Environmental Statement CHAPTER 10 FLOOD RISK...Management Plan and Preliminary Flood Risk Assessment. 10.3.13 The LBE Strategic Flood Risk Assessment concludes that the risk of flooding

Deephams Sewage Works Upgrade Environmental Statement

Chapter 10 - Flood Risk Page 10.1 of 10.34 June 2014

CHAPTER 10 – FLOOD RISK

10.1 Introduction and Key Issues

10.1.1 This chapter describes the likely effects that the construction and operation of the Upgrade will have on flood risk. The potential effects on ground and surface water quality as a result of the Upgrade are considered in Chapter 8 - Contaminated Land and Chapter 18 - Water Resources.

10.1.2 Deephams Sewage Works is situated within the catchment of the Lower Lee and is adjacent to the Lee Navigation and its tributaries including Salmons Brook. The Environment Agency Flood Maps indicate that a small part of the Site is at risk from fluvial flooding from the Lower Lee and tributaries. As described throughout this chapter, this assessment also considers the risk of flooding from other sources including surface water, groundwater and artificial waterbodies (reservoirs).

10.1.3 As a small part of the site lies in an area of fluvial flood risk and since the Upgrade covers a site greater than one hectare, a Flood Risk Assessment (FRA) has been prepared to accompany the planning application for the works. This has been used to inform this chapter of the ES.

10.1.4 The elements of the Upgrade that are of particular relevance to this assessment are:

● Local landscape / topographical modifications affecting fluvial floodplain and overland flow routes across the site;

● change in proportion of permeable / impermeable areas – which may affect surface water runoff generation on site; and

● increased members of public on site at visitor centres – increased consequence in the event of any flooding (e.g. of reservoir failure).

10.1.5 The assessment covers effects of the Upgrade on the flood risk to the site as well as to surrounding areas.

10.2 Consultation

10.2.1 Consultation has been undertaken with the Environment Agency through the series of Stakeholder Forums carried out by Thames Water from 2011 to 2014, and through meetings and pre-application discussions on specific issues such as addressing potential groundwater impacts within the contaminated land assessment. Consultation has also been carried out with Ian Russell, Senior Engineer at LBE on surface water runoff on site and flood risk issues, including integrating the findings of the LBE’s Salmons Brook Flood Alleviation Scheme (FAS).

10.2.2 The provisional scope of this chapter was outlined in Thames Water’s EIA Scoping Report (Appendix 2.1). In its EIA Scoping Opinion dated 25 April 2014 (Appendix 2.2), LBE clarified the scope of this chapter as well as outlined what the separate FRA should assess. These clarifications are as follows:

● It is a requirement under the NPPF that risks associated with all potential sources of flooding should be considered in the FRA, so the FRA report will need to consider that there could be potential impacts on the groundwater flows/groundwater flooding as a result of the proposed new altered structures and these should be considered in the FRA and ES;

● The EIA and FRA should address the impact on the facility itself including impacts on its functionality, the efficiency of existing and future flood risk and



flood alleviation proposals and the adequacy of design and mitigation measures to address these issues; and

● The EIA and FRA should determine if there may be any impact of the proposed construction and design/ operations at the works on the integrity of the river and reservoir structures, e.g. any potential impact on ground stability and the stability of the retaining/ impounding structures.

10.2.3 A meeting to discuss water resources and flood risk was held with the Environment Agency on 11 June 2014. The Environment Agency also provided comments on a draft version of Chapter 18 - Water Resources on 23 June 2014. As part of this response they stated the following:

● Anticipated increased flow due to climate change and anticipated population growth could impact on water levels on the Salmons brook. We are aware that suspected high water levels in the Salmons brook downstream of the Deephams effluent channel may be causing water to increase in the Montagu Road area especially during the summer months. This matter should be investigated in order to establish a possible cause. In addition, it is important that the increases in flow do not cause increases in flood risk

● Reference is made to changes in flow pathways although it is not clear how those changes will occur and what the impacts could be. We consider it necessary for the document to expand on this so that it is possible to understand what the implications of the changes in flow pathways will be.

10.3 Legislation and Planning Policy

National

10.3.1 The NPPF, and associated Technical Guidance, replaces previous national planning policy adopted since 2004 (notably PPS25 Development and Flood Risk).

