Nothing is more essential to reservoirs than the dams that hold them in. For these, we calculate Probable Maximum Flood – because it could happen.
Dams contain the body of water in a reservoir, with a spillway to ensure water levels are controlled. When the upstream flood is larger than the capacity of a dam spillway a reservoir fills and spills over the top of the dam. This overtopping is a common method of dam failure and can mean catastrophe.
In the case of Category A dams, overtopping or breaching can even endanger lives in a community so we must bring risk down to an absolute minimum. We calculate the Probable Maximum Precipitation (PMP) upstream, and from this we can estimate the Probable Maximum Flood (PMF) that the dam must control.
Estimating extreme floods in the UK
Estimating the largest flood that could theoretically occur in a given catchment, either as a statistically derived design event (a T-year event of a flood occurring within a specific return period) or as maximum precipitation leading to maximum flood, is bound to be complicated.
In the UK, estimating flood events up to 1,000-year return periods is currently achieved using the Flood Estimation Handbook (FEH) methods including the Revitalised Flood Hydrograph (ReFH) rainfall-runoff models (available in the WHS ReFH2.3 software) and the FEH13 rainfall depth-duration frequency model.
For more extreme events current recommendations are to use the restated Flood Studies Report (FSR)/FEH rainfall-runoff model with either the FEH13 or FSR rainfall models.
For the PMF, current methods use a bespoke Probable Maximum Precipitation event (first developed within the FSR) with the restated FSR/FEH rainfall-runoff model.
As well as developing ReFH2.3, WHS undertakes UK reservoir assessments to guide dam design using current best practice.
When commissioned to find PMF and 10,000-year peak flows for dam spillway design in UK reservoirs, we follow the latest methods specified by The Institution of Civil Engineers guidance (ICE 2015).
- Catchment descriptors taken from the FEH Web Service
- Rainfall-runoff models from FSR/FEH
- Rainfall depth-duration frequency models from FEH13 and FSR
- Snowmelt based on Hough and Hollis (1997) using Met Office data
We use the best methods available, including methods such as snowmelt to ensure our estimates are as accurate as possible.
WHS also models dam breaches.
Our work is not limited to PMF and hydrology to assess reservoirs. WHS has undertaken numerous dam-breach modelling assessments in the UK, including the flood risks of failure of several dams in Scotland, using SEPA guidance.
Methods used include:
- hydrograph derivation using FSR/FEH
- 2D hydraulic modelling using a Digital Terrain Model (DTM)
- flood map production showing flood extent, depth and associated hazards.
Back to the future
PMF and PMP derivation is based on methods dating back 45 years to the 1975 FSR.
Crucial as this has been in ensuring Category A dams are safe, the future is uncertain and methods must remain appropriate, both in terms of risks and the effort required.
We are contributing to the Environment Agency’s project to assess current methods of estimating PMP and PMF and identify potential future approaches.
The last word
A study in collaboration with the University of Bath (Pucknell et al, 2020) adapted the PMF methodology to ReFH 2 and showed promising results.
WHS has an excellent understanding of current methods for extreme flood estimations and we aim to improve these further.