Sustainable drainage on a steeply sloping site

On a site that falls steeply, the drainage problem changes from storing water to slowing it down. SuDS steep slope design has to deal with velocity, erosion and the fact that storage wants to be level while the ground is not, and it has to do all that without pushing water into the ground in a way that destabilises the slope. Get the sequence wrong and you end up with a scoured swale, a silted control chamber and a drainage authority asking for the whole strategy again.

The three questions a reviewer will ask are straightforward. How fast is the water moving at each point, what stops it eroding what it runs over, and where does it go when the system is exceeded.

Why gradient narrows the options

Most SuDS components assume a gentle fall. Swales work best within a modest longitudinal gradient range, and the SuDS Manual published by CIRIA sets out the ranges and the mitigation needed outside them. Above that, flow accelerates, grass gives way at the invert, and the feature turns into a channel.

Permeable paving has the same problem in a different form. On a slope, water travels through the sub-base to the low end and sits there, so the storage volume you calculated across the whole area is only available at the bottom. The fix is internal barriers, or baffle walls, dividing the sub-base into cells, and the drawing has to show them with levels rather than leaving them as a note.

Level storage is the hardest thing to find. A basin needs a flat base, and on a 1 in 8 slope a basin of any size means cut, fill and retaining. That pushes designers towards tanks, and tanks on slopes bring their own problems with uplift and access.

Velocity, erosion and energy dissipation

Velocity is the design variable to control first. Grass lined features are usually designed to keep velocities below a threshold that depends on how well established the grass is, and the SuDS Manual gives the values to design to. Above the threshold you need either a harder lining or a way of taking energy out of the flow.

The practical measures on a steep site are:

• Check dams across a swale at intervals set by the gradient, each creating a short level pool and a small drop.
• Stone or rip rap aprons where a pipe or channel discharges, sized for the outlet velocity rather than to a standard detail.
• Cascades and stepped channels where the fall is large and the flow is concentrated.
• Level spreaders to convert a concentrated flow into sheet flow before it crosses a grassed area.
• Reinforced grass or cellular confinement where the surface has to take occasional high velocity flow.

Check dams are the workhorse and they are often drawn badly. Each one needs an invert level, a crest level, a spillway detail and a defined arrangement for the flow that passes over it, otherwise water finds the edge and cuts round the end. Show them in longitudinal section as well as on plan.

Sediment is the quiet problem. Steep catchments deliver silt, and silt fills the first pool, the first cell of the permeable sub-base and the orifice of the flow control. Design the sequence so there is a sacrificial feature early, and say in the maintenance plan who empties it and how often.

Terracing storage down the slope

The alternative to one large basin is several small ones stepped down the site, each with its own control, connected in series. It costs more in structures and gives you something worth having: storage that fits the ground, shorter flow paths and a system where a failure in one cell does not empty the whole volume downhill.

Three rules make a terraced system work.

1. Give each cell its own outflow control and check the performance of each control across the range of heads it will see, because a vortex control designed for one head behaves differently at another.
2. Design the connection between cells for the exceedance flow as well as the design flow, since the upper cell will overtop at some return period.
3. Set the freeboard on the basis of the wave and surge a steeply arriving flow produces, not the still water level.

Flow splitters and level spreaders are what keep the cells loaded evenly. Where a terraced system serves a residential layout, put the cells in areas that are usable when dry, because a fenced concrete box on a slope is the feature residents complain about first.

Infiltration on a slope and the stability question

Infiltration is where drainage and geotechnics collide. Putting water into the ground upslope of a slope, a retaining wall or a cutting raises pore water pressure, and raised pore pressure reduces the effective stress that holds the slope together. On clay slopes and on made ground, that is a real risk rather than a theoretical one.

Approved Document H already requires soakaways to be kept clear of buildings, and on a slope the separation needed is a geotechnical judgement rather than a fixed distance. Where infiltration is proposed on a sloping site, a Geotechnical Assessment should address slope stability with the infiltration in place, and it should be prepared by a suitably qualified engineer because the council and the drainage authority will both look for that.

Three positions are common. Infiltration accepted at the top of the slope with a stability check. Infiltration refused entirely, with attenuation and a controlled discharge instead. Or infiltration accepted only below the slope, which usually means the water has to be conveyed down the site safely first.

Check the geology before you design. British Geological Survey mapping will tell you whether landslip deposits are recorded in the area, and a recorded slip nearby changes the conversation entirely. Groundwater behaviour on slopes also drives flood risk, which we look at in groundwater flooding evidence and how to assess it.

Exceedance routing downhill

Every system is exceeded eventually, and on a slope the exceedance flow arrives somewhere fast. The plan the drainage authority wants shows where water goes when the design event is exceeded, drawn as arrows with levels, and it should demonstrate that the route does not run through a doorway, into a basement, across a driveway that falls into an integral garage, or onto a neighbouring property at the bottom.

The tools are road and kerb levels, threshold levels set above the flow path, deliberate low points in boundary treatments, and where necessary a formal overland channel. Two details do most of the work: setting finished floor levels with meaningful freeboard above the adjacent external level, and making sure that access ramps to any lower ground floor have their own high point.

Do not leave the downstream position unexamined. A scheme that safely conveys exceedance flow off site and onto the road below has exported the problem, and a lead local flood authority will say so.

Retaining structures and drainage together

Retaining walls and drainage interact in three ways that need coordinating rather than sequencing.

Water behind the wall has to be collected and taken away, so the filter drain and its outfall belong on the drainage layout as well as on the structural detail.

Attenuation tanks sited behind or below a retaining structure change the loading assumptions, and a tank that fills changes them again. Get the structural engineer to sign off the position.

Uplift matters. A large empty tank in ground with a high water table will try to float, and the calculation has to consider the worst case of an empty tank in saturated ground.

What the drainage authority needs to see

On major development the lead local flood authority is a statutory consultee, and on a steep site its comments concentrate on the same short list: longitudinal sections of every feature, velocity calculations at the design and exceedance events, details of every check dam and energy dissipation measure, a stability statement where infiltration is proposed, the exceedance flow plan, and a maintenance plan that acknowledges the gradient.

Maintenance is the part that gets forgotten. Grass steeper than roughly one in three cannot be mown safely with normal equipment, so either flatten the slopes to something maintainable or specify a planting regime that does not need mowing. Say who has access to each feature and how they reach it.

A Drainage Strategy structured around those headings answers the consultation before it is raised, and where the site also sits in a flood zone the Flood Risk Assessment carries the sequential and exception test work alongside it. Where a watercourse crosses the slope, resist the temptation to pipe it, for the reasons set out in culverts and the presumption against burying a stream.

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. This is general guidance rather than legal advice, so take slope stability and drainage design advice from qualified engineers for a specific site.