10.3.2 Sections 10 and 11 of the NPPF are of particular reference to this assessment. Section 10, ‘Meeting the challenge of climate change, flood risk and coastal change’ (paragraph 100) states that inappropriate development in areas at risk of flooding should be avoided by directing development away from areas of highest risk, but where development is necessary, making it safe without increasing flood risk elsewhere. Section 11, ‘Conserving and Enhancing the Natural Environment’ (paragraph 109) states that development should be prevented from contributing or put at unacceptable risk from, or been adversely affected by unacceptable levels of water pollution.

10.3.3 The Technical Guidance to the NPPF sets out the requirements for a site specific FRA (and ultimately the assessment within the ES). For example, the NPPF Technical Guidance states that a FRA must take into account the potential impacts of climate change.

10.3.4 The NPPF associated Planning Practice Guidance, published in March 2014, also requires an assessment of flood risk to consider all forms of flooding, and lists six forms of flooding that should be considered: Flooding from Rivers (Fluvial Flooding), Flooding from the Sea (Tidal Flooding), Flooding from Land (Pluvial Flooding), Flooding from Groundwater, Flooding from Sewers, and Flooding from Other Artificial Sources (i.e. reservoirs, canals, lakes and ponds).

10.3.5 The Government’s water strategy for England, Future Water was published in February 2008. This strategy sets out the Government’s long-term vision for water and the framework for water management in England. It aims to permit the supply of secured water supplies whilst ensuring an improved and protected water environment. Future Water brings together the issues of water demand, water



supply, water quality in the natural environment, surface water drainage and river / coastal flooding into a single coherent long term strategy, in the context of the need to reduce greenhouse gas emissions. The strategy also considers the issue of charging for water. The water environment and water quality have great economic, biodiversity, amenity and recreational value, playing an important role in many aspects of modern day society, and thus the functions provided must be sustainably managed to ensure they remain available to future generations without compromising environmental quality.

10.3.6 The Flood and Water Management Act 2010 encourages developers to include sustainable drainage systems (SuDS) in their proposals where practicable. SuDS provide a way to attenuate runoff from a site to the rate agreed with the Environment Agency to avoid increasing flood risk, but they are also important in reducing the quantities and concentration of diffuse urban pollutants found in the runoff. The type of SuDS proposed depends on local circumstances (e.g. ground conditions) and in the following order of preference as set out in the Building Regulations 2000: Approved document H:

● Soakaway (therefore to groundwater) or some other adequate infiltration system. When that is not practical;

● Watercourse. When that is not practical; and if the above are not possible; ● Surface water sewer.

Local

10.3.7 The London Plan was published by the Mayor of London in July 2011 and Revised Early Minor Alterations to the London Plan (REMA) were published in October 2013. Policy 5.12 of the London Plan relates to flood risk management and in particular, for new development / re-development requires that development proposals must comply with the FRA and management measures as set out in PPS25 (now superseded) and have regard to measures set out in Thames Estuary 2100 and the Thames Catchment Flood Management Plan. Policy 5.13 relates to Sustainable Drainage and requires development to use SuDS unless there are practical reasons for not doing so, and should aim to reduce runoff to greenfield runoff rates ensuring that surface water is managed as close to the source as possible. It also outlines a SuDS hierarchy plan.

10.3.8 The London Regional Flood Risk Appraisal (RFRA) was developed to meet the requirements of PPS25 for flood risk to be considered at all scales within the planning framework and was published by the Mayor in October 2009. It is a high level risk appraisal which includes a broad assessment of the risk across the region and makes a series of recommendations with the overall aim of ensuring that overall flood risk does not increase and that by addressing existing problems, overall risk is reduced. The findings of the RFRA have informed the flood risk management policies in the London Plan. The RFRA highlights that Deephams Sewage Works lies within the floodplain of the Lower Lee.

Local Planning Policy Guidance

10.3.9 The Council adopted the LBE’s Core Strategy (part of the Local Plan) in November 2010. Core policy 28 of the strategy relates to managing flood risk through development and states that the council will take a risk-based approach to development based on the requirements of PPS25 (now superseded). The Core Strategy notes that the council will support flood risk management projects including the Salmons Brook FAS (see below).

