344 



dodge. DEBRIS CONTROL 



[Ch. 19 



consists of a bowl-shaped pit excavated in the surface of the debris 

 cone; an embankment, usually U-shaped, constructed from the pit 

 material and located along the two sides and downstream end of the 

 pit; an inlet structure at the upstream end of the pit; and an overflow 

 spillway at the downstream end. 



Debris basins are usually designed to hold only the debris estimated 

 to result from a single major storm, and the storage provided is there- 



Fig. 2. Typical debris basin, La Crescenta area, San Gabriel Mountains. Hay 

 Canyon debris basin, inlet structure in background, outlet structure at right 



foreground. 



fore considerably less in relation to drainage area than that provided 

 by debris barriers. Because the basin is small, the shape of the basin 

 and the relative positions of inlet and outlet require careful con- 

 sideration in order that debris will not be discharged over the spillway 

 before all the debris-storage capacity of the basin has been utilized. 

 The inlet consists of a chute or drop structure tied in to the canyon 

 walls and extending from the natural stream bed to the bottom of the 

 excavated pit. The structure serves both to direct flow into the basin 

 and to prevent filling of the basin by headward erosion of the stream 

 bed. The outlet structure consists of an overflow spillway with crest 

 set at the elevation required to provide the desired debris storage. 



Debris basins provide immediate and positive control of debris. 

 Because they are designed to store debris from one flood only, ac- 

 cumulations must be removed. As long as the deposited material is 

 removed, however, the basins can control debris indefinitely. 



