Education Funding Agency
Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
Issue | 9 September 2014
This report takes into account the particular instructions and requirements of our client. It is not intended for and should not be relied upon by any third party and no responsibility is undertaken to any third party.
Job number 232207-70
Ove Arup & Partners Ltd The Arup Campus Blythe Gate Blythe Valley Park Solihull B90 8AE United Kingdom www.arup.com Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
Contents
Page
1 Introduction 1 1.1 Proposed Development 1 1.2 Local Stakeholders and Operating Authorities 2 1.3 Data Sources 3
2 Flood Risk Planning Context 4 2.1 National Planning Policy Framework 4 2.2 Flood and Water Management Act 2010. 6
3 The Sequential and Exception Tests 7 3.1 The Sequential Test 7 3.2 The Exception Test 7
4 Existing Conditions 8 4.1 Site Description 8 4.2 Site Topography 8 4.3 Existing Surface Water Features 9 4.4 Existing Sewage Infrastructure 9 4.5 Existing Hydrogeology 10
5 Operating Authorities 12 5.1 Staffordshire County Council (SCC) 12 5.2 Environment Agency (EA) 13 5.3 United Utilities (UU) 13 5.4 Severn Trent Water (STW) 14
6 Flood Risk Assessment 15 6.1 Fluvial Flood Risk 15 6.2 Groundwater Flood Risk 15 6.3 Overland Flow/ Pluvial Flood Risk 16 6.4 Infrastructure Failure 18 6.5 Artificial Drainage System 19
7 Conclusions 22
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
Appendices
Appendix A Consultation
Appendix B Drawings
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
1 Introduction
As part of the Priority School Building Programme (PSBP), the Education Funding Agency (EFA) proposes to rebuild the Clough Hall Technology College in Stoke-on-Trent. Arup have been commissioned by EFA to undertake a Flood Risk Assessment (FRA), to support a planning application for the proposed redevelopment site of the Clough Hall Technology College. The proposed development site is located off Fourth Avenue in Stoke-on-Trent, Staffordshire. The report has been prepared with reference to the National Planning Policy Framework (NPPF)1, the NPPF Technical Guidance2 and follows the methodology prescribed in CIRIA document C624: Development and Flood Risk, Guidance for the Construction Industry3. This report was prepared by Arup on behalf of EFA in connection with the planning application for the proposed development site in Staffordshire. It takes into account our client's particular instructions and requirements and addresses their priorities at the time. This report was not intended for and should not be relied on by any third party not involved in the planning application.
1.1 Proposed Development It is proposed to redevelop the site to provide a modern two storey building within the existing building footprint. The new school building will be constructed in phases to coincide with the demolition of the existing building. The new building footprint will be approximately 2,430m². The remaining site is proposed to be redeveloped as playing fields and green landscaping, interspersed with hardstand (car parks, foot paths, etc.). There will be a slight reduction in impermeable area extents, from 2.95 Ha to 2.40 Ha. The car park will be located at the north of the site and vehicular access to the site will be retained in its current position, from Fifth Avenue. It is proposed that there will be additional pedestrian accesses at the north, east and south of the site. In terms of onsite drainage, only the area in the vicinity of the new school building, at the southeast of the site, is affected by the development. A preliminary design for this is shown in Appendix B. The existing and proposed layouts of the site are shown in Figure 1 and Figure 2.
1 Department for Communities and Local Government (DCLG) (2012). National Planning Policy Framework (NPPF) 2 DCLG (2012). NPPF Technical Guidance 3 CIRIA (2004). C624 – Development and Flood Risk
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
Figure 1: Existing layout of the site
Figure 2: Proposed layout of the site
1.2 Local Stakeholders and Operating Authorities With regards to development planning, residual flood risk and water related issues; there are a number of key local stakeholders and/or approving authorities associated with the development of the proposed site. These are described as follows:
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
• The Environment Agency (EA) - They have wide ranging powers for main rivers and groundwater bodies under the Water Resources Act (1991)4 and the Environment Act (1995)5. Under the Flood and Water Management Act (FWMA) (2010)6 they have a responsibility to produce a national strategy towards managing flood risk and are a statutory planning consultee for development and flood risk issues; • Staffordshire County Council (SCC) - The Council are the Lead Local Flood Authority (LLFA) on local flood risk matters under the FWMA (2010) and as such have the primary responsibility for local flood risk matters. In addition, when the FWMA is fully enacted the National Standards for Sustainable Drainage will be released and the Sustainable Drainage Approval Bodies (SABs) will be formed. This will allow SCC to assess and approve sustainable drainage design associated with proposed development and take over the adoption of approved sustainable drainage schemes; and, • Severn Trent Water (STW) - The sewerage undertaker and primary supplier of potable water with powers under The Water Industry Act 19917. • United Utilities (UU) – The sewerage undertaker with powers under The Water Industry Act 19918.
