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Residential Development at Llanmaes Flood Consequence Assessment Welsh Government Project number: 60509148 November 2019

Residential Development at Llanmaes Flood Consequence Assessment

60509148/BRRP0002

Prepared for: Welsh Government

AECOM 2

Quality information Prepared by Checked by Approved by

Tom Gibson Senior Engineer

Richard Moore Flood Risk Consultant

Mark Davin Associate Director

Revision History Revision Revision date Details Authorised Position

1 17/08/2018 Draft Issue

2 11/09/2018 Final Issue

3 29/11/2019 Updated to include details of flood map challenge

Prepared for: Welsh Government Tom Gibson Senior Engineer E: [email protected]

Prepared by: AECOM Limited 3rd Floor Portwall Place Portwall Lane Bristol BS1 6NA UK T: +44 (117) 901 7000 aecom.com

© 2019 AECOM Limited. All Rights Reserved.

This document has been prepared by AECOM Limited (“AECOM”) for sole use of our client (the “Client”) in accordance with generally accepted consultancy principles, the budget for fees and the terms of reference agreed between AECOM and the Client. Any information provided by third parties and referred to herein has not been checked or verified by AECOM, unless otherwise expressly stated in the document. No third party may rely upon this document without the prior and express written agreement of AECOM.


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Table of Contents 1. Introduction ............................................................................................................................................... 4

1.1 Commission ................................................................................................................................ 4 1.2 Policy Context ............................................................................................................................. 4 1.3 Aim and Objectives ..................................................................................................................... 5

2. Site Description ......................................................................................................................................... 6 2.1 Location ...................................................................................................................................... 6 2.2 Environmental Setting ................................................................................................................ 6 2.3 Local Water Features ................................................................................................................. 6 2.3.1 Existing Flood Defence Structures ............................................................................................. 7 2.4 Topography ................................................................................................................................. 8 2.5 Geology ...................................................................................................................................... 8

3. Policy Context ......................................................................................................................................... 10 3.1 National Policy .......................................................................................................................... 10 3.1.1 TAN15....................................................................................................................................... 10 3.2 Evidence Base .......................................................................................................................... 11 3.2.1 Strategic Flood Consequence Assessment (SFCA) ................................................................. 11 3.2.2 Preliminary Flood Risk Assessment ......................................................................................... 11 3.2.3 Local Flood Risk Management Strategy ................................................................................... 11 3.3 SuDS Guidance ........................................................................................................................ 11

4. Flood Risk – To Development ................................................................................................................. 12 4.1 Overview .................................................................................................................................. 12 4.2 Tidal .......................................................................................................................................... 12 4.3 Fluvial ....................................................................................................................................... 12 4.4 Surface Water ........................................................................................................................... 13 4.5 Sewer Flooding ......................................................................................................................... 14 4.6 Groundwater ............................................................................................................................. 14 4.7 Artificial Sources ....................................................................................................................... 15

5. Flood Risk – From the Development ...................................................................................................... 16 5.1 Overview .................................................................................................................................. 16 5.2 Surface Water Management ..................................................................................................... 16

6. Flood Map Challenge .............................................................................................................................. 17 6.1 Overview .................................................................................................................................. 17

7. Conclusion .............................................................................................................................................. 18 7.1 Overview .................................................................................................................................. 18 7.2 Flood Sources .......................................................................................................................... 18 7.3 Surface Water Management ..................................................................................................... 18

Appendices ........................................................................................................................................................... 19 Appendix A – Proposed Site Layout ...................................................................................................................... 20 Appendix B – WinDES Calculations ...................................................................................................................... 21 Appendix C – Flood Map Challenge ...................................................................................................................... 22


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1. Introduction

1.1 Commission

AECOM Limited (AECOM) has been commissioned by Welsh Government to undertake a Flood Consequence Assessment (FCA) to inform the masterplanning for the development of two residential development plots situated between the newly constructed Ffordd Bro Tathan and Eglwys Brewis Road, each referred to in this report as Plot A (west) and Plot B (east), as shown in Figure 1-1.

1.2 Policy Context

In March 2017, AECOM produced an FCA in conjunction with fluvial and pluvial hydraulic models to assess the impact of the proposed Ffordd Bro Tathan and to identify mitigation options.

Construction of the new road was completed in November 2019 and Ffordd Bro Tathan now links the B4265, north of the Llantwit Major Railway line to the St. Athan MoD site. The road crosses two watercourses, Boverton Brook and Llanmaes Brook and a third called Nant y Stepsau to the east of this that discharges downstream of Ffordd Bro Tathan. The proposed residential plots, as shown in Figure 1-1, are located immediately south of Ffordd Bro Tathan.

The Development Advice Map (DAM) presented on the Natural Resources Wales (NRW) website (Figure 1-1), shows that the majority of Plots A and B are located within DAM Zone A (shown as no colour in Figure 1-1). For Plot A, a small proportion of the site is located within DAM Zone C2 and DAM Zone B, at the southern and eastern boundaries. For Plot B, a small area towards the east of the site adjacent to the Boverton Brook is within DAM Zone C2.

Figure 1-1: Natural Resources Wales Development Advice Map

(Adapted from NRW flood risk maps. Last accessed 09/08/18)

Areas located in DAM Zone A are defined by NRW as areas with little or no risk of fluvial/tidal flooding whereas areas located in DAM Zone C2 are based on NRW’s extreme flood outline, equal or greater than 0.1% Annual Exceedance Probability (fluvial or tidal) and are considered areas of the floodplain without significant flood defence infrastructure. Areas located within DAM Zone B are classified as areas known to have been flooded in the past.

Legend

Site Boundary

Ffordd Bro Tathan

DAM Zone B

DAM Zone C2

PLOT A

PLOT B

Boverton Brook

Llanmaes Brook

Nant y Stepsau


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This report has been prepared in accordance with the Technical Advice Note 15 (TAN15): Development and Flood Risk1. Welsh Government’s TAN15 policy guidance, alongside the NRW DAMs, have been used to inform this assessment. As the majority of the development area is within DAM Zone A, the Justification Test is not applicable and there is no need to consider flood risk further. For the areas that are within DAM Zone C2, only ‘less vulnerable’ development should be considered subject to the application of the Justification Test and including the acceptability of consequences.

With reference to the proposed site layout plans presented in Appendix A, it should be noted that the proposed development is limited to the DAM Zone A areas only and no development is proposed within areas at risk of flooding. The illustrative masterplans confirm that the proposed residential development and associated infrastructure will be outside of the flood extents. Accordingly, the residential development and means of access thereto will not be at risk of flooding and there is no need to apply the Justification Test for the proposed development.

1.3 Aim and Objectives

The aim of this FCA is to consider the flood risk posed to, and arising from, the Proposed Development. In order to achieve this, the following objectives are required to be met:

• Collect and review existing flood risk data including topographic data, surface water drainage, online NRW information, scheme proposals and the VoGC Strategic Flood Consequence Assessment (SFCA);

• Assess and interpret available information to identify potential sources of flood risk including groundwater, surface water and infrastructure failure;

• Complete preliminary runoff calculations to inform potential drainage options;

• Propose recommendations for appropriate flood risk mitigation measures (where applicable); and,

• Produce an FCA report in full accordance with TAN15 to accompany the subsequent planning application.

