Dobbs Weir Road, Essex

DOBBS WEIR ROAD, ESSEX

FLOOD RISK ASSESSMENT Final Report v3.2

May 201 3

Weetwood Suite 1, Park House Broncoed Business Park Wrexham Road Mold CH7 1HP

T: 01352 700045 E:[email protected] www.weetwood.net

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Report Title: Dobbs Weir Road, Essex Flood Risk Assessment Final Report v3.2

Client: L.W. Developments Ltd

Date of Issue: 24 May 2013

Prepared by: Adam Edgerley BSc (Hons) Technical Engineer

Checked by: Rebecca Ellis BSc (Hons) Associate Director

Approved by: Andrew Grime BEng MBA CEng C.WEM MICE FCIWEM Managing Director

This document has been prepared solely as a Flood Risk Assessment for L.W. Developments. Weetwood Services Ltd accepts no responsibility or liability for any use that is made of this document other than by L.W. Developments Ltd for the purposes for which it was originally commissioned and prepared.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

CONTENTS

Page Signature Sheet i Contents ii & iii List of Tables, Figures & Appendices iv

1 INTRODUCTION ...... 1

1.1 background ...... 1 1.2 Site Location ...... 1 1.3 Existing and Proposed Development ...... 2 1.4 Site Levels ...... 2 1.5 Access and Egress ...... 2

2 NATIONAL PLANNING POLICY FRAMEWORK (NPPF) ...... 4

2.1 Flood Zone Designation ...... 4 2.2 Sequential Test ...... 6 2.3 Exception Test ...... 6

3 FLOOD RISK ...... 7

3.1 Fluvial – River Lee System ...... 7 3.2 FLuvial - River Lynch ...... 12 3.3 Nazeing Drain ...... 13 3.4 Reservoirs, Canals and Other Artificial Sources ...... 13 3.5 Groundwater ...... 14 3.6 Surface Water ...... 15 3.7 application of the sequential and exception tests ...... 16

4 MITIGATION MEASURES ...... 17

4.1 Flood Mitigation ...... 17 4.2 Compensatory Storage / Flood Risk Elsewhere ...... 18 4.3 Access and Egress ...... 19 4.4 Flood Warning ...... 22

5 SURFACE WATER ...... 23

5.1 Requirements for Surface Water Drainage ...... 23 5.2 Site Areas ...... 23 5.3 Surface Water Runoff from the Existing Site ...... 23 5.4 Surface Water Runoff from Developed Site ...... 24

6 SUMMARY ...... 28

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

7 RECOMMENDATIONS ...... 29

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

LIST OF TABLES

Table 1: Modelled Flood Levels (m AOD) for the River Lee and RLN ...... 8 Table 2: Modelled Flood Levels (m AOD) for the RLFRC ...... 10 Table 3: Modelled Flood Levels (m AOD) for Reservoir Units near Site ...... 10 Table 4: Modelled Flood Levels (m AOD) for the Nazeing Mead and Reservoir Unit .. 11 Table 5: Modelled Flood Levels (m AOD) for the River Lynch ...... 12 Table 6: Flow Across Dobbs Weir Road (Velocity Calculation) ...... 20 Table 7: Access & Egress Routes - Hazard Rating ...... 21 Table 8: Site Areas ...... 23 Table 9: Total Peak Runoff Rate – Developed Site ...... 25

LIST OF FIGURES

Figure 1: Site Location ...... 2 Figure 2: Environment Agency Flood Map ...... 5 Figure 3: SFRA Flood Map ...... 6 Figure 4: Flood Routes at Dobbs Weir Road ...... 7 Figure 5: River Lee Modelled Node Locations ...... 8 Figure 6: Richard White Sluices (RLFRC) ...... 9 Figure 7: Node Locations – River Lynch ...... 13 Figure 8: Environment Agency Risk of Flooding from Reservoirs Map ...... 14 Figure 9: BGS Groundwater Flooding Hazard Map ...... 15 Figure 10: Diagram Illustrating Flooding Mechanism at Site ...... 18 Figure 11: Dobbs Weir Road Flow Depths ...... 20 Figure 12: Dobbs Weir Road Flow Area ...... 20 Figure 13: Environment Agency Flood Warning Map ...... 22

LIST OF APPENDICES

Appendix A: Development Proposals Appendix B: Topographic Survey Appendix C: Sequential Test Appendix D: Micro Drainage Outputs for Greenfield Runoff Appendix E: MicroD rainage Storage Volume Calculation

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

1 INTRODUCTION

Weetwood has been instructed 1 by L.W. Developments to undertake a Flood Risk Assessment (FRA) for the proposed redevelopment of a site located off Dobbs Weir Road, Hoddesdon, in accordance with the requirements of the National Planning Policy Framework (NPPF) and its supporting Technical Guidance.

1.1 BACKGROUND Weetwood has previously undertaken two FRAs for the proposed development site. The first FRA 2 was submitted to the Environment Agency (EA) on 22 January 2009. The EA's response to this was received in their letter dated 20 February 2013 3 and indicated the information available was insufficient to determine the level of flood risk to the site from the River Lee. At this time the EA were undertaking further modelling to resolve inaccuracies in the floodplain representation in the River Lee model.

Once the revised model had been completed, a second FRA 4 was prepared in support of a planning application for the development of residential houses. The EA generally agreed 5 with the content of the FRA, stating that all the relevant sources of flood risk had been correctly identified and that the proposed finished floor level was acceptable. However, the EA requested further information concerning the availability of safe access and egress, and the provision of compensatory storage.

This updated FRA has been produced in support of a planning application to provide the additional information requested.

1.2 SITE LOCATION The site is located at Ordnance Survey National Grid Reference TL 388 079, as shown in Figure 1. The site is approximately 0.18 hectares (ha) in area.

1 Instructed to proceed received 3 April 2013 2 Weetwood report entitled “Flood Risk Assessment: Dobbs Weir Road, Essex” dated 19 January 2009, ref 1282/R01 3 Letter from Sandeep Shankar (EA) to Sarah Cowie (Weetwood) dated 20 February 2009, ref NE/2009/107713/01- L01 4 Weetwood report entitled “Flood Risk Assessment: Dobbs Weir Road, Essex” dated 12 May 2010, ref 1282/R02 submitted 16 September 2010 5 Letter from Joe Martyn (EA) to Sarah Cowie (Weetwood) dated 21 September 2010, ref NE/2010/110326/01-L01

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Essex Road

River Lee Flood River Lynch Relief Channel

Dobbs Weir Road

River Lee Proposed development site

Sedge River Lee Green Navigation Eldon Road

Figure 1: Site Location

1.3 EXISTING AND PROPOSED DEVELOPMENT The site comprises an area of previously developed land which is now vacant and heavily overgrown. It is currently occupied by a storage bunker, a residential dwelling and sheds. The site is located within an existing residential area.

