Caulmert Limited Engineering, Environmental & Planning Consultancy Services
Residential Development at Penygroes
DWE Developments Ltd.
Flood Consequence Assessment
Caulmert Limited Unit 14 Ffordd Richard Davies St Asaph Business Park Denbighshire LL17 0LJ Tel: 01745 530890 Email: [email protected] Web: www.caulmert.com
Document Reference: 3693-CAU-XX-XX-RP-C-0300.S4-P1
September 2019
DWE Developments Ltd. Proposed Residential Development Land at Penygroes, Gwynedd Flood Consequence Assessment
APPROVAL RECORD
Site: Land at Penygroes, Gwynedd
Client: DWE Developments Limited
Project Title: Flood Consequence Assessment
Document Title: Report
Document Ref: 3693-CAU-XX-XX-RP-C-0300.A0-C1
Report Status: FINAL
Project Director: Jonathan Sykes
Project Manager: David High
Caulmert Limited: Unit 14 Ffordd Richard Davies St Asaph Business Park LL17 0LJ
Tel: 01745 530890
Author Steve Barber-Bailey Date 3rd September 2019
Reviewer David High Date 4th September 2019
Approved Jonathan Sykes Date 5th September 2019
DISCLAIMER
This report has been prepared by Caulmert Limited with reasonable skill, care and diligence in accordance with the instruction of the above named client and within the terms and conditions of the Contract with the Client.
The report is for the sole use of the above named Client and Caulmert Limited shall not be held responsible for any use of the report or its content for any purpose other than that for which it was prepared and provided to the Client.
Caulmert Limited accepts no responsibility of whatever nature to any third parties who may have been made aware of or have acted in the knowledge of the report or its contents.
No part of this document may be copied or reproduced without the prior written approval of Caulmert Limited.
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DWE Developments Ltd. Proposed Residential Development Land at Penygroes, Gwynedd Flood Consequence Assessment
TABLE OF CONTENTS
0 1.0 INTRODUCTION ...... 1 1.1 Terms of Reference ...... 1 2.0 LEGISLATION AND POLICY FRAMEWORK ...... 1 2.2 Planning Context – Local Policy ...... 2 3.0 ASSESSMENT APPROACH ...... 2 3.1 Methodology ...... 2 4.0 BASELINE CONDITIONS ...... 4 4.1 Site Description and Context ...... 4 4.2 Fluvial Features ...... 5 4.3 Culverted Watercourse ...... 5 4.4 Other Features ...... 7 4.5 Topography ...... 8 4.6 Geology ...... 8 4.7 Hydrogeology ...... 8 4.8 Flood Defences ...... 9 4.9 Flood Zone – NRW’s Indicative Flood Map ...... 9 4.10 Previous Studies ...... 9 5.0 PROPOSED DEVELOPMENT ...... 10 5.2 Proposed Development’s Drainage...... 10 5.3 Development Classification ...... 11 5.4 Consultation with SuDS Approval Body ...... 11 6.0 KEY IMPACTS & LIKELY SIGNIFICANT EFFECTS ...... 11 6.1 Assessment of Risk ...... 11 6.2 Flooding – Fluvial ...... 11 6.3 Tidal Flooding ...... 11 6.4 Groundwater Flooding ...... 11 6.5 Flooding from Surface Water ...... 11 6.6 Flooding from Other Sources ...... 12 6.7 Site Development Impacts ...... 13 6.8 Surface Water Exceedance ...... 13 6.9 Justification Test ...... 14 7.0 MITIGATION, ENHANCEMENT AND RESIDUAL EFFECTS ...... 14 8.0 SUMMARY ...... 14 9.0 REFERENCES ...... 15
TABLES Tables 1 - 4: Calculations for the estimated Flow from a Small Catchment
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Appendices
Appendix A: Topographic Survey Appendix B: The Proposed Development
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1.0 INTRODUCTION
1.1 Terms of Reference
1.1.1 Caulmert Ltd was commissioned by DWE Developments Limited (the Developer) to undertake a Flood Consequences Assessment (FCA) for a proposed residential development at land in Penygroes.
