Long term storage against attenuation in drainage design
Attenuation storage and long term storage answer different questions in the non-statutory SuDS standards. Mixing them up is the most common reason a drainage strategy comes back from the lead local flood authority.
Long term storage against attenuation is the distinction that decides whether a drainage strategy survives its first reading at the lead local flood authority. Attenuation storage exists to hold the peak back so the discharge rate stays within the permitted figure. Long term storage exists to deal with the extra volume of water a development creates over the whole event, which a flow control cannot fix because the water has to go somewhere eventually.
Rate and volume are separate tests in the non-statutory technical standards for sustainable drainage systems, and a scheme can pass one and fail the other. Most strategies that come back for revision have sized the attenuation properly and either ignored volume or quietly counted the same cubic metres twice.
What each volume controls
Attenuation answers this question: in the critical duration event for the design return period, with the climate change allowance applied, what storage is needed so that the outflow never exceeds the agreed rate. The output is a volume in cubic metres and a stage-storage-discharge relationship.
Long term storage answers a different one: how much more water runs off the site now than ran off it before, in the 1 in 100 year 6 hour event, and where is that extra volume held so it leaves slowly enough not to add to downstream flood risk.
The second question only has a meaningful answer once you have fixed the baseline. For a greenfield site the baseline is the greenfield runoff volume for the same event. For a previously developed site it is the volume discharged before redevelopment, with the standards asking for the result to be as close as reasonably practicable to greenfield.
Why volume control exists on brownfield sites
Peak rate control alone can leave a catchment worse off. Hold a large volume back and release it at the greenfield peak rate and you have extended the duration of the discharge, which matters when every site upstream is doing the same thing and the receiving watercourse is still full when the next band of rain arrives.
Volume control is the answer to that cumulative effect, which is why the standards treat it separately and why flood authorities will not accept a strategy that addresses only rate. On brownfield sites the volume test is also where betterment is delivered: the site was once entirely impermeable and discharged everything, and the redevelopment is the opportunity to reduce both the rate and the volume.
Where constraining volume to the greenfield figure is not reasonably practicable, the standards contemplate discharge at a rate that does not adversely affect flood risk, which in practice is the long term storage arrangement with a very low release rate.
Calculating the attenuation volume
The sequence is familiar, and each step has a decision in it that an officer will look for.
- Fix the impermeable area, with an allowance for future paving of private areas where local policy requires it.
- Select the rainfall data and state the source, because which rainfall dataset you cite changes the answer.
- Apply the current climate change allowance for peak rainfall intensity for the relevant river basin district and the development's lifetime, and check the published allowances rather than reusing an older figure.
- Add an urban creep allowance where the authority's policy sets one.
- Establish the greenfield runoff rate for the site, usually the mean annual flood estimate adjusted for the return period, and state the method used.
- Run the critical duration analysis across durations and return periods, rather than the 6 hour event alone, because the critical duration for storage is often far longer.
- Report the storage volume for each return period with the half drain down time.
Half drain down time matters because a basin that is still full when the next event arrives is not providing the storage the model assumed.
Calculating long term storage
The method is a volume comparison rather than a routing exercise.
Work out the total runoff volume from the developed site for the 1 in 100 year 6 hour event, with the climate change allowance applied where local policy requires it. Work out the corresponding greenfield volume for the same event by a recognised method, and name the method. The difference is the volume that has to be managed.
Then decide how to deal with it, in this order of preference. Reduce it, by infiltrating at source through permeable paving, soakaways, rain gardens or trees in structural soils. Infiltrate at depth where the ground and the groundwater table allow. Or hold it in long term storage and release it very slowly.
Infiltration needs evidence. Infiltration testing to a recognised method, at the depth and location of the proposed feature, with the groundwater level established over a wet period and a check on source protection zones and contamination. An assumed infiltration rate is not evidence, and a strategy built on one gets a holding objection.
Where the two can share a structure
A single detention basin commonly provides both. The lower zone holds the long term storage volume and discharges at a very low rate or infiltrates. The upper zone provides attenuation and discharges through the main flow control.
