Gabion Dam

A gully is a channel that running water has cut into a slope, and once one starts it deepens and works its way uphill. A gabion dam is a low wall built across that channel out of rock packed into wire mesh baskets, which is what a gabion is. It holds the channel floor at a set level so the water stops cutting downward, and silt dropped by the slowed flow settles behind it until a level, plantable bed has built up. Water filters through the rock instead of pooling against it, so it stores no water, and survives flows that would push over a solid wall.

What the wall is set into decides whether it lasts. It is keyed into a trench 20 to 40 cm (8 to 16 in) deep on a bed of gravel and sand, and its wings run at least 50 cm (20 in) into each bank, so water cannot cut around the ends or burrow beneath. A spillway in the crest carries flow over the middle, and the foundation is built longer than that spillway so falling water lands on protected ground. Baskets up to about 3 m (10 ft) high stand on their own; taller work needs an engineer. The guidance this comes from scopes the practice to a continuous gully draining about 20 hectares (50 acres) and no more than about 1 km (0.6 mi) long.

The usual mistake is to build in the gully and leave the slopes above it untouched, because the flow that cut the channel simply arrives again. The recommended order is to slow runoff across the catchment first, divert surface water above the gully head, then build, then plant. Structural work here holds only where planting follows it in the growing season after construction; within a few years the vegetation is doing the work and the wall has become a backstop. Dams are normally built as a series down the channel, in the dry season so rain cannot take an unfinished one, and then kept by inspection, prompt repair, clearing of woody growth, and refilling of baskets as the wire corrodes.