Figure 4-22, from Inman (1957) and including data from two earlier 

 studies, shows the velocity needed to start motion in a sediment bed of 

 a given grain size. These results are in general agreement with other 

 studies relating critical velocity to grain size. Also shown in this 

 figure are maximum velocities above which ripples tend to be smoothed 

 off, in qualitative agreement with conditions for bed forms in unidirec- 

 tional flows. (Southard, 1972.) 



From Figure 4-22, it appears that maximum wave-induced bottom veloc- 

 ities between 0.4 and 1.0 foot per second are sufficient to initiate sand 

 motion imder waves. In field studies, Inman (1957) found that ripples are 

 always present whenever computed maximum velocities exceed 0.33 foot per 

 second, and Cook and Gorsline (1972) report ripples above a velocity range 

 of 0.5 to 0.6 foot per second. Equation 4-19 can be used to determine the 

 combination of wave conditions and depth that produces any given critical 

 value of Umax at the bottom. Figure 4-23 shows the relation between depth 

 and wave height, for given wave periods, for a critical velocity, vmax = 

 0.5 foot per second. 



4.523 Seaward Limit of Significant Transport . Figures 4-20 through 4-23 

 and a knowledge of offshore wave climate suggest that waves can move bot- 

 tom sediments over most of the Continental Shelf (to depths of 100 to 400 

 feet or more) during some time of the year. Geologic studies indicate 

 that fine material has been winnowed from the surficial sediments over 

 much of the shelf. (Shepard, 1963; and Dietz, 1963.) The question is, 

 what is the maximum depth to which the rate of sand movement is signifi- 

 cant in coastal engineering? This section discusses field data that 

 supply partial answers to this question. 



a. Bathymetry . Dietz (1963) and others point out that waves rework 

 nearshore sands, smoothing out irregularities by longshore and onshore- 

 offshore transport. This smoothing produces a quasi -equilibrium surface 

 in the nearshore zone which forms relatively straight contours, nearly 

 parallel to the shoreline. 



Most bathymetric charts with closely spaced contours as illustrated 

 by Figures 4-24 and 4-25 show that isobaths near the shore run parallel 

 to the shoreline; further offshore, the contours may indicate linear 

 shoals (Duane, et al., 1972), or other irregular submarine features. 



Following the idea of Dietz (1963), the depth below which shore- 

 parallel contours give way to irregular contours is assumed to mark the 

 local transition between the nearshore zone where sands are moved by the 

 waves in significant quantities and the offshore zone where sand is moved 

 in lesser quantities. Possible exceptions to this shore-parallel contour 

 rule are the contours around river deltas and inlets, or where reefs and 

 ledges occur in the nearshore zone. 



4-65 