1 Technical Advice Note (TAN 15). Available from: https://gov.wales/docs/desh/publications/040701tan15en.pdf. Last Accessed: 30/07/18.

https://gov.wales/docs/desh/publications/040701tan15en.pdf


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2. Site Description

2.1 Location

The proposed development plots are located within the administrative district of the Vale of Glamorgan, approximately 15km west of Barry and 17km south east of Bridgend. Plot A is 8.23 ha, located adjacent to the B4265 to the south of the newly constructed Ffordd Bro Tathan. Plot B is 3.96 ha, located to the east of this between Ffordd Bro Tathan and Eglwys Brewis Road (Figure 2-1). The approximate Ordnance Survey National Grid Reference (OSNGR) centred on Plot A and Plot B is SS 98476 68881 (postcode CF61 2GS) and SS 99199 69185 (postcode CF62 4NR) respectively.

This FCA will focus on these two plots to clearly document the associated impacts and outline mitigation options.

Figure 2-1: Site Location Map

Reproduced from OS Data © Crown Copyright & Database Rights 2018 Ordnance Survey 100031673

2.2 Environmental Setting

As seen in Figure 1-1 the majority of Plot A and Plot B are within DAM Zone A, with small sections adjacent to Llanmaes Brook and Boverton Brook, within DAM Zone C2. Located approximately 3.9km south-west of the development plots is the tidal Severn Estuary. Due to the distance from the estuary, elevation of the channel and surrounding topography, no risk of flooding to the site exists from this source.

Both Plot A and B are currently considered as greenfield sites (fields) with a small area in the east of Plot B used as arable land.

2.3 Local Water Features

There are a number of local water features, depicted in Figure 2-2 that flow through and adjacent to the proposed development plots:

Ffordd Bro Tathan

PLOT B

PLOT A


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• Boverton Brook is a tributary of the River Hoddnant with the confluence located downstream of the town of Boverton. It is classified as Main River upstream of the eastern boundary of Plot B. Prior to this, Boverton Brook is an Ordinary Watercourse;

• Llanmaes Brook is a tributary of Boverton Brook. It is classified as Main River and flows north to south through Llanmaes village;

• A series of field drains and ordinary watercourses feed into the Boverton Brook, these branch out over agricultural fields to the north of Plot B; and,

• Nant y Stepsau to the east of Plot B is classified as Main River but flows eastwards, away from Plot A and 2.

Figure 2-2: Boverton Brook, Llanmaes Brook and Nant y Stepsau Catchments

Reproduced from OS Data © Crown Copyright & Database Rights 2018 Ordnance Survey 100031673

2.3.1 Existing Flood Defence Structures

Two flood storage areas are located on Llanmaes Brook to help alleviate the risk of flooding to the town of Boverton downstream. The Frampton Lane Flood Storage is located in the upper Llanmaes Brook catchment near the village of Frampton (NGR 297354, 169647). The outfall is regulated from the flood storage area by a flapped culvert that allows water to back up and inundate the storage area in times of high flows.

A second flood storage area had been located between the confluence of Llanmaes Brook and Boverton Brook and the village of Llanmaes (NGR 298565, 168969). The scheme consisted of a small earth embankment across Llanmaes Brook that was culverted beneath. Flows through the culvert were restricted by a 0.50m diameter orifice which allowed for small-scale attenuation of water upstream of the structure.

This earth bund was replaced with a more significant flood defence in 2019 as part of the Ffordd Bro Tathan works. The road embankment replaces the earth bund and the defences now comprise a ‘low-flow’ culvert installed on the line of Llanmaes Brook and a series of ‘high-flow’ culverts above. A series of photos showing the flood defences are presented in Figure 2-3.

It should be noted that NRW’s DAM map presented in Figure 1-1 is based on the original flood defences prior to construction of Ffordd Bro Tathan and have not yet been updated to take account the impct of the new flood defences. This matter is discussed in further detail in Section 6 of this report.


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Low-flow culvert upstream High-flow culverts Low-flow culvert downstream

Figure 2-3: Llanmaes Brook Flood Defence at Ffordd Bro Tathan

2.4 Topography

Figure 2-4 displays the contour lines surrounding and within the site boundary. Plot A sits between 45m AOD and 35m AOD and has a 10m sloping gradient from north to south towards the Boverton Brook. Plot B sits at 40m AOD in a largely flat area adjacent to the road to the south.

Figure 2-4: OS Contour Map

Reproduced from Ordnance Survey digital map data © Crown copyright 2018. All rights received. Licence No. 0100031673 2017

2.5 Geology

The 1:50,000 scale geological map of Bridgend - Sheet 2622 shows that the site is underlain by Porthkerry Member Limestone and Mudstone Formation. These rocks are interbedded and formed approximately 190 - 200 million years ago, in the Jurassic Period.

The solid geology is overlain by small regions locally around the Boverton Brook of head and alluvium (formed of clay, silt, sand and gravel).

A detailed Ground Investigation (GI) was carried out prior to the development of Ffordd Bro Tathan between November 2016 and January 2017. As part of this study, a series of trial pits were excavated and boreholes 2 BGS (British Geological Survey), Geology of Britain Viewer (1974): Grid Reference 299491,169228. Available at: http://mapapps.bgs.ac.uk/geologyofbritain/home.html

40m AOD

35m AOD

45m AOD 40m AOD

http://mapapps.bgs.ac.uk/geologyofbritain/home.html


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drilled. Table 2-1 shows the anticipated geology for the site. Srata thicknesses are based on the four rotary boreholes formed as part of this GI. Groundwater was encountered at 0.7-1.3m bgl.

NRW classifies zones around potable groundwater abstraction points as Groundwater Source Protection Zones (SPZ) and these are designed to limit potential pollution activities. The site is not in a SPZ and there are no SPZ limits within 1km of the site.

Welsh Government holds two groundwater abstraction licences within 1km of the site, at RAF St Athan. There is one discharge consent to groundwater on the site and nine other discharge consents within 1km of the site.

The Geo Environment Report3 indicates that the site is located within a Secondary A Aquifer. The Porthkerry Limestone form a permeable layer capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. Therefore, it is unlikely there will be any significant restrictions on surface water runoff to ground.

Table 2-1 Summary of Geology

Stratum Depth To Stratum (bgl)

Geological Map Description/ Anticipated

Presence Thickness

Made Ground Ground Level Soft slightly gravelly silty Clay 0.20m - 0.65m

Probable Alluvium

0.20m - 0.45m

Silty/ gravelly/ sandy Clay. Appears to be only present

in proximity to the watercourses in the area

0.10m - 2.20m

Distinctly Weathered Porthkerry Member 0.35m - 1.30m Interbedded limestone with

clay bands 1.7m*

Partially Weathered Porthkerry Member 0.70m - 2.40m Interbedded limestone with

clay bands 0.10m* - 7.80m

Porthkerry Member 2.40m Interbedded limestone with clay bands 5.60m*

*Base of Stratum not proven, greater thickness may be present.

Source: AECOM (2017) St Athan Northern Access Road, Factual Ground Investigation Report.

3 AECOM (2016) St Athan Northern Access Road Phase 1 Geo-Environmental Assessment.


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3. Policy Context

3.1 National Policy

3.1.1 TAN15

TAN15 provides guidance which supplements the policy set out in Planning Policy Wales (PPW)4 in relation to development and flooding. A precautionary framework is set out which advises caution in respect of new development in areas at high risk of flooding and this is used as a guide for planning decisions. The overall aim of the precautionary framework is to direct new development away from those areas that have a high risk of flooding; and development will only be justified in these areas if it meets the criteria and tests specified in this guidance.

The operation of the precautionary framework is governed by DAMs made up of three zones (Table 3-1) which are used to trigger the appropriate planning test and definitions of vulnerable developments. The DAMs are based on the best available information considered adequate to determine when flood risk needs to be taken into consideration with future development.

