Flood Risk Assessment

FLOOD RISK ASSESSMENT

LAND AT DEE BANK INDUSTRIAL ESTATE BAGILLT

Job No. 203021/FRA July 2020

AA Environmental Ltd Units 4 to 8 Cholswell Court Shippon, Abingdon Oxon OX13 6HX T01235 536042 F01235 523849 [email protected] Wwww.aae-ltd.co.uk

Report for: GTR Aggregates and Recycling Ltd GTR Aggregates and Dee Bank Industrial Estate Recycling Limited Boot End Bagillt Dee Bank Industrial Estate Flintshire Bagillt CH6 6HJ Issued by

…………………….. Kristian Wood MSc BSc (Hons) Reviewed by

…………………….. Ed Brown BSc (Hons) Approved by

…………………………………….. Matthew Lawman MSc BSc (Hons) Issue Date

8th July 2020

Issue AA Environmental Limited (Registered Office) Final Revision A Units 4 to 8 Cholswell Court Shippon, Abingdon Oxon OX13 6HX T 01235 536042 F 01235 523849 E [email protected] W www.aae-ltd.co.uk

Company No. 84743322

Table of Revisions

Issue Description of status 16/04/2020 First version 15/06/2020 Revision A – include comments from Natural Resources Wales

AA Environmental Limited Bagillt WTS 203021 Flood Risk Assessment

Contents Page No

1.0 INTRODUCTION 1

2.0 SITE DESCRIPTION AND BASELINE CONDITIONS 2

3.0 FLOOD RISK ASSESSMENT AND MITIGATION 3

4.0 CONCLUSIONS 6

DRAWINGS

203021/D/001 Site Location Plan

203021/D/002 Existing Site Layout Plan

203021/D/004 Revision A Proposed Site Layout Plan

203021/PH1 Flood Zone Extent Plan

APPENDICES

Appendix A Surface Water Drainage Strategy (P1, MJA Consulting Ltd, July 2020)

Appendix B Flint Point to Greenfield Initial Assessment Study (JBA, January 2015)

AA Environmental Limited Bagillt 203021 Flood Risk Assessment

1.0 INTRODUCTION

Overview

1.1 AA Environmental Limited (AAe) has been commissioned by GTR Aggregates and Recycling Ltd (GTR) to produce a Flood Risk Assessment (FRA) in support of the proposed change in operation and infrastructure at land at Dee Bank Industrial Estate, Bagillt.

1.2 The waste recycling facility covers an area of approximately 0.86 hectares and comprises of an open yard, with 5 buildings and several concrete bays atop concrete surfacing with areas of hardcore surfacing. The Dee Bank Industrial Estate borders the site to the north west and a disused pumping station to the south east, the main Chester to Holyhead railway line atop a bund to the north east, with agricultural fields and the Dee Estuary tidal flats beyond. The A548 road runs parallel to the south west boundary. The site location is shown in 203021/D/001 and existing contours are shown in Appendix A.

1.3 In compliance with the Planning Policy Wales (PPW), this report assesses flood risks associated with all types of flooding, to and from the development, and demonstrates how these risks have been managed through design and mitigation controls.

Methodology and Data Sources

1.4 The guidance for Flood Risk Assessment is set out in Technical Advice Note 15: Development and Flood Risk and LPGN 29 Management of Surface Water for New Development. The scope and detail of the assessment should be appropriate to the scale and potential impact of the development.

1.5 Further guidance is provided in CIRIA Research Paper 6241.

1.6 For the purposes of the assessment, data has been obtained from the following sources:

• Natural Resources Wales' (NRW) Flood Risk Map Viewer online resource (Detailed Map View & Development Advice Map); • DEFRA Magic Online Resource Map; • British Geological Survey Geological and Borehole Map Series (1:50,000); and • Landmark Information Envirocheck report.

Local Policy Guidance

1.7 Local policy guidance that has been reviewed in the preparation of this report is as follows:

• The Flintshire County Local Flood Risk Management Strategy (2013); and • The Flintshire County Preliminary Flood Risk Assessment (2011)

Limitations

1.8 The findings of this report are based upon information from a range of third party data sources. Professional care and diligence has been undertaken when obtaining and using this data, however AAe cannot be held responsible for the quality and accuracy of the data relied on.

