Making space for water
Urban flood risk and integrated drainage
Hogsmill (Environment Agency, Thames Region)
Pilot Summary
The objectives of the River Hogsmill IUD study were to better understand the level of flood risk from all sources in the catchment, identify its causes and potential ways of reducing the risk. This was achieved by building partnerships with key stakeholders who manage an element of surface water drainage, exploring ways of modelling surface water runoff and its interaction with river flooding and developing sustainable solutions to manage flood risk.
The Pilot approached the problem through a three pronged attack; production of Integrated Urban Drainage Plan (which could be considered an early form of Surface Water Management Plan (SWMP); development of a GIS internet based tool to visually display all sources of flood risk to improve the communication, understanding and management of flood risk amongst responsible organisations; and gather evidence to help inform future flood risk policy.
The IUD Pilot has provided a quantification of existing flood risk across the whole catchment, showing that in a 1% annual probability flood there are 1356 houses at risk from surface water flooding, 3902 from fluvial flooding, and that the annual average damage from surface water flooding is 45% that of fluvial flooding. The type of risk varies significantly with location and type of flooding; in Epsom and Ewell 90% of the risk to property is from overland and surface water, whilst in Kingston this is only 4% of the risk, with the major risk from fluvial flooding
The IUD Pilot has set-up both a Flood Risk Working group which comprises key stakeholders with responsibilities for flood risk management and a Flood Risk Tool to facilitate communication of flood risk between partner organisations. These should lead to better informed and co-ordinated decision making for spatial planning and development and as such has fed into the Local Authority Strategic Flood Risk Assessments (SFRA). The Flood Risk Tool will also provide a good source of information for Local Authority Emergency Planners. More accurate information will allow for better prepared emergency plans and procedures. Subject to funding and other constraints it should also promote identification and implementation of measures to reduce existing flood risk. It is the desire of the Project Team for a controlled version of the Flood Risk Tool to be made available on-line for public use, to inform and educate the public and local developers.
The main barriers to spatial planning remain a) cooperation between local authorities, and b) means to encourage, or enforce, use of SUDS in all new development.
Implications for future management of urban drainage systems
We suggest that surface water management plans should comprise three elements:
- Surface water flood risk assessment (led by the Local Planning Authority with support from appropriate partners and the Environment Agency)
- Flood risk management communication plan (led by flood risk forums)
- Surface water action plan (led by Local Planning Authority)
Voluntary cooperation in producing SWMPs is considered unlikely to work on a National basis. New legislation is therefore required. We propose that local/regional flood risk forums are set up with enforced cooperation between stakeholders, following the model of the Civil Contingencies Act 2004. For a flood risk forum to be effective there must be a commonly agreed methodology for evaluating options to reduce flood risk across different assets. In relation to promoting the use of SUDS on all sites we consider that national policy is required to set out standards for design and construction.
In relation to management of surface water pipe drainage systems we suggest that water companies should be required to adopt a risk-based approach in considering adequacy of a given system, including specific consideration of the consequences of above design standard events (ref CIRIA Design for Exceedance, 2006) and thus quantifying residual risk.
Findings of the Pilot
- All sources of flood risk in the catchment were quantified, concluding that 35% is from surface water flooding and that climate change is likely to more than double the existing Average Annual Damages (AAD) caused by surface water flooding.
- Geological features are important. The upper catchment underlain by chalk relies primarily on natural infiltration to control surface runoff and as a result of this is vulnerable to flooding of dry valleys in the extreme higher intensity rainfall events, which are predicted to become more frequent with Climate Change. In the lower catchment, which is located on London Clay, the risk of river flooding in Kingston is significantly determined by drainage arrangements in the upstream authority (Epsom & Ewell). Runoff from the Chalk will increase more with climate change than runoff from the Clay, therefore increasing flood risk in the urbanised area.
- The Pilot study highlighted interdependence to both local authorities and provided significant benefits in encouraging people to think outside their political boundaries.
- The need for improved communication and management of flood risk across the various functions of local authorities and other partner organisations, as well as to developers, was identified. This led to the development of a web based “Flood Risk Tool” and a shared understanding of flood risk within the Hogsmill Catchment.The tool provides users with the ability to determine the overall flood risk at a site of interest and is transferable to other catchments.
- Methods of modelling surface water flooding are still immature. Accurate prediction of the extent of flooding in the existing scenario requires both high quality digital terrain models and spatial extent of infrastructure. However, using fairly simple GIS mapping procedure to identify low spots and depressions provided a useful understanding of predicted surface water flooding areas.
Outputs
Page last modified: 25 June 2008
Page published: 5 March 2007