Runup, inundation, and flow velocity of non-decaying dam-break bores on coarse-grained slopes

Abstract

This study investigates the swash flows generated by non-decaying dam-break bores traveling over permeable coarse-grained and impermeable beaches. Experiments were conducted to measure runup height, inundation depth, flow velocity, and variations in groundwater. The experiments covered a wide range of bore strengths, which is beneficial for systematically examining bore hydrodynamics, including their effects on runup and inundation. Then, comparisons were extended to the measured results on permeable and impermeable slopes. In this context, empirical formulations, originally developed for smooth and rough impermeable slopes, were refined with reduction factors derived from measurements on coarse-grained beaches, thus allowing for quantifying beach permeability effects. The reduction factors derived from the measured runup heights and inundation depths demonstrated a dependency on the bore types. Velocity fields within the surf and swash zones were captured using time-resolved particle image velocimetry, from which bed shear stresses were subsequently calculated. Four distinct stages of flow behavior were identified: uprush, first downrush, stagnant phase, and second downrush. During the flow reversal phase, it was observed that velocities near the bed reversed direction sooner than those nearer to the free surface, a phenomenon attributed to the boundary layer effect. In permeable slope cases, exfiltration processes became significant during the latter two stages of swash motion. The bed shear stresses, estimated using two distinct formulations, generally exhibited comparable trends. The mechanisms governing groundwater flow also differed between the two permeable beach types: on the gravel slope, bore-induced infiltration advances laterally and rapidly merges with the groundwater level; on the coarse sand beach, infiltration was primarily driven by gravitational drainage during the uprush phase.