Table 3-1: Flood Zone Designations (source: TAN 15)

Flood Zone Definition Use within the precautionary framework

A Little or no risk of fluvial/ tidal flooding Justification Test is not applied and do not need to consider further

B Areas known to have flooded historically evidenced by sedimentary deposits.

Used as part of the precautionary approach to indicate where site levels should be checked against the extreme (0.1% annual probability) flood. No need to consider flood risks further if site levels are greater than the extreme flood level

C Based on NRW extreme flood outline (0.1% annual probability)

Indicates that flooding issues should be considered as an integral part of the decision making by the application of the Justification Test, including FCA

C1 Areas of Zone C which are developed and served by significant infrastructure, including flood defences

Indicates that development can take place subject to the application of the Justification Test, including acceptability of consequences

C2 Areas of Zone C without significant flood defence infrastructure

Indicates that only ‘less vulnerable’ development should be considered, subject to the application of the Justification Test, including acceptability of consequences. Emergency services and highly vulnerable development should not be considered.

The precautionary framework identifies the vulnerability of different land uses to flooding and classifies proposed uses accordingly as detailed in Table 3-2. This is because certain flooding consequences may not be acceptable for particular development types.

Table 3-2 Development Categories (source: TAN 15)

Flood Zone Definition Use within the precautionary framework

Emergency Services Hospitals, ambulance stations, fire stations, police stations, coastguard stations, command centres, emergency depots and buildings used to provide emergency shelter in time of flood.

Highly vulnerable development All residential premises (including hotels and caravan parks), public buildings (e.g. schools, libraries, leisure centres), especially vulnerable industrial development (e.g. power stations, chemical plants, incinerators), and waste disposal sites.

Less vulnerable development General industrial, employment, commercial and retail development, transport and utilities infrastructure, car parks, mineral extraction sites and associated processing facilities, excluding waste disposal sites.

Table 3-2 highlights that residential premises such as Plots A and B are classified as ‘Highly Vulnerable’. The majority of Plots A and B are within DAM Zone A and therefore the Justification Test is not applicable and there is no need to consider flood risk any further. For those areas in DAM Zone C2, without robust evidence to the

4 Welsh Government (2016) Planning Policy Wales, Edition 10 (December, 2018);


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contrary, construction is not permitted and therefore future development will need to take these restrictions into consideration.

3.2 Evidence Base

3.2.1 Strategic Flood Consequence Assessment (SFCA)

No SFCA is available from VoGC at the time of writing.

3.2.2 Preliminary Flood Risk Assessment

VoGC developed a Preliminary Flood Risk Assessment5 (PFRA) which examined the areas within the Vale of Glamorgan that have historically suffered from flooding and areas that are at risk of flooding. This report highlighted that areas in Llanmaes and areas around Froglands Farm suffered from flooding in October 1998. The area which flooded around Froglands Farm is directly adjacent to the western boundary of Plot B.

3.2.3 Local Flood Risk Management Strategy

In 2012, VoGC developed a Local Flood Risk Management Strategy (LFRMS)6, which highlights the responsibilities of VoGC as Lead Local Flood Authority (LLFA) with respect to flooding from surface water, ordinary watercourse and groundwater. This report also outlines the priority for investigation of surface water catchments. Both Llanmaes and St. Athan are stated as having a medium investigation priority, with 18 and 22 properties identified as being at risk of surface water flooding respectively. The timescale for delivery of measures to reduce the risk of surface water flooding is 50 years for medium priority areas.

3.3 SuDS Guidance

Information with regard to the appropriate mechanisms for considering Sustainable Drainage Systems (SuDS) is currently contained in the ‘Interim Code of Practice for Sustainable Drainage Systems’ (2004)7 and the Non-statutory standards for SuDS in Wales (updated 2017)8. From the 7th January 2019, the SuDS process became statutory so that any planning undertaken after this point must demonstrate compliance to these regulations. Planning authorities may consider imposing a condition requiring developers to examine the SuDS option and provide the planning authority with details and options. If it is demonstrated that SuDS could work on a site, and subject to the appropriate agreements being in place with regard to adoption, then the planning authority would require SuDS to be implemented. Developers will need to give good reason why SuDS could not be implemented. If a conventional drainage system does not improve the status quo or has a negative impact then this can be a valid reason for refusal.

5 Vale of Glamorgan Preliminary Flood Risk Assessment, 2011. 6 Vale of Glamorgan Council Local Flood Risk Management Strategy, 2012. Available online: http://www.valeofglamorgan.gov.uk/en/living/highways_and_engineering/Flooding/Flood-and-Coastal-Erosion/Local-Flood-Risk-Management-Strategy.aspx 7 Interim Code of Practice for Sustainable Drainage Systems. National SUDS Working Group, July 2004. Available from: https://www.susdrain.org/files/resources/other-guidance/nswg_icop_for_suds_0704.pdf. Last Accessed: 09/05/18. 8 https://gov.wales/topics/environmentcountryside/epq/flooding/drainage/?lang=en

Residential Development at LlanmaesFlood Consequence Assessment

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4. Flood Risk – To Development

4.1 Overview

TAN15 requires that all potential flood sources that could affect the proposed development be considered. This

chapter includes flooding from rivers and the sea, directly from rainfall on the ground surface, rising groundwater,

overwhelmed sewers and drainage systems. Flooding from reservoirs, canals, lakes and other artificial sources

should also be considered. There should be demonstration of how these should be managed so that the

development remains safe throughout its lifetime, taking into account climate change.

4.2 Tidal

Tidal flood sources include both the sea and estuaries. The assessment of tidal flood risk takes into account the

site’s distance from the Severn Estuary (approximately 3.9km) and minimum ground levels on site (approximately

35m AOD). This assessment identifies that there is no tidal flood risk posed to the development site given the

distance and therefore it is not considered further within this FCA.

4.3 Fluvial

As discussed in Section 1.2, small areas within both Plots are within DAM Zone C2, which are areas classified as

having a high risk of fluvial flooding without significant defence infrastructure. For Plot A this area is where the site

boundary extends over the Boverton Brook to the south and the Llanmaes Brook to the east; and for Plot B this is

where the site boundary extends over the Boverton Brook to the east of the site.

A series of field drains act as conduits for overland flow from the surrounding agricultural fields and outfall in to

the Boverton Brook. Areas within the Proposed Development site have suffered from fluvial flooding in the past,

specifically land around Froglands Farm. This area is a low point and key conveyance route, and the capacity of

the bridge located immediately downstream of Froglands Farm may be exceeded at times of high flow.

An approximate outline of the flood extent from the October 1998 flood event has been taken from the Vale of

Glamorgan PFRA and is shown in Figure 4-1 below.

Figure 4-1: Extent of Flooding 28th October 1998

(Source: Vale of Glamorgan PFRA, 2011)


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The publicly available NRW Flood Map is provided in Figure 4-2. The Flood Map presents the undefended fluvial flood extents for land at a high (3.33% AEP), medium (3.33% AEP to 1% AEP) and low risk (1% AEP to 0.1% AEP).

This shows that the risk to Plot A is at high to low risk where the site boundary is in close proximity to the Boverton Brook in the south; and Llanmaes Brook in the east. The risk to Plot B is at high to low risk adjacent to the Boverton Brook site towards the east of the site.

