PE21 7JE LEVEL 2 FLOOD RISK ASSESSMENT

Transcription

1 IE14/014/ymc Proposed Marstons Public House Swineshead Road Wyberton Fen Boston Lincolnshire PE21 7JE LEVEL 2 FLOOD RISK ASSESSMENT SUFFOLK: Registered Office: 7 Museum Street Ipswich Suffolk IP1 1HQ T F E ipswich@chick.co.uk (Registered No: ) ESSEX: 8 Atlantic Square Station Road Witham Essex CM8 2TL T E witham@chick.co.uk NORFOLK: 23 St Stephens Road Norwich NR1 3SP T F E norwich@chick.co.uk

2 FLOOD RISK ASSESSMENT [LEVEL 2 SCOPING STUDY] Proposed Restaurant / public house Swineshead Road Wyberton Fen Boston Lincolnshire PE21 7JE Client: Marston s Inns & Taverns PLC 1

3 Document Control Report prepared by: Y M Crowther HNC RMaPS On behalf of JPC Environmental Services A Division of JP Chick & Partners Limited 7 Museum Street Ipswich IP1 1HQ Signed: Dated: 23 / 09 / 2014 Report reviewed by: R M Crowther AIEMA On behalf of JPC Environmental Services A Division of JP Chick & Partners Limited Signed: Dated:..23 / 09 / ISSUE RECORD Issue Date Details Initial / 09 / 2014 Report issued in electronic format to: Barnes Construction - Andy Bates JWA Architects Stewart Prior Cerda Planning Michael Robson RMC 2

4 CONTENTS DOCUMENT CONTROL 1.0 EXECUTIVE SUMMARY 2.0 INTRODUCTION 2.01 Brief 2.02 Scope 2.03 Location 2.04 Site Description 2.05 Development Proposal 3.0 BACKGROUND TO FLOOD RISK AND REGULATORY CONTEXT 3.01 Current Guidance 3.02 Sequential Test 3.03 Exception Test 4.0 DESK STUDY 4.01 Sources of Information 4.02 Potential Sources of Flooding 4.03 Environment Agency 4.04 Boston Strategic Flood Risk Assessment (Aecom, 2010) 4.05 River Witham Catchment Flood Management Plan Boston Flood Alleviation Project 4.07 The Wash Shore Management Plan 4.08 Envirocheck Flood Screening Report 5.0 SURFACE WATER DRAINAGE 5.01 Current Drainage Strategy 5.02 Future Drainage Strategy 5.03 Pre & Post Development Runoff Rates 3

5 5.04 Borehole Permeability Testing 6.0 RESIDUAL RISK 6.01 General 6.02 Flood Warnings 6.03 Safe Access & Egress 6.04 Flood Evacuation 6.05 Compensatory Storage 7.0 SUMMARY / RECOMMENDATIONS 7.01 Potential Flood Risk 7.02 Recommendations APPENDICES A Site Location Plan B Topographical Survey C Architect s Layout Plan D Environment Agency Correspondence & Flood Modelling Outputs E Key extracts from the Boston SFRA F River Witham Catchment Flood Management Plan 2009 G Key extracts from the Wash Shore Management Plan H Envirocheck Flood Screening Report I Pre & Post Development Runoff Rates J Infiltration Test Results K Flood Resilient Construction Techniques Information Sheet 4

6 EXECUTIVE SUMMARY Site Name: Site Address: Client: Architect: Present Site Use: Proposed Site Use: Marston s Inns & Taverns PLC JWA Architects Agricultural / horticultural land Construction of new restaurant / public house with association car parking Objectives: To ascertain the potential extent of fluvial / tidal flood zones and explore whether these might impact on either the site or likely routes of access and egress. To make recommendations in respect of any or flood mitigation measures or drainage improvements that may be required to minimise the impact of the development. To make recommendations on additional investigations or hydraulic modelling, if considered necessary to better assess the level of risk. Sequential Test: According to the Environment Agency s indicative flood map, the site is located within Flood Zone 3 and therefore has an annual probability of 0.5% or greater, of flooding in any given year. This is classified as a High Probability of flooding. However it should be noted that this assessment does not take into account the protection afforded by existing flood defences. The Strategic Flood Risk Assessment (SFRA) undertaken in 2010 by Aecom Water for Boston Borough Council also places the site within flood zone 3. In terms of sensitivity, the development proposal is classified as More Vulnerable. Findings: Site levels range between 2.11m aod and 2.27m aod. The principal source of flood risk is tidal, via the nearby River Witham and the South Forty Foot Drain. The site benefits from a number of flood defences, offering protection against both fluvial and tidal flood events. While not considered to be formally defended the river channel offers a nominal level of protection rated at 1% (1 in 100 yrs). The tidal defences offer a varied standard of protection rated at 2% (The Haven) and 0.67% (1 in 150 yrs) around The Wash. The SFRA report confirms that the most significant risk of flooding is from a tidal event overtopping and/or a breach these earth embanked defences. The most significant historical flood event occurred in Whilst this impacted land to the East, the site itself remained unaffected. 5

