Tregroes Waffles Extension
Total Page：16
File Type：pdf, Size：1020Kb
Francis Sant Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report ETL 291 Drain Drain 287 285 WALTERS ROAD WALTERS 283 1 2 3 Fran Felin Bike Racks YNYSALLAN ROAD M 4 14.26m BM 1200 Wide Cycle Path 1 FB Gas Main with 30m wide Easement 23 22 18 24 21 20 25 19 26 27 28 31 30 29 Drain Drain (disused) Railway Drain May 2018 Final Project Development At Felin Fran, Llansamlet, Swansea Document Flood Consequence Assessment Report Status Final Revision O Report Number R/WWG/D/1748 Date May 2018 Client Mark Saunders Consultant Francis Sant Limited Office 14, Queen St. Carmarthen Carmarthenshire SA31 1JT Author Ceirion Herbert Telephone 01267 233833 e-mail [email protected] This document has been produced in accordance with Francis Sant Limited Quality Management System Signed …………………………………………………………………… Date …………………………………………………………………… Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report CONTENTS 1 Introduction 1 2 The Development 3 3 Flood Risk 6 4 Flood Proofing Measures 13 5 Conclusions and Recommendations 14 R/WWG/D/1748 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report 1 INTRODUCTION It is proposed to develop thirty one houses on a site at Felin Fran, Llansamlet, Swansea. The client has been advised, however, that the entrance into the site lies within Zone C2 flood area as defined by the development advice maps (see Figure 1.1) referred to in Technical Advice Note 15, Development and Flood Risk (TAN 15). This means that the area is considered to be at risk of flooding during at least the 1 in 1000 year event and is not afforded protection from recognised significant flood defences. Location of Proposed Development Figure 1.1 - Development Advice Flood Map The development advice maps are based on Natural Resources Wales Flood Risk Maps (see Figure 1.2) supplemented by sediment data, held by the British Geological Survey (BGS), of historical flooding. The maps adopt the precautionary principle and are based on the best known information available at the time. However, a detailed examination of a site can refine an area’s risk of flooding. It is noted that the location of the proposed houses is expected to remain flood free even during a 1 in 1000 year event. Potentially the site could be at risk of flooding from Nant Bran. With this in mind, the client recognises the need to undertake a Flood Consequence Assessment to evaluate the risk and implication of any flooding on the site. The assessment should be in accordance with the requirements of Section 7 and Appendix 1 of TAN 15. Francis Sant have been employed to undertake this task based on information currently available (ie. no additional modelling). R/WWG/D/1748 1 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report Location of Proposed Development Figure 1.2 - NRW Flood Map Section 2 of this report describes the location of the site while the method and risk of flooding from various sources are considered in Section 3. Potential flood proofing measures are discussed in Section 4 and the conclusions of the study, along with the main recommendations, are provided in Section 5. R/WWG/D/1748 2 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report 2 THE DEVELOPMENT Location The proposed site (coordinates E269150 N198720) is located off Walters Road, Llansamlet, Swansea and is situated on a strip of land between the M4 motorway and a branch railway line (see Figure 2.1, 2.2 and Plate 2.1). Currently, access to the site is gained along a small lane that also services 3 existing houses. At the junction with Walters Road the lane crosses over Nant Bran via a small concrete slab bridge. The Bran runs parallel to Walters Road at this point. Walters Road Runs beneath both the M4 and the railway line. Site Figure 2.1 – Location Plan R/WWG/D/1748 3 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report Site Figure 2.2 – Plan of Area Proposed Development Plate 2.1 – Aerial View of Site Topography LIDAR data indicates that the existing ground level of the site generally ranges between 13.6m AOD and 22m AOD (see Figure 2.3). R/WWG/D/1748 4 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report 21.2 2221.8 20.6 13.6 14.6 ETL 291 Drain Drain 287 285 WALTERS ROAD WALTERS 283 1 2 3 Fran Felin Figure 2.3 – LIDARBike Racks Data