Surf zone hydrodynamics and sediment transport

Ole Madsen

Professor Ole Secher Madsen and Graduate student Yoshimitsu Tajima in the Department of Civil and Environmental Engineering are working on a three-year project aimed at the development of a predictive model for the longshore sediment transport rate within the surf zone along long straight beaches. The project, currently in its second year, is supported by the US Army Corps of Engineers' Coastal and Hydraulics Laboratory which also supplies experimental data obtained in their Longshore Sediment Transport Facility.

The hydrodynamics model predicts breaking wave characteristics, near-bottom velocities, and the driving forces for long-shore and cross-shore currents. Combined wave-current bottom boundary layer theory is employed with a predicted movable bed roughness to determine the bottom shear stress and the vertical structure of the wave-induced currents.

The sediment transport model consists of a bed load component, which accounts for bottom slope, wave nonlinearity and wave-induced current effects; and a suspended sediment component, which is based on a reference concentration derived from the bed load and yields predictions of the concentration and transport of suspended sediments in the water column.

Agreement of model predictions with experimental observations, as demonstrated in the accompanying Figure, is encouraging, in particular when it is recognized that all model coefficients are predetermined from experimental data obtained elsewhere rather than by fitting the data against which predictions are being compared.

Figure 1

Reference
Tajima, Y., O.S. Madsen, Shoaling, breaking and broken wave characteristics, Proc. 28th International Conference on Coastal Engineering, in press.

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