Before revision: Flood control channels are a series of large and empty basins which let water flow in and out (except when a flood is actually present) open-air channels that extend below the street levels of some larger cities, so that if and when a flood occurs, the flood will run into the channels, and proceed to be drained to the proper body of water.
After revision: Flood control channels are a series of 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 water occurs, these flood will run into the channels, and eventually drain into a river or other body of water.