1 Development and Flood Risk: Guidance for the Construction Industry – CIRIA, London, 2004 AA Environmental Limited 1 Bagillt WTS 203021 Flood Risk Assessment

2.0 SITE DESCRIPTION AND BASELINE CONDITIONS

Site Layout and Proposed Use

2.1 The site is located in Bagillt, within the Dee Bank Industrial Estate. adjacent to the Dee Estuary. The site is located circa 9 km north west of Connah’s Quay. The current ground level is at circa 6 m AOD and has a relatively flat gradient. The site location is shown in drawing 203021/D/001. The sites existing site layout and topography is presented in drawing 203021/D/002.

2.2 A railway line runs parallel to the north eastern boundary of the site. The railway is on embankment, some 2.78 m above the site level. To the north west, there is the existing industrial estate. To the south west, there is the existing A548 highway. To the south east, there is an open water drain which discharges to a main river, ultimately joining the Dee Estuary to the north east.

2.3 The proposed development is to increase the size of the existing waste treatment enclosure and construct and operate a soil washing plant to manufacture high quality recycled aggregate. The proposed site layout is presented in drawing 203021/D/004 Revision A. The existing and proposed development is classified as ‘Highly Vulnerable development’ in flood risk terms.

Geology and Hydrogeology

2.4 The British Geological Survey mapping classifies the bedrock geology as Etruria Formation - Mudstone, sandstone and conglomerate. The superficial deposits are Tidal Flat Deposits - Clay, silt and sand. The bedrock geology is classified as a Secondary A aquifer and the superficial is classified as unproductive strata. Based on the historical usage of the site as a Lead Works, and subsequently concrete batching and waste treatment, it is considered likely that the concrete surface is underlain by Made Ground.

Hydrology

2.5 With reference to the NRW’s Flood Risk Map Viewer online resource, the majority of the site is located within Flood Zone 3, with the southern part of the site in Flood Zone 2. The site is at risk from tidal flooding from Dee Estuary.

2.6 The nearest main river is an unnamed watercourse circa 30 m to the north of the site (parallel to the north of the railway line). The river ultimately discharges to the Dee Estuary some 280 m north east. There is an open surface water drain circa 10 m south east of the site which also discharges to the main river.

2.7 The majority of the site is not at risk of surface water flooding with a small area in the north of risk which is at low risk with an estimated depth between 0.15 and 0.3 m. The site has grown from a number of previous land uses and the existing drainage is complex, but it is understood comprises of discharge to sewer, via petrol interceptor and holding tanks.

AA Environmental Limited 2 Bagillt WTS 203021 Flood Risk Assessment

3.0 FLOOD RISK AND MITIGATION

Assessment of Flood Sources

3.1 All potential sources of flooding must be considered for any proposed development. A summary of the potential sources of flooding and a review of the severity of risk posed by each source at the site is presented in Table 1.

Table 1. Sources of Flooding Flood source Description Significant Fluvial/ tidal The majority of the site is within Flood Zone 3 (1:200 year event), with the Yes inundation southern extent in Flood Zone 2 only. This is presented in photo plate 203021/PH1. Additional climate change allowances are likely to designate the whole site within Flood Zone 3.

The 2018 modelled data for a 1:200 year event level is 6.4 m AOD. The 1:1000 year event level is 6.61 m AOD. These are taken from nodal point at grid reference 322649E, 375610N from the online government datasets.

The site ground level is at circa 6 m AOD. The railway embankment to the north is at circa 8.8 m AOD. Although not explicitly defined as a flood defence, the embankment and raised mud flats to the north of the railway do act as a defence from the estuary. There is the risk of breach through access tunnels under the railway line. The nearest tunnel is to the east known as Milwr Tunnel.

An assessment study was undertaken by Natural Resources Wales in 2015 following a breach in the flood defence in January 2014 (shown in Appendix B). 6.2 m AOD was the highest surveyed wrack mark. The flood event caused three breaches of the mud flat defence and the Fish Pond Farm tunnel and subway further north of the proposed site. There was no breach of the Milwr Tunnel nor any flooding of the site during the event. This was closely correlated within the updated model. Figure 1 shows LIDAR data of the tunnel to be at circa 6 m AOD.

This is corroborated by the Development Advice Map, used as a trigger mechanism for TAN15, which designates the site as C1. This means the site is served by significant infrastructure including flood defences.

