AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 1: EASTFIELDS ESTATE, MITCHAM
1) SFRA SITE AREA
Site ID 1
Site Address Eastfields Estate, Mitcham
Site Area 5.88 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
Part of the council’s emerging Estates Local Plan
Current Use Residential (C3)
Proposed Use Residential (C3)
Vulnerability More Vulnerable
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The River Graveney is located approximately 1km to the north east of the site. The site is within Flood Zone 1, which is defined as
Low Probabiltiy of flooding from rivers.
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding area
Surface Water
flooding:
(uFMfSW)
High Risk
1 in 30 year (3.33% AEP)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater
flooding:
(BGS Susceptibility to
Groundwater
Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Parts of the current site and surrounding area are shown to be at high risk of surface water flooding. The central part of the site is at
low risk of surface water ponding. The areas at higher risk of surface water ponding are within the cul de sac areas of the estate.
There is also ponding to the east and south of the site.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple
source
records
Other
0 0 0 4 0 0
(Contains Ordnance Survey data © Crown copyright
and database right 2016. Contains Environment
Agency data © Environment Agency and database
right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 1: EASTFIELDS ESTATE, MITCHAM
3) RECOMMENDATIONS
In accordance with the NPPF, More Vulnerable development is considered compatible within Flood Zone 1 and does not require the
application of the Exception Test. However, given the risk of surface water flooding to this site, the principles of the Exception Test
should still be considered when developing on this site, namely:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The following information and recommendations are therefore provided for consideration.
Development
Layout and
Sequential
Approach
An assessment of surface water flow paths should be made prior to site design, to
encourage the location of buildings and more vulnerable aspects of the development away
from those areas at risk of surface water ponding.
Measures to manage surface water on the site using above ground storage should be
considered early in the site masterplan to enable inclusion of attenuation SuDS where
possible.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate
mitigation (BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements,
basement extensions and conversions, such as plant, car parking etc, must provide safe
internal access to higher floors situated above levels derived from the fluvial modelling.
Further ground investigations, such as site specific boreholes, would be required at this site
to confirm the likelihood of groundwater occurrence.
Section 9.2
Flood
Resistance
Where there may be a future risk of surface water flooding on the site, flood resistant
construction measures may be employed, such as raising property thresholds, and the use
of landscaping to manage surface water and fluvial floodwater to reduce the risk of
floodwater entering properties.
Section 9.4
Flood Resilience Where parts of proposed buildings may be affected by surface water floodwaters, e.g.
undercroft parking areas, flood resilient design techniques should be employed to minimise
damage to buildings and structures. The use of concrete flooring and waterproof building
materials could be considered.
Section 9.5
Flow Routing Potential overland flow paths should be determined and appropriate solutions proposed to
minimise the impact of the development, for example by configuring road and building
layouts to preserve existing flow paths and improve flood routing, whilst ensuring that flows
are not diverted towards other properties elsewhere.
Section 9.12
Surface Water
Management
Current risk of flooding
The site is located within Critical Drainage Area (CDA) Group7_012, which is an area with
localised flooding issues. The potential development must not increase flood risk to other
areas, especially within in the CDA.
The site is within Drainage Catchment 23, which covers the east part of the borough, and
drains towards the River Wandle and Colliers Wood. This area may have historically drained
to the Figges Marsh ditch.
The uFMfSW indicates that the site and surrounding area is at high risk of surface water
flooding. The uFMfSW identifies ponding within and surrounding the site. There are reported
incidents of sewer flooding from Thames Water within the post code area in which the site is
located.
Indicative existing runoff rate: 27.2l/s (1 in 1 year), 102.2l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 11.8l/s
Level 2
Appendix B
SuDS Suitability
Reference to the Surface Water Management Plan Appendix C2 Figure 4 identifies that
infiltration of surface water into the ground is potentially uncertain and requires further
investigation prior to the development of a Drainage Strategy for the site.
Techniques which should be considered include green roofs, filter strips, detention basins
and ponds, as well as permeable surfacing in combination with tanked systems.
The proposed development for Eastfields Estate should actively consider the creation of a
small linear park on the north eastern boundary of the estate, to incorporate a swale or linear
water feature to be facilitated by the deculverting of the existing historic watercourse that
flows underground in this vicinity, as stated in the London Borough of Merton Draft Estates
Local Plan1.
Section 10.3
and 10.9
1 London Borough of Merton (2016) Draft Estates Local Plan, Stage 2 Consultation (1st February 2016 – 18th March 2016)
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 1: EASTFIELDS ESTATE, MITCHAM
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the Mayor’s Design and Construction SPG (2014) and Merton’s Local Plan
policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the preferred
standards set out in the London Plan policy 5.13 and Design and Construction SPG (April
2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be
made and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Permeable paving ~ £30-50/m2.
Concrete storage tank £449-518/m3.
Section 10.4
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 2: LONESOME PRIMARY SCHOOL
1) SFRA SITE AREA
Site ID 2
Site Address Grover Road, Mitcham, CR4 1SD
Site Area 1.81 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
No
Current Use Primary school (D1)
Proposed Use School/ school facilities (D1)
Vulnerability More Vulnerable (Educational Establishment)
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
Lonesome Primary School is at risk of flooding from the 0.1% AEP (1 in 1000 year) flood event of the River Graveney, which is
approximately 1km to the northeast. At this point the River Graveney flows from the southeast to the northwest as mostly an open
channel; there are a few sections that are culverted.
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
0 % 0 % 17 % 83 % 0 %
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding area
Surface Water
flooding:
(uFMfSW)
Medium Risk
Between 1 in 100 year
(1% annual probability)
and 1 in 30 year (3.3%
annual probability)
The uFMfSW data does not show the susceptibility
of individual properties to surface water flooding.
The uFMfSW also does not take into account the
details of the existing drainage system.
Groundwater
flooding:
(BGS Susceptibility to
Groundwater
Flooding)
Medium Risk
Potential for
groundwater flooding
to occur at surface, but
no historic records of
groundwater flooding
The dataset cannot be used on its own to indicate
risk of groundwater flooding and should not be
used to inform planning decisions at a site scale. It
is suitable for use in conjunction with a large
number of other factors, e.g. records of previous
incidence of groundwater flooding, to establish
relative risk of groundwater flooding.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 1 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 2: LONESOME PRIMARY SCHOOL
3) RECOMMENDATIONS
In accordance with the NPPF, More Vulnerable development is considered compatible within Flood Zone 1 and 2, and does not
require the application of the Exception Test. However, given the risk of surface water and groundwater flooding to the area
surrounding the site, the principles of the Exception Test should still be considered when developing on this site, namely:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The following information and recommendations are therefore provided for consideration.
Development
Layout and
Sequential
Approach
This site partially intersects Flood Zone 2 of the River Graveney; the rest of the site is Flood Zone
1. The site is currently used for primary education and therefore is classed as More Vulnerable.
New building development (e.g. classrooms, educational facilities) should avoid the areas of
Flood Plain 2 where possible and build in Flood Zone 1. The Water Compatible facilities of the
school (e.g. sports ground, recreational ground) can be allocated anywhere in the floodplain.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate mitigation
(BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements, basement extensions
and conversions, such as plant, car parking etc, must provide safe internal access to higher
floors situated above levels derived from the fluvial modelling. Further ground investigations,
such as site specific boreholes, would be required at this site to confirm the likelihood of
groundwater occurrence.
Section 9.2
Finished Floor
Levels
This site does not intersect with the 1% AEP (1 in 100 year) flood event with an allowance for
climate change, therefore with regard to policy there is no requirement for finished floor levels.
