Reservoir safety: a new model to estimate long-term period rainfall

Developing a new statistical model of point rainfall depth-duration-frequency (DDF) for the UK to replace the current Flood Estimation Handbook (FEH) DDF model.

• From: Environment Agency, Flood and Coastal Erosion Risk Management Research and Development Programme, and Department for Environment, Food & Rural Affairs
• Published: 18 February 2021

• Category: Understanding risks from all sources of flooding
• Topics: Asset monitoring, Climate change, Flood forecasting, Flood risk assessment, Meteorology, Modelling, Reservoirs
• Project status: Completed
• Project code: FD2613
• Date of completion: 31 March 2008

Documents

Reservoir safety: a new model to estimate long return period rainfall - technical report 1 (18 MB) PDF

Reservoir safety: a new model to estimate long return period rainfall - technical report 2 (11 MB) PDF

Reservoir safety: a new model to estimate long return period rainfall - appendices (31 MB) PDF

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Details

This project investigates alternative ways of estimating extreme rainfall relevant to reservoir safety for the whole of the UK. It presents the results of a major project to develop a new statistical model of point rainfall depth-duration-frequency (DDF) for the UK. This is to replace the current Flood Estimation Handbook (FEH) DDF model.

Background

Reservoir engineers have expressed concern about the apparently high estimates produced by the FEH DDF model. In many locations, the FEH model was giving 10,000 year estimates considerably higher than the Flood Studies Report (FSR) probable maximum precipitation (PMP). This is used to calculate the likely maximum flood as a statutory part of the spillway design procedure for major reservoirs.

Outcome

In this project, the framework of the FEH approach to rainfall modelling has been retained, but several key elements have been substantially revised. In particular, the Focused Rainfall Growth Curve Extension (FORGEX) methodology has been reformulated and the dataset of annual maximum rainfalls, to which the final model is fitted, has been updated.

The new model estimates rainfall depths falling over durations ranging from one hour to 192 hours (eight days) for return periods ranging from two years to 10,000 years. However, in many locations it can produce indicative estimates for higher return periods (up to 100,000 years).

This research has led to several revisions to the FORGEX methodology. A new model of rainfall DDF has also been developed, which can be applied at any location in the UK where there’s sufficient rainfall data available. The report sets out an implementation programme for the new model and makes suggestions for future research.

This project ran from 2004 to 2008 at a cost of £347,980.

Published 18 February 2021