Ridgepoint Dam

Most dams sit below the ground that feeds them. A ridgepoint dam does the opposite: its wall closes off a shallow saucer on a ridge, a spur or a plateau, catching runoff from two slopes that fall away on either side. Building high buys two things. Flat ground takes less earth to enclose, so a ridge wall holds more water for the soil moved than a wall across a steep valley, and what it stores sits above the rest of the land, so a valved pipe low in the wall waters the ground below without a pump. Its crescent footprint is the shape of the ridge nose it closes off.

The catch is that a dam at the top of a slope has the smallest catchment of any dam on a holding, which decides whether it ever fills. Published guidance disagrees and both figures matter: one asks for a catchment 5 to 20 times the water surface in grassland and 20 to 40 times in woodland, another for 10 to 20 times whatever the cover. Below the range a dam fills poorly, swings up and down and grows weed, or dries out in a long dry spell, so release through a drought is what this siting is least able to promise. The fix is to bring catchment to the wall: a graded channel cutting across ground the dam does not naturally drain, feeding it in at height. The sum is done before the digging.

Ridge and plateau soil is often the thinnest and stoniest on a holding, so it is tested rather than assumed: it needs real clay to hold water, and a cutoff trench at least 30 cm (12 in) deep into material water will not pass runs its full length. Pools are dug 1.5 to 2.4 m (5 to 8 ft) deep on average and 3 to 3.7 m (10 to 12 ft) at the deepest, and the spillway is sized for a downpour of record size. Upkeep is the same as any earth dam, with one job the others do not have: the feeder channel silts and grows over like any other earthwork, and a dam that has lost its feeder shows it only as a level that stops recovering.