Laurel hill flood risk assessment by 10bristol

FLOOD RISK ASSESSMENT FOR

PROPOSED RESIDENTIAL AND COMMERCIAL DEVELOPMENT AT LAND AT LAUREL HILL, CRIBBS CAUSEWAY, BRISTOL ON BEHALF OF

DICK LOVETT GROUP LIMITED MAY 2014 [ISSUE 3]

Head Office: Unit 2 York House Edison Park Dorcan Way Swindon Wiltshire SN3 3RB Tel. 01793 619965 Fax. 01793 619967 Email: cec@ColeEasdon.com www.ColeEasdon.com

Cole Easdon Consultants Limited Civil Engineering ~ Highways ~ Transportation ~ Flood Risk Bristol • Swindon • Warrington Incorporated in UK as Cole Easdon Consultants Ltd No. 202 7005

Cole Easdon Consultants (CEC) DOCUMENT ISSUE RECORD

Client:

Dick Lovett Group Limited

Project:

Proposed Residential and Commercial Development at Land at Laurel Hill, Cribbs Causeway, Bristol

Job Number:

4111

Document Title:

Flood Risk Assessment

Issuing Office:

Swindon

Issue / Revision:

Issue 1

Issue 2

Issue 3

Description / Status:

Draft for Client Comment

Formal Issue

Date:

February 2014

April 2014

May 2014

N. Parajuli MEng

S. Coates BA (Hons)

R. Bowley BSc CEng MCIWEM

4111 – Land to east of Cribbs Causeway - FRA – Issue 1

4111 – Land at Laurel Hill, Cribbs Causeway - FRA – Issue 2

4111 – Land at Laurel Hill, Cribbs Causeway - FRA – Issue 3

Prepared: Signature: Document Check: Signature: Technical Check: Signature:

Authorised: Signature: File Reference:

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CONTENTS SECTION

HEADING

PAGE NO.

1.0

INTRODUCTION

2.0

THE EXISTING SITE

3.0

FLOOD RISK ISSUES

4.0

FOUL WATER DRANAGE DISPOSAL

5.0

DISCUSSION AND CONCLUSIONS

APPENDICES Appendix 1 – CEC Plans and Figures 4111/500/Figure 1

Site Location Plan

4111/500/Figure 2

Flood Map

4111/500

Existing Site Layout

4111/SK501

Preliminary Drainage Layout

Appendix 2 – Drawings by Others 1459-100 A

Commercial Masterplan (by NC Architects)

BRS.4751_06 F

Concept Plan (By Pegasus)

PRCCCB-590

As Built Levels, Drainage and Services (by Halcrow)

Appendix 3 – Sewer Records Appendix 4 – EA Flood Data Appendix 5 - Calculations

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INTRODUCTION

1.1

This Flood Risk Assessment (FRA) has been prepared by Cole Easdon Consultants (CEC) on behalf of Dick Lovett Group Limited in respect of a planning application for the proposed residential and commercial development at land to the rear of Dick Lovett, Cribbs Causway, Bristol. Refer to CEC Figure 4111/500/Figure 1 [Site Location Plan] contained in Appendix 1. Development Proposals

1.2

The development proposal comprises two separate parcels of land being developed for residential and commercial use. The residential development comprises the erection of up to 110 No. properties, with associated access road, parking and landscaping. The commercial parcel comprises buildings and car parking facilities as an extension of the existing Dick Lovett car dealership facility at Cribbs Causeway. Refer to Appendix 2 for proposed master plans. Need for Study

1.3

This report intends to provide the LPA and EA with the main relevant points with respect to flooding in order to determine any planning application. The study has been undertaken as per guidance provided within the National Planning Policy Framework (NPPF) and the accompanying technical guidance to that document. The NPPF requires development sites exceeding 1 ha within Flood Zone 1 to be accompanied by a site specific FRA.

1.4

Accordingly, this study has been prepared to: i)

assess flood risk to the development from fluvial sources;

ii)

assess flood risk to the development from other potential sources, including ditches, sewers, groundwater, overland flows and artificial sources;

iii)

ensure that the proposed development will fully comply with the requirements of the Environment Agency's policy on the safeguarding of floodplains, as detailed within the NPPF;

iv)

investigate sustainable surface water disposal for the site; and

consider appropriate foul water drainage proposal for the site.

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In Section 2.0, we describe the characteristics of the proposed development site and surrounding area. In Section 3.0, we assess flood risk issues. In Section 4.0, we discuss foul water drainage proposal for the site. Finally, conclusions are presented in Section 5.0.