Furthermore, there is no set guidance for the setting of finished floor levels of development in
relation to surface water flood risk. However, the site is at medium risk of surface water flooding
and there has been one recorded surface water flood previously. Therefore it is good practice to
raise finish floor levels a minimum of 300mm above the 1% AEP (1 and 100 year) flood event,
especially where More Vulnerable development intersects the floodplain.
Section 9.3
Safe
Access/Egress
In the event of a 0.1% AEP (1 in a 1000 year) event it is possible that Grove Road will be be
flooded in the west of the site, therefore occupants of the site should evacuate to the east of
Grove Road.
Section 9.7
Surface Water
Management
Current risk of flooding
The site is located within Critical Drainage Area (CDA) Group7_012, which is an area with
localised flooding issues. The potential development must not increase flood risk to other areas
in the CDA.
The site is within Drainage Catchment 23, which is predominantly within London Borough of
Merton, and which drains much of Mitcham into the River Graveney. The uFMfSW indicates that
the site and surrounding area is at medium risk of surface water flooding. There is one reported
incidents of surface water flooding held by Merton Council in this location.
Indicative Existing Runoff Rate: 8.4l/s (1 in 1 year), 31.4l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 5l/s
Level 2
Appendix B
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a site
investigation to determine precise local conditions) infiltration of surface water into the ground is
uncertain at this site location and requires further investigation. This will need to be confirmed
prior to the development of a Drainage Strategy for the site.
If infiltration SuDS are not appropriates then techniques which should be considered include
green roofs, filter strips, detention basins and ponds, as well as permeable surfacing in
combination with tanked systems.
If infiltration SuDS are appropriate then infiltration basins and/or trenches could be considered.
Section 10.3
and 10.9
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 2: LONESOME PRIMARY SCHOOL
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the Mayor’s Design and construction SPG (2014) and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Infiltration trench £55-65 /m3.
Infiltration basin £10-15 /m3.
Section 10.4
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 3: ST MARK’S ACADEMY, MITCHAM
1) SFRA SITE AREA
Site ID 3
Site Address St Mark’s Church of England Academy, Acacia Road, Mitcham, CR4 1SF
Site Area 6.741 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
No
Current Use School (D1)
Proposed Use School/ school facilities (D1)
Vulnerability More Vulnerable
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The site is at risk of flooding from the River Graveney.
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
0 % 0 % 16 % 84 % 0 %
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding area
Surface Water
flooding:
(uFMfSW)
Medium Risk
Between 1 in 100 year
(1% annual probability)
and 1 in 30 year (3.3%
annual probability)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater
flooding:
(BGS Susceptibility to
Groundwater
Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 2 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 3: ST MARK’S ACADEMY, MITCHAM
3) RECOMMENDATIONS
In accordance with the NPPF, More Vulnerable development is considered compatible within Flood Zones 1 and 2 and does not
require the application of the Exception Test. However, given the risk of fluvial and surface water flooding to this site, the principles
of the Exception Test should still be considered when developing on this site, namely:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The following information and recommendations are therefore provided for consideration.
Development
Layout and
Sequential
Approach
The northern part of the development site intersects with Flood Zone 2 of the River
Graveney. The rest of the site is Flood Zone 1. The site is currently used for a school which is
classed as More Vulnerable. It is important to direct new development away from the
floodplain and within Flood Zone 1 where possible.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate
mitigation (BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements,
basement extensions and conversions, such as plant, car parking etc, must provide safe
internal access to higher floors situated above levels derived from the fluvial modelling.
Further ground investigations, such as site specific boreholes, would be required at this site
to confirm the likelihood of groundwater occurrence.
Section 9.2
Finished Floor
Levels
The site is above the 1% AEP (1 in 100 year) flood event including an allowance for climate
change, therefore there are no requirements in terms of policy for finished for levels.
However, it may be prudent to consider raising finished floor levels or threshold levels,
especially as the site is at medium surface water flood risk and there are two records of
previous surface water flood incidents in the area.
Section 9.3
Safe
Access/Egress
In the scenario of a very extreme event access/egress via Grove Road will be limited;
therefore occupants of the site should evacuate via Acacia Road. Section 9.7
Surface Water
Management
Current risk of flooding
The site is located within Critical Drainage Area (CDA) Group7_012, which is an area with
localised flooding issues. The potential development must not increase flood risk to other
areas in the CDA.
The site is within Drainage Catchment 23, which is predominantly within London Borough of
Merton, and which drains much of Mitcham.
The uFMfSW indicates that the site and surrounding area is at medium risk of surface water
flooding. There are two reported incidents of flooding held by Merton Council in this
location.
Indicative Existing Runoff Rate: 31.1l/s (1 in 1 year), 116.7l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 13.5l/s
Level 2
Appendix B
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a
site investigation to determine precise local conditions) infiltration of surface water into the
ground is potentially uncertain and requires further investigation.
If unsuitable the techniques which should be considered include green roofs, filter strips,
detention basins and ponds, as well as permeable surfacing in combination with tanked
systems. If infiltration SuDS are suitable then infiltration basins and/or trenches could be
considered.
Section 10.3
and 10.9
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the mayor’s Design and construction SPG and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London
Plan policy 5.13 and Design and Construction SPG (April 2014), then justification must be
provided.
Arrangements for the future maintenance and adoption of the drainage system must be
made and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 3: ST MARK’S ACADEMY, MITCHAM
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Infiltration trench £55-65 /m3.
Infiltration basin £10-15 /m3.
Section 10.4
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITES 4, 5, 6 & 7: SIBTHORPE CAR PARK, FAIR GREEN EAST, RALEIGH GARDENS CAR PARK, LOVE LANE
1) SFRA SITE AREA
Site ID 4 5 6 7
Site Address Sibthorpe Car Park, Mitcham Fair Green East Raleigh Gardens Love Lane
Site Area 0.29 ha 0.52 ha 0.10 ha 0.39 ha
Local Plan:
Site allocation or
designation (Sites
and Policies Plan
2014)
Site proposal 69: No No No
Current Use Mix of town centre uses including some car parks
Proposed Use Mix of town centre uses including some car parking
Vulnerability More Vulnerable
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The sites are all located within Flood Zone 1, which is defined as Low Probabiltiy of flooding from rivers.
Flood risk from all other sources Limitations
Risk of flooding to
the potential
development site and
surrounding area
Surface Water flooding:
(uFMfSW)
Medium – High Risk
1 in 100 year (1% AEP) - 1 in
30 year (3.33% AEP)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater flooding:
(BGS Susceptibility to
Groundwater Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Parts of the current site and surrounding area are shown to be at high risk of surface water flooding. There is ponding of water in the
central part of the site and against the buildings. There is also ponding to the east and south of the site.
Historic records of flooding
Historic records of flooding
from each source within a
100m radius of potential
development site
Fluvial
records
Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 4 0 1 1 0
(Contains Ordnance Survey data © Crown
copyright and database right 2016.
Contains Environment Agency data ©
Environment Agency and database right
2016).
4
5
6
7
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITES 4, 5, 6 & 7: SIBTHORPE CAR PARK, FAIR GREEN EAST, RALEIGH GARDENS CAR PARK, LOVE LANE
3) RECOMMENDATIONS
In accordance with the NPPF, More Vulnerable development is considered compatible within Flood Zone 1 and does not require the
application of the Exception Test. However, given the risk of surface water flooding in the area in which these sites are located, the
principles of the Exception Test should still be considered when developing each site, namely:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The following information and recommendations are therefore provided for consideration.
Development
Layout and
Sequential
Approach
An assessment of surface water flow paths should be made prior to the design of each site, to
encourage the location of buildings and more vulnerable aspects of the development away from
those areas at risk of surface water ponding.