Development proposals are for the construction of 4 residential dwellings (Appendix A ). Residential development is classified as ‘more vulnerable development’ in Table 2 of the NPPF Technical Guidance.

1.4 SITE LEVELS A topographic survey of the site has been undertaken by APR Services and is provided in Appendix B . Site levels are approximately 26.20 metres Above Ordnance Datum (m AOD).

1.5 ACCESS AND EGRESS Access to the site is provided via Dobbs Weir Road to the north and Eldon Road to the east (Figure 1). Dobbs Weir Road leads to Sedge Green to the east and Essex Road to the west. Eldon Road is a cul-de-sac which leads to Dobbs Weir Road.

Levels on Dobbs Weir Road to the west are approximately 29.33m AOD. These levels fall to 26.70m AOD adjacent to the site, before rising to 27.45m AOD to the east. ©Weetwood 2 1282/FRA_v3.2 www.weetwood.net 24 May 2013

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Levels on Eldon Road fall from 26.60mAOD at the junction with Dobbs Weir Road to 25.97mAOD in the south.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

2 NATIONAL PLANNING POLICY FRAMEWORK (NPPF)

The aim of the NPPF and its supporting Technical Guidance is to ensure that flood risk is taken into account at all stages in the planning process and is appropriately addressed.

2.1 FLOOD ZONE DESIGNATION Table 1 of the NPPF provides the definitions for each of the flood zones, which are summarised as follows: • Flood Zone 1: Low Probability. Land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year. • Flood Zone 2: Medium Probability . Land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding or between a 1 in 200 and 1 in 1000 annual probability of flooding from the sea in any year. • Flood Zone 3a: High Probability. Land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year. • Flood Zone 3b: The Functional Floodplain. Land where water has to flow or be stored in times of flood. The identification of the functional floodplain should take account of local circumstance and not be defined solely on rigid probability parameters. However, land which would flood with an annual probability of 1 in 20 or greater in any year should provide a starting point for consideration and discussion.

2.1.1 Environment Agency Flood Map According to the Environment Agency (EA) Flood Map ( Figure 2) the site is located in Flood Zone 3.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Legend FZ3 (1 in 100 yr / 200yr) FZ2 (1 in 1000 yr) Flood defences River Lynch Areas benefiting from flood defences Main River

River Lee Flood Relief Channel

River Lee

Nazeing Drain River Lee Navigation

Figure 2: Environment Agency Flood Map (Source: Environment Agency website)

2.1.2 Strategic Flood Risk Assessment A SFRA was published by Epping Forest District Council (EFDC) in April 2011.

According to Flood Map 4 of the SFRA (Figure 3), the site is located in Flood Zone 2.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Figure 3: SFRA Flood Map (Source: Epping Forest District Council 2011 SFRA: Map 4)

2.2 SEQUENTIAL TEST The aim of the Sequential Test (as outlined in Chapter 10 of the NPPF and paragraphs 3-5 of the Technical Guidance) is to encourage development to be located in areas at the lowest probability of flooding.

The Sequential Test has been addressed by Weetwood Ltd in consultation with EFDC ( Appendix C ).

It has been confirmed 6 that the Sequential Test is sufficient and would be officially agreed if submitted along with any future application.

2.3 EXCEPTION TEST Given that development proposals are for ‘ more vulnerable development’ and could be located in Flood Zone 2, the Exception Test may not be required. This is discussed further in Section 3.7 based on the conclusions of the detailed assessment of flood risk set out in Section 3.

6 Email from G Courtney (Epping Forest DC) to A Edgerley (Weetwood), 22 May 2013

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

3 FLOOD RISK

3.1 FLUVIAL – RIVER LEE SYSTEM The River Lee system in the vicinity of the site is highly complex. Flow is divided into three channels; the River Lee, the River Lee Navigation (RLN) and River Lee Flood Relief Channel (RLFRC), which all flow in a predominately southerly/south-westerly direction. The site is also located in close proximity to Nazeing Mead Lake (to the east and south), which is a flood storage area in hydraulic connectivity with the RLFRC.

Given the location of the site in relation to the surrounding watercourses, floodwaters from fluvial sources may reach the site via any one of four different routes, as detailed below and illustrated in Figure 4.

1) Overtopping from the River Lee and/or RLN with floodwaters flowing east along Dobbs Weir Road 2) Overtopping from the RLFRC with floodwaters flowing west along Dobbs Weir Road 3) Overtopping of the flood storage area to the north of the site adjacent to Dobbs Weir Road with flood waters flowing south 4) Nazeing Mead Lake overtopping with floodwaters encroaching north

Figure 4: Flood Routes at Dobbs Weir Road

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

The EA has provided 7 modelled levels for the River Lee system at node points (Figure 5) in the vicinity of the site. The flood storage area north of Dobbs Weir Road is modelled as reservoir unit Dobbs1spu, while the residential area containing the site is located within a second reservoir unit Prop_sp1u.

Figure 5: River Lee Modelled Node Locations

3.1.1 Flood Route 1 – Overtopping of River Lee and RLN onto Dobbs Weir Road Overtopping from the River Lee or RLN could result in floodwaters flowing onto Dobbs Weir Road and then continuing east towards the site.

The EA has provided modelled flood levels upstream of Dobbs Weir Road, adjacent to the junction between the River Lee and RLN ( Table 1). Levels along Dobbs Weir Road in the vicinity of the River Lee and RLN are at 29.30m AOD. Table 1: Modelled Flood Levels (m AOD) for the River Lee and RLN Return Period Event (years) Node 1 in 100 1 in 100 plus climate change 1 in 1000 WGA79U 27.13 27.19 27.29

Given that the Dobbs Weir Road is raised in excess of 1 metre above any of the modelled flood levels in Table 1, the risk of overtopping of either watercourse is negligible up to the 1 in 1000 year event.

7 Letter from Lisa Llewellyn (EA) to Mr Eden (Planning and Development Consultant) dated 4 June 2008, ref NE16914_LL, and subsequent email from Rhys Hobbs (EA) to Sarah Cowie (Weetwood), received 23 November 2009

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Floodwaters from the River Lee and RLN which come out of bank upstream of Dobbs Weir Road are likely to flow east into reservoir unit Dobbs1spu (Figure 5). Flood risk from overtopping of reservoir unit Dobbs1spu is considered in Section 3.1.3 .

3.1.2 Flood Route 2 – Overtopping of the RLFRC The RLFRC is located approximately 0.12km to the west of the proposed development site.

The EA has provided modelled flood levels for the RLFRC at Node DG3 located upstream of the Richard White Sluice gates ( Figure 5), a major flow control structure. Given that the site is located downstream of the sluices, applying the modelled flood levels from Node DG3 to determine flood risk to the site is not considered viable. Floodwaters overtopping the channel upstream of the sluices are likely to flow into reservoir unit Dobbs1spu. Flood risk from overtopping of reservoir unit Dobbs1spu is considered in Section 3.1.3 .

