FLOOD RISK ASSESSMENT PROPOSED 9 DWELLINGS AT FARMBOURGH VIEW, BATH ROAD, FARMBOROUGH BA2 1BX

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1 FLOOD RISK ASSESSMENT PROPOSED 9 DWELLINGS AT FARMBOURGH VIEW, BATH ROAD, FARMBOROUGH BA2 1BX Client: Mr E Davis Job No: J-319 Winway Homes Developments Ltd Date: 10 th October Parry Close Southdown Park V1 Bath BA2 1JR 1

2 CONTENTS 1.0 Introduction Contact Details Project Outline Site Description Existing Land Use Existing Vulnerability Classification Proposed Land Use Proposed Vulnerability Classification Comparison of Impermeable Areas Hydrology General Overview of the Hydrology in the Area Overview of All Sources of Flooding Strategic Flood Risk Assessment Confirmation of the Flood Zone Site Investigations Proposed Development Discussion Compatible Development Safe Egress Safe Haven Sequential and Exception Test Equipment Provision/Flood Mitigation measures Finished Floor Levels Flood Storage Compensation Residual Risk Flood Defence Consent Safe Warning and Evacuation Plan Equipment Provision Flood Plan and Kit

3 5.0 Sustainable Urban Drainage System (SUDS) (appendix a) Calculation of rural runoff QBAR Long Term Storage SUDS Strategy Area 1 (Appendix B) SUDS Strategy Area Conclusions References Table 1 Overview of Flooding... 9 Table 2 Infiltration Test Results Table 3 Calculation of Impermeable Areas Figure 1 - Open Channel at Bridge Garden Road... 8 Figure 2 2.0m wide culvert at Timsbury Road... 9 Figure 3 Trial Pit Locations Appendix A Appendix B Calculations Drawing EN 001.pdf Drawing C13894 C030A, SKC007 A Impermeable Areas.pdf and Microdrainage This report has been produced in good faith, with all reasonable skill, care and diligence based on the information provided and accessible at the time of its preparation and within the scope of the work agreed with the client. Please be advised that a separate Structural Report will be required during the Building Design stage to ensure that the building is sufficiently stable during periods of inundation if this FRA concludes that flooding will occur. This will be up to the Client to organize separately to this FRA. The Micro-Drainage Calculations and SuDS design for Conygre Brook Park and the Drainage Design for Farmborough View have been undertaken by others and we take no responsibility for their part of the project. We disclaim any responsibility to the client and others in respect of any matters outside the scope of the above. This report is provided for the sole use of the named client and is confidential to them and their professional advisors. 3

4 1.0 INTRODUCTION 1.1 CONTACT DETAILS The full site address is: Proposed Development at Land known as Farmborough View, Old Lane, Farmborough, BA2 0BW. The NGR is: ST Planning Application Reference: 14/03923/FUL Planning Authority: Bath and North East Somerset Council, Guildhall, High Street, Bath BA1 5AW Planning Officer: Chris Griggs Trevarthen Planning Agent: David Hadfield, Hatfield Associates Architects, The Old Brushworks, 56 Pickwicks Road, Vorsham, Wiltshire, SN13 9BX FRA Consultant: Louisa Inch, Premier Water Solutions Ltd, 2 Cogos Park, Mylor Bridge, Cornwall, TR11 5SF. 1.2 PROJECT OUTLINE The proposal is to demolish the existing bungalow on the site and erect 9 no. two and three bedroom two storey dwellings with associated parking and construction of vehicular and pedestrian access. The planning permission has suggested using soakaways as a methodology to reduce the runoff and volume from the site. BaNES Drainage Engineer has formally commented that they support the principle of using soakaways and further detail is required in order to discharge any planning condition: The strategy should review the existing drainage system. This has been highlighted as soakaways but there is no information regarding the location, condition or connections to these soakaways. Crucially, an assessment needs to be made about how much surface water they can accommodate. At this stage it is unclear whether or not current soakaways are in a sufficient condition to accommodate new drainage. A calculation should be presented estimating the likely surface water discharge rates and volumes from the proposed hard standing areas. Calculations should be based on the critical storm return periods (1in30 year and 1in 100year + 30% for climate change). A clear explanation detailing which drainage features will account for which areas and what volumes they will accommodate. Any proposed soakaways should be designed to BRE Digest 365 standard. This will necessitate an infiltration test and an estimate of inflow into each proposed soakaway. A similar exercise should be undertaken to assess the capacity of the ground for porous paving as there may be a requirement to include a storage volume below the porous paving. An assessment of ground infiltration capacity will determine a suitable storage volume and as such reduce the risk of surface water flooding. Building regulations Part H, Section 2 Drainage of paved areas should be reviewed. the applicant should include drawings showing the proposed drainage system in plan (including all pipes, connections and manholes etc) as well as detailed design drawings of any surface water features such as soakaways or pervious paving. It should be clear which areas connect to which soakaways and their relative sizing/ capacity. the drainage strategy should make clear who will own and maintain the drainage system 4

