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Scoping the Risk Assessment Process for Small Reservoirs - FD2640

An assessment of the available data sources has shown that individual reservoir parameters (such as dam height and storage volume) are likely to be costly and time consuming to acquire, and may be of questionable accuracy. In contrast reservoir locations, ground topography and details of flooding receptors are relatively easy to obtain and are generally of good quality.

Three methodologies that allow an assessment to be made of the risk posed by small raised reservoirs were scoped and tested. The analysis demonstrated a preferred methodology being an `Intermediate method` whereby the risk assessment is based on detailed modelling but with assumptions made regarding reservoir details. Whilst this method cannot produce individual risk assessments for each reservoir as it uses generic breach hydrographs, it provides a detailed assessment of the potential level of risk of each reservoir location.

The safety of reservoirs above a certain volume in England and Wales is currently governed by the provisions contained in the Reservoirs Act 1975, which aims to reduce the risks posed to public safety from a reservoir or dam failure which may lead to severe flooding. The provisions of the Reservoirs Act currently only apply to “Large Raised Reservoirs” (LRRs) which hold, or are capable of holding, more than 25,000m3 of water above the natural level of any part of the land adjoining the reservoir. The provisions are not risk based, and provides for all LRRs to be subjected to the same level of statutory engineering provision. Furthermore, the legislation does not provide for smaller reservoirs to be appropriately supervised through their construction or operational phases and essential features of the legislation have not changed significantly since 1930s.
As part of the draft Flood and Waters Bill an update to the legislation is proposed which aims to introduce a more proportionate, targeted, and risk-based approach which better reflects the danger that reservoir failure may post to human life.

As part of this new approach, the minimum size of reservoirs covered by the legislation has been suggested to be dropped to 10,000m3, a suggestion that is supported by the Environment Agency. However, there is also the ability to include others below this should significant risks become apparent and to raise minimum if it appears too low. Furthermore, the opportunity to exempt certain types of reservoirs is being explored in secondary legislation.

The purpose of this project is to scope the process and means to undertake a risk assessment for small raised reservoirs (<10,000m3). The project will need to determine what information is required, as well as what is available, in order to undertake these risk assessments which need to include risks to society and the environment. The project will undertake case studies to verify the promoted methodology although a risk assessment for all small reservoirs is not required at this stage. This project will then feed into the evidence base for determining an appropriate minimum sized reservoir which to be used in revised legislation.

Project Documents
• Final Report : FD2640 SID5 Final Report   (72k)
• Final Report - SID5A : FD2640 SID5a Project Report   (44k)
• Technical Report : FD2640 Technical Report   (2902k)
• Technical Summary : FD2640 Technical Summary   (39k)
Time-Scale and Cost
From: 2010

To: 2010

Cost: £34,380
Contractor / Funded Organisations
Mott MacDonald
Environmental Protection              
Flood and Coastal Defence              
Flood Defence              
Policy Development              
Fields of Study
Flood and Coastal Erosion Risk Management