Swale conveyance capacity against what is drawn

A swale drawn on a layout and a swale that carries the design flow are often different things, and the gap shows up in three places: the longitudinal gradient, the roughness assumed for the vegetation, and whether the feature is being asked to convey and store at the same time. Checking swale design capacity therefore starts with the section and the invert levels rather than with the volume table.

The quickest test on any submitted drainage strategy is to compare the swale's stated capacity against a Manning calculation using a realistic roughness. Where the reported capacity assumes closely mown grass and the maintenance schedule proposes two cuts a year, the numbers do not belong to the same feature.

What a swale does and does not do

A swale is a shallow vegetated channel. It conveys water along a route, it can detain water where check dams or a raised outlet are provided, and it treats water by filtration through vegetation and by settling out sediment.

Three broad types appear in submissions. A conveyance swale moves water from A to B at the design flow. A dry swale has a filter layer and an underdrain beneath it, so it drains down between events and gives better treatment. A detention swale uses check dams or an outlet control to hold volume.

What a swale does not do is provide conveyance and attenuation volume from the same cross-section at the same time. Water occupying the channel as storage is not available as freeboard for conveyance, and a drawing that counts the full cross-section twice is overstating the design.

Conveyance against storage functions

Establish which function the swale is performing in the calculations. If the drainage model treats it as a pipe-equivalent conduit, the storage credited in the volume table has to come from somewhere else. If it is credited as storage, the conveyance route for the design event and for exceedance still has to be demonstrated.

The awkward case is a swale performing both in sequence: conveying in smaller events and filling behind a control in larger ones. That is a legitimate design, and it needs the water level shown at each event so the reviewer can see the freeboard remaining in the largest case.

Ask for the long section. A plan with a hatched green strip tells you nothing about whether the invert falls consistently along its length.

Gradients, side slopes and freeboard

Longitudinal gradient sets the capacity and the velocity. Too flat and the channel ponds, holds water between events and grows the wrong vegetation. Too steep and the velocity erodes the grass, which then loses both its treatment function and its roughness assumption. The SuDS Manual gives recommended ranges for gradient, side slope and velocity, and those ranges should be taken from the manual rather than from memory.

Side slopes are a maintenance and safety question as much as a hydraulic one. A slope shallow enough for a ride-on mower to cut safely is what keeps the vegetation in the condition the calculation assumed. Steeper slopes get strimmed occasionally or left, and the capacity falls.

Freeboard is the item most often missing. A swale designed to run full at the design event has no allowance for a blocked inlet, settlement of the invert, or a build-up of cuttings, and all three happen.

An illustrative conveyance calculation

The following worked example is illustrative. It shows the method and the sensitivity rather than a design.

Take a trapezoidal swale with a 1.0 metre base, side slopes of 1 in 3, a flow depth of 0.3 metres and a longitudinal gradient of 1 in 200. The flow area is 0.57 square metres and the hydraulic radius is about 0.197 metres.

The change in maintenance regime alone removes about 40 per cent of the capacity. That is the number worth putting in front of a designer who has assumed a mown roughness and proposed twice-yearly cutting, and it is the reason the maintenance schedule and the hydraulic model have to be consistent.

Then check the depth. If the swale is formed 0.45 metres deep, the 0.3 metre design flow leaves 0.15 metres of freeboard. If it is formed 0.30 metres deep, it leaves none.

Check dams and their effect

Check dams raise the water level locally, slow the flow and create storage pockets. They are useful on steeper gradients where velocity would otherwise erode the channel.

They also reduce conveyance capacity, because the water has to pass over or through each crest. A swale with check dams at close spacing behaves as a chain of small ponds rather than as a channel, and modelling it as a channel overstates what it can pass.

Design points to look for: the crest level of each dam relative to the invert and the adjacent ground, erosion protection immediately downstream of each crest, a route for the flow that exceeds the crest, and access to remove the sediment that collects behind them. Sediment behind check dams is the main maintenance burden on this type of feature.

Maintenance and vegetation management

The maintenance schedule is part of the hydraulic design, not an appendix to it.

1. Cutting frequency and, critically, removal of cuttings rather than leaving them in the channel.
2. Silt removal at the inlet, with a stated trigger such as a depth of accumulation.
3. Re-levelling of the invert where settlement or scour has occurred.
4. Replanting of bare areas, with a species mix suited to intermittent inundation.
5. Access route for mowing and for a vacuum tanker where silt removal is needed.

Who does the work matters as much as what the work is. Swales in adopted highway verges and swales in private open space have different maintaining parties, and a feature sitting across the boundary between them tends to be maintained by neither. Name the party in the Drainage Strategy and align the planting with the Landscape Strategy so the two documents do not specify different regimes.

Where the drawn swale cannot work

Several layout problems make a swale undeliverable, and they are visible on the drawing before any calculation.

A swale interrupted by driveway crossings, where each crossing needs a pipe that becomes the hydraulic control. A swale drawn along a road with a consistent kerb line but no consistent fall, so the invert has to rise. A swale whose outfall invert sits above the swale invert, which nothing will fix. A swale squeezed to a width that leaves no side slope on a constrained frontage. A swale drawn through root protection areas, where the excavation cannot be made. And a swale that is also the designated exceedance route for the extreme event while being sized for a much smaller one.

Our note on drainage evidence for a single new dwelling covers the small-site version of these problems, where a swale is often proposed and rarely the right feature.

Reviewer checks on the calculation

Working through a submitted swale design, ask for these:

1. The long section with invert levels at stated intervals and the calculated water level at each design event.
2. The Manning roughness used, with the vegetation condition it corresponds to, cross-referenced to the cutting frequency in the maintenance schedule.
3. The freeboard at the largest event modelled, stated as a figure.
4. Confirmation that the cross-section is not credited as both conveyance and storage in the same event.
5. The velocity at the design flow, checked against the erosion limit for the proposed vegetation.
6. The inlet and outlet details, including any pipe crossings and their capacity.
7. The maintaining party and the access arrangement.

Keep the resulting rates and volumes consistent with the Flood Risk Assessment, because a mismatch between the two documents is what holds up discharge of the drainage condition.

This is general guidance, not legal advice. Drainage design and review is work for a suitably qualified engineer, and for specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.