Figure 4-2: NRW Risk of Flooding from Rivers and Sea Map


Taking the above information into account, the risk posed from fluvial flood sources to the Proposed Development plots, outside of the boundary of the immediate watercourses, is considered to be medium to low. For Plot A, the risk is greatest adjacent to the Boverton Brook and over and adjacent to the Llanmaes Brook. For Plot B the risk is greatest to the east of the site adjacent to the Boverton Brook. However, as discussed in Section 1.2 of this report, the proposed development has been directed away from the areas at risk of flooding, as illustrated in the proposed site layout plans presented in Appendix A. It should also be noted that as per NRW’s DAM maps, the flood risk maps presented in this report do not take into account the construction of Ffordd Bro Tathan.

4.4 Surface Water

The village of Llanmaes is located upstream of Plot A and has historically suffered from surface water flooding. VoGC is currently investigating the benefits of constructing a flood alleviation scheme in Llanmaes. The town of Boverton is located downstream of the both Proposed Development plots and has suffered frequent fluvial flooding in the past. A key aspect of this FCA is to demonstrate that the development plots have no detrimental effect on the flood risk to the town of Boverton and the immediate surroundings.

Overland flow results from rainfall that fails to infiltrate the surface. This is exacerbated where the permeability of the ground is low due to the type of soil and geology (such as clayey soils) or urban development. Surface water flow may also occur in areas where steep topography can rapidly convey water that has failed to penetrate the surface or where the ground may already be saturated.

The BGS’ historic boreholes9 and AECOM Ground Investigation Report10 indicate that clay soils are present at the site in areas surrounding the watercourses. Therefore, infiltration of rainwater may be limited, resulting in higher surface water runoff rates compared with more permeable soils (such as sandy soils).

9 BGS borehole record SS96NE51 and ST06NW54. Available at: http://mapapps.bgs.ac.uk/geologyofbritain/home.html.

Legend

High

Medium

Low

Site Boundary


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Figure 4-3 shows NRW’s Surface Water Flood Risk Map11. The dark orange shading (high risk) shows areas that have a chance of flooding of greater than 3.33% AEP. The light orange shading (medium risk) shows areas have a chance of flooding between 1% AEP and 3.33% AEP. The yellow shading (low risk) shows areas have a chance of flooding between 0.1% AEP and 1% AEP.

Figure 4-3: NRW Risk of Flooding from Surface Water


This shows that for Plot A the majority of the site has no risk except to south of the Boverton Brook adjacent to the Eglwys Brewis Road and that for Plot B there is no risk for the majority of the site with very small pockets of low risk present adjacent to the Boverton Brook in the east of the site.

4.5 Sewer Flooding

Flooding can occur as a result of infrastructure failure e.g. blocked sewers or failed pumping stations. Sewer flooding can occur when the system surcharges due to the volume or intensity of rainfall exceeding the capacity of the sewer, or if the system becomes blocked by debris or sediment.

Due to the lack of historical flood information on sewer flooding in this area combined with the proposed drainage options and current land use, flood risk from sewers is considered low and so will not be considered further within this report.

4.6 Groundwater

Groundwater flooding occurs where groundwater levels rise above ground surface levels. The geology has a major influence on where this type of flooding takes place; it is most likely to occur in low-lying areas underlain by permeable rocks (aquifers).

The superficial and bedrock geology underlying the site has been classified as a minor aquifer, it is an important source of base flow to rivers, but not considered significant for water supply at a strategic scale.

Boreholes available from the GI report that were constructed to aid the planning application for the proposed Ffordd Bro Tathan scheme indicate that groundwater is typically encountered at depths between 0.70m and

10 AECOM, 2017. St Athan Northern Access Road, Factual Ground Investigation Report. 11 Source: Natural Resources Wales Flood Map for Surface Water. Available from: https://flood-warning-information.service.gov.uk/long-term-flood-risk. Last accessed 09/05/18.

Legend

High

Medium

Low

Site Boundary

https://flood-warning-information.service.gov.uk/long-term-flood-risk

https://flood-warning-information.service.gov.uk/long-term-flood-risk


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1.30m bgl across the site. However, the regions of clay soil located above these groundwater bearing soils are likely to form an impermeable layer, preventing shallow groundwater from rising above ground surface levels.

Based on the information derived from the borehole log combined with no specific flood incidents, the risk from groundwater is considered to be low.

4.7 Artificial Sources

Artificial flood sources include raised channels such as canals or storage features such as ponds and reservoirs. The NRW Flood Risk from Reservoirs Map (Figure 4-4) indicates that the nearest area at risk to the proposed scheme is located along the River Thaw, approximately 1.8km away.

Figure 4-4: NRW Risk of Flooding from Reservoirs Flood Map


As the Proposed Development plots are located outside of maximum flood extent associated with reservoir inundation, combined with the low likelihood of a reservoir breach occurring, the risk from this flood source is very low and not considered further within this FCA.


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5. Flood Risk – From the Development

5.1 Overview

National policy highlights how built development tends to increase the risk of flooding by increasing surface water runoff. Development often increases the area of impermeable surfaces thereby promoting rapid runoff to surface water sewers or watercourses rather than percolation into the ground. The effect has potential to lead to an increase in both total volume and peak water flows, contributing to flooding.

5.2 Surface Water Management

In order to comply with national policy, surface water runoff rates up to and including the 1% AEP rainfall event, including climate change (+30%) have been calculated for the proposed site using the mean annual flood flow equation from the Institute of Hydrology Report No. 124 (IH 124) within the WinDES software (Table 5-1 and Appendix B). These calculations are based on an 8.23 ha area for Plot A and a 3.96 ha area for Plot B.

It should be noted that runoff rates have been calculated assuming an overall impermeable area of 40% for each development plot. As can be seen from Table 5-1 and Table 5-2, the storage volume for the 1% AEP plus 30% climate change event, range between 3,091 m3 and 4,244 m3 for Plot A and 1,492 m3 and 2,046 m3 for Plot B. It is likely however these will change depending on the size of the Proposed Development at each plot. In line with national policy, it is the 1% AEP plus 30% climate change which needs to be considered for the Proposed Development.

Table 5-1: WinDES Quick Storage Estimates (FSR Rainfall) – Plot A

AEP Climate Change Discharge Rate

(l/s) Storage Volume

Minimum (m3)

Maximum (m3)

100% 0 1.70 1006 1562

10% 0 2.74 1497 2248

3.33% 0 3.39 1785 2608

1% 30 4.20 3091 4244

Table 5-2: WinDES Quick Storage Estimates (FSR Rainfall) – Plot B

AEP Climate Change Discharge Rate

(l/s) Storage Volume

Minimum (m3)

Maximum (m3)

100% 0 0.82 490 755

10% 0 1.32 720 1081

3.33% 0 1.63 865 1261

1% 30 2.02 1492 2046

Further assessment will need to be completed at detailed design stage, once the development layout is finalised. This will include further runoff and storage calculations specific to the areas of the site that are being developed. This in turn will help inform potential SuDS options (i.e. permeable paving, above ground storage features such as ponds or swales, etc.) depending on what is suitable and appropriate for the type and size of the proposed development at each plot.

It is noted that a Drainage Strategy12 has been prepared for the proposed development, and this FCA should be read in conjunction with that report.

12 Residential Development at Llanmaes – Drainage Strategy Report ref. A093950-14, WYG, October, 2019.


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AECOM 17

6. Flood Map Challenge

6.1 Overview

As discussed in Sections 2.3 and 4.3 of this report, NRW’s current DAM and Flood Risk Maps do not take into account the construction of Ffordd Bro Tathan which included new flood defences on Llanmaes Brook, as detailed in Section 2.3.1 of this report. These works will have a benefit to the proposed development land, particularly Plot A, and will reduce the area of land situated within DAM Zones B and C2.

