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1 AX0276-RPT - Whitestone Garage - Flood Risk Assessment
DECEMBER 2023
Axis Consulting Engineers Flood Risk Assessment Whitestone Garage AX0276
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2 AX0276-RPT - Whitestone Garage - Flood Risk Assessment Contents 1. Introduction ... 3 2. Topography ... 5 3. Historic use ... 6 4. Existing surface water management ... 7 5. Existing foul water management ... 8 6. Proposed development ... 9 7. Comparison of surface areas ... 11 8. Flooding from rivers (fluvial) and tidal sources ... 11 9. Flooding from surface water (pluvial) ... 12 10. Flood risk from sewers ... 12 11. Other sources... 12 12. Proposed surface water management ... 12 13. Proposed foul water management ... 13 14. Probability of Flooding ... 14 15. Strategies for flood mitigation ... 18 16. Recommendation ... 20 17. Conclusion ... 20 References ... 22
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3 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 1. Introduction 1.1 Axis Consulting Engineers have been commissioned to undertake a Flood Risk Assessment on the site of the old Whitestone Garage in Ballasalla, to support a planning application for development (PA 23/00774/B).
Figure 1 - Location of the site 1.2 The proposed 0.37 ha development is located on the approach to Ballasalla from the Douglas direction. At this stage the application is for approval in principle and sets out provision for a retail facility. 1.3 The site currently consists of an area of hardstanding, with a set of buildings that formed the previously operating car showroom and garages. The site is bordered by residential properties in Oatfield Rise to the north, two properties to the east, a ditch on the west (with the Clagh Vane estate on the other side) and the A5 Douglas Road to the south.
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4 AX0276-RPT - Whitestone Garage - Flood Risk Assessment
Figure 2 - current site, with the garage buildings, car showroom and associated forecourt/parking areas 1.4 A culvert runs along the northern boundary, from an inlet approximately 50m from the northeastern edge of the site and runs into the open ditch section on the western boundary. At the Douglas Road end, this then continues parallel the road to a culvert under the main road towards the southwest of the site (see Figure 3).
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5 AX0276-RPT - Whitestone Garage - Flood Risk Assessment
Figure 3 - wider location plan, showing site area (red), the watercourse (open sections solid blue line, culverted sections dashed dark blue line). Arrows indicate direction of flow, with 1m contours shown and road gullies as square symbols. 2. Topography 2.1 The development site falls towards the southern corner the site, as shown in Figure 3. 2.2 Levels on the site vary from around 32.7 (northern corner) to 31.3 (A5 southern tie in) mAOD. 2.3 The approximate calculated catchment for the culvert at the eastern edge of the site is 20.5 hectares. The majority of this catchment is agricultural land, with a small number of properties within this area.
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6 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 3. Historic use 3.1 The 1869 Ordnance Survey [1] shows the site as a field, with the watercourse in place as a marked ditch. .
Figure 4 - extract from the 1869 Ordnance Survey, with the site boundary shown in red. 3.2 The adjacent Oatfield Rise and the Whitestone Garage first appear on the 1980s Ordnance Survey map, at which time the culvert across the rear of the site would have been constructed.
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7 AX0276-RPT - Whitestone Garage - Flood Risk Assessment
Figure 5 - 1980s Ordnance Survey, with the site boundary shown in red. 4. Existing surface water management 4.1 The hardstanding areas and buildings are positively drained. Gullies collect the flows from areas of hardstanding, with an additional drainage channel across the entrance onto the A5. The building roofs are collected with suitable guttering and rainwater pipes. 4.2 The rainwater pipes on the western side of the buildings were observed to route into the adjacent watercourse. It is assumed other site surface water drainage also discharges into the ditch (with the vegetation cover it was not possible to inspect for all outfalls).
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8 AX0276-RPT - Whitestone Garage - Flood Risk Assessment
Figure 6 - Existing downpipes discharging to ditch.
Existing foul water management 5.1 There is an existing foul sewer running down Douglas Road. It is assumed that site foul flows discharge into this sewer.
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9 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 6. Proposed development 6.1 The proposed development is for approval in principle to establish a retail area with associated parking. The access is off the A5 Douglas Road, at the southern end of the site.