7 According to the South Forty Foot Drain Model (September 2009) the key fluvial flood levels are 2.69m (1 in 100) and 2.73 (1 in 1,000). These figures include an allowance for climate change. According to the Northern Area Tidal Model (2006) the key Still Water tidal flood levels are 5.93m (1 in 200) based on a breach at Grand Sluice and 6.27 (1 in 1,000) based on a breach at Hobhole. These figures do not include an allowance for climate change. RMS modelling indicates that the site is NOT at risk from surface water (pluvial) flooding during the 1 in 75 or 1 in 100 yr events, but that the more extreme 1 in 1,000 yr event would result in the flooding of lower lying aspects (approx. 50% of the site). The Environment Agency RFfSW mapping identifies a small area of Low Risk to the west of the site. The Environment Agency have no record of any historical flooding on-site or in the immediate area. The proposed building will have a design life of 75 years, and therefore the climate change allowance factored into the fluvial modelling (100 years) makes the levels conservative. Searches indicate that there is no risk of groundwater flooding affecting the site. The Boston Flood Alleviation Project will ensure that the defences are maintained and kept up to a good standard. The Wash Shoreline Management Plan recommends a policy of Hold the Line for the length of shoreline around The Wash. This will ensure that the coastal frontage is continually protected. The site is supported by a good road infrastructure, and a comprehensive network of public transport. These provide excellent opportunities for safe access and egress. Risk Assessment: Hazard mapping provided by the Environment Agency suggests a minimum hazard rating of Danger to Most, increasing to a Danger for All when considering the more extreme events and impact of climate change. Based on recent local developments, a typical finished floor level of 3.30m aod may be required, to mitigate the residual risk of flooding. This offers a freeboard of 570mm above the 1 in 1,000 year fluvial flood level, however this is 1.03m above current ground levels. It is not considered feasible to fully protect against the predicted tidal flood levels. According to the SFRA, the site would not be at risk of flooding unless climate change is applied and or flood defences breached. In terms of surface water run-off, the proposals will increase the rate of run-off by l/s for a 1 in 100 year storm event. The volume of runoff associated with a 1 in 100 year storm of 6 hour duration will increase by 263m 3. The location of the site is on agricultural land accessed from a roundabout on the A52. In terms of safe access and egress during an extreme flood event, the site and 6

8 surrounding area are within the extent of expected flooding. However if warning is received early enough then it should be possible to reach areas of safety North of the site within a few minutes. Recommendations: Based on the information gathered as part of this Level 2 Scoping Study, JPC Environmental Services would make the following recommendations: - A finished floor level of not less than 3.03m aod is recommended, however further consultation is required with the local authority as nearby development have adopted floor level of 3.30m aod. The new restaurant / public house should be registered with Flood line Warnings Direct, the free flood warning service provided by the Environment Agency. A site specific Flood Response Plan should be formulated in consultation with the Emergency Planning Officer, and adopted. Staff should be trained in the evacuation procedures and need to be made aware of the warning / evacuation process. Due to the risk of on-site flooding, it would be prudent to consider the inclusion of some basic flood resilience measures such as: - Piled foundations - Where timber framed construction is adopted, knock-out panels should be included to provide improved opportunities for drying out the fabric of the building. - Non-absorbent insulation should be used within all floor and wall construction. - The electrical consumer unit should be locating at first floor level with wiring descending to the ground floor. - Electrical sockets / telecom and/or data ports should be a minimum of 1.0m above floor level. 7

9 2.0 INTRODUCTION 2.01 Brief JPC Environmental Services were appointed by Marston s Inns & Taverns Plc to undertake a Level 2 Flood Scoping Study in relation to the construction of a new Restaurant and Public House. Hereafter this will be referred to as the site The purpose of this desk based research was to: To identify potential sources and probable extent of flooding, and determine the likely risk of such events occurring. To determine the extent of the flood zones within the site by evaluating the topographical survey and the outputs from a 2-D model held by the Environment Agency. To make recommendations regarding the extent / type of flood resilient construction methods and a safe / sustainable floor level. To assess the potential impact of the development on surface water run-off and make recommendations on how this can be mitigated This Flood Risk Assessment, written in accordance with and meeting the requirements of, Planning Policy Statement 25 (PPS 25) Development and Flood Risk, has been requested by the local planning authority, (Boston Borough Council) and will be submitted to the Environment Agency for their comments / approval Authority to carry out this work was by written instruction from Barnes Construction Ltd This report shall be for the private and confidential use of Marston s Inns & Taverns, for whom it was undertaken. It should not be reproduced in whole or in part, or relied upon by a third party for any use without the express written authority of JPC Environmental Services Scope The main elements of the investigation were as follows: - To establish the potential extent of fluvial / tidal flood zones on site. To explore whether these might adversely impact either the site or likely routes of access and egress. 8

10 To explore any history of pluvial or surface water flooding not directly associated with a watercourse. To make recommendations in respect of any or flood mitigations measures or drainage improvements that may be required to minimise the impact of the development. To make recommendations on additional investigations or hydraulic modelling if considered necessary Location Address: Swineshead Road Wyberton Fen Boston PE21 7JE Grid References: Easting: m Northing: m Ordnance Survey Tile: TF A detailed map of the location is presented within Appendix A Site Description The site, which is situated West of Boston, currently comprises a parcel of agricultural field The site is located adjacent to a nearby roundabout and is bounded by the A52 Swineshead Road to the North, Old Hammond Beck to the South East and a Nursery to the West The site is located on the edge of a mixed commercial/residential area, to the West of Boston town centre and South of the A The land to the North has been recently developed to provide a number of commercial / retail units, further North of this area is the South Forty Foot Drain (Black Sluice Navigation) Land to the South beyond Old Hammond Beck is Agricultural. Whilst land immediately to the West is agricultural, further to the West there are a number of 9

11 established residential properties and commercial ribbon development either side of the A52. The land to the East adjacent to Old Hammond Beck Bank Road also comprises residential ribbon development A recent topographical survey of the site, provided by the client s Architect indicates that ground in the area of the proposed building footprint varies between 2.11m aod and 2.27m aod A copy of the survey is presented within Appendix B Development Proposal We understand that it our client s intention to develop the site as a restaurant / public house. The two storey building will be positioned to the West of the site, with customer parking to the South East. See extract below. Extract from architect s layout drawing While much of the land to the South East of the Marston s restaurant will be laid to hard surfaced car park, the land immediately around the building will be soft 10

