CHAPTER 12.0 SURFACE WATER, FLOOD RISK AND DRAINAGE€¦ · on information presented in the...
Transcript of CHAPTER 12.0 SURFACE WATER, FLOOD RISK AND DRAINAGE€¦ · on information presented in the...
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.
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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
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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
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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
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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
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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.
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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).
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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
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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
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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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-19
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.
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-20
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.
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-21
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)
Surface Water, Flood Risk and Land Drainage
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.
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-22
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.
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-23
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
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-24
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
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-25
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
Permitted but not yet implemented
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)
Permitted but not yet implemented
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-26
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
Application not yet determined
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
Application not yet determined
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
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-27
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.
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-28
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
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-29
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.
Surface Water, Flood Risk and Land Drainage
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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
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-31
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
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-32
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.
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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.
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-34
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
Receptor Effect (Existing Scenario) Effect (HWRC Scenario)
Operation
River Medway Negligible Negligible
Existing pond Negligible Negligible
Existing lagoon Negligible Negligible
Grid Connection
2565-01 / Proposed Extension to the Allington IWMF PEIR Main Report July 2020 12-35
Receptor Effect (Existing Scenario) Effect (HWRC Scenario)
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.