Flood Risk Assessment - West Lindsey District Council
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Flood Risk Assessment - West Lindsey District Council
Mr Ali Demirkiran Proposed Re-development Trinity Street Gainsborough Lincolnshire Flood Risk Assessment Prepared by EWE Associates Ltd Final RevA September 2011 EWE Associates Ltd Windy Ridge Barn Thealby Lane Winterton Scunthorpe North Lincolnshire DN15 9TG t: 01724 733349 M: 07875 972270 e: lea.favill@eweassociates.com This document has been prepared solely as a Flood Risk Assessment for Mr Ali Demirkiran. EWE Associates Ltd accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared. Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ CLIENT DETAILS Mr Ali Demirkiran 5 Paddock Drive Sunnyside Rotherham S66 3ZE CONTRACT This report describes work commissioned by Mr Ali Demirkiran following written instruction on 31st August 2011. Lea Favill of EWE Associates Ltd carried out the work. Date: 7th September 2011 Prepared by: ……………………………………… ……………………Lea Favill Director REVISION HISTORY Draft Report Rev0 issued 7th September 2011 - 1No copy issued to Mr Ali Demirkiran (client) - 1No copy issued to Mr Thomas Slater (TAS Building Design) Final Report RevA issued 14th September 2011 – minor amendments - 1No copy issued to Mr Ali Demirkiran (client) - 1No copy issued to Mr Thomas Slater (TAS Building Design) _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 2 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ EXECUTIVE SUMMARY The site is presently a two storey building with a retail development located at ground floor level and a residential dwelling at first floor level. There is a small garden area to the rear of the building which is walled along all boundaries. To the front of the building is the pavement which accesses the building from Trinity Street. The paved area is at a level of 5.36mOD. During the day of the site inspection there was evidence of positive drainage in the form of gutters and downpipes to all roof areas and gully’s to all paved areas. As such the built part of the site is considered to be 100% impermeable. The site is located within an existing residential/retail area and is to the south of the Gainsborough town centre. The total site covers an area of 132m². The existing roofed area is 76m² and is considered to be 100% impermeable. The existing ground floor level of the building is 5.413mOD. The proposal involves the redevelopment of the ground floor retail area to provide a fast food unit. The first floor level will remain as a residential dwelling with minor internal modifications. It is considered that the proposed development will not increase the built area and as such it is also considered that the impermeable area will not increase. The development site lies within Zone 3 of the Environment Agency Flood Map (version 2.8.2). The site is within Flood Zone 3a, being the zone with risk of 1 in 100 year (1% Annual Exceedence Probability) or greater for river flooding and 1 in 200 year (0.5% AEP) or greater for tidal/coastal flooding. The development site is within an existing developed area and is less than 1 hectare. The site according to the Environment Agency flood maps benefits from flood defences. The River Trent is defended to the appropriate 1 in 200 year standard and therefore the site is not at risk from the defences overtopping. The River Trent is not defended to the appropriate 1 in 200 year plus climate change standard and therefore the flood defences could be overtopped. The SFRA estimates the 1 in 200 year plus climate change flood level at 7.130mOD which would result in the defences being overtopped by 0.330m. The flood mapping included within the SFRA shows that during this event the site could be inundated. However, there is no indication of the flood depth at the site. The site is located close to the extent of the flood envelope which appears to be in line with Prospect Terrace which is to the east of the site. As such it is assumed that the flood depth would be in the order of 0.5m or less. During a breach event within the River Trent which was modelled as part of the SFRA the breach hazard maps which show that the site is located within an area which would be danger to most if the defences breached. The flood depth at the site is estimated as between 0.5m and 1m and the velocity is between 0 and 0.2m/s. During the extreme 1 in 1,000 year flood event where the flood defences could be overtopped by approximately 0.5m the site is considered to be located within an area which is danger to some. It is proposed that the ground floor part of the building is redeveloped to provide a fast food unit. Further more disabled access will be required into the ground floor building directly off Trinity Street therefore raising of the ground floor level would be impractical. Also existing ceiling heights are marginal which would make raising floor levels impractical. As such it is recommended that the internal ground floor level remains as at present and that the ground floor level is suitable for the intended use. Consideration should be given to flood resilience measures and flood warning to reduce the risk to the building and its occupants during the extreme flood event. The residential part of the building is located within the first floor level and above which will remain dry even during an extreme flood event. It is considered that during the 1 in 200 year plus climate change overtopping and breach scenarios that the site could be flooded. It is also considered that during the extreme 1 in 1,000 year event that the site could also be flooded. The worst case scenario is during a breach event which could result in flood depths of between 0.5m and 1m, hence the site is considered to be located in an area which is danger to most. As such the site will be accessible to the emergency services. The habitable part of the development is within the first floor area which is considered to be a safe area from the flood water. For health and safety reasons there is a preference to evacuate, as failure to do so, would put the sites occupants and others (e.g. rescuers) at risk of injury. Therefore, it is highly recommended that the occupants are advised to devise a flood plan and evacuation route from the site to a safe area within Flood Zone 1, should a flood warning be issued by the Environment Agency. