For flood purposes alone it would be necessary to provide reservoirs of
sufficient capacity to contain the run-off waters resulting from the
largest storms. With such provisions it would be necessary to entirely
empty the reservoir as soon as possible after a storm had passed and
leave its full capacity available for the next storm. It is therefore
better, wherever possible, to provide a reservoir capacity considerably
larger than that represented by the run-off from the heaviest storms, so
that water may be stored for use as power or domestic supply. With such
provision it is necessary merely to draw from the reservoir water to a
depth equivalent to the stream run-off in the drainage area above.
The second plan for prevention of flood damages involves provisions for
letting the flood water out rapidly by removing obstructions to its flow
by straightening and deepening the channels and providing long
embankments, dikes, or levees which rise above the ordinary river level
to a height exceeding that of the stream during its highest floods.
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