and the elevations specified. Downstream of the dam within bank flows are represented by 1D model elements and out of bank flow is modeled in the 2D domain. For the purpose of this reservoir inundation modeling, the geometry of the SFRM Parent Model has been modified to represent a rapid and total breach of the dams. The geometry of the undefended model is used to represent these breach scenarios, to lower the model topography down to the assumed natural ground level of these areas. At New Mills Dam the natural ground level is at approximately 20.1mAOD and at Idless approximately 16.8mAOD. Immediately downstream of the dams the 1D model network is removed from the model and the flow of water following the breach is represented purely in 2D. The bed slope of the two rivers in the 2D model has been defined using a z-line based on the surveyed cross-section bed levels. The capacity of the channels of the Rivers Kenwyn and Allen is limited compared to the volume of water released in the dam breaks and therefore this 2D representation is considered adequate. On the River Kenwyn the 1D river network is removed from section KE04346 to KE03296 inclusive, which is between New Mils Dam and just upstream of Bosvigo Lane. For this to occur, two structures are removed from the model. Three small culverts are removed at the ford just downstream of New Mills dam (Section KE04209c) and a small footbridge at Millbrook (Section KE03382b).

Figure 2.1 Representation of New Mills Dam in the Truro Parent Model

4 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

The 1D river network for the River Allen is removed from section AL03089 to AL01086 inclusive, which is between Idless Dam and the Viaduct at the outskirts of Truro. An arch bridge (Section AL02809b) is removed from the model. The removal of these three structures is considered to be an appropriate representation. If either of the dams were to fail, these structures are likely to be destroyed by the resulting flood wave and associated debris due to their size and locations.

2.3 Tidal Boundary In the Truro SFRM model, a mean high water spring tide was applied in the ISIS section of the model. For the purpose of this commission, there is not the requirement to represent the tide gate and therefore the ISIS component of the model was removed. Rather than applying a tide curve as the downstream boundary, a constant water level boundary of 0mAOD is applied. The purpose of this study is to model the inundation following the breach of the dams, and the joint probability of this coinciding with the tide is not considered. The constant water level of 0mAOD is thought to provide a realistic water level that does not cause flooding from the tide in Truro.

2.4 Breach approach The dams on both rivers are designed as flood alleviation reservoirs to store water during times of heavy rainfall and river flows. This implies that for the reservoirs to reach capacity, the flows in the downstream channels will be significant, and areas of natural flood storage may already contain water. Therefore it is not suitable to model the breach by applying an initial water level at the reservoirs without considering the likely conditions downstream when the reservoirs become full. The approach to be taken is to breach each reservoir independently rather than together. This will allow greater understanding of flow pathways and inundation extent from each reservoir without interaction between the two rivers. Each model scenario is comprised of two model runs. The first model is run to fill each reservoir to capacity. This model is then used as the basis for the breach scenario. In the second part of the scenario, the model geometry is changed by applying the undefended model topography at the dam being breached. A variable boundary condition is applied at the crest of the dam and this allows the rate at which the dam is breached to be specified. In this commission a rapid and total failure scenario is adopted. The dams are breached from the dam crest vertically down. The dam is breached across its entire length and width across the valley. It is assumed that the breach scenario is such that the top metre of the dam is removed instantaneously and then the remainder is lowered linearly over the subsequent five minutes. The crests of the dams are lowered by approximately 4.9m and 2.9m at New Mills and Idless respectively.

2.5 Hydrology For the two breach scenarios, there are three elements to the hydrology that are considered, i) the inflows to the reservoir before it is breached, ii) the continued inflows following the breach, and iii) the inflows to the reservoir not being breached. To fill the reservoir that is going to be breached, the 1% AEP flow is applied continuously to the flow node upstream of the dam. This approach is taken as it will fill the reservoirs quickly. In the SFRM commission it was found that the 1.33% AEP hydrograph leads to Idless overtopping and the 1% AEP hydrograph at New Mills does not fill the reservoir.

5 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

When the reservoir has reached capacity, the dam is breached and then the recessional limb of the 1% AEP hydrograph is applied (Figure 2.2). For the flow nodes on the river where the dam is breached the 1% AEP hydrographs are applied. For all other flow nodes, the 50% AEP hydrographs are applied. This means that the modelled inundation is solely from one reservoir rather than trying to understand and interpret the results from a combined breach scenario.

