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

Abstract

In mountainous rivers, large relatively immobile grains partly control the local and reach‐averaged flow hydraulics and sediment fluxes. When the flow depth is similar to the size of these grains (low relative submergence), heterogeneous flow structures and plunging flow cause spatial distributions of bed surface elevations, textures, and sedimentation rates. To explore how the bed surface responds to these flow variations we conducted a set of experiments in which we varied the relative submergence of staggered hemispheres (simulated large boulders) between runs. All experiments had the same average sediment transport capacity, upstream sediment supply, and initial bed thickness and grain size distribution. We combined our laboratory measurements with a 3‐D flow model to obtain the detailed flow structure around the hemispheres. The local bed shear stress field displayed substantial variability and controlled the bed load transport rates and direction in which sediment moved. The divergence in bed shear stress caused by the hemispheres promoted size‐selective bed load deposition, which formed patches of coarse sediment upstream of the hemisphere. Sediment deposition caused a decrease in local bed shear stress, which combined with the coarser grain size, enhanced the stability of this patch. The region downstream of the hemispheres was largely controlled by a recirculation zone and had little to no change in grain size, bed elevation, and bed shear stress. The formation, development, and stability of sediment patches in mountain streams is controlled by the bed shear stress divergence and magnitude and direction of the local bed shear stress field.

Boulder Array Effects on Bedload Pulses and Depositional Patches

“…The control of other factors in the present experiments also indicates that < H >/ d c is the parameter most likely to explain this behavior. This finding may have significant implications for the timing of bedload conveyance through high gradient gravel‐bed rivers, which often receive sporadic sediment inputs from steep landscapes. The findings of this study overall support the interconnection of flow modification by the presence of immobile boulders and the role of boulders on bedload transport, which has been suggested in previous research (e.g., Monsalve & Yager, ; Papanicolaou & Tsakiris, ). These results can aid investigations of morphodynamic evolution in high gradient gravel‐bed rivers by providing improved understanding of local bed morphology as well as of bedload entrapment and conveyance.…”

“…The findings of this study overall support the interconnection of flow modification by the presence of immobile boulders and the role of boulders on bedload transport, which has been suggested in previous research (e.g., Monsalve & Yager, 2017;Papanicolaou & Tsakiris, 2017). These results can aid investigations of morphodynamic evolution in high gradient gravel-bed rivers by providing improved understanding of local bed morphology as well as of bedload entrapment and conveyance.…”

“…The local bed morphology developed in the LRS tests (Figure ) was instead marked by bedload deposition predominantly upstream of individual boulders, which presents a direct contrast with the HRS tests. This type of upstream depositional morphology is not well documented in literature, and to the authors' knowledge very few researchers (e.g., Monsalve & Yager, ; Papanicolaou et al, ; Papanicolaou & Kramer, ) have connected deposition upstream of boulders with partial submergence flow conditions, whereas other studies have typically connected this behavior with imbrication (e.g., Brayshaw, ; Laronne & Carson, ). We attribute the deposition of the material upstream of the boulders to the necklace structure of the horseshoe vortex (HSV) that develops at the leading edge of the boulders, where the flow usually oscillates between the zero velocity and backflow mode (Paik et al, ).…”

“…In contrast, the limited available data at boulder LRS conditions suggest that in the case of Fr < 1 the necklace developed by the bimodal state of the HSV creates entrapment of sediment at the stoss; in the case of Fr > 1 it is the LWCs and potentially a von‐Karman vortex street, which controls deposition and effective area of deposition (Papanicolaou et al, ; Shamloo et al, ). Research has further suggested that these different vortex structures in the boulder region under HRS and LRS conditions have different impacts on the bed shear stress directionality and magnitude (Monsalve & Yager, ; Papanicolaou et al, ; Papanicolaou & Tsakiris, ). Thus, < H >/ d c is believed to influence bedload depositional locations through its impacts on the local hydrodynamics and vortex topology developing around boulders.…”

Stabilising large grains in self-forming steep channels

“…The validity of using a single characteristic grain size as a descriptor of a whole system's state is, however, fundamentally flawed. We know that surficial adjustment, bedforms, and macroforms modulate bed material sediment transport rate, acting to dissipate energy and provide stability to the overall channel (Cherkauer, 1973;Montgomery and Buffington, 1997;Venditti et al, 2017). For example, it has been thought that grains may stabilise through rotation (Masteller and Finnegan, 2017), their organisation into cells (Church et al, 1998;Monsalve and Yager, 2017), and the formation of alternate bars and patches (Lisle et al, 1991;Dietrich et al, 2006;Nelson et al, 2010).…”