Land off Lane, Flood Consequence Assessment

August 2020

www.jbaconsulting.com

Wales & West Housing Tŷ Draig St Davids Park CH5 3DT

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx i

JBA Project Manager Howard Keeble 2nd Floor Mersey Bank House WARRINGTON WA1 1WA

Revision History Status Revision Date Originated Checked Approved Ref/Date J Landells- H Keeble H Keeble Molloy S3 P01 01/05/2020 Technical Technical Assistant Director Director Engineer J Landells- H Keeble H Keeble Molloy A3 C01 03/06/2020 Technical Technical Assistant Director Director Engineer J Landells- K Keating K Keating Molloy A3 C02 Technical Technical Assistant Director Director Engineer

Contract This report describes work commissioned by Emma Hancock, on behalf of & West Housing, by an email dated 31st January 2020. Joseph Landells-Molloy of JBA Consulting carried out this work.

Prepared by ...... Joseph Landells-Molloy MEng

Assistant Engineer

Reviewed by ...... Howard Keeble MPhil BEng BSc CEng CEnv CSci CWEM MICE MCIWEM MCMI IMaPS

Technical Director

Krista Keating BSc MSc CEnv CSci MCIWEM C.WEM

Technical Director

Purpose This document has been prepared as a Draft Report for Wales & West Housing. 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.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx i

JBA Consulting has no liability regarding the use of this report except to Wales & West Housing.

Copyright © Jeremy Benn Associates Limited 2020.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx ii

Carbon Footprint A printed copy of the main text in this document will result in a carbon footprint of 132g if 100% post-consumer recycled paper is used and 168g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex. JBA is aiming to reduce its per capita carbon emissions.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx iii

Contents 1 Introduction 1 1.1 Overview 1 1.2 Scope 1 2 Development Location and Description 1 2.1 Location 1 2.2 Description 2 2.3 Topography 3 3 Review of Flood Risk 4 3.1 Fluvial & Tidal 4 3.1.1 Development Advice Map (DAM) 4 3.1.2 Planning Requirements 5 3.1.3 Breach Scenario 7 3.1.4 NRW Consultation 10 3.2 Pentre Drain South West 11 3.3 Surface Water 12 3.4 Groundwater 13 4 Safe Site Access and Egress 14 5 Outline Drainage Strategy 15 5.1 Sustainable Drainage Systems Standards for Wales 15 5.2 Surface Water Management 16 5.3 SuDS Maintenance 17 5.4 Water Quality 17 6 Foul Drainage 18 7 Conclusions 19 8 Appendices 20 A 2020 Ground Investigation Report 21 B Drainage Layout 22 C Micro Drainage Modelling Results 23 D Greenfield Runoff Rates 24 E Welsh Water Sewer Plan 25 F Welsh Water Response 26 G NRW Consultation 29

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx iv

List of Figures Figure 2-1 - Wider Location Plan 2 Figure 2-2 - Proposed Development Layout 3 Figure 2-3 - Site Topography 4 Figure 3-1 - NRW Development Advice Map (DAM) 5 Figure 3-2 - Summary of TAN15 Policy Requirements (Section 9, TAN15 2004) 6 Figure 3-3 - 2017 Flintshire SFCA Modelled Breach Locations 7 Figure 3-4 - Climate Change 0.5% AEP Tidal Breach Depth Map 8 Figure 3-5 - Climate Change 0.1% AEP Tidal Breach Depth Map 9 Figure 3-6 - Climate Change 0.5% AEP Tidal Breach Velocity Map 10 Figure 3-7 - Flood Risk from Pentre Drain South West 12 Figure 3-8 – NRW’s updated Flood Map for Surface Water 13 Figure 4-1 - Primary Egress Route (fluvial/tidal sources) 14

List of Tables Table 2-1 - Site Description 2 Table 5-1 - Standard S1 Discussion 15 Table 5-2 - Calculated Greenfield Runoff Rates 16 Table 5-3 - Required Volumes of Attenuation 17

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx v

1 Introduction

1.1 Overview This Flood Consequence Assessment (FCA) has been prepared following instruction from Emma Hancock, on behalf of Wales & West Housing, by an email dated 31st January 2020. It aims to demonstrate that the proposed development of 24 residential units at Land off Mancot Lane is compliant with the requirements of both Technical Advice Note 15: Development and Flood Risk1 (TAN15) and Sustainable Drainage Systems Standards for Wales2.

1.2 Scope This FCA has been undertaken with consideration of TAN15 and is based on a desktop review of available information, including tidal River Dee breach mapping from the 2017 Flintshire Strategic Flood Consequence Assessment (SFCA). In addition to reviewing flood risk at the site, an Outline Drainage Strategy has been undertaken to quantify attenuation requirements and inform preparation of the separate SuDS Approving Body (SAB) Pre-Application Form.

2 Development Location and Description

2.1 Location Land off Mancot Lane is located in Mancot, Deeside as indicated by the red line boundary in Figure 2-1. It is bounded by existing residential development to the south and by Mancot Lane to the west which is proposed for site access. Pentre Drain South West is located to the north of the site and adjacent to Road (B5129), it converges with Queensferry Drain and flows northwards before bending eastwards, parallel to A494 and outfalling into the tidal River Dee. It is noted that Flintshire SAB could require discharge to a surface water body (Pentre Drain South West) in accordance with National SuDS Standards for Wales, therefore, third party land ownership issues must be confirmed and considered prior to SAB pre-application. Based on a review of Google Earth, there is one field between areas of site ownership and Pentre Drain South West, equivalent to a horizontal distance of between 125 and 150 metres depending on the alignment of potential conveyance infrastructure through third- party land. It is noted that Pentre Drain South West and Queensferry Drain are NRW Main Rivers.

