68 einstein-johnson. SEDIMENT TRANSPORT [Ch. 3 



tions on this type of bed-load equation may be expected to appear 

 in the next few years. 



PRACTICAL APPLICATION OF BED-LOAD EQUATIONS 



After a bed-load equation has been developed from flume experi- 

 ments, it is very important to determine its applicability to natural 

 streams by making measurements in such streams. Such measure- 

 ments of sediment load should combine bed-load measurements (Ein- 

 stein, 1944) with a suspended-load-measuring program in order to 

 determine the bed-load function of the stream. For a comparison 

 it is necessary, therefore, to integrate the specific load as obtained 

 from the bed-load equation over the entire cross section. The inte- 

 gration of the transport over the cross section may be made on a basis 

 of either local or average specific load; that is, either the hydraulic 

 conditions from which the load is calculated may be averaged and 

 the load calculated from this average flow, or the load may be calcu- 

 lated locally and then integrated over the cross section. Both methods 

 lead to practical difficulties and call for various assumptions to be 

 made. At the present stage of development in the field of sediment 

 transportation, it is not clear in which cases one or the other method 

 will give the best results. 



The predominant significance of wash load in the silting of most 

 reservoirs has been mentioned. In very few cases would this problem 

 justify the separate and detailed study of bed-material load. Its 

 main significance appears in problems of stream-channel stability, 

 for which it has been shown that the wash load has no influence. If 

 a stream is depositing annually a certain amount of sediment in its 

 channel bed, flood damage must be expected to develop. The usual 

 question in such a problem is whether or not any countermeasures are 

 economically feasible to prevent all or part of the flood damage. This 

 problem may be approached by either of two methods or by a combi- 

 nation of them. The rate of sediment supply may be reduced by re- 

 taining the material at its point of origin or in sediment-retention 

 basins; or the sediment capacity of the channel in question may be 

 increased by channel rectification and elimination of unnecessary flow 

 resistance (Einstein, 1944), or by a combination of the two methods. 



ROUGHNESS 



A discussion of the laws of sediment transportation would not be 

 complete without some remarks about the roughness conditions along 



