Wylfa Newydd Project A5025 On-line Highway Improvements

Transcription

1 ENERGY WORKING FOR BRITAIN Wylfa Newydd Project Flood Consequences Assessment

2 Wylfa Newydd Project Flood Consequence Assessment Contents 1 Introduction Purpose of this document Background Horizon Nuclear Power Wylfa Limited The Wylfa Newydd Project The A5025 Highway Improvements Modelling and climate change allowance Policy and planning Planning Policy Wales Technical Advice Note Flood Mapping Local planning policy Baseline site context Climate Topography Surface water features Geology and groundwater Services Reservoirs Drainage design Existing drainage Requirements of the design Effects of the Proposed Development Drainage strategy general principles Run-off collection Run-off conveyance Run-off attenuation Run-off discharge Drainage at the Temporary Construction Compound Flood Consequences Assessment Flood Consequence Assessment methodology Sensitivity of receptor Severity of flooding Likelihood of occurrence Section 2 Valley Junction to north of Llanynghenedl TAN 15 mapping Surface water flood mapping Increased run-off due to impermeable surfaces Assessment of flood risk from structures Groundwater Reservoirs Page i

3 Flood Consequence Assessment Wylfa Newydd Project 6.7 Effects of climate change Summary of risks Section 4 North of Llanynghenedl to south of Llanfaethlu TAN 15 Mapping Surface water flood mapping Increased run-off due to impermeable surfaces Assessment of flood risk from structures Groundwater Reservoirs Effects of climate change Summary of risks Section 6 North of Llanfaethlu to north of Llanrhuddlad TAN 15 Mapping Surface water flood mapping Increased run-off due to impermeable surfaces Assessment of flood risk from structures Groundwater Reservoirs Effects of climate change Summary of risks Section 8 North of Cefn Coch to Power Station Access Road Junction TAN 15 Mapping Surface water flood mapping Increased run-off due to impermeable surfaces Assessment of flood risk from structures Groundwater Reservoirs Effects of climate change Summary of risks Conclusions Risks to the Proposed Development Additional Mitigation requirements Glossary References Page ii

4 Wylfa Newydd Project Flood Consequence Assessment List of Tables Table 5-1 Screening of potential flood sources and receptors Sections 2, 4, 6 and 8 of the Proposed Development Table 6-1 Section 2 flood risk summary table Table 7-1 Section 4 flood risk summary table Table 8-1 Section 6 flood risk summary table Table 9-1 Section 8 flood risk summary table List of Figures Figure 1 Location Plan Figure 2 Section 2 Flood Consequence Assessment study area and water features Figure 3 Section 4 Flood Consequence Assessment study area and water features Figure 4 Section 6 Flood Consequence Assessment study area and water features Figure 5 Section 8 Flood Consequence Assessment study area and water features Figure 6 Location of attenuation ponds in Section Figure 7 Section 2 TAN 15 Development Advice Map Figure 8 Section 4 TAN 15 Development Advice Map Figure 9 Section 6 TAN 15 Development Advice Map Figure 10 Section 8 TAN 15 Development Advice Map Figure 11 Section 8 locations of culvert lengthening Page iii

5 Flood Consequence Assessment Wylfa Newydd Project [This page is intentionally blank] Page iv

6 Wylfa Newydd Project Flood Consequence Assessment 1 Introduction 1.1 Purpose of this document Horizon is a UK energy company developing a new generation nuclear power station to help meet the country's need for stable and sustainable low carbon energy. Horizon's ultimate parent company is Hitachi Ltd., a Japanese corporation and the parent company of the multi-national Hitachi group of companies. Horizon is part of the Horizon Nuclear Power Limited group of companies and has premises in Gloucestershire and a site office on the Isle of Anglesey This Flood Consequences Assessment (FCA) has been undertaken for the (the Proposed Development), and will form part of the planning application documentation The objectives of this FCA are to: identify possible mechanisms by which the Proposed Development could flood; identify any aspects of the design, which could exacerbate flooding elsewhere; undertake a formal assessment of the risks posed to the Proposed Development from all identified flood risk sources and mechanisms; confirm that the Proposed Development would not exacerbate flooding elsewhere; consider the level and acceptability of any residual flood risk; and produce an FCA compliant with Planning Policy Wales (PPW) [RD1] and Technical Advice Note 15 (TAN 15) [RD2] In Wales, PPW [RD1] and TAN 15 [RD2] provide the national policy framework for the assessment and management of flood risk for new developments. Taken together, they establish a presumption against development in areas at the highest risk of flooding, setting a framework for the sequential assessment of the suitability of sites for development. They also set out an assessment methodology (the FCA methodology) for the systematic evaluation of flood risk and the need to integrate mitigation and flood resilience into the design of development This FCA has been written in accordance with the planning policy requirements for the Proposed Development, in association with TAN15 [RD2] guidance. 1.2 Background Land adjacent to the Existing Power Station at Wylfa Head, west of Cemaes on the north coast of the Isle of Anglesey, is considered by the UK Government to be suitable for the construction of a new nuclear power station. Horizon Nuclear Power Wylfa Limited (Horizon) proposes to construct and operate a new nuclear power station, known as Wylfa Page 1

7 Flood Consequence Assessment Wylfa Newydd Project Newydd, on this land. This power station would deliver approximately 2,700 megawatts of electricity, enough power for around five million homes. 1.3 Horizon Nuclear Power Wylfa Limited Horizon is a UK energy company developing a new generation nuclear power station to help meet the country's need for stable and sustainable low carbon energy. Horizon's ultimate parent company is Hitachi Ltd., a Japanese corporation and the parent company of the multi-national Hitachi group of companies. Horizon is part of the Horizon Nuclear Power Limited group of companies and has premises in Gloucestershire and a site office on the Isle of Anglesey. 1.4 The Wylfa Newydd Project Horizon is applying to the Secretary of State for a Development Consent Order (DCO) under the Planning Act 2008 [RD3], to construct, operate and maintain a new nuclear power station on land west of Cemaes on the Isle of Anglesey The Wylfa Newydd Project (the Project) comprises the Wylfa Newydd DCO Project and the Enabling Works The Wylfa Newydd DCO Project is defined as those parts of the Project which are to be consented by the DCO, comprising: the Power Station; other on-site development; the Marine Works; the Off-Site Power Station Facilities; and the Associated Development: Power Station: the proposed new nuclear power station, including two UK Advanced Boiling Water Reactors to be supplied by Hitachi-GE Nuclear Energy Ltd., supporting facilities, buildings, plant and structures, and radioactive waste and spent fuel storage buildings; Other on-site development: including landscape works and planting, drainage, surface water management systems, public access works including temporary and permanent closures and diversions of public rights of way, new Power Station Access Road and internal site roads, car parking, construction compounds and temporary parking areas, laydown areas, working areas and temporary works and structures, temporary construction viewing area, diversion of utilities, perimeter and construction fencing; Marine Works: comprising a Cooling Water System intake and outfall, Marine Off-Loading Facility and breakwater structures; Off-Site Power Station Facilities: comprising the Alternative Emergency Control Centre (AECC), Environmental Survey Laboratory (ESL) and a Mobile Emergency Equipment Garage (MEEG); and Associated Development: comprising works included in the DCO which facilitate the delivery of the Nationally Significant Infrastructure Project, and which principally include: a Site Campus providing accommodation for construction workers; a temporary Park and Ride Page 2

8 Wylfa Newydd Project Flood Consequence Assessment facility at Dalar Hir for construction workers; a temporary Logistics Centre at Parc Cybi; A5025 Off-line Highway Improvements; and an electrical connection to the National Grid substation Horizon will prepare planning applications under the Town and Country Planning Act 1990 (as amended) (the TCPA ) [RD4] for the Enabling Works, to be submitted to the Isle of Anglesey County Council (IACC) as the determining local planning authority The Enabling Works comprise the TCPA works and the Site Preparation and Clearance (SPC) TCPA works The following terms are used in this document when describing the geographical areas related to the Project: Power Station Site - the indicative areas of land and sea within which the majority of the permanent Power Station buildings, plant and structures would be situated. It would include the two nuclear reactors, steam turbines, the Cooling Water System intake and pumphouse, outfall structures, breakwaters and the Marine Off-Loading Facility, as well as other ancillary structures. Wylfa Newydd Development Area - the indicative areas of land and sea including the areas surrounding the Power Station Site that would be used for the construction and operation of the Power Station. This area is representative of the maximum area that would be physically affected by Power Station Main Construction activities and used to form the setting and features of the operational Power Station The proposed Off-Site Power Station Facilities would be located at one site located at Llanfaethlu, south of the Power Station Site, adjacent to the A The proposed Logistics Centre site would be located to the south-east of Holyhead at Parc Cybi, close to Junction 2 of the A55. The proposed Park and Ride facility would be located at Dalar Hir to the north of Junction 4 of the A55. The A5025 Off-line Highway Improvements will take place at a number of locations along the A The A5025 Highway Improvements Construction of the Power Station would require very substantial transport needs for materials, large components and staff. Studies undertaken by Horizon in identified that the stretch of the A5025 between Valley and the proposed Power Station Access Road Junction has physical and operational constraints in relation to its width, alignment, overtaking opportunities and surfacing condition As a result of these studies Horizon has committed to improve this section of the A5025 in order to mitigate the predicted impacts of increased traffic associated with construction activities that would be undertaken at the Wylfa Newydd Development Area, and from operational Power Station traffic, that would travel along this part of the highway network. Page 3

9 Flood Consequence Assessment Wylfa Newydd Project Horizon therefore intends to deliver a series of on-line and off-line improvements (collectively termed the A5025 Highway Improvements) between Valley and the proposed Power Station Access Road Junction as part of its wider transport strategy for the Project, the objectives being to: upgrade the route, both in terms of standard of construction and road geometry, such that it can support increased levels of traffic and improve safety and accessibility; ensure that all relevant abnormal loads can pass along the full length of the A5025; reduce any potential increase in road accident risk; reduce any adverse impacts on local communities; reduce any adverse impacts on the environment; and seek opportunities where possible to achieve improvements for local communities and the environment through road design measures The 16.19km stretch of the A5025 identified for on-line improvement has been divided into eight sections (see figure 1). Section 1 A5 east of Valley junction to north of Valley Junction (A5/A5025) a length of 1.06km; Section 2 north of Valley Junction (A5/A5025) to north of Llanynghenedl a length of 2.46km; Section 3 north of Llanynghenedl to north of Llanfachraeth a length of 2.28km; Section 4 north of Llanfachraeth to south of Llanfaethlu a length of 2.7km; Section 5 south of Llanfaethlu to north of Llanfaethlu a length of 1.43km; Section 6 north of Llanfaethlu to north of Llanrhuddlad a length of 3.36km; Section 7 north of Llanrhuddlad to north of Cefn Coch a length of 1.3km; and Section 8 north of Cefn Coch to the proposed Power Station Access Road Junction a length of 1.6km The Proposed Development involves the implementation of on-line improvements largely within the existing highway boundary of the A5025. In summary, these comprise: improvement of the existing pavement through the application of a surface dressing through Sections 1, 3, 5 and 7; reconstruction and localised widening of the existing pavement through Sections 2, 4, 6 and 8; modifications and improvements to existing signage and road markings through Sections 1 8; and Page 4

10 Wylfa Newydd Project Flood Consequence Assessment construction of a Temporary Construction Compound (incorporating a temporary pavement recycling facility) within Section 7, immediately adjacent to the A The A5025 Off-line Highway Improvements would be implemented around the time of completion of the Proposed Development (subject to the grant of consent) through Sections 1, 3, 5 and 7. In summary, these would involve the construction of bypasses to reduce the effects of future traffic on existing communities, the formation of new junction arrangements (including the provision of the proposed Power Station Access Road Junction), and localised improvements to existing bends The A5025 Off-line Highway Improvements form an integral component of the Wylfa Newydd DCO Project application, and do not form part of the planning application for the Proposed Development. 1.6 Modelling and climate change allowance This FCA relates solely to the Proposed Development and is specific to the works associated with pavement reconstruction and widening within Sections 2, 4, 6 and 8 (see figure 1). The application of surface dressing to the existing pavement within Sections 1, 3, 5 and 7 would not alter the current flood risk due to the nature of the works, which would remain within the existing highway boundary; therefore, these sections of the A5025 have been scoped out of the FCA Guidance in TAN 15 indicates that a FCA should be commensurate with the nature and scale of the proposed development. The Proposed Development would not alter the existing route of the A5025, and there would only be a very small increase in the area of hardstanding, which will be dealt with through appropriate drainage provision. Detailed survey and complex hydraulic modelling was therefore considered unnecessary to identify and assess either the risk of flooding to or the impacts on flood risk from the. Reliance has instead been placed on flood risk maps published by Natural Resources Wales (NRW) and the Welsh Government [RD6] along with a quantitative and qualitative assessment of the impacts on run-off and the benefits of drainage provision TAN 15 [RD2] states that consideration must be given to the impacts climate change may have on the risk of flooding over the lifetime of a development; to ensure that development does not take place where flooding would be unacceptable either now or in the future The Welsh Government has provided guidance (CL-03-16) [RD5] on how the UK climate changes projections (UKCP09) [RD19] should be used to determine the future flood consequences for developments in Wales and must be incorporated in all FCAs produced after December The guidance outlines a potential increase in main river flows by up to 75% (Upper end estimate) by the 2080s, however, it is recommended in the central estimate of 30% by the 2080s be used for FCAs. Welsh Government guidance on climate change allowances for planning purposes [RD5] has been used where appropriate in assessments of how climate change may affect flood risk. Page 5

