Attenuation storage sizing: durations, uplifts and volume
Attenuation volume is set by a matrix of return periods and durations, not a single storm. The uplifts, the critical duration and the drain down check decide the number.
Attenuation storage sizing comes down to one number, the volume that has to be held back so the discharge from the site stays within the agreed rate through the worst credible event. Getting to that number means testing a matrix of return periods and durations, applying the right rainfall uplifts, and checking that the store empties fast enough to be ready for the next storm. Reviewers reject far more drainage strategies for an unexplained matrix than for an undersized tank.
The arithmetic is only half of it. The other half is showing the working in a form the lead local flood authority can audit without rebuilding your model.
The return periods that have to be tested
Three events do the work, and each answers a different question.
The 1 in 1 year event tests the flow control and the conveyance. Nothing should surcharge to ground level. If it does, the pipe network is undersized regardless of what the storage does.
The 1 in 30 year event tests whether any part of the site floods. Under the Defra non-statutory technical standards for sustainable drainage systems, flooding of any part of the development other than designed storage areas is not acceptable in events up to this return period.
The 1 in 100 year event with the climate change allowance is the sizing case. No property on or off the site should flood, and there should be no increase in flooding elsewhere. This is the run that sets the storage volume in almost every scheme.
Some authorities ask for additional runs, commonly a 1 in 2 year for water quality and habitat considerations or a sensitivity run at a higher allowance. Ask at pre-application rather than assuming, and record what was agreed in the Drainage Strategy.
Climate change and urban creep
Two uplifts apply and they do different things.
The peak rainfall intensity allowance comes from the Environment Agency and is published by river basin district and management catchment, for defined epochs and percentiles. The allowance for a 2070s central estimate in one catchment is not the allowance for another, and the figures have been revised more than once. Take them from the current gov.uk page at the time of design, state the catchment, the epoch and the percentile in the report, and give the date you took them. A reviewer who cannot see which allowance you used will ask, and that is a month of correspondence.
Urban creep allows for the paving over of gardens and the extension of driveways during the life of the development. The usual approach adds a percentage to the impermeable area for residential schemes, with the percentage varying by density, and CIRIA C753 sets out the reasoning. Apply it to the contributing area before the model runs rather than adding a lump to the storage afterwards.
Where the development lifetime justifies it, be explicit about the epoch you have designed for. A residential scheme with a 100 year lifetime is normally assessed against the later epoch, and using the 2050s figure for a housing site is a common reason for a holding objection.
Finding the critical duration
The critical duration is the storm length that produces the largest storage requirement, and it is not the same as the storm that produces the highest peak flow.
Run a full range. For a small attenuated site that means durations from around 15 minutes to 24 hours as a minimum, and for schemes with tight discharge rates and large volumes it means going considerably further, sometimes to several days. The pattern is predictable once you have run a few: as the permitted discharge rate falls, the critical duration lengthens, because the store fills over hours rather than minutes.
Run summer and winter profiles. Winter profiles combined with longer durations often produce the controlling volume on an attenuated site, while summer profiles control the pipe network.
State the rainfall dataset. FEH13, FEH99 and the older FSR rainfall models give different depths for the same point, and a reviewer comparing your volume against their own quick check will get a different answer if they do not know which you used. Give the grid reference, the dataset and any adjustment factor applied.
| Run | Purpose | What failure looks like |
|---|---|---|
| 1 in 1 year, all durations | Network and flow control check | Surcharge to ground level |
| 1 in 30 year, all durations | On-site flooding check | Water outside designed storage areas |
| 1 in 100 year plus allowance | Storage sizing | Flooding of property, or off-site increase |
| Half drain time check | Recovery before the next event | Store still part full after the design period |
Half drain time and back to back events
An attenuation store that takes three days to empty is not available for the storm that arrives on day two. The convention is that the store should half empty within about 24 hours of the end of the design event, and many lead local flood authorities apply that as a requirement.
The consequence for sizing is direct. A very low discharge rate reduces the outflow and lengthens the drain down, so the rate that looks best for greenfield runoff compliance can produce a store that never recovers. Where the calculated greenfield rate is very low, most authorities will accept a minimum practical discharge rate on the basis that a smaller orifice blocks, and that trade needs stating in the report rather than being applied silently.
