342 dodge. DEBRIS CONTROL [Ch. 19 



viding at each drop a relatively narrow notch or opening that would 

 permit fine materials to be sluiced down the channel instead of being 

 deposited in each pool. Such openings should not, however, be large 

 enough to reduce materially the ponding effect on large flows. 



Unless substantially constructed, the structures might fail during a 

 major flow and release all the stored debris, causing a greater debris 

 flow than would result under natural conditions. In very steep can- 

 yons (and most debris-producing areas are steep) , the cost of treatment 

 with adequately constructed drop structures might be excessive be- 

 cause the drop structures would have to be placed in close succession 

 to reduce stream velocities effectively. 



Check Dams and Barriees 



Check dams and barriers are somewhat similar, and the distinction 

 made here is purely arbitrary. Check dams usually consist of rela- 

 tively low barriers constructed across a drainage channel to intercept 

 debris. The storage provided by such construction is usually small, 

 however, and re-excavation frequently impracticable. As a method 

 of debris control, check dams have only limited application. They 

 could be used perhaps to provide immediate relief in connection with 

 methods of watershed treatment already discussed which, in themselves, 

 require several years to become fully effective. As with drop struc- 

 tures, poorly constructed check dams may cause increased damage 

 by collapsing during a flood. 



Barriers may be considered as check dams of major proportions. As 

 such, they are designed and constructed as dams. Those known to 

 the writer are located in narrow canyons and are generally concrete- 

 arch structures with overflow lips at the center of the arch. Debris 

 carried into the reservoirs formed by the dams is deposited in the still 

 pools, and the cleared water is discharged over the spillways. Opera- 

 tion is thus entirely automatic. The Los Angeles County Flood Con- 

 trol District has constructed two such structures, and the U. S. Forest 

 Service one, in the canyons along the south face of the San Gabriel 

 Mountains. Storage not occupied by debris has incidental use for flood- 

 peak reduction and water conservation. Original storage capacities of 

 these basins are from 6 to 60 acre-feet. Larger barriers that have 

 been constructed in the Sacramento River Basin are North Fork Debris 

 Dam on North Fork of American River, and Upper Narrows Debris 

 Dam on Yuba River in California, constructed by the Corps of En- 

 gineers. The North Fork Debris Dam provides storage for 17,000 acre- 

 feet of debris. 



