Getting rainfall and run-off data for a drainage note

A small drainage note lives or dies on where its rainfall data came from. Four numbers do most of the work: the rainfall depth for the design event, the greenfield run-off rate for the site, the impermeable area being drained, and the climate change uplift applied to the rainfall. Cite the source and the date of each one and a lead local flood authority officer can check the calculation in ten minutes. Leave them unattributed and the note comes back, whatever the arithmetic says.

This is about assembling and citing those inputs for a minor scheme, not about designing a drainage system, which is a different job for a different person.

Data a small drainage calculation needs

Six inputs, and each has a defensible public or licensed source.

• Site area, in hectares, measured from the red line boundary.
• Existing and proposed impermeable area, in square metres, measured from the drawings.
• Rainfall depth or intensity for the design events, from a recognised rainfall model for the site's grid reference.
• Greenfield run-off rate, estimated by a published method.
• Climate change allowance for peak rainfall intensity, from the current published allowances.
• Soil or infiltration information, from published mapping first and from testing before anything is designed.

Write them into a table at the front of the note with the source, the date and the version of each. That single table is what turns a set of figures into evidence.

Rainfall depth and return periods

Rainfall depth for a given duration and return period comes from a depth duration frequency model. The current generation of these models is derived from the national rain gauge record and is queried by grid reference, so the figures are site specific rather than regional. Access is normally through a licensed web service, and the output should be saved and appended to the note showing the grid reference, the model version and the date of the query.

Return periods are conventionally expressed as annual exceedance probabilities or as one in so many years. For surface water design on a small site the events that matter are the one in one year event, used for the long term storage and interception requirement, and the one in one hundred year event, used for the peak rate and attenuation volume. The non-statutory technical standards for sustainable drainage systems set the expectation that peak run-off from a developed site should not exceed the greenfield rate for those events.

Two pitfalls. Do not mix a rainfall depth for one duration with a run-off rate derived for another, because the critical duration for a small site is often short and has to be tested rather than assumed. And record whether the figures you are using are depths in millimetres or intensities in millimetres per hour, since transcribing one as the other is the most common error in these notes.

Estimating greenfield run-off for a small site

Greenfield run-off is the rate at which the site would shed water in its undeveloped state, and it is the benchmark the developed site is measured against.

The established approach for ungauged sites estimates the mean annual flood, usually written as QBAR, from catchment area, standard average annual rainfall and a soil index, then scales it to the required return periods. The method is published and reproducible, which is why authorities accept it. Where the site is very small, below the area the method was derived for, the convention is to calculate the rate for a larger minimum area and apply it pro rata, and to say in the note that you have done so.

Three things to record every time:

1. The area used in the estimate, and whether it is the whole site or the part draining to the outfall.
2. The soil index or hydrology of soil type value, and its source.
3. The standard average annual rainfall value, and its source.

Authorities frequently apply a minimum discharge rate below which they will not require attenuation, on the basis that very low rates block small pipes. That minimum is set locally, so read the lead local flood authority's published guidance rather than assuming a figure. Our note on the greenfield run-off rate section of a drainage strategy goes further into how that section is written up.

Measuring impermeable area from a drawing

This is the input most often got wrong, and it is the easiest to get right.

Measure from the proposed site plan, at a stated scale, and colour up what you measured on a copy that goes in the appendix. Include roofs, including any overhang, and all hard surfaces that drain to the system. Exclude areas that drain to a separate outfall, and treat permeable paving according to how it is constructed rather than by name, because permeable paving over an impermeable liner with a drain is not an infiltration surface.

Do the same for the existing site, because the comparison between existing and proposed impermeable area is what determines whether a betterment requirement applies. On a redevelopment site with a large existing hardstanding, the honest existing figure may be the single most useful number in the note.

Where residential development is proposed, authorities commonly require an allowance for future paving over of gardens and driveways, usually described as urban creep. The percentage applied varies between authorities, so take it from the local standard and cite it.

Climate change uplift and where to find the figure

Peak rainfall intensity allowances are published by the Environment Agency and are revised periodically. They vary by river basin district, by management catchment in some cases, by the allowance category, and by the epoch the development is designed for. They are not a single national percentage, and a note that applies one is out of date.

Take the figure from the current published allowances page, record the date you accessed it, and state the river basin district, the allowance category and the epoch you used. Where the scheme has a long design life or serves a vulnerable use, the higher allowance category is normally expected, and the lead local flood authority will say which it wants.

The same page carries peak river flow allowances, which matter for a site affected by fluvial flooding rather than for surface water design. Do not confuse the two sets of figures.

For sites in Flood Zone 2 or 3, or of one hectare or more, the surface water work sits inside a Flood Risk Assessment rather than standing alone, and our note on reading a flood map for planning and extracting it correctly covers how to evidence the zone.

Presenting a calculation a reviewer can follow

Structure the note so that a reviewer never has to guess.

Open with the input table described above. Then set out the existing situation, the proposed situation, the discharge hierarchy considered in order, the selected discharge route with the reason the higher options were rejected, the calculation itself with units on every figure, and the resulting storage volume and control device. Append the rainfall output, the greenfield run-off output, the coloured impermeable area plan and any infiltration test results.

Two presentational habits earn goodwill. Show the discharge hierarchy explicitly, because authorities want to see that infiltration was considered before a surface water sewer connection, and evidenced refusal from the undertaker where a connection was sought. And state the critical duration your calculation identified rather than only the storage volume, since that is what shows the duration range was actually tested.

If the note supports a Drainage Strategy for anything beyond a very small scheme, keep the two consistent in area, phasing and outfall, because they will be read together.

When to hand the work to a drainage engineer

There is a point at which a well cited note stops being enough, and it arrives sooner than most people expect.

Hand over where the site relies on infiltration and the ground conditions are uncertain, where attenuation has to be provided below ground in a constrained layout, where there is an existing culvert, watercourse or ordinary watercourse consent issue, where the site is in a critical drainage area, or where the lead local flood authority has asked for modelling rather than calculation. Also hand over for any major application, since the lead local flood authority is a statutory consultee and the submission will be assessed by a specialist.

One honest limit worth writing into the note. Published soil and geology data supports a preliminary view on infiltration and nothing more. If the strategy depends on soakaways, the infiltration testing has to be done, and the layout needs a fallback in case the results come back badly.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

PlanScope can draft the supporting drainage and flood risk documents in the structure an authority expects, marking clearly every figure that needs confirming from site data. For specialist technical reports, qualified professional input may be required.

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