The effect of lateral confinement on gravel bed river morphology

Abstract

In this paper, we use a physical modeling approach to explore the effect of lateral confinement on gravel bed river planform style, bed morphology, and sediment transport processes. A set of 27 runs was performed in a large flume (25 m long, 2.9 m wide), with constant longitudinal slope (0.01) and uniform grain size (1 mm), changing the water discharge (1.5-2.5 L/s) and the channel width (0.15-1.5 m) to model a wide range of channel configurations, from narrow, straight, embanked channels to wide braided networks. The outcomes of each run were characterized by a detailed digital elevation model describing channel morphology, a map of dry areas and areas actively transporting sediment within the channel, and continuous monitoring of the amount of sediment transported through the flume outlet. Analysis reveals strong relationships between unit stream power and parameters describing the channel morphology. In particular, a smooth transition is observed between narrow channels with an almost rectangular crosssection profile (with sediment transport occurring across the entire channel width) and complex braided networks where only a limited proportion (30%) of the bed is active. This transition is captured by descriptors of the bed elevation frequency distribution, e.g., standard deviation, skewness, and kurtosis. These summary statistics represent potentially useful indicators of bed morphology that are compared with other commonly used summary indicators such as the braiding index and the type and number of bars.

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

“…Experiment 1 seems to have a greater bedload transport rate than would be expected for the range of active widths, which is either an artifact of the low detection rates for that experiment (i.e. The dimensionless morphological active width While previous measurements of the morphological active width are relatively limited, Bertoldi et al (2009a), Ashmore et al (2011), andLugo et al (2015) showed that the variation of active width with discharge or total stream power may collapse to a general function using dimensionless stream power (ω*) and active width ratio (non-dimensionalized using wetted width). The correlation of the morphological active width with directly measured bedload transport rate in braided rivers at constant discharge and under varying flow conditions has not previously been demonstrated.…”

“…To date, field data on braided river channel topography is limited by low spatial and temporal frequency and a lack of simultaneous bedload transport rates (Bertoldi et al, 2010;Church and Ferguson, 2015;Williams et al, 2016b;Vericat et al, 2017). These data and corresponding analysis can inform, assess, and assist in developing numerical models of braiding morphodynamics and bedload flux (Bertoldi et al, 2009a;Williams, 2012;Church and Ferguson, 2015;Lugo et al, 2015), which require calibration and validation with high-resolution 3-dimensional surveys of braided rivers (Williams et al, 2016a;Redolfi et al, 2017). These data and corresponding analysis can inform, assess, and assist in developing numerical models of braiding morphodynamics and bedload flux (Bertoldi et al, 2009a;Williams, 2012;Church and Ferguson, 2015;Lugo et al, 2015), which require calibration and validation with high-resolution 3-dimensional surveys of braided rivers (Williams et al, 2016a;Redolfi et al, 2017).…”

“…Also, while the research here focused on braided river morphologies, Lugo et al (2015) used lateral confinement experiments in uniform sand to show that a general relationship between active width (in that case, instantaneous), and dimensionless stream power may not be restricted to braiding rivers, but seems to apply to transitioning (i.e. wandering) rivers as well.…”

“…Given the positive relationship between active width and ABI, and the fundamental association with morphodynamic processes, the active width is expected to be a meaningful parameter for linking complex braided river morphodynamics and the corresponding bedload transport flux (Bertoldi et al, 2009a;Ashmore et al, 2011;Lugo et al, 2015;Redolfi et al, 2017). Research has already demonstrated that active width, ABI, and bedload transport rates are positively correlated with dimensionless unit stream power (ω*):…”

“…Therefore, the focus here is on the connection between morphological activity and the rate of bedload transport in braided rivers. By expanding the relatively limited number of morphological active width measurements to date, along with introducing direct measurements of bedload flux, this research has the potential to inform future morphodynamic models, both conceptual and numerical, which cannot currently account for the inherent complexity and morphologically-driven bedload transport of braided rivers (Lugo et al, 2015;Recking et al, 2016;Williams et al, 2016b). The result is a large novel dataset of morphological change and bedload flux across five formative discharge experiments as well as three simulated event hydrographs, using spatially continuous topographic data taken at high temporal frequency.…”