12 landscaped, with a patio / beer garden to the North and a children s play area to the East The car park will provide parking for approximately 58 cars including 3 disabled spaces. There will also be a dedicated yard area for deliveries to the side The ground floor will be entirely laid out as a restaurant, bar and kitchen facilities, with the first floor flat providing a small office, staff facilities and living accommodation for the Site Manager The site access will be situated to the North East of the new restaurant, off an existing roundabout on the A52. This is located on the southern bank of New Hammond Beck, due South of Chain Bridge For more detailed information on the layout of the proposed development, a copy of which is presented within Appendix C. 11

13 3.0 BACKGROUND TO FLOOD RISK AND REGULATORY CONTEXT 3.01 Current Guidance In relation to flood risk, planning policy in England is currently guided by the new National Planning Policy Framework (NPPF) and the associated guidance relating to flood risk, published in March 2012, replacing Planning Policy Statement 25: Development and Flood Risk (PPS 25) The purpose of this planning framework is to ensure that flood risk issues are taken in to account at every stage of the planning process and that new residential and commercial developments are steered towards less vulnerable locations (zone 1) in preference to higher risk areas (zone 3) At a district or county level this takes the form of a Strategic Flood Risk Assessment, which seeks to inform local government development strategies. At this level potential development sites are classified in terms of their suitability for various types of future use. Where land in flood zone 1 is not available this is based on the identified level of risk, coupled with the sensitivity of the end use At all levels this policy relies on a series of predicted flood zones, which are defined by the Environment Agency. These zones are: - Zone 3b Functional flood plain annual probability of flooding >5%, or affected by flood events having a return period of up to 1 in 20 years Zone 3a area affected by floods with an annual probability of >1% fluvial flooding or >0.5% tidal. Return periods of up to 1 in 100 year fluvial or 1 in 200 year tidal flooding. Zone 2 Extreme flood plain with an annual probability of between 1% - 0.1% fluvial or 0.5% - 0.1% tidal. Return periods of up to 1 in 100 and 1 in 1000 for fluvial flooding or between a 1 in 200 and 1 in 1000 for tidal flooding. Zone 1 Outside the flood plain Land with <0.1% probability of tidal or fluvial flooding. Return periods greater than 1 in every 1,000 years In addition to exploring the potential risk and impact of flooding on the development, the site specific FRA s are required to assess the potential impact of the development on existing sites and the local hydrology. This is designed to ensure that new developments, which typically include extensive areas of impermeable surfacing, do not exacerbate flooding elsewhere. 12

14 3.02 Sequential Test The new NPPF advises local authorities, developers and consultants to follow a sequential, risk based approach to identifying land suitable for development At the strategic level local authorities zone areas for appropriate development, for example More Vulnerable uses such as residential development are preferred in flood zone 1, while Less Vulnerable development such as commercial premises may be appropriate in flood zone 2, where zone 1 sites are not available. Flood zone 3 is typically reserved for Water Compatible activities PPS 25 advises that When seeking planning permission for individual developments on sites allocated in development plans through the application of the Sequential Test, informed by a SFRA, developers need not apply the Sequential Test, but should apply the sequential approach to locating development within the site In the case of windfall sites, which have not been considered as part of the wider SFRA process, it may be necessary for developers and consultants to provide evidence that the Sequential Test has been undertaken. Such evidence must demonstrate that the type of development selected is appropriate to the level of potential flood risk pursuant to the site, and that alternative available sites have been considered The Environment Agency s flood map places the site within flood zone 3a. The proposed use of the site for restaurants would lead to a classification of Less Vulnerable, and would be suitable within the flood zone. However, the inclusion of residential accommodation increases the classification to More Vulnerable. It should be noted however that this is ancillary to the principal use as a food orientated restaurant / public house. Previous discussions with the Environment Agency have suggested that this may be suitable, however such case are decided at a local level, taking account of multiple factors. Extract from Technical Guidance to the NPPF March

15 3.03 Exception Test In the event that lower risk sites are not available then the Exception Test must be satisfied. This test provides a method for managing flood risk while still allowing necessary development to take place. There are three elements to the Exception Test, all of which must be satisfied These are: a) Sustainability it must be proven that the development confers wider benefits to community at large that outweigh the potential flood risk b) Brownfield land the site should be developable, previously developed land or alternatively there are no reasonable alternative previously developed sites available c) Safe a site specific FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere and where possible will reduce flood risk overall As the site is situated within flood zone 3, according to the Environment Agency s flood map, the NPPF indicates that the Exception Test may need to be satisfied. (see table below). Extract from NPPF Technical Guidance 14

16 While the restaurant aspect of the proposed development is acceptable within the flood zone, the living accommodation is considered to be higher risk In line with the sequential approach the more vulnerable accommodation has been located at first level above the restaurant. The residential aspects will therefore remain safe and dry during even the most extreme 0.1% probability (1 in 1,000 year + CC) flood event. 15

17 DESK STUDY 4.01 Sources of Information As part of the desk based research, JPC Environmental Services consulted the following sources of information: Environment Agency s indicative flood mapping Product 3 & 8 information supplied by the Environment Agency Boston Strategic Flood Risk Assessment (Aecom, 2010) River Witham Catchment Flood Management Plan Boston Flood Alleviation Project The Wash Shoreline Management Plan British Geological Survey online reference mapping Envirocheck Flood Screening Report 4.02 Potential Sources of Flooding In line with guidance contained in National Planning Policy Framework (NPPF) and the retained Practice Guide to Planning Policy Statement 25 (PPS 25), we have explored the various potential sources of flooding which could impact the site both before and after the proposed development. This assessment has identified and will go on to evaluate the following flood risk issues: Fluvial flooding Tidal flooding South Forty Foot Drain (Black Sluice Navigation) Old & New Hammond Becks Via the River Witham Via South Forty Foot Drain Boston Haven (& Witham Outfall Channel) Wash Banks Surface water flooding (pluvial flooding) Fluvial Flooding Fluvial flooding occurs when excessive rainfall takes place within a river s catchment resulting in high levels of run-off, which in turn causes water levels within the river to rise. This can occur over an extended period or, in the case of flash flooding, due to 16