11 Flood Consequence Assessment Wylfa Newydd Project 2 Policy and planning PPW [RD1] provides the national policy framework for the assessment and management of flood risk for developments in Wales, and references a range of European and national legislation that relates to flood risk. This is supplemented by TAN15 [RD2] and local planning policy. 2.1 Planning Policy Wales The objective of PPW [RD1] is to avoid the construction of development within areas defined as being at risk of flooding, with planning authorities adopting a precautionary approach when formulating development plan policies, including the principle that it is likely that climate change will increase the risk of coastal and river flooding. A strategic approach to flood risk that considers the catchment as a whole is encouraged PPW [RD1] states that developments should not be at risk of flooding itself and should not increase the risk of flooding elsewhere. Additionally, it indicates that hard-engineered flood defences are likely to be unsustainable in the long-term; and new development should be avoided in flood hazard zones Only essential transport and utilities infrastructure are considered acceptable within unobstructed floodplains, and then only when such infrastructure is designed to remain operational during times of flooding and with no net loss of floodplain storage or increase in flooding elsewhere. 2.2 Technical Advice Note TAN15 [RD2] provides technical guidance that supplements the policy set out in PPW [RD1] in relation to development and flooding. It advises on development and flood risk relating to sustainability principles and provides a framework within which risks arising from both river and coastal flooding, and from additional run-off from development in any location, can be assessed. This incorporates climate change scenarios TAN15 [RD2] provides guidance on flood consequences that may not be acceptable for particular types of development. The location of the development needs to be justified, in line with TAN15 [RD2] and flood risk areas, and the consequence needs to be acceptable given the vulnerability and use of the receptor The guidance defines a threshold for the frequency of flooding, below which development should not be allowed. This threshold for General Infrastructure is equivalent to the 1% Annual Exceedance Probability (AEP) event, or an event with a 1 in 100 chance of occurring in any given year, for fluvial flooding. For tidal flooding this is equivalent to the 0.5% AEP event, or an event with a 1 in 200 chance of occurring in any given year. Additionally, the depth of flooding, regardless of development type, should not be greater than 0.6m for any return period. These thresholds automatically apply to all developments in Flood Zone A and B and to those in Flood Zone C once the justification test has been passed. Page 6

12 Wylfa Newydd Project Flood Consequence Assessment It is also a requirement of TAN15 [RD2] that future users and occupiers of the site are adequately aware of the flood risk, the consequences of flooding, that effective flood warnings are provided, emergency flood plans are available and that safe access and egress is available. There is also a requirement that the site is designed to facilitate movement of goods/possessions away from flooding, to minimise structural damage and to facilitate recovery TAN15 [RD2] also states that development should not increase flooding offsite; however, it acknowledges that there may be practical difficulties in achieving this aim. 2.3 Flood Mapping There are two initial reference maps for assessing the level of fluvial and tidal flood risk associated with land: the TAN15 [RD2] and the NRW flood map of fluvial flood risk [RD5]. These are broadly similar, although the NRW flood map provides additional detail in relation to flood probability. TAN15 [RD2], which shows the fluvial and coastal flood zones as issued by the Welsh Government, is discussed below as the classifications from this directly relate to planning policy. However, where relevant, information from the NRW maps [RD6] is referred to The TAN15 [RD2] flood zones are defined as follows: Zone A: considered to be at little or no risk of fluvial or coastal/tidal flooding; Zone B: an area known to have been flooded in the past; Zone C1: an area with an annual probability of flooding from tidal and fluvial sources greater than 0.1%, but which has significant infrastructure including flood defences; and Zone C2: an area with an annual probability of flooding from tidal and fluvial sources greater than 0.1% and without significant flood defence infrastructure. 2.4 Local planning policy The adopted Anglesey Local Development Plans are the Gwynedd Structure Plan [RD7] and the Ynys Môn Local Plan [RD8], both of which pre-date TAN15 [RD2]. Policies within these Plans relate to the need to take account of the increased danger of tidal and river flooding. The IACC resolved to stop work on the Stopped Unitary Development Plan [RD9] in December 2005; however, this unadopted plan has weight when dealing with current planning applications. Relevant flood risk policies [RD9] are as follows. Environmental Policy EN9 development should be limited to essential transport and utilities infrastructure in the natural floodplain. Infrastructure Policy SG2 development (including the raising of land) will only be permitted where it would not result in risk to human life and Page 7

13 Flood Consequence Assessment Wylfa Newydd Project damage to property within areas of indicative flood risk and/or where it would result in flooding, including tidal inundation, either on or off-site. Infrastructure Policy SG6 proposals for development which would result in an unacceptable adverse impact on the water environment due to additional surface water run-off will not be permitted Additionally, the IACC has released supplementary planning guidance specifically for the Project [RD10]; however, this does not contain any information relating to flood risk The Anglesey and Gwynedd Joint Local Development Plan, Topic Paper 8: Strategic Flood Consequence Assessment (Level 1) [RD11] forms a key part of the evidence base for planning. The document helps to determine appropriate development policies and land allocations that avoid or minimise flood risk from all sources, and helps to assess any future development proposals in line with the precautionary framework in PPW [RD1] and TAN15 [RD2]. This document [RD11] and the IACC s Preliminary Flood Risk Assessment [RD12] include information on surface water, groundwater, ordinary watercourses and small reservoir flooding. Information on the IACC flood strategy and the council s objectives in managing flood risk are provided in the Anglesey Local Flood Risk Management Strategy [RD13]. Page 8

14 Wylfa Newydd Project Flood Consequence Assessment 3 Baseline site context 3.1 Climate Twenty years of rainfall data is available from the NRW-operated Llyn Alaw rain gauge, which lies to the east of the Proposed Development. RAF Valley is located approximately 3km to the southeast of the A5025/A5 junction. The RAF Valley rain gauge comprises more than 50 years of digitised hourly data (1960 to 2014). Before this date, data is only available in analogue form. The Met Office average annual rainfall data for the RAF Valley gauge for the period 1981 to 2010 (the most recent climate period) shows an average annual rainfall at RAF Valley of 841mm/yr. This is below the UK average of 1154mm/yr. The long-term data indicates that rainfall is typically higher in the late autumn/early winter and lowest in late spring/early summer. 3.2 Topography The study area (the extent of which is defined in chapter 9 of the A5025 Online Highway Improvements Environmental Report) largely comprises undulating pastoral land. There are several small villages and isolated farmsteads along the existing A5025. Within Section 2, the highway rises gently from around 5m above Ordnance Datum (AOD) at the southern end to 11m AOD at the northern end of Section 2 at Llanynghenedl. Through Section 4, the road rises again from approximately 20m AOD to over 50m AOD just south of Llanfaethlu. North of Llanfaethlu, the landscape becomes steeper and rises to over 80m AOD at Llanrhuddlad and then drops more steeply to 60m AOD within Section 6. Section 8 starts at 30m AOD at Cefn Coch, dips slightly at Caerdegog bend and then rises up to 40m AOD at the northern end of the Proposed Development. 3.3 Surface water features The study area includes four Water Framework Directive (Council Directive (EU) 2000/60/EC) (WFD) [RD14] water bodies (Afon Cleifiog, Afon Alaw, Tan R Allt and Afon Cafnan). The western extent of the Afon Alaw is a transitional water feature. There are also several non-wfd watercourses, drainage ditches and a lake (Llyn Llygeirian). The smaller watercourses and drains are typically tributaries to the WFD water bodies and as a result form part of the WFD catchments. The main water features, as illustrated in figures 2 to 5, are described below (from south to north). Afon Cleifiog: located to the south east of the A5025 (Section 2), which has two tributaries: Cleifiog Fawr, which originates to the south of Llanynghenedl and Cleifiog Isaf, which rises south of Bodedern. Both flow in a south-westerly direction parallel to the A5025 towards Valley. The two streams join and both the A5 and the A55 cross the resulting river. Afon Alaw (fluvial): a watercourse that is fed by a series of tributaries (including the Tan R Allt) that mostly rise to the east of the A5025, with Page 9

15 Flood Consequence Assessment Wylfa Newydd Project the main watercourse flowing from Llyn Alaw approximately 6km to the north-east of Llanfachraeth. Afon Alaw (transitional): a transitional water body that continues from the Afon Alaw and flows into Holyhead Bay at Traeth y Gribin. Tan R Allt: a watercourse crossed by the A5025 north of Llanfachraeth (Section 4). The Tan R Allt flows in a southerly direction parallel to the A5025 and is joined by a number of minor tributaries, in the vicinity of Sections 4 and 6, and discharges to the Alaw estuary downstream of the A5025. This includes a relatively significant channel that flows in a southerly direction parallel to, and to the east of, the A5025 for around 6km between the village of Llanrhuddlad and its confluence with the Tan R Allt. Afon Cafnan: a watercourse which is crossed by the A5025 in the vicinity of Cefn Coch (Section 7) and into which several smaller watercourses flow in the vicinity of Sections 7 and 8. Small watercourses and ditches: a significant number of small watercourses and ditches across the area in the vicinity of the A5025, which would act as preferential flow pathways to the rivers listed above. Llyn Llygeirian: a water body located to the east of the A5025 with Section 7 designated as a Site of Special Scientific Interest (SSSI) due to its ecological interest The most significant of these watercourses in terms of upstream catchment areas are the Afon Alaw (57km 2 ) and Tan R Allt (20km 2 ). 3.4 Geology and groundwater The majority of the Proposed Development is underlain by Glacial Till, which is classed as an unproductive stratum. The deposits generally have low permeability, with negligible significance for water supply or river base flow, but are important in terms of pluvial and fluvial flood risk response Beneath, and adjacent to, watercourses across the study area, alluvium is present in the form of a mixture of clay, silt, sand and gravel, and this is classed as a Secondary A Aquifer. Alluvium includes permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. The areas of superficial Secondary A Aquifer are generally classified as being of high vulnerability The underlying bedrock largely comprises the New Harbour Group and Gwna Group which are both Secondary B Aquifers with similar hydrogeological characteristics, in which fracture flow dominates. These strata generally store and yield limited amounts of groundwater. 3.5 Services There are a number of services that run alongside, or under, the A5025, including water mains and sewers, both of which have the potential to affect Page 10

16 Wylfa Newydd Project Flood Consequence Assessment flood risk. However, as the Proposed Development would not affect these services and, as the services are already present such that any risk from them would not change due to the Proposed Development, they have been scoped out of this report. 3.6 Reservoirs Llyn Alaw is located to the east of the A5025 on the Afon Alaw. It is a manmade reservoir situated to the north west of the village of Llannerchymedd and is the largest waterbody on Anglesey. The dam is across the Afon Alaw at Bodnolwyn Wen. The reservoir is 4.3km long with a surface area of 3.6km2. The A5025 crosses the Afon Alaw north of Valley. Page 11

17 Flood Consequence Assessment Wylfa Newydd Project 4 Drainage design The initial drainage strategy for the Proposed Development was devised by Mott MacDonald (see annex A). Since the initial drainage strategy, AECOM has further developed the drainage design in consultation with the IACC and NRW. AECOM s addendum to the initial drainage strategy (see annex B) and the key elements of relevance to the FCA are summarised below. 4.1 Existing drainage Formal drainage assets are limited along the A5025. The current system relies on run-off from the road discharging over-the-edge into adjoining land, ditches or watercourses and, in some cases, into gullies installed at low points. In some cases the gullies are directly connected to ditches and watercourses. However, the connectivity of some of the gullies is unclear and it is possible that some gullies may be connected to soakaways There are no reports of significant carriageway flooding in the past. Where localised flooding has repeatedly occurred, reactive measures have been put in place by the IACC to reduce the risk of localised flooding, mainly through the installation of additional gullies at low points where road run-off tends to accumulate. 4.2 Requirements of the design The IACC would adopt the proposed drainage assets on completion of the Proposed Development. Consultation with the IACC, as part of the designdevelopment process between 2014 and 2017 (see chapters 2 and 3 of volume 1 and appendix 3.2 of volume 3 of the A5025 On-line Highway Improvements Environmental Report respectively) has ensured that its requirements have been incorporated into the final design of the Proposed Development, summarised as follows. The proposed drainage and maintenance regime should be similar to the existing system and the drainage schemes should follow the Sustainable Drainage Systems approach (SuDS). Minimise the use of piped drainage systems. This implies that run-off will not be concentrated at one discharge point, but will instead be distributed, attenuated and slowed down before being discharged. Minimise asset maintenance requirements. Reduce the risk of flooding to the road and third parties focusing, in particular, on areas with flooding history or known drainage problems. Reduce the impact of the road improvement works on flooding and the water environment These requirements have been addressed in the design of the Proposed Development with, for example, attenuation ponds incorporated into Section 2, and formal drainage being installed along the roadside where there is currently none in order to manage the risk of flooding. Filter drains have been incorporated into the design to meet SuDS requirements. Page 12