10.3.10 Section 8.5 of the LBE Submission Draft Development Management DPD relates to Flood Risk and includes a number of policies which are broadly in line with the



requirements of the NPPF and Technical Guidance but have been refined to take into account more specific information from LBE’s Strategic Flood Risk Assessment and Surface Water Management Plan (see below). In summary the DMD:

● Outlines the requirements for a site specific FRA including additional requirements for non-fluvial flooding;

● outlines how the development will manage surface water as close to its source as possible and follow the SuDS hierarchy plan of the London Plan;

● States that all new development that results in increased flood risk should be accompanied by appropriate flood mitigation measures to make development and its users safe; and

● States that new development must make space for water and not harm the integrity of flood defences.

10.3.11 A Level 1 Strategic Flood Risk Assessment was produced for LBE and completed in 2008. The LBE was also part of a wider North London Strategic Flood Risk Assessment (2008)1 specifically to support the North London Waste Plan.

10.3.12 The LBE Strategic Flood Risk Assessment is required to consider all sources of flooding but is mostly focussed on the risk of fluvial flooding. The risk of flooding from other sources has subsequently been assessed in the Surface Water Management Plan and Preliminary Flood Risk Assessment.

10.3.13 The LBE Strategic Flood Risk Assessment concludes that the risk of flooding is a serious consideration in Enfield and makes the following recommendations for the management of risk:

● Reduce flood risk through spatial planning; ● Reduce flood risk through FRA and site design; ● Reduce flood risk from non-fluvial sources; ● Enhance and restore the river corridor; ● Reduce surface water runoff from new developments; and ● Improve flood awareness and emergency planning.

10.3.14 The North London Strategic Flood Risk Assessment used data from a variety of sources (including the LBE Strategic Flood Risk Assessment) to define flood risk. There are a number of hydraulic models of the main rivers across the Strategic Flood Risk Assessment study area which were used to inform that assessment of risk. The North London Strategic Flood Risk Assessment identified fluvial flood risk as the most significant flood risk to the area.

Other Legislation, Guidance and Planning Policy

10.3.15 The Environment Agency published the Thames Catchment Flood Management Plan2 (CFMP) in 2009. The CFMP presents an overview of existing flood risk in the catchment and sets out the Environment Agency’s preferred plan for sustainable flood risk management over the next 50-100 years. The CFMP is undertaken at a catchment scale and therefore does not provide detailed information for the Deephams Sewage Works. Sub-area 5 of the CFMP is known as ‘Urbanised places with some flood defences’ which includes the Lower Lee Tributaries in Enfield and covers the Site.

10.3.16 For Sub-area 5, policy option 6 is preferred and the CFMP recommends taking “action with others to store water or manage run-off in locations that provide overall flood risk reduction or environmental benefits, is indicative of our vision of managing runoff, safeguarding open space and potential flood storage”.

1 Mouchel (2008) North London Strategic Flood Risk Assessment

2 Environment Agency (2009) Thames: Catchment flood management plan



10.3.17 The Environment Agency has also produced a Flood Risk Management Strategy for the Lower Lee which includes Salmons Brook and Pymmes Brook. Delivery of a Lower Lee Flood Risk Management Strategy is one of the actions from the Thames CFMP. The Salmons Brook Flood Alleviation Scheme is part of this wider strategy and is discussed further below (see paragraph 10.5.9).

10.3.18 As Lead Local Flood Authority under the Floods and Water Management Act (2010), LBE is required to complete a Preliminary Flood Risk Assessment. The Preliminary Flood Risk Assessment is the first step for the LBE in meeting the Flood Risk Regulations (2009)3. The Preliminary Flood Risk Assessment provides a high level overview of flood risk from all sources within the borough area including surface water, groundwater, ordinary watercourses and canals. The Preliminary Flood Risk Assessment4 for the LBE was completed in 2011 as part of a co-ordinated programme of work across Greater London facilitated by the Drain London Forum and the Greater London Authority (GLA).