18 intense rainfall over a relatively short duration. When the level of surface water runoff exceeds the river s capacity this generally leads to localised overtopping or in some cases one or more breaches may occur In this instance the nearest watercourses are Old Hammond Beck, New Hammond Beck and South Forty Foot Drain. Old Hammond Beck is the closest and forms the site s South East boundary, flowing from South West to North East. New Hammond Beck, which flows from West to East, is located on the opposite side of the A52, to the North of the site. The largest of the watercourses is the South Forty Foot Drain which runs broadly parallel to the New Hammond Beck While the close proximity of these watercourses suggests that fluvial flood events may have a significant impact on the site, the flow within the drains is artificially managed, via a series of pumping stations. These pumps are controlled by local Internal Drainage Board. Many of the drains in this part of the country are hundreds of years ago, having been constructed to drain the low lying fenland As a result of their managed status, water levels within these watercourses is not prone to significant fluctuation and therefore we do not consider these to be the primary or most significant source of flood risk. Tidal Flooding Tidal flooding occurs when an exceptionally high tide, almost always accompanied by a storm tide surge, overtops and/or breaches the tidal defences along the coastline or tidal estuary Direct tidal flooding tends to produce higher flood levels than fluvial events and there is an increased likelihood of hard or earth embanked sea defences being overtopped or breached, particularly when water levels rise quickly, allowing floodwaters to pour inland. In this instance Boston is located within the tidal stretches of The Haven and the River Witham and approximately 10-12km from the Wash. Due to the low lying nature of the site and the close proximity of the coast, there is a relatively high probability that the site may become flooded during an extreme event The potential extent of tidally influence flooding can also be exacerbated by the inability of the River Witham to discharge via its tidal outfall at Boston (Grand Sluice). The River Witham s discharge can be restricted for significant periods of time during a high tide. Such events can threaten both coastal and inland areas. 17

19 Although, this assessment is confirmed by the Environment Agency s indicative flood mapping it is worth noting that during the most severe flood event of recent times, in 1953, the site and Boston area was not affected. However, Boston was affected by the more recent storm surge event of December 2013, although again the site remained dry Tidal flooding is subsequently considered to be the most likely source of significant flood risk, to affect the site, and will be explored in more detail within subsequent sections of this report. Surface Water (Pluvial) Flooding Pluvial flooding is closely related to Fluvial flooding, in that it typically occurs when excessive rainfall occurs within a catchment, to such an extent that it is unable to be absorbed by the underlying soils. Water that is unable to soak in to the soil accumulates and migrates in line with the local topography, eventually making its way the nearest watercourse or more usually in built up areas the nearest storm sewer The site is situated in a relatively rural location, with large expanses of permeable greenfield land around it. While the underlying geology is typically fine grained alluvial silts / clays which are not especially permeable, the landscape is crisscrossed with drainage to direct surface water quickly into the network of Main Drains Because of the Borough s relatively flat topography, many of the local drainage systems are either pumped into the receiving watercourse, or discharge under gravity into a watercourse (Drain) which is itself pump-drained We therefore consider that surface water flooding does not represent as significant source of potential flood risk, as fluvial or tidal flooding Environment Agency This section of the report contains Environment Agency information Environment Agency and database rights The Environment Agency is responsible for the maintenance of all Main Rivers, for keeping records of significant fluvial and tidal flood events, and for establishing an indicative flood map, which displays the extent of anticipated flooding in four distinct 18

20 probability zones. These zones were previously described in section 3, paragraph To confirm the extent of both fluvial and tidal flood zones JPC Environmental Services have consulted with the Environment Agency and received both a detailed flood map and a series of modelled flood levels. See image below The above map extract shows that the site is located within tidal flood zone 3. The annual risk of flooding is therefore >0.5% (1 in 200 yrs) from the sea or 1% (1 in 100 yrs) from fluvial flooding. It should be noted however that the above map does not take into account the protection afforded by established flood defences in the area and as such is very much the worst case scenario The fluvial flood levels associated with the above model nodes are set out in the table below. These are measured in metres above Ordnance Datum Newlyn (modn) 19

21 The above table shows that the current maximum 1 in 100 year flood level is considered to be 2.73m aod. During the more extreme 1 in 1,000 year return period, the maximum flood level has been modelled at 2.76m aod. These figures include climate change, based on a default design life of 100 years This flood level would result in approximately 0.5m of flooding within the site In addition to fluvial levels, the Environment Agency has included Tidal flood levels derived from the North East Coastal Model. These levels, which incorporate a number of coastal and inland breach scenarios, are set out below: When comparing the undefended levels to the on-site ground levels, it is evident that during even the 0.5% probability event (1 in 200 year the design level), the site could potentially be inundated to a depth of 3.82m In addition to the model outputs, the Environment Agency has provided condition information regarding the various fluvial and tidal flood defences There are no formal fluvial flood defences protecting the site, but the natural channel is maintained by the Environment Agency providing a nominal protection against a flood event with a 1% change of occurring any year (1 in 100). The channel is inspected regularly and any potential defects rectified The Haven tidal defences protecting the site consist of earth embankments. They are in good condition and provide protection again a flood event with a 2% chance of occurring in any year (1 in 50). The defences are inspected regularly to identify potential defects early The Wash tidal defences protecting this site consist of earth embankments. They are in good condition and provide protection against a flood event with a 0.67% chance of occurring in any year (1 in 150). We inspect these defences regularly to ensure that any potential defects are identified early The Environment agency have issued to us modelled breach hazard mapping for the area which is in line with the Strategic Flood Risk Assessment which is discussed in more detail later in this report. 20

