The goals of these ongoing studies are to understand better the physics driving sediment transport and morphological change on the beach face (foreshore). Studies utilize field instrumentation, lab techniques, remote sensing and numerical modelling.
In an early study, the temporal and spatial characteristics of swash zone sediment suspension were investigated. The field site was located at Gleneden Beach, Oregon. The beach face is intermediate to steep (slope = 1:12 to 1:10). Gleneden is an energetic beach with wave heights during the experiment approaching 2 meters. During the experiment, three cross-shore arrays of instrumentation were installed (FIGURE 1). The arrays contained a pressure sensor, two impellor current meters and a 19-channel fiber optic backscatter sensor (FOBS) to measure the suspended sediment concentration.
FIGURE 1. Instrument layout during field experiment at Gleneden Beach, Oregon.
Spatial and temporal characteristics were also observed by looking at all three cross-shore locations simultaneously (FIGURE 2). High suspended sediment concentrations (SSC) are observed in the leading edge of the swash motion which can be traced (for example) starting at roughly 0 time at the seaward location, continuing to the middle, and then to the landward locations. As the swash velocities decrease the amount of suspended sediment is drastically reduced, especially in the middle and seaward locations. During backwash, the amount of suspended sediment increases but is confined to the vicinity of the bed, where strong vertical gradients in suspended sediment exist. High suspended sediment concentrations are observed near t = 50 s at the middle location during the backwash of a long period (» 30 second) event. This points to the potential for infragravity motions to drive higher suspended sediment concentrations in the swash zone.
FIGURE 2. Temporal and spatial variability in suspended sediment concnetration.
Many previous works have tried to describe swash zone sediment transport using an energetics-type sediment transport model. This study addressed those formulations and found the correlations to be moderate to poor. It was hypothesized that the turbulence contained in the bore and shoreward propagating swash, not contained in the simple energetics formulation, likely plays a dominant role in sediment suspension and transport, atleast during the uprush phase. In response to this, a fomrulation relating the energy dissipation across a bore, analogous to a hydraulic jump in a reference frame moving with the bore. A much higher correlation was found (FIGURE 3). Unfortunatley the newer formulation is only applicable in the direct vicinity of the bore and cannot be used at other locations within the swash zone.
FIGURE 3. Correlation between suspended sediment transport and bore dissipation (a) high side of bore, (b) average across bore.
It turns out that during the time bore collapse a short-lived fluid acceleration may occur. This acceleration may serve as a rough proxy for the bore turbulence and allow for an additional term to be added to the energetics sediment transport formulation. FIGURE 4. shows a phase plane diagram of the suspended load (SSC integrated over depth) as a function of acceleration. The upper panel shows a time-normalized swash event and the circular arrow indicates the direction of swash flow in the lower panel. The lower left panel is where accelerating uprush is expected. It is clear that the largest suspended loads occur during this time.
FIGURE 4. SSC as a function of acceleration for normalized swash events.
FIGURE 5. shows the effect of adding an acceleration term to the energetics model. While for the normalized ensemble-averaged swash, both models appear to perform well, the correlation is slightly improved when the additional term is used. The use of acceleration also shifts the peak in uprush sediment transport to occur sooner in agreement with observations.
FIGURE 5. One-to-one correlations for ensemble-averaged swash events for the energetics model (A) and the energetics model with an acceleration extension (B).
For more infomration on this work, see Puleo et al. Journal of Geophysical Research, V105, 17021-17044 (2000) and Puleo et al. Journal of Geophysical Research, V108, 14-1 - 14-12 (2003).
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