Port Dinorwic Residential Development, Y Felinheli, Gwynedd Dinorwic Marina Ltd Flood Consequences Assessment Report & Drainage Strategy October 2020 62 Albany Road CARDIFF, CF24 3RR t: 029 2030 2521 Dinorwic Marina Ltd, Port Dinorwic Residential Development Y Felinheli, Gwynendd Flood Consequences Assessment Report & Drainage Strategy Document Control Sheet Project Number 2037 Project Title Port Dinorwic Residential Development Client Dinorwic Marina Ltd Document Title Flood Consequences Assessment Report & Drainage Strategy Prepared by Checked by Verified by Approved by Dimitrios Gkraikis Stephen Davis Principal Civil Engineer Director Revisions Revision Date Details Authorised by - 29 Oct 2020 First Issue Page | 2 Dinorwic Marina Ltd, Port Dinorwic Residential Development Y Felinheli, Gwynendd Flood Consequences Assessment Report & Drainage Strategy CONTENTS 1. INTRODUCTION ........................................................................................................... 4 1.1 Background ............................................................................................................................. 4 1.2 Existing Site............................................................................................................................. 4 1.3 Fluvial Flood and Tide Levels ................................................................................................. 5 1.4 Development Proposals .......................................................................................................... 5 2. FLOOD CONSEQUENCES ASSESSMENT .................................................................. 5 2.1 Legislation ............................................................................................................................... 5 2.2 Flood Zones – NRW Maps ...................................................................................................... 6 2.3 Flood Risk Vulnerability Classification .................................................................................... 6 2.4 Flood Risk Vulnerability Classification and Flood Zone Compatibility .................................... 6 2.5 Site Specific Sources of Flood Risk ........................................................................................ 6 2.6 Tide Level Calculations ........................................................................................................... 6 2.7 Proposed Development Finished Floor Levels ....................................................................... 8 2.8 Emergency Access/Egress – Flood Hazard Rating ................................................................ 9 3. DRAINAGE STRATEGY .............................................................................................. 10 3.1 Foul/Surface Water Drainage................................................................................................ 10 4. CONCLUSIONS ........................................................................................................... 10 4.1 Summary ............................................................................................................................... 10 4.2 Conclusion ............................................................................................................................. 11 APPENDICES A SITE SURVEY B PROPOSED SITE LAYOUT C NRW FLUVIAL FLOOD/TIDAL FLOOD MAP D NRW SURFACE WATER FLOOD MAP E WELSH WATER PLANS F DRAINAGE STRATEGY Page | 3 Dinorwic Marina Ltd, Port Dinorwic Residential Development Y Felinheli, Gwynendd Flood Consequences Assessment Report & Drainage Strategy 1. INTRODUCTION 1.1 Background 1.1.1 PHG Consulting has been commissioned by Dinorwic Marina Ltd to undertake a flood consequences assessment report for a site located in the Marina area of Y Felinheli (Port Dinorwic), Gwynedd. 1.1.2 This report is to support a planning application for a residential use development. 1.1.3 The site is undeveloped, facing Nant Y Garth, currently used as boatyard. 1.1.4 The report addresses all possible sources of flood risk and demonstrates how the risks are mitigated to make the development safe without increasing flooding elsewhere. 1.2 Existing Site 1.2.1 The site is of rectangular shape with a total area of circa 600m2. It is located at the National Grid Reference SH528679. The site is facing Porth-Y-Llechen to the north boundary and past that is Nant Y Garth. To the east and west the site is bound by neighbouring developed sites. To the back of the site, at the south, there is an existing retaining wall and past that a tarmac road that is about 6m higher. Figure 1 below shows the location plan. Figure 1: Site location plan Page | 4 Dinorwic Marina Ltd, Port Dinorwic Residential Development Y Felinheli, Gwynendd Flood Consequences Assessment Report & Drainage Strategy 1.2.2 The site topography slopes south east to north west. The site is generally flat with levels between 5.95mAOD to 5.75mAOD at a gradient of circa 1in130. The existing Site Survey is shown in Appendix A. 1.2.3 The site is currently occupied as a boatyard and the whole plan area of the site is impermeable and tarmac paved. 