Discharge rates: greenfield, brownfield and betterment

The permitted surface water discharge rate is the single number that shapes a drainage design, and on a greenfield site it is normally the greenfield runoff rate for the equivalent return period. On previously developed land the position is different: the Defra non-statutory technical standards for sustainable drainage expect the rate to be as close as reasonably practicable to the greenfield rate, and in any case no greater than the existing rate. That gap between "as close as practicable" and "no worse than now" is where most negotiation with a lead local flood authority happens.

Get the rate agreed before the layout is fixed. A scheme designed around 12 litres per second does not survive being told the figure is 5.

What a greenfield rate means

A greenfield rate estimates how much surface water would run off the site if it were undeveloped, for a given storm return period. It is an estimate derived from catchment characteristics, not a measurement, and two competent engineers can produce different numbers from the same site.

The common reference point is QBAR, the mean annual flood flow, which approximates the 1 in 2.33 year event. Designs are then checked at the 1 in 1 year and 1 in 100 year rates, because the technical standards control peak flow at both ends of that range and the 1 in 1 year control is what protects watercourse morphology from constant scouring.

Rates are expressed in litres per second, often per hectare, and the contributing area matters as much as the method. Use the developed area that actually drains to the control, not the red line area, and say which you have used.

Methods used to estimate it

Two approaches dominate. The IH124 method, from Institute of Hydrology Report 124, is the long-standing approach for small rural catchments and is still widely accepted for sites below about 50 hectares. The Flood Estimation Handbook statistical method is the more modern route and is generally preferred where the catchment descriptors are reliable.

Both are sensitive to the same inputs: area, soil type expressed through the SPR or SOIL index, and annual average rainfall. A soil classification taken from a desk source rather than from the site investigation is the usual reason two estimates diverge.

There is also a floor in practice. Flow controls below roughly 5 litres per second are prone to blockage, so lead local flood authorities commonly set that as a minimum permitted rate for a vortex or hydrobrake control. On a small site the calculated greenfield rate can be well below that, which means the practical rate is set by buildability rather than hydrology.

Brownfield sites and the existing positive connection

On previously developed land the starting point is what currently leaves the site, and that requires evidence rather than assertion. A positive connection to a public sewer or a watercourse has to be demonstrated, usually through a CCTV survey, a drainage record from the undertaker, and a site walkover that confirms the connection is live.

The existing rate is then calculated from the impermeable area draining to that connection, with a runoff coefficient. Claiming the whole site as impermeable because it is covered in broken hardstanding is the mistake authorities look for first. Verges, scrub and areas that drain to ground do not count.

Where no positive connection can be proved, the authority will often treat the site as greenfield for the purpose of setting the rate. That outcome is common on cleared sites where the old drainage was grubbed up, and it is worth investigating the records before accepting it.

Betterment expectations on redevelopment

The technical standards do not put a percentage on betterment, and neither does national policy. Lead local flood authorities fill that gap with their own published expectations, and the figures vary. Read the authority's current SuDS guidance and the local plan policy rather than carrying a figure across from the last scheme in a different county.

The argument for a lower level of betterment has to be evidenced, not asserted. What persuades is a demonstration that the site cannot physically accommodate more attenuation: proven ground conditions that rule out infiltration, a shallow outfall invert that limits tank depth, a tight site where the attenuation volume already occupies the only undeveloped ground.

What does not persuade is cost. Attenuation storage is expensive and that is not a drainage argument. It becomes a viability argument, which belongs in a different document and carries a different evidential burden.

Where a rate becomes undeliverable

Three situations recur. The calculated rate is below the 5 litres per second practical minimum, so the control cannot be built reliably. The outfall invert is too high for the storage volume needed to achieve the rate. Or the receiving system is a surface water sewer that the undertaker will not accept any flow into at all.

Each has a route through it. A rate below the practical minimum is usually resolved by agreeing the minimum and compensating with volume control. A shallow outfall points to pumped discharge, which authorities accept reluctantly and condition tightly. A refused connection means a different discharge hierarchy decision, and discharge to an ordinary watercourse brings its own consent, as land drainage consent for works to an ordinary watercourse explains.

Raise all three early. A lead local flood authority asked for a view at pre-application stage is far more flexible than one presented with a fixed layout at week six of a thirteen week determination.

Agreeing the rate before design

The sequence that works is short. Establish the existing situation with survey evidence. Calculate the greenfield rate by a named method with the inputs stated. Propose a rate with the betterment position reasoned. Put all of it in a two page note and ask the lead local flood authority to confirm before the engineer starts sizing storage.

For a major application the lead local flood authority is a statutory consultee on surface water drainage, so its position will be in the officer's report whatever you do. Getting it in writing first turns a consultation response into a formality.

An illustrative example

A 1.8 hectare former depot drains through a proven 225mm connection to a surface water sewer. Measured impermeable area is 1.1 hectares, giving an existing rate well above the greenfield figure calculated by IH124. The engineer proposes discharge at the greenfield 1 in 100 year rate, achieved with a geocellular tank and a vortex control set at 7 litres per second, and demonstrates volume control through a lined basin on the only area of open ground. The authority accepts the rate at pre-application stage. This worked example is illustrative and the figures are not transferable to another site.

Evidence the authority wants

The discharge rate section of a Drainage Strategy should be short and fully evidenced. In order of how often it is missing:

1. The method used, named, with every input listed.
2. The contributing area, distinguished from the red line area, shown on a plan.
3. For brownfield sites, proof of the existing positive connection and the impermeable area calculation.
4. The proposed rate, the control device and its diameter.
5. Climate change and urban creep allowances applied, with the source of the uplift stated.
6. The exceedance route when the control blocks.

Where the site also sits in Flood Zone 2 or 3, the rate sits inside a wider Flood Risk Assessment and has to be consistent with the finished floor levels and the sequential approach taken on the layout. Surface water risk on the site itself is a separate question, and reading the surface water flood risk map without overstating it covers how far that mapping can be pushed.

Check the current climate change allowances before you fix anything, because they are revised periodically and a strategy built on a superseded uplift will be sent back.

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