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 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 ...... 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 km 2 ...... 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 km 2 ...... 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

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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.

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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 km 2 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 defined as greater than…1 200 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 km 2 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.

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Figure A1 Five touching blue squares within 3x3 km grid

Figure A2 Indicative flood risk areas for England

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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: , & and , 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.

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Figure A4 Potential flood risk areas based on EA and Cornwall Council analyses

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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 can be followed up on a case-by-case basis, the standard procedure would not pick up a grid square containing a single, but vital, critical service. For this reason the grid-based analysis was rerun using one critical service as the threshold criterion. 2. The grid-based approach contains an arbitrary reference. The geographical location of each grid square depends upon the grid origin, which is set by the Ordnance Survey grid system. If the grid approach were applied to a homogeneous dataset then this might not affect the results. Communities across Cornwall, however, are not homogeneously distributed and a single community may be split across two or more 1 km 2 grid squares.

For these reasons a sensitivity analysis was conducted using the grid-based and “5/9” cluster approach. The analysis was run with four different grid systems. This is illustrated in Figure A5. Grid A represents the original grid based on the Ordnance Survey grid origin. For Grid B the origin is shifted 500 m in the east-west plane. For Grid C the origin is offset by 500 m in the north-south plane and for Grid D it is shifted 500 m in both directions. It should be noted that each grid is equally valid – no grid can be classified as being of greater accuracy or importance than any of the other grids, and so the results of the sensitivity analyses are also equally valid.

For each of the four grid systems the analyses were conducted using two critical services as the threshold and again using one critical service. Thus eight sets of runs were conducted to provide the sensitivity analysis. The results are shown in Figures A6 to A13. These show the resulting blue squares and also highlight the clusters meeting the “5/9” specification.

Table A2 summarises the results of the grid-based sensitivity analysis. For each of the 8 scenarios modelled, Table A2 list the number of grid cells (“blue squares”) triggered due to each of (i) number of people, (ii) number of non-residential properties and (iii) number of critical services, the total triggered by all three criteria and also the number of “5/9” clusters identified.

Tables A3 and A4 provide a scoring system for ranking the clusters identified through the offset grid approach. From the 8 analyses (4 grids and 2 critical services criteria) the number of times that a “5/9” cluster is identified is counted. 15 locations are identified by this approach, ranging from appearing on all 8 analyses (Camborne and Penzance) to appearing on only one of the analyses ( and ). The locations of the grid clusters are ranked in order in Table A4.

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(0, 1000)

Grid A (500, 1000)

(0, 0) (1000, 0) Grid B

(0,1500) (500, 0) (1500, 0)

Grid C (500, 1500)

Grid D (0, 500) (1000, 500)

(500, 500) (1500, 500)

Figure A5 Origins of the each of the grids used in the sensitivity analysis

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Figure A6 Grid squares and clusters derived using Grid A and 2 critical services

Figure A7 Grid squares and clusters derived using Grid A and 1 critical service

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Figure A8 Grid squares and clusters derived using Grid B and 2 critical services

Figure A9 Grid squares and clusters derived using Grid B and 1 critical service

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Figure A10 Grid squares and clusters derived using Grid C and 2 critical services

Figure A11 Grid squares and clusters derived using Grid C and 1 critical service

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Figure A12 Grid squares and clusters derived using Grid D and 2 critical services

Figure A13 Grid squares and clusters derived using Grid D and 1 critical service

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Table A2 The effect of offsetting the grid and varying the number of critical services Grid A B C D X Offset (m) 0 500 0 500 Y Offset (m) 0 0 500 500 Criteria No of 1 km 2 grid cells Residential ≥ 200 people 95 55 50 48 Non-residential ≥ 20 properties 97 85 90 241 Critical services ≥ 1 161 157 162 157 Critical services ≥ 2 28 37 29 33 All criteria (CS ≥ 1) 236 199 213 320 All criteria (CS ≥ 2) 143 107 111 249 No of clusters ≥ 5/9 (CS ≥ 1) 8 6 9 13 No of clusters ≥ 5/9 (CS ≥ 2) 7 4 3 12

Table A3 Comparison of clusters identified using the offset grids No of grid cluster identifications Cluster Score Critical services ≥ 1 Critical services ≥ 2 4 2 6 4 1 5 Camborne 4 4 8 1 1 2 Helston 1 0 1 2 1 3 1 1 2 1 1 2 Par 4 3 7 Penryn 1 1 2 Penzance 4 4 8 Redruth 2 2 4 St Austell 4 3 7 2 1 3 Wadebridge 1 0 1

