9h» Direct measurements may be made with considerable accuracy under 

 certain conditions. Delta measurements by auccessive hydrographic surveys 

 are adequate to determine the amount contributed by those streams which 

 carry sediment to the ocean or lake only duiing flash floods lasting a 

 relative short period of times Similar comparative surveys are adequate 

 to determine the amount contributed by streams which bring material to 

 the shore continuously j, or over an extended period of timej, where those 

 streams terminate in navigable channels or other natural settling basins. 

 Some cozTection may be necessary to account for sediment deposited out- 

 side of the channels or basin area^ for material moving out of the area 

 betiireen surveys by natural littoral drift, and for material removed from 

 the area artificially as maintenance of navigation channels and basinse 



95o Should investigation show that the principal sources of beach- 

 building materials are the drainage basins tributary to the shore under 

 consideration, a detailed study of the continental geology of these 

 basins may be required if direct measurements are impracticable » Such 

 a study would include hydrology^ physiography^ stratigraphy, and sediment 

 supply deduced from measured or estimated rates of terrestrial sedimenta- 

 tion. 



96c For mountainous watersheds,, the Forei^t Service has developed 

 empirical methods for estimating the sediiaentation rate (Anderson 19U9). 

 The determination was for a specific area of known geologic character- 

 isticSj, but the results are valid for other areas if corrections are made 

 for several variablesj vegetation, hydrology,, rock type_, etc. 



97. Even if there are excellent data on terrestrial sedimentation 

 rates., it may be very difficult to estimate how m.uch material reaches the 

 shore. The measurement of losses must be indirect. If the streams are 

 degrading or appear to be at grade_, it can be assumed that all source 

 m.aterial ultimately reaches the shore. But if the streams are aggrading 

 deposition rates along the channel must be estimated and losses si±)tracted 

 from the total sadimentaticn to determine the net sediment supply to the 

 beaches. 



98. The most accepted approach in the computation of the sediment 

 carrying capacit;/ of a stream is the direct measurement of the wash-load 

 of a river by suspended-load sampling (lo'wa University 19U8). This 

 method is expensive and time consuming, taking from 1 to 10 years of 

 continuous ;,b"erTation to predict the wash-load of a river^ depending 



on the regularity of its flows. Accordingly 5 a different method of 

 approach has been developed based on grain size of the bed materials 

 (Einstein 19^0,, 19^1). Its basic concept is that bed material always 

 m.oves according to the capacity of the stream. The capacity rates for- 

 bad material may be computed, by formulas which were developed to permit 

 the prediction of the individual bed-load rates of the different bed 

 components in terms of stream discharge. The solutions are long and 

 laborious but are not difficult to follow. However., the method would 

 probably be used only if a determination of tiie stream carrying capacity 



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