1.3 Data Sources The key data used in compiling this FRA is listed below; • EA indicative flood mapping as indicated on their website9; • EA groundwater mapping also available on their website; • Halcrow (July 2008) ‘Newcastle-under-Lyme Borough Council Level 1 Strategic Flood Risk Assessment’ • Consultation with EA; • Consultation with SCC; • STW and UU asset management records; • Consultation with STW and UU; • Greenhatch Group underground utilities surveys; • Mott MacDonald (September 2013) ‘Geoenvironmental Desk Study’; and • Mott MacDonald (October 2013) ‘Flood Risk and Drainage Issues - Broad Overview’.
4 Her Majesty’s Stationary Office (HMSO) (1991). Water Resources Act 5 HMSO (1995). Environment Act 6 HMSO (2010). Flood and Water Management Act 7 HMSO (1991). Water Industry Act 8 HMSO (1991). Water Industry Act 9 www.environment-agency.gov.uk
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
2 Flood Risk Planning Context
2.1 National Planning Policy Framework The Department for Communities and Local Government published the NPPF on the 27th March 2012. This is a wholesale reform of the planning system and replaces all existing Planning Policy Guidance (PPG’s) and Statements (PPS’s) in an effort to make the planning system less complex and more accessible. It transfers more responsibility onto individual planning authorities and states that there should normally be a practice in favour of ‘sustainable development’. The aim of NPPF is to ensure that flood risk is taken into account at all stages in the planning process, to avoid inappropriate development in areas at risk of flooding and to direct development away from areas at highest risk. It does this by formulating a risk-based approach towards flooding to be adopted at all levels of planning. NPPF requires that the “Sequential Test” is applied during the planning process. The Sequential Test aims to ensure that preference for developable land is given to land that has the lowest risk of flooding, based on the data available. The starting point for the ‘Sequential Test’ is the system of ‘Flood Zoning’. This is a system that assesses the risk posed by rivers and in coastal areas estuaries and the sea. This information is collected and made available by the EA and the LPA. The Flood Zoning system adopted in England is described in Table 1 below, as defined in NPPF technical guidance. Table 1: Flood Zoning System used across England as defined in NPPF Flood Zone Definition Zone 1 This zone comprises land assessed as having a less than 1 in 1000 annual Low Probability probability of river or sea flooding in any year (<0.1% Annual Exceedance Probability (AEP)).
Zone 2 This zone comprises land assessed as having between a 1 in 100 and 1 in Medium 1000 annual probability of river flooding (1% - 0.1% AEP) or between a 1 in Probability 200 and 1 in 1000 annual probability of sea flooding (0.5% - 0.1% AEP) in any year.
Zone 3a This zone comprises land assessed as having a 1 in 100 or greater annual High Probability probability of river flooding (>1% AEP) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5% AEP) in any year.
Zone 3b This zone comprises land where water has to flow or be stored in times of Functional flood. Strategic flood risk assessments (SFRAs) should identify this flood Floodplain zone as land which would flood with an annual probability of 1 in 20 (5% AEP) or greater in any year or is designed to flood in an extreme (0.1% AEP) flood, or at another probability to be agreed between the LPA and the Environment Agency, including water conveyance routes.
The Sequential Test requires that development should only be considered within Flood Zone 2, if there are no appropriate development sites in Flood Zone 1. Development in Flood Zone 3 should only be considered if development is not possible in Flood Zone 2, assuming development in Flood Zone 1 has also been ruled out.
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
The application of the ‘Sequential Test’ relies on Regional Planning Bodies (RPB’s) Regional Flood Risk Appraisals (RFRA’s) which informs the Regional Spatial Strategies (RSS) and the LPA’s Strategic Flood Risk Assessments (SFRA’s) which informs the Local Development Framework (LDF) by identifying areas suitable for development. Therefore the ‘Sequential Test’ has to be undertaken at an early stage of the development process (in some cases before land is purchased) and should involve close consultation with the LPA. NPPF also encourages those involved in development to consider the flood vulnerability of the proposed development to the impact of flooding. The vulnerability of different types of development is listed in Table 2 within NPPF (not shown). This is relevant for considering what type of development is appropriate for a site (based on its Flood Zone) and also how a development site should be laid out if there are different Flood Zones encountered within a site. The compatibility of development in terms of its vulnerability and Flood Zoning is described Table 3 of the NPPF as shown in Table 2 below.
