Here Is a Risk of Flooding

i ABSTRACT This research examines the potential contribution of ‘soft-engineered’ rural structures to flood-risk reduction, and seeks to persuade statutory authorities that such an approach is worthy of practical demonstration. Communities which flood are under particular threat both from weather extremes, and from the cumulative effect of past development. However, until the threat from flood damage translates into loss, construction of expensive 'hard' assets such as upland dams is unlikely to be resourced. Against this background, high friction floodplain features which attenuate flow and increase flood storage are of interest, particularly as, in lowland areas, riparian buffer strips are already promoted via government incentive and best practice guidance as environmentally beneficial. The research was carried out at the confluence of the Rivers Severn and Vyrnwy which lies on the Welsh/English national border in a productive alluvial valley dominated by flooding. A two dimensional model was constructed from both remotely sensed and survey data; and calibrated against hydrometric archives. Variable resolution point-clouds covering c.32 km2 were analysed by terrain feature and re-assembled in a modular condensed structure. This novel technique allowed not only land and flow surfaces to be discretized productively, but also model length, point-density and boundary conditions to be optimised for effective computation. Multiple experimental ‘broad-hedge-weirs’ (of dense, stiff vegetation) were introduced as features and their hydraulic performance was analysed in terms of both form and roughness. The effect of alternative weir configurations on attenuation storage volume was measured. The research both confirms the extent and form of flood-risk within the study site and demonstrates a sustainable technique of managing flood-waters which would otherwise exceed thresholds of loss. It shows that significant increases in reach scale storage (up to 1.3 million m3 in 5. 3 million m3) and flood-wave attenuation are gained with minimal lateral impact on the floodplain wet perimeter Keywords: Flooding, Exceedance, Attenuation, 2D modelling, Riparian, Sustainable, Biomass. i ACKNOWLEDGEMENTS It is a pleasure to thank those who have contributed to this research, but difficult to name everyone. Particular thanks are due to the academic staff of Aberystwyth University for their support and expert knowledge of river modelling techniques and of the Upper Severn catchment. The Environment Agency Midlands Flood Risk and data teams are thanked for their help in assembling the data, and for their authoritative contribution to liaison meetings. Members and landowners of the Powysland Internal Drainage Board (PIDB) are thanked for information regarding land drainage, for allowing access to the River Severn and for the mounting of benchmarks on their land. The modelling code (Version 0.95a January 15 2010, © Copyright 2006) was run courtesy River 2D authors P Steffler, A Ghanem, J. Blackburn and Z Yang, University of Alberta, Fisheries and Oceans Canada, and United States Geological Survey. The programme is in the public domain. The authors and the above organisations state that they assume no responsibility of liability for the use or applicability of the programme, nor are they obliged to offer technical support. John Swogger [email protected] provided artwork for Figure 15 from an original sketch. George Arthur Whitworth [email protected] July 2011 ii CONTENTS ABSTRACT............................................................................................................. i ACKNOWLEDGEMENTS ...................................................................................... ii CONTENTS............................................................................................................ ii LIST OF FIGURES ................................................................................................ iv LIST OF TABLES.................................................................................................. vi 1 INTRODUCTION ............................................................................................... 1 1.1 Background ..................................................................................................... 1 1.2 Research Aims, Objectives and Structure .................................................... 7 1.3 Literature Review ............................................................................................ 9 1.4 Complexity of Flood Management ............................................................... 18 1.5 Summary........................................................................................................ 22 2 STUDY SITE..................................................................................................... 23 2.1 The Upstream Catchment............................................................................. 24 2.2 The Study Site Floodplain ............................................................................ 26 2.3 Downstream Defence.................................................................................... 31 2.4 Data Resource ............................................................................................... 33 3 METHODOLOGY ............................................................................................ 35 3.1 Hydraulic Modelling ...................................................................................... 35 3.2 Terrain Data Development............................................................................ 39 3.3 CFD 2D Numerical Analysis ......................................................................... 48 3.4 The Modelled Domain ................................................................................... 52 3.5 Summary of Numerical Method.................................................................... 57 4. SOFTWARE FIELD TESTS ............................................................................ 58 4.1 Channel Calibration (Model 1): 100 cumecs Transient Flow ..................... 58 4.2 Overbank Storage (Model 2): 400 cumecs Steady-State Flow .................. 61 4.3 1998 Hydrograph (Model 3): ‘Transient’ flow at High Resolution ............. 71 5 DISCUSSION ................................................................................................... 73 5.1 Summary of Field Tests................................................................................ 73 5.2 Results ........................................................................................................... 74 5.3 Further Development .................................................................................... 76 6. CONCLUSIONS .............................................................................................. 78 Glossary .............................................................................................................. 81 Bibilography........................................................................................................ 83 APPENDIX 1 ............................................................................................................ iii LIST OF FIGURES Figure 1: Total precipitation from ‘heavy’ precipitation days............................................... 2 Figure 2: The number of days per winter on which there is a risk of flooding..................... 2 Figure 3: Adjacent urban doorstep levels built to different climate regulation. ................... 5 Figure 4: Typical floodplain hedgerow hydraulic feature ................................................... 6 Figure 5: Field Evidence of hydraulic headloss .................................................................. 7 Figure 6: Generic river regulation (courtesy Jones, JAA, 1997)....................................... 10 Figure 7: Yarm river management.................................................................................... 11 Figure 8: Flood-plain flow through vetiver hedges (courtesy Dalton, 1995) . ................... 12 Figure 9: Attenuation achieved by woodland plantations (courtesy Nisbet, 2006). .......... 13 Figure 10: Perspective view of hybrid hydraulic computation mesh of an urban site. ...... 14 Figure 11: Comparison of velocity patterns of different approaches to CFD flow............. 15 Figure 12: Comparative hydrographs of the treatment of the porosity of buildings. ......... 15 Figure 13: Indicative sketches of mass and momentum conservation principles............. 16 Figure 14: Traditional graphic of sediment equilibrium..................................................... 17 Figure 15: The threads and theme of the project. ............................................................ 18 Figure 16: The importance of the topic............................................................................. 22 Figure 17: Upper Severn south-west of the Vyrnwy confluence....................................... 23 Figure 18: Severn Catchment Relief and rainfall characteristics...................................... 24 Figure 19: Urban channel modifications at Newtown (courtesy Jones, 1997). ................ 25 Figure 20: Abermule–Caerhowel sub-reach..................................................................... 27 Figure 21: Caerhowel–Munlyn sub-reach......................................................................... 27 Figure 22: Munlyn–Buttington sub-reach ........................................................................

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