10.3.19 The LBE Preliminary Flood Risk Assessment used data from local and national sources including LBE, the Environment Agency, Thames Water, emergency measures and other risk management authorities (e.g. Transport for London) to develop an understanding of flood risk across the borough. Data was collected on past and future (potential) floods. For the LBE Preliminary Flood Risk Assessment data were collated centrally by the Greater London Authority as part of the Drain London project. The LBE Preliminary Flood Risk Assessment does not require the development of new information and so can be based on limited datasets.

10.3.20 The LBE Preliminary Flood Risk Assessment is accompanied by an assessment/reporting spreadsheet which forms the basis of the Environment Agency’s reporting to the European Commission on the implementation of the Floods Directive. After approval of this Preliminary Flood Risk Assessment (by the Environment Agency) the Flood Risk Regulations require the borough to carry out two further stages:

● Flood hazard maps and flood risk maps by June 2013; and ● Flood risk management plans by June 2015.

10.3.21 A review and update of the LBE Preliminary Flood Risk Assessment will be undertaken in the next cycle of the Flood Risk Regulations, starting in 2017.

10.3.22 As part of the Drain London project, LBE has prepared a Surface Water Management Plan. LBE was part of the Group 4 working group for delivery of Surface Water Management Plans in the Drain London project, together with neighbouring boroughs of Haringey, Hackney, Tower Hamlets, Newham and Waltham Forest.

10.3.23 As part of the work 18 Critical Drainage Areas were identified. The Deephams Sewage Works does not lie within one of these Critical Drainage Areas. The closest Critical Drainage Area to the Deephams Sewage Works covers an area to the west of Meridian Way, adjacent to the Deephams Sewage Works.

10.3.24 The Surface Water Management Plan identifies opportunities for measures to be implemented across the borough to reduce the impact of surface water flooding. Mitigation opportunities are not restricted to the Critical Drainage Areas and there are opportunities for generic measures to be implemented such as

3 The requirement for preparation of PFRAs was in place under the EU Flood Risk Management Regulations (the Floods

Directive) prior to implementation of the Floods and Water Management Act. The transposition of the EU Floods Directive into UK law is referred to as the ‘Flood Risk Regulations’. 4 London Borough of Enfield (2011) Preliminary Flood Risk Assessment London Borough of Enfield



recommendations for widespread use of water conservation measures, soakaways, permeable paving etc.

10.3.25 The Surface Water Management Plan concludes with a long-term Action Plan to assist LBE in the management of surface water flood risk across the borough. This is a ‘living’ document which should be reviewed and updated regularly.

10.3.26 The Surface Water Management Plan Action Plan recommended three policies to be implemented within LBE. These are:

● Policy 1: All developments acoss the borough (excluding minor house extensions less than 30m2) which relate to a net increase in impermeable area are to include at least one ‘at source’ SuDS measure (e.g. waterbutt, rainwater harvesting tank, bioretention planter box etc). This is to assist in reducing the peak volume of runoff discharging from the site.

● Policy 2: Developments of more than one property or covering an area greater than 0.1ha are to reduce un-attenuated post development runoff rates by 50% through the use of SuDS, with on site attenuation provided for the 1 in 100 year event with an allowance for climate change (in line with PPS25 and UKCIP guidance).

● Policy 3: Developments located in Critical Drainage Areas of more than one property or greater than 0.1ha are to reduce runoff rates to pre-development Greenfield runoff rates (calculated in accordance with IoH 124), with on site attenuation provided for the 1 in 100 year event with an allowance for climate change (in line with PPS25 and UKCIP guidance). It is recommended that a SuDS treatment train is used to assist in this reduction.

10.3.27 One of the specific actions identified in the Surface Water Management Plan related to the area to the west of Meridian Way Critical Drainage Area was to carry out a feasibility study/further investigation using a refined Drain London model working with the Environment Agency to identify a joint fluvial/surface water solution in the Montagu Road area. This has subsequently resulted in the Salmons Brook Flood Alleviation Scheme (currently being constructed).

10.3.28 The Catchment Flood Management Plan, Preliminary Flood Risk Assessment, Surface Water Management Plan and Strategic Flood Risk Assessment referred to above will all underpin the Local Flood Risk Management Strategy that the LBE is required to produce as a Lead Local Flood Authority under the Flood and Water Management Act.