———————————————————————————————————————————— 1 Technical advice note (TAN) 15: development and flood risk | GOV.WALES 2 https://gov.wales/sites/default/files/publications/2019-06/statutory-national-standards-for-sustainable- drainage-systems.pdf

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 1

Figure 2-1 - Wider Location Plan

2.2 Description Table 2-1 - Site Description

Site Name Land off Mancot Lane Site Area (ha) 1.55 Existing Land Use Greenfield Proposed Land Use Residential Proposed Vulnerability Classification Highly Vulnerable OS-NGR SJ 322 674 Local Planning Authority (LPA) Flintshire County Council

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 2

Figure 2-2 - Proposed Development Layout

It is understood that the developer is proposing to erect 24 residential units (see Figure 2-2), eight of which are bungalows (single-storey). In accordance with TAN15, this constitutes a change of use of the land from greenfield to highly vulnerable development (residential). It is understood that northern areas of the site are to accommodate a children’s play area and associated footpaths, else this area will remain as open green space. It is proposed that SuDS attenuation features are also located within this area.

2.3 Topography In accordance with National SuDS Standards for Wales, if infiltration drainage has been appropriately discounted, Flintshire SAB require that discharge to watercourse be considered. LiDAR indicates that the area falls away to the north east towards the tidal River Dee (see Figure 2-3), however, the gradient of topography shallows within the red line boundary which would likely have implications for gravity discharge of surface water runoff to Pentre Drain South West. In Figure 2-3, the spot level of 5.53 metres Above Ordnance Datum (mAOD) indicates existing ground levels within northern areas of the site where attenuation features are to be located. Associated ground levels for Pentre Drain South West are between 5.22mAOD and 5.10mAOD. Comparison of site levels and Drain levels indicates a difference in levels of approximately 0.43m, equivalent to a gradient of 1 in 470. To maintain greenfield runoff rates surface water attenuation basins will be required. It is highlighted that attenuation features would need to be excavated to provide any

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 3

meaningful storage volume. Therefore, it is not practical for proposed development to discharge via gravity to Pentre Drain South West. A pumped solution would be required. As a result, discharge to ground or sewer should be considered.

Figure 2-3 - Site Topography

3 Review of Flood Risk

3.1 Fluvial & Tidal

3.1.1 Development Advice Map (DAM) The Natural Resources Wales’ (NRW) Development Advice Map (DAM) is presented in Figure 3-1 and indicates that six of the proposed residential properties are located within DAM Zone C1 and a further four in DAM Zone B. DAM Zone maps are based on Natural Resources Wales extreme flood outlines (Zone C) and the British Geological Survey 10k Superficial Geology data (Zone B). It is noted that NRW’s Development Advice Map is based on the modelled undefended scenario. DAM Zone C1 represents "areas of the floodplain which are developed and served by significant infrastructure, including flood defences" and covers 51% of the total site area. Areas in Mancot benefit from the tidal River Dee flood defences as defined in Figure 2-1, therefore, could be at risk from a potential tidal breach scenario. It is noted that the remaining 14 residential units are located within DAM Zone A and “considered to be at little or no risk of fluvial or tidal/coastal flooding” hence the justification test is not applicable and there is no need to consider flood risk further in this location.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 4

Figure 3-1 - NRW Development Advice Map (DAM)

3.1.2 Planning Requirements In accordance with TAN15, new development should be directed away from zone C and towards suitable land in Zone A, otherwise to Zone B, where river or coastal flooding will be less of an issue. However, in Zone C, the tests outlined in sections 6 and 7 (of TAN15) will be applied, recognising, however, that highly vulnerable development and Emergency Services in Zone C2 should not be permitted. It is recognised that a preferred solution would be for residential units to be located within areas of land defined as Zone A (i.e. considered to be at little or no risk of fluvial or tidal/coastal flooding) only, however, this would have negative implications for residential yields. However, given that the River Dee is tidally influenced at this location, land raising to elevate all residential units above flood level should be considered. In accordance with TAN15, for highly vulnerable development in DAM Zone C1, the planning requirements defined in Figure 3-2 (an extract from TAN15) must be demonstrated for proposals to be considered acceptable in terms of flood risk.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 5

Figure 3-2 - Summary of TAN15 Policy Requirements (Section 9, TAN15 2004)

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 6

3.1.3 Breach Scenario Based on the 2017 Flintshire SFCA, the site is not considered to be at risk from the modelled breach scenarios (of the River Dee flood defences) during present day tidal flood events at the locations defined in Figure 3-3 below. Flintshire County Council and NRW specified potential breach locations to be modelled at locations near to some of Flintshire County Council’s concept development sites and at areas that are considered to be more prone to risk, such as outfall structures. It is noted that no further breach modelling has been undertaken as part of this FCA given the proximity of the existing breach locations to the development site.

Figure 3-3 - 2017 Flintshire SFCA Modelled Breach Locations

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 7

In relation to flood risk at Land off Mancot Lane, Figure 3-4 indicates that the proposed development is not at risk of a tidal breach event at Queensferry during the climate change enhanced 0.5% Annual Exceedance Probability (AEP) event. 100 years climate change was used in the 2017 SFCA presumably based on Welsh Government Guidance. Predicted flooding within the wider site boundary, including areas to the north and a highly localised area of the new, dedicated highway is to a depth of less than 300 mm.

