Maendy Primary School Flood Consequence Assessment

Final

April 2021 www.jbaconsulting.com

Morgan Sindall Construction & Infrastructure 5 Cae Gwyrdd, Greenmeadow Springs Tongwynlais Cardiff CF15 7AB

Maendy Primary School FCA i

JBA Project Manager Ed Hartwell BSC MSc MCIWEM C.WEM FRGS JBA Consulting Arlington House Park Five Harrier Way Sowton Exeter EX2 7HU

Revision History Revision Ref/Date Amendments Issued to C01 April 2021 Final Report David Humphrey

Contract This report describes work commissioned by David Humphrey, on behalf of Morgan Sindall Construction & Infrastructure Ltd, by an email dated 16 December 2020. Morgan Sindall Construction & Infrastructure Ltd’s representative for the contract was David Humphrey. Ed Hartwell and Gwyn Jones of JBA Consulting carried out this work.

Prepared by ...... Gwyn Jones MESci MSc

...... Assistant Analyst

...... Ed Hartwell BSC MSc MCIWEM C.WEM FRGS

...... Principal Analyst

Reviewed by ...... Faye Tomalin BSc (Hons) MSc C.WEM MCIWEM

Chartered Senior Analyst

...... George Baker BEng AIEMA CEnv IEng MCIWEM C.WEM

Associate Director

Purpose This document has been prepared for Morgan Sindall Construction & Infrastructure Ltd. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared. JBA Consulting has no liability regarding the use of this report except to Morgan Sindall Construction & Infrastructure Ltd.

Copyright © Jeremy Benn Associates Limited 2021.

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Contents 1 Introduction 1 1.1 Terms of reference 1 1.2 FCA Requirements 1 2 Site description 2 2.1 Site summary 2 2.2 Existing land use and site topography 3 2.3 Ground conditions and drainage 3 2.4 Development proposal 4 3 Review of existing flood data 5 3.1 Historic Flood Risk 5 3.2 Existing flood zone mapping (fluvial and tidal) 6 3.3 Surface water and small watercourses 7 3.4 Reservoirs 8 3.5 Groundwater 8 3.6 Sewers 9 4 Planning Policy and Flood Risk 10 4.1 Planning context 10 4.2 Local Development Plan 10 4.3 DAM zoning and vulnerability classification 11 4.4 Justification Test 12 5 Flood Risk Assessment 13 5.1 Assessment of flood risk 13 5.2 Model details 13 5.3 Model results – baseline 14 5.3.1 Baseline – 1% AEP plus climate change 14 5.3.2 Baseline – 0.1% AEP 15 5.4 Model results – post development 16 5.4.1 Post-development – 1% AEP plus climate change 16 5.4.2 Post-development – AEP 0.1% 17 5.5 Model results – Blocked culvert 18 5.5.1 Baseline – 1% AEP plus climate change and blocked culvert 18 5.5.2 Baseline – 0.1% AEP and blocked culvert 19 5.5.3 Post-development – AEP 1% plus climate change and blocked culvert 20 5.5.4 Post-development – 0.1% AEP and blocked culvert 21 5.6 Temporary School Buildings 22 5.6.1 Temporary buildings – 1% AEP plus climate change 23 5.6.2 Temporary buildings – 0.1% AEP 24 5.7 Third party impacts 25 5.7.1 1% AEP plus climate change 25 5.7.1 0.1% AEP 26 5.7.2 1% AEP plus climate change and blocked culvert 27 5.7.3 0.1% AEP and blocked culvert 28 5.7.4 Temporary buildings – 1% AEP plus climate change 29 5.7.5 Temporary buildings – 1% AEP plus climate change 30 5.8 Assessment of Acceptability Criteria 31 6 Conclusions and recommendations 33 A Topographic Survey I B Proposed site layout II

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List of Figures Figure 2-1 – Site location 2 Figure 2-2 - Satellite image of current site 3 Figure 2-3 - Proposed Layout 4 Figure 3-1 – NRW fluvial flood risk map 6 Figure 3-2 – NRW surface water and small watercourse flood risk map 7 Figure 3-3 – NRW reservoir flood risk map 8 Figure 4-1 – NRW Development Advice Map 11 Figure 5-1 - Baseline 1% AEP + CC Flood Depths 14 Figure 5-2 - Baseline 0.1% AEP Flood Depths 15 Figure 5-3 - Post-Development 1% AEP + CC Flood Depths 16 Figure 5-4 - Post-Development AEP 0.1% Flood Depths 17 Figure 5-5 - Baseline 1% AEP + CC + Blocked Culvert Flood Depths 18 Figure 5-6 - Baseline 0.1% AEP + Blocked Culvert Flood Depths 19 Figure 5-7 - Post-Development AEP 1% + CC + Blocked Culvert Flood Depths 20 Figure 5-8 - Post-Development AEP 0.1% + Blocked Culvert Flood Depths 21 Figure 5-9 - Temporary school building layout 22 Figure 5-10 - Temporary buildings 1% AEP plus climate change flood depths 23 Figure 5-11 - Temporary buildings 0.1% AEP flood depths 24 Figure 5-12 - 1% AEP + CC Flood Detriment 25 Figure 5-13 - 0.1% AEP Flood Detriment 26 Figure 5-14 - 1% AEP + CC + Blocked Culvert Flood Detriment 27 Figure 5-15 - 0.1% AEP + Blocked Culvert Flood Detriment 28 Figure 5-16 - Temporary Buildings -1% AEP + CC Flood Detriment 29 Figure 5-17 - Temporary Buildings -1% AEP + CC Flood Detriment 30

