Duncan Faulkner discusses the overdue update of UK reservoir flood estimation procedures

Reservoirs are vital assets, but they can also be liabilities. The safety of reservoirs is taken very seriously by the water industry, and the UK has been mercifully free of catastrophic flooding from reservoir breaches for many years.

There have been some near misses though. Recent years have seen several concerning incidents of damage to spillways during extreme floods in England and Scotland. There has been a trend towards heavier rainfall and larger flood flows in some upland areas where many reservoirs are located.

Dated procedures

JBA Cruachan Dam and Reservoir
Cruachan Dam and Reservoir

Design floods for high-risk reservoirs in the UK and Ireland are currently estimated using a method that, in many respects, has not changed since 1975. Although estimates of design rainfall for reservoirs in the UK have recently been updated, other aspects of the design method are dated. Meanwhile, methods for river flood estimation have passed through several generations of updates since the 1970s.

I’ve been involved in flood studies on a wide range of reservoirs, from small flood storage dams and ornamental ponds to large hydro-electric dams. After leading JBA’s hydrological analysis on a number of reservoir safety projects, I have become convinced that an update to reservoir flood estimation procedures is overdue in the UK.

Reservoir flood estimation: Time for a re-think

In 2016, Jeremy Benn and I published a paper on aspects of the design flood estimation procedure that are in need of an update in the proceedings of the British Dam Society’s biennial conference. We identified six main aspects that need to be reviewed and improved:

  1. Probable maximum precipitation (PMP) estimates are dated and have been exceeded.
  2. The Flood Studies Report rainfall-runoff model, used for estimating design floods for reservoirs, has been superseded.
  3. Flood response times may be much quicker than thought in extreme events on some catchments. This may explain some of the accounts of floods on small catchments having exceeded the estimate of the probable maximum flood.
  4. There is inadequate guidance on spatial variation in snowmelt for reservoir design flood estimation.
  5. There is inconsistency over the allowance for frozen ground in reservoir flood estimation.
  6. Despite the routine application of climate change uplifts for floods used in the design of flood defences and new development, design floods for reservoirs do not usually allow for climate change.

Click here to read the paper abstract.

Do you think this is important?

If, like me, you think it is important to review the reservoir flood estimation procedures, join us in London on Thursday 7 March at the British Hydrological Society and British Dam Society’s one day conference – UK reservoir spillway flood hydrology. Taking place at the Institution of Civil Engineers in Westminster, we have nine presentations lined up from a range of perspectives including dam owners, inspecting engineers, consultant hydrologists, researchers and a geomorphological angle.

Click here to book your place on the UK reservoir spillway flood hydrology event today.

Want to know more?

Email Duncan Faulkner for more information on the event or about any of our hydrology and reservoir work and expertise. You can also find out more about our work in this area on our flood and water resources web pages.

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