Flood Consequence Assessment
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Machynlleth General Recreational Land FLOOD CONSEQUENCE ASSESSMENT 8923-FCA-003 DECEMBER 2020 CONFIDENTIAL Machynlleth General Recreational Land FLOOD CONSEQUENCE ASSESSMENT REPORT (RV2) CONFIDENTIAL PROJECT NO. 70068923 OUR REF. NO. 8923-FCA-003 DATE: DECEMBER 2020 WSP The Mailbox Level 2 100 Wharfside Street Birmingham B1 1RT Phone: +44 1213 524 700 Fax: +44 121 352 4701 WSP.com QUALITY CONTROL Issue/revision First issue Revision 1 Revision 2 Revision 3 Remarks Draft for Comment Draft for Comment First Issue Date December 2020 December 2020 December 2020 Prepared by Jack Grant Jack Grant Jack Grant Signature Checked by Alison Caldwell Alison Caldwell Alison Caldwell Signature Authorised by Richard Wilks Richard Wilks Richard Wilks Signature Project number 70068923 70068923 70068923 Report number Rv0 Rv1 Rv2 File reference 8923-FCA-001 8923-FCA-002 8923-FCA-003 Flood Consequence Assessment WSP Project No.: 70068923 | Our Ref No.: 8923-FCA-003 December 2020 Machynlleth General Recreational Land CONTENTS EXECUTIVE SUMMARY 1 1. INTRODUCTION 3 2. SITE SETTING 5 3. POLICY CONTEXT 9 4. ASSESSMENT OF FLOOD RISK 12 5. CONCLUSIONS 17 Flood Consequence Assessment WSP Project No.: 70068923 | Our Ref No.: 8923-FCA-003 December 2020 Machynlleth General Recreational Land TABLES Table 1 - Stakeholder Consultation Summary 4 Table 2 - BGS Borehole Summary 8 Table 3 - Flood Risk Overview 12 FIGURES Figure 1 – Site Location 5 Figure 2 – Baseline Drainage Features 6 Figure 3 – BGS Map Extract (Bedrock Designation) 7 Figure 4 – BGS Map Extract (Superficial Deposits Designation) 7 Figure 5 – NRW Development Advice Map Extract 13 Figure 6 – Natural Resources Wales Flood Risk from Rivers and Sea Map Extract 14 Figure 7 – Surface Water Flood Risk Map 15 APPENDICES - DRAWINGS - CORRESPONDENCE WSP Flood Consequence Assessment December 2020 Project No.: 70068923 | Our Ref No.: 8923-FCA-003 Machynlleth General Recreational Land EXECUTIVE SUMMARY This Flood Consequence Assessment has been undertaken to accompany the proposed planning application for the proposed site for general recreation purposes in Machynlleth, hereafter referred to as ‘recreational land’. This has been prepared largely in accordance with the guidelines set out in the Technical Advice Note (TAN) 15: Development and Flood Risk published in July 2004 along with other relevant local and national guidance including relevant legislation and policies in Schedule 3 of Flood and Water Management Act (2010) and the Sustainable Drainage (Approval and Adoption) (Wales) Order 2019 (SNSSuDS) and associated regulations. Item Overview Site Location The site is located in Machynlleth, Wales. An approximate postcode is: SY20 8DR 7PE and OS Coordinates are: 274339, 300587. Development Proposals The proposed development consists 1.14ha of recreational land to the west of Machynlleth. Development Advice Mapping The site is located wholly within Development Advice Mapping (DAM) Zone(s) Zone C2. Vulnerability Classification(s) Less Vulnerable Fluvial Flood Risk High Tidal Flood Risk Very Low Surface Water Flood Risk Low Groundwater Flood Risk Low Sewer Flood Risk Low Artificial Flood Risk Low Justification Test Given the sites location within Zone C2 (Development Advice Map), in accordance with current national guidance, the site is required to undertake the Justification Test. Given the proposed use of the recreational land for general recreation purposes, that no changes are proposed to existing ground levels and that the potential consequences of a flooding event for this particular type of development are considered to be negligible, it can be deemed that the development is suitable. Flood Consequence Assessment WSP Project No.: 70068923 | Our Ref No.: 8923-FCA-003 December 2020 Machynlleth General Recreational Land 1. INTRODUCTION 1.1. BACKGROUND 1.1.1. WSP has been appointed by Powys County Council to prepare a Flood Consequence Assessment (FCA) to support the planning application for recreational land in Machynlleth (approximate Post Code: SY20 8DR). 1.1.2. The objective of the study is to demonstrate that the site may be developed safely, without exposing the development to an unacceptable degree of flood risk or increasing the flood risk to third parties. The primary objectives are to: . Identify the likely potential sources of flooding which may affect the site; and, . Undertake an appraisal of the flood risk posed to the site and potential impact of the development on flood risk elsewhere. 1.2. LIMITATIONS 1.2.1. WSP has prepared this report in accordance with the instructions of their client, Powys County Council, for their sole and specific use. Any person who uses any information contained herein do so at their own risk. © WSP UK Ltd 2020. 