Table 2: Flood Risk Vulnerability & Compatibility Flood Zone Essential Water Highly More Less Infrastructure Compatible Vulnerable Vulnerable Vulnerable Flood Zone 1 Flood Zone 2 Exception Test Flood Zone 3a Exception Test Exception Test Flood Zone 3b Exception Test “Functional Floodplain” Key: Development is appropriate, Development should not be permitted, ‘Exception Test’ will be required.
This illustrates how higher vulnerability land uses should be directed to lower flood risk locations sites and vice versa. Should the sequential approach show it is not possible for the development to be located in Zones of lower flood risk it may be possible to, using the ‘Exception Test’ demonstrate that development is still feasible by the management of flood risk. The Exception Test within NPPF requires the demonstration that: • The development provides wide sustainability benefits that outweigh the flood risk; and • A FRA must be provided. A FRA is also required for any development irrespective of flood zone, where the development is in excess of 1 hectare. This is due to the potential flood risk caused by the increase in surface water discharge. A NPPF compliant FRA should be undertaken to consider the following: • If the development is likely to be affected by flooding;
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
• Examining the risk posed by all potential sources of flooding while also considering the impact of climate change (in most cases the risk should be less than 1% in any given year); • The development will not increase flood risk elsewhere from any potential source, with climate change considered once more; • The development is designed to be safe with flood protection considered where necessary as well as a design that considers emergency access and egress arrangements; • The development process should seek to reduce flood risk; and • Management and funding arrangements to ensure the site can be developed and occupied safely throughout its proposed lifetime are in place.
2.2 Flood and Water Management Act 2010. The FWMA is a direct result of the recommendations made by Sir Michael Pitt, taken from his report on the severe flooding experienced across the country in 2007 and was given Royal Assent in April 2010. It provides for better, more comprehensive management of flood risk for people, homes and businesses, helps safeguard community groups from unaffordable rises in surface water drainage charges and protects water supplies to the consumer.
The Act is being implemented by a series of ministerial orders. Currently orders have been commenced which: • Require the EA to develop a national flood risk and coastal management strategy; • Establish Lead Local Flood Risk and Coastal Management authorities; • Establish Regional Flood Risk and Coastal Management Committees; and • Providing for changes to administration and financing of flood risk and water management.
However, the most significant change facilitated by the FWMA is the establishment of the Sustainable Drainage National Standards and the SABs. The National Standards will prescribe how surface water management design will have to be approached during the course of new development and redevelopment proposals, adopting sustainable methods and practices. The SABs who will sit within the Lead Local Authority will then be tasked with scrutinising proposed surface water management strategies and evaluating them in terms of their technical feasibility and sustainability credentials. The documentation related to commencement of this part of the Act was issued for consultation on December 2011 and ended on the 13th March 2012. The full commencement of this act is still outstanding as are the release of the National Standards. An announcement in April 2013 indicated that the ministerial order for this part of the act would be implemented in April 2014. Latest indications are that enactment will actually take place in April 2015, although a draft of the standards has been released and is now available from DEFRA.
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
3 The Sequential and Exception Tests
3.1 The Sequential Test The Sequential Test, as described in Section 2.1, aims to steer development to the areas of lowest risk.
Figure 3: Flood map for planning (rivers and sea)10
The EA Flood Zone mapping as shown in Figure 3 indicates that the proposed development site is located outside Flood Zone 2 and 3. Land use within the development is classified as ‘More Vulnerable’ as defined in Table 2 of the NPPF Technical Guidance. Table 3 in NPPF Technical Guidance indicates that it is acceptable to place this type of development within Flood Zone 1. However, as the total area of the site is greater than 1 hectare (3.4ha) a FRA is required.
3.2 The Exception Test As the site lies wholly outside Flood Zone 2 and 3, the Exception Test is not required.
10 http://maps.environment- agency.gov.uk/wiyby/wiybyController?lang=_e&topic=floodmap&layer=default&ep=map&layer Groups=default&scale=10&x=383632&y=354210#x=383632&y=354210&lg=1,&scale=11
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
4 Existing Conditions
4.1 Site Description Clough Hall Technology School lies on the south western side of Kidsgrove, about 9km north of Stoke-on-Trent and 4km south east of Alsager. The school grounds are irregular in shape, with existing residential development to the north, west and south. First Avenue lies to the south with Fifth Avenue on the western side. The northern portal of the Harecastle Tunnel on the Trent and Mersey Canal lies on the eastern side. Existing buildings and hardstanding covers much of the school ground with small grassed areas on the western side.