1.1.2 The requirement for an FCA in support of this Proposed Development is at the request of the Group Environment Officer of the County Council.
1.1.3 The assessment has been carried out in accordance with Technical Advice Note 15, Development and Flood Risk (TAN15) and based on the information which was provided by the Developer for the Proposed Development.
2.0 LEGISLATION AND POLICY FRAMEWORK
2.1.1 The susceptibility of land to flooding is a material consideration in determining planning applications. The potential consequences for occupiers, either of the development or elsewhere, in terms of personal safety and financial risk can be serious. Consideration must be made of the specific risk of flooding to the development being proposed over its expected lifetime, and its possible effects on flood risks elsewhere in terms of its effects on flood flows and flood storage capacity and runoff implications.
National Policy Guidance
2.1.2 National policy guidance relevant to flood risk and the project includes the following:
• Planning Policy Wales (Edition 9 – Nov 2016) • Technical Advice Note 15: Development and Flood Risk
2.1.3 Planning Policy Wales provides an overview of the land use planning policies in Wales. It is supplemented by a series of Technical Advice Notes. TAN 15, published in 2004, sets out the policy on development and flood risk. It highlights the key role of the planning system in managing flood risk
2.1.4 TAN15 sets out a precautionary framework to guide planning decisions for new development in areas of flood risk with the aim of directing new development away from those areas that are at a high risk of flooding, taking account of development vulnerability, and making an assessment of current and future conditions.
2.1.5 Orders were laid before the National Assembly in October 2018, as part of a suite of statutory instruments relating to sustainable drainage systems (SuDS). These make provisions under Schedule 3 to the Flood and Water Management Act 2010, which make SuDS a mandatory requirement for all new developments from January 2019. National standards are available that advises on the most effective way of embedding SuDS principles in new developments.
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2.1.6 Sustainable drainage is one of a range of measures designed to reduce flood risk, protect water quality, as well as encourage biodiversity and amenity. Instead of connecting surface water drainage from new developments directly to public sewers and watercourses, developers are encouraged to provide SuDS wherever possible.
2.2 Planning Context – Local Policy
2.2.1 The Anglesey & Gwynedd Joint Local Development Plan Strategic Flood Consequence Assessment (SFCA) (Stage 1) embraces the guiding principles of flood management that are to:
• Embrace a collaborative, local approach to managing the risk and consequences of flooding and coastal erosion, and the provision and funding of innovative schemes to reduce those risks; • Increase the understanding of the risks of flooding and ensure everyone is aware of these risk, and understands their responsibility to protect their own property from flooding; • Ensure better, more frequent, communication between everyone involved in the management of flood risk; • Ensure new development fully embraces sustainable drainage principles and does not increase flood risk; • Develop and maintain effective emergency plans and responses to flood incidents; • Encourage effective maintenance of all structures and watercourses.
3.0 ASSESSMENT APPROACH
3.1 Methodology
3.1.1 The principal aim of a FCA is to establish the following:
• Whether a proposed development is likely to be affected by current or future flooding from any source; • Whether it will increase flood risk elsewhere; • Whether the measures proposed to deal with these effects and risks are appropriate; • Whether the site will be safe to enable the passing of the justification test.
3.1.2 The content of a FCA should be appropriate to the scale and nature of the development. The FCA prepared for a project should cover various aspects relating to flood risk, flood management and drainage strategy.
3.1.3 The potential impacts of the project on flood risk have been assessed with regard to relevant tests provided in national legislation and, where relevant, in national, regional and local policies. Flood risk will be assessed from tidal and river systems, and from other sources.
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Flood Risk – Tidal and River
3.1.4 An assessment of the risk of flooding of the existing land has been made by reference to Natural Resources Wales (NRW) Developer Advice Map and a comparison has been made between the current and projected flood risk and how this is affected by the project.
