Keypoint Dam

Walk up a valley and the ground changes shape: the steep slope above sheds water, the gentler floor below collects it. The place where one becomes the other is called the keypoint, and it is the highest point in that valley where an earth bank can hold a useful body of water. A keypoint dam is a dam built just below it. It is built for height rather than for size: water held high can be released and led anywhere lower under its own weight, so every metre of elevation is delivery that never needs a pump or fuel.

The shape of the land decides, not the rainfall, and the method has been used on ground rising only 30 cm over 1.5 km (1 ft over 5000 ft). The bank needs clay or workable subsoil, compacted into a trench cut along its centre line and down into material water cannot pass through. Skimp on it and the dam seeps from the day it fills, and the only repair is to drain it and rebuild. A pipe through the wall with a screened inlet and a valve is not optional either: without it the dam can only overflow, releasing water when the weather decides rather than when it is wanted. Sizing the spillway for a rare flood and leaving a margin of bank above it is calculation rather than judgement by eye, and past about 11 m (35 ft) of height the structure moves into a regulated class with its own permits.

The design life is a century and the obligations are annual, and the distance between those two is where dams are lost. Grass on the bank and spillway is mown so leaks and burrows stay visible and so trees never start, because roots eventually open a seepage path through the core and a trunk much over 20 cm (8 in) across cannot be taken out without damaging the bank. Burrowing animals do the same damage faster, discouraged by rock laid below the waterline and by filling each hole found. The rest is inspection at first filling, after any unusual storm, and once a year.