DRAIN LONDON APPENDIX E - OPTIONS ASSESSMENT
CDA DESCRIPTION
London Borough: Critical Drainage Area ID: Group 7_022 (Clapham Junction) Wandsworth (Lead)
Lambeth (Other) Description: This CDA broadly follows the route of Falcon Brook which is now a culverted. The CDA spans across both Lambeth and Wandsworth but the Local Flood Risk Zones are located in Wandsworth. Flow originates in Streatham Hill in Lambeth and then flows west along Atkins Road, ponds in Kenilford Road, then flows west along Dinsmore Road, and Oldridge Road. At the western point of Oldridge Road, surface water flows north along the old course of Falcon Brook, along Northcote Road, under Clapham Junction underpass to Falcon Road where water ponds up to a depth of 1.5m to the east of Falcon Road.
The northern and southern parts of the CDA are located within iPEG areas, with small localised iPEG areas in the centre.
Flood Risk Categorisation: Surface Water, Groundwater and Sewer Property Count: 4,828 residential and 405 non-residential properties >0.03m 181 residential and 12 non-residential properties >0.5m Critical Infrastructure: • Battersea Fire Station • Christchurch CE Primary School • Holy Ghost RC Primary School • Falcon Road Medical Centre • St Peter's Children Centre Validation: • Following heavy rain in July 2007, flooding was experienced at: - Christchurch CE Primary School, Falcon Road, Northcote Library (water from street flooded the basement and caused damage to walls, floors and lift). - Grant Road (where water from Clapham Junction station and track, was routed through tunnel to Grant Road). • There are five recorded incidents of groundwater flooding in the CDA. • There are over 100 sewer flooding incidents within the wider postcode areas, in which the CDA is located. Local Flood Risk Zones • Falcon Road – pluvial modelling and historic flood records show significant surface water flooding in the Falcon Road area, to the east of Clapham Junction Railway Station, along the course of the ‘lost’ Falcon Brook • Kenilford Road – pluvial modelling shows that surface water from Atkins Road ponds in this area during heavy rainfall due to the local topography Figures: Figure 3.8.9a – Surface Water Flood Depth (1% AEP) Figure 3.8.9b – Surface Water Flood Hazard (1% AEP) DRAIN LONDON APPENDIX E - OPTIONS ASSESSMENT
STAGE 1 - MEASURES IDENTIFICATION
Critical Drainage Area ID: Group 7_022 (Clapham Junction)
Measure Opportunity Assessment Description Location / Specific Details Comments
To be identified on site-by-site basis when opportunities arise but likely to be limited opportunity for Green Roof a Generic Measure Throughout CDA - large buildings. implementation of measure within the CDA.
Likely to be limited due to geology. Further investigation is needed to assess the infiltration potential due Soakaways a Generic Measure Throughout CDA to geology.
Throughout CDA for new developments. Swales could be located on the western edge of Clapham Further investigation will be required to determine drainage arrangements and discharge for swales - Swales a Generic Measure Common to restrict surface water from flowing onto Battersea Rise. may be limitied due to geology.
Likely to be limited due to geology. Further investigation is needed to assess the infiltration potential due Permeable Paving a Generic Measure Throughout CDA to geology. RCE
SOU Rainwater Harvesting G Generic Measure. For all new development and retrospectively. Throughout CDA To be identified on site-by-site basis.
A strategically located detention basin could be constructed to manage the surface water from the Detention Basins r N/A Limited space in the CDA for these to be implemented. upstream catchment of the CDA.
A strategically located pond could be constructed to manage the surface water from the upstream Ponds and Wetlands R N/A Limited space in the CDA for these to be implemented. catchment of the CDA.
Other 'Source' Measures N/A
Northcote Road acts as a major flow path in this CDA and by increasing the capacity of the existing Increasing Capacity in Drainage Systems g The existing drainage system capacity could be increased to accommodate storm water drainage system could potentially alleviate a lot of flooding in this CDA. Falcon Road and Ingrave Street Requires further investigation could potentially benefit. Northcote Road acts as a major flow path in this CDA and by increasing the capacity of the existing Separation of Foul and Surface Water Sewers G The combined sewers within this CDA could be separated to allow for more capacity. drainage system through separation could potentially alleviate a lot of flooding in this CDA. Falcon Road Requires further investigation and Ingrave Street could potentially benefit. Generic Measure. More regular inspection of the current sewer system to remove debris and improve Improved Maintenance Regimes G Throughout CDA. To be identified on site-by-site basis. conveyance.
An small park is located between Meyrick Road and Ingrave Street which could be used for flood Managing Overland Flows (Online Storage) a Creating storage areas Some but limited opportunities in CDA. storage. Surface water could be directed here from Ingrave Street. WAY Northcote Road acts as a major flow path in this CDA and by designating it as a flowpath could
PATH Managing Overland Flows (Preferential Flowpaths) G Streets could be lowered or kerbs raised to create a flow path. potentially alleviate a lot of flooding in this CDA. Falcon Road and Ingrave Street could potentially be To be identified on site-by-site basis. designated.
Land Management Practices R N/A N/A Not applicable due to CDA being heavily urbanised.
Deculverting Watercourse(s) R N/A N/A The CDA contains no main rivers or ordinary watercourses.