18 Wylfa Newydd Project Flood Consequence Assessment 4.3 Effects of the Proposed Development The increased areas of hardstanding associated with pavement widening would provide increased volumes of run-off and a change in flow paths associated with changes in highway contouring. Although the increase in hard surfaces is small compared to the size of the catchment of any receiving watercourses, the increased run-off may have localised effects. The highway drainage is designed to mitigate these effects locally. 4.4 Drainage strategy general principles The current design criteria have been developed from the initial Mott MacDonald drainage strategy (see annex A), taking into account the requirements of the IACC and NRW. A major change has been the inclusion of attenuation for Section 2 of the Proposed Development, in order to protect sensitive receptors at Valley (see figure 6) The key objectives of the drainage strategy for the Proposed Development are: the speedy removal of surface water for safety and minimum nuisance; provision of effective sub-surface drainage to maximise longevity of the highway surface and its associated earthworks; and reduction of the impact of run-off on the receiving environment. Run-off collection Run-off collection methods applied in the design include: over-the-edge drainage where an embankment exists, or where water can flow beyond the verge; gullies; over-the-length filter drains, with or without a perforated pipe, on the verge. Setting the verge below the carriageway would allow more runoff from the road to flow onto the verge and into the filter material; use of block drains that include short sections of surface water channel; and kerb drains, which are only considered in exceptional cases, where kerbs currently exist or where their application is considered to be absolutely necessary. Run-off conveyance Run-off conveyance methods applied in the design include: filter drain with a perforated pipe; ditches on the side of the road; and Page 13

19 Flood Consequence Assessment Wylfa Newydd Project kerb drains, which are only considered in exceptional cases, where kerbs currently exist or where their application is considered to be absolutely necessary. Run-off attenuation Methods considered to attenuate run-off from widening of the road comprise the following: attenuation ponds, where the risk of flooding in the receiving watercourse is high; storage within the filter material beneath the verge (filter drains, with or without a perforated pipe); and increasing the capacity of existing or new ditches on the side of the road. Run-off discharge Run-off discharge methods considered include (in order of preference): discharging to soakaway (where feasible); discharging to a watercourse; discharging to a gully at the longitudinal low point (provided that the hydraulic connectivity of the gully is clearly understood and the capacity is sufficient); and discharging to a shallow pond, where no other option exists The methods described above have been applied in the design, giving due consideration to the existing conditions and constraints. The following have been considered in implementing the strategy: Where possible, run-off collected at the longitudinal low point of the road would be discharged in the order of preference described above. The design would seek to provide attenuation to cater for the increase in road area. The attenuation volume provided would depend on the specific site constraints. The target return period considered would be between 1 in 1 and 1 in 5 years. For sections of the road where the verge width is narrow, no drainage would be provided. All existing gullies would be used or replaced. If the location of a low point in the road changes, the gullies would be moved to the new low point. All connections to the existing gully would thereby be retained and new gully tails may be required if gullies are relocated a significant distance away. Filter drains with a perforated pipe would be used where the verge is sufficiently wide and it is possible to discharge to a watercourse, soakaway or gully at the longitudinal low point of the highway. Page 14

20 Wylfa Newydd Project Flood Consequence Assessment Alternatively, a filter drain without a pipe would be used to collect and store run-off in that section of the highway. A block drain would be installed where more run-off is expected as a result of the change in highway contours. An appropriate run-off collection method would be implemented at super-elevation points on the new highway. Where existing culverts are affected by pavement widening, they would be extended and, if necessary, replaced. 4.5 Drainage at the Temporary Construction Compound Topsoil at the Temporary Construction Compound would be stripped and a layer of crushed aggregate placed such that drainage at the site would be broadly similar to the current greenfield area. The crushed aggregate would be spread evenly and compacted to establish a porous hardstanding that would permit drainage to ground. No formal drainage system would be required, except to prevent overland flow of any sediment contaminated runoff into adjacent watercourses during intense periods of rainfall. Page 15

21 Flood Consequence Assessment Wylfa Newydd Project 5 Flood Consequences Assessment 5.1 Flood Consequence Assessment methodology Industry guidance [RD15] recommends that a FCA should consider all possible sources of flooding for a given site. This is also reflected in Appendix 1 of TAN 15 [RD2] guidance on flood risk. A number of specific sources exist but many of these can easily be discounted. Table 5-1 has been used to identify these sources and remove them from the assessment at an early stage. This initial screening stage references the TAN 15 [RD2] development advice maps and the NRW surface water flood map [RD6]. Flood type Page 16 Screening of potential flood sources and receptors Sections 2, 4, 6 and 8 of the Proposed Development Source Pathway Tidal Irish Sea The A5025 is located sufficiently far from the coast to avoid flooding by the sea and does not fall within the Flood Zone of tidal risk. Fluvial and pluvial Groundwater Drainage Reservoir flooding Pluvial / fluvial flooding Road improvements Increased culvert lengths required due to road widening New culverts Temporary Construction Compound Risk of flooding associated with river crossings, neighbouring watercourses and surface water flows. Increased run-off due to the introduction of impermeable surfaces. Raising of upstream water levels. Changes in flow, increasing flood risk upstream or downstream. Potential for increased run-off to adjacent watercourses is limited as the crushed aggregate surface for the Temporary Construction Compound would allow infiltration so that run-off does not increase. Consider further? No Yes Yes Yes Yes Groundwater Flooding from groundwater. Yes Road drainage system Failure of reservoir Surcharge or failure of drainage systems associated with the Proposed Development. Flooding from reservoirs has been recognised as a hazard in Section The risk assessment methodology used within this report is set out in annex C and is based on PPW [RD1] and TAN 15 [RD2]. The guidance recommends that flood risk be assessed through consideration of both the significance of potential effects and the likelihood of occurrence. The significance of effect is dependent on two factors: the sensitivity of potential No Yes Yes

22 Wylfa Newydd Project Flood Consequence Assessment receptors and the severity of the flooding. Thus, the three criteria on which flood risk is assessed are: sensitivity of the receptors; severity of flooding (i.e. the magnitude of the hazard); and likelihood (i.e. probability) of occurrence. Sensitivity of receptor The Proposed Development is discussed in sections 6 to 9 of this report with respect to flood risk for each assessed road section. Receptors are identified in each of those sections and their sensitivity to flooding specified, however, in general receptors have been divided into two groups; on-site receptors in the form of the Proposed Development itself, and off-site receptors, such as all adjacent built developments and agricultural land. Under TAN 15 [RD2] guidance and the methodology presented in annex C, all off-site built development receptors are consider to have a very high sensitivity to flooding. Undeveloped, agricultural land is considered to have a medium sensitivity to flooding. Severity of flooding Appendix 1 of TAN 15 [RD2] identifies acceptable thresholds of flooding for different types of development and also presents indicative consequences of flooding that may be acceptable subject to adequate warnings and preparation. This guidance has been used to define the magnitude of flooding that fall within the categories negligible to very high hazard. Further information on the typical criteria against which the category is defined is presented in annex C. Likelihood of occurrence The probability of occurrence is fully defined within annex C, and the classification of this criteria is discussed for the different flood sources in sections 6 to 9 below. Page 17

23 Flood Consequence Assessment Wylfa Newydd Project 6 Section 2 Valley Junction to north of Llanynghenedl This section discusses the flood risk to Section 2 of the A5025 and any increase in flood risk due to the Proposed Development. Reference is made to the TAN 15 development advice maps [RD2] and the NRW surface water flood maps [RD6], and uses the methodology detailed in annex C. 6.1 TAN 15 mapping Much of the route of Section 2 of the Proposed Development falls within Flood Zone A, classed as having little or no flood risk. However, at the southerly end of the section, the road crosses the floodplain of the Afon Valley, Glan Morfa and Afon Cleifiog (see figures 7 to 10). The Glan Morfa watercourse is culverted beneath the A5025 at National Grid Reference (NGR) At this location the road passes through Flood Zone C2, within the NRW 1 s extreme flood outline and indicating an area with a 0.1% AEP of flooding or greater that is not served by significant flood defence infrastructure In addition, smaller watercourses have been identified as being crossed by the A5025 between Tyddyn-y-gof and Bronallt (NGR ) and north of Penterfyn (NGR ). These watercourses have not been included in the TAN 15 flood mapping [RD2] as their catchments are less than 3km 2. The watercourses may constitute a risk of flooding to the highway The existing A5025 and the Proposed Development are considered to have a very high sensitivity to flood risk. Where the Proposed Development lies in Flood Zone C2 the magnitude of the hazard is assessed as medium, by virtue of the duration and potential depth of flooding, and the significance of effect is therefore considered to be high. The likelihood of flooding in these areas is medium, therefore, the overall risk from this source is considered high Since the improvements to the A5025 at all locations within Section 2 would be at the same level or slightly higher than the existing highway, the risk of flooding from fluvial sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from fluvial sources elsewhere from the Proposed Development would also remain unchanged. 1 Although the Development Advice Map is a Welsh Government publication it refers to Environment Agency mapping Page 18

24 Wylfa Newydd Project Flood Consequence Assessment Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 6.2 Surface water flood mapping NRW s surface water flood map [RD6] indicates areas that may be at risk of surface water flooding. These are primarily associated with the watercourses that the highway crosses, at Ty n-y-coed and Bronalt, and there is additional surface water flooding indicated between Tyddyn-y-gof and Bronallt where ponding on the edges of the road is suggested both to the north and south The risk of flooding in these areas is classed, in places, as high risk on NRW flood mapping [RD6] and is expected to happen on average every 30 years, which is equivalent to an annual exceedance probability (AEP) of 3.3%. However, the Proposed Development would not increase the existing level of flood risk since the road level and alignment will not be altered. Therefore, the risk remains high The existing A5025 and the Proposed Development is considered to have a very high sensitivity to flood risk. Where the Proposed Development is shown to be affected by surface water flooding the magnitude of the hazard is assessed as medium, by virtue of the duration and potential depth of flooding, and the significance of effect is therefore considered to be high. The likelihood of flooding in these areas is high, as flooding is expected to happen on average every 30 years, which is equivalent to an annual exceedance probability (AEP) of 3.3%, therefore, the overall risk from this source is considered high Since the improvements to the A5025 at all locations within Section 2 would be at the same level or slightly higher than the existing highway, the risk of flooding from surface water sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from surface water sources elsewhere from the Proposed Development would also remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 6.3 Increased run-off due to impermeable surfaces The localised widening of the A5025 would lead to a very slight (approximately 0.3ha) increase in the area of impermeable surfaces. These surfaces would increase the run-off produced, as rainfall would be unable to percolate into the ground. The drainage system described in section 4 of this report would be installed based, where appropriate, on the existing infrastructure. Page 19

25 Flood Consequence Assessment Wylfa Newydd Project NRW has advised that attenuation should be provided for Section 2 of the Proposed Development due to concerns that the drainage, if un-attenuated, could increase the existing flood risk in Valley (Section 1). Three locations have been identified at the two main low points in Section 2 where attenuation ponds would be provided to cater for run-off from the widened section of the road. These attenuation ponds will provide sufficient storage to cater for the 1 in 100 year storm event plus a 20% allowance for climate change. This is to accommodate an increased amount of run-off as a result of the increase in hardstanding at the widened section. The design of the attenuation ponds restricts flow to the median annual greenfield run-off rate, equivalent to approximately the 1 in 2 year storm event Incorporation of the drainage system will ensure that increased impermeable surfaces would not increase surface water flood risk elsewhere and would not contribute to an increased risk from other sources. No additional mitigation is proposed. 6.4 Assessment of flood risk from structures The A5025 crosses three watercourses via existing culverts within Section 2. These are on the Glan Morfa at NGR and on two unnamed watercourses at NGR and NGR The existing flood extents in the vicinity of and upstream of these structures, as indicated on NRW s fluvial and surface water flood maps [RD6], is restricted to agricultural land of medium sensitivity to flooding. Although information on the depth of flooding is unavailable, it is likely that the magnitude of hazard is between medium and high and therefore the significance of effect is assessed as moderate. Given the high likelihood of occurrence, the existing risk associated with these structures is considered to be high Changes to this section of the highway resulting from the Proposed Development would not affect the existing culverts through lengthening or changes in flow capacity. Furthermore, surface water run-off from the Proposed Development would be channelled through proposed carrier/filter drains that would outfall into watercourses at locations downstream of the existing culverts. Consequently, the Proposed Development is not expected to have any effect on the flood risk upstream or downstream associated of these structures and the risk of flooding is expected to remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 6.5 Groundwater Increased impermeable surfaces and ground compaction during the construction period may reduce rainwater reaching the groundwater table, potentially altering local recharge rates, groundwater levels and groundwater flow directions and reducing the risk of flooding from groundwater. However, since the majority of the Proposed Development within Section 2 is underlain by low permeability Glacial Till, which limits natural recharge, it is considered that there would be little difference in the movement of water between surface and groundwater, associated with the Proposed Development. Page 20