22 In relation to historical flood events, the Environment Agency have advised that they hold no records of any flooding in this area A copy of the information received from the Environment Agency is presented within Appendix D Boston Borough Council Strategic Flood Risk Assessment Aecom Water (October 2010) Boston Borough Council commissioned an update to the 2002 SFRA to account for the impact of climate change. This contains mapping, in accordance with PPS While PPS 25 has itself since been withdrawn, the climate change allowances ascribed to the East Coast during a series of 25 year tranches, have been adopted by its successor, the National Planning Policy Framework (NPPF). The Practice Guide associated with PPS 25 also remains valid guidance for the assessment of flood risk The SFRA states that due to the location of Boston being 10km West of the Wash Coastline, lying on the River Witham and Boston Haven (the river s tidal outfall channel) together with various drainage channels, Boston is at risk of both fluvial and tidal / coastal flooding. However, of these, tidal flooding from the overtopping and breaching of defences is the most significant source of flood risk Due to the land being reclaimed from the sea, there are numerous drainage channels throughout this area, see extract from SFRA study area below: The Site Figure 1 Significant watercourses in Boston 21

23 Relative Probability of Flooding. Within the SFRA, hydraulic modelling of the 1% (1 in 100 year) and 0.1% (1 in 1000 year) fluvial flood event and 0.5% (1in 200 year) and 0.1% (1 in 1000 year) tidal flood event was carried out to determine the relative probability of flooding at the proposed development site taking present flood defences into account. Flooding was assumed to occur when a flood embankment overtopped. Where the defence was an earth embankment it was assumed that breaching would follow. Figure 2 indicates that the site is classified as having a low relative probability of tidal flooding taking present flood defences into account. The Site FFigure 2. Relative Flood Risk Map taken from Boston Borough Council SFRA (2010). Residual Risk of Flooding Residual risks of flooding arise from either extreme events with exceptionally high return periods (200+ years) or events which, due to their unpredictable nature, are not readily amenable to quantitative analysis, for example premature structural failure, serious operational or equipment failures or freak accidents which cannot be foreseen. The principal residual flood risk in the Boston area would be a result of premature failure of the flood embankments well before the defences were overtopped The Catchment Flood Management Plan (CFMP) advises that tidal flooding from the Haven and overtopping of the tidal defences could inundate large parts of Boston. Within the Boston Borough Council SFRA, a 2-D model was developed for the 2115 climate change scenario to estimate the impact of a breach. 22

24 The resulting future Flood Hazard Mapping (Figure 3) shows the entirety of the proposed development site is classed as being at high risk of residual flooding with danger for most should a breach of the present flood defences occur. The report notes that this represents a worst case scenario and cannot be attributed to a particular magnitude event. The Site Figure 3 - Flood Hazard Map (for 200 year event plus climate change) taken from Boston Borough Council SFRA (2010) Key extracts from the SFRA are presented within Appendix E 4.05 River Witham Catchment Flood Management Plan (2009) The probability of river and tidal flood risk has been reduced significantly in the Boston area through the construction of defences; however the area is still at substantial risk from tidal flooding due to the overtopping of these defences. There is also a flood risk associated with the network of inland rivers and drains. Flood risk measures rely heavily on raised embankments, tidal doors to prevent tidal waters entering inland and the Black Sluice Pumping Station. Historical Flooding Records show that during the tidal surge in December 2013 Boston Haven area was flooded due to a rare tidal storm event flooding approximately 300 homes. Storm surges are created when strong winds push water up against a coastline. Low 23

25 atmospheric pressure associated with weather systems causes the sea level to rise even further. A storm surge in the North Sea can raise the average sea level by 2 3 metres over a large area. Serious coastal flooding often occurs when storm surges coincide with large spring tides. The most recent storm surge event, which occurred in December 2013, did not reach the site Key extracts from the CFMP are presented in Appendix F Boston Flood Alleviation Project Around the Wash, earth embankments give a standard of protection rated at 0.67% or 1 in 150 years. Currently the tidal storm defences in Boston provide a standard of protection for a tidal event of just 2% in any year (or 1 in 50 years). The Environment Agency is currently planning a flood alleviation scheme for Boston, consisting of a multifunctional barrier combined with the refurbishment of existing defences The Flood alleviation scheme has been developed as follows: Phase 1 - Black Sluice Lock completed March Connects River Witham (Lincoln) to Black Sluice Navigation (Boston to Donington Bridge) via 35km of new navigation. Phase 2 - Boston Barrier, improving navigation safety and reliability and reducing tidal flood risk. Phase 3 - Donington Bridge to Surfleet Seas End Link, connecting Black Sluice Navigation to RiverGlen. Feasibility work underway. Phase 4 - Surfleet to Crowland Link, enhancing connection from River Glen to River Welland. Phase 5 - Crowland to Peterborough Link, connecting River Welland to River Nene Phase 6 - Nene-Ouse Link, improving connection across the Middle Level Navigations (Peterborough to Ely, Cambridge and Bedford) The 90.2million multi-functional Boston Barrier will increase the level of protection to 0.33% or storms having a return period of 1 in 300 years (annual chance for the next 70 years). It will also provide safe inland navigation during the boating season (April to October) by retaining daytime low tide water levels upstream of the barrier. The barrier will not retain water levels during a tidal or fluvial flood event Work will be undertaken on the banks of the Haven downstream of the Barrier, to raise existing flood defences. Some of the existing downstream defences, like sea banks, are already better than those in Boston and there is no immediate need to 24