Other 'Pathway' Measures N/A
The area to the north of the railway line including a school has been modelled to flood to depths greater Improved Weather Warning a Dependent on location and storm event duration. than 1m. Evacuation could be achieved from these buildings/properties following the dissemination of an Given the nature of the flooding in the CDA, it is unlikely this measure could be effectively implemented. appropriate flood warning.
Planning Policies to Influence Development G Generic Measure Throughout CDA For all new development.
To the north of the railway line. Where depths are greater than 1m flood resilient measures are likely to This measure will need to be deployed in parallel with an efficient flood warning system and community Temporary or Demountable Flood Defences g Household / building level demountable flood barriers. be more appropriate. education.
Will be dependent on engagement opportunities with community. In areas with a large migration of Social Change, Education and Awareness g Generic Measure Throughout CDA population it will be difficult to undertake / pass on information from one property owner to other
G Community Flood Plan For Falcon Road Area RECEPTOR
Improved Resilience and Resistance Measures g Property level resilience measures For Falcon Road Area To be identified on site-by-site basis.
g Building level resilience measures For critical infrastructure predicted / historically flooded in north of CDA To be identified on site-by-site basis.
Other 'Receptor' Measures N/A DRAIN LONDON APPENDIX E - OPTIONS ASSESSMENT
STAGES 2 and 3 - OPTIONS IDENTIFICATION AND SHORT LISTING
Critical Drainage Area ID: Group 7_022 (Clapham Junction)
Standard Measures Short listing Options SOURCE PATHWAY RECEPTOR
Option No. Option (Scheme Category) Comments Social Overall Technical Technical Economic Objectives Environmental Appropriate Appropriate Measures Available? Take ForwardTake Option to Detailed Assessment? Green Roof Green Soakaways Swales Permeable Paving Harvesting Rainwater Basins Detention Ponds and Wetlands Measures 'Source' Other inIncreasing Capacity Systems Drainage Separation of Foul Sewers and Water Surface Improved Regimes Maintenance Managing Overland Flows (Online Storage) Managing Overland Flows Flowpaths) (Preferential Land Management Practices Watercourse(s) Deculverting Measures 'Pathway' Other ImprovedWarning Weather Planning Policies to Influence Development Temporary or Demountable Flood Defences Social Change, Education and Awareness Improved Measures Resilience and Resistance Measures 'Receptor' Other
1 Do Nothing 2 -1 -2 0 -2 -3 In line with PAG the 'do nothing' option (no intervention and no maintenance) and 'do minimum' (continuation of current practise) should be taken forward to the detailed options assessment. 2 Do Minimum 2 0 -1 0 -1 0
This option will be relatively easy to implement by increasing the regularity of the existing maintenance regime. It is however only likely to see 3 Improved Maintenance G N/A 2 1 1 0 1 5 localised flooding benefits.
To implement this option into new developments would be relatively simple. Once an area has been identified as being in a CDA policies to 4 Planning Policy G 2 2 0 1 1 6 manage the surface water on the site are already in place. These could be reiterated in forthcoming policy documents.
This has the potential to achieve environmental benefits as well as flood alleviation but cannot easily be applied retrospectively. The best 5 Source Control, Attenuation and SUDS a a a a G R 1 2 1 1 2 7 opportunities exist through retrofitting for small scale SuDS such as rainwater harvesting for larger buildings and utilising existing green spaces interspersed between buildings.
6 Flood Storage / Permeability r R N/A a R N/A 0 -1 1 2 1 3 Limited opportunities for these schemes in CDA and as such technically difficult to implement.
This is technically possible, but the cost of carrying out sewer separation over a large geographic area is not viable. The cost-benefit ratio is 7 Separate Surface Water and Foul Water Sewer Systems G 1 -2 0 0 1 0 likely to be negative.
8 De-culvert / Increase Conveyance R N/A 0
Designating Northcote Road as a preferential flow path would have flood alleviation benefits but no additional environmental benefits. Measures 9 Preferential / Designated Overland Flow Routes G N/A 1 1 0 -1 1 2 would also need to be put in place to receive / attenuate flows along road.
This option could protect many properties from flooding through the installation of flood barriers on the doors of properties. But there may be 10 Community Resilience a g g G N/A 2 1 1 0 1 5 local resistance to the uptake of the barriers and the success of the barriers relies on human intervention and the dissemination of appropriate flood warnings. It is also a costly exercise to fit multiple properties with demountable barriers and/or property level resilience measures. This option could be considered for schools and infrastructure predicted to flood in the CDA, particularly to depths of greater than 0.5m (in 11 Infrastructure Resilience g G 2 1 1 0 1 5 north of CDA). However, it is likely to be achieved in combination with improved education / awareness and small scale SuDS measures such as rainwater harvesting.
A local increase in drainage capacity on Northcote Road, Falcon Road and Ingrave Street is technically feasible and will achieve local flood 12 Other - Improvement to Drainage Infrastructure g N/A 2 1 1 0 1 5 alleviation and potentially more widespread flood alleviation. However, there would be no environmental benefits.
Given the complex nature of the flooding in this CDA it is recommended that a Drainage Capacity Study is undertaken to identify existing 13 Other or Combination of Above drainage arrangements and identify the most cost beneficial schemes for mitigating flood risk in the CDA.