SECT. 3] 



BEACH AND NEARSHORE PROCESSES 



537 



amplitude should produce a draw-down or negative slope in the mean surface 

 level as the depth diminishes. However, when the waves break, the water level 

 rises and the slope becomes positive. 



A rise of the water level nearshore is also produced by winds blowing on- 

 shore. Model experiments performed in the Geography Department at 

 Cambridge University (King, 1959, p. 211) demonstrated that onshore winds 

 caused offshore currents to flow along the bottom. Where combined with wave 

 action, the wind-induced currents increased the rate of offshore sand transjjort 

 of the waves in the surf zone, and decreased or reversed the onshore transport 



(a) RIPPLE PROFILE 



''-'b '^'npple 



^ripple 



(b) SAND DISPERSION BY WAVES (PLAN) 



WAVES 



(c) WAVES AND WIND 



WAVES 



^ 



WIND 



BEACH 



Fig. 10. (a) Path of a sand grain in a migrating ripple, observer moving forward with the 

 ripple form; (b) dispersion pattern of marked sand introduced at "0" under the 

 influence of waves; and (c) dispersion pattern under influence of waves and wind. 



outside of the breaker point. Rise of the water level nearshore may be of 

 considerable importance in the real three-dimensional case. 



10. Migration of Sand Grains by Wave Action 



The average speed, Ug, at which individual grains migrate from a given initial 

 position on the sea bed is likely to be much less than the mean speed. Ub, of mass 

 transport. For owing to the presence and movement of bed ripples any in- 

 dividual grain is alternately exposed on the ripple surface and buried within it. 



Consider the ripple cross-section sketched in Fig. 10a. To an observer moving 

 with the ripple form, as the form slowly advances in the direction of sediment 

 drift, marked grains would appear to circulate forwards over its surface at the 



