Before revision: Flood control channels are large and empty basins where surface water can flow through but is not retained (except during flooding), or dry channels that run below the street levels of some larger cities, so that if and when a flash flood occurs the water will drain along these channels into a river or other bodies of water.
After revision: Flood control channels are large and empty basins where surface water can flow through but is not retained (except during flooding), or dry channels that run below the street levels of some larger cities, so that if and when a flash flood occurs the excess water can drain out along these channels into a river or other bodies of water.