skip to main content

Title: History‐Dependent Threshold for Motion Revealed by Continuous Bedload Transport Measurements in a Steep Mountain Stream

To explore the causes of history‐dependent sediment transport in rivers, we use a 19‐year record of coarse sediment transport from a steep channel in Switzerland. We observe a strong dependence of the threshold for sediment motion (τc) on the magnitude of previous flows for prior shear stresses ranging from 104 to 340 Pa, resulting in seasonally increasingτcfor 10 of 19 years. This stabilization occurs with and without measureable bedload transport, suggesting that small‐scale riverbed rearrangement increasesτc. Following large transport events (>340 Pa), this history dependence is disrupted. Bedload tracers suggest that significant reorganization of the bed erases memory of previous flows. We suggest that the magnitude of past flows controls the organization of the bed, which then modifiesτc, paralleling the evolution of granular media under shear. Our results support the use of a state function to better predict variability in bedload sediment transport rates.

more » « less
Author(s) / Creator(s):
 ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Page Range / eLocation ID:
p. 2583-2591
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ*c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion,p) and intergranular friction (ϕf). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes inpandϕfcan induce large changes inτ*cwithout needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values ofτ*ccan be partly explained by differences inpbetween rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation ofτ*cfrom an assumed constant value. Given that bed structure is known to vary with applied shear stresses and upstream sediment supply, we conclude that a constantτ*cis unlikely. Values ofτ*care not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.

    more » « less
  2. Abstract

    Stream hydromorphology regulates in‐stream water flow and interstitial flow of water within streambed sediments, the latter known as hyporheic exchange. Whereas hyporheic flow has been studied in sand‐bedded streams with ripples and dunes and in gravel‐bedded streams with pool‐riffle morphology, little is known about its characteristics in plane bed morphology with subdued streambed undulations and sparse macroroughness elements such as boulders and cobbles. Here, we present a proof‐of‐concept investigation on the role of boulder‐induced morphological changes on hyporheic flows based on coupling large‐scale flume sediment transport experiments with computational fluid dynamics. Our results show that placement of boulders on plane beds increase the reach‐scale hyporheic median residence time,τ50, by 15% and downwelling flux,qd, by 18% from the plane bed. However, reach‐scale hyporheic exchange changes are stronger withτ50decreasing by 20% andqdincreasing by 79% once the streambed morphology reached equilibrium (with the imposed upstream sediment and flow inputs on boulders). These results suggest that hyporheic flow is sensitive to the geomorphic response from bed topography and sediment transport in gravel‐bedded streams, a process that has been overlooked in previous work.

    more » « less
  3. Abstract

    Vegetation provides habitat and nature‐based solutions to coastal flooding and erosion, drawing significant interest in its restoration, which requires an understanding of sediment transport and retention. Laboratory experiments examined the influence of stem diameter and arrangement on bedload sediment transport by considering arrays of different stem diameter and mixed diameters. Bedload transport rate was observed to depend on turbulent kinetic energy, with no dependence on stem diameter, which was shown to be consistent with the impulse model for sediment entrainment. Existing predictors of bedload transport for bare beds, based on bed shear stress, were recast in terms of turbulence. The new turbulence‐based model predicted sediment transport measured in model canopies across a range of conditions drawn from several previous studies. A prediction of turbulence based on biomass and velocity was also described, providing an important step toward predicting turbulence and bedload transport in canopies of real vegetation morphology.

    more » « less
  4. Abstract. Despite a rich history of studies investigating fluid dynamics over bedforms and dunes in rivers, the spatiotemporal patterns of sub-bedform bedload transport remain poorly understood. Previous experiments assessing the effects of flow separation on downstream fluid turbulent structures and bedload transport suggest that localized, intermittent, high-magnitude transport events (i.e., permeable splat events) play an important role in both downstream and cross-stream bedload transport near flow reattachment. Here, we report results from flume experiments that assess the combined effects of flow separation–reattachment and flow re-acceleration over fixed two-dimensional bedforms (1.7 cm high; 30 cm long). A high-speed camera observed bedload transport along the entirety of the bedform at 250 frames per second. Grain trajectories, grain velocities, and grain transport directions were acquired from bedload images using semiautomated particle-tracking techniques. Downstream and vertical fluid velocities were measured 3 mm above the bed using laser Doppler velocimetry (LDV) at 15 distances along the bedform profile. Mean downstream fluid velocity increases nonlinearly with increasing distance along the bedform. However, observed bedload transport increases linearly with increasing distance along the bedform, except at the crest of the bedform, where both mean downstream fluid velocity and bedload transport decrease substantially. Bedload transport time series and manual particle-tracking data show a zone of high-magnitude, cross-stream transport near flow reattachment, suggesting that permeable splat events play an essential role in the region downstream of flow reattachment.

    more » « less
  5. Abstract

    Turbidity current and coastal storm deposits are commonly characterized by a basal sandy massive (structureless) unit overlying an erosional surface and underlying a parallel or cross‐laminated unit. Similar sequences have been recently identified in fluvial settings as well. Notwithstanding field, laboratory and numerical studies, the mechanisms for emplacement of these massive basal units are still under debate. It is well accepted that the sequence considered here can be deposited by waning‐energy flows, and that the parallel‐laminated units are deposited under transport conditions corresponding to upper plane bed at the dune–antidune transition. Thus, transport conditions that are more intense than those at the dune–antidune transition should deposit massive units. This study presents experimental, open‐channel flow results showing that sandy massive units can be the result of gradual deposition from a thick bedload layer of colliding grains called sheet flow layer. When this layer forms with relatively coarse sand, the non‐dimensional bed shear stress associated with skin friction, the Shields number, is larger than a threshold value approximately equal to 0·4. For values of the Shields number smaller than 0·4 the sheet flow layer disappeared, sediment was transported by a standard bedload layer one or two grain diameters thick, and the bed configuration was characterized by downstream migrating antidunes and washed out dunes. Parallel laminae were found in deposits emplaced with standard bedload transport demonstrating that the same dilute flow can gradually deposit the basal and the parallel‐laminated unit in presence of traction at the depositional boundary. Further, the experiments suggested that two different types of upper plane bed conditions can be defined, one associated with standard bedload transport at the dune–antidune transition, and the other associated with bedload transport in sheet flow mode at the transition between upstream and downstream migrating antidunes.

    more » « less