SITE 1: EASTFIELDS ESTATE, MITCHAM · SITE 1: EASTFIELDS ESTATE, MITCHAM Drainage Strategy and...

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).




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)




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



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:



No

Current Use Primary school (D1)

Proposed Use School/ school facilities (D1)

Vulnerability More Vulnerable (Educational Establishment)



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 %





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:


Groundwater

Flooding)

Medium Risk

Potential for

groundwater flooding

to occur at surface, but

no historic records of


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 from each


radius of potential

development site


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).




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:


flood risk” and




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.


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



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







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.



Section 10.6







Infiltration trench £55-65 /m3.

Infiltration basin £10-15 /m3.

Section 10.4



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:



No

Current Use School (D1)

Proposed Use School/ school facilities (D1)




The site is at risk of flooding from the River Graveney.



Flood Zone


0 % 0 % 16 % 84 % 0 %





Surface Water

flooding:

(uFMfSW)

Medium Risk




annual probability)





system.

Groundwater

flooding:


Groundwater

Flooding)

Medium Risk





flooding









Historic records of

flooding from each


radius of potential

development site


records

Groundwater

records

Sewer

records

Multiple source

records

Other

0 2 0 0 0 0







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:


flood risk” and




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.





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



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.



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




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.



Section 10.6












Section 10.4



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




The sites are all located within Flood Zone 1, which is defined as Low Probabiltiy of flooding from rivers.


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)





system.

Groundwater flooding:


Groundwater Flooding)

Medium Risk





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.



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




3) RECOMMENDATIONS


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:


flood risk” and




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.



(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


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)


Raleigh Gardens Car Park

Indicative existing runoff rate: 0.5 l/s (1 in 1 year), 1.8 l/s (1 in 100 year)


Love Lane

Indicative existing runoff rate: 1.8 l/s (1 in 1 year), 6.8 l/s (1 in 100 year)


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







policy 5.13, the mayor’s Design and construction SPG (2014) and Merton’s Local Plan policies.







Section 10.6







Section 10.4



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:



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)



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.



Flood Zone


0 % 0 % 40 % 60 % 0 %





Surface Water

flooding:

(uFMfSW)

Medium Risk




annual probability)





system.

Groundwater

flooding:


Groundwater

Flooding)

Medium Risk





flooding









Historic records of

flooding from each


radius of potential

development site


records

Groundwater

records

Sewer

records

Multiple source

records

Other

0 0 0 0 0 0







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).













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:


flood risk” and




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.



(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


The site is within Drainage Catchment 37, which is within London Borough of Merton, and which

drains much of Mitcham Common and Morden Park.


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




SuDS Suitability


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




policy 5.13, the Mayor’s Design and construction SPG and Merton’s Local Plan policies.







Section 10.6









Section 10.4



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:



Yes. Designated employment Area

Current Us Employment Use (B use Class)

Proposed Use Employment Use (B Use Class)




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.



Flood Zone


3 % 2 % 57 % 38 % 0 %





Surface Water

flooding:

(uFMfSW)

Medium Risk




annual probability)





system.

Groundwater

flooding:


Groundwater

Flooding)

Medium Risk





flooding









Historic records of

flooding from each


radius of potential

development site


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).







Flood Hazard Rating

Maximum Flood Depth

Maximum Velocity




database right 2016. Contains Environment Agency

data © Environment Agency and database right 2016).











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.





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




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




Surface Water

Management


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)


Level 2

Appendix B

SuDS Suitability


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.



Section 10.3

and 10.9











Section 10.6







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:


flood risk” and



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.



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:



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)





Flood Zone


0 % 0 % 4 % 96 % 0 %





Surface Water

flooding:

(uFMfSW)

High Risk

1 in 30 year (3.33% annual

probability)





system.

Groundwater

flooding:


Groundwater

Flooding)

Medium Risk





flooding









Historic records of

flooding from each


radius of potential

development site


records

Groundwater

records

Sewer

records

Multiple source

records

Other

0 0 0 0 0 0










Flood Hazard Rating

Maximum Flood Depth

Maximum Velocity



(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).








4) RECOMMENDATIONS


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:


flood risk” and




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.








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 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


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)


Level 2

Appendix B

SuDS Suitability



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














Section 10.6









Section 10.4



SITE 11: RAVENSBURY ESTATE

1) SFRA SITE AREA

Site ID 11

Site Address Ravensbury Estate, Mitcham, CR4

Site Area 4.46 ha

Local Plan:



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.




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.



Flood Zone


0 % 81 % 15 % 4 % 0 %


Risk of flooding

to the potential

development site

and surrounding

area


(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.




Medium Risk


flooding to occur at surface,

but no historic records of


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.



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










Flood Hazard Rating

Maximum Flood Depth

Maximum Velocity
















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.








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




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


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


• Where proposals include floodable outbuildings or garages, design the external walls to


Section 9.12

Flood Warning

and Evacuation

Plan



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


Morden. Residents of the site should ensure they are signed up to the Environment Agency



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)


Level 2

Appendix B

SuDS Suitability



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




policy 5.13, the mayor’s Design and construction SPG 2014 and Merton’s Local Plan policies.







Section 10.6

6 London Borough of Merton (2016) Draft Estates Local Plan, Stage 2 Consultation (1st February 2016 – 18th March 2016)










Section 10.4




flood risk” and



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.



SITE 12: RAVENSBURY GARAGES

1) SFRA SITE AREA

Site ID 12

Site Address Mitcham, CR4

Site Area 0.162 ha

Local Plan:



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)



The River Wandle flows from east to west in various open channels, one of which lies just south of the development site.



Flood Zone


0 % 6 % 17 % 77 % 0 %




site and surrounding

area


(uFMfSW)

Low Risk

1 in 1000 year (0.1%

annual probability)





system.




Medium Risk





flooding









Historic records of

flooding from each


radius of potential

development site


records

Groundwater

records

Sewer

records

Multiple source

records

Other

0 1 0 0 0 0










Flood Hazard Rating

Maximum Flood Depth

Maximum Velocity
















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.








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


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


Where proposals include floodable outbuildings or garages, design the external walls to


Section 9.12

Flood Warning

and Evacuation

Plan



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


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.


The closest designated emergency rest centre for this site is Vestry hall, which is located

approximately 1km southeast from the site.

Section 9.14




Surface Water

Management


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.


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)


Level 2

Appendix B

SuDS Suitability



is potentially unsuitable for the site. This will need to be confirmed prior to the development of

a Drainage Strategy for the site.



Section 10.3

and 10.9




policy 5.13, the mayor’s Design and construction SPG (2014) and Merton’s Local Plan policies.







Section 10.6







Section 10.4




flood risk” and



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.



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:



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



The River Pickle flows to the northwest of the development site in both culverted and open channels.



Flood Zone


0 % 0 % 18 % 82 % 0 %





Surface Water

flooding:

(uFMfSW)

High Risk

1 in 30 year (3.33% annual

probability)





system.

Groundwater

flooding:


Groundwater

Flooding)

Medium Risk





flooding









Historic records of

flooding from each


radius of potential

development site


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).







Flood Hazard Rating

Maximum Flood Depth

Maximum Velocity
















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:


flood risk” and




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.








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


Wimbledon’. Residents of the site should ensure they are signed up to the Environment

Agency Flood Warning system.


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


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.



Level 2

Appendix B




SuDS Suitability


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.



Section 10.3

and 10.9




policy 5.13, the Mayor’s Design and construction SPG (2014) and Merton’s Local Plan policies.







Section 10.6







Section 10.4

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