The 1% AEP peak flow is applied Following the model breach the at the upstream flow node until the recessional limb of the 1% AEP reservoir is full to capacity hydrograph is applied /s) 3 Flow (m Flow RESERVOIR BREACH

Time(Hours)

Figure 2.2 Breach model hydrology

2.6 Restart files The model to breach the reservoirs is run in two stages, i) to fill the reservoir to capacity, and ii) to breach the dam. The model to fill the reservoir is run until it reaches capacity. When the reservoir reaches capacity, TUFLOW writes restart files. The restart files store data on water depths and velocities for the time step when they were written. The restart files can then be used as the starting condition for breach model. When the reservoir is breached, it is assumed that the series of culverts, weirs and open channel sections that normally allow water to pass through the dam are destroyed. As a result, the water levels and velocities have to be removed for the 1D ESTRY restart file (.erf). This is done by editing the restart file in a text editor. The amended restart files are then applied as the initial conditions for the restart model which simulates the actual breach collapse.

6 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

3 Discussion of results

3.1 Introduction This chapter of the report presents the results of the reservoir inundation modelling. The results for the inundation of New Mills and Idless reservoirs will be discussed separately and will focus on flood depth, velocity and hazard. The speed of inundation will be discussed and the extent of the flooding outlined. Mapped model outputs are presented in Appendix A, and a summary of these outputs in given in Table 3.1. The model output has been reduced to a 1 minute interval for this commission due to the rapid inundation observed. Table 3.1 Summary of mapped outputs Drawing Number Description CS032116-RES-1 New Mills reservoir inundation – Flood depth CS032116-RES-2 New Mills reservoir inundation – Flood velocity CS032116-RES-3 New Mills reservoir inundation – Flood hazard CS032116-RES-4 New Mills reservoir inundation – Flood propagation CS032116-RES-5 Idless reservoir inundation – Flood depth CS032116-RES-6 Idless reservoir inundation – Flood velocity CS032116-RES-7 Idless reservoir inundation – Flood hazard CS032116-RES-8 Idless reservoir inundation – Flood propagation

The hazard caused by flood waters to people and buildings is a combination of the depth and velocity of the flood waters and the type of debris that is likely to be entrained by the flood waters. The equation used for calculation of the UK hazard rating is as follows5: HR = d (v + 0.5) + DF Where HR = Flood hazard rating; d = Depth of flooding (m); v = Velocity of flood waters (ms-1); and DF = Debris factor. In the present commission an Urban debris factor has been selected, and this reflects the type of debris that is likely to be transported by the flood waters, i.e. cars and trees. The Flood Hazard Rating should be interpreted in line with the guidance in Table 3.2.

5 Defra / Environment Agency (2006) – R&D Outputs: Flood Risks to People, Phase 2 (FD2321/TR1 & FD2321/TR2)

7 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

Table 3.2 Flood Hazard Rating5: Flood Hazard Rating Degree of Flood Hazard Description Caution: flood zone with shallow <0.75 Low flowing water or deep standing water Danger: flood zone with deep or fast 0.75 – 1.25 Moderate flowing water Danger: flood zone with deep fast 1.25 – 2.5 Significant flowing water Extreme Danger: flood zone with deep > 2.5 Extreme fast flowing water