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 3 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ The residents of the new development should be made aware of the potential for flooding from the River Trent and the likely effect this could have on the site. It is noted that during evacuation, the residents should be directed towards Flood Zone 1. The proposed escape route from the site entails directing residents eastwards along the A631 Thorndyke Way adjacent to the railway underpass. It is considered that this area lies within flood zone 1 and is safe from the flood water whilst being accessible in the dry to emergency services. Due to the nature of the proposed development it is concluded that there is a risk of flooding due to extreme overtopping at the proposed development site. The redevelopment provides a good opportunity to install flood proofing techniques and flood warning which will reduce the overall flood risk at the site. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 4 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ CONTENTS 1. INTRODUCTION------------------------------------------------------------------------------------------------------- 7 Terms of Reference --------------------------------------------------------------------------------------------------------------------- 7 Approach to the Assessment---------------------------------------------------------------------------------------------------------- 7 Application of Sequential & Exceptions Test -------------------------------------------------------------------------------------- 8 2. DETAILS OF THE SITE---------------------------------------------------------------------------------------------- 9 Site Location ------------------------------------------------------------------------------------------------------------------------------ 9 Site Details--------------------------------------------------------------------------------------------------------------------------------- 9 Site Description------------------------------------------------------------------------------------------------------------------------- 10 Site Photographs----------------------------------------------------------------------------------------------------------------------- 10 3. INITIAL ASSESSMENT -------------------------------------------------------------------------------------------- 11 Environment Agency Flood Map --------------------------------------------------------------------------------------------------- 11 Past Flooding History ----------------------------------------------------------------------------------------------------------------- 11 SFRA Flooding History --------------------------------------------------------------------------------------------------------------- 11 Environment Agency Flooding History -------------------------------------------------------------------------------------------- 11 Possible Flooding Mechanisms----------------------------------------------------------------------------------------------------- 12 4. FLOOD RISK ASSESSMENT ------------------------------------------------------------------------------------ 13 Requirements of the Environment Agency--------------------------------------------------------------------------------------- 13 River Trent ------------------------------------------------------------------------------------------------------------------------------- 13 5. MITIGATION MEASURES ---------------------------------------------------------------------------------------- 15 Raising Floor Levels/Land Raising------------------------------------------------------------------------------------------------- 15 Emergency Access & Egress ------------------------------------------------------------------------------------------------------- 15 Flood Warning -------------------------------------------------------------------------------------------------------------------------- 15 Flood Resilience Measures---------------------------------------------------------------------------------------------------------- 16 6. CONCLUSION-------------------------------------------------------------------------------------------------------- 18 APPENDICES: APPENDIX A: - EXISTING FLOOR PLANS APPENDIX B: - PROPOSED LAYOUT PLANS LIST OF FIGURES Figure 2.1: Existing Site viewed from Trinity Street. ------------------------------------------------------------------------------------- 10 Figure 3.1: Environment Agency Flood Zones -------------------------------------------------------------------------------------------- 11 Figure 5-1: Flood Warning Map of the area------------------------------------------------------------------------------------------------ 16 LIST OF TABLES Table 1-1: Flood Risk Vulnerability and Flood Zone ‘Compatibility’ -------------------------------------------------------------------- 8 Table 2-1: Location Plan-------------------------------------------------------------------------------------------------------------------------- 9 Table 2-2: Site Details----------------------------------------------------------------------------------------------------------------------------- 9 _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 5 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ Table 4-1 Danger to people from overtopping relative to distance from defence------------------------------------------------- 14 _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 6 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ 1. INTRODUCTION Terms of Reference This report was commissioned by Mr Ali Demirkiran to support a planning application for the redevelopment of the existing building at 168 and 168A Trinity Street, Gainsborough. The building is located the south of the centre of Gainsborough. The location of the site is shown on Table 2-1. The development site lies within Zone 3 of the Environment Agency Flood