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: [email protected]
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