10.4 Assessment Methodology

10.4.1 The assessment outlined within this chapter has been informed by the site specific FRA which accompanies the Deephams Sewage Works Upgrade planning application. The assessment methods of the FRA are outlined within that report and include quantitative flood risk hydraulic modelling and numeric surface water flood risk assessment.

Impact Evaluation

10.4.2 A baseline review of flood risk has been undertaken, considering all sources of flooding using a range of qualitative and quantitative data (including results of modelling as outlined in the FRA). The outcomes of this baseline assessment have been used to identify a number of receptors against which the effects of the Upgrade can be assessed. The importance of the receptor (based on its sensitivity, value and location) has been classified as high, medium, low or negligible based on



the criteria in Table 10.1. Where appropriate, the flood risk vulnerability classification from the Technical Guidance to the National Planning Policy Framework has been used to classify the sensitivity of the receptors.

Table 10.1: Criteria for assessment of receptors

Sensitivity, value, importance

High Located on or adjacent to site

Essential infrastructure or highly vulnerable landuse (as defined in Technical Guidance to NPPF)

Medium Located within the area of influence of the Upgrade, near to the Site

More vulnerable landuse (as defined in Technical Guidance to NPPF)

Low Located within the area of influence of the Upgrade, but not near the Site

Less vulnerable landuse (as defined in Technical Guidance to NPPF)

Negligible Located outside the area of influence of the Upgrade

Water compatible landuse (as defined in Technical Guidance to NPPF)

Nature of Impacts

10.4.3 The potential effects of the Upgrade are considered as high, medium, low or negligible based on the criteria set out in Table 10.2. The nature of the effect is considered separately and collectively in terms of the magnitude, probability, reversibility, duration and direction of the effect of the Upgrade. Effects may be either adverse or beneficial.

Significance of Impacts

10.4.4 The significance of effect is determined using the matrix presented in Table 10.3. The significance of a given impact or effect is based on a combination of the importance of the receptor (see Table 10.1) and the nature of a given potential impact (see Table 10.2). Impacts can be beneficial, adverse or no effect (i.e. negligible) and their significance major, moderate or minor.

10.4.5 Generally, a higher level of significance is attached to large scale effects on sensitive or highly valued receptors. The matrix has been followed with professional judgement applied to ensure an appropriate level is attributed.



Table 10.2: Nature of Effects

Magnitude Probability Reversibility Duration

High Large-scale (regional to catchment) effects on flows, water levels and/or flood extent.

Significant impact on the risk (probability * consequence) of flooding.

High likelihood of direct effects on flows, levels or flood extent.

Effects on flows, levels or flood extent are irreversible

Long term effects on flows, levels or flood extent.

Medium Medium-scale (sub-catchment / tributary) effects on flows, water levels and/or flood extent.

Significant impact on the risk (probability * consequence) of flooding.

Medium likelihood of direct effects OR high likelihood of indirect effects on flows, levels or flood extent.

Effects on flows, levels or flood extent are partially reversible

Medium term effects on flows, levels or flood extent.

Low Small-scale (local / less than sub-catchment) changes to flows, water levels and/or flood extent.

No significant impact on the risk (probability * consequence) of flooding.

Low likelihood of direct effects OR medium likelihood of indirect effects on flows, levels or flood extent.

Effects flows, levels or flood extent are mostly reversible

Short term effects on flows, levels or flood extent.

Negligible Little or no changes to flows, levels or flood extent.

Low likelihood of direct or indirect effects on flows, levels or flood extent.

Effects on flows, levels or flood extent are fully reversible

At most temporary effects on flows, levels or flood extent.

Direction

Adverse Negative effects on flood risk (i.e. increase in flood risk)

Beneficial Positive effects on flood risk (i.e. flood risk reduction)



Table 10.3: Significance of Effects

Receptor Value, Scale and Sensitivity

High Medium Low

Magnitude of effect, including Duration, Frequency and Reversibility

High Major Major Moderate or Minor

Medium Major Moderate Minor

Low Moderate or Minor Minor Negligible

Climate Change (Future Baseline)

10.4.6 The risk of flooding on the Deephams Sewage Works site will increase as a consequence of climate change.

10.4.7 The National Planning Policy Framework requires consideration of the impact of climate change over the lifetime of the development. DEFRA (2006)5 provides figures for the estimated increase in rainfall and river flow; these are summarised in Table 10.4. This suggests that river flows should be increased by 20% for developments with a lifetime of up to 100 years (2115). Similarly, with regard to the rainfall runoff assessment, rainfall should be increased by 30% for developments with a lifetime of up to 100 years (2115). These increases are included in the assessment and have been accounted for within the modelling work that supports the site specific FRA.

