338 dodge. DEBRIS CONTROL [Ch. 19 



FACTORS THAT INFLUENCE THE PROBLEM 



The successful solution of debris-control problems depends on ac- 

 curate appraisal of physical conditions in the problem area and the de- 

 velopment of an adequate method of control that is both suited to 

 those physical conditions and within the economically justified cost. 



Debris Quantities 



Control of debris by trapping or storing material carried in chan- 

 nels involves estimation of the amount of debris that may be expected 

 either to result from a single large flood or to accumulate over a period 

 of years. No practical method exists for gaging debris while in transit. 

 The only source of data therefore is from comparison surveys of areas 

 where debris is deposited. Such records are extremely meager. In 

 Los Angeles Qounty, in the area at the base of the San Gabriel Moun- 

 tains, the average debris production resulting from a major flood in 

 1934, as estimated from deposits over the area, was 88,000 cubic yards 

 per square mile of drainage area. This flood occurred a few months 

 after most of the mountain drainage area involved had been denuded 

 by fire. Four years later, in 1938, another major flood in the same 

 area produced debris, as measured in 8 debris basins constructed in the 

 interim, at an average rate of about 75,000 cubic yards per square mile 

 of drainage area, with a maximum rate of 124,000 cubic yards per 

 square mile. Mean annual rates at the same basins, as tentatively in- 

 dicated from only about 10 years of record, average 16,000 cubic yards 

 per square mile of drainage area, with a maximum of 22,300 cubic 

 yards per square mile. These rates, from a combined drainage area of 

 about 6.5 square miles, are the highest known to the writer. Debris- 

 production rates from nearby but larger drainage areas are substan- 

 tially smaller (Dodge, 1947) . Such data are useful in planning debris- 

 control programs for similar areas (Douma, 1947). 



However, no method exists for applying this information directly to 

 the estimation of debris-production potentialities of areas where the 

 physical conditions are appreciably different. In such cases, the only 

 alternative is to consider the physical conditions that lead to debris 

 production in the problem area and, by comparison with conditions in 

 an area for which records exist, arrive at an approximate estimate. 



Drainage-Area Conditions 



The principal physical conditions that, in combination, usually 

 characterize debris-producing areas are: 



