Spatial Variations in Excess Shear Stress in a Gravel-Bed River Bend

Abstract

Detailed topographic and hydraulic measurements were obtained in a bend of the Colorado River in Rocky Mountain National Park, Colorado, to characterize flow through the reach with a two-dimensional model. When compared with the threshold Shields stress required for particle entrainment (τ * c ), estimates of Shields stress (τ * ) obtained from the model provided a distribution of excess stress through the reach for a range of discharges. Magnitudes of excess stress have been related (elsewhere) to transport phase: ratios of τ * /τ * c between 1 and 2 imply partial mobility; τ * /τ * c ≥ 2 suggests a fully mobile bed. This study found that at ~bankfull flow, areas of excess stress (1 < τ * /τ * c < 2, and τ * /τ * c ≥ 2) approach roughly 60% and 10% of the bed, respectively. For the peak discharge observed during the field campaign, areal coverage of excess stress probably increased to approximately 85 and 15%, respectively, suggesting that all of the bed was experiencing at least some degree of sediment mobility. These results suggest that small regions of the channel contribute disproportionately to the overall sediment flux at high flows, that the size of these regions increases with stage, and that this effect amplifies with channel curvature.

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

“…A particular case of local sediment deposition is the formation of sediment patches, which are distinct areas of the bed that scale in length around 0.1–1 channel widths (Nelson et al, ) with a relatively narrow grain size distribution (GSD) compared to that of the entire reach (Dietrich et al, ; Laronne et al, ). Sediment patches in turn create areas of different roughness and affect the local hydraulics but are commonly omitted from flow and sediment transport models (Ferguson, ; Ferguson & Church, ; Lisle et al, ; Nicholas, ) and consequently the model's performance is reduced (Clayton, ). Sediment patch locations and characteristics also have very important implications for riverine habitat availability, for instance salmonid spawning can occur in local zones of gravel textures that are finer than the entire reach (Buffington & Montgomery, ; Buxton et al, ; Hassan et al, ; Kondolf & Wolman, ).…”

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

“…In our experiments we accounted for both of these effects by holding sediment supply and the roughness configuration constant and using the near‐bed , which implies that same q b for different is likely explained by spatial variations in both shear stress (Figure b) and bed grain size distribution. Accounting for this spatial variability in shear stress is therefore key for accurate sediment transport predictions (Cienciala & Hassan, ; Clayton, ; Dietrich & Whiting, ; Monsalve et al, ; Segura & Pitlick, ).…”

“…Changes in turbulence intensities (Bathurst, ; Canovaro, Paris, & Solari, ; Nelson, McLean, & Wolfe, ; Papanicolaou et al, ; Papanicolaou, Bdour, & Wicklein, ; Thompson, ), local pressure distributions (Pournazeri, Li, & Haghighat, ; Zeng, Constantinescu, & Weber, , ), and water surface elevations (Vallé & Pasternack, ; Yager et al, ) also can occur. Flow variability has important consequences for the spatial distribution of shear stresses (Afzalimehr & Rennie, ; Clayton, ; Clayton & Pitlick, ; Cooper et al, ; Ferguson, ; Maddux, McLean, et al, ; Papanicolaou, Tsakiris, & Kramer, ), water aeration (Chanson, , ; Straub, Killen, & Lamb, ; Straub & Lamb, ), mixing (Chanson, ; Claxton, Bates, & Cloke, ; Tonina & Buffington, ), and transport of scalar properties such as water temperature and dissolved oxygen, all of which are key factors for aquatic habitat quality and riverine ecosystems (Cienciala & Hassan, ; Ouellet, Secretan, & Morin, ). In particular, the spatial distribution of shear stress has significant influence on sediment transport rates, sediment deposition patterns, and channel stability (Lisle et al, ; Monsalve et al, ; Nelson, Dietrich, & Venditti, ; Segura & Pitlick, ; Yager & Schmeeckle, ).…”

Input‐variable sensitivity assessment for sediment transport relations

“…Note that equations and assume that both the channel width and the specific discharge are independent variables although, commonly, the specific discharge is computed as where is the flow discharge. This is true for straight channels and is considered a good assumption for natural river reaches with riffles but not for pools and river bends where the specific discharge per unit width, and thus the shear stresses, vary within the cross section as has been recently observed by Clayton [] in a bend of the Colorado River. Here the variables are assumed to be independent of each other to assess their first‐order specific contributions.…”