Applicability of bed load transport models for mixed‐size sediments in steep streams considering macro‐roughness

Abstract. Bedrock channel slope and width are important parameters for setting bedload transport capacity and for stream-profile inversion to obtain tectonics information. Channel width and slope development are closely related to the problem of bedrock channel sinuosity. It is therefore likely that observations on bedrock channel meandering yields insights into the development of channel width and slope. Active meandering occurs when the bedrock channel walls are eroded, which also drives channel widening. Further, for a given drop in elevation, the more sinuous a channel is, the lower is its channel bed slope in comparison to a straight channel. It can thus be expected that studies of bedrock channel meandering give insights into width and slope adjustment and vice versa. The mechanisms by which bedrock channels actively meander have been debated since the beginning of modern geomorphic research in the 19th century, but a final consensus has not been reached. It has long been argued that whether a bedrock channel meanders actively or not is determined by the availability of sediment relative to transport capacity, a notion that has also been demonstrated in laboratory experiments. Here, this idea is taken up by postulating that the rate of change of both width and sinuosity over time is dependent on bed cover only. Based on the physics of erosion by bedload impacts, a scaling argument is developed to link bedrock channel width, slope and sinuosity to sediment supply, discharge and erodibility. This simple model built on sediment-flux-driven bedrock erosion concepts yields the observed scaling relationships of channel width and slope with discharge and erosion rate. Further, it explains why sinuosity evolves to a steady-state value and predicts the observed relations between sinuosity, erodibility and storm frequency, as has been observed for meandering bedrock rivers on Pacific Arc islands.

Section: Channel Bed Slopementioning

confidence: 99%

Alluvial cover controlling the width, slope and sinuosity of bedrock channels

Turowski

2018

“…Despite over a century of quantitative study (Gilbert, 1914), it often remains challenging to predict gravel transport rates to much better than an order of magnitude because of the complexity of grain interactions with the flow and the surrounding grains (e.g., Schneider et al, 2015;Nitsche et al, 2011;Rickenmann, 2001;Wilcock and Crowe, 2003;Chen and Stone, 2008). Predictive models for complex systems often derive utility from their simplicity, as is the case with the widely-used Meyer-Peter and Müller (1948) …”

Section: Motivationmentioning

confidence: 99%

“…Flow characteristics influencing τ * c include particle Reynolds number, flow depth relative to grain size, the intensity of turbulence, the history of prior flow both above and below transport thresholds, and the partitioning of stress into form drag and skin friction (e.g., Shvidchenko and Pender, 2000;Ockelford and Haynes, 2013;Schneider et al, 2015;Valyrakis et al, 2010;Celik et al, 2010). Most flowdependent controls are not independent of the bed surface controls.…”

Section: Previous Work: Mechanistic Controls On τ * Cmentioning

confidence: 99%

“…Conversely, another common approach (and that taken here) is simply to use higher τ * c (e.g., Bunte et al, 2013;Lenzi et al, 2006), treating τ * c as a physically meaningful fitting parameter to predict transport. Using field data, Schneider et al (2015) recently compared gravel transport predictions based on (a) form drag corrections and (b) higher reference stresses. For the most part, they found that both approaches could provide similar accuracy.…”

Section: Form Drag Vs Parsimonymentioning

confidence: 99%

See 1 more Smart Citation

Gravel threshold of motion: a state function of sediment transport disequilibrium?

Johnson

2016

Abstract. In most sediment transport models, a threshold variable dictates the shear stress at which nonnegligible bedload transport begins. Previous work has demonstrated that nondimensional transport thresholds (τ * c ) vary with many factors related not only to grain size and shape, but also with characteristics of the local bed surface and sediment transport rate (q s ). I propose a new model in which q s -dependent τ * c , notated as τ * c(q s ) , evolves as a power-law function of net erosion or deposition. In the model, net entrainment is assumed to progressively remove more mobile particles while leaving behind more stable grains, gradually increasing τ * c(q s ) and reducing transport rates. Net deposition tends to fill in topographic lows, progressively leading to less stable distributions of surface grains, decreasing τ * c(q s ) and increasing transport rates. Model parameters are calibrated based on laboratory flume experiments that explore transport disequilibrium. The τ * c(q s ) equation is then incorporated into a simple morphodynamic model. The evolution of τ * c(q s ) is a negative feedback on morphologic change, while also allowing reaches to equilibrate to sediment supply at different slopes. Finally, τ * c(q s ) is interpreted to be an important but nonunique state variable for morphodynamics, in a manner consistent with state variables such as temperature in thermodynamics.

“…underwater microphones (Barton et al, 2010;Camenen et al, 2012;Rigby et al, 2015). It is well known that bedload transport rates often show very large variability for given flow conditions (Gomez, 1991;Leopold and Emmett, 1997;Ryan and Dixon, 2008;Recking, 2010), and that prediction of (mean) bedload transport rates is still very challenging, particularly for steep and coarse-bedded streams (Bathurst et al, 1987;Nitsche et al, 2011;Schneider et al, 2015Schneider et al, , 2016. For such conditions, direct bedload transport measurements are typically difficult to obtain, or may be impossible to make during high-flow conditions (Gray et al, 2010).…”

Section: Introductionmentioning

confidence: 99%

Bedload transport measurements with impact plate geophones in two Austrian mountain streams (Fischbach and Ruetz): system calibration, grain size estimation, and environmental signal pick-up

Rickenmann

Fritschi

2017

Self Cite

Abstract. The Swiss plate geophone system is a bedload surrogate measuring technique that has been installed in more than 20 streams, primarily in the European Alps. Here we report about calibration measurements performed in two mountain streams in Austria. The Fischbach and Ruetz gravel-bed streams are characterized by important runoff and bedload transport during the snowmelt season. A total of 31 (Fischbach) and 21 (Ruetz) direct bedload samples were obtained during a 6-year period. Using the number of geophone impulses and total transported bedload mass for each measurement to derive a calibration function results in a strong linear relation for the Fischbach, whereas there is only a poor linear calibration relation for the Ruetz measurements. Instead, using geophone impulse rates and bedload transport rates indicates that two power law relations best represent the Fischbach data, depending on transport intensity; for lower transport intensities, the same power law relation is also in reasonable agreement with the Ruetz data. These results are compared with data and findings from other field sites and flume studies. We further show that the observed coarsening of the grain size distribution with increasing bedload flux can be qualitatively reproduced from the geophone signal, when using the impulse counts along with amplitude information. Finally, we discuss implausible geophone impulse counts that were recorded during periods with smaller discharges without any bedload transport, and that are likely caused by vehicle movement very near to the measuring sites.