1.6

The following publicly available documents have also been reviewed as part of this assessment: 

National Planning Policy Framework (March 2012);



Technical Guidance to the NPPF (March 2012);



South Gloucestershire Council Strategic Flood Risk Assessment (SFRA) – Level 1 (Scott Wilson, February 2009);



South Gloucestershire Council Strategic Flood Risk Assessment (SFRA) – Level 2 (JBA Consulting, December 2011);

 1.7

South Gloucestershire Council Preliminary Flood Risk Assessment (PFRA) (August 2011).

The site topographical survey (referenced to an Ordnance Datum) undertaken by D & H Surveys Limited (Plans 981, Sheet 1 and 2 Rev C) has been provided to Cole Easdon Consultants and has been reviewed as part of this study.

This topographical survey is

included within CEC Drawings 4111/500 [Existing Site Layout] and 4111/SK501 [Preliminary Drainage Layout] within Appendix 1. 1.8

The following abbreviations are used in this report: EA – Environment Agency NPPF – National Planning Policy Framework SFRA – Strategic Flood Risk Assessment SuDS – Sustainable Drainage System PFRA – Preliminary Flood Risk Assessment AOD – Above Ordnance Datum BGS – British Geological Survey

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THE EXISTING SITE Site Location and Topography

2.1

The site is located within the northern fringes of the city of Bristol, some 10.0km north of the city centre. The site is located to the eastern side of the A4018 Cribbs Causeway, some 1.0km southwest of the Junction 17 of the M5. Refer to CEC Figure 4111/500/Figure 1 [Site Location Plan] contained within Appendix 1.

2.2

The application site is located to the rear of the existing Dick Lovett car showrooms at Cribbs Causeway just to the south of an existing highway The Laurels off the A4018 Cribbs Causeway.

2.3

The site lies within mixed urban and rural settings. The River Henbury Trym forms the southeastern site boundary. The site is adjoined by the Dick Lovett car showrooms and other commercial units at Cribbs Causeway to the northwest, by residential properties to the northeast, by fields to the southeast with Filton Airfield located beyond, and by recently approved car body repair centre for Dick Lovett to the southeast.

2.4

The total development site extends over an area of approximately 7.00 ha, comprising 2.50 ha of residential development parcel and 4.50 ha of commercial development parcel. Out of the commercial parcel, the north western site extent (approx. 0.90 ha) comprising existing built up area (buildings and car parking) is excluded in this study. Refer to CEC Drawing No. 4111/500 [Existing Site Layout] within Appendix 1. The site predominantly exists as grass fields.

2.5

The site has a general fall in the southerly direction towards the Henbury Trym. Site levels vary from approximately 53.00mAOD (Above Ordnance Datum) within the northern region to 40.00mAOD within the southern region adjacent to Henbury Trym. Henbury Trym exists as an approximately 1.5m deep and 1.5m wide channel at the location of the site. Refer to CEC Plan 4111/500 [Existing Site Layout] within Appendix 1. Nearby Watercourses/Drainage Features

2.6

The Henbury Trym is located along the southeastern site boundary. The Henbury Trym rises from its source near Cribbs Causeway commercial estate and flows southwesterly between the site and the Filton Airfield towards the Bristol city. The watercourse flows into the tributary of the River Trym, namely Hazel Brook downstream of the A4018, approximately

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FLOOD RISK ASSESSMENT PROPOSED RESIDENTIAL AND COMMERCIAL DEVELOPMENT AT LAND AT LAUREL HILL, CRIBBS CAUSEWAY, BRISTOL ___________________________________________________________________________________ 0.70km to the southwest of the site. The Henbury Trym is designated as a main river at this location. 2.7

The Cribbs Reservoir (also known as Lysander Road Delaying Reservoir) is situated on the upper part of the Henbury Trym catchment, 200m upstream of the site. This reservoir functions to attenuate surface water runoff from Cribbs Causeway Regional Shopping Centre (RSC). A proportion of the runoff from the Filton Airfield drains into this catchment.

2.8

Several shallow drainage ditches runs across the site and its vicinity, which generally outfall into the Henbury Trym. These ditches will be retained where possible. Existing Drainage/Sewers

2.9

Sewer records obtained from Wessex Water indicates the presence of a 375mm diameter public foul water sewer within the south eastern extent of the site, adjacent to the Henbury Trym. Two 225mm diameter foul and surface water sewers enter the residential site from the north, runs westerly and then southerly direction. The foul sewer joins the abovementioned 375mm dia. foul sewer within the south eastern extent of the site whilst the surface water sewer discharges into the Henbury Trym. Refer to Appendix 3.