List of Tables Table 2-1 - Site summary 2 Table 3-1 - Summary of flood risk 5 Table 4-1 - Development categories defined by TAN-15 10 Table 4-2 - TAN 15 Justification Test 12 Table 5-1 - Assessment of TAN-15 Acceptability Criteria 31

Abbreviations AEP Annual Exceedance Probability BGS British Geological Survey CC Climate change DAM Development Advice Map DCWW Dŵr Cymru/Welsh Water ESTRY 1D solver component of TUFLOW FCA Flood Consequence Assessment Flood Modeller 1D hydraulic modelling software ha Hectares JBA Jeremy Benn Associates m AOD metres Above Ordnance Datum NRW Natural Resources Wales TAN Technical Advice Note TUFLOW Two-dimensional Unsteady FLOW (a hydraulic model)

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1 Introduction

1.1 Terms of reference JBA Consulting (JBA) were commissioned by Morgan Sindall Construction and Infrastructure Ltd to prepare a Flood Consequence Assessment (FCA) to support the proposed development of a new primary school at the site of Maendy Primary School, Cwmbran. The development is for an educational facility.

1.2 FCA Requirements This FCA follows Welsh Government guidance on development and flood risk set out in the Technical Advice Note 15: Development and Flood Risk1 (TAN15). Where appropriate the following aspects of flood risk will be addressed in all planning applications over its expected lifetime in flood risk areas: • The likely mechanisms of flooding • The likely source of flooding • The depths of flooding through the site • The speed of inundation of the site • The rate of rise of flood water across the site • Velocities of floodwater across the site • Overland flow routes • The effect of access and egress and infrastructure, for example public sewer outfalls, combined sewer overflows, surface water sewers and effluent discharge pipes from waste water treatment works • The impacts of development in terms of flood risk on neighbouring properties and elsewhere on the floodplain.

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1 Welsh Assembly Government. Planning Policy Wales. Technical Advice Note 15: Development and Flood Risk. July 2004.

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2 Site description

2.1 Site summary The proposed development site is situated on the current site of Maendy Primary School at 36 Wayfield Crescent in Cwmbran, Torfaen, South Wales. The approximately 6.3-hectare (ha) site is bordered on the west by the A4051 and on the south by Maendy Way, as shown in Figure 2-1. Further site details are provided in Table 2-1. The Afon Lwyd main river is located approximately 700m to the east of the site. The Blaen Bran ordinary watercourse is located approximately 570m to the north, where it passes underneath A4051 in a culvert. A section of the Monmouthshire Canal is located approximately 175m to the west of the site.

Table 2-1 - Site summary

Site name Maendy Primary School Site Area 6.3 ha Existing land use Primary School Purpose of development New Primary School Postcode NP44 1NH OS NGR 3219 1999 Local Planning Authority Torfaen County Borough Council Lead Local Flood Authority Torfaen County Borough Council

Figure 2-1 – Site location

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2.2 Existing land use and site topography The site is currently occupied by Maendy Primary School. The school is currently comprised of two larger buildings – the northern Infant School and the southern Main building - alongside several smaller structures such as lockups and sheds, as seen in Figure 2-2. The topographic survey can be seen in Appendix A. The ground elevation falls away significantly across the site, with a slope from high ground in the south west of the site (at approximately 70m AOD) to a low point in the north eastern corner of the site (at approximately 66m AOD). The ground levels immediately to the northwest and south of the site are higher than the site itself, with the roads being embanked above the surrounding area.

Figure 2-2 - Satellite image of current site

2.3 Ground conditions and drainage The British Geological Survey (BGS) Geology of Britain viewer2 was consulted to identify the local geological conditions. The site is underlain by the Raglan Mudstone formation which comprises of interbedded mudstone and sandstone of fluvial origin. There are superficial deposits of river terraces and alluvium, comprising of clay, silt, sand and gravel. The soils on site were determined from the Cranfield University Soilscape viewer3. The site is on the border between freely draining floodplain soils and slightly acid loamy and clayey soils with impeded drainage.

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2 BGS Geology of Britain viewer http://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/viewer.html 3 Cranfield University Soilscape Viewer http://www.landis.org.uk/soilscapes

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As the site is comprised of playing fields, school buildings, and tarmacked areas, the site is likely to drain by a mix of natural infiltration, overland flow to surface water sewers, and evapotranspiration.

