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2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 i

CHAPTER 12.0 SURFACE WATER, FLOOD RISK AND DRAINAGE

12.0 SURFACE WATER, FLOOD RISK AND DRAINAGE .................................. 12-1

Introduction ................................................................................................... 12-1

Legislation, Policy and Guidance .................................................................. 12-2

Consultation .................................................................................................. 12-4

Parameters Used for Assessment ................................................................. 12-6

Assessment Methodology ............................................................................. 12-6

Baseline ........................................................................................................ 12-9

Assessment of Effects ................................................................................ 12-16

Mitigation .................................................................................................... 12-27

Residual Effects .......................................................................................... 12-31

Summary .................................................................................................... 12-35

APPENDICES (bound separately in Volume 3)

Appendix 12-1 .............................................. Flood Risk and Water Supply Assessment

Appendix 12-2 ...................................................................................... Drainage Report

Please note that a full list of acronyms is provided the contents to this PEIR and should

be referred to when reading this Chapter.

2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-1

12.0 SURFACE WATER, FLOOD RISK AND DRAINAGE

12.1 Introduction

12.1.1 This chapter presents an assessment of the likely significant effects of the

Proposed Extension upon the water environment during the construction,

operating and decommissioning phases.

12.1.2 The Chapter addresses the following effects:

• Surface water bodies;

• Flood risk;

• Water supply; and

• Wastewater treatment and sewerage.

12.1.3 Matters relating to groundwater are comprehensively covered in Chapter 10.0

Geology, Hydrogeology, Contaminated Land and Ground Stability.

12.1.4 This Chapter describes the methods used to assess the likely significant effects;

the baseline conditions that exist at the Site and within the surrounding area; the

mitigation measures required to prevent, reduce or off-set any significant

negative effects; and the likely residual effects after these measures have been

adopted.

12.1.5 This Chapter utilises the site-specific Flood Risk and Water Supply Assessment

(hereafter referred to as the FRA) prepared by Weetwood Services Limited

(Weetwood) for the Proposed Extension (Appendix 12-1).

12.1.6 The FRA was in undertaken in accordance with the requirements of the NPS for

Energy (EN-1) and the NPS for Renewable Energy Infrastructure (EN-3) (July

2011), the revised NPPF (updated on 19 February 2019) and the NPPG

(updated on 1 October 2019).

12.1.7 This chapter includes an overview of the surface and foul water drainage

arrangements for the Existing Station and the drainage strategy for the

Proposed Extension and extended Generating Station. This overview is based


on information presented in the Drainage Report prepared by Idom Merebrook

Ltd (Idom). The Drainage Report is included as Appendix 12-2.

Competence

12.1.8 This chapter has been prepared by Weetwood Services Ltd. It has been written

by a specialist with over 14 years of experience working in the water

environment and with extensive experience of managing and co-ordinating both

small and large projects and drafting water environment chapters for

environmental statements for a range of proposed developments and planning

submissions.

12.2 Legislation, Policy and Guidance

12.2.1 In preparing this section of the PEIR, relevant international and national

legislation and policy guidance documents have been considered as presented

in Table 12.1. A more detailed overview of the relevant policy requirements has

been provided within the FRA contained within Appendix 12-1.

Table 12.1: Relevant Legislation, Policies and Guidance Documents

Guidance

International Legislation

• Water Framework Directive 2000/60/EC

• EC Dangerous Substances Directive 2006/11/EC and daughter directives

• Drinking Water Directive 98/83/EC

National Legislation, Policies and Guidance

• National Policy Statements (NPS) EN1 and EN-3

• National Planning Policy Framework [NPPF] (updated on 19 February 2019) and the National Planning Practice Guidance (NPPG) (updated on 1 October 2019)

• Water Industry Act 1991

• Water Act 2003 (as amended)

• Flood and Water Management Act 2010

• The Water Environment (Water Framework Directive) (England and Wales) Regulations


Guidance

• Control of Pollution (Oil Storage) (England) Regulations (2001)

• Surface Waters [Dangerous Substances (Classification)] Regulations 1998

• Control of Substances Hazardous to Health (COSHH) Regulations (2002)

• Environment Act 1995 (as amended)

• Land Drainage Act 1991 (as amended)

• Sustainable Drainage Systems: Non-statutory Technical Standards for SuDS, DEFRA 2015

• The Building Regulations - Drainage and Waste Disposal, Approved Document H HM Government, published in 2010, Amended 2015

• The SUDS Manual (C753), CIRIA, 2015

• Sustainable Drainage Systems – Hydraulic, structural and water quality advice (C609), CIRIA, 2004

• Control of Water Pollution from Construction Sites (C532), CIRIA, 2001

• Control of Pollution from Highway Drainage Discharges, Report 142, CIRIA, 1994

• CIRIA Designing for Exceedance in Urban Drainage - Good Practice

• Sewers for Adoption, 6th and 7th Editions, A Design and Construction Guide for Developers

Local Policies and Guidance

• Policy CP10 Flood Protection, Tonbridge and Malling Borough Council Core Strategy, September 2007

• Policy LP17 Flood Risk, Tonbridge and Malling Local Plan Proposal, January 2019

• Policy DM3 Natural Environment, Maidstone Borough Local Plan October 2017.

• Kent County Council Preliminary Flood Risk Assessment, Kent County Council, Final Report, September 2011

• Kent Local Flood Risk Management Strategy 2017-2023, Kent County Council, June 2013

• Tonbridge and Malling Borough Council Level 1 Strategic Flood Risk Assessment, Final Report, August 2016

• Maidstone Borough Council Level 1 Strategic Flood Risk Assessment Addendum Report, Final Report, October 2016

• Maidstone Borough Council Surface Water Management Plan, 2013


12.3 Consultation

12.3.1 A scoping report was submitted to PINS in November 2019 (refer Appendix 6-

1) and a scoping opinion was received from PINS in December 2019 (refer

Appendix 6-2).

12.3.2 A response to the Scoping Opinion is provided in Table 12.2, together with

details of further consultation undertaken with the EA.

Table 12.2: Response to Scoping Opinion and Further Consultation

Consultee Comment Response to Consultation

Scoping Responses

PINS ID4.5.1 Effects on groundwater dependant terrestrial ecosystems (GWDTE). The Inspectorate stated that the ES should include an assessment of the effects of the Proposed Extension on groundwater dependant terrestrial ecosystems

This is covered in the Ecology chapter of the PEIR. (Chapter 11).

ID4.5.2. Changes to the natural drainage Patterns. PINS stated that changes to the natural drainage arising from the Proposed Extension should be covered within the ES.

A drainage strategy has been prepared and is presented in the Drainage Report (Appendix 12-2). Changes to natural drainage are assessed within this Chapter.

ID4.5.3. Effects on water resources (both private and public water supply). PINS stated that ES should clearly explain the interaction between the proposed development and water resources, including private and public water supply any likely significant effects should be assessed in the ES.

A water supply assessment has been completed within the FRA (Appendix 12-1) and is assessed within this Chapter.