Measures to manage surface water on the sites should be considered early in the site
masterplans to enable inclusion of attenuation SuDS where possible.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate mitigation
(BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements, basement
extensions and conversions, such as plant, car parking etc, must provide safe internal access to
higher floors situated above levels derived from the fluvial modelling. Further ground
investigations, such as site specific boreholes, would be required at this site to confirm the
likelihood of groundwater occurrence.
Section 9.2
Flood
Resistance
Where there may be a future risk of surface water flooding on the sites, flood resistant
construction measures may be employed, such as raising property thresholds, and the use of
landscaping to manage surface water and fluvial floodwater.
Section 9.4
Flood
Resilience
Where parts of proposed buildings may be affected by surface water floodwaters, e.g.
undercroft parking areas, flood resilient design techniques should be employed to minimise
damage to buildings and structures. The use of concrete flooring and waterproof building
materials could be considered.
Section 9.5
Flow Routing Potential overland flow paths should be determined and appropriate solutions proposed to
minimise the impact of the development, for example by configuring road and building layouts to
preserve existing flow paths and improve flood routing, whilst ensuring that flows are not
diverted towards other properties elsewhere.
Section 9.12
Surface Water
Management
Current risk of flooding
The sites are within Drainage Catchment 24, which covers the east part of the borough, and
drains towards the River Wandle and Colliers Wood.
The uFMfSW indicates that the sites and surrounding area is at high risk of surface water
flooding. The uFMfSW identifies ponding within and surrounding the sites. There are reported
incidents of sewer flooding from Thames Water within the post code area in which the sites are
located.
Sibthorpe Car Park
Indicative existing runoff rate: 1.3 l/s (1 in 1 year), 5 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 5 l/s
Fair Green East
Indicative existing runoff rate: 2.4 l/s (1 in 1 year), 9 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 5 l/s
Raleigh Gardens Car Park
Indicative existing runoff rate: 0.5 l/s (1 in 1 year), 1.8 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 5 l/s
Love Lane
Indicative existing runoff rate: 1.8 l/s (1 in 1 year), 6.8 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 5 l/s
Level 2
Appendix B
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that infiltration of surface water into the
ground is potentially uncertain and requires further investigation prior to the development of a
Drainage Strategy for the site.
Techniques which should be considered include green roofs, filter strips, detention basins and
ponds, as well as permeable surfacing in combination with tanked systems.
Section 10.3
and 10.9
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITES 4, 5, 6 & 7: SIBTHORPE CAR PARK, FAIR GREEN EAST, RALEIGH GARDENS CAR PARK, LOVE LANE
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the mayor’s Design and construction SPG (2014) and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Permeable paving ~ £30-50/m2.
Concrete storage tank £449-518/m3.
Section 10.4
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 8: GRACE BUSINESS CENTRE, WILLOW LANE
1) SFRA SITE AREA
Site ID 8
Site Address Willow Lane, Mitcham, Greater London, CR4 4TQ
Site Area 17.91 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
Designated Employment Area
Current Use Employment / waste management uses (B / sui generis Use Classes)
Proposed Use Employment / waste management use (B / sui generis use class)
Vulnerability Less Vulnerable (Commercial)
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The River Wandle flows to the southwest of the development site. There are various tributaries and ordinary watercourses
surrounding the river at this point, all of which flow in open channels.
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
0 % 0 % 40 % 60 % 0 %
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding area
Surface Water
flooding:
(uFMfSW)
Medium Risk
Between 1 in 100 year
(1% annual probability)
and 1 in 30 year (3.3%
annual probability)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater
flooding:
(BGS Susceptibility to
Groundwater
Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 0 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 8: GRACE BUSINESS CENTRE, WILLOW LANE
3) LEVEL 2 ASSESSMENT
The fluvial hazard, depth and velocity outputs used in the Level 2 SFRA assessment and mapped below are based on the Environment
Agency modelling of the River Wandle (2015) and are provided for the 1% AEP plus Climate Change event2.
Flood Hazard Rating
Maximum Flood Depth
Maximum Velocity
2 It is important to note that any future site specific FRA would be required to assess this site against the updated climate change allowances (London
Borough of Merton Level 2 SFRA, Section 1.4).
(Contains Ordinance Survey data © Crown copyright and database right
2015. Contains Environment Agency data © Environment Agency and database right 2015).
(Contains Ordinance Survey data © Crown copyright and database right 2015. Contains Environment Agency data © Environment Agency and database right 2015).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 8: GRACE BUSINESS CENTRE, WILLOW LANE
4) RECOMMENDATIONS AND POLICIES
In accordance with the NPPF, Less Vulnerable development is considered compatible within Flood Zone 1 and 2, and does not
require the application of the Exception Test. However, given the risk of fluvial and surface water flooding to the area surrounding the
site, the principles of the Exception Test should still be considered when developing on this site, namely:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The following information and recommendations are therefore provided for consideration.
Development
Layout and
Sequential
Approach
Almost half the site is within Flood Zone 2 of the River Wandle, the rest is defined as Flood Zone
1. The area of greatest hazard is towards the south of the site. The site is likely to remain for
commercial purposes, therefore Less Vulnerable. Nonetheless, the most vulnerable aspects of
the re-development should be located on the north eastern part of the site, in Flood Zone 1,
while accepting that the site is not in single ownership and will be developed separately. Where
possible, the least vulnerable developments, such as car parks and public open space should be
located to the southwest, in Flood Zone 2.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate mitigation
(BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements, basement extensions
and conversions, such as plant, car parking etc, must provide safe internal access to higher
floors situated above levels derived from the fluvial modelling. Further ground investigations,
such as site specific boreholes, would be required at this site to confirm the likelihood of
groundwater occurrence.
Section 9.2
Finished Floor
Levels
There is a small section in the south that is within the outline of the 1% AEP (1 in 100 year) flood
event including an allowance for climate change. Any More Vulnerable development within this
zone must have a minimum freeboard of 300mm above this level in accordance with the NPPF.
For Less Vulnerable development, there are no requirements in terms of policy for finished floor
levels, however, for this area flood resistance measures should be considered to avoid
inundation of flood waters.
For the rest of the site outside of the 1% AEP flood event with climate change there are no
requirements for finished floor levels. Regardless, it is good practice to raise floor levels in order
to reduce risk from extreme (0.1% AEP (1 in 1000 year)) flood events and from surface water
flooding, which is at Medium risk in this area.
Section 9.3
Flood
Resistance
The area in the south is at Low-Moderate Hazard. It is therefore recommended that flood
resistant construction methods should be considered, including use of construction materials
with low permeability, raising property thresholds, using landscaping to manage surface water
and fluvial floodwater.
Section 9.4
Safe
Access/Egress
In the event of a 1 in 1000 year flood, access along Wates Way and Willow Lane could be limited.
Therefore occupants of the site should seek refuge via Carshalton Road, which can be accessed
from the north or south eastern corners.
Section 9.7
Floodplain
Compensation
Storage
Part of this site is located within the outline of the 1% Annual Exceedance Probability (1 in 100
year) flood event including an allowance for climate change. Within this area, new development
must not result in a net loss of flood storage capacity. If there is an increase in development
footprint in this area then the loss in floodplain storage must be compensated for elsewhere in
the site, in an area that does not already flood.
Section 9.9
Flood Warning
Areas
Flood Warning Areas
The local area is covered by the Environment Agency Flood Warning Areas for ‘River Wandle at
Morden’. Residents of the site should ensure they are signed up to the Environment Agency
Flood Warning system.
