Contour Dam

A contour dam is a wall of compacted earth built across a narrow part of a valley, so that runoff from the ground above collects behind it as a pond. Built high on the slope, it sits above everything it serves, so water reaches the land below by gravity alone. A pipe laid through the wall lets that water out on demand. One pond usually serves several purposes at once, and they compete: stored water, stock, irrigation, fire fighting, fish and swimming each want a different water level.

Shape decides the site. A narrow valley with steep sides and a floor flat enough to flood a wide area holds the most water for the least wall. Ground where water would spread out thin is avoided: a wide shallow surface loses a quarter to nearly a third of what it holds to evaporation in a year. Building one is not casual earthmoving. A trench is cut the full length of the wall and packed with clay to form a core that stops seepage, and the banks either side are built from soil carrying 20 to 30 percent clay. The part that keeps the dam standing is buried and invisible once the work is done, which is why this is a designed and supervised build with a hazard class settled first.

This is the one water component that can kill somebody. Seepage is the root failure: water finding a way through, under or around the wall. Its doorways are burrowing animals, a stock path that wears the grass off the spillway, and trees whose roots rot into channels and whose cover hides the wet patch that was the warning. A sound pond still seeps about 25 mm (1 in) a month and loses more again to evaporation in heat, but the two together rarely pass 300 mm (12 in) a month. More than that is a real leak. Silt eats capacity from the day the wall closes. Upkeep is an inspection at first fill, after any big storm and at least yearly, with grass kept vigorous and woody growth off the bank so a wet spot still shows.