2.10

A 150mm diameter foul water drain serving the adjoining Dick Lovett site enters the development site from the southwestern boundary, turns 900 to join the abovementioned 375mm dia. public foul water sewer within the southern region adjacent to the Henbury Trym. A 150mm dia. surface water drain exists to the south west of the site (within the car body repair centre site) which runs to discharge into the Henbury Trym downstream of the site. The existing drainage information is based on Drawing No. PRCCCB-590 [As Built Levels, Drainage and Services] by Halcrow included within Appendix 2. These are not indicated within the Wessex Water sewer records. There are possibly further drains serving commercial units to the north. Existing Ground Conditions

2.11

British Geological Survey (BGS) Map available online indicates the area to be underlain by Mercia Mudstone Group. No detailed ground investigation works have been undertaken at the development site. Site investigation works undertaken within the adjacent sites to the north and west indicate presence of weathered Mercia Mudstone comprising locally fissured and indistinctly structured slightly sandy, silty clay (0.70 â&#x20AC;&#x201C; 2.6m thick) increasing in strength with depth.

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The EA’s Aquifer Map indicates Mercia Mudstone bedrock beneath the area as ‘Secondary B’ Aquifers. These are predominantly lower permeability strata which may store and yield limited amounts of groundwater due to localised fissures, thin permeable horizons and weathering. The EA’s Groundwater Source Protection Zone Map indicates that there are no local sources of abstraction, nor any groundwater protection zones in the vicinity of the site.

2.13

Groundwater is expected to be at shallow depth considering the proximity to the watercourse within the southern extent. Site investigation works undertaken at a site to the immediate north did not record any groundwater to a depth of 3.07m beneath the surface.

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FLOOD RISK ISSUES

3.1

This section of the study reviews historical flooding events within the local catchment area and presents an assessment of flood risk to the proposed development from: A) external sources; and B) surface water discharge from the proposed development.

3.2

Recommended flood risk mitigation measures appropriate to the level of perceived risks are included within this assessment, and summarised in Table 3.3 on page 13. Flood History

3.3

The South Gloucestershire Council Level 2 SFRA includes historical flood information for Cribbs/Pathway area indicates highway flooding incident at the M5 near Cribbs Causeway commercial area, and a flooding incident at Passage Road. The site is not known to have been affected by flooding in past.

Assessment of Flood Risk to the Development Site from External Sources Ai)

3.4

Assessment of Flood Risk from Fluvial Sources

The Environment Agency flood map indicates that the site is located within Flood Zone 1 (low risk) with less than a 1 in 1000 (0.1%) annual probability of fluvial flooding. The extreme southeastern site boundary (adjacent to the Henbry Trym) possibly encroaches upon the EA Flood Zone 2 (medium risk zone) with an annual probability of fluvial flooding of between a 1 in 100 and 1 in 1000 (1% - 0.1%). Refer to CEC Figure 4111/500/Figure 2 [Flood Map] within Appendix 1.

3.5

Modelled data have been obtained from the EA that provide flood data up to the 1 in 100 year plus climate change (1% + CC) events. These relate to the River Trym SOP Study (1D HEC-RAS) model that was produced in 2010. Refer to EA flood data within Appendix 4. These indicate 1% flood levels varying between 42.40m and 40.10m, and 1% + CC flood levels of between 42.45m and 40.15m. Comparison with ground levels indicate the 1% floodplain (Flood Zone 3) is contained within the watercourse channel except within the extreme southern extent where it slightly overtops the bank. The 1% + CC floodplain is also contained within the banks within the south eastern extent, whilst encroaches marginally into the site as it progresses downstream. Refer to CEC Plan 4111/500 [Existing Site Layout] within Appendix 1.

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The development proposals provided within Appendix 2 indicate that the development will largely remain out of 1% and 1% + CC floodplains. With a 10-15m wide green/ecological buffer zone maintained adjacent to the Henbury Trym, it is likely that the development will mostly remain within Flood Zone 1. The proposed car parking is likely to slightly extend into the 1% + CC floodplain within Flood Zone 2.

3.7

The proposed site usage as ‘residential’ is classified as ‘more vulnerable’ whilst ‘commercial’ is classified as ‘less vulnerable’ within the Technical Guidance to the NPPF. Both land uses are considered appropriate within Flood Zones 1 and 2. As such, the proposed site use remains compatible with flood risk vulnerability associated with the site. Fluvial Flood Risk Mitigation Measures

3.8

No mitigation measures are required. However, it is advised that no ground raising is involved within the car parking area located within the 1% + CC floodplain to ensure that the existing floodplain storage or flood flow routes remain unchanged. A minimum of 8m buffer will be maintained from the top of the bank to the development zone. All proposed above or below ground storage structures are to be located outside the 1% + CC floodplain extent. Aii)

3.9

Assessment of Flood Risk from Surface Water (Overland Flow)