2.4 Development proposal The proposed development is for the demolition of the two existing buildings of Maendy Primary school and construction of a new ‘Passivhaus’ primary school in the centre of the site. The entrance of the site will also be moved from Wayfield Crescent on the north-east boundary to Maendy Way on the southern boundary, as seen in Figure 2-3. A finished floor level of 68.0m AOD is proposed for the new school building (situated in the centre of the site). Parking will be located to the south of the building along with the Flying Start and Childcare building. In the north of the site there will be an all-weather playing field and a grass playing field. The development plans are contained in Appendix B.

Figure 2-3 - Proposed Layout

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3 Review of existing flood data A review of the existing flood risk data relating to the site has been undertaken. The main sources of risk have been published by Natural Resources Wales (NRW) and within the Torfaen County Borough Council Flood Risk Management Plan4. Table 3-1 summarises these findings.

Table 3-1 - Summary of flood risk

Source of Flooding Onsite Description Presence Fluvial and Tidal Yes The site is partially located in the High, Medium, and Low risk zones of flooding from rivers – see section 3.2 The site is not at risk of tidal flooding. Surface Water and Yes The site is partially located in the Low and Medium risk Small Watercourses zones of flooding from surface water and small watercourses – see section 3.3 Reservoirs Yes The site is at risk of flooding from a breach of the Blaen Bran reservoir – see section 3.4 Groundwater No The site is not shown to be at risk of groundwater flooding - see section 3.5 Sewers No The site is not at apparent risk from flooding from sewers – see section 3.6.

3.1 Historic Flood Risk There is no record of historic flood events at the site and the site is not contained within NRW’s historic flood extents dataset.

———————————————————————————————————————————— 4 Torfaen County Borough Council Flood Risk Management Plan (2015), https://www.torfaen.gov.uk/en/Related- Documents/Roads-Highways-and-Pavements/Drainage/Torfaen-Flood-Risk-Management-Plan.pdf

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3.2 Existing flood zone mapping (fluvial and tidal) The NRW flood risk map for flooding from rivers (Figure 3-1) shows that the majority of the northern part of the site is located within the high risk flood zone (greater than 3.3% Annual Exceedance Probability (AEP) (1 in 30-year) chance of flooding in a given year from fluvial flood sources). In the centre of the site, between the two existing buildings there are areas within the medium risk flood zone (between 1% AEP (1 in 100-year) and 3.3% AEP (1 in 30- year) chance of flooding in a given year from fluvial flood sources) and low risk flood zone (between 0.1% AEP (1 in 1000-year) and 1% AEP (1 in 100-year) chance of flooding in a given year from fluvial flood sources). The southern half of the site is considered at very low risk of flooding from fluvial sources. The risk of fluvial flooding to the site is discussed further in section 5. As the site is located inland, there is no risk of tidal flooding.

Figure 3-1 – NRW fluvial flood risk map

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3.3 Surface water and small watercourses Small areas of the site are located in the Low risk zone of flooding from surface water and small watercourses in the NRW flood risk map (Figure 3-2). These areas are concentrated around the footprints of the existing buildings and a flow path crossing the north-east corner of the site. This means that these areas have a 0.1%-1% AEP of flooding from these sources in any given year. There is also a small section of the site near the boundary with Wayfield Crescent that is within the Medium (1%-3.3% AEP) risk zone. The rest of the site is at very low risk. Figure 3-2 – NRW surface water and small watercourse flood risk map

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3.4 Reservoirs The NRW flood risk map (Figure 3-3) shows the majority of the northern extent of the proposed development site is at risk from flooding from a breach of the Blaen Bran reservoir approximately 2.7km to the north-west. However, due to the regular maintenance and inspections carried out on reservoirs under the Reservoirs Act 1975, the risk of a reservoir failure is very low.

Figure 3-3 – NRW reservoir flood risk map

3.5 Groundwater Groundwater flooding is caused by unusually high groundwater levels. It occurs as excess water emerging at the ground surface or within manmade structures such as basements. Groundwater flooding tends to be more persistent than surface water flooding, in some cases lasting for weeks or months, and can result in significant damage to property. The risk of groundwater flooding depends on the nature of the geological strata underlying the site, as well as on the local topography. According to the Torfaen County Borough Council Flood Risk Management Plan5, the “South Wales coal field is regarded as a minor aquifer and the permeable carboniferous limestone is deemed a major aquifer, despite the underlying geology presenting ample stratigraphy to allow for high levels of groundwater, the topography of the area having a high relief, channels the groundwater to the valley floors quickly limiting the amount of percolation and increasing the amount of surface runoff”. The management plan states that numerous studies have concluded that “groundwater is not

———————————————————————————————————————————— 5 Torfaen County Borough Council Flood Risk Management Plan (2015), https://www.torfaen.gov.uk/en/Related- Documents/Roads-Highways-and-Pavements/Drainage/Torfaen-Flood-Risk-Management-Plan.pdf

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typically a hazard”, and as such the site is not considered to be at a significant risk of groundwater flooding.