Flood Risk – Other Sources
3.1.5 An assessment of the risk of flooding from other possible sources has also been made. The current risk of flooding from each source has been assessed upon the existing and proposed land uses. This has then been compared to theorised risks associated with the application site.
Baseline Methodology
3.1.6 The methodology involved in undertaking the assessment of the baseline conditions is as shown in this section.
Flood Risk – Existing Land Use
(a) Topographical information of existing land use; (b) Flood level data from NRW and other responsible bodies; (c) Flooding data from reported and anecdotal sources; (d) A review of current impact (area, severity, consequence, mitigation) of flooding by various sources of water upon existing land use.
3.1.7 The methodology for undertaking the assessment of the effects of the project is as follows:
Flood Risk – Proposed Land Use
(a) A review of the proposed development levels; (b) A review of areas of the project that fall within existing flood zones; (c) A quantification of the predicted impact of existing flood risk upon the project; (d) A determination of those areas that might realise an increase or decrease in flood risk.
Limitations
3.1.8 This assessment is primarily based on a desktop study, and has been supported by a single site walkover visit.
3.1.9 Some of the aspects considered in this study are subject to change with time. Therefore, if the development is delayed or postponed, consideration should be given to reviewing such issues to confirm that no changes have taken place, either at the site or within legislation.
3.1.10 The FCA is based on the end use specified in the text. If this end use is changed then consideration should be given to re-visiting the findings of this document to ensure that they remain valid.
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DWE Developments Ltd. Proposed Residential Development Land at Penygroes, Gwynedd Flood Consequence Assessment
4.0 BASELINE CONDITIONS
4.1 Site Description and Context
4.1.1 The site is located some 500m to the north of the centre of Penygroes village. It is a brownfield site and is currently occupied by an industrial unit (garage/depot/warehouse) and associated hardstanding. There is a small area of dense unkempt vegetation to the rear of the plot. The OSGR of the centroid is 247203, 353560. The total plan area of the site is 3,590m2. The location of the site is in Figure 1.
Site Location
Figure 1: Site Location Plan (Ordnance Survey 1:25,000)
4.1.2 Access to the site is from the former A487 Lôn Caernarfon, and opposite a row of residential terraced housing known as Tredaffydd. There is existing residential development to the north (Bro Llwyndu) and to the south (Mor Awel) of the site, and a residential unit ‘Llys Eifion’ to the east.
4.1.3 The majority of site comprises impermeable surfaces of roofs and hardstanding. The western hardstanding abuts the public highway. The parcel of unmanaged ground to the rear of the site comprises approximately 1,200m2 of vegetated ground. There is a culvert crossing this area with a short length of open channel close to the east end of the industrial unit. This is considered in detail in section 4.3 below.
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4.2 Fluvial Features
4.2.1 The principal fluvial feature closest to the site is the Afon Llyfni that is approximately 900m south of the site. At Penygroes this river has a drainage catchment area of 30.7km2, which is predominantly from within the Nantlle Valley south of the site. The catchment includes Llyn Nantlle Uchaf, and a number of smaller lakes from flooded quarries along the valley. The Afon Llyfni outfalls to Caernarfon Bay at Pontllyfni.
4.2.2 The development site falls outside the Afon Llyfni catchment and within the drainage catchment of the adjacent Afon Llifon. This river outfalls into Caernarfon Bay some 4km to the west of Penygroes and 1km to the north of the Afon Llyfni outfall. The Llyfni catchment is small and measures less than 1km2 at Penygroes.
4.3 Culverted Watercourse
4.3.1 A site walkover visit revealed a short section of open channel within the site area. This channel is believed to be contiguous with upstream and downstream culverts across the site that originate from ditchcourse(s) to the east of the Bro Llwyndu residential estate.