6.2 The car parking and associated hardstanding areas are circa 1750 m2. 6.3 As the application is for approval in principle only at this stage, it has not yet been established whether areas of the building footprint and hardstanding areas will be re-used, or whether the site will be reconstructed. This detail will be developed in subsequent stages. 6.4 The building areas are circa 640 m2 (with circa 170m2 indicated for potential subsequent extension). It has not been established at this stage which areas are planned for landscaping surfaces - this should be confirmed as part of the subsequent design stages and discussions with Flood Risk Management when agreeing appropriate attenuation and s20 license (if new outfalls or changes are proposed). Compatibility with policy
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10 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 6.5 The site is within the existing town boundaries. The proposals align with the Strategic Plan [2] that sets out that “Developments should make the best use of resources by optimising the use of previously developed land”. 6.6 Redeveloping sites ahead of new development area aligns with the “Sequential Approach” to flood management set out in the Draft Planning Policy Statement [3]. 6.7 Whilst the draft planning policy statement gives only a high level overview of vulnerability classifications, assessing against UK Planning Policy Statement 25 [4], the proposed development would relate to a ‘Less Vulnerable’ category in accordance with Annex D. 6.8 The Draft Planning Policy Statement sets out that “Development within Flood Zones ... may be permitted, provided it can be demonstrated that the site has the appropriate level of flood protection, and that the development will not increase the probability of flooding to the surrounding environment.”
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11 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 7. Comparison of surface areas 7.1 As the application is at approval in principle stage, the extents of permeable and impermeable surfaces have not been established. 7.2 Noting the extents of the current site hardstanding areas, with the buildings footprint and areas of asphalt covering the full site with the exception of a rear grassed access area, it is anticipated that impermeable area will be either the same or less than in the existing condition. 8. Flooding from rivers (fluvial) and tidal sources 8.1 The primary flood risk within the area is fluvial, from the adjacent watercourse. 8.2 A previous study [5] by the Isle of Man Water and Sewerage Authority (WSA) on the Ronaldsburn Stream identified the areas adjacent to this watercourse as a flood risk. 8.3 The primary driver for the flood risk is the lack of culvert capacity and sensitivity testing for culvert blockages. 8.4 The culvert adjacent to Oatfield Rise (300mm pipe) is at the greatest risk of flooding, and there have been historic incidents of flooding occurring in the vicinity of the culvert screen. A flood event was recorded in 2002 (noted in the WSA commissioned report [5]), and anecdotal reports from residents suggest the immediate area around the culvert surcharges around twice yearly, with relatively frequent cleaning of the debris screen required (Figure 7).
Figure 7 - culvert inlet to the north of the site, adjacent to Oatfield rise and the football club.
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12 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 9. Flooding from surface water (pluvial) 9.1 The site is currently hard standing. There is positive drainage which likely discharges to the adjacent ditch. The site also falls to the Douglas Road end. There is channel drainage across the entrance to the road. 9.2 It is not known the capacity of the existing drainage system; however, the proposals indicate a reconfiguration of this area, and it is therefore anticipated at detailed design drainage surveys would be undertaken. 10. Flood risk from sewers 10.1 There is no known risk from the existing sewers in the area as advised by Manx Utilities. 11. Other sources 11.1 Risk of groundwater flooding is minimal due to the close proximity to the watercourses and the raised site level. The superficial geology in the area is mostly drift deposits which are relatively impermeable, with bands of sands and gravels. 12. Proposed surface water management 12.1 Surface water will be positively drained from buildings and areas of hardstanding, as in the present condition, and discharged into the adjacent watercourse. 12.2 Details of the outfall and any associated attenuation will be agreed with Flood Risk Management as part of any Section 20 application during detailed design, and existing arrangements surveyed to verify potential for re-use.
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13 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 13. Proposed foul water management 13.1 Foul services are proposed to connect into the existing sewer on Douglas Road. It is anticipated the existing building connects to this sewer and it may be possible to reuse the site connection subject to confirmation of size and suitability. 13.2 Levels, falls and exact manhole arrangements will be set in the detailed design.
Figure 8 - extract from plans provided by Manx Utilities. Brown indicates foul system, blue indicates surface water system and red indicates combined system.
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14 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 14. Probability of Flooding 14.1 The site lies in an area of flood risk, due to the watercourse that comes from near Glashen Farm, traverses the northern and western edges of the site, and continues adjacent and under the Douglas Road to form the subsequent Ronaldsburn Stream. 14.2 The Flood Hub’s flood risk map [4] show the site at risk of fluvial flooding (Figure 9), in 30 year storm events and in more severe events.
Figure 9 - extract from the Flood Hub’s flood risk maps, with the site shown by the grey dot. 14.3 In addition, a WSA commissioned study [4] reviewed the Ronaldsburn area in particular and identified the following area as at risk of fluvial flooding.
