Ch. 19] ECONOMIC CONSIDERATIONS 339 



Topography. Debris-producing areas are usually rugged and pre- 

 cipitous, covered only with sparse vegetation. Main drainage courses 

 are narrow, steep, and conducive to the turbulent high-velocity runoff 

 necessary for transportation of large quantities of debris. 



Geology. Geologic features conducive to debris production are slide 

 areas, shattered and deeply weathered rock, and coarse, porous, rocky 

 soils. 



Hydrology. Debris-producing areas are usually those having low an- 

 nual rainfall, classed as semi-arid. In such areas, long periods of little 

 rainfall result in only sparse vegetation and permit the weathering and 

 accumulation of debris in quantity on the watershed and in the drain- 

 age channels. Such areas are subject to occasional short-duration high- 

 intensity rainfall that produces flash floods having high peak discharges 

 capable of collecting and transporting large quantities of debris. 



Fire. In areas having the characteristics described above, the only 

 retarding influence on excessive erosion is the vegetal cover. When the 

 plant growth, even though sparse, is destroyed, the rate of debris pro- 

 duction resulting from a given rainfall is enormously increased, and 

 it diminishes only gradually as new growth becomes established. In 

 appraising the debris-production potentialities of any area, therefore, 

 the possibility of complete denudation by fire should always be con- 

 sidered. 



Economic Considerations 



Economic factors are as varied as the developments to be protected. 

 Obviously, the cost of control works should not exceed the value of 

 benefits to be derived, except where intangible benefits such as pro- 

 tection of life are an important part of the problem. In highly de- 

 veloped urban and suburban areas, potential damage is tremendous, 

 and control measures must be positive. Consider, for example, the 

 flood on January 1, 1934, in the La Crescenta area. This flood, from 

 a number of individual canyons having a total drainage area of about 

 7.5 square miles, lasted less than 15 minutes, deposited about 660,000 

 cubic yards of debris over the area, killed 42 people, and destroyed 

 500 homes. The total damage was estimated at $5,000,000. Although 

 such floods are infrequent (a factor that encourages development of 

 such areas) , costly control works are well justified. 



In Utah, where the development to be protected is generally rural, 

 most of the damage results from debris deposition on farm land. The 

 benefits from debris control are consequently not so great as in urban 

 areas, and control measures are required that are less costly and per- 

 haps less positive. 