Figure 4: Aerial Imagery of Clough Hall Technology College, Stoke-on-Trent The school buildings are located in three blocks: in the north, east and south of the site. Between the north and south buildings in the centre of the site are an artificial sports pitch and a tennis court. There are two points for vehicular access to the site; one from Fifth Avenue from the west and one from Fourth Avenue at the south.
4.2 Site Topography The topographical survey of the site is available in Appendix B, Drawing No. “19147_OGL_REV1”. Within the proposed development site boundary, the highest point is approximately 140m Above Ordnance Datum (AOD) at the southeast of the site, 5m from the existing building. The lowest point is approximately 131m AOD, found at the northwest of the site, approximately 25m from the existing building. The overall slope of the site is approximately 3% from southeast to northwest.
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
4.3 Existing Surface Water Features The nearest watercourse to the site is the Trent and Mersey Canal, which runs approximately 20m northeast of the school boundary. The canal then enters Harecastle Tunnel which directs the canal to the southeast. The EA’s flood maps show that the school site is not located within Flood Zone 2 or 3. Furthermore, the canal is significantly lower than the proposed school site. The EA have been contacted as part of the routine compilation of data for this FRA and their response indicates that there is an ordinary watercourse, which becomes culverted at Grove Avenue, approximately 100m south of the proposed development site. It is understood that this culvert runs to Millstone Avenue, approximately 300m west of the site. Figure 5 shows the location of surface water features near the site.
Figure 5: Location of surface water features in close proximity to the site
4.4 Existing Sewage Infrastructure The site is bounded by residential streets, which are served by an existing public sewer network. The United Utilities asset record (available in Appendix B) shows there is an existing 150mm diameter combined water sewer running roughly north through the site. This continues north out of the site before diverting northwest along Third Avenue. The UU asset record also indicates the existence of: • A 100mm foul water sewer running adjacent to the eastern boundary of the site; • A 100mm combined water sewer running north within Fifth Avenue, to the west of the proposed development site; and
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
• 100mm and 150mm combined water sewers running with First Avenue to the south of the site. Figure 6 shows the approximate location of the existing infrastructure.
Figure 6: UU asset management record11 An underground utilities survey of the site (October 2013) undertaken by Greenhatch Group is available in Appendix B. This indicates that surface water runoff is collected in an onsite combined drainage network and is likely to be discharged to the public combined sewer running north through the site.
4.5 Existing Hydrogeology The British Geological Survey (BGS) mapping12 indicates proposed development site is underlain by Pennine Middle Coal Measures Formation, comprised of mudstone, siltstone and sandstone. BGS mapping indicates that this is overlain with glaciofluvial superficial deposits comprised of sands and gravels. The BGS hydrogeology mapping13 indicates that the proposed development site is found in a moderately productivity aquifer, while the EA mapping14 indicates that the solid and superficial deposits are Secondary A aquifers. This indicates that the underlying stratum has the potential to contain relatively high volumes of water. However, water in close proximity to this site is not abstracted for public water supply. This data does not provide any indication of groundwater levels.
11 United Utilities, ‘Extract from Map for Public Sewers’ 14/06/2013 12 http://mapapps.bgs.ac.uk/geologyofbritain/home.html 13 http://mapapps.bgs.ac.uk/hydrogeologymap/hydromap.html 14 Error! Hyperlink reference not valid. http://maps.environment- agency.gov.uk/wiyby/wiybyController?value=kidsgrove&lang=_e&ep=map&topic=groundwater &layerGroups=default&scale=9&textonly=off&submit.x=0&submit.y=0#x=383694&y=354157& lg=4,3,&scale=7
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
From available BGS borehole logs, the closest available record (SJ85SW586) approximately 200m northeast of the site was excavated to a depth of 15 feet (approximately 4.5m) without striking water. The SFRA carried out on behalf of Newcastle-Under-Lyme Borough Council indicates that extensive mining has been undertaken in the region and two mines are present under the school site. Mine workings often accumulate water and this is believed to be occurring at the site, although there are no indications of water levels being close the surface. At this stage of proposed development project, no onsite groundwater records are available.
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
5 Operating Authorities
Throughout the pre planning process, consultation and input has been sought from the key stakeholders. A record of this consultation is provided in Appendix A.
