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Eunomia Research & Consulting Ltd

Waste Transfer Station, Broadway Lane, South Cerney, Cotswolds

Flood Risk Assessment & Surface Water Drainage Strategy

Document Ref: SJC/616528/JRC Prepared: Steve Cox Revision: 2 Managing Director

Date: 19 January 2015

Checked: James Calvert Chartered Engineer

Waste Transfer Station, Broadway Lane, South Cerney, Cotswolds © MLM Consulting Engineers Ltd Flood Risk Assessment & Surface Water Drainage Strategy

Project Revision Sheet

Revision No Date Status Changes Author Approved

0 08/01/15 Draft - SJC JRC

1 16/01/15 Final Surface water drainage strategy updated. JRC JRC

2 19/01/15 Final Text updated to suit Client comments JRC JRC

616528-REP-CIV-FRA


616528-REP-CIV-FRA

Contents Page 1 Introduction 1 2 Site Description 2 3 Flood Risk 6 4 Surface Water and Foul Drainage Strategy 8 5 Conclusions 9


Appendices Appendix A – Existing Site HM Land Registry – Site Plan Environment Agency online mapping - Flood Map for Planning (Rivers and Seas) Environment Agency online mapping - Risk of Flooding from Reservoirs Environment Agency online mapping – Risk of Flooding from Surface Water MK Surveys drawing 20144 Sheet 1 – Topographical and Utility Survey MK Surveys drawing 20144 Sheet 2 – Topographical and Utility Survey MK Surveys drawing 20144 Sheet 3 – Floor plans/Section MK Surveys drawing 20144 Sheet 4 - Elevations Appendix B – Proposed Site Eunomia drawing 2 – Depot Configuration Plan – Option 1 Appendix C – Thames Water Sewer Records Foul Water Sewer Plan Appendix D – Surface Water and Foul Drainage MLM drawing 616528/100 – Foul and Surface Water Drainage Strategy MicroDrainage Surface Water Calculations Appendix E – Environment Agency Flood Data

616528-REP-CIV-FRA


1 Introduction

1.1 MLM Consulting Engineers Limited has been appointed by Eunomia Research & Consulting Ltd to undertake a Flood Risk Assessment (FRA) for the proposed extension and change of use of an existing vehicle maintenance depot to form a Waste Transfer Station, Broadway Lane, South Cerney, Cotswolds.

1.2 This report has been prepared for the sole use of Eunomia Research &

Consulting Ltd and the contents should not be relied upon by others without the express written authority of MLM Consulting Engineers Limited. If any unauthorised third party makes use of this report they do so at their own risk and MLM owes them no duty of care or skill.

1.3 This report has been completed in accordance with the National Planning

Policy Framework (NPPF) and its accompanying Planning Practice Guidance (PPG). The report is an assessment of flood risk to the development, from on and off site sources, and to off site receptors caused by the redevelopment of the site. The report includes details of the surface water drainage strategy for the site which sets out how the proposals will not increase off site flood risk.

1.4 The site is shown to lie in Flood Zone 1 (low risk). Flood Zone 1 is the area

described as having less than a 1 in 1000 (0.1 %) annual probability of fluvial flooding.

1.5 The proposed use is classed as ‘less vulnerable’ in accordance with Table 2

of the PPG. Table 3 of the PPG shows that all development is appropriate in Flood Zone 1 and therefore the proposed redevelopment of the site is appropriate in flood risk terms.

1.6 This report concludes that in flood risk context, the proposals are safe and

appropriate and do not cause increased flood risk.

616528-REP-CIV-FRA Page 1


2 Site Description

Existing Site

2.1 The existing site is currently being used as a vehicle maintenance depot. The site extent shown by the red line on Eunomia Depot Configuration Plan is split into two sections with an access road for an existing building dividing the two. The larger of the two sites (to the east) is approximately 1.6 ha with the smaller site (to the west) being approximately 0.3 ha.

2.2 The larger site is predominantly surrounded by the Cotswold Water Park to

the north, east and west with the exception of the southern boundary which is bounded by Broadway Lane. The smaller site has the existing building access road to its north and east, Broadway Lane to the south and west.

2.3 The site is centred on approximate Ordnance Survey (OS) grid reference

405289, 196134.

2.4 A topographical survey of the site, see Appendix A, shows that the site is relatively flat with ground levels at approximately 87.7 to 88.3 mAOD.

