Taff Vale Redevelopment

Transcription

1 Taff Vale Redevelopment Taff Street, Pontypridd CF37 4TD Final report for On behalf of Rhondda Cynon Taff County Borough Council May 2017 Hydrock Reference: C C

2 Issued by: Project: Title: Client: Status: Hydrock Consultants Limited Over Court Barns Over Lane Almondsbury Bristol BS32 4DF Tel: Fax: Taff Vale Redevelopment Darnton B3 Final DOCUMENT CONTROL SHEET Date: May 2017 Document Production Record Issue Number: P1.2 Name Signature Prepared Iain Hissett BSc, MSc Flood Risk Consultant Checked Jon Cracknell BSc, MSc Senior Flood Risk Consultant Approved David Lloyd BSc, PhD Technical Director Flood Risk Document Revision Record Issue number Date Revision Details P th May 2017 Draft report P rd May 2017 Final Report taking on client review comments Hydrock Consultants Limited has prepared this report in accordance with the instructions of the above named client for their sole and specific use. Any third parties who may use the information contained herein do so at their own risk.

3 CONTENTS 1.0 INTRODUCTION SITE INFORMATION Location Existing Land Use General Topography Proposed Development SOURCES OF FLOOD RISK Fluvial Flooding Tidal Flooding Surface Water Flooding Groundwater Flooding Sewer Flooding Infrastructure Failure Flooding HISTORIC FLOODING TAN15 AND THE PROPOSED DEVELOPMENT Planning Policy Requirements Flood Resistance Natural Resources Wales Flood Warning Service Flood Emergency Plan CLIMATE CHANGE Peak Rainfall Intensity Peak River Flow DRAINAGE ENVIRONMENTAL PERMIT FOR FLOOD RISK ACTIVITIES SUMMARY REFERENCES APPENDIX A SITE LOCATION APPENDIX B TOPOGRAPHICAL SURVEY APPENDIX C PROPOSED LAYOUT Hydrock Consultants 3

4 FIGURES Figure 1 Site Location... 6 Figure 2 NRW Fluvial and Tidal Flood Map (Approximate Site Location in Red Outline)... 9 Figure 3 View Downstream from Right/East Bank, North of Site Figure 4 NRW Surface Water Flooding (Approximate Site Location in Red Outline) Figure 5 NRW Reservoir Flooding (Approximate Site Location in Red Outline) Figure 6 Development Advice Map (Approximate Site Location in Red Outline) TABLES Table 1 Site Reference Information... 6 Table 2 Summary of Potential Sources of Flood Risk... 8 Table 3 Peak Flood Level at Model Node TWWBCS Table 4 NRW Flood Zones Hydrock Consultants 4

5 1.0 INTRODUCTION This (FCA) report has been prepared by Hydrock Consultants Limited (Hydrock) on behalf of Darnton B3 in support of a Planning Application to be submitted to Rhondda Cynon Taff County Borough Council for a proposed development on land formally occupied by the Taff Vale shopping precinct site (now demolished), on Taff Street, Pontypridd. The report aims to address the requirements of Technical Advice Note 15: Development and Flood Risk (TAN15) and Natural Resources Wales (NRW) policy, through justifying the location of the proposed development and assessing the identified flooding consequences. This report considers these requirements through: Assessing whether the proposed development is likely to be affected by flooding. Assessing whether the proposed development is appropriate in the suggested location. Detailing any measures necessary to mitigate any flood risk identified, to ensure that the proposed development and occupants would be safe and that flood risk would not be increased elsewhere. Hydrock Consultants 5

6 2.0 SITE INFORMATION 2.1 Location The site is located between Taff Street to the west, the Maues Parcio Gas Road Car Park to the south, the right/west bank of the River Taff to the east, and the historic footbridge and A4223 (Bridge Street) road bridge immediately upstream to the northeast. The site location is shown in Figure 1 and in Appendix A, with full address and Ordnance Survey Grid Reference provided in Table 1. Table 1 Site Reference Information Site Address Taff Street, Pontypridd, CF37 4TR Grid Reference , ST Figure 1 Site Location Site Location Hydrock Consultants 6