5.1 Staffordshire County Council (SCC) Under the FWMA, SCC is the LLFA. This means they are responsible for developing, maintaining and applying a strategy for local flood risk management in their areas and for maintaining a register of flood risk assets. They also have lead responsibility for managing the risk of flooding from surface water, groundwater and ordinary watercourses. In addition, when the final ministerial order for the FWMA act is passed SCC will become the SAB. The SAB will assess all drainage designs submitted as part of development proposals against a set of National Standards for Sustainable Drainage (these are currently being redrafted by DEFRA following consultation of their first draft completed in 2012). The aim of this process is to encourage the use of natural open features integrated into the landscape which then not only enable discharges from the site to be controlled but also encourage natural infiltration processes, can be used to control the quality of the water leaving a site due to the cleansing action of biological processes and also provide an amenity and biodiversity benefit. The SAB approval process will work in parallel to the planning process but will be completely separate. Consequently developments with planning permission will not be able to commence construction until the design of their drainage system meets SAB approval. Once the design is in accordance with National Standards and is approved the SAB will then adopt the system (operate and maintain). Adoption will take place after construction is complete and tests have confirmed that it has been constructed appropriately and is operating successfully. SCC was approached for the following information to help compile this FRA: • Data on historic flooding of the site or adjacent areas; • Information of the groundwater regime including water levels, flow direction, yields and quality; • Verification of the available EA flood risk maps and any available up-to-date flood mapping data; and • Confirmation if there is a model of this watercourse available. SCC responded and advised that the existing Strategic Flood Risk Assessment15 (SFRA) should provide the majority of the information required. They also provide the following information: • Newcastle-under-Lyme Borough Council’s SFRA contains records of flooding incidents in the area with property damage along The Avenue in 2007; and
15 Halcrow (2008), Newcastle-under-Lyme Borough Council, Level 1 Strategic Flood Risk Assessment
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
• The do not hold any information on groundwater further to the Environment Agency’s areas susceptible to groundwater flooding which shows this area to be between 50 and 75%. A full account of correspondence is found in Appendix A.
5.2 Environment Agency (EA) Under the FWMA, the EA assumes responsibility for taking a strategic overview of the management of all sources of flooding and coastal erosion; operational responsibility for managing the risk of flooding from main rivers, reservoirs, estuaries and the sea; and, responsibility as coastal erosion risk management authority. As part of its strategic overview role, the EA has published a National Flood and Coastal Risk Management Strategy for England16. The strategy provides a lot more information designed to ensure that the roles of all those involved in managing risk are clearly defined and understood. The EA have been contacted in order to obtain information that they may hold on record concerning flood risk and water quality for the site and the surrounding area. In addition they have been contacted in order to understand their requirements in terms of flood risk and water management for the proposed site. The EA were also approached for the following information to help compile this FRA: • Data on historic flooding of the site or adjacent areas; • Information of the groundwater regime including water levels, flow direction, yields and quality; • Verification of the available EA flood risk maps and any available up-to-date flood mapping data; and • Confirmation if there is a model of this watercourse available The EA responded on 6 August 2014, indicating the following points: • The proposed development site lies in Flood Zone 1; • The EA do not hold any modelling or historic data in the vicinity of the site; and • There are no EA maintained flood defences in the area and there is no information available for the standard of protection offered by other flood defences or natural channels in this area. A full account of correspondence is found in Appendix A.
5.3 United Utilities (UU) Under the FWMA, Water and Sewerage Companies are responsible for managing the risks of flooding from water and foul or combined sewer systems providing drainage from buildings and yards.
16 Environment Agency/Defra, ‘Understand the risks, empowering communities, building resilience: The national flood and coastal erosion risk management strategy for England’ (2011)
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
United Utilities (UU) is the water company within this area responsible for the public sewerage network and has been contacted obtain records of their assets. Evidence of this consultation is found in Appendix A. The public sewer asset location plans supplied can be found in Appendix B.
5.4 Severn Trent Water (STW) Potable water in the vicinity of the site is supplied by Severn Trent Water (STW). STW has been contacted to obtain records of their assets at the proposed development site. A full account of correspondence is found in Appendix A. Following this, a drawing was received on 28 August 2014 containing their asset records. This is available in Appendix B.
