CHAPTER 10 HYDROLOGY AND DRAINAGE - dppukltd.com · 10.1 This Environmental Statement (ES) chapter...

CHAPTER 10

HYDROLOGY AND DRAINAGE

Environmental Statement – Hydrology and Drainage

10-1

10.0 HYDROLOGY AND DRAINAGE

Introduction

10.1 This Environmental Statement (ES) chapter assesses the likely significant effects of the proposed

development of Land north of Llantrisant Road, Creigiau, with specific regard to hydrology and

flood risk, water quality, surface and foul water drainage.

10.2 This chapter is supported by the outline drainage strategy included in Appendix 10.1, which has

been assessed as a tertiary mitigation measure for the management of potential impacts on

water and drainage receptors. Information relevant to this water environment assessment can

also be found in other chapters of this ES, in particular Chapter 8 Ecology and Nature

Conservation.

10.3 The Hydrology & Drainage ES Chapter has been prepared by Eleanor Maxfield BSc, Senior Flood

Risk Analyst at JBA Consulting, and has been technically reviewed by Mark Cope FGS MIEMA CEnv

REIA Principal Environmental Consultant at JBA Consulting.

Regulatory and Policy Context

Flood and Water Management Act 2010

10.4 The Flood and Water Management Act (FWMA) 2010 places a duty on Local Authorities to

prepare a Local Flood Risk Management Strategy (LFRMS) that sets out how the Lead Local Flood

Authority (LLFA) will proactively seek to minimise flood risk and prepare our communities. It

places duties on Cardiff City Council as LLFA to provide better, more comprehensive management

of flood risk for people, homes and businesses by promoting an integrated approach to planning

and managing all forms of flood risk.

10.5 Schedule 3 of the FWMA requires surface water drainage for new developments to comply with

statutory national standards for sustainable drainage systems (SuDS). The statutory standards

were published by Welsh Government on 15 October 2018 along with the Statutory Instruments.

Schedule 3 to the FWMA 2010 places a duty on local authorities as SuDS Approving Body (SAB) to

approve, adopt and maintain systems compliant with section 17 of the Schedule. Under Schedule

3, as of 7th January 2019, all new developments of more than 1 dwelling house or where the

construction area is 100m2 or more, require SuDS for surface water.

Water Environment (WFD) (England and Wales) Regulation 2017

10.6 The EU Water Framework Directive (WFD) (2000/60/EC) is transposed into law in England and

Wales by the Water Environment (Water Framework Directive) (England and Wales) Regulations

2017. The WFD requires all watercourses that are not designated as Artificial or Heavily Modified

Water Bodies (A/HMWB) to achieve both Good Chemical Status (GCS) by 2027 and Good

Ecological Status (GES) by 2021. River Basin Management Plans (RBMP) prepared by the

competent authority, outline the actions required to enable these water bodies to achieve GES.


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All HMWB’s are required to achieve Good Ecological Potential (GEP) and Good Chemical Status by

the deadlines given above, through the implementation of a series of mitigation measures. The

proposed development is within the Severn River Basin Management Plan1, which is led by the

Environment Agency. The South East Valleys Management Catchment2 forms a sub-unit of the

Severn RBMP area, managed by Natural Resources Wales (NRW), and includes the location of the

proposed development site.

10.7 New activities and schemes that affect the water environment have the potential to adversely

impact biological, hydro-morphological, physico-chemical and/or chemical quality elements (WFD

quality elements), leading to a deterioration in water body status. They may also render

proposed improvement measures ineffective. Under the WFD, activities and schemes must not

cause deterioration in water body status or prevent a water body from meeting GES/GEP by

invalidating improvement measures.

Planning Policy Wales (PPW) 2018

10.8 Chapter 6 of PPW (Edition 10, February 2018)3 provide guidance to the issues relating to water

and flood risk. With regards to water services, the Welsh Government aims to secure the

provision of water services whilst minimising adverse impacts on the environment, amenity,

health and communities, in light of the consequences of climate change. This should be achieved

by ensuring that the infrastructure is adequate to accommodate proposed development,

ensuring sustainable drainage systems are an integral part of the design approach for new

development, and ensuring the protection of surface water quality.

10.9 Sections of PPW of particular relevance to sustainable water management are quoted below:

Section 6.6.6 ‘The ability of the planning system to protect water features and foster

sustainable water management as key attributes of attractive and resilient places to live is closely

aligned with securing the multiple benefits of green infrastructure. Embracing integrated

approaches should make a contribution toward achieving the requirements imposed by EU Water

Framework Directive along with Welsh Government policy for the integrated planning and

management of water both in urban and rural areas.’

Section 6.6.9 ‘Development proposals should incorporate design for surface water

management, based on principles which work with nature to facilitate the natural functioning of

the water cycle.’

10.10 With regards to development and flood risk, planning authorities should take a precautionary

approach of positive avoidance in areas of flooding from the sea or from rivers. Surface water

flooding should affect the choice of location and layout of scheme designs. Sections of PPW with

particular relevance to flood risk are quoted below:

1 EA Severn River Basin Management Plan. https://www.gov.uk/government/publications/severn-river-basin-district-river-basin-

management-plan 2 NRW South East Valleys Management Catchment Summary. https://naturalresources.wales/media/679387/2016_updated-south-east_valleys_catchment_summary_nrw.pdf 3Planning Policy Wales, Edition 10. December 29018. http://www.planning-policy-wales-edition-10.pdf

https://www.gov.uk/government/publications/severn-river-basin-district-river-basin-management-plan


https://naturalresources.wales/media/679387/2016_updated-south-east_valleys_catchment_summary_nrw.pdf


http://www.planning-policy-wales-edition-10.pdf/


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Section 6.6.25 ‘Development should reduce, and must not increase, flood risk arising from river

and/or coastal flooding on and off the development site itself. The priority should be to protect the

undeveloped or unobstructed floodplain from development and to prevent the cumulative effects

of incremental development.’

Section 6.6.27 ‘Planning authorities should be aware of the risk of surface water flooding,

usually caused by heavy rainfall, and ensure developments are designed and planned to minimise

potential impacts. Development should not cause additional run-off, which can be achieved by

controlling surface water as near to the source as possible by the use of SuDS.’

Section 6.6.17 ‘New developments of more than one dwelling or where the area covered by

construction work equals or exceeds 100 square metres also require approval from the SuDS

Approval Body (SAB) before construction can commence.’

10.11 Welsh Government Technical Advice Note 15: Development and Flood Risk (TAN15, July 2004,) is

referred to in PPW for further policy advice on development and flood risk. TAN-15 states that

‘the aim should be for new development not to create additional run-off when compared with the

undeveloped situation’. TAN-15 requires that consideration must be given to the impacts of

climate change on flooding over the lifetime of the development.

Cardiff City Council Local Development Plan (LDP) 2016 - 2026

10.12 The Cardiff City Council LDP4 includes specific requirements with regard to sustainable

management of the water environment and flood risk. These are outlined below.

10.13 EN14: Flood Risk. This policy seeks to avert development from areas at risk of flooding or where

it would increase the risk of flooding or cause additional run off from development,

developments must incorporate environmentally sympathetic SuDS design:

‘Development will not be permitted…where it would increase the risk of flooding from fluvial

and/or tidal flooding or from additional run-off from the development in any location…

The developer should demonstrate that they have considered the need to incorporate

environmentally sympathetic flood risk mitigation measures such as Sustainable Urban Drainage

Systems (SUDS).’

10.14 Policy EN 14 states that new developments will only be permitted where the Council is satisfied

that they will not result in adverse effects on the quality and/or quantity of surface water

resources. Cardiff City Council are opposed to the culverting of watercourses because of the

adverse ecological, flood risk and other effects that are likely to arise.

4 Cardiff Adopted LDP 2006 – 2026. January 2016. https://www.cardiff.gov.uk/ENG/resident/Planning/Local-Development-

Plan/Pages/default.aspx

https://www.cardiff.gov.uk/ENG/resident/Planning/Local-Development-Plan/Pages/default.aspx



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10.15 EN11: Protection of Water Resources. This policy states that development will not be permitted

that would cause unacceptable harm to the quality or quantity of underground, surface or coastal

waters.

10.16 The site is allocated as Strategic Site E on the LDP Proposals Plan. Cardiff City Council Strategic

Flood Consequences Assessment (SFCA) forms an evidence base document for the LDP and was

updated in 2011. The SFCA includes a review of the application site (named as site 102LGR in the

SFCA). Overall a moderate level of risk is identified, however successful risk mitigation is

anticipated with no significant issues identified for the development of the site with regards to

flood risk and drainage.

