DOUGHTY ROAD, GRIMSBY Flood Risk Assessment and Surface Water Drainage Strategy DECEMBER 2017

Transcription

1 Flood Risk Assessment and Surface Water Drainage Strategy DECEMBER 2017

2 CONTACTS SARAH DEMETRIOU Graduate Flood Modeller dd e sarah.demetriou@arcadis.com Arcadis. 2 Glass Wharf Temple Quay Bristol BS2 0FR Arcadis (UK) Limited is a private limited company registered in England & Wales (registered number ). Registered Office at Arcadis House, 34 York Way, London, N1 9AB, UK. Part of the Arcadis Group of Companies along with other entities in the UK. Copyright 2015 Arcadis. All rights reserved. arcadis.com

3 Doughty Road, Grimsby Author Sarah Demetriou Checker Nick Bosanko Approver Hywel Roberts Report No _1.1_A1 & B1 Date DECEMBER 2017 VERSION CONTROL Version Date Author Changes This report dated 19 December 2017 has been prepared for North East Lincolnshire Council (the Client ) in accordance with the terms and conditions of appointment (the Appointment ) between the Client and Arcadis (UK) Limited ( Arcadis ) for the purposes specified in the Appointment. For avoidance of doubt, no other person(s) may use or rely upon this report or its contents, and Arcadis accepts no responsibility for any such use or reliance thereon by any other third party.

4 CONTENTS 1 INTRODUCTION General Aims and Objectives Terminology BACKGROUND INFORMATION Site location and Description Surrounding Area Hydrology Site Topography Existing Drainage Geology and Hydrogeology Proposed Site Layout CONSULTATION PLANNING POLICY CONTEXT NPPF and Flood Risk Local Policy The Sequential and Exception Tests ASSESSMENT OF FLOOD RISK Overview Climate Change Flood Map for Planning Fluvial Tidal Benefit of Coastal Flood Defences Overtopping Assessment Breach Assessment Discussion Surface Water Groundwater Sewers Reservoirs, Canals and other Artificial Sources FLOOD MITIGATION AND MANAGEMENT Flood Resistance and Resilience Access and Egress SURFACE WATER DRAINAGE STRATEGY Overview Proposed Receptor of Surface Water Runoff Existing Runoff and Allowable Discharge Rates Surface Water Strategy CONCLUSIONS AND RECOMMENDATIONS REFERENCES... 19

5 APPENDICES Anglian Water Sewer Asset Plans Environment Agency Information Anglian Water Sewer Flood Records Preliminary Surface Water Calculations

6 1 Introduction 1.1 General Arcadis (UK) Limited ( Arcadis ) has been commissioned by North East Lincolnshire Council to undertake a Flood Risk Assessment (FRA) to support residential development aspirations for the site at Doughty Road, Grimsby. This is a pre-planning exercise and this FRA is preliminary in nature, which can be used, at a later stage, to inform the development layout of the site. The FRA will need to be subsequently updated to reflect the development proposals. The Environment Agency (EA) Flood Map for Planning indicates that the development site is located almost entirely within Flood Zone 2 (medium probability of fluvial/tidal flooding). A very small part of the site is located in Flood Zone 3 (medium probability of fluvial/tidal flooding). Therefore, and in accordance with the National Planning Policy Framework (NPPF) and supporting Planning Practice Guidance (PPG), an FRA will be required to inform a planning application at the site. 1.2 Aims and Objectives The overall aim of this FRA is to demonstrate that the site can be used for residential development, without exposing it to an unacceptable degree of flood risk or increasing the flood risk to third parties. The specific objectives of this FRA are to: Consult with the Lead Local Flood Authority (LLFA; i.e. North East Lincolnshire Council in partnership with ENGIE) to gather baseline flood risk data and local knowledge Consult with the EA to gather site specific flood risk data. Review relevant strategic documents (e.g. Strategic Flood Consequences Assessment, Preliminary Flood Risk Assessment, Flood Risk Management Strategy) to gather further baseline flood risk data. Undertake an appraisal of the various potential sources of flood risk posed to the site and surrounding area. Provide a conceptual strategy for mitigating flood risks at the site, where required. Provide a conceptual surface water drainage strategy for mitigating any increase in runoff resulting from the development. Prepare an FRA report, including any recommendations for further work. 1.3 Terminology Flood risk is a product of both the likelihood and consequences of flooding. Throughout this document, flood events are defined according to their likelihood of occurrence. Floods are described according to an annual chance, meaning the chance of a particular flood occurring in any one year. This is directly linked to the probability of a flood. For example, a flood with an annual chance of 1-in-100 (a 1-in-100 chance of occurring in any one year), has an annual exceedance probability (AEP) of 1%. 1

