the error inherent in the measurement. However, soundings separated by 

 decades suggest that the linear shoals superposed on the profile do show small 

 but real shifts in position (Moody, 1964, p. 143). Charts giving depths on 

 the continental shelves may include soundings that differ by decades in date. 



Plotted profiles usually use vertical exaggeration or distorted scales to 

 bring out characteristic features. This exaggeration may lead to a false 

 impression of the actual slopes. As plotted, the three profiles in Figure 4- 

 26 have roughly the same shape, but this sameness has been obtained by 

 vertical exaggerations of 2x, lOx, and 50x. 



Sand level changes in the beach and near shore zone may be measured quite 

 accurately from pipes imbedded in the sand (Inman and Rusnak, 1956; Urban and 

 Galvin, 1969; Gonzales, 1970). 



2. Onshore-Offshore Transport . Quantitative engineering guidance has 

 been more firmly established for rates of longshore transport than for rates 

 of onshore-offshore transport. This seems mainly due to the complexity 

 involved in the respective processes and in adequate analyses : simple 

 considerations using small-amplitude wave theory are applicable to longshore 

 transport (see Ch. 4, Sec. V,3), while the need for a higher order treatment 

 in considering onshore-offshore transport is well established but still 

 problematical (Wells, 1977; van de Graaff and Tilmans, 1980). With nearshore 

 waves propagating usually at only a slight angle with respect to a shore- 

 normal line, an appreciable unidirectional longshore current and net sediment 

 transport are driven by fairly steady longshore wave thrust. In contrast, net 

 onshore-offshore transport results from usually small differences betveen 

 oscillating sediment movements near to and opposite the wave direction. 



Onshore-offshore transport is sensitive to the detailed structure of the 

 reversing flow within the wave cycle and to any net flow. Also, besides the 

 intensely agitated surf zone, relatively gentle processes out to the seavard 

 limit of sediment motion must be considered. The integrated effect of complex 

 onshore-offshore transport processes, continuously varying along the active 

 profile, determines erosion and accretion along the profile and at the 

 shoreline (in regions of steady longshore sediment transport). 



Appreciable analytical and laboratory efforts have been devoted to 

 onshore-offshore transport in terms of separate bedload and suspended-load 

 components. However, significant uncertainties remain, and no formulation for 

 transport rate has established validity in prototype situations. 



Many laboratory studies have measured rates of sediment transport as 

 bedload collinear with various oscillatory flows. One problem in correlating 

 results is the complication associated with sediment movement possibly 

 occurring during only portions of the wave cycle. Available prediction 

 procedures for bedload or total transport by vaves (Bagnold, 1963; Einstein, 

 1971; Swart, 1976; Madsen and Grant, 1976; Sleath, 1978; Bailard and Inman, 

 1981) proceed from radically different analytical presumptions, consider 

 various selections of available data, and usually present complicated 

 empirical curves needed for calculations. Predicted transport rates by 

 different procedures can disagree by more than an order of magnitude, and no 

 procedure can be recommended presently. 



4-65 



