A dished channel is a shallow, saucer-shaped gutter formed in a paved surface that collects surface water and leads it to a gully or other outfall. You find them along roads, on car parks, on service yards, in driveways and increasingly in gardens too. The Dutch term is molgoot.
What sets it apart from other channel types is the shape. A dished channel has a smooth, hollow profile in which the water gathers. A channel block or kerb-and-channel is usually flat and laid to a fall; a concealed drain and a slot drain carry the water away below ground instead.
Forms of construction
- Laid in setts or block paving. Several courses side by side, with the middle course or courses set lower than the outer ones. Usually an odd number of courses, so the low point falls exactly in the middle. The traditional version, and the easiest to repair
- Precast concrete units. Individual channel units bedded and vibrated into mortar or a bound bedding. Quick to lay and dimensionally accurate
- Cast in situ with a slipform paver. On long stretches of road a machine presses the concrete directly into the required dished profile as it travels along the alignment
Laying: what matters
- Crossfall. The surfacing on both sides must fall towards the channel. A crossfall in the order of 1.5 to 2.5 per cent is usual; any flatter and water stands on the carriageway
- Longitudinal fall. The channel itself needs a fall to the outfall — reckon on at least about 0.2 per cent. Without it water stands, and standing water means silt, algae, mosquitoes and ice in winter
- A firm base. Bed the units or setts on lean concrete or mortar. Laid on sand alone a dished channel settles within a few years and loses its fall
- Connection to the gully. The channel should run in smoothly and finish a few millimetres above the gully grating edge, so the water actually falls in rather than running past
- Trafficking. Where the channel sits in a carriageway or driveway, choose units with a load class matched to the traffic
- Maintenance. Leaves, grit and moss collect precisely in the channel. Sweep it periodically and keep the gully clear, otherwise heavy rain simply washes straight over it
Related terms
- Paving
- Gully
- Kerb
- Slot drain
- Fall
- Rainwater drainage
- Soakaway
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