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
6 Flood Risk Assessment
6.1 Fluvial Flood Risk Flooding from rivers, streams and other natural inland watercourses is usually caused by prolonged or intense rainfall generating high rates of surface water runoff throughout the catchment. This overwhelms the capacity of the fluvial system as a flood flow and as a result, spills into available floodplain storage areas. In the first instance the risk posed by fluvial sources has been assessed using the EA’s flood zone mapping. From this it has been have deemed that this site is subject to minimal risk from rivers or seas and is not located in either Flood Zone 2 or 3. The EA have been contacted as part of the routine compilation of data for this FRA and their response indicates that there is an ordinary watercourse, which becomes culverted at Grove Avenue, approximately 100m south of the proposed development site. It is understood that this culvert runs to Millstone Avenue, approximately 300m west of the site. An examination of the overland flow paths on the EA’s Flood Risk from Surface Water (FMfSW)17, indicates that although flood waters may flow along Fifth Avenue, water flowing along this flow path would not enter the school site but would instead continue northwards and accumulate along Third Avenue. Staffordshire County Council have been contacted as part of the compilation of data for this study and have indicated the existence of a linked 1D/2D model of the area, developed as part of an upcoming Surface Water Management Plan (SWMP). Although the school site is considered to be at minimal risk from flooding due to the topography of the area, further examination of this model is required by the project drainage design team at detailed design. The development is unlikely to change the flood risk posed by this development to any other location
6.2 Groundwater Flood Risk Flooding can occur in locations where groundwater is present at shallow depths. Prolonged periods of rainfall can result in elevated groundwater levels that can lead to the groundwater level reaching the surface. This can pose a flood risk to developments, particularly basements and cellars. In addition, the emergence of groundwater will prevent infiltration occurring and so will promote the occurrence of overland flow. In addition, groundwater may leak into existing surface water drainage systems of poor integrity, reducing their ability to accommodate surface water runoff. The data outlined in Section 4.5 indicates that although groundwater may be present and could be found in large volumes, there is no evidence that
17 http://watermaps.environment- agency.gov.uk/wiyby/wiyby.aspx?lang=_e&topic=ufmfsw&layer=0&x=383632&y=354210&scal e=10&location=ST7+1DP#x=383631&y=354209&scale=11
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
groundwater levels are elevated and are an imminent threat to buildings or infrastructure at the surface. Further investigations into groundwater levels are recommended as this information will inform the design of excavation and buried structures.
6.3 Overland Flow/ Pluvial Flood Risk Overland flow is a description for water flowing over the surface, which has yet to enter a natural drainage channel, an artificial drainage system or the natural substrate. It is often a result of very intense short lived rainfall events but can also be produced during mild rainfall events when drainage systems are at capacity or the ground is already saturated. This can result in the inundation of low-lying areas. It is also related to sewer flooding, excessive groundwater and infrastructure failure. The existing overland flow routes around and within the site have been established based on the EA’s Flood Risk from Surface Water (FRfSW) mapping available on their website.
Figure 7: Flood risk from surface water mapping18 Figure 7 indicates that surface water can flow into and accumulate in the site via the southern access road to the site (Fourth Avenue), although depths will not be significant. Further risk of surface water flooding exists to the northwest of the site, along Third Avenue, although the FRfSW mapping indicates that there is no link between this accumulation of surface water and the school site. Fifth Avenue is identified as a flow path for surface water flooding but the FRfSW does not indicate that flood waters from Fifth Avenue can access the school site. The SFRA19 states that surface water flooding has historically been an issue in the Kidsgrove area, with several properties being affected in 2007. It is understood
18 http://watermaps.environment- agency.gov.uk/wiyby/wiyby.aspx?topic=ufmfsw&scale=11&ep=map&layerGroups=default&lang =_e&y=354059&x=383548#x=383548&y=354059&scale=11
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
that the existing school buildings were not affected, as the majority of the flooding occurred over 400m northeast of the site, at Liverpool Road on the opposite side of the canal. The main area identified as being at risk in the SFRA is the junction between Gloucester Road and Newchapel Road, which lies over 1km northeast of the site. Staffordshire County Council has been contacted as part of the compilation of data for this study and they has indicated that the flooding in 2007 also caused some property damage along The Avenue, which lies approximately 200m northeast of the site. There are no overland flow paths linking this area to the proposed development site. SCC also indicates the existence of a linked 1D/2D model of the area, developed as part of an upcoming Surface Water Management Plan (SWMP). Although the school site is considered to be at minimal risk from flooding due to the topography of the area, further examination of this model is required by the project drainage design team at detailed design. Figure 8 indicates that the existing flow of surface waters is from south to north across the site. The proposed layout does not interfere with the existing overland flow routes, but does have the potential for allowing additional surface water to flow across the site and potentially off site into the gardens of the houses off Third Avenue. Consequently special attention has been made in the layout of surface water management infrastructure to ensure that this water is collected and not allowed to flow offsite. This has been done by incorporating a channel drain to intercept this flow path. This is shown in Drawing ‘232207-70-AR-01-XX-DR-C- 00002’ available in Appendix B.
