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.
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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
Figure 2.3 – LIDAR Data of the Site
Proposals It is intended to construct 31 houses along with a new access road and bridge (see Figure 2.4).
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 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.
Site
Figure 3.9 - Bedrock Geology
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Site
Figure 3.11 - Superficial Geology
Surface Water The NRW surface water maps (see Figure 3.12) indicate that some water could flow along the access road towards the Bran near the site entrance. This water could be accommodated either by the highway drain or the new access road itself while ensuring that the finished floor level of the properties is higher than the access road.
Surface water is also expected to flood the area near the access road bridge. The measure proposed to accommodate fluvial flooding will also ensure that third parties are not detrimentally affected by the development while also ensuring that the water can be drained into Nant Bran.
Figure 3.12 - NRW Surface Water Map
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Sustainable Drainage As part of the development is to be constructed on a permeable surface it could result in an increase in the rainfall runoff rate. Therefore, a sustainable drainage system will be required adhering to the requirements of the Welsh Government Standards and the SUDS Manual.
Sewage Backflow. The proposed development will require a new connection into the main sewer system. Care should be taken to ensure that ground water can't enter and that the sewage is able to drain away. Considering the fall to the main sewer on Walters Road there is unlikely to be a backflow into the development during flood conditions.
Risk From Reservoir Failure The site is not considered to be at risk from reservoir failure.
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4 FLOOD PROOFING MEASURES
No special flood proofing measures are required for the properties. However, residents should be made aware of the flood risk at the entrance into the site.
It would also be prudent for the properties to registered on the flood alert scheme provided by NRW.
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5 CONCLUSIONS AND RECOMMENDATION
Conclusions The client wishes to build 31 new houses on a site at Felin Fran, Llansamlet Swansea. However, the entrance into the site lies within a Zone C2 flood area, in accordance with the development advice maps, and is considered to be at risk of flooding during at least the 1 in 1000 year event with no recognised flood defences. Francis Sant were, therefore, requested to evaluate the consequence of flooding.
NRW's 2D hydraulic model suggest that the proposed houses are expected to remain flood free during the Q100, Q100 + CC and the Q1000 events, however, the entrance into the site will be affected. Care should be taken, therefore, in the design of the proposed new access bridge and access road to ensure that the conveyance beneath the bridge and across the access road is nor reduced. Water should also be allowed to flow into the Nant Bran, downstream of the new bridge and upstream of the railway bridge, from both the access road and Walters Road.
The site is not at risk from ground water flooding, tidal flooding, or reservoir failure and the measures proposed to account for the fluvial flooding at the entrance will also be sufficient to account for surface water flooding.
A summary of the adherence of the development with the requirements of Section 7 and Appendix A1 of TAN 15 is summarised in Table 5.1.
Table 5.1 - Adherence With Section 7 and Appendix A1 of TAN 15 Requirement Comment 1 Is the development flood free during a Q100 Yes. event 2 Is the flood depths at the development less than Yes. Flood free 600mm during a Q1000 event 3 Is the flood velocity at the development less than Yes. Flood free 0.3m/s during a Q1000 event 4 Is the flood depths on the access route less than Potential Emergency Access Route Available 600mm during a T1000 event although depth near the access bridge is likely to exceed 600mm. 5 Is the flood velocity on the access route less Potential Emergency Access Route Available than 0.3m/s during a Q1000 event although velocity along Walters Road is greater than 0.3m/s 6 Is the maximum rate of rise of flood waters less Houses expected to be flood free than 0.3m/hr 7 Is the maximum speed of inundation of flood risk No. Houses expected to be flood free area less than 2hrs 8 Will the development have an impact on third No With careful design of the access bridge parties and road. 9 Will there be a loss of flood storage capacity No
The need for a sustainable drainage system, to mitigate against any increase in rainfall runoff, will need to be considered. If required, the system should be designed in accordance with the Welsh Government Standards and the SUDS manual.
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Recommendations In developing the site it is recommended that: • the new bridge and access road is carefully designed to ensure that water conveyance is not reduced and that the risk of flooding to the properties upstream of the bridge is not increased; • residents are made aware of the risk of flooding at the site entrance; • the property is registered on the flood warning scheme provided by NRW; • the need for a sustainable drainage system is assessed.
R/WWG/D/1748 15 Francis Sant May 2018