3.6 Sewers According to the Torfaen County Borough Council Flood Risk Management Plan6, “foul and freshwater flood risk is the responsibility of the water undertaker which falls into the responsibility of Dwr Cymru/Welsh Water (DCWW)”. No Historic Sewer flood incidents have been identified in the vicinity of the site in either the Torfaen County Borough Council Flood Risk Management Plan or the Torfaen County Borough Council Flood Risk Management Strategy7.

———————————————————————————————————————————— 6 Torfaen County Borough Council Flood Risk Management Plan (2015), https://www.torfaen.gov.uk/en/Related- Documents/Roads-Highways-and-Pavements/Drainage/Torfaen-Flood-Risk-Management-Plan.pdf

7 Torfaen County Borough Council Flood Risk Management Strategy (2013), https://www.torfaen.gov.uk/en/Related- Documents/Roads-Highways-and-Pavements/Drainage/TorfaenLocalFloodRiskManagementStrategy.pdf

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4 Planning Policy and Flood Risk

4.1 Planning context TAN-15 was introduced in 2004 by the Welsh Assembly Government. It provides technical guidance related to development planning and flood risk using a sequential characterisation of risk based on the Welsh Government's Development Advice Map (DAM). The initial requirement is to identify the DAM zones and vulnerability classification relevant to the proposed development. TAN-15 assigns one of three flood risk vulnerability classifications to a development as shown in Table 4-1. The educational and public nature of the proposed development is categorised as Highly Vulnerable Development.

Table 4-1 - Development categories defined by TAN-15

Development category Types Emergency services Hospitals, ambulance stations, fire stations, police stations, coastguard stations, command centres, emergency depots and buildings used to provide emergency shelter in time of flood. Highly vulnerable All residential premises (including hotels and caravan parks), public development buildings (e.g. schools, libraries, leisure centres), especially vulnerable industrial development (e.g. power stations, chemical plants, incinerators), and waste disposal sites. Less vulnerable General industrial, employment, commercial and retail development, development transport and utilities infrastructure, car parks, mineral extraction sites and associated processing facilities, excluding waste disposal sites.

4.2 Local Development Plan Policy Objective CF3 of the Torfaen County Borough Council’s Adopted Local Development Plan (2006-2021)8 seeks to improve the quality and accessibility of community facilities. This development would satisfy this objective.

———————————————————————————————————————————— 8 https://www.torfaen.gov.uk/en/Related-Documents/Forward-Planning/LDP-summary-English.pdf

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4.3 DAM zoning and vulnerability classification Figure 4-1 shows that sections of the site lie in Zones A, B and C2 of NRW’s Development Advice Map (DAM).

Figure 4-1 – NRW Development Advice Map

The southern third of the site is Zone A, the central third is in Zone B – including the existing Main building and a section of the Infant School, and the northern third is in Zone C2, including a section of the existing Infant School. Zone A is defined as areas “considered to be at little or no risk of fluvial or tidal/coastal flooding” and does not require justification and further consideration of the flood risk. Zone B is defined as “areas known to have been flooded in the past evidenced by sedimentary deposits” and require site levels to be checked against the 0.1% AEP flood extent. Zone C2 is defined as “areas of the floodplain without significant flood defence infrastructure” and should not be used for highly vulnerable developments such as schools. Although this development is classed as Highly Vulnerable, taking a sequential approach the site design the proposed buildings will be located away from areas of Zone C2. It is proposed that all buildings will be located within Zones A and B, with only playing fields located in the C2 area at the northern extent of the site. (see section 2.4). NRW guidance9 recognises that allowing flooding to ancillary areas, such as open spaces and recreational areas, is an appropriate approach to manging flood risk. The flood risk to the proposed development will be explored in further detail in section 5.

———————————————————————————————————————————— 9 Operational Guidance Note - Flooding to ancillary areas. Natural Resources Wales

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It should be stressed that parts of the existing school buildings currently lie within the Zone C2 area, and as a result it is the intention that the new development will reduce the flood risk overall and align the school with current flood risk regulations.

4.4 Justification Test Table 4-2 below summarises how the proposal meets the requirements set out within the Justification Test. All of the criteria have been satisfied with regards to the proposed development. The justification test requires the satisfaction of the following criteria: • Its location is necessary to assist a local authority regeneration initiative or strategy, or contribute to key employment objectives, necessary to sustain an existing settlement or region. • The site meets the definition of previously developed land (i.e. it is not a Greenfield site) and concurs with the aims of Planning Policy Wales (i.e. the presumption in favour of sustainable development). • A Flood Consequence Assessment has been produced to demonstrate that the potential consequences of a flood event up to the extreme flood event (1 in 1000 chance of occurring in any year) have been considered and meet the Acceptability Criteria in order to be considered acceptable.