EIA Scoping Opinion

10.17 An EIA scoping report was submitted to Cardiff City Council Planning in January 2019 outlining

the proposed scope of assessment. An EIA Scoping Opinion was received from the Planning

Authority in March 2019. The response included a statutory consultation response provided by

NRW dated 8th February 2019 is provided in Appendix 10.2.

10.18 In isolation, the proposed development is unlikely to constitute EIA development. However, an

EIA is required on the basis of Schedule 3 part 1(b) the “cumulation with other development”, in

particular, with consideration of the larger strategic site D (J33) in the LDP located immediately

south of the site. The scope of this EIA is therefore broadly based on the assessments required

for the J33 development applications, to enable the impacts as a whole to be considered. The

Council’s formal response to scoping for the ‘Hydrology and Drainage’ chapter has not raised any

further issues to those outlined in the scoping report.

10.19 A further development site is located 3km south-east of proposed development, called the

Plasdwr development (strategic site C in north-west Cardiff). The cumulative impacts of sites D

and E with sites C has been scoped out of this assessment on the basis of proximity from impacts

and likely significant effects. The drainage of the Plasdwr development site flow into the River Ely

some distance downstream of the contributing flows from the proposed development and are

separated by different catchments.

10.20 The scope of the assessment of likely significant effects of the development proposals presented

herein is limited to water receptors within the site boundary, and the watercourse and

hydrological regime in the downstream catchment of the Nant Henstaff and Nant Coslech, both

tributaries of the River Ely.

Consultation

10.21 Early consultation was carried out with Cardiff City Council and verbal confirmation that hydraulic

modelling of the Nant Henstaff tributary of the River Ely would not be necessary as part of the

proposed development application. The drainage department at Cardiff City Council (David Brain,

Ian Titherington Pers. Comm.) reinforced the need for a compliant Schedule 3 SuDS scheme

ensuring that specific site constraints had been discussed. The sustainable drainage strategy


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masterplan was tabled in a pre-application meeting, the response was favourable with some

points raised regarding design and quantification, these have been addressed in the final

proposed outline drainage strategy. During these meetings with Cardiff City Council’s drainage

team, the broad parameters of the approach to the, drainage strategy, EIA and SuDS approval

were agreed.

10.22 Consultation on the foul drainage strategy has been carried out with Dwr Cymru/Welsh Water

(DCWW) (Matthew Lord). DCWW have provided agreement in principle for foul connection to the

existing Coslech Sewage Treatment Works via a section 98 sewer requisition in the public

highway (approximately 1.25km). DCWW has suggested further hydraulic modelling should be

carried out to confirm capacity. s such further consultation with DCWW is ongoing.

10.23 The scoping response from Cardiff City Council made specific reference to the response from

DCWW regarding limited capacity of the sewerage network at Creigiau Waste Treatment Works

(CWTW) and the requirement for foul drainage towards a CWTW with adequate capacity.

10.24 The response from NRW (provided in Appendix 10.2) made no comments on flood risk, hydrology

or drainage.

Methodology of Assessment

10.25 The assessment undertaken herein has been developed from the scope outlined in the EIA

Scoping Report, which including the following:

• Review of legislation, policies and guidelines relevant to water resources, water quality and

flood risk;

• Examination of the baseline conditions for the study area;

• Identification of receptors and determination of their sensitivity or relative importance;

• Characterisation of impacts to receptors from the proposed development during the

construction and operational phase;

• Assessment of Likely Significant Effects (LSE);

• Identification of proposed mitigation measures to avoid, reduce, offset or compensate for

the LSE; and

• Assessment of residual effects taking into account the likely effectiveness of mitigation

measures.

10.26 The likely cumulative effects of the proposals for the application site together with proposals for

the J33 LDP site D has also been assessed with regards to flood risk and drainage.

10.27 The water assessment has been adapted from industry recognised practice guidance provided in

Volume 11 of the DMRB (Highways Agency, 2009) and the detailed assessment methodologies

set out in Section 3, Part 10, Annex I.


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Identification of receptors

10.28 Water environment receptors have been identified based on the value of the feature or resource

using the criteria described in Table 10.1 (adapted from the DMRB HD 45/09 Table A4.35). The

sensitivity is determined through a combination of professional judgement and the

environmental value of receptors, as recognised or determined by criteria from the WFD

classifications and flood risk vulnerability classifications specified in TAN15. Each receptor may be

categorised as being of low, moderate or high or very high importance or sensitivity. Examples of

the classification of receptors are outlined in Table 10.1.

Table 10.1 Estimating the value of water environment receptors (adapted from DMRB HD 45/09 Table A4.36).

Value/ Sensitivity

Criteria Examples

Very high Attribute has a high quality and rarity on regional or national scale.

WFD Class ‘High’; Source Protection Zone 1; Floodplain or defence protecting > 100 residential properties; European designated salmonid fishery; Site or species protected by European legislation; Principal aquifer providing a regionally important resource or supporting a site or species protected by European legislation

High Attribute has a high quality and rarity on local scale.

WFD Class ‘Good’; Source Protection Zone 2; Floodplain or defence protecting > 1 residential/industrial properties; Site or species protected by national legislation; Principal aquifer providing locally important resource or supporting river ecosystem.

Medium Attribute has a medium quality and rarity on local scale.

WFD Class ‘Moderate’; Source Protection Zone 3; Floodplain or defence protecting < 10 industrial properties; Site or species of local importance; Aquifer providing water for agricultural or industrial uses, but with limited importance to surface water.

Low Attribute has a low quality and rarity on local scale.

WFD Class ‘Poor’; Unproductive strata; Floodplain with low probability of flooding residential/industrial properties.

5 Design Manual for Road and Bridges. Volume 11 Section 3 Environmental Assessment Techniques. November 2009.

http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol11/section3/hd4509.pdf 6 Design Manual for Road and Bridges. Volume 11 Section 3 Environmental Assessment Techniques. November 2009.

http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol11/section3/hd4509.pdf




10-7

10.29 Impact magnitude has been qualified following the criteria outlined in Table 10-2 (adapted from

the DMRB HD 45/09 Table A4.4).

10.30 Assessment of impact magnitude has been based on the risk of flooding from all sources to the

site. Further consideration has been given to flooding impacts caused from the site during both

construction and operational phases of the proposed development. The assessment of flood risk

impacts with regards to surface water drainage is based on the policy requirements to limit the

post-development surface water discharge rate to that of the existing greenfield site, during

storm events up to the 1% annual exceedance probability (AEP) plus a 30% allowance for climate

change.

10.31 Assessment of impacts on water quality receptors is based on the potential to impact

downstream water bodies protected under the WFD and Welsh Government requirement that

policies within the LDP help deliver WFD requirements through biodiversity and water quality

benefits. Where water quality is assessed on a watercourse that is not classified under the Water

Environment (WFD) Regulations 2017, the WFD classification headings have been applied to

make an assessment of overall water quality that is consistent with the WFD approach (e.g.

inclusion of biological, chemical, hydro-morphological supporting quality elements).

Table 10-2 Estimating the magnitude of an impact on an attribute (adapted from the DMRB HD 45/09 Table A4.4)

Magnitude Criteria Examples

Major Results in loss of or major improvement in attribute quality and/or integrity.

Loss or extensive change to a fishery; Potential high risk of pollution to groundwater; Increase in peak flood level >100 mm;

Physical disturbance of a designated aquifer;

Excavation works within area of contaminated ground (in the vicinity of controlled waters);

Storage of fuel/oil or other liquid chemicals within 50m of surface watercourse or within 10m of surface water drain;

Major excavation or demolition works within 50m of surface watercourse or within 10m of surface water drain;

Stockpiling of spoil and/or storage of building materials within 50m of surface watercourse or within 10m of surface water drain;

Diversion culverting of greater than 100m of a watercourse or diversion of greater than 100m of a sewer;

Additional permanent land-take within a floodplain within a flood risk area.

Moderate Results in negative or positive effect on integrity of attribute, loss of or improvement in part of

Partial loss in productivity of a fishery;

Partial loss or change to an aquifer;

Increase in peak flood level >50 mm;

Storage of oil/fuel/other liquid chemicals within


10-8

Magnitude Criteria Examples

attribute. area underlain by designated aquifer or

Groundwater Source Protection Zone, where aquifer protected by drift cover up to 5m;

Transport of spoil and construction materials along new haul roads or other roads requiring crossing of surface watercourse;

Temporary land-take within a flood risk area;

Diversion/culverting of less than 100m of watercourse.