7 2 Background Information 2.1 Site location and Description The former industrial depot is located in Grimsby (National Grid Reference TA ). The site location is presented in Figure 1. The site consists almost entirely of hardstanding and roof surfaces. Numerous industrial buildings are present across the site. The site has an existing access from Doughty Road. Access Road Retail buildings Grimsby Town train station Local businesses Figure 1: Aerial photo of site showing key features (Copyright 2014 Microsoft and its suppliers) 2.2 Surrounding Area The site is within a predominantly commercial area with retail properties to the east and local businesses to the south of the site. Grimsby town train station is located approximately 300m to the south-west of the site and the tracks run alongside to the north-western boundary of the site. The coastline is located approximately 2km north of the site. The EA have confirmed that concrete floodwalls, supplemented by saltmarshes, line the coast in this area, which defend the site and wider area from tidal flooding. It is understood that these defences offer protection to the site for up to a 1 in 100 year annual probability event. 2.3 Hydrology There are no watercourses present on site. The nearest watercourse to the site is the River Freshney, which is located approximately 400m north, where it drains into Alexandra Dock before flowing into the Humber Estuary. 2

8 2.3 Site Topography A topographic survey of the site is not available. However, LiDAR (Light Detection And Ranging) data have been downloaded from the Environment Agency Geostore (Ref.1) and used to create a Digital Terrain Model (DTM) for the site. The DTM (Figure 2), shows that ground level across the site slope down from northeast to southwest. Figure 2 also shows that the wider topography in fairly flat, sloping gently down to the southeast. The site slopes from a maximum of approximately 4.5m Above Ordinance Datum (AOD) to a minimum of approximately 2.5m AOD. Doughty Road, on the south-western boundary, has a minimum elevation of 0.5m AOD; Peaks Parkway, north of the site, has a minimum elevation of -2.5m AOD. These two roads are very low, where they pass beneath the existing rail infrastructure. Figure 2: LiDAR Data (site location outlined in red) - levels shown in m AOD 2.4 Existing Drainage It is assumed that the existing site is positively drained, which discharges to the local public sewer network. Sewer plans have been provided by Anglian Water and are included within Appendix A. There are a number of combined sewers located around and through the site. Two large (900mm) combined sewers pass through the site. 2.5 Geology and Hydrogeology The 1 in 50,000 scale British Geological Survey (BGS) (Ref. 2) online mapping shows that the solid geology beneath the site consists of the Flambourgh Chalk Formation. The online mapping indicates that the superficial deposits are present and consist of tidal flat deposits Sand and Gravel. The Cranfield University Soilscapes (Ref. 3) website shows that Loamy and clayey soils of coastal flats with naturally high groundwater occupy the site. 3

9 The solid geology present beneath the site is classified a Principal Aquifer. These are layers of rock or drift deposits that have high intergranular and/or fracture permeability- meaning they usually provide a high level of water storage. They may support water supply and/or river base flow on a strategic scale. Given the depth of the chalk beneath the site (approx. 20m), infiltration as a means of surface water management is not considered to be practical. However, this should be verified as part of a site investigation and is discussed in further detail in Section Proposed Site Layout It is understood that the site is being considered for residential development. No further details are available at this stage. 4

10 3 Consultation Flood data was obtained from the EA, which is enclosed in Appendix B. This information has been used to inform the assessment of fluvial and tidal flood risk in Section 5 of this report. As part of the preparation of this FRA, consultation with the Lead Local Flood Authority (LLFA) was undertaken. Information provided by the LLFA for the site has been extracted from their below: No record of significant local flood risk issues on or around the site. However there are areas with a very high local flood risk both upstream and downstream. A number of trunk sewers run through the site that will influence the layout of any new development. These sewers are prone to significant siltation build ups which I feel may be influenced by its current use. A fully sustainable surface water drainage scheme would be required and any positive discharge needed would have to be as close to the greenfield run off rate for the site as is practicable. Ground permeability may be poor. This information has been used to inform the remainder of this FRA. Anglian Water were consulted regarding historic sewer flooding records. They have confirmed that they hold no records in the vicinity of the site (see Appendix C). 5

11 4 Planning Policy Context 4.1 NPPF and Flood Risk The NPPF (Ref.4) and its accompanying PPG (Ref.5) set out Government planning policy for England. The principal aim of the NPPF is to achieve sustainable development. This includes ensuring that flood risk is considered at all stages of the planning process, avoiding inappropriate development in areas at risk of flooding and directing development away from those areas where risks are highest. Where development is necessary in areas of flooding, the NPPF aims to ensure that it is safe, without increasing flood risk elsewhere. Early adoption of, and adherence to, the principles set out in the NPPF and its PPG, with respect to flood risk, should ensure that detailed designs and plans for developments take due account of flood risk and the need for appropriate mitigation, if required. 4.2 Local Policy The North East Lincolnshire Council Local Flood Risk Management Strategy (LFRMS) (Ref. 6) outlines Roles and Responsibilities, Preferred Strategies, and Sustainable Development. The following measures are proposed by the Council to ensure that new development does not increase local flood risk and contributes to a reduction where possible: The North East Lincolnshire Pre-Submission Draft Local Plan (Ref. 7) includes Policy 32 on flood risk, which is replicated on the following page. Policy 32 is consistent with the general requirements of the NPPF. From a flood context, Section 5 and 6 of this report address these policy requirements. The need to manage surface water, through the application of Sustainable Drainage Systems (SuDS), is also identified in Policy 32. This is discussed in Section 7 of this report. 6