Table 4-2 - TAN 15 Justification Test

TAN 15 Justification Criteria Comments Achievable? Its location is necessary to assist The proposed development will contribute to Yes a local authority regeneration delivering Torfaen County Borough’s CF3 Policy initiative or strategy, or Objective of the Adopted Local Development contribute to key employment Plan (2006-2021), which seeks to improve the objectives, necessary to sustain quality and accessibility of community facilities. an existing settlement or region The site meets the definition of The site is currently occupied by an existing Yes previously developed land (i.e. it primary school. This meets the definition of is not a Greenfield site) and previously developed land. concurs with the aims of Planning Policy Wales (i.e. the presumption in favour of sustainable development) A Flood Consequence Section 5 details the site flood risk assessment Yes Assessment has been produced using JBA flood modelling data. The model to demonstrate that the potential results demonstrate the flood risk at the site can consequences of a flood event up be acceptably managed for the lifetime of the to the extreme flood event (1 in proposed development. 1000 chance of occurring in any year) have been considered and meet the [Acceptability Criteria] … in order to be considered acceptable

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5 Flood Risk Assessment

5.1 Assessment of flood risk As identified in Section 3, the primary source of flood risk identified at the development site is from fluvial sources. The following section therefore assesses these risks in more detail.

5.2 Model details A Flood Modeller / TUFLOW / ESTRY model of the Afon Lwyd and its tributaries was requested from NRW in December 2020 and has been used under an NRW licence (ATI 19304b), dated 11 January 2021. A model review was carried out to ascertain the confidence of in the model within the vicinity of the proposed development site. This review identified the following key points: • The main source of flood risk to the site is from the Blaen Bran. Water backs up from the culvert under Cwmbran Drive and floods out of the channel in the park land to the west of the road. It then flows down Cwmbran Drive entering the proposed development site in the northwest corner. • There is concrete panel fencing around the culvert inlet that is not represented within the model, but would help to reduce the amount of flooding entering the road. • A slight instability was identified in modelled flood depths along Cwmbran Drive. This was found to be caused by fluctuations in the depth varying Manning’s roughness. Having reviewed the model, two model scenarios were created; • Baseline: the model grid in TUFLOW was updated with topographic survey data for the site and the roughness approach along Cwmbran Drive between Blaen Bran and Maendy Way was changed to a fixed roughness of n=0.02 to reduce the instabilities that were occurring. • Proposed – The updates from the baseline scenario were retained and the model was further updated with the proposed site topography (see Appendix B) and roughness values on site were updated to account for the changes in the land uses. A fixed Manning’s roughness of 0.3 was used for buildings, the playing fields were represented as general surfaces with a variable Manning’s roughness of n=0.3 below 0.1m depth of flow and n=0.03 above 0.2m depth of flow and the hard standing areas, including carparking and footpaths were represented with a variable Manning’s roughness of n=0.3 below 0.1m depth of flow and n=0.02 above 0.2m depth of flow. This is in line with the approach taken for the rest of the model. Finally, due to the impacts of the Blaen Bran culvert under Cwmbran Drive on the flood risk to the site two blockage scenarios were created (for the baseline and development scenarios) to provide an understanding on the flood risk to the site for both the existing (baseline) situation and the proposed development, should the culvert block. A blockage proportion of 67% was applied to the pipe (which was modelled in ESTRY) as per the NRW guidance for a medium risk blockage10. All other components of the modelling have been left as they were in the original model. The model was run for the 1% AEP plus a climate change uplift of 25% to all flows, following the central uplift for the 2080s estimate for the Severn River Basin District

———————————————————————————————————————————— 10 NRW Guidance Note - Flood Risk Management: Modelling blockage and breach scenarios (2015)https://cdn.cyfoethnaturiol.cymru/media/692247/gn43-modelling-for-breach-and-blockage-scenarios- accessible.pdf

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as per the Welsh Government’s guidance11; and the 0.1% AEP events using Flood Modeller version 4.5 and TUFLOW version 2018-03-AE-iDP-w64.

5.3 Model results – baseline This section details the modelled flood extents and depths of the baseline 1% AEP plus climate change and 0.1% AEP flood events.

5.3.1 Baseline – 1% AEP plus climate change In the 1% AEP plus climate change event, the northern extent of the site is generally predicted to flood to no more than 0.3m. A band of flood water crossing the north-eastern corner of the site is predicted to flood to a maximum depths of approximately 0.31m, as shown in Figure 5-1. The current Infant School is partly flooded to a depth of 0.01m.

Figure 5-1 - Baseline 1% AEP + CC Flood Depths

———————————————————————————————————————————— 11 Welsh Government Guidance Note - Guidance for Flood Consequence Assessments: Climate Change Allowances (2016) https://gov.wales/climate-change-allowances-and-flood-consequence-assessments-cl-03-16

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5.3.2 Baseline – 0.1% AEP In this scenario, the northern extent of the site is generally predicted to flood to no more than 0.4m. A band of flood water crossing the north-eastern corner of the site is predicted to flood to a maximum depth of approximately 0.42m, as shown in Figure 5-2. The current Infant School is partly flooded to a maximum depth of 0.02m.