1.2.2. The conclusions and recommendations contained herein are limited by the availability of background information and the planned use for the site. 1.2.3. Third party information has been used in the preparation of this report, which WSP UK Ltd, by necessity assumes is correct at the time of writing. Whilst all reasonable checks have been made on data sources and the accuracy of the data, WSP UK Ltd accepts no liability for same. 1.3. CONSTRUCTION (DESIGN AND MANAGEMENT) REGULATIONS 2015 1.3.1. The revised Construction (Design and Management) Regulations 2015 (CDM Regulations) came into force on April 2015 to update certain duties on all parties involved in a construction project, including those promoting the development. One of the designer’s responsibilities under clause 9 (1) is to ensure that the client organisation, in this instance Powys County Council, is made aware of their duties under the CDM Regulations. 1.4. SCOPE OF ASSESSMENT 1.4.1. The assessment has been undertaken in accordance with the overarching national requirements for Flood Consequence Assessment for proposed developments including, but not limited to, the following: . TAN 15: Development and Flood Risk . Development and Flood Risk (C624) . The SuDS Manual (CIRIA C753) . Sustainable Drainage (SuDS) Statutory Guidance 1.4.2. This Flood Consequence Assessment is solely to be used for the proposed recreational land in Machynlleth. Flood Consequence Assessment WSP Project No.: 70068923 | Our Ref No.: 8923-FCA-003 December 2020 Machynlleth General Recreational Land Page 3 of 18 1.5. CONSULTATION Written Consultation 1.5.1. Ahead of production of this report, consultation was undertaken as summarised in Table 1. Table 1 - Stakeholder Consultation Summary Stakeholder Date Received Comments Powys County Council Lead 04/12/2020 Initial consultation has been undertaken with Powys Local Flood Authority (LLFA) County Council who confirmed there to be no flood records in the proposed recreational land. Natural Resources Wales 10/12/2020 Initial consultation has been undertaken with NRW who provided the latest Flood Zone data and also confirmed the expected publishing date of the updated TAN15 to be the middle of 2021. 1.5.2. A copy of the consultations is contained in Appendix B. WSP Flood Consequence Assessment December 2020 Project No.: 70068923 | Our Ref No.: 8923-FCA-003 Page 4 of 18 Machynlleth General Recreational Land 2. SITE SETTING 2.1. LOCATION 2.1.1. The site is located to the south-east of Machynlleth with a site location plan shown in Figure 1 and also included in Appendix A. Figure 1 – Site Location 2.2. DEVELOPMENT PROPOSALS 2.2.1. The development proposal is for land used for general recreation purposes over an approximate areal extent of 1.14ha. 2.3. EXISTING TOPOGRAPHY 2.3.1. Available EA 1m DTM LiDAR identifies that the site falls from east to west with levels of approximately 7.18mAOD identified in the south east corner of the site falling to approximately 6.08mAOD on the western boundary of the site. 2.4. EXISTING DRAINAGE NETWORK 2.4.1. Garswin Stream (which is defined as a Main River by Natural Resources Wales) flows east to west, through the northern portion of the site, with School Stream bordering the southern and western boundary of the site with agricultural fields bordered the east of the site. School Stream discharges into the Garswin Stream to the north west of the site before flowing into the Afon Dyfi, a Natural Resources Wales Main River, which flows from east to west, approximately 1.5km south west of the site. These are shown indicatively in Figure 2. Flood Consequence Assessment WSP Project No.: 70068923 | Our Ref No.: 8923-FCA-003 December 2020 Machynlleth General Recreational Land Page 5 of 18 Figure 2 – Baseline Drainage Features 2.5. GEOLOGICAL AND HYDROGEOLOGICAL CONTEXT Geology 2.5.1. Reference to the British Geological Society (BGS) published mapping identifies the site to be underlain by bedrock of Devil's Bridge Formation (Mudstone and Sandstone) with predominantly overlying superficial Alluvial clay, silt and gravel deposits with a small section to the south east of the site overlain by Alluvial Fan gravel deposits. 2.5.2. An extract of the BGS Bedrock Designation mapping is shown in Figure 3 and BGS Superficial Deposits Mapping is shown in Figure 4. WSP Flood Consequence Assessment December 2020 Project No.: 70068923 | Our Ref No.: 8923-FCA-003 Page 6 of 18 Machynlleth General Recreational Land Figure 3 – BGS Map Extract (Bedrock Designation) Figure 4 – BGS Map Extract (Superficial Deposits Designation) 2.5.3. There are a number of boreholes registered approximately 250m east of the site, the deepest of which is 3m. These provide an indication of expected ground conditions on-site in addition to the details provided by the BGS mapping. The following general ground profile is recorded in Table 2. Flood Consequence Assessment WSP Project No.: 70068923 | Our Ref No.: 8923-FCA-003 December 2020 Machynlleth General Recreational Land Page 7 of 18 Table 2 - BGS Borehole Summary Description Approximate Depth [mBGL] Sandy clay topsoil 0.0 – 1.2 Dense, dark brown clayey coarse gravel 1.2 - 2.8 Hydrogeology 2.5.4. According to the Source Protection Zone map provided by Natural Resources Wales, the site does not lie within any Source Protection Zone.