Swale Regrading

A swale is a shallow channel dug along the level across a slope so that rain running off the ground collects in it and soaks in instead of carrying on downhill. Water held still drops whatever soil it is carrying, so the channel slowly fills with the sediment it caught. Regrading lifts that fill out and cuts the channel back to the shape it was built to. A silted swale is a swale that has been working.

The one thing that must be right is the bottom. A swale spreads water because its base is level along its whole length; restored with any fall in it, the channel carries water along instead and becomes a drain. The work needs an accurate reference for that original level, ground dry enough to shape and compact, and a route a machine can travel without cutting the berm, the mound of soil on the downhill side. No numbered practice standard treats swale desilting on its own, so there is no schedule to follow: the trigger is measured loss of cross-section, found by inspecting after heavy storms and at least once a year.

What comes out is worth keeping, as the sediment is rich in silt and clay and can be spread along the downhill mound as recovered topsoil. Mowing or grazing the channel slows the rate it silts at, and a sediment trap upstream keeps the deposit in one place. The regrade is a sequence rather than one act - reshape, compact, sow, hold the designed width - because bare ground in a channel that concentrates flow is the most dangerous state it can be in. A fast-growing nurse crop holds the soil until permanent cover closes over it, and a sowing that fails leaves the site worse than before it was touched. Left alone, a silted channel loses capacity and overflows. Tillage, wheel tracks and livestock wearing at the edges are where gullies begin, and the job ranges from an afternoon with a shovel to hired earth-moving equipment depending on how long it was left.