Flood Consequence Assessment. Proposed Commercial Development, Tank Farm Way, Sully

Transcription

1 Flood Consequence Assessment Proposed Commercial Development, Tank Farm Way, Sully Client O Reilly Concrete Group Larchfield Kingscourt County Cavan Ireland By RVW Consulting 6 Neptune Court Vanguard Way Ocean Park Cardiff CF24 5PJ Tel: Fax: D A Stimpson A Britton 26 May 2017 Revision Author Approved Date

2 Flood Consequence Assessment RVW Consulting Limited CONTENTS 1.0 Introduction 2.0 Brief 3.0 General 4.0 Acceptability Criteria 5.0 Conclusion 6.0 Planning Drawings 7.0 Tan 15 Development Advice Map and Flood Level Information Project Number C6639

3 Flood Consequence Assessment RVW Consulting Limited 1.0 INTRODUCTION RVW Consulting Limited has been commissioned by O Reilly Concrete Group to undertake a flood consequence assessment for a Commercial Development at Tank Farm Way, Sully. The assessment has been undertaken to support a planning application to construct a new precast concrete products facility at the Tank Farm Way development site. Where possible, the assessment has been developed in conjunction with Natural Resources Wales. This assessment has been prepared broadly in line with the recommendations set out in Technical Advice Note 15 (TAN 15) Development and Flood Risk, published by the National Assembly for Wales in July BRIEF We have been instructed to provide an initial assessment of the flood risks associated with the site, and whether the development can be justified in terms of TAN 15. As part of the assessment we have visited the site to consider the likely implications of the proposed development. We have also contacted Natural Resources Wales to seek their views regarding the flood risks associated with sites in this area. In addition, this assessment contains flood level information for the site that has been provided by Natural Resources Wales. Project Number C6639

4 Flood Consequence Assessment RVW Consulting Limited 3.0 GENERAL Background O Reilly Concrete Group is promoting a project to construct a new precast concrete products facility at tank Farm Way, Sully. The outline development proposals have been discussed with the Local Planning Authority, who is satisfied, in principle, with the scheme. Site Location The proposed site is located off Tank Farm Way, Sully in the centre of an industrial and commercial development to the west of Sully village. The site is bounded by the Hexion UK site to the east, the Sully Brook to the south and west, and a former development site to the north. The site currently comprises an industrial unit, access roads and car parking areas, service yards, and storage areas. The Sully Brook is located directly adjacent to the southern and western boundaries, with the Cadoxton River being located 100m to the west of the site. History The site was previously occupied by an industrial engineering company, but is now vacant. Development Proposals The development proposals for the project comprise the construction of a precast concrete products facility, with offices, and associated car parking, access roads, landscaping, and drainage. There are no basements proposed for the development. These details are illustrated in the planning drawings contained in Section 6.0. Drainage The development proposals will not increase the extent of the impermeable area offered by the site. To ensure that there will be no flood risk due to surface water generation, surface water flows from the site will be disposed using the existing connections to the industrial estate drainage network. As the receiving network is maintained by the industrial estate operator, the risk of surface water drainage flooding at the site is considered to be very low. Foul flows generated by the development proposals will also be connected to the industrial estate drainage network, using the existing connections. Project Number C6639

5 Flood Consequence Assessment RVW Consulting Limited As the receiving network is maintained by the industrial estate operator, the risk of surface water drainage flooding at the site is considered to be very low. Flood Defences The site is located in an area that does not benefit from formal flood defences. Site Topography The proposed site is very flat, with an average ground level of 6.30m AOD. The site possesses existing ground levels ranging from circa 6.20m to 6.60m AOD. Proposed Levels The proposed FFL of the new building extension will be the same as the existing building, and will be set at circa 6.50m AOD. Proposed external levels will remain the same for the development site. Flood Levels The main source of flood risk to the surrounding area is fluvial flooding from the River Cadoxton and the Sully Brook, and tidal inundation from Sully Bay. The development site is located within the 1% fluvial flood zone, and the 0.5% tidal flood zone. The current year (2017) undefended flood level for the 1% fluvial flood scenario is 6.54m AOD, and 7.07 for the 0.1% fluvial flood scenario. The flood level for the 1% + climate change fluvial flood scenario is 6.76m AOD. The existing/proposed site levels range from 6.20m to 6.60m AOD, and this sets the maximum flood depths for the site at circa 0.34m, 0.56m and 0.87m. Whilst an extension building is proposed, the flood storage characteristics will remain unchanged for the development site as the finished floor level will be the same as existing external levels, and the building will become flooded during times of extreme flood events. As a result, no compensatory flood storage is proposed at the site. A copy of the TAN15 flood map, and flood level information, for the area is contained in Section 7.0. Flood History There is no record of flooding at the site. Project Number C6639

