Rising Sun. Scaffold Hill. Flood Risk Assessment

Transcription

1 Rising Sun Scaffold Hill Flood Risk Assessment August 2010

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3 Contents 1.0 Introduction Site Description Legislative Framework Flood Risk Surface Water Drainage Development Potential Conclusions Appendix A: Topographical Survey Spatial Geomatics Drawing No Rev B (Sheets 1-7) Appendix B: Drawings Drawing No /2001 Proposed Development Site and Features IDP Drawing No. N81:2091/RS-SK01 Indicative Masterplan Appendix C: Environment Agency Correspondence Letter (Ref: NA/2010/105245/01-L01) 07 July (Ref: /RC07) 13 July 2010 Appendix D: Existing Surface Water Runoff Calculations Appendix E: Micro Drainage Source Control Outputs 3

4 1.0 Introduction W.A. Fairhurst & Partners were appointed by The Northumberland Estates to undertake a Flood Risk Assessment of the site at Scaffold Hill in North Tyneside. The proposal is for residential development with some small scale local facilities. The aim of this Flood Risk Assessment report is to evaluate the current proposals with regard to flood risk and drainage, identifying potential flood risk both to and from the development site. Fairhurst have carried out the following: i. Assessment of the development potential of the site in the line with the Planning Policy Statement 25 Development and Flood Risk, (PPS25) ii. An assessment of the surface water runoff. 4

5 2.0 Site Description The development site is located between the A19 and Holystone Way in North Tyneside; the approximate National Grid Reference is NZ The site is currently undeveloped Greenfield land, comprising an area of approximately 63 ha. The southern part of the site is occupied by the Rising Sun Country Park. A detailed topographic survey has been carried out for the site and this can be seen in Appendix A. The proposed new built development will be restricted to that part of the site which lies to the north of the overhead pylon route, comprising an area of approximately 22 ha. To the south of this area improvements will focus on recreation. The extent of the new built development is shown in Drawing 86072/2001, in Appendix B. An indicative Masterplan for the development is also shown in IDP Drawing No. N81:2091/RS-SK01. There are a number of open water features on site, including Wallsend Dene which flows along the south eastern boundary and a pond within the Rising Sun Country Park. These are highlighted on Drawing 86072/

6 3.0 Legislative Framework The key aims of PPS 25 Development and Flood Risk are 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. Where new development is, exceptionally, necessary in such areas, policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall. A risk-based approach should be adopted at all levels of planning. Applying the source pathway-receptor model to planning for development in areas of flood risk requires: a strategic approach which avoids adding to the causes or sources of flood risk, by such means as avoiding inappropriate development in flood risk areas and minimising run-off from new development onto adjacent and other downstream property, and into the river systems; managing flood pathways to reduce the likelihood of flooding by ensuring that the design and location of the development maximises the use of SUDS, and takes account of its susceptibility to flooding, the performance and processes of river/coastal systems and appropriate flood defence infrastructure, and of the likely routes and storage of floodwater, and its influence on flood risk downstream; and reducing the adverse consequences of flooding on the receptors (i.e. people, property, infrastructure, habitats and statutory sites) by avoiding inappropriate development in areas at risk of flooding. Flood risk assessment should be carried out to the appropriate degree at all levels of the planning process, to assess the risks of all forms of flooding to and from development taking climate change into account and to inform the application of the sequential approach. A sequential risk-based approach to determining the suitability of land for development in flood risk areas is central to the policy statement and should be applied at all levels of the planning process. In areas at risk of river or sea flooding, preference should be given to locating new development in Flood Zone 1. If there is no reasonably available site in Flood Zone 1, the flood vulnerability of the proposed development can be taken into account in locating development in Flood Zone 2 and then Flood Zone 3. Within each Flood Zone new development should be directed to sites at the lowest probability of flooding from all sources. Flood risk has been categorised as High, Medium and Low based on the probability of inundation. Extract from the Flood Zones set out in PPS 25 highlight the likely response to planning applications within these zones. 6