7 2.2 Existing Land Use The site was previously occupied by the Taff Vale Shopping Precinct. The site has now been cleared for future development and all that remains are the concrete slabs and other areas of hardstanding. The original site layout is shown on the Topographical Surveys in Appendix B. 2.3 General Topography The site levels generally fall from approximately 60m AOD in the northwest and 59m AOD in the southwest to approximately 53.5m AOD in the northeast and 54.5m AOD in the southeast of the site before a steep bank down to the right/west bank of the River Taff. The Topographical survey of the site is included in Appendix B. 2.4 Proposed Development The proposed scheme comprises the redevelopment of the site to provide three new office and commercial use blocks, a basement carpark, highway improvements to the Crossbrook Street and new, street level, public open space. Hydrock Consultants 7

8 3.0 SOURCES OF FLOOD RISK The following section assesses the potential flood risk to the site from various sources. A summary of conclusions from this assessment are given in Table 2. Table 2 Summary of Potential Sources of Flood Risk Potential Source of Flooding Fluvial Tidal Surface Water Groundwater Sewer Infrastructure Failure Overall Risk Ranging from Low to potentially High Negligible Low Medium Low Low 3.1 Fluvial Flooding The site lies with the right bank of the River Taff along its eastern boundary which flows in a southerly direction to its confluence with the Rhondda River approximately 400m south of the site before flowing onwards to Cardiff and out into the Severn Estuary. NRW Flood Mapping (Figure 2) shows eastern portions of the site to be within Flood Zone 3 (high risk) and Flood Zone 2 (medium risk), while the west of the site is within Flood Zone 1 (low risk). The Flood Zones are defined as follows: Flood Zone 1 (no colour) comprises land assessed as having a 0.1% Annual Exceedance Probability (AEP) of fluvial or tidal flooding in any given year, equivalent to the 1 in 1,000 year return period event. Flood Zone 2 (light blue) comprises land assessed as having a 0.1-1% AEP of fluvial or % AEP of tidal flooding in any given year, equivalent to the 1 in 1, or 1 in 1, year return period event respectively. Flood Zone 3 (dark blue) comprises land assessed as having a 1% AEP of fluvial or 0.5% AEP of tidal flooding in any given year, equivalent to the 1 in 100 or 200 year return period event respectively. The Welsh Government Development Advice Mapping in Section 5.0 and Table 4 is based on this fluvial flood risk. Hydrock Consultants 8

9 Figure 2 NRW Fluvial and Tidal Flood Map (Approximate Site Location in Red Outline) Source: NRW The mapping is understood to be based on the River Taff Flood Risk Mapping Update Study in November 2007, which is a one dimensional (1D) channel only ISIS model. Modelled flood levels taken from the Study adjacent to the site are summarised in Table 3 below. Table 3 Peak Flood Level at Model Node TWWBCS24 Return Period (AEP) Peak Flood Level (m AOD) 1,000yr (0.1%) yr (1%) + 20% CC yr (1%) Source: NRW Pontypridd Flood Alleviation Scheme The Pontypridd Flood Alleviation Scheme (FAS) undertaken in 1992/3 constructed flood defences at the site following extreme historic flooding (see Section 4.0), and this significantly reduces the fluvial flood risk posed to the site. The scheme included a flood defence wall, substation defences, a formal earth embankment and formal wall defences that provide protection from the River Taff to m AOD. This therefore provides protection to the site in excess of the 0.1% (1,000yr) AEP level, effectively re-classifying the site as Flood Zone 1 (low risk). Hydrock Consultants 9