According to the NRW Flood Risk Map Viewer, there have been no historical occurrences of flooding on the site. The Flintshire Preliminary FRA (2011) indicates there was a flood in 2000, where 3 residential and 2 non-residential properties flooded. This is not linked to the site.

There is also a local open watercourse immediately 5 m east of the site, which drains to the north east underneath the railway, through Milwr Tunnel, into the Dee Estuary. There is no evidence of flooding from this source.

The site is within a Flood Alert and Flood Warning Area. Surface water The NRW Flood Risk Map Viewer designates an area in the north east of the Very Unlikely drainage ponding site to be at low risk of surface water flooding, associated with the 1,000 year and/or inundation return period. (pluvial flooding) Currently, surface water across the site drains to a sealed system ultimately to foul sewer.

The MJA Consulting detailed drainage strategy proposed for the site is attached in Appendix A. Surface water run- There is no change in surfacing of the site and the sustainable drainage Possible off and systems proposed would see an improvement in the run-off mechanism with attenuation controlled discharge at greenfield run off rates. Groundwater The site is defined by the Envirocheck Report as being in an area where Unlikely inundation there is potential for groundwater flooding at surface. There are no historic records at the site.

AA Environmental Limited 3 Bagillt WTS 203021 Flood Risk Assessment

Fluvial/Tidal Flooding

3.2 The majority of the site is located within Flood Zone 3a, with the south eastern edge of the site being in Flood Zone 2. The site is served by significant infrastructure including flood defences. In flood risk terms, the proposal is classed as both ‘Highly Vulnerable development’ and is deemed an acceptable development. The site is an existing facility making infrastructure improvements and is justifiable within its development area.

3.3 The site ground levels will not change at the site and the soil washing plant and open-sided building infrastructure are floodable. The tidal flood risk and propagation risk remain the same as the existing approved layout.

3.4 There is also the drain to the east of the site. Previous modelling does not show the downstram tunnel (Milwr Tunnel) or adjacent watercourse to be the main risk of breach and there are no historical flood events connected to this source. The possibility of flooding caused by blockages in fluvial systems and surface water drains is anticipated to be low. The drain to the east is a small channel and it would be more likely to become blocked further upstream where it is culverted prior to crossing the A548. The depth to the water level within this surface water ditch is approximately 2.5 - 3 m below the existing ground level.

Surface Water Flooding

3.5 The NRW Flood Risk Map Viewer designates an area in the north east of the site to be at low risk of surface water flooding. The site drainage will ensure surface water propagates from the site and does not create standing water.

3.6 The proposed surface water drainage strategy is shown in Appendix A. The proposals provide sufficient capacity (up to 100 year storm plus 40 % climate change) to manage the surface water runoff from the site and that rainwater harvesting and water recycling are implemented. The washing plant requires between 30-200 m3 of water per day, dependent upon through put.

3.7 The rainwater harvesting will ensure the majority of this water requirement can be provided by the surface run off at the site. The attenuation proposals will reduce the run-off rates to 40% of estimated existing rates. The attenuated run-off will be discharged to sewer. GTR will submit an application to connect to the sewer in the A548.

Groundwater Flooding

3.8 The site is defined as an area where there is potential for groundwater flooding at surface. There have been no historic records at the site.

3.9 The risk of groundwater flooding is highest when there are basements or sunken bays constructed. There are neither sunken bays nor basements. The proposal is a change of use only and there will be no change in site levels, with the site remaining capped by concrete hardstanding. In the event of groundwater emergence, works will be suspended in the impacted area to prevent pollutants becoming suspended and migrating from the site and safety risks to site operatives.

Foul Sewer Flooding

3.10 The existing sewerage infrastructure will remain unchanged. There are no records that the site and surrounding area has been affected by foul sewer flooding in the past. Any further mitigation to that set out in this FRA will be agreed with

Safe Access & Egress

3.11 The site is within Flood Zone 3 and 2. There will be no change to the existing ability of staff to make access/egress during a flood event.

AA Environmental Limited 4 Bagillt WTS 203021 Flood Risk Assessment

3.12 The site is within the Flood Alert and Warning Area. The main Site Team will be signed up to the Flood Alerts system and the site will have a site-specific Flood Management Plan. The Plan will outline evacuation routes and procedures during a flood event. The Plan will form part of the site’s induction briefing when a new operative start.