Figure 3-4 - Climate Change 0.5% AEP Tidal Breach Depth Map

It is noted that the most extreme modelled flood event was the climate change enhanced 0.1% AEP event which is presented in Figure 3-5. In this event six properties are predicted to flood to a depth less than 300 mm based on existing ground levels. The maximum flood level within residential areas of the development is predicted to be 5.96 mAOD. It is recommended that finished floor levels (FFLs) are set above this level including an allowance for freeboard, assumed to be 200mm at this stage. It is noted that development is usually raised above surrounding ground levels and this shallow flooding could be accommodated within kerb and property threshold levels. To ensure that all property thresholds remain above this level, localised ground raising is required. Ground raising of up to 0.56m, is limited to a 0.29 ha area of the site. Overall, it is estimated that this will result in a relative reduction in flood storage volume within the site of 666m3 at the proposed threshold level of 6.16mAOD. Through the design process, the open green space to the north of the site will be reprofiled to ensure that there is no net reduction in flood storage volume.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 8

Figure 3-5 - Climate Change 0.1% AEP Tidal Breach Depth Map

In accordance with A1.15 of TAN15, velocity of floodwaters should be considered. Predicted outputs from the 2017 SFCA breach scenario modelling for the 0.5% AEP + climate change event are presented in Figure 3-6. This figure highlights that floodwater velocities at the site are generally predicted to be less than 0.1 m/s in northern areas of the site, increasing to 0.2 m/s across a small area at the eastern site boundary. It can be concluded that floodwater velocity and overall flood hazard (taking into account predicted flood depths) are very low.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 9

Figure 3-6 - Climate Change 0.5% AEP Tidal Breach Velocity Map

3.1.4 NRW Consultation Outputs from the NRW consultation exercise are included in Appendix G. The 2017 SFCA breach assessment has been previously undertaken by JBA working with both Flintshire County Council and NRW. At the time this work was undertaken, a method statement was produced and agreed with NRW. This included dialogue between NRW and JBA modellers, whereby there was agreement to assess a consistent set of breach locations and to adopt a standardised geometry for breaches. The breach locations modelled by JBA for the 2017 assessment were selected by Flintshire County Council. The six site locations are the same as those considered in 2015 by NRW, who in addition also considered other breach locations, including Shotton. The differences in technical detail between the 2015 and 2017 assessments have been investigated and are noted below: • The volume of floodwater passing into the floodplain will be sensitive to the way defence breaches are modelled. The 2015 assessment completed by NRW considered two alternative approaches; firstly, breaches being present in the Tidal Dee embankment from the start of the model run and secondly, breaches forming and developing during a tidal event. NRW have reported the conservative set of levels based on a breach being present from the start of the run. The 2017 outputs represent the alternative scenario whereby a breach develops during the tidal event. Given the condition of the Tidal Dee defences, their residual life, and the assigned ‘Hold The Line’ policy within the North West England and Shoreline Management Plan 2, this representation is considered appropriate and is justified. This variation in modelling approach will have a significant impact on the predicted flood extent and levels within the Tidal Dee floodplain and is expected to be the main reason for the difference in outputs between the two assessments.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 10

• Updated defence level representation drawing on the survey collected in 2015 as part of the North Wales Tidal Defence Survey project. • Updated representation of floodplain topography (drawing on the most recent Digital Terrain Model), enhanced representation of floodplain structures and reconfiguration of 2D roughness values set using OS MasterMap data. In their consultation letter, NRW reference the outputs from the 2015 model, but given the above updates made in 2017, that have improved the definition of the modelling and the justification provided, it is suggested that it would be more appropriate to reference and use the 2017 outputs. Furthermore, since the time that both assessments were completed, the Coastal Flood Boundary dataset used to define extreme tidal levels in the River has been updated. At the mouth of the estuary the change in datasets is -0.09m and -0.1m for the 0.5% and 0.1% AEP event tidal levels respectively (at chainage 1152m). In this respect, given the reduction in extreme sea levels, the predicted 2017 levels will be conservative.

3.2 Pentre Drain South West Pentre Drain South West is located approximately 115m north of the site boundary and approximately 200m north of development. Given the local topography, the watercourse will not pose a fluvial flood risk to the site. Based on LiDAR data, the lowest flow route into the site boundary has an elevation of 5.43 mAOD. The contoured local topography, shown in Figure 3-7, illustrates that if the channel capacity of Pentre Drain South West was exceeded (either as a result of an extreme event or blockage), this would cause localised flooding in the field between the development site and the watercourse. However, the general ground topography would mean that the predominant flow route for floodwater would be northward, over the existing highway and away from the site. Ground levels along Chester Road at the junction with Mancot Lane based on LiDAR data are 0.1m lower than the minimum level into the site (of 5.43 mAOD) and are 0.83m lower than the proposed threshold level of 6.16mAOD. Furthermore, to support the conclusion that the development site is not at risk of flooding from Pentre Drain South West, an assessment of the incoming catchment area and local floodplain storage volumes has been undertaken. The FEH Web Service indicates that the incoming catchment area is small at approximately 0.61km2, although it is acknowledged that the general flat low-lying topography will introduce greater uncertainty in this and other local catchment boundaries. Based on a level of 5.43 mAOD a potential flood storage volume of over 7,500m3 is estimated in the fields to the north of the site before overtopping northwards over Chester Road is initiated.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 11

Figure 3-7 - Flood Risk from Pentre Drain South West

3.3 Surface Water NRW’s updated Flood Map for Surface Water (uFMSW) is presented in Figure 3-8 and indicates that the proposed development is at low risk if surface water flooding, however, there is an area in the 3.33% (high risk) and 1% AEP (medium risk) of surface water ponding in the northern area of the site which is to remain free from significant hardstanding development. It is noted that surface attenuation features could be sized with additional capacity to accommodate this area of ponding. It is understood that northern areas will accommodate a children’s play area and associated footpaths, and surface attenuation features. It is noted that the children’s play area should be designed to take surface water risks into account. SuDS and attenuation features should be formed outside of this mapped surface water flood extent to prevent inundation and interaction with off-site impacts.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 12