12 4.3 The Sequential and Exception Tests The NPPF identifies four Flood Zone classifications, detailed in Table 1. Flood Zone Definition 1 Low Probability Land having a less than 1 in 1,000 annual probability of river or sea flooding 2 Medium Probability 3a High Probability Land having between a 1 in 100 and 1 in 1,000 annual probability of river flooding; or land having between a 1 in 200 and 1 in 1,000 annual probability of sea flooding. (Land shown in light blue on the Flood Map for Planning) Land having a 1 in 100 or greater annual probability of river flooding; or Land having a 1 in 200 or greater annual probability of sea flooding. (Land shown in dark blue on the Flood Map for Planning) 3b The Functional Floodplain Table 1: Flood Zones (Source: PPG NPPF Table 1) This zone comprises land where water has to flow or be stored in times of flood. Local planning authorities should identify in their Strategic Flood Risk Assessments areas of functional floodplain and its boundaries accordingly, in agreement with the Environment Agency. (Not separately distinguished from Zone 3a on the Flood Map for Planning) The NPPF specifies that the suitability of all new development in relation to flood risk should be assessed by applying the Sequential Test to demonstrate that there are no reasonably available sites in areas with a lower probability of flooding that would be appropriate to the type of development proposed. The NPPF provides guidance on the compatibility of each land use classification in relation to each of the Flood Zones as summarised in Table 2. The EA Flood Map for Planning shows that almost the enstire site is located within Flood Zone 2; a very small part of the site is located in Flood Zone 3. The proposed residential use of the site is classified as a more vulnerable development in accordance with the NPPF. Since the site has not been allocated within the Local Plan, it is assumed that there is a requirement to undertake the Sequential Test given the Flood Zone 2 and Flood Zone 3 status of the site. This will need to be undertaken to inform the planning 7

13 application. Given the partial Flood Zone 3 status of the site and with reference to Table 2, the application of the Exception Test is required. This is discussed in Section 6.. Flood Zone Essential Infrastructure Water Compatible Highly Vulnerable More Vulnerable Less Vulnerable Zone 1 Zone 2 Exception Test required Zone 3a Exception Test required Exception Test required Zone 3b Key: Exception Test required Development is appropriate Development should not be permitted Table 2: Flood Risk Vulnerability Classification (Source: PPG NPPF Table 3) 8

14 5 Assessment of Flood Risk 5.1 Overview In line with the requirements of the NPPF, this section of the FRA considers flood risk from the range of possible sources listed in Table 3. Source of Flooding 1 Flooding from rivers (Fluvial) 2 Flooding from the sea (Tidal) 3 Flooding from land (Surface Water) 4 Flooding from groundwater 5 Flooding from sewers 6 Flooding from reservoirs, canals and other artificial sources Table 3: Sources of Flooding Description Flood water originating from a nearby watercourse when the amount of water exceeds the channel capacity of that watercourse This can occur as a result of extreme high tides, following tidal surges, or as a result of waves overtopping costal defences, for example Flooding caused by intense rainfall exceeding the available infiltration and/or drainage capacity of the ground Flooding caused when groundwater levels rise above ground level following prolonged rainfall Flooding originating from surface water, foul or combined drainage systems, typically caused by limited capacity or blockages Failure of infrastructure that retains or transmits water or controls its flow 5.2 Climate Change The anticipated effects of climate change are well understood. It is a key planning policy requirement that development proposals are demonstrated to be safe for their lifetime; this is further discussed in Section 5.5. With respect to flooding, climate change will influence both sea level (i.e. tidal flood risk) and river flow (i.e. fluvial flood risk) and is discussed below, where appropriate. 5.3 Flood Map for Planning The EA Flood Map for Planning (Ref.8) identifies likely flood risk from both fluvial and tidal sources. This indicates that the site is almost entirely located within Flood Zone 2 (see Figure 3) and is therefore at a medium risk of flooding. Flood Zone 2 is defined as land having between a 1 in 100 and 1 in 1,000 annual probability of river flooding; or land having between a 1 in 200 and 1 in 1,000 annual probability of sea flooding. A very small part of the site, on the south boundary, is located in Flood Zone 3. This is defined as l and having a 1 in 100 or greater annual probability of river flooding; or Land having a 1 in 200 or greater annual probability of sea flooding. 5.4 Fluvial The EA have confirmed that the site is located within Flood Zone 2 and Flood Zone 3 as a result of tidal flooding only (see Appendix B). Consequently, fluvial flooding is considered to have no impact on the site and does not warrant any further discussion in this FRA. It is considered that there is low risk of fluvial flooding on site. 5.5 Tidal As outlined above, Flood Zone 2 and Flood Zone 3 at the site is a result of the risk from tidal flooding. The EA have provided still water tidal levels for the South Humber Estuary. These are the peak water levels estimated to occur at the coastline for a range of annual exceedance probability events and are presented in Table 4. They are referred to as still water levels because they take no account of the impact from waves. 9