26 Wylfa Newydd Project Flood Consequence Assessment It is considered that the existing magnitude of hazard associated with this source is very low and the significance of effect therefore low from this source. The likelihood of occurrence is assumed to be very low, given the nature of the ground conditions, hence the overall risk of flooding from groundwater is assessed as remaining low and no additional mitigation is required. 6.6 Reservoirs The majority of Section 2 is shown to be at risk of flooding from the Llyn Alaw reservoir. If the dam of the reservoir were to fail then flood water would follow the Afon Alaw valley. Although the Afon Alaw river crossing on the A5025 is in Section 3, some of the water has been shown on NRW maps to follow the Afon Cleifiog southwards from Cleifiog Isaf towards Valley The modelling of reservoir flood risk by NRW constitutes the worst-case scenario and, whilst it is unlikely that the extent of flooding would be this large, the magnitude of the hazard would undoubtedly be high. The sensitivity of the Proposed Development and off-site receptors is considered very high, resulting in a high significance of potential effect if failure were to occur Reservoir flooding is, however, extremely unlikely to happen because of the management regime implemented under the Reservoirs Act 1975 [RD16] where all large reservoirs must be inspected regularly and any essential safety work undertaken when required. The likelihood of occurrence is therefore considered to be very low and it is noted that the likelihood of flooding from this source would not be affected by the Proposed Development Overall, the level of flood risk of Section 2 from reservoirs is considered to be low. This would not be affected by the activities associated with the Proposed Development and the risk from this source is assessed as remaining unchanged The Off-line Highways Improvements is part of a wider transport strategy to associated with the development of the Wylfa Newydd DCO Project, including the main power station site and associated development sites. There is expected to be a temporary increase in traffic volume associated with the construction of these facilities that will marginally increase the consequences of a failure, however, this would not change the overall risk In all cases, the existing risk of flooding from groundwater will remain low and no additional mitigation is required. 6.7 Effects of climate change Climate change allowances have been considered throughout this assessment. The conclusions presented above indicate that the Proposed Development will not have a detrimental impact on existing flood risks from any source. As there is not expected to be any impacts on the flooding mechanisms associated with these sources there is also not expected to be Page 21

27 Flood Consequence Assessment Wylfa Newydd Project any impact on the flood risks when an allowance for climate change has been considered The drainage design has incorporated a 20% increase in rainfall intensity when developing the drainage system and attenuation pond sizing. Allowing for climate change in this manner means that the proposed drainage system will ensure no increase in discharge rates downstream over the lifetime of the development when the effects of climate change are considered In conclusion, the effect of climate change on the sources of flooding in this assessment is not expected to be worsened as a result of the Proposed Development and no additional mitigation is required. Page 22

28 Wylfa Newydd Project Flood Consequence Assessment 6.8 Summary of risks Assuming installation of the drainage strategy summarised in section 4 of this report, the following risks are predicted as set out in table 6-1. Flood type Source Pathway Receptor Sensitivity Fluvial and pluvial Reservoirs Fluvial flooding Surface water Surface water - road widening Culvert capacity Groundwater Groundwater Flood water from reservoir Risk of flooding from neighbouring watercourses Surface water flow routes across the road Increased run-off due to the introduction of additional impermeable surfaces and drainage strategy Raising of upstream water levels Risk of flooding due to ground water expressed at surface Flood water following dam failure A5025 Section 2 A5025 Section 2 Off-site areas Off-site areas A5025 Section 2 A5025 Section 2 Section 2 flood risk summary table Magnitude of hazard Significance of effect Likelihood of occurrence Flood risk Very High Medium High Medium High High Very High Medium High High High High Withdevelopment flood risk Medium Low Low High Moderate Moderate Medium Medium to High Moderate High High High Very High Very Low Low Low Low Low Very High High High Very Low Low Low Page 23

29 Flood Consequence Assessment Wylfa Newydd Project 7 Section 4 North of Llanynghenedl to south of Llanfaethlu This section discusses the flood risk to Section 4 of the A5025 and any increase in flood risk due to the Proposed Development. Reference is made to the TAN 15 development advice maps [RD2] and the NRW surface water flood mapping [RD6], and uses the methodology detailed in annex C. 7.1 TAN 15 Mapping Much of the route of Section 4 of the Proposed Development falls within Flood Zone A, classed as having little or no flood risk. However, at the southern end of Section 4 the A5025 crosses the Tan R Allt. At this location the road passes through Flood Zone C2, within the NRW s 2 extreme flood outline and indicating an area with a 0.1% AEP of flooding or greater that is not served by significant flood defence infrastructure. This is the only location within Section 4 that is at risk of flooding from fluvial sources The existing A5025 and the Proposed Development are considered to have a very high sensitivity to flood risk. Where the Proposed Development lies in Flood Zone C2 the magnitude of the hazard is assessed as medium, by virtue of the duration and potential depth of flooding, and the significance of effect is therefore considered to be high. The likelihood of flooding in these areas is medium, therefore, the overall risk from this source is considered high Since the improvements to the A5025 at all locations within Section 4 would be at the same level or slightly higher than the existing highway, the risk of flooding from fluvial sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from fluvial sources elsewhere from the Proposed Development would also remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 7.2 Surface water flood mapping NRW s surface water flood map [RD6] indicates areas that may be at risk of surface water flooding. The only part of Section 4 that is shown to be at risk is the section of the highway crossing the Tan-yr-allt floodplain, which has a high risk of surface water flooding associated with the channel of that 2 Although the Development Advice Map is a Welsh Government publication it refers to Environment Agency mapping Page 24

30 Wylfa Newydd Project Flood Consequence Assessment watercourse. Elsewhere, there are isolated, narrow and shallow areas of ponding shown in the vicinity of the Tan-yr-allt, between the bend at Bytheicws and Plas Newydd The risk of flooding at Tan-yr-allt is indicated on NRW flood mapping [RD6] as happening on average every 30 years, which is equivalent to an annual exceedance probability (AEP) of 3.3%. Elsewhere, the risk of flooding is indicated to occur on average between every 100 and every 1000 years, which is equivalent to an annual exceedance probability (AEP) of between 1% and 0.1%. The Proposed Development would not increase the existing level of risk from this source since the road level and alignment will not be altered The existing A5025 and the Proposed Development is considered to have a very high sensitivity to flood risk. Where the Proposed Development is shown to be affected by surface water flooding at Tan-yr-allt the magnitude of the hazard is assessed as medium, by virtue of the duration and potential depth of flooding, and the significance of effect is therefore considered to be high. The likelihood of flooding in these areas is high, as flooding is expected to happen on average every 30 years, which is equivalent to an annual exceedance probability (AEP) of 3.3%, therefore, the overall risk from this source is considered high Elsewhere, the magnitude of hazard is assessed as low, by virtue of shallow depths, and the significance of effect is therefore considered to be moderate. The likelihood of flooding in these areas is medium, as flooding is expected to happen between every 100 and every 1000 years, which is equivalent to an annual exceedance probability (AEP) of between 1% and 0.1%, therefore, the overall risk from this source is considered moderate Since the improvements to the A5025 at all locations within Section 4 would be at the same level or slightly higher than the existing highway, the risk of flooding from surface water sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from surface water sources elsewhere from the Proposed Development would also remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 7.3 Increased run-off due to impermeable surfaces The localised widening of the highway would lead to an increase in the area of impermeable surfaces of approximately 0.03ha. These surfaces would increase the run-off produced, as rainfall would be unable to percolate into the ground. The drainage strategy, described in section 4 of this report, would be installed based, where appropriate, on the existing infrastructure. Page 25

31 Flood Consequence Assessment Wylfa Newydd Project A short section of new carriageway would be constructed as part of the Proposed Development within Section 4, which is required to improve an existing bend at Bytheicws. A new drainage system would be provided for this section of road in accordance with the drainage strategy Incorporation of the drainage system will ensure that increased impermeable surfaces would not increase surface water flood risk elsewhere and would not contribute to an increased risk from other sources. No additional mitigation is proposed. 7.4 Assessment of flood risk from structures The A5025 crosses the Tan-yr-allt within Section 4 via an existing culvert at NGR The existing flood extents in the vicinity of and upstream of these structures, as indicated on NRW s fluvial and surface water flood maps [RD6], is restricted to agricultural land of medium sensitivity to flooding and a minor road to the east, which is considered to have a high sensitivity. Although information on the depth of flooding is unavailable, it is likely that the magnitude of hazard is between medium and high to agricultural land and medium to the minor roads and therefore the significance of effect is assessed as moderate. Given the high likelihood of occurrence, the existing risk associated with these structures is considered to be high Changes to this section of the highway resulting from the Proposed Development would not affect the existing culverts through lengthening or changes in flow capacity. Furthermore, surface water run-off from the Proposed Development would be channelled through proposed carrier/filter drains that would outfall into watercourses at locations downstream of the existing culverts. Consequently, the Proposed Development is not expected to have any effect on the flood risk upstream or downstream associated with these structures and the risk of flooding is expected to remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 7.5 Groundwater Increased impermeable surfaces and ground compaction during the construction period may reduce rainwater reaching the groundwater table, potentially altering local recharge rates, groundwater levels and groundwater flow directions and reducing the risk of flooding from ground water. However, since the entire route of the Proposed Development within Section 4 is underlain by low permeability Glacial Till, limiting natural recharge, it is considered that there would be little difference in the movement between surface and groundwater It is considered that the existing magnitude of hazard associated with this source is very low and the significance of effect therefore low from this source. The likelihood of occurrence is assumed to be very low, given the nature of the ground conditions, hence the overall risk of flooding from groundwater is assessed as remaining low and no additional mitigation is required. Page 26

32 Wylfa Newydd Project Flood Consequence Assessment 7.6 Reservoirs There are no reservoirs that would impact the A5025 within Section 4 if they were to breach and, therefore, there would be no risk of flooding from reservoirs. 7.7 Effects of climate change Climate change allowances have been considered throughout this assessment. The conclusions presented above indicate that the Proposed Development will not have a detrimental impact on existing flood risks from any source. As there is not expected to be any impacts on the flooding mechanisms associated with these sources there is also not expected to be any impact on the flood risks when an allowance for climate change has been considered In conclusion, the effect of climate change on the sources of flooding in this assessment is not expected to be worsened as a result of the Proposed Development and no additional mitigation is required. Page 27

33 Flood Consequence Assessment Wylfa Newydd Project 7.8 Summary of risks Assuming installation of the drainage strategy summarised in section 4 of this report, the following risks are predicted as set out in table 7-1. Flood type Fluvial and pluvial Source Pathway Receptor Sensitivity Fluvial flooding Surface water Surface Water road widening Culvert capacity Groundwater Groundwater Risk of flooding from neighbouring watercourses Surface water flow route across the road Tan-yr-allt Surface water ponding on the road Increased run-off due to the introduction of impermeable surfaces. Raising of upstream water levels Risk of flooding due to ground water expressed at surface A5025 Section 4 A5025 Section 4 A5025 Section 4 Section 4 flood risk summary table Magnitude of hazard Significance of effect Likelihood of occurrence Flood risk Very High Medium High Medium High High Very High Medium High High High High Withdevelopment flood risk Very High Low Moderate Medium Moderate Moderate Off-site areas Medium Low Low High Moderate Moderate Off-site areas agricultural land Off-site areas minor road A5025 Section 4 Medium Medium to High Moderate High High High High Medium Moderate High High High Very High Very Low Low Low Low Low Page 28

34 Wylfa Newydd Project Flood Consequence Assessment 8 Section 6 North of Llanfaethlu to north of Llanrhuddlad This section refers to the flood risk to Section 6 of the A5025 and any increase in flood risk due to the Proposed Development. Reference is made to the TAN 15 development advice maps [RD2] and the NRW surface water flood maps [RD6], and uses the methodology detailed in annex C. 8.1 TAN 15 Mapping Much of the route of Section 6 of the Proposed Development falls within Flood Zone A, classed as having little or no flood risk. There is a single watercourse crossing at Ty n-cae within Section 6. The catchment area of this watercourse is less than 3km 2 and, therefore, the risk of flooding from this watercourse has not been included in the modelling used to produce the TAN15 flood maps [RD2]. The watercourse may constitute a risk of flooding to the highway, however, there have been no recorded flood events The existing A5025 and the Proposed Development are considered to have a very high sensitivity to flood risk. Where the Proposed Development lies in Flood Zone A the magnitude of the hazard is assessed as low, as whilst is no identifiable impact of flooding it is conservatively assumed that there is a potential flood risk from the watercourse at this point. The resulting significance of effect is therefore considered to be moderate. The likelihood of flooding is also conservatively assumed to be high, therefore, the overall risk from this source is considered high Since the improvements to the A5025 at all locations within Section 6 would be at the same level or slightly higher than the existing highway, the risk of flooding from fluvial sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from fluvial sources elsewhere from the Proposed Development would also remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 8.2 Surface water flood mapping NRW s surface water flood map [RD6] indicates areas that may be at risk of surface water flooding. There is one location where surface water flow paths have been identified as crossing the A5025 within Section 6. At the very south of the section, at Bodhelen, a flow path runs from a pond to the west of the highway eastwards, over the A5025 towards the tributary of the Afon Llanrhuddlad The risk of flooding at Bodhelen is classed as moderate risk on NRW flood mapping [RD6] and is expected to happen on average between every 30 years Page 29