10 This work was carried out in conjunction with improvements to the sewer and highways drainage (CC, 2009, Section Flood Defences, p11) Hydrological Review and Pontypridd River Taff Bridges At the time of writing, Hydrock has obtained the ISIS 1D hydraulic model of the River Taff from NRW, and are in the process of linking this to a two dimensional (2D) TUFLOW model, the results of which, when available, will be provided as an update to this report. This will include the following: Revised 1% (100yr) AEP and 0.1% (1,000yr) AEP design maximum events (based on updated hydrometric datasets and hydrological modelling techniques). Revised climate change allowances for the 1% (100yr) AEP event. Flood hazard mapping results (based on maximum flood depths and velocities). Two bridges cross the River Taff in the centre of Pontypridd immediately upstream of the site (Figure 3): An upstream clear span Old Bridge that was constructed in 1756 and includes three cylindrical voids on each side to reduce the weight and pressure on the crown after several previous bridges were either washed away or collapsed due to vehicle vibrations. This bridge is still in operation today, though only for foot traffic. The steepness of the old bridge meant that it was difficult for horse carts to cross, and subsequently a downstream triple arch Victoria Bridge was constructed in Bridge Street crosses this bridge. Flood levels in the River Taff adjacent to the site during high flow events are likely to be controlled, to an extent, by the two stanchions and lower soffit of the downstream Victoria Bridge. This bridge (and not the Old Bridge which is unlikely to have the same impact) is included in the hydraulic model. Any major blockage of the arches to the Victoria Bridge is likely to reduce predicted design flood levels at the site (if at all), and Hydrock are in the process on confirming with NRW that blockage analysis is therefore not necessary in this situation. Hydrock Consultants 10

11 Figure 3 View Downstream from Right/East Bank, North of Site By Chris Andrews, CC BY-SA 2.0, Tidal Flooding The site is significantly elevated above sea level (>50m AOD) and due to its geographical location is at negligible risk of flooding from this source. 3.3 Surface Water Flooding Surface water flooding occurs as the result of an inability of intense / prolonged rainfall to infiltrate to ground. This often happens when the maximum soil infiltration rate or storage capacity is exceeded, or in urban areas as a result of man-made impermeable surfaces. Such flows either drain into existing land drainage features or follow the general topography which can concentrate flows and lead to localised ponding / flooding. A simplified rainfall run-off model has been used to determine the surface water flood risk in the NRW mapping (Figure 4). This modelling does not fully take into account urban drainage networks nor does it differentiate the risk from watercourses (which is considered a fluvial risk and addressed in Section 3.1). The Surface Water Map shows the majority of the site is at a very low risk, with only a small area in the southeastern corner of the site predicted to be at a low to medium risk from this source. However, this area appears to be hydraulically connected to the River Taff, and so is considered a fluvial flood risk issue. Consequently, the whole site is considered to be at a very low risk of surface water flooding. Hydrock Consultants 11

12 Figure 4 NRW Surface Water Flooding (Approximate Site Location in Red Outline) Source: NRW 3.4 Groundwater Flooding British Geological Survey mapping shows the site to be underlain by Alluvium superficial deposits comprising Clay, Silt, Sand and Gravel, which overlie the Brithdir Member, comprising Sandstone. The site is not at specific risk of groundwater flooding from higher ground, and any rainfall in the surrounding area is likely to be intercepted by urban drainage networks rather than infiltrating to ground. However, given that the east of the site is categorised as Flood Zone 3, groundwater levels in this area could potentially be comparable to channel water levels and as such these areas may experience a high water table (i.e. close to the surface) even if the site is not affected by fluvial flooding. Measures to address this risk are included in Section 5.2. There is consequently considered to be a potentially medium risk of groundwater flooding to the site. 3.5 Sewer Flooding There is no specific mention of sewer flooding at the site or in Pontypridd in the Strategic Flood Consequence Assessment (SFCA) (SW, 2008). Furthermore, with relatively recent improvements to the local sewers and highways drains (see Section 3.1.1), the site is concluded to be at a low risk of flooding from this source. Hydrock Consultants 12