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15 AX0276-RPT - Whitestone Garage - Flood Risk Assessment
Figure 10 - extract from WSA commissioned report, site boundary in solid red. 14.4 The WSA commissioned report [5] assessed culvert capacity, risk of blockage and modelled catchment flows to provide the above analysis. 14.5 The primary flood risk is associated around the inlet to the Oatfield Rise culvert, the report identified the following capacity and predicted flow:
Figure 11 - extract from WSA commissioned report. 14.6 It should be noted that the report identified further survey works would be required to confirm overland flow paths through the area, however this does broadly match anecdotal evidence from local residents who suggested previous flood events (including the 2002 event listed in the report) have seen flow from Oatfield Rise culvert Clagh Vane culvert
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16 AX0276-RPT - Whitestone Garage - Flood Risk Assessment the watercourse travel through the grounds of the adjacent property Nane Darragh (and possibly parts of Monaltrie), and then onwards through the site area down to the southern corner of the site. 14.7 The above flood maps, and levels surveyed on site (Appendix A - Levels in Area), suggest two properties are at risk of flooding (with corresponding floor levels) - Abbeydale (36.2 mAOD), Nane Darragh (34.9m AOD). A further property, Monaltrie, is vulnerable to flooding (35.2 mAOD, possibly airbrick at 35.0 mAOD) subject to the exact flow path which additional surveys would be required to establish. Whitestone Cottages has a significantly elevated floor level and is therefore not anticipated to be at risk of flooding, although there is a risk to garden areas and associated outbuildings. The adjacent Oatfield Rise and Cooyrt Drive areas are not anticipated to be at risk of flooding.
Figure 12 - properties adjacent to the site 14.8 The development site plays a role in relation to potential flooding to these properties. If flows in flood condition were restricted from onward flow through the site, the probability of these properties flooding would increase. 14.9 Previous modelling indicates overland flows would exit the site onto Douglas Road. Despite the presence of the adjacent ditch, other than site drainage (which could only be relied on to convey a small proportion
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17 AX0276-RPT - Whitestone Garage - Flood Risk Assessment of flows), there is no route into this ditch due to the presence of a solid wall and the site topography channelling flows towards the road. It is not believed there are grips on the road although it is anticipated that highway drainage is connected to the culvert. 14.10 The existing building on the site has a threshold of around 32.7m AOD. This is slightly raised from adjacent ground but noting the fall of the site (north to south) there is anticipated to be some risk to the property. The property is believed to have a solid floor, and a consistent floor level across the site. 14.11 Douglas Road is therefore at risk of flooding in the current condition. Previous modelling suggests properties adjacent to the road and around Homefield Park may be at risk of onwards flooding. 14.12 As part of this assessment, levels have been taken at adjacent properties and across the garage site. This broadly support the results of the above modelling output. 14.13 The levels at the culvert inlets were 30.7m AOD (Douglas A5 culvert) and 34.8m AOD (Oatfield Rise). The ditch carrying the watercourse at the western edge of the site is around 950mm lower than the adjacent bank. 14.14 There is also flood risk associated with the culvert under Douglas Road (Figure 11), however the culvert is estimated to have the capacity for the 1 in 100 year flow (although due to the issues identified previously, with the Oatfield Rise culvert surcharging only a proportion of total flows will enter the Douglas Road culvert, with the rest travelling overland). If this culvert were to surcharge it is anticipated flows would generally travel down Douglas Road, which is at a lower level (circa 31.3 mAOD) than the site and which the general area falls toward.
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18 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 15. Strategies for flood mitigation Increased threshold heights 15.1 Noting the risk of overland flow it is recommended that any floor level has a raised threshold entry to mitigate risk of property damage - if the site is reconstructed it would be recommended to achieve a freeboard of 600mm [7] above predicted flood level (which could be assessed using the flows from the WSA commissioned report [5]). Noting the relatively moderate size of the catchment area, and that the building is raised from wider surrounding topography, it may be prudent to relax this freeboard requirement to 300mm. 15.2 The development threshold is currently slightly raised over external areas (however is unlikely to provide a freeboard of more than circa 100mm), therefore if the existing building is retained additional property level protection should be considered. 15.3 The overland flows modelled in the WSA commissioned report [5] are anticipated to be relatively modest (at a maximum of 0.29 m3/s) and the existing hardstanding areas will easily convey flows across the site. The risk to car parking areas is relatively low with any flooding in extreme events likely to be less than 200mm in depth. A full topographical survey would confirm the existing flow path in the site and inform the required threshold level, which could be established to allow a sufficient freeboard above the maximum predicted flood level. Provide property level protection 15.4 If the existing threshold height is insufficient to establish required freeboard above maximum flood level, and subsequent design stages decide to re-use the site rather than rebuilding, the existing building could be protected with property level protection. This could be in the form of walls to channel flows away from the building, or protection to doors. A detailed property risk assessment should be prepared if this is pursued to ensure protection has been adequately considered. Consider overland flows 15.5 An important consideration is that any grading changes to the site, or any new solid walls should consider the impact on the overland flows and ensure that the site is still capable of receiving these flows. Any solid walling at the northern boundary or equivalent bunds would be anticipated to worsen the flooding condition of the adjacent properties. 15.6 There may also be an opportunity for improving the routing of overland flows if the site is being regraded (Figure 13). These flows could be diverted into the existing ditch and culvert that run parallel to the Main Road, rather than onto the road itself, reducing a source of highway flooding and risk to properties across the road.