2.5 The nearest significant watercourse (shown as Main River on the EA data in

Appendix E) is shown to lie approximately 150 m from the southern boundary of the site.

2.6 The existing development is classed as ‘less vulnerable’ in accordance with

Table 2 of the Flood Risk element of the PPG.

2.7 The existing larger site is currently used as a vehicle maintenance depot and storage area, this is the site to be used, extended and changed. The smaller site to the south-west (Area F as shown in Fig 1 below) is an existing car park not affected by the scheme.

2.8 The facility includes a steel portal frame building, approximately 15.6 m

wide and 42 m long. 2.9 The building has a concrete apron directly in front of the building (Figure 1

- Area A) laid to fall towards a central drainage channel with gullies along the front of the building.

2.10 There is a permeable block paved parking area off to the north (Figure 1 -

Area C). This area is surrounded by a palisade steel fence and gates to the compound. The steel fence around the north edge of the site is set in from the redline boundary, the actual site boundary appears to be in dense trees and shrubs.



Figure 1 – Annotated site plan

2.11 Across from the main access road from the building is an area of wasteland

not currently in use (see Figure 1 - Area E). This reaches across to the edge of the lake, but it divided from it by dense trees and vegetation at the water’s edge. The redline boundary appears to be the edge of the lake.

2.12 At the north-east corner of the site is a fenced concrete compound (Figure

1 - Area D) currently used as a vehicle fuelling facility. The redline boundary is beyond the fence in the dense trees and shrubs.

2.13 To the left (north-west) of the main entrance gates is an area of hard

standing and made ground (Figure 1 - Area F). 2.14 To the right of the main entrance gates is an area of broken concrete and

tarmac hard standing used for storage (Figure 1 - Area G). This area backs onto the adjacent saw mill, separated from this site by a small earth bund as shown on the topographical survey.

Existing Drainage

2.15 The RWPs from the main building gutters are assumed to be designed discharge to outfall culvert in the lake. The concrete apron in front of the building (Figure 1 - Area A) falls to gullies which link together and are assumed to be designed to drain to the outfall culvert to the lake.



2.16 MLM have been advised that there is a discharge consent in place for the outfall to the lake. There are two manholes in the fuelling compound (Figure 1- Area D) which contain a hydro-brake and sampling point which then are assumed to outfall to the lake via a culvert.

2.17 The four gullies to the north-east of the main building are assumed to be

designed to drain to the outfall culvert in the lake (Figure 1 - Area B). It appears from the MK Surveys Utility Drawing that these drains discharge into a stone blanket beneath the porous paving. It also appears that the porous paving discharges to the piped outfall system.

2.18 The second yard to the north east is drained by permeable paving. This is

used as a parking area for HGVs. 2.19 Beany Block drains are present along the access road in front of the main

building and adjacent property. These appear to discharge to the lake outfall in Area D.

2.20 A petrol interceptor is located within the fuelling compound (Figure 1 -

Area D). 2.21 Gullies are present in the hard standing area on the left of the entrance

(Figure 1 - Area F) but their outfall is unknown. 2.22 No drainage was located in the hard standing on the right of the entrance

(Figure 1 - Area G). 2.23 The main route for foul drainage from the main building off site was

located in the access road. From a manhole at the end of the access road it crosses the adjacent property before connecting to another manhole in the corner of the site. From this location the drainage leaves the site to an adopted pumping station along the road.

2.24 SVPs were located on three corners of the building. The exact route of the

drainage is uncertain but appears to connect to the foul drain in the access road. The route from the SVP on the western corner could not be determined.

2.25 Existing foul drainage appears to be working; there was no evidence of

surface sewage or contamination noted. 2.26 The on-site foul and surface water drainage systems are generally shown

on the topographical survey and the off-site foul pumping station is shown on the Thames Water sewer plans in Appendix A.

Proposed Site

2.27 It is proposed to extend and utilise the site as a Waste Transfer Station. 2.28 The site currently has a vehicle maintenance and crew welfare/office

building (built in 2001), an area for vehicle parking and a fuel store. We propose to retain the existing facilities and use the site for vehicle parking for 33 refuse collection vehicles (including residual and recycling) and for 25 other vehicles such as transit vans, which support the waste and street cleansing services. In addition we propose to build a waste transfer station on the site, which will comprise the following facilities:




• Glass recycling bays (open, i.e. without a roof);

• A building containing recycling bays for paper, kitchen waste, cardboard and mixed cans and plastics. This will have sorting and baling equipment, to sort and bale the card, cans and plastics, prior to them being transported in bulk to reprocessors. It will also have a ‘lean-to’ roof extension on one end to cover the area where the covered food skips will be kept. This building will be a similar design and height as the existing vehicle maintenance building;

• A building for the tipping and bulking of residual and organic waste - this waste would then be transferred to bulk road transport vehicles for transfer to disposal/recovery sites. This building will be a similar design to the vehicle maintenance building and be around 11 m high (to ensure that walking floor trailers can be safely loaded).

