Drainage engineer competence for a drainage strategy
There is no statutory competent person scheme for drainage design. What the lead local flood authority reviews, and the questions to ask an engineer before appointing them.
There is no statutory competent person scheme for drainage design, so drainage strategy engineer competence is judged by whoever reviews the document. On a major application that is the lead local flood authority, and its consultees have seen enough weak submissions to recognise one in about four minutes. They look for the discharge rate and its derivation, the evidence behind the infiltration assumption, and whether the storage volume is a calculation or a guess.
Self-builders often ask an architect or a groundworker to produce the strategy. That works for the simplest sites and fails everywhere else.
What the lead local flood authority reviews
Lead local flood authorities are the county or unitary authorities given surface water functions under the Flood and Water Management Act 2010, and since April 2015 they have been a statutory consultee on surface water drainage for major development. Their review is narrower and more technical than most applicants expect.
They want to see the discharge destination chosen in the right order, infiltration first, then a watercourse, then a sewer. They want the proposed discharge rate justified against greenfield rates or against a betterment on the existing rate for brownfield sites. They want storage volumes calculated for the appropriate return periods with a stated climate change allowance. They want exceedance routes drawn, so that when the design event is exceeded the water goes somewhere other than through a front door.
Most refusals to support an application come from an absence of evidence rather than from a disputed design. A strategy that asserts infiltration will work, without a test result, is the commonest example.
Civil engineering competence against flood risk expertise
These are related skills and they are not the same skill.
A civil or drainage engineer designs the network: pipe sizes, gradients, storage structures, flow controls, water quality treatment stages. A flood risk specialist assesses the site against fluvial, tidal, surface water and groundwater sources, models flood levels and sets finished floor levels. Some practitioners do both competently. Many do one and assume the other.
Look for chartered status with a relevant institution, evidence of drainage schemes taken through to adoption or discharge of conditions in comparable authorities, and professional indemnity cover appropriate to the scheme. Ask directly which parts of the work the practice does in house and which it subcontracts.
That kind of scoping conversation is the same one described in what competent person means in planning legislation, where the label carries a statutory meaning in some regimes and none at all in others.
Hydraulic modelling capability
For anything beyond a handful of plots, storage volumes should come from a hydraulic model rather than from a rational method calculation on a spreadsheet.
The model needs to be built with the right rainfall inputs, the right runoff coefficients for the actual surface types, and a flow control that matches the device being specified. A design that assumes a 2 litre per second discharge without checking that a suitable control device exists and will not block is a design that fails at detailed design stage.
Ask to see a sample of modelling output from a previous scheme. You are looking for results presented at a range of return periods and durations, with the critical duration identified, not a single headline volume. An engineer who cannot explain which storm duration governed the storage volume has not interrogated their own model.
Ground investigation feeding the infiltration design
Infiltration is the preferred discharge route in national guidance and the one most often assumed without evidence.
The test regime matters. Soakage testing carried out to a recognised method gives an infiltration rate for a specific location and depth, and results vary sharply across a site. Groundwater level is the other half of the answer, because a soakaway needs clearance above the highest expected water table, and a single summer borehole reading tells you very little about February.
Contamination and source protection add constraints. Infiltrating surface water through made ground on a former industrial site can mobilise contaminants, and authorities in source protection zones will resist it. The geotechnical and contamination work therefore has to be commissioned early enough to inform the drainage design, which is the sequencing point made in geotechnical competence under Eurocode 7.
| Evidence the reviewer looks for | Why it is asked for |
|---|---|
| Infiltration test results at proposed soakaway locations and depths | supports or removes the infiltration assumption |
| Groundwater monitoring covering a wet period | sets the clearance above the water table |
| Greenfield runoff rate calculation with method stated | justifies the proposed discharge rate |
| Modelled storage volumes across return periods and durations | shows the critical duration was identified |
| Exceedance flow routing plan | shows where water goes beyond the design event |
Adoption and maintenance advice
A strategy that does not say who will own and maintain each component is incomplete, and the question is not academic on a small site.
The realistic options are adoption by the sewerage undertaker under a section 104 agreement where the assets meet its standards, adoption by the local authority for components within public space, or a private management arrangement with a funded maintenance schedule. Each has different design consequences, because adoption standards constrain what can be built.
Note that the sustainable drainage approval regime in Schedule 3 to the Flood and Water Management Act 2010 has not been brought into force in England, and government has stated an intention to bring forward implementation. Check the current position before relying on any statement about it, including this one, and write the strategy against the arrangements that apply now.
Coordinating with the flood risk assessment author
Where a site needs both documents, the drainage strategy and the Flood Risk Assessment must agree with each other. Reviewers check.
The recurring conflicts are finished floor levels quoted differently in the two documents, a climate change allowance applied in one and not the other, a discharge point shown at a location the flood assessment identifies as within the floodplain, and exceedance routes that run across land the flood assessment treats as flood storage.
If two firms are producing the documents, give one of them the coordination role in writing and share the drawing set between them. If one firm produces both, ask who is checking the other's numbers internally.
Scoping the work before the layout is fixed
The most expensive drainage problems are layout problems. Attenuation needs space, infiltration features need the right ground in the right place, and gravity drainage needs fall across the site.
Bring the drainage engineer in at concept stage with a topographical survey and a first sketch layout. An afternoon spent identifying the low point and the discharge destination before the plots are arranged saves a redesign later, and it costs far less than discovering at reserved matters that the only viable attenuation location is under plot seven.
For a Drainage Strategy PlanScope can structure the narrative against national policy and the local plan, with missing site data flagged rather than invented. The calculations and the design judgement remain the engineer's. Reviewers who can take on that sign-off can be found through the professional directory.
This is general guidance, not legal advice; check the position with your lead local flood authority or a qualified engineer 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.
Frequently asked questions
Who can write a drainage strategy for a planning application?
No statutory competence scheme applies, so the test is practical. Look for chartered status with a relevant engineering institution, hydraulic modelling capability, previous schemes taken through to discharge of conditions in comparable authorities, and suitable professional indemnity cover. Ask which parts of the work are done in house and which are subcontracted.
When is the lead local flood authority a statutory consultee?
Lead local flood authorities have been statutory consultees on surface water drainage for major development since April 2015. For minor development they are usually not consulted, but the local planning authority will still apply its own drainage policy, so a proportionate strategy is often required on small sites as well.
Do I need infiltration testing before submitting a drainage strategy?
If the strategy relies on infiltration, yes. An assumed infiltration rate without test results at the proposed soakaway locations and depths is the most common reason a strategy is not supported. Groundwater monitoring covering a wet period is needed too, because a soakaway must have clearance above the highest expected water table.
What discharge rate will the authority accept?
For greenfield sites the starting point is the calculated greenfield runoff rate, with the calculation method stated. For previously developed sites the expectation is usually a betterment on the existing rate. Discharge destination should follow the hierarchy of infiltration first, then a watercourse, then a sewer, with each option evidenced as unavailable before moving on.
Has the SuDS approval regime in Schedule 3 started?
Schedule 3 to the Flood and Water Management Act 2010 has not been brought into force in England, and government has stated an intention to bring forward implementation. Check the current position before relying on any statement about it, and prepare the strategy against the arrangements that apply now through planning conditions and adoption agreements.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.