The Impact of Coarse‐Grain Protrusion on Near‐Bed Hydrodynamics

Abstract

In steep rivers, sediment is often transported over immobile cobbles and boulders. Previous studies of such conditions have observed that the entrainment rate of the mobile sediment strongly depends on the level of protrusion of the immobile grains. Here experiments are conducted in a laboratory flume in order to quantify how different levels of protrusion of large aggregates above a fixed fine-sediment bed, modeled as a patch of hemispheres, modify the local hydrodynamics near the fine-sediment bed. Five protrusion levels defined by P = k∕R = {0%, 20%, 40%, 60%, 80%} were investigated, where k is the protrusion height and R the radius. For small protrusion (P = 20%), enhanced shear stress and turbulence intensity on the fine-sediment bed is observed as the mixing layer generated at the hemisphere top impacts the fine-sediment bed. Moreover, sweep events generated near the top of the hemispheres reach the fine-sediment bed. For large protrusions (P ≥ 60%), the mixing layer generated near the top of the hemispheres does not reach the fine grains with the consequence that the shear stress drops. The remaining turbulence near the fine-sediment bed, although enhanced by the wakes generated by the hemispherical caps, is quasi-isotropic. The transition between these two distinct near-bed flow regimes is found to be around P = 40%, corresponding to the protrusion levels observed by Grams and Wilcock (2014, https://doi.org/10.1002/2013JF002925) above which the erosion of fine sediment ceases.

Fine‐Sediment Erosion and Sediment‐Ribbon Morphodynamics in Coarse‐Grained Immobile Beds

“…As shown by Raus et al. (2019) for a pattern of hemispheres in an open‐channel flow or by Liu et al. (2019) and Ferreira et al.…”

“…Furthermore, for a square pattern of spherical caps over an immobile flat rough bed, Raus et al. (2019) observed the development of a shear layer at the top of the spheres for P = 0.2, which reaches the underlying bed causing locally enhanced turbulence. The increase in erosion rate observed for P c 1 < P < P c 2 can be connected to an increase in shear‐layer intensity, which Raus et al.…”

“…By defining the angle of attack for a protruding spherical cap on the basis of the ratio between the protruding height and the planar radius measured at half height, as done in the case of hills (Ferreira et al, 1995), the critical protrusion level P c1 corresponds to an angle of attack of approximately 25°-30°, for which the development of horseshoe vortices was observed for cylinders, cones and spherical caps investigated for varying inclinations, side-slopes and protrusion levels, respectively (Euler et al, 2014;Okamoto et al, 1977;Raus, 2019). Furthermore, for a square pattern of spherical caps over an immobile flat rough bed, Raus et al (2019) observed the development of a shear layer at the top of the spheres for P = 0.2, which reaches the underlying bed causing locally enhanced turbulence. The increase in erosion rate observed for P c1 < P < P c2 can be connected to an increase in shear-layer intensity, which Raus et al (2019) showed to increase with P. It is further expected that the intensity of the horseshoe vortex increases as the induced scour hole deepens, as observed in scouring processes around vertical cylinders (Dey & Raikar, 2007;Muzzammil & Gangadhariah, 2003), contributing to the erosion-rate enhancement.…”

“…Overall, the effect of coarse immobile roughness elements on the underlying fine‐sediment surface has been characterized mostly from a hydrodynamic point of view; the immobile roughness elements increase the drag on the flow causing increased bed resistance (Rickenmann & Recking, 2011) and a reduction of the shear stress acting on the underlying surface (Le Bouteiller & Venditti, 2015; Marshall, 1971; Raupach, 1992; Yager et al., 2007). At the same time, the coherent structures shed by the immobile roughness elements, such as horseshoe vortices developing in their stoss side and arc‐shaped shear layers encompassing a recirculation region in their lee, induce highly heterogeneous shear stress conditions (Fang et al., 2017; Papanicolaou et al., 2012; Pattenden et al., 2005; Raus et al., 2019). These create localized scour regions and sediment shadows in the stoss and lee side of the immobile roughness elements (Dixen et al., 2013; Euler & Herget, 2012; McKenna Neuman et al., 2013).…”

Emergent Hydrodynamics and Skimming Flow Over Mussel Covered Beds in Rivers

“…This change in flow regime as a response to mussel density suggests an emergent phenomenon or a hydrodynamic transition that results in skimming flow. Numerous studies have also demonstrated such transitions between flow regimes and regions due to the presence of submerged aquatic vegetation (Ghisalberti, 2009; Nepf & Ghisalberti, 2008) or relatively coarse, immobile sediment (Raus et al, 2019). Moreover, a nondimensional measure of roughness density λ = 0.1 and an inflection point in the vertical velocity profile at this roughness density can be used to identify skimming flow and to differentiate between sparse and dense canopy limits (Nepf, 2012).…”