Minor Results in some measurable change in attribute’s quality or vulnerability.

Potential low risk of pollution to ground water;

Increase in peak flood level >10 mm;

Storage of oil/fuel/other chemicals, within area underlain by designated aquifer where drift cover >5m;

Reversible impacts on flood defence works;

No works within 250m of surface watercourse.

Negligible Results in effect on attribute but of insufficient magnitude to affect the use or integrity.

No measurable change risk of pollution to ground water or in peak flood level;

No construction within area underlain by designated aquifer, or sufficient protection offered by drift deposits. No land take within floodplain. No construction over watercourses or within 50m or watercourse;

No increase in area of hardcover and/or no increase in volume of surface drainage;

Potential for spillage of chemicals from operational site activities on a site that is mainly hardstanding.

Significance criteria

10.32 Following the methodology outlined in the Creigiau Scoping Report together with industry

recognised DMRB guidelines (Highways Agency, 2009), an assessment of LSE has been

undertaken. Significance is qualified by appraising the value of the of the receptor against the

magnitude of likely impacts, as outlined in Table 10-3. In line with changes brought about in the

2017 EIA Regulations, the magnitude of impact terminology has been updated to include

intensity, complexity and/or irreversibility of impacts on receptors. Where a LSE would fall

between significance scores a professional judgement has been made on which to apply.

10.33 Significance of effects is assessed for the construction and operational phases, both with and

without mitigation, and taking into account the scheme design and planned mitigation (which

includes the drainage strategy). LSE can be either beneficial or adverse, direct or indirect,

temporary or permanent. Intense impacts that would only persist for the duration of the

construction works or a short time afterwards are considered temporary, while impacts that are


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complex, irreversible or would persist for a long time during the operation of the development,

and/or for a period of time afterwards, are considered permanent.

10.34 Residual effects refer to any LSE that would remain following the implementation of mitigation

measures proposed.

Table 10-3 Estimating the significance of potential effects

Magnitude, intensity, complexity and/or irreversibility of impact

Major Moderate Minor Negligible No

change

Importance and/or

sensitivity of receptor

Very high Very large Large/

Very large

Moderate/ Large

Slight Neutral

High Large/

Very large

Moderate/

Large Slight/Moderate Slight Neutral

Medium Moderate/

Large Moderate Slight Neutral/Slight Neutral

Low Slight/

Moderate Slight Neutral/Slight Neutral/Slight Neutral

Negligible Slight Neutral/Slight Neutral/Slight Neutral Neutral

Baseline Conditions

Topography and land use

10.35 The proposed development site is located approximately 10km west of Cardiff city centre at

national grid reference 308099 180699. The site comprises 38.9 hectares of undeveloped

greenfield land adjacent and to the village of Creigiau. The site is mostly comprised of semi-

improved and wetland agricultural grassland, with hedgerows and plantation woodland along the

boundaries and a large area of ancient woodland in the centre. The former railway corridor

dissects the eastern part of the site.

10.36 The north of the site is formed by residential housing in the south of Creigiau village. The south is

bounded by Llantrisant Road (A4119), beyond which is currently open countryside but is the

location of the planned housing development located to the north of Junction 33 of the M4

motorway. The eastern boundary of the site is formed by Cardiff Road with further housing and

agricultural fields.

10.37 The orography of the site is characterised by sloping hillsides and localised high points. The

highest ground is at the eastern end of the site and known as Robin Hill (approximately 81m

Above Ordnance Datum, AOD), from which levels generally fall away to the north, west and

south. The northern boundary is approximately 65m AOD. The land to the west of the ancient

woodland also marks a second high point at approximately 62m AOD, with land falling away to

the east, north and west. The land between the railway corridor and ancient woodland is

relatively level. The lowest point of the site is approximately 52m AOD along the southern


10-10

boundary of the site where ancient woodland meets the A4119 and an unnamed watercourse is

culverted under the road.

10.38 The geology and hydrogeology of the site is described within this chapter.

Hydrology and surface drainage

10.39 There are a number of existing watercourses on or close to the site, as illustrated in Figure 10-1

below.

Figure 10-1 Location of main rivers and watercourses in the vicinity of the proposed development site

10.40 The Upper Nant Coslech is located immediately west of the site, although the catchment of which

is partly within the site boundary. The Upper Nant Coslech flows from the north through the

centre of Creigiau before passing the site boundary and flowing in a southerly direction under the

A4119 via a culvert. This watercourse collects runoff from fields in the west of the proposed

development site. The watercourse is a tributary of the Nant Henstaff, which is a tributary of the

River Ely that flows through the west of Cardiff and into Cardiff Bay.


10-11

10.41 An unnamed watercourse flows through the centre of the site, starting at the northern boundary

where a surface water drainage outfall discharges onto the site from the housing estate to the

north. This forms the head of this watercourse. The location of this outfall is shown in Figure 10-2

below. The Cardiff City Council Phase 2 SFCA assessment of the site observed that the head of

this watercourse was in poor condition at the points of discharge, with water backing up, ponding

of causing very wet ground conditions.

10.42 The watercourse flows in a southerly straight line through the western edge of the area of

ancient woodland and leaves the site through a culvert under the A4119 past Henstaff Court and

into the Nant Henstaff. This watercourse is characterised as an artificial straight channel that

serves an agricultural drainage purpose. The unnamed watercourse is shown in Figure 10-3

looking downstream towards the south of the site and ancient woodland. This watercourse

collects drainage from the centre of the site.

Figure 10-2 Head of unnamed watercourse in the north of the site boundary


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Figure 10-3 Unnamed watercourse from the north of the site looking south (downstream) towards ancient woodland.

10.43 The Nant Henstaff flows onto the site at the north-east corner from Cardiff Road and contains

contributing flows from two DCWW surface water drainage outfalls along the northern boundary

of the site from the housing estate to the north. A number of small field drainage channels also

contribute to this watercourse. Figure 10-4 shows one of the surface water outfalls that

contributes flows to this watercourse from the housing estate.

10.44 The Nant Henstaff flows in a south-westerly direction through the eastern part of the site, along

the disused Trehafod Branch Line before skirting along the south-east boundary of Castell-y-

Mynach ancient woodland in the centre of the site. The Nant Henstaff exits the site via a culvert

underneath the A4119. The watercourse joins the flows of the unnamed watercourse described

above and flows into the Nant Coslech to the south-west (designated as a main river) before

joining the River Ely.


10-13

Figure 10-4 Surface water outfall at the source of the Nant Henstaff watercourse on site

10.45 The Nant Henstaff is designated as a Site of Importance for Nature Conservation (SINCs) as a

small unimproved tributary stream with diverse riparian habitat including an in-line pond of high

ecological quality and adjacent wet woodland and mires with a population of Monkshood7.

However, the Nant Henstaff is heavily modified and re-profiled where it flows through the site

and is characterised as an artificial drainage ditch. This is evident in Figure 10-5, Figure 10-6 and

Figure 10-7 where field drainage for agricultural purposes and artificially straight sections of

channels or right angle bends are evident. Limited geomorphological or habitat features are

present in this watercourse, which is currently of limited biodiversity value. This watercourse

collects drainage from approximately a third of the site.

7 Land South of Creigiau Phase 1 Habitat Assessment 2019.


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Figure 10-5 Nant Henstaff showing artificial features and agricultural purpose

Figure 10-6 Nant Henstaff showing limited geomorphological diversity and incoming flows from field drainage


10-15

Figure 10-7 Nant Henstaff showing field drainage inflows

Foul drainage

10.46 There is an existing sewerage network in the residential area along the northern site boundary.

This includes the DCWW Ffordd Dinefwr Sewage Pumping Station (SPS).

10.47 The DCWW Creigiau Wastewater Treatment Works (CWTW) discharges into the Nant y Glasywg,

which is the catchment located to the west of the proposed development that joins the River Ely.

DCWW Water have advised that capacity in the public sewer network is limited and CWTW does

not have the required capacity to accommodate further flows without potential upgrades.

10.48 The DCWW Coslech Wastewater Treatment Works discharges into the Nant Coslech

approximately 1.3km to the south-east of the proposed development site. This is upstream of the

confluence of the Nant Coslech with the River Ely. Discussions with DCWW have identified that

the Coslech Wastewater Treatment Works has come capacity for sewerage flows.

Flood risk

10.49 The site is located within Zone A of the Welsh Government Development Advice Map. This zone

comprises land assessed as having less than a 0.1% AEP (1 in 1000 year chance of flooding) and so

is categorised as an area with little or no risk of fluvial flooding. The NRW flood map is illustrated

in Figure 10-8. There is no risk of flooding from tidal or reservoir sources.