Map (version 2.8.2). The site is within Flood Zone 3a, being the zone with risk of 1 in 100 year (1% Annual Exceedence Probability) or greater for river flooding and 1 in 200 year (0.5% AEP) or greater for tidal/coastal flooding. The development site is within an existing developed area and is less than 1 hectare. The site according to the Environment Agency flood maps benefits from flood defences. It is usual for the Agency to raise an objection to development applications within the floodplain or Zone 2 or 3 of the flood map until the question of flood risk has been properly evaluated. The Agency will also object to developments where the total site area is in excess of 1 hectare until suitable consideration has been given to surface water runoff. Approach to the Assessment As there are two sources of flood risk – River Trent and surface water runoff – it is necessary to determine flood water levels at the site for the desired return periods emanating from the two sources. Consideration has also been given to the site flooding from ponding and overland flow within the site. AECOM completed a strategic flood risk assessment for West Lindsey District Council during July 2009. The assessment provides additional modelling information especially for the Gainsborough part of the district which has been used within this assessment. The River Trent is Main River; the Environment Agency holds modelled flood data which may assist in predicting the design flood level for the River Trent adjacent to the proposed development site. The proposed development is for a re-development of an existing building located within an impermeable site. The existing building/site is already supported by both roof drainage and drainage to the paved areas directly adjacent to the building. It is considered that the proposal will not result in any significant increase in impermeable area and as such it is considered to be minor. As such no further consideration will be given to this mechanism. A walk over of the site was conducted by Mr Lea Favill, a senior river engineer on 6th September 2011; during the visit a photograph survey of the site and adjacent river was undertaken. A spot level survey of the site was undertaken by EWE Associates Ltd. The survey was related to ordnance survey datum. The requirements for flood risk assessments are generally as set out in Annex E of PPS25. The detail and complexity of the study required should be appropriate to the scale and potential impact of the development. For the purposes of this study, the following have been considered:- Available information on historical flooding in the area. - Site level information. - Details of structures, which may influence hydraulics of the watercourse and consideration of the effect of blockage of structures. - Estimates of design levels, equivalent to a 200-year (coastal/tidal) and a 100-year (fluvial) return period flood event. - Allowances for increased flows resulting from the effects of climate change. - Allowances for sea level rise resulting from the effects of climate change. Assess the existing runoff characteristics and the potential impact the proposed development will have on the runoff. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 7 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ Further guidance is also provided in the CIRIA Research Project 624 “Development and Flood Risk: Guidance for the Construction Industry”. Application of Sequential & Exceptions Test The development site lies partly within Zone 3 of the Environment Agency Flood Map (version 2.8.2), being the zone with risk of 1 in 100 year (1% AEP) or greater for river flooding and 1 in 200 year (0.5% AEP) or greater for tidal/coastal flooding. The proposed development consists of residential at first floor level and as such is considered to be more vulnerable. Table 1-1: Flood Risk Vulnerability and Flood Zone ‘Compatibility’ Flood Risk Vulnerability classification Essential Infrastructure Water compatible Highly Vulnerable More Vulnerable Less Vulnerable Flood Zone Zone 1 Zone 2 Exception Test required Zone 3a Exception Test required Exception Test required Zone 3b Exception Test required Development is appropriate Development should not be permitted It is considered therefore that a sequential test and exceptions test will not be required for this development project. The proposed development involves the rebuilding of the existing building whilst providing considerable betterment and as such is considered as minor as defined by footnote 7 of PPS25 as such the development is exempt from the sequential test. Footnote 7 is shown below:Definition of minor development: – Minor non–residential extensions: Industrial/Commercial/Leisure etc. extensions with a footprint less than 250m2. – Alterations: development that does not increase the size of buildings eg alterations to external appearance. – ‘Householder’ development: eg sheds, garages, games rooms etc. within the curtilage of the existing dwelling in addition to physical extensions to the existing dwelling itself. This definition EXCLUDES any proposed development that would create a separate dwelling within the curtilage of the existing dwelling eg subdivision of houses into flats. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 8 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ 2. DETAILS OF THE SITE Site Location Table 2-1: Location Plan Ordnance Survey Licence no.: WL1005160 Site Details Site Name Existing Land Use Proposed Development Grid Reference County Local Planning Authority Internal Drainage Board Others Post Code Table 2-2: Site Details 168 & 168A Trinity Street Gainsborough Commercial/Residential Retail/Residential SK 81678 89157 Lincolnshire West Lindsey District Council Not Applicable Not Applicable DN21 1JW _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 9 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ Site Description The site is presently a two storey building with a retail development located at ground floor level and a residential dwelling at first floor level. There is a small garden area to the rear of the building which is walled along all boundaries. To the front of the building is the pavement which accesses the building from Trinity Street. The paved area is at a level of 5.36mOD. A photograph of the site viewed from Trinity Street is provided below in Figure 2.1 which shows the current retail use of the ground floor. During the day of the site inspection there was evidence of positive drainage in the form of gutters and downpipes to all roof areas and gully’s to all paved areas. As such the built part of the site is considered to be 100% impermeable. The site is located within an existing residential/retail area and is to the south of the Gainsborough town centre. The total site covers an area of 132m². The existing roofed area is 76m² and is considered to be 100% impermeable. The existing ground floor level of the building is 5.413mOD. The existing floor plans for the ground, first and second floors are provided at Appendix A. The proposal involves the redevelopment of the ground floor retail area to provide a fast food unit. The first floor level will remain as a residential dwelling with minor internal modifications. The detailed plans of the development are provided at Appendix B of this report. It is considered that the proposed development will not increase the built area and as such it is also considered that the impermeable area will not increase. Site Photographs Figure 2.1: Existing Site viewed from Trinity Street. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 10 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ 3. INITIAL ASSESSMENT Environment Agency Flood Map Figure 3.1: Environment Agency Flood Zones Past Flooding History A search on the British Hydrological Society Chronology of British Hydrological Events website1 found no record of past flooding within the Gainsborough area. Although the River Trent has flooded in the past, there are no flood records for flooding since the flood defence improvements which have recently been completed. Undertaking an internet based search for flooding in the area provided no further information. SFRA Flooding History The SFRA contained no references to the site being flooded. Environment Agency Flooding History The Environment Agency provided no further information. 1 http://www.dundee.ac.uk/geography/cbhe/ 5 Development and Flood Risk Guidance for the Construction Industry, CIRIA C624, London 2004 _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 11 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ Possible Flooding Mechanisms As there is a single source of flood risk – River Trent – it is necessary to determine flood water levels at the site for the desired return periods emanating from this source. The first is from the River Trent which is to the west of the proposed development. The River Trent is defended by flood walls which are elevated above the adjacent natural ground level. As such, consideration will only need to be given to overtopping and a breach of the defences. The proposed development is for a change of use and as such the existing impermeable area will not be affected by the proposed development. The existing building is already supported by both roof drainage and drainage to the paved areas directly adjacent to the building. The building footprint is approximately 76m² and as such it is considered to be minor. Furthermore there is no history of localised flooding or reports of sewer incapacity. As such no further consideration will be given to this mechanism. Due to the local topography the site itself is relatively flat. There are no depressed areas which could encourage ponding. There is no higher ground to any side of the site which could promote the overland flow of water across the site. As such both of these mechanisms have not been considered further. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 12 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ 4. FLOOD RISK ASSESSMENT Requirements of the Environment Agency The Environment Agency, as part of its development control procedures, generally require finished floor levels to be set above the 1% AEP plus 20% for climate change flood water level at the site. The development is residential in nature, as such it is considered that access and egress from the development site will be essential during times of extreme floods. River Trent The River Trent is defended by flood walls, which are approximately 150m west of the proposed development site. The flood defences inline with the proposed development have recently been improved with the works completed in the last five years. The works involved the construction of a twin continuous piled wall which provides a 4m wide walkway along the front of the River Trent. Above the walkway the piled defences are extended a further 1m to provide a flood wall which has been cladded with masonry on the land side. The defence are of a robust construction and relatively new. The Environment Agency provided estimated 1 in 200 year water levels for the River Trent inline with the proposed development site. The 1 in 200 year flood level at this location has been estimated at 6.411mAOD. The ground level directly to the rear of the flood wall is at a level of 6.008mOD. The top of the flood wall is at a level of 6.8mOD. The ground floor level of the existing building within the site is at a level of 5.413mOD and Trinity Street in line with the building is at a level of 5.4mOD. Therefore it is considered that the existing flood wall is elevated above the estimated 1 in 200 year flood level and the flood water will remain in channel. Increase in estimated flood level due to Climate Change PPS25 states that ‘…Flood risk assessment should be carried out to the appropriate degree at all levels of the planning process, to assess the risks of all forms of flooding to and from development taking climate change into account. The future users of the development must not be placed in danger from flood hazards and should remain safe throughout the lifetime of the plan or proposed development and land use.’ As the proposed development is a residential land use, consideration has therefore been given to take into account the potential effects of climate change over the next 100 years in accordance with PPS25. Gainsborough is located approximately 20 miles upstream of its confluence with the Humber Estuary. The tidal limit for the River Trent is at Torksey which is only 3 miles upstream of the site. As such it is considered that the tidal influence at Gainsborough would be significantly reduced and therefore the increase in flood level due to climate change is considered to be highly conservative. The SFRA included additional modelling of the River Trent for several climate change scenarios including the 1 in 200 year event. The modelling estimated that the 1 in 200 year plus climate change flood level in line with the A631 Gainsborough Road Bridge would be 7.130mOD. As such the flood defences which are elevated to a level of 6.80mOD could be overtopped by 0.330m during this event. The flood mapping included within the SFRA shows that during this event the site could be inundated. However, there is no indication of the flood depth at the site. The site is located close to the extent of the flood envelope which appears to be in line with Prospect Terrace which is to the east of the site. As such it is assumed that the flood depth would be in the order of 0.5m or less. Breach Failure of the defences The SFRA contains breach modelling for the flood cell within Gainsborough. From the SFRA it appears that the modelling was undertaken with a flood level equivalent to the top of the existing flood wall (6.8mOD) and as such is likely to be less than the 1 in 200 year plus climate change breach scenario. However, due to the scale of the development it is considered that the SFRA breach data is the most current data and as such fit for purpose. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 13 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ The modelling included breach hazard maps which show that the site is located within an area which would be danger to most if the defences breached. The flood depth at the site is estimated as between 0.5m and 1m and the velocity is between 0 and 0.2m/s. It is therefore considered that the site could be safely accessed by the able bodied and the emergency services. Overtopping of the defences during Extreme flood events Consideration has been given to the extreme flood event and its likely impact on the site and its occupants. The site is defended to the 1 in 200 year standard and as such the 1 in 1000 year flood event has been considered. From the SFRA the 1 in 1,000 year plus climate change flood level has been estimated at 7.248mOD which is 0.448m above the top of the flood wall. As such this has been conservatively adopted as the extreme flood level. Adopting the simple approach as recommended in DEFRA report FD2320/TR2 and utilising table 12.1 of that report, the danger classification is considered to be within the danger for some zone, as the site is 150m form the river and the flood water depth at the site is approximately 0.5m. The table is shown below at Table 4-1. Table 4-1 Danger to people from overtopping relative to distance from defence Distance from breach(m) Head above crest level(m) 0.5 1 2 3 100 250 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Danger for some Danger for most Danger for all Note: Table copied from Table 12.1, Section 12, FD2320/TR2 _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 14 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ 5. MITIGATION MEASURES Raising Floor Levels/Land Raising The River Trent is defended to the appropriate 1 in 200 year standard and therefore the site is not at risk from the defences overtopping. The River Trent is not defended to the appropriate 1 in 200 year plus climate change standard and therefore the flood defences could be overtopped. The SFRA estimates the 1 in 200 year plus climate change flood level at 7.130mOD which would result in the defences being overtopped by 0.330m. The flood mapping included within the SFRA shows that during this event the site could be inundated. However, there is no indication of the flood depth at the site. The site is located close to the extent of the flood envelope which appears to be in line with Prospect Terrace which is to the east of the site. As such it is assumed that the flood depth would be in the order of 0.5m or less. During a breach event within the River Trent which was modelled as part of the SFRA the breach hazard maps which show that the site is located within an area which would be danger to most if the defences breached. The flood depth at the site is estimated as between 0.5m and 1m and the velocity is between 0 and 0.2m/s. During the extreme 1 in 1,000 year flood event where the flood defences could be overtopped by approximately 0.5m the site is considered to be located within an area which is danger to some. It is proposed that the ground floor part of the building is redeveloped to provide a fast food unit. Further more disabled access will be required into the ground floor building directly off Trinity Street therefore raising of the ground floor level would be impractical. Also existing ceiling heights are marginal which would make raising floor levels impractical. As such it is recommended that the internal ground floor level remains as at present and that the ground floor level is suitable for the intended use. Consideration should be given to flood resilience measures and flood warning to reduce the risk to the building and its occupants during the extreme flood event. The residential part of the building is located within the first floor level and above which will remain dry even during an extreme flood event. Emergency Access & Egress It is considered that during the 1 in 200 year plus climate change overtopping and breach scenarios that the site could be flooded. It is also considered that during the extreme 1 in 1,000 year event that the site could also be flooded. The worst case scenario is during a breach event which could result in flood depths of between 0.5m and 1m, hence the site is considered to be located in an area which is danger to most. As such the site will be accessible to the emergency services. The habitable part