Analysis of Surface Water Flood Risks Within the Cornwall Lead Local Flood Authority (LLFA) Area
Total Page：16
File Type：pdf, Size：1020Kb
Cornwall Council Preliminary Flood Risk Assessment ANNEX 6 – Analysis of Surface Water Risk June 2011 TABLE OF CONTENTS TABLE OF CONTENTS ..............................................................................................i LIST OF FIGURES......................................................................................................i LIST OF TABLES........................................................................................................i 1 INTRODUCTION............................................................................................... 1 2 ENVIRONMENT AGENCY METHODOLOGY ................................................... 2 3 CORNWALL COUNCIL METHODOLOGY ........................................................ 6 3.1 Grid-based approach ................................................................................. 6 3.2 Community-based approach .................................................................... 13 LIST OF FIGURES Figure A1 Five touching blue squares within 3x3 km grid.................................................... 3 Figure A2 Indicative flood risk areas for England................................................................. 3 Figure A3 Potential flood risk areas based on EA analysis.................................................. 4 Figure A4 Potential flood risk areas based on EA and Cornwall Council analyses ............. 5 Figure A5 Origins of the each of the grids used in the sensitivity analysis .......................... 7 Figure A6 Grid squares and clusters derived using Grid A and 2 critical services .............. 8 Figure A7 Grid squares and clusters derived using Grid A and 1 critical service ................ 8 Figure A8 Grid squares and clusters derived using Grid B and 2 critical services .............. 9 Figure A9 Grid squares and clusters derived using Grid B and 1 critical service ................ 9 Figure A10 Grid squares and clusters derived using Grid C and 2 critical services ............ 10 Figure A11 Grid squares and clusters derived using Grid C and 1 critical service .............. 10 Figure A12 Grid squares and clusters derived using Grid D and 2 critical services ............ 11 Figure A13 Grid squares and clusters derived using Grid D and 1 critical service .............. 11 Figure A14 Polygons incorporating clusters of properties potentially at flood risk............... 14 Figure A15 Results of parish-based analysis using the same procedure as applied to the community-based analysis ...................................................................... 22 LIST OF TABLES Table A1 Flood risk threshold used to identify future consequences of flooding................ 2 Table A2 The effect of offsetting the grid and varying the number of critical services...... 12 Table A3 Comparison of clusters identified using the offset grids .................................... 12 Table A4 Priority locations from grid-based analysis ........................................................ 12 Table A5 Top 30 ranked communities in terms of numbers of people at risk................... 15 Table A6 Top 30 communities in terms of the number of people at risk per km2 ............. 16 Table A7 Top 30 ranked communities in terms of numbers of businesses at risk............ 17 Table A8 Top 30 communities in terms of the number of businesses at risk per km2 ...... 18 Table A9 Top 30 communities in terms of the total number of properties at risk.............. 19 Table A10 Top 31 communities in terms of the number of critical services at risk ............. 20 Table A11 Top 50 communities ranked by total aggregate risk.......................................... 21 i 1 INTRODUCTION This annex to the Preliminary Flood Risk Assessment (PFRA) describes an analysis of surface water flood risks within the Cornwall Lead Local Flood Authority (LLFA) area. Section 2 reviews the national analysis conducted by the Environment Agency, using the methodology set by Defra to determine the extents of indicative flood risk areas in England and reflects upon the results for Cornwall. In Section 3 this analysis is applied specifically to Cornwall. Section 3.1 describes the implications of the grid-based approach and also includes a sensitivity analysis using this approach. In Section 3.2 an alternative community-based approach is used and the results are presented. In order to identify and prioritise potential local flood risk areas, the results of both approaches need to be taken into account and these quantitative assessments are used along with knowledge of known flood risks in Section 6 of the PFRA Report. 