ID4.5.4. Water framework directive (WFD). PINS stated that the Scoping Report states that a WFD assessment will be undertaken, but makes no reference to PINS Advice Note 18: The Water Framework Directive. The Applicant should have regard to advice contained in PINS Advice Note 18 when undertaking the WFD assessment.

The Scoping Report does not state that a WFD Assessment would be undertaken or is needed, but rather that the River Medway has been assessed [by the EA] under the WFD, and that the EIA would be informed by the findings of the EA’s WFD assessment. This chapter makes reference to the WFD Assessment for the River Medway.

ID4.5.5. Flood risk. PINS stated that (i) the ES should assess impacts from foul water flooding where significant effects are likely to occur, and should consult with Southern Water regarding measures to prevent foul water flooding; and (ii) The Applicant should ensure that climate change allowances used to inform the assessment are sufficiently up to date and reflect relevant guidance from the EA.

A drainage strategy has been prepared and is presented in the Drainage Report (Appendix 12-2). The illustrative drainage strategy has been informed by and the key principles agreed with Southern Water. Foul water drainage is assessed within this chapter. Climate change has been taken into account in the illustrative drainage strategy as presented in the Drainage Report


Consultee Comment Response to Consultation

(Appendix 12-2) and to assess the future effect of climate change on fluvial flood risk as presented in the FRA (Appendix 12-1). In both cases, the appropriate climate change allowances have been used in accordance with current EA guidance.

ID4.5.6. Sustainable Drainage Systems (SuDS). PINS stated that if SuDS are to be used in the proposed development, a detailed description should be included in the ES.

The use of SUDS is presented in the Drainage Report (Appendix 12-2) and is also covered in this chapter.

Environment Agency

Groundwater and Contaminated Land - The site is situated in a source protection zone 1 (SPZ1) and therefore, in accordance with our approach to groundwater protection, we would provide outright approval for clean roof water into the ground only. Inside a SPZ1, all sewage effluent discharges to ground would require an environmental permit. Generally, we would only agree to the release of sewage effluent to ground, if we were satisfied that it was not reasonable to make a connection to the public foul sewer. This would require evidence as to why the proposed development cannot connect to the foul sewer.

An illustrative drainage strategy is presented in the Drainage Report (Appendix 12-2). The strategy confirms that only roof runoff will be discharged to an existing pond and that all foul water would be conveyed via a trapped gully and pipe system with Class 1 petrol/oil interceptors to an attenuation lagoon before being pumped off-site to the public sewer system. All domestic foul water arising from the Proposed Extension would be treated by an on-site package treatment plant with treated effluent from the plant re-used on site as process water. All trade effluent will be reused. No foul water would be disposed of to the ground.

The Scoping Report also raises the possibility of pollution arising from workers during the construction phase of the project. Further information is required regarding these risks and how they intend to manage them. For instance, the number of workers that will be on site simultaneously and details regarding the sewage facilities provided for them.

The potential effects of pollution arising from site workers during the construction phase are assessed in this chapter, and in Chapter 10.0. Further details are also included in the draft CEMP (Appendix 5-4).

Additional Consultation

Environment Agency

As the Site is located in Flood Zone 1, the Environment Agency has advised that it has no comments on the Proposed Extension relating to flood risk.

Irrespective, the submitted FRA report (Appendix 12-1) assesses the risk of flooding from all known sources.


12.4 Parameters Used for Assessment

12.4.1 The overall approach to the use of the Rochdale Envelope and the Parameters

that have been assumed for the assessment of likely significant environmental

effects arising from the Proposed Extension is set out in Chapter 5.0 of this

PEIR.

12.4.2 In terms of the assessment of significant effects on the water environment, the

main parameters in the context of the Proposed Extension relate to the extent

of additional impermeable areas that would be generated by the Proposed

Extension i.e. the overall roof area of the main building, the extent of

hardstanding, and the runoff generated from these surfaces due to rainfall.

12.4.3 As noted previously a Drainage Report has been prepared in support of the

PEIR (Appendix 12-2). The report has informed the preparation of both the FRA

and this Chapter.

12.4.4 The Drainage Report includes an indicative drainage design supported by

hydraulic calculations. The indicative design has been based on the greatest

impermeable area that could be generated (in accordance with the Proposed

Works Packages) and the maximum roof area of the Main Building (which has

fixed parameters). The works packages are explained in detail in Chapter 5 of

the PEIR and a works plan drawing is provided as Figure 5-13.

12.4.5 Given that the maximum parameters have been used in the underlying drainage

assessment and calculations, it is considered that the assessment of effects has

been based upon a reasonable worst-case assessment scenario.

12.5 Assessment Methodology

Methodology

12.5.1 This chapter has been prepared following a detailed review of currently available

information (references are provided in Section 1.3 of the FRA - Appendix 12-

1).


12.5.2 The information has been used to inform the baseline conditions at the Site and

identify any potential constraints or significant effects the Proposed Extension

may have on surface water, flood risk and drainage.

12.5.3 This chapter also presents the findings of the FRA (Appendix 12-1) which

assesses the risk of flooding from all sources, including from rivers, reservoirs,

canals and other artificial sources, groundwater and surface water. The FRA

also assesses the impact of the extended Generating Station on water supply.

12.5.4 The Drainage Report (Appendix 12-2) presents a strategy for the management

of surface water runoff and foul water that would arise from the Proposed

Extension. The drainage strategy forms part of this assessment.

Significance Criteria

12.5.5 The criteria used to assess the significance of effects from the Proposed

Extension on surface water, flood risk and drainage are set out in Tables 12.3

to 12.5.

12.5.6 To assess the potential significance of the effect of the Proposed Extension on

the identified receptors, the characteristics of each identified effect at the

construction, decommissioning and operational stages has been considered.

12.5.7 The level of effect has been determined from the importance of the receptor, the

magnitude of the effect and, where appropriate, the likelihood of the effect

occurring. Potential effects may be assessed to be adverse or beneficial.

12.5.8 Mitigation measures have been developed for identified effects using technical

guidance, best practices, and professional experience. Where the magnitude of

a potential effect (or effects) is assessed to be Negligible, no mitigation

measures are deemed to be necessary and the receptor is screened out.

12.5.9 The level of potential effect following the application of identified mitigation

measures (i.e. the residual effect) has then been assessed regarding the extent,

magnitude and duration of the effect and performance against environmental

quality standards, again regarding the criteria presented in Table 12.3 to Table


12.5. The magnitude of the residual (i.e. post mitigation) effects has been

assessed as described above.

12.5.10 Effects that are Moderate, Major or Severe (in bold in Table 12.5) are

considered significant in EIA terms.

Table 12.3 Estimating Receptor Importance

Importance Criteria

Very High Receptor has a high quality and/or rarity on a regional or national scale

High Receptor has a high quality and/or rarity on a local scale

Medium Receptor has a medium quality and/or rarity on a local scale

Low Receptor has a low quality and/or rarity on a local scale

Table 12.4 Criteria for Estimating the Magnitude of Change on a Receptor

Magnitude Descriptor

Major A considerable effect (by extent, duration or magnitude) resulting in a complete loss

of resource or receptor in terms of surface water, flood risk management and land

drainage. If adverse, could result in a breach of legislation or exceedance of

statutory objectives and planning policy.