of the Site YNYSALLAN ROAD M 4 14.26m BM Proposals 1200 Wide Cycle Path It is intended to construct 31 houses along with a new access road and bridge (see 1 FB Figure 2.4). Gas Main with 30m wide Easement 23 22 18 24 21 20 25 19 26 27 28 31 30 29 Drain Drain (disused) Railway Drain Figure 2.4 - Proposed Plan of Site R/WWG/D/1748 5 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report 3 FLOOD RISK Flooding can occur from several sources, some of which are considered in this section. A flood map (Figure 3.1), obtained from NRW, shows that most of the site is expected to remain flood free even during the 1 in 1000 fluvial event (Q1000 event or 0.1% event). The entrance into the site, however, is affected even during the Q100 event. The maps are believed to be based on the results of a 2D TUFLOW model. SITE Figure 3.1 – NRW Flood Map of the Area Fluvial The results from the latest NRW 2D TUFLOW model suggests that other than the entrance into the site the development is expected to remain flood free during the considered events (Q100, Q100 + CC and Q1000 see Figures 3.2 to Figure 3.9). Access to the site is gained from Walters Road over the Nant Bran. The access area is at risk of flooding from water flooding directly from the stream and also from water flowing south westwards along Walters Road, having overtopped the channel upstream of the M4. Water that overtops the right hand bank of the Bran, just upstream of the access bridge, is likely to flow overland in a localised trough and re-enter the stream below the bridge. There are two large arches beneath the railway embankment near the site entrance, one to accommodate the road and the other to accommodate the stream. Water flowing along the road from north of the M4 could possibly flow beneath the railway either via the road or possibly it could flow into the Bran upstream of the railway. Some water could also flow towards the four houses off Walters Road immediately upstream of the access bridge. R/WWG/D/1748 6 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report As part of the development it is proposed to rebuild the access bridge. In doing this it is important that the conveyance is not reduced either via the stream or overland. This can be achieved by: • increasing the potential conveyance beneath the bridge (lowering the bed level and widening the span; • allowing water to continue to flow into the Bran from Walters Road downstream of the bridge but upstream of the railway bridge; • allow water to flow over land either beneath or over the access road on the right hand bank of the stream; • Allow water flowing overland on the right-hand bank to connect into the Bran downstream of the proposed bridge. Table 3.1 Velocity and Depth Key Region Depth Region Velocity 0-0.1m 0 - 0.15m/s 0.1m-0.2m 0.15m/s - 0.3m/s 0.2m-0.3m 0.3m/s - 0.45m/s 0.3m-0.6m 0.45m/s - 0.6m/s 0.6m-1m 0.6m/s - 1m/s 1m-2m 1m/s - 2m/s >2m > 2m/s Development Figure 3.2 - Q100 Depth R/WWG/D/1748 7 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report Development Figure 3.3 - Q100 Velocity Development Figure 3.4 - Q100 With Climate Change - Depth (Key on Table 3.1) Development Figure 3.5 - Q100 With Climate Change - Velocity (Key on Table 3.1) R/WWG/D/1748 8 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report Development Figure 3.6 - Q1000 Depth (Key on Table 3.1) Development Figure 3.7 - Q1000 Velocity (Key on Table 3.1) Access Bridge Figure 3.8 - Q1000 Depth At Site Entrance (Key on Table 3.1) R/WWG/D/1748 9 Francis Sant May 2018 Development At Felin Fran, Llansamlet, Swansea Flood Consequence Assessment Report Access Bridge Figure 3.9 - Q1000 Velocity At Site Entrance (Key on Table 3.1) If it becomes necessary to leave the site during an event access along Walters Road is unlikely to be possible due to the expected flood velocity. As an emergency, however, and ambulance could use the hard shoulder of the adjacent M4 to pick up any patience if necessary. Tidal The site is not expected to be at risk from tidal flooding. Ground Water The bedrock geology consists of Mudstone, Siltstone and Sandstone of the Grovesend Formation (see Figure 3.10) overlain by superficial consisting of Diamicton of Devensian Till (see Figure 3.11). As such, with due regard to the terrain, the risk of ground water flooding is assessed to be low.