Figure 6: Richard White Sluices (RLFRC) The EA previously provided a more complete set of modelled flood levels for this area 8, which included an additional Node downstream of the Richard White Sluices. Since the overall structure and mechanism of the sluices has not changed since publication of the previous modelled flood levels, their effect on flood levels is expected to remain the same. Although the modelled flood levels from this Node have now been superseded, the results showed that modelled levels downstream of the sluices fall compared to the upstream levels. The observed differences have been utilised and applied to the updated modelled levels from Node DG3 in order to derive a flood level for the RLFRC downstream of the sluice gate ( Table 2).

8 Letter from Lisa Llewellyn (EA) to Mr Eden (Planning and Development Consultant) dated 4 June 2008, ref NE16914_LL

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Table 2: Modelled Flood Levels (m AOD) for the RLFRC Return Period Event (years) Node 1 in 100 1 in 100 plus climate change 1 in 1000 DG3 26.86 27.00 27.28 Downstream 25.14 25.37 25.90 of Sluice Gate

Site levels are generally in the region of 26.20m AOD. Therefore, flood risk to the site from overtopping of the RLFRC downstream of the Richard White Sluices is considered negligible up to the 1 in 1000 year flood event.

3.1.3 Flood Route 3 - Overtopping of Flood Storage Area Over Dobbs Weir Road Floodwaters overtopping the River Lee, RLN or RLFRC upstream of Dobbs Weir Road are likely to flow into the open area of land between the River Lee and RLFRC, to the north of Dobbs Weir Road. This area of open land acts as a flood storage area, and is represented in the EA River Lee model as reservoir unit Dobbs1spu (Figure 4 and Figure 5). Dobbs Weir Road is represented in the EA model as a spill unit. If flood levels in reservoir unit Dobbs1spu exceed levels along Dobbs Weir Road, overtopping may occur causing floodwaters to flow south across the road into reservoir unit Prop_sp1u. As such this may subsequently affect the site.

The EA have provided the modelled flood levels in Table 3 for reservoir units Dobbs1spu and Prop_sp1u. Table 3: Modelled Flood Levels (m AOD) for Reservoir Units near Site Return Period Event (years) Node 1 in 100 1 in 100 plus climate change 1 in 1000 Dobbs1spu 26.89 27.01 27.21 Prop_sp1u 25.73 25.87 26.40

Given the location of node Prop_sp1u, it is considered that these modelled outputs would be most representative of the flood levels expected at the site (Figure 5). Given that site levels are approximately 26.20m AOD, the modelled flood levels in Table 3 suggest that the site would remain dry up to the 1 in 100 year plus climate change event with only minimal inundation in a 1 in 1000 year event (i.e. up to 0.2 m). However, the site may be situated in a flow path for potential floodwaters that may overtop reservoir unit Dobbs1spu.

3.1.3.1 Flow Paths Overtopping Dobbs1spu As detailed in Section 1.5 , levels along Dobbs Weir Road adjacent to the site are generally 26.70m AOD; however, Appendix B indicates a raised strip of land directly to the north with levels between 26.90 and 26.96m AOD. Therefore, any overtopping from Dobbs1spu onto the site during the 1 in 100 year event is likely to be insignificant. As such, the flood risk to the site during the 1 in 100 year event is considered negligible.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Beyond the 1 in 100 year flood event Table 3 suggests that some floodwaters would be expected to overtop Dobbs Weir Road during the 1 in 100 year plus climate change and 1 in 1000 year flood events to maximum depths of 0.11m and 0.31m respectively. The site could therefore be located in a potential flow path of floodwaters overtopping Dobbs1spu and may therefore be at some risk of flooding during the 1 in 100 years plus climate change and 1 in 1000 year flood events.

The flood risk to the site from this source will be mitigated though implementation of the measures proposed in Section 4 of this report.

3.1.4 Flood Route 4 – Flooding from the South from Nazeing Mead Lake Floodwaters from Nazeing Mead Lake (and the lower section of the RLN) may flow north towards the site during an extreme flood event, causing floodwaters to accumulate in reservoir unit Prop_splu. The EA has provided modelled flood levels for Nazeing Mead Lake and reservoir unit Prop_sp1u (Table 4). No modelled levels have been provided for the lower section of the RLN to the east of the site, so flood risk from this channel must be assessed using flood levels within Prop_sp1u as a proxy. Table 4: Modelled Flood Levels (m AOD) for the Nazeing Mead and Reservoir Unit Return Period Event (years) Node 100 100 plus climate change 1000 Prop_sp1u 25.73 25.87 26.40 DG1D 24.29 25.02 26.39

As previously detailed, site levels are generally at 26.20m AOD. Therefore, floodwaters are unlikely to inundate the site from the south up to the 1 in 100 year plus climate change flood event. However, the site may be at some risk of flooding during the 1 in 1000 year flood event to depths of approximately 0.20m.

The flood risk to the site from this source will be mitigated though implementation of the measures proposed in Section 4 of this report.

3.1.5 River Lee Flood Defences Although the EA Flood Map ( Figure 2) suggests that the site does not benefit from suitable flood defence infrastructure, it is important to recognise that the RLFRC does act as a flood defence for the site. Such flood defence infrastructure would not be capable of catastrophic failure in the same way as raised earth bunds or flood walls, which can breach. The system could only fail through overtopping due to lack of capacity. The above flood levels take account of this, as they have been provided for the defended scenario 9, and should therefore provide an accurate assessment of the flood risk to the site during the quoted return periods.

9 Letter from Joe Martyn (EA) to Sarah Cowie (Weetwood) dated 21 September 2010, ref NE/2010/110326/01-L01

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3.2 FLUVIAL - RIVER LYNCH The River Lynch is located 0.68m to the west of the proposed development site, before its confluence with the River Lee located 0.73km to the southwest.

The EA have provided the modelled flood levels for this watercourse (Table 5) and the corresponding node locations are shown in Figure 7. Given that site levels are approximately 26.20m AOD, the site is not considered to be at risk of flooding from this source up to the 1 in 100 year plus climate change event.

Modelled flood levels are shown to marginally exceed site levels during the 1 in 1000 year event; however given that both the River Lee and RLN are situated between the site and the River Lynch, floodwaters are unlikely to reach the site.

The site is therefore not considered to be at risk of flooding from this source. Table 5: Modelled Flood Levels (m AOD) for the River Lynch Return Period Event (years) Node 100 100 plus climate change 1000 LY.011 25.54 25.70 26.30 LY.010 25.54 25.70 26.30 LY.009 25.54 25.70 26.30 LY.008D 25.54 25.70 26.30 LY.008U 25.54 25.70 26.30 LY.007 25.54 25.70 26.30 LY.006 25.54 25.70 26.30 LY.005 25.54 25.70 26.30 LY.004 25.54 25.70 26.30 LY.003 25.54 25.70 26.30 LY.002 25.54 25.70 26.30 LY.001 25.54 25.70 26.30

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Legend LY.009 Node point LY.010 LY.008D LY.008U

LY.011

LY.007

LY.006

LY.005

LY.004

LY.003 River Lee

LY.002 LY.001

Figure 7: Node Locations – River Lynch

3.3 NAZEING DRAIN Nazeing Drain is located approximately 0.05km to the east of the site and flows in a predominately southerly direction before discharging into Nazeing Mead Lake ( Figure 2).