Local topography falls in a southerly direction towards the Henbury Trym located adjacent to the site. Land rises to the north and is occupied by commercial complex and residential properties. These should have their own drainage systems to pick up surface water runoff. Should these flood under extreme events, floodwater would direct towards the A4018, and towards the site. Such flows entering the site would flow towards the Henbury Trym following topography. The layout should make provision for suitable overland flow routes to route such overland flows, such as via roadways and green corridors, without affecting any properties. Such flow routes are shown within CEC Plans 4111/500 [Existing Site Layout] and 4111/SK501 [Preliminary Drainage Layout] within Appendix 1. Surface Water Flood Risk Mitigation Measures

3.10

As mentioned above, the development layout should incorporate suitable overland flow routes to route any potential overland runoff entering the site safely towards the Henbury Trym. Refer to CEC Plan 4111/SK501 [Preliminary Drainage Layout] within Appendix 1. Setting finished floor levels within the buildings sufficiently above the adjoining ground (at least 150mm) will provide additional protection to properties.

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Assessment of Flood Risk from Existing Sewers/ Highway Drains

There are foul and surface water sewers and drains within the site and its vicinity. Should these foul or surface water drains/sewers flood, floodwater would direct southerly towards the Henbury Trym following natural site topography.

3.12

As mentioned above in Paragraphs 3.9 & 3.10 above, suitable overland flow routes should be incorporated within the layout to route such floodwaters safely towards the Henbury Trym. The layout should allow for 3m easement on either side of the sewers crossing the site, or consider suitable diversion route in agreement with the Wessex Water or relevant owner as applicable. Refer to CEC Plan 4111/SK501 [Preliminary Drainage Layout] within Appendix 1. Sewer Flood Risk Mitigation Measures

3.13

The development should incorporate suitable overland flow routes to safely route floodwaters towards the Henbury Trym to the south without affecting any properties. The foul sewer/drain within the site should be provided with a 3m easement on each side to allow for maintenance. Aiv)

3.14

Assessment of Flood Risk from Groundwater

The site is underlain by Weathered Mercia Mudstone comprising fissured sandy silty Clay, with potential to hold water within the localised fissures. Groundwater beneath the site should remain in continuity with the water level within the adjoining watercourse, and is likely to occur at shallow depths, particularly within the low lying southern region. However, groundwater should remain relatively deeper beneath the higher ground to rise to the surface, considering relatively gentler flow gradient and low groundwater fluctuation characteristics within such strata.

3.15

Under extreme events, it is likely that groundwater would emerge to the surface within the low lying southern region. Should groundwater rise to the surface, it would flow towards the Henbury Trym, and would not affect large part of the site. Groundwater Flood Risk Mitigation Measures

3.16

No mitigation measures are required. Setting finished floor levels sufficiently above the existing levels and at least 150mm above the adjoining finished ground levels will afford additional protection from rising groundwater.

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Assessment of Flood Risk from Artificial Sources

The Cribbs Reservoir is located 200m upstream of the site on Henbury Trym. The Cribbs Reservoir is an impounding structure with 5m maximum depth and 36,000m3 capacity. It was originally designed to attenuate excess surface water flows from the Cribbs Causeway RSC development. The structure however does not function to reduce flood risk to downstream area. The SFRA identifies the Cribbs Reservoir as one of the potential source of artificial flooding within the site vicinity. The SFRA mentions that when the reservoir is at capacity, the high rate of flow that spills from the reservoir is sufficient to cause flooding. However, no such incidents have been recorded in the past.

3.18

Should a breach or failure occur, the consequences are likely to be significant. Such events would affect areas around Passage Road, area upstream of the A4018 and Tormarton Crescent in the vicinity of the site. The southern extent of the development site is shown to extend within the reservoir failure floodplain extent.

3.19

However, reservoir flooding is extremely unlikely to happen. The EA relate that there has been no loss of life in the UK from reservoir flooding since 1925. The responsibility for managing flood risk from reservoirs lies with the owners of each reservoir, while the EA have a duty to ensure that this responsibility is enforced. As the enforcement authority for the Reservoirs Act 1975 in England, the EA ensure that reservoirs are inspected regularly and essential safety work is carried out. Through the enforcement of regular inspection and maintenance, the risk of flooding as a result of reservoir failure is considered low. Reservoir â&#x20AC;&#x2DC;Failureâ&#x20AC;&#x2122; Flood Risk Mitigation Measures

3.20

No mitigation measures are required.

Assessment of Flood Risk Arising from Surface Water Discharge from the Proposed Development Bi)

3.21

Surface Water Runoff Control

The introduction of hard areas within the new development will increase site impermeability, resulting in potential increase in surface water runoff when compared to the existing greenfield site condition. In order to mitigate flood risk posed by post development runoff, adequate control measures will be required within the site.

