Exciting new research project on the probable maximum flood (PMF)

We are pleased to announce that our recent bid for an exciting new research project on extreme floods has been successful.

The Environment Agency has commissioned us to lead a project, which aims to assess the suitability of methods for estimating the probable maximum precipitation (PMP) and probable maximum flood (PMF) in the UK.

The PMF is used to check the safety of Category A dams, where a breach of the dam could endanger lives in downstream communities. Current guidance requires that Category A dam spillways are able to discharge the flow of the PMF event without endangering the safety of the dam. The PMF is calculated from a theoretical PMP.

UK methods used to derive PMP and PMF have not been significantly updated since publication of the Flood Studies Report in 1975. Several rainfall and catastrophic flow events have been observed exceeding existing PMP and PMF estimates. The first phase of the project aims to review options for alternative approaches to PMP and PMF estimation and recommend a way forward for implementation in a future phase 2.

The project follows a critique of existing methods of flood estimation for reservoirs published by JBA’s Duncan Faulkner and Jeremy Benn last year:

Faulkner, D. and Benn, J. (2019). Reservoir flood estimation: the way ahead. Dams and Reservoirs 29:4, 139-147. Abstract  

Design floods for reservoirs in the UK and Ireland are currently estimated using a method that in many respects has not changed since the Flood Studies Report of 1975. Although estimates of design rainfall in the UK have been updated, other aspects of the design method and the estimation of probable maximum precipitation (PMP) are dated. Methods for river flood estimation have moved on since the 1970s and there are new and longer-term sources of hydro-meteorological data. Research studies have shown instances of both PMP and probable maximum flood (PMF) estimates using existing methods being exceeded. In this paper we give an overview of aspects of the design flood estimation procedure that are in need of an update and others that tend not to be correctly applied, such as allowance for snowmelt. We identify discrepancies between the different methods used to calculate percentage runoff and time to peak for the 10,000-year flood and the PMF. We discuss the pros and cons of adopting the newer revitalised flood hydrograph rainfall-runoff model for reservoir safety work in the UK. We offer suggestions for development of an up-to-date method for reservoir flood estimation that builds on existing methods, in an international context.


Want to know more?

We are delighted to be supported on this project by the following sub-contractors: Wallingford HydroSolutions, University of Leeds, Newcastle University, University of Bath and Mott MacDonald.

Email Duncan Faulkner for more information about this project.

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