Model the drain down explicitly and put the result in the output tables. Reviewers look for it.
Basins, tanks and the volume each needs
Storage form changes the volume you need as well as the cost.
Open basins and ponds carry a permanent or seasonal water level, side slopes and freeboard, so the gross excavation is larger than the attenuation volume. They also deliver water quality treatment, amenity and habitat value, and they can be maintained by a person with a strimmer rather than a jetting crew.
Geocellular crates and tanks store close to their nominal volume with a void ratio typically above 90 per cent, which is why they are chosen on tight sites. The costs sit elsewhere: inspection and maintenance access, adoption difficulty, groundwater and contamination constraints, and a replacement liability under the road or car park in 40 years.
Ground conditions decide whether infiltration can reduce the volume at all. Where infiltration is proposed, in-situ testing to a recognised method is needed, along with groundwater level monitoring and a check on Source Protection Zones and contamination. A Geotechnical Assessment that reports actual soakage test results, rather than a soil description and an assumed rate, is what a reviewer needs to accept an infiltration component.
Freeboard and exceedance
Freeboard above the design water level in an open basin is normally around 300 millimetres, and the exact figure should follow the authority's local guidance. Include it in the level design and show it on the sections.
Exceedance is the event beyond the design case. Show where water goes when the 1 in 100 year plus allowance is exceeded, or when the outfall blocks. That means an overland flow route plan with arrows, levels and a destination that is not a doorway. Finished floor levels set above the modelled water level, with a stated margin, do the rest.
Do not present exceedance as a paragraph of reassurance. A drawing with contours and flow arrows answers it and text does not.
Presenting outputs a reviewer can audit
The reviewer's task is to reproduce your conclusion without your software. Give them what that needs.
- A network schedule with node references, invert levels, cover levels, pipe sizes, gradients and contributing areas.
- The full results matrix for every return period and duration run, including the cases that were never critical.
- The rainfall dataset, grid reference, climate change allowance with epoch and percentile, urban creep percentage and the date each was obtained.
- The discharge rate calculation, including the greenfield method used and any minimum practical rate applied, with the reasoning.
- Storage volumes broken down by component, with the half drain time result.
- The exceedance plan and the finished floor level schedule.
Cross-refer all of it to the Flood Risk Assessment rather than repeating it, and keep the model files available in case the authority asks for them.
One honest uncertainty. Practice on minimum discharge rates, urban creep percentages and acceptable drain down periods varies between lead local flood authorities, and there is no single national figure for any of them. Agree the parameters in writing before the model is built, because rebuilding a storage design after a holding objection is expensive and slow.
This is general guidance, not legal advice. Drainage design for planning should be prepared or checked by a suitably qualified engineer, and for specialist technical reports qualified professional input may be required.
To see which drainage and flood documents a site is likely to need, browse the PlanScope report types.
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Frequently asked questions
Which return periods must a drainage strategy test?
Typically the 1 in 1 year for the network and flow control, the 1 in 30 year for flooding of any part of the site outside designed storage, and the 1 in 100 year with the climate change allowance for flooding of property and off-site effects. Some lead local flood authorities ask for additional runs, so confirm the scope at pre-application.
How do I find the critical storm duration?
Run the full range of durations for each return period and take the one producing the largest storage requirement. As the permitted discharge rate falls, the critical duration lengthens, so tightly controlled sites often need runs well beyond 24 hours. Run both summer and winter profiles, since winter profiles frequently control attenuated volumes.
What climate change allowance should I apply?
Take the peak rainfall intensity allowance from the current Environment Agency guidance for your river basin district and management catchment, choosing the epoch appropriate to the development lifetime and the percentile the authority expects. State the catchment, epoch, percentile and the date you obtained the figures in the report.
Why does half drain time affect the storage volume?
A store that has not emptied cannot accept the next storm. The usual convention is that attenuation storage should half empty within around 24 hours of the design event. A very low discharge rate lengthens drain down, so the rate chosen to match greenfield runoff can produce a store that never recovers between events.
Can infiltration reduce the attenuation volume needed?
Only where in-situ testing supports it. A reviewer needs actual soakage test results at the depth and location of the proposed feature, groundwater monitoring data, and confirmation that Source Protection Zone and contamination constraints do not rule infiltration out. A soil description with an assumed rate will not be accepted.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.