Experimental study of bore-driven swash–swash interactions on an impermeable rough slope

Self Cite

The aim of the present investigation is to clarify the role and evaluate the importance of air entrainment in the swash zone by carrying out a set of detailed laboratory experiments. Experiments involved generating a single, highly repeatable, large-scale, bore-driven swash event on a sand-rough impermeable beach with slope 1:10. Measurements that yield the characteristics of the entrained air, including the void fractions, bubble size and bed-parallel bubble velocity, and the hydrodynamics are obtained at five crossshore locations in the swash zone using an optical probe and a combined PIV/LIF system. The results show that two distinct bubble clouds enter the swash zone, the first is the result of local entrainment at the wave tip and the second are the remaining bubbles of the air that is entrained at the second plunge point of wave breaking before the wave starts climbing the beach. The void fractions are up to 0.20 and bubble size up to 20mm which are similar to air entrainment after wave breaking in the surf zone and deep water. Local entrainment of air continues in the swash zone, but void fractions decrease rapidly with distance up the slope and no air is present after flow reversal. Energy dissipation in the swash zone attributable to entrained air is at least of order 1% of the total energy dissipation. This is smaller than in the surf zone or deep water because of the smaller volume of entrained air and the greater total energy dissipation in the swash zone.

Section: Discussionmentioning

confidence: 99%

“…Cowen et al, 2003, Barnes et al, 2009, Pujara and Liu, 2014, Jiang and Baldock, 2015, Ruju et al, 2016 and swash-swash interactions (e.g. Lo et al, 2013, Pujara et al, 2015, Chen et al, 2016.…”

Section: Introductionmentioning

confidence: 99%

Entrained air in bore-driven swash on an impermeable rough slope

Dai

Kikkert

Chen

et al. 2017

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“…A swash bore overtaking a preceding bore during the uprush is typically termed "wave capture" while an incident bore arriving during a preceding backwash leads to a "wave-backwash interaction" (Hughes and Moseley, 2007;Cáceres and Alsina, 2012). Wave-backwash interactions can be classified as "weak", when the incident wave has higher momentum than the backwash flow and continues to propagate towards the beach, or "strong", when the incident wave and backwash flow have similar momentum and the incident wave is halted or washed seaward (Hughes and Moseley, 2007;Cáceres and Alsina, 2012;Chen et al, 2016). Detailed observations and numerical simulations show that such interactions lead to strong velocity shearing, flow separation and vortex formation (Sou and Yeh, 2011;Pujara et al, 2015b;Chen et al, 2016;Higuera et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

Sand transport processes and bed level changes induced by two alternating laboratory swash events

Zanden

Caceres

Eichentopf

et al. 2019

Sand transport processes and net transport rates are studied in a largescale laboratory swash zone. Bichromatic waves with a phase modulation were generated, producing two continuously alternating swash events that have similar offshore wave statistics but which differ in terms of wave-swash interactions. Measured sand suspension and sheet flow dynamics show strong temporal and spatial variability, related to variations in flow velocity and locations of wave capture and wave-backwash interactions. Suspended and sheet flow layer transport rates in the lower swash zone are generally of same magnitude, but sheet flow exceeds the suspended load transport by up to a factor four during the early uprush. The bed level near the inner surf zone is relatively steady during a swash cycle, but changes of O(cm/s) are measured near the mid swash zone where wave-swash interactions lead

“…Waveswash interactions (Caceres and Alsina, 2012;Hughes and Moseley, 2007) (sometimes termed swash-swash interactions) occur when waves arrive at the shoreline during a preceding uprush, hence adding momentum ("wavecapture") or during the preceding backwash, inducing opposing momentum (termed "wave-backwash interaction"). The type and degree of wave-swash interaction is highly important for swash zone hydrodynamics, as the momentum of the incident wave affects the shoreline motion (Erikson et al, 2005) and swash velocities (Chen et al, 2016;Pujara et al, 2015), but also for swash zone sediment transport where certain wave-swash interactions events have been shown to promote large sediment re-suspension events and offshore sediment transport (Alsina et al, 2012;Blenkinsopp et al, 2011;Caceres and Alsina, 2012).…”

Section: Introductionmentioning

confidence: 99%

The influence of wave groups and wave-swash interactions on sediment transport and bed evolution in the swash zone

Alsina

Zanden

Caceres

et al. 2018

Large scale laboratory measurements of sediment dynamics in the swash zone are presented. Two bichromatic wave group conditions were generated, having the same energy content but different wave group period (T g = 15.0 s and 27.7 s). For the shortest wave group, due to bore focussing, the shoreline fluctuates predominantly at the T g time scale, showing a large runup and the presence of wave-swash interactions with strong momentum exchange. In contrast, for the longer wave groups, the swash excursion is dominated by the individual waves. The uprush generally promotes onshore sediment advection with consequent erosion at the rundown location but accretion close to the runup. On the contrary, the backwash promotes seaward sediment advection and accretion at the rundown location. The presence of repeated wave-swash interactions modifies these patterns slightly. A wave overrunning a previous uprush promotes a reduction in onshore sediment advection while weak wave-backwash interactions reduce seaward advection. Consequently, the measured sediment dynamics shows stronger intra-swash cross-shore sediment advection for the swash events produced by the short wave groups. Measurements of the sheet flow layer near the shoreline show that for the shortest wave groups the vertical structure of the concentration