Figure 8: Proposed overland flow paths New surface water infrastructure will be integrated into the design of the development to capture runoff from the new impermeable areas, connecting to the existing surface water network (east of the proposed site).
19 Halcrow (July 2008) Newcastle-under-Lyme Borough Council Strategic Flood Risk Assessment’
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
The risk posed by overland flow to the actual school buildings has been recognised and addressed by raising the proposed building finished floor levels (FFL). The surrounding area will be re-graded to divert flow away from the proposed building where overland flow will then be captured by the proposed surface water network.
6.4 Infrastructure Failure Where significant infrastructure exists that retains, transmits or controls the flow of water, flooding may result if there is a structural, hydraulic, geotechnical, mechanical or operational failure. This may be infrastructure that has been specifically designed and implemented as a water controlling structure, such as a water main or a dam. It may also be a structure such as a road or rail embankment that acts as an informal flood defence during severe storm events. For the proposed development site the following infrastructure has been assessed in terms of flood risk: • Public Water Supply pipelines; • Combined Sewer System pipelines; and, • Other existing water bodies. The existing infrastructure consists of: • A 3 inch (approximately 75mm) potable distribution main, running within Third Avenue to the north of the site; • A 3 inch (approximately 75mm) potable distribution main, running within Fifth Avenue, to the west of the site; • One 300mm and one 150mm potable distribution main running withing First Avenue, to the south of the site; • 3 inch (approximately 75mm) potable distribution main, running into the school site, via Fourth Avenue to the south; • A 150mm public combined gravity sewer, flowing north through the proposed development site; • A 225mm public combined gravity sewer running adjacent to the north boundary, within Third Avenue; • A 150mm public combined gravity sewer, running in alignment with Fifth Avenue to the west of the site; • A 100mm public foul gravity sewer running parallel with the eastern boundary of the site, connecting to the combined sewer to the north of the site; and • A 150mm public combined gravity sewer running within First Avenue to the south of the site. The existing infrastructure to the north and west of the proposed development site, identified above, are all relatively small assets. Consequently the volume of water released in the event of failure is likely to be low. Furthermore, the location of these assets in relation to the site is such that there are no obvious flow paths onto or across the site.
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
The existing potable water main and combined gravity sewer located within First Avenue and Fourth Avenue to the south of the site are also relatively small assets and consequently the volume of water released in the event of failure is likely to be low. The location of these assets is such that water from these sources could potentially flow onto and across the site. However, the overland flow paths identified in Section 6.3 indicate that this water would not be directed to the school building. Finally, failure of the existing combined sewer running north through the site may result in foul effluent inundating the site. However, the inflows to the public foul water network are strictly controlled and new connections are only permitted if it can be demonstrated that the system is able to accommodate them. These assets are then upgraded if demand exceeds capacity. Consequently there are processes and systems which safeguard against foul water flooding from separate foul water systems, under normal operational conditions. The EA mapping shows that the site is not at risk of flooding from reservoirs20 The local topography indicates that the Trent and Mersey Canal and the Harecastle Tunnel are set at an elevation below the proposed development site and so poses minimal flood risk to the proposed development.
6.5 Artificial Drainage System Artificial drainage systems designed to manage surface water runoff can pose a flood risk if the system is overwhelmed. This may occur if the amount of surface water runoff exceeds the system capacity or if the system becomes blocked or surcharged by the receiving watercourse. Artificial drainage systems designed to manage foul water (and combined effluent) can pose a flood risk and public health risk if the system is overwhelmed. This may occur if the amount of foul water allowed to discharge exceeds the system capacity. Existing artificial drainage systems (foul, combined and surface) pose a flood risk to the site in the event of failure. This would result in water (possibly contaminated) exploiting the overland flow routes explored in Section 6.3.
6.5.1 Foul Water Sewerage The flood risk posed by the foul water infrastructure required by the proposed development site will be minimised by designing it in accordance with the relevant guidance and codes and continuing the consultation with STW that has already been undertaken. This will ensure foul water discharges from the site do not overwhelm the capacity of the public foul water sewerage system. There is expected to be a 0.51 l/s increase in the foul flow rate compared with the existing discharges. The existing connection to the public sewerage network is to be retained.