Table 10.4: Climate Change Indicative Sensitivity Ranges

Parameter 1990-2025 2025-2055 2055-2085 2085-2115

Peak rainfall intensity

+5% +10% +20% +30%

Peak river flow +10% +20% +20% +20%

Limitations of Assessment

10.4.8 The assessment of flood risk is partially based on quantitative modelling. The accuracy of flood models is dependent on a number of factors, such as the accuracy of survey data and assumptions that have been made during its construction (e.g. with regard to peak flow estimation). A 2-D fluvial flood risk model has been used within the assessment, which typically increases the accuracy of a flood risk model (compared to 1-D models).

10.5 Baseline

Landscape and Topography

10.5.1 Ground levels across the Deephams Sewage Works site generally vary from 11-13m AOD. The northwest corner of the Site is currently higher at around 15m AOD although this area will be reduced in elevation as a result of the AMP5 works prior to the Upgrade, to around 12.4m AOD.

10.5.2 Overall, the Site is reasonably flat with local variations in elevations, as a result of differing land uses (e.g. kerbs, buildings), playing a critical role in directing surface water flows. The Site lies within the catchment of Salmons Brook which flows in a south-easterly direction.

5 DEFRA (2006) Flood and Coastal Defence Appraisal Guidance, FCDPAG3 Economic Appraisal. Supplementary Note to

Operating Authorities – Climate Change Impacts



Hydrology

10.5.3 The LBE contains a significant length of watercourses including Pymmes Brook, Salmons Brook and Turkey Brook, all of which are major tributaries of the River Lee and flow in the vicinity of Deephams Sewage Works. Figure 18.2 in Chapter 18 - Water Resources provides an overview map of the watercourses that flow through or near to the Deephams Sewage Works. Enfield Ditch and Enfield Ditch Tributary (tributaries of Salmons Brook) flow around the Site.

10.5.4 Figure 18.1 in Chapter 18 - Water Resources provides a larger scale map of the catchment. Deephams Sewage Works is located to the west of Chingford Reservoirs, a non-impounding reservoir owned and operated by Thames Water with a capacity of 16.5 million m3. Chingford Reservoirs is composed of two reservoirs separated by the A110 Lea Valley Road. At its closest the Deephams Sewage Works is approximately 100m from the embankment around the southern most of the two reservoirs, William Girling Reservoir.

10.5.5 To the north of William Girling Reservoir is King George’s Reservoir, also a non-impounding reservoir owned and operated by Thames Water and with a capacity of 13.97 million m3.

Flood Risk

Historic Flooding

10.5.6 LBE was affected by fluvial flooding in the catchment-wide flood event of March 1947 with fluvial flooding observed near the current location of the Deephams Sewage Works site. Following this event, the River Lee Flood Relief Channel was constructed and became operational in 1976 and there has been no fluvial flooding on a similar scale in LBE since then, although the Strategic Flood Risk Assessment reports that the flood relief channel almost reached full capacity in 1987, 1993 and 2000.

10.5.7 On construction, the River Lee Flood Relief Channel was designed to protect against an event of similar magnitude to the 1947 event, believed to be approximately a 1 in 70 year flood.

10.5.8 The LBE Preliminary Flood Risk Assessment includes a summary of past flooding from surface water, sewer or groundwater sources. The Preliminary Flood Risk Assessment reports flooding in the Montagu Road area (approximately 0.5km west of the south west corner of Deephams Sewage Works) in October 2000 when fluvial flows on Salmons Brook exceeded channel capacity and overtopped the banks. This event was classed as having ‘significant harmful consequences’ (over 200 properties were flooded).