35 Flood Consequence Assessment Wylfa Newydd Project and 100 years, which is equivalent to an annual exceedance probability (AEP) of between 3.3% and 1%. However, the Proposed Development would not increase the existing level of flood risk since the road level and alignment will not be altered. Therefore, the risk remains moderate Elsewhere a watercourse is shown to be crossed at National Grid Reference (NGR) , however, there is no associated flood risk indicated by NRW s surface water flood map. The flood extents that are shown on the surface water flood maps do not appear to follow the watercourses particularly well and for the purposes of this assessment it is assumed that there is an existing high risk as is similar for other watercourse crossings. Flooding is therefore expected to happen on average every 30 years, which is equivalent to an annual exceedance probability (AEP) of between 3.3%. Despite this, the Proposed Development would not increase the existing level of flood risk since the road level and alignment will not be altered. Therefore, the risk remains unchanged The existing A5025 and the Proposed Development is considered to have a very high sensitivity to flood risk. Where the Proposed Development is shown to be affected by surface water flooding at Bodhelen, the magnitude of the hazard is assessed as low, by virtue of the expected shallow depth of flooding, and the significance of effect is therefore considered to be moderate. The likelihood of flooding is high, as flooding is expected to happen on average more frequently than every 100 years, which is equivalent to an annual exceedance probability (AEP) of 1%, therefore, the overall risk from this source is considered high Where the Proposed Development crosses the watercourse, the magnitude of the hazard is assumed to be medium to high, by virtue of the expected depths and durations of flooding, and the significance of effect is therefore considered to be high. The likelihood of flooding in these areas is high, as flooding is expected to happen on average every 30 years, which is equivalent to an annual exceedance probability (AEP) of 3.3%, therefore, the overall risk from this source is considered high Since the improvements to the A5025 at all locations within Section 6 would be at the same level or slightly higher than the existing highway, the risk of flooding from surface water sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from surface water sources elsewhere from the Proposed Development would also remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 8.3 Increased run-off due to impermeable surfaces The localised widening of the highway would lead to an increase of approximately 0.3ha in the area of impermeable surfaces. These surfaces Page 30

36 Wylfa Newydd Project Flood Consequence Assessment would increase the run-off produced, as rainfall would be unable to percolate into the ground. The drainage strategy described in section 4 of this report would be installed based, where appropriate, on the existing infrastructure Incorporation of the drainage system will ensure that increased impermeable surfaces would not increase surface water flood risk elsewhere and would not contribute to an increased risk from other sources. No additional mitigation is proposed. 8.4 Assessment of flood risk from structures The A5025 crosses one watercourse via existing culverts within Section 6 at NGR At Ty n-cae, properties are sited immediately upstream of the culvert on the right bank. As indicated in section 8.2, there is no associated flood risk indicated by NRW s surface water flood map in this area, however, flooding is therefore expected to happen on average every 30 years, which is equivalent to an annual exceedance probability (AEP) of between 3.3% Changes to this section of the highway resulting from the Proposed Development would not affect the existing culvert through lengthening or changes in flow capacity. Furthermore, surface water run-off from the Proposed Development would be into proposed replacement gullies that would connect into the existing highway drainage. Consequently, the Proposed Development is not expected to have any effect on the flood risk upstream or downstream associated with this structure and the risk of flooding is expected to remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 8.5 Groundwater Increased impermeable surfaces and ground compaction during the construction period may reduce rainwater reaching the groundwater table, potentially altering local recharge rates, groundwater levels and groundwater flow directions. However, as all of Section 6 of the Proposed Development is underlain by low permeability Glacial Till, which would limit natural recharge, it is considered that there would be little difference in the movement between surface and groundwater It is considered that the existing magnitude of hazard associated with this source is very low and the significance of effect therefore low from this source. The likelihood of occurrence is assumed to be very low, given the nature of the ground conditions, hence the overall risk of flooding from groundwater is assessed as remaining low and no additional mitigation is required. 8.6 Reservoirs There are no reservoirs that would impact the Proposed Development within Section 6 if they were to breach, and therefore there would be no risk of flooding from reservoirs. Page 31

37 Flood Consequence Assessment Wylfa Newydd Project 8.7 Effects of climate change Climate change allowances have been considered throughout this assessment. The conclusions presented above indicate that the Proposed Development will not have a detrimental impact on existing flood risks from any source. As there is not expected to be an impacts on the flooding mechanisms associated with these sources there is also not expected to be any impact on the flood risks when an allowance for climate change has been considered In conclusion, the effect of climate change on the sources of flooding in this assessment is not expected to be worsened as a result of the Proposed Development and no additional mitigation is required. Page 32

38 Wylfa Newydd Project Flood Consequence Assessment 8.8 Summary of risks Assuming installation of the drainage strategy summarised in section 4 of this report, the following risks are predicted as set out in table 8-1. Flood type Fluvial and pluvial Source Pathway Receptor Sensitivity Fluvial flooding Surface water Surface Water road widening Culvert capacity Groundwater Groundwater Risk of flooding from neighbouring watercourses Surface water flow route across the road Increased run-off due to the introduction of impermeable surfaces. Raising of upstream water levels Risk of flooding due to ground water expressed at surface A5025 Section 6 A5025 Section 6 - Bodhelen A5025 Section 6 - Ty n-cae Section 6 flood risk summary table Magnitude of hazard Significance of effect Likelihood of occurrence Flood risk Very High Low Moderate High High High Very High Low Moderate High High High Very High Medium to High High High High High Off-site areas Medium Low Low High Moder ate Off-site areas agricultural land Off-site areas properties A5025 Section 6 Medium Medium to High Moderate High High High Very High Low Moderate High High High Very High Very Low Low Low Low Low Withdevelopment flood risk Moderate Page 33

39 Flood Consequence Assessment Wylfa Newydd Project 9 Section 8 North of Cefn Coch to Power Station Access Road Junction This section discusses the flood risk to Section 8 of the A5025 and any increase in flood risk due to the Proposed Development. Reference is made to the TAN 15 development advice maps [RD2] and the NRW surface water flood mapping [RD6], and uses the methodology detailed in annex C. 9.2 TAN 15 Mapping All of the route of Section 8 of the A5025 is within Flood Zone A, classed as having little or no flood risk. There are two locations where the A5025 crosses watercourses. The first crossing is over an unnamed watercourse, just north of Cefn Coch at NGR The second is at NGR This coincides with the junction of a minor road with the A5025, where the watercourse is culverted beneath the highway junction The catchment areas of these watercourses are less than 3km 2 and, therefore, the risk of flooding has not been included in the modelling used to produce the TAN15 flood maps [RD2]. The watercourse may constitute a risk of flooding to the highway, however, there have been no recorded flood events The existing A5025 and the Proposed Development are considered to have a very high sensitivity to flood risk. Where the Proposed Development lies in Flood Zone A the magnitude of the hazard is assessed as low, as whilst is no identifiable impact of flooding it is conservatively assumed that there is a potential flood risk from the watercourses crossed in this section. The resulting significance of effect is therefore considered to be moderate. The likelihood of flooding is also conservatively assumed to be high, therefore, the overall risk from this source is considered high Since the improvements to the A5025 at all locations within Section 8 would be at the same level or slightly higher than the existing highway, the risk of flooding from fluvial sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from fluvial sources elsewhere from the Proposed Development would also remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 9.3 Surface water flood mapping NRW s surface water flood map [RD6] indicates areas that may be at risk of surface water flooding. These are primarily associated with the watercourses that the highway crosses, at and there is additional small areas of surface water ponding adjacent to the A5025, such as on the 90 bend at Caerdegog Uchaf. Page 34

40 Wylfa Newydd Project Flood Consequence Assessment The risk of flooding in these areas is classed as high risk on NRW flood mapping [RD6] and is expected to happen on average at leat every 30 years, which is equivalent to an annual exceedance probability (AEP) of 3.3%. However, the Proposed Development would not increase the existing level of flood risk since the road level and alignment will not be altered. Therefore, the risk remains high The existing A5025 and the Proposed Development is considered to have a very high sensitivity to flood risk. Where the Proposed Development is shown to be affected by surface water flooding at watercourse crossings the magnitude of the hazard is assessed as medium, by virtue of the duration and potential depth of flooding, and the significance of effect is therefore considered to be high. The likelihood of flooding in these areas is high, as flooding is expected to happen on average every 30 years, therefore, the overall risk from this source is considered high Elsewhere, the areas of ponding are on agricultural land and therefore have a sensitivity to flooding of medium magnitude. The magnitude of hazard is assessed as medium, by virtue of frequent ponding and likely duration, and the significance of effect is therefore considered to be moderate. The likelihood of flooding in these areas is high, as flooding is expected to happen at least every 30 years, therefore, the overall risk from this source is considered high Since the improvements to the A5025 at all locations within Section 8 would be at the same level or slightly higher than the existing highway, the risk of flooding from surface water sources to the Proposed Development would remain unchanged The application of a surface dressing, reconstruction and localised widening of the existing pavement within the existing highway boundary and improvements to signage and road markings would have no effect on existing flood mechanisms and flood levels elsewhere, meaning that the impact on the risk of flooding from surface water sources elsewhere from the Proposed Development would also remain unchanged Given there would be no change in flood risk to or from the Proposed Development, no additional mitigation is proposed. 9.4 Increased run-off due to impermeable surfaces The localised widening of the A5025 would lead to an increase of approximately 0.08ha in the area of impermeable surfaces. These surfaces would increase the run-off produced, as rainfall would be unable to percolate into the ground. The drainage strategy described in section 4 of this report would be installed based, where appropriate, on the existing infrastructure Incorporation of the drainage system will ensure that increased impermeable surfaces would not increase surface water flood risk elsewhere and would not contribute to an increased risk from other sources. No additional mitigation is proposed. Page 35

41 Flood Consequence Assessment Wylfa Newydd Project 9.5 Assessment of flood risk from structures The A5025 crosses two watercourses within Section 8 via existing culverts at NGR and NGR The existing flood extents in the vicinity of and upstream of these structures, as indicated on NRW s fluvial and surface water flood maps [RD6], is restricted to agricultural land of medium sensitivity. Although information on the depth of flooding is unavailable, it is likely that the magnitude of hazard is between medium and high to agricultural land and therefore the significance of effect is assessed as moderate. Given the high likelihood of occurrence, the existing risk associated with these structures is considered to be high Road widening is proposed at these locations (see figure 11). The existing culverts would require lengthening to accommodate the road widening, which could influence the upstream flood risk since hydraulic losses occurring over a longer culvert could result in a marginal rise in upstream river levels [RD17]. The extended sections will maintain the existing culvert size, to avoid any direct impacts on conveyance, and, where the culvert entrance is altered as part of the process, a low head loss design will be considered for the new culvert inlet Incorporating this mitigation will minimise if not avoid the potential effect on upstream flood levels in upstream agricultural areas. At this stage, it is assumed that a neutral impact on flood risk would be achieved, resulting in no change in overall risk to the Proposed Development of other receptors. 9.6 Groundwater Increased impermeable surfaces and ground compaction during the construction period, may reduce rainwater reaching the groundwater table, potentially altering local recharge rates, groundwater levels and groundwater flow directions. However, as all of Section 8 of the Proposed Development is underlain by low permeability Glacial Till, which would limit natural recharge, it is considered that there would be little difference in the movement between surface and groundwater It is considered that the existing magnitude of hazard associated with this source is very low and the significance of effect therefore low from this source. The likelihood of occurrence is assumed to be very low, given the nature of the ground conditions, hence the overall risk of flooding from groundwater is assessed as remaining low and no additional mitigation is required. 9.7 Reservoirs There are no reservoirs that would impact the Proposed Development within Section 8 if they were to breach, and therefore there is no risk of flooding from reservoirs. 9.8 Effects of climate change Climate change allowances have been considered throughout this assessment. The conclusions presented above indicate that the Proposed Development will not have a detrimental impact on existing flood risks from any source. As there is not expected to be an impacts on the flooding mechanisms associated with Page 36

42 Wylfa Newydd Project Flood Consequence Assessment these sources there is also not expected to be any impact on the flood risks when an allowance for climate change has been considered In conclusion, the effect of climate change on the sources of flooding in this assessment is not expected to be worsened as a result of the Proposed Development and no additional mitigation is required. Page 37

43 Flood Consequence Assessment Wylfa Newydd Project 9.9 Summary of risks Assuming installation of the drainage strategy summarised in section 4 of this report, the following risks are predicted as set out in table 9-1. Flood type Fluvial and pluvial Source Pathway Receptor Sensitivity Fluvial flooding Surface water Surface Water road widening Culvert capacity Groundwater Groundwater Risk of flooding from neighbouring watercourses Surface water flow route across the road Increased run-off due to the introduction of impermeable surfaces. Raising of upstream water levels Risk of flooding due to ground water expressed at surface A5025 Section 8 A5025 Section 8 Off-site areas Off-site areas Off-site areas agricultural land A5025 Section 8 Section 8 flood risk summary table Magnitude of hazard Significance of effect Likelihood of occurrence Flood risk Very High Low Moderate High High High Very High Medium Medium to High Medium to High High High High High Moderate High High High Withdevelopment flood risk Medium Low Low High Moderate Moderate Medium Medium to High Moderate High High High Very High Very High Low Low Low Low Page 38