4.3.2 The topographic survey shows that there is an 800mm diameter culvert feeding into the short section of open channel. The site walkover discovered that the open channel returns to culvert and flows along the south of the existing warehouse building, flowing east-west towards Lôn Caernarvon. The culvert flows beneath Lôn Caernarvon and beneath the row of terraced houses opposite. A plan of the expected route of the culvert is shown in Figure 2.
Site Location
Approximate route
of existing culvert
Figure 2: Approximate Route of Existing Culvert
4.3.3 Investigation has taken place to identify the drainage catchment that outfalls at the short section of open channel. Within the broad catchment area there is a number of smaller sub- catchments that take account of minor changes in ground levels and other local factors. Site
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DWE Developments Ltd. Proposed Residential Development Land at Penygroes, Gwynedd Flood Consequence Assessment
observation has made it possible to isolate the sub-catchment that drains towards the site as having a plan area of 28.8 hectares. Water enters the culvert via a chamber at the upstream end that allows wetland to be drained into it.
4.3.4 Calculations have been made to estimate the flow rates that can be expected from the sub- catchment. A number of computational techniques were used to assess basic annual flow rates based on the Institute of Hydraulics Report 124 Flood Estimation for Small Catchments.
(1) HR Wallingford Greenfield Runoff Estimation for Sites (online at uksuds.com)
QBAR = 139 l/s based on SAAR of 1605mm and SOIL type 2 (SPR/SPRHOST = 0.3)
Based of calculated growth curve factors the flow rates for different return periods:
Flow Rate (m3/s)
QBAR 0.14
1 in 1 year 0.12
1 in 30 years 0.25
1 in 100 years 0.30
Table 1: Greenfield Runoff Rates (HR Wallingford – Greenfield Runoff Tool)
(2) Flood Estimation for Small Catchments – IoH Report 124 (hand calculations)
QBARrural = 147 l/s based on SAAR of 1605mm and SOIL type 2 (SPR/SPRHOST = 0.3)
The value of QBARrural was recalculated for alternative SPR/SPRHOST figures 0.2 and 0.4. This was carried out to identify the sensitivity of the calculated flow as a result of potential variation in soil characteristic.
The same growth curve factors used in Table 1 were adopted to estimate the flow rates for different return periods:
Flow Rate (m3/s) Flow Rate (m3/s) Flow Rate (m3/s)
(SPR – 0.2) (SPR – 0.3) (SPR – 0.4)
QBARrural 0.06 0.15 0.28
1 in 1 year 0.05 0.13 0.24
1 in 30 years 0.11 0.26 0.49
1 in 100 years 0.13 0.32 0.60
Table 2: QBARrural Estimation for Small Catchments (IoH Report 124)
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4.3.5 The values presented in Tables 1 and 2 are broadly similar for comparable return periods. There is variation in flow rates when a range of different soil characteristics are used to represent actual site conditions. For example, the 1 in 30 year flow rates using SPR values of 0.2 and then 0.4 results in a near five-fold increase on flow. A precautionary value of SPR – 0.3 is considered adequate for this catchment.
4.3.6 A consideration of the predicted effects of Climate Change should be made in order to assess the potential increase in flow over the lifetime of the proposed development. An allowance of 40% for climate change is considered reasonable which takes account of recent guidance for FCA compilation.
Flow Rate (m3/s) Flow Rate (m3/s)
(SPR – 0.3) 40% Uplift
QBARrural 0.15 0.21
1 in 1 year 0.13 0.18
1 in 30 years 0.26 0.36
1 in 100 years 0.32 0.45
Table 3: QBARrural Estimation for Small Catchments plus 40% uplift for Climate Change
4.3.7 The existing culvert will need to be accommodated through the proposed development. It is believed that a diverted culvert can be thread through the potential layout to avoid building foundations. It will be necessary to demonstrate that the diverted culvert is hydraulically similar to the existing culvert so that the upstream and downstream fluvial conditions remain unchanged.