Section 9.14
Surface Water
Management
Current risk of flooding
The site is within Drainage Catchment 37, which is within London Borough of Merton, and which
drains much of Mitcham Common and Morden Park.
The uFMfSW indicates that the site and surrounding area is at medium risk of surface water
flooding. There are no reported incidents of flooding held by Merton Council in this location.
Indicative Existing Runoff Rate: 84 l/s (1 in 1 year), 315 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 35.8 l/s
Level 2
Appendix B
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 8: GRACE BUSINESS CENTRE, WILLOW LANE
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a site
investigation to determine precise local conditions) infiltration of surface water into the ground is
potentially uncertain and requires further investigation. This will need to be confirmed prior to the
development of a Drainage Strategy for the site.
If the ground is unsuitable for infiltration SuDS then the techniques which should be considered
include green roofs, filter strips, detention basins and ponds, as well as permeable surfacing in
combination with tanked systems.
If the infiltration of surface water into the ground is possible then infiltration SuDS (e.g. infiltration
basins, infiltration SuDS) should be considered.
Section 10.3
and 10.9
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the Mayor’s Design and construction SPG and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Infiltration trench £55-65 /m3.
Infiltration basin £10-15 /m3.
Section 10.4
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 9: EAGLE TRADING ESTATE, WILLOW LANE
SFRA SITE AREA
Site ID 9
Site Address Eagle Industrial Area and Wates Way, Willow Lane, Mitcham, CR4 4
Site Area 9.60 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
Yes. Designated employment Area
Current Us Employment Use (B use Class)
Proposed Use Employment Use (B Use Class)
Vulnerability More Vulnerable
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The site in located to the immediate east of the River Wandle which flows adjacent as an open channel from the south to north. Once
passing the site the watercourse splits into multiple channels, some as the main river and others as ordinary watercourses.
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
3 % 2 % 57 % 38 % 0 %
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding area
Surface Water
flooding:
(uFMfSW)
Medium Risk
Between 1 in 100 year
(1% annual probability)
and 1 in 30 year (3.3%
annual probability)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater
flooding:
(BGS Susceptibility to
Groundwater
Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 0 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and database right 2016. Contains Environment Agency data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 9: EAGLE TRADING ESTATE, WILLOW LANE
3) LEVEL 2 ASSESSMENT
The fluvial hazard, depth and velocity outputs used in the Level 2 SFRA assessment and mapped below are based on the Environment
Agency modelling of the River Wandle (2015) and are provided for the 1% AEP plus Climate Change event3.
Flood Hazard Rating
Maximum Flood Depth
Maximum Velocity
3 It is important to note that any future site specific FRA would be required to assess this site against the updated climate change allowances (London
Borough of Merton Level 2 SFRA, Section 1.4).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 9: EAGLE TRADING ESTATE, WILLOW LANE
4) RECOMMENDATIONS AND POLICIES
Development
Layout and
Sequential
Approach
Although the majority of the site is within Flood Zones 1 and 2, there is a small area adjacent to
Wates Way designated Flood Zone 3a and 3b adjacent to the River Wandle. A sequential
approach to site layout should be used and development should be set back from the edge of
the River Wandle by at least 8m.
Modelling of the River Wandle identifies that during the flood event with 1% AEP (1 in 100 year)
including an allowance for climate change water comes out of bank to the south of the
adjacent to Wates Way. Flood depths for this modelled scenario may reach 0.25 – 0.5m.
The proposed use is compatible with Flood Zone 1 and 2. Development should be avoided in
the area adjacent to the River Wandle designated as Flood 3a and 3b.
Measures to manage surface water on the site should be considered early in the site
masterplan to enable inclusion of attenuation SuDS where possible.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate
mitigation (BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements,
basement extensions and conversions, such as plant, car parking etc, must provide safe
internal access to higher floors situated above levels derived from the fluvial modelling. Further
ground investigations, such as site specific boreholes, would be required at this site to confirm
the likelihood of groundwater occurrence.
Section 9.2
Finished Floor
Levels
The site does not flood during 1% AEP including climate change, therefore there is no
implication for finished floor levels in terms of fluvial flood risk. However, surface water flows
should be considered as part of drainage strategy; there is an area in the centre of the site that
is at high risk of surface water flooding, here finished floor levels could be increased to ensure
that surface waters do not cause flooding.
Section 9.3
Flood
Resistance
The majority of the site has a flood risk depth of <0.1m. For flood depth up to 0.6 metres it is
recommended that flood resistant measures are implemented to keep the water out. This can
be achieved by:
• Using materials and construction with low permeability.
• Land raising.
• Landscaping e.g. creation of low earth bunds (subject to this not increasing flood risk to
neighbouring properties).
• Raising thresholds and finished floor levels e.g. porches with higher thresholds than
main entrance.
• Flood gates with waterproof seals.
• Sump and pump for floodwater to remove waste faster than it enters
Section 9.4
Safe
Access/Egress
Access to the site is provided via Willow Lane to the north east of the site where there is no
hazard during the 1% AEP (1 in 100 year) flood event. A Flood Warning and Evacuation Plan
(FWEP) should be provided to ensure residents of the site evacuate the site using the safest
route.
Section 9.7
Floodplain
Compensation
Storage
This site is not located within the outline of the 1% AEP (1 in 100 year) flood event including an
allowance for climate change. Therefore this site does not need to consider the net loss of
flood storage capacity.
Section 9.9
Flow Routing New development should not adversely affect flood routing and thereby increase flood risk
elsewhere (including surrounding area). On this site, opportunities should be sought to make
space for water, such as:
• Removing boundary walls or replacing with other boundary treatments such as hedges,
fences (with gaps).
• Create under-croft car parks or consider reducing ground floor footprint and creating an
open area under the building to allow flood water flow.
• Where proposals include floodable outbuildings or garages, design the external walls to
enable the free flow of floodwater.
Section 9.12
Flood Warning
and Evacuation
Plan
Due to the proximity to the River Wandle, it is advised that a Flood Warning and Evacuation Plan
(FWEP) should be prepared for the site, detailing how flood warning will be provided how the
safety of occupants and access to/from the development will be ensured.
Flood Warning Areas
The local area is covered by the Environment Agency Flood Warning Areas for ‘River Wandle at
Morden’. Residents of the site should ensure they are signed up to the Environment Agency
Flood Warning system.
Emergency Rest Centres
The closest designated emergency rest centre for this site is the Canons Leisure Centre
located north to the site within the Merton Borough.
Section 9.14
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 9: EAGLE TRADING ESTATE, WILLOW LANE
Surface Water
Management
Current risk of flooding
The site is within Drainage Catchment 37, which is within London Borough of Merton. The sites
sits upstream of this drainage catchment in a low-lying area adjacent to the river.
The uFMfSW indicates that the site and surrounding area is at low risk to surface water
flooding, however, there are some patches of medium to high risk in the centre of the site.
There are no reported incidents of flooding held by Merton Council in this location.
Indicative existing runoff rate: 45 l/s (1 in 1 year), 169 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 19.2 l/s
Level 2
Appendix B
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a site
investigation to determine precise local conditions) infiltration of surface water into the ground
is potentially unsuitable for the site or it is uncertain whether the site is suitable for infiltration
SuDS. This will need to be confirmed prior to the development of a Drainage Strategy for the
site.
Techniques which should be considered include green roofs, filter strips, detention basins and
ponds, as well as permeable surfacing in combination with tanked systems.