Figure 5-2 - Baseline 0.1% AEP Flood Depths

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5.4 Model results – post development This section details the modelled flood extents and depths of each post-development flood scenario. See section 2.4 for details of the proposed site layout. The proposed school buildings remain flood free in every scenario.

5.4.1 Post-development – 1% AEP plus climate change In this scenario, the northern extent of the site is generally predicted to flood to no more than 0.5m. A band of flood water crossing the northeastern corner of the site is predicted to flood to a maximum depth of approximately 0.55m as shown in Figure 5-3. The proposed school buildings are flood-free.

Figure 5-3 - Post-Development 1% AEP + CC Flood Depths

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5.4.2 Post-development – AEP 0.1% In this scenario, the northern extent of the site is generally predicted to flood to no more than 0.5m. A band of flood water crossing the northeastern corner of the site is predicted to flood to a maximum depth of approximately 0.57m as shown in Figure 5-4. The proposed school buildings are flood-free.

Figure 5-4 - Post-Development AEP 0.1% Flood Depths

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5.5 Model results – Blocked culvert The following runs have been modelled with the culvert on the Blaen Bran under Cwmbran Drive (A4051) 67% blocked, as per the NRW guidance for a medium blockage12. In general the majority of the additional out of bank flows caused by the blockage follow the route of the Blaen Bran in an easterly direction towards the Afon Lwyd and therefore the impact of a blockage at the Cwmbran Drive culvert on flood risk at Maendy Primary School is minimal.

5.5.1 Baseline – 1% AEP plus climate change and blocked culvert In this scenario, the northern extent of the site is generally predicted to flood to no more than 0.3m. A band of flood water crossing the north-eastern corner of the site is predicted to flood to a maximum depth of approximately 0.4m as seen in Figure 5-5. The current Infant School is partly flooded to a maximum depth of 0.015m.

Figure 5-5 - Baseline 1% AEP + CC + Blocked Culvert Flood Depths

———————————————————————————————————————————— 12 NRW Guidance Note - Flood Risk Management: Modelling blockage and breach scenarios (2015)https://cdn.cyfoethnaturiol.cymru/media/692247/gn43-modelling-for-breach-and-blockage-scenarios- accessible.pdf

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5.5.2 Baseline – 0.1% AEP and blocked culvert In this scenario, the northern extent of the site is generally predicted to flood to no more than 0.5m. A band of flood water crossing the north-eastern corner of the site is predicted to flood to a maximum depth of approximately 0.52m as seen in Figure 5-6. The current Infant School is partly flooded to a maximum depth of 0.02m.

Figure 5-6 - Baseline 0.1% AEP + Blocked Culvert Flood Depths

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5.5.3 Post-development – AEP 1% plus climate change and blocked culvert In this scenario, the northern extent of the site is generally predicted to flood to no more than 0.5m. A band of flood water crossing the north-eastern corner of the site is predicted to flood to a maximum depth of approximately 0.56m as seen in Figure 5-7. The proposed school buildings are flood-free.

Figure 5-7 - Post-Development AEP 1% + CC + Blocked Culvert Flood Depths

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5.5.4 Post-development – 0.1% AEP and blocked culvert In this scenario, the northern extent of the site is generally predicted to flood to no more than 0.5m. A band of flood water crossing the north-eastern corner of the site is predicted to flood to a maximum depth of approximately 0.6m (Figure 5-8). The proposed school buildings are flood-free.

Figure 5-8 - Post-Development AEP 0.1% + Blocked Culvert Flood Depths

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5.6 Temporary School Buildings During the construction of the proposed school, it will be necessary to construct temporary school buildings to accommodate the continued teaching of pupils. This temporary school will be located within the northern extent of the Maendy Primary School site (see Figure 5-9) to ensure that there is sufficient room to the southern extent to demolish the existing southern building, store materials and provide access to construct the new ‘Passivhaus’ Primary school, outside of the predicted flood extent. The proposed temporary school buildings are located within the modelled flood extents of the 1% AEP plus climate change and the 0.1% AEP events. Modelled average flood levels within the proposed temporary building footprint do not exceed 67.5m AOD in the 1% AEP + climate change scenario and 67.58m AOD in the 0.1% AEP event. As these building will be portacabin-style constructions with raised floors, it is proposed that the temporary buildings will have a finished floor level of at least 67.7m AOD, above the modelled flood levels. The temporary school building being located to the north allows maintained access through the existing school gates and maintains much of the existing areas for car parking and play. The current Infant School, Reception, and Additional Learning Needs resource base (the northern building) are required to continue operating in the existing building, therefore not allowing the requirements for the six classrooms of children to be accommodated within this building. The temporary school shall be required through the construction phase only. This equates to the September 2021- September 2023 academic years, prior to the completion and handover of the new school building to Torfaen County Borough Council. Upon completion of all works to the southern extent of the site, including the new access from Maendy Way, the temporary school buildings to the north of the site shall be removed, along with the demolition of the existing Infant block. These areas will then be reworked to provide drained, soft and accessible playing areas to the north.

