DRAIN LONDON APPENDIX E - OPTIONS ASSESSMENT
CDA DESCRIPTION
London Borough: Critical Drainage Area ID: Group 7_024 (Tooting Bec) Wandsworth (Lead)
Lambeth (Other) Description: This CDA centres on an area of ponding along the railway line between Streatham Common Station and Balham Station. Surface water is directed here from Lambeth across Tooting Bec Common and ponds along the railway line as it is in a cutting.
The west of the CDA is locations within an iPEG area. Flood Risk Categorisation: Surface Water, Groundwater and Sewer Property Count: 513 residential and 20 non-residential properties >0.03m 60 residential properties >0.5m Critical Infrastructure: • Railway line between Streatham Common Station and Balham Station. • Electricity Substation (Byrne Road) Validation: • There are records of surface water flooding in Tooting Bec Common in 2007. • There are two records of groundwater flooding in the CDA. Local Flood Risk Zones Streatham Common to Balham Railway Cutting – pluvial modelling shows that surface water from Tooting Bec Common flows on to the railway cutting and causes flooding to the railway track and downstream properties along Byrne Road. Figures: Figure 3.8.11a – Surface Water Flood Depth (1% AEP) Figure 3.8.11b – Surface Water Flood Hazard (1% AEP) DRAIN LONDON APPENDIX E - OPTIONS ASSESSMENT
STAGE 1 - MEASURES IDENTIFICATION
Critical Drainage Area ID: Group 7_024 (Tooting Bec)
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 Soakaways g Generic Measure Throughout CDA due to geology.
Further investigation will be required to determine drainage arrangements and discharge for swales - Swales a Generic Measure Throughout CDA may be limitied due to geology.
Likely to be limited due to geology and space available to implement option. Further investigation is Permeable Paving a Generic Measure Throughout CDA needed to assess the infiltration potential due to geology. RCE 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 Detention Basins A Possible locations for a detention basin could be in Tooting Bec Common. Likely to have limited space in the CDA for these to be implemented. from/to the railway line.
A strategically located pond could be constructed to manage the surface water from/to the Ponds and Wetlands A Possible locations for a pond could be in Tooting Bec Common. Likely to have limited space in the CDA for these to be implemented. railway line.
Other 'Source' Measures N/A
The existing drainage system under the railway could be improved to manage an increased Network Rail may need to review drainage along the railway cutting as this is predominantly where the Increasing Capacity in Drainage Systems G Throughout CDA amount of storm water. surface water ponds.
Limited opportunity for implementation of this measure. Could only be undertaken on a small area of Separation of Foul and Surface Water Sewers r The combined sewers within this CDA could be separated to allow for more capacity. Throughout CDA the CDA as the majority of flooding is along the railway line cutting.
Generic Measure. More regular inspection of the current sewer and Network Rail owned Improved Maintenance Regimes g Throughout CDA To be identified on site-by-site basis. drainage system to remove debris and improve conveyance.
Managing Overland Flows (Online Storage) r Generic Measure Tooting Bec Common See Detention Basin comments.
Limited opportunity for implementation of this measure. There are no obvious flow paths along roads Managing Overland Flows (Preferential Flowpaths) r Streets could be lowered or kerbs raised to create a flow path. Urban area to the east of CDA PATHWAY PATHWAY that could easily be adapted as preferential flow paths.
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
Improved Weather Warning a Dependent on location and storm event duration. N/A Not considered an appropriate measure for this CDA given the size and nature of the flooding.
Planning Policies to Influence Development A Generic Measure In urban area to east of CDA. For all new development.
Temporary or Demountable Flood Defences A Household / building level demountable flood barriers. For basement or ground floor properties in the area in the east of the CDA. Limited opportunity for implementation of this measure.
Will be dependent on engagement opportunities with community. In areas with a large migration of Social Change, Education and Awareness A Generic Measure Throughout CDA
RECEPTOR RECEPTOR population it will be difficult to undertake / pass on information from one property owner to other.
Improved Resilience and Resistance Measures G Property level resilience measures Byrne Road area 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_024 (Tooting Bec)
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 3 Improved Maintenance g N/A 2 1 1 0 1 5 see localised benefits. The benefits of this option depend on the existing maintenance regime and the current drainage system. Confirmation will need to be sought with Network Rail. 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 A 2 2 0 0 0 4 manage the surface water on the site are already in place. These could be reiterated in forthcoming policy documents. However, likely to have limited opportunity / impact in this CDA due to the nature of the flooding.
Implementing small scale SuDS measures in Tooting Bec Common and/or residential properties in the area are likely to offer the best 5 Source Control, Attenuation and SUDS a g a a G N/A R 1 1 1 1 2 6 opportunity for flood alleviation in the CDA. None of the options are considered technically difficult.
Using Tooting Bec Common as a flood storage area, would have flood alleviation benefits and environmental benefits. There may by local 6 Flood Storage / Permeability AA N/A r R N/A 1 0 1 1 2 5 resilience to this option depending on the different uses of these areas.
7 Separate Surface Water and Foul Water Sewer Systems r
8 De-culvert / Increase Conveyance R N/A 0
9 Preferential / Designated Overland Flow Routes r N/A a
This option could protect many properties from flooding through the installation of flood barriers on the doors of properties. But there may be local resistance to the uptake of the barriers and the success of the barriers relies on human intervention and the dissemination of 10 Community Resilience a A A G N/A 2 1 1 0 2 6 appropriate flood warnings. It is also a costly exercise to fit multiple properties with demountable barriers and/or property level resilience measures. Resilience of key infrastructure (e.g. railway lines that operate as key transport routes into/out of central London) will provide economic 11 Infrastructure Resilience A G N/A 1 1 1 0 2 5 (Network Rail) and social benefits.
Whether this is technically feasible will need to be ascertained in liaison with Network Rail but potentially this option would be affective at 12 Other - Improvement to Drainage Infrastructure G N/A 2 0 1 1 1 5 alleviating flooding but would need to be undertaken predominantly along the railway line cutting.
Flood storage (pond would provide greater environmental benefits), with some source control SUDS and planning policy for new 13 Other or Combination of Above a G A G g A 1 1 2 2 2 8 development, alongside improved maintenance and/or improvements to Network Rail drainage system along railway tracks.