10-16

Figure 10-8 Natural Resource Wales (NRW) Development Advice Map of flood risk from fluvial and tidal sources8

10.50 The potential for flooding downstream of the site, for which the site watercourses form a

contributing flow, was assessed using the NRW map of flood risk from rivers and seas. The NRW

map does not extend to the upstream sections of the Nant Henstaff and Nant Coslech, but does

includes flood risk from downstream of the M4 motorway. The map shows that flooding in the

Nant Coslech up to the confluence with the River Ely is largely confined to the river banks. There

is some low risk to the Coslech Wastewater Treatment Works, classified as a ‘less vulnerable’ in

TAN-15. There are no other receptors affected by fluvial flooding immediately downstream of the

development site. The masterplan for the J33 Development Site E (as submitted)9, does not show

any development planned in flood risk areas adjacent to the watercourse that could be affected

by upstream flows from the Nant Henstaff.

8 NRW Flood Risk Map Viewer.

https://maps.cyfoethnaturiolcymru.gov.uk/Html5Viewer/Index.html?configBase=https://maps.cyfoethnaturiolcymru.gov.uk/Geocortex/Essentials/REST/sites/Flood_Risk/viewers/Flood_Risk/virtualdirectory/Resources/Config/Default&layerTheme=2 9 17/02305/MJR | DISCHARGE OF CONDITION 23 (STRATEGIC SUSTAINABLE SURFACE WATER DRAINAGE MASTERPLAN) OF

14/00852/DCO. https://planningonline.cardiff.gov.uk/online-applications/files/01FBBA6574AAD06DDBFCC1602EAF6A52/pdf/17_02305_MJR-70031844-WSP-PB-XX-XX-DR-CE-SK04_PROPOSED_PHASING_AND_DRAINAGE_CATCHMENTS6406659-1959494.pdf

https://maps.cyfoethnaturiolcymru.gov.uk/Html5Viewer/Index.html?configBase=https://maps.cyfoethnaturiolcymru.gov.uk/Geocortex/Essentials/REST/sites/Flood_Risk/viewers/Flood_Risk/virtualdirectory/Resources/Config/Default&layerTheme=2


https://planningonline.cardiff.gov.uk/online-applications/files/01FBBA6574AAD06DDBFCC1602EAF6A52/pdf/17_02305_MJR-70031844-WSP-PB-XX-XX-DR-CE-SK04_PROPOSED_PHASING_AND_DRAINAGE_CATCHMENTS6406659-1959494.pdf




10-17

Figure 10-9 NRW long term flood risk map showing fluvial flood risk downstream of development site10

10.51 The flood risk from surface water is generally very low across the site, as illustrated in Figure

10-10. This means that most of the site has a current chance of flooding of less than 1 in 1000

(0.1% AEP) from surface water runoff. The only areas of the site with a risk from surface water

flooding are either in the existing watercourses or the immediate area adjacent to the

watercourse. There is also a small area susceptible to surface water flooding in the east of the

site at the location of a small topographic depression/pond.



Low risk of flooding

to WWTW

No flooding to properties




10-18

Figure 10-10 NRW long term flood risk map showing surface water flood risk at the development site11

10.52 The NRW surface water flood maps indicates that there are no sensitive receptors (as defined in

TAN-15) affected by flooding up to the 0.1% AEP surface water flood event in the area

downstream of the proposed development, that could be affected by flows from the proposed

development. Surface water flooding is constrained to the banks and floodplain of the

watercourses and does not affect properties or developments. This is illustrated in Figure 10-11

below.






10-19

Figure 10-11 NRW map of surface water flood risk downstream of development site

10.53 Flood risk from the sewerage network is referred to in the Cardiff City Council Phase 2 SFCA

assessment of the development site. A risk is identified from sewer surcharging at the head of the

unnamed watercourse through the centre of the site. The DCWW Solutions for Total Asset

Management (STAM) database indicate that the wet well of the Ffordd Dinefwr SPS becomes

surcharged during storm events, spilling via an outfall pipe to the unnamed watercourse. Flood

reports from other system flooding are noted to be limited for the area, with only three isolated

incidents reported. Two of these reports relate to surface water flooding along Cardiff Road to

the east of the site.

10.54 Groundwater flooding is not considered to be a risk to the proposed development site. The

geological sequence in the area typically comprises superficial deposits of Till, underlain by Blue

Lias Formation, Penarth Group, and Blue Anchor Formation (mudstones and siltstones) of limited

permeability and not designated as an aquifer.


10-20

Water quality

10.55 There are no known water quality data records available for the Nant Henstaff and the other

ditches on-site. The water quality of the Nant Henstaff as it flows through the site will be

influenced by: surface water entering the watercourse upstream before entering the site at the

north-east boundary, contributing runoff from overland flow across the site itself, and the DCWW

surface water drainage outlets discharging onto site from the northern boundary.

10.56 Likewise, water quality data records are not known to be available for the unnamed watercourse

flowing through the centre of the site before discharging into the Nant Henstaff approximately

250m south of the site boundary. The water quality of this watercourse will be influenced by: the

condition of the surface water entering the site from the DCWW drainage outflow at the head of

the watercourse, the contributing overland flow, and the potential for sewer overflow from the

DCWW Ffordd Dinefwr SPS.

10.57 Information regarding water quality in the upper sections of the Nant Coslech as it flows past the

western site boundary is limited. However, the watercourse flows through Creigiau and is likely to

contain contributing flows of surface water runoff from developed areas.

10.58 There are not anticipated to be any significant sources of water pollution from the site itself, as

land uses are a mixture of agricultural pasture and woodland (semi-natural and ancient

woodland). There is some potential for nutrient loading as a result of grazing livestock. However,

the site is not located in a Nitrate Vulnerable Zone, and the total contributing area of pastural

land is small, and so the risk of nutrient pollution is low.

10.59 The surface water entering the site from the three drainage outlets along the northern boundary

could contain sources of highway pollution from the road network. However, the area of

highway drainage is limited to the small residential area to the north only, with limited residential

traffic (i.e. comprised of cul de sacs). The potential for road pollution sources is also therefore

limited.

10.60 It is noted that the watercourses are generally found to be of limited ecological quality as a result

of low hydro-morphological and species diversity (refer to Chapter 8 Ecology and Nature

Conservation for more information). The Nant Henstaff is lacking in any aquatic or marginal

vegetation other than very sparse small sweet grass. The stream has more species diversity

alongside the railway embankment but remains limited. In the unnamed watercourse marginal

vegetation is present but limited to the northern 50m section of channel (see Figure 10-3),

primarily consisting of fool’s water cress, yellow flag iris and great willowherb with occasional

small sweetgrass12. Where the unnamed watercourse runs adjacent to the woodland, the

marginal aquatic species become sparse.

10.61 The Ground Conditions chapter (Chapter 11) indicates that the risk of contamination from the

site that could impact the watercourses is low.

12 Phase 1 Habitat Survey


10-21

WFD status of downstream watercourses

10.62 The NRW Water Watch Wales website13 and Environment Agency (EA) Catchment Data Explorer14

provides WFD classification data for the nearby waterbodies. The watercourses on site contribute

to the WFD status of the River Ely from Nant Clun to Allot Gardens, as shown in Figure 10-12. The

flows from the watercourses on site contribute to this WFD waterbody and are within the WFD

catchment. The unnamed watercourse in the centre of the site joins the Nant Henstaff 250m

south of the site. The Nant Henstaff joins the Nant Coslech at the confluence located

approximately 1.5km south-west of the proposed development. The watercourse just outside of

the western site boundary is the upper Nant Coslech and therefore joins flows from the Nant

Henstaff at this confluence. The Nant Coslech becomes a main river at this confluence and joins

the River Ely a further 1.5km downstream, where it becomes the WFD waterbody.

Figure 10-12 WFD Waterbody catchment. River Ely – Nant Clun to Allot Gardens

13 NRW Wales Watch website. http://waterwatchwales.naturalresourceswales.gov.uk/en/ 14 Environment Agency Catchment Data Explorer: Ely Operational Catchment. https://environment.data.gov.uk/catchment-planning/OperationalCatchment/3171

http://waterwatchwales.naturalresourceswales.gov.uk/en/

https://environment.data.gov.uk/catchment-planning/OperationalCatchment/3171

https://environment.data.gov.uk/catchment-planning/OperationalCatchment/3171


10-22

10.63 The River Ely WFD waterbody from the confluence of Nant Clun to Allot Gardens is within the

South East Valleys area of the River Severn Catchment. This section of the River Ely covers the

downstream extent of the river as it flows south-east into Cardiff and discharges into Cardiff Bay.