Residual Flood Risk - Flood Defence Breach / Wave Overtopping

3.13 The risk of wave overtopping is deemed very low given the size of flood defences. The residual risk may come from flood water breach through tunnels beneath the railway line.

3.14 In the unlikely event there is a floodwater breach from the Milwr Tunnel to the east, there is the potential for tidal flooding at shallow depths. In a worst case scenario, this could conservatively be at circa 300 mm depth at a 1:200 year event and up to 610 mm depth during a 1:1000 year event. The site ground levels will not change at the site and the soil washing plant and open- sided building infrastructure are floodable. The tidal flood risk and propagation risk remain the same as the existing approved layout.

3.15 The tunnels/subways will act as a restrictive barrier slowing down inundation rate. This will ensure sufficient time for all operatives on site to evacuate off site. There would be no additional displacement of water during extreme estuary flooding events.

AA Environmental Limited 5 Bagillt WTS 203021 Flood Risk Assessment

4.0 CONCLUSIONS

4.1 The site is located within Flood Zone 3 and 2. The site is located in Bagillt, in the south of the Dee Bank Industrial Estate. The Dee Estuary is located circa 320 m north east of the site boundary.

4.2 The proposals include demolishing dilapidated structures in the east of the site, extend the existing waste enclosure, reducing particulate emission risk and to construct and operate a soil washing plant to manufacture high quality recycled aggregate. Both the existing and proposed land uses are classified as ‘Highly Vulnerable development’ in flood risk terms.

4.3 The proposed alterations in the enclosures and plant operating at the site will neither change the basic flood characteristics or the risk to users of the site and surrounding land.

4.4 The construction of the soil washing plant generates an opportunity to implement significant betterment in terms of rainwater harvesting run off with capacity to use up to 200 m3 of water a day. In addition, the existing drainage will be decommissioned, and a bespoke solution constructed. The new drainage system will attenuate runoff to 40% of the existing rates. The run-off rates will accord to Flintshire’s drainage requirements.

4.5 In addition, the following flood risk mitigation measures will be implemented:

• GTR will enrol in the flood warning service to ensure the site can close down safely in significant flood events; • Recycled aggregate will be primarily generated through a closed loop soil washing facility which utilises surface water run off. This will be able to harvest up to 200 m3 of water a day. Washdown of the yard will also utilise clean water harvested from the structures; and • Any required mitigation to protect the sewers will be agreed and implemented in consultation with Welsh Water.

4.6 There is no increase in foul sewer, groundwater flooding, residual flood defence breach risk and the safe dry access / egress route from the site remains the same.

4.7 Provided the following design and mitigation measures are implemented, the proposed development is considered acceptable in flood risk terms in accordance with PPW, NRW and EA guidance.

203021/FRA AA Environmental Limited

July 2020

AA Environmental Limited 6 Bagillt WTS 203021 Flood Risk Assessment

Drawing 203021/D/001 Site Location Plan

AA Environmental Limited Bagillt WTS 203021 Flood Risk Assessment

KEY:

Site Boundary Filling Station

Dee Bank Industrial Estate

Sluice

ers uld Bo

Boulders S u b w a y

MP 194.25

A 548 Drawn Rev. Details Date Chkd.

Project Subway 203021 Waste Recycling Facility Bagillt

Title Site Location Plan

AA Environmental Ltd Units 4-8 Cholswell Court Belm Shippon Abingdon ont Oxon OX13 6HX

T:(01235) 536042 F:(01235) 523849 [email protected] www.aae-ltd.co.uk

Scale Date Apr'20 Drg. No. Rev. Drawn Chkd. 1:1000(A3) 203021/D/001 EB ML

Drawing 203021/D/002 Existing Site Layout Plan

AA Environmental Limited Bagillt WTS 203021 Flood Risk Assessment

KEY:

Site Boundary

5 .90 .9 5 1 02 B 6.

