Longshore Sand Transport Calculated by Time-Dependent Shear Stress

Based on longshore sand transport experiments performed in a large basin, measured sand transport rates obtained for spilling and plunging waves are compared to the Bodge and Dean and Watanabe distributed load formulas, representative of typical formulas applied in engineering practice. Neither formula estimates the measurements satisfactorily. The Bodge and Dean formula is sensitive to changes in energy flux and does not include threshold shear. The Watanabe formula includes critical shear, but transport estimates are made from time-averaged values of bottom shear stress, which did not exceed critical shear stress at most cross-shore locations. A new transport formula is introduced based on time-dependent shear stress calculated from the total velocity that includes the wave orbital velocity. The new formula gives reasonable estimates for both spilling and plunging breaker types. A conclusion is that it is essential to represent the time-dependent, or fluctuating, component of fluid motion in predictive equations of the longshore sand transport rate.