10.64 The WFD Cycle 2 waterbody status of the River Ely from the confluence of Nant Clun to Allot

Gardens is given in Table 10.4 below. The overall status is ‘Bad’ with an objective of ‘Good’ by

2027.

Table 10.4 WFD classification items for River Ely – Nant Clun to Allot Gardens

WFD Classification Item 2015 Objective year for good status

Overall water body Bad 2027

Ecological Bad 2021

Biological quality element Bad 2021

- Macrophytes and Phytobenthos Combined

Good 2015

- Fish Moderate 2021

- Invertebrates Bad 2021

Hydro-morphological supporting element Supports good

2015

Physico-chemical quality elements Good 2015

Specific pollutants High 2015

Chemical Fail 2017

Priority substances Fail 2027

Other pollutants Good 2021

Priority hazardous substances Fail 2021

10.65 The EA Catchment Data Explorer cites the reasons for not achieving good status and

deterioration in status from WFD Cycle 1 are given in Table 10.5 below. The reasons relate to the

invertebrate element of the biological classification, and the priority substances and priority

hazardous substances element of the chemical classification. This table shows that there are

multiple sources of poor status including physical modifications, diffuse agricultural pollution,

point source sewerage pollution and unknown pollution incidents.

Table 10.5 Reason for not achieving good WFD status. River Ely – Nant Clun to Allot Gardens

Source Activity Category Classification Element

Point source Incidents Sector under investigation

Invertebrates

Unknown (pending investigation)


Sector under investigation

Fluoranthene

Diffuse source Unknown (pending investigation)

Agriculture and rural land management

Macrophytes and Phytobenthos


10-23

Source Activity Category Classification Element

Combined

Point source Sewage discharge (continuous)

Water Industry Phosphate




Tributyltin Compounds

Physical modification Barriers - ecological discontinuity


Fish

Point source Sewage discharge (continuous)

Water Industry Macrophytes and Phytobenthos Combined




Benzo(a)pyren

Cumulative effects baseline

10.66 The scope of this ES includes consideration of cumulative impacts of the proposed development

with adjacent planned developments allocated in the Cardiff City Council LDP. In particular, J33

allocation site D. Therefore, the baseline relating location and contributing flows from the J33 site

into WFD waterbodies is set out as follows to inform the cumulative effects assessment.

10.67 The J33 development site is located immediately south of the proposed development. The Nant

Henstaff flows along the western boundary of the J33 site and through a small portion on the

south-east corner, prior to joining the Nant Coslech and into the River Ely. The Nant y Glaswg also

flows through a small portion of the east of the J33 site, leaving the site via a culvert under the

M4. The drainage strategy that forms part of the approved development application15 for the J33

site shows that approximately one third of the site drains into the Nant Henstaff (joining flows

from the proposed development) and two thirds drain into the Nant Dowlais catchment.

10.68 The Nant Dowlais forms a separate WFD waterbody called the Nant Dowlais from the source to

the confluence with the River Ely. This waterbody includes contributing flow downstream into the

River Ely waterbody from the confluence of Nant Clun to Allot Garden. The location of these

waterbodies and watercourses is given in Figure 10-13 below. The overall WFD status of the Nant

Dowlais – source to confluence with the River Ely is Moderate, with a Moderate objective. The

reasons for not achieving good status result from diffuse pollution from agriculture and land use

management, affecting fish and macrophyte and phytobentos classification elements.

15 17/02305/MJR Drainage Strategy Figure 264066590000. 26th September 2017. https://planningonline.cardiff.gov.uk/online-

applications/files/F144F583A2D1110442D54CFA2A92C7DD/pdf/17_02305_MJR-DRAINAGE_STRATEGY_FIGURE_264066590000.PDF-1959505.pdf

https://planningonline.cardiff.gov.uk/online-applications/files/F144F583A2D1110442D54CFA2A92C7DD/pdf/17_02305_MJR-DRAINAGE_STRATEGY_FIGURE_264066590000.PDF-1959505.pdf




10-24

Figure 10-13 WFD catchment for J33 development site.

Water environment baseline summary

10.69 Table 10.6 summarises the relative value of baseline receptors on the basis of their importance

and sensitivity as scoped in to this assessment in relation to flooding, drainage and water quality.

As outlined in the methodology section, this will be used in combination with the assessment of

the magnitude of impacts to determine the likely significant effects arising from the proposed

development.


10-25

Table 10.6 Summary of baseline water environment receptors

Receptor type

Receptor Importance/ sensitivity

Flood risk receptor

Floodplain with low probability of flooding residential (highly vulnerable) and industrial (less vulnerable) properties on site or downstream.

Low

Flood risk receptor

Downstream Coslech WWTW (less vulnerable). Medium

Water quality

Un-named watercourse on site (non-designated). Low

Water quality

Nant Henstaff SINC on site (non-statutory designated site of local importance).

Medium

Water quality

Upper Nant Coslech on the western site boundary. Low

WFD status River Ely confluence of Nant Clun to Allot Garden Low

Proposed Development

10.70 The proposed development assessed is outlined in detail in Chapter 4 of this ES. In summary the

proposals are to develop the site for residential use with public and private open spaces, parking,

access and associated infrastructure. The site masterplan shows that the proposed development

site totals 38.9 hectares and including approximately 650 new residential units. The site is split

into three separate parcels/phases of development.

10.71 The development proposal includes a comprehensive outline drainage strategy developed by JBA

Consulting in April 2019. The outline drainage strategy documents are located in Appendix 10.1.

The outline drainage strategy can be regarded as tertiary mitigation, as defined in the Institute of

Environmental Management and Assessment (IEMA) 2016 guidance on Environmental Impact

Assessment Guide to Delivering Quality Development16. Tertiary mitigation includes actions that

have been undertaken regardless of the EIA process, due to other legislative requirements or

standard practices. The outline drainage strategy measures are described in more detail in

section 10.102 below with reference to mitigation and residual impacts.

10.72 The off-site foul water drainage strategy for the proposed development has been developed by

Hydrock and is provided in Appendix 10.3. DCWW has given approval in principle for a foul

16 Institute of Environmental Management and Assessment (IEMA) (2016). Environmental Impact Assessment Guide to Delivering Quality

Development. https://www.iema.net/assets/newbuild/documents/Delivering%20Quality%20Development.pdf

https://www.iema.net/assets/newbuild/documents/Delivering%20Quality%20Development.pdf


10-26

connection to the existing Coslech WWTW via a section 98 sewer requisition in the public

highway (approximately 1.25km) with upgrades to capacity as necessary. The Coslech WWTW is

also the planned location for connection of the foul drainage from the J33 development site. The

environmental controls on the Coslech WWTW discharge to the River Ely (confluence of Nant

Clun to Allot Garden) WFD water body form tertiary mitigation for the potential impacts of

increased nutrient loading on the receiving watercourses. These tertiary mitigation measures are

discussed in more detail in section 10.107 below with reference to mitigation and residual

impacts.


10-27

Potential Impacts and LSE

10.73 An assessment of the LSE on water environment receptors from the construction and operation

of the proposed development has been undertaken.

10.74 Climate change is expected to increase future flood risk from both fluvial and surface water

sources. Climate change may also lead to negative impacts on water environment receptors such

as fish, invertebrates and aquatic flora as a result of impacts on water temperature, shading,

invasive species and summer flows. The impact of climate change does not affect the assessment

of impacts for the construction phase, but has been considered in the assessment of the

operational impacts during the lifetime of the scheme.

10.75 Cumulative impacts of the proposed development with the J33 development has been

considered separately after this section.

Construction Phase

10.76 Potential impacts on the water environment during construction are expected to be temporary

and would relate to:

• Sedimentation through silt mobilisation entering on-site watercourses;

• Increased overland flow in watercourses;

• Reduction in water quality due to pollution events; and

• Impediments to flow in watercourses causing increased flow.

10.77 These impacts are discussed below. With reference to the methodology provided in Paragraphs

10.25 to 10.33, Table 10.7 provides a summary the likely significant effect of the construction

phase (without mitigation measures in place) utilising the criteria provided in Table 10-3, taking

account of the value of the receptor as given in Table 10.1 and Table 10.6, and the magnitude of

the potential impact as given in Table 10-2.

