FEH13 rainfall against older depth curves in drainage design

Choosing between FEH13 rainfall and the older depth-duration-frequency curves changes the storage volume a drainage design has to provide, and on a constrained site that difference decides whether the attenuation fits. The current Flood Estimation Handbook rainfall model, published in 2013, replaced the 1999 FEH model, which had in turn replaced the Flood Studies Report rainfall statistics of 1975. Most lead local flood authorities now expect the current model for new designs. The trap is assuming the change in depth runs one way everywhere, because it does not, and the only reliable answer is to extract both sets of parameters for the actual site and compare them.

What each dataset is built from

The Flood Studies Report rainfall statistics were derived from gauge records available up to the early 1970s, with a national growth curve approach and regional adjustment. They are simple to apply and were embedded in a generation of drainage software and design manuals.

The 1999 FEH model rebuilt the analysis on a longer record and produced a gridded depth-duration-frequency model, so depths came from the site's own grid square rather than from a regional curve. The 2013 revision refitted that model on a further 14 years of data and changed the way extreme events are pooled, which is where the differences in the tail of the distribution come from.

All three produce a rainfall depth for a given duration and return period. None of them tells you the profile, which comes separately, and none of them accounts for climate change.

Where the newer model gives larger depths

For many English locations FEH13 returns larger depths than FEH99 at long return periods and short durations, which is the combination that sizes attenuation for a 1 in 100 year design. In other locations, and at other durations, it returns smaller depths. The pattern is regional and it interacts with duration, so a rule of thumb picked up on one scheme will mislead on the next.

That has a practical consequence for fee-earning work. Where a site is tight, extract the parameters early, run the critical duration analysis against both datasets and find out at concept stage whether the storage requirement moves by a few cubic metres or by a third. Finding out after the layout is fixed is how basins end up in the middle of a plot.

The rainfall depth is only one input. Greenfield runoff rates estimated by the IH124 method or the FEH statistical method, the areal reduction factor, urban creep, the summer and winter profiles and the climate change allowance all move the answer, and a change of rainfall dataset can be swamped or amplified by any of them.

Effect on attenuation volume

Storage volume responds to the difference between inflow and the permitted discharge rate across the critical duration event. Because the permitted rate is usually fixed by the greenfield calculation or by a sewer undertaker's limit, extra rainfall depth converts almost directly into extra volume.

Test the full range of durations for each dataset rather than carrying the critical duration over from one run to the other. A change in the depth-duration relationship can move the governing event from a short intense storm to a longer one, and the volume at the new critical duration is the number that matters.

Which dataset the authority expects

The Defra non-statutory technical standards for sustainable drainage systems set the peak flow and volume requirements but do not prescribe a rainfall dataset. The expectation comes from the lead local flood authority's own guidance, published as part of its role as statutory consultee on major applications, and from the sewerage undertaker where a connection is proposed.

So the question is answered by reading two documents before the modelling starts: the lead local flood authority's drainage guidance for developers, and the undertaker's design and construction requirements. Where the local guidance is silent, use the current FEH model and say in the strategy that you have done so. Nobody has ever been criticised for using the current dataset.

The Drainage Strategy is where this is recorded, and on a site in Flood Zone 2 or 3 it sits alongside the Flood Risk Assessment rather than inside it, because the two documents answer different questions for different consultees.

Transitional arguments on older consents

Schemes consented years ago carry a drainage strategy approved under the dataset of the day. When a condition is discharged or a reserved matters application comes forward, the lead local flood authority sometimes asks for the design to be reworked on current rainfall data.

Whether that request is reasonable depends on what the permission actually secures. If the condition requires a detailed drainage design in accordance with the approved strategy, and the approved strategy specifies discharge rates and storage volumes, the parameters are fixed by the permission and a rerun goes beyond it. If the condition simply requires a scheme to be submitted and approved, the authority is entitled to apply its current standards to what you submit.

The pragmatic route on a live site is usually to run the current dataset, report the comparison, and show whether the consented storage still satisfies the volume requirement. Where it does, the argument disappears. Where it does not, you are better off knowing before construction than during it, and a section 73 application to vary the drainage parameters is cleaner than a retrospective argument about condition wording.

Documenting the dataset and testing between them

A drainage strategy that says "FEH rainfall data used" will come back with questions. Record enough for a reviewer to reproduce the calculation.

1. The rainfall model and its version, with the date the parameters were extracted.
2. The grid reference or catchment descriptors used, and whether a point or catchment average was taken.
3. The areal reduction factor applied and its basis.
4. The storm profiles used for summer and winter, and the durations tested.
5. The peak rainfall climate change allowance, quoting the management catchment, the epoch and whether the central or upper allowance was applied.
6. The urban creep allowance and where the local requirement for it is published.
7. The greenfield runoff method, with the descriptors and the resulting QBAR and 1 in 1 year rates.

Then add a sensitivity table. One page showing storage volume under the current dataset and under the dataset the previous design used, at the critical duration for each, answers the reviewer's question before it is asked and demonstrates that the design is not sitting on the edge of a threshold. Where the two answers are close, say so. Where they differ materially, explain which you have designed to and why.

Climate change allowances are reissued periodically and the Environment Agency guidance is the version to check on the day you run the model, not the version quoted in the last report you wrote. Storage assumptions also interact with reuse, which is covered in rainwater harvesting and whether it counts as attenuation, and the proportionate approach for very small schemes is set out in drainage evidence for a single new dwelling.

PlanScope drafts a Drainage Strategy structured to the sections a lead local flood authority looks for and flags every site-specific figure it cannot verify rather than filling it in, which is the right behaviour for rainfall parameters that have to come from the model run. For specialist technical reports, qualified professional input may be required.

Enter the site postcode on PlanScope to see which flood and drainage documents the site is likely to need.

This is general guidance, not legal advice, so check the position with your lead local flood authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.