Diversion Drain

When rain falls faster than a slope absorbs it, the runoff gathers into a single stream and reaches the bottom as a hazard to a road, building or field. A diversion drain is a shallow channel cut across the slope above whatever is at risk, with the soil dug out of it packed along the lower side into a ridge. It differs from a swale, which is dug dead level so water stands and soaks in: a diversion has a slight fall along its length, so water keeps moving and leaves. It holds nothing; what it catches, it carries away.

A diversion is defined by its outlet, not its channel, and that outlet - a stable waterway, a protected area, a pond - has to exist before the channel is cut. Fall along it is kept to about 3 per cent, and what limits it is speed, not volume: bare earth scours above 0.4 to 0.6 m (1.3 to 2 ft) a second, grass at 1.8 m (6 ft) and rock lining at 3 m (10 ft), so the lining decides the grade allowed. The channel is built larger than the runoff it carries, with 10 to 30 cm (4 to 12 in) of spare depth above the design flow. The ridge is fill, compacted in layers and built about a tenth higher than finished height because it will settle.

Grass sown in the channel is part of the build, not a later tidy: a bare one cannot carry the flow it was sized for. Left alone it fails in a fixed order. The outlet erodes first and the notch works back up the channel, so one built before its outlet is finished is a gully-making machine aimed downhill. Then the ridge is overtopped where it settled low and the whole flow concentrates there. Last and quietest, sediment and brush close off the capacity storm by storm until one arrives that the channel cannot carry. Upkeep is cheap: inspect yearly and after any large storm, clear deposits, restore lost ridge height, reseed bare patches and cut out woody growth before it roots into the fill. Keep machinery off when the ground is wet.