1.3 Fluvial Flood and Tide Levels 1.3.1 The site is within the Marina area and is facing the Nant Y Garth which discharges to the Menai Strait. Due to the proximity of the Marina to the estuary mouth, the flood levels within the vicinity are predominantly affected by the Tide Levels. 1.3.2 According to the available stations for Tide Levels, the closest station is located close to the estuary mouth at Menai Strait. Calculations were undertaken to determine the T200 and the T1000 tide level including climate change using the Menai Strait station tide levels. 1.4 Development Proposals 1.4.1 The proposal comprises the construction of a residential use building with 11 flats within four floors in total. The proposed development Layout is included in Appendix B. 1.4.2 The scheme is also subject to new SAB (SuDS Approval Body) legislation and the proposed scheme will need to adhere to it. In addition to the proposed planning application a pre-SAB and full SAB application and approval process will also run in parallel. The SAB is Gwynedd Council. 2. FLOOD CONSEQUENCES ASSESSMENT 2.1 Legislation 2.1.1 TAN 15 is the planning guidance used in Wales to assess flood risk. A requirement of TAN 15 is that developers making planning applications on sites that are potentially at risk from flooding should consult with Natural Resources Wales and produce a Flood Consequence Assessment (FCA) to support their proposals. This assessment has been conducted in accordance with TAN 15: Development and Flood Risk 2.1.2 Definition of Flood Risk Flood risk is the product of the likelihood or chance of a flood occurring (flood frequency) and the consequence or impact of the flooding (flood consequence). 2.1.3 Flood Frequency 2.1.4 Flood frequency is identified in terms of the return period and annual probability. For example, a 1 in 100-year flood event has a 1% annual probability of occurring. Table 1 below provides a conversion between return periods and annual flood probabilities. Table 1: Flood Probability Conversion Table Return Period (years) 2 5 10 20 50 100 200 1000 Annual Flood Probability (%) 50 20 10 5 2 1 0.5 0.1 Page | 5 Dinorwic Marina Ltd, Port Dinorwic Residential Development Y Felinheli, Gwynendd Flood Consequences Assessment Report & Drainage Strategy 2.2 Flood Zones – NRW Maps 2.2.1 The proposed development area is located outside the flood zones according to the NRW Flood Maps included in Appendix C. 2.3 Flood Risk Vulnerability Classification 2.3.1 The proposed development is residential so is classed as highly vulnerable in TAN 15. 2.4 Flood Risk Vulnerability Classification and Flood Zone Compatibility 2.4.1 The site to be developed according to the maps is outside the floodplain in Flood Zone 1 and the type of development is allowed. 2.4.2 Nonetheless, since the Nant Y Garth is a tide locked watercourse, the Tide Levels, including climate change allowance due to the rising of the sea level, will be taken into consideration. 2.5 Site Specific Sources of Flood Risk 2.5.1 The site is in Flood Zone 1 according to the NRW’s Flood Map, included in Appendix C. The proposed development is facing Nant Y Garth and is circa 300m from the Menai Strait. 2.5.2 Part of the site is at Low Surface Water Flood Risk according to the NRW’s map included in Appendix D. Given the site’s direct proximity to the watercourse, the risk is not taken into consideration as the Tide Level is the predominant source of flood risk. Any surface water will naturally flow from south to north direction towards the watercourse since the site slopes from 5.89mAOD to the south to 5.60mAOD to the north at the watercourse. 2.6 Tide Level Calculations The available data from the UK.GOV web site regarding the Tide Levels are as shown below: The downloaded “Coastal Design Sea Levels” information was used to obtain the T200 and T1000 with a confidence level of 97.5%. On top of that, the climate change was applied in line with the latest guidance from Welsh Government Document “Adapting to Climate Change: Guidance for Flood and Coastal Erosion Risk”, December 2017. Page | 6 Dinorwic Marina Ltd, Port Dinorwic Residential Development Y Felinheli, Gwynendd Flood Consequences Assessment Report & Drainage Strategy 2.6.1 The data “Coastal Design Sea Levels” was analysed to determine the closest tide level station available in relation to the Estuary Mouth of Nant Y Garth in the