Hydrodynamics and sediment transport under a dam-break-driven swash: An experimental study

Abstract

An experimental study is presented consisting of swash zone hydrodynamics, sediment transport flux estimates, and bed changes under a single dam-break-driven swash event over a steep (1:7), movable sand bed. The study provides resolved measurements of instantaneous water depths, cross-shore flow velocities, sheet flow and suspended sediment concentrations, and morphological response. An ensemble-averaging over 24 repetitions of the swash event allows convergence in the time-averaged measured swash quantities. The sand bed profile was re-constructed after each run with a re-smoothing procedure that allowed for minimum variations that are within the profiler system accuracy (<0.006 m). High-resolution measurements are employed to reconstruct the spatial-temporal evolution of the flow velocity field. Temporal extrapolation and spatial logarithmic/exponential fits are employed to fill velocity gaps in the measured data for the estimation of instantaneous and depth-integrated sediment flux. The spatial gradients of instantaneous sediment flux indicate that the sheet flow sediment dominates over the suspended load during the most relevant phases for sediment transport. Upon bore arrival and initial uprush, the relative contribution of the sheet flow load is larger than during the backwash phase and is decreasingly dominant onshore. Depth-integrated instantaneous sediment flux and total sediment transport estimates, based on two different approaches, are compared with equivalent total sediment transport estimates obtained from the observed morphological changes and the sediment continuity equation. The close agreement between the various approaches indicates that existing models and empirical estimates for the sheet load concentration profile can be used as first order estimates for sediment transport in the swash zone.