15 Reference Location Annual Exceedance Probability Tidal Level 1 in 10 1 in in in 1000 H060 Grimsby Table 4: Still Water Tidal Levels These still water levels are used by the EA to derive the Flood Zone 2 and Flood Zone 3 extents. This is undertaken through hydraulic modelling, whereby the flood defences are removed, and inland propagation of floodwater is simulated. The still water levels in Table 4 do not, therefore, relate to on-site flood levels. As a result of this hydraulic modelling, the site is almost entirely located in Flood Zone 2. This means that, if flood defences were not present, the site would be expected to flood in a 1 in 1,000 annual probability event. This is a conservative approach adopted by the EA for the derivation of Flood Zone 2 and Flood Zone 3. In reality, coastal flood defences are present (as described in Section 2), which reduce tidal flood risk at the site. It is understood that these defences provide protection for up to a 1 in 100 year tidal event. The EA derive Flood Zone 2 and Flood Zone 3 in this way because these defences can fail, either through a breach or following overtopping (i.e. water levels exceeding the level of the defence). Therefore, for areas located behind flood defences, a residual risk is present. If the benefit of the flood defences was not excluded on the Flood Map for Planning (i.e. in Flood Zone 2 and Flood Zone 3), this residual flood risk would not be fully identified and the particular development site may therefore not subjected to an FRA as part of a planning submission. Figure 3 EA Flood Map for Planning (Contains Ordnance Survey data Crown copyright and database right 2017) Benefit of Coastal Flood Defences Overtopping Assessment The EA have undertaken hydraulic modelling to demonstrate the benefit offered by the coastal flood defences. This is achieved by retaining the flood defences in the hydraulic model and allowing the still water levels to overtop these features. Water that is able to overtop will then propagate inland and flood low lowing 10

16 areas. This is often referred to as actual flood risk because it better represents the more likely flooding mechanism. The EA have confirmed that the site is not impacted by tidal flooding in the present day conditions (i.e. no climate change) up to and including the 1 in 1,000 annual probability event, with the benefit of the defences included. This is because water that overtops the defences, is unable to penetrate sufficiently far inland to flood the site. However, with the addition of the anticipated impact of climate change on sea level, flooding of the site is anticipated. The EA have provided a series of tidal climate change flood maps at the site for the following overtopping scenarios: 1 in 200 year annual probability event (2115) 1 in 1000 year annual probability event (2115) It is assumed that the EA hydraulic modelling study was undertaken in 2015 and 100 years of climate change was adopted from that point. Flood maps from these scenarios are enclosed in Appendix B. The 1 in 200 year annual probability event (2115) overtopping map shows that the entire site is impacted with significant depths of floodwater observed. The mapping suggests that flood depths up to 1m in the north of the site are estimated. Flood depths across the rest of the site are up to 1.6m. The site is surrounded almost entirely by land with flood depths exceeding 1.6m. Unsurprisingly, the 1 in 1,000 year annual probability event (2115) results in a greater impact on the site and surrounding area Breach Assessment The EA have also undertaken hydraulic modelling to assess the impacts of a series of breaches in the coastal flood defences. This modelling assumes that no overtopping of the defences would occur in the event of a breach. This modelling confirms that the site is entirely outside of the breach extent for all present day scenarios, up to and including the 1 in 1000 annual probability event. However, with the addition of climate change, flooding of the site is observed. For the 1 in 200 year plus climate change event, small areas in the north and south of the site experience flood depths of up to 0.25m. However, the surrounding roads are shown to be flooded to significant depths (over 1.6m) Discussion Various flood modelling scenarios are available from the EA to inform the assessment of flood risk at the site with respect to tidal flooding. Some of these scenarios have been shown to have no impact on the site, whilst others are anticipated to result in flooding. Typically, a design flood event is established, for which the safety of the site is compared against. The NPPG definition of a design flood has been replicated below: Climate change must also be considered as part of the flood design event. The PPG (Ref 5) states that the lifetime of residential development should be considered for a minimum of 100 years, unless there is specific justification for considering a shorter period. Therefore, for tidal flooding, and using the information available from the EA, the design event is the 1 in 200 year annual probability event (2115). 11

