Which rainfall theory should I use when designing and simulating drainage networks for the UK – FSR or FEH? At the support desk for InfoDrainage, we frequently hear this question.
For drainage designs to be valid, they have to meet the no-surcharge, no-flood criteria. Pipes have to be sized adequately and effective storage solutions need to be built. To do that, it’s vital to understand and accurately represent the inflows entering a drainage network.
This makes it very important to use the right rainfall methodology for each site that you are working on. Get this wrong, and it could spell disaster. Your model could predict no flooding, but the reality might be a completely different story. Remember: different rainfall theories will predict different rainfall levels.
In the UK, we are rather lucky to have two established rainfall theories:
- FSR: Flood Studies Report (FSR)
- FEH: Flood Estimation Handbook (two iterations; FEH99 and FEH13)
The debate between FSR and FEH rainfall is a long one – still ongoing. There are pros and cons for each, and there is no UK-wide legislation stipulating which one should be used. So, what is deemed acceptable by LLFAs across the UK varies significantly.
How the two methods compare
We’ve created this handy chart to show you how the methods compare:
Note that the percentage differences between FSR and FEH data will vary across the UK. Sometimes the difference will be small, and sometimes it may be more significant. The obvious pro for using FEH13 data versus FSR data is that it’s based on more recent rainfall data, enhanced statistical analysis, and a greater number of rainfall gauges.
We recommend checking which rainfall theory will give you the worst case scenario on a site-by-site basis. You can do this in InfoDrainage as it allows both methodologies to be used. FSR is embedded directly, and FEH data can be loaded in via download from the FEH web service. Early consultations with your LLFA will help you understand which rainfall theory they require their drainage designs to be designed in.
Different approving authorities may require you to use a particular methodology. In the absence of guidance from the approving authority, you should refer to the references for FSR and FEH and the various limitations of each method to make an engineering judgement on the most suitable methodology; often the rainfall method generating the largest intensities is chosen to ensure a conservative design. Rainfall can vary significantly for different areas, durations, seasons, and return periods. It may be prudent to check with both rainfall methods.
Heat map of the UK
This ‘heat map’ of the UK is based on a single return period, single storm duration, and single seasonal profile. The relationship between FSR and FEH data is more complex than simply the geographic location, and we can show that the FSR/FEH relationship (for a fixed location) varies with return period. It also varies with seasonal profile and storm duration. The dark red area shows where FEH gives ~40% higher rainfall intensity compared to FSR. The dark blue area shows FEH giving ~30% lower rainfall intensity compared to FSR.
How the difference plays out in an example
The table below illustrates the difference in peak rainfall intensity predicted between FSR and FEH for Oxford, UK for a 60-minute, summer storm.
Common questions and myths about FEH
Can you use FEH data for sub-hourly events?
We asked our neighbours in Wallingford – the FEH team at the Centre for Ecology and Hydrology (CEH) – about this. They told us that both FEH13 and FEH99 were based on hourly and daily rain gauge data and did not focus on sub-hourly events.
CEH said that, however, FEH Vol. 2 states that ‘some extrapolation beyond this range is justified, for example for durations as short as half an hour’ and elsewhere ‘it is unlikely that the processes governing half-hour extreme rainfalls are very different to those governing 1-hour extremes’.
They also stated that FEH99 data taken from the CD-ROM allowed the results to be extrapolated down to a duration of 30 minutes. This was found to be roughly equivalent to the FSR results. The FEH CD-ROM used to give a warning if a user tried to estimate rainfall from FEH99 for durations of less than 30 minutes. This is because the DDF model was being used to extrapolate depths below the duration at which data was available for the study. However, despite the warning it still provided 15-minute estimates in the export file.
CEH found that there was a user requirement for sub-hourly data. As FSR included sub hourly events, CEH used the ratios of FSR depths for hourly events to the required sub-hourly event to calculate the FEH13 data for sub-hourly events. For FEH13, CEH extrapolated the model down to 5 minutes. There is no warning message when extracting data for durations of less than an hour. We asked CEH for their advice. They told us that sub-hourly values should be used with caution as is the case for FEH99. While an improvement on the FEH99 estimates below one hour, they said that the sub-hourly durations provided for the FEH13 model are therefore outside of the original FEH13 model development.
Myth: you cannot use FEH data for a small site
This may have been the case when the only option was to use FEH99 data which was purely catchment based. Since the FEH13 release, the CEH has enabled users to select point-based rainfall data instead of catchment based. Rainfall can vary a lot across a catchment.
Myth: FEH data makes models unstable
We would expect that unstable analysis is more likely to be caused by network layout rather than being attributed directly to FEH data, but this will obviously be model-specific.
Common causes of instability are:
- V steep pipes (eg, steeper than 1:10)
- ‘Hydraulically’ short pipes (relative to pipe diameter)
- Offline loops
- Location of storage systems relative to flow controls
- Junction type manholes
- Overall lack of storage in the network
More questions? Explore our docs.
Stlll deciding which one to use? You’ll find out even more in our docs about FEH Rainfall and FSR Rainfall options and how to pick the right one using the Rainfall Ribbon in InfoDrainage. Want to go even further? Explore the UK government’s most recent revitalization advice on using FEH and FSR to estimate rainfall runoff.