Given that ground levels appear to rise to the west of Nazeing Drain and fall to the east, potential floodwaters are likely to be directed away from the site and into Nazeing Mead Lake to the east.

Irrespective, any residual concern of flood risk from this source will be mitigated through the implementation of measures proposed in Section 4 of this report.

3.4 RESERVOIRS, CANALS AND OTHER ARTIFICIAL SOURCES Reservoir or canal flooding may occur as a result of the facility being overwhelmed and/or as a result of dam or bank failure.

The EA Risk of Flooding from Reservoirs Map (Figure 8) indicates that the site may be at some risk of flooding from such sources. However, the reservoir identified is located 1.06km from the site. Any floodwaters from this reservoir would be intercepted by various local watercourses before reaching the site. In addition, the reservoir identified appears to be the water retention facility at Ryemead Sewage Purification Works, and will therefore be maintained to a very high standard, with little probability of failure of retaining walls.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Legend

Flooding from reservoirs

Figure 8: Environment Agency Risk of Flooding from Reservoirs Map (Source: Environment Agency website) Irrespective, any residual risk of flooding will be mitigated though implementation of the measures proposed in Section 4 of this report.

3.5 GROUNDWATER Groundwater flooding generally occurs during intense, long-duration rainfall events, when infiltration of rainwater into the ground raises the level of the water table until it exceeds ground levels. It is most common in low-lying areas overlain by permeable soils and permeable geology, or in areas with a naturally high water table.

According to the Soilscapes maps produced by the National Soils Research Institute 10 soil conditions at the site and within the surrounding area are described as ‘Loamy and clayey floodplain soils with naturally high groundwater’. However, the maps contained in Volume 3 of the Wallingford Procedure 11 indicate that the site lies in an area described as ‘ Soil Type 2’ and therefore comprising permeable soils, which is also supported by Map 13 of the EFDC SFRA. As such, there may be the propensity for groundwater flooding at the site.

10 Soilscapes www.landis.org.uk/soilscapes/ 11 The Wallingford Procedure: Design & Analysis of Urban Storm Drainage, Hydraulic Research Ltd, 1981

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This is reiterated by the British Geological Survey (BGS) Groundwater Flooding Hazard map ( Figure 9), which classifies the susceptibility to groundwater flooding as moderate to significant.

Legend Susceptibility to flooding from groundwater Significant

Moderate

Low

Figure 9: BGS Groundwater Flooding Hazard Map (Source: FindMaps) The risk of flooding from this source will be mitigated through the implementation of the measures proposed in Section 4 of this report.

3.6 SURFACE WATER Surface water flooding comprises pluvial, sewer and highway drains and gullies.

3.6.1 Pluvial Pluvial flooding results from rainfall-generated overland flow, before the runoff enters any watercourse or sewer, or where the sewerage/drainage systems and watercourses are overwhelmed and therefore unable to accept surface water. Pluvial flooding is usually associated with high intensity rainfall events but may also occur with lower intensity rainfall where the ground is saturated, developed or otherwise has low permeability resulting in overland flow and ponding within depressions in the topography.

As detailed previously, according to the Soilscapes maps soil conditions are described as ‘Loamy and clayey floodplain soils with naturally high groundwater’. However, the maps contained in Volume 3 of the Wallingford

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Procedure 12 indicate that the site lies in an area described as ‘ Soil Type 2’ and therefore comprising permeable soils, which is also supported by Map 13 of the SFRA. As such it is unlikely that the site is at risk of significant pluvial flooding.

The flood map for surface water (Map 6 of the EFDC SFRA) indicates that the site is at negligible risk of flooding from this source.

The propensity for pluvial flooding at the site is therefore considered to be low. However, any residual concern regarding flood risk from this source will be addressed through the mitigation measures as detailed in Section 4 and the surface water drainage strategy in Section 5.

3.6.2 Sewer Sewer flooding can occur when the capacity of the sewer system is overwhelmed by heavy rainfall, becomes blocked or is of inadequate capacity, resulting in flooding of land and/or property. Normal discharge of sewers and drains through outfalls may be impeded by high water levels in receiving waters.

Thames Water has been consulted to ascertain whether it holds any records of sewer flooding at the site. A response is awaited.

3.6.3 Highway Drains and Gullies Essex County Council has confirmed 13 that they hold no record of highway flooding in the vicinity of the site.

3.7 APPLICATION OF THE SEQUENTIAL AND EXCEPTION TESTS The above detailed assessment of flood risk suggests that the proposed development may not be at risk of flooding during the 1 in 100 year event and could therefore be located in Flood Zone 2. As such, this reaffirms the SFRA Flood Map ( Figure 3). The Sequential Test is addressed in Appendix C and has been produced on this basis. The Exception Test is not required for residential development in Flood Zone 2.

12 The Wallingford Procedure: Design & Analysis of Urban Storm Drainage, Hydraulic Research Ltd, 1981 13 Email from Essex County Council to Weetwood, 11 January 2013

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

4 MITIGATION MEASURES

4.1 FLOOD MITIGATION The flood risk to the site from the River Lee System and groundwater will be mitigated though the implementation of the measures proposed within the following section of this report. These mitigation measures will also address any residual concern of flood risk from other sources.

4.1.1 Finished Floor Levels Ideally, finished floor levels should be set at a minimum of 27.31m AOD to provide a freeboard of 300mm above the 1 in 100 year plus climate change flood level expected at reservoir unit Dobbs1spu (Table 3). However, this is not considered feasible at this site for the following reasons: • Planning considerations for eaves heights and building appearance, which must match the existing street scene. It is noted that the design of the existing dwellings surrounding the site leaves them more vulnerable to the effects of flooding; nevertheless new dwellings must not appear out of place if planning permission is to be granted • New buildings must comply with the Disability Discrimination Act. This has a requirement for level access. While it is possible to use a ramped access route, for a site of this size the ramps would need to be unsuitably steep in order to fit between the building threshold and the road

Therefore, it is proposed to set finished floor levels as high as feasibly possible, which has been confirmed 14 as 27.11m AOD. This affords a 100mm freeboard above the 1 in 100 year plus climate change flood level at reservoir unit Dobbs1spu ( Table 3 and Figure 5) and has previously been agreed 15 by the EA. Furthermore, the proposed finished floor level is significantly above the modelled flood levels at the adjacent Node, Prop_sp1u during a 1 in 1000 year event (i.e. 26.40m AOD - Table 3).

