Parameterization of intrawave ripple‐averaged sediment pickup above steep ripples

Abstract

Near‐bed sediment pickup is critical for predictions of intrawave suspension and in turn net sediment transport in coastal models. In the present study, numerical results from a two‐dimensional Reynolds‐averaged Navier‐Stokes model are used to assess the functional relationship of intrawave ripple‐averaged sediment pickup above steep ripples. The numerical model provides intrawave time histories of ripple‐averaged near‐bed velocities and turbulence, which are qualitatively interrogated to determine pickup functional relationships. Several specific sediment pickup formulations are implemented within the numerical model: expressions relating pickup to near‐bed velocity or near‐bed turbulent kinetic energy via the bed shear stress; and expressions relating pickup to near‐bed shear production of turbulent kinetic energy. These are then tested via model‐data comparisons of near‐bed suspended sediment concentration. The results show that the traditional functions relating sediment pickup to near‐bed velocity cannot lead to reasonable intrawave suspension predictions above vortex ripples under a ripple‐averaged framework. Instead, relating sediment pickup to near‐bed turbulence quantities, such as turbulent kinetic energy or shear production of turbulent kinetic energy, significantly improves the numerical predictions for these conditions.

Sand suspension and fluxes by wave groups and equivalent monochromatic waves

“…These differences complicate the interpretation of the measurements in the present experiment, especially the phase behaviour of SSC at intra-wave time scale and the net short-wave-related flux, which are both strongly affected by the degree of velocity skewness (see e.g. van der Werf et al, 2007;Amoudry et al, 2016).…”

“…Such more gentle ripples have been termed 'post-vortex ripples' (Clifton & Dingler, 1984), and 'dynamically plane' beds (Davies & Villaret, 2002); the term 'post-vortex ripples' is adopted in the present study. The vertical transfer of momentum and suspended sand grains over post-vortex ripples is due to turbulent diffusive processes rather than organised vortices (Davies & Villaret, 2002;Amoudry et al, 2016) and the sand pickup rate is lower than over a vortex rippled bed (O'Hara Murray et al, 2011;Davies & Thorne, 2016). Note that 12% of the measured irregular ripples have a steepness higher than 0.10 and could thus be classified as vortex ripples.…”

Surf Zone Turbulence and Suspended Sediment Dynamics—A Review

“…In these coherent vortices, turbulence spreads upward by convection, rather than through diffusion [50]. Experimental evidence [51] and RANS modeling [52] have shown that vortex ejection enhances near-bed k and amplifies TKE to a level of about 4-8 ripple heights, in the model example corresponding to 25 cm above the bed.…”

“…In these coherent vortices, turbulence spreads upward by convection, rather than through diffusion [50]. Experimental evidence [51] and RANS modeling [52] have shown that vortex ejection enhances near-bed k and amplifies TKE to a level of about 4-8 ripple heights, in the model example corresponding to 25 cm above the bed. Turbulent production caused by flow separation around bedforms scales with u and with bed roughness, with the latter depending on both sediment grain size and bedform steepness [50] and/or bedform height [54].…”

“…In numerical models, pick-up functions are often parameterized using proxies for c and w', such as u and quadratic stress formulations. However, Amoudry et al [52] found that these proxies did not provide reasonable results over rippled beds since pick-up maxima are predicted at times of maximum velocity. Instead, an approach involving shear production of k resulted in considerable improvement of their numerical model predictions.…”