4.3.8 An initial estimate of the flow capacity of the 800mm diameter culvert can be made from physical parameters of the pipe. Based on the measured upstream and downstream pipe invert levels within the site, and the distance between them the pipe gradient has been calculated to be 1 in 150. Assuming a nominal internal friction value of ks = 0.6mm, the pipe full capacity is 1.2m3/s (reference – HR Wallingford Hydraulic Design Tables 6th Edition). The theoretical flow rate of the culvert is greater than the flow predicted from the sub-catchment. Nevertheless a full account of local variables should be taken into account when considering a culvert diversion around or through the proposed development.
4.4 Other Features
4.4.1 There are no other water features located in close proximity to the proposed development. Nevertheless, contemporary Ordnance Survey maps show a number of ‘springs’ and ‘sinks’ close to the site, and an indication of ‘marshy vegetation’ to the east. None of these features is believed to adversely affect the drainage of the proposed development site.
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4.5 Topography
4.5.1 Penygroes is a 19th Century village established along the course of the medieval road from Caernarfon to Clynnog, and along an 1820s road that partly superseded it. The village lies on the western or southwestern flanks of Mynydd y Cilgwyn (347m) and the lower Clogwyn Melyn (220m).
4.5.2 A topographic survey of the application site (Appendix A) shows that the ground within the area of development is generally flat. The east-west length of the site is approximately 100m. The highest elevation at the east is 121.3m, and the lowest at the western end of 117.7m, giving an overall gradient of 1:28.
4.5.3 The topographic survey shows the ground floor level of the existing industrial unit is approximately 118.3m. There are no drainage features depicted on the survey, although there are a collection of chambers within the hardstanding that require further investigation in case they house tanks or interceptors.
4.6 Geology
4.6.1 The site’s bedrock geology, based the BGS Geology of Britain viewer (bgs.ac.uk), generally comprises the Padarn Tuff Formation - an igneous bedrock formed approximately 600 million years ago in the Ediacaran Period. Superficial deposits are listed as Glaciofluvial Terrace Deposits, Devensian – essentially sand and gravel from the Quaternary Period.
4.6.2 The feasibility of surface water disposal by ground infiltration based upon the broad geology described above is considered to be moderate. According to the Soilscape online viewer, the site falls within Soilscape 6, typified by freely draining slightly acid loamy soils. The potential for ground infiltration is considered fair but will be dependant on local conditions.
4.7 Hydrogeology
4.7.1 The area in and around Penygroes is listed by Envirocheck as not in a Source Protection Zone. It has a Secondary ‘A’ bedrock aquifer designation, and is a minor aquifer that is variably permeable.
4.7.2 Site-specific ground investigation was carried out at the site in April 2019. Groundwater was encountered and varied between 0.87m below ground level (bgl) and 1.33mbgl. Groundwater levels were monitored over a period of three months and was recorded between 0.42mbgl and 1.00mbgl in all monitoring wells. No falling head tests were carried out during the ground investigation due to the relatively high groundwater levels. It was noted that groundwater levels were likely to vary due to seasonal and other effects.
4.7.3 According to the BGS Flood GFS Data, the site has a limited potential for groundwater flooding.
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4.8 Flood Defences
4.8.1 There are no flood defences present within or close to the site.
4.9 Flood Zone – NRW’s Indicative Flood Map
4.9.1 According to the NRW’s Developer’s Advice Map the site is wholly within Flood Zone A. The NRW zonings refer to the possibility of river, or river and sea, flooding and ignore the presence of flood defences. Flood Zone A represents the lowest risk of flood and comprises land that has been assessed as having a less than 1 in 1,000 (<0.1%) annual probability of river or sea flooding.
4.9.2 The closest Flood Zone B and Flood Zone C locations relative to the site are associated with the Afon Llyfni, which is 900m southeast of the site. An extract from the Developer Advice Map is reproduced in Figure 3.
