Laboratory observations of inner surf and swash-zone hydrodynamics on a steep slope

Abstract

Laboratory data of free surface elevation and fluid velocities (cross-shore and vertical velocities) were obtained using a laser-Doppler velocimeter (LDV) for the case of a periodic wave plunging over an impermeable, steep (1:10) slope with a fixed bottom roughness (nominal diameter=2.2 mm). The measurements were conducted over 14 cross-shore locations from the outer surf zone to the swash zone with approximately 10 vertical points at each location. The vertical resolution included measurements in the bottom boundary layer and some points above the trough level. Various hydrodynamic quantities, including ensemble-averaged turbulent kinetic energy (TKE) (〈 k〉) and turbulence intensity (〈 u′ 2〉 and 〈 w′ 2〉), were estimated to better understand inner surf and swash zone hydrodynamics induced by strongly plunging waves. In the outer region of the bottom boundary layer, the ratio of the time-averaged horizontal turbulence intensity ( u ′ 2 ¯ ) to the time-averaged vertical turbulence intensity ( w ′ 2 ¯ ) agreed well with the case of plane wake turbulence ( u ′ 2 ¯ / w ′ 2 ¯ = 1.31 ) . The average ratio near the bottom was larger ( u ′ 2 ¯ / w ′ 2 ¯ = 5.91 ) which is closer to flows typified by the inner region of a boundary layer ( u ′ 2 ¯ / w ′ 2 ¯ = 6.20 ) . In the surf zone, the ensemble-averaged horizontal fluid velocity (〈 u〉) near the bottom leads in phase compared to 〈 u〉 in the upper layer, and 〈 u〉 at the bore front exceeds the theoretical wave celerity. At the impinge point, a strong return flow occurred ( u min = - 50.98 cm / s ) which was greater in magnitude than the onshore directed velocity ( u max = 40.09 cm / s ) . In the surf zone, 〈 k〉 was largest just below trough level with a forward shift in phase of the peak intensity. The turbulence energy was mostly dissipated after the passage of the crest at this location. 〈 u〉 near the bottom was leading in phase with a sharp vertical gradient in 〈 u〉 indicating that boundary layer processes may have been important. In the swash zone, the vertical gradient of 〈 u〉 was relatively small compared to the vertical gradient of 〈 u〉 in the surf zone and may be due to the effect that the strong downrush had on turbulent mixing. Finally, the time-averaged estimate of TKE ( 〈 k 〉 ¯ ) was vertically uniform over the inner surf and swash zone.