Contour Bench

A contour bench is a narrow level shelf cut sideways into a hillside so a person, an animal or a vehicle can reach ground the natural slope will not carry. It is made by cutting into the hill above and laying that spoil out below, so it has a cut face above the tread and a built-up fill face under its outer edge. Following the contour - the line across a slope that neither climbs nor falls - is what keeps it level and the digging to a minimum. Access is the whole product; nothing else the bench does is a reason to build one.

What decides whether it lasts is drainage rather than the surface underfoot. Grades are normally kept to 10 percent or less, and on erodible soil trouble starts at 5 or 6. The tread is tilted outward by about 3 percent on purpose, so it sheds a thin film of water continuously instead of gathering a stream, and it is interrupted by cross drains at a spacing set by the grade rather than by eye - roughly every 75 m (250 ft) at a 2 percent grade and every 15 m (50 ft) at 10 percent. The steeper it runs, the more often the water has to be let off. Where wheels must pass, the interruption is a broad-based dip, a long shallow trough a vehicle drives through.

Unlike an earthwork built to catch water, a bench cut across a slope is a net source of erosion until it is drained. It moves the runoff it meets rather than absorbing it, so it must discharge somewhere stable, and the cut face, the fill face and the drains are inspected separately. The failures are quiet: a plugged culvert ponds water and weakens the ground under the tread before anything shows on top, and ruts or a berm of spoil along the outer edge tip the outslope inward until the bench works as a ditch down the hill. The commonest mistake is a single grading pass by somebody who does not know where the dips are, leaving a smoother track that drains not at all. How long a bench lasts is not a published figure; it depends almost entirely on whether that upkeep happens.