Sedimentation

10.78 During construction there will be considerable disturbance to the existing soils and superficial

geology while preparing foundations for dwellings and formations for highways. The disturbance

of the soils and the subsequent surface water runoff could cause increased sediment loads into

receiving watercourses on site (unnamed watercourse and Nant Henstaff) and the Upper Nant

Coslech to the west of the site. The increased sediment load could have an impact on the

ecological quality status of the downstream WFD water body.

Flooding, increased flow and impediments

10.79 The compaction of soils during construction could give rise to increased surface overland flows

into the receiving watercourses and a reduction in the natural attenuation of the soil on site.

Likewise, on-site watercourses may be temporarily culverted during construction to facilitate

further works. If the temporary culverts are inadequate, the impediment to flow could cause


10-28

localised flooding issues at the proposed development and on adjacent land. Higher flows from

these sources could lead to an increased risk of flooding downstream of the development site.

10.80 However, it was noted in the baseline section on flood risk that modelled flood extents

downstream of the development do not show flooding to receptors up to the 0.1% AEP event,

other than the sewage treatment works experiencing flooding during the 1% - 0.1% AEP event.

This includes development allocated in the J33 development site. Therefore, the potential impact

of increased flows during construction on flood risk receptors is minimal due to the very low risk

currently experienced and relates only to the sewage treatment works or for very extreme events

(0.1% AEP event and above). Increased flows as a result of the construction phase could lead to

increase erosion of the watercourses onsite and the Nant Henstaff immediately downstream of

the site. The Nant Henstaff on the site has limited hydro-morphological diversity or stable

watercourse features and is susceptible to further erosion. The Nant Henstaff is designated as an

SINC, partly due to watercourse features downstream of the site, such as riparian vegetation,

mires and an on-line pond. Localised increase in flows from the development site could have a

negative impact on the hydro-morphological features in the watercourse with a corresponding

impact on ecology and water quality if not mitigated.

Pollution

10.81 During construction of the proposed development there is a risk of uncontrolled discharge of

pollutants that could impact the water quality of the receiving watercourses and the downstream

waterbody, with negative ecological impacts likely. The main source of pollution risk is from

accidental spillages of hydrocarbons from plant operations or cementitious materials from

construction.

Summary of likely significant effects

10.82 Table 10.7 provides a summary of the LSE on water environment receptors during the

construction phase, without any mitigation measures in place.

Table 10.7 Significance of potential effects from construction without mitigation

Receptor Importance/

sensitivity

Potential Impact

Impact Magnitude

LSE

Flood risk receptor:

Floodplain with low probability of flooding residential (highly vulnerable) and industrial (less vulnerable) properties downstream

Low Flooding

Minor

Adverse

Neutral/slight


Downstream Coslech Sewage Treatment works (less vulnerable)

Medium Flooding Minor

Adverse

Slight


10-29


sensitivity

Potential Impact

Impact Magnitude

LSE

Water quality:

Un-named watercourse on site (non-designated)

Low Increased flow Minor

Adverse

Neutral/slight

Sedimentation Moderate

Adverse

Slight

Pollution Major

Adverse

Slight/moderate

Water quality:

Nant Henstaff SINC on site (non-statutory designated site of local importance)

Medium Increased flow Moderate

Adverse

Moderate


Adverse

Moderate

Pollution Major

Adverse

Moderate/large

Water quality:

Upper Nant Coslech on the western site boundary

10.83 Low Increased flow Minor

Adverse

Neutral/slight


Adverse

Slight

Pollution Major

Adverse

Slight/moderate

WFD status:

River Ely confluence of Nant Clun to Allot Garden

Low Increased flow Negligible

Adverse

Neutral/slight

Sedimentation Minor

Adverse

Neutral/slight

Pollution Moderate

Adverse

Slight

Operational Phase

10.84 Potential operational phase impacts on the water environment would relate to:

• Water supply and foul drainage loading;

• Sedimentation and pollution;

• Flood risk and increased flow;

10.85 These impacts are discussed below. With reference to the methodology provided in Paragraphs

10.25 to 10.33, Table 10.8 provides a summary the likely significant effect of the operational

phase (without mitigation measures in place) utilising the criteria provided in Table 10-3, taking

account of the value of the receptor as given in Table 10.1 and Table 10.6, and the magnitude of

the potential impact as given in Table 10-2.


10-30

Water supply and foul drainage

10.86 The strategic level impact of the proposed development on the capacity of the water supply

(abstraction) and foul drainage (discharges) systems has not been considered in detail within this

ES chapter. The assessment of this issue is carried out by DCWW as the water and sewerage

provider. It is the responsibility of DCWW to ensure that their functions are carried out as part of

strategic assessment of water resource capacity taking into account the environmental impacts,

including environmental consent for abstraction and discharges. The detailed environmental

assessment of these impacts is therefore not within the scope of this assessment.

10.87 The nearby DCWW Coslech WWTW discharge into watercourses that contribute flow to the River

Ely confluence of Nant Clun to Allot Garden WFD waterbody. This waterbody is of poor status for

a number of reasons. Sewage discharge is listed as one of the reasons for not achieving good

status, due to phosphate (although it is noted that phosphate is listed as Good status for the

waterbody). This suggests that maintaining water quality limits for discharge from the WWTW is

particularly significant on this watercourse and ensuring that sewage overflow events are

controlled.

10.88 The proposed development intends to increase the foul flows connected to the Coslech WWTW.

DCWW have indicated that the WWTW has capacity but may require potential upgrade works to

the pumping station and downstream rising main pipes. Furthermore, the existing phosphate and

nutrient classifications for the waterbody are Good. However, the increase in foul load to the

WWTW has a potential for increased risk of overflow pollution events from the WWTW as a

result of greater load, or during construction of new sewerage infrastructure. Further impacts to

the WFD waterbody could result if the additional foul loading requires a variation to the effluent

discharge permit conditions and an increase in nutrient flows to the watercourse beyond the

existing water quality capacity.

Sedimentation and pollution

10.89 The proposed development involves the permanent construction of access roads and parking

areas that could increase the sediment and pollutant load to the receiving watercourses and

downstream waterbodies. The risk of this occurring during operation is lower than during

construction but could still give rise to a moderate impact on all receiving waterbodies.

Flood risk and increased flow

10.90 The proposed development will include large areas of impermeable surfaces which will generate

increased volumes of surface water runoff to the receiving watercourses compared to the

present greenfield land use. This higher runoff rate and volume could result in increased flood

risk on site and downstream. Without mitigation, the potential impact could cause flooding to the

properties onsite and also increase the baseline flood extents downstream to include vulnerable

receptors. These receptors include proposed development of properties at the J33 site, existing

properties along the Nant Coslech watercourse, the Coslech Wastewater Treatment Works and

other vulnerable receptors affected by flows in the River Ely.


10-31

10.91 The proposed development is likely to include several permanent crossings of the watercourses

and on-site ditches. If the proposed culverts are inadequate, the impediment to flow could cause

localised flooding within the proposed development site and on adjacent land. The impacts are

not expected to be widespread as the proposed development layout leaves a large buffer

between the watercourses and housing, and as a result of the limited number of downstream

properties affected by existing flood risk.

10.92 As with the construction phase impacts, increased flows during rainfall events as a result of the

significant increase in impermeable area could also lead to increased erosion of the

watercourses. This could result in a negative impact on the hydro-morphological features in the

Nant Henstaff with a corresponding impact on ecology and water quality if not mitigated. The


10.93 Table 10.8 provides a summary the LSE on water environment receptors during the operational

phase without mitigation measures in place.

Table 10.8 Significance of potential effects from operational phase without mitigation


sensitivity

Potential Impact

Impact Magnitude

LSE



Low Flooding

Major

Adverse

Slight/moderate



Medium Flooding Moderate

Adverse

Moderate

Water quality:


Low Increased flow Minor

Adverse

Neutral/slight


Adverse

Slight

Pollution Moderate

Adverse

Slight

Water quality:


Medium Increased flow Moderate

Adverse

Moderate


Adverse

Moderate

Pollution Moderate

Adverse

Moderate

Water quality: Low Increased flow Minor Neutral/slight


10-32


sensitivity

Potential Impact

Impact Magnitude

LSE


Adverse


Adverse

Slight

Pollution Moderate

Adverse

Slight

WFD status: River Ely confluence of Nant Clun to Allot Garden

Low Wastewater effluent

Major

Adverse

Slight/moderate

Increased flow Minor

Adverse

Neutral/slight


Adverse

Slight

Pollution Moderate

Adverse

Slight


10-33

Proposed Mitigation and Residual LSE

10.94 The LSE during construction and operation highlighted in the section above can potentially be

mitigated with the implementation of appropriate mitigation measures.