6 Flood Consequence Assessment RVW Consulting Limited TAN 15 Classification The specific guidance with regards to the classification of commercial/office premises contained in the TAN 15 Document (Figure 2) confirms that the proposed development could be classified as less vulnerable development. The TAN 15 Development Advice Map for the area indicates that the site would be categorised as a Zone C2 area, which states that it is without significant flood defence infrastructure. Flood Risk There is a risk of flooding at the development site from the 1%, 1% + climate change, and 0.1% flood event scenarios. This risk could be mitigated by the implementation of formal flood plans and procedures as part of an operating plan for the premises. This would include the erection of suitable warning signs to inform people entering the site, and the preparation of safe and effective flood warning and evacuation plans agreed with the local authority and emergency services, including subscription to Floodline Warnings Direct (as itemised in Appendix 6 of TAN 15). Floodline Warnings Direct is a free 24-hour service to registered users providing automated warnings to telephone, mobile, fax or pager. The 24 hour Floodline number is a telephone information service operated by the Environment Agency in England and Wales. Trained operators provide a 24/7 service to provide immediate advice on flood risk. They also provide a Quickdial number that allows direct access to any recorded flood warnings for the local area. Flood warnings are usually issued by Natural Resources Wales with an effective lead-time of between 12 and 24 hours before the onset of flooding around the site. Natural Resources Wales provides a comprehensive flood warning service for the River Taff. Warnings are issued to the Emergency Services, the Media and, other than for Flood Watch, to individual properties within Flood Risk Areas via the Floodline Warnings Direct service. The proposals indicate that the development will not provide for any overnight sleeping accommodation on the premises. The proposed development of the site will not result in the displacement of floodwater to the detriment of other properties. Project Number C6639

7 Flood Consequence Assessment RVW Consulting Limited 4.0 ACCEPTABILITY CRITERIA In support of the planning application, the following information is offered. 1. The proposed development will generate no flood risk, due to surface water run-off, as flows will continue to be discharged to a drainage system that is managed by the industrial estate operator. 2. There will be no loss of flood storage. 3. Flood defences are those that currently exist, and are deemed adequate for existing usage. 4. The Client is aware of the potential for flooding in this area and will implement flood management procedures, if required. 5. If considered necessary, end users and occupiers will be informed of the potential flood risk at the site so that they are aware of their duties under the Health and Safety at Work legislation, and can be duly made aware of any appropriate procedures to protect people and property. 6. Escape routes onto adjacent higher land, which is outside the extent of the flood risk zone, are available. 7. If considered necessary, formal flood plans and procedures can be established as part of an operating plan for the premises. This could include the erection of suitable warning signs to inform people entering the site, and the preparation of safe and effective flood warning and evacuation plans agreed with the local authority and emergency services, including subscription to Floodline Warnings Direct (as itemised in Appendix 6 of TAN 15). 8. Flood proofing and mitigation will be considered within the design of the development, if considered necessary. This could be undertaken in accordance with the guidance that is available from the Environment Agency using their online publication Prepare your property for flooding. Project Number C6639

8 Flood Consequence Assessment RVW Consulting Limited 5.0 CONCLUSION The size and nature of the proposed development can be categorised as less vulnerable development in accordance with TAN 15. Due to the nature of the development, the proposals do not aggravate or increase the risk of flooding to the existing site or to surrounding properties. The Client has been advised of the potential for flooding in this area and is aware of the residual risk. In summary, based upon the published guidelines and relevant provisions of the technical advice note, the proposed development meets the acceptability criteria. Project Number C6639

9 Flood Consequence Assessment RVW Consulting Limited 6.0 PLANNING DRAWINGS Project Number C6639

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11 Flood Consequence Assessment RVW Consulting Limited 7.0 TAN 15 DEVELOPMENT ADVICE MAP AND FLOOD LEVEL INFORMATION Project Number C6639

