Fluid acceleration effects on suspended sediment transport in the swash zone

Abstract

Suspended sediment concentrations and fluid velocities measured in the swash zone of a high‐energy steep beach were used to investigate the importance of fluid accelerations to suspended sediment transport. Swash flow acceleration was nearly constant at about one‐half downslope gravitational acceleration with two important exceptions. We observed strong, short‐lived periods of accelerating uprush at the beginning of the swash cycle and decelerating backwash at the end of the swash cycle (magnitudes of both approximately twice that of the expected downslope gravitational acceleration). Interestingly, spikes in suspended load followed the anomalies in acceleration in a way that was not apparent from the nearly symmetric (in magnitude) ensemble averaged velocity time series. Suspended load values were largest during accelerating uprush associated with the shoreward propagating turbulent bore or swash front. During backwash, suspended loads were generally not as large. Correspondingly, suspended sediment transport rates obtained from the sediment concentration and velocity measurements showed best comparisons with a modified sediment transport model that includes a physical mechanism for enhancing transport rates due to flow acceleration. The modified sediment transport model reduced the overall root‐mean square prediction error by up to 35% and shifted the predicted peak in uprush sediment transport rate earlier in the swash cycle, resulting in a better fit to the observations. These findings suggest that the inclusion of the acceleration term may account for physical mechanisms that include bore turbulence and horizontal pressure gradients typically associated with the accelerating portion of uprush.