5 Premier Water Solutions Ltd was awarded the contract of preparing the Flood Risk Assessment and was appointed by way of an from Mr Davis. The Flood Risk Assessment (FRA) has been written to satisfy the requirements of The National Planning Policy Framework (TNPPF). It should be noted that other consulting engineers have designed the drainage for Conygre Brook Park and Farmborough View and Premier Water Solutions has written the Flood Risk Assessment and therefore can take no liability for any work completed by others. 5

6 2.0 SITE DESCRIPTION The following description comes from a combination of viewing Google Earth historical images, survey information, web searches and plans. It is proposed to demolish the Trelawney bungalow sandwiched between Old Lane and Bath Road, and erect 9 no. new dwellings. It is proposed to keep the existing access of Bath Road and install 14 parking spaces and associated hard standing. There will not be any access from Old Lane onto the site. There will be retaining walls between the rear gardens of plots 1-9 and Old Lane. The site is roughly triangular with the narrow edge following the junction between Old Lane and Bath Road to the west of the site and the eastern boundary abuting the next door bungalow boundary. As described the northern boundary will be Bath Road and the southern Old Lane. The site slopes from Bath Road towards Old Lane from a ground elevation at the site entrance of 124.5mAOD to the rear garden behind plot 4 at 123.5mAOD and then again onto Old Lane at mAOD. The site is compact which will limit the use of some Sustainable Urban Drainage techniques. 2.1 EXISTING LAND USE Google Earth records that the Trelawney bungalow has been in existence for at least 13 years according to the images of 1999, 2001, 2005, 2006 and 2009 but it is likely that it is much longer than this. 2.2 EXISTING VULNERABILITY CLASSIFICATION According to the NPPF the Populars would have a vulnerability classification of More Vulnerable. 2.3 PROPOSED LAND USE It is proposed to demolish the Trelawney bungalow and erect 9 new dwellings, formalise the site access of Bath Road, and create parking spaces, gardens for residential use. 2.4 PROPOSED VULNERABILITY CLASSIFICATION More Vulnerable. 2.5 COMPARISON OF IMPERMEABLE AREAS The existing impermeable area of The Trelawney bungalow including the house, pathways, parking and tarmac entrance area = 782m2. The proposed impermeable areas including the new dwellings, access and manoeuvring is 578m 2. Therefore there will be a decrease in impermeable area of 204m 2. (Figures taken from Hatfield and Associates). 6

