Laboratory Observation of Turbulence and Wave Shear Stresses Under Large Scale Breaking Waves Over a Mild Slope

Abstract

Laboratory measurements of turbulent flow and shear stresses in a surf zone under monochromatic breaking waves in a large‐scale wave flume are presented. Waves with various surf similarity parameters were generated to break at nearly identical water depths over a 1/100 mild slope. Free surface elevations were measured along the plane beach. Velocities in a water column located at the breaking region were measured by acoustic Doppler velocimeters. Results from wave shoaling coefficient, breaking index, and images are inconsistent with the breaker type categorizations from the surf similarity parameter. This suggests that a modification of the surf similarity parameter for very mild slopes may be necessary. For the plunging breakers, the ensemble‐averaged results indicate that intense turbulence kinetic energy was initially generated by large eddies at the surface then transported to the bottom frequently. The instantaneous results demonstrate that the intermittency of turbulence kinetic energy near the bed was caused by large eddies generated by wave overturning and relatively smaller obliquely descending eddies. Moreover, the magnitude of the wave shear stress (WSS) is one order of magnitude larger than that of the turbulent shear stress, indicating that the effects of the bottom slope, wave breaking, and bed friction are significant in the surf zone. The sign of the time‐averaged WSS switched between positive and negative, depending on the occurrences of phase lag and phase lead of the vertical coherent velocity. It suggests that a change may be required in Zou et al.'s (2006, https://doi.org/10.1029/2005JC003300) theory on the formulation of wave‐breaking induced WSS.

Surf Zone Turbulence and Suspended Sediment Dynamics—A Review

“…Wave-averaged <k > was similar at the two beaches (for similar γ) and scaled with the relative wave height, but maximum k during individual wave cycles (k max ) were larger for plunging waves than for spilling (Figure 6b), which indicates that instantaneous levels of k are larger, but of shorter duration for plunging waves. This is consistent with Hsu et al [73] who noted that for plunging breakers, instantaneous k(t) is typically many times larger than mean and phase-averaged <k>, while k may be up to a factor 5 larger for plunging breakers compared to shoaling waves [63].…”

“…Wave-averaged <k'> was similar at the two beaches (for similar γ) and scaled with the relative wave height, but maximum k' during individual wave cycles (k'max) were larger for plunging waves than for spilling (Figure 6b), which indicates that instantaneous levels of k are larger, but of shorter duration for plunging waves. This is consistent with Hsu et al [73] who noted that for plunging breakers, instantaneous k(t) is typically many times larger than mean and A different perspective is offered in Figure 6a. Here, data from roughly 60,000 waves were extracted from about 500 irregular-wave time series using a zero-downcrossing algorithm; the crest-to-trough height (H z ), mean water depth (h z ), and k were calculated for each wave and turbulence data were then binned within γ z -ranges.…”

“…Turbulent kinetic energy may be non-dimensionalized using Froude-scaling, 𝑘 = 𝑘/𝑔ℎ which makes intuitive sense because the rate of turbulence production should scale with wave celerity [59]. For irregular waves in field and large-scale laboratory settings, <k'> ≈ 0.02-0.06 [42,72,73] which is typically smaller than for regular laboratory waves of similar root-mean-square height [25,56]. The difference arises because each regular wave injects turbulence while only some waves are breaking in an irregular wave field.…”

“…For nonbreaking wave conditions (γs = Hs/h < 0.3), <k'> incre towards the bed due to the roughness created by small (mainly anorbital) wave ripp fine-grained sand and <k'> was similar at the top and the bottom of the water col Weakly breaking waves (0.3 < Hs/h < 0.5) exhibited a mixture of the two shapes with being largest at the highest measurement elevation and likely even larger near th surface. In the horizontal dimension, <k> tends to be maximum at the transition from the outer to the inner surf zone [73], i.e., at the transition from breaking waves to surf bores. This was illustrated by measurements with regular plunging breakers in a wave flume over a sandy bed [63].…”

“…In the horizontal dimension, <k> tends to be maximum at the transition from the outer to the inner surf zone [73], i.e., at the transition from breaking waves to surf bores. This was illustrated by measurements with regular plunging breakers in a wave flume over a sandy bed [63].…”