17 Table 5 summarises the maximum flood depths observed on site in this design event for the overtopping and breach scenarios. The overtopping scenario results in significantly greater flood depths on site. Maximum Flood Depth (m) Overtopping Breach 1 to Table 5: 1 in 200 year annual probability event (2115) Flood Depths The overtopping climate change scenario modelling undertaken by the EA assumes that the existing flood defences will remain at their present design standard in the future. In reality, it is possible that the flood defences will be raised to at least maintain their existing standard of protection for Grimsby into the future. However, no such works are known and for the purposes of this FRA, this cannot be relied upon. It has therefore been assumed that the overtopping scenario represents the design event which the site must be protected from. These requirements are an essential element of this FRA and are discussed in Section Surface Water Flooding from land (often known as surface water flooding) occurs when extreme rainfall exceeds the infiltration or drainage capacity of the ground surface. The flood risk relates to both the conveyance of waters through the site or as a result of the ponding of these waters in depressions in the topography. An extract from the Risk of Flooding from Surface Water map (Ref. 8) is presented in Figure 4. It shows that the majority of the site is at a Very Low Risk of flooding and means that each year, this area has a chance of flooding each year of less than 1 in Figure 4: Risk of Flooding from Surface Water map Crown copyright and database rights 2017 OS Two minor areas of the site are shown to be at a Low Risk of surface water flooding whereby the annual probability of surface water flooding is between 1 in 1000 and 1 in 100. From the topography of the site 12

18 (Section 2.3), these areas of Low Risk correspond to depressions in the site topography and it is likely that water ponds here during extreme rainfall. The EA (Ref. 9) provide flood depth information for surface water flooding, which shows that these areas experience shallow ponding (<0.3m) and is not considered to be a significant risk. Furthermore, this is likely to be a result of runoff form the site itself and is likely to be mitigated through the provision of a new surface water drainage strategy. Where Doughty Road and Peaks Parkway pass beneath the railway infrastructure, a High Risk of surface water flooding is evident (annual probability of flooding is greater than 1 in 30), but this will have no direct impact on development aspirations. It is concluded that the risk from this source is low. 5.7 Groundwater Groundwater flooding occurs when groundwater rises to the ground surface. This may happen during winter and/or after prolonged or heavy rain storms. The North and North East Lincolnshire Strategic Flood Risk Assessment (Ref. 9) presents limited information on groundwater flooding. It suggests that groundwater flooding is most likely to occur in low-lying areas underlain by permeable rocks, or aquifers. Therefore, the lower lying area to the south of the site, may be more susceptible to groundwater flooding. BGS boreholes logs located on or near the site do not identify any shallow groundwater. It is considered that there is low risk of groundwater flooding on site. However, it is recommended that as part of any intrusive site investigation, groundwater levels below the site should be determined to provide further verification. 5.8 Sewers The North East Lincolnshire Preliminary Flood Risk Assessment (Ref. 10) lists several areas of Grimsby which are known to be at risk from combined sewer flooding. None of these roads are within the immediate vicinity of the site therefore the site is not considered to be within a high-risk area. Also, as discussed in Section 3, Anglian Water have no records of sewer flooding in the proximity of the site. If in the unlikely event that the local sewers did experience some flooding in the proximity of the site, it is anticipated that the majority would flow off site to the south, towards the low-lying area. It is considered that there is low risk in terms of sewer flooding on site. 5.9 Reservoirs, Canals and other Artificial Sources The EA has produced mapping that shows the maximum area that might be flooded if large reservoirs were to fail and release the water they hold. The map (Ref. 8) shows that the development site is not located within the maximum extent of flooding from any reservoir in the area. There are no canals or other artificial sources located within the vicinity of the site. It is concluded that there is low risk of flooding from artificial sources. 13

19 6 Flood Mitigation and Management 6.1 Flood Resistance and Resilience The majority of the sources of flooding at the site have been identified to be low and flood risk mitigation measures are not required. However, tidal flood risk was found to be more significant. In accordance with the requirements of the national planning policy, and more specifically, the Exception Test, the site must be considered safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall. Similar requirements are identified in the North East Lincolnshire Pre-Submission Draft Local Plan (Ref. 7). It is the purpose of this section to demonstrate how this may be achieved. It is important to note that the word safe is meant in the context of both people using the site (i.e. residents, staff and the public) and the structural safety of the buildings. However, given the preliminary nature of this FRA, the discussion below is high level only. It should be used to inform further consultation with the EA, through the pre-application process. For the purposes of this preliminary FRA we have assumed a worst-case scenario and adopted the overtopping scenario as the design event (i.e. 1 in 200 year annual probability (2115) overtopping event). However, it is recommended that the EA are consulted as part of the preparation of a site layout plan to confirm the appropriate design event scenario that should be assessed. During the 1 in 200 year plus climate change design event, peak flood depths are estimated to be greater than 1.6m in parts of the site. A series of potential options exists to ensure that the development remains safe for the duration of its lifetime. The key options have been summarised below: Raising ground floor levels above the design flood level floor levels would need to be raised by up approximately 1.9m (includes a 0.3m freeboard above the estimated peak depths). More modest raising of ground floor levels, with flood resilience to minimise the impact of flooding. This could be combined with the use of duplex residential units, with sleeping accommodation at an upper level. Provision of Less Vulnerable uses (i.e. commercial or parking) on the ground floor, with residential uses on the first floor and above. During the preparation of a development layout, these possible options will need to be discussed with both the EA and the design team to identify a viable solution for the site Surface Water Flooding Surface water flood risk on the site was assessed to be low. However, some local off-site areas of surface water flooding were identified. It is recommended that vehicular access into the site should be positioned outside of the area susceptible to surface water flooding, if possible. 6.2 Access and Egress The NPPF advises that access considerations should include the voluntary and free movement of people during a design flood. During this design event (i.e. 1 in 200 year annual probability (2115) overtopping event), flood hazard in the vicinity of the site is considered too great to allow for safe access and egress from the development during flooding. However, the site is situated in an area where the EA provide flood warnings. Given the site is located at the periphery of the tidal floodplain, it is assumed that sufficient time will be available for occupants to evacuate following a warning from the EA prior to the onset of flooding. Once planning has been granted, a Flood Risk Management Plan should be prepared for the site detailing steps to take following a warning from the EA including recommended evacuation routes. In addition, site occupiers will need an area of safe refuge located above potential flooding in the event that evacuation is not possible. This may be available, as a result of the flood mitigation measures incorporated into the site. However, if this is not available, an internal route to a safe refuge is likely to be required. This approach should be agreed with the EA, as part of the development of a site layout.. 14