4.1.2 Flood Resistant & Resilient Construction In order to reduce the risk of flooding of the new buildings, taking into account the fact that the finished floor levels cannot be set a full 300mm above ground levels, it is proposed to incorporate a range of flood resistant construction measures into the design of the building.

The measures will be designed in line with guidance provided in the Communities and Local Government Document, ‘Improving the Flood Performance of New Buildings: Flood Resilient Construction’16 . These include

14 Telephone conversation between Tim Edens (Planning and Development Consultant) and Sarah Cowie (Weetwood) on 8 February 2010 15 Letter from Joe Martyn (EA) to Sarah Cowie (Weetwood) dated 21 September 2010, ref NE/2010/110326/01-L01 16 Improving the Flood Performance of New Buildings: Flood Resilient Construction. Department for Communities and Local Government. May 2007.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

design features and finish materials to minimise the entry of water and/or reduce the damage in the unlikely event of the development being inundated.

4.2 COMPENSATORY STORAGE / FLOOD RISK ELSEWHERE Compensatory storage is normally required where development will result in a loss of floodplain storage, with an associated displacement of floodwaters and increase in flood risk elsewhere.

The following factors should be taken into account when considering the requirement for compensatory storage at this site: • The site may be considered to be located in Flood Zone 2 • The River Lee floodplain is extensive in the vicinity of the site. As such, the volume of water displaced by the proposed development would be negligible compared with the volume of floodwater inundating the River Lee floodplain during the conditions required to cause the site to flood

The slope of the site and surrounding area would suggest that floodwaters would pass through the site as sheet flow. As such, flood levels would vary at the site from north to south; however the depth may remain consistent. This is illustrated in Figure 10 (note that the predicted flood levels are taken from Table 3).

DIAGRAM NOT TO SCALE

Volume displaced = flow depth x area of buildings 100yrsCC Flood 100yrs Flood Level Level = Road Level = FFL = 27.11mAOD = 26.89mAOD 27.01mAOD 26.896mAOD 100yrsCC Flood 114mm Level = 25.87mAOD

Sheet Flow Through Site

SITE

Reservoir Unit Prop_sp1u Reservoir Unit Dobbs1spu Dobbs Weir Road (spill unit) Figure 10: Diagram Illustrating Flooding Mechanism at Site Site levels fall from 26.68m AOD in the north to 26.22m AOD in the south. Assuming the depth of sheet flow across the site during the 1 in 100 year plus climate change event is approximately 0.11m (Section 3.1.3.1 ), flood levels would fall from 26.79m AOD to 26.33m AOD across the site.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Recognising this variation in flood levels and the complexity of the flooding mechanisms, it was agreed with the EA 17 that compensatory storage should be provided on a volume for volume basis rather than a level for level basis. This volume has been calculated below.

The existing development at the site consists of a single residential unit and a high earth bund to the south of the site ( Appendix B ), totalling approximately 131m 2. These are both to be removed as part of the redevelopment. Development plans ( Appendix A ) suggests that the total area of new buildings at the site is approximately 424m 2. Therefore, compensatory storage will be provided for the additional 293m2 of new building footprint. If the site were to be uniformly flooded to a depth of 0.11m, the additional footprint of new buildings would displace 32.23m3 of floodwater.

Given that the development plans show approximately 845m2 of gardens, lowering this area by 40mm would provide 33.8m 3 of storage, which should be more than sufficient to compensate for any loss of floodplain storage.

4.3 ACCESS AND EGRESS Development plans ( Appendix A ) suggest that access and egress will be provided via Eldon Road to the east, located off Dobbs Weir Road to the north. Dobbs Weir Road leads to Sedge Green to the east and Essex Road to the west (Figure 1).

Sedge Green is located entirely in Flood Zone 1 ( Figure 2), whilst Essex Road provides access to areas of the defended floodplain. Levels along Dobbs Weir Road are generally above modelled flood levels, although some overtopping of Dobbs Weir Road may occur if the River Lee flood defence system became overwhelmed.

Figure 11 has been provided by the EA which is a scaled diagram showing the peak flood levels over Dobbs Weir Road from Dobbs1spu in the 1 in 100 year, 1 in 100 year plus climate change and 1 in 1000 year flood events. It should be noted that the vertical scale is greatly exaggerated in comparison to the horizontal scale .

17 Letter from Joe Martyn (EA) to Sarah Cowie (Weetwood) dated 21 September 2010, ref NE/2010/110326/01-L01

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Figure 11: Dobbs Weir Road Flow Depths Table 6 provides the flow rate (m 3/s) over the road for each flood event, as provided by the EA. The flow area has been calculated by reference to Figure 12 . The flow areas have been measured from this drawing, and scaled as required. The flow velocity has been calculated by simply dividing the flow rate by the flow area.

Additional flow area in 1 in 100 years Additional flow area in 1 in 1,000 years event plus climate change event

1 in 100 years event flow area Figure 12: Dobbs Weir Road Flow Area

Table 6: Flow Across Dobbs Weir Road (Velocity Calculation)

Return Period Event Flow Rate Flow Area Flow Velocity (years) (m 3/s) (m 2) (m/s) 1 in 100 2.18 9.18 0.24 1 in 100 plus climate change 14.87 42.05 0.35 1 in 1000 71.85 128.89 0.56

As previously stated in Section 3.1.3 , levels along the raised land directly to the north Dobbs Weir Road adjacent to the site are between 26.90 and 26.96m AOD. As such no overtopping of Dobbs Weir Road is expected during the 1 in 100 year flood event; however overtopping may occur during the 1 in

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

100 year plus climate change and 1 in 1000 year flood events to depths of 0.11m and 0.31m respectively.

Figure 11 and Figure 12 suggest that a flow route is located in the western section of Dobbs Weir Road at approximately 26.76m AOD. Given the modelled flood levels from the adjacent node (Dobbs1spu), the maximum flood depths expected along the access/egress route to the west during the 1 in 100 year, 1 in 100 year plus climate change and 1 in 1000 year flood events are 0.13m, 0.25m and 0.45m respectively (Table 7).

Another flow area is located at the east of Dobbs Weir Road leading to Sedge Green. This is situated east of the bridge over the RLFRC with levels of 26.59m AOD. Given the modelled flood levels from the adjacent node (Dobbs1spu), maximum flood depths expected along the access/egress route to the east during the 1 in 100 year, 1 in 100 year plus climate change and 1 in 1000 year flood events are 0.30m, 0.42m and 0.62m respectively ( Table 7).

Flood Risk to People Phase 2 18 provides a formula for calculating the hazard rating along an access route. In this formula, the hazard rating is given by:

Hazard Rating = (V+C) x D

Where V is the velocity of the floodwaters, C is the debris factor (here taken as 0.5 19 ) and D is the flood depth. A hazard rating of 0-0.75 is considered safe, while a hazard rating of 0.75-1.25 may pose a risk to some, such as children. A hazard rating of 1.25-2.50 would present a hazard to most, while a rating of 2.50-20.00 would be dangerous for all users.