7 3.0 HYDROLOGY 3.1 GENERAL OVERVIEW OF THE HYDROLOGY IN THE AREA The site is located on a small site sandwiched between Bath Road and Old Lane. There are no formal watercourses nearby or any specific drainage features across the site. It is thought that the existing bungalow and hard standing areas drained to soakaways located on the site, the location of which, condition and size are unknown. There are highway gullies located on the northern edge of Bath Road and hence a formal highway drainage system is provided. There is no highway drainage system located within Old Lane. During rainfall it is expected that highway water is directed towards the northern kerb line and into the drainage system; it should be noted that there is the potential of some highway water egressing onto the site during exceptional events as the site is at a lower elevation to the highway. Rainwater within the site would drain via the formal soakaway or directly through the ground surface since the topography is falling towards Old Lane, it would be normal for the sub surface water flow to follow the same direction. It is expected that due to the retaining walls that there would be some runoff from the site either directly or indirectly onto Old Lane. Once onto Old Lane the road is steep and therefore the water would more than likely remain along the road surface with the direction of flow following the road downhill to Timsbury Road and beyond. The site lies within Flood Zone 1 (Low Risk). Inspection of the Environment Agency maps show that the site is not at risk of flooding from fluvial or tidal flooding; is not at risk of flooding from reservoir flooding, is not located within a Groundwater Protection Zone, but is located within a Minor Aquifer with high leaching potential. The source of the Conygre Brook is from two springs at Barrow Vale, to the east of Blackberry Hill, approx. 2.5km upstream of the site. This means that the Conygre Brook has a relatively small catchment area, which reduces the risk of flooding considerably. As a rule of thumb the larger the catchment area the higher the risk of flooding as there is a greater land mass to collect any rainwater. The Conygre Brook is an open natural channel until being culverted and channelled in a man-made system as it passes under the A39 Bath Road (approx. 240m to the west of the site). The brook changes direction and flows to the south and is conveyed in an open channel to the SE and along Bridge Gardens. Figure 1 shows this open channel. 7

8 Figure 1 - Open Channel at Bridge Garden Road It crosses underneath Bridge Garden Road at ST , flowing behind the properties off Bellifants Road before remerging at the in the agricultural field at ST and flowing through a 2.0m wide culvert under Timsbury Road. Figure 2 shows this culvert. 8

9 Figure 2 2.0m wide culvert at Timsbury Road The design of which should not impede the flow of water and therefore it would be unlikely for floodwater to back up and over top towards the site. This is further justified by the Environment Agency indicative flood maps. 3.2 OVERVIEW OF ALL SOURCES OF FLOODING Table 1 Overview of Flooding Source of Flooding Risk Rating Further Information Mitigation measures Mitigated Risk Rating Fluvial Low EA Indicative Flood Map checked and the site is outside of the Extreme Flood Outline. Low Tidal Low EA Indicative Flood Map checked and the site is outside of the Extreme Flood Outline. Low Reservoirs Low EA Indicative Flood Map checked and the site is outside of the Extreme Flood Outline. Low Surface Water High SUDS Design and Micro-Drainage Low 9

10 Results Provided Groundwater Medium Groundwater was not encountered in our intrusive test pits but due to the high clay content and the steep nature of the site, spring lines could be present during heavy rainfall. Medium Sewers/infrastructure /canals Low There are no adopted sewers in the vicinity of Trelawney Bungalow. There is an adoptable sewer which takes flow from No. 2 Bath Road potentially picking up 17 properties; this sewer currently crosses land now known as Conygre Brook Park. There is new drainage to be installed for Conygre Brook Park but this is at a lower elevation than the development site, and will not cause an impact on the development site. Low Highways High There is a possibility that highway can egress onto the site and therefore it is recommended to ensure that measures such as aco drains or raised kerbs are used to keep highway water on the highway. Medium 3.3 STRATEGIC FLOOD RISK ASSESSMENT The SFRA has been consulted on line ( ) and the Level 1 study identified areas at high risk of fluvial flooding in areas such as Bath, Keynsham, Midsomer Norton and Radstock. Further assessments were undertaken as a Level 2, which provided flood depths/velocities and other flooding information for these key urbanised areas, however these do not seem to mention the development area or Farmborough. 3.4 CONFIRMATION OF THE FLOOD ZONE Given the evidence provided by the SFRA, the Environment Agency s Indicative Flood Maps and PWS classifies this site to be Flood Zone 1 (low risk of flooding). 3.5 SITE INVESTIGATIONS Site Investigations have been undertaken by Ground Investigation on 26 th August Infiltration testing in accordance with BRE 365 were completed in test holes 1-4 as shown in figure 3. 10