What the submission has to demonstrate is that the two volumes are not the same cubic metres counted twice. That means showing, in the model output, that the long term storage zone is occupied at the start of the design event or is otherwise unavailable, and stating how long it takes to empty. Where the long term storage drains down over a period of days, say what the assumed starting condition is.
The other credible arrangement is to separate them physically: long term storage in a permeable paving sub-base or an infiltration feature at the head of the system, attenuation in a basin or tank downstream. That is easier to explain and easier to maintain, and it keeps the two calculations visibly independent.
Do not forget interception. The standards and the SuDS Manual both expect the first few millimetres of rainfall to be intercepted so that small events produce no runoff at all, and the features that achieve it are usually the same ones doing the volume reduction.
Discharge rates and the blockage problem
The arithmetic of long term storage pushes towards very small discharge rates, and reality pushes back. Below roughly 2 litres per second a conventional orifice is small enough to block with leaf litter, silt or a plastic bag, and a blocked control is a design that does not work in the event it was designed for.
Three practical responses. Use a vortex control, which achieves a low flow rate with a larger aperture. Set a minimum rate agreed with the authority, accepting slightly less than the theoretical ideal in exchange for reliability. Or avoid the control altogether by infiltrating.
Whichever route, show the sensitivity. Modelling a partially blocked flow control is increasingly requested, because it reveals whether the exceedance route is safe when the device underperforms. Where blockage would put water into a building, the design needs changing rather than the model.
Showing both in the strategy
Lay the numbers out so a drainage engineer at the flood authority can check them in ten minutes. The Drainage Strategy should carry:
| Item | What to state |
|---|---|
| Areas | Existing and proposed impermeable area, with urban creep |
| Baseline | Greenfield rate and volume with the method named, and the pre-development rate on brownfield |
| Rainfall | Dataset, location, climate change allowance and source |
| Proposed rates | Discharge rate by return period, and the control device |
| Attenuation | Volume by return period, critical duration, half drain down |
| Long term storage | Volume, release rate or infiltration rate, drain down period |
| Double counting | How the two volumes are kept separate in the model |
| Exceedance | Overland flow routes for events beyond the design standard |
| Maintenance | Who maintains what, and the adoption or management route |
Where the site also sits in Flood Zone 2 or 3, the drainage work feeds the Flood Risk Assessment rather than sitting beside it, and the two documents need the same finished levels and the same climate change assumptions.
One honest uncertainty: greenfield runoff estimation for small or heavily modified catchments carries real error, and two competent engineers can produce different baseline figures from the same site. State the method, show the inputs, and the conversation with the flood authority is about assumptions rather than about who is right.
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Frequently asked questions
What is the difference between attenuation and long term storage?
Attenuation storage holds water back so that the peak rate of discharge in a given event does not exceed the permitted rate. Long term storage deals with the extra volume of runoff a development generates, holding it and releasing it very slowly or by infiltration. One is about litres per second, the other about cubic metres.
Where do the volume control requirements come from?
The non-statutory technical standards for sustainable drainage systems, published in 2015, set standards for peak flow control and for volume control. For a previously developed site the runoff volume in the 1 in 100 year 6 hour event should be as close as reasonably practicable to the greenfield volume and should not exceed the volume discharged before redevelopment.
Can one basin provide both volumes?
Yes, provided the strategy shows the volumes are not double counted. The usual arrangement is long term storage in the lower part of the structure with a very low discharge rate, and attenuation above it. The modelling has to show the long term storage volume is not available to attenuate the same event, and the drain down time has to be stated.
Why do flood authorities resist very low discharge rates?
A tiny orifice blocks. Below roughly 2 litres per second the control device becomes unreliable in practice, so authorities ask for a minimum rate, a larger equivalent aperture through a vortex control, or a design that does not depend on a small opening. The alternative is infiltration, where the ground and the groundwater level allow it.
Does long term storage apply if the site drains to a sewer?
Volume is still an issue, because the sewerage undertaker's capacity is finite and most adopted policies apply the standards whatever the destination. The hierarchy puts infiltration first, then a surface water body, then a surface water sewer, then a combined sewer. Agree the destination and the rate with the undertaker and the flood authority before you fix the layout.
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.