7 Zone 1 Low Probability Definition This zone comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%). Appropriate uses All uses of land are appropriate in this zone. FRA requirements For development proposals on sites comprising one hectare or above the vulnerability to flooding from other sources as well as from river and sea flooding, and the potential to increase flood risk elsewhere through the addition of hard surfaces and the effect of the new development on surface water run-off, should be incorporated in a FRA. This need only be brief unless the factors above or other local considerations require particular attention. Policy aims In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development, and the appropriate application of sustainable drainage techniques Zone 2 Medium Probability Definition This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% 0.1%) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% 0.1%) in any year. Appropriate uses The water-compatible, less vulnerable and more vulnerable uses of land and essential infrastructure in Table D.2 are appropriate in this zone. Subject to the Sequential Test being applied, the highly vulnerable uses are only appropriate in this zone if the Exception Test is passed. FRA requirements All development proposals in this zone should be accompanied by a FRA. Policy aims In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area through the layout and form of the development, and the appropriate application of sustainable drainage techniques. Zone 3a High Probability Definition This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year. Appropriate uses The water-compatible and less vulnerable uses of land are appropriate in this zone. The highly vulnerable uses should not be permitted in this zone. The more vulnerable and essential infrastructure uses should only be permitted in this zone if the Exception Test is passed. Essential infrastructure permitted in this zone should be designed and constructed to remain operational and safe for users in times of flood. FRA requirements All development proposals in this zone should be accompanied by a FRA. 7

8 Zone 3a (cont.) Policy aims In this zone, developers and local authorities should seek opportunities to: i. reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques; ii. relocate existing development to land in zones with a lower probability of flooding; and iii. create space for flooding to occur by restoring functional floodplain and flood flow pathways and by identifying, allocating and safeguarding open space for flood storage. Zone 3b The Functional Floodplain Definition This zone comprises land where water has to flow or be stored in times of flood. SFRAs should identify this Flood Zone (land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the Environment Agency, including water conveyance routes). Appropriate uses Only the water-compatible uses and the essential infrastructure listed in Table D.2 that has to be there should be permitted in this zone. It should be designed and constructed to: remain operational and safe for users in times of flood; result in no net loss of floodplain storage; not impede water flows; and not increase flood risk elsewhere. Essential infrastructure in this zone should pass the Exception Test. FRA requirements All development proposals in this zone should be accompanied by a FRA. Policy aims In this zone, developers and local authorities should seek opportunities to: i. reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques; and ii. relocate existing development to land with a lower probability of flooding. Table 1 (continued): Extract from PPS 25 Annex D 8

9 PPS 25 sets out the compatibility of developments within each Flood Zone. summarised in Table 2. This is Flood Risk Vulnerability classification Essential Infrastructure Water Compatible Highly Vulnerable More Vulnerable Less Vulnerable Flood Zone Table 2: Zone 1 Zone 2 Zone 3a Zone 3b Exemption Test Required Exemption Test Required Development Acceptable Development not suitable Exemption Test Required Exemption Test Required Flood Risk Vulnerability and Flood Zone Compatibility Where required an exception test must be passed in order for developments of that nature to be justified within the Flood Zone. For the Exception Test to be passed the following must be demonstrated: a) it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. b) the development should be on developable previously-developed land or, if it is not there are no reasonable alternative sites on developable previouslydeveloped land; and c) a FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall. 9

10 4.0 Flood Risk The Environment Agency (EA) flood map shows the development site to be within Flood Zone 1 - Low Probability, as shown in Figure 1. This is outside of the area which is at risk from extreme fluvial or tidal flooding and the site is therefore not at risk from inundation in a 1 in 1000 year event. A more detailed version of the EA flood map is given in Appendix C. Figure 1: Extract from EA flood map, North Tyneside = Flooding from river or sea without defences (1 in 100 year event) = Extent of extreme flood (1 in 1000 year event) = Location of development The EA have been consulted with a Pre-Planning Application Enquiry. The responses to the enquiry and a subsequent request for further information can be seen in Appendix C. The EA has confirmed that the site is in Flood Zone 1 and that this Flood Risk Assessment should therefore concentrate on surface water drainage. The EA were unable to provide any information on historical flooding at the site. A review has been carried out of the Tyne & Wear Strategic Flood Risk Assessment (SFRA) which covers the North Tyneside area. The site and the surrounding area are not mentioned within the SFRA. It is therefore concluded that the site is not currently at significant risk from other sources of flooding such as ground water, sewers or surface water runoff. 10