20 7 Surface Water Drainage Strategy 7.1 Overview It is well understood that one of the effects of development is typically to reduce the permeability of a site and consequently change its response to rainfall. Therefore, a suitable surface water drainage strategy is required to ensure that the surface water runoff regime is managed appropriately and that the proposed development does not increase flood risk on the site and/or to surrounding areas. The NPPF states that flood risk to land and property must not be increased as a result of development. The associated Planning Practice Guidance (PPG) states that flood risk should not increase for events up to and including a 1 in 100 year return period, with an appropriate allowance for climate change. A fundamental principle of sustainable development in terms of flood risk management is the reduction of surface water run-off from new developments. Surface water drainage arrangements for any development site must ensure that volumes and peak discharge rates, leaving the site, are managed to ensure that there is no detrimental impact associated with the development. The proposed surface water management strategy should be derived based upon the principles of Sustainable Drainage System (SuDS), in accordance with NPPF and the North East Lincolnshire Council SuDS Guide. The surface water drainage strategy presented in this FRA is preliminary in nature and will need to be refined during the preparation of a proposed site layout. It identifies a likely option for the management of surface water runoff, based on information that was available at the time. 7.2 Proposed Receptor of Surface Water Runoff National and local planning policy and guidance promotes the use of the drainage hierarchy for the discharge of surface water from development sites. This stipulates that surface water should be discharged as high up the following hierarchy as reasonably practicable: Infiltration into the ground; To a watercourse; To a surface water sewer; To a combined sewer. As discussed in Section 2, chalk is present beneath the site, which would normally be suitable for infiltration as a means of surface water management. However, information available suggests that the chalk in present at a depth in excess of 10m and is considered unlikely to be suitable. It is recommended that this is verified as part of a ground investigation. There are no watercourses on site and it may therefore be necessary to discharge surface water to a sewer, with a surface water sewer preferred, over a combined sewer. It is likely that the existing positive drainage that serves the site discharges to a public sewer. As discussed in Section 2, two combined sewers pass through the site, but a connection will need to be agreed with Anglian Water. 7.3 Existing Runoff and Allowable Discharge Rates The ICP SuDS Method has been used within Micro Drainage to calculate the existing greenfield runoff rates for the undeveloped site. The parameters utilised are detailed in Table 6. Whereas the calculated rates are presented in Table 7. The summary sheet is included in Appendix D. The greenfield runoff rates were calculated based on the amount of developable area within the site, instead of the gross area. The developable area excludes the green corridors and landscaped areas, as these would drain more naturally. Given that no site layout is available at this stage, it has been assumed that the developable area consists of 90% of the site. 15

21 Parameter Value Unit Developable Area ha SAAR 636 mm Soil Class Region 5 - Urban 0 % Table 6: ICP SuDS Parameters Return Period Peak Greenfield Discharge Rate (l/s) QBAR in in in Table 7: ICP SuDS Results A series of building and hardstanding areas occupy the site, which are assumed to discharge to the adjacent public sewer network. Therefore, these greenfield runoff rates are not considered to be a reflection of the existing runoff rates generated by the site. The surface water strategy outlined below has been based upon the restriction of surface water runoff to these greenfield rates. However, it may be possible to negotiate a greater discharge rate with Anglian Water and the LLFA, given the current use of the site. 7.4 Surface Water Strategy In order to restrict runoff from the site to greenfield rates, following development, attenuation storage will be required. A Micro Drainage quick storage estimate was undertaken to determine the amount of attenuation storage that would be required to accommodate surface water runoff from the developed part of the site for all events up to and including a 1 in 100 year plus climate change event (40% increase in peak rainfall intensity). This has based on restricting runoff to the QBAR greenfield runoff rate (6.4 l/s). The QBAR rate has been adopted to ensure that the long-term storage volume has been accommodated so that the volume of surface water that is discharged from the site (as well as the rate) has also been managed (Ref. 11). The Micro Drainage quick storage estimate was based on the parameters identified in Table 8. The impermeable area has been calculated based upon an industry standard 60% impermeable ratio (i.e. that 60% of the developable area would have an impermeable cover). The Micro Drainage calculation sheets are enclosed in Appendix D. This has shown that an attenuation storage volume of approximately 629m 3 would be required to restrict runoff rates to the greenfield QBAR rate during the 1 in 100 year rainfall plus climate change event. Should it be possible to agree a greater discharge rate into the public sewer, with Anglian Water, this volume of attenuation storage required could be reduced. The SuDS Manual (Ref. 12) identifies various measures that can be used to provide this attenuation. However, there is generally a preference for above ground attenuation features (such as ponds, swales, 16