13 3.6 Infrastructure Failure Flooding The site is shown by NRW s Risk of Flooding from Reservoirs mapping (Figure 5) to be within an area at risk from this source. This is however understood to be a worst case scenario, and it is unlikely that any actual flooding would be this extensive. Furthermore, the upstream reservoirs that potentially could give rise to this risk (the Beacons, Cantref, Neuadd, Pentwyn and Pontsticill Reservoirs) are subject to a rigorous inspection and maintenance regime, and as such this risk is considered as residual and acceptable. There is no known risk of flooding from any other artificial source and so the site is concluded to be at low risk of flooding from infrastructure failure. Figure 5 NRW Reservoir Flooding (Approximate Site Location in Red Outline) Source: NRW Hydrock Consultants 13

14 4.0 HISTORIC FLOODING A desk based search for flooding in Pontypridd reveals a substantial record of historic events, dominated by fluvial flooding from the River Taff: Flooding in 1748 and one shortly after is understood to have washed away two bridges (Skempton and Chrimes, 2002) (PCW, 2014). December 1979, understood to have been approximately a 0.1% (1,000yr) AEP event (CC, 2009) (CC, 2009). December 2012, including a large earth embankment collapse (BBC, 2012). January 2013 (ITV, 2013). Hydrock Consultants 14

15 5.0 TAN15 AND THE PROPOSED DEVELOPMENT 5.1 Planning Policy Requirements The site is demonstrated in Section 3.0 to be potentially at high risk of fluvial flooding from the River Taff, and at medium risk of groundwater flooding, but at a low risk of flooding from all other sources. The Welsh Government Development Advice Map (Figure 6) shows the site to be within Flood Zone B and Flood Zone C1. These flood Zones are defined in Table 4. The proposed development is considered less vulnerable with respect to flood risk (Welsh Government, 2004, Figure 2, p7). There is consequently a need for the proposed scheme to meet the requirements of the Justification Test as advocated within TAN15 (WG, 2004, Section 6 Justifying the location of the development, p8). In this instance, the proposed scheme is considered to meet the requirements of the Justification Test, through: The Rhondda Cynon Taff County Borough Council state that the Local Development Plan supports the regeneration of Pontyridd by providing a policy context that encourages new residential and commercial development within the town centre (RCTCBC, 2011, p98). The site comprises previously developed land. The consequences of potential flooding are also considered acceptable, subject to the application of flood resistance measures to mitigate residual risk, registration of NRW s Flood Warning Service, and, application of Flood Emergency Plan measures discussed further in Sections 5.2, 5.3 and 5.4 respectively. Hydrock Consultants 15

16 Figure 6 Development Advice Map (Approximate Site Location in Red Outline) Source: Welsh Government Table 4 NRW Flood Zones Description Considered to be at little or no risk of fluvial or tidal/coastal flooding Areas known to have been flooded in the past evidenced by sedimentary deposits. Based on Environment Agency extreme flood outline, equal to or greater than 0.1% (river, tidal or coastal) Areas of the floodplain which are developed and served by significant infrastructure, including flood defences. Areas of the floodplain without significant flood defence infrastructure. Source: TAN 15 (WA, 2004, p5) Zone A B C C1 C2 Used to indicate that justification test is not applicable and no need to consider flood risk further Used as part of a precautionary approach to indicate where site levels should be checked against the extreme (0.1%) flood level. If site levels are greater than the flood levels used to define adjacent extreme flood outline there is no need to consider flood risk further Used to indicate that flooding issues should be considered as an integral part of decision making by the application of the justification test including assessment of consequences. Used to indicate that development can take place subject to application of justification test, including acceptability of consequences. Used to indicate that only less vulnerable development should be considered subject to application of justification test, including acceptability of consequences. Emergency services and highly vulnerable development should not be considered. Hydrock Consultants 16