Figure 3-8 – NRW’s updated Flood Map for Surface Water

3.4 Groundwater Based on a 2020 ground investigation report for this proposed development (refer to Report No. 19WWH002/GI), shallow groundwater was encountered across the site. In trial pits of depths varying between 1.40 and 1.60 metres below ground level (mbgl), there was observed slow groundwater ingress at the base of each pit rising to up to 1.00 mbgl. In Section 16.3 (Control of Groundwater), the report recommends that further site investigation will be required if sewer construction is proposed at a greater depth than those of the trial pits. Whilst this does not constitute a risk to development levels, it will influence soakaway potential and the potential depth of surface water attenuation facilities.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 13

4 Safe Site Access and Egress Based on the proposed development layout, the majority of properties are located on a new, dedicated highway off Mancot Lane, with six properties accessed directly off Mancot Lane itself. 2017 SFCA modelled breach scenarios indicate that the site can be safely accessed and egressed via Mancot Lane heading south-west during all flood events, however, the proposed new highway within the development is predicted to flood to depths less than 300 mm in the climate change 0.1% AEP event based on existing ground levels. It could be necessary to land raise both properties and road levels above this predicted flood extent to enable safe access and egress during all flood events. Based on the uFMFSW, Mancot Lane is at medium risk of flooding, equivalent to the 1% AEP event, to the north and south of the site, however, this flooding is largely contained within kerb lines, therefore, depths of flooding on the road are likely to be relatively shallow and pavements are unlikely to flood. In the event of an emergency, this depth of flooding is unlikely to prevent emergency access to the properties. It is, therefore, recommended that a safe access and egress route following Mancot Lane to the south as defined in Figure 4-1 below is proposed during a tidal breach scenario. Under surface water flood conditions this route is dry in the 3.33% AEP event.

Figure 4-1 - Primary Egress Route (fluvial/tidal sources)

It is noted that the areas of the site that are located within DAM Zone C1 are also located within the NRW Flood Warning Area: Bank. It is recommended that the six properties in Zone C1 sign up to the NRW Flood Warning Service.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 14

5 Outline Drainage Strategy It is proposed that surface water runoff will be attenuated in a detention basin to greenfield runoff rates (with discharge to the adjacent existing surface water sewer beneath Mancot Lane). However, in accordance with Sustainable Drainage Systems Standards for Wales3, surface water runoff destination, in order of priority, is as follows: collected for use, infiltrated to ground, discharged to a surface water body, discharge to a surface water sewer and finally, discharge to a combined sewer; as much of the runoff as possible (subject to technical or cost constraints) should be discharged to each destination before a lower priority destination is considered, this is discussed in more detail in Table 5-1. Table 5-1 - Standard S1 Discussion

Priority Level Surface Water Runoff Comments Destination 1 Collected for use Demand for non-potable water is limited 2 Infiltrated to ground 2020 ground investigation concluded that existing ground conditions are unsuitable for soakaway 3 Discharged to a surface water Discharge to Pentre Drain body South West discounted based on levels (see Section 2.3), where proposed detention basin bed levels (including 250mm freeboard) are too low for discharge to the Drain 4 Discharged to a surface water Discharge to a surface water sewer, highway drain, or sewer is proposed based on a another drainage system fully attenuated system Further consultation is required 5 Discharged to a combined N/A sewer

It is noted that the potential site demand for non-potable water is limited. Based on Report No. 19WWH002/GI (see Appendix A) - a 2020 ground investigation report at the site including infiltration testing, concluded that due to the presence of a significant depth of cohesive strata, standing surface water, and high groundwater tables, the use of infiltration is not deemed a suitable surface water drainage option. A provisional drainage layout drawing is provided in Appendix B. Micro Drainage modelling results are provided in Appendix C.

5.1 Sustainable Drainage Systems Standards for Wales This Outline Drainage Strategy has been undertaken in accordance with SuDS Standards for Wales, however, it remains a high-level strategy that will need to be reviewed as part of a formal FCA submission. To prevent development worsening flood risk elsewhere, surface water runoff must be managed on site.

———————————————————————————————————————————— 3 https://gov.wales/sites/default/files/publications/2019-06/statutory-national-standards-for-sustainable- drainage-systems.pdf

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 15

In accordance with SuDS Standards for Wales, proposed measures for surface water management must comply with the following criteria: Table G2.1 (Interception of runoff) Interception methods that can be assumed to be compliant for zero runoff for the first 5mm rainfall for 80% of events during the summer and 50% in winter: Detention basins Areas of the site drained to detention basins with a flat unlined base (without specific provision for routing low flows directly to the outlet) can be assumed to comply where the drained impermeable surface area is less than 5 times the vegetated surface area receiving the runoff for any soil type. Runoff volume control Where controlling runoff to greenfield volumes is considered unachievable, then the runoff volume should be reduced as much as possible and any additional volume should be stored and released at a low rate which will not increase downstream flood risk (normally 2 l/s/ha is considered an appropriate rate) using either of the following approaches: 1. The additional runoff volume (i.e. the difference between the predicted development runoff volume and the estimated greenfield runoff volume, often called Long-Term Storage) should be discharged from the site at a rate of 2 l/s/ha or less, while still allowing greenfield runoff peak flow rates to be applied for the greenfield runoff volume. 2. All the runoff from the site for the 1:100 year event should be discharged at either a rate of 2 l/s/ha or the average annual peak flow rate (i.e. the mean annual flood, QBAR), whichever is the greater.