22 detention basins), rather than below ground features (such as tanks). Given the proposed connection to a public sewer, the SuDS used will be influenced by Anglian Water and their adoption criteria. The majority of the attenuation storage is likely to be located towards the lowest part of the site on the southern boundary to allow a gravity based arrangement. Given the size of the site, source and site control measures should be considered to help mimic a more natural surface water regime. Parameter Value Unit Impermeable area ha Discharge rate 6.4 l/s Infiltration rate 0 m/hr Climate change provision 40 % Table 8: Attenuation Storage Parameters 17

23 8 Conclusions and Recommendations Arcadis (UK) Limited ( Arcadis ) has been commissioned by North East Lincolnshire Council to undertake a Flood Risk Assessment (FRA) to support to support the residential development aspirations of the the site at Doughty Road, Grimsby. This is a pre-planning exercise and this FRA is preliminary in nature, which will be used, at a later stage, to inform the development layout of the site. The site is located in Flood Zone 2, which is attributed to tidal flooding. A very small part of the site is located in Flood Zone 3, which is also attributed to tidal flooding. Data obtained from the Environment Agency (EA), suggests that the site is considered safe from present day tidal flooding given the benefit provided by local coastal defences. However, if the anticipated impacts of climate change materialise, the site is shown to be affected by flooding during a 1 in 200 year event. In the 1 in 200 year annual probability (2115) overtopping event flood depths of up to 1.6m are estimated across the site. This design event assumes that the existing defences remain at their current design standard into the future. It is likely that surface water runoff from the site will be discharged to a combined public sewer. Surface water runoff from the site must be managed up to and including the 1 in 100 year rainfall plus climate change event. In order to achieve greenfield runoff rate from the site, it has been estimated that 629m 3 of attenuation storage will be required. The following recommendations should be used to inform the final FRA, prior to submission with a planning application: As part of the development of a site layout plan, pre-application consultation with the EA should be undertaken. This should agree the appropriate design event and the necessary flood mitigation to ensure the safety of the proposed development. A new vehicular access into the site should be positioned outside of the area susceptible to surface water flooding, if possible. Groundwater levels below the site should be determined, if an intrusive site investigation is undertaken. A Flood Risk Management Strategy should be prepared to outline procedures to be undertaken following receipt of an EA flood warning. 18

24 9 References 1. Environment Agency, Geostore. Accessed November 2017 via: 2. British Geological Survey (BGS), Geology of Britain Viewer. Accessed March 2017 via 3. Cranfield University, Soilscapes Interactive Map. Accessed March 2017 via 4. Department for Communities and Local Government, National Planning Policy Framework 5. Department for Communities and Local Government, Planning Practise Guidance: Flood Risk and Coastal Change. 6. North East Lincolnshire, Local Flood Risk Management Strategy. 7. Environment Agency, Flood Map for Planning. Accessed November 2017 via: 8. Environment Agency, Flood Risk Map. Accessed November 2017 via: 9. North and North East Lincolnshire, Strategic Flood Risk Assessment 10. North East Lincolnshire, Preliminary Flood Risk Assessment. 11. Environment Agency, Rainfall runoff management for developments. Accessed November 2017 via pments_-_revision_e.sflb.ashx 12. CIRIA, The SuDS Manual. Accessed November 2017 via 19

