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1 November 2007 Suspended Sediment in the Swash Zone: Heuristic Analysis of Spatial and Temporal Variations in Concentration
Michael G. Hughes, Troels Aagaard, Tom E. Baldock
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Swash zone sediment transport is generally measured in an Eulerian reference frame, but this approach tends to give insufficient information on spatial transport characteristics. This paper utilizes a novel technique to map measured suspended sediment concentrations (SSC) onto the x–t plane, thus providing a visualization of SSC at all locations throughout a swash cycle. The widely adopted energetics or bed shear stress approaches to modeling suspended sediment transport in the swash predict that SSC varies directly with the horizontal velocity, u, to the power of either 2 or 3. A calibrated ballistic swash model was used to predict SSC(x,t) ∝ u2 and SSC(x,t) ∝ u3. The qualitative comparison between field measurements and transport model predictions in x–t space provides the opportunity to examine the transport model performance throughout the swash zone. The agreement between observed and modeled SSC patterns was generally poor. Similar to previous observations, SSC was larger during the uprush than the backwash. During the uprush, however, the models predict a maximum in SSC at the moving shoreline and a rapid drop-off behind the leading swash edge, whereas observed SSC was maintained at high levels for most of the uprush phase. During backwash, there was very little correspondence at all between the patterns of predicted and observed SSC; in particular, observed SSC was unexpectedly small in the mid- and late stages of the backwash. A general consensus is emerging that existing swash zone sediment transport models that assume SSC is in equilibrium with the horizontal flow velocity are inadequate. This study suggests that this is probably due to presuspended sediment being advected into the swash zone, the effects of bore-generated turbulence being advected into the swash zone, and both bed shear stress and Reynolds stresses being out of phase with the horizontal flow velocity.

Michael G. Hughes, Troels Aagaard, and Tom E. Baldock "Suspended Sediment in the Swash Zone: Heuristic Analysis of Spatial and Temporal Variations in Concentration," Journal of Coastal Research 2007(236), 1345-1354, (1 November 2007).
Received: 6 June 2005; Accepted: 1 October 2005; Published: 1 November 2007

Beach morphodynamics
numerical models
sediment transport
suspended sediment
wave runup
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