Terrace

A terrace is a bank of earth, or a matched bank and channel, built across a hillside along the level. Rain that cannot soak in runs downhill, and the further it runs the faster it goes and the more soil it takes with it. A terrace cuts that run into short pieces. Water reaching one is held behind the bank long enough to soak in, and the soil it was carrying settles there instead of leaving the field. The flat strip between two banks, called a bench, is ground that can be worked.

Whether it is the right structure depends on the slope and on what lies under it. The bench form suits hillsides of roughly 7 to 25 degrees, by hand or by machine; gentler than that, a simpler contour bank answers better. There has to be enough soil to cut and fill without striking rock, because terracing thin ground buys erosion control by destroying the soil profile it was meant to save. And a terrace conveys as much as it holds: each bench falls slightly along its length, half to one percent, so surplus water walks to a drain at the toe and out to a protected outlet. A bench with nowhere to send its water is unfinished, and most terraces that fail, fail at the outlet.

The cost is labour, which is why the practice is kept for valuable ground: a person moving earth by hand shifts 3 to 4 cubic metres (4 to 5 cubic yards) in an eight-hour day, and 12 to 16 with animal power and a scraper. Upkeep never stops either - inspect after heavy rain, build the bank back where tillage has worn it down, keep the toe drains open, let grass hold the step faces between benches but cut it back, and repair small breaks at once. Capacity vanishes silently: sediment filling the channel and a lowered bank shrink the store while the terrace still looks sound, so a storm past the ten-year event it was sized for goes over the top, and one blocked or frozen outlet can carry a whole chain of benches away in sequence. On rock such as schist or gneiss, water soaking in closer to the rock makes a landslide more likely rather than less, so a terrace there raises the risk it is meant to lower.