11 5.0 Surface Water Drainage The development is within Flood Zone 1, therefore in accordance with PPS25 and the advice of the EA, this Flood Risk Assessment focuses on the management of surface water. The surface water strategy for the development site will be developed in accordance with PPS25, Sewers for Adoption 6th Edition and The Building Regulations Part H. 5.1 Existing Surface Water Runoff The EA/DERFA R&D Technical Report W5-074 Preliminary Rainfall Runoff Management for Developments states that for developments which are less than 50 ha in size the Institute of Hydrology Report 124 (IH124) Flood Estimation for Small Catchments should be used to calculate the peak Greenfield runoff rates. This advice is replicated in The SUDS Manual (CIRIA C697). The IH124 method uses the following equation to calculate Greenfield runoff: Where: Q BAR, rural = * AREA 0.89 * SAAR 1.17 * SOIL 2.17 AREA = Catchment Area (km 2 ) SAAR = Standard Average Annual Rainfall (mm) SOIL = Soil Index (from Wallingford Procedure maps) Technical Report W5-074 states that Where developments are smaller than 50 ha the analysis for determining the peak Greenfield discharge rate should use 50 ha in the formula and linearly interpolate the flow rate value based on the ratio of the development to 50 ha. On this basis the Greenfield runoff rate for the proposed development has been calculated to be 90.2 l/s. The full calculation can be seen in Appendix D. 5.2 Proposed Surface Water Drainage Principles The Building Regulations Part H sets out a hierarchy for the choice of discharge point for a rainwater system. In order of priority, the possibilities are given as: 1. an adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable, 2. a watercourse; or where that is not reasonably practicable, 3. a sewer. A geo-environmental desk study has been carried out which summarised that the site is anticipated to be underlain by relatively impermeable Glacial Till and variably permeable made ground over middle coal measures. As such, the main groundwater body is expected at depth in the coal measures bedrock strata with only minor perched groundwater present, if any, at shallow depth. It is therefore concluded that infiltration is unlikely to be a suitable means of discharging surface water runoff arising from the site. There are a number of open water features on site, including Wallsend Dene which flows along the south eastern boundary and a pond within the Rising Sun Country Park. The pond is a SUDS feature, created to attenuate flows from the adjacent Holystone development before discharging into Wallsend Dene via a ditch. It is therefore possible that a similar 11

12 arrangement could be implemented for this proposed development. This is discussed further in the following section. PPS25 states that Surface water arising from a developed site should, as far as is practicable, be managed in a sustainable manner to mimic the surface water flows arising from the site prior to the proposed development Technical Report W5-074 states that to deal with increased runoff due to development the limiting discharge for the 30 and 100 year return periods will be constrained to the mean annual peak rate of runoff for the Greenfield site (Referred to as Q BAR in IH Report 124). It is therefore proposed that surface water runoff from the site will be discharged at a flow rate no greater than 90.2 l/s, for all events up to and including the 100 year return period event, incorporating a 20% increase in rainfall intensity to account for climate change. 5.3 Surface Water Attenuation To ensure that surface water runoff does not pose a risk to the proposed development, or increase the risk of flooding elsewhere, surface water attenuation will be provided on site in the form of Sustainable Urban Drainage Systems (SUDS) to store all flows above the mean annual Greenfield runoff rate, up to the design event (100 year return period, with a 20% increase in rainfall intensity to account for climate change). The Source Control module of the industry standard software Micro Drainage has been used to determine the attenuation storage volume which could be required, based on an estimation of the impermeable areas on site, post development. The indicative site Masterplan, which can be seen in Appendix B, has been used to determine the extent of impermeable areas. It is conservatively estimated that 50% of the site will be impermeable, equating to an area of approximately 11 ha. It is likely that a detailed assessment will indicate that the area of impermeable land is in fact less than this. However, at this stage a precautionary approach is considered to be prudent. Outputs from Micro Drainage, based on a discharge of 90.2 l/s, indicate that to store the runoff from a 100 year return period event, with a 20% allowance for climate change, the volume of attenuation required would be approximately 5800 m 3. This is based on the 600 minute winter storm, which was determined to be the critical duration event in Micro Drainage. The Micro Drainage outputs can be seen in Appendix E. To provide the required volume of storage it may be possible to extend the pond located in the south of the site, within the Rising Sun Country Park. There is sufficient space surrounding the existing pond to facilitate an increase in storage capacity without adverse effect elsewhere. The pond is currently used as a SUDS facility to attenuate surface water runoff from the adjacent Holystone development before discharging into the Wallsend Dene. It would be possible to adjust the flow control arrangement on the outlet of the pond to account for the allowable discharge restrictions for the proposed development. Alternatively, there are large areas of open space elsewhere within the southern part of the site which could be utilised to create a new SUDS attenuation pond. The topography in the south eastern section of the site is well suited for the provision of a SUDS pond which could discharge to the adjacent watercourse, as indicated in Drawing 86072/2001 in Appendix B. 12