11 Figure 3 Trial Pit Locations In summary the trial pit logs contained the following information: Trial Pit 1 - Made Ground m; Clay 0.65m 1.7m (d) Trial Pit 2 Made Ground m; Clay 1.7m 2.6m (d) Trial Pit 3 Made Ground 0.0m 1.7m; Clay 1.7m 3.6m (d) Trial Pit 4 Made Ground 0.0m 1.8m; Clay 1.8m 3.0m (d) No groundwater was encountered in any of the test holes. Table 2 Infiltration Test Results Trial Pit Trial 1 (m/s) Trial 2 (m/s) Trial Pit x 10-6 N/A Trial Pit x 10-6 N/A Trial Pit x x 10-4 Trial Pit x x 10-5 From these results the following observations are made: 1) No soakaway can be located within made ground (discounting soakaways across the site) 2) There is very little infiltration in Trial Pit 1 or 2 (discounting permeable paving at the front of the plots) 11

12 3) There is some infiltration in Trial Pit 3 and 4 but given the Made Ground, retaining walls and the 5m offset, it would not be possible to drain the whole site into soakaways. 12

13 4.0 PROPOSED DEVELOPMENT DISCUSSION The client wishes to demolish Trelawney and erect 9 dwellings with access from Bath Road. The site known as Trelawney has an existing impermeable area of 782m 2 and as a result of the development there will be 578m 2 impermeable area, thus there will be a reduction of 204m COMPATIBLE DEVELOPMENT According to the NPPF Technical Guide (table 3), More Vulnerable development is appropriate development for Flood Zone SAFE EGRESS There is safe egress to Bath Road during the 1 in 100 year flood event. 4.4 SAFE HAVEN Not required. 4.5 SEQUENTIAL AND EXCEPTION TEST Bath and north East council do not need to apply the Sequential Test. 4.6 EQUIPMENT PROVISION/FLOOD MITIGATION MEASURES None Required. 4.7 FINISHED FLOOR LEVELS Levels should be raised by 300mm above the existing ground level to reduce any impact of overland flow or from highway sources. 4.8 FLOOD STORAGE COMPENSATION The proposals will not encroach into the flood plain nor raise ground levels therefore flood storage compensation is not required. 4.9 RESIDUAL RISK Finished Floor Levels should be located at least 300mm higher than surrounding ground levels to prevent against overland flow if possible FLOOD DEFENCE CONSENT None required. 13

14 4.11 SAFE WARNING AND EVACUATION PLAN Not required EQUIPMENT PROVISION None Required FLOOD PLAN AND KIT None Required. 14

15 5.0 SUSTAINABLE URBAN DRAINAGE SYSTEM (SUDS) (APPENDIX A) Please refer to drawing EN 001.pdf which shows the proposed on-site drainage. Due to the site constraints listed in the site investigations in section 3.5 the use of soakaways and permeable paving has been discounted. Infiltration testing provide to be unsuccessful in the area to the front of the plots, which is where the permeable paving would have been sited. With permeable paving, the water needs to infiltrate through the base between each storm to ensure that the voids within the sub base are empty. If there was no infiltration through the base the voids would remain full for a considerable time period afterwards, when the rainstorm occurred the water would sheet off the surface towards the nearest drainage gully and would either flow off site towards Old Lane or towards the plots. Infiltration was found to be good in the ground to the rear of plots 1-9, however, when considering the 5m exclusion zone between the nearest dwelling and soakaway and the retaining wall, there was insufficient room This is because of the retaining wall to the rear of the plots leading down to Old Lane will prevent the use of soakaways. Water Introducing water behind a retaining wall at point source can increase the hydrostatic pressure which can either lead to the collapse of the retaining wall or water egressing directly onto Old Lane. Therefore in order to provide a SuDS solution which, will satisfy the authorities, it was proposed to take the drainage from Farmborough View across Old Lane and into the proposed drainage scheme at Conygre Brook Park. Table 3 Calculation of Impermeable Areas Area Description Area (m2) Farmborough View 562 Proposed Area 1-10 dwellings, Access from Old 3673 Lane, Access and garages Proposed Area 2-2 dwellings, access from Bridge 201 Gardens and off road parking 5.1 CALCULATION OF RURAL RUNOFF QBAR Undertaken in Micro Drainage and based upon a site area of 1 ha. Existing greenfield runoff rate QBAR = 7.5 l/s 5.2 LONG TERM STORAGE To consider long term storage we have used a total runoff from the site of 3 l/s which is equivalent to a runoff of 3/l/ha. 5.3 SUDS STRATEGY AREA 1 (APPENDIX B) Please ref to drawing C13894 C030 A Drainage Layout and C13894 Conygre Brook Park Storm Drainage Calc - To drain 4235m 2 of impermeable area via traditional 150mm plastic pipework and manholes, adopted where permitted by Wessex Water (S104) (roofs and private garages) and Somerset County Council (S38) (gullies and connecting pipework for the adopted road) to concrete tank constructed of pre cast concrete box culverts of dimensions 3.5m (w) x 3m (d) x 27m (l). The hydro brake attenuation device will limit the surface water to 2 l/s, in times of exceedence flows there will be an overflow weir from the tank with a high level connection into the downstream pipework. The upstream pipework, concrete tank, attenuation device and downstream pipework will be offered for adoption to Wessex Water under a Section 104 Agreement once planning permission is awarded. 15

