Drainage Strategy and SuDS

Work down the hierarchy, and show your working

Surface water should go into the ground first, then to a watercourse, then to a surface water sewer, and only then to a combined sewer. A lead local flood authority will expect to see each option considered in that order and rejected on evidence, not skipped.

Where infiltration is plausible, that evidence means testing to BRE Digest 365 rather than an assumption about the underlying geology. Where it is not — high groundwater, clay, contamination — saying so clearly and moving down the hierarchy is a perfectly good answer.

Rates, volumes and the storms that matter

Greenfield sites are normally held to greenfield runoff rates. Previously developed sites are usually expected to improve on what is there now, and the scale of that betterment varies between authorities — it is worth checking rather than assuming the usual figure.

Attenuation is then sized across the full range of return periods, not just the headline event, because the critical duration is often not the one people expect. The one hundred year event with a climate change allowance sets the volume; the more frequent events determine whether the system actually works day to day.

Choosing SuDS somebody will actually maintain

A drainage strategy is only as good as its upkeep. Specifying a treatment train that needs quarterly attention on a site with no management company is how systems silt up and stop working within a few years.

We design to what the site can realistically support, and set out a maintenance plan that names who is responsible for each component and how often. It is also worth remembering that SuDS earn their keep on amenity and biodiversity as well as water — a swale is easier to justify than a buried tank, and often cheaper.

Plan for the storm that beats the design

Every drainage system has an event that exceeds it. Exceedance routing is about deciding in advance where that water goes — across a car park and into a basin, rather than through the front doors of the ground floor flats.

Showing the exceedance route on a plan is increasingly expected, and it is one of the cheapest things you can get right early, while the levels on the site are still being set.