Divergent behavior of the swash zone in response to different foreshore slopes and nearshore states

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Marine Geology





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bed elevation change, sediment advection, suspension potential, swash zone

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Ultrasonic distance sensors were used to measure changes in bed elevation to a resolution of ∼ 0.1 mm following individual swash excursions in the high-tide swash zone of an intermediate foreshore. Changes in bed elevation are related to depth-averaged Eularian swash velocities, estimated by assuming mass balance of the swash discharge also measured using the ultrasonic distance sensors. Early in the study (May 21-25, 2008), the alongshore uniform foreshore had a cross-shore slope of 0.18 and was fronted by a nearshore bar that forced wave breaking at low tide. Towards the end of the study (June 5-9, 2008), the foreshore had a slope of 0.12 and was fronted by a nearshore terrace that developed as the innermost bar migrated landward. Despite similar inshore wave heights and periods during these times, the steeper foreshore eroded by 0.69 m3 m- 1, while the more gently-sloped foreshore accreted by 0.21 m3 m- 1. The divergent behavior of the swash zone is attributed to the welding of the innermost bar to the foreshore creating a shallow nearshore terrace that saturated the inshore wave field. This in turn led to a faster uprush velocity that promoted the landward advection of sediment picked-up over the shallow terrace. It is argued that accretion in the upper swash zone was the result of a larger supply of sediment at the base of the foreshore, and a transformation of the wave field by the shallower depths of the welded feature. In this respect, swash hydrodynamics and morphological change can only be made if the swash zone is considered within the context of the broader nearshore state. © 2010 Elsevier B.V.