10.5.9 In order to mitigate this risk of flooding, the Salmons Brook Flood Alleviation Scheme (FAS) is currently being constructed. The FAS involves works to the immediate west of the site (on the other side of Meridian Way) on the Montagu Recreation Ground. The FAS has a design standard of 1 in 75 years and components in/near Montagu Road include:

● Flood walls along both sides of Salmons Brook from Plevna Road to Montagu Road;

● Montagu Recreation Ground bypass culvert – box culvert and open channel extending from the existing culvert under Montagu Road to the open channel through Montagu Road Recreation Ground; and

● Montagu Road Recreation Ground flood storage – the creation of a new reservoir over the left and right bank of the river between Montagu Road and the railway line. This will provide 40,000m3 of storage with an embankment



crest level of 12.82m AOD and will become a statuory reservoir under the Reservoirs Act 1975.

Tidal Flood Risk

10.5.10 Deephams Sewage Works is located approximately 13km from the Tidal Thames (with a number of lock structures preventing tidal surge travelling up the River Lee towards the site). The site is therefore not deemed to be at risk of tidal flooding and so the effect of flooding from sea and tidal influences is not considered within the remainder of the report.

Fluvial Flood Risk

10.5.11 The flood map6 of the site, based on Environment Agency data, is shown in Figure 10.1. The map demonstrates that most of the Deephams Sewage Works site lies within Flood Zone 1 and is considered to have a low probability of flooding (less than 1 in 1,000 annual probability of flooding) from rivers (noting that the Site is not at risk of tidal flooding). A small area along the eastern boundary of the Site is shown to be in Flood Zone 2 and has a medium probability of flooding from rivers or the sea (between a 1 in 100 and 1 in 1,000 annual probability of flooding from rivers).

10.5.12 The Enfield Ditch Tributary and the Outflow Channel are not included in the Environment Agency hydraulic model that has been used to derive this Flood Map.

10.5.13 In order to assess the impact of the Upgrade on fluvial flood risk, Hyder Consulting has undertaken hydraulic modelling of the Site and surrounding watercourses7. The Hyder hydraulic model was developed using the following key sources of data:

● Lower Lee Model – developed by Halcrow in 2010 under the Environment Agency Strategic Flood Risk Management Framework. The model extends from Fieldes Weir to the confluence of the Lower Lee and the River Thames, including various tributaries such as Salmons Brook.

● The Salmons Brook Flood Alleviation Scheme Environment Agency Model (2012) - this model was built to support the planning application for the FAS scheme, now being constructed and considered be part of the baseline agaonst which the Impact of the Upgrade Is assessed. This model was based in part on the Lower Lee Model but includes a more detailed model of the Salmons Brook and an updated representation of the Enfield Ditch Tributary (along the northern boundary) Enfield Ditch (along the eastern boundary of the Deephams Sewage Works) and of the Brimsdown Ditch (along the western boundary).

10.5.14 The Enfield Ditch Tributary and the Outflow Channel have been included within the Hyder model.

10.5.15 The updated baseline Hyder model has been run for the 1 in 35, 1 in 100 and 1 in 100 plus climate change (see Section 10.4.8) events. Flood extents for these events are shown in Figure 10.2. When comparing the 1 in 100 baseline model extents to the Environment Agency Flood Zones it can be seen that generally the extents are very similar, with the overall risk to the Deephams STW site remaining low, following the revised modelling exercise.

6 Hyder A630 Deephams STW Upgrade – Hydraulic Modelling and Flood Risk Issues Report, D1-Rp-A630 Deephams-

UA002439-1012-2012-07-18-v1.5 7 Hyder Consulting (2014) A630 Deephams STW Upgrade Hydraulic Modelling and Flood Risk Issues, June 2014 on behalf of

Thames Water.

June 2014

Figure Title:

Project Title:

Deephams SewageWorks Upgrade

Environment Agency flood zones at

Deephams Sewage Works For Information Only

Figure 10.1

Legend

Flood Zone 2

Flood Zone 3

500m0

Note: contains Environment Agency information © Environment Agency and database right

Approximate Site boundary

Outflow Channel

Enfield Ditch Tributary

Environmental Statement CHAPTER 10 FLOOD RISK...Management Plan and Preliminary Flood Risk Assessment. 10.3.13 The LBE Strategic Flood Risk Assessment concludes that the risk of flooding

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