3.2 New Mills The crest of the New Mills reservoir is at 25mAOD, this gives the reservoir a capacity of 197,250m3. The capacities of the dams were calculated using volume storage calculations function of the software package 12D. The rapid and total failure of New Mills dam results in severe and extensive flooding in Truro. The depths of flood inundation are greatest outside of Truro, particularly upstream of Bosvigo Lane. This area may have areas of natural floodplain storage. Along St George’s Road predicted flood depths of 2.2m occur. In Victoria Park model results show depths of 2m, and on Lemon Quay depths of approximately 1m. The extent of flooding in Truro is confined by steep valley sides. This stops the flood waters from spreading out and leads to significant depths of flooding. Following the dam breach significant velocities are modelled throughout the inundated areas. It is identified that the road network provides significant flow routes for water that is travelling out of bank, in particular along St George’s Road, Kenwyn Street and River Street. On Lemon Quay water is predicted to flow back into the River Kenwyn, but also across to the River Allen. The water flowing across to the River Allen uses Prince’s Street as a significant flow route and water flows into the River Allen opposite the St Clement Street car park and from Quay Street. The A390 road bridge that passes across the downstream end of Lemon Quay acts as a significant obstruction to flow. The road bridge acts as a barrier and allows water to collect behind it before it can drain into either the River Kenwyn or River Allen. This road acting as a barrier means that there is no out of bank flooding downstream of this location. Significant velocities, exceeding 1ms-1 are predicted throughout the study area. Upstream of Truro velocities are typically in the range of 3 – 5ms-1. The magnitudes of velocities reach 3.5ms-1 along St George’s Road, 2.5ms-1 along River Street and 3ms-1 along Kenwyn Street. As flood waters reach Lemon Quay, the velocities reduce significantly from approximately 3ms-1 to 0.5 ms-1. The high flood depths and velocities combined with urban debris lead to extreme hazard being predicted throughout the majority of the study area. The extreme flood hazard extends from New Mills to Lemon Quay. As flood waters enter Lemon Quay the flood waters can spread out and the hazard drops from extreme to significant. At the Kenwyn at Truro gauging station significant depths and flows are observed. The model results show that the maximum depth reaches 3.76m and a maximum flow of

8 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

262m3s-1. The maximum flow, which incorporates both within channel and out of bank flows, is significantly greater than the highest event in the annual maximum series recorded in January 1998 which has a magnitude of 30.49m3s-1. The propagation of the dam breach is extremely rapid. The modelled maximum flow is predicted in approximately 10 minutes at Bosvigo Lane, the edge of the urban extent of Truro, in approximately 21 minutes at Lemon Quay and 40 minutes downstream of the tide gate. The short time between dam failure and peak flow has serious implications for flood warning and evacuation procedures and this is discussed further in Chapter 4. The extent of the flooding can be used to understand how the breach of New Mills dam has an effect on essential infrastructure. Essential infrastructure is classified in Table D.2 of PPS256 as including essential transport infrastructure, including mass evacuation routes, and strategic utility infrastructure. The modelled flood extents show that the breach of New Mills dam is unlikely to affect key utility infrastructure. The key transportation infrastructure leading in and out of Truro including the railway station and the A39 and A390 roads are not affected. At a more localised level, St Georges Road, Kenwyn Street, Frances Street, River Street, Lemon Quay, Back Quay, Princes Street, Quay Street and Green Street are affected by the inundation and this should be considered when preparing any evacuation plans. Hospitals and Schools are classified as ‘more vulnerable’ by PPS25 and these are not affected by the reservoir inundation.

3.3 Idless The crest of Idless reservoir is at 19.67mAOD resulting in a reservoir capacity of 97,800m3. The capacity of Idless reservoir is significantly smaller than New Mills and therefore the resulting flood extent from breaching Idless reservoir is much smaller. This does not mean that the consequences of Idless dam breaching are less significant. Flood depths are variable throughout the area of inundation. The extent of flooding is confined due to the steep valley sides. Depths greater than 1m are predicted between Idless dam and the edge of the urban area of Truro. The model results show that water is collected and stored upstream of the viaduct with predicted depths of water between 1-2m. At the cathedral water is 1.5m and at the St Clement Street car park depths reach 1.15m. High velocities are predicted to occur throughout the area of inundation. Velocities greater than 2.5ms-1 are shown in the model results immediately downstream of Idless dam. Velocities of 2ms-1 are predicted upstream of the viaduct and water accelerates to 3ms-1 to the downstream side. At the cathedral model velocities reach 2ms-1 and across St Clement Street car park velocities are 1ms-1, and this is where much of the out of bank flow returns to the channel. Modelled flood hazard is classified as extreme for the entire distance between Idless dam and the council offices downstream of the viaduct. In the remainder of the city centre, flood hazard is classified as significant other than a small area around the cathedral. The propagation of flooding as a result of the rapid and total failure of Idless dam is extremely rapid. The model results show that the maximum flow is observed in approximately 27 minutes downstream of the viaduct, in 30 minutes at the cathedral and