26 raise these defences. The strategy has also identified the need to raise the defences along the Haven in approximately 60 years time to reduce the risk of flooding from future sea level rises caused by climate change In 2011 there was extensive public consultation to decide the preferred location for the Boston Barrier. As a result, there was overwhelming support for the location just downstream of Black Sluice lock The Government Department for the Environment, Food and Rural Affairs (Defra) has reviewed Boston Barrier Strategy and given it permission to move to the next stage. This means the flood defence scheme, which will reduce tidal flood risk to 20,000 properties in Boston, remains on track to be completed by The Wash Shoreline Management Plan (SMP) The Wash SMP covers the area of coastline from Gibraltar Point to Old Hunstanton. This plan has taken into account the Humber Estuary coastal Authorities Group SMP2, the Old Hunstanton to Kelling Hard SMP2 and the Catchment Flood Management plans which assess the management of flood risks from rivers The Wash SMP covers from Gibraltar Point to North West King s Lynn the shoreline is characterised by a sea bank that divides salt marsh and mud flat from extensive low-lying agricultural land. Most of the agricultural land has been reclaimed from the sea over past centuries, and contains almost 50% of England s grade one and 10% grade two land. It supports a regionally and nationally valuable agriculture industry Following finalisation by the Environment Agency, local authorities and consultants, the Wash SMP was published in The coastline was separated into policy units, and each unit of managed coastline was evaluated and assigned one of four policy options, which were: - Hold the existing line Advance the line Managed realignment No active intervention The policy unit affecting the site is Policy Zone 1: River Steeping at Gibraltar Point to Wolferton Creek. Based on established settlements, habitation and land use around The Wash, the SMP is to continue to defend against tidal flooding. The risk is greater than for the other Policy Development Zones in this Plan and for other SMPs around 25

27 the country. It is beyond the scope of this Plan to determine a required standard of protection, but it can make decisions about the relative level of flood risk in the face of climate change. For this Policy Development Zone therefore, one of the critical decisions is whether to sustain the existing activity level (accepting gradual increase of risk), increase the activity level to sustain the existing level of risk, or aim to reduce flood risk. Sea level rise and potential future loss of foreshore width and height would increase pressure on the defences In the short-term, the policy option is to hold the existing sea bank alignments. In the medium and long-term, ideally the existing alignments should continue to be held. However there is a chance that climate change will cause a significant loss of salt marsh and mud flat in front of the sea banks. If this occurs, localised landward realignment should be considered as an alternative to holding the line. Extract from The Wash SMP Key extracts from the Wash SMP are presented in Appendix G British Geological Survey According to British Geological Survey online reference mapping the bedrock geology of the site lies within Ampthill Clay Formation Mudstone, Sedimentary Bedrock formed approximately million years ago in the Jurassic Period. Local environment previously dominated by shallow seas. The Superficial deposits are: Tidal Flat Deposits, 1- Clay and silt. Superficial deposits formed up to 3 million years ago in the Quaternary Period. Local environment previously dominated by shoreline Envirocheck Report Until recently, the Environment Agency did not hold information on surface water flooding. To address this shortfall, and obtain useful information on flood related insurance claims, JPC Environmental Services purchased an Envirocheck Flood Report. The insurance claims information is supplied by Crawford & Co. 26

28 The RMS dataset, which explores the potential of surface water flooding, uses a combination of land height, predicted rainfall and other variables to predict areas that are likely to be affected by surface water flooding during three key return periods, 1 in 75 years, 1 in 100 years and 1 in 1,000 years. This resulting information is displayed in map form of the attached report, which is presented in Appendix H The RMS Flood Risk mapping for the 1 in 75 year storm event shows that the proposed site is not at risk from surface water flooding. This nearest flooding is an area to the South. There are areas subject of surface water flooding adjacent to the South Forty Drain North of the site as well as North East in the residential areas of Boston. 1 in 75 year return period During a 1 in 100 year storm event, the extent of pluvial flooding around the site is likely to increase, although the site remains dry. The flooding area to the North of the South Forty Foot Drain has become more widespread, although immediately North (new supermarket) remains dry. 1 in in year year return period 27

29 During the more extreme 1 in 1,000 year storm event, the extent of pluvial flooding has again increased and is expected to impact the majority area, with the exception of a few isolated pockets of dry land to the North including the new supermarket. The site is now clearly shown within the flooded area, with approximately 50% of the site affected. 1 in 1,000 year return period The Flood Risk Location Map within the appendices of the report is an extract of the EA flood mapping and as a consequence shows that the site is situated within flood zone In addition to the flood modelling data, the report also includes details of flood related insurance claims within the locality, highlighting areas where water damage may have occurred. The report advises that the overall flood risk rating for the site is Low Finally, within section of the report covering British Geological Survey information, the report advises that there is no risk of groundwater flooding within the search area. The report also notes that the underlying geology suggests that the site is vulnerable to coastal flooding. 28

30 5.0 SURFACE WATER DRAINAGE 5.01 Current Drainage Strategy This site is undeveloped Greenfield land, and therefore has no formal drainage system other than the network of land drainage ditches Future Drainage Strategy To minimise the risk associated with new areas of impermeable surfacing, new developments are required to take into consideration the increased rate and volume of surface water run-off The new development would be expected to limit surface water discharges to Greenfield rates of run-off, via the use of sustainable drainage techniques, in line with a sustainable drainage hierarchy Below is a review of the key drainage techniques recognised as part of the sustainable drainage hierarchy, below: Rainwater harvesting by storing rainwater for later use potential discharges are reduced as is the use of treated tap water. Unfortunately rainwater harvesting remains a fairly expenses option and one best used on developments where non-potable water usage is high water. This approach has been dismissed. Infiltration techniques solutions such as permeable paving or soakaways rely on granular soils, where this ample opportunity for infiltration. The underlying soils in this site makes this unsuitable. Attenuation (above ground) temporary storage of rainwater in ponds or open water features for gradual release to a nearby watercourse. Such ponds present environmental / habitat benefits in addition to their water management characteristics. Attenuation (below ground) storage tanks or sealed water features with gradual release to a watercourse less beneficial to the wider environment but often required where space is at a premium Discharge rainwater direct to a watercourse Initial consultation with the Inland Drainage Board suggests that this approach would not be permitted. Discharge rainwater to a surface water drain Not available Discharge rainwater to the combined sewer Not permitted 29