5.2 Surface Water Management Based on an a calculated impermeable area of 0.494 ha and calculated greenfield runoff rates (see Table 5-2 below and Appendix D), Table 5-3 defines the required volumes of attenuation for critical flood events. Table 5-2 - Calculated Greenfield Runoff Rates

Rainfall Event (% AEP) Greenfield Runoff Rate (l/s) Qbar 2 100 2 3.33 4 1 5 0.5 5

In accordance with SuDS Approval Body (SAB) requirements, all the runoff from the site for the 1% AEP event is assumed to discharge at the average annual peak flow rate (i.e. the mean annual flood, Qbar). Further to this, the invert area of the proposed detention basin(s) is assumed to be 0.099 ha (approximately 6% of the total site area) where the drained impermeable surface area (assumed to be 0.494 ha) is less than 5 times the vegetated surface area receiving the runoff (0.099 ha). In accordance with SuDS Standards for Wales and the required storage for the 3.33% AEP + 40% CC event (see Table 5-3), the depth of the proposed basin should be 0.325 m excluding any freeboard allowance to attenuate the design event. At this stage, a 250mm freeboard is envisaged.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 16

Table 5-3 - Required Volumes of Attenuation

Design flood Critical Inflow Outflow Attenuation Time to event storm volume volume required empty (incl climate duration m3 m3 m3 (assuming change) Hrs no infiltration) Hrs 3.33% AEP + 5.5 298 69 229 18.1 20% CC

3.33% AEP + 6.5 362 82 280 22.2 40% CC

1% AEP + 20% 6.5 414 82 332 (103 26.3 CC exceedance storage) 1% AEP + 40% 7.5 497 95 403 (123 31.9 CC exceedance storage)

It is noted that discharge to the existing surface water sewer beneath Mancot Lane is subject to Welsh Water consultation and confirmation. Whilst initial discussions were undertaken by the client and Welsh Water, the results to Ground Investigation were still pending.

5.3 SuDS Maintenance 1 in 3 side slopes to the detention basin have been assumed for safe access for operatives. Further to this, the basin is shallow (approx. 500mm) and designed to be dry under normal conditions – reducing potential risk to operatives It is assumed that the detention basin will be managed and maintained by a third-party appointed by the developer. Owing to the proximity of the proposed playground facilities, the basin will need to be designed for safety and egress. This may include consideration of barrier planting, clear pathways, shallow banks and fencing if required.

5.4 Water Quality In meeting S2 interception requirements, this has reduced the number of runoff events discharged from the site, therefore, reducing contaminant loadings. Based on the proposed land use type, the Pollution Hazard Level is classified as Low. It is recognised that Welsh Water may have additional water quality requirements. Using the Simple Index Approach, as defined in CIRIA’s The SuDS Manual, a detention basin is considered to provide sufficient pollution mitigation for the proposed land use type. It is proposed to set the invert level of the basin outfall above the basin invert to provide the potential to provide adequate residency time for the first 5mm of rainfall.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 17

6 Foul Drainage It is noted that the proposed development site is crossed by a 225mm and 300mm diameter foul gravity public sewer which has been incorporated into the development layout to include for an appropriate standoff of three metres either side. Welsh Water’s sewer network plan is provided in Appendix E. Based on previous consultation with Welsh Water (see Appendix F), it is understood that it is unlikely that the local public sewerage system has sufficient capacity for the anticipated foul flows from the development. However, Welsh Water indicated that foul flows could be pumped to the siphon which is located to the west of the site. Consent for this connection is not yet in place. Development plans include for a pumped discharge (see Figure 2-2) – no further reference is made in this FCA. Further to this, Welsh Water indicated that no problems are envisaged with the Waste Water Treatment Works for the treatment of domestic discharges from this site.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 18

7 Conclusions This Flood Consequence Assessment (FCA) has been prepared following instruction from Wales & West Housing, by an email dated 31st January 2020. 24 residential units, eight of which are bungalows (single-storey) are proposed for the Land off Mancot Way site. In accordance with TAN15, this constitutes a change of use of the land from greenfield to highly vulnerable development (residential). It is understood that northern areas of the site are to accommodate a children’s play area and associated footpaths, else this area will remain as open green space. SuDS features are proposed to be accommodated here also. NRW mapping indicates that six of the proposed residential plots are located within DAM Zone C1 and a further four are located in DAM Zone B. DAM Zone C1 could be at risk from a potential tidal breach scenario (River Dee). Flintshire County Council undertook tidal Dee breach modelling as part of the 2017 SFCA. As part of this study, the most extreme modelled flood event was the climate change 0.1% AEP and, under this event, six properties are predicted to flood to a depth less than 300 mm based on existing ground levels. In this event, the maximum flood level within residential areas of the development is predicted to be 5.96 mAOD. It is recommended to set finished floor levels (FFLs) above this level including allowance for freeboard, assuming to be 200mm at this stage. To ensure that all property thresholds remain above this level, localised ground raising is required. Ground raising of up to 0.56m, is limited to a 0.29 ha area of the site. Overall, it is estimated that this will result in a relative reduction in flood storage volume within the site of 666m3 at the proposed threshold level of 6.16mAOD. Through the design process, the open green space to the north of the site will be reprofiled to ensure that there is no net reduction in flood storage volume. NRW’s uFMSW indicates that the proposed development is at low risk if surface water flooding, however, localised areas of flooding are to be avoided in terms of attenuation features. 2020 ground investigations highlighted that groundwater could impact excavations to depths greater than 1.60 mbgl. Whilst this does not constitute a risk to development levels, it will influence soakaway potential and the potential depth of surface water attenuation facilities. A safe access and egress route are achievable via Mancot Lane in a south-westerly direction under tidal breach conditions. It is proposed to pump foul flows to a siphon located to the west of the site. Infiltration testing has been discounted based on 2020 ground investigations, therefore, in accordance with SAB requirements, runoff from the site for the 1% AEP event is to be attenuated in surface attenuation features and assumed to discharge at Qbar to a public surface water sewer beneath Mancot Lane (subject to Welsh Water consultation and confirmation). Discharge to Pentre Drain South West has been discounted based on levels, where proposed detention basin bed levels (including 250mm freeboard) are too low for discharge to the Drain. (See Table 5-1 summary) In conclusion, the site can be safely developed providing that units are raised to a safe development level about the extreme climate change 0.1% AEP tidal breach event and that surface water is fully attenuated on site.

