Incipient motion of coarse particles under regular shoaling waves

Abstract

Incipient motion of coarse particles under regular shoaling waves is studied. Experiments are performed to investigate the effects of bed fluid acceleration on coarse particle stability. By varying wave height, wave period and water depth combinations of similar peak orbital velocities and weak to strong intra-wave accelerations were created. The particles used in these experiments have two different sizes both of a cm order-of-magnitude. The data confirm that acceleration plays a role for the initiation of motion, since combinations of similar orbital velocity and varying acceleration magnitude resulted in no motion, some motion and motion as acceleration increased. Qualitatively we found that initiation of motion occurs at or is very close to the maximum shear stress due to the combined effects of drag/lift and acceleration as introduced by Nielsen and Callaghan (2003). However, quantitatively their formulation does not lead to convincing discrimination between motion and no motion. We expect this to be due to the assumption that the coefficients for drag/lift and acceleration in their formulation are taken equal. From literature and from plotting our data against the Keulegan-Carpenter number we expect that the coefficients strongly vary caused by flow separation effects.To arrive at a more convincing discrimination between motion and no-motion we introduced a new fluid acceleration descriptor for nonlinear shoaling waves. The combination of this descriptor with a Reynolds number Re g clearly delineates the regions with particle motion and without particle motion and has the potential to serve as a descriptor of the incipient motion of coarse particles under nonlinear regular waves.

Performance of submerged nearshore sand-filled geosystems for coastal protection

“…wave characteristics, and local water depth), and the interplay between both. The complex and stochastic nature of incipient motion led researchers to investigate the influence of other features such as turbulence (Zanke, 2003); density (Nielsen, 2009), sorting (Mitchener and Torfs, 1996), and packing (Komar, 1998); bed slope angle (Dey, 2003); and wave shape (Terrile et al, 2006).…”

A numerical–empirical approach for evaluating morphodynamic processes on gravel and mixed sand–gravel beaches

“…However, Hughes and Baldock (2004) have suggested that the appearance of strong onshore accelerations in some field data may be an artefact, related to the difficulty of measuring velocities accurately in very shallow water. Moreover, results found in a recent investigation by Terrile et al (2006) show that fluid acceleration is indeed important for the initiation of sediment motion. Nevertheless, further studies are necessary in order to identify and describe the effects of acceleration-dependent transport.…”

Nearshore dynamics of artificial sand and oil agglomerates

“…While these types of formulations have been found to be generally accurate in the surf zone (Deigaard et al, 1991;Soulsby et al, 1993), recent research suggests that incipient motion under some conditions may be driven by pressure gradients or a combination of pressure gradients and shear stress forcing (Sleath, 1999;Foster et al, 2006;Terrile et al, 2006;Nichols and Foster, 2009;Frank et al, 2014Frank et al, , 2015. In particular, pressure gradient forcing can become important under large accelerations, such as occur under moderately energetic waves with shorter periods (T < 5 s; Frank et al, 2015).…”