5 .9 5 5 .9 9 Section Lines

8 .9 AH1 5 6.04 6.133

6 .0 9 m4 2.0 . 6 OFFICE & 8 2 .9 5 ce WORKSHOP n 6 9 e 4 . 6.03 6.05 .0 RH 5 5 6.03 6.12 F 6 12.96 .9 e 0 6 d .0 a 5 is 6 ll .00 a 6

. 1 P FL SITE ENTRANCE 4 6.09 6 .0 6.04 8 8 .9 5.97 5 6.03 6.00 6 TP .1 6 6.11 .6 6.01 6 6.08 5 09 6.

6 6.05 .3 STORAGE BAYS 0 6.05 5.97 6.02 5.98 6. 02 5.99 00 6.

5.96 5.97 6.05 6.05 6.27 5.99 EH TP 10.64 6.02 6 6 AA . 4 1 . 3 6 01 5.99 6. 6 .0 Concrete STAFF 6 LP 7 .1 W 4 6.00 6.03 E PARKING 5 I . G 5 3 6.08 .7 9 4 H 6.03 5 5 8 G . 0 6.23 6. 6 B6.54 STORAGE BAYS R ID G 6.04 E 6.01 FL 8.78 3 6 .1 7 6 6 7 . . 6 . 0 4 . 5 6 7 0 6.28 9 17 6. AH2 6.03 6.582 STAFF 6.34 6.08 PARKING6.31 6.26 6 6.01 .4 6.05 0 5 .8 G 5 MH 6.03 AH3 6.099 9 .0 6 C.L. 6.18m 5.86

6 . 5 4 . 7 9 2 5 6.47 6.10 .9 Concrete 5.99 EXTERNAL PROCESSING 7

6 . 4 4 AND STORAGE

6 MH .1 8 6.24 9 .0 5.86 6.16 6 STORAGE BAYS C.L. 5.94m 6.03 S to 6.01 n 5.95 e 5 .9 7 W 7 6 .0 a . 1 4 5 9 5.99 l .9 l 6 2 .3 Concrete 5.94 6 m .8 4 RH 15.43 6.22 5.86

6.02 5 .9 5.94 8 AH4 6.27 6.067 6 .1 5.83 5 6.00 6 .2 6

0 .0 RH 7 6 13.48 . FL 5 5 6.05 6.23 5.99 5.94 5.96 C EH 6 5.89 12.82 l . o 7 ENCLOSURE/ s 4 6.28 e COVERED BAY m B 2 o MH Concrete e a c r C.L. 5.89m n d e e 6.22 F 6.00 t d 5.90 e T e EH h im 14.06 5.93 S 6.01 b in e WASTE T r

F TRANSFER 7 6.24 .1 7 e 6 .2 n STATION 6 c Concrete e 5.90 6 . 6 6.21 6.03 BB 1 6 6.04 . EH 8 8 13.75 .0 m 6

EH 10.62 6.17 7 .0 6 Drawn Rev. Details Date 6.08 RH 14.88 Chkd.

RH 4 FL .1 6.39 12.15 6 AH5 6.17 6.090 9 .0 STORAGE 6 Project 6.19 FL BUILDING 6.12

4 6 .3 . 6 203021 8 3 STORAGE AND 6.12 PROCESSING Waste Recycling Facility BUILDING Bagillt

FL 6.33 6.33

Title 7 .0 0 7 m .0 4 8 . 2 Existing Site Layout 6.35 e FEEDSTOCK AND SKIP c 6.35 n UNLOADING AREA e F MH e Concrete d C.L. 6.29m a is ll a A 7 . 1 P 2 A 548 0 .5 C 6 lo 6.43 3 s 0 6.33 5 .7 . e 6 6 B o AH6 AA Environmental Ltd a 6.410 Units 4-8 rd e Cholswell Court d 1 .6 T 6 Shippon Abingdon im b Oxon OX13 6HX e 6.43 r 2 7 6 . . F 3 6 e 0 T:(01235) 536042 n c F:(01235) 523849 e 4 .7 1 6 [email protected] .8 m www.aae-ltd.co.uk 3 .7 6

7 6 . .1 3 7 0 Scale Date Apr'20 Drg. No. Rev. Drawn Chkd. 1:500(A3) 203021/D/002 JNT ML

Drawing 203021/D/004 Revision A Proposed Site Layout Plan

AA Environmental Limited Bagillt WTS 203021 Flood Risk Assessment

KEY:

