Volumetric run-off coefficient in drainage design

The volumetric run-off coefficient is the proportion of rainfall landing on a surface that leaves it as run-off, and it is the number that drives long term storage volume in a drainage strategy. It is not the same thing as the peak rate control that policy sets. A volumetric runoff coefficient of 0.95 applied to a roof means 95 per cent of the rain that lands on it has to go somewhere, and the storage has to be sized for that volume rather than for the flow rate at the outfall.

Confusing volume with rate is the most common error in a small scheme's drainage calculations, and it produces a system that passes the discharge rate check and floods anyway.

What the coefficient does in a model

In surface water modelling, each contributing area is given an area, a surface type and a coefficient. The coefficient converts rainfall depth into run-off depth. Multiply by the area and you have a volume.

Two different coefficients appear in UK practice and they do different jobs. The percentage run-off used in peak flow calculations accounts for losses during an event, including initial wetting of the surface, depression storage and infiltration. The volumetric run-off coefficient used in volume calculations is higher, because over the whole of a long duration event those losses are a smaller proportion of a larger total.

That is why a single number cannot be carried across from a peak calculation into a volume calculation. The strategy has to say which coefficient is applied to which calculation.

Values used for roofs and paving

For impermeable surfaces in volume calculations, UK practice commonly adopts a coefficient of 0.95, which should be confirmed against the industry SuDS guidance for the particular case. The reasoning is that a roof or a sealed carriageway loses very little to infiltration, and once it is wet it sheds almost everything.

Pervious and semi-pervious surfaces are harder. A permeable block paving construction over a granular sub-base does not behave as an impermeable area at all, and it should be modelled as a component with its own storage and infiltration rather than given a coefficient. Gravel, reinforced grass and green roofs each need their own treatment, and claiming a low coefficient for them without modelling the build-up is the kind of shortcut a reviewer will query.

Pervious areas that drain to the system, such as a garden that falls towards a road gully, should be included. Pervious areas that drain away from the site should not, and the strategy should state which assumption has been made and show it on the catchment plan.

Why winter profiles give higher volumes

Long term storage is assessed against winter rainfall rather than summer rainfall, which surprises people who expect summer thunderstorms to be the worst case.

Summer events in the UK rainfall profiles are short and intense, so they produce high peak flows and modest volumes. Winter events are longer and less intense, with wetter antecedent conditions, so they produce lower peaks and considerably larger volumes. Peak rate controls are therefore usually sized by a summer profile and storage volume by a winter one.

The long term storage requirement in UK practice is the difference between the volume of run-off from the developed site and the volume that would have left the site in its greenfield condition, for a long duration winter event, commonly the 1 in 100 year 6 hour event. Confirm the event and profile expected by the lead local flood authority, because the figure moves with it.

Effect on long term storage sizing

This is where the coefficient does its work.

Peak rate controls limit what leaves the site per second. Volume controls limit what leaves the site in total over the event. A site can comply with a greenfield run-off rate and still send far more water downstream than the greenfield site would have done, because it sends it for longer. Long term storage is the provision that addresses that, and it is often the item that forces an additional tank or a larger basin.

The sensitivity is direct. Raising the coefficient from 0.90 to 0.95 raises the volume requirement by about five per cent, which on a large impermeable area is a real quantity. Which is why the coefficient should be stated, justified and applied consistently, and why reviewers check whether a strategy quietly used a lower value to make the storage fit the available space.

Where urban creep is applied

Urban creep is the gradual increase in impermeable area after a development is occupied, as front gardens become parking, patios are laid and extensions are built.

The allowance is applied by increasing the impermeable area used in the calculation before the coefficient is applied. Residential development commonly carries an allowance in the order of 10 per cent, with the figure varying by density and by authority, so check the local requirement rather than assuming a national number. Flats with no private external space usually attract a smaller allowance or none, and detached housing with large plots the largest.

Apply it to the areas that can realistically change. Adding an urban creep allowance to a roof makes no sense, and adding it to an adopted highway makes less.

Climate change is a separate allowance again, applied to rainfall intensity rather than to area, using the current published allowances for the relevant river basin district and development lifetime. The two are cumulative and both have to be stated.

An illustrative volume calculation

This worked example is illustrative, and the numbers are chosen to show the method rather than to describe a real site.

Take a roof of 1,000 m² and a rainfall event depth of 50 millimetres over the duration considered. With a volumetric run-off coefficient of 0.95, the run-off volume is 1,000 multiplied by 0.05 metres multiplied by 0.95, which is 47.5 cubic metres.

Now add a car park of 600 m² at the same coefficient, with a 10 per cent urban creep allowance applied to a 400 m² residential curtilage area within the same catchment. The curtilage becomes 440 m² for calculation purposes, and at the same coefficient contributes 20.9 cubic metres. The three areas together produce around 97 cubic metres from that event depth before any allowance for climate change is applied.

The point of setting it out that way is that each step is visible and checkable. A strategy that reports a single storage volume with no breakdown cannot be checked by anyone, including the engineer who wrote it.

What a strategy should state

The Drainage Strategy should carry a table rather than a paragraph, giving for each contributing area:

1. The area in square metres and the surface type.
2. The volumetric run-off coefficient applied, with the source of the value.
3. Any urban creep allowance, with the percentage and the reason.
4. The rainfall event, duration and profile used.
5. The climate change allowance applied to rainfall intensity.
6. The resulting volume, and the greenfield comparison for long term storage.

Alongside that, a catchment plan showing every area coloured by surface type with its reference number, so the table and the drawing can be read together.

Where the lead local flood authority publishes standing advice setting out its expected parameters, follow it and say you have, because that removes an entire round of comments. The role of that kind of published advice is explained in the note on standing advice from planning consultees. Where a drainage condition later needs changing because the parameters move, the route is set out in the note on varying a planning condition under section 73.

Draft a drainage strategy on PlanScope, then have a drainage engineer check the coefficients, the allowances and the greenfield comparison before submission.

This is general guidance, not legal advice; check the position with your local planning authority, the 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.