The hazard rating at the site and the two flow routes in the road are calculated in Table 7. Table 7: Access & Egress Routes - Hazard Rating Maximum Flood Hazard Rating Return Flow Flood Depths (m) =(V+C)D Period Velocity Level Flow Flow Flow Flow Event At At (m/s) (mAOD) Route Route Route Route (years) Site Site to west to East to West to East 1 in 100 0.24 26.89 0.19 0.13 0.30 0.14 0.09 0.22 1 in 100 plus 0.35 27.01 0.31 0.25 0.42 0.26 0.21 0.36 climate change 1 in 0.56 27.21 0.51 0.45 0.62 0.54 0.47 0.66 1000

Table 7 indicates that the maximum hazard rating is calculated at 0.66. As such, access and egress to the site can be considered safe up to the 1 in 1000 year flood event.

18 R&D Outputs: Flood Risks To People: Phase 2 FD2321/TR1, published by Defra in March 2006 19 See Table 3.1 of Guidance note for Flood Risks To People: Phase 2 FD2321/TR2. Conditions north (upstream) of the site are mainly rural (greenbelt) for which the recommended debris factor is 0, but land further north and to the west is urban (debris factor = 1). A debris factor of 0.5 has been used as a conservative estimate of conditions at the site.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

4.4 FLOOD WARNING The site is included in an EA flood warning area ( Figure 13 ).

Legend Flooding Warning Area

Figure 13: Environment Agency Flood Warning Map (Source: Environment Agency website) The EA offers a free flood warning service called Floodline Warnings Direct (FWD).

Users of the site may register with the EA to receive flood warnings by telephone, email, text or fax. More information can be found either by visiting the EA’s website ( www.environment-agency.gov.uk ) or by calling Floodline on 0845 988 1188. A free iPhone app called Flood Alert can also be used to monitor flood warnings.

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L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

5 SURFACE WATER

5.1 REQUIREMENTS FOR SURFACE WATER DRAINAGE The NPPF and its supporting Technical Guidance requires developers and local authorities to seek opportunities to reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of Sustainable Drainage Systems (SuDS).

As a minimum this is seen as a requirement that the peak flow rates of surface water leaving the developed site are no greater than the rates prior to development. Opportunities to reduce surface water runoff, and the associated flood risk, should also be identified and climate change taken into consideration.

Recognising the above, and the requirements of the EA 20 , Building Regulations Approved Document H, the Code for Sustainable Homes Technical Guide (Category 4) and the requirement placed upon local planning authorities within the NPPF to promote the use of SuDS, surface water runoff from the proposed site should demonstrate: • No increase in existing flow rates discharged to watercourse/public sewer • The use of SuDS as the preferred method of dealing with surface water • How runoff up to the 1 in 100 year event plus an allowance for climate change will be dealt with without increasing flood risk elsewhere

5.2 SITE AREAS The existing and proposed impermeable and permeable areas at the site are shown in Table 8. This indicates that the extent of impermeable area at the site will increase by 0.08ha following redevelopment. Table 8: Site Areas

Existing Site Redeveloped Site Impermeable area (ha) 0.01 0.09 Permeable area (ha) 0.17 0.09 TOTAL (ha) 0.18 0.18

5.3 SURFACE WATER RUNOFF FROM THE EXISTING SITE There are no known existing surface water drainage systems at the site. Although the site comprises previously developed land, this is largely permeable and it is therefore assumed that infiltration is the primary mechanism of surface water disposal at the existing site.

20 Preliminary Rainfall Runoff Management for Developments, R&D Technical Report W5-074/A/TR/1 Revision C, 2005

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

The 1 in 2 year greenfield runoff rate per hectare has been calculated using the ICP SUDS method within MicroDrainage. Details of the MicroDrainage input parameters and the output results are provided in Appendix D. This suggests that the site (0.18ha) currently discharges surface water at a rates of: • 0.27l/s during the 1 in 2 year storm event • 0.61l/s during the 1 in 30 year storm event • 0.88l/s during the 1 in 100 year storm event

5.4 SURFACE WATER RUNOFF FROM DEVELOPED SITE Table 8 indicates that impermeable areas at the site will increase following redevelopment. The following sections describe how surface water runoff from the redeveloped site may be managed in accordance with the requirements of the NPPF and its supporting Technical Guidance.

5.4.1 Disposal of Surface Water Building Regulations Approved Document Part H sets out a hierarchy of preferred methods for the disposal of surface water runoff 21 . These are listed below in order of preference: 1. Disposal by infiltration - As detailed previously, according to the Soilscapes maps produced by the National Soils Research Institute, soil conditions at the site ‘ Loamy and clayey floodplain soils with naturally high groundwater’ . However, the maps contained in Volume 3 of the Wallingford Procedure 22 indicate that the site lies in an area described as ‘Soil Type 2’ and therefore comprising permeable soils, which is also supported by Map 13 of the SFRA. As such, there is a possibility that disposal of surface water by infiltration methods may be appropriate. This should be confirmed with percolation tests undertaken to BRE 365 23 . 2. Disposal to a watercourse - There are no watercourses within the vicinity of the site into which surface water could be discharged without crossing third party land. Discharge to watercourse is therefore not considered feasible. 3. Disposal to a public sewer - Should infiltration not be possible on site, surface water could be disposed of to the public sewer system. Given that the site is located within an area of residential development, it is assumed that there would be a sufficient public sewer network to connect to. The details of this connection and the allowable rate of discharge will need to be agreed with Thames Water.

5.4.2 Surface Water Discharge Rate The site currently comprises predominantly permeable land with no known formal drainage system.

21 Building Regulations Approved Document H Section 3 page 45 22 The Wallingford Procedure: Design & Analysis of Urban Storm Drainage, Hydraulic Research Ltd, 1981 23 BRE Digest 365, Soakaway Design, 1991

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

Given that the current greenfield runoff rate is 1.5l/s/ha for the 1 in 2 year storm event (Appendix D ), total runoff rates from impermeable areas at the site (0.09ha) should ideally be restricted to 0.14l/s. However, these rates are extremely low and would be unfeasible to achieve without compromising the risk of blockage of the drainage system.

This report will aim to reduce runoff rates to the minimum limit of discharge rate of 5.0l/s, as outlined in paragraph 17 of the Preliminary Rainfall Runoff Management for Developments (January 2012) and also within Sewers for Adoption (7 th edition). Table 9 shows the total runoff rates from the site following development.