Moderate

Limited effects which may affect the quality or integrity of surface water, flood risk

management and land drainage so that there may be a loss to part of the receptor.

Minor

Slight, very short or highly localised effects in terms surface water, flood risk

management and land drainage unlikely to affect the integrity of the resource.

Negligible Effects that would have no meaningful impact in terms of surface water, flood risk

management and land drainage

Table 12.5 Estimating the Significance of Potential Effects

Magnitude of Change

Major Moderate Minor Negligible

Imp

ort

an

ce o

f

Recep

tor

Very High Severe Major Moderate Negligible

High Major Moderate Minor Negligible

Medium Moderate Minor Negligible Negligible

Low Minor Negligible Negligible Negligible


Scope

12.5.11 The scope of the assessment comprises an assessment of the quality of surface

water bodies, sources of flood risk at or within the vicinity of the Site, existing

land drainage, water supply and foul water arrangements.

12.5.12 Informed by the above, the effect of the Proposed Extension and ultimately the

extended Generating Station on surface water bodies, flood risk and drainage,

and on water supply has been assessed. This assessment has been informed

by consultation with the Environment Agency (EA) and with Southern Water and

South-East Water (by Idom).

12.5.13 Matters relating to groundwater are addressed ibn Chapter 10.0 (Geology,

Hydrogeology, Contaminated Land and Ground Stability).

Limitations

12.5.14 The scope of the assessment has been based upon a review of available

desktop information within the study area to identify the baseline conditions and

development receptors. This has been supported by detailed assessments

where necessary.

12.5.15 An assessment of the potential effects of the Proposed Extension has been

undertaken utilising the best data, methods and scientific knowledge available

at the time of writing.

12.6 Baseline

Data Sources

12.6.1 The presence, location and quality of surface water bodies at and within the

vicinity of the Site, and the risk of flooding from known sources have been

assessed utilising Ordnance Survey (OS), Government, EA and British

Geological Society (BGS) data and mapping and the other sources of

information listed in Table 12.1.


12.6.2 The assessment of flood risk has also been informed by topographic data

derived from EA LIDAR data.

12.6.3 The current surface and foul water drainage arrangements at the Site are

provided in the Drainage Report (Appendix 12-2).

Environmental Baseline

Existing Scenario

Surface Water Bodies

12.6.4 The River Medway flows in an overall north-westerly direction to the north of the

Site. At its closest, the river is approximately 270 m north-east of the Site

boundary. The River Medway is tidally influenced in this location, with Allington

sluice and lock forming the tidal limit 760 m east of the Site (Figure 2, Appendix

12-1).

12.6.5 There is a significant amount of intervening infrastructure between the Site and

the River Medway, including the M20 to the north, and a railway to the north /

east.

12.6.6 The River Medway has been assessed under the WFD and is split into two

separate waterbodies; the Medway at Maidstone (ID: GB106040018440) which

ends east of the M20, and the Medway (ID: GB530604002300). Both are

classified as heavily modified waterbodies.

12.6.7 The current overall WFD status of the Medway at Maidstone waterbody is

Moderate, with the current ecological status being Moderate and the current

chemical status being Good. The target date for achieving a Moderate overall

status, a moderate chemical status and a Good ecological status, was by the

end of Cycle 1 in 2015. These objectives were met. The waterbody is protected

under the Nitrates Directive and Drinking Water Directive.

12.6.8 The current WFD status of the Medway waterbody is Moderate, with the current

ecological status being Moderate and the current chemical status being Fail.


The target date for achieving Moderate overall status, Moderate chemical status,

and Good ecological status was by the end of Cycle 1 in 2015. These objectives

were met, although the chemical status has since deteriorated. The waterbody

is protected under the Bathing Water Directive, Nitrates Directive, Shellfish

Directive and Conservation of Wild Birds Directive.

12.6.9 There are no other WFD defined surface waterbodies within the vicinity of the

Site, or upon which the Proposed Extension would have the potential to impact.

12.6.10 The only other surface waterbodies located within the Site are an existing pond

(the existing pond) located within the north-west corner of the Site, and an

existing surface water attenuation lagoon (the existing lagoon) located adjacent

to the north-east corner of the Existing Station.

Flood Risk Management

12.6.11 There are no records of historic flood events on the EA Historic Flood Map and

Recorded Flood Outline databases, or in KCC’s 2011 Preliminary Flood Risk

Assessment.

12.6.12 According to the EA Flood Map for Planning (Figure 5, Appendix 12-1 FRA) the

Site is not within the flood outline for the 1 in 1,000 annual probability event and

is therefore located in Flood Zone 1. Flood Zone 1 is defined as having a ‘low

probability’ of river or sea flooding.

12.6.13 The effect of sea level rise and increases in peak flow on flood risk to the year

2115 was modelled in accordance with the EA’s climate change allowance

guidance in the 2016 Tonbridge and Malling Level 1 SFRA report. The SFRA

report confirms that the Site is unaffected by the increased extent of flooding

due to the future effect of climate change to the year 2115 (Figure 7, Appendix

12-1 FRA).

12.6.14 The EA Flood Risk from Surface Water map (Figures 9-12, Appendix 12-1

FRA) indicates:

• There is a risk of surface water accumulation around the Existing Station.

The depth of surface water is indicated to be generally less than 300mm


for a 30-year storm event, increasing to 300-900mm for a more severe

storm event (i.e. 100-year and 1,000-year). The extent of the surface water

flooding increases as the severity of the storm event increases.

• Surface water would accumulate in the low-lying part of the western

section of the Site in the vicinity of the existing pond. The depth of surface

water is indicated to be up to 900mm for the 100 and 1,000-year storm

events with the depth exceeding 900mm for the 1,000-year event. Again,

the extent of the surface water flooding increases as the severity of the

event increases.

• A surface water pathway across the western part of the Site is indicated

to be generated during the 1,000-year storm event. The surface water is

generated from land to the south of the Site and conveyed across the A20

London Road and the Site in a northerly direction. The pathway terminates

in the low-lying area in the vicinity of the existing pond.

• Flow velocities are indicated to be generally low (i.e. less than 0.25 m/s)

for the 30 and 100-year, increasing to more than 0.25 m/s during the

1,000-year event.

12.6.15 The EA Risk of Flooding from Reservoirs map indicates that the Site is not at

risk of flooding due to reservoir failure.

12.6.16 There are no canals or other impounded waterbodies located with the immediate

vicinity of the Site.

12.6.17 The BGS Groundwater Flooding Hazard map (Figure 13, Appendix 12-1)

indicates that the susceptibility of the Site to groundwater flooding is generally

Low, but with some areas, primarily the lowest lying land in the western part of

the Site, having a higher (generally Moderate) susceptibility.

Surface Water Drainage

12.6.18 The surface water drainage system for the Existing Station is described in the

Drainage Report (Appendix 12-2) and can be summarised as follows.

12.6.19 Runoff from the roof of the main building and from the perimeter of the Existing

Station drains to the existing pond. The existing pond has been assessed (see


Appendix 12-2) as having sufficient capacity to accommodate surface water

runoff generated from the roof of the main building of the Existing Station and

from the land drainage system for the 1 in 100 plus 40% climate change event.

