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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 3. (March)

First Page: 613

Last Page: 613

Title: Shallow Sand Bars and Nearshore Processes: ABSTRACT

Author(s): R. A. Davis, Jr., W. T. Fox

Article Type: Meeting abstract


Detailed daily topographic maps of beach and inner nearshore areas indicate a cyclic pattern of processes and responses in this environment. This pattern is the result of complex interaction between shoreline configuration, nearshore sand bars, and environmental variables such as barometric pressure, wind velocity, breaker height, and longshore currents. The key indicator in this pattern is barometric pressure. As it changes there are corresponding changes in coastal processes which thereby cause morphologic changes in the beach and inner nearshore area.

A model can be constructed which is characterized by the following sequence:

(1) During high-pressure and low-energy conditions, a shallow discontinuous sand bar is present with somewhat regularly spaced rip channels. The shoreline is sinuous with protuberances (cusps) behind the sand bars and embayments adjacent to rip channels. Slow-moving longshore currents and small waves prevail. Waves break on sand bars and cause their shoreward migration. Shoreline sinuosity is increased as protuberances grow and embayments are slightly eroded.

(2) Falling barometric pressure results in increased wind velocity and subsequently in greater wave height. The resulting rapid longshore currents are deflected by the sinuous shoreline and rip currents are formed. These rip currents excavate channels in the bars and new sand bars are formed as sediment accumulates in the relatively low-energy areas between rip channels. As a result there is apparent migration of the bar form.

(3) When wind, waves, and longshore currents decline, conditions return to those described in (1) above, but with the bar displaced alongshore with respect to its original position.

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