with the accuracy desired and the degree of regularity of the area, (see 

 Beach Erosion Board Technical Memorandum No. 32). Profiles may be run 

 by any standard hydrographic method. Where the amount of such work to be 

 done is large and continuing, sonic equipment and DUKWS may be used to ad- 

 vantage. Care should be taken to insure accurate vertical control and 

 measurement as small vertical errors result in large quantity errors, 



90. Measurement of accumulations on the updrif t side of jetties or 

 long groins provides a good basis for estimating the rate of littoral 

 drift. Depth of water to which the structures extend^ and the character 

 of material trapped, must be considered in evaluating the impoundment rate 

 in comparison with total littoral drifto Short groins provide a poor 



means for measuring the rate of littoral drift because the amount of material 

 trapped is usually a small and indeterminable part of the entire quantity 

 of littoral drift. Shoaling in entrance channels may provide a measurement 

 of littoral drift in locations where maintenance dredging is done frequently. 

 However, this method can seldom be used because of the difficulty of 

 separating shoaling caused by reversals in the direction of littoral drift 

 from that caused by the predominant drift, 



91. Sources of Littoral Material . - A stable shore line is one in 

 which the supply of littoral material to the area under consideration is 

 approximately equal to losses of littoral material from the area. On an 

 accreting shore line the supply of material exceeds the losses., and the 

 reverse is true of an eroding shore line. Accordingly, the need for pro- 

 tective works and the choice of type of protective works to be provided are 

 dependent on the net balance between supply and loss of littoral material, 



92. The three main natural sources of littoral material to any beach 

 segment are: (a) material moving into the area by natural littoral drift 

 from adjacent beach areas; (b) contributions by streainsi and (c) contributions 

 by bluffs as the result of natural bluff erosion. In addition, there may 

 occasionally be some long range net movement of material onshore apart from 

 normal seasonal or other periodic fluctuations. These might occur^ for ex- 

 aiaple^i with permanent or semipermanent changes in lake level. Considering 

 coasts as a vjholej maintenance of beaches must be attained at the expense 



of erosion of the land mass. For any individual segment of beach, the 

 largest source of material moving into the area is generally littoral drift 

 eroded from the adjoining updrift segment unless some major sediment 

 bearing streaia enters the segment in question or cliff or dune erosion is 

 sufficiently rapid to provide appreciable supply, 



93. Contributions by Streams , - The amounts of the various contributions 

 to the littoral supply by sand carrying streams can be determined approx- 

 imately by these general methods; (l) direct measurements c, (2) studies of 

 terrestrial sedimentation, and (3) computation of the sediment carrying 

 capacity of the streams. To date, the only method upon which any great 

 degree of reliance can be placed is that of direct measurement. 



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