Table A4 Priority locations from grid-based analysis Location Level Score Camborne, Penzance 1 8 Par, St Austell 2 7 Bodmin 3 6 Bude 4 5 Redruth 5 4 Liskeard, Truro 6 3 Hayle, Looe, Newquay, Penryn 7 2 Helston, Wadebridge 8 1

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3.2 Community-based approach The analysis described in Section 3.1 is entirely consistent with the EA/Defra approach outlined in the official guidance (Defra 2010, EA 2010 a, b, c, d) but goes a little further by also providing a sensitivity analysis on the grid origin location and number of critical services threshold.

However, local needs may be better served by using a community-based and natural cluster approach. This approach was felt to be more robust than the grid-based approach as it is not dependent on an arbitrary reference point.

The properties located within potential flood zones were mapped for the whole of Cornwall. This was done for all sources of flooding: surface water, fluvial and tidal. The map was examined and natural clusters of properties at risk were identified. For all those clusters containing at least 10 properties, a polygon was drawn to encompass those properties. These polygons were drawn to broadly cover the clusters, without excessively following urban extents or flood zone boundaries.

This approach led to the identification of 186 communities in Cornwall where flooding from surface water, fluvial or tidal sources presents a potential risk to at least 10 properties. These polygons are shown in Figure A14. Of these 186 communities with risks of flooding from all sources, 6 did not contain properties with surface water flood risks (FMfSW, 1 in 200 year, deep water scenario), so 180 communities did have surface water risks.

Summaries of the results are provided in Tables A5 to A11

Table A5 lists the top 30 communities ranked in terms of the numbers of people at surface water flood risk.

In order to provide a density-based approach that can be equated with the 1 km 2 grid-based approach the number of people potentially at flood risk within each community was divided by the area of the polygon representing that community. Table A6 lists the top 30 ranked communities using this approach. This highlights some of the smaller communities.

A similar approach can be taken with non-residential properties. The results of this are provided in Table A7 in terms of the absolute numbers of non-residential properties and in Table A8 in terms of the densities of non-residential properties potentially at surface water flood risk. Table A9 lists the 30 top ranked communities in terms of the total number of properties at risk (residential + non-residential).

Table A10 provides a similar breakdown in terms of critical services and the type of critical service involved. 31 communities are listed in Table A10 as this is the number of communities identified as having at least 2 critical services within the locally agreed surface water flood risk zone. At this stage no attempt has been made to assess the individual critical services, but this analysis will allow that step to be made once the PFRA work is taken to the stage of developing local strategies and local Flood Risk Management Plans.

In order to provide an overall assessment of risk an aggregate scoring system was developed based on people, businesses and critical services.

Score = No of people + (10 × no of non-residential properties) + (100 × no of critical services)

This weighting was chosen to reflect the weightings embedded within the EA/Defra criteria listed in Table A1 (200:20:2). The top 50 ranked communities resulting from this process are listed in Table A11.

A similar analysis to that described above was applied to 213 parishes across Cornwall. The results are not listed here in detail but Figure A15 presents the results of this analysis geographically.

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Figure A14 Polygons incorporating clusters of properties potentially at flood risk

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Table A5 Top 30 ranked communities in terms of numbers of people at risk Number of Rank Community residential No of people properties 1 Redruth 887 2076 2 Camborne 879 2057 3 Bodmin 741 1734 4 Truro 656 1535 5 Penzance 637 1491 6 St Austell 621 1453 7 Liskeard 444 1039 8 Hayle 428 1002 9 Helston 403 943 10 St Ives 335 784 11 Launceston 309 723 12 300 702 13 268 627 14 Newquay 262 613 15 244 571 16 Penryn 240 562 17 Bude 229 536 18 221 517 19 Flexbury 215 503 20 Wadebridge 215 503 21 209 489 22 Polperro 209 489 23 Mevagissey 199 466 24 Looe (West) 190 445 25 Falmouth 171 400 26 Portreath 171 400 27 Mousehole 155 363 28 155 363 29 Pool & Tolvaddon 153 358 30 Millbrook 143 335

