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DFJ-JBAU-XX-XX-RP-Z-0001-A3-C01-Flood_Consequence_Assessment.docx i
Land off Mancot Lane,
Deeside Flood Consequence Assessment
May 2020
www.jbaconsulting.com
Wales & West Housing
Tŷ Draig
St Davids Park
EWLOE
Flintshire
CH5 3DT
DFJ-JBAU-XX-XX-RP-Z-0001-A3-C01-Flood_Consequence_Assessment.docx i
JBA Project Manager
Howard Keeble
2nd Floor
Mersey Bank House
WARRINGTON
Cheshire
UNITED KINGDOM
WA1 1WA
Revision History
Status Revision Ref/Date
Date Originated Checked Approved
S3 P01 01/05/2020
J Landells-
Molloy
Assistant
Engineer
H Keeble
Technical
Director
H Keeble
Technical
Director
A3 C01 03/06/2020
J Landells-
Molloy
Assistant Engineer
H Keeble
Technical
Director
H Keeble
Technical
Director
Contract
This report describes work commissioned by Emma Hancock, on behalf of Wales & 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
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.
JBA Consulting has no liability regarding the use of this report except to Wales & West
Housing.
Copyright
© Jeremy Benn Associates Limited 2020.
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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.
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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 3.2 Surface Water 10 3.3 Groundwater 10 4 Safe Site Access and Egress 11 5 Outline Drainage Strategy 12 5.1 Sustainable Drainage Systems Standards for Wales 12 5.2 Surface Water Management 13 5.3 SuDS Maintenance 14 5.4 Water Quality 14 6 Foul Drainage 15 7 Conclusions 16 8 Appendices 17
A 2020 Ground Investigation Report 18
B Drainage Layout 19
C Micro Drainage Modelling Results 20
D Greenfield Runoff Rates 21
E Welsh Water Sewer Plan 1
F Welsh Water Response 1
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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 – NRW’s updated Flood Map for Surface Water 10 Figure 4-1 - Primary Egress Route (fluvial/tidal sources) 11
List of Tables
Table 2-1 - Site Description 2 Table 5-1 - Standard S1 Discussion 12 Table 5-2 - Calculated Greenfield Runoff Rates 13 Table 5-3 - Required Volumes of Attenuation 14
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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 Chester 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
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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
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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
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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 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.
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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.
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Figure 3-2 - Summary of TAN15 Policy Requirements (Section 9, TAN15
2004)
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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.
Figure 3-3 - 2017 Flintshire SFCA Modelled Breach Locations
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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
0.5% Annual Exceedance Probability (AEP) event. 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 0.1% AEP
which is presented in Figure 3-5 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. 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.
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Figure 3-5 - Climate Change 0.1% AEP Tidal Breach Depth Map
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3.2 Surface Water
NRW’s updated Flood Map for Surface Water (uFMSW) is presented in Figure 3-6 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.
Figure 3-6 – NRW’s updated Flood Map for Surface Water
3.3 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.
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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: Hawarden Bank. It is recommended that the six
properties in Zone C1 sign up to the NRW Flood Warning Service.
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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
Destination
Comments
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
body
Discharge to Pentre Drain
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
sewer, highway drain, or
another drainage system
Discharge to a surface water
sewer is proposed based on a
fully attenuated system
Further consultation is required
5 Discharged to a combined
sewer
N/A
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.
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3 https://gov.wales/sites/default/files/publications/2019-06/statutory-national-standards-for-sustainable-
drainage-systems.pdf
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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.
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Table 5-3 - Required Volumes of Attenuation
Design flood
event
(incl climate
change)
Critical
storm
duration
Hrs
Inflow
volume
m3
Outflow
volume
m3
Attenuation
required
m3
Time to
empty
(assuming
no
infiltration)
Hrs
3.33% AEP +
20% CC
5.5 298 69 229 18.1
3.33% AEP +
40% CC
6.5 362 82 280 22.2
1% AEP + 20%
CC 6.5 414 82 332 (103
exceedance
storage)
26.3
1% AEP + 40%
CC 7.5 497 95 403 (123
exceedance
storage)
31.9
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.
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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.
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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. 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. It is further noted that critical flood risk at the site is tidal
(hence tidal breach modelling), therefore, ground level raising is acceptable without
increasing flood risk elsewhere.
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.
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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
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A 2020 Ground Investigation Report
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B Drainage Layout
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C Micro Drainage Modelling Results
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D Greenfield Runoff Rates
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E Welsh Water Sewer Plan
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F Welsh Water Response
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DFJ-JBAU-XX-XX-RP-Z-0001-A3-C01-Flood_Consequence_Assessment.docx
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