44 Wylfa Newydd Project Flood Consequence Assessment 10 Conclusions 10.1 Risks to the Proposed Development Sections 2, 4, 6 and 8 of the Proposed Development all have some level of risk of fluvial flooding, particularly at river crossings. In Section 4, there are additional fluvial flood risks associated with the crossing of the Afon Alaw floodplain. However, since in all cases the highway would be at the same level, or raised to some degree, the risk of flooding would not be worse than the current risk Surface water flood risk, in the majority of cases, is confined to the channels and floodplains at the crossing points of smaller watercourses. The exception to this is within Section 8 where there is an area of ponding to the south of the road and within Sections 2 and 4 where there is minor ponding on the road. The drainage strategy incorporated into the design of the Proposed Development would manage the risk of flooding from surface water The drainage strategy for the Proposed Development would ensure that additional run-off from impermeable surfaces does not impact downstream flood risk. The design would provide different attenuation volumes depending on the specific site constraints at each section. However, the target return periods are between the 1 in 1 and 1 in 5 year events. In Section 2, two attenuation ponds, restricting run-off to a 1 in 2-year discharge rate and providing attenuation for a 100-year event plus an allowance for climate change, have been included within the design of the Proposed Development to further reduce the flood risk at Valley Locations have been identified within Section 8 where the Proposed Development would require existing culverts to be lengthened to accomodate the full width of the widened road. The lengthening of culverts is expected to have a neutral impact on flood levels in agricultural land upstream and will not therefore have a significant effect on off-site receptors Section 2 falls within the flood risk zone of the reservoir Llyn Alaw. Since reservoir failure is considered very unlikely, due to the high specifications of the structure along with regular inspections and maintenance, the risk has been assessed as being low Groundwater flooding is considered to be low for all Sections due to the impermeable nature of the superficial deposits along the A5025, leading to little interaction between groundwater and surface water Additional Mitigation requirements No additional mitigation measures have been identified as being necessary to supplement the design of the Proposed Development. Page 39

45 Flood Consequence Assessment Wylfa Newydd Project 11 Glossary A Full title Abbreviation Description A5025 Off-line Highway Improvements A5025 On-line Highway Improvements Additional mitigation Above Ordnance Datum Advanced Boiling Water Reactor - The A5025 Off-line Highway Improvements would be implemented through Sections 1, 3, 5 and 7. These would involve the construction of bypasses, the formation of new junction arrangements (including the provision of the proposed Power Station Access Road Junction), and localised improvements to existing bends. - Improvements to the existing A5025 between the A5 east of Valley junction to the proposed Power Station Access Road Junction, to include reconstruction and localised widening of the existing pavement and application of a surface dressing. The proposals also comprise a Temporary Construction Compound including a temporary pavement recycling facility, and other associated works such as drainage infrastructure, boundary treatments, planting, new signage and road markings. - Measures that have been identified through the assessment process to reduce environmental effects further. AOD Above the mean sea level at Newlyn in Cornwall calculated between 1915 and 1921, taken as a reference point for the height data on Ordnance Survey maps. - A third generation evolution of the boiling water reactor design. The Power Station will use the Advanced Boiling Water Reactor design provided by Hitachi-GE, adapted for use in the UK. Alluvium - A deposit of clay, silt and sand left by flowing floodwater in a river valley or delta. Annual Exceedance Probability AEP Flood frequency is expressed in terms of an annual exceedance probability (AEP), which is the inverse of the annual maximum return period. For example, the 100-year flood (a flood likely to occur once every 100 years) can be expressed as the 1% AEP flood, which has a 1% chance of being exceeded in any year. Attenuation - The process of water retention on site and slow release in a controlled flow to surface water. Page 40

46 Wylfa Newydd Project Flood Consequence Assessment Full title Abbreviation Description Alternative Emergency Control Centre Associated Development AECC A facility that is physically separate from but local to the Power Station and forms part of the Power Station. This would provide back-up command and communications facilities that would be used to manage an incident at the Power Station Site in the extremely unlikely event that the primary facilities on the Power Station Site were not available. - Works included in the DCO which facilitate the delivery of the Nationally Significant Infrastructure project, and which principally include: the Site Campus; a temporary Park and Ride facility at Dalar Hir for construction workers; A temporary Logistics Centre at Parc Cybi; A5025 Off-line Highway Improvements; and an electrical connection to the National Grid substation. Attenuation pond - A pond designed to slow the passage of water from surface run-off to the ground/drainage system. B Baseline - A reference level of existing environmental conditions against which a project is measured and controlled. Bedrock - Solid bedrock formations underlying superficial deposits (if present). C Carriageway - The width of the highway pavement that can be used by motorised vehicles and non-motorised users. Catchment - A drainage/basin area within which precipitation drains into a river system and eventually into the sea. Cooling Water System - The once-through system that removes, using Cooling Water, the proportion of heat energy produced by the Nuclear Reactors which cannot be converted into electricity, and for this Project includes the intakes, pumphouses, breakwaters, seal pits and outfall structures as well as connecting pipelines and tunnels, for each Generating Unit. Culvert - A tunnel carrying a stream or open drain under a road, railway or development area. D Development Consent Order DCO The consent for a Nationally Significant Infrastructure Project required under the Planning Act Page 41

47 Flood Consequence Assessment Wylfa Newydd Project E Full title Abbreviation Description Embedded mitigation - Measures to avoid or reduce environmental impact that can reasonably be incorporated into the design of the scheme that is submitted to the relevant decision making as part of the application for the proposals. Enabling Works - The works required to remove and clear parts of the Wylfa Newydd Development Area of vegetation, topsoil, existing services, utilities and other features and structures, including ecology mitigation, in order to allow the earthworks and Main Construction stage activities to commence. This work would also include the installation of any new services or utilities required to support the Main Construction stage activities. Environment Agency Environmental assessment Environmental Survey Laboratory Environmental Report Existing Power Station F Flood Consequences Assessment - The executive non-departmental public body with responsibility for environmental regulation in England. - A method and a process by which information about environmental effects is collected, assessed and used to inform decision-making. - A facility that is physically separate from but local to and part of the Power Station. - A non-statutory document which reports the findings of the environmental assessment process. - The existing Magnox nuclear power station at Wylfa. FCA The process of assessing potential flood risk to a site and identifying whether there are any flooding or surface water management issues that may warrant further consideration or may affect the feasibility of a development. Assessment requirements are set out in Planning Policy Wales Technical Advice Note 15: Development and Flood Risk (TAN 15). Floodplain - A floodplain is flat or nearly flat land adjacent to a stream or river, stretching from the banks of its channel to the base of the enclosing valley walls and (under natural conditions) experiences flooding periods of high discharge. Fluvial - A term that relates to rivers and streams and the processes that occur within them. Forward flow - The amount of flow that is allowed to pass forward following an attenuating feature. Page 42

48 Wylfa Newydd Project Flood Consequence Assessment Full title Abbreviation Description Fracture flow - The flow of groundwater through rock fractures. G Glacial till - Unsorted glacial sediment derived from erosion and entrainment of material by a glacier. Groundwater - All water which is below the surface of the ground in the saturation zone (below the water table) and in direct contact with the ground or subsoil. Gwna Group - Rock comprising grit, phyllite, quartzite, limestone, lava and tuffs several hundred metres thick below Anglesey and north Wales. H Horizon Nuclear Power Wylfa Limited I Isle of Anglesey County Council J Joint Local Development Plan K Horizon IACC The applicant. The local authority governing the area within which the Wylfa Newydd Project is intended to be constructed. - The local development plan being prepared by IACC and Gwynedd Councils. Kilometre km A unit of length. L Logistics Centre - An Off-Site facility at which deliveries can be consolidated into fewer loads and the timing of traffic movements to the Wylfa Newydd Development Area can be controlled during both the Enabling Works and Main Construction stages of development of the Power Station. Longitudinal low point M Main Construction - A low point on a longitudinal section - Construction activities within the Wylfa Newydd Development Area that would result in the completion of the Power Station, including final levelling and deep excavations for the Power Station foundations, civil construction activities, Page 43

49 Flood Consequence Assessment Wylfa Newydd Project Full title Abbreviation Description Meteorological Office Met. Office Metre m A unit of length. Millimetre mm A unit of length. commissioning of both Units and site finishing. The United Kingdom s national weather service Mitigation - Measures intended to avoid, reduce and, where possible, remedy significant adverse environmental effects. Mobile Emergency Equipment Garage N National Grid Reference National Policy Statement National Policy Statement for Energy EN-1 National Policy Statement for Nuclear Power Generation EN-6 Nationally Significant Infrastructure Project Natural Resources Wales New Harbour Group - An Off-Site facility for the managed storage of vehicles and equipment for responding to any incidents that might arise during the operational phase of the Power Station. NGR NPS NPS EN-1 NPS EN-6 Map reference system used by the Ordnance Survey Statements prepared and designated by the Secretary of State under the Planning Act 2008, which establish national policy for Nationally Significant Infrastructure Projects, including energy, transport and water, waste water and waste and against which applications for Development Consent Orders are assessed. The National Policy Statement designated in July 2011, which sets out the overarching national policy for delivery of major energy infrastructure projects. The National Policy Statement designated in July 2011 which sets out national policy on new nuclear power stations. - A type of project listed in the Planning Act 2008, which must be consented by a DCO. NRW The public body whose stated purpose is to ensure that the natural resources of Wales are sustainably maintained, enhanced and used, now and in the future. It absorbed the regulatory and advisory duties of the Environment Agency Wales, Countryside Council for Wales and the Forestry Commission in Wales. - Rock type comprising schist and lava up to 2000m thick in the west and northern parts of Anglesey. Page 44

50 Wylfa Newydd Project Flood Consequence Assessment O Full title Abbreviation Description Off-Site - Areas of land needed for the Wylfa Newydd Project that would be outside the Wylfa Newydd Development Area. Off-Site Power Station Facilities Ordnance Survey P Park and Ride facility - The emergency and other facilities that would be physically separate from the Power Station Site and would complement the facilities within the Power Station Site, including an Alternate Emergency Control Centre, Environmental Survey Laboratory and Mobile Emergency Equipment Garage. - The UK national mapping agency currently operating as a government owned company. - The park and ride is a temporary facility where workers can park their vehicles securely and transfer to shuttle buses, which would take them to the Power Station Site. The site is designed to include a zone for buses to collect and drop off passengers, with a management office and parking for staff (working at the park and ride facility). Pavement - Materials comprising the sub-base, base course and surface course placed on a sub-grade to support the traffic load. Planning Policy Wales PPW Guidance prepared by the Welsh Government setting out the current land use planning policy for Wales, to be taken into account by local planning authorities when preparing Development Plans. Plant - The machinery or infrastructure used to construct or support the operation of a given development or facility. Pluvial - A term that relates to rainfall and its characteristics. Power Station - The proposed new nuclear power station, including two UK Advanced Boiling Water Reactors to be supplied by Hitachi-GE Nuclear Energy Ltd., supporting facilities, buildings, plant and structures, and radioactive waste and spent fuel storage buildings. Power Station Access Road Power Station Access Road Junction - The proposed new access road linking the A5025 to the Power Station Site. - The proposed junction connecting the A5025 and the Power Station Access Road. Power Station - The indicative areas of land and sea within Page 45

51 Flood Consequence Assessment Wylfa Newydd Project Site Full title Abbreviation Description Proposed Development which the majority of the permanent Power Station buildings, plant and structures would be situated. It would include the two nuclear reactors, steam turbines, the cooling water system intake and pumphouse, outfall structures, breakwaters and the Marine Off- Loading Facility, as well as other ancillary structures. - The. Project - The Wylfa Newydd Project. R Return Period - An average repeat interval between events of the same magnitude Run-off - Precipitation that flows as surface water from a site, catchment or region to the sea. S Secondary A aquifers Secondary B aquifers Secretary of State Permeable layers capable of supporting water supplies at local rather than strategic scale, and in some cases forming an important source of baseflow to rivers - Primarily lower permeability layers which may store and yield limited amounts of groundwater due to localised features. - The cabinet minister who (among other things) ultimately determines applications for Development Consent Orders. Sediment - Organic and inorganic material that has precipitated from water to accumulate on the floor of a water body, watercourse or trap. Soakaway - A pit or channel, typically filled with hard core, into which water can drain slowly out into the surrounding soil. Site Campus - The Site Campus is a temporary facility that would house up to 4,000 construction workers in modular type accommodation blocks, providing an independent living space for each worker, with shared campus-style amenities. Site of Special Scientific Interest SSSI Sites designated as being of special interest for their flora, fauna or geological or physiographical features and protected under the Wildlife and Countryside Act Site Preparation - The term used for the works required to clear Page 46

