Before revision: Flood control channels are large and empty basins which let water flow in and out (except during flooding) or dry channels that run below the street levels of some larger cities, so that if and when a flood occurs, the water will run into these channels , and eventually drain into a river or other body 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 water will drain along these channels into a river or other bodies, of water.