• There will be a roofed area to keep baled recyclates awaiting collection by reprocessors.

• Staff parking – there will be around 110 employees at the site and we intend to allow for staff parking for 72 cars.

2.29 The total tonnage of dry recycling passing through the facility per annum

would be in the region of 10,000 tonnes. In addition, there would be around 3,000 tonnes of separately collected kitchen waste. Residual waste would total around 12,000 tonnes per annum and garden waste would total 10,000 tonnes. The annual total of all wastes passing through the site would be around 35,000 tonnes.

2.30 The existing fuelling area is to be retained as a fuelling & vehicle wash

area.

2.31 The previously undeveloped land south east of the access road (Area E) is to be extended and developed to provide new facilities including include a weighbridge, bin store and furniture space, bale store, baling area (for card, plastic cans and paper) and various waste containers. The proposed site layout is shown on Eunomia drawing 2, dated 06.12.14 in Appendix B.

2.32 The smaller of the two sites does not have any development proposals. 2.33 Waste treatment development is classed as ‘less vulnerable’ in accordance

with Table 2 of the PPG. The proposed redevelopment does not affect the vulnerability classification. As the site is located in Flood Zone 1 the proposed development is shown to be appropriate in accordance with Table 3 of the PPG and no Sequential Test (or Exception Test) is required.

2.34 British Geological Survey mapping shows the site as being underlain by

Kellaways Clay Member (Mudstone), no superficial deposits are recorded.



3 Flood Risk Tidal and Fluvial Flooding

3.1 The site is shown on the Environment Agency (EA) flood map (see Appendix A) to lie in the low probability flood zone (Flood Zone 1). The closest watercourse lies approximately 150 m to the south and is shown as Main River on the EA data in Appendix E.

3.2 According to the Environment Agency data (see Appendix E) the site is not

protected by any formal defences and there are no flood alleviation works planned for the area. Modelled flood level information has been provided and reviewed, it is noted that this data is taken from the Thames (MRL to St Johns Lock) Flood Mapping Study completed in January 2014 which was carried out using 2D modelling software.

3.3 The modelling data identifies 5 different floodplains surrounding the site.

3.4 Floodplains 4 and 5 have the highest levels; both are 87.69 for the 1% AEP

with climate change allowance. The 0.1% AEP levels are 87.72 and 87.73 respectively. These floodplains are separated from the site by Broadway Lane which is at a higher elevation than the maximum flood levels thus protecting the site from flooding from floodplains 4 and 5.

3.5 Floodplain 3 is shown to lay to the south east of the site and has predicted

flood levels of 87.36 mAOD for the 1% AEP with climate change allowance and 87.39 mAOD for the 0.1% AEP level. According to the EA flood mapping this does not encroach onto or close to the site and this appears to be due to the existing land levels between the flood zone and the site being higher than the flood level. The site therefore does not appear to be at significant risk of flooding from floodplain 3.

3.6 Floodplain 1 and 2 are immediately adjacent to the site to the north west

and south east respectively. The predicted flood levels are 85.47 and 85.07 mAOD respectively for the 1% AEP with climate change allowance. The 0.1% AEP levels are shown as being the same as the 1% AEP with climate change allowance. The lowest site level is 86.67 mAOD therefore the site is not at significant risk of flooding from floodplain 1 or 2.

3.7 Historical flooding maps shown in the EA data show that the site has not

flooded during past recorded events including the 1947 flood event. Surface Water from Off-Site 3.8 There is always a potential risk of surface water flooding from very high

intensity rainfall events exceeding the capacity of drainage systems. The EA surface water mapping (see Appendix A) shows that the site is not at risk of surface water flooding. Also from a review of OS contour mapping and the topographical survey there are no specific significant surface water flood risks to the site. The site is therefore not considered to be at significant risk of flooding from this source.



