Jerolmack, Douglas J

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Now showing 1 - 10 of 38
  • Publication
    The Pulse of Calm Fan Deltas
    (2008-07-01) Kim, Wonsuck; Jerolmack, Douglas J
    At the heart of interpreting the history of Earth surface evolution preserved in the rock record is distinguishing environmental (allogenic) forcing from internally generated (autogenic) “noise.” Allogenic deposits classically have been recognized by their cyclic nature, which apparently results from periodic changes in base level, sediment supply, or tectonics. Autogenic deposits, which are quite variable in their origin and scale, are caused by the nonlinearity of sediment transport and might be expected to have a random or scale-free (fractal) signature. Here we describe a robust mechanism that generates cyclic deposits by an autogenic process in experimental fan deltas. Sheet flow over the fan surface induces deposition and an increase in fluvial slope and curvature to a point where the surface geometry is susceptible to a channelization instability, similar to channel initiation on hillslopes. Channelized flow results in incision and degrading of the fan surface to a lower slope, releasing a pulse of sediment that pushes the shoreline forward. Sheet flow resumes once the surface is regraded, and the cycle repeats in a surprisingly periodic fashion to produce cyclic foreset accretions. We use simple scaling and a one-dimensional fan evolution model to (1) demonstrate how time-varying flow width can cause pulses in sediment discharge at the shoreline in agreement with experiments and (2) reinterpret cyclic deposits reported in the field. Alternating sheet and channelized flows are known to operate on noncohesive fans in nature. Our results suggest that rather than reflecting variation in environmental forcing, many observed cyclic sedimentation packages may be a signature of the autogenic “pulse” of fan deltas under calm environmental conditions.
  • Publication
    Rheology of Sediment Transported by a Laminar Flow
    (2016-12-19) Houssais, Morgane; Ortiz, Carlos P; Durian, Douglas J; Jerolmack, Douglas J
    Understanding the dynamics of fluid-driven sediment transport remains challenging, as it occurs at the interface between a granular material and a fluid flow. Boyer, Guazzelli, and Pouliquen [Phys. Rev. Lett.107, 188301 (2011)] proposed a local rheology unifying dense dry-granular and viscous-suspension flows, but it has been validated only for neutrally buoyant particles in a confined and homogeneous system. Here we generalize the Boyer, Guazzelli, and Pouliquen model to account for the weight of a particle by addition of a pressure P0 and test the ability of this model to describe sediment transport in an idealized laboratory river. We subject a bed of settling plastic particles to a laminar-shear flow from above, and use refractive-index-matching to track particles' motion and determine local rheology—from the fluid-granular interface to deep in the granular bed. Data from all experiments collapse onto a single curve of friction μ as a function of the viscous number Iv over the range 3 × 10−5≤ Iv ≤ 2, validating the local rheology model. For Iv < 3 × 10−5, however, data do not collapse. Instead of undergoing a jamming transition with μ → μs as expected, particles transition to a creeping regime where we observe a continuous decay of the friction coefficient μ ≤ μs as Iv decreases. The rheology of this creep regime cannot be described by the local model, and more work is needed to determine whether a nonlocal rheology model can be modified to account for our findings.
  • Publication
    How River Rocks Round: Resolving the Shape-Size Paradox
    (2014-02-12) Domokos, Gabor; Jerolmack, Douglas J; Sipos, Andras Á; Török, Ákos
    River-bed sediments display two universal downstream trends: fining, in which particle size decreases; and rounding, where pebble shapes evolve toward ellipsoids. Rounding is known to result from transport-induced abrasion; however many researchers argue that the contribution of abrasion to downstream fining is negligible. This presents a paradox: downstream shape change indicates substantial abrasion, while size change apparently rules it out. Here we use laboratory experiments and numerical modeling to show quantitatively that pebble abrasion is a curvature-driven flow problem. As a consequence, abrasion occurs in two well-separated phases: first, pebble edges rapidly round without any change in axis dimensions until the shape becomes entirely convex; and second, axis dimensions are then slowly reduced while the particle remains convex. Explicit study of pebble shape evolution helps resolve the shape-size paradox by reconciling discrepancies between laboratory and field studies, and enhances our ability to decipher the transport history of a river rock.
  • Publication
    Hydrodynamic and Suspended Sediment Transport Controls on River Mouth Morphology
    (2014-01-01) Falcini, Federico; Piliouras, Anastasia; Garra, Roberto; Guerin, Adrien; Jerolmack, Douglas J; Rowland, Joel; Paola, Chris
    River mouths building into standing bodies of water have strikingly varied growth habits. This presents a compelling pattern formation problem that is also of great practical relevance for subsurface prediction and managing coastal wetlands. Here we present a generalized 2.5-dimensional potential vorticity (PV) theory that explains sedimentation patterns of a sediment-laden stationary jet by coupling an understanding of vorticity with suspended sediment concentration fields. We explore the physical meaning of this new sediment-PV definition, and its impact on outflow depositional patterns, by analyzing data from a shallow wall-bounded plane jet experiment and by discussing new theoretical insights. A key result is that lateral advection and diffusion of suspended sediment are directly proportional to jet vorticity, a feature that reveals the mechanistic process that forms elongated channels by focused levee deposition. The new PV theory constitutes a more generalized mathematical framework that expands the Rouse theory for the equilibrium of suspended sediment.
  • Publication
    Interactions Between Bed Forms: Topography, Turbulence, and Transport
    (2006-06-01) Jerolmack, Douglas J; Mohrig, David
    Results are presented examining the interaction between two sandy bed forms under low–sediment transport conditions in a small laboratory flume. The initial artificially made bed forms were out of equilibrium with the flow field. Temporal evolution of bed forms was monitored using time-lapse photography in order to characterize bed form adjustment to the imposed flow. Velocity measurements were collected using an acoustic Doppler velocimeter to characterize both mean flow and turbulence associated with different bed form geometries. Sandy bed forms all had identical initial geometries; however, the initial distance between bed form crests was varied between experiments. Overall deformation of the bed varied as a function of initial bed form spacing; however, bed forms evolved unpredictably as periods of relatively slow change were punctuated by periods of rapidly changing geometry. Subtle changes in bed form trough geometry