Accordingly, a Flood Map Challenge has been submitted to NRW with a view to updating the DAM and Flood Risk Maps. These are updated on a quarterly basis and until the Flood Map Challenge has been accepted, and the revised maps published, any planning application relating to these plots is likely to receive comment from NRW. A copy of the Flood Map Challenge and accompanying reports are provided in Appendix C.

NRW has advised that if at the time that a planning application is made and the Flood Map Challenge has been accepted, but the maps have not been updated, then they would advise the Local Planning Authority accordingly and provide a technical response on that basis. It would then be up to the Local Planning Authority to determine how they wish to proceed and either accept the information or respond based on the DAMs published at the time of the planning application.

It should be noted that at the time of writing, the Flood Map Challenge is yet to be accepted by NRW.


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AECOM 18

7. Conclusion

7.1 Overview

This FCA has assessed flood sources to and from the Proposed Development plots in the context of the existing and Proposed Development. The Proposed Development is classified as a highly vulnerable development (residential), however given that the majority of each plot is located within DAM Zone A, development is permitted. There are areas within each plot that coincide with DAM Zone C2 and DAM Zone B where in close proximity to the Boverton and Llanmaes Brooks and consequently development should be avoided in these areas. However, it is noted that the DAM map is due to be updated to take account of the recently constructed Ffordd Bro Tathan, which is expected to result in additional land within each plot being situated within DAM Zone A.

7.2 Flood Sources

The following potential sources of flooding which could affect the Proposed Development plots have been considered and assessed as follows:

• There is no risk of tidal flooding;

• The risk of fluvial flooding to the majority of each plot is considered to be low;

• The risk of surface water flooding to the majority of the site is considered to be low;

• The risk of sewer flooding is considered to be low;

• The risk of groundwater flooding is considered to be low; and,

• The risk of flooding from artificial sources is considered to be very low.

7.3 Surface Water Management

Preliminary runoff and storage calculations have been undertaken; however these assume an overall impermeable area of 40% for each plot which will need to be confirmed at detailed design stage. The Proposed Development plots are likely to consist of new build residential units occupying a potentially larger area (>40%) therefore runoff and attenuation calculations should be refined once further details regarding the development are available.


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AECOM 19

Appendices


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AECOM 20

Appendix A – Proposed Site Layout

A093950-14drf01v17.vwx

Land East of B4265

WYG GroupFloor 5, Longcross Court, 47 Newport Road, Cardiff, CF24 0ADTel: +44 (0)29 2082 9200 Fax: +44 (0)29 2045 5321 Email: [email protected] www.wyg.com© WYG Environment Planning Transport Limited 2019

creative minds safe hands

Based upon the Ordnance Survey map with the permission of The Controller of Her Majesty's Stationery Office, © Crown copyright.WYG Environment Planning Transport Limited 2019, licence no: AR 1000 17603

Key

North

Scale 1:2,000@A3October 2019

A093950-14[B]13a_rev B

Framework masterplanFramework 13a

0 25 50 75 100 125 metres

Secondary access routes (indicative)

Site boundary

Primary access routes (indicative)

Adjoining future development site

Trees and hedgerows

Amenity green space

Garden

Attenuation basin (indicative)

Swale/rain garden (indicative)

Key pedestrian route

Green corridor

Existing bus stop

9.0

Path (um)

Track

HEOLMERIONETH

ROAD

HARDIN

G CLOSE

GASKELL CLOS

PERCY SMITH ROAD

GLANYMOR

CS

CHURCH MEADOW

34.7m

MP 8.75

L E A P

Eglwys Brewis

Llantwit MajorB4265

Boverto

n Brook

k

Eglwys Brewis Road

Va

Ffordd Bro Tathan

North

Land North of West Camp

WYG GroupFloor 5, Longcross Court, 47 Newport Road, Cardiff, CF24 0ADTel: +44 (0)29 2082 9200 Fax: +44 (0)29 2045 5321 Email: [email protected] www.wyg.com© WYG Environment Planning Transport Limited 2019

creative minds safe hands

Based upon the Ordnance Survey map with the permission of The Controller of Her Majesty's Stationery Office, © Crown copyright.WYG Environment Planning Transport Limited 2019, licence no: AR 1000 17603

Key

A093950-14drf01v17.vwx

Scale 1:2,000@A3October 2019

A093950-14[B]13b_RevB

Framework masterplanFramework 13b

0 25 50 75 100 125 metres

Primary access routes (indicative)

Site boundary

Secondary access routes (indicative)

Key pedestrian route

Adjoining future development site

Trees and hedgerows

Amenity green space

Green corridor

Garden

Attenuation basin (indicative)

Existing bus stop

Swale/rain garden (indicative)

Track

42.4m

GP

40.2m

40.2m

39.9m

39.9m

CS

40.2m

RIDGE ROAD

WREN ROAD

39.6m

CS

L E A P

Eglwys Brewis Road

Eagle Road

West Camp

Brook

Boverton

Ffordd Bro Tathan


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AECOM 21

Appendix B – WinDES Calculations

St. Athan – Land Development Flood Consequence Assessment

60509148/BRRP0002


AECOM 20/22

St. Athan – Land Development Flood Consequence Assessment

60509148/BRRP0002


AECOM 21/22


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AECOM 22

Appendix C – Flood Map Challenge

Flood Map Challenge Form

Page 1 of 3

Proposed amendment

Flood Map considered for amendment Yes/No Historic Flood Map No Flood Zones Yes Development Advice Map (C1/C2) Yes Other (please state) No Amendment details: Reference

Watercourse LLanmaes Brook and Boverton Brook

Geographic location

St Athan, Vale of Glamorgan, United Kingdom

Reaches (if known)

LLanmaes Brook and Boverton Brook Easting 297334

299195 Northing 169635 170157

Amendment reason: New modelling information

Yes New historic information No

New topographic information

Yes New defence information Yes

Reason Model update for FCA and planning permission.

Supporting evidence

Model ref: Name: St Athan Author: Jessica Pearce Date: Aug-19 Consultant:Wallingford Hydro Solutions

Contacts: Name Thomas MacClough Address One Callaghan Square, Cardiff, CF10 5BT Date NRW FRA Contact Filippo Scimone

Contact details

Filippo Scimone, Flood Risk Analysis Specialist Advisor, Natural Resources Wales, Haverfordwest, Tel: 03000 65 3561, Email: [email protected]

This is to certify that I have read the NRW Guidance Note – GN 029 Flood Map Challenge (GN29) and by making a formal flood map challenge I or, where I have indicated below that I am acting on behalf of a company or other organisation, the company or other organisation on whose behalf I am acting (the “Company”) hereby:

1. Grant(s) NRW an irrevocable royalty-free non-exclusive license to use all models and other information submitted to NRW (Materials) and any and all intellectual property rights subsisting in the Materials (Materials IP) for the purposes of updating the Flood Map [(as defined in GN29)] and for any other purpose together with the right to grant sublicenses to third parties;

2. Warrant(s) that:

a. I am/the Company is able to grant the licence being granted in paragraph 1 above;


Page 2 of 3

b. (in the case of a challenger who is an individual) I am the sole creator of the Materials;

c. the Materials are original and have not been copied wholly or substantially from any other material, work, or source;

d. (in the case of a challenger who is an individual) the Materials were not created in the course of my employment or pursuant to any agreement which transferred the Materials or Materials IP to any third party or (where a flood map challenge is being submitted on behalf of the Company), the Materials or Materials IP were created solely by employees of the Company in the normal course of their employment or by a third party pursuant to an agreement that transferred ownership of the Materials and Materials IP to the Company;

e. I am/the Company is the sole legal and beneficial owner of the Materials and the Materials IP; and

f. Exploitation of the Materials and Materials IP by NRW or any person authorised by NRW will not infringe any third-party rights.