52 Wylfa Newydd Project Flood Consequence Assessment Full title Abbreviation Description and Clearance the Power Station site in preparation for Main Construction of the Power Station. Stratum - A layer or series of layers of rock in the ground Study area - The spatial area within which environmental effects are assessed (i.e. extending a distance from the development footprint in which significant environmental effects are anticipated to occur). This area varies between the different environmental topic areas. Superelevation Sustainable Drainage System T Technical Advice Note Temporary Construction Compound W Water Framework Directive WFD water bodies Wylfa Newydd Development Area Wylfa Newydd DCO Project - The vertical distance between the heights of inner and outer edges of highway pavement SuDS TAN A sequence of water management practices and facilities designed to drain surface water in a manner that will provide a more sustainable approach than the practice of routing run-off through a pipe to a watercourse. A series of topic-specific policy documents that supplement Planning Policy Wales. - An area where construction materials are stored and construction site offices occur along with parking areas for construction workers and machinery. WFD Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy. - A discrete section of a river, groundwater area, lake or coast that is a defined management unit under the WFD. - The indicative areas of land and sea including the areas surrounding the Power Station Site that would be used for the construction and operation of the Power Station. This area is representative of the maximum area that would be physically affected by Power Station Main Construction activities and used to form the landscaped setting of the operational Power Station. - Those parts of the Wylfa Newydd DCO Project which are to be consented by a Development Consent Order - comprising: the Power Station; other on-site development; Page 47

53 Flood Consequence Assessment Wylfa Newydd Project Full title Abbreviation Description Marine Works; the Off-Site Power Station Facilities; and the Associated Development. Wylfa Newydd Project - The Wylfa Newydd DCO Project and Enabling Works. Page 48

54 Wylfa Newydd Project Flood Consequence Assessment 12 References RD1 RD2 ID Reference The Welsh Government, (2016). Planning Policy Wales (PPW). Edition 9. [Online]. Available from: Welsh Government, (2004). Technical Advice Note (TAN) 15: Development and Flood Risk. [Online] Available from: pdf RD3 Planning Act [Online] [Accessed June 2017] Available at: RD4 RD5 RD6 RD7 RD8 RD9 RD10 Town and Country Planning Act [Online] [Accessed June 2017] Available at: _en.pdf The Welsh Government, (2016). CL Climate change allowances for Planning Purposes. [Online] [Accessed June 2017] Available from: rs/2016/cl climate-change-allowances-for-planningpurposes/?lang=en Natural Resources Wales, (2016). Flood Risk Map. [Online]. Available from: [ Isle of Anglesey County Council, (1993). Gwynedd Structure Plan. [Online]. Available from: Isle of Anglesey County Council, (1996). Ynys Môn Local Plan. [Online] Available from: Isle of Anglesey County Council, (2005). Stopped Unitary Development Plan (unadopted). [Online]. Available from: Isle of Anglesey County Council, (2014). New Nuclear Build at Wylfa: Supplementary Planning Guidance. [Online]. Page 49

55 Flood Consequence Assessment Wylfa Newydd Project Available from: RD11 RD12 RD13 RD14 Isle of Anglesey County Council, (2013). Anglesey and Gwynedd Joint Local Development Plan, Topic Paper 8: Strategic Flood Consequence Assessment (Level 1). [Online] Available from: Isle of Anglesey County Council. Preliminary Flood Risk Assessment, (2009). [Online]. Available from: Isle of Anglesey County Council, (2013). Anglesey Local Flood Risk Management Strategy. [Online]. Available from: European Parliament, (2000), EU Water Framework Directive. 2000/60/EC. [Online]. Available from: RD15 Lancaster, JW, M Preene and CT Marshall, (2004). Development and flood risk - guidance for the construction industry. Report C624. CIRIA RD16 HMSO The Reservoirs Act HMSO RD17 Balkham, M, Fosbeary, C, Kitchen, A, Rickard, C, (2010). Culvert Design and Operation Guide. Report 689. CIRIA RD18 RD19 The Town and Country Planning (Environmental Impact Assessment) (Wales) Regulations [Online] [Accessed June 2017] Available at: Meteorological Office UK Climate Projections. [Online] [Accessed: April 2017] Available from Page 50

56 Wylfa Newydd Project Flood Consequence Assessment Annex A Mott MacDonald A5025 Drainage Strategy Page 51

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94 Wylfa Newydd Project Flood Consequence Assessment Annex B Annex B On-line A5025 Highway Improvements: Drainage Strategy B.1 Introduction B.1.1 B.1.2 B.1.3 B.1.4 B.1.5 The proposed involve works to 16.19km of the existing A5025 from Valley to the proposed Power Station Access Road Junction. Construction of the Power Station would require very substantial transport needs for materials, large components and staff. Studies undertaken by Horizon in identified that the stretch of the A5025 between Valley and the proposed Power Station Access Road Junction has physical and operational constraints in relation to its width, alignment, overtaking opportunities and surfacing condition. As a result of these studies Horizon has committed to improve this section of the A5025 in order to mitigate the predicted impacts of increased traffic associated with construction activities that would be undertaken at the Wylfa Newydd Development Area, and from operational Power Station traffic, that would travel along this part of the highway network. The A5025 Highway Improvements are separated into: On-line improvements involving reconstruction and localised widening of the existing pavement and application of a surface dressing. Off-line improvements involving the construction of bypasses, the formation of new junction arrangements (including the provision of the proposed Power Station Access Road Junction), and localised improvements to existing bends. Since the original A5025 Drainage Strategy was written by Mott MacDonald in February 2012, AECOM has developed the design in consultation with Isle of Anglesey County Council (IACC) and Natural Resources Wales (NRW). This document is an addendum that updates the A5025 On-line Highway Improvements section of the original Mott MacDonald drainage strategy (included in annex A) and should be read in conjunction with the strategy for more background information on the existing drainage and hydrology. B.2 Overview of B.2.1 B.2.2 The consist mainly of carriageway widening from the existing average width of 6.5m (6m in some areas) to a minimum width of 7.3m ( x 0.3 hardstrips) with a 450mm wide verge generally increasing to 700mm wide where additional land is required. Where the existing bend radius is less than 400m, the minimum carriageway width will be increased to a maximum of 8.5m to enable HGVs to pass through. On-line sections will have a 60mph speed limit, except where the road goes through villages. Visibility splays will be improved at intermediate junctions and Private Means of Access (PMA) enabled in line with TD 42/95 (where practical). Other Page 93

95 Flood Consequence Assessment Annex B Wylfa Newydd Project B.2.3 improvement measures in line with TA 85/01, include traffic signs, road markings and surface dressing. Investigations show that the existing pavement has a zero residual life. It is therefore proposed to replace it by recycling the existing pavement material using cold recycling methods. B.3 Drainage Strategy B.3.1 B.3.2 B.3.3 B.3.4 Existing A5025 On-line Highway Drainage Formal drainage assets are limited along the A5025. The current system relies on run-off from the road discharging over the edge into adjoining land, ditches or watercourses and in some cases into gullies installed at low points. In some cases the gullies are directly connected to ditches and watercourses, however the connectivity of some of the gullies is unclear and it is possible that some gullies may be connected to soakaways. There are no reports of significant carriageway flooding in the past. Where localised flooding has repeatedly occurred, reactive measures have been put in place by IACC to reduce the risk of localised flooding. This is mainly achieved by installing additional gullies at low points, where run-off from the road tends to accumulate. IACC Requirements IACC will ultimately adopt the proposed drainage assets and be responsible for their maintenance. It is therefore essential that the Council s requirements are taken into consideration in developing the drainage design. The drainage strategy has been developed in close consultation with IACC. Based on the consultations that have taken place, IACC s requirements are summarised as follows: the proposed drainage and maintenance regime should be similar to the existing system; minimise the use of piped drainage systems (this implies that run-off will not be concentrated at one discharge point, but will instead be distributed, attenuated and slowed down before being discharged); minimise asset maintenance requirements; reduce the risk of flooding to the road and third parties, focusing in particular on areas with flooding history or known drainage problems; and reduce the impact of the road improvement works on flooding and the water environment. Impact of the The main impact of the works is considered to be a small increase in run-off rates and volumes and change in run-off flow paths, due to the proposed carriageway widening and change in road contours. Page 94

96 Wylfa Newydd Project Flood Consequence Assessment Annex B B.3.5 It is important to note that the relatively small increase in road area by 7-15% is insignificant when considered within the wider catchment of the watercourses they are located within. Table 1 below provides context in this regard by comparing the increase in road area with the area of the catchment for all 4 on-line sections. The comparison shows the increase in road area to be relatively small compared to the catchment area. The catchment wide impact of the scheme is considered to be insignificant. The local impact at each section can be mitigated by implementing the range of proposed methods discussed in Drainage Strategy General Principles section. B.3.6 In its current state the pavement of the A5025 is in a poor condition. It is possible that some water percolates through the weakened pavement when it rains and replacing the existing pavement may result in some increase in run-off. However, information received from IACC suggests that the existing road surface has undergone regular surface dressing in the past. It is therefore considered reasonable to assume that the permeability of the new road surface will be comparable to that of the existing. Table B3-1 Comparison of increase in road catchment with the wider watercourse catchment for each on-line section Chainage Section 2 Road Catchment Increase (ha) Watercourse Catchment within which road section is located Cleifiog (Valley) Alaw Estuary Tributary 130 Section Tan R'Alt 2390 Section Tan R'alt Llyn Garreglwyd/Tan R'alt Tan R'alt Afon Cafnan 990 Section Afon Cafnan 990 Watercourse Catchment Area (ha) B.3.7 Mott MacDonald s Drainage Strategy Summary Mott MacDonald produced a drainage strategy for the road improvement works at an early stage of the project. (A copy of the strategy is included in annex A.) It is understood that the drainage principles were discussed and verbally agreed with the statutory consultees. The flow chart below summarises the approach proposed in the Mott MacDonald s Drainage Strategy report. It has been used as the starting point in developing this Page 95

97 Flood Consequence Assessment Annex B Wylfa Newydd Project drainage strategy, with amendments made as appropriate, as the road design progressed and taking into account feedback from consultations with IACC and NRW. Figure B3-1 Drainage Strategy Block Diagram (except at Chainage (Ch) 3800 to Ch 3900 and Ch 5650 to Ch 5900) No positive drainage currently in place Where road falls towards a receiving watercourse Where road falls away from a receiving No drainage work required Installing new filter drain or ditch at the low spots of the road Existing gullies currently in place Existing culverts affected by the proposed on-line improvement works Relocate gullies to the proposed kerblines Culvert extension as appropriate Connected to existing discharge points B.3.8 Permeability Testing and Drainage Investigations On 30 August 2016, AECOM and Horizon commenced permeability tests to BRE Digest 365 in the verge at 7 low points along the A5025. The aim was to establish whether the permeability of the ground would be suitable for highway run-off to drain by infiltration. Results of one location in Section 2 at Ch 2700 showed suitable permeability. Existing gullies and ditches were observed on-site and required further investigation. B.3.9 These further investigations were conducted during week commencing 18 October 2016 and involved gully testing and walking the length of existing ditches to determine their eventual discharge location. B.3.10 Drainage Strategy General Principles The Drainage Strategy has been developed building upon Mott MacDonald s drainage strategy, various site visits and taking into consideration IACC requirements. Some changes have been made in regard to the strategy proposed by Mott MacDonald following further consultation and other design considerations. The proposed changes have been made in recognition of the need to provide on-line attenuation to account for the road widening, to reduce peak flows, to slow down and direct road run-off away from sensitive receptors. B.3.11 There are 3 major objectives in the drainage of trunk roads (DMRB HD 33/06 - Volume 4, Section 2): Page 96

98 Wylfa Newydd Project Flood Consequence Assessment Annex B B.3.12 B.3.13 B.3.14 B the speedy removal of surface water to provide safety and minimum nuisance; 2. provision of effective sub-surface drainage to maximise longevity of the pavement and its associated earthworks; and 3. minimisation of the impact of the run-off on the receiving environment. The principles of the strategy adopted by AECOM are based on the sourcepathway-receptor model. Accordingly, the drainage system is designed to take the run-off (source) from the surface of the road, convey the run-off to a point (pathway) where it can be discharged to the receiving environment (receptor). In standard road drainage systems run-off is normally collected (run-off collection) by gullies, kerb drains, channel drains, or (in rural areas) over the length of filter drains3 or simply over the edge of the carriageway into ditches. Run-off thus collected is conveyed in pipes (run-off conveyance), channels or ditches and discharged to a watercourse or via infiltration based systems such as soakaways (run-off discharge). In order to limit the impact of the run-off on the receiving environment, attenuation ponds, storage tanks or oversized pipes are often used (run-off attenuation). In accordance with DMRB (Volume 4 Section 2 Part 3 HD 33/16) carrier drains are normally designed to accommodate a one-year storm in-bore without surcharge. The design is then checked against a five-year storm intensity to ensure that surcharge levels do not exceed the levels of chamber covers. If a storm event with a higher return period occurs, or the collection system does not work properly (for example due to gullies being blocked), the excess run-off generated flows towards the longitudinal low point of the road. Both the DMRB and NRW recommend assessing the impact of run-off from a road on a fluvial surface water body based on the one-in-100-year storm event. In traditional design, the above components that constitute a drainage system are clearly linked. As discussed in Section 3.1, the existing A5025 drainage system is not clearly defined. In some sections of the A5025 road, the edge of verge is higher than the carriageway. This appears to be due to vegetation growth and accumulation of sediment over time. As a result of the above, the amount of run-off accumulating at the longitudinal low points of the road is expected to be more than that of a road drainage designed to DMRB standards. Run-off Collection B.3.16 Run-off collection methods applied in the design include: over the edge drainage - where an embankment exists, or where water can flow beyond the verge; gullies; 3 Linear trench filled with a permeable material on the verge. Page 97