Figure 5-9 - Temporary school building layout

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5.6.1 Temporary buildings – 1% AEP plus climate change In this scenario, the entirety of the temporary building layout is predicted to flood, to a maximum of 0.74m on the access road, as shown in Figure 5-10 below. The footprints of the temporary buildings are predicted to flood to no more than 0.44m. Figure 5-10 - Temporary buildings 1% AEP plus climate change flood depths

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5.6.2 Temporary buildings – 0.1% AEP In this scenario, the entirety of the temporary building layout is predicted to flood, to a maximum of 0.85m on the access road, as shown in Figure 5-11 below. The footprints of the temporary buildings are predicted to flood to no more than 0.5m. Figure 5-11 - Temporary buildings 0.1% AEP flood depths

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5.7 Third party impacts TAN-15 requires the demonstration that a proposed development will not worsen the flood risk to others (third parties). Ground raising risks displacing flood water, either by disrupting flow paths, reducing floodplain storage or a combination of both. Therefore, to assess this risk and determine the impact of predicted flood levels, comparisons of pre-and post- development flood depths for each scenario were undertaken in line with NRW guidance13. Results have been symbolised to show differences of equal to or greater than ±0.05m when compared to the respective baseline scenario.

5.7.1 1% AEP plus climate change Figure 5-12 shows that in this scenario the development causes no significant detriment outside of the site, with small sections to the east of the site boundary experiencing a >0.05m reduction in flood depths. Several areas in the north of the site experience a >0.05m increase in flood depths however this is where the existing ground level has been modified and levelled for the playing fields.

Figure 5-12 - 1% AEP + CC Flood Detriment

———————————————————————————————————————————— 13 Modelling for Flood Consequence Assessments (ref GN028), Natural Resources Wales. January 2020

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5.7.1 0.1% AEP Figure 5-13 shows that in this scenario the development causes no significant detriment outside of the site, with small sections to the east of the site boundary experiencing a >0.05m reduction in flood depths. Several areas of the northern site experience a >0.05m increase in flood depths due to ground levelling as part of the proposed development.

Figure 5-13 - 0.1% AEP Flood Detriment

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5.7.2 1% AEP plus climate change and blocked culvert Figure 5-14 shows in this scenario that the development causes no detriment outside of the site, with small sections to the east of the site boundary experiencing a >0.05m reduction in flood depths. Several areas of the northern site experience a >0.05m increase in flood depths due to ground levelling as part of the proposed development.

Figure 5-14 - 1% AEP + CC + Blocked Culvert Flood Detriment

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5.7.3 0.1% AEP and blocked culvert Figure 5-15 shows that in this scenario the development causes no significant detriment outside of the site, with small sections to the east of the site boundary experiencing a >0.05m reduction in flood depths. Several areas of the northern site experience a >0.05m increase in flood depths due to ground levelling as part of the proposed development.

Figure 5-15 - 0.1% AEP + Blocked Culvert Flood Detriment

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5.7.4 Temporary buildings – 1% AEP plus climate change Figure 5-16 below shows that in this scenario the temporary development causes no significant detriment outside of the site, with small sections to the east of the site boundary experiencing a >0.05m reduction in flood depths. Several areas of the northern site experience a >0.05m increase in flood depths due to ground levelling as part of the temporary development. Figure 5-16 - Temporary Buildings -1% AEP + CC Flood Detriment

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5.7.5 Temporary buildings – 1% AEP plus climate change Figure 5-17 below shows that in this scenario the temporary development causes no significant detriment outside of the site, with small sections to the east of the site boundary experiencing a >0.05m reduction in flood depths. Several areas of the northern site experience a >0.05m increase in flood depths due to ground levelling as part of the temporary development. Figure 5-17 - Temporary Buildings -1% AEP + CC Flood Detriment

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5.8 Assessment of Acceptability Criteria Table 5-1 below details the acceptability criteria required from TAN-15. It is deemed that all criteria have been met by the development.