The existing pond is understood to be hydraulically connected to the underlying

ground and therefore provides a pathway to the underlying aquifer.

12.6.20 Runoff from remaining impermeable surfaces is collected and conveyed by a

trapped gully and pipe system to the existing lagoon. Water is pumped from the

existing lagoon to the public sewer in Laverstoke Road at a maximum rate of

100 l/s via an existing dedicated sewer connection.

12.6.21 The division of drainage described above ensures that only clean runoff is

discharged into the existing pond whilst potentially contaminated runoff is

discharged to the existing lagoon and thereafter the public sewer system.

12.6.22 Hydraulic modelling of the existing surface water drainage system (see

Appendix 12-2 for further details) identifies that some localised flooding may be

expected during the 1 in 100 storm event (and greater) due to the capacity of

the existing surface water drainage system serving the Existing Station being

exceeded.

12.6.23 This is presumed to be because the existing system was designed

approximately fifteen years ago and predates current standards.

Notwithstanding this, current standards allow localised flooding to occur for

events exceeding the 1 in 30 event, as long as the excess runoff is appropriately

managed on site and does not render a development unsafe or increase the risk

of flooding to third party land.

Foul Water Drainage

12.6.24 All domestic and trade effluent from the Existing Station is currently pumped to

the public foul sewer in Laverstoke Road.

12.6.25 The foul pumping station is located to the east of the Existing Station, and has

a maximum operating flow rate of 10 l/s. The Existing Station has a trade effluent

licence to discharge 50 cu m/day at a maximum rate of 36 l/s.


Water Supply

12.6.26 The Existing Station benefits from an existing connection to the South-East

Water supply network, with a maximum permitted supply of 35 cu m/hr.

12.6.27 The current consumption of the Existing Station during normal operation

equates to 11.7 cu m/hr, rising to 23.4 cu m/hr during peak usage, e.g. during

boiler filling after a maintenance outage.

The HWRC Scenario

12.6.28 A separate planning application for a HWRC at the north-eastern edge of the

Site was submitted to KCC in December 2019 (application reference

KCC/TM/0284/2019). The application is due to be determined in July 2020.

12.6.29 As such, and for the reasons explained in more detail in Chapter 6.0 of the PEIR,

it is necessary for the assessment of likely significant effects on the water

environment to also consider a second scenario, where the HWRC has been

granted planning permission and thereafter implemented.

12.6.30 The HWRC would comprise of a new recycling facility with a new access from

Laverstoke Road. Construction of the HWRC would entail removal in part and

the re-engineering of the existing perimeter bund on the eastern Site boundary,

which would include the loss of existing tree cover and new native woodland

planting.

12.6.31 The planning application of the HWRC was supported by a Drainage

Assessment Report and a Flood Risk Assessment report.

12.6.32 According to the illustrative drainage strategy presented in the Drainage Report,

surface and foul water drainage from the proposed drainage system would

discharge into public sewers located in Laverstoke Road; an approach agreed

by Southern Water.


12.6.33 The HWRC surface and foul water systems would be separate to the drainage

systems at the Existing Station and the earthworks associated with construction

of the HWRC would not affect drainage of the Site.

12.6.34 As such, the HWRC development would have no adverse impact on the surface

water or foul drainage infrastructure associated with the Existing Station, rather,

the introduction of the HWRC facility on the eastern bund would reduce the

amount of land draining into the existing land drainage system for the Site,

thereby limiting flows to the existing pond.

Grid Connection

12.6.1 As set out in Chapter 5.0, the connection to the local electricity grid would be

undertaken by the District Network Operator (DNO) under their statutory powers

under the Electricity Act 1989. These works would be brought forward separately

by the DNO and as such do not fall to be consented under the DCO process.

Nonetheless, the Chapters of this PEIR have given consideration to the likely

impacts associated with the installation of the Grid Connection, because the

connection is a critical requirement for the Proposed Extension to recover

energy.

12.6.2 The Existing Station is connected to the Maidstone Sub-Station via an

underground connection that runs east around the 20/20 Business Park and

then south through a number of land-uses and suburban areas to the north

Maidstone (Route A). The existing grid route is the subject of a number of

potential environmental and physical constraints, which has led the Applicant to

consider whether an alternative route may be preferable. Accordingly, there are

two potential grid connection routes for the Proposed Extension, as follows:

• Route A: the existing route from the Existing Station, running through

undeveloped and suburban areas towards the northern edge of Maidstone;

• Route B: an alternative route that follows the A20, and roads within

Maidstone town centre.

12.6.3 This assessment will consider the potentially significant environmental effects

associated with construction / decommissioning and operation of both routes on

the water environment.


Development Receptors

12.6.4 Table 12.6 lists the identified environmental receptors and their assessed

importance using the criteria presented in Table 12.3.

Table 12.6 Development Receptors

Receptor Nature of Effect Importance of Receptor

River Medway Pollution risk Medium - on the basis of it having Moderate WFD

status as discussed in para 12.67-12.6.8

Existing pond

Pollution risk and

Flood risk

Medium – on the basis that the pond has is medium

quality on a local scale

Existing lagoon Flood risk Low – on the basis that runoff stored in the lagoon is

pumped to the public sewer system minimising the risk

of the lagoon flooding

12.7 Assessment of Effects

Assessment of Construction Phase Effects Against Existing Scenario

(Without Mitigation)

Surface Water, Flood Risk and Land Drainage

12.7.1 During the construction phase there would be a number of activities which could

reduce surface water quality as a result of physical contaminants or could have

an effect on flood risk. These include:

• Materials handling, storage, stockpiling, spillage and disposal;

• Earthworks involving relocation of the existing western screen bank on site

and manipulation of ground levels and re-engineering of existing ground;

• Excavation and foundation construction within the Site and site preparation

• Installation of temporary and permanent infrastructure and roads;

• Construction of drainage runs and utility duct runs;

• Formation of public spaces, public realm and associated restoration and

landscaping; and

• Movement and use of static and mobile plant / construction vehicles.


12.7.2 Construction activities could lead to the disturbance and mobilisation of physical

contaminants (i.e. dust, sediments and muds). During periods of heavy rainfall,

vehicle movements resulting in damage to soil structure may generate increased

sedimentation within surface water runoff. Whilst during periods of dry, windy

weather, wind-blown dusts generated by the excavation of soils have the

potential to directly reduce the quality of surface water features.

12.7.3 Contaminants, spilled contaminants and suspended sediments have the

potential to affect surface water bodies via surface runoff. There is also a risk of

pollution from foul water from temporary site worker accommodation and

sanitary facilities.

12.7.4 Potential ponding of surface water and accidental runoff to the surrounding area

may occur whilst the surface water drainage system is being constructed.

12.7.5 Off-site flood risk may increase due to increased run-off due to soil compaction

on the Site.

12.7.6 The identified effects could lead to an increase in run-off to the existing pond.

The run-off may also be polluted.