Construction Phase

10.95 During construction the mitigation measures for the impacts of sedimentation and pollutants will

be implemented by adherence to strict protocols relating to the method of construction. A

Construction Environmental Management Plan (CEMP) will be developed and agreed with the

appropriate stakeholders.

10.96 The CEMP should be developed in accordance with best practice guidance such as the Pollution

Prevention Guidelines (Environment Agency) and Control of Water Pollution from Construction

Sites (CIRIA). The CEMP should incorporate on-going monitoring by the environmental clerk of

works throughout the construction phase. The CEMP should incorporate measures to reduce the

potential effect on water quality. These measures include:

• Minimising sediment load in runoff by covering/sealing exposed surfaces;

• Preventing runoff from unaffected areas mixing with construction runoff;

• Control of potentially contaminated construction runoff;

• Controls on the use of potential pollutants such as cementitious materials; and

• Correct storage and handling of on site chemicals and hydro-carbons, with adequate

hardstanding, bunding and drip trays.

10.97 The use of temporary settlement and detention basins will mitigate the risk of increased flows

and sediment loads from construction activities. This will help to mitigate impacts on

downstream flood risk as well as sedimentation. Exclusion zones around particularly sensitive

areas should be implemented to further reduce the risk and any works within these areas

undertaken with the necessary regulatory consents.

10.98 The use of temporary culverts during construction should be kept to a minimum and removed

when no longer necessary. Culverts should be adequately sized with safe routes for discharge

should blockage occur.

10.99 The likely significance of potential effects on hydrology and drainage during construction taking

account of the CEMP mitigation measures stated above is summarised in Table 10.8. All impacts

are neutral or slight and are temporary in nature. The significant of these impacts is considered

recoverable by the sensitive receptors.


10.100 Table 10.9 below provides a summary of the LSE on water environment receptors during the

construction phase with mitigation measures in place.


10-34

Table 10.9 Significance of potential effects from construction phase with mitigation


sensitivity

Potential Impact

Impact Magnitude

Mitigated LSE



Low Flooding

Negligible Neutral/slight



Med Flooding Negligible Neutral/slight

Water quality:


Low Increased flow Negligible Neutral/slight

Sedimentation Minor Neutral/slight

Pollution Minor Neutral/slight

Water quality:


Medium Increased flow Negligible Neutral/slight

Sedimentation Minor Slight

Pollution Minor Slight

Water quality:





WFD status:

River Ely confluence of Nant Clun to Allot Garden




Operational Phase

10.101 During the operation of the proposed development, the impacts on hydrology and drainage will

be mitigated by the implementation of the outline drainage strategy and foul drainage strategy

included in Appendix 10.1 and 10.3.

10.102 The drainage strategy uses sustainable drainage systems to attenuate surface runoff from

impermeable developed areas up to the 1% AEP plus a 30% allowance for climate change and

urban creep. These measures mitigate the potential for increased flood risk to on-site and off-site

receptors. SuDS have been incorporated into the drainage strategy in accordance with the Welsh

Government statutory standards for sustainable drainage systems17. Offsite surface water runoff

is restricted to greenfield runoff rates. Therefore, flows off-site will be maintained at or below the

present rate.

17 Statutory standards for sustainable drainage systems – designing, constructing, operating and maintaining surface water drainage

systems. https://gweddill.gov.wales/docs/desh/publications/181015-suds-statutory-standards-en.pdf

https://gweddill.gov.wales/docs/desh/publications/181015-suds-statutory-standards-en.pdf


10-35

10.103 Attenuation is in the form of off-line and on-line swales and attenuation basins with further SuDS

features such as bioretention areas and filter strips. These assets provide natural water quality

filtering, ensuring that potential pollutants are removed from the development runoff and

therefore mitigating this impact. The use of trapped gullies and catchpits on highways will also

reduce the risk of silt and other contaminants being discharged into the receiving watercourses.

Mandatory maintenance requirements for SuDS will ensure that these assets are maintained and

provide the necessary functioning throughout their lifetime.

10.104 These features will provide a water quality filtering affects for the surface water entering the site

from the surface water outfalls from the residential area to the north (that discharges into the

unnamed watercourse and Nant Henstaff). The SuDS features therefore provide a water quality

improvement to the receiving watercourses.

10.105 The watercourses on site are also proposed to be restored to a more natural realignment to

provide channel sinuosity, improved hydro-morphological features, and diverse habitat areas

including marginal vegetation and marshy areas, as well as native species planting. This will result

in an improvement to the hydrological regime of the site by holding back water in the newly

created habitat areas and facilitating localised infiltration where possible.

10.106 The provision of improved water environment through the implementation of the SuDS features

increases the capacity of the watercourses to adapt to future environment change, in particular

climate change. By increasing the species and habitat diversity, it provides ecosystem resilience

and a net benefit to the water environment.

10.107 The drainage strategy is therefore likely to result in environmental benefits to the water

environment with positive impacts on the water quality receptors of the receiving watercourses

and the downstream WFD waterbody. Given the existing poor status of the River Ely from the

confluence of Nant Clun to Allot Gardens, the mitigation proposals will contribute a small

contribution towards improving this WFD status, with particular reference to ecological quality

elements.

10.108 Tertiary mitigation of the potential impacts to water quality of the River Ely (confluence of Nant

Clun to Allot Garden) WFD water body as a result of an increase in foul loading to the Coslech

WWTW is assumed to be carried out by DCWW in negotiation with the developer. This will be

completed via the necessary upgrades to the sewerage network and pumping station to manage

the risk of pollution or overflow events. Further to this, regulatory limits and management

controls will be placed on the discharge of polluting substance (ecological and chemical

elements) from the WWTW under environmental permitting consent (managed by DCWW).

10.109 It is assumed that the necessary updated to the sewerage network and WWTW will be completed

as part of the development proposals. It is also assumed that DCWW will adhere to all statutory

requirements and environmental consent regarding the water quality of the receiving waterbody,

in their function as the sewerage provider. If such tertiary mitigation measures are carried out,

additional foul discharge to the WFD watercourse as a result of the proposed development will

be within the environmental limits required to mitigate potential adverse impacts.


10-36

Summary of LSE

10.110 Table 10-10 below provides a summary of the LSE on water environment receptors during the

operational phase with mitigation measures in place.

Table 10-10 Significance of potential effects from operational phase with mitigation


sensitivity

Potential Impact Impact Magnitude

Mitigated LSE



Low Flooding




Med Flooding Negligible Neutral/slight

Water quality:


Low Hydro-morphological

Major

Beneficial

Slight/moderate

Ecological Major

Beneficial

Slight/moderate

Sedimentation and Pollution


Erosion Negligible Neutral/slight

Water quality:


Medium Hydro-morphological

Major

Beneficial

Moderate/large

Ecological Major

Beneficial

Moderate/large


Negligible Slight

Erosion Negligible Slight

Water quality:


Low Hydro-morphological

Major

Beneficial

Slight/moderate

Ecological Major

Beneficial

Slight/moderate

Pollution Negligible Neutral/slight


WFD status:

R Ely confluence of Nant Clun to Allot Garden

Low Wastewater effluence


Hydro-morphological


Ecological Moderate

Beneficial

Slight


10-37





10-38

Cumulative Effects Assessment

10.111 The scope of this ES includes consideration of cumulative impacts of the proposed development

with adjacent planned developments allocated in the LDP, in particular, J33 allocation site D. As

detailed in the cumulative effects baseline section, the J33 site is located to the south of the

proposed development. The Nant Henstaff flows from the proposed development to the J33 site

and runs along the western boundary and through a small section of the site. Approximately one

third of the surface water flows from the J33 development will discharge into the Nant Henstaff.

10.112 The outline drainage strategy for the proposed development ensures that surface water does not

cause a risk to flooding downstream of the site. It is assumed that the J33 outline drainage

strategy incorporates the same statutory requirements and can be regarded as tertiary

mitigation. Consideration of the two sites together therefore does not result in a cumulative

adverse impact on surface water flood risk because runoff rates and volumes are restricted to

pre-development greenfield rates for both sites.

10.113 The sustainable drainage system features detailed in the outline drainage strategy for the

proposed development provide a beneficial moderate/large likely significant effect on the Nant

Henstaff and a slight/moderate likely significant effect on the other on site watercourses. It is

anticipated that the J33 drainage strategy will incorporate similar natural surface water SuDS

features to ensure that the water quality leaving the site has a negligible impact on downstream

watercourses. The cumulative impact on the Nant Henstaff water quality and downstream WFD

waterbody from these two sites is therefore considered negligible and the likely significant

effects are neutral/slight.