13 The EA has been consulted as part of the Pre-Planning Application Enquiry to seek an opinion on the suitability of providing surface water attenuation as described above. The EA response can be seen in Appendix C and is summarised below. From a flood risk perspective, this would be in line with our recommendations for Sustainable Urban Drainage Systems and therefore we would support any proposal to do this. However, from a nature conservation perspective we would require more details about what is currently existing on site and if there are any newts in the area. An ecological survey of the area would be required before any proposals were undertaken... We recommend you consult Natural England for further details about protected species and any consents required. An ecological study will therefore be undertaken as part of the development design and the results will be used to inform the planning of the surface water drainage system. Close consultation will be maintained with both the EA and Natural England throughout the design process to ensure that the required standards are met. To reduce the size of any final attenuation facility, it may be possible to implement a SUDS management train, employing a series of drainage techniques to slow and attenuate water through the development, as suggested in The SUDS Manual (CIRIA C697). The SUDS Manual details a wide range of drainage techniques; those which may be suitable are as follows: rainwater harvesting; swales; pervious pavements & car parking spaces; detention basis in open landscaped areas; geocellular systems. The feasibility and suitability of any given SUDS solution will be fully considered at the detailed design stage of the development. However, it is considered that the information provided demonstrates that adequate procedures can be put in place to ensure that flood risk will not increase as a result of the proposed development, either at the site or elsewhere. 13

14 6.0 Development Potential The proposed development is within Flood Risk Zone 1. No detailed sequential test is therefore required to identify sites of lower risk. This is in line with the guidance set out in PPS25 and is summarised in Figure 2. The proposed development can be classified as More Vulnerable according to Annex D of PPS25. From table D3 in PPS25, the proposed development may be considered suitable for this site, and no exception test is required. Figure 2: PPS25 Sequential Test Summary 14

15 7.0 Conclusions The Flood Risk Assessment for the development proposals at Scaffold Hill in North Tyneside has been prepared in accordance with PPS 25 Development and Flood Risk. The development site is not within the Environment Agency s indicative flood envelopes and is therefore classed as being within Flood Risk Zone 1 i.e. low risk. Based on the compatibility of developments within each Flood Zone, set out within PPS 25, the site is suitable for all types of developments. The site is currently undeveloped Greenfield land, therefore the existing rate of surface water runoff has been calculated using the Institute of Hydrology Report 124 method. This is in line with advice given in the EA/DERFA R&D Technical Report W5-074 Preliminary Rainfall Runoff Management for Developments and The SUDS Manual (CIRIA C697). To ensure that surface water runoff does not pose a risk to the proposed development, or increase the risk of flooding elsewhere, surface water attenuation will be provided on site to store all flows above the mean annual Greenfield runoff rate, up to the design event. The possible effects of climate change have been considered by acknowledging the requirement to make an allowance for increased rainfall in the calculation of the surface water discharge rates and design of the drainage system over the lifespan of the development in line with PPS 25. A number of methods for providing attenuation have been identified, including a single attenuation storage facility and a management train approach. It has been demonstrated that, even if a management train approach is not found to be suitable for the site, a single attenuation storage facility is feasible and would achieve the required objectives. It is therefore concluded that the development can be carried out without increasing flood risk either to the site or elsewhere. 15

16 APPENDIX A Topographical Survey Spatial Geomatics Drawing No Rev B (Sheets 1-7)

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