3. Shall indemnify and hold NRW harmless against any loss or damage or liability or cost (including reasonable legal expenses) suffered or incurred by NRW resulting from any breach of any of the above warranties.

Signed by …………………………….……………………………………………

Print Name Thomas MacClough………….………………………………………

[If relevant] Duly authorised signatory for and on behalf of

Welsh Government…..….……………………………………………

Position Engineer………………….……………………………………………

Date 05/09/2019……………….……………………………………………


Page 3 of 3

Recommendation for change to map To be completed by Flood Risk Analysis team. Include details on how acceptable the information provided is to change the extent, defence location, insertion of dry island or comment on likelihood of property flooding.

Recommend that the Flood Map is updated to amend Flood Zones 2, 3 to reflect new model information from the XXXXXXXXX Study (YYYY).

Decision To be completed by the Flood Risk Analysis team leader

Request granted [Yes / No] Date of decision [dd/mm/yyyy]

Reason granted / rejected

Map change date: internal [dd/mm/yyyy] Map change date: web [dd/mm/yyyy]

St Athan Fluvial Model Updates

September 2019

Technical Note

Project St Athan Northern Access Road

Project Number WHS1701

Title St Athan Fluvial Model Updates

Description This technical note details the updates made to the existing St Athan

Hydraulic Model in order to satisfy the requirements for a flood map

challenge following the construction of the St Athan Northern Access

Road.

This document should be read in conjunction with the associated

hydrology report (Appendix 1) and the original hydraulic modelling

report produced by AECOM (Appendix 2).

Date 03/09/2019

Version 1.0

1 Introduction

Wallingford HydroSolutions (WHS) have been commissioned by Alun Griffiths to update the existing

fluvial hydraulic model of St Athan, which was last updated by AECOM in 2016 in order to prepare

the model for use in a flood map challenge. The existing Natural Resources Wales (NRW) flood map

does not include the St Athan Northern Access Road and associated flood mitigation measures which

were constructed in 2019.

Liaison with NRW has confirmed the requirements for the model updates to ensure the model is

appropriate for a flood map challenge. This report details the updates made to the existing model in

line with the requirements specified by NRW:

A review and update of the hydrology estimates informing model inflows.

The model has been updated to run using the latest version of TuFLOW

Bridges and weirs have been set to using the latest model unit types

The model extent has been reduced to position the downstream boundary at NGR: SS 9830 6860

The model includes all water courses that may impact upon the site

The St Athan Northern Access Road and associated culverts, underpasses and flood defences have

been included within the model

Sensitivity runs for the 1.0% AEP event.

The updates made to the model are detailed within this technical note, this should be read in

conjunction with the original hydraulic modelling report produced by AECOM in 2016, attached as

Appendix 2. The model will be run for the 3.33%, 1%, 1%+CC and 0.1% AEP scenarios, the

associated mapped outputs are provided in Appendix 3.

2 Model History

In 2016 AECOM updated an ESTRY-TUFLOW model of the Boverton Brook and Llanmaes Brook to

inform a Flood Risk Assessment for the proposed Northern Access Road in St Athan. The original

model was provided to AECOM by NRW and was constructed in 2014 using ESTRY-TUFLOW software,

the model included the Boverton Brook and Llanmaes Brook and was used to inform flood risk maps

for Boverton

In 2016 the model was extended approximately 1km upstream along the Boverton Brook to assess

the impact of the proposed Northern Access Road on flood risk. The hydrology was not updated

during this model extension. The model was run using ESTRY and TUFLOW version 2016-03-AC-

ISP-w64 and simulated the Boverton Brook from NGR: 299195, 170157, and the Llanmaes Brook

from NRG: 297334, 169635. The model extended downstream to the coast at NGR: 295633, 167457,

the 2016 model extent is shown in Figure 1 and the hydraulic modelling report is available in

Appendix 2.

Figure 1- 2016 AECOM model extent

Following the completion of the FCA the Northern Access Road was approved for construction which

began in 2019, as such the model is required to be updated to reflect the “as built” road design.

Wallingford HydroSolutions have obtained the 2016 model and updated it in line with the

requirements outlined in Section 1. The model log detailing the updates made is available in Appendix

4.

3 Hydrology

Updated hydrology estimates have been completed using the latest NRFA datasets and data from

the Boverton gauging station. A summary of the catchments and the 2019 hydrology estimates vs

the 2013 estimates are provided Figure 2 and Table 1. The main hydrology report is available as

Appendix 1.

For the 100 year and 1,000 year events, the updated 2019 estimates are generally higher.

Figure 2. Location of FEH Web Service Catchments and Inflow Locations

Table 1 - Summary of 2019 flow estimates vs 2013 flow estimates

2yr (m3/s) 50yr (m3/s) 100yr (m3/s) 1000yr (m3/s)

Site

Code

NRW

2013

WHS

2019

Change

(%)

NRW

2013

WHS

2019

Change

(%)

NRW

2013

WHS

2019

Change

(%)

NRW

2013

WHS

2019

Change

(%)

8 3.1 2.84 -8.44 7.83 7.58 -3.25 9.27 9.09 -2.00 16.48 16.58 0.62

9 2.87 2.74 -4.56 7.24 7.3 0.84 8.57 8.76 2.15 15.07 15.98 6.07

10 1.15 1.11 -2.69 2.89 2.97 2.83 3.42 3.56 4.16 6.16 6.51 5.62

11 1.91 1.89 -1.40 4.83 5.03 4.19 5.72 6.03 5.53 10.08 11.01 9.28

12 0.65 0.67 4.33 1.63 1.8 10.24 1.93 2.16 11.66 3.38 3.94 16.47

13 1.21 1.24 2.99 3.04 3.31 8.83 3.6 3.97 10.24 6.35 7.25 14.20

4 Model Updates

4.1 Overview

The primary model updates relate to the inclusion of the completed Northern Access Road and the

associated flood defence schemes. Table 2 summaries the data sources used to inform the geometry

of the Northern Access Road and associated flood relief culverts and structures (attached in Appendix

5).

Table 2- Northern Access Road and flood protection data sources

Feature Data Source

Northern Access Road NAR-BUR-ALL-ALL-M3-C-StringModel_190704.dwg

Agricultural Underpass 4187700-001-Approval In Principle Agricultural Underpas-6

Frogland Wall 4187700-004-Approval In Principle Frogland Wall -4

Llanmaes Low Flow culvert 4187700-002-Approval In Principle Low Flow & Flood Culverts-7

Llanmaes High Flow culverts 4187700-002-Approval In Principle Low Flow & Flood Culverts-7

East Low Flow Culvert 4187700-003-Approval In Principle East Culverts -5

East High Flow Culverts 4187700-003-Approval In Principle East Culverts -5

4.2 Software and Run Parameters

The hydraulic model has been run using the latest version of ESTRY-TUFLOW, 2018-03-AE

TUFLOW_iSP_w64, utilizing HPC software in a GPU environment.

4.3 Updated Model Units

A number of bridges within the hydraulic model were updated during the preparation of the model

for a flood map challenge to reflect the latest software update. The bridges within the model were

updated to BB bridges as introduced in the 2016-03-AA model, the following bridges in the 1D

network have been updated accordingly.

