. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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 acti
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - co
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