relationship is not the same at all localities since it is also influenced 

 by water level variability and wave exposure. 



680 Evidence of the slope sorting of beach materials is illustrated 

 on Figures 37 and 38. The slope sorting is ascribed generally to the 

 differential velocity in the oscillating wave currents in shallow water, 

 whereas the velocity of water particles is theoretically uniform about 

 the orbital path of the particles in deep water ^ moving in the direction 

 of wave propagation while the crest is passing and in the opposite 

 direction with passage of the trough, the uniformity ceasing when 'the 

 wave is deformed by diminishing depth, Deforraation takes the form of 

 steepening the crests, shortening them in relation to lengths of the troughs, 

 so that the particles move forvjard with the wave crest in less time than 

 they return with the trough. Figure 39 illustrates laboratory measurements 

 of this differential velocity. The effect of this phenomenon is to trans- 

 port coarser pai'ticles of bed material shoreward. Where the slope becomes '^ 

 sufficiently steepj, gravity counterbalances the cuxrent effect and an 

 equilibrium condition results. Currents introduced by reflected waves 

 likewise affect the balance of forces in this region near the shore. 



69. Because of slope sorting, it may be stated that material in 

 littoral transport moves generally within a depth range compatible with 

 its size or resistance to transport. The actual path or rate of transport 

 of individual particles or groups of particles cannot be stated from, 

 present knowledge. While density and particle shape are factors in 

 transportability, median grain size is a satisfactory parameter for 

 evaluating generally the transportability of littoral material. 



70, Littoral characteristics. - The existence of an erosion 

 problein is prima facie evidence that one of two conditions exist. The 

 first is that the water level and the land have not become adjusted in 

 terms of shore slope 5 as in the case of exceptionally high lake levels 

 or storm tidesj and upland material is being eroded to establish equili- 

 brium slopes. The second and more common condition, which may exist 

 C'jmcurrently with that cited above, is that material being removed from 

 the area exceeds the rate of supply. Normal supply and loss occurs 

 through the process of littoral drift. Analysis of an erosion problem 

 preparatory to fuxictional planning of remedial m.easures thus requires 

 that conclusions be reached concerning the characteristics of littoral 

 drift. These characteristics as ideally defined for a particular shore 

 segment ares (a) the predominant drift direction; \.b) direction variability 5 

 (c) character of littoral materials and depths in which they are trans- 

 portedi (d) rate of supply to the area and source of material supplied; 

 and (e) rate of loss from the area and the manner in which loss occurs. 



It will not always be possible to reach well supported conclusions for 

 these fi/e features , nevertheless careful investigation will provide 

 knowledge in place of opinion in m.any cases. 



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