25 Anglian Water Sewer Asset Plans

26 (c) Crown copyright and database rights 2017 Ordnance Survey Date: 24/11/17 Scale: 1:1250 Map Centre: , Data updated: 01/10/17 Our Ref: Wastewater Plan A3 This plan is provided by Anglian Water pursuant its obligations under the Water Industry Act 1991 sections 198 or 199. It must be used in conjunction with any search results attached. The information on this plan is based on data currently recorded but position must be regarded as approximate. Service pipes, private sewers and drains are generally not shown. Users of this map are strongly advised to commission their own survey of the area shown on the plan before carrying out any works. The actual position of all apparatus MUST be established by trial holes. No liability whatsoever, including liability for negligence, is accepted by Anglian Water for any error or inaccuracy or omission, including the failure to accurately record, or record at all, the location of any water main, discharge pipe, sewer or disposal main or any item of apparatus. This information is valid for the date printed. This plan is produced by Anglian Water Services Limited (c) Crown copyright and database rights 2017 Ordnance Survey This map is to be used for the purposes of viewing the location of Anglian Water plant only. Any other uses of the map data or further copies is not permitted. This notice is not intended to exclude or restrict liability for death or personal injury resulting from negligence. Foul Sewer Surface Sewer Combined Sewer Final Effluent Rising Main* Private Sewer* Decommissioned Sewer* Outfall* Inlet* Manhole* Sewage Treatment Works Public Pumping Station Decommissioned Pumping Station *(Colour denotes effluent type) Doughty R

27 Manhole Reference Liquid Type Cover Level Invert Level Depth to Invert 0011 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C Manhole Reference Liquid Type Cover Level Invert Level Depth to Invert 9103 C C C C C C C S S S Manhole Reference Liquid Type Cover Level Invert Level Depth to Invert Our Ref:

28 Environment Agency Information

29 Bosanko, Nick From: Coastal L&N, PSO Sent: 15 December :45 To: Bosanko, Nick Cc: Lincs & Northants, Customer Enquiries; Sarah Demetriou Subject: RE: Product 4 Data Duchess Street and Doughty Road Grimsby - CCN/2017/66343 & CCN/2017/66345 Hello Nick The flood zones may be partially informed by undefended rivers in the area but in terms of taking existing defences into account, the sites in question aren t affected by fluvial flooding. Our extents mapping shows that were the River Freshney to the north of the site to flood it would most likely do so on the left bank, away from the area of Grimsby your two sites are in. Kind regards Rob Robert Eames Partnerships and Strategic Overview Officer, Lincolnshire and Northamptonshire Area Environment Agency Ceres House, 2 Searby Road, Lincoln, LN2 4DT rob.eames@environment-agency.gov.uk +44 (0) From: Bosanko, Nick [mailto:nick.bosanko@arcadis.com] Sent: 14 December :26 To: Coastal L&N, PSO <PSO_Coastal@environment-agency.gov.uk> Cc: Lincs & Northants, Customer Enquiries <LNenquiries@environmentagency.gov.uk>; Sarah Demetriou <Sarah.Demetriou@arcadis.com> Subject: RE: Product 4 Data Duchess Street and Doughty Road Grimsby - CCN/2017/66343 & CCN/2017/66345 Hi Rob Thank you for the below. I do have one last query please. All the information you have provided refers to tidal flooding. Are we correct to assume that the two sites are unaffected by fluvial flooding? Many thanks, Nick From: Coastal L&N, PSO [mailto:pso_coastal@environment-agency.gov.uk] Sent: 14 December :58 To: Bosanko, Nick <nick.bosanko@arcadis.com> Cc: Lincs & Northants, Customer Enquiries <LNenquiries@environmentagency.gov.uk>; Sarah Demetriou <Sarah.Demetriou@arcadis.com> Subject: RE: Product 4 Data Duchess Street and Doughty Road Grimsby - CCN/2017/66343 & CCN/2017/66345 Hello Nick Yes, the flood zones are derived through modelling. You are correct in that the levels are projected inland with the flood defences removed from the models. The water levels are taken according to the scenario for flood zone 2

30 Anglian Water Sewer Flood Records

31 Bosanko, Nick From: Anglian Water Sent: 05 December :04 To: Sarah Demetriou Cc: Subject: Doughty Road, GRIMSBY - West Marsh Flood Risk Query Response Sarah Demetriou, Thank you for your Flood Risk Query you submitted for Doughty Road, GRIMSBY - West Marsh. Our response to this is: Anglian Water is able to confirm that we have no records of flooding in the vicinity that can be attributed to capacity limitations in the public sewerage system. It is possible that other flooding may have occurred that we do not have records of, other organisations such as the Local Authority, Internal Drainage Board or the Environment Agency may have records. Should you have any questions relating to this please contact Option 1. Your reference for this enquiry is Kind Regards Growth and Planning Services Team 1

32 Preliminary Surface Water Calculations

33 Arcadis Consulting (UK) Ltd Page 1 HCL House Fortran Road NE Lincs St Mellons Business Pakr Doughty Road Cardiff CF3 0EY Greenfield Date Designed by Jo Evans File Checked by Nick Bosanko XP Solutions Source Control ICP SUDS Mean Annual Flood Input Return Period (years) 100 SAAR (mm) 636 Urban Area (ha) Soil Region Number Region 5 Results l/s QBAR Rural 6.4 QBAR Urban 6.4 Q100 years 22.6 Q1 year 5.5 Q30 years 15.3 Q100 years XP Solutions

34

35 Arcadis (UK) Limited 2 Glass Wharf Temple Quay Bristol BS2 0FR T: +44 (0) arcadis.com