6 Planning Policy Statement 25: Development and Flood Risk, December 2006, Communities and Local Government

9 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

49 minutes downstream of the tide gate. The onset of flooding is extremely rapid, and as with the failure of New Mills dam, this has serious implications for flood warning and evacuation of properties and areas at risk from inundation. Due to its smaller size, the extent of flooding is smaller for Idless dam then New Mills, but the flood extent still envelopes many properties, the cathedral and areas of the city centre. In terms of essential infrastructure the inundation from Idless dam is unlikely to effect the major transportation routes in and out of Truro, but localised flooding is predicted to affect St Clement Street, Oak Way and High Cross.

3.4 Model operation manual update As part of the Truro SFRM commission2, a model operation manual was produced for the Truro Parent Model. The model operation manual is a vital asset that can be used by the Environment Agency and their consultants when using and updating the Parent Model. The model manual describes the baseline model developed for the SFRM commission and details all of the files required to run the model. The model operation manual includes a schedule of base data that has been used to develop the model and including information on ownership of any datasets. The model manual also contains a shortened version of the modelling log. It is imperative to the future value and use of the Truro Parent Model that this modelling log is kept up-to-date to ensure any changes made to the base model are effectively communicated to and are available to all model users. The model operation manual for the Truro Parent Model has been updated as part of the commission and is presented in Appendix B. A description of the changes made to the Parent Model for the purposes of reservoir mapping are explained, based on information in Chapter 2 of this report. No further base datasets were used to create the reservoir inundation model. The modelling log used to develop the reservoir inundation model has been appended to the existing log in the model operation manual to ensure that all model information remains up-to-date and available to all model users. The key model builds for the reservoir inundation modelling are described in Table 3.3. Table 3.3 Reservoir inundation model Builds Model Build Model Name Description No. 261 TRU_RES_NM_Fill_261.tcf Model to fill New Mills reservoir to capacity 262 TRU_RES_NM_BCH_262.tcf Model of rapid and total breach of New Mills dam 263 TRU_RES_ID_Fill_263.tcf Model to fill Idless reservoir to capacity 264 TRU_RES_ID_BCH_264.tcf Model of rapid and total breach of Idless dam

10 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

4 Conclusions and recommendations

4.1 Introduction The previous chapters have discussed the approach taken to adapt the Truro Parent Model to allow reservoir inundation modelling to be undertaken, and have described the model results for the rapid and total failure of New Mills and Idless dams. This chapter focuses on the implications of the model results, reviews the objectives of this commission, outlines the scope for future studies and provides an overview of the key conclusions of the reservoir inundation modelling.

4.2 Implications of model results The model results have shown that the rapid and total failure of both New Mills and Idless dams results in extensive and high hazard flooding throughout the study area. High flood depths and velocities are predicted through Truro and this has serious implications. The modelling has shown that the onset of flooding is extremely rapid following the breach of either dam. Flood waters would reach the edge of the urban area of Truro within 10 minutes for New Mills dam and 27 minutes for Idless dam. The potential rapid inundation means that there could be limited or no time to provide a flood warning following a breach at either of these locations. Consequently, there may not be time to initiate and complete any evacuation plan. It should be noted that no engineering assessment of the likely failure mechanism has been carried out. A rapid and total failure of the dams is a worst case scenario and may not represent the reality of a structural failure. We recommend that the Panel Engineer reviews the findings of this report and if necessary revises the model representation of the dam failure. The depths and velocities of flood inundation are likely to have consequences for the structural integrity of buildings and other infrastructure. The speed of the flood wave propagation posses serious risk to life due to the limited or absence of time to be able to provide a flood warning and implement an evacuation plan. The implications of either New Mills or Idless dams failing when they are full to capacity have been highlighted. However, it should also be noted that the likelihood of this event occurring is relatively small. The breach of both New Mills and Idless dams can be considered in terms of the flood risk equation shown in Figure 4.1. The dams are flood alleviation structures and therefore only impound water during flood events and remain empty for the majority of the time. New Mills has a less than 0.5% AEP of reaching capacity and Idless overtops at the 1.33% AEP event. The probability of a dam being at capacity and a rapid and total failure occurring is low, but the consequences are severe. The inundation mapping has shown that large areas of the study area are enveloped by flood waters that have high depths, velocities and flood hazard, the onset of this flooding is also extremely rapid. The flooding has been shown to effect several roads within the centre of Truro as discussed in Sections 3.2 and 3.3 of this report and should be taken into account if evacuation plans are created. In the reservoir inundation modelling undertaken, an urban debris factor has been used. In the modelling undertaken no consideration is given to the transportation of debris resulting from the dam failure. It is likely that dam construction material would be transported downstream and this is likely to result in a greater flood hazard.

