Making space for water
Urban flood risk and integrated drainage
Torbay (Environment Agency, South West Region)
Pilot Summary
Defra’s strategy ‘Making Space for Water’ identified the need to improve the management of urban drainage and urban flooding. The Torbay Integrated Urban Drainage (IUDM) Pilot Study was one of fifteen studies commissioned to look at ways of improving the delivery of IUDM.
Led by the Environment Agency, working in partnership with Torbay Council and South West Water, the Torbay Study has focussed on the historical flooding that affects the Union Street and Fleet Street areas of Torquay. It has looked in detail at:
- Identifying key flood problems using computer modelling.
- Recommending the most effective and sustainable solutions, taking account of climate change.
- Gauging public awareness and attitudes to urban flooding.
- Identifying barriers that prevent the partners from working effectively together, and suggesting ways to overcome these.
The current drainage system is mainly highway drainage and public sewers, with all parts of the drainage network eventually flowing into the public combined sewerage system. Although this system does become hydraulically overloaded, this is assessed as occurring at high return periods. Consequently the properties affected do not appear on the water company’s DG5 register, and are not targeted for improvement under current Ofwat requirements. So, under the current duties on each partner organisation, and in line with existing design standards, without a change in policy there is no existing driver that could be used to promote major capital improvements. However, to test IUDM solutions, the pilot study Group agreed on a target design standard for the town centre area of 1 in 100 year protection of property from internal flooding.
The flood risk problems were investigated using a range of models and historic data. A combination of a 1D sewer network model and a 2D overland flow model was found to best represent the interaction of above and below ground systems and the ‘exceedance flooding’ that Torbay suffers from. This joint assessment by partners of the flood risk was made possible by data and model sharing. The combined model has been used to assess the impact of design rainfall events at return periods of up to 1 in 100 years. This showed that the range of predicted flooding to property in Torquay town centre ran from 50 properties during a 1 in 30 year event, up to 97 properties during 1 in 100 year event.
The partnership study has shown that a range of measures will be needed to achieve flooding and water quality goals, and that integrated solutions are the most efficient way to improve the management of urban drainage. To this end, an “Action Plan” has been proposed, which includes consideration of construction of a new large diameter drainage pipe in Union Street and Fleet Street. Funding will be sought for this pipe, to include the submission of a bid in PR09 by South West Water. The basis of this Action Plan is that this pipe would form part of a wider suite of IUDM solutions in the catchment. Other measures sought will include separation of surface water flows from the combined sewer network into a new piped surface water drainage system, or the implementation of sustainable drainage systems (SUDS) that return surface runoff to the ground.
Assessment of the preferred option, which would form the basis of a PR09 bid, showed that it was predicted to reduce the number of properties at risk of flooding during a 1 in 100 year event from 97 to 1, and would have a benefit cost ratio of 2.82. A benefit cost ration of 2.82 is low when compared to the levels normally achieved for fluvial flood risk management schemes. One possible reason why this value is relatively low is that it reflects the high cost per property of providing flood protection from surface water runoff in a fully urbanised location. If this is the case then the method of appraisal of surface water flooding schemes may need further study.
To deliver the Action Plan, the three partner organisations have worked together, sharing data and knowledge, and developing an agreed approach to promote IUDM in the catchment. The partners have recognised the benefits of this joint working, and this has led to an agreement to extend the joint cooperative working on urban drainage management in Torbay beyond the completion of this pilot study project.
The pilot study has also sought to engage with the general public, and with business owners who have been affected by flooding in the past (including a flood event that occurred in August 2007). Engagement with the general public was delivered through running a public exhibition, and requesting that visitors complete a questionnaire about attitudes and opinions on urban flooding. However the exhibition was poorly attended, and the results difficult to interpret. Work with a specialist consultant on public engagement confirmed that the delivery of a full scale public consultation, with statistically meaningful results, is time consuming and costly, and was therefore beyond the means of the pilot study. Engagement with retail business management was more successful. It found that there is active local management of the problem, including the installation of flood resistance and resilience measures, but that this work would benefit from having additional expert advice and co-ordination available locally.
Other key findings of the study included:
- A national standard agreement on joint working, data sharing, data status, etc, would simplify future collaborative projects.
- Clarity of roles and responsibilities is required if multi agency IUDM schemes are to be delivered. This may need to include coordination of investment planning.
- The use of above ground 2D flood flow modelling provided the final element of the catchment modelling needed to verify a suite of modelling tools that accurately replicates urban surface water flooding. It provided a high level of detail on the impacts of flooding, and allowed detailed analysis of proposed IUDM options, including designed overland flow channels.
- The necessary funding triggers or incentives do not exist to resolve existing urban flooding problems satisfactorily. These should be established by Government, with the aim of ensuring that urban drainage systems perform to a higher level of service, meeting the expectations of modern society, and delivering financial benefits to the nation.
Outputs
Page last modified: 25 June 2008
Page published: 5 March 2007