1 2 ENVIRONMENT AGENCY METHODOLOGY The EA have applied the methodology set out by Defra (2010) to identify indicative flood risk areas. This methodology involved the following procedures. The properties layer from within the EA National Receptors Database (NRD), combined with the OS MasterMap buildings attributed features, was overlaid by the Flood Map for Surface Water FMfSW 1 in 200 year return period event resulting in deep (>0.3m) flooding layer, to select those properties whose buildings are intercepted by this flood zone. The selected properties were assessed according to whether they are residential or not. The non-residential properties were then assessed as to whether they constitute critical services. The results were then overlaid by a 1 km2 grid and the number of people (at 2.34 people per residential property), number of businesses (non-residential properties) and number of critical services lying within each grid square were compiled. The flood risk thresholds listed in Table 15 of the PFRA Report were then applied. If a grid square contained more than this threshold (201 people; 21 business or 2 critical services) it was then flagged as a “blue square” – the “blue squares map”. For ease of reference Table 15 of the PFRA Report is reproduced as Table A1, below. Table A1 Flood risk threshold used to identify future consequences of flooding ‘Significant harmful consequences’ Scenario 1 defined as greater than… 200 people Flooded to a depth of 0.3m during a rainfall event 20 businesses with a 1 in 200 year chance of occurring (or 0.5% 1 critical service AEP) The EA then used the 1 km2 blue grid squares to identify clusters of grid squares whereby if 5 or more grid squares meeting the threshold criteria are present and touching within a 3 × 3 km square of 9 grid squares (a “5/9” approach) then this represents a cluster. This is illustrated in Figure A1. Any moving 3 × 3 km square meeting the “5/9” criterion creates an aggregated cluster. If an aggregated cluster using the “5/9” approach results in a total of 30,000 people at risk, then this identifies an indicative flood risk area. Using this methodology, 10 indicative flood risk areas were identified within England, Figure A2. 1 i.e 201 people (85 residential properties), 21 non-residential properties or 2 critical services. 2 Figure A1 Five touching blue squares within 3x3 km grid Figure A2 Indicative flood risk areas for England 3 The results of this analysis were provided to Cornwall Council in GIS format (EA 2010b). This consisted of the “blue squares map”, based on the FMfSW and criteria listed above, and also clusters derived from the “blue squares map”, as GIS layers. These are shown for Cornwall in Figure A3. It can be noted from Figure A3 that there is not an exact correlation between the “blue squares map” and the clusters, though the clusters are purportedly based on the blue squares. Figure 3 shows that there are 3 clusters identified in Cornwall using this approach: Penzance, Camborne & Redruth and St Austell, though there are other areas identified that do not quite meet the threshold and cluster criteria. Clusters based on the same analysis but using the Areas Susceptible to Surface Water Flooding (AStSWF) map were also provided by the EA (2100d) but did not yield any clusters within Cornwall. Figure A3 Potential flood risk areas based on EA analysis The GIS analysis used by the EA was repeated by Cornwall Council, the only notable difference being that the Council used the latest version of the NRD database (v.1.1), whereas it is understood that the EA had used an earlier version (v.1.0) though the difference is considered here to be insignificant. The result is shown in Figure A4, along with the EA clusters and the result can be seen to be slightly different, though broadly similar to the EA analysis shown in Figure A3. 4 Figure A4 Potential flood risk areas based on EA and Cornwall Council analyses 5 3 CORNWALL COUNCIL METHODOLOGY In order to fully understand and appreciate the implications of the EA/Defra approach to assessing surface water flood risk described in Section 2, a sensitivity analysis was conducted by Cornwall Council using the same grid-based approach. An alternative community-based approach was also undertaken using natural clusters rather than “5/9” clusters. 3.1 Grid-based approach Figure A6 shows the blue squares resulting from the Cornwall grid-based analysis using the EA/Defra criteria listed in Table A1, (the same blue squares as shown along with the EA derived clusters in Figure A4) along with the “5/9” clusters derived from it. Cornwall Council had two main concerns regarding this approach: 1. The requirement for two critical services to be within the threshold may be misrepresentative of the importance of those critical services: two nursing homes would outrank a hospital or two electricity sub-stations would outrank a school, for example. Whilst these issues