Section 10.3
and 10.9
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the mayor’s Design and construction SPG and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Section 10.4
5) EXCEPTION TEST CONSIDERATIONS
The NPPF states that there are two parts to the Exception Test that must be passed for development to be allocated or permitted:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The majority of the site is within Flood Zone 1 or 2, and safe access is likely to be achievable from the western edge of the site. The
main development constraints for this development site are ensuring that development is at least 8m from the River Wandle, which
borders the north western boundary, and ensuring careful consideration of surface water management across the site. The potential
impacts of flooding should be mitigated through careful site layout, landscaping and incorporation of SuDS, to reduce the risk of
increasing flood risk elsewhere. Therefore, on this basis, it is likely that this site would pass the Exception Test.
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 10: ABBEY INDUSTRIAL
1) SFRA SITE AREA
Site ID 10
Site Address Abbey Industrial Estate, Willow Lane, CR4 4NA
Site Area 13.88 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
Designated Employment Area
Current Use Industrial / storage / waste management (B and Sui Generis Use Class)
Proposed Use Industrial / storage / waste management (B and Sui Generis Use Class)
Vulnerability Less Vulnerable (Commercial)
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
0 % 0 % 4 % 96 % 0 %
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding area
Surface Water
flooding:
(uFMfSW)
High Risk
1 in 30 year (3.33% annual
probability)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater
flooding:
(BGS Susceptibility to
Groundwater
Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 0 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 10: ABBEY INDUSTRIAL
3) LEVEL 2 ASSESSMENT
The fluvial hazard, depth and velocity outputs used in the Level 2 SFRA assessment and mapped below are based on the Environment
Agency modelling of the River Wandle (2015) and are provided for the 1% AEP plus Climate Change event4.
Flood Hazard Rating
Maximum Flood Depth
Maximum Velocity
4 It is important to note that any future site specific FRA would be required to assess this site against the updated climate change allowances (London
Borough of Merton Level 2 SFRA, Section 1.4).
(Contains Ordinance Survey data © Crown copyright and database right
2015. Contains Environment Agency data © Environment Agency and database right 2015).
(Contains Ordinance Survey data © Crown copyright and database right 2015. Contains Environment Agency data © Environment Agency and database right 2015).
(Contains Ordinance Survey data © Crown copyright and database right 2015. Contains Environment Agency data © Environment Agency and database right 2015).
(Contains Ordnance Survey data © Crown copyright and database right 2016.
Contains Environment Agency data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and database right 2016.
Contains Environment Agency data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and database right 2016.
Contains Environment Agency data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 10: ABBEY INDUSTRIAL
4) RECOMMENDATIONS
In accordance with the NPPF, More Vulnerable development is considered compatible within Flood Zone 2 and does not require the
application of the Exception Test. However, given the risk of fluvial and surface water flooding to this site, the principles of the
Exception Test should still be considered when developing on this site, namely:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The following information and recommendations are therefore provided for consideration.
Development
Layout and
Sequential
Approach
The development site is predominately within Flood Zone 1, with 4% of the area within Flood
Zone 2 of the River Wandle. The hazard mapping shows that the entire site is at “No Hazard”.
The development proposal is commercial, therefore Less Vulnerable. Nonetheless
development should be avoided in the area which intersects with Flood Zone 2.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate
mitigation (BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements,
basement extensions and conversions, such as plant, car parking etc, must provide safe
internal access to higher floors situated above levels derived from the fluvial modelling. Further
ground investigations, such as site specific boreholes, would be required at this site to confirm
the likelihood of groundwater occurrence.
Section 9.2
Finished Floor
Levels
The site is located outside of the 1% AEP (1 in 100 year) flood event including allowance for
climate change and therefore there is no requirement to raise finished floor levels.
Consideration should be made of the risk of surface water flooding when planning threshold
locations and levels.
Section 9.3
Safe
Access/Egress
In the event of an extreme 0.1% AEP (1 in 1000 year) flood event it is possible that the access
route will be flooded, therefore it is necessary to provide a Flood Warning and Evacuation Plan
(FWEP) for residents of the site.
Section 9.7
Flood Warning
and Evacuation
Plan
A Flood Warning and Evacuation Plan (FWEP) must be prepared for the site, detailing how flood
warning will be provided how the safety of occupants and access to/from the development will
be ensured.
Flood Warning Areas
The local area is covered by the Environment Agency Flood Warning Areas for ‘River Wandle at
Morden’. Residents of the site should ensure they are signed up to the Environment Agency
Flood Warning system.
Emergency Rest Centres
The closest designated emergency rest centre for this site is Cannons Leisure Centre,
approximately 0.5km north of the site.
Section 9.14
Surface Water
Management
Current risk of flooding
The site is within Drainage Catchment 25, which is within the London Borough of Merton.
The uFMfSW indicates that the site and surrounding area is at high risk of surface water
flooding. There are no previous records of any flood incidents.
Indicative Existing Runoff Rate: 65 l/s (1 in 1 year), 243.5 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 27.8 l/s
Level 2
Appendix B
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a site
investigation to determine precise local conditions) infiltration of surface water into the ground
is uncertain and requires further site investigation.
If the ground is unsuitable for infiltration then the techniques which should be considered
include green roofs, filter strips, detention basins and ponds, as well as permeable surfacing in
combination with tanked systems. If the ground is suitable for infiltration SuDS then infiltration
basins and/or trenches should be considered.
Section 10.3
and 10.9
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 10: ABBEY INDUSTRIAL
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the mayor’s Design and construction SPG and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Infiltration trench £55-65 /m3.
Infiltration basin £10-15 /m3.
Section 10.4
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 11: RAVENSBURY ESTATE
1) SFRA SITE AREA
Site ID 11
Site Address Ravensbury Estate, Mitcham, CR4
Site Area 4.46 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
Part of Merton Council’s emerging Local Plan.
Current Use Residential
Proposed Use 91 fully refurbished homes, 101 replacement homes for existing residents and around 97 new
homes. Six different types of homes including one, two and three bedroom flats, plus two, three
and four bedroom houses. Comprehensive refurbishment of Ravensbury Court. Private outdoor
space for every new home. Parking for all homes in a mix of on-street, on plot parking and parking
courts.
Vulnerability More Vulnerable
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The River Wandle flows adjacent to the southern site boundary as an open channel flowing in a southwest direction. There are several
meandering channels of the River Wandle in the surrounding area.
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
0 % 81 % 15 % 4 % 0 %
Flood risk from all other sources Limitations
Risk of flooding
to the potential
development site
and surrounding
area
Surface Water flooding:
(uFMfSW)
Medium Risk
Between 1 in 100 year (1%
annual probability) and 1 in 30
year (3.3% annual probability)
The uFMfSW data does not show the susceptibility
of individual properties to surface water flooding.
The uFMfSW also does not take into account the
details of the existing drainage system.
Groundwater flooding:
(BGS Susceptibility to
Groundwater Flooding)
Medium Risk
Potential for groundwater
flooding to occur at surface,
but no historic records of
groundwater flooding
The dataset cannot be used on its own to indicate
risk of groundwater flooding and should not be
used to inform planning decisions at a site scale. It
is suitable for use in conjunction with a large
number of other factors, e.g. records of previous
incidence of groundwater flooding, to establish
relative risk of groundwater flooding.
Historic records of flooding
Historic records of flooding
from each source within a
100m radius of potential
development site
Fluvial
records
Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 1 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 11: RAVENSBURY ESTATE
3) LEVEL 2 ASSESSMENT
The fluvial hazard, depth and velocity outputs used in the Level 2 SFRA assessment and mapped below are based on the Environment
Agency modelling of the River Wandle (2015) and are provided for the 1% AEP plus Climate Change event5.
