340 dodge. DEBRIS CONTROL [Ch. 19 



In the Sacramento River areas, the production of debris can be 

 controlled by limitation of hydraulic mining operations. Conversely, 

 debris-control structures are installed primarily to permit continuation 

 of mining operations and are justified by the return from those opera- 

 tions. 



One other economic factor sometimes missed is that debris cones are 

 natural water-percolating areas and are frequently important in main- 

 taining underground water supplies. In semi-arid areas, reduction of 

 water supply by debris-control works may be more serious than the 

 damage from debris. 



METHODS OF CONTROL 



The methods of debris control most generally used are briefly de- 

 scribed below. The applicability and effectiveness of any of these 

 methods depend on the physical characteristics of the area involved. 



Watershed Treatment 



The most direct approach to the problem is concerned with stabiliza- 

 tion of the drainage area for the purpose of reducing erosion to a 

 minimum. Such methods consist in establishing or encouraging plant 

 growth, terracing, and preventing fires. These methods can be used 

 singly or in combination. 



Artificial establishment or increase in plant growth depends mainly 

 on rainfall supply, soil type, and soil fertility ; the availability of plant 

 types that can thrive under the given conditions ; and, lastly, adequate 

 protection of the cover growth from fire. 



Terracing to form long, narrow, on-contour storage basins, serves 

 to trap runoff on the watershed and prevent its concentration into 

 debris-carrying flow. The trapped runoff is dissipated by percola- 

 tion into the ground and by evaporation. By increasing the runoff re- 

 tained on the watershed, terracing may also result in improvement of 

 cover growth. Terracing is not applicable in areas that are rugged 

 and rocky and have only a thin soil mantle. Terrace basins can be 

 dangerous if not made large enough. The basins are, in effect, small 

 reservoirs having considerable aggregate storage, and failure of an up- 

 stream basin may result in progressive failure of lower basins, thus 

 causing a flood greater than might have come from the untreated area. 

 Determination of required basin capacity should therefore be based 

 on careful study of the drainage-area hydrology, and it should include 

 consideration of the possibility of two storms occurring in close succes- 

 sion. 