12.7.7 In respect of a potential increase in inflows, the existing pond is assessed to

have capacity to accommodate an increase in in-flows, but localised flooding

may occur if the spare capacity in the existing pond is exceeded. The magnitude

of the change is assessed to be Moderate adverse and the significance of the

effect is assessed to be Minor adverse.

12.7.8 Regarding pollutant loading, the existing pond retains a permanent body of

water and is likely to have the ability to buffer short-term inputs of pollutant load.

The magnitude of the change is assessed to be Moderate adverse and the

significance of the effect is assessed to be Minor adverse.

12.7.9 As previously stated, the existing pond provides a pollutant pathway to the

aquifer. An assessment of the potential effect of aquifer pollution via the pond is

covered in Chapter 10.0.


12.7.10 The existing lagoon only receives run-off from the drainage system serving the

hardstanding at the Existing Station. As such, it is unlikely that run-off from the

construction of the Proposed Extension would be conveyed to the existing

lagoon. On this basis, no flooding is expected in relation to the receptor and the

magnitude of change and significance of the effect is assessed to be Negligible.

12.7.11 Although the River Medway is located in relatively close proximity to the Site,

there is no known hydraulic connectivity. As such, the magnitude of change and

significance of effect on water quality due to any of the identified effects is

assessed to be Negligible.

Water Supply

12.7.12 The construction process is not expected to have a significant demand for water

and no environmental effects would be expected.

Wastewater Treatment and Sewerage

12.7.13 Domestic foul water generated during the construction process would be stored

in sealed vessels and periodically removed from site. Leakage of domestic

sewage could result in pollution of the aquifer; this is covered in Chapter 10.0.

Summary

12.7.14 The likely effects during the construction of the Proposed Extension are

summarised in Table 12.7.

12.7.15 The assessment of construction phase effects without the implementation of

mitigation measures has identified minor adverse effects on the existing pond

and negligible effects on all other receptors. None of the effects are deemed to

be significant in EIA terms.


Table 12.7 Effect Significance during Construction Phase (Pre-Mitigation)

Receptor Potential

Effect

Importance

of Receptor

Magnitude of

Change

Significance

of Effect

River Medway Pollution risk Medium Negligible Negligible

Existing pond Pollution risk

Flood risk

Medium

Medium

Moderate

Moderate

Minor adverse

Minor adverse

Existing lagoon Flood risk Low Negligible Negligible

Decommissioning Phase Effects (without mitigation)

12.7.16 It is unclear at what point decommissioning and demolition of the Proposed

Extension would occur and what changes there would be in methods and

approach to decommissioning facilities in the intervening period.

Notwithstanding, this could arise, and it is necessary to consider whether this

would have the potential to give rise to likely significant effects.

12.7.17 Due to the uncertainty regarding the timing and approach to decommissioning,

for the purpose of this assessment, it has been assumed that similar techniques

and / or approaches would be used as the construction phase. Thus, the impacts

from decommissioning would be equivalent to those associated with the

Construction Phase.

12.7.18 However, in taking this approach, it is also recognised that the DCO application

is for an extension to an Existing Station and the assessment of construction

phase impacts only relates to the construction activity associated with the

Proposed Extension. When decommissioning occurs, it may apply to the

extended Generating Station. Such operations would be of a slightly different

scale to the effects assessed for the construction phase of the Proposed

Extension. Nevertheless, it is reasonable to assume that similar construction

techniques and mitigation measures would be applied, and that the outcome of

the construction phase assessment remains a reasonable proxy for the

assessment of decommissioning phase effects.

12.7.19 The effects of decommissioning are assumed to be similar to those identified in

Table 12.7, i.e. Negligible for all receptors except for the existing pond where, in

the absence of mitigation, the effects are assessed to as being minor adverse

and not significant in EIA terms.


Construction and Decommissioning of Grid Connection

12.7.20 As stated in Sub-section 12.6 above, two potential grid connection routes are

being considered for the Proposed Extension (Route A and Route B). This

section assesses the potential construction and decommissioning phase

impacts each route would have upon the water environment.

Construction Phase

12.7.21 Route A runs in a generally easterly direction from the Site for approximately

3.3km before heading in a south-westerly direction to the point of connection.

The main consideration in respect of the water environment is that the route will

require a new crossing of the tidal River Medway, approximately 900m east of

the Site.

12.7.22 Whilst the form of crossing is not known, it will by necessity require significant

works to be undertaken near the river in a location that is, according to the EA

Flood Map for Planning, in Flood Zone 3, i.e. in a location at a high risk of river

flooding. As such, the works would be at a significant risk of flooding. In addition

to this, any works near a watercourse have the potential to pollute the

watercourse through accidental spillages and mobilisation of sediments.

12.7.23 Route B would be buried within a trench located within the verge and/or

carriageway of the A20 and town centre roads and the only crossing of a

watercourse would be via an existing bridge (over the River Medway).

Construction activity associated with the installation of this route would have

very minor potential to impact upon the water environment.

12.7.24 Based on the above, it is concluded that the potential effects on the water

environment of Route A would be greater than the effects of Route B.


Decommissioning of Grid Connection

12.7.25 At the end of its operational life, it is anticipated that the ducting for the Grid

Connection would be left in situ, such that there would be no decommissioning

works and therefore no potential significant effects upon the water environment.

Assessment of Construction / Decommissioning Effects - HWRC

Scenario

12.7.1 Should the planning application for the HWRC facility be granted planning

consent, it would be built and operated to meet the obligations of a waste

contract with KCC.

12.7.2 Construction of the HWRC is likely to commence in the Autumn of 2020 and it

is envisaged that the facility would be fully operational by the Summer of 2021.

The construction phase for the Proposed Extension is not expected to

commence until 2022 and as such, the HWRC would be operational during the

construction of the Proposed Extension.

12.7.3 The HWRC would benefit from its own sealed drainage system for both surface

water and foul water flows and, as such, its presence does not have the potential

to alter the conclusions of the construction phase assessment for the Existing

Scenario.

Assessment of Operation Phase Effects against Existing Scenario

(without mitigation)


12.7.4 If not mitigated, the increase in the area of impermeable surfaces associated

with the Proposed Extension would increase the rate and volume of run-off. The

risk of the run-off being contaminated would increase due to spillage of

contaminants and from flushing of pollutants from the impermeable surfaces.


12.7.5 There is no known hydraulic connectivity (pathway) between the Site and the

River Medway. As such, the magnitude of change and significance of effect on

water quality due to any of the identified effects is assessed to be Negligible.

12.7.6 In respect of the existing pond, the increase in runoff generated during storm

events and inflowing into the existing pond would be substantial and could lead

to localised flooding. The magnitude of the change is assessed to be Major and

the significance of the effect is assessed to be Moderate adverse.

12.7.7 The existing pond is likely to have the ability to buffer short-term inputs of

pollutant load. Nevertheless, increased inflow of polluted runoff would have an

effect on the quality of the water retained in the existing pond. The magnitude of

the change is assessed to be Major and the significance of the effect is

assessed to be Moderate Adverse.

12.7.8 In the absence of mitigation, it is feasible that surface water runoff from

impermeable surfaces on the Proposed Extension could be conveyed to the

existing lagoon, via the drainage system serving the Existing Station, potentially

resulting in localised flooding. The magnitude of the change is assessed to be

Major and the significance of the effect is assessed to be Minor Adverse.