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13 ATIX-12630a Tank Farm Way, Sully E: N: Current Flood Map Figure 1 shows the current Flood Map (version ) at this location. The Flood Map represents a combination of the undefended fluvial and tidal flood extents derived from detailed local models and national generalised model data. Undefended scenarios are provided as being a possible worst case scenario in the event of defence failure. Please note that the current tidal flood outlines shown on the Flood Map in Figure 1 are based on an NRW Tidal Projection Mapping study (2013) that uses sea level nodes within the Severn Estuary. Each node represents a set of extreme sea levels which were generated by the Environment Agency in for current day (in the studies case, 2008). These levels were projected in-land over a digital terrain model to produce depth and elevation grids as well as tidal mapped outlines for both the 0.5% ( 200) AEP (annual exceedance probability) and the 0.1% ( 1000) AEP; including climate change and upper confidence intervals (+/-95%). These outlines offer a more precautionary picture of tidal flood risk and therefore supersede the localised hydraulic modelled outlines described in Section 2 below. More information on the Flood Map can be obtained from the Natural Resources Wales website 2. Local Flood Risk Mapping Study Model Summary The results summarised in the tables below are taken from two hydraulic models developed as part of the Cadoxton Strategic Flood Risk Assessment. These are: a. A multi-domain 1D-2D ESTRY/TUFLOW model assessing fluvial and tidal flood risk from the River Cadoxton and its tributaries the East Brook, Sully Brook and Cold Brook from just upstream of Dinas Powys to the Cadoxton sea outfall 2. The fluvial outputs below are provided from this model. b. A Tidal Inundation model extending the coastal boundary to include Barry Docks 3. The above fluvial/tidal model was adopted and modified to allow broad-scale tidal flooding within the lower Cadoxton River catchment. This model is subject to substantially more tidal inundation flooding and as such has been used to provide tidal outputs below. Results Site of Interest The polygon shown in the figures represents the site in question and has been used to query the height, depth, velocity & hazard grids to provide the results in Tables 1-6 below. NULL values indicate that the site is flood free during a particular scenario. The elevation results have been interpolated to include the climate change increment from the DEFRA guidance on extreme sea level data to show current day scenarios (see Tables 4 & 6 below). 95% confidence bounds for these values were also derived using the confidence intervals for the Newport Extreme Sea Level node. Example depth grids for the defended fluvial 100+CC and 1000, and the defended tidal 200 (2114) and 1000 (2089) including confidence intervals - are reproduced in Figures 2, 3, 5 & 6 below. Page 1 of 6

14 Example hazard grids for the defended fluvial 1000, and the defended tidal 200 year (2089) including Confidence Interval, are represented in Figures 4 & 7. The hazard rating below relates to the Hazard to People Classification using the hazard matrix 4. Flood Hazard Rating (HR) Colour Code Hazard to People Classification Less than 0.75 Very low hazard Caution 0.75 to 1.25 Danger for some includes children, the elderly and the infirm 1.25 to 2.0 Danger for most includes the general public More than 2.0 Danger for all includes the emergency services Table 1: Defended Fluvial Level Data CC 1000 Model Grid Size (m) Wet Cells Depth, mean (m) NULL Depth, max (m) NULL Elevation, mean NULL Elevation, max NULL Velocity, mean NULL Velocity, max NULL Hazard, mean NULL Hazard, max NULL Table 2: Undefended Fluvial Level Data Model Grid Size (m) 1 1 Wet Cells Depth, mean (m) Depth, max (m) Elevation, mean Elevation, max Velocity, mean Velocity, max Hazard, mean Hazard, max Page 2 of 6

15 Table 3: Defended Tidal Level Data with Climate Change (excluding upper confidence Intervals) Model Grid Size (m) Wet Cells Depth, mean (m) NULL NULL 0.09 NULL Depth, max (m) NULL NULL 0.25 NULL Elevation, mean NULL NULL 6.07 NULL Elevation, max NULL NULL 6.14 NULL Velocity, mean NULL NULL 0.08 NULL Velocity, max NULL NULL 0.49 NULL Hazard, mean NULL NULL 0.55 NULL Hazard, max NULL NULL 1.03 NULL Table 4: Interpolated Tidal Results (2016) Elevation, max NULL NULL 6.17 NULL Table 5: Defended Tidal Level Data with Climate Change (including upper confidence Intervals) Model Grid Size (m) Wet Cells Depth, mean (m) NULL Depth, max (m) NULL Elevation, mean NULL Elevation, max NULL Velocity, mean NULL Velocity, max NULL Hazard, mean NULL Hazard, max NULL Page 3 of 6

16 Table 6: Interpolated Tidal Results (2016) Elevation, max NULL Results Emergency Access to Site The access route provided was used to query the height, depth, velocity and hazard grids to provide the results in Tables NULL values indicate the site is flood free during this scenario. Table 7: Defended Fluvial Level Data CC 1000 Model Grid Size (m) Wet Cells Depth, mean (m) NULL Depth, max (m) NULL Elevation, mean NULL Elevation, max NULL Velocity, mean NULL Velocity, max NULL Hazard, mean NULL Hazard, max NULL Table 8: Undefended Fluvial Level Data Model Grid Size (m) 1 1 Wet Cells Depth, mean (m) Depth, max (m) Elevation, mean Elevation, max Velocity, mean Velocity, max Hazard, mean Hazard, max Page 4 of 6