Surface Water from On-Site

3.9 There is always a potential risk of surface water flooding from very high intensity rainfall events exceeding the capacity of drainage systems. In this instance the site is relatively flat, falling gently from south-west to north-east by about 0.5 m over the length of the access road, with the existing HGV parking area at the northern corner being slightly lower. Whilst there may be a risk of some surface water ponding in places on the site during such an event, it is not considered to be a risk of significant flooding from this source.

3.10 The proposed development of the site will increase the impermeable area

and the volume of surface water run-off generated by the site. Proposed drainage is dealt with later in this report.

Infrastructure Flooding

3.11 If surcharging or blockage of the sewers / drains and the foul pumping

station in the vicinity of the site did occur it is possible that there may be localised surface ponding in areas on or surrounding the site. The risk of significant flooding to the site from sewers or drains is assessed as low.

Water Bodies 3.12 The site is surrounded to the north, east and west by water bodies (lakes

associated with Cotswold Water Park) which do not appear likely to pose a significant risk to the site as detailed above. The EA Reservoir Inundation map (see Appendix A) shows that the site is not at risk of flooding from reservoir failure.

Groundwater 3.13 British Geological Survey mapping shows the site as being underlain by

Kellaways Clay Member (Mudstone), no superficial deposits are recorded. The underlying geology is expected to be generally of low permeability. It is considered unlikely that groundwater would express at the surface of the site, given the lower land and water levels in the adjacent lakes to the south, east and north. The site is therefore not considered to be at significant risk of flooding from groundwater.


4 Surface Water and Foul Drainage Strategy

Surface Water 4.1 The existing ground is either fill or made ground; it may therefore

potentially be contaminated making it unsuitable for infiltration drainage. The drainage strategy drawing shows how a new positive piped system can be designed and installed. This is shown with paved areas draining via trapped gullies into a piped system which in turn passes through a class 1 bypass separator before discharging into the lake through a new outfall alongside the existing outfall. The roof run-off discharges into the new system but downstream of the separator. The system has been checked using MicroDrainage to show that all flows for the 100 year return period event plus climate change are accommodated although some minor surface ponding is shown to occur on the new car park area in such an event of short duration and high intensity.

4.2 If the run-off were discharging into a fluvial watercourse then it would be

appropriate to restrict run-off from the new development to greenfield run-off rate. However, since the development discharges into a lake and as the volume of water discharging from the site would not significantly raise the water level in the lake (i.e. the site area is small in comparison with the lake) there would be no benefit in flood risk terms in doing so. Direct discharge has therefore been designed. As well as a separator and trapped gullies, other pollution control measures such as catchpits and a shut-off valve will be included.

4.3 To show how this could be achieved an illustrative drainage strategy

drawing together with supporting MicroDrainage calculations have been produced and are included in Appendix D.

Foul Water

4.4 Some of the proposed areas of the site such as the residual waste and

garden waste area are in buildings. Where internal gullies e.g. For wash down purposes are included in the buildings that could contain potentially polluting materials or processes, run-off from within these areas may be polluted and unsuitable for discharge to the surface water system. Any such small areas have been shown to drain to the existing foul drainage system on the site.



5 Conclusions

5.1 The proposed site is shown to lay in the low probability flood risk area (Flood Zone 1) according to the Environment Agency flood maps.

5.2 There is a risk of localised surface water ponding on the site due to blocked

or overwhelmed drainage systems but there does not appear to be a risk of significant flooding from this source.

5.3 Other flood risks to the site have been assessed as low and the site is

therefore considered to be at low risk from all sources of flooding.

5.4 Investigations should be undertaken to determine whether infiltration drainage would be viable. Assuming that infiltration drainage is not viable, it is proposed to discharge surface water from the impermeable areas to the existing surface water drainage system, using attenuation and restricting discharge from the new impermeable area to 5 l/s. The capacity of the outfall pipe will need to be investigated and may need to be replaced or supplemented with a separate pipe.



Appendix A – Existing Site

HM Land Registry – Site Plan Environment Agency online mapping - Flood Map for Planning (Rivers and Seas)

Environment Agency online mapping - Risk of Flooding from Reservoirs Environment Agency online mapping – Risk of Flooding from Surface Water

MK Surveys drawing 20144 Sheet 1 – Topographical and Utility Survey MK Surveys drawing 20144 Sheet 2 – Topographical and Utility Survey

MK Surveys drawing 20144 Sheet 3 – Floor plans/Section MK Surveys drawing 20144 Sheet 4 - Elevations

616528-REP-CIV-FRA

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