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 19

8 Appendices A – 2020 Ground Investigation Report B - Drainage Layout C – Micro Drainage Modelling Results D - Greenfield Runoff Rates E – Welsh Water Sewer Plan F – Welsh Water Response G – NRW Consultation

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 20

A 2020 Ground Investigation Report

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 21

B Drainage Layout

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 22

C Micro Drainage Modelling Results

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 23

D Greenfield Runoff Rates

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 24

E Welsh Water Sewer Plan

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 25

F Welsh Water Response

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 26

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 27

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 28

G NRW Consultation

DFJ-JBAU-XX-XX-RP-Z-0001-A3-C02-Flood_Consequence_Assessment.docx 29

Ein cyf/Our ref: CAS-117178-J8B6 Eich cyf/Your ref: N/A

Maes Y Ffynnon, Penrhosgarnedd, Bangor, Gwynedd LL572DW

Justin Paul ebost/email: J10 Planning [email protected] 1 - 3 Upper Eastgate Row Ffôn/Phone: 03000 65 4114 Eastgate Row North Chester CH1 1LQ By email: [email protected]

09/07/2020

Dear Justin Paul,

STATUTORY PRE-APPLICATION CONSULTATION - TOWN AND COUNTRY PLANNING (DEVELOPMENT MANAGEMENT PROCEDURE) (WALES) ORDER 2012 AS AMENDED.

PROPOSAL: Proposed residential development for 25 no. affordable dwellings, public open space with new pedestrian links, landscaping, means of highway access and pumping station

LOCATION: Land adjacent to Mancot Lane, Flinsthire, CH5 2AJ

Thank you for providing a requisite notice to us under Article 2D of the above Order. We received a copy of your proposed application on 18/06/2020.

Based on the information provided, we have significant concerns with the proposed development. To overcome these concerns, we would recommend to the planning authority that the following requirements should be met before permission is granted and the condition listed below is attached to the permission. Otherwise, we would object to the planning application.

Requirement (Flood Risk) 1: updated Flood Consequences Assessment.

Requirement (Foul Drainage) 2: further information is required to demonstrate that either the foul drainage will be discharged to the main sewerage system or that it is not reasonable to connect to the mains.

Condition (Protected Species): submission and implementation of an approved ecological compliance audit (ECA) scheme to the satisfaction of the LPA.

Croesewir gohebiaeth yn y Gymraeg a’r Saesneg Correspondence welcomed in Welsh and English

Flood risk

The site lies partially within Zone C1 as defined by the Development Advice Map (DAM) referred to under Technical Advice Note (TAN) 15 Development and Flood Risk (July 2004), and is shown to be located partially within the 0.5% (1 in 200) and 0.1% (1 in 1,000) annual exceedance probability (AEP) event flood outlines on the Natural Resources Wales Flood Risk Map.

The development proposal is for the construction of 24 dwellings at the site, which is currently greenfield.

A Flood Consequences Assessment (FCA) has been prepared in support of the statutory pre-application consultation by JBA Consulting (ref: DFJ-JBAU-XX-XX-RP-Z-0001-A3-C01- Flood_Consequence_Assessment.docx, May 2020). We provide the following comments on the FCA:

1. The FCA has identified tidal flood risk from the River Dee as the primary source of flood risk at the site. The FCA has assessed a breach scenario located at Queensferry, using the outputs of the Flintshire County Council Strategic Flood Consequences Assessment (SFCA) (2017), which JBA also prepared. Based on the SFCA outputs, there is some flooding of the site in the 0.5% AEP breach event with an allowance for climate change, limited to the public open space area and a small section of the highway. Flood depths are shown to be 300 mm or less. A design flood level for this event is not provided. The FCA proposes to set finished floor levels 200 mm above the flood level for the 0.1% AEP breach event with an allowance for climate change (5.96 m AOD), which would be 6.16 m AOD.

However, we have some concerns regarding the data used for the breach scenario within the FCA, and at this stage are unable to fully comment on the acceptability of the proposed mitigation measures. Based on the outputs from our Tidal Dee Flood Mapping Study (2015), the flood extent for a breach at Queensferry is greater than the outline provided within the FCA, with a flood level of approximately 6.23 m AOD based on the worst-case modelled breach scenario for the 0.5% AEP breach event with an allowance for climate change. It is not clear if this is due to a slightly different breach location or modelling methodology. Additionally, it does not appear that Queensferry is the most sensitive breach location for the development site. A breach at Shotton results in higher flood levels, in the region of 6.47 m AOD for the 0.5% AEP breach event with an allowance for climate change, based on our 2015 model. We would therefore welcome clarification as to why there is such a difference between the SFCA outputs and our outputs. We would typically expect a conservative approach to be taken, which uses the worst-case breach flood level for the site. In this case this appears to be the Shotton breach from our Tidal Dee Flood Mapping Study. If this FCA were to be submitted in support of a planning application, we would raise significant concerns, as it does not appear that the correct design flood level has been used to inform the mitigation measures. The FCA needs to be updated to address this issue, and to re-evaluate the proposed mitigation measures based on the worst-case breach scenario for the site. In order to comply with A1.14 of TAN15,

www.naturalresourceswales.gov.uk www.cyfoethnaturiolcymru.gov.uk Page 2 of 7

our expectation is that the site is designed to be flood free in the 0.5% AEP breach event with an allowance for climate change.