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Table A6 Top 30 communities in terms of the number of people at risk per km 2 2 No of people Rank Community Area (km ) No of people 2 per km 1 0.2 119 526 2 Mousehole 0.8 363 475 3 Chyandour 0.8 314 416 4 Flushing 0.7 253 360 5 Flexbury 1.4 503 352 6 Camborne 6.6 2057 311 7 Penzance 4.8 1491 309 8 Cosawes Wood (nr Ponsanooth) 0.3 94 306 9 Perranporth 2.3 702 303 10 Millendreath 0.2 66 293 11 St Just 1.1 302 282 12 Portreath 1.4 400 279 13 Looe (West) 1.6 445 271 14 Wheal Rose 0.5 136 263 15 Mevagissey 1.8 466 262 16 St Ives 3.2 784 243 17 Padstow 2.1 517 242 18 Brea 0.8 185 229 19 Helston 4.5 943 211 20 St Blazey 3.0 627 209 21 Redruth 10.6 2076 196 22 Troon 1.0 183 176 23 Looe (East) 1.4 253 175 24 Callington 3.5 571 164 25 St Austell 8.9 1453 164 26 Liskeard 6.6 1039 158 27 Rosecraddoc (Holiday village) 1.2 194 157 28 Chacewater 1.5 239 155 29 Penryn 3.6 562 154 30 Calenick 0.3 42 154

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Table A7 Top 30 ranked communities in terms of numbers of businesses at risk Number of non- Rank Community residential properties 1 Truro 476 2 Bodmin 289 3 Redruth 257 4 St Austell 205 5 Liskeard 193 6 Camborne 189 7 Penzance 186 8 Hayle 147 9 St Ives 135 10 Newquay 128 11 Polperro 125 12 Callington 123 13 Wadebridge 115 14 Launceston 107 15 Penryn 106 16 Lostwithiel 94 17 Perranporth 89 18 Par & Tywardreath 89 19 Bude 83 20 Helston 75 21 Falmouth 74 22 St Blazey 73 23 Penjerrick 73 24 Mevagissey 62 25 St Agnes 62 26 60 27 56 28 Looe (East) 52 29 46 30 Looe (West) 45

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Table A8 Top 30 communities in terms of the number of businesses at risk per km 2 No of non- No of non- 2 residential Rank Community Area (km ) residential properties properties 2 per km 1 Penjerrick 0.7 119 103 2 Millendreath 0.2 363 58 3 Cawsand 0.2 314 57 4 Mousehole 0.8 253 56 5 Mullion Cove 0.7 503 56 6 Par Moor 0.4 2057 42 7 St Ives 3.2 1491 42 8 Truro 12.1 94 39 9 Penzance 4.8 702 39 10 Perranporth 2.3 66 38 11 Looe (East) 1.4 302 36 12 Callington 3.5 400 35 13 Mevagissey 1.8 445 35 14 Kingsand 0.4 136 35 15 Wheal Rose 0.5 466 35 16 Bridge 0.8 784 34 17 St Just 1.1 517 31 18 Liskeard 6.6 185 29 19 Chacewater 1.5 943 29 20 Penryn 3.6 627 29 21 Nancherrow 0.4 2076 29 22 Camborne 6.6 183 29 23 1.6 253 28 24 Polperro 4.5 571 28 25 Looe (West) 1.6 1453 27 26 Roserrow 0.4 1039 27 27 Charlestown 0.4 194 27 28 Long Rock 1.1 239 26 29 St Teath 1.4 562 26 30 Bodmin 11.4 42 25

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Table A9 Top 30 communities in terms of the total number of properties at risk No of No of non- Total no of Rank Community residential residential properties properties properties 1 Redruth 887 257 1144 2 Truro 656 476 1132 3 Camborne 879 189 1068 4 Bodmin 741 289 1030 5 St Austell 621 205 826 6 Penzance 637 186 823 7 Liskeard 444 193 637 8 Hayle 428 147 575 9 Helston 403 75 478 10 St Ives 335 135 470 11 Launceston 309 107 416 12 Newquay 262 128 390 13 Perranporth 300 89 389 14 Callington 244 123 367 15 Penryn 240 106 346 16 St Blazey 268 73 341 17 Polperro 209 125 334 18 Wadebridge 215 115 330 19 Bude 229 83 312 20 Lostwithiel 209 94 303 21 Mevagissey 199 62 261 22 Padstow 221 34 255 23 Falmouth 171 74 245 24 Looe (West) 190 45 235 25 Flexbury 215 17 232 26 Par & Tywardreath 134 89 223 27 Portreath 171 36 207 28 Mousehole 155 43 198 29 Pool & Tolvaddon 153 42 195 30 Porthleven 155 35 190

