Worked greenfield run-off rate for a 0.6 hectare site

A greenfield runoff rate calculation for a 0.6 hectare site usually lands somewhere between 2 and 5 litres per second, and which end of that range you get depends less on the site than on how you handle a formula that was never built for plots this small. Lead local flood authorities check the method before they check the answer. So the worked example below sets out every input, states where each one came from, and shows the two defensible ways of scaling the calculation down.

Everything here is illustrative. The soil and rainfall values are assumptions chosen to make the arithmetic visible, and a real submission needs values read for the actual site.

The inputs the calculation needs

The standard approach for small sites uses the Institute of Hydrology Report 124 method, which estimates the mean annual flood flow, QBAR, from three variables:

• catchment area in square kilometres
• standard average annual rainfall, SAAR, in millimetres
• a soil index derived from the hydrology of soil types classification

The formula is QBAR = 0.00108 multiplied by area raised to the power 0.89, multiplied by SAAR raised to the power 1.17, multiplied by the soil index raised to the power 2.17. The result is a flow in cubic metres per second, which you convert to litres per second by multiplying by 1,000.

For this example the site is 0.6 hectares, which is 0.006 square kilometres. Assume SAAR of 700 mm and a soil index of 0.45, both of which sit in the middle of the national range.

Where soil and rainfall data come from

SAAR and soil index values are read from national datasets rather than guessed. Most practitioners take them from a proprietary hydrological dataset covering the site grid reference, and the report should state the source, the grid reference used and the date of the lookup. Where a Flood Estimation Handbook catchment descriptor set is available, the statistical method is preferred over the older report, and the lead local flood authority will say so if it has a view.

Record the values in a short table in the drainage report. Reviewers spend more time checking whether the inputs are plausible for the location than recalculating the arithmetic, and an unsourced soil index invites a question that delays a discharge of conditions.

Setting out the illustrative calculation

Applied directly to 0.006 square kilometres, the three terms come out as 0.0105 for the area term, 2,142 for the rainfall term and 0.177 for the soil term. Multiplied together with the coefficient, QBAR is 0.0043 cubic metres per second, or about 4.3 litres per second.

That figure has a problem. The report the method comes from was calibrated on gauged catchments, and it is unreliable below about 50 hectares. Applying it to half a hectare extrapolates well outside the data.

The convention that has grown up around this is to calculate QBAR for a 50 hectare catchment using the same SAAR and soil values, then scale the result by area. Run the same formula for 0.5 square kilometres and QBAR comes out at roughly 221 litres per second, which is 4.4 litres per second per hectare. Multiply by 0.6 and the site rate is about 2.6 litres per second.

Two methods, the same site, a difference of 1.7 litres per second. On a scheme of this size that difference changes the attenuation volume by tens of cubic metres and can change the orifice size to something that blocks. State which approach you used and why, rather than presenting one number as fact.

A practical floor also applies. Many drainage teams will not accept a controlled discharge below about 5 litres per second through a single small orifice, because the aperture blocks in service. Where the greenfield rate falls under that, the usual answer is a vortex control device sized to the calculated rate, or an agreed minimum with the flood authority recorded in writing.

Applying the climate change allowance

The secondary question every reviewer asks is what happens in the 1 in 100 year event with an uplift for climate change. Note that peak rainfall intensity allowances are published by the Environment Agency and vary by management catchment and by epoch. The single national 40 per cent figure that circulated for years no longer reflects the published allowances, so quoting it without a source is now a reliable way to attract a holding objection.

Look up the allowance for the relevant management catchment and the design lifetime of the development, cite the table and the date you read it, and apply the central or upper end allowance as the guidance directs for the vulnerability of the development.

The allowance changes the rainfall input to the storage calculation. It does not change the greenfield rate, which describes what the undeveloped site would have discharged.

Comparing proposed against greenfield rates

The test is set by the non-statutory technical standards for sustainable drainage systems. Peak runoff from the developed site, for the 1 in 1 year and the 1 in 100 year events, should be no greater than the rate from the site before development, and where the site was previously developed the standards look for a reduction towards greenfield so far as reasonably practicable.

Set the comparison out as a table with four rows: greenfield 1 in 1 year, greenfield 1 in 100 year, proposed 1 in 1 year, proposed 1 in 100 year. Growth curve factors convert QBAR into event rates, and those factors vary by location, so name the source.

And say plainly what the proposed discharge will be. A reviewer wants one number to condition.

Attenuation volume that follows

Storage volume comes from routing the rainfall through the network, but a simple arithmetic check tells you whether the design is in the right territory. For illustration, assume 0.36 hectares of impermeable area, a 6 hour 1 in 100 year event with the climate change allowance giving 45 mm of rainfall, and a discharge of 2.6 litres per second.

Rainfall of 45 mm falling on 3,600 square metres gives an inflow of 162 cubic metres. Outflow over six hours at 2.6 litres per second is about 56 cubic metres. The difference, roughly 106 cubic metres, is the order of storage the site needs for that one duration.

Real design tests a range of durations, because the critical storm for a small attenuation basin is often longer than six hours. The hand calculation is a sanity check on the model output, nothing more, and every drainage report should say which software and which rainfall model produced the final figures. A Drainage Strategy submitted without that statement of method usually comes back with questions.

Presenting the result for a consultee

Lead local flood authorities read these reports quickly and look for the same handful of things. Give them:

1. The site area and impermeable area, with a plan showing both.
2. The greenfield rate, the method, the inputs and their sources.
3. The chosen discharge rate and the control device proposed.
4. Storage volumes for each modelled duration, with the critical case identified.
5. Exceedance routes for events beyond the design standard.
6. Confirmation of who will maintain the system and under what arrangement.

Cross-reference the figures to the Flood Risk Assessment rather than repeating them, because inconsistent numbers between the two documents are the single most common reason for a drainage condition to stall.

What to do where data is unavailable

Some sites defeat the standard method. A site with no viable outfall, one underlain by made ground with no measured infiltration, or a plot whose soil index reads as urban because the surrounding area is built up.

In those cases, say so in the report and set out the alternative. Infiltration testing to the appropriate standard where soakaways are proposed. An agreed betterment percentage against a measured existing rate where the site is previously developed and the greenfield figure is theoretical. A pre-application enquiry with the flood authority, minuted, where the method itself is in doubt.

Honesty about the limits of a calculation is treated better than a confident number a reviewer cannot reproduce. Write down the assumption, name the person who agreed it, and date it.

This is general guidance, not legal advice, and drainage design for a real site needs a suitably qualified engineer. PlanScope can draft the reporting structure around your calculations, and professional review is strongly advised for the hydraulic content. Start by checking which documents your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.