Jeremy G. Venditti

Simon Fraser University

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Michael Church

University of British Columbia

R. W. Bradley

Simon Fraser University

Eric Deal

ETH Zurich

Santiago Benavides

Massachusetts Institute of Technology

J. Taylor Perron

Massachusetts Institute of Technology

Ken Kamrin

University of California, Berkeley

Matthew Rushlow

Massachusetts Institute of Technology

L. S. Sklar

San Francisco State University

Ray Kostaschuk

Simon Fraser University

Tingan Li

Simon Fraser University

Cooperative Institutions

Simon Fraser University

University of British Columbia

Planetary Science Institute

United States Geological Survey

University of California, Berkeley

The University of Texas at Austin

University of Minnesota

California Institute of Technology

Massachusetts Institute of Technology

University of Washington

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Why do large, deep rivers have low-angle dune beds?

Geology (2019)

Ray Kostaschuk Jeremy G. Venditti

Abstract Dunes are the most common bedform in sand-bedded rivers. Small, high-angle dunes (HADs) dominate in shallow (<2.5 m) flows and have lee sides with steep downstream-facing slipfaces (>24°) and reversed upslope-oriented flow in the separation vortex. In contrast, large, deep (>2.5 m) rivers have low-angle dunes (LADs) with gentle lee-side slipfaces (<24°; often <10°), little or no flow separation, and more-pronounced downslope currents. Avalanches driven primarily by particle interaction maintain slipfaces of HADs. On LADs, excess pore pressures that occur during the failure of thick, loosely packed deposits can produce liquefied avalanches that flow and stop on gentle gradients. As lee-side angles decrease over LADs, downslope currents increase in strength, accelerating avalanches, transporting bedload, and creating smaller, superimposed dunes and uniform concave slopes on the lee side. Nearly a century of research on dunes in shallow laboratory flows has assumed that dune morphodynamics are scale invariant, which is not true.

Beach morphodynamics

Bedform

Sand dune stabilization

10.1130/g46460.1

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Citations (28)

Bedform spurs: a result of a trailing helical vortex wake

Sedimentology (2017)

Travis Swanson David Mohrig Gary Kocurek M. M. Perillo Jeremy G. Venditti

Abstract The formative conditions for bedform spurs and their roles in bedform dynamics and associated sediment transport are described herein. Bedform spurs are formed by helical vortices that trail from the lee surface of oblique segments of bedform crest lines. Trailing helical vortices quickly route sediment away from the lee surface of their parent bedform, scouring troughs and placing this bed material into the body of the spur. The geometric configuration of bedform spurs to their parent bedform crests is predicted by a cross‐stream Strouhal number. When present, spur‐bearing bedforms and their associated trailing helical wakes exert tremendous control on bedform morphology by routing enhanced sediment transport between adjacent bedforms. Field measurements collected at the North Loup River, Nebraska, and flume experiments described in previous studies demonstrate that this trailing helical vortex‐mediated sediment transport is a mechanism for bedform deformation, interactions and transitions between two‐dimensional and three‐dimensional bedforms.

Bedform

Flume

10.1111/sed.12383

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Citations (21)

Bed form initiation from a flat sand bed

Journal of Geophysical Research Atmospheres (2005)

Jeremy G. Venditti Michael Church Sean J. Bennett

Bed form initiation in unidirectional flow is examined on a flat bed composed of a homogeneous 0.5 mm sand. Velocity profiles taken prior to bed form development indicate that the examined flows are typical of fully turbulent, uniform, open channel flows. Under these conditions, two separate modes of bed form initiation are observed: defect and instantaneous initiation. Defect initiation occurs at lower flow stages, where sediment transport is sporadic and patchy, and is characterized by defect propagation associated with flow separation. Instantaneous initiation occurs at larger flow strengths, where sediment transport is general and widespread. This form of bed form initiation begins with the imprinting of a cross‐hatch pattern on the flat sediment bed, which leads to chevron‐shaped forms that migrate independently of the initial pattern. The chevrons eventually align to form incipient crest lines. This mode of bed form initiation does not appear to be linked to turbulent structures, but integral scales derived from velocity measurements prior to bed form development are similar to the initial bed form length scales.