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Table A10 Top 31 communities in terms of the number of critical services at risk No of critical Breakdown of critical Rank Community services services 1 Truro 17 Sch=8, NH=2, H=3, E=4 2 Bodmin 12 Sch=5, FA=1, H=1, E=4 3 St Austell 9 Sch=5, E=4 4 Penzance 6 Sch=4, NH=1, STW=1 5 St Ives 5 Sch=2, NH=1, P=1, E=1 6 Newquay 5 Sch=2, P=1, STW=2 7 Looe (East) 5 Sch=3, FA=1, E=1 8 Camborne 4 Sch=2, NH=1, FA=1 9 Wadebridge 4 Sch=1, STW=1, E=2 10 Par & Tywardreath 4 STW=2, E=2 11 Redruth 3 Sch=2, E=1 12 Liskeard 3 NH=1, H=1, E=1 13 Helston 3 Sch=2, H=1 14 Bude 3 Sch=1, E=2 15 Portreath 3 Sch=1, NH=1, STW=1 16 Porthleven 3 Sch=2, E=1 17 Camelford 3 Sch=1, STW=2 18 Launceston 2 Sch=1, NH=1 19 Perranporth 2 FA=1, E=1 20 Callington 2 Sch=2 21 Penryn 2 Sch=1, E=1 22 St Blazey 2 P=1, E=1 23 Looe (West) 2 FA=1, E=1 24 Pool & Tolvaddon 2 E=2 25 Chyandour 2 Sch=1, E=1 26 Porthtowan 2 STW=2 27 Port Isaac 2 Sch=2 28 Troon 2 Sch=1, E=1 29 Mullion Cove 2 STW=2 30 Ladock 2 STW=2 31 Rilla Mill 2 STW=2

Key Sch = School NH = Nursing home H = Hospital P = Police station FA = Fire or ambulance station E = Electricity installations STW = Sewage treatment works

• 31 communities ≥ 2 critical services • the next 33 entries have 1 critical service • and the remaining 122 entries have zero.

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Table A11 Top 50 communities ranked by total aggregate risk No of non- No of No of Rank Community residential critical Score people properties services 1 Truro 1535 476 17 7995 2 Bodmin 1734 289 12 5824 3 Redruth 2076 257 3 4946 4 St Austell 1453 205 9 4403 5 Camborne 2057 189 4 4347 6 Penzance 1491 186 6 3951 7 Liskeard 1039 193 3 3269 8 St Ives 784 135 5 2634 9 Hayle 1002 147 1 2572 10 Newquay 613 128 5 2393 11 Wadebridge 503 115 4 2053 12 Callington 571 123 2 2001 13 Launceston 723 107 2 1993 14 Helston 943 75 3 1993 15 Penryn 562 106 2 1822 16 Perranporth 702 89 2 1792 17 Polperro 489 125 0 1739 18 Bude 536 83 3 1666 19 Par & Tywardreath 314 89 4 1604 20 St Blazey 627 73 2 1557 21 Lostwithiel 489 94 0 1429 22 Looe (East) 253 52 5 1273 23 Falmouth 400 74 1 1240 24 Mevagissey 466 62 1 1186 25 Looe (West) 445 45 2 1095 26 Camelford 161 60 3 1061 27 Portreath 400 36 3 1060 28 Porthleven 363 35 3 1013 29 St Agnes 271 62 1 991 30 Pool & Tolvaddon 358 42 2 978 31 Mousehole 363 43 1 893 32 Padstow 517 34 0 857 33 Millbrook 335 44 0 775 34 Port Isaac 185 38 2 765 35 Penjerrick 16 73 0 746 36 St Just 302 33 1 732 37 Porthtowan 227 29 2 717 38 St Columb Major 150 46 1 710 39 Chyandour 314 19 2 704 40 Chacewater 239 45 0 689 41 Flexbury 503 17 0 673 42 Lanner 180 39 1 670 43 Fowey 108 56 0 668 44 Mullion Cove 35 39 2 625 45 Stratton 311 30 0 611 46 Troon 183 20 2 583 47 Barripper & 101 38 1 581 48 321 15 1 571 49 Mylor Bridge 143 30 1 543 50 St Dennis 229 30 0 529

• Score = people + (10×non-residential)+(100×Critical Services) • To reflect the weighting used in EA “Blue Squares” analysis

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Figure A15 Results of parish-based analysis using the same procedure as applied to the community-based analysis

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