Making space for water
Urban flood risk and integrated drainage
Brent North (Thames Water)
Executive Summary
This pilot study was set up under the auspice of the North Brent Flood Working Group to understand the cause of flood risk and to identify possible strategies to mitigate this risk in the Wealdstone Brook catchment of North Brent and Harrow in North London. The Working Group comprises local stakeholders in flood risk, Thames Water, the Environment Agency, the London Boroughs of Harrow and Brent, and local residents under the chairmanship of Barry Gardiner (MP for North Brent). The Working Group acted as a Steering Group for the Project and contributed much valuable input to the study.
Within the study it has become clear that the problem of flood risk relates to historic factors. The area includes separate surface water and foul sewers as well as local and main rivers. The sewerage system was installed in the early 20th century when much of the area was green field. Since then the area has become heavily urbanised with a mix of commercial and residential development. There is continued pressure on local government to continue to develop the area.
The study team used network and river models provided by the Stakeholders to analyse the sewers, rivers and overland flow. It was found that the most representative analysis was obtained by integrating all the network models so that the interactions of each of the networks on the other systems were replicated. The study has identified that the flooding usually manifests itself as flooding from foul and surface water sewers. Flooding from the foul sewers is largely due to excessive volumes of surface water diverted into this network due to lack of capacity in the surface water network. This capacity shortage is related to the original surface water network design standards, changing customer expectations over the years, and continued development exacerbated by urban creep (increasing areas of impermeable paving within existing developed areas) preventing water infiltration to ground in the area.
Various individual strategies were investigated to mitigate flood risk. Upgrading of the foul or surface water or river system would not, on its own, resolve the issues. Provision of surface water attenuation tanks to prevent water entering the sewers and river would need to be very extensive and numerous large tanks would be needed to provide significant improvements. This was not considered to be practical in this type of area. Large scale retrospective fitting of SUDS runoff attenuation features would not be suitable for this type of area with large areas of roads, roofs and paved gardens.
The most feasible solution envisages integrating many aspects of urban management. This would involve a strategy of re-separation of the foul and surface water systems, and upgrade of the surface water network as needed to resolve increased local flood risk caused. Additional flow would be passed to the river, so local river attenuation, possibly in Council parks or public open space would be needed. SUDS techniques should be adopted wherever possible for municipal developments. Incentives should also be made available to encourage householders to implement measures to alleviate the impact of runoff from their property.
This strategy has the following benefits:
- Almost all predicted foul sewerage flooding will be removed. Some relatively minor, local foul sewerage improvements may still be needed, but the trunk sewer is generally adequate.
- Both river and sewer flooding are addressed in the long term. Sizing of the flood attenuation areas will need to take account of predicted increases in flows due to climate change and increased paving. Sewer upsizing will need address local circumstances at time of construction.
- Local disruption will be alleviated through upsizing of only one pipe network.
- The water quality of the river will be improved by removing foul sewage overflows and reducing misconnections of foul sewage to the surface water sewers.
- River quality will be further improved by de-culverting the channel and re-routing it through vegetative treatment processes, such as reed beds.
- Significant lengths of the existing piped culvert will be transformed into an open, natural watercourse, providing recreation and amenity to the local community.
- The planned storage to be provided for rivers will manage flood risk for major storm events.
- The storage areas and the re-profiled river channel and its bankside areas, will create wildlife habitats, increasing biodiversity in the area. This will enhance the Green Infrastructure in the area in line with the aspirations of the Southeast Regional Assemblies RSS South East Plan.
- Capacity will be made available within existing foul sewers for continuing growth in foul flows.
- Peak flows to Mogden sewage treatment works will be reduced, with resulting improvements to treatment and reduced treatment costs and carbon footprint.
Detailed flood mitigation schemes have not been developed for the area. But the budget estimate for construction of flow attenuation areas by the river as described, and locating and resolving sewer misconnections is £17.1 million. Upgrade of the surface water sewers could then be carried out in a prioritised programme. The estimated total cost of this is £108.4 million. Costs for changes to the wastewater sewers would be small.
However a number of issues were identified that need to be resolved before a solution like this is practical.
- Additional space is required for attenuation of river flows if target 1 in 100 year river levels are to be achieved.
- There is a dependency between capital programmes of more than one Stakeholder. Funding will need to be co-ordinated to ensure a sustainable programme of work.
- Maintaining separation of flows to sewer is partly the responsibility of Local Authorities through building control. They need to be given powers and resources to ensure this can be maintained in the long term including taking retrospective action on infringers.
- Better control needs to be given for new connections to sewers, probably with the right of connection removed for both foul and surface water flows.
- Use of public open space for river attenuation raises issues of ownership, accountability, maintenance, and health and safety.
Outputs
Page last modified: 25 June 2008
Page published: 5 March 2007