The remainder of the site, comprising permeable areas, will continue to runoff at greenfield rates. Table 9: Total Peak Runoff Rate – Developed Site

Runoff Rate (l/s) Return Period Impermeable areas Permeable areas Total 1 in 2 year 5.00 0.14 5.14 1 in 30 year 5.00 0.31 5.31 1 in 100 year 5.00 0.44 5.44

5.4.3 SuDS Options and Storage Calculation In order to restrict runoff rates from the proposed impermeable areas to 5.0l/s, attenuation storage will be required. SuDS elements may be used to provide this required storage.

SuDS aim to mimic natural drainage and can achieve multiple objectives such as removing pollutants from urban runoff at source, controlling surface water runoff from developments, and ensuring that flood risk is not increased downstream. Combining water management with green space can provide amenity and biodiversity enhancement. Typical SuDS components include surface or subsurface storage with flow limiting devices, roadside swales, detention basins and infiltration areas or soakaways.

The surface water storage facilities described in the following sections have been modelled using the Detailed Design module of MicroDrainage Source Control.

The required storage volume has been sized to store the 1 in 100 year storm event including a 30% increase in rainfall intensity in order to allow for climate change in accordance with Table 5 of the NPPF Technical Guidance.

5.4.3.1 Storage Volume Calculation The parameters used in the storage calculation along with the MicroDrainage Source Control output results are provided in Appendix E. This indicates that a storage volume of 26.7m3 would be required.

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

The calculations have assumed a 1.5m deep ‘storage tank’, which fills to a depth of 1.0m with a 0.5m freeboard using a Hydro-brake outfall control device. The size and depth of the storage facility can vary and it would be possible to reduce this depth, provided the area is increased accordingly to accommodate the required volume.

5.4.3.2 Storage Options To accommodate surface water flows up to the 1 in 100 year plus climate change storm event, permeable paving could be utilised using an aggregate sub-base with 30% voids. Therefore, an area comprising 200m 2 would require a sub-base depth of approximately 0.44m to achieve the required storage volume. Given that development plans suggest a total area of driveways, patios and parking spaces of approximately 400m 2 there should be ample space within the development site.

Alternatively, the attenuation of surface water could be achieved using oversized pipes and/or swales.

Ideally, the methods outlined above should be designed to infiltrate into the ground (subject to percolation testing) or alternatively discharge into the public sewer network subject to agreement with Thames Water.

5.4.4 Maintenance of SuDS In the past local planning authorities and water companies have been reluctant to adopt SuDS. With no arrangements in place that require local planning authorities or water companies to adopt SuDS their maintenance has subsequently been the responsibility of the developer.

The Flood and Water Management Act (2010) is currently being implemented through a series of Commencement Orders. Section 32 introduces Schedule 3: Sustainable Drainage. This introduces: • New standards for the design, construction, operation and maintenance of new rainwater drainage systems • A new ‘approving body’ (generally a unitary, county or county borough local authority) • A requirement for the approving body to approve most types of rainwater drainage systems before any construction work with drainage implications can start, subject to: (i) the system being constructed in line with an approved drainage plan to national standards; (ii) the approving body being satisfied the drainage system has been built and functions in accordance with the drainage plan, and (iii) the system being a sustainable drainage system, as defined by regulations.

However, this provision is awaiting commencement following further work by DEFRA on arrangements for adoption and maintenance of SuDS, including technical guidance. At present it is envisaged that implementation of these arrangements will be sometime in 2014.

In the meantime, other options for maintenance of SuDS include:

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

• SuDS elements within the curtilage of residential dwellings (e.g. soakaways, permeable paving) will be the responsibility of the owner of the property • The pipe network, designed to Sewers for Adoption (7 th edition) standard, may be adopted by the sewerage undertaker

5.4.5 Final Drainage Layout The purpose of this FRA is to demonstrate that a surface water drainage strategy is feasible for the site given the development proposals and the land available. The proposals provide the opportunity for the inclusion of SUDS elements, ensuring that there will be no increase in surface water runoff from the proposed development.

This FRA has demonstrated that, not only can the required storage be accommodated within the site layout, but that various options are feasible and ample land is available, providing flexibility for the final drainage solution. A final decision on the types of storage to be provided will be made at the detailed drainage design stage.

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

6 SUMMARY

There are proposals for the redevelopment on an area of land located off Dobbs Weir Road, Essex for residential use.

According to the EA Flood Map the proposed development site is located within the 1 in 100 year fluvial flood outline and is therefore defined as being situated within Flood Zone 3 under the NPPF and its supporting Technical Guidance. However, a detailed analysis of the existing information has shown that the site may be located within Flood Zone 2. Residential development is considered appropriate within Flood Zone 2 subject to addressing the requirements of the Sequential Test. The Exception Test should not be required.

The main source of flood risk to the site is from the River Lee System. There may also be some risk of groundwater flooding. Other sources of flood risk to the site are considered negligible. The site is protected by the River Lee flood defence system in up to the 1 in 100 year flood event.

In order to mitigate the risk of flooding from the River Lee and any residual risk from groundwater, it is recommended that the finished floor levels of the proposed residential dwellings be set at 27.11m AOD. This affords a 100mm freeboard above the 1 in 100 year plus climate change flood level and has previously been agreed by the EA. Flood resilient and resistant construction measures will be incorporated into the design of the new buildings. Compensatory storage will be provided on a level for level basis to mitigate against any loss of floodplain storage in the 1 in 100 years plus climate change flood event.

Safe access and egress can be provided for the site along Dobbs Weir Road. However, to reduce the potential risks involved from flooding of Dobbs Weir Road, it is recommended that occupants should be informed of the level of risk and encouraged to register for flood warnings.

Following redevelopment, the overall impermeable areas at the site are expected to increase. A scheme for the provision and implementation of a surface water regulation system following the principles set out in this FRA should be submitted to and approved in writing by the local planning authority, prior to the commencement of development.

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

7 RECOMMENDATIONS

This FRA has demonstrated that the proposed development may be completed without conflicting with the requirements of the NPPF and its supporting Technical Guidance subject to the following:

• Garden areas (equating to a total area of 845m2) to be lowered by 40mm to provide suitable compensatory storage

• Finished floor levels to be set at a minimum of 27.11m AOD

• The latest best practice flood resilient construction techniques to be incorporated into the design of the buildings

• The detailed drainage design, developed in accordance with the principles set down in this FRA, should be submitted to and approved by the local planning authority prior to the commencement of development

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

APPENDIX A: Development Proposals

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

APPENDIX B: Topographic Survey

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

APPENDIX C: Sequential Test

L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex

APPENDIX D: MicroDrainage Outputs for Greenfield Runoff

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ICP SUDS Mean Annual Flood

Input

Return Period (years) 100 Soil 0.300 Area (ha) 1.000 Urban 0.000 SAAR (mm) 600 Region Number Region 6

Results l/s

QBAR Rural 1.5 QBAR Urban 1.5

Q100 years 4.9

Q1 year 1.3 Q30 years 3.4 Q100 years 4.9

APPENDIX E: MicroDrainage Storage Volume Calculation

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Summary of Results for 100 year Return Period (+30%)

