Sand suspension and fluxes by wave groups and equivalent monochromatic waves

Abstract

Sand transport dynamics under non-breaking regular waves and wave groups are studied in a large-scale wave flume. The water surface elevation, flow velocity, suspended sand concentration (SSC) distributions, and bedforms were measured over a horizontal test section of medium sand. The experiment involved five wave groups that had equal root-mean-square water surface elevation and mean period, but which differed in terms of modulation, wave group length, and short wave sequencing. In addition, three monochromatic wave conditions were generated which were equivalent to the wave groups in terms of root-mean-square water surface elevation or maximum wave height. Temporal and vertical distributions of SSC suggest a more effective sediment pumping for symmetric and backward leaning 'waxing' wave groups.Vertical distributions of the SSC phase lag indicate a more rapid vertical pumping for the shortduration wave group. Overall, the differences between wave group conditions are small and the time-averaged SSC and sediment diffusivity profiles are very similar. The total net suspended sand flux consists of current-related, long-wave-related and short-wave-related contributions that are all of similar magnitude. The net total suspended sand flux was always offshoredirected, but its magnitude depends on the vertical and temporal variation of SSC at short-and long-wave time scales. The results are used to discuss the applicability of the 'equivalent wave concept' for time-averaged SSC distributions and net suspended sand transport.

Near‐Bed Sediment Transport Processes During Onshore Bar Migration in Large‐Scale Experiments: Comparison With Offshore Bar Migration

“…This might explain why test 51, being far offshore from the bar crest and under less energetic forcing (A1), still features a relatively wide layer of large time‐averaged concentration near the bed (Figure 5d, ζ ′ ≤ 0.035 m). Other experiments in oscillating water tunnels (Davies & Thorne, 2005) and large‐scale wave flumes (van der Zanden et al., 2019b) similarly indicate wide layers of large time‐averaged concentration over rippled beds. Furthermore, the influence of convective mixing processes might explain the different concentration profile shape (Figure 5) when comparing this test to the others.…”

“…(2012), and van der Zanden et al. (2019b)). This indicates the importance of suspension or “storage” of sediment in the water column (e.g., Kobayashi, 2001) that might be advected.…”

“…Under the first short waves of each group (e.g., 𝐴𝐴 𝐴𝐴∕𝑇𝑇𝑟𝑟 = {0.06 ; 0.11; 0.16; 0.41; 0.46; 0.51; 0.76; 0.81; 𝐴𝐴 0.86} ) sediments largely settle down before suspension by subsequent waves. In the second halves of the groups (e.g., 𝐴𝐴 𝐴𝐴∕𝑇𝑇𝑟𝑟 = {0.25 − 0.35 ; 0.55 − 0.7; 𝐴𝐴 0.9 − 0.05} ), however, suspended sediments seem to not settle down fully before re-suspension by subsequent short waves (see also Villard et al (2000), O'Hara Murray et al (2012), andvan der Zanden et al (2019b)). This indicates the importance of suspension or "storage" of sediment in the water column (e.g., Kobayashi, 2001) that might be advected.…”

“…Additionally, wave breaking has been observed to inject turbulence into the water column (e.g., Cox & Kobayashi, 2000, Sumer et al., 2013; Zhou et al., 2017), potentially influencing diffusive processes via its effects on turbulent viscosity (e.g., Deigaard et al., 1986). Furthermore, sediment pumping (Villard et al., 2000), potentially through the build‐up of turbulence over consecutive short waves in a group (e.g., Holmedal et al., 2004; Kassem et al., 2015), has been observed as a process of sediment suspension under wave groups (e.g., van der Zanden et al., 2019b).…”

Near‐Bed Sediment Transport During Offshore Bar Migration in Large‐Scale Experiments

“…This indicates a larger impact of sediment advection and sediment diffusivity. These may be of special importance between the wave groups (t/ T r = 0.3-0.45 and 0.85-0.95), where suspended concentrations remain large at high vertical elevations (suspension) during consecutive waves (see also Villard et al, 2000;O'Hara Murray et al, 2012;van der Zanden, van der A, Thorne, et al, 2019). The increased suspension might be partly induced by larger velocity magnitudes and, most likely, by larger turbulent kinetic energy (van der Zanden, van der A, Larsen et al, 2020).…”