17 5.2 Flood Resistance As explained in Section 3.4, during periods of prolonged rainfall and high flows along the River Taff, groundwater levels in the area are likely to be comparable to those within the adjacent channel. Noting the 0.1% (1,000yr) AEP peak flood level of 56.44m AOD it is recommended that the basement level include tanking walls with an impermeable membrane to discourage the residual risk of any groundwater ingress. The following further flood resistance measures are also recommended at this basement level: Services be brought in at a high level, and sockets/sensitive equipment etc. set at the highest levels practicable. An emergency sump pump(s) installed at the lowest point(s) within the basement. A threshold entrance level for vehicular access to the basement be set no lower than the 0.1% design flood level of 56.44mAOD. A passive flood door be fitted to the sub-station in the southeastern corner of the basement, so that this room can be sealed if required. All ground floor building finished floor levels should, where practicable, be elevated a minimum of 150mm above immediately surrounding ground levels to ensure any design exceedance flows, should they occur, are directed away from any buildings (in line with best practice). 5.3 Natural Resources Wales Flood Warning Service NRW operates a free Flood Warning Service that can be accessed and registered for by phone, text or alerts online at: Current flood warnings can be viewed directly at: NRW also operates a hydrometric river level gauging station at the Old Bridge (upstream of the site and confluence with the Rhondda) which can be viewed at: Flood Emergency Plan Given that the site is within a wider area that may experience flooding it is recommended that occupiers register with NRW s Flood Warning Service, and consider evacuating the basement and ground floor upon receipt of a flood warning, which are issued between half an hour and one day in advance of potential flooding. Even if there is no immediate risk to the site, local news and weather reports should be monitored to check for disruption to transport infrastructure in the local area. Hydrock Consultants 17

18 Further details for emergency planning at the site can be found at: g.aspx. Hydrock Consultants 18

19 6.0 CLIMATE CHANGE The impacts of climate change on peak rainfall determination have been considered in the Surface Water Drainage Strategy and will be considered in the proposed River Taff hydrological review (which will be included in the proposed update to this FCA report discussed in Section 3.1.1). 6.1 Peak Rainfall Intensity Contingency for the impact of climate change on peak rainfall intensity has been taken as 30% in accordance with TAN15 (WG, 2004, Section 2.5, p2) and this is considered in a separate report by Hydrock (Hydrock, 2017b). 6.2 Peak River Flow Consideration of the impact of climate change on the River Taff 1% (100yr) AEP peak flow will be included by applying a 25% increase to flow (based on the River Severn catchment) in accordance with NRW s Technical Guidance: Flood Estimation (NRW, 2016b, p3). Hydrock Consultants 19

20 7.0 DRAINAGE A surface and foul water drainage strategy is included in a separate report by Hydrock (Hydrock, 2017b). It is recommended that this document be reviewed in conjunction with this report. Hydrock Consultants 20

21 8.0 ENVIRONMENTAL PERMIT FOR FLOOD RISK ACTIVITIES The River Taff through Pontypridd is designated Main River and development extends to within 8m of the right bank. As such a NRW Environmental Permit for Flood Risk Activities (FRAP) is likely to be required for any works proposed within this easement. The submission of this Permit is outside the scope of this FCA and more information on this can be found at: Hydrock Consultants 21

22 9.0 SUMMARY This (FCA) report has been prepared by Hydrock Consultants Limited (Hydrock) on behalf of Darnton B3 in support of a Planning Application to be submitted to Rhondda Cynon Taff County Borough Council for a proposed development on the site of the former Taff Vale Shopping Precinct, Pontypridd. The report findings can be summarised as follows: The site was previously the Taff Vale Shopping Precinct, but has now been cleared for future development. All that remains are the concrete slabs and other areas of hardstanding. The proposed development consists of the redevelopment of the site to provide three new office and commercial use blocks, a basement carpark, highway improvements to the Crossbrook Street and new, street level, public open space. Whilst the majority of the site is within Flood Zone 1 (low risk) of fluvial flooding, there are areas within Flood Zone 2 (medium risk) and Flood Zone 3 (high risk) of the River Taff, which poses the principle flood risk to the site. There is a medium risk of groundwater flooding, but a low risk from all other sources. Flood defences constructed prior to and as part of a 1992/3 Pontypridd Flood Alleviation Scheme provide protection from the River Taff to a level of m AOD, above the 0.1% (1,000yr) AEP, effectively re-classifying the site as Flood Zone 1 (low risk). Modelling of the River Taff to take account of revised climate change allowances introduced in December 2016 is in progress and an update to this report will include a hydrological review and revised hydraulic modelling to address this change in policy. There is a long record of flooding in Pontypridd, the latest high flow event understood to be in January Welsh Government Development Advice Maps show the site to be within Flood Zone B (areas known to have flooded in the past) and Flood Zone C1 (high risk but benefitting from defence infrastructure). The proposals are considered to meet the Justification Test, and the consequences of potential flooding are also considered acceptable. Notwithstanding, it is recommended that the basement includes flood resistance measures to mitigate residual risk of defence failure. Given that the site is within a wider area that may experience flooding during more extreme events, it is recommended that future users register with the NRW s Flood Warning Service. Climate Change, in accordance with Welsh Government and NRW guidelines, is considered in the separate Drainage Strategy and is currently being modelling for inclusion, when available, within a proposed FCA update. A Surface and Foul Water Drainage Strategy has been prepared for the site and this is detailed in a separate report by Hydrock. Hydrock Consultants 22