Flood Hazard Rating
Maximum Flood Depth
Maximum Velocity
5 It is important to note that any future site specific FRA would be required to assess this site against the updated climate change allowances (London
Borough of Merton Level 2 SFRA, Section 1.4).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 11: RAVENSBURY ESTATE
4) RECOMMENDATIONS AND POLICIES
Development
Layout and
Sequential
Approach
A sequential approach to site layout should be used. Almost the entirety of the development
site is located within the Flood Zone 3a of the River Wandle, with small areas of Flood Zone 2.
There is a small area of Flood Zone 1 in the south of the site. Where the River Wandle borders
the southern site boundary, all development should be at least 8 metres from the river. The
hazard mapping shows that the centre of the site is at significant flood hazard, with lower
hazard around the edge. Therefore development with the greatest vulnerability (i.e. residential)
should be located in the areas of “Low Hazard”. The development with less vulnerability (i.e. car
parking, outside space) should be concentrated in the areas of greatest hazard, towards the
centre of the site.
Measures to manage surface water on the site should be considered early in the site
masterplan to enable inclusion of attenuation SuDS where possible.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate
mitigation (BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements,
basement extensions and conversions, such as plant, car parking etc, must provide safe
internal access to higher floors situated above levels derived from the fluvial modelling. Further
ground investigations, such as site specific boreholes, would be required at this site to confirm
the likelihood of groundwater occurrence.
Section 9.2
Finished Floor
Levels
For the current development site (without mitigation), modelling of the River Wandle identifies
that during the flood event with 1% AEP (1 in 100 year) including an allowance for climate
change there is an area in the centre of the site that is it at risk of flooding to depths of up to 1
metre. The surrounding area is at risk of flooding to depths up to 0.5m and the northern
section of the site is at risk of flooding up to 0.25 metres.
For the More Vulnerable development finished floor levels should be set a minimum of 300mm
above the 1% annual probability flood level (1 in 100 year) including climate change. All new
building development is residential; therefore all sleeping accommodation should be restricted
to the first floor or above where the new development intersects Flood Zone 2 or 3.
However, there are no policy requirements with regard to finished floor levels for Less
Vulnerable development. Therefore where new development intersects the areas of
“Significant Hazard”, the ground floor level should be occupied by Less Vulnerable
development, for example, car parking.
Section 9.3
Flood
Resistance
Where flood risk depth is approximately 0.6 metres; development should attempt to keep
water out. This can be achieved with appropriate construction materials and building design.
In the north and south where flood depth is less than 0.3 metres the strategy should implement
flood resistance measures such as:
• Using materials and construction with low permeability.
• Land raising.
• Landscaping e.g. creation of low earth bunds (subject to this not increasing flood risk
to neighbouring properties).
• Raising thresholds and finished floor levels e.g. porches with higher thresholds than
main entrance.
• Flood gates with waterproof seals.
• Sump and pump for floodwater to remove waste faster than it enters.
Section 9.4
Flood Resilience In the centre of the site the flood risk is in excess of 0.6 metres. Therefore the strategy should
be to allow floodwaters into the property but to implement careful design in order to minimise
damage and allow rapid re-occupancy. These measures are acceptable only when a suitable
flood warning and evacuation plan is in place. The techniques that could be utilised include:
• Use materials with either, good drying and cleaning properties, or, sacrificial materials
that can easily be replaced post-flood.
• Design for water to drain away after flooding.
• Design access to all spaces to permit drying and cleaning.
• Raise the level of electrical wiring, appliances and utility metres.
• Coat walls with internal cement based renders; apply tanking on the inside of all
internal walls.
• Ground supported floors with concrete slabs coated with impermeable membrane.
• Tank basements, cellars or ground floors with water resistant membranes.
• Use plastic water resistant internal doors.
Section 9.5
Safe
Access/Egress
Access to the site is via Morden Road. In the event of widespread flooding associated with the
River Wandle, there is potential that dry routes out of the local area to a safe place of refuge
may be limited. It will therefore be necessary to prepare a Flood Warning and Evacuation Plan
(FWEP), described further below.
Section 9.7
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 11: RAVENSBURY ESTATE
Floodplain
Compensation
Storage
Where proposed development results in an increase in building footprint, the developer must
ensure that it does not impact upon the ability of the floodplain to store water and that it does
not impact upon floodwater flow conveyance.
This site is located within the outline of the 1% annual probability (1 in 100 year) flood event
including an allowance for climate change. Within this area, new development must not result
in a net loss of flood storage capacity. Flood compensation must be provided on a level for
level, volume for volume basis on land which does not already flood and is within the site
boundary. There is an area of Flood Zone 1 in the south of the site which could be utilised for
this purpose.
Section 9.9
Flow Routing New development should not adversely affect flood routing and thereby increase flood risk
elsewhere (including surrounding area). On this site, opportunities should be sought to make
space for water, such as:
• Removing boundary walls or replacing with other boundary treatments such as hedges,
fences (with gaps).
• Create under-croft car parks or consider reducing ground floor footprint and creating an
open area under the building to allow flood water flow.
• Where proposals include floodable outbuildings or garages, design the external walls to
enable the free flow of floodwater.
Section 9.12
Flood Warning
and Evacuation
Plan
A Flood Warning and Evacuation Plan (FWEP) must be prepared for the site, detailing how flood
warning will be provided how the safety of occupants and access to/from the development will
be ensured and what will be done to protect development and contents. The FWEP should
consider arrangements for the evacuation of basement car parks. Where possible, the FWEP
should also detail the length of time before the site becomes inaccessible by emergency
vehicles.
Flood Warning Areas
The local area is covered by the Environment Agency Flood Warning Areas for ‘River Wandle at
Morden. Residents of the site should ensure they are signed up to the Environment Agency
Flood Warning system.
Emergency Rest Centres
The closest designated emergency rest centre for this site is the Morden Baptist Church, 1km
away.
Section 9.14
Surface Water
Management
The site is within Drainage Catchment 25, which is within London Borough of Merton, which
drains nearby residential and playing field areas upstream.
The uFMfSW indicates that the site surrounding area has a low risk of surface water flooding.
Towards the centre of the site there is a high risk of surface water flooding. Along Hatfield
Close. There is one reported incident of flooding held by Merton Council in this location.
Indicative Existing Runoff Rate: 20.6 l/s (1 in 1 year), 77.1 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 8.9 l/s
Level 2
Appendix B
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a site
investigation to determine precise local conditions) infiltration of surface water into the ground
is potentially unsuitable for the site. This will need to be confirmed prior to the development of
a Drainage Strategy for the site.
There are a number of mitigation measures which should be considered for this site, including
an open network of swales, permeable paving surfaces, areas of landscaping, front and rear
gardens and above and belowground storage (i.e. maximise void spaces above and below
ground as a form of attenuation).6
Section 10.3
and 10.9
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the mayor’s Design and construction SPG 2014 and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
6 London Borough of Merton (2016) Draft Estates Local Plan, Stage 2 Consultation (1st February 2016 – 18th March 2016)
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 11: RAVENSBURY ESTATE
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Section 10.4
5) EXCEPTION TEST CONSIDERATIONS
The NPPF states that there are two parts to the Exception Test that must be passed for development to be allocated or permitted:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
Almost the entire development site is within a Flood Zone 2 or 3a with the centre of the site being at significant risk. Therefore the
most vulnerable buildings should be built around the edge of the site. If More Vulnerable development is required in the area at
significant risk then finished flood levels must be risen and all sleeping accommodation should be located on the first floor or above.
There is potential that dry routes out of the local area to a safe place of refuge may be limited and it is therefore necessary to prepare
a FWEP for residents / occupants of the site detailing steps to evacuate the site prior to the onset of flooding. Resilient and resistant
construction techniques should be used where appropriate and SuDS should be incorporated into building design to reduce the risk
of increasing flood risk elsewhere. Therefore, on this basis, it is likely that this site would pass the Exception Test.