of the development is within the first floor area which is considered to be a safe area from the flood water. For health and safety reasons there is a preference to evacuate, as failure to do so, would put the sites occupants and others (e.g. rescuers) at risk of injury. Therefore, it is highly recommended that the occupants are advised to devise a flood plan and evacuation route from the site to a safe area within Flood Zone 1, should a flood warning be issued by the Environment Agency. The residents of the new development should be made aware of the potential for flooding from the River Trent and the likely effect this could have on the site. It is noted that during evacuation, the residents should be directed towards Flood Zone 1. The proposed escape route from the site entails directing residents eastwards along the A631 Thorndyke Way adjacent to the railway underpass. It is considered that this area lies within flood zone 1 and is safe from the flood water whilst being accessible in the dry to emergency services. Flood Warning The proposed development site is within the current Environment Agency flood warning area as detailed on the Environment Agency Website and shown below in Figure 5-1. Due to the nature of the development consideration should be given to including the Development on the Environment Agency’s AVM (Automatic Voice Messaging) _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 15 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ system or the new Multi-media flood warning system. A warning would give occupants the opportunity to move to a safe area during extreme events. Figure 5-1: Flood Warning Map of the area Flood Resilience Measures Consideration should be given to flood proofing the building to a level equivalent to the estimated breach level to reduce the residual damages if an extreme flood was to occur. Flood proofing is a technique by which buildings are designed to withstand the effects of flooding. There are two main categories of flood proofing, which are dry proofing and wet proofing. Dry proofing methods are designed to keep water out of the building, and wet proofing methods are designed to improve the ability of the property to withstand the effects of flooding once the water has entered the building. Both would be required in this case, due to the possible failure of the dry proofing methods. Where wet proofing is required it is important that a flood response plan should be prepared and practised regularly, so that any contents of the building can be moved to design flood level if required or are built to withstand immersion in water or are designed to be easily replaceable. The differential pressures across load bearing walls and the flotation effect that will occur during flood events should be taken into account when considering dry proofing techniques. For most existing properties this means that dry flood proofing should only be considered if the expected flood depth is under 0.5m, which in this particular case is expected to be between 0.5m and 1m in depth during a breach event and as such dry proofing is unlikely to be successful. It is therefore considered that flooding in excess of 0.5m will cause some damage to the building; however, this is considered to be acceptable as the ground floor will be non-habitable. The following table summarises the recommendations for flood proofing measures which can be incorporated within the design of buildings5: _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 16 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ Feature Considerations To Improve Flood Proofing External Walls Careful consideration of materials: use low permeability materials to limit water penetration if dry proofing required. Avoid using timber frame and cavity walls. Consider applying a water resistant coating. Provide fitting for flood boards or other temporary barriers across openings in the walls. Internal Walls Avoid use of gypsum plaster and plasterboards; use more flood resistant linings (e.g. hydraulic lime, ceramic tiles). Avoid use of stud partition walls. Floors Avoid use of chipboard floors. Use concrete floors with integrated and continuous damp proof membrane and damp proof course. Solid concrete floors are preferable; if a suspended floor is to be used, provide facility for drainage of sub-floor void. Use solid insulation materials. Fitting, Fixtures and Services If possible, locate all fittings, fixtures and services above design floor level. Avoid chipboard and MDF. Consider use of removable plastic fittings. Use solid doors treated with waterproof coatings. Avoid using double-glazed window units that may fill with flood water. Use solid wood staircases. Avoid fitted carpets. Locate electrical, gas and telephone equipment and systems above flood level. Fit antiflooding devices to drainage systems. Demountable flood defences External doorways should be fitted with demountable flood barriers which are 500mm high. This will reduce the ingress of flood water significantly. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 17 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ 6. CONCLUSION Due to the nature of the proposed development it is concluded that there is a risk of flooding due to extreme overtopping at the proposed development site. The redevelopment provides a good opportunity to install flood proofing techniques and flood warning which will reduce the overall flood risk at the site. _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 18 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ Appendix A: - Existing Floor Plans _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 19 Mr Ali Demirkiran Proposed Re-development, Trinity Street, Gainsborough – Flood Risk Assessment Final Report RevA September 2011 Reference: 2011/817 _________________________________________________________________________________________________________________ Appendix B: - Proposed Layout Plans _________________________________________________________________________________________________________________ EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire, DN15 9TG T: 0845 8377783 20