Ch. 19] RESEARCH NEEDED 345 



Emergency Debris Basins 



Emergency debris control may occasionally be required for highly 

 developed and vulnerable areas lying below a potential debris-pro- 

 ducing canyon that is completely denuded by fire shortly before the 

 beginning of the flood season. If insufficient time is available for 

 financing and constructing a permanent debris basin, a large measure 

 of protection against a single flood can quickly be provided by excavat- 

 ing, at the canyon mouth, a pit without inlet or outlet structures. 

 In order to avoid the increased damage that would result from sudden 

 release of stored debris, all the storage provided should be below the 

 natural ground surface. Excavated material should not be used to 

 form an embankment around the pit to increase its capacity ; it should 

 be disposed of in a location free from attack by flood flows. Without 

 an inlet structure, the pit may be partially filled by headward erosion 

 during minor flows, and, without an outlet structure, the basin storage 

 may be rendered ineffective if discharged flow creates a gully at the 

 downstream side. These defects can be minimized to some extent by 

 segregating the larger boulders encountered in the excavation and us- 

 ing them to form rock inlet and outlet sills. Gullying at the down- 

 stream side of the basin can also be minimized if the basin is ex- 

 cavated along the downstream edge on a contour so that initially, at 

 least, overflow will be in a thin sheet rather than concentrated at one 

 point. 



RESEARCH NEEDED 



All the described methods of debris control are susceptible to im- 

 provement. The control of debris by watershed treatment is probably 

 the least expensive in some areas. However, most debris-producing 

 areas are rugged and semi-arid, and natural plant growth is sparse. 

 Furthermore, in any area, the protection afforded by plant cover is 

 subject to sudden and complete nullification by fire. It is suggested 

 that one field for further study include the discovery and cultivation 

 of plant types capable of achieving dense growth in unfavorable areas; 

 development of inexpensive processes of establishing cover growth; 

 and, possibly, discovery of plant types that would develop a dense sur- 

 face network of roots that could hold the material in place even if all 

 growth aboveground were destroyed by fire and that could continue to 

 afford such protection until new growth became established. 



With the exception of watershed treatment, the methods of debris 

 control discussed involve an estimate of the debris-producing capacity 

 of the problem area. In Los Angeles County, before-and-after surveys 



