The drainage hierarchy section of a drainage strategy

The drainage hierarchy is the order in which a scheme has to try to get rid of its surface water, and a Drainage Strategy that skips a tier without evidence will come back from the lead local flood authority with a holding objection. The order runs: into the ground, then to a surface water body, then to a surface water sewer or highway drain, then to a combined sewer as the last resort. Each step down has to be justified by something you tested or something a third party told you in writing.

The failure mode is always the same. A drainage note says infiltration is unsuitable because the ground is clay, no test was done, and the whole strategy rests on an assertion.

The order the hierarchy requires

National guidance on flood risk sets the priority, and the Defra non-statutory technical standards for sustainable drainage systems set what the design has to achieve once the destination is chosen. For buildings, the same order appears in Approved Document H of the Building Regulations, which is why a building control officer and a drainage engineer occasionally arrive at different answers on the same site and both think they are right.

Discharge into the ground comes first because it mimics the natural water cycle and takes the flow out of the network entirely. A surface water body comes second. A surface water sewer or highway drain third. A combined sewer last, and on many schemes the undertaker will refuse it outright.

Write the strategy so the reader can see all four tiers and the reason each one was accepted or set aside. A short table at the front of the section saves the flood authority officer twenty minutes and saves you a fortnight.

Testing infiltration properly before discounting it

Infiltration is rejected on most small sites, and on perhaps half of those it is rejected without evidence.

The standard method is the soakage test described in BRE Digest 365, which requires a trial pit or borehole filled to a defined level on three separate occasions, with the time taken to fall from 75 per cent to 25 per cent of the effective depth recorded each time. The slowest of the three results is used for design. One test on one afternoon is not the method.

Three site conditions kill infiltration regardless of soil type, and each needs stating:

• groundwater within a metre of the proposed base of the soakaway, assessed at seasonally high levels rather than on the day of the trial pit
• made ground or contamination, where infiltration would mobilise something into the aquifer, which is why the ground gas and contamination findings belong in the same conversation as the drainage design
• a source protection zone or an aquifer designation where the Environment Agency restricts discharge to ground

Where infiltration works partially, say so. Partial infiltration combined with a restricted discharge is a common and perfectly acceptable answer, and it usually produces a smaller attenuation tank than either option alone.

Discharge to a watercourse and the consents behind it

A watercourse discharge needs three things that are not planning permission, and all three take longer than people plan for.

You need the riparian owner's agreement to connect, which on a ditch along a neighbouring field means finding and persuading a landowner. You need land drainage consent from the lead local flood authority for works in or affecting an ordinary watercourse, under the Land Drainage Act 1991. If the watercourse is a main river, the Environment Agency's flood risk activity permit applies instead.

Applicants routinely draw an outfall to a ditch on the drainage layout, obtain permission, and then discover the ditch is culverted under a garden and has no capacity. Walk the receiving watercourse before you design to it, and photograph the proposed outfall location.

Sewer connection as the last resort

There is a right to connect foul and surface water drainage to a public sewer under section 106 of the Water Industry Act 1991, and undertakers can refuse or impose conditions where the connection would be prejudicial to the system. Surface water into a combined sewer is now refused as a matter of course by most undertakers where any alternative exists.

Submit a pre-development enquiry to the sewerage undertaker early. The response tells you the capacity position, the permitted discharge rate and whether off-site reinforcement is needed, and it is the document the flood authority will want to see. Where foul drainage is also constrained, that answer feeds the Foul Sewage Assessment rather than the surface water strategy, and the two enquiries can be made together.

Where the connection will be offered for adoption, the undertaker's design and construction requirements apply, and those are more prescriptive than the planning standards. Design to them from the start rather than redesigning after technical approval.

Restricted discharge rates and the storage that follows

Greenfield sites are normally restricted to the greenfield runoff rate, often expressed as the mean annual flood flow, calculated from published methods using catchment descriptors. Brownfield sites are normally required to achieve a betterment against the existing positive connection, and the percentage varies by authority and by local plan policy, so read the policy rather than assuming a figure.

Storage volume then follows from the difference between what falls on the site and what you are allowed to release. The design is checked for the 1 in 1 year, 1 in 30 year and 1 in 100 year events, with an allowance for climate change added to the rainfall intensity. Those climate change allowances are revised periodically by the Environment Agency, so use the current published allowance for the relevant river basin district and management catchment, and state in the report which version you used and when you downloaded it.

Two details the flood authority checks and applicants forget. The half drain time, which governs whether the storage is empty before the next storm. And the urban creep allowance for future paving over of private gardens, which many authorities require on residential schemes.

Exceedance routes and overland flow

Every system has a design capacity, and the strategy has to say what happens when it is exceeded. That means showing on a plan where water goes in a storm larger than the design event, and demonstrating that it goes somewhere that does not flood a habitable room or discharge onto a neighbour.

Finished floor levels set above the surrounding ground, falls away from buildings, a designed overland route across car parking or open space, and kerb or level changes that steer flow are the usual moves. On a sloping site, also consider flow arriving from upslope, which is the part most commonly missed on a small infill plot.

If the site sits in Flood Zone 2 or 3, or is one hectare or more, the surface water design has to sit inside a Flood Risk Assessment rather than standing alone, and the two documents must agree on levels.

Evidencing each step for the flood authority

Say who maintains the system for its lifetime and how that is secured, whether by adoption, by a management company or by a planning obligation. Schedule 3 to the Flood and Water Management Act 2010, which would introduce a separate SuDS approval body in England, has not been brought into force, so maintenance is still handled through planning and adoption agreements. Check the current position, because the bringing into force of that Schedule has been announced more than once.

This is general guidance, not legal advice. Local standards and discharge rates vary by authority, so check the position with your lead local flood authority before relying on it.

Browse the PlanScope report types to see how the drainage strategy fits with the rest of your evidence base. Reports should be reviewed before submission, and we do not guarantee planning approval.