LLAN_0009BW – updated to BBW

LLAN_0008BW – updated to BBW

BOVE_0050BW – updated to BBW

BOVE_0041BW – updated to BBW

BOVE_0048BW - updated to BBW

4.4 Model Extent

The river channel was modelled in 1D in ESTRY and sufficiently captures all hydraulic influences on

the area of interest. The upstream extent of the Boverton Brook lies approximately 1km north of the

proposed access road with the upstream of the Llanmaes Brook situated 2km north of the road. The

downstream boundary is positioned approximately 500m downstream of the road at NGR: 298301,

168595 as advised by NRW. Figure 3 shows the modelled extent.

Figure 3: Model extent

4.4.2 Modelled watercourses

Whilst updating the hydrology estimates, no additional watercourses of a significant catchment size

within the vicinity of the Northern Access Road were identified. Therefore, the modelled watercourses

remain unchanged from the 2016 model and the model includes all watercourses which may impact

upon the site.

4.4.3 Downstream Boundary

As per NRW’s request, the model extent has been reduced with the downstream boundary positioned

at NGR: 298301, 168595. A HQ (Head/Flow) boundary has been applied at this position to allow

water to flow out of the 1D domain. The HQ boundary utilised the baseline model results from the

2016 model at node BOVE_0036a.1 to produce a stage-discharge curve which was applied to the

boundary based on modelled stage and flow, shown in Figure 4.

Figure 4- Downstream boundary, Head-Discharge Relationship

30.2

30.4

30.6

30.8

31

31.2

31.4

31.6

31.8

32

0 1 2 3 4 5 6 7 8

Head (

H m

AO

D)

Discharge (Qm3/s)

4.5 Northern Access Road

4.5.1 The “As Built” Northern Access Road

The “as built” northern access road has been added into the 2D domain based upon the finalised

CAD drawing provided by Burroughstitled NAR-BUR-ALL-ALL-M3-C-StringModel_190704 (available

as Appendix 5).

The northern access road was incorporated into the 2D domain by converting the provided 3D string

lines into an ascii grid at a resolution of 0.5m. The derived ascii grid is shown in Figure 5. This method

was chosen over importing the elevation points from the CAD file within a z shape as the model

required a large amount of time to triangulate the points.

Figure 5- St Athan Northern Access Road as built topography

4.5.2 “As Built” Flood Bunds

The flood bunds situated to the immediate north and south of the road were incorporated into the

model within the ascii grid of the road.

The flood bunds situated further north of the road at NGR: 299450, 169350 and NRG: 299590,

169661 were incorporated using 2D z shapes containing the elevations of the finalised structures.

2d_zsh_final_design_flood_bunds_R_001.shp

2d_zsh_final_design_flood_bunds_P_001.shp

The crests of the flood bunds have been enforced by reading in a z line thick to ensure the topography

is correctly represented.

2d_zsh_final_design_flood_bund_crest_L_001.shp

2d_zsh_final_design_flood_bund_crest_P_001.shp

4.6 Approved in Principle structures

Alun Griffiths provided WHS with “Approval in Principle” design drawings of the four structures

associated with the construction of the northern access road, the structures have been incorporated,

into the relevant 1D/2D domains of the hydraulic model. The structures and the associated design

drawings are listed (available as Appendix 5):

Agricultural underpass, 4187700-001-Approval In Principle Agricultural Underpas-6

Frogland Wall, 4187700-004-Approval In Principle Frogland Wall -4

East Culverts, 4187700-003-Approval In Principle East Culverts -5

Low Flow and Flood Culverts, 4187700-002-Approval In Principle Low Flow & Flood Culverts-7

4.6.1 Agricultural Underpass

The Agricultural Underpass was incorporated into the model as a 1D ESTRY culvert within the 2D

domain. The design drawings indicate that the underpass is built from a precast box section with a

geometry of 3x3m, as such, the underpass was modelled as a rectangular culvert with 3x3m

dimensions.

Setting out points from the “Approval in Principle” drawings identify an inlet invert level of 38.516m

AOD, and an outlet invert level of 38.106, as per the drawings the underpass was modelled with a

length of 20.30m and connected to the 2D domain using SX connections.

1d_nwke_agricultural_underpass_001.shp

2d_bc_HX_L_AGR_UNDPASS_001.shp

4.6.2 Frogland Wall

Frogland Wall was added into the 2D domain as a DTM modification. Setting out points along the

centre line of the wall, extracted from the approved in principle drawings, were converted to z shape

points to which a z line THICK was snapped to represent the flood defence.

(2d_zsh_frogland_wall_P_001)

(2d_zsh_frogland_wall_L_001)

4.6.3 East Culverts

Flood Bund Culvert

Node BOV_EXT_T4_C Type Circular Culvert (C)

Source of Data 4187700-003-Approval In Principle

East Culverts -5

Modelled Yes

NGR 299562, 169288

BOV_EXT_T4_C is a circular culvert under a flood bund, it has been modelled in ESTRY as

a circular culvert.

The “approval in principle” plan of the culvert is shown below.

The dimensions and invert levels for the culvert have been obtained from the approved in principle drawings, upstream invert: 41.219m AOD, downstream invert: 40.877m AOD.

The culvert has a diameter of 1.2m and a modelled length of 22.507m based upon the

setting out points identified within the drawing.

Height Contraction Coefficient (Height_Con) = 0.6

Width Contraction Coefficient (Width_Con) = 0.9

Entry loss, 0.5. Exit loss, 1.0.

The culvert has been assigned a manning’s value of 0.013 representing a concrete

structure.

Approved in Principle drawing (Full drawing available in Appendix 5)

Low Flow Eastern Culvert

Node BOV_EXT_EC_C Type Circular Culvert (C)

Source of Data 4187700-003-Approval In Principle East Culverts -5

Modelled Yes

NGR 299571, 169231

BOV_EXT_T4_C is a low flow culvert and has been modelled in ESTRY as a circular culvert.

The “approval in principle” plan of the culvert is below within dashed red boundary.

The dimensions and invert levels for the culvert have been obtained from the approval in

principle drawings, upstream invert: 40.730m AOD, downstream invert: 40.866m AOD.

The culvert has a diameter of 2.1m and a modelled length of 32.492m based upon the setting out points identified within the drawing.





structure.


High Flow Eastern Culverts

Node BOV_EXT_CA BOV_EXT_CB

Type Circular Culvert (C)

Source of Data 4187700-003-Approval In Principle East Culverts -5

Modelled Yes

NGR 299562, 169240

BOV_EXT_CA and BOV_EXT_CB are two high flow culverts, modelled in ESTRY as circular

The “approval in principle” plan of the culverts is below within dashed red boundary.

BOV_EXT_CA, u/s invert: 41.943m AOD, d/s invert: 40.871m AOD.

BOV_EXT_CB, u/s invert: 41.951m AOD, d/s invert: 40.870m AOD.

The culvert has been connected to the 2D domain using an SX connection, a z shape was

used to appropriately set the elevation of the ground at the outlet to below the downstream invert, with the elevation set to 40.86m AOD (2d_zsh_topo_change_R_001). The SX connection at the inlet to the culvert was assigned a z flag.

Both culverts have a diameter of 1.2m and a modelled length of 32.555m.





structure.


4.6.4 Llanmaes Low Flow and Flood Culverts

Low Flow Culvert

Node LLAN_LFR_C Type Rectangular Culvert (R)

Source of Data 4187700-002-Approval In Principle Low

Flow & Flood Culverts-7

Modelled Yes

NGR 298519, 169039

LLAN_LFR_C is a low flow flood relief rectangular culvert under the northern access road,

it has been modelled in ESTRY as an R culvert.

Upstream invert: 37.606m AOD, downstream invert: 36.602m AOD. This is based upon

the setting out levels plus 300mm to allow for the formation of a natural bed due to the inclusion of fish passage mitigation baffles.