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19 AX0276-RPT - Whitestone Garage - Flood Risk Assessment
Figure 13 - current approximate flow paths (navy arrows) through the site, utilising coarse LiDAR data with infill surveyed points. Red - site boundary. Blue - Watercourse (culverted where dashed). Orange arrows show potential opportunity to divert flows back into the watercourse if the site was regraded.
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20 AX0276-RPT - Whitestone Garage - Flood Risk Assessment Reconstruct or alter culverts 15.7 It is noted that the flood risk is largely associated with the undersized and blockage prone culverts (and primarily the culvert bordering Oatfield Rise). The WSA commissioned report [5] identified various options for improving this situation, such as reconstruction of the culverts, additional bunds to create storage and improving the screen to reduce risk of blockages. 15.8 It is not anticipated that this is feasible as part of this project due to the various landowners involved and substantial works required. The problem area is outside the site boundary and this assessment demonstrates that flood risk within the site can be safely managed, therefore provided there is no worsening of flood risk in the wider area any further improvements are outside the scope of this development. 16. Recommendation 16.1 The proposed development should ensure that building thresholds have adequate freeboard (400mm freeboard for commercial buildings) over predicted flood levels or provide property level protection. Noting the status of the application as approval in principle, confirmation will be required at the following design stage depending on whether the existing buildings and areas of hardstanding are modified or whether the site is redeveloped in its entirety. 16.2 Adequate consideration should be given to the overland flows noting the risk to adjacent properties. Provision should be made for ensuring flows can convey across the site. 16.3 The development is at approval in principle stage, and it has not yet been established whether existing hardstanding areas and surface water systems will be retained within the site. In subsequent design stages this should be confirmed, and levels of attenuation agreed with the Flood Risk Management team to ensure compatibility with policy and that surface water flows are managed appropriately. 17. Conclusion 17.1 The proposed redevelopment sits in an area at risk of flooding due to the culvert adjacent to Oatfield Rise, which has insufficient capacity and is prone to blockages in its current arrangement. 17.2 The flood risk is primarily from overland flows associated with this watercourse during flood events, where flows pass through the site and onto Douglas Road. The site has a relatively constant fall and therefore is not believed to provide storage.
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21 AX0276-RPT - Whitestone Garage - Flood Risk Assessment 17.3 The flood risk within this site can be safely managed through appropriate threshold levels and/or property level protection. The threshold level and available freeboard should be established during subsequent design stages. 17.4 The flood risk to adjacent properties will remain unchanged provided detailed proposals do not inhibit the ability of the site to convey excess overland flows from the northern corner, and through the site. 17.5 If detailed proposals identify the site will be fully reconstructed, there may be opportunity for some improvement to the wider area by conveying overland flows back into the watercourse rather than onto Douglas Road. This would reduce flood risk to the highway and adjacent properties to the south of the site. The culverted watercourse adjacent to Clagh Vane is believed to have sufficient capacity although some surcharging may be witnessed in 1 in 100 year flood events including climate change allowance. 17.6 Noting the length of the culvert, land ownership issues, and relevant influence on the site on local flood risk it is not practical to improve the wider capacity issues with the Oatfield Rise culvert. 17.7 Site discharge arrangements are anticipated to be similar to the current arrangements, with surface water discharging to the watercourse and foul discharging to the sewer on Douglas Road. The requirement and sizing for attenuation should be confirmed with Flood Risk Management at subsequent design stages following establishment of impermeable areas and confirmation of whether existing systems are suitable and/or will be reused.