2. The FCA has not assessed the potential flood risk posed to the site from Pentre Drain, including the potential impact that culvert blockage could have on the site. The FCA should be updated to include this.

3. In relation to A1.15 of TAN15, the FCA states that the flood depths along the internal access are expected to be below 300 mm for the 0.1% AEP breach event with an allowance for climate change. However, as outlined above, there is some uncertainty as to the modelled data used to inform the FCA. The flood levels for the 0.5% AEP breach events with an allowance for climate change from our Tidal Dee Breaches Study (2015) for the Queensferry and Shotton breaches are higher than the level quoted from the SFCA for the 0.1% AEP event (5.96 m AOD). This section of the FCA also needs updating to reflect the above, so that it can established whether the proposal complies with A1.15 of TAN15. It should also be noted that we would expect flood velocity data to be provided when assessing compliance with A1.15 of TAN15, which is not currently presented in the FCA.

4. The FCA states the critical flood risk at the site is “tidal...therefore, ground level raising is acceptable without increasing flood risk elsewhere”. Based on the current proposal, some minor land raising within the flood extent for the 0.1% AEP breach event with an allowance for climate change would be required, but the FCA should still comment on the potential impacts of this. However, it is likely that additional mitigation measures will be required to mitigate the correct design flood event, which would result in additional ground raising/reprofiling. The impact of the proposals on flood risk elsewhere will therefore need to be re-assessed if further ground raising is required.

To summarise the above, if this FCA were submitted in support of a planning application we would raise significant concerns. The FCA should be updated to address the above points. The primary issue is that the worst-case breach scenario needs to be used as the design event for the 0.5% AEP breach event with an allowance for climate change, and the mitigation measures will need to be revisited to demonstrate that the site is designed to be flood free in this event. The updated FCA also needs to include an assessment of the flood risk posed by Pentre Drain, and provide the data required to assess compliance with A1.15 of TAN15. Finally, the assessment relating to the impact of the proposals on flood risk elsewhere needs to be revisited if the proposed mitigation measures are amended.

Foul Drainage

The location of the development is in Dŵr Cymru Welsh Water’s (DCWW) sewage catchment and therefore we would expect the site to connect to the mains sewerage system. The drainage plans state that a connection to DCWW’s network has not been confirmed. A connection needs to be confirmed to ensure drainage is dealt with appropriately.

www.naturalresourceswales.gov.uk www.cyfoethnaturiolcymru.gov.uk Page 3 of 7

We refer you to Welsh Government Circular 008/2018 on the use of private sewerage in new development, specifically paragraphs 2.3 - 2.5 which stress the first presumption must be to provide a system of foul drainage discharging into a public sewer. Only where having taken into account the cost and/or practicability it can be shown to the satisfaction of the local planning authority that connection to a public sewer is not feasible, should non-mains foul sewage disposal solutions be considered.

We therefore advise that you should thoroughly investigate the possibility of connecting to the foul sewer by taking the following steps:

• Formally approach the sewerage undertaker regarding a connection under Section 106 or a requisition under Section 98 of the Water Industry Act (WIA) 1991. • Serve notice for connection under Section 106 of the WIA 1991 if the sewerage undertaker has refused connection. • Provide details of the reasons given by the sewerage undertaker if it has refused connection under section 98 or section 106 of the WIA 1991 and confirmation that they have appealed against this decision. • Demonstrate that it is not reasonable to connect to the public foul sewer. • Where it is not reasonable to connect to the public foul sewer, demonstrate that they have considered requesting that the sewerage undertaker adopt their proposed system.

You should be aware that should a connection to the mains sewer not be feasible, you will also need to demonstrate that the proposal would not pose an unacceptable risk to the water environment. Welsh Government Circular 008/2018 advises that a full and detailed consideration be given to the environmental criteria listed under paragraph 2.6 of the Circular, in order to justify the use of private sewerage.

Should a connection to the mains sewer not be feasible, you will need to apply for an environmental permit or register an exemption with us. As stated above, we expect developers discharging domestic sewage to connect to the public foul sewer where it is reasonable to do so. We will not normally grant a discharge permit for a private sewage treatment system where it is reasonable to connect to the public foul sewer. We also expect discharges of trade effluent to connect to the public foul sewer where it is reasonable to do so and subject to the sewerage undertaker granting a trade effluent consent or entering into a trade effluent agreement.

Septic tanks and small sewage treatment works may be registered as exempt from the requirement to obtain an environmental permit if certain criteria are met. Please note, should a permit be required, further information may be required as part of that application and the Applicant is therefore advised to hold pre-application discussions with our Permitting Team on 0300 065 3000, at the earliest opportunity, to try to ensure that there is no conflict between any planning permission granted and the permit requirements. It is important to note that a grant of planning permission does not guarantee that a permit will be granted, should a proposal be deemed to be unacceptable (either because of environmental risk or because upon further investigation, a connection to mains sewer was feasible). The

www.naturalresourceswales.gov.uk www.cyfoethnaturiolcymru.gov.uk Page 4 of 7

Applicant should ensure that they have all the required permissions, consents, permits and any other approvals in place prior to commencement of works on the site.

More information, including a step by step guide to registering and the relevant application forms are available on our website. Where private sewage treatment/disposal facilities are utilised, they must be installed and maintained in accordance with British Standard 6297 and Approved Document H of the Building Regulations. We also refer the Applicant to Guidance for Pollution Prevention 4 on the NetRegs website, which provides further information.