16 5.4 SUDS STRATEGY AREA 2 Appendix B drawing C13894 C030 A Drainage Layout. To drain 201m 2 of impermeable area from the 2 dwellings and hardstanding to a flow control chamber to provide surface water storage of dimensions 2.7m diameter pre cast concrete ring to be adopted by Wessex Water, with discharge limited to 1 l/s by 20mm Orifice plate. The peak storage requirement for the 1 in 100 year event + 30% is 8.19m 3. The attenuated discharge will connect into the downstream discharge pipework from Area 1, with connection into the Conygre Brook. 16

17 6.0 CONCLUSIONS The site has been confirmed as being located within Flood Zone 1 (low risk of flooding) from tidal and fluvial sources. The site is further categorised as being at low risk from all sources of flooding, except highway (high) and groundwater (medium) where spring lines could emerge during extreme rainfall events due to the topography and underlying geology. Infiltration methods were discounted for a surface water solution due to the underground clay and retaining walls to the south of the site at Old Lane. A SuDS solution has been designed to take the surface water from Farmborough View underneath Old Lane and into the drainage system at Conygre Brook Park. A surface water management system has been designed to limit the discharge in Conygre Brook Park to 3 l/s which is equivalent to 3 l/s/ha. The rural QBAR for the site is 7.5 l/s and has been reduced to 3 l/s consider long term storage. The majority of the site will drain to a concrete tank of dimensions 3.5m (w) x 3m (d) x 27m (l) and will attenuate the discharge from 4235m 2 to 2 l/s. It is proposed for Wessex Water to adopt the pipework in Farmborough View and in Conygre Brook Park where possible, the concrete culvert attenuation storage tank, and downstream pipework. Somerset County Council will adopt the highway pipework in the adopted road at Conygre Brook Park. Area 2 which is where the 2 dwellings (201m2) at the south of the site are located, will drain to a separate 1.5m diameter pre cast concrete manhole storage tank with orifice plate limiting the discharge to 1 l/s with connection into the Wessex Water downstream pipework from Area 1. The attenuated discharge will connect directly into the Conygre Brook and will require a land drainage Consent from Somerset County Council as Lead Flood Authority under the Flood and Water Management Act This report has been written to satisfy the National Planning Policy Framework March

18 7.0 REFERENCES Document Date Location National Planning Policy Framework National Planning Policy Framework A Technical Guide Indicative Flood Maps Indicative Reservoir Maps Indicative Flood Warning Maps Maps Strategic Flood Risk Assessment Report (Level 1) March 2012 March 2012 October 2014 October 2014 October 2014 October

19 Appendix A Drainage Layout EN 001.pdf for Farmborough View 19

20 Appendix B Drawing C13894 C030A Drainage Layout.pdf, SKC007 A Impermeable Areas.pdf and calculations C