Because legal requirements and high costs restrict new dam construction, other methods of water development may be more suitable. In some places only a round concrete box structure is needed to divert part of the flow of a stream through a pipe to a downstream turbine.
On any given stream, the best site for a diversion is usually quite obvious: a natural waterfall, a swift current or a steep slope. The terrain should be steep enough (10% slope or more) so the diversion pipe doesn’t have to be too long to obtain sufficient head.
Because a diversion has no backup storage for periods of low flow, potential power is sometimes calculated by determining the runoff in a year of normal rainfall which is exceeded on half the days of the year. The example watershed (page 1) has a stream flow of 8.4 cubic feet per second or less for half the year. With a 12' head, this stream can operate a 5 KW generator at full capacity for half the year and at about 50% capacity for the remainder. Some people install batteries to provide electrical storage for periods of low flow.
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