Site Location
Flood Zone C
Figure 3: Extract from NRW’s Developer Advice Map (June 2019) (Reference - https://maps.cyfoethnaturiolcymru.gov.uk)
4.10 Previous Studies
Anglesey & Gwynedd Joint Local Development Plan - Strategic Flood Consequence Assessment (Level 1) (2015)
4.10.1 The Joint Local Development Plan contains in Topic Paper 8 a Level 1 Strategic Flood Consequence Assessment. It is designed to detail the roles and responsibilities of
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organisations within Anglesey & Gwynedd that contribute to managing flood risk, along with providing information to enable the local delivery of reducing overall flood risk.
4.10.2 The Topic Paper notes previous flooding within Penygroes, although this is attributable to parts of the village that contribute to the Afon Llyfwy to the south of the site. There is no commentary on flood events at or close to the site, or to events that contribute to the Afon Llifon catchment.
4.10.3 Appendix 3 of the Topic Paper shows Penygroes on Map 27 (A, B and C). The site is depicted as a ‘Housing Allocation not started’. Map 27C shows small areas of intermediate surface water flooding adjacent to the site to the north and west.
Adjacent Planning Applications
4.10.4 Gwynedd Council’s online viewer (www.gwynedd.llyw.cymru) details of number of nearby planning applications. None of these has merited the compilation of an accompanying FCA.
5.0 PROPOSED DEVELOPMENT
5.1.1 The proposed development is for the erection of twelve residential units and associated off- road access and parking. There is proposed to be two semi-detached buildings and a pair of detached units served by an adoptable highway immediately off Lôn Caernarfon. A further two blocks of three units extends east from the notional turning head of adoptable highway. A plan of the proposed development is shown in Appendix B.
5.1.2 The serviceable life of the residential units is anticipated to be 100 years.
5.2 Proposed Development’s Drainage
5.2.1 Foul and surface water from the proposed development will drain separately from the site to respective points of discharge. Reference should be made to the proposed Drainage Strategy document that provides commentary on the derivation of an appropriate system of drainage for the application site.
5.2.2 Surface water from the proposed development should accord with the Sustainable Drainage Systems Standards for Wales (SDSSfW) and with CIRIA C753 to manage the quality and quantity of rainwater runoff close to where it falls, and to allow its use in a manner that provides amenity benefits to site users and encourages biodiversity.
5.2.3 The proposed surface water management from the proposed development would provide betterment over drainage of the current land use. Surface water from the developed site would be directed to the existing culvert at a reduced rate of discharge via a system of storage volumes and hydraulic controls.
5.2.4 Foul water drainage would connect with an existing public sewer in Lôn Caernarvon. Dŵr Cymru has given an approval in principle to connect at existing manhole reference SH47531596 (Reference: pre-development advice dated 7th February 2019).
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5.3 Development Classification
5.3.1 The Proposed Development falls under the classification of ‘highly vulnerable development’ as defined in Figure 2 of TAN15. This type of development may be permitted within Flood Zone A. There is no requirement to provide further justification for the proposed development at this location.
5.4 Consultation with SuDS Approval Body
5.4.1 Discussions have taken place in February 2019 with the Environmental Officer of Gwynedd County Council to review the surface water drainage at the site. Concern was expressed at the routing of surface water exceedance flows through the development in case of blockage of the culvert.
6.0 KEY IMPACTS & LIKELY SIGNIFICANT EFFECTS
6.1 Assessment of Risk
6.1.1 In accordance with TAN15 flood risk must be assessed for all sources of flooding. The development of any site must be carried out in such a way as to mitigate any potential flood risk both on and off site. An assessment should be made of all possible sources of flooding and of the risk associated with each.
6.2 Flooding – Fluvial
6.2.1 The site is not at risk of fluvial flood.
6.3 Tidal Flooding
6.3.1 The site is remote from tidal influence. It is not affected by tidal regime.
6.4 Groundwater Flooding
6.4.1 The Anglesey and Gwynedd SFCA states that groundwater flooding is not thought to be a significant source of flooding throughout the area.