10.114 The foul drainage from the proposed development and the J33 development are both planned to

connect to the Coslech Wastewater Treatment Works. Consultation with DCWW is ongoing and it

is anticipated that DCWW will provide steer on any necessary upgrades to the system to ensure

capacity for both developments. Opportunities have been identified by the project team for

coordinated construction and shared costs for system upgrades between the two development

sites. By managing the two connections in combination (as a cumulative increase in capacity), this

allows DCWW to specify the necessary mitigation measures to ensure suitable system capacity

and environmental controls on discharge. The cumulative impacts as a result of combined foul

flows are therefore considered negligible and the likely significant effects are neutral/slight.

Assumptions

10.115 The following assumptions have been made in this assessment:

• The scheme will be implemented and constructed in accordance with masterplan and

outline drainage strategy as given at the time of writing (April 2019).

• The hydraulic modelling used in the NRW fluvial and surface water mapping can be

assumed to represent the most likely scenario for flood risk to and from the proposed

development site.


10-39

• The WFD Cycle 2 information for the relevant waterbodies is the latest information

available regarding water quality data.

• Construction mitigation measures will follow standard industry best practice be

implemented as outlined this chapter, with particular focus on the CEMP measures, for

example:

• CIRIA Report C532: Control of water pollution from construction sites – guidance for

consultants and contractors (CIRIA, 2001);

• CIRIA Report C624: Development and Flood Risk – guidance for the construction industry

(CIRIA, 2004);

• CIRIA Report C741: Environmental good practice on site (CIRIA, 2015);

• Environment Agency: Pollution prevention guidance (Environment Agency, 2014);

• All environmental regulatory consents will be gained prior to construction and adhered to

throughout.

• DCWW will continue to manage the necessary mitigation measures from water abstraction

and sewerage discharge under their existing responsibilities and will meet all consenting

requirements.

10.116 These assumptions are considered to be appropriate for the type of assessment, the proposed

development and the importance of the baseline water environment.


10-40

Summary and Conclusion

10.117 The long term impact of the proposed development with regards to hydrology and drainage is

considered to be neutral or slight if mitigation measures are carried out. In particular, these

relate to a robust and monitored CEMP to mitigate potential impacts during construction and the

implementation of the proposals outlined in the drainage strategy to manage surface water

runoff volumes, rates and water quality. The drainage strategy measures have the potential to

provide a positive impact on the water quality of the watercourses on site through improvements

to hydro-morphological and habitat features. Table 10-11 summarises the potential impacts,

their likely significant effects, mitigation measures to be implemented and residual effects.

10.118 The outline drainage strategy for the proposed development and the J33 outline drainage

strategy would generate runoff rates and volumes are restricted to pre-development greenfield

rates for both development sites. Collectively the drainage strategies can be regarded as tertiary

mitigation, meaning that it would be unlikely that there would be any significant cumulative

effects on surface water flooding. Similarly both developments are likely to need to contribute

towards upgrades to the Coslech Wastewater Treatment Works, and so it would be unlikely that

there would be significant cumulative effects arising from combined foul flows.


10-1

Table 10-11Potential impacts, likely significant effects, mitigation and residual effects

Potential impact Receptor Nature of Effect

Pre-mitigation LSE

Mitigation measures Residual LSE

Construction Phase

Flooding Floodplain, with low probability of flooding highly and less vulnerable receptors

Temporary

Direct

Neutral/slight

Adverse

Detention basins to store increased runoff during construction. Minimise culvert use during construction. Define safe routes for exceedance overland flow.

Neutral/slight

Coslech Wastewater Treatment Works

Slight

Adverse

Neutral/slight

Erosion/

increased flow

Nant Henstaff SINC Temporary

Direct

Moderate

Adverse

Detention basins to store increased runoff during construction. Minimise culvert use during construction. Define safe routes for exceedance overland flow.

Neutral/slight

Un-named watercourse

Upper Nant Coslech WFD waterbody River Ely confluence of Nant Clun to Allot Garden

Neutral/slight

Adverse

Neutral/slight

Sedimentation Nant Henstaff SINC Temporary

Direct

Moderate

Adverse

Implementation of approved CEMP, use of detention basins, settlement ponds, best practice construction methods for silt management. Exclusion zones around sensitive areas.

Slight adverse



Slight

Adverse

Neutral/slight

Pollution Nant Henstaff SINC

Temporary

Direct

Moderate/large

Adverse

Implementation of approved CEMP with best practice construction methods including procedures for fuel/oil/concrete storage and use.

Slight adverse


Upper Nant Coslech

Slight/moderate Adverse

Neutral/slight


10-2

WFD waterbody River Ely confluence of Nant Clun to Allot Garden

Slight

Adverse

Neutral/slight

Operational Phase

Flooding Floodplain, with low probability of flooding highly and less vulnerable receptors

Permanent

Direct

Slight/moderate

Adverse

Implementation of SuDS drainage strategy to attenuate flows to greenfield runoff rates and volumes in the 1% AEP event plus 30% climate change allowance.

Neutral/slight

Coslech Wastewater Treatment Works

Moderate

Adverse

Neutral/slight

Erosion/

increased flow

Nant Henstaff SINC Permanent

Direct

Moderate

Adverse

Implementation of SuDS drainage strategy to attenuate flows to greenfield runoff rates (controlling high flows) and provide watercourse restoration and hydro-morphological diversity.

Neutral/slight



Neutral/slight

Adverse

Neutral/slight

Sediment and Pollution


Direct

Moderate

Adverse

Implementation of SuDS drainage strategy including water quality filtering from attenuation basins, swales, bioretention features, filter strips and highway catchment pits. SuDS maintenance regime.

Neutral/slight



Slight

Adverse

Neutral/slight

Watercourse hydro-morphology restoration

Nant Henstaff SINC

Permanent

Direct

Moderate/large

Beneficial

No mitigation necessary. Hydro-morphological restoration implemented as part of SuDS drainage strategy.

Moderate/large Beneficial


Upper Nant Coslech WFD waterbody R Ely

Slight/moderate

Beneficial

Slight/moderate Beneficial


10-3

confluence of Nant Clun to Allot Garden

Watercourse habitat creation


Direct

Moderate/large

Beneficial

No mitigation necessary. Water environment habitat creation implemented as part of SuDS drainage strategy.

Moderate/large Beneficial


Upper Nant Coslech WFD waterbody R Ely confluence of Nant Clun to Allot Garden

Slight/moderate

Beneficial

Slight/moderate Beneficial

Foul discharge WFD waterbody River Ely confluence of Nant Clun to Allot Garden

Permanent

Indirect

Slight/moderate DCWW WWTW cumulative system and capacity impacts assessed by DCWW, relevant improvements specified and carried out. DCWW discharge in compliance with environmental requirements and permits.

Neutral/slight


10-1

Reference List

• BSG Ecology, 2019. Land North of Llantrisant Road, Creigiau, Phase 1 Habitat Survey.

• Cardiff City Council, 2016. Cardiff Adopted LDP 2006 – 2026.

https://www.cardiff.gov.uk/ENG/resident/Planning/Local-Development-

Plan/Pages/default.aspx

• DPP Planning, 2018. Land north of Llantrisant Road, Creigiau (LDP Allocation - Site E) EIA

Scoping Report.

• Environment Agency. Severn River Basin Management Plan.

https://www.gov.uk/government/publications/severn-river-basin-district-river-basin-

management-plan

• Highways Agency, 2009. Design Manual for Road and Bridges. Volume 11 Section 3

Environmental Assessment Techniques.


• Shear Design 2013. Drainage Strategy at proposed Residential Development Land South of

Creigiau.

• Natural Resources Wales, 2016. South East Valleys Management Catchment Summary.

https://naturalresources.wales/media/679387/2016_updated-south-

east_valleys_catchment_summary_nrw.pdf

• Natural Resources Wales. Flood Risk Map Viewer.

https://maps.cyfoethnaturiolcymru.gov.uk/Html5Viewer/Index.html?configBase=https://

maps.cyfoethnaturiolcymru.gov.uk/Geocortex/Essentials/REST/sites/Flood_Risk/viewers/F

lood_Risk/virtualdirectory/Resources/Config/Default&layerTheme=2

• Welsh Government, 2018. Policy Wales Edition 10. http://www. planning-policy-wales-

edition-10.pdf

• Welsh Government, 2018. Statutory standards for sustainable drainage systems –

designing, constructing, operating and maintaining surface water drainage systems.













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