Site Boundary

5 .90 .9 25 1 B 6.0 Section Lines 5 5 .9 .9 5 9 8 9 . 6.04 5 WATER TANK

6 .0 4 9 .0 OFFICE & 8 6 .9 WORKSHOP 5 6 4 5 9 6.03 . 6.03 6.12 6.05 .0 .9 5 6 6 0 .0 6 5 6 .00 . 1 6 4 6.09 . 6.04 8 0 5.97 .9 8 6 6.03 5 . TP 6.00 1 6.01 6 LORRY PARKING 6.08 6.11 9 5.66 .0 6 6 .3 6.05 0 6.05 5.97 6.02 5.98 6.0 2 00 5.99 6. TP 5.96 STAFF 6.27 5.99 5.97 6.05 6.05 6 6.02 PARKING 6 . AA 4 1 . 3 1 6 5.99 6 .0 . 6 6 0 . 7 1 TWIN 15 m6.00 4 6.03 5 WEIGHBRIDGES 5 .3 4 6.08 .7 9 5 AND TICKET 8 6.03 5 . 0 6.23 6.54 6. 6 OFFICE STAFF 6.01 6.04 PARKING 8.78 6 6 6 13 7 . . 7 . 4 . 6 . 0 0 5 6 7 6.28 9 7 .1 6.34 6 6.03 VEHICLE 6.31 6.08 6.26 WASH 6 . 6.01 4 6.05 5.85 BAY 0 6.03 9 .0 6 6 RECOVERED 5.86 . 5 4 . 9 5 7 MATERIAL2 6.47 6.10 .9 6 STORAGE 5.99 7 . 4 4 6 .1 6.24 8 9 6.16 .0 6.03 5.86 6 QUARANTINE 6.01 BAY 5.95 5 .9 6 7 . 7 . 5 CDE WASH PLANT 0 4 . 1 9 9 5.99 6 (REFER TO DRAWING 5.946381-L3-T1) 6 .8 4 6.22 5.86 5 6.02 .9 5.94 8 6 6.27 .1 5.83 6.00 6.26 5

0 6 .0 . 7 5 6.05 5 6.23 5.99 5.96 5.94

6 . SUB-SURFACE 7 5.89 4 6.28 WATER TANK (10 x 15 m)

6.22 6.00 5.90 14.06 5.93 6.01 WASTE TRANSFER 7 6.24 1 7 . .2 STATION 6 6 6 . 5.90 6 6.21 6.03 BB 6 6.04 13.75 6.08

Drawn 6.17 7 .0 Rev. Details Date 6 Chkd. 6.08 14.88 FEEDSTOCK 4 Project .1 6.39 6 9 6.17 .0 6.19 6 6.12 203021

6 4 . .3 8 6 Waste Recycling Facility 3 6.12 Bagillt

6.33 6.33

Title BALING 7.00 7 . Proposed Site Layout 0 8 6.35 6.35

A 7 A 548 .1 FEEDSTOCK 2 0 .5 6.43 6 0 6.33 3 AA Environmental Ltd .7 .5 6 6 Units 4-8 Cholswell Court Shippon Abingdon 1 .6 Oxon OX13 6HX 6

6.43 T:(01235) 536042 7 2 .3 .6 F:(01235) 523849 0 6 [email protected] www.aae-llp.com 6.74 3 .7 6 Scale Date Drg. No. Rev. 7 6 Jun'20 .3 .1 0 7 Drawn Chkd. 1:500 (A3) 203021/D/004 A JNT ML

Photoplate 203021/PH1 Flood Zone Extent

AA Environmental Limited Bagillt WTS 203021 Flood Risk Assessment

Comment Project Extract taken from Natural Resources Wales 203021 Geocortex Viewer. Dee Bank, Bagillt

Reference 203021/PH1

Date 15/04/2020

Originator EB

AA Environmental Limited Units 4-8 Cholswell Court Shippon, Abingdon OX13 6HX T: (01235) 536042 F: (01235) 523849 [email protected] www.aae-ltd.co.uk

Appendix A Surface water Drainage Strategy (P1, MJA, July 2020)