6.4.2 There is limited further information available on groundwater flooding. However, should groundwater flooding occur, it is likely to result in a shallow depth of water within low spots only and be of short duration.
6.5 Flooding from Surface Water
6.5.1 The NRW’s flood map for surface water provides a general indication of areas that may be at risk of surface water flooding. It takes broad account of topography, underground drainage, and typical storms that are likely to cause flooding. According to this map there is predominantly a ‘very low’ risk of flooding throughout the site, although a small area of ‘low’
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flood risk is shown at the western extent of the site. The area of ‘low’ risk is likely to be symptomatic of the existing landuse of the site.
6.5.2 The flood map also depicts areas of ‘medium’ and ‘high’ flood risks within the Bro Llwyndu housing estate to the north of the site. By observation there is no pathway between Bro Llwyndu and the site, and is therefore not considered to affect the development site.
6.5.3 These predicted areas of flood risk should not materially affect the proposed development, although effective mitigation should be reviewed to ensure a pathway remains closed. It is noted that the rear of the dwellings that back onto the site are raised in comparison to existing site levels. Effective mitigation may be required to endure that drainage onto the site from the north does not cause detriment.
6.5.4 An extract from the surface water flood map is shown at Figure 4. NRW defines a ‘low’ flood risk as an area that has an annual chance of flooding between 1 in 1000 (0.1%) and 1 in 100 (1%). A ‘medium’ flood risk between 1 in 100 (1%) and 1 in 30 (3.3%), and a ‘high’ flood risk is greater than 1 in 30 (3.3%).
Medium and High Flood Risk Low Flood Risk
Site Location
Figure 4: Extract from Surface Water Flooding map viewer (June 2019) (maps.cyfoethnaturiolcymru.gov.uk)
6.6 Flooding from Other Sources
6.6.1 There is residual flood risk from blockage of the existing and proposed culverted watercourse. Mitigation of this risk should be considered as part of the detailed design of the development.
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The predicted paths of exceedance flows should be identified so that there is no detriment to the proposed residential units and does not affect third party property.
6.6.2 Residual risk also occurs when runoff from intense rainfall exceeds the capacity of the existing system, which could lead to localised flooding. These types of incidents usually result in shallow levels of ponding water, and the risk of flooding in such circumstances is considered to be low.
6.6.3 According to the NRW maps there is no risk to the site from reservoir flooding.
6.6.4 The risk of flooding from public sewerage and from highway drainage is low. This is because drain surcharge and overland flow on the public highway would flow past the site by virtue of relative road levels.
6.7 Site Development Impacts
6.7.1 The erection of the proposed development should reduce the amount of impermeable area within the site. There is therefore an opportunity for surface water to be used as an asset within the development site with the deployment of effective SuDS design, installation and management.
6.7.2 The existing culvert passing through the site is to be replaced by a new asset on a new alignment. There is opportunity to provide hydraulic variation to upstream and/or downstream flow conditions. Overland exceedance flow routes should be accommodated that do not impinge on residential dwellings.
6.7.3 Drainage systems that are currently in place for the existing landuse will be removed and the ground locally remediated. The risk of pollution to the environment should be reduced following the removal of the legacy drainage.
6.8 Surface Water Exceedance
6.8.1 The existing surface water culvert through the site will be subject to diversion. Its new route will run through the rear and side garden/parking area of Plot-10 before leaving the development site and reconnecting with the existing culvert.
6.8.2 Exceedance flow routes will be created at two places within the site. It is proposed to form a higher exceedance route to the side and rear of Plot 8. This should cater for flows that may be created from a blockage of the culvert, diverting flow onto the adjacent access road to the south. The paved area in front of plots 5 to 10 would also provide attenuation storage during the 1 in 100 year rainfall event and so the proposed ground floor levels of dwelling units, which will be approximately 275mm above the ground level on the flood routing should not require any further consideration during exceedance.