31 The preferred methods for managing any site s surface water discharge retain the water within the site. This is either for re-use, or in the case of soakaways and permeable paving, makes use of natural infiltration to channel surface water into the underlying soils where it can slowly recharge groundwater resources at depth Unfortunately the cohesive nature of the underlying geology, comprising tidal flat deposits over Amphill Clay (Mudstone) make infiltration based techniques unsuitable We understand that the preferred solution will incorporate an attenuated discharge, although a detailed drainage scheme is to be subject to the findings of this report Pre & Post Development Runoff Rates The Pre-development run-off rates have been calculated as follows (based on a 15 minute storm): 1 in 1 year = 0.8 l/s 1 in 30 year = 2.3 l/s 1 in 100 year = 3.3 l/s based on no impermeable surfacing The Post development run-off rates have been calculated as follows (based on a 15 minute storm): 1 in 1 year = l/s 1 in 30 year = l/s 1 in 100 year = l/s based on 2885m2 of impermeable surfacing The detailed calculations for both pre and post development run-off rates are presented within Appendix I The additional areas of impermeable surfacing will increase the 1 in 100 year surface water discharge by l/s In terms of the volume of run-off arising from a 1 in 100 year storm, of 6 hour duration, the post-development catchment has the potential to produce an increased discharge of 263m 3. 30

32 5.04 Infiltration Testing In August 2014, Harrison Group were commissioned to carry out a ground investigation, comprising trial pitting and deep percussive boreholes As part of this combined geotechnical and geo-environmental investigation, two of the trial pits were selected for in-situ infiltration testing, in accordance with BRE Digest Each trial pit was extended to a depth of 2.10m, and filled with gravel to prevent collapse. Water levels were recorded at key intervals. Over a period of 26 hours, the water level in TP6 dropped 340mm, while the level in TP4 dropped by just 130mm in 25 hours. Both of these tests failed to meet the minimum Building Control requirement of 50% in 24 hours As neither test achieved 75% empty it was not possible to calculate an infiltration rate in accordance with BRE 365. Based on these results it is evident that the on-site soils within this location are of low permeability, and therefore not suitable for infiltration techniques such as soakaways A copy of the infiltration test results are presented in Appendix J. 31

33 6.0 RESIDUAL RISK 6.01 General While the site is located in close proximity to several IDB drains, including the South Forty Foot Drain, these watercourses form part of an artificially managed system. The main residual risks associated with these are: a failure of the pumping infrastructure; an inability for them to discharge in the tidally influenced river / estuary downstream, due to tide levels; and the route they provide for tidal flood waters to flow inland Of these, the most significant risk is associated with tidal flooding While this risk is mitigated by established defences, there remains an unquantifiable risk of tidal flooding via a defence failure or overtopping. This risk will also increase over time, as sea levels rise The breach scenarios modelled for the Environment Agency indicate that the site could see flooding between 0 and 0.25m during the design (1 in 200 yr) flood event. During the more extreme (1 in 100 yr) plus climate change flood event, the potential depth of flood waters on-site could reach m Flood Warnings In addition to the existing flood defences the residual risk outlined above is mitigated by an automated flood warning service provided by the Environment Agency. The site is situated within the flood warning area known as Tidal flooding in areas to the south of Boston, including Chain Bridge, Wyberton, Frampton and Skeldyke All registered properties receive advanced warning of adverse weather conditions and particularly high tides that might result in possible flooding The site is located within the Flood Alert area shown overleaf. 32

34 The Site Environment Agency Flood Warning areas While the amount of advanced warning is not guaranteed, the service aims to provide flood warnings up to 6 hours in advance of an impending flood, based on prevailing weather patterns and known tide levels Flood line Warnings Direct issues automated flood warnings via telephone, , fax and text message. The public house will therefore receive advance warning of potential flooding, allowing them to maintain a period of increased awareness or evacuate the site as necessary It should be noted that, according to the Environment Agency s website, the flood status for this area last changed in December See below. 33

35 The current status can be obtained via the link below: Safe Access & Egress As a result of the site s inland location, well behind the coastal defences, it is almost certain that any significant flood event will be preceded by news of flooding along the East Coast, well before floodwaters reach the site Unfortunately due to the low lying nature of Lincolnshire countryside, most of the surrounding road infrastructure is also situated within the predicted flood plain. However, by the time a flood warning is received it is unlikely that floodwaters will have encroached onto the highway, and therefore safe egress from the site should be possible The site benefits from almost immediate access to the A52, located to the North of the site, which itself provides quick access to the A17 and higher ground to the West. See map extract below. The Site Customers and staff should exit the site onto the roundabout, and take the first exit onto the A52. After 5.5miles they will reach a crossroads with the A17, at which they 34