AA Environmental Limited Bagillt WTS 203021 Flood Risk Assessment

6.54

6.47 6.46

6.47

6.44 6.02

6.23

6.49 6.13 6.05 TP PARKING STAFF 6.31

6.27

6.55 6.02 6.01

6.14 A 548 A 6.74 6.47 5.96

5.98 6.54 6.16 6.26 6.02

5.98 6.00 6.34 LORRY PARKING

6.03

WATER TANK 5.95 6.66 6.01

6.28 5.97 6.07 6.01

6.09

6.05 6.02 6.12 WEIGHBRIDGES 6.04 6.04 AND TICKET 5.90 TWIN 15 m

6.24 6.00

6.09 OFFICE 6.83 5.99 6.03 TRANSFER STATION

6.03

WASTE 6.01 5.91

6.00

5.99

Cellular Polypropylene Tank Polypropylene Cellular 6.05 5.96 6.17 6.08 (200m 20m x 10m

5.98

6.27 6.04 TANK ATTENUATION WATER

6.16 7.00

6.13

SUB-SURFACE

7.08 6.03 5.99

6.08 6.09

6.18 ) x 1.2m deep 1.2m x ) 14.06 2 6.39 6.08 6.28 5.97

6.22

6.05

7.12 5.96 5.99 5.97 6.10 6.24 TP 6.03

5.98 5.98 6.09

6.23 7.30 6.17 6.01 6.35 6.00 6.70 VEHICLE 6.04 WASH FEEDSTOCK BAY 6.03 6.05 QUARANTINE 6.19 6.22 6.09 6.04 BAY

6.03

5.86 7.30 6.62 6.43 6.33

7.16 PARKING STAFF 6.11 6.21 RECOVERED MATERIAL 6.08 STORAGE

6.73 5.99 5.77 5.92 5.95 6.43 6.35 14.88

6.74 BALING 6.09 13.75 6.33

5.75 6.05 6.40 6.00 WORKSHOP OFFICE & 5.99 6.33 5.94 PV PV PV

6.61 6.06 5.90

6.53 5.96 5.99

6.50 6.05 6.14 5.66 5.90 5.83 6.03

FEEDSTOCK 5.90 6.12 6.03 5.89

6.02 6.08 6.12 5.86

5.93 6.08 6.30 5.86 6.34

6.09 5.94 SUB-SURFACE WATER TANK 6.14 (10 x 15 m)

6.17 (REFER TO DRAWING 6.04 5.94 CDE WASH PLANT 6381-L3-T1) 6.07

6.01 5.39 6.00

6.08 6.05

6.17

6.27 5.85

5.97

7.01

‰

5.97 6.84

7.00 6.26 6.15 8.78

Tel: Ipsum Court, 24 The Quadrant, CIVIL & STRUCTURAL ENGINEERS MJA Email: Oxon, OX14 3YS Abingdon Science Park, 01235 555173 Subway [email protected] CONSULTING Sluice

North MP 194.25 MP

Appendix B Flint Point to Greenfield Initial Assessment Study (JBA, January 2015)

AA Environmental Limited Bagillt WTS 203021 Flood Risk Assessment

NOTE TO FILE JBA Project Code 2014s1703 Contract Flint Point to Greenfield Initial Assessment Study Client Natural Resources Wales (Cyfoeth Naturiol Cymru) Day, Date and Time 13 January 2015 Author Tim Diesner Subject Flint Point to Greenfield Initial Assessment Study

1 Calibration of Greenfield Hydraulic Model 1.1 Introduction The aim of this File Note is to document the work undertaken to calibrate the Greenfield hydraulic model. The existing model of Greenfield was developed quickly following the January 2014 tidal event using the local information available at the time. Since the existing model was developed, detailed survey of the hydraulic structures across the site and channel cross-section survey has been collected. The additional and updated survey data (received December 2014) has been used to improve the representation of the watercourses and structures in the model. This document details the calibration of the model following the model updates undertaken. Figure 1 presents an overview of the site, and shows the location of the breaches that were present in the tidal defence during the January 2014 event. Figure 1: Location Map for Greenfield Study

1.2 Tidal levels A tidal profile for the event has been configured using the following process;  An analysis of local data at Hilbre Island and Liverpool Gladstone Dock has been undertaken for a period when both gauges were in operation between 1998 and 2003.  A regression analysis of the concurrent data series was completed to determine a relationship between the surge residual values at Liverpool and Hilbre Island. As the astronomical tides are deterministic these were not included in the analysis. The data was declustered to remove all surge residuals below a threshold of 0.1m to concentrate the analysis on the positive extremes.