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 12: RAVENSBURY GARAGES
1) SFRA SITE AREA
Site ID 12
Site Address Mitcham, CR4
Site Area 0.162 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
No, but allocated as a potential estate regeneration in Merton’s draft Estates Local Plan 2016.
Planning permission granted (planning application number 16/P1968).
Current Use Garages
Proposed Use 21 Residential units (C3 use): Comprising of 14 flats in two buildings rising to part three, part four
storeys and 7 dwelling houses. Provision of associated vehicular access, parking, cycle, refuse
storage and landscaping.
Vulnerability More Vulnerable (residential)
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The River Wandle flows from east to west in various open channels, one of which lies just south of the development site.
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
0 % 6 % 17 % 77 % 0 %
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding
area
Surface Water flooding:
(uFMfSW)
Low Risk
1 in 1000 year (0.1%
annual probability)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater flooding:
(BGS Susceptibility to
Groundwater Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 1 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 12: RAVENSBURY GARAGES
3) LEVEL 2 ASSESSMENT
The fluvial hazard, depth and velocity outputs used in the Level 2 SFRA assessment and mapped below are based on the Environment
Agency modelling of the River Wandle (2015) and are provided for the 1% AEP plus Climate Change event7.
Flood Hazard Rating
Maximum Flood Depth
Maximum Velocity
7 It is important to note that any future site specific FRA would be required to assess this site against the updated climate change allowances (London
Borough of Merton Level 2 SFRA, Section 1.4).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency data ©
Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 12: RAVENSBURY GARAGES
4) RECOMMENDATIONS AND POLICIES
Development
Layout and
Sequential
Approach
The majority of the site is outside the floodplain; however there is a small area to the north that
intersects with Flood Zone 2 and 3a of the River Wandle. The proposed development is mainly
residential (More Vulnerable) and should be steered away from this location. The River Wandle
flows adjacent to the southern site boundary therefore all development should be set back at
least 8 metres from the designated main river.
The rest of the site intersects Flood Zone 1, therefore is not at flood risk and all types of
development are permitted.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate
mitigation (BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements,
basement extensions and conversions, such as plant, car parking etc, must provide safe
internal access to higher floors situated above levels derived from the fluvial modelling. Further
ground investigations, such as site specific boreholes, would be required at this site to confirm
the likelihood of groundwater occurrence.
Section 9.2
Finished Floor
Levels
The majority of the site is within Flood Zone 1, therefore there are no requirements for finished
floor levels. However, if it is necessary to build in the north where the site intersects Flood
Zones 2 and 3a, then finished floor levels for this More Vulnerable development should be set a
minimum of 300mm above the 1% AEP flood level (1 in 100 year) including climate change.
Any sleeping accommodation in this location should be restricted to the first floor or above.
There is no requirements to raise finished floor levels of any Less Vulnerable development in
these locations (e.g. car parking).
Section 9.3
Flood
Resistance
The use of flood resistant construction methods, which keep floodwaters out, should be
considered for any new development in the northern part of the site. This includes the use of
construction materials with low permeability, raising property thresholds, using landscaping to
manage surface water and fluvial floodwater.
Section 9.4
Safe
Access/Egress
Access to the site is provided via Morden Road to the north of the site. In the event of
widespread flooding associated with the River Wandle, there is potential that dry routes out of
the local area to a safe place of refuge may be limited. It will therefore be necessary to prepare
a Flood Warning and Evacuation Plan (FWEP), described further below.
Section 9.7
Floodplain
Compensation
Storage
Where proposed development results in an increase in building footprint, the developer must
ensure that it does not impact upon the ability of the floodplain to store water and that it does
not impact upon floodwater flow conveyance.
This site is located within the outline of the 1% annual probability (1 in 100 year) flood event
including an allowance for climate change. Within this area, new development must not result
in a net loss of flood storage capacity. Floodplain compensation must be provided on a level
for level, volume for volume basis on land that does not already flood and is within the site
boundary.
Section 9.9
Flow Routing New development should not adversely affect flood routing and thereby increase flood risk
elsewhere (including surrounding area). If new development is required in the north of this site
then opportunities should be sought to make space for water, such as:
Removing boundary walls or replacing with other boundary treatments such as hedges,
fences (with gaps).
Create under-croft car parks or consider reducing ground floor footprint and creating an
open area under the building to allow flood water flow.
Where proposals include floodable outbuildings or garages, design the external walls to
enable the free flow of floodwater.
Section 9.12
Flood Warning
and Evacuation
Plan
A Flood Warning and Evacuation Plan (FWEP) must be prepared for the site, detailing how flood
warning will be provided how the safety of occupants and access to/from the development will
be ensured and what will be done to protect development and contents. Where possible, the
FWEP should also detail the length of time before the site becomes inaccessible by emergency
vehicles.
Flood Warning Areas
The local area is covered by the Environment Agency Flood Warning Areas for ‘River Wandle at
Morden. Prior to first occupation, the developer must ensure that future residents are signed
up to the Environment Agency Flood Warning system. This should be reviewed regularly for the
lifetime of the development.
Emergency Rest Centres
The closest designated emergency rest centre for this site is Vestry hall, which is located
approximately 1km southeast from the site.
Section 9.14
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 12: RAVENSBURY GARAGES
Surface Water
Management
Current risk of flooding
The site is within Drainage Catchment 25, which is within London Borough of Merton. The River
Wandle flows along the southern edge of this drainage catchment. The site is located in the
centre of the southern edge of this drainage catchment, therefore will receive flood waters
from the upstream residential areas.
The uFMfSW indicates that the site and surrounding area is at medium risk of surface water
flooding. There is one reported incident of flooding held by Merton Council in this location.
Indicative Existing Runoff Rate: 0.7 l/s (1 in 1 year), 2.8 l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 5 l/s
Level 2
Appendix B
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a site
investigation to determine precise local conditions) infiltration of surface water into the ground
is potentially unsuitable for the site. This will need to be confirmed prior to the development of
a Drainage Strategy for the site.
Techniques which should be considered include green roofs, filter strips, detention basins and
ponds, as well as permeable surfacing in combination with tanked systems.
Section 10.3
and 10.9
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the mayor’s Design and construction SPG (2014) and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Section 10.4
5) EXCEPTION TEST CONSIDERATIONS
The NPPF states that there are two parts to the Exception Test that must be passed for development to be allocated or permitted:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
Most of the development site is located in Flood Zone 1; however, there is a small area in the north of the site that intersects a Flood
Zone 2 and 3a. New development should be located away from this area of greater flood risk. There is potential that dry routes out of
the local area to a safe place of refuge may be limited and it is therefore necessary to prepare a FWEP for residents / occupants of
the site detailing steps to evacuate the site prior to the onset of flooding. The potential impacts of flooding should be mitigated
through careful site layout, resilient construction techniques, and the incorporation of SuDS to reduce the risk of increasing flood risk
elsewhere. Therefore on this basis it is likely that this site would pass the Exception Test.
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 13: PRINCE GEORGE INDUSTRIAL
1) SFRA SITE AREA
Site ID 13
Site Address Prince George’s Road, Colliers Wood, London, SW19 2JD
Site Area 4.55 ha
Local Plan:
Site allocation or designation
(Sites and Policies Plan 2014)
Yes. Designated Employment Area
Current Use Employment uses (within the B Use Classes sui generis)
Proposed Use Employment Use (B Use Class)
Vulnerability Less Vulnerable
2) SUMMARY OF LEVEL 1 FLOOD RISK
Flood risk from rivers
The River Pickle flows to the northwest of the development site in both culverted and open channels.