6.8.3 Similar criterion would apply to a proposed to the lower area of the site and closer to Lôn Caernarfon. Exceedance flows would flow along the lower section of the proposed site access
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DWE Developments Ltd. Proposed Residential Development Land at Penygroes, Gwynedd Flood Consequence Assessment
road to then flow onto the street opposite the Tredafydd terraced buildings. Exceedance flow would then flow south along Lôn Caernarfon. This is the route currently used by surface water runoff from the existing landuse.
6.8.4 The provision of a restricted flow discharge and of surface water attenuation, and a considered approach to the management of exceedance flows, the risk of flooding of the proposed dwelling units is considered to be low. In addition, the proposed reduction in the rate of flow within the existing culvert consequential to the development is likely to reduce the risk of downstream flooding.
6.9 Justification Test
6.9.1 The justification test is not required as the proposed development lies wholly within Flood Zone A.
7.0 MITIGATION, ENHANCEMENT AND RESIDUAL EFFECTS
7.1.1 The proposed diversion of the culverted watercourse will need to make sure that the pre- and post-installation hydraulic conditions are not detrimental to affected land and property.
7.1.2 Potential surface water flood routes from Bro Llwyndu are identified and effective mitigation put in place that ensures the risk to the proposed development is acceptable.
7.1.3 Legacy drainage components of the existing site should be identified and grubbed up as appropriate prior to the construction of the proposed development.
7.1.4 There is a residual risk to the proposed development of flood from a blockage within the culverted watercourse. The design of the diversion should take account of adequate access provision, and be robustly designed for a blockage scenario.
7.1.5 The culverted watercourse and the installed SuDS features should be adequately operated and maintained for the lifetime of the development.
8.0 SUMMARY
8.1.1 The site falls within TAN15 Flood Zone A. Guidance indicates that new residential development should compatible with this flood designation.
8.1.2 The proposed development is not anticipated to be flooded by fluvial conditions.
8.1.3 The FCA has established that the proposed development:
• is not affected by current flooding; • will not increase flood risk elsewhere.
8.1.4 It will be necessary to divert an existing culverted watercourse through or around the proposed development
Caulmert Ltd 3693-CAU-XX-XX-RP-C-0300 14 September 2019
DWE Developments Ltd. Proposed Residential Development Land at Penygroes, Gwynedd Flood Consequence Assessment
9.0 REFERENCES
1) Welsh Assembly Government (2004), Planning Policy Wales, Technical Advice Note -15 Development and Flood Risk.
2) NRW Flood Map https://maps.cyfoethnaturiolcymru.gov.uk.
3) Welsh Government (2018), Statutory Standards For Sustainable Drainage Systems – Designing, Constructing, Operating And Maintaining Surface Water Drainage Systems.
4) Anglesey and Gwynedd Joint Local Development Plan (2016), Strategic Flood Consequence Assessment.
5) Flood Consequence Assessments: Climate change allowances (https://gweddill.gov.wales)
6) Reference Websites – jncc.defra.gov.uk; lle.gov.wales; old-maps.co.uk; nora.nerc.ac.uk; bgs.ac.uk; magic.defra.gov.uk.
7) Phase I and Phase II Geo-Environmental Report at the Former Industrial Buildings, Penygroes – Caulmert Ltd. (Reference: 3693-CAU-XX-XX-RP-V-3001.S0-P0.1 - April 2019).
8) Proposed Housing Development, Tredafydd Garage Site, Penygroes - Drainage Strategy (Revision P1) – Caulmert Ltd. (August 2019).
Caulmert Ltd 3693-CAU-XX-XX-RP-C-0300 15 September 2019
Appendix A Topographic Survey
Appendix B The Proposed Development
Registered Office: InTec, Parc Menai, Bangor, Gwynedd, LL57 4FG Tel: 01248 672666 Fax: 01248 672601 Email: [email protected] Web: www.caulmert.com