Storm Max Max Max Max Status Event Level Depth Control Volume (m) (m) (l/s) (m³)

15 min Summer 0.722 0.722 4.5 19.3 O K 30 min Summer 0.844 0.844 4.6 22.5 O K 60 min Summer 0.869 0.869 4.7 23.2 O K 120 min Summer 0.811 0.811 4.5 21.6 O K 180 min Summer 0.734 0.734 4.5 19.6 O K 240 min Summer 0.656 0.656 4.5 17.5 O K 360 min Summer 0.509 0.509 4.5 13.6 O K 480 min Summer 0.344 0.344 4.5 9.2 O K 600 min Summer 0.233 0.233 4.5 6.2 O K 720 min Summer 0.170 0.170 4.5 4.5 O K 960 min Summer 0.118 0.118 4.1 3.2 O K 1440 min Summer 0.086 0.086 3.0 2.3 O K 2160 min Summer 0.065 0.065 2.2 1.7 O K 2880 min Summer 0.054 0.054 1.7 1.4 O K 4320 min Summer 0.042 0.042 1.2 1.1 O K 5760 min Summer 0.035 0.035 1.0 0.9 O K 7200 min Summer 0.031 0.031 0.8 0.8 O K 8640 min Summer 0.027 0.027 0.7 0.7 O K 10080 min Summer 0.025 0.025 0.6 0.7 O K 15 min Winter 0.819 0.819 4.5 21.9 O K 30 min Winter 0.967 0.967 4.9 25.8 O K 60 min Winter 1.000 1.000 5.0 26.7 O K 120 min Winter 0.920 0.920 4.8 24.6 O K Storm Rain Flooded Discharge Time-Peak Event (mm/hr) Volume Volume (mins) (m³) (m³)

15 min Summer 133.672 0.0 22.5 17 30 min Summer 86.013 0.0 29.0 31 60 min Summer 52.662 0.0 35.5 50 120 min Summer 31.184 0.0 42.1 84 180 min Summer 22.679 0.0 45.9 118 240 min Summer 18.004 0.0 48.6 152 360 min Summer 12.930 0.0 52.4 220 480 min Summer 10.231 0.0 55.2 276 600 min Summer 8.526 0.0 57.5 328 720 min Summer 7.344 0.0 59.5 382 960 min Summer 5.799 0.0 62.6 492 1440 min Summer 4.152 0.0 67.2 734 2160 min Summer 2.968 0.0 72.1 1100 2880 min Summer 2.338 0.0 75.7 1468 4320 min Summer 1.668 0.0 81.1 2200 5760 min Summer 1.312 0.0 85.0 2872 7200 min Summer 1.088 0.0 88.2 3632 8640 min Summer 0.934 0.0 90.8 4384 10080 min Summer 0.821 0.0 93.0 5136 15 min Winter 133.672 0.0 25.3 17 30 min Winter 86.013 0.0 32.5 31 60 min Winter 52.662 0.0 39.8 52 120 min Winter 31.184 0.0 47.1 90 ©1982-2013 Micro Drainage Ltd Weetwood Page 2 Suite 1 Park House Broncoed Bus Park Wrexham Rd Mold Date 02/05/2013 13:29 Designed by adamedgerley File Checked by Micro Drainage Source Control 2013.1.1

Summary of Results for 100 year Return Period (+30%)

Storm Max Max Max Max Status Event Level Depth Control Volume (m) (m) (l/s) (m³)

180 min Winter 0.806 0.806 4.5 21.5 O K 240 min Winter 0.691 0.691 4.5 18.5 O K 360 min Winter 0.460 0.460 4.5 12.3 O K 480 min Winter 0.220 0.220 4.5 5.9 O K 600 min Winter 0.135 0.135 4.3 3.6 O K 720 min Winter 0.110 0.110 3.8 2.9 O K 960 min Winter 0.088 0.088 3.1 2.3 O K 1440 min Winter 0.066 0.066 2.2 1.7 O K 2160 min Winter 0.050 0.050 1.6 1.3 O K 2880 min Winter 0.042 0.042 1.3 1.1 O K 4320 min Winter 0.033 0.033 0.9 0.9 O K 5760 min Winter 0.028 0.028 0.7 0.7 O K 7200 min Winter 0.024 0.024 0.6 0.6 O K 8640 min Winter 0.022 0.022 0.5 0.6 O K 10080 min Winter 0.020 0.020 0.4 0.5 O K Storm Rain Flooded Discharge Time-Peak Event (mm/hr) Volume Volume (mins) (m³) (m³)

180 min Winter 22.679 0.0 51.4 128 240 min Winter 18.004 0.0 54.4 164 360 min Winter 12.930 0.0 58.6 238 480 min Winter 10.231 0.0 61.9 278 600 min Winter 8.526 0.0 64.4 320 720 min Winter 7.344 0.0 66.6 374 960 min Winter 5.799 0.0 70.1 490 1440 min Winter 4.152 0.0 75.3 726 2160 min Winter 2.968 0.0 80.8 1100 2880 min Winter 2.338 0.0 84.8 1464 4320 min Winter 1.668 0.0 90.8 2132 5760 min Winter 1.312 0.0 95.2 2856 7200 min Winter 1.088 0.0 98.7 3624 8640 min Winter 0.934 0.0 101.7 4352 10080 min Winter 0.821 0.0 104.2 5080

Rainfall Details

Rainfall Model FSR Winter Storms Yes Return Period (years) 100 Cv (Summer) 0.750 Region England and Wales Cv (Winter) 0.840 M5-60 (mm) 20.000 Shortest Storm (mins) 15 Ratio R 0.450 Longest Storm (mins) 10080 Summer Storms Yes Climate Change % +30

Time Area Diagram

Total Area (ha) 0.090

Time (mins) Area From: To: (ha)

0 4 0.090

Model Details

Storage is Online Cover Level (m) 1.500

Tank or Pond Structure

Invert Level (m) 0.000

Depth (m) Area (m²)

0.000 26.7

Hydro-Brake® Outflow Control

Design Head (m) 1.000 Hydro-Brake® Type Md13 Invert Level (m) 0.000 Design Flow (l/s) 5.0 Diameter (mm) n/a

Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s)

0.100 3.5 1.200 5.4 3.000 8.6 7.000 13.1 0.200 4.5 1.400 5.9 3.500 9.2 7.500 13.5 0.300 4.2 1.600 6.3 4.000 9.9 8.000 14.0 0.400 3.7 1.800 6.6 4.500 10.5 8.500 14.4 0.500 3.7 2.000 7.0 5.000 11.0 9.000 14.8 0.600 3.9 2.200 7.3 5.500 11.6 9.500 15.2 0.800 4.5 2.400 7.7 6.000 12.1 1.000 5.0 2.600 8.0 6.500 12.6