Grassed Spillway

When rain arrives faster than the ground can take it in, it gathers into one line and starts to cut. A grassed spillway chooses where that happens: a wide, shallow, grass-covered channel shaped to carry concentrated flow at a speed the grass can survive, down to an outlet that will not wash out. It does two jobs: carrying the storm a pond's main outlet cannot, and occupying the low draw in a field where a gully would otherwise cut. It stores nothing; it moves water off ground rather than into it.

It is sized to carry the peak runoff of a storm of the kind expected once in ten years, without the water moving fast enough to tear the grass out. The cross-section is broad and shallow - side slopes flatter than 2 across to 1 down, or a parabolic dish at least 10 m (32 ft) wide and 30 cm (1 ft) deep. The outlet decides everything: a drop-off or an abrupt steepening where the channel discharges is a named cause of gullies, because a head cut starts at that lip and works upstream until the channel is eaten away from the bottom. Costing the job without an armoured outlet is not building it.

The dangerous window is between shaping the ground and the grass taking hold, when the channel is bare graded soil in the lowest line of the catchment. A quick nurse crop of oats or rye, fabric strips across the channel, or biodegradable matting hold it until the sward closes. After that it is a vegetative practice rather than a structure: mowing keeps the designed cross-section, because tall growth slows the flow, lets sediment settle and reshapes the channel - but mowing waits until ground-nesting birds have finished, so capacity and habitat trade against each other. Thin cover, machinery driven along the channel and livestock paths are the other ways it goes. Inspect it yearly and after any unusually large storm; stop paying attention and it reverts to the gully it replaced.