Please note, lack of capacity or plans to improve capacity in the sewer is not a valid reason for a sewerage undertaker to refuse connection under Section 106 of the Water Industry Act 1991 and we may refuse to issue an environmental permit for private treatment facilities in such circumstances.

Protected Species

We have reviewed the following ecological submission: Roe, R. (2018); Land at Mancot Lane, Mancot, Flintshire; Kingdom Ecology (Unpublished).

We advise that any future consent includes the imposition of a condition that requires the submission and implementation of an approved ecological compliance audit (ECA) scheme to the satisfaction of the LPA.

The purpose of the Ecological Compliance Audit is to evidence compliant implementation of all ecological avoidance, mitigation and compensation works, either proposed or subject to the provisions of reserved matters conditions. The Audit shall identify Key Performance Indicators (KPI’s) that are to be used for the purposes of assessing and evidencing compliant implementation of proposals.

We consider biosecurity to be a material consideration owing to the nature and location of the proposal. Ecological surveys confirmed the presence of Himalayan balsam. This species is listed under the provisions of the Invasive Alien Species (Enforcement and Permitting) Order 2019.

We therefore advise that any consent includes the imposition of a condition requiring the submission and implementation of a Biosecurity Risk Assessment to the satisfaction of the LPA.

We consider that this assessment must include: (i) appropriate measures to control any INNS on site; and (ii) measures or actions that aim to prevent INNS being introduced to the site for the duration of construction and operational phases of the scheme.

Land Contamination

The following reports have been reviewed:

www.naturalresourceswales.gov.uk www.cyfoethnaturiolcymru.gov.uk Page 5 of 7

• Caulmert Ltd, Phase 1 Geo-environmental Desk Study Report, Mancot Lane, Deeside, Ref 3480-CAU-XX-XX-RP-V-0300.A0.C1 Dated October 2018 • Betts Geo, Ground Investigation Report for Mancot Lane, Mancot, Deeside, 19WWH002/GI Dated February 2020

We have no adverse comments to provide and consider that the controlled waters at this site are not of the highest environmental sensitivity, therefore we will not be providing detailed site-specific advice or comments with regards to land contamination issues for this site.

It is recommended that the requirements of Planning Policy Wales and the Guiding Principles for Land Contamination (GPLC) should be followed.

These comments are based on our assumption that gross contamination is not present at this location. If, during development, gross contamination is found to be present at the site the Local Planning Authority may wish to re-consult Natural Resources Wales.

Other Matters

Please note, if further information is prepared to support an application, it may be necessary for us to change our advice in line with the new information.

Our comments above only relate specifically to matters included on our checklist, Development Planning Advisory Service: Consultation Topics (September 2018), which is published on our website. We have not considered potential effects on other matters and do not rule out the potential for the proposed development to affect other interests.

In addition to planning permission, you are advised to ensure all other permits/consents/licences relevant to the development are secured. Please refer to our website for further details.

Further advice on the above matters could be provided prior to your planning application being submitted, however there would be a charge for this service. Additional details are available on our website.

Advice for the Developer

Pollution Prevention

During the construction phase you should take any precaution to prevent contamination of surface water drains and local watercourses. Oils and chemicals should be stored in bunded areas and spill kits should be readily available in case of accidental spillages. For further guidance please refer to GPP 5 and PPG 6 at the following link: http://www.netregs.org.uk/environmental-topics/pollution-prevention-guidelines-ppgs-and- replacement-series/guidance-for-pollution-prevention-gpps-full-list/

www.naturalresourceswales.gov.uk www.cyfoethnaturiolcymru.gov.uk Page 6 of 7

Environmental Management

All works at the site must be carried out in accordance with PPG6: ‘Working at construction and demolition sites’ which are available on the following website: http://www.netregs.org.uk/environmental-topics/pollution-prevention-guidelines-ppgs-and- replacement-series/guidance-for-pollution-prevention-gpps-full-list/

The activity of importing waste into the site for use as, for example hardcore, must be registered with Natural Resources Wales as an exempt/permittable activity under the Environmental Permitting Regulations 2016. The developer should contact Natural Resources Wales to discuss the necessity for an exemption/permit for any material imported to and exported from the site.

If during construction/excavation works any contaminated material is revealed, then the movement of such material either on or off site should be done in consultation with Natural Resources Wales. Any waste excavation material or building waste generated in the course of the development must be disposed of satisfactorily and in accordance with Section 34 of the Environmental Protection Act 1990. Carriers transporting waste from the site must be registered waste carriers and movement of any Hazardous Waste from the site must be accompanied by Hazardous waste consignment notes.

If you have any queries on the above, please do not hesitate to contact us.

Yours sincerely,

Tristan Williams Cynghorydd - Cynllunio Datblygu / Advisor - Development Planning Cyfoeth Naturiol Cymru / Natural Resources Wales

www.naturalresourceswales.gov.uk www.cyfoethnaturiolcymru.gov.uk Page 7 of 7

Offices at Coleshill Doncaster Dublin Edinburgh Exeter Glasgow Haywards Heath Isle of Man Limerick Newcastle upon Tyne Newport Peterborough Saltaire Skipton Tadcaster Thirsk Wallingford Warrington

Registered Office 1 Broughton Park Old Lane North Broughton SKIPTON North Yorkshire BD23 3FD United Kingdom

+44(0)1756 799919 [email protected] www.jbaconsulting.com Follow us:

Jeremy Benn Associates Limited

Registered in England 3246693

JBA Group Ltd is certified to: ISO 9001:2015 ISO 14001:2015 OHSAS 18001:2007