SECT. 3] BEACH AND NEARSHORE PROCESSES 535 



observed by divers at depths exceeding 52 m (Inman, 1957). Everywhere out- 

 side of the surf zone, where the effect of wave action is moderate, the sandy 

 bottom is rippled. In and near the surf zone the intense bottom shear produced 

 by the higher orbital velocities destroys the ripples and the bottom is smooth. 

 A seasonal fluctuation in the level of the sand, caused by differences between 

 summer and winter waves, is clearly discernible at depths greater than 10 m, 

 as shown in Fig. 9. The long low waves cause a net shoreward transport of sand 

 so that the beach builds seawards and the offshore profile steepens. High steep 

 waves erode the beach and transport material offshore, causing a decrease in 

 the beach slope. 



Model experiments (Bagnold, 1947) showed that waves of small wavelength 

 to height ratio caused merely a to-and-fro motion of the horizontal sand bed 

 offshore of the plunge line, with little or no net transport to or from the shore. 

 Waves of the same height and longer wavelength caused general shoreward 

 transport, accompanied by a general shoreward drift of the lowest water layer. 

 However, while the shoreward bed-water drift continued up the sloping bed 

 towards and as far as the plunge line, the corresponding di'ift of sand was 

 appreciably reduced by the upward bed inclination. As a result, sand tended to 

 accumulate at the foot of the slope and thus to reduce its steepness. Owing to 

 the slowness of the movement, a steady-state profile was not reached in the 

 time available. It is noteworthy, however, that no progressive building out of 

 the beach face was detectable. All shoreward movement of sand stopped at 

 the plunge line. 



It has been shown (Bagnold, 1946; Inman, 1957) that the wavelength of 

 sand ripples is approximately equal to the orbital diameter (horizontal particle 

 displacement) of the waves generating the ripples, up to a limiting maximum 

 ri232:)le size beyond which the ripple cannot grow. The limiting maximum ripple 

 wavelength is a function of sand size and varies from about 15 cm for very 

 fine sand to 125 cm for coarse sand. Thus the long, low waves that produce an 

 onshore transport of sand along ocean beaches have orbital diameters many 

 times greater than the wavelength of the bottom ripples, and the sand moved 

 by each wave may cross over several ripples. In models, on the other hand, the 

 orbital diameter of the small waves does not commonly exceed the limiting 

 ripple size, and the ripples have the same wavelength as the orbital diameters. 

 For this condition Russell ^ has observed that long waves in models may cause 

 a net offshore transport of sediment even though the bottom is horizontal. 

 The rapid onshore acceleration accompanying the passage of a wave crest 

 forms a vortex of suspended sand over each ripple crest, with relatively little 

 onshore displacement of sand. The offshore motion accompanying the wave 

 trough, although more gradual, results in a net offshore transport of sand 

 already j^laced in suspension by the preceding onshore motion. Steep waves 

 on the other hand, because there is less asymmetry between the crest and 

 trough orbital velocities, merely cause a to-and-fro motion of sediment with no 



1 Personal communication from R. C. H. Russell, Hydraulics Research Station, Walling- 

 ford. England. 