20 http://watermaps.environment- agency.gov.uk/wiyby/wiyby.aspx?lang=_e&topic=reservoir&layer=default&scale=11&x=383548 &y=354059#x=383548&y=354059&scale=11
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
The detailed design of this system has not yet been undertaken and will proceed in conjunction with consultation with United Utilities to ensure flood risk from the public sewerage network is not increased.
6.5.2 Surface Water Management Strategy A surface water management strategy for the proposed development site has been developed based on contemporary sustainable drainage principles. This means that the surface water discharge from the site will mimic the existing conditions for all rainfall events up to and including the 1 in 100yr+CC. The surface water management strategy adopts the following hierarchy: 1. Ground infiltration; 2. Local watercourse; 3. Surface water sewer; and 4. Combined sewer. Due to the proposed use of the development, as a school, ground infiltration will be permitted on sports fields and other pervious areas only. Surface water generated by hardstanding will not be drained to the sports fields for infiltration; it will be captured and attenuated in the proposed surface water attenuation. However, there is little scope for providing open surface water features to act as attenuation, which would allow infiltration to continue, due to the health and safety implications of having areas of open water within a school site with limited space available. The potential for incorporating storage mechanisms that still allow water to infiltrate under the site will be investigated at later stages of design. This will require obtaining infiltration testing. The closest watercourse is the Trent and Mersey Canal. Although there is potential to discharge to the canal, it has been decided that as the impact further downstream is not as well understood, it will be more appropriate to discharge to the existing sewer system. The proposed development site is considered to be a brownfield site, where the existing building will be replaced by a new, modern building for the same purpose and overall hardscape/greenscape ratio will be maintained similar to existing. Based on this, a preliminary surface water management strategy has been determined, taking into account the following points: • The existing outgoing 150mm public sewer capacity has been calculated to be approximately 48 l/s under surcharged conditions. As the proposed drainage design applies to approximately half of the impermeable area within the proposed development site, the maximum outgoing flow from this system will be limited to 24 l/s. The detailed design of this system has not been undertaken at this time and is to be approved by UU prior to development. The existing pipe is severely undersized and as part of detailed design, a drainage exceedance assessment will be carried out to check if flooding could potentially occur within the school grounds to remain as-is (i.e. that will not be part of this development). This will include an estimate of combined sewer inflows from upstream areas. In any case, controlled 24 l/s will provide significant betterment to the existing uncontrolled situation;
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
• It is proposed to upgrade the outgoing public sewer to 225mm for the length between the site drainage connection and Third Avenue. This is to be approved by UU during detailed design; • Development should be designed so that there is no flooding to the development in a 1 in 30 year event and so that there is no property flooding in a 1 in 100 year plus climate change event; and • A rainfall event of 1 in 100 year plus climate change (20% increase of rainfall intensity) will be accommodated in the onsite attenuation storage. It has been estimated that approximately 445 m³ volume will be required within the system, though this is subject to agreement of discharge rates with UU. Drawing ‘232207-70-AR-01-XX-DR-C-00002’ indicates a potential configuration of the proposed surface water network, available in Appendix B. Using the above parameters, a preliminary surface water management plan has been designed using MicroDrainage WinDes software. The model demonstrates there is no surface flooding for 1 in 30yr and 1 in 100yr+cc events.
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Education Funding Agency Priority Schools Building Programme Flood Risk Assessment: Clough Hall Technology College
7 Conclusions
This FRA assessed the flood risk posed from all sources of flooding. The key aspects of the analysis undertaken as part of this FRA are: • Flooding from fluvial sources has been considered and it has been demonstrated that the site is at low risk from this source of flooding. Further investigation of the SCC linked 1D-2D flood model is required by the project drainage design team; • The data available indicates that the site is at low risk from groundwater flooding although further investigation of onsite groundwater levels are recommended to inform the detailed design of the proposed development; • The risk from overland flow paths has been identified both on and off site. This has led to the decision to increase finished floor levels to prevent inundation of buildings. The landscaping of areas around the building has also been steered by the need to control surface waters and direct water towards the surface water management infrastructure; • The potential for local infrastructure to cause flooding in the event of failure has been considered. A significant risk has not been identified; and • A preliminary surface water management strategy has been defined that reduces the quantity of surface water being discharged from the proposed areas of hard standing by at least 20% than for the existing school up to the 1 in 100 year return period rainfall event with an additional allowance for climate change. In order to achieve this, a volume of attenuation is required.
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Appendix B Drawings