Dams

The benefits of dams
The prime function of a dam is to store water behind it. Combined, the 10 dams managed by Watercare can hold 96 million cubic metres of water in their reservoirs. Besides holding sufficient supplies of water, dams serve another purpose: when water is stored in a lake its quality improves. Sediment particles settle down on the bed of the lake and bacteria die off substantially, especially after a period of 30 days. Another function of dams is that, within the unused space between the water level and spillway overflow level, storm runoff can be stored. Thus dams reduce the effects of floods.

Location of dams
It makes sense to locate dams in high rainfall areas close to cities. The Hunua Ranges and Waitakere Ranges have a high elevation and receive 1800mm of rain annually, about 1.5 times the amount that Auckland City gets. Their high location also means water can flow to the city by gravity.

Structure of dams
Dams in Auckland are either compacted earth or concrete. The earlier ones in the Waitakeres were made of concrete while the later, post-war dams were built with earth and rock.