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Surface water runoff from the development site will be managed sustainably, taking into account the requirements of the Local Authority, the EA, the NPPF technical guidance and any site specific constraints as follows: 

runoff from the proposed development will not exceed runoff from the existing site



condition;



Sustainable Drainage Systems (SuDS) will be implemented wherever possible;



surface water runoff will be managed as close to source as is possible; and



surface water runoff will be managed on site for storm events up to and including the 1 in 100 year event plus 30% to allow for the climate change (as suggested within the NPPF Technical Guidance).

Existing Site Drainage 3.23

The site exists as undeveloped greenfield land with no formal drainage arrangements. The site currently drains into the Henbury Trym via overland flow routes and onsite drainage ditches. The existing site generates 1 in 100 year runoff of approximately 24.0l/s and 27.4 l/s from the respective residential and commercial sites. Refer to Appendix 4 for calculations. The existing greenfield runoff rates have been calculated using Microdrainage in accordance with ICP for SUDS – FSR Method as shown in Table 3.1 below. Table 3.1:

Greenfield Runoff Calculation Greenfield Runoff (l/s)

Site

Area (ha)

Residential

1 in 1 year

1 in 30 year

1 in 100 year

Mean Annual(Qbar)

2.50

7.8

19.00

24.10

9.90

Commercial

3.60

11.20

27.30

34.70

14.30

Total

6.10

19.00

46.30

58.80

24.20

Proposed Site Drainage 3.24

The development is likely to include approximately 1.25 ha impermeable area within the residential development site (assumed 50% impermeable) and 2.90 ha impermeable area within commercial site (assumed 80% impermeable). This is likely to significantly increase surface water runoff rates from the development site when compared to the existing greenfield site condition without any mitigation measures. However, surface water runoff from the development site will be limited to the existing greenfield runoff rates, thereby maintaining status quo.

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The site is underlain by Mercia Mudstone (sandy silty Clay) with limited potential for infiltration. Further, the proximity to the watercourse indicates the possibility of shallow groundwater beneath the site. It is understood that the adjoining sites utilise attenuation systems with controlled outfalls into the Henbury Trym. Infiltration drainage has therefore been discounted at this stage. This should be confirmed by site specific soakage testing and groundwater monitoring works before the detailed design stage.

3.26

Accordingly, surface water storage features such as attenuation basins or swales, (tanked) permeable paving, cellular systems etc will be utilised to attenuate surface water runoff within the site. Surface water runoff generated within residential and commercial sites will be dealt separately within each site itself. Refer to CEC Plan 4111/SK501 [Preliminary Drainage Layout] within Appendix 1. Attenuation basins will be located within open/green space adjacent to the Henbury Trym which will provide a percentage of the required storage volume whilst the remaining storage will be provided within tanked permeable paving, cellular storage systems etc suitably located throughout the site. A minimum of 8m buffer will be maintained from the top of the bank to the development zone and a minimum of 3m of easement will be available for the existing sewers.

3.27

The on-site attenuation systems will discharge into the Henbury Trym at a controlled rate using a Hydrobrake or similar flow control device. Total discharge into the Henbury Trym will be limited to a maximum of 24.0 l/s from residential site and 35.0 l/s from commercial site for events up to the 1 in 100 year and 30%. Accordingly, 600m3 and 1,600m3 of storage will be required within the site.

3.28

The on-site attenuation systems will be designed to accommodate the 1 in 100 year + 30% storm event. Accordingly, storage as shown in table 3.2 will be required on-site. Refer to Appendix 4 for calculations. Table 3.2:

Storage Calculation

Site

Area (ha)

Impermeable Area (assumed 60%)

Limiting Runoff (l/s)

Storage Volume (m3)

Residential

2.50

1.25

24.00

600.00

Commercial

3.60

2.90

36.00

1,600.00

Total

6.10

4.15

60.00

2,200.00

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In case of an extreme event exceeding the design standard or onsite drainage system failure/blockages, flooding is likely. Floodwaters under such circumstances would flow towards the attenuation basins located at low lying region, and finally into the Henbury Trym following natural site topography. The layout should incorporate suitable overland flow routes (roadways, green areas) to route such exceedance flows safely towards the storage features and watercourse, away from properties. Refer to CEC Plan 4111/SK501 [Preliminary Drainage Layout] in Appendix 1 for design exceedance flow route. Bii)

3.30

Adoption

All the onsite surface water piped systems are intended to be adopted by Wessex Water, the sewerage undertaker for the area. The attenuation basins located within public area will be offered for adoption by the Local Authority, or alternatively managed by a private management company. All other SuDS facilities will be managed by a private management company. Biii)

3.31

Water Quality

The attenuation basins could be designed with permanent wet pools at the bottom to offer water quality improvement opportunities. Tanked permeable paving and cellular systems will utilise pre-treatment devices (e.g. silt chamber, oil separator etc) prior to discharging into the Henbury Trym. Utilisation of such SuDS features will ensure that surface water discharge entering the Henbury Trym is of acceptable quality.

