Before revision: Flood control channels are a series of large and empty (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: basins which let water flow in and outFlood control channels are a series of large and empty (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.