Ch. 19] DEBRIS BASINS 343 



Maintenance of debris-storage capacity behind barriers by excava- 

 tion is usually not feasible, because the narrow, rugged canyons re- 

 strict access of equipment, and because there is usually no suitable 

 adjacent area where the excavated material can be placed and be free 

 from attack by subsequent flood flows. Sluicing of fine materials 

 through suitable openings during periods of low flow will help to 

 maintain storage. However, the sluiced material will be carried down- 

 stream by subsequent floods and may cause damage. Consequently, 

 sufficient storage should usually be provided for the entire debris in- 

 flow during the life of the structure. Within the limits of storage space 

 available, debris barriers afford positive control of debris. If the down- 

 stream channel is steep and erodible, the clear water discharged from 

 the dam will pick up material from the stream bed and carry it to the 

 lower channel reaches. Resulting degradation below the dam may 

 cause caving of banks and undermining of bridge structures. In the 

 lower reaches, the resulting aggradation will generally be somewhat less 

 than would occur under natural conditions, because the dams effect 

 some reduction in peak flow, particularly for small floods. In southern 

 California, where urban and suburban development usually extends 

 up to the canyon mouths and downstream channels are restricted, 

 downstream-channel improvement frequently becomes a necessary ad- 

 junct to construction of debris barriers. However, the effect of North 

 Fork and Upper Narrows dams in northern California on downstream 

 regimen is negligible, because the natural sediment load (as distinct 

 from that resulting from mining operations) is relatively light. 



Debris Basins 



At the mouths of the canyons that drain the precipitous southern 

 face of the San Gabriel Mountains, the Los Angeles County Flood Con- 

 trol District and the Corps of Engineers have constructed 25 debris 

 basins. The approved comprehensive plans of the two agencies provide 

 for eventual construction of about 50 more such basins (see Fig. 2). 



Debris basins are like debris barriers in that a pool is provided for 

 settlement and retention of debris while the cleared water is discharged 

 over a spillway. Debris basins are distinctly different, however, in 

 location, design, construction, and maintenance. 



The best location for a debris basin is usually at the canyon mouth, 

 in such a position that the basin is outside the canyon for ease of ac- 

 cess, but close enough that the inlet can easily be tied to the canyon 

 sides to prevent flood flows from flanking the basin. The site should 

 also include an adjacent area suitable for debris disposal. The basin 



