Understanding Defence Failures and Coastal Flood Events: a Case Study Approach
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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF ENGINEERING AND THE ENVIRONMENT Civil and Environmental Engineering Understanding Defence Failures and Coastal Flood Events: a Case Study Approach by Matthew Wadey Thesis submitted for the degree of Doctor of Philosophy October 2013 UNIVERSITY OF SOUTHAMPTON 1 2 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING AND THE ENVIRONMENT Civil and Environmental Engineering Doctor of Philosophy UNDERSTANDING DEFENCE FAILURES AND COASTAL FLOOD EVENTS: A CASE STUDY APPROACH By Matthew Wadey Extreme sea level events are a current global threat, whilst sea-level rise (SLR) and climate change over the 21st century will increase the frequency and severity of flooding in most coastal regions. Numerical model simulations can help to understand and predict coastal floods (e.g. flood mapping and forecasting) but in comparison to flood sources (waves and water levels) coastal flood pathways (defence failures and inundation) are presently less integrated within these models. This thesis develops and demonstrates a methodology to rapidly simulate and understand the consequences of coastal flood events, with an emphasis upon regions where the risks of flooding are not well understood and could change quickly with SLR. The Solent on the south coast of England is the case study, and is prone to frequent flooding. This region is currently differentiated from the UK east and west coasts by experiencing smaller storm surges, and is characterised by undefended sections of shoreline and small floodplains. Within the Solent is Portsmouth, a city of national flood significance (only London and Hull contain more people considered at risk of coastal flooding in the UK). However, life threatening floods have not occurred in living memory. An integrated modelling approach is developed, coupling loads and defence failures with two-dimensional simulations of floodplain inundation. Observations collated from a real storm surge and flood event are shown to generate a validation data set, which indicates that this model can predict floodplain water levels to a good level of accuracy, whilst highlighting implications of such data collection. Solent-wide analysis includes simulations of hypothetical coastal flood events based upon scenarios that cover the full range of coastal loadings (realistic waves and water levels) and defence failures (overflow, outflanking, overtopping and breaching). More detailed case-studies are also applied at two sites within the region (including Portsmouth). This analysis generates peak flood water depths and an overview of impacts across this spectrum of possible floods. This research improves the existing knowledge of coastal flooding in the case study, and highlights a number of generic concepts that should be applied to others. For example the combination of flood simulation methods with real flood event analysis is essential for optimising the interpretation of model outputs whilst supporting inferences about flood consequences associated with extreme loading events (including how these may change with SLR). Simple methods estimated that >24,000 properties are within a 1 in 200 year flood event outline; and incorporating defence failures, flood dynamics, validation and detailed case studies substantially refine the assessment of places likely to experience damages. Breach defence failures generate the worst flood impacts, although in the Solent this failure mechanism is presently less of a threat than outflanking, overflow and wave overtopping. The modelling system includes easily interpreted outputs, whilst being computationally fast; therefore with potential applications including supporting land-use and defence planning, and real-time flood forecasting and warning. 3 4 Contents LIST OF FIGURES 7 LIST OF TABLES 8 DECLARATION OF AUTHORSHIP 10 ACKNOWLEDGEMENTS 11 ABBREVIATIONS AND DEFINITIONS 12 DEFINITIONS/GLOSSARY 13 1. INTRODUCTION 18 1.1 Coastal flood events 18 1.2 Role, timeliness and relevance of this research 19 1.3 Aims and objectives 20 1.4 The Solent case-study 21 1.5 Thesis structure 23 2. LITERATURE REVIEW 25 2.1 Coastal flood sources 25 2.2 Coastal flood pathways 34 2.3 Receptors and consequences 46 2.4 Understanding and forecasting coastal flooding 52 2.5 Changes to coastal flood risk 59 2.6 Summary 62 3. COASTAL FLOODING CASE STUDY: THE SOLENT 65 3.1 Regional overview 65 3.2 Description of the detailed case study sites 70 3.3 Summary 75 4.1. Overview of the hydraulic flood simulations 77 4.2. Data sources and integration 78 4.3. Integrating loads, defence failures and inundation modelling 87 4.4. Numerical simulations of floodplain flow 99 4.5. Surface features and receptors 100 4.6. Scenarios for coastal flood simulations 101 4.7. Summary 105 5. MODEL VALIDATION & UNDERSTANDING COASTAL FLOOD EVENTS 109 5.1. The 10 March 2008 Storm Surge 111 5.2. The 14 and 17 December 1989 West Solent floods 120 5.3. Historic water levels & flood events in the Solent 124 5.4. Summary 126 6. COASTAL FLOOD SIMULATIONS: HYPOTHETICAL EVENTS IN THE SOLENT 131 6.1. Solent-wide coastal flood simulations 132 6.2. Detailed case study 1: Portsmouth 138 6.3. Detailed case study 2: the Pennington flood compartment 150 5 6.4. Summary 154 7. DISCUSSION 161 7.1. Overview of the methodological position 161 7.2. Review of flood simulation methods 165 7.3. Historic floods, data sets and validation 167 7.4. Detailed analysis and higher resolution studies: findings and applications 171 7.5. Coastal flood events in the Solent 172 8. CONCLUSIONS 181 8.1. Achievement of aims and objectives 181 8.2. Recommendations for further research 186 7. REFERENCES 191 APPENDICES 210 Appendix A – Additional information of case study datasets 210 Appendix B - Exposure to coastal flooding in the Solent 212 Appendix C - Breach analysis - Fragility curves 214 Appendix D – Evaluation of the 10 March flood event photo data set 222 Appendix E – Wave Overtopping 234 6 List of Figures Figure 1.1 Position of this research within the well-known ‘source-pathway-receptor’ model ............. 19 Figure 1.2 Coastal floodplain and sub-regions used to describe flood modelling. ................................. 22 Figure 2.1 Coastal sea level effects caused by tides, storm surge and wave processes .......................... 26 Figure 2.2 The global distribution of tropical cyclones .......................................................................... 27 Figure 2.3 (a) UK Environment Agency regional divisions and ‘Class A’ tide gauges, the Solent case study area is highlighted; (b) example visual output of a numerical storm surge simulation ................. 29 Figure 2.4 Wave transformation at the coast .......................................................................................... 31 Figure 2.5 Sea level analysis using the skew surge joint probability method ........................................ 33 Figure 2.6 Sea wall failure modes .......................................................................................................... 35 Figure 2.7 A breach in the defences in the southwest Netherlands, 1953 .............................................. 38 Figure 2.8 A typical breach outflow hydrograph.................................................................................... 40 Figure 2.9 Example of a sea defence’s fragility ..................................................................................... 41 Figure 2.10 Inundation map example (location: Towyn, Wales) ........................................................... 44 Figure 2.11 Coastal flood mortality based upon large-scale events ....................................................... 48 Figure 2.12 The main components of a flood risk assessment methodology ......................................... 54 Figure 2.13 Example of UK Depth-Damage-Duration Data for Residential Properties (sector mean) . 55 Figure 2.14 Example of spatial distribution of EAD in the Thames Estuary ......................................... 55 Figure 2.15 Stages within the flood forecasting and warning process ................................................... 57 Figure 3.1 The Solent case study region. ................................................................................................ 66 Figure 3.2 The city of Portsmouth; (a) key locations; (b) floodplain topography. ................................. 67 Figure 3.3 Sea level time-series recorded at three locations across the Solent during a spring tide ...... 68 Figure 3.4 Flooding in Old Portsmouth on the 16th December 1989 ...................................................... 72 Figure 3.5 The Pennington detailed case study site, showing defence