36 should turn right and then continue in a north westerly direction out of the flood warning area Flood Evacuation JPC Environmental Services have produced this FRA as part of the planning application process as requested by Boston Borough Council We would also recommend that a Flood Response Plan is produced to assist in the safe and organised evacuation of the site, should a flood warning be received. It is likely that this will form part of a planning condition from the Local Planning Authority Compensatory Storage There is no requirement for compensatory storage on this site as the principal source of flooding is tidal, via The Haven & South Forty Foot Drain. Due to the vast potential volumes associated with tidal flooding, it is generally deemed inappropriate Mitigation Measures As this risk assessment has identified a residual risk of significant flooding, it is recommended that flood resilient construction techniques are incorporated within the construction of the new building. These measures will be designed to protect the occupants of the Manager s apartment, reduce the extent of water damage and facilitate any post-flood repairs / drying out In terms of finished floor levels it is usual for these to be based on the 1% inc cc fluvial design level, plus a freeboard of 300mm. This would result in a finished floor level of 3.03m aod. However we understand from reviewing recent developments in the area, that a floor level of 3.30m aod has been required. While 3.30m would afford an enhanced freeboard of 570mm, it is below the predicted tidal flood levels of 5.93m aod (0.5% inc cc) to 6.35m 0.1% inc CC) In relation to the office / living accommodation at first floor level, a floor to ceiling height 2.85m is shown. This places the first floor at a height of 6.15m aod. It is therefore recommended to incorporate flood resilient construction for the entire ground floor providing protection close to the 0.1% inc cc level. 35

37 The flood resilient construction techniques recommended for this development will include the following: - Piled foundations with a 225mm concrete beam and block suspended floor has been adopted to provide robust floor construction. The resulting sub-floor void will incorporate a 50mm concrete blinding laid to falls over 1200mm gauge polythene. This blinding will be laid at a gradient of 1 in 150 towards gullies and drainage channels which pass through the adjacent sleeper walls. This sub-floor drainage terminates in a modest sump, so that any flood water penetrating beneath the new building can be pumped out if necessary. Where timber fixings are considered to be at risk from exposure to flood water, stainless steel connectors and fixings will be specified. In terms of the timber frame, Acetylated wood products will be utilised for all ground floor studwork as an alternative to the traditional C16 timber studs. This will reduce the risk of fungal decay if moisture content of the frame remains above 20% for a significant period, following internal flooding. As masonry will be used to enclose the timber framed building it will not be possible to gain access to the internal wall construction from the building s exterior, in the event that drying out is required. However, the ground floor timber frame design will incorporate a series of separately flamed knock out panels. In the event of a significant flood, which results in internal flooding, these knock out panels can be removed to improve access for drying out or for total replacement of the knock out panel itself. The knock out panels will be sheathed with exterior grade plywood (EN636-3 to BSEN636) rather than OSB3 to prevent distortion due to water absorption. Between the external sheathing and the interior wall lining the inter-stud insulation will be a third party approved close cell insulation product rather than mineral wool. While paper faced gypsum plasterboard will be utilised for the internal wall lining, this will laid horizontally and considered a sacrificial finish. In the event of internal flooding internal disturbance and reconstruction will then be minimised. 36

38 Non-absorbent, closed cell insulation will be utilised within all floor and wall construction to a height of at least 6.15m aod. The electrical consumer unit will be located at first floor level with wiring descending to ground floor. Associated sockets will be located a minimum of 1.0m above the finished floor level to further reduce the risk of water damage. The proposed finished floor level will be set a minimum of 3.30m aod with external pavement and ground levels no greater than 3.15m aod where they abut the building. In drawing up the above measures we have consulted with the Communities and Local Government report: Improving the Flood Performance of New Buildings Flood Resilient Construction, which was published in May 2007; and the TRADA Information Sheet WIS2/3-64: Timber Frame Design for Flood-Prone Sites, published May Consideration should be given to the inclusion of a flood resilient floor covering, such as ceramic tile or stone to reduce the future cost of any flood damage Please note that the above is not an exhaustive list. A comprehensive list of flood resilient construction methods, which should be considered during the detailed design, is presented within Appendix K. 37

39 7.0 SUMMARY / RECOMMENDATIONS Summary of Potential Flood Risk The site is at risk from tidal flooding, which may arise as overtopping or a breach in the current coastal flood defences When comparing the modelled flood levels to the existing on-site ground levels, it is evident that the depth of external flooding could reach a maximum depth of m during an extreme tidal event, although hazard mapping indicates a more likely depth of between 1.0 and 2.0m for the 0.1% event (2115) The additional potential risk of surface water flooding, associated with the development s new areas of impermeable surfacing will be mitigated by an attenuated discharge In the event of a flood warning, the business will close to the public and the site will be evacuated. This will be facilitated by a Flood Response Plan. In the event that staff or customers become trapped by incoming flood waters, the Manager s apartment will provide a safe haven until flood waters recede Recommendations Based on the information gather as part of this desk based risk assessment, JPC Environmental Services would recommend the following: A finished floor level of not less than 3.03m aod is recommended, however further consultation is required with the local authority as nearby developments have adopted floor level of 3.30m aod. The new restaurant / public house should be registered with Floodline Warnings Direct, the free flood warning service provided by the Environment Agency. A site specific Flood Response Plan should be formulated in consultation with the Emergency Planning Officer, and adopted. Staff should be trained in the evacuation procedures and need to be made aware of the warning / evacuation process. Due to the risk of on-site flooding, it would be prudent to consider the inclusion of some basic flood resilience measures such as: - Piled foundations (reduced risk of scour) 38

40 - Where timber framed construction is adopted, knock-out panels should be included to provide improved opportunities for drying out the fabric of the building. - Non-absorbent insulation should be used within all floor and wall construction. - The electrical consumer unit should be locating at first floor level with wiring descending to the ground floor. - Electrical sockets / telecom and/or data ports should be a minimum of 1.0m above floor level The opinions and recommendations expressed within this report are based on the results of desk based research and information provided by third party agencies. No additional hydraulic modelling has been undertaken as part of this assessment. 39

41 IG14/062 APPENDIX A Site Location Plan

42 JPC Environmental Services (A division of J P Chick & Partners Limited) 7 Museum Street Ipswich Suffolk IP1 1HQ Project: New Marston s Restaurant / Public House Swineshead Road BOSTON Lincs Date: Sept 2014 Drawing Number: JPC ES SK01 Scale: NTS Job Number: IG14/062 Drawing Title: Site Location Plan