Water Supply

12.7.9 The Existing Station benefits from an existing connection to the South-East

Water’s water supply network, with a maximum consented flow of 35.0 cu m/hr.

12.7.10 Water consumption during normal operation is 11.7 cu m/hr, with a peak of 23.4

cu m/hr when a boiler is refilled after a maintenance outage.

12.7.11 The Proposed Extension is expected to have a normal operational water

demand of 7.0 cu m/hr, rising to a peak of 30.0 cu m/hr when a firefighting water

tank is filled.

.

12.7.12 The combined water demand of the Existing Station and the Proposed

Extension during normal operation would be approximately 18.7 cu m/hr. This is

well within the consented supply from South-East Water.


12.7.13 The peak demand for water from the Generating Station would not be expected

exceed the consented supply because the tank refilling would be appropriately

scheduled and would be undertaken outside of normal operations.

Wastewater Treatment and Sewerage

12.7.14 The management of trade effluent at the Existing Station is to be restructured

so that all process water is recycled and re-used. The same approach would

also be used at the Proposed Extension. As such, no trade effluent will be

discharged off-site to the public sewer, reducing the loading on the public sewer

system by up to 50 cu m/day.

12.7.15 A package treatment plant would be installed to treat domestic foul water

generated by staff at the Proposed Extension. Treated effluent from the package

treatment plant would be re-used as process water within the Proposed

Extension, with solids retained in the package treatment plant periodically

removed from the site by tanker.

12.7.16 As such, no domestic foul water from the Proposed Extension would be

discharged to the public sewer system.

Summary

12.7.17 The likely effects of the Proposed Extension during operation, without the benefit

of mitigation measures are summarised in Table 12.8.

Table 12.8 Effect Significance during Operation Phase (Pre-Mitigation)

Receptor Potential

Effect

Importance

of Receptor

Magnitude of

Change

Significance

of Effect

River Medway Pollution risk Medium Negligible Negligible

Existing pond Pollution risk

Flood risk

Medium

Medium

Major

Major

Moderate

adverse

Moderate

adverse

Existing lagoon Flood risk Low Major Minor adverse


Grid Connection

12.7.18 As noted previously, the electrical grid connection would comprise an

underground cable. Accordingly, during the operational phase, it would not have

any potential to give rise to significant effects upon the water environment.

Assessment of Operational Phase Effects - HWRC Scenario

12.7.19 As noted previously within this chapter and within the FRA contained within

Appendix 12-1, the HWRC would have an independent drainage system for the

management of surface and foul water, which would be separate to the drainage

systems serving the Existing Station and the system that would be put in place

to serve the Extended Generating Station.

12.7.20 Whilst there would be no interaction with the existing surface and foul drainage

infrastructure, the HWRC would be constructed on a reduced platform level

upon the eastern bund. The surface water run-off from this area is currently

directed into the French drain that runs around the Site and discharges into the

existing pond.

12.7.21 The development of the HWRC (which covers an area of over 2ha) would have

the effect of reducing the amount of water that is collected in the land drainage

system and directed to the existing pond for attenuation and infiltration.

12.7.22 In light of the above, it is not considered that the HWRC would introduce any

additional effects that would alter the conclusions of the operational phase

assessment for the Proposed Extension, summarised in Table 12.8 above.

Cumulative Effects

12.7.23 As described in Chapter 6.0, the cumulative assessments undertaken within

each chapter of this PEIR has been carried out by considering which scenario

(i.e. development of the Proposed Extension with or without the HWRC) would

give rise to the ‘worst-case’ for the assessment of that environmental topic. Once

that has been established, it has then been assessed on that basis in

conjunction with the cumulative scheme set out below. Where the two scenarios


give rise to different worst-case effects, two cumulative scenarios are assessed

separately.

12.7.24 For the purposes of the assessment of potentially significant effects on the water

environment, it has been judged that the ‘Existing Scenario’ would represent the

worst case. This is on the basis that the introduction of the HWRC would result

in an overall reduction in the amount of water that would need to be managed in

the land drainage systems and result in a consequential reduction in the amount

of water that would need to be the subject of attenuation / infiltration in the

existing pond.

12.7.25 The approach to the cumulative assessment is discussed in Chapter 6.0. The

schemes were identified for inclusion in the cumulative assessment on the basis

that they are within 1 km of the Site. All six identified developments are for

residential use; three benefit from planning permission (not yet implemented),

whilst the planning applications for the other three have not yet been

determined. The schemes are summarised in Table 12.9 below.

Table 12.9 Cumulative Schemes

Project name / location

Details of development Status

Springfield Mill, Sandling Road, Maidstone, Kent, ME14 2LD

Demolition of existing buildings, and development of 295 residential units (use class C3), including 218 x 1-2 bed apartments and 77 x 2-4 bed houses, including associated car parking, public realm and landscaping works, Grade II Listed Rag Room to be preserved and re-used for community (D1), office (B1) or residential (C3) use

17/502432/FULL (8 Jun 2018)

1.7 km from Site boundary

Permitted but not yet implemented

Former Somerfield Distribution Centre, Station Road, Aylesford, Kent, ME20 7QR

Construction of 92 dwellings with associated parking, roads, landscaping and public open space (Revised scheme to that approved under application TM/13/03109/FL)

17/03350/FL (21 Jan 2019)

980 m from Site boundary


Land West Of Hermitage Lane And East Of Units 4A 4B And 4C Mills Road Quarry Wood Industrial Estate Aylesford Kent

Demolition of all existing buildings and structures and redevelopment to provide a new Centenary Village. Redevelopment of the site to provide 24 Assisted Living Apartments, 40 Dwellings, Community Hub, Access Roads, Landscaping and Parking (Phase 2). Outline planning permission for up to 35 Dwellings (all matters reserved) (Phase 3).

17/03513/FL (29 Mar 2018)



600 m from Site boundary

KCC Springfield Library HQ Sandling Road Maidstone ME14 2LD

Demolition of the existing County Central Library and associated buildings, and erection of six-to-sixteen storey residential development of 170no. Apartments and 85no. Car parking spaces at the former KCC Springfield Library site, Sandling Road, Maidstone.

17/504568/FULL

1.8 km from Site boundary

Application not yet determined

Land South Of London Road And East Of Hermitage Lane, Aylesford, Kent

Erection of up to 840 dwellings (including affordable homes) with public open space, landscaping, sustainable drainage systems, land for a Primary School, doctors surgery and for junction improvements at Hermitage Lane/A20 junction, and a link road between Poppy Fields roundabout and Hermitage Lane. Vehicular accesses into the site from Poppy Fields Roundabout and Hermitage Lane. All matters reserved with the exception of means of access.

17/01595/OAEA

80 m from Site


Land South West Of London Road And West Of Castor Park, Allington, Maidstone, Kent

Permission for layout, access and scale for a residential scheme of 106 units comprising mix of 1, 2, 3 and 4 bedroom dwellings (including bungalows, houses and apartments), associated access and infrastructure.

