Swale design: gradients, side slopes and check dams

A swale works as a conveyance and treatment feature when its longitudinal gradient is shallow enough to keep flow slow, its side slopes are gentle enough to mow and walk on, and check dams are used where the ground falls faster than the design wants. Swale design is mostly geometry, and the geometry is what lead local flood authorities check. Get the long section and the cross sections right and most of the rest follows.

The usual failure is a swale drawn as a landscape gesture on a plan with no long section behind it, discharging into a pipe at the low point and treating nothing.

What a swale does and what it does not

A swale conveys surface water slowly along a vegetated channel, allowing sediment to settle and some infiltration where the ground permits. It removes a proportion of the pollutant load through sedimentation and filtration through the grass sward.

It does not provide much storage. A swale holds very little volume compared with a basin or a tank, so where attenuation is needed the swale is a conveyance and treatment stage feeding something that stores. Strategies that claim a swale as the attenuation volume usually fall apart on the calculation.

It also does not work as a drain for a catchment that delivers water faster than the channel can pass it. If the inflow is a single piped discharge at a high flow rate, the swale erodes at the inlet and becomes a gully. Distributed inflow over the edge, or an energy dissipation detail at a piped inlet, is part of the design rather than a refinement.

Longitudinal gradient and conveyance

The design envelope commonly used in UK practice is a longitudinal gradient of around half a per cent to five per cent, and those figures should be confirmed against the industry SuDS guidance for the particular case.

Below about half a per cent the channel does not drain, and standing water, poor grass and complaints follow. Above a few per cent the velocity rises, treatment falls away because sediment does not settle, and erosion of the invert begins. That upper end is where check dams come in.

Size the channel for the design event with a freeboard, and state the design flow, the velocity and the depth of flow on the drawing. Treatment performance depends on a shallow depth of flow over the grass for the smaller, frequent events, so the geometry that matters most is the one that handles the first few millimetres of rainfall rather than the one that handles the extreme event.

Two further points on levels. The outfall level fixes everything upstream, so establish it first. And the swale invert must sit above the groundwater level for most of the year, or the channel is a ditch with a permanent wet base and a different maintenance regime.

Side slopes, safety and machine maintenance

Side slopes are usually specified no steeper than one in three, with one in four preferred where the swale sits in public open space. The reasoning is practical rather than regulatory: a ride-on mower cannot safely work a steeper slope, and a slope that cannot be mown by machine will be mown by hand, which means it will not be mown.

Gentle slopes also matter for people. A shallow swale in a residential street reads as a grassed verge and is crossed casually by children, dog walkers and anyone parking. A steep sided channel with a sharp break of slope at the top is a trip hazard and attracts objections at consultation.

Specify the top of slope, the invert width, the depth and the side slope on a typical cross section, and provide at least two further cross sections where the geometry changes.

Check dams, spacing and ponding

Check dams are low barriers across the swale that break the gradient into a series of shallow steps. They slow the flow, encourage sedimentation and create temporary ponding after rain.

The spacing rule that works is to set each dam so that the ponded water behind it reaches roughly the toe of the dam upstream. On a steeper swale that means dams close together; on a gentle one they may not be needed at all. Draw them on the long section rather than noting them as a specification item, because the spacing is the design.

Construction options are timber boards, stone or gabion, or a grass covered earth bund. Each needs a scour protection detail on the downstream face, because the overspill point is where erosion starts. Where the dam is in a public area, keep the retained depth shallow.

An illustrative example: a 60 metre swale along the edge of an access road falling at about three per cent. The design shows an invert width of 600 millimetres, side slopes at one in four, four timber check dams with a retained depth of 150 millimetres and stone scour protection downstream of each, with the ponding from each dam reaching the toe of the one above. Grass crossings at two driveway entrances use shallow culverts with headwalls. That package is checkable. A plan showing a green ribbon is not.

Planting and establishment

Grass is the usual surface, and the choice is between seeding and turf. Turf establishes faster and resists early erosion, which matters where the swale will receive flow before the sward is established. Seeding costs less and gives a better long-term sward, but needs protection in the first season.

Avoid fertiliser, because nutrient runoff defeats the water quality purpose. Specify a cutting regime that keeps the sward at a height suitable for filtration rather than a lawn finish, and say who will carry it out.

Where the swale is wet for long periods, select species accordingly and accept that the result is a wet swale with a different appearance and a different maintenance schedule. Do not specify an amenity grass mix for a channel that will hold water.

Driveway and footway crossings

Crossings are where swales fail in practice. Each one is either a culvert, which creates a blockage point and a maintenance liability, or a level crossing with the swale continuing through, which only works at shallow depths.

Minimise the number of crossings in the layout, which usually means grouping driveways or running the swale on the opposite side of the road. Where a culvert is unavoidable, provide headwalls, a trash screen where the diameter is small enough to block, and an access chamber upstream. Then name the party responsible for clearing it.

Vehicle crossings of a swale also need their own load case, so coordinate with the highway design and the position of any service runs.

Drawings and sections to submit

The Drainage Strategy should carry, as drawings:

1. A plan showing the swale alignment, inflow points, check dam positions, crossings and the outfall.
2. A long section with existing and proposed invert levels, gradients between dams, water levels for the design events and the outfall level.
3. Typical and additional cross sections giving invert width, depth, side slopes and freeboard.
4. Details for the inlet, the check dams, each crossing and the outfall structure.
5. A maintenance plan naming the responsible party, the inspection frequency and the cutting regime.

Where the outfall is to a ditch rather than a sewer, the consent regime is separate and the timing is set out in the note on land drainage consent. Where surface water flood risk is part of the case for the swale, read the mapping carefully in the way described in the note on reading the surface water flood risk map, and keep the drainage conclusions consistent with the site's Flood Risk Assessment.

Draft a drainage strategy on PlanScope, then have a drainage engineer check the long section, the gradients and the crossings before submission.

This is general guidance, not legal advice; check the position with your local planning authority, the 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.