11 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

Figure 4.1 Flood risk equation

Flood risk = Probability x Consequence

Consequence = Hazard x Vulnerability

4.3 Truro Parent Model development The Truro Parent Model has been used and adapted in the present commission to undertake reservoir inundation modelling. One of the fundamental drivers for the Environment Agency adopting a Parent Model for Truro was to realise efficiencies across the range of Environment Agency functions. This reservoir inundation study is one of three studies that have made use of the Truro Parent Model to help realise this aim of the modelling strategy for Truro. The completion of these further modelling studies is shown in the Model Tree in Figure 4.2.

0.5m resolution Channel & Defence and As-build LIDAR structure cross- crest level drawings for (2008) sections survey flood alleviation (2008) (2008) schemes (2008)

Truro Parent Model Version 1 August 2008 (SW859)

Truro Surface Water Truro Reservoir Truro Blockage Model Inundation Model Assessment September 2008 September 2008 September 2008 (SW73) (SW877) (SW876)

Figure 4.2 Model Tree for the Truro Parent Model

12 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

4.4 Recommendations for further work The reservoir inundation modelling work undertaken in this commission precedes any guidance being issued by Defra on the requirements of inundation studies and therefore a relatively conservative approach has been taken by applying a rapid and total failure scenario. When the guidance on the requirements for these studies is issued by Defra, which is expected imminently, it is recommended that the approach adopted in this study is compared. Any outstanding actions following this review should be addressed due to the consequences of failure of either New Mills or Idless dams. One of the recommendations of the Pitt Review is for flood plans to be completed for all large raised reservoirs. The reservoir inundation modelling undertaken as part of this study has produced inundation maps for both dams in Truro which is one of the requirements of the flood plan. The other requirements of the flood plans are the definition of an on-site plan to provide guidelines on how to respond to a potential breach to reduce flood risk downstream, and to devise a communications plan that establishes how reservoir undertakers and emergency services should communicate with each other. These further requirements of flood plans should be implemented following this study. Through the development of the reservoir inundation modelling there have been lessons learned. Modelling a breach of a dam is likely to be simpler if the dam crest is aligned with the model grid. Representing the model breach solely in the 2D model domain is thought to be appropriate, and having 1D model elements to represent the channels through the urban areas is required to correctly define channel conveyance.

4.5 Review of objectives and conclusions The aim of this study was 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; • 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. The Truro Parent Model has been used to model a rapid and total failure of New Mills and Idless dams. The failure of each dam is comprised of two model runs, one to fill the reservoir to capacity and a second to model the failure of the dam. For New Mills this is model runs 261 and 262, and for Idless this is models 263 & 264. Mapping of model results has been provided in Appendix A of this report. Maps show the modelled flood depths, velocities and hazard for the rapid and total failure of New Mills and Idless dams. Mapping has also been produced to show the propagation of the peak flow caused by the failure of the dams. To fulfil the requirements of using the Truro Parent Model, the model operation manual has been updated as part of this commission to include details of how the model has been adapted to undertake reservoir inundation modelling and to keep the modelling log up-to-date for all model users. The reservoir inundation modelling for New Mills and Idless dams in Truro has shown that the rapid and total failure of these assets leads to high depths and velocities

13 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008 resulting in a flood hazard rating of extreme throughout the majority of the study area. The consequences of the failure occurring is extreme and has severe consequences for risk to life and for the structural integrity of buildings and other infrastructure.

14 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

Appendix A

15 Environment Agency Cornwall Truro Reservoir Inundation Modelling 06/10/2008

Appendix B

16