19/00376/OAEA

140m from site


12.7.26 In respect of flood risk and drainage, to comply with planning policy, proposals

for new development must demonstrate that the development will be safe from

flood risk and will not increase off-site flood risk for its lifetime i.e. taking the

future effects of climate change into account. This is accomplished through

appropriate flood risk mitigation measures if / as required and by controlling

surface water run-off such that the peak rates and runoff volume do not exist,

and where possible are less than the pre-development rates and volumes.

12.7.27 The EA and LLFA must confirm at the application stage that these requirements

have been met through the submission of appropriately detailed technical

proposals. Invariably, drainage proposals are also conditioned pending

submission of detailed proposals that comply with the aforementioned

requirements.

12.7.28 In respect of foul water, it must be demonstrated that the development will not

have a negative impact on the quality of a receptor and the management of foul

water from a new development is scrutinised at planning stage by the relevant

sewerage undertaker. Where capacity or treatment issues are identified with the


existing sewerage infrastructure, these are raised by the sewerage undertaker

potentially resulting in constraints to be placed on delivery such as phased

delivery etc.

12.7.29 In respect of water quality more generally, all proposed development is required

to comply with building regulations e.g. in respect of oil interceptors, and relevant

drainage technical guidance (e.g. Sewers for Adoption; The SuDS Manual) to

ensure that the appropriate level of quality treatment is provided to ensure that

the new development would not have a negative impact on the quality of a

receptor.

12.7.30 Similar provision would also be in place during the construction phase where

policy and legislation requires developers to demonstrate (amongst other

matters) nil detriment in terms of water quality and WFD status / potential and

no increase in flood risk to the site or elsewhere. Without demonstrating

compliance with these stringent requirements, planning permission will not be

granted, and construction cannot commence.

12.7.31 It is therefore reasonable to conclude that the three permitted developments

would comply with planning policy, would not increase (and may reduce) flood

risk elsewhere, and would not have a negative impact on receptor waterbodies.

Likewise, it is also the case that planning consent would only be granted for the

three outstanding applications if they also comply with planning policy.

12.7.32 It is concluded that there will be no cumulative environmental effects arising from

the Proposed Extension in combination with the identified developments during

either the construction or operational phases.

12.8 Mitigation

Construction / Decommissioning Mitigation

12.8.1 Potential impacts on the water environment during the construction /

decommissioning phases would be managed by a range of operational, control

and monitoring measures as set out below.


12.8.2 The following standard measures would be adopted as a matter of course to

avoid, minimise and mitigate any adverse effects on the water environment:

• A Construction Environmental Management Plan (CEMP) would be

prepared and agreed with the local planning authority. The CEMP will set

out the methods, including the minimum requirements as agreed between

the Applicant and the local planning authority, by which construction will be

managed to avoid, minimise and mitigate any adverse effects on the water

environment. A draft CEMP has been prepared and submitted with this

PEIR (Appendix 5-4)

• All construction works will be designed in accordance with their latest

relevant guidelines including the ADAS Technical Note on Workmanship

and Materials for Drainage Schemes (1995).

• Contractors undertaking earthworks would develop risk assessments and

method statements covering all aspects of their work that have the potential

to cause physical damage to structures (e.g. water supply and sewerage

infrastructure), mobilise large quantities of soil / sediment or block open

watercourses. Earth moving operations would be undertaken in accordance

with BS 6031:2009 Code of Practice for Earthworks.

• Works affecting soils would follow MAFF’s Good Practice Guide for

Handling Soils (2000) which provides comprehensive advice on soil

handling including stripping, soil stockpiling and reinstatement.

• Works would comply with DEFRA guidance in the Construction Code of

Practice for the Sustainable Use of Soils on Construction Sites (2009) which

provides guidance on the use, management and movement of soil on site.

This action should prevent the mobilisation of sediment and prevent

pollution of watercourses.

• Good practice guidance on erosion and pollution control would be followed,

e.g. CIRIA Environmental Good Practice on Site (C692) and Control of

Water Pollution from Construction Sites (C532).

• The Applicant and their principal contractor would avoid the storage of plant

/ machinery fuel or material (including soil stockpiles) alongside

watercourses unless unavoidable. Construction works should be

programmed as far as is practicable to minimise soil handling and temporary

soil storage.

• The refuelling of plant / machinery, storage of fuels and chemicals and

overnight storage of mobile plant would be within the designated


contractors’ compound areas. The compounds would contain appropriate

facilities for the storage of fuels and chemicals (i.e. bunded and locked

storage containers) and would also be equipped with spill kits.

12.8.3 The adoption of best practice construction methods and construction

management processes would mitigate many of the identified potential

environmental effects during the construction phase. The Applicant may use

alternative procedures compliant with their own Environmental Management

System. However, the broad approach and content would as a minimum be

comparable.

12.8.4 Surface water runoff during the construction phase will be carefully controlled

with temporary infrastructure to allow it to drain to the existing drainge system

serving the Existing Station without increasing flood risk or pollution.

12.8.5 Foul water from temporary staff welfare facilities would be contained within

sealed storage vessels (e.g. septic tank) and disposed of off-site to minimise the

risk of accidental spillage of pollutants.

12.8.6 In terms of the Decommissioning Phase, it is proposed (see Chapter 5.0) that

the need to prepare a decommissioning plan would be the subject of a DCO

Requirement. The plan would set out a series of measures aimed at mitigating

the impact of decommissioning. It is anticipated that the measures would be

similar to the construction phase mitigation measures outlined above.

12.8.7 No further mitigation would be required when accounting for the HWRC in the

baseline scenario.

Operational Mitigation

12.8.8 Potential impacts on the water environment during the operational phase would

be mitigated through implementation of the measures described below.



12.8.9 As set out in the FRA (Appendix 12-1), the risk of flooding from all identified

sources (effectively from surface water and groundwater) would be mitigated by

setting the level of the ground floor of any new buildings at least 0.15 m above

adjacent ground levels. This could be accomplished by raising floor levels in

relation to the existing adjacent ground level or by raising the development

platform relative to the surrounding land.

12.8.10 This measure would, in conjunction with the implementation of an appropriately

designed surface water drainage scheme (outlined below), allow surface water

to be conveyed across the Site without increasing flood risk.

12.8.11 A surface water drainage strategy for the Proposed Extension is presented in

the Drainage Report (Appendix 12-2). The overarching design principle is that

surface water runoff from the Proposed Extension would, insofar as is possible,

be managed in a sustainable manner to mimic the surface water runoff from the

Existing Station. An indicative drainage layout is provided in Appendix 3 of the

Report, Appendix 12-2).

12.8.12 Runoff from the roof of the main building of the Proposed Extension would

discharge to the existing pond via a new dedicated ‘roof water only’ (i.e. clean

surface water) drainage network. In addition, a land drain around the perimeter

of the Proposed Extension would convey intercepted surface water run-off to the

pond, as is currently the case.

12.8.13 Surface water run-off generated from all other impermeable surfaces within the

Proposed Extension would link in to the existing surface water drainage system,

and discharge into either the existing lagoon, or a new attenuation lagoon (the

proposed lagoon) located to the north-west of the main building of the Proposed

Extension and to the east of the existing pond. Flows would be balanced to

optimise the use of the storage of the proposed lagoon. As this runoff may be

polluted (hydrocarbons), the conveyance network would be a trapped gully and

pipe system, with Class 1 petrol/oil interceptors to manage the potential pollution

risk.

