Hydrologist or drainage engineer for a drainage strategy

For most sites the honest answer is that a drainage strategy needs a civil engineer who can design the network, and it needs hydrological input to set the rates that network has to meet. On a single plot the same person does both. On anything above about 20 dwellings, or on any site where infiltration is being claimed, the two skills separate and the lead local flood authority will notice which one is missing.

The confusion is understandable. Hydrology and drainage engineering overlap at exactly the point a planning application cares about: how much water leaves the site, how fast, and where it goes.

What a drainage strategy has to demonstrate

A Drainage Strategy submitted with a planning application has to answer four questions, in this order.

1. Where does surface water discharge, working down the hierarchy from infiltration, to a watercourse, to a surface water sewer, and only then to a combined sewer.
2. At what rate, limited to the greenfield equivalent for greenfield sites and to a betterment on previously developed land.
3. How much storage is needed to hold back the difference, tested across the design return periods including an allowance for climate change.
4. Who will own and maintain each component for the life of the development.

Question one is geology and hydrogeology. Question two is hydrology. Question three is hydraulic modelling. Question four is adoption and legal drafting. No single professional institution covers all four, which is why the competence question keeps coming up.

Hydrological analysis against hydraulic design

Hydrology deals with how a catchment behaves. That means rainfall depth and duration statistics, the proportion of rain that becomes runoff, baseflow, groundwater levels and their seasonal range, and the interaction between the site and any watercourse it drains to. A hydrologist reading a site will look at soil hydrology, catchment descriptors and historic flooding before touching a pipe size.

Drainage engineering deals with conveyance and storage. Pipe gradients and diameters, manhole spacing, flow control devices, attenuation basin geometry, freeboard, exceedance routing when the design event is exceeded, and the hydraulic model that ties them together. The output is a drawing set and a schedule, not a rainfall analysis.

The overlap sits in the middle rows, and most competent consultancies employ both. What matters for the application is that the named author can explain every assumption in the report, because the lead local flood authority will ask about at least one of them.

Infiltration testing and who reads the result

Infiltration is where appointments go wrong. A site investigation contractor digs trial pits and runs falling head tests. The results come back as a rate in metres per second, and someone has to decide whether they support a soakaway.

The test method matters. BRE Digest 365 sets out the soakaway design method used across England, and it expects three successive fillings of the same trial pit with the rate taken between 75 per cent and 25 per cent of the effective storage depth. A single fill on a dry summer day is not a test. Nor is a small-diameter percolation test of the kind used for a septic tank drainage field, which measures something different at a different scale.

Reading the result needs three things together: the infiltration rate, the groundwater level including its likely seasonal maximum, and the ground contamination position. A rate that looks workable in September can be meaningless in February if the winter water table sits within a metre of the base of the proposed soakaway. And infiltration through made ground on a site with a contaminative history can be refused outright by the Environment Agency where the site lies in a source protection zone.

That combination is why a geotechnical or hydrogeological specialist normally interprets infiltration data rather than the drainage engineer who will use it. The wider question of what competent person means in planning legislation applies here too, because no statute names a drainage strategy author, while several name the person who must sign ground investigation work.

Greenfield runoff estimation and its assumptions

Greenfield runoff rates are estimated, not measured, and the estimate carries assumptions that change the answer by a wide margin on small sites.

The two methods in general use are the IH124 approach, derived from Institute of Hydrology Report 124, and the Flood Estimation Handbook statistical method with its revitalised rainfall runoff extension. IH124 was developed from catchments much larger than a typical residential site, and applying it to two hectares stretches it. The Defra non-statutory technical standards for sustainable drainage systems set the policy expectation that peak runoff should not exceed the greenfield rate, but they do not prescribe an estimation method.

Three assumptions deserve stating explicitly in the report. Which SPR or soil class was used and where it came from. Whether the site area used is the gross red line or the net contributing area. And whether a minimum discharge rate has been applied, since most flow control devices cannot reliably pass less than about five litres per second without blocking, and lead local flood authorities differ on whether they accept that floor.

Say which you used. A strategy that presents a single number with no derivation invites a holding objection, and the objection is usually right.

Adoption design and the undertaker's standards

If the network is to be adopted by the sewerage undertaker under section 104 of the Water Industry Act 1991, the design has to meet the current Design and Construction Guidance that sits under the Sewerage Sector Guidance, which replaced the older adoption standard in 2020. Those requirements cover cover depths, materials, access, and which SuDS components the undertaker will adopt at all.

Adoption by the local authority or by a management company is the alternative, and it changes the design. A basin that a water company will not adopt may be perfectly acceptable to a management company with a funded maintenance schedule. Set out the adoption route in the strategy, because the maintenance answer is the one planning officers condition and the one that survives into the section 106 agreement.

Schedule 3 to the Flood and Water Management Act 2010, which would introduce a separate SuDS approving body in England, remains subject to commencement. Check the current position before you assume a design will go through that route.

Where one discipline is not enough

Four situations reliably need more than one specialist. A site relying on infiltration into variable ground, where geotechnical interpretation drives the whole strategy. A site discharging to an ordinary watercourse where a land drainage consent and a hydraulic assessment of the receiving channel are both needed. A site in Flood Zone 2 or 3, where the drainage strategy has to sit alongside a flood risk assessment and the two must use the same climate change allowance. And any site where groundwater flooding is a known local issue, which is a hydrogeological question before it is a drainage one.

Watch the date on the inputs as well. Infiltration tests, groundwater monitoring and topographic surveys all age, and checking whether a survey is still valid before resubmission saves an avoidable objection. Ground investigation carries its own competence rules, covered in more detail under geotechnical competence and Eurocode 7.

Scoping the appointment on a small site

For a scheme of one to nine dwellings, a single civil engineer with surface water experience is normally proportionate, provided the appointment is written properly. Ask for these deliverables by name rather than commissioning a drainage strategy in the abstract.

• A discharge hierarchy assessment with evidence for each rejected option, including the undertaker's response if a sewer connection was sought.
• Infiltration testing to BRE Digest 365 with trial pit logs and groundwater observations, interpreted by someone named in the report.
• A greenfield runoff estimate stating the method, the soil class and the area used.
• Storage volume for the design events with the climate change allowance applied and stated.
• An exceedance plan showing where water goes when the design event is exceeded.
• A maintenance schedule with responsibility and frequency for each component.

Ask who is signing it and what their chartered status is, in water and environmental management or civil engineering. Then ask whether their professional indemnity cover extends to drainage design, which is not a given for a generalist practice. The PlanScope professional directory is one route to a reviewer where a draft needs a qualified eye before it goes in.

Enter your postcode on PlanScope to see which reports your site is likely to need before you commission anything.

This is general guidance, not legal advice. Check the position with your 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. For specialist technical reports, qualified professional input may be required.