Title

Bed elevation changes in the upper-swash zone

Document Type

Conference Paper

Publication Date

2009

Publication Title

Journal of Coastal Research

Issue

SPEC. ISSUE 56

First Page

64

Last Page

68

Keywords

Bed elevation change, Swash zone, Ultrasonic distance sensor

Abstract

Changes in bed elevation within the upper-swash zone are measured using ultrasonic distance sensors that can measure swash depths and changes in bed elevation at a resolution of ∼0.1 mm. Changes in bed elevation are examined relative to the depth-averaged Eularian flow velocity estimated using a cross-shore array of pressure transducers and distance sensors in the foreshore. Bed elevation change over the observed tidal cycle ranged from a net loss of 0.01 m mid-foreshore to a net gain of 0.18 m at the uppermost station in the swash zone. No statistically significant relationship is observed between the change in bed elevation and the higher order moments of the velocity as would be expected from an energetics model for either bedload or suspended load. Accretion occurs through thin (grain-scale) sediment drapes deposited following the relatively rapid uprush of swash bores formed through breaking across the fronting nearshore terrace at low tide. Erosion results from deeper swash with a strong backwash velocity that becomes increasinlgy common through high tide as wave breaking becomes increasingly focused on the lower to mid-foreshore. The balance of erosion and deposition is shown to to vary in response to water level fluctuatinos at tidal and sub-tidal scales.

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