Effects of Bed Forms and Large Protruding Grains on Near‐Bed Flow Hydraulics in Low Relative Submergence Conditions

Abstract

In mountain rivers, bed forms, large relatively immobile grains, and bed texture and topographic variability can significantly alter local and reach‐averaged flow characteristics. The low relative submergence of large immobile grains causes highly three‐dimensional flow fields that may not be represented by traditional shear stress, flow velocity, and turbulence intensity equations. To explore the influence of large protruding grains and bed forms on flow properties, we conducted a set of experiments in which we varied the relative submergence while holding the sediment transport capacity and upstream sediment supply constant. Flow and bed measurements were conducted at the beginning and end of each experiment to account for the absence or presence of bed forms, respectively. Detailed information on the flow was obtained by combining our measurements with a 3‐D numerical model. Commonly used velocity profile equations only performed well at the reach scale when shallow flow effects and the roughness length of the relatively mobile sediment were considered. However, at the local scale large deviations from these profiles were observed and simple methods to estimate the spatial distribution of near‐bed shear stresses are likely to be inaccurate. Zones of high turbulent kinetic energy occurred near the water surface and were largely controlled by the immobile grains and plunging flow. The reach‐averaged shear stress did not correlate to depth or slope, as commonly assumed, but instead was controlled by the relative boulder submergence and degree of plunging flow. For accurate flow predictions in mountain rivers, the effects of bed forms and large boulders must be considered.

Bed Surface Adjustments to Spatially Variable Flow in Low Relative Submergence Regimes

“…These local changes in elevation and GSD did not correlate with slope or relative submergence, possibly because of the limited range of flow depths in our experiments. Relative submergence based on the flow depth upstream of the hemispheres (an alternative definition) may instead be important for bed morphology changes because it controls the extent of plunging flow over obstacles and this is discussed in Monsalve et al (). Given that most changes around the hemispheres were relatively similar between runs (see section 3.2 and Figures ), we now focus on how all runs together changed through time.…”

Breaking from the average: Why large grains matter in gravel‐bed streams

“…Empirically, the best flow resistance relations are associated not with the D 50 but with grains much larger than average – typically D 84 (Hey, ) or D 90 (Bray, ) – as these grains most strongly influence the near‐bed channel hydraulics (Recking, ; Monsalve et al. ).…”

“…where C i is the correction factor corresponding to grain size D i , wherein i signifies the percentile of the cumulative GSD. Empirically, the best flow resistance relations are associated not with the D 50 but with grains much larger than average -typically D 84 (Hey, 1979) or D 90 (Bray, 1980) -as these grains most strongly influence the near-bed channel hydraulics (Recking, 2009;Monsalve et al, 2017). That a correction factor is necessary even when using D 84 or D 90 is attributable to the fact that roughness in natural channels is imparted not only from the surface of the bed but also from self-organizing bedforms and structures that develop within the channel (Clifford et al, 1992;Millar, 1999;Wilcox et al, 2006).…”

Controls Over Particle Motion and Resting Times of Coarse Bed Load Transport in a Glacier‐Fed Mountain Stream

“…Only part of the flow power in mountain streams is available to entrain and transport sediments (Hohermuth & Weitbrecht, 2018; Rickenmann & Recking, 2011), as some energy is dissipated in various ways, including local hydraulic plunging and jumps over steps (Comiti et al, 2009; Green et al, 2013; Monsalve et al, 2017). In such complex hydraulic conditions, well‐known and established flow resistance approaches and empirical bed load formulas are unlikely to be reliable (e.g., Yager et al, 2018).…”