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FLOOD RISK ASSESSMENT PROPOSED RESIDENTIAL AND COMMERCIAL DEVELOPMENT AT LAND AT LAUREL HILL, CRIBBS CAUSEWAY, BRISTOL ___________________________________________________________________________________ Table 3.3: Source of Potential Flooding to the Development Site

Assessment of Flood Risk to the Development Site Arising from External Sources Flood Risk

Mitigation/Comments



Fluvial

LowMedium



Overland Flow

Low Medium



 

Sewers/Highway Drains

LowMedium



Groundwater

LowMedium



Artificial Bodies (Cribbs Reservoir)

Low



The site lies mainly within the EA’s Flood Zone 1 (low risk zone), with the southern extent slightly extending into Flood Zone 2 (medium risk). However, significant space adjacent to the Henbury Trym within potential Flood Zone 2 will be maintained as a green/ecological corridor. The NPPF considers site usage as residential as ‘more vulnerable’ and commercial as ‘less vulnerable’. Such developments are considered acceptable within Flood Zones 1 and 2. The development will be entirely located outside of 1% floodplain (Flood Zone 3), and largely out of 1% + CC floodplain. A small segment of car parking area extends into the 1% + CC floodplain. Fluvial flood risk mitigation measures as follows have been recommended: - no land raising within the within the car parking area located within the 1% + CC floodplain. - a minimum of 8m of buffer zone to be maintained from the top of the banks to the development zone. - all storage devices to be located out of 1% + CC floodplain. Potential exists for excess overland runoff from the north to enter the site under extreme events. Such flows would flow towards the Henbury Trym following topography. Mitigation measures as follows have been recommended: - incorporation of suitable overland flow routes (roadways, green space) within the development layout to route any potential overland runoff entering the site safely towards the Henbury Trym. - setting finished floor levels within the buildings sufficiently above the adjoining ground (at least 150mm). Should sewers/drains within the vicinity flood, floodwater would direct southerly towards the Henbury Trym. Mitigation measures as follows have been recommended: - incorporation of suitable overland flow routes (roadways, green space) within the development layout to route any potential overland runoff entering the site safely towards the Henbury Trym. - setting finished floor levels within the buildings sufficiently above the adjoining ground (at least 150mm). - a 3m easement to be maintained from each side of the sewers to allow for maintenance access. Groundwater is likely to occur at shallow depths within the low lying southern region. Should groundwater rise to the surface, it would flow towards the Henbury Trym. Precautionary measure of setting finished floor levels sufficiently above the existing levels and at least 150mm above the adjoining finished ground levels will afford additional protection from rising groundwater. The site is located within the reservoir floodplain extent associated with the Cribbs Reservoir, and is at risk of flooding should the reservoir failure or breach occur. However, the EA considers reservoir flooding is extremely unlikely to happen. Through the enforcement of regular inspection and maintenance, the risk of flooding as a result of reservoir failure is considered low.

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FOUL WATER DRAINAGE PROPOSAL

4.1

The residential site with some 110 No. properties will generate a design foul water flow rate of 5.10 l/s based on 4,000 litres/unit dwelling/day in accordance with Sewers for Adoption, 7th Edition â&#x20AC;&#x201C; A Design and Construction Guide for Developers. The commercial site will generate a foul dry weather flow of 0.21 l/s based on an assumed 6,000m2 total building area and at a rate of 300 litres/day per 100m2 for commercial premises. This gives a design foul water flow rate of 1.40 l/s (6 times dry weather flow + 10%) from commercial site. The estimate total foul discharge from the development is 6.5 l/s.

4.2

It is expected that with agreement with the area sewerage undertaker, it will be possible to discharge foul flow from both sites to the existing foul water drains and sewers located within site at suitable locations as governed by layout and topography. Refer to CEC Plans 4111/500 [Existing Site Layout] and 4111/SK501 [Preliminary Drainage Layout] in Appendix 1 and sewer records within Appendix 3 for existing sewer information.

