Planted swales, or bioswales, convey water at the surface while they reduce its velocity, trap sediment and can encourage infiltration.
Planted swales, or bioswales, convey water at the surface while they reduce its velocity, trap sediment and can encourage infiltration. They differ from a buried drain because they make the path of the water visible and planted. They differ from a rain garden because they have a longitudinal conveyance function.
Conveying without causing erosion
The longitudinal gradient needs to keep the water moving without creating excessive velocity. Small check dams, sills or changes of section can increase retention and reduce erosion.
Concentrated inlets should be protected with stone, robust vegetation or energy dissipators.
Section, base and safety
The width, the depth and the gradient of the side slopes should allow for maintenance and safety. Beside roads, visibility and set-back are essential. In pedestrian spaces, gentle side slopes reduce risk.
The base can infiltrate, store above a drainage layer or convey to another system. The choice depends on the soil and the aim.
Planting by zones
Species on the bottom should tolerate alternating wet and dry; the sides need to withstand dry periods. Fast cover helps to stabilise the soil, but it should not block the flow.
In industrial parks and on access roads, a planted swale can integrate drainage and landscape, provided it is not treated as a decorative bed with no hydraulic inspection.
Operation in rain and in dry weather
A planted swale should be observed at two moments: during an event, to confirm velocity, erosion and overflow; and afterwards, to measure how long water remains. Prolonged puddles may indicate clogging, a wrong level or a blocked outlet.
In dry weather, the swale is still part of the garden. The selection should maintain sufficient cover without requiring disproportionate watering, and maintenance access cannot depend on the ground being dry.
Critical points
The most common mistakes are using excessive gradients; planting species that flop and obstruct the flow; not providing crossings and access; and confusing a planted swale with a deep drain.
Applying it to the project
At Diaplant, planted swales are integrated into the design of parks, access roads and business areas, coordinating the path of the water, safety, planting and maintenance.
The bioswale turns the conveyance of water into visible infrastructure. Its performance depends on the section, the velocity, the soil and the vegetation.
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