Making space for water
Urban flood risk and integrated drainage
Lewes (Black & Veatch, East Sussex County Council, Environment Agency, Lewes District Council and Southern Water)
Pilot Summary
The Lewes Integrated Urban Drainage (IUD) Pilot Study is one of 15 studies funded by Defra with the objective of developing and testing best practice for the management of urban flood risk and integrated drainage. Lewes was selected as a pilot study site because of the extensive and complex nature of the flooding experienced in the town in recent times. The four principal stakeholder organisations in the area, East Sussex County Council, the Environment Agency, Lewes District Council and Southern Water fully participated in the project. The bulk of the work was undertaken by Black & Veatch under the guidance of these stakeholder organisations.
The methodology for the study comprised the following steps:
- Identification of flooding problems and mechanisms;
- Identifying interactions between drainage system components;
- Quantifying the flood risks;
- Prioritising flooding issues;
- Investigating ownership and responsibility issues;
- Proposing an integrated strategy to alleviate flooding in Lewes; and
- Reporting the findings in terms of both the specific needs of Lewes and their significance in the national context.
The historical problems in Lewes were recognised to have been focussed on flooding from the River Ouse and the combined sewer system that serves the central parts of the town. In particular the major flooding event of October 2000 in which more than 800 properties were inundated causing financial losses of approximately £130 millions was a major stimulus to the project. Past investigations meant that the configuration and performance of these two elements of the overall system were well understood and flood improvement works had been planned and, in some cases, implemented. Examples of this included the enhanced river defences in the Malling Brooks area and the new Storm Interceptor Tunnel which had allowed the numerous combined sewer overflows discharging to the River Ouse and its tributaries to be closed. The more integrated approach of the pilot study confirmed the effectiveness of these measures and showed that the levels of protection were raised and the scope for interaction between them was much reduced. Nonetheless it is estimated that some 300 properties in Lewes remain at risk of flooding mainly from the river more frequently than once in 100 years, whilst up to 50 properties may be subject to sewer flooding at a frequency of less than once in 5 years.
Information on the other facets of the Lewes drainage system was, and is, more sparse. Records and anecdotal information were available from a variety of sources about the occurrence of flooding incidents associated with highway drains, surface water sewer systems, springs and ephemeral streams. However, data relating to the configuration and performance of these systems was generally not available at the level of detail which would allow a fully integrated investigation as to the causes and mechanisms by which these failures occur. Investigations of these parts of the overall drainage system of Lewes during the course of the study have therefore had to be of a more subjective nature.
Adoption of a more integrated approach to examining the overall performance of the Lewes urban drainage system has not lead to any major improvements in terms of the levels of protection offered by the system. However, the investigations have provided a better insight into how some of the key parts of the system operate and their robustness under stress, whether that be due to extreme rainfall events, climate change or breakdowns. This has allowed a series of schemes comprising both capital investment and operation/maintenance activities to be identified aimed at addressing the known remaining flooding issues in Lewes. A methodology has been developed to prioritise these schemes on the basis of their relative costs and benefits. It is believed that this prioritisation approach could have application on a wider basis for other IUD studies. The schemes, with prioritisation scores are presented in Table 3 of the report.
The purely technical aspects of quantifying flooding mechanisms and risks and identifying measures to improve these aspects of performance were only part of the overall purpose of the pilot study. Of equal importance was the process of getting the stakeholders together and discussing the issues in a positive and constructive manner. This environment has allowed problems which were previously “too-difficult-to-tackle” to be successfully progressed. The involvement of an objective, independent third party in the form of the consultant who undertook most of the work was also considered to be helpful in facilitating the process of finding the most appropriate solutions.
The main product of the pilot study was the Lewes Integrated Urban Drainage Plan (presented in Section 4 of the report) which brings together a prioritised investment plan spanning all parts of the drainage system (Table 3) with procedural and policy proposals and recommendations. The total investment requirement to implement the plan is estimated to be in excess of £12.3 millions with annual operating costs of around £110,000.
The key parts of the procedural and policy proposals can be summarised as follows:
- A plan is required to manage system exceedance flows in the town, i.e. when the system capacity is exceeded by an extreme event.
- The condition and function of the Cliffe Culverts system needs be understood and a rehabilitation and subsequent management scheme put in place.
- The location and ownership of all sewer systems draining to, and flood walls along, the River Ouse needs to be determined and a maintenance programme put in place.
- Gully maintenance should be enhanced within high risk areas.
- The general public, including riparian owners, need to be made aware of the importance of drainage assets and how they should be treated.
- Key infrastructure should be identified and suitable levels of flood resilience provided.
- Data collection programmes should be initiated for those parts of the system where basic data are not currently available.
A programme of activities (Appendix A) is planned following conclusion of the pilot study which will address some of these considerations.
The study concludes that, whilst recent capital investment has improved the flooding situation in Lewes, significant numbers of properties remain at risk of flooding on an unacceptably frequent basis and that, if nothing is done, this situation will get worse with the effects of climate change and the deteriorating condition of drainage infrastructure. The integrated strategy which has been developed in this pilot study provides an opportunity to alleviate these risks. It should provide a useful tool for targeting future investment and managing potential future development. However, the policy and procedural issues which have been identified also need to be implemented for the full benefit of the strategy to be realised.
Outputs
Page last modified: 25 June 2008
Page published: 5 March 2007