23 Any works within 8m of the top of bank will likely require a NRW Environmental Permit for Flood Risk Activities (FRAP). It is concluded that the Application meets the requirements of TAN15 and Welsh Government planning policy. Hydrock Consultants Limited Hydrock Consultants 23

24 REFERENCES Author Date Description Atkins Nov 2007 River Taff Flood Risk Mapping Update Addendum Report for Environment Agency Wales BBC Dec 2012 Flooding: Residents in Pontypridd escape wall collapse (available at ) Cascade Consulting May 2009 Riverside Shopping Centre, Pontypridd Flood Consequence Assessment Final Report Taff Vale Ltd. Centre for Ecology and Hydrology (CEH) 2009 The Flood Estimation Handbook CD-ROM 3. Centre for Ecology & Hydrology, Wallingford, Oxon, UK. Hydrock Consultants Ltd. May 2017a Flood Risk Review (Ref HYD-XX-XX-CO-G-6000 P1) Hydrock Consultants Ltd. May 2017b Surface and Foul Water Drainage Strategy (Ref. TVR-HYD- 00-XX-RP-S-000-Taff Vale Redevelopment-Drainage Strategy) ITV Jan 2013 Flash floods turned Pontypridd streets into rivers (available at 27/flash-floods-turned-pontypridd-streets-into-rivers/) Natural Resources Wales (NRW) Natural Resources Wales (NRW) People s Collection Wales (PCW) May 2016a Aug 2016b Uploaded Nov 2014 Good Practice Guide, Technical Guidance: Flood Estimation, GPG 102 Hydrology and Water Resources Management v1.0 s: Climate Change Allowances Flooding Pontypridd (available online at Pontypridd Town Council Unknown History of Pontypridd Rhondda Cynon Taff County Borough Council (RCTCBC) Rhondda Cynon Taff County Borough Council (RCTCBC) Mar 2011 Feb 2012 Adopted Local Development Plan (available online at ingcontrol/localdevelopmentplans/localdevelopmentpl ans.aspx) The History of the Old Bridge Hydrock Consultants 24

25 Rhondda Cynon Taff County Borough Council (RCTCBC) Jan 2013 Local Flood Risk Management Strategy Rhondda Cynon Taff County Borough Council (RCTCBC) Nov 2015 Flood Risk Management Plan, Volume 1 Ruddock, Ted 2008 Arch Bridges and their Builders , Chapter 5: "The Theory of Arches and Pontypridd Scott Wilson (SW) Oct 2008 Strategic for Rhondda Cynon Taff Skempton, A. W.; Mike Chrimes 2002 A Biographical Dictionary of Civil Engineers in Great Britain and Ireland: 1500 to 1830, p Welsh Government (WG) Jul 2004 Technical Advice Note 15: Development and Flood Risk (TAN15) Welsh Government (WG) Jan 2016 Planning Policy Wales Edition 8 Hydrock Consultants 25

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