The culvert has a width of 1.5m, a height of 1.5m and a modelled length of 70.83m based upon the setting out points identified within the drawing and to allow for the formation of

a natural bed.

Height Contraction Coefficient (Height_Con) = 0.6,



The culvert has been assigned a manning’s value of 0.015, representing a natural bed with

concrete walls and soffit.


Llanmaes High Flow Culverts

Node LLAN_FRC_1, LLAN_FRC_2, LLAN_FRC_3, LLAN_FRC_4

Type Rectangular Culvert (R)

Source of Data 4187700-002-Approval In Principle Low Flow & Flood Culverts-7

Modelled Yes

NGR 1) 298533, 169010 2) 298537, 169010

3) 298544, 169011 4) 298548, 169012

LLAN_FRC_1, LLAN_FRC_2, LLAN_FRC_3, LLAN_FRC_4 are four flood relief culverts, all

are rectangular in shape and run under the northern access road. The culverts all have the

same dimensions and upstream and downstream invert levels. They have been modelled in ESTRY as an R culverts.

Upstream invert: 39.508m AOD, downstream invert: 39.143m AOD.

The culverts have been connected to the 2D domain using SX connections assigned with a z flag, to appropriately set the ground elevation at the invert and outlet locations.

The culverts have a width of 2.7m, height of 1.225m and a modelled length of 27.431m based upon the setting out points identified within the drawing.




The culverts have been assigned a manning’s value of 0.013 representing a concrete structure.


5 Modelled Results

The modelled results for the 1.0% AEP and 0.1% AEP fluvial events, representing Flood Zones 3 and 2 respectively are shown below alongside the current NRW flood zones.

The following figures show the mapped flood extent for the 1% AEP flood event alongside the previous NRW Flood Zone 3 mapping.

Figure 6 Modelled extent vs Current NRW Flood Zone 3 whole model domain

Figure 7- Modelled extent vs Current NRW Flood Zone 3 Llanmaes

Figure 8- Modelled extent vs Current NRW Flood Zone 3 Eastern Culverts

The following figures show the mapped flood extent for the 0.1% AEP flood event alongside the previous NRW Flood Zone 2 mapping.

Figure 9- Modelled extent vs Current NRW Flood Zone 2 whole model domain

Figure 10- Modelled extent vs Current NRW Flood Zone 2 Llanmaes Culverts

Figure 11- Modelled extent vs Current NRW Flood Zone 2 Eastern Culverts

6 Sensitivity Analysis

Sensitivity analysis has been carried out on varying flow (Q+/-10%), manning’s roughness (n+/-

20%), and downstream boundary gradient (b+/-20%). All sensitivity analysis has been carried out

on the 1% AEP event as per NRW’s request. The sensitivity of the model to these changes has been

reviewed by identifying the change in flood level at numerous sample points within the modelled

flood extent. The positioning of the sensitivity sample points are shown in Figure 12.

Figure 12- Sensitivity Sample Point

The chosen sensitivity sample points have been positioned downstream of the culvert features added

during the construction of the Northern Access Road.

6.2 Flow

The sensitivity of the model to changes in flow (Q +/-10%) was tested. The testing indicated that

the model is only minimally sensitive to changes in the flow with the maximum variation in modelled

depth reaching 126mm. This variation is deemed acceptable given the relative change in modelled

inflows.

Point BSC

Depth (m)

Q+10%

Depth (m)

Q-10%

Depth (m)

Max Difference

(m)

1 0.656 0.757 0.529 - 0.127

2 0.008 0.016 0.001 + 0.007

6.3 Manning’s n

The sensitivity of the model to changes in Manning’s (n+/-20%) roughness coefficient was tested.

The testing indicates that the model is only minimally sensitive to changes in Manning’s roughness

value with a maximum change in modelled flood depths of 88mm.

Point BSC

Depth (m)

n+10%

Depth (m)

n-10%

Depth (m)

Max Difference

(m)

1 0.656 0.693 0.609 -0.047

2 0.008 0.096 0.088 +0.088

6.4 Downstream Boundary

The sensitivity of the model to change in the boundary conditions was assessed. The boundary slope

was changed by +/-20%. The results indicate that the model is only minimally sensitive to changes

in the downstream boundary slope with modelled flood depths varying by a maximum of 95mm.

Point BSC

Depth (m)

dsby +10%

Depth (m)

dsby -10%

Depth (m)

Max Difference

(m)

1 0.656 0.751 0.751 +0.095

2 0.008 0.100 0.100 +0.092

The results of the sensitivity testing indicate that the model is stable with minimal variation in modelled flood depths at the chosen sensitivity points in response to changing model parameters.

7 Model Stability and Limitations

7.1 Time Step

With the HPC solver, one of the main indicators of model stability is the time step selected by

TuFLOW. This has been reviewed and plotted in Figure 13.

Ideally, the time step should not fall below one tenth of the time step that would have been used

with the classic solver. For this model, a time step of between 1 and 0.5 would have been selected

for a 2m grid, hence the HPC time step should not fall below 0.1 and 0.05 seconds. The plot of the

evolution of the 2d time step falls within these criteria.

In addition to the 2d timestep, there are three control numbers which must be satisfied;

A Nu value of 1.0 or greater may indicate that the velocity is unusually high, or the cell size is too

small for the modelled velocity.

A Nc value of 1.0 or higher can be caused large depths in relation to the cell size.

A Nd value of 0.3 or higher, might suggest that there is poor boundary setup, or insufficient SX

cells linked to a 1D structure or the cell size is too small.

The model stability indicators for dt and Nu are acceptable. Nc and Nd reach their respective limits

of 1.0 and 0.3 for a portion of the model run, however TUFLOW prevents the values from exceeding

these limits by decreasing the time step. As the time step remains within acceptable limits for the

duration of the model this is considered satisfactory.

Figure 13 - TUFLOW HPC Indicators

0

0.2

0.4

0.6

0.8

1

1.2

0 20000 40000 60000 80000 100000 120000

Valu

e (

dt

(s))

(N

u,

Nc a

nd N

d)

iStep

St Athan 1% AEP Flood Event

TuFLOW HPC Srability Indicators

dt Nu Nc Nd

7.2 Checks and Warning Messages

A number of Check and Warning messages are present in the TUFLOW log file upon completion of

the model runs, which for the 0.1% AEP flood event are summarised in Table 3. These have been

reviewed and they are all considered to be acceptable.

Table 3- TUFLOW Check and Warning messages

ID Count Comment

Check 2370 128 Ignoring coincident point found in ORIGINAL layer. Many of these

check points were carried forward from the original 2016 AECOM,

additional ones are surrounding the flood bund.

Check 2099 27 Ignored repeat application of boundary to 2D cell. This is

acceptable.

Check 2118 14 Lowering of cell elevation due to Z flag on SX links. This is

intended.

Check 1037 12 Interpolate channel sections along the Boverton Brook and

Llanmaes Brook. This is intended.

Check 1152 6 Cross sections appropriately informed by HW tables.

Warning 2118 4 Lowering of cell elevation due to Z flag on SX links.

Warning 1100 3 Warning adopted from AECOM 2016 model, channel geometry/

survey data not available to check.

Warning 1132 1 Channel LLAN_0016c assigned an unusually high form loss per

unit length via the Exit_Loss attribute- warning adopted from

AECOM 2016 model.

Check 1284 1 Connecting a 1D H boundary to 2D HX link- check at downstream

boundary of the model

St. Athan NAR – Land Development Flood Consequence Assessment

60509148/BRRP0002


AECOM 22


60509148/BRRP0002


AECOM 23

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