Tujunga Wash is an example of a concrete flood control channel.
Flood control channels are a series of large and empty (except when a flood is actually present) open-air channels that extend a ways 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. Flood control channels are sometimes built on the former courses of waterways as a way to reduce flooding. They are not to be confused with watercourses which are simply confined between levees. These structures may be made entirely of concrete, with concrete sides and an exposed bottom, with riprap sides and an exposed bottom, or completely unlined. They often contain grade control sills or weirs to prevent erosion and maintain a level streambed. By definition, flood control channels range from the size of a street gutter to a few hundred or even a few thousand feet wide in some rare cases. Flood control channels are found in most heavily developed areas in the world. One city which contains many of these channels is Los Angeles, as they became mandatory with the passage of the Flood Control Act of 1941, passed in the wake of the Los Angeles Flood of 1938.