Effect of point bar development on the local force balance governing flow in a simple, meandering gravel bed river

Abstract

[1] The patterns of depth, velocity, and shear stress that direct a river's morphologic evolution are governed by a balance of forces. Analyzing these forces, associated with pressure gradients, boundary friction, channel curvature, and along-and across-stream changes in fluid momentum driven by bed topography, can yield insight regarding the establishment and maintenance of stable channel forms. This study examined how components of the local force balance changed as a meandering channel evolved from a simple, flat-bedded initial condition to a more complex bar-pool morphology. A numerical flow model, constrained by measurements of velocity and water surface elevation, characterized the flow field for four time periods bracketing two floods. For each time increment, runs were performed for discharges up to bankfull, and individual force balance components were computed from model output. Formation and growth of point bars enhanced topographic steering effects, which were of similar magnitude to the pressure gradient and centrifugal forces. Convective accelerations induced by the bar reduced the cross-stream pressure gradient, intensified flow toward the outer bank, and routed sediment around the upstream end of the bar. Adjustments in the flow field thus served to balance streamwise transport along the inner bank onto the bar and cross-stream transport into the pool. Even in the early stages of bar development, topographically driven spatial gradients in velocity played a significant role in the force balance at flows up to bankfull, altering the orientation of the shear stress and sediment transport to drive bar growth.Citation: Legleiter, C. J., L. R. Harrison, and T. Dunne (2011), Effect of point bar development on the local force balance governing flow in a simple, meandering gravel bed river,

The influence of a vegetated bar on channel-bend flow dynamics

“…1; see Supplement for more detail). The RMSE ranges obtained through calibration are consistent with values reported in other studies that have used FaSTMECH (e.g., Legleiter et al, 2011;Mueller and Pitlick, 2014;Segura and Pitlick, 2015), providing 10 confidence in model performance. Relaxation coefficients were set to 0.5, 0.3, and 0.1 for ERelax, URelax, and ARelax, respectively, through trial and error.…”

“…They additionally found that flow over the heads of bars resulted in cross-stream 5 components of velocity (v) and boundary shear stress ( ) directed toward the concave bank. Whiting (1997) hypothesized that convective accelerations arising from flow steering would be most important at low flows, whereas Legleiter et al (2011) showed that steering from bars continued to be important with increasing discharge. We found that vegetation began to impact channel-bend hydraulics for flows greater than the Q2, when plants began to be inundated.…”

Modeling the influence of flow on invertebrate drift across spatial scales using a 2D hydraulic model and a 1D population model

“…Full details about the site can be found in C.A.D.W.R. (2005) as well as Albertson et al (2011, and Legleiter et al (2011). In brief, the Robinson Reach was restructured and rescaled in 2001 as part of a larger effort to improve Chinook salmon habitat and channel-floodplain functionality following 150 years of gravel mining that had occurred at the site.…”

“…In order to take advantage of existing hydraulic data sets, the MIKE21 FM model was calibrated and validated using a 2D flow model developed for the entire Robinson Reach, which included 10 bends of nearly uniform dimensions and curvature (see Harrison et al, 2011;Legleiter et al, 2011) for additional site description). Bed topography was surveyed across the active channel and roughly 10 m of the floodplain on either bank using a total station, with a mean cross section spacing of 7 m (20% of the channel width) and an average distance of 2 m between points along a transect.…”

Sediment supply as a driver of river meandering and floodplain evolution in the Amazon Basin

“…Elevated shear stresses near the outer bank promote the fluvial entrainment of the bank material primarily responsible for bank erosion 19 , and the displacement of the zone of maximum shear stress from the inner to the outer bank shifts the zones of maximum bed-material transport and bank scour accordingly 15 . As a result, the bar aggrades, growing laterally and vertically 20 until its surface is infrequently inundated, promoting the continued advection of high-momentum fluid by increasing flow curvature and asymmetric shoaling.…”