Ch. 19] DROP STRUCTURES 341 



Control of debris by improvement of cover growth and by terracing 

 is generally not applicable to the rugged and rocky San Gabriel Moun- 

 tains near Los Angeles. Such methods have, however, been reported 

 to be successful along the west face of the Wasatch Mountains in 

 northern Utah (Bailey and Craddock, 1947). The protection of cover 

 growth from fire is, or course, effective in any area. 



The foregoing methods of control may require many years to be- 

 come fully effective. Unless terraces trap all runoff from the drainage 

 area, detritus collected in the natural stream channels will continue 

 to be carried down by floods. Furthermore, as the stream beds are 

 cleaned out and lowered, bank caving is likely to increase. Plant 

 growth usually requires many years to become well established ; it can 

 be destroyed in a single fire. In general, where the drainage area is 

 suitable, these methods are applicable if the cost of more positive 

 measures is prohibitive and if immediate control is not necessary. 



Drop Structures 



In areas where the bulk of debris comes from the beds and sides of 

 canyons or gullies that have cut through otherwise stable drainage sur- 

 faces, a series of drop structures in the channel may be employed to 

 halt debris flow. Drop structures are more generally used in land- 

 conservation projects to halt headward erosion of gullies. They are 

 mentioned here as a method having limited application for debris con- 

 trol. 



The function of drop structures for debris control would be to re- 

 duce flood velocities and, hence, debris erosion and transportation. 

 Vertical falls and stilling basins at the drops would provide for con- 

 trolled absorption of excess energy (Morris and Johnson, 1943). The 

 bottoms of the structures should be placed at stream bed with the crests 

 projecting above stream bed. Generally the crest of one drop should 

 be at the elevation of the bottom of the next drop upstream. Initially 

 the structures would form a series of pools. These pools would gradu- 

 ally fill with the finer material carried by the reduced flood-flow 

 velocities until the deposits reached the overflow-crest elevation. Be- 

 yond that point, the deposits would continue to build up on increasing 

 slopes upstream from the crests until the original stream slope was 

 approximated. The increasing slope would result in increasing flow 

 velocities and, with the bottom protected from erosion, aggravated 

 erosion would be likely to result along the banks. Thus, if the de- 

 posits were not removed, the structures might eventually become detri- 

 mental. The useful life of such structures might be increased by pro- 