Proportion of potential
development site within
Flood Zone
Flood Zone 3b Flood Zone 3a Flood Zone 2 Flood Zone 1 Area Benefiting of Defences
0 % 0 % 18 % 82 % 0 %
Flood risk from all other sources Limitations
Risk of flooding to the
potential development
site and surrounding area
Surface Water
flooding:
(uFMfSW)
High Risk
1 in 30 year (3.33% annual
probability)
The uFMfSW data does not show the
susceptibility of individual properties to surface
water flooding. The uFMfSW also does not take
into account the details of the existing drainage
system.
Groundwater
flooding:
(BGS Susceptibility to
Groundwater
Flooding)
Medium Risk
Potential for groundwater
flooding to occur at
surface, but no historic
records of groundwater
flooding
The dataset cannot be used on its own to
indicate risk of groundwater flooding and should
not be used to inform planning decisions at a site
scale. It is suitable for use in conjunction with a
large number of other factors, e.g. records of
previous incidence of groundwater flooding, to
establish relative risk of groundwater flooding.
Historic records of flooding
Historic records of
flooding from each
source within a 100m
radius of potential
development site
Fluvial records Surface water
records
Groundwater
records
Sewer
records
Multiple source
records
Other
0 3 0 0 0 0
(Contains Ordnance Survey data © Crown copyright and database right 2016. Contains Environment Agency data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 13: PRINCE GEORGE INDUSTRIAL
3) LEVEL 2 ASSESSMENT
The fluvial hazard, depth and velocity outputs used in the Level 2 SFRA assessment and mapped below are based on the Environment
Agency modelling of the River Wandle (2015) and are provided for the 1% AEP plus Climate Change event8.
Flood Hazard Rating
Maximum Flood Depth
Maximum Velocity
8 It is important to note that any future site specific FRA would be required to assess this site against the updated climate change allowances (London
Borough of Merton Level 2 SFRA, Section 1.4).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
(Contains Ordnance Survey data © Crown copyright and
database right 2016. Contains Environment Agency
data © Environment Agency and database right 2016).
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 13: PRINCE GEORGE INDUSTRIAL
4) RECOMMENDATIONS AND POLICIES
In accordance with the NPPF, Less and More Vulnerable development is considered compatible within Flood Zone 1 and 2 and does
not require the application of the Exception Test. However, given the risk of fluvial and surface water flooding to the area surrounding
the site, the principles of the Exception Test should still be considered when developing on this site, namely:
1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh
flood risk” and
2) “demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”.
The following information and recommendations are therefore provided for consideration.
Development
Layout and
Sequential
Approach
The majority of this site is designated Flood Zone 1, however, to the west there is interception
with Flood Zone 2 of the River Pickle. If development were to occur on the site, More
Vulnerable development should be located to the east of the site in Flood Zone 1.
Self-contained residential basements and bedrooms at basement level are not permitted in
areas that have ‘potential for groundwater to occur at the surface’ without appropriate
mitigation (BGS Susceptibility to Groundwater Flooding). Less Vulnerable basements,
basement extensions and conversions, such as plant, car parking etc, must provide safe
internal access to higher floors situated above levels derived from the fluvial modelling. Further
ground investigations, such as site specific boreholes, would be required at this site to confirm
the likelihood of groundwater occurrence.
Section 9.2
Finished Floor
Levels
There is a small area in the west that is within the 1% AEP (1 in 100 year) flood event with
allowance for climate change. In accordance with the NPPF any More Vulnerable development
in this location must be a minimum of 300mm above this level. For Less Vulnerable
development there is no requirements in terms of policy, instead the building can be designed
using appropriate flood resistance and resilience measures.
There is no set guidance for the setting of finished floor levels of development in relation to
surface water flood risk. However, this site is at high risk of surface water flooding therefore it
would be good practice to raise all floors levels the specified 300mm regardless.
Section 9.3
Flood
Resistance
In the west there is an area that is at risk of flooding up to 0.6 metres during the 1% AEP flood
event including climate change. Therefore it is recommended that flood resistant construction
methods should be considered, including use of construction materials with low permeability,
raising property thresholds, using landscaping to manage surface water and fluvial floodwater.
Section 9.4
Safe
Access/Egress
In the event of a 0.1% AEP (1 in 1000 year) flood event, it is possible that Christchurch Road,
Prince George’s Road and West Road are part flooded. Therefore occupants/residents of the
site should seek refuge via Western Road in an easterly direction.
Section 9.7
Floodplain
Compensation
Storage
Part of this site is located within the outline of the 1% annual probability (1 in 100 year) flood
event including an allowance for climate change. Within this area, new development must not
result in a net loss of flood storage capacity. A large part of the site area is located outside of
the floodplain; therefore compensatory storage should be implemented here providing there is
floodplain connectivity.
Section 9.9
Flood Warning
and Evacuation
Plan
Flood Warning Areas
The local area is covered by the Environment Agency Flood Warning Areas for ‘River Wandle at
Wimbledon’. Residents of the site should ensure they are signed up to the Environment
Agency Flood Warning system.
Emergency Rest Centres
Due to the proximity of this site to the Merton council’s administration boundary, the closest
designated emergency rest centre for this site is the South Mitcham Community Centre.
Section 9.14
Surface Water
Management
Current risk of flooding
Although the site is not at significant risk of fluvial flooding, the uFMfSW indicates that the site
and surrounding area is at high risk of surface water flooding. The site is located within Critical
Drainage Area (CDA) Group7_010, which is an area with localised flooding issues. The potential
development must not increase flood risk to other areas in the CDA.
The site is within Drainage Catchment 24, which is within the Merton Borough and drains the
Mitcham area.
There are three previous reported incidents of flooding held by Merton Council in this location.
Indicative Existing Runoff Rate: 20.9l/s (1 in 1 year), 78.3l/s (1 in 100 year)
Indicative Greenfield Runoff Rate: 9.1l/s
Level 2
Appendix B
AECOM London Borough of Merton Level 2 Strategic Flood Risk Assessment
60471781 Final Report January 2017
SITE 13: PRINCE GEORGE INDUSTRIAL
SuDS Suitability
Reference to the SWMP Appendix C2 Figure 4 identifies that (prior to the completion of a site
investigation to determine precise local conditions) the site is located over two areas of
infiltration SuDS suitability. The majority of the site overlays ground unsuitable for infiltration
SuDS. This will need to be confirmed prior to the development of a Drainage Strategy for the
site.
Techniques which should be considered include green roofs, filter strips, detention basins and
ponds, as well as permeable surfacing in combination with tanked systems.
Section 10.3
and 10.9
Drainage Strategy and Approvals
Merton Council will require a Drainage Strategy to be prepared outlining the surface water
management for the site, runoff rates and consideration of SuDS in line with the London Plan
policy 5.13, the Mayor’s Design and construction SPG (2014) and Merton’s Local Plan policies.
Where it is not possible to achieve greenfield runoff rates in accordance with the London Plan
policy 5.13 and Design and Construction SPG (April 2014), then justification must be provided.
Arrangements for the future maintenance and adoption of the drainage system must be made
and detailed in the Drainage Strategy.
Any potential diversions and/or discharges into a sewer or main river must be agreed with
Thames Water or Environment Agency, respectively.
Section 10.6
Indicative Unit Costs
Green roofs ~ £90/m2.
Permeable paving ~ £30-50/m2.
Filter strips £2-4m2.
Detention basin £15-50m3.
Concrete storage tank £449-518/m3.
Section 10.4