12.8.14 The combined storage of the existing and proposed lagoons, and associated

pipe network would provide the necessary capacity to convey and store run-off


generated during the 1 in 100 plus 40% climate change event, with only minor

flooding (less than 5 cu m). If the capacity of the drainage system were to be

exceeded, the excess runoff would be appropriately managed on Site.

12.8.15 Implementation of the proposed drainage strategy would ensure that only

unpolluted run-off would be discharge to the existing pond.

12.8.16 There would be no increase in the maximum rate that surface water is pumped

off-site to the public sewer network. Southern Water has agreed the discharge

rate of 100 l/s.

12.8.17 An assessment of the impact of the Proposed Extension indicates that there

would be a significant reduction in the area contributing to run-off to the existing

pond by virtue of it being conveyed to the existing and proposed lagoons as per

the illustrative drainage strategy presented in the Drainage Report (Appendix

12-2).

12.8.18 The existing pond and the existing and proposed lagoons would also provide

additional water quality benefits via the settlement of pollutants, adsorption by

the soil (existing pond only), and biological activity and by acting as a buffer in

the event of accidental spills by preventing a direct discharge of high

concentrations of pollutants to the receiving surface water sewer.

12.8.19 Additional Sustainable Drainage System (SuDS) components may be

incorporated into the final drainage design to provide further quality treatment.

12.9 Residual Effects

Construction / Decommissioning Phase Effects

12.9.1 The assessment of effects identified that, in the absence of mitigation, there is

potential for minor adverse effects on the existing pond during the construction

and decommissioning phases.

12.9.2 Whilst not significant in EIA terms, the assessed effects would be controlled

through the range of mitigation measures listed in Section 12.8 and the effective

implementation of the measures would ensure that the residual construction and


decommissioning phase effects on the water environment would be Negligible

for all receptors and therefore not significant in EIA terms.

12.9.3 Table 12.10 provides a summary of the residual effects resulting from the

construction of the Proposed Extension, following implementation of the

mitigation measures.

Table 12.10 Residual Effects – Construction Phase

Receptor Potential

Effect

Importance

of Receptor

Significance

of Effect;

Pre-

mitigation

Mitigation Residual

Significance

of Effect

River

Medway

Pollution risk Medium Negligible None

required

N/A

Existing

pond

Pollution risk

Flood risk

Medium

Medium

Minor

adverse

Minor

adverse

Operational

control and

monitoring

measures

including a

CEMP.

Negligible

Negligible

Existing

lagoon

Flood risk Low Negligible N/A

12.9.4 As set out previously, the introduction of the HWRC would not influence the

construction phase effects of the Proposed Extension. Accordingly, the residual

cumulative effects of the Proposed Extension under the HWRC scenario would

be identical to those under the existing scenario, which have been assessed as

negligible and not significant in EIA terms.

12.9.5 In terms of the Grid Connection, it has been concluded that if unmitigated, Route

A has the potential to have significant effects on the water environment and that

in respect of the water environment, Route B is preferable.

Operational Phase Effects

12.9.6 The assessment of effects identified that, in the absence of mitigation, there is

potential for significant adverse effects on the existing pond during the

operational phase.


12.9.7 The assessed effects would be controlled through the mitigation measures listed

in Section 12.8 and effective implementation of the measures would ensure that

the residual operational phase effects on the water environment would be

Negligible for all receptors and therefore not significant in EIA terms.

12.9.8 Table 12.11 provides a summary of the residual effects resulting from the

construction of the Proposed Extension, following implementation of the

mitigation measures.

Table 12.11 Residual Effects – Operational Phase

Receptor Potential Effect

Importance of Receptor

Significance of Effect;

Pre-mitigation

Mitigation Residual Significance

of Effect

River Medway

Pollution risk Medium Negligible None required N/A

Existing pond

Pollution risk Flood risk

Medium Medium

Moderate adverse

Moderate adverse

Surface water drainage scheme

including SuDS, with separate systems for

clean runoff and potentially

contaminated runoff

Negligible

Existing lagoon

Flood risk Low Minor adverse

Negligible

12.9.9 The residual effects associated with the operation of the Proposed Extension on

the water environment would remain unchanged in the HWRC Scenario. Under

this scenario there would be a slight improvement in the extent of land drainage

required. No effects would be significant in EIA terms.

12.9.10 No significant effects are predicted on the water environment during the

operation of the proposed grid connection.

Cumulative Effects

12.9.11 As set out above, all the schemes identified in the cumulative assessment would

be subject to compliance with the requirements of local and national policy on

the protection of the water environment. Under relevant policy and legislation,

the schemes are required to demonstrate (amongst other matters) nil detriment

in terms of water quality and WFD status/potential and no increased flood risk

to the site or elsewhere. Without demonstrating compliance with these stringent

requirements, planning permission for ‘other development’ should not be /

should not have been granted, and those developments commenced.


12.9.12 The ‘cumulative schemes identified’ would all need to be the subject of

mitigation, to achieve the aims of policy and legislation. This would result in

residual effects of the construction and operational phases being classified as

not significant or beneficial. When combined with the negligible / beneficial and

not significant residual effects that have been identified for the construction and

operational phases of the Proposed Extension, it can be concluded that there

would be no significant adverse cumulative effects.

Summary of Residual Effects

12.9.13 Tables 12.12 and 12.13 set out a summary of the residual effects resulting from

the construction/decommissioning and operation of the Extended Generating

Station following implementation of the mitigation measures outlined in the

previous sections.

Table 12.12 Summary of Residual Effects: Construction / Decommissioning

Receptor Effect (Existing Scenario) Effect (HWRC Scenario)

Construction / Decommissioning

River Medway Negligible Negligible

Existing pond Negligible Negligible

Existing lagoon Negligible Negligible

Grid Connection

Construction of Grid Connection (Routes A and B)

Not Significant Not Significant

Cumulative Effects

All Receptors Negligible (not significant) Negligible (not significant)

Table 12.13 Summary of Residual Effects: Operational


Operation

River Medway Negligible Negligible

Existing pond Negligible Negligible

Existing lagoon Negligible Negligible

Grid Connection



Construction of Grid Connection (Routes A and B)

Not Significant Not Significant

Cumulative Effects

All Receptors Not significant Not significant

12.10 Summary

12.10.1 Subject to the implementation of identified mitigation measures outlined within

this chapter, the Proposed Extension would not have significant effects on the

environment from a surface water, flood risk, and drainage perspective during

the construction / decommissioning and operational phases. Those conclusions

would not change when the Proposed Extension is considered in the context of

the HWRC Scenario.

12.10.2 Significant adverse cumulative effects are not anticipated on account of the

construction phase and operational phase mitigation measures being employed

in connection with the Proposed Extension and the other cumulative schemes.

12.10.3 No residual significant environment effects on the water environment have been

identified during either the construction, decommissioning or operation of either

of the proposed Grid Connection routes.