Half drain time and why attenuation designs get rejected

Half drain time is the time an attenuation feature takes to release half of the volume it stored during the design event. Lead local flood authorities check it because a tank that holds water for two days is not available for the next storm, and most rejections of attenuation design turn on that single number rather than on the storage volume itself.

The volume calculation is usually right. The recovery behaviour is usually where the design falls over.

What half drain time measures

After the peak of a design storm, the attenuation feature is full and discharging through a flow control at a restricted rate. Half drain time records how long the stored volume takes to fall to half its maximum.

It is a proxy for two things at once. It tells the reviewer whether the storage will have recovered before a second event arrives, and it tells them something about treatment performance, because water that sits in a basin or tank for a sensible period allows sediment to settle.

The number is reported by the hydraulic model for each return period modelled. Reviewers look at the critical duration event for the 1 in 100 year return period with the relevant climate change allowance applied, which is the one that produces the largest stored volume and the longest recovery.

Why back to back events matter

English rainfall does not arrive in tidy isolated storms. A system designed to the 1 in 100 year event that takes 40 hours to half empty is, for most of a wet week in November, already part full when the next rainfall arrives.

That is why the design criterion exists and why reviewers will not accept an assurance that the storage is adequate "for the design event". The question is whether the system returns to its starting condition quickly enough to perform again.

Where a site drains to a watercourse that is itself in flood, the problem compounds. Tide locking on a coastal outfall, or a high tailwater level in a receiving sewer, stops the flow control working at its design head and lengthens the drain time well beyond the modelled figure. If the outfall can be drowned, model it drowned.

The criteria applied in practice

There is no single national number, which is the honest answer and the reason designers get caught out moving between authorities.

The widely used industry position, set out in the CIRIA SuDS Manual, is that attenuation storage should half empty within a period measured in hours rather than days, commonly expressed as a range of around 12 to 24 hours. Many lead local flood authorities adopt that range in their published design guidance, some apply 24 hours as a flat maximum, and a few set tighter figures for particular catchments.

Read the authority's own guidance before you size anything. It is published, it is usually a short document, and it saves a resubmission.

Flow control devices and minimum apertures

The tension in most designs is simple. A low permitted discharge rate demands a small opening, and a small opening blocks.

Authorities respond by setting a minimum physical aperture for orifice plates, commonly quoted at 75 millimetres, and by preferring vortex flow controls where the required discharge would otherwise need something smaller. A vortex control achieves a low discharge with a larger physical opening, which is why it appears on almost every constrained site.

Two consequences follow for the drain time. A vortex control's discharge relationship is not a simple orifice curve, so the manufacturer's head-discharge data has to be used in the model rather than an orifice approximation. And where the authority accepts a minimum practical discharge rate, often quoted around 5 litres per second, that floor may be higher than the calculated greenfield rate, which helps the drain time and has to be justified in the text rather than slipped into the model.

Maintenance access to the control chamber is checked at the same time. Our note on trash screens, headwalls and maintenance access covers what the drawings need to show.

Storage that never empties in time

Certain arrangements fail the check predictably.

Deep tanks with a single low-level control drain slowly by definition, because the head falls quickly and the discharge falls with it. Long shallow swales connected in series behave the same way once the downstream cell is full. Permeable paving sub-bases relying on infiltration through a liner that was never tested will sit full for days. And geocellular storage placed below the receiving sewer invert cannot drain at all without pumping, which nobody wants to condition.

The other common failure is an infiltration assumption carried through from a feasibility study. Where a design relies on infiltration, the reviewer wants in situ testing at the proposed depth and location, with the groundwater level recorded and a factor of safety applied. An assumed rate lifted from a soil map is the fastest route to a holding objection.

Evidence to include in the calculations

Submit the working, not the conclusion. A Drainage Strategy that states a half drain time without the output behind it will come back.

1. The model summary tables for each return period and each storm duration tested, with the critical duration identified.
2. The stored volume, the peak discharge and the half drain time reported for each, with the criterion you are designing to stated alongside.
3. The flow control specification, including the manufacturer's head-discharge curve and the physical aperture.
4. The climate change allowance used, with its source, the catchment it relates to and the date the figure was taken.
5. The outfall condition modelled, including any tailwater or tide locking assumption and why it is appropriate.
6. Infiltration test results where infiltration is relied on, with locations shown on the drainage layout.
7. The maintenance schedule, since a design that cannot be maintained will not perform as modelled.

What a consultee looks for line by line is set out in our note on what a consultee checks in a drainage model output.

Revising a design that fails the check

Work through the options in order of cost to the scheme rather than order of engineering elegance.

Splitting the storage into cells with separate controls is usually the first move, because it lets the upper cells drain at a higher rate while the lowest cell holds back the restricted discharge. Raising the control invert increases the available head. Widening and shallowing the storage does the same thing geometrically. Where the permitted discharge rate is the binding constraint, ask the authority whether a higher rate is acceptable in a catchment where the receiving system has capacity, and be ready to evidence that it does.

Reducing impermeable area is the option people skip and the one that helps everywhere at once. Permeable surfacing on parking bays, rain gardens on the frontage and a green roof on the bin store all cut the volume before the control ever sees it.

If the drain time still fails after that, the site is telling you something about its levels. That conversation costs far less at planning stage than at discharge of conditions, when the layout is fixed and the groundworks are priced.

A Flood Risk Assessment and drainage strategy drafted to the right structure, then reviewed by the drainage engineer, is the usual route through. Browse all 66 report types to see what else the site triggers.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.