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DISCUSSION AND CONCLUSIONS

5.1

This Flood Risk Assessment has been prepared on behalf of Dick Lovett Group Limited in respect of a planning application relating to proposed residential and commercial development on land to the rear of Dick Lovett site in Cribbs Causeway, Bristol. The study discusses flood risk issues associated with the development proposal in line with the requirements of the Local Authority, the Environment Agency and guidance contained within the NPPF. Assessment of Flood Risk from External Sources

5.2

The development site is essentially located with Flood Zone 1 (low flood risk zone) with 0.1% probability of fluvial flooding. The south eastern extent of the site potentially extends into the Flood Zone 2 associated with the Henbry Trym located along the southeastern site boundary. A 10-15m of ecological buffer will maintained adjacent to the Henbury Trym within the potential Flood Zone 2 area.

5.3

Modelled flood data indicates that the site entirely remains out of 1% floodplain (Flood Zone 3) and largely out of 1% + CC floodplain, with a small segment of car parking area within the south eastern extent extending into the 1% + CC floodplain within Flood Zone 2.

5.4

The proposed site usage, residential classified as ‘more vulnerable’ and commercial classified as ‘less vulnerable’, both are considered acceptable within Flood Zones 1 and 2.

5.5

The site remains at slight risk of flooding from overland runoff and drainage systems associated with land use (commercial premises) to the north. Groundwater emergence is likely to be an issue within the low lying southern extent of the site.

5.6

Mitigation measures as follows have been recommended to mitigate against fluvial, overland flow, sewer flooding and any residual flood risks: 

no land raising within the within the car parking area located within the 1% + CC floodplain.



a minimum of 8m of buffer zone to be maintained from the top of the banks to the development zone.



all storage devices to be located out of 1% + CC floodplain.

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QMF 12.25 ISSUE 4

Cole Easdon Consultants (CEC)

MAY 2014

incorporate suitable overland flow routes (roadways, green areas) to route any potential overland runoff/sewer overflows entering the site safely towards the Henbury Trym



set finished floor levels within the buildings sufficiently above the adjoining ground (at least 150mm).



provide a 3m easement on each side of the sewers to allow access for maintenance.

Assessment of Flood Risk Arising from Surface Water Discharge from the Proposed Development 5.7

The development occupies 6.10 ha of an undeveloped greenfield land, which is largely permeable. The existing site generates 1 in 100 year runoff of 24.1 l/s and 34.7 l/s from the respective residential and commercial sites. The site currently drains into the Henbury Trym via overland flow routes and onsite drainage ditches.

5.8

The development will increase site impermeability with the introduction of significant hard areas (assumed 4.15 ha) and as such is likely to significantly increase surface water runoff compared to the existing site runoff situation without adequate runoff control measures.

5.9

The development will utilise attenuation based SuDS (such as attenuation basins/swales, permeable paving and cellular systems) to control surface water runoff to the predevelopment greenfield rates. All SuDS will be designed for the 1 in 100 year + 30% climate change event in accordance with NPPF requirements.

5.10

Discharge into the Henbury Trym will be limited to the existing greenfield runoff rates of 24.0 l/s (residential site) and 35.0 l/s (commercial site). Flow will be controlled using Hydrobrake or similar flow control device. Accordinlgy, some 600m3 of storage will be required within the residential site and 1,600m3 will be required within the commercial site.

5.11

Consequently, the development will not increase surface water runoff into the adjoining watercourse, and will not exacerbate flood risks within the locality.

5.12

The attenuation basins could be designed with permanent wet pool to offer water quality improvement opportunities. Tanked permeable paving and cellular systems will utilise pretreatment devices (e.g. silt chamber, oil interceptor) prior to discharging into the

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Cole Easdon Consultants (CEC)

MAY 2014

FLOOD RISK ASSESSMENT PROPOSED RESIDENTIAL AND COMMERCIAL DEVELOPMENT AT LAND AT LAUREL HILL, CRIBBS CAUSEWAY, BRISTOL ___________________________________________________________________________________ watercourse. Utilisation of such SuDS features will ensure that surface water discharge entering the Henbury Trym is of acceptable quality. Foul Water Drainage 5.13

The development is likely to generate design foul flow of 6.5 l/s based on 5.1 l/s from residential site and1.4 l/s from the commercial site. It is proposed to discharge foul water flow from the sites into the existing foul water drains and sewers located within the site at suitable locations as dictated by site layout and topography. Conclusions

5.14

This study has been undertaken in accordance with the principles of the NPPF document. We conclude that providing the development adheres to the conditions advised within this report, the proposals can be accommodated without increasing flood risk within the locality in accordance with objectives set by Central Government and the EA.