Crest

Bedform

10.1029/2004jf000149

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Citations (121)

Morphodynamics of small‐scale superimposed sand waves over migrating dune bed forms

Water Resources Research (2005)

Jeremy G. Venditti Michael Church Sean J. Bennett

The kinematics and morphodynamics of low‐amplitude, small‐scale sand waves developed over migrating dunes are examined using data drawn from laboratory experiments. We refer to the superimposed features as “sand sheets,” a general descriptive term for low‐amplitude bed waves that are not easily classified as ripples, dunes, or bars. Within the experiments, the sheets formed downstream of the reattachment point at a distance that was invariant with dune size. Some sheets lacked slip faces composed of sand grains avalanching down a slope near the angle of repose. Over equilibrium dunes, three to four sand sheets were observed per 100 s. Sheet thickness was 10% of the height of the dune upon which they were superimposed; they migrated at 8 to 10 times the dune rate; they had nearly constant lengths over the full range of dune lengths and flow conditions; and they had aspect ratios of ∼0.025. Dunes and sand sheets represent distinct scales of sediment transport with different migration rates. However, sediment transport rates, calculated from the sand sheet and dune morphologies, are nearly identical. For transport equivalence to occur, sand sheets migrating at 10 times the dune rate must be 0.1 times the size, which is consistent with the morphological observations. Superimposed bed waves on dunes are often considered simply as additional roughness elements, but these results indicate that such bed waves are the agency by which the dune bed form itself moves downstream.

Beach morphodynamics

Bedform

Sand dune stabilization

Angle of repose

Length scale

10.1029/2004wr003461

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Citations (122)

Mud in rivers transported as flocculated and suspended bed material

Nature Geoscience (2020)

Michael P. Lamb Jan de Leeuw Woodward W. Fischer Andrew J. Moodie Jeremy G. Venditti

Settling

Suspended load

Flume

Silt

Sedimentation

Siltation

Bedform

10.1038/s41561-020-0602-5

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Citations (83)

Implications of sediment transport by subglacial water flow for interpreting contemporary glacial erosion rates

EGUGA (2017)

Flavien Beaud Gwenn E. Flowers Jeremy G. Venditti

Source

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Efficacy of bedrock erosion by subglacial water flow

Earth Surface Dynamics (2016)

Flavien Beaud Gwenn E. Flowers Jeremy G. Venditti

Abstract. Bedrock erosion by sediment-bearing subglacial water remains little-studied; however, the process is thought to contribute to bedrock erosion rates in glaciated landscapes and is implicated in the excavation of tunnel valleys and the incision of inner gorges. We adapt physics-based models of fluvial abrasion to the subglacial environment, assembling the first model designed to quantify bedrock erosion caused by transient subglacial water flow. The subglacial drainage model consists of a one-dimensional network of cavities dynamically coupled to one or several Röthlisberger channels (R-channels). The bedrock erosion model is based on the tools and cover effect, whereby particles entrained by the flow impact exposed bedrock. We explore the dependency of glacial meltwater erosion on the structure and magnitude of water input to the system, the ice geometry, and the sediment supply. We find that erosion is not a function of water discharge alone, but also depends on channel size, water pressure, and sediment supply, as in fluvial systems. Modelled glacial meltwater erosion rates are 1 to 2 orders of magnitude lower than the expected rates of total glacial erosion required to produce the sediment supply rates we impose, suggesting that glacial meltwater erosion is negligible at the basin scale. Nevertheless, due to the extreme localization of glacial meltwater erosion (at the base of R-channels), this process can carve bedrock (Nye) channels. In fact, our simulations suggest that the incision of bedrock channels several centimetres deep and a few metres wide can occur in a single year. Modelled incision rates indicate that subglacial water flow can gradually carve a tunnel valley and enhance the relief or even initiate the carving of an inner gorge.