Table 5-1 - Assessment of TAN-15 Acceptability Criteria

TAN-15 acceptability criteria Comments Achievable? Developer is required to The proposed school buildings remain flood Yes demonstrate that the site is free during the 1% AEP plus climate change designed to be flood free for the event. The proposed playing fields to the lifetime of development for a 1 in northern extent of the site are predicted to 100(1%) chance (fluvial) flood flood in this event. However, these areas event including an allowance for can be designed to be out of use in the climate change, in accordance with event of a flood warning, with suitable Tan-15 Table A1.14. space to the remaining site area to accommodate suitable and safe refuge areas, with access and egress from the site maintained in this event. In respect of the residual risk to The proposed school buildings and access Yes the development it should be road to Maendy Way remain flood free designed so that in an extreme (1 during the 0.1% AEP event. The playing in 1000 chance) event there would fields flood to a localised maximum depth be less than 600mm of water on of 600mm. access roads and within property, the velocity of any water flowing across the development would be less than 0.4m/seconds. No flooding elsewhere Post-development results show that no Yes detriment is caused by the development to third-party land. Flood defences must be shown by The development site does not rely upon Yes the developer to be structurally any formal flood defences in the area. adequate particularly under extreme overtopping conditions (i.e. that flood with a 1 in 1000 chance of occurring in any given year). Effective flood warnings are The site is not located within any specific provided at the site. NRW flood alert areas but is close to the Afon Lwyd at Llanyrafon and Afon Lwyd at Croesyceiliog flood alert areas. The developer must ensure that The building occupants should sign up to Yes future occupiers of the the NRW Flood Warning Service to provide development are aware of the warning in the event of an extreme flood flooding risks and consequences. event. It is recommended that a Flood Emergency Plan is prepared to ensure that future occupiers of the site are aware of the flood risk associated to the playing field areas of the site.

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Escape/evacuation routes are The access/egress road to Maendy Way is Yes shown by the developer to be flood free under all modelled events. operational under all conditions. The development is designed by The proposed school buildings remain flood Yes the developer to allow the occupier free in all events. the facility for rapid movement of goods/possessions to areas away from flood waters. Development is designed to The proposed school buildings remain flood Yes minimise structural damage free in all events. The playing fields should during a flooding event and is flood be designed to drain well to ensure that proofed to enable it to be returned there is no standing water following flood to its prime use quickly in the events. aftermath of the flood.

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6 Conclusions and recommendations • JBA Consulting (JBA) were commissioned by Morgan Sindall Construction & Infrastructure Ltd to prepare a Flood Consequence Assessment (FCA) to support the proposed development application of Maendy Primary School, Cwmbran. • The proposed development is for the demolition of the two existing buildings of Maendy Primary school and construction of a new ‘Passivhaus’ primary school in the centre of the site. • The proposed development lies within Zones A, B, and C2 of NRW’s Development Advice Map (DAM). Zone A are areas not considered to be at risk of flooding. Zone B is defined as “areas known to have been flooded in the past evidenced by sedimentary deposits” and require site levels to be checked against the 0.1% AEP flood extent. Zone C2 is defined as areas of the floodplain without significant flood defence infrastructure. Less vulnerable development can take place in Zone C2 subject to the application of the Justification Test and acceptability of flood consequences. Zone C2 falls across the northern third of the site, with this section of the site intended to be used as playing fields, and the proposed school buildings located in Zone A and Zone B in the southern two-thirds of the site. • The proposed development meets the Justification Test as it will meet Torfaen County Borough Council’s CF3 Policy Objective of the Adopted Local Development Plan (2006-2021), which seeks to improve the quality and accessibility of community facilities. • The site is at high risk of fluvial flooding but low risk of surface water, groundwater, reservoir and sewage flooding. • Detailed hydraulic modelling was undertaken to assess the risk of fluvial flooding to the site. This shows the proposed building remains flood free in all modelled scenarios, while the playing fields in the northern end of the site were flooded in all scenarios. • Detailed hydraulic modelling was undertaken to assess the risk of fluvial flooding to the temporary buildings, situated to the north of the site while construction of the proposed school is being undertaken. This shows the temporary buildings will flood to a maximum of 440mm in the 1% AEP plus climate change and 500mm in the 0.1% AEP events. As these buildings will be portacabin-style constructions, it is proposed that the finished floor levels will be at least 67.7m AOD; above the modelled flood levels. • In all flood events up to and including the 0.1% AEP event, safe access and egress can be provided from the completed new school building via the proposed new access road to Maendy Way. • Due to its location, a Flood Response Plan has been prepared for the temporary building which sets out the contingency measures that have been designed to maximise the safety of persons at the school. Should flooding be considered to be imminent, pupils and staff within the temporary building should relocate to the infant block where flood risk is lowest. From here the evacuation process can then be initiated. • The development would reduce the overall flood risk to the school by moving the school buildings out of DAM Zone C2 and modelled flood extents. • It is recommended that the building owners and occupiers should sign up to the NRW Flood Warning Service to ensure the playing fields are evacuated in the event of an extreme event, and that a Flood Emergency Plan is prepared to ensure that future occupiers of the site are aware of the flood risk associated to the playing field areas of the site.

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• It is concluded that on the grounds of flood risk, the proposed development at Maendy Primary School meets the principles and requirements set out in TAN-15 and the aims of Planning Policy Wales.

As previously noted, parts of the existing school buildings currently lie within the Zone C2 area, and as a result it is the intention that the new development will reduce the flood risk overall and align the school with current flood risk regulations.

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Appendices

A Topographic Survey

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B Proposed site layout

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