99 Flood Consequence Assessment Annex B Wylfa Newydd Project over the length of filter drains 4 (with or without a perforated pipe) on the verge. Setting the verge below the carriageway will allow more run-off from the road to flow onto the verge, into the filter material; block drains - short sections of surface water channel (applied only at PMAs); and kerb drains (only considered in exceptional cases, where kerbs currently exist or where their application is considered to be absolutely necessary). B.3.17 B.3.18 B.3.19 Run-off Conveyance Run-off conveyance methods applied in the design include: filter drain with a perforated pipe; open channel ditches on the side of the road; and kerb drains (only considered in exceptional cases, where kerbs currently exist or where their application is considered to be absolutely necessary). Run-off Attenuation Methods considered to attenuate run-off from the road are stated below. Note that attenuation is only required to mitigate the impact of widening the road on the environment. attenuation ponds, where the risk of flooding in the receiving watercourse is high; storage within the filter material beneath the verge (filter drains, with or without a perforated pipe); and increasing the capacity of existing or new ditches on the side of the road. Run-off Discharge Run-off discharge methods considered include (in order of preference): Discharging to soakaway (where feasible); discharging to a watercourse; discharging to a gully at the longitudinal low point (provided that the hydraulic connectivity of the gully is clearly understood and the capacity is sufficient); and discharging to a shallow attenuation pond, where no other option exists. 4 For the purpose of this report a distinction is made between a filter drain with a perforated pipe in the base of the trench and a filter drain without a perforated pipe. Page 98

100 Wylfa Newydd Project Flood Consequence Assessment Annex B B.3.20 B.3.21 B.3.22 B.3.23 Implementing the Drainage Strategy The methods described above will be applied in the design, giving due consideration to the existing conditions and constraints. The following shall be considered in implementing the strategy: Where possible, run-off collected at the longitudinal low point of the road will be discharged in the order of preference described in Section 3.5 above. The design will seek to provide attenuation to cater for the increase in road catchment. The attenuation volume provided will depend on the specific site constraints. The target return period considered would be between one-in-one and one-in-five years. For sections of the road where the verge width is narrow, no drainage will be provided and existing drainage will be utilised. All existing gullies will be either re-used or replaced. If the location of a low point in the road changes, the gullies will be moved to the new low point. All connections to the existing gully will thereby be retained and new gully tails may be required if gullies are relocated a significant distance away or the existing gully tail is removed as part of the excavation for the widened carriageway. Filter drains with a perforated pipe will be used where the verge is sufficiently wide and it is possible to discharge to a water course or gully at the longitudinal low point of the road. If there is no available outfall, a strip of filter material will be used to collect and store run-off which will drain away by infiltration. A concrete dished channel will be installed at PMAs where more run-off is expected as a result of the change in road contours. An appropriate run-off collection method will be implemented at superelevation points on the new road. Where existing culverts are affected by carriageway widening, they will be extended using the same cross-sectional area and gradient, and if necessary replaced. Where no positive drainage is proposed, grassed verges are to be maintained to ensure highway run-off can drain over the edge. Exceptions During the Integrated Drainage and Flood Meeting on 31 March 2016, NRW noted that attenuation should be provided for the on-line section of the road (Section 2) north of the proposed Valley bypass, due to concerns that the online drainage (if un-attenuated) could increase the existing flood risk in Valley. A site visit was jointly undertaken by staff from Horizon, IACC and AECOM to investigate possible locations for attenuation ponds. Following the site visit, 3 Page 99

101 Flood Consequence Assessment Annex B Wylfa Newydd Project B.3.24 B.3.25 locations were identified (subsequently reduced to two) at the two main low points in Section 2 (Ch 280 and Ch 700), north of Valley, and these are included in the updated drainage design drawings. Attenuation will be provided in these ponds to cater for run-off from the entire road catchment for the one-in-100-year storm event plus 30% allowance for climate change. There is a short off-line bypass on Section 4 (Ch 800), which is required to improve the bend radii. A new positive drainage system will be provided for this new section of road using the methods described in Section 3.5 and will discharge to the main river using the existing outfall. (This will form part of the proposed A5025 Off-line Highway Improvements). In areas where superficial aquifers within Source Protection Zones (SPZ) exist, use of infiltration based systems should be avoided. The suitability of infiltration based systems in other areas will be agreed with NRW following the ground investigation results. B.4 Summary B.4.1 The Drainage Strategy presented in this document has been developed in close consultation with IACC and NRW. In developing the strategy, AECOM considered the Drainage Strategy produced previously by Mott McDonald at an earlier stage of the project and IACC s requirements. With the exception of part of Section 2 in Valley and a short length in Section 4, the proposed drainage relies on using the existing drainage methods of over the edge and other methods considered to be appropriate for rural roads. The design aims to reduce the impact of the on the environment and meet the requirements of IACC and NRW. Page 100

102 Wylfa Newydd Project Flood Consequence Assessment Annex C Annex C Assessment methodology C.1 Introduction C.1.1 In order to allow for the wider assessment of flood risk, a generalised assessment methodology has been developed. C.2 Assessment methodology C.2.1 In line with the risk-based approach detailed by the Welsh Government and recommended elsewhere in industry guidance, the key to the assessment is that the designation of significance (or risk) is based upon the consideration of: the sensitivity of the receptor takes into account the nature of the development or receptor and its likely response to increased risk; the severity of the potential hazard takes into account the potential nature of the flooding; and the probability of occurrence (i.e. likelihood) takes into account both the presence of the hazard and receptor, and the integrity of the pathway. C.3 Classification of sensitivity of the receptor C.3.1 When considering developments, the classification of sensitivity is based (where possible) directly on the technical guidance set out within TAN 15 [RD2]. When considering off-site impacts, there is a general assumption that all developments are highly sensitive. This assumption can, however, typically be relaxed when considering a water compatible development or undeveloped land. Given this, the sensitivity of the receptor is ranked as shown in table C-1. Table C-1 Classification of sensitivity of receptor Sensitivity of receptor Development Off-site Very High High Medium Emergency services* developments Highly vulnerable* developments Less vulnerable* developments All built developments unless mitigating circumstances exist Key access routes Other access routes Undeveloped land Page 101

103 Flood Consequence Assessment Annex C Wylfa Newydd Project Sensitivity of receptor Development Off-site Low Very Low Water compatible 5 developments Flood attenuation features * For definitions of terms, please see figure 2 of TAN 15 [RD2] - - C.4 Classification of the magnitude of potential hazard C.4.1 C.4.2 Magnitude of hazard High Medium Low To classify the severity of potential effects, it is necessary to look at the nature and scale of the individual impacts. These include, but are not confined to, the extent of flooding, the depth of flooding, the duration of flooding and the velocity of flood waters. For developments the assessment is based on the likely post-development situation; for off-site receptors, it is based solely on the likely deterioration. Given this, the severity of the potential hazard is then ranked as shown below in table C-2. Table C-2 Classification of the magnitude of the potential hazard Development Any one of the following criteria achieved: flood depths greater than 1m; flood flow velocities greater than 0.45m/s; or likely flood duration in excess of 24 hours. Any one of the following criteria achieved: flood depths between 0.3m and 1m; flood flow velocity greater than 0.15m/s; likely flood duration in excess of one hour; or any restrictions to access and egress. All of the following criteria achieved: flood depths below 0.3m; Off-site Any marked (>10%) increase in flood depth, flood flow velocity or flood duration Any change in flood extent that impacts additional properties, including access to those properties Any other measurable increase of flood depths, durations, flow velocities or extent Likely, but unquantifiable small increases of flood depths, durations, flow 5 Category not outlined within TAN15 [RD2], but would include any types of development that clearly, by their nature, often need to be in a floodplain such as buildings associated with water-sports or pumping stations for low-lying areas. Page 102

104 Wylfa Newydd Project Flood Consequence Assessment Annex C Magnitude of hazard Very Low Negligible Development likely flood duration below one hour; and flood proofing measures planned. Planned or permitted flooding that does not adversely impact the built development No potential for flooding, or no identifiable impact of flooding Off-site velocities or extent - No likely increase in flood severity at any off-site location C.5 Significance of potential effect C.5.1 The magnitude of the hazard and the sensitivity of the receptor are combined using a matrix (shown below in table C-3) to determine the significance of the potential effect, if realised. Table C-3 Matrix for determining the significance of the potential effect SENSITIVITY OF RECEPTOR VERY LOW LOW MEDIUM HIGH VERY HIGH MAGNITUDE OF POTENTIAL HAZARD HIGH Low Moderate Moderate High High MEDIUM Very Low Low Moderate Moderate High LOW Very Low Very Low Low Moderate Moderate VERY LOW Negligible Very Low Very Low Low Low NEGLIGIBLE Negligible Negligible Negligible Negligible Negligible C.6 Classification of probability of occurrence C.6.1 To classify the probability of occurrence for a potential effect, it is necessary to understand how regularly a given event or outcome will occur. This can be assessed in a number of ways, including assessments based on historical data, quantitative analysis or experience from other similar sites. Often this assessment is be based on standard guidance. The probability of the potential effect is then ranked as shown below in table C-4. Table C-4 Clasification of probability of occurrence Probability of occurrence Potential effect High Medium Any consequence would likely appear in the medium-term and inevitably in the long-term (i.e. the lifetime of the development). Equivalent to an annual probability of flooding of greater than 1% (0.5% for tidal). Circumstances are such that an event is possible in the mediumterm and likely over the long-term, although not necessarily inevitable. Page 103

105 Flood Consequence Assessment Annex C Wylfa Newydd Project C.6.2 Probability of occurrence Low Very Low Potential effect Equivalent to an annual probability between 0.1% and 1% (0.1% and 0.5% for tidal). It is unlikely that any consequence would arise within the lifetime of the development. Equivalent to an annual probability of less than 0.1%. It is unlikely that any consequence will ever arise. In circumstances where sites are defended, determining an accurate assessment of probability of flood occurrence is complex, and assumptions that defences will not fail are unlikely to be acceptable. In such cases, assessments cannot be prescriptive and site-specific assessments would be undertaken. Factors that would be considered include construction, age, condition, maintenance, exposure and other external pressures. C.7 Risk assessment C.7.1 Once the significance of the potential effect and likelihood of occurrence have been assessed, these are then combined using a risk matrix (see table C.5) to assess the flood risk of each potential effect. Table C-5 Risk matrix LIKELIHOOD OF OCCURRENCE VERY LOW LOW MEDIUM HIGH SIGNIFICANCE OF POTENTIAL EFFECT C.7.2 C.7.3 HIGH Low Moderate High High MODERATE Low Low Moderate High LOW Very Low Low Low Moderate VERY LOW Negligible Very Low Low Low NEGLIGIBLE Negligible Negligible Negligible Negligible Typically flood risks assessed as low, very low or negligible are considered acceptable. If the assessment results in moderate or high risk, this is considered significant i.e. equivalent to a significant environmental effect under the Town and Country Planning (Environmental Impact Assessment) (Wales) Regulations 2017 [RD18], and additional mitigation measures would be required to facilitate development. In some situations, the risk assessment procedure will result in an artificially low assessment of risk. This is particularly the case in situations where consequences of very rare flooding (i.e. breach scenarios) are so extreme that any residual risk, however low, would not be allowed. In such instances, the assessed risk would be elevated. Such decisions must always be accompanied by detailed justification. Page 104

106 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 1 Location Plan Page 105

107 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 106

108 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 2 Section 2 Flood Consequence Assessment study area and water features Page 107

109 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 108

110 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 3 Section 4 Flood Consequence Assessment study area and water features Page 109

111 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 110

112 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 4 Section 6 Flood Consequence Assessment study area and water features Page 111

113 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 112

114 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 5 Section 8 Flood Consequence Assessment study area and water features Page 113

115 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 114

116 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 6 Location of attenuation ponds in Section 2 Page 115

117 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 116

118 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 7 Section 2 TAN 15 Development Advice Map Page 117

119 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 118

120 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 8 Section 4 TAN 15 Development Advice Map Page 119

121 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 120

122 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 9 Section 6 TAN 15 Development Advice Map Page 121

123 Flood Consequence Assessment Annex C Wylfa Newydd Project [This page is intentionally blank] Page 122

124 Wylfa Newydd Project Flood Consequence Assessment Annex C Figure 10 Section 8 TAN 15 Development Advice Map Page 123