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22 AX0276-RPT - Whitestone Garage - Flood Risk Assessment References
[1] O. Survey, "1868/9 Map of the Isle of Man," [Online]. Available: https://maps.nls.uk/os/6inch- england-and-wales/isle_of_man.html. [2] C. Office, "Isle of Man Strategic Plan," 2016. [Online]. Available: https://www.gov.im/media/1350906/the-isle-of-man-strategic-plan-2016-approved-plan- 15_03_16.pdf. [3] DLGE, "Draft Planning Policy Statement," 2017. [Online]. Available: https://www.gov.im/media/633689/050809devel.pdf. [4] H. a. C. Department for Levelling Up, "Planning Policy Statement 25: Development and Flood Risk
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23 AX0276-RPT - Whitestone Garage - Flood Risk Assessment Appendix A - Levels in Area
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45 PNT
36.384
41 CLW_2 41 CLW_2
35.58 35.58
46 PNT
36.291
45 PNT
46 PNT
36.341
36.138
44 PNT
36.29
40 CLW_2 40 CLW_2 47 PNT
44 PNT
35.54 35.54
36.091 47 PNT
35.957
40 CLW_2 40 CLW_2 36.135
42 OTF 43 OTF
35.466 35.466
34.896 34.828
PNT 48 PNT PNT
35.931
35.806 35.806 PNT
PNT
35.768
36.133
36.156
PNT
35.702
PNT
36.207
PNT Threshold
35.108
PNT Threshold
PNT
35.389
34.983
PNT Threshold
PNT PNT Entrance Entrance
35.5 PNT
35.496
35.407
34.933
PNT threshold
35.432
PNT Entrance
35.422
35.445
PNT Entrance PNT Entrance 35.434
PNT
35.333
PNT Garage Threshold
PNT
35.521 PNT Entrance
35.623
35.638
PNT Entrance PNT Entrance
35.6
22 WALL_1 22 WALL_1
35.089 35.089
23 PNT
32.685
6 BLDG_1 6 BLDG_1
29 PNT
21 WALL_1 21 WALL_1
34.128 34.128
30 PNT
32.685
32.667 32.667
32.641
23 PNT
33.011
31 PNT
32.685
5 BLDG_1 5 BLDG_1
32.644 32.644
8 BLDG_1 8 BLDG_1
20 WALL_1 20 WALL_1
33.839 33.839
32.597 32.597
7 BLDG_1 7 BLDG_1
24 PNT
33.028
34.98
PNT Airbrick
35.209
PNT 35.234 Threshold
PNT Garages Threshold
PNT
PNT
35.687
35.722
PNT
35.786
PNT
PNT
35.781 PNT
35.789
35.687
PNT
PNT
35.6 35.65
31 PNT
32.499 32.499
32.568
9 BLDG_1 9 BLDG_1
32.578 32.578 12 BLDG_1 12 BLDG_1
10 BLDG_1 10 BLDG_1
32.587 32.587 13 BLDG_1 13 BLDG_1
32.702 32.702
19 WALL_1 19 WALL_1 32.51 32.51
16 BLDG_1 16 BLDG_1 32.473 32.473
11 BLDG_1 11 BLDG_1
32.81 32.81
14 BLDG_1 14 BLDG_1
32.598 32.598
15 BLDG_1 15 BLDG_1
32.641 32.641
33.21 33.21
32 PNT
32.512
18 WALL_1 18 WALL_1
33.252 33.252
17 BLDG_1 17 BLDG_1
33 PNT
32.363 32.363
32.365
38 CLW_1 38 CLW_1
31.584 31.584
4 BLDG_1 4 BLDG_1
32.671 32.671
34 PNT
32.347
37 CLW_1 37 CLW_1
31.145 31.145
35 PNT
32.125
37 CLW_1 37 CLW_1
36 PNT
31.029 31.029
31.956
2 BLDG_1 2 BLDG_1 1 BLDG_1 1 BLDG_1
31.937 31.937 32.229 32.229
31.697 31.697
LINE_1 LINE_1 Road tie in
LINE_1 LINE_1 Road tie in
CLW_1 39 CLW_1 CLW_1 39 CLW_1
31.32 31.32
30.716 30.716
29 PNT
32.525
35.181
24 PNT
33.314
PNT
34.534
PNT
33.873
28 PNT
32.384
26 PNT
32.616
25 PNT
32.772
PNT 25 PNT
33.187 33.187
PNT
33.24
33.031
PNT gulley
PNT
32.746
3 BLDG_1 3 BLDG_1
32.236 32.236
LINE_1 LINE_1 PNT Road tie in
32.405 32.405 32.403
PNT 27 PNT
32.25 32.25
PNT
32.013
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