The influence of sediment thickness on energy delivery to the bed by bedload impacts

Turowski, Jens M.; Bloem, Jean-Pierre

doi:10.1080/09853111.2015.1047195

Cited by 14 publications

(34 citation statements)

References 58 publications

(89 reference statements)

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“…Work at large spatial scales (reach scale to catchment scale) has mostly focused on long‐term bedrock river evolution driven by changes in tectonics and climate [e.g., Lavé and Avouac , ; Whittaker et al , ; Montgomery and Korup , ; Yanites et al , ; Jansen et al , ], while on intermediate scales (subreach scale to reach scale), studies have mostly sought to characterize site‐specific dominant processes [e.g., Lamb et al , ; Ouimet et al , ; Valla et al , ; Wilson et al , ]. The tools and cover effects have been shown to be of major importance at least on intermediate scales [ Turowski et al , ; Turowski and Rickenmann , ; Johnson et al , , ; Hobley et al , ; Jansen et al , ; Cook et al , ; Beer and Turowski , ; Turowski and Bloem , ]. Furthermore, local erosion can be related to factors including the sinuosity of the stream channel [ Stark et al , ] or the channel's macroroughness [ Johnson and Whipple , ; Yager et al , ; Inoue et al , ; Zhang et al , ].…”

Section: Introductionmentioning

confidence: 99%

Spatial patterns of erosion in a bedrock gorge

2017

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Understanding the physical processes driving bedrock channel formation is essential for interpreting and predicting the evolution of mountain landscapes. Here we analyze bedrock erosion patterns measured at unprecedented spatial resolution (mm) over 2 years in a natural bedrock gorge. These spatial patterns show that local bedrock erosion rates depend on position in the channel cross section, height above the streambed, and orientation relative to the main streamflow and sediment path. These observations are consistent with the expected spatial distribution of impacting particles (the tools effect) and shielding by sediment on the bed (the cover effect). Vertical incision by bedrock abrasion averaged 1.5 mm/a, lateral abrasion averaged 0.4 mm/a, and downstream directed abrasion of flow obstacles averaged 2.6 mm/a. However, a single plucking event locally exceeded these rates by orders of magnitude (∼100 mm/a), and accounted for one third of the eroded volume in the studied gorge section over the 2 year study period. Hence, if plucking is spatially more frequent than we observed in this study period, it may contribute substantially to long‐term erosion rates, even in the relatively massive bedrock at our study site. Our observations demonstrate the importance of bedrock channel morphology and the spatial distribution of moving and static sediment in determining local erosion rates.

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Section: Introductionmentioning

confidence: 99%

Spatial patterns of erosion in a bedrock gorge

2017

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“…Beer et al, 2016;Turowski et al, 2008a). Using a combination of experiments and modelling, it has been argued that the fraction of covered bed area is an adequate proxy for the reduction in erosion due to the shielding effect of sediment on the reach scale (Turowski and Bloem, 2016). Consequently, cover C is commonly defined as the covered bed area fraction, i.e.…”

Section: Lateral Erosion and Bedrock Channel Widthmentioning

confidence: 99%

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

Turowski

2018

Earth Surf. Dynam.

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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.

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“…Amongst those, abrasion, plucking, and macroabrasion depend directly on the amount of material that is removed from the bedrock by rolling, sliding, or impacting particles transported by the flow, which itself depends mainly on the amount of energy that is transmitted to the bedrock by moving particles (Foley, 1980). This transfer of energy from the impacting particle to the bedrock has been recently directly measured experimentally by Turowski and Bloem (2015), as a function of the thickness of the sediment layer covering the bedrock. As can be expected, the fraction of the incipient kinetic energy that is effectively transmitted to the bedrock decreases when the sediment thickness increases.…”

Section: Introductionmentioning

confidence: 99%

“…(28) in order to take into account only the Shields number and the sediment flux, which are easier to compute, or to measure experimentally, than the frequency of impacts and their energy. Indeed, if a saltating particle hits a mobile particle shielding the bedrock, a small fraction of its incipient energy could still be transmitted to the bedrock (Turowski and Bloem, 2015), though this event would not be counted as an eroding impact with Eq. (1).…”

Section: Cover Effectmentioning

confidence: 99%

Bedrock incision by bedload: insights from direct numerical simulations

Aubert¹,

Langlois²,

Allemand³

2016

Earth Surf. Dynam.

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Abstract. Bedload sediment transport is one of the main processes that contribute to bedrock incision in a river and is therefore one of the key control parameters in the evolution of mountainous landscapes. In recent years, many studies have addressed this issue through experimental setups, direct measurements in the field, or various analytical models. In this article, we present a new direct numerical approach: using the classical methods of discrete-element simulations applied to granular materials, we explicitly compute the trajectories of a number of pebbles entrained by a turbulent water stream over a rough solid surface. This method allows us to extract quantitatively the amount of energy that successive impacts of pebbles deliver to the bedrock, as a function of both the amount of sediment available and the Shields number. We show that we reproduce qualitatively the behaviour observed experimentally by Sklar and Dietrich (2001) and observe both a "tool effect" and a "cover effect". Converting the energy delivered to the bedrock into an average long-term incision rate of the river leads to predictions consistent with observations in the field. Finally, we reformulate the dependency of this incision rate with Shields number and sediment flux, and predict that the cover term should decay linearly at low sediment supply and exponentially at high sediment supply.

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The influence of sediment thickness on energy delivery to the bed by bedload impacts

Cited by 14 publications

References 58 publications

Spatial patterns of erosion in a bedrock gorge

Spatial patterns of erosion in a bedrock gorge

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

Bedrock incision by bedload: insights from direct numerical simulations

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