Cole Easdon Consultants Limited May 2014

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QMF 12.25 ISSUE 4

Cole Easdon Consultants (CEC)

MAY 2014

Appendix 1

Appendix 2

N C Architects 23 High Street, Wroughton, Swindon, Wiltshire. SN4 9JX 0m 0m 0m 0m 0m 0m

MINI

PORSCHE CENTRE

EXISTING BUSINESS

Telephone

01793 845183 01793 845186 Fascimile 01793 845896 E-mail admin@nc-architects.com Scale 1:50 Scale 1:100 Scale 1:200 Scale 1:500 Scale 1:1250 Scale 1:2500

4m 8m 16m 40m 100m 200m

BMW

EXISTING INDUSTRIAL ESTATE

PLANNED REDEVELOPMENT OF EXISTING BMW BODYSHOP AND USED CAR CANOPY

1.5 ACRE SITE FOR DEALERSHIP IN LIEU OF APPROVED CHRYSLER JEEP DEALERSHIP

CUSTOMER PARKING APPROX. 26 SPACES

93469

EXISTING MOTORRAD DEALERSHIP

Approx. 4000 sq.m building floor area

CUTOMER PARKING APPROX. 75 SPACES

BUIL DING

LAPSED PERMISSION FOR FARMHOUSE EXTENSION AND COURTYARD

FRO NTA GE

1.8 ACRE SITE FOR SUI GENERIS, B1, B2 & B8 USE 2.7M HIGH PALLISADE FENCE TO BOUNDARY

BODYSHOP/PREP CENTRE

BUIL DING

1.7 ACRE SITE FOR STAFF CAR PARKING

FRO NTA GE

BUIL DING

FRO NTA GE

Approx. 235 Spaces

2.6 ACRE SITE FOR SUI GENERIS, B1, B2 & B8 USE

10/04/2014 10:05:51

10M WIDE GREEN CORRIDOR

A 03-04-14 Red line area amended. Staff car park shown. New connection road to north of farmhouse CLIENT

PROJECT

DICK LOVETT COMPANIES

BMW-MINI USED CAR CRIBBS CAUSEWAY

COMMERCIAL MASTER PLAN

SHEET SIZE

DRAWING NUMBER

1459-100

REVISION

Appendix 3

Appendix 4

Node Location Map centred on BS10 7TT - created 09/04/2014 Ref: SW/4949

Scale: 1:10,000

Legend 1022

Main River Bank Top ePlanning Tool

544 494

882 795 711 428

306

396

Modelled Flood Level Nodes A table that references the node locations/unique identifiers is also attached, giving associated flood levels, NGRs and further information for the river channel and model.

Contact Us: National Customer Contact Centre, PO Box 544, Rotherham, S60 1BY. Tel: 08708 506 506 (Mon-Fri 8-6). Email: enquiries@environment-agency.gov.uk

NODE DATA

SW/4949

Definitions

AEP = Annual Exceedance Probability - e.g. 1% AEP has a 1% chance of occuring in any one year (1:100 chance) CC = Climate change (20% additional flow)

Notes

This model was produced to assess our flood risk management assets and the results are fit for this purpose. We have MODERATE confidence in its input data, and subsequently its results. The reason that we have MODERATE confidence in the model and its results is because the model requires verification against a known flood event. You will need to contact our Development and Flood Risk Team to discuss whether the flood levels from this model are suitable for your FRA or whether they require you to carry out further work to update the modelling.

Page 1 of 10

NODE TITLE DATE SCENARIO TYPE SOFTWARE NOTES 50% AEP Fluvial Level 50% AEP Fluvial Flow 20% AEP Fluvial Level 20% AEP Fluvial Flow 10% AEP Fluvial Level 10% AEP Fluvial Flow 5% AEP Fluvial Level 5% AEP Fluvial Flow 5%+CC AEP Fluvial Level 5%+CC AEP Fluvial Flow 4% AEP Fluvial Level 4% AEP Fluvial Flow 3.3% AEP Fluvial Level 3.3% AEP Fluvial Flow 2% AEP Fluvial Level 2% AEP Fluvial Flow 1.3% AEP Fluvial Level 1.3% AEP Fluvial Flow 1% AEP Fluvial Level 1% AEP Fluvial Flow 1%+CC Fluvial Level 1%+CC Fluvial Flow 0.5% AEP Fluvial Level 0.5% AEP Fluvial Flow 0.1% AEP Fluvial Level 0.1% AEP Fluvial Flow

1022 River Trym 2010 01/03/2010 Undefended 1D HECRAS Please see p1 42.73 1.94 42.84 2.52 43.01 2.96 No modelled data No modelled data No modelled data No modelled data 43.14 3.60 No modelled data No modelled data 43.23 4.17 No modelled data No modelled data 43.38 4.83 43.46 No modelled data 43.44 5.60 No modelled data No modelled data No modelled data No modelled data