17 Table 9: Defended Tidal Level Data with Climate Change (excluding upper confidence Intervals) Model Grid Size (m) Wet Cells Depth, mean (m) Depth, max (m) Elevation, mean Elevation, max Velocity, mean Velocity, max Hazard, mean Hazard, max Table 10: Interpolated Tidal Results (2016) Elevation, max Table 11: Defended Tidal Level Data with Climate Change (including upper confidence Intervals) Model Grid Size (m) Wet Cells Depth, mean (m) Depth, max (m) Elevation, mean Elevation, max Velocity, mean Velocity, max Hazard, mean Hazard, max Page 5 of 6

18 Table 12: Interpolated Tidal Results (2016) Elevation, max Table 13: Sea level rise, mm per year Assumed vertical land movement Additional Information We do not hold any historic flooding information for the area of interest. The local authority may be able to provide information on issues such as localised flooding from sewers, drains and culverts. 4. References Environment Agency (2011) Coastal flood boundary conditions for UK mainland and islands SC JBA Consulting (2015) Cadoxton Flood Risk Study Hydraulic Model User Report JBA Consulting (2015) Cadoxton Flood Risk Study Tidal Inundation Model Model User Report and Results Discussion Final Supplementary note on flood hazard ratings and thresholds for development planning and control purpose (May 2008) Flood and Coastal Defence Appraisal Guidance: FCDPAG3 Economic Appraisal. Supplementary Note to Operating Authorities Climate Change Impacts; October 2006; Department for Environment, Food and Rural Affairs. 5. Notes The scope of the model is the mapping of flood risk, it is not intended for detailed design. The model should be considered as the starting point for more detailed modelling, commensurate with the consequences of flooding at the site of interest. NRW models are available under licence agreement for the purpose of further development. Contact Natural Resources Wales Data Distribution team for details of terms, conditions and pricing. If the data is used in support of an FCA, please include the reference number. Please refer to NRW standard terms and conditions. Flood Risk Analysis 18/04/2017 Page 6 of 6

19 Legend Site Location Flood Zone 3 ( 100 year undefended fluvial and 200 year undefended tidal extents) Flood Zone 2 ( 1000 year undefended fluvial and tidal extents) Areas benefiting from defences!! Defences Flood Storage Area Main Rivers Access Project Drawing Tank Farm Way, Sully, CF64 5RP [Ref:ATI-12630a] Figure 1: Current Floodmap [v201704] ± Date 18th April 2017 Scale 1:5,000

20 Legend Site Location Flood Depth (m) >0.3 >0.6 >1.0 >2.0 Access Project Tank Farm Way, Sully, CF64 5RP [Ref:ATI-12630a] ± Drawing Figure 2: Fluvial Depth Grid for 1% AEP with climate change ( %) year event with Defences Date 18th April 2017 Scale 1:5,000

21 Legend Site Location Flood Depth (m) >0.3 >0.6 >1.0 >2.0 Access Project Tank Farm Way, Sully, CF64 5RP [Ref:ATI-12630a] ± Drawing Date Figure 3: Fluvial Depth Grid for 0.1% AEP ( 1000) year event with Defences 18th April 2017 Scale 1:5,000

22 Legend Site Location Flood Hazard < > 2.0 Access Project Tank Farm Way, Sully, CF64 5RP [Ref:ATI-12630a] ± Drawing Date Figure 4: Fluvial Hazard Grid for 0.1% AEP ( 1000) year event with Defences 18th April 2017 Scale 1:5,000

23 Legend Site Location Flood Depth (m) >0.3 >0.6 >1.0 >2.0 Access Project Tank Farm Way, Sully, CF64 5RP [Ref:ATI-12630a] ± Drawing Figure 5: Tidal Depth Grid for 0.5% AEP ( 200) year event - defended including upper confidence intervals (2114) Date 18th April 2017 Scale 1:5,000

24 Legend Site Location Flood Depth (m) >0.3 >0.6 >1.0 >2.0 Access Project Tank Farm Way, Sully, CF64 5RP [Ref:ATI-12630a] ± Drawing Figure 6: Tidal Depth Grid for 0.1% AEP ( 1000) year event - defended including upper confidence intervals (2089) Date 18th April 2017 Scale 1:5,000

25 Legend Site Location Flood Hazard < > 2.0 Access Project Tank Farm Way, Sully, CF64 5RP [Ref:ATI-12630a] ± Drawing Figure 7: Tidal Hazard Grid for 0.1% AEP ( 1000) year event - defended including upper confidence intervals (2089) Date 18th April 2017 Scale 1:5,000