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A stronger correlation was found between the data at Hilbre Island and Liverpool with a final R2 value of 0.72.  Using the relationship developed, a new series of surge residuals were calculated for Hilbre Island for the period 2004-2014 (for the period when the tidal station at Hilbre Island has been closed). Astronomical tidal data was extracted for Hilbre Island for the same period which was then combined with the surge data to create a virtual tidal record for the 2004-2014 period. This enabled the extraction of tidal graphs for individual storm events.  The tidal graph for January 2014 was extracted from the generated record and used to set a downstream boundary condition within the River Dee ISIS model (at Hilbre Island). The Tidal Dee model was run, to represent the propagation of the tide up the estuary with the predicted tidal levels at Greenfield extracted and used to set the boundary condition within the Greenfield ESTRY-TuFLOW model. The original model developed for Greenfield used the data recorded at the Liverpool Gladstone Dock during the January 2014 event, to set the boundary condition within the model, resulting in a modelled peak water level of 5.859 mAOD. This level has been increased for the present study, using the above process, with the peak tidal level of 6.144 mAOD predicted at Greenfield.

1.3 Breach representation The representation of the breaches that were present in the existing flood defence at the time of the event have been updated from the original simplified representation used within the original model (reduced crest level along the 2D downstream model boundary). As an alternative, a 1D representation of the breaches, using weir units in ESTRY has been incorporated into the updated model. This update has been made to allow the efficiency and roughness associated with the breaches to the set. Within the weir units, a weir coefficient of 1.0 has specified at each breach. The weirs have been connected to the 2D domain using SX connections on the inland side of the tidal defence with a head-time boundary set to define the tidal levels for the January 2014 event connected to the seaward side of the weirs. Re-running the model simulation resulted in reduced flow through the breaches compared to the original modelling. 1.4 Blockage representation Blockage proportions on Coed Bagillt (Walwen Drain,) and Fishpool Farm Drain have been set using the surveyed blockage proportions that were found to be significant. A blockage proportion of 95% has been applied at each culvert. No blockage was applied at New Brighton Drain as there was no evidence of blockage surveyed at this structure.

1.5 Modelled Flood Extent The updated model has been run for the January 2014 tidal event. The predicted output from the model is shown in Figure 3 and can be compared to the observed extent of flooding shown in Figure 2. Comparison of the modelled flood outline for the event against the observed data shows;  The model correctly predicts the observed flooding within Greenfield Marsh.  As per the observed information, no flooding from New Brighton Drain is predicted.  The flooding inland of the railway at Fishpool drain reflects the observed flooding, although the extent of the spread of floodwater predicted within the model is larger than what was observed during the event.  Only limited flooding is shown to originate from Coed Bagillt watercourse, as observed.

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 The predicted model outputs are now shown to more closely correlate with the observed data compared to the original model outputs produced in January 2014, that were found to significantly overestimate the extent of flooding. The main reason for the improved outputs is likely to be the better definition of the breaches and culverts under the railway within the model.

Figure 2: Observed Flood Extent - January 2014 Event

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Figure 3: Modelled Flood Extent - January 2014 Event

1.6 Modelled flood levels Wrack levels were collected by NRW following the January 2014 event. Three wrack marks were surveyed at around 6.2 mAOD on the seaward side of the tidal defence and presumably reflect the peak tide level in combination with wave action within the estuary. The wrack marks at around 4.8 mAOD occur behind the defence and so could be reflective of the still water level reached across Greenfield Marsh. Maximum water levels of approximately 4.96 mAOD were simulated in the calibrated model in “The Marsh” area (see Figure 4). The degree of fit between the predicted and observed wrack level information is considered to be acceptable given the uncertainties associated with wrack data.

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Figure 6: Maximum Water Levels Modelled - January 2014 Event

Flood Level (m AOD)

Surveyed Wrack Marks

1.7 Summary The existing model of Greenfield has been updated and calibrated against the January 2014 event. The outputs from the updated model are shown to generally match the observations and wrack level information collected during the event, providing some confidence in the performance of the model. The reported blockage proportions at Coed Bagillt watercourse and Fishpool Farm drain in this note have been removed from the design event and option model runs that assume that the culverts under the railway remain free from blockage.

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