Bedrock

Meltwater

10.5194/esurf-4-125-2016

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Citations (55)

The gravel-sand transition: Sediment dynamics in a diffuse extension

Journal of Geophysical Research Earth Surface (2015)

Jeremy G. Venditti Natalia Domarad Michael Church Colin D. Rennie

As gravel-bedded rivers fine in the downstream direction, they characteristically exhibit an abrupt transition from gravel- to sand-bedded conditions. The prevailing theory for why abrupt gravel-sand transitions emerge is based on bed load sorting of a bimodal sediment. The abruptness is thought to be a consequence of sand overwhelming the gravel-sand mixture once it reaches a critical coverage on the bed. The role suspension plays in the development of gravel-sand transitions has not been fully appreciated. The Fraser River, British Columbia, is an archetypical abrupt gravel-sand transition with a "diffuse extension" composed of a sand bed with some patches of gravel. We examine flow, shear stress, and suspended sediment flux in the diffuse extension to better understand sediment dynamics where the sand bed emerges. Sand is carried in suspension upstream of the primary abrupt gravel-sand transition, but in the diffuse extension, sand is moved as both bed load and suspended load. We do not observe downstream gradients in shear stress or suspended sand flux through the diffuse extension that would suggest a gradual "rain out" of sand moving downstream, which raises the question, how is the sand bed formed? Sediment advection length scales indicate that with the exception of very fine sand that moves as wash load in the diffuse extension, fractions coarser than the median sand size cannot be carried in suspension for more than one channel width. This suggests that sand is deposited en masse at the beginning of the diffuse extension, forming a sediment slug at low flood flows that is smeared downstream at high flood flows to form the sand reach.

Suspended load

Suspension

10.1002/2014jf003328

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Citations (53)

Experiments on Constriction‐Pool‐Widening Morphology in Bedrock Canyons

Journal of Geophysical Research Earth Surface (2024)

Kyle M. Kusack Tingan Li Jeremy G. Venditti

Abstract Bedrock rivers often alternate between relatively wide unconstrained reaches and conspicuously narrow deep incised bedrock reaches (canyons). These bedrock canyons exhibit a constriction‐pool‐widening (CPW) morphology that consists of a lateral constriction, a deeply scoured pool formed downstream of the constriction, and a channel widening at or near the pool exit. To explore how CPWs are formed in bedrock canyons, we hypothesize that the lateral constriction at the canyon entrance forces a CPW to form allogenically with subsequent CPWs propagating further downstream. Our hypothesis was tested experimentally in a flume channel with a forced lateral constriction at the canyon entrance. Our experiment shows that the forced constriction can cause a primary CPW to form allogenically because the backwater upstream of the forced constriction causes sediment deposition that creates an elevation drop, promoting flow and sediment to plunge toward the bed and carve a primary pool. Channel widening occurs at the primary pool exit because sediment deposit forms that deflects sediment into the banks, causing lateral erosion. Downstream of the primary widening, channel width declines and a new lateral constriction forms, which causes the formation of pools and widening downstream, resulting in downstream CPW propagation. In our experiment, the bedrock channel evolved until a persistent alluvial cover formed, reaching a steady state morphology without further vertical erosion until perturbed by higher discharge. Our experiment shows that discharge variation is necessary for a channel to evolve in the absence of uplift.

Morphology

Bedrock

Constriction

10.1029/2024jf007808

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Citations (1)

The Impact of Intermittency on Bed Load Sediment Transport

Geophysical Research Letters (2022)

Santiago Benavides Eric Deal Matthew Rushlow Jeremy G. Venditti Qiong Zhang

Abstract Sediment transport by wind or water near the threshold of grain motion is dominated by rare transport events. This intermittency makes it difficult to calibrate sediment transport laws, or to define an unambiguous threshold for grain entrainment, both of which are crucial for predicting sediment transport rates. We present a model that captures this intermittency and shows that the noisy statistics of sediment transport contain useful information about the sediment entrainment threshold and the variations in driving fluid stress. Using a combination of laboratory experiments and analytical results, we measure the threshold for grain entrainment in a novel way and introduce a new property, the “shear stress variability”, which predicts conditions under which transport will be intermittent. Our work suggests strategies for improving measurements and predictions of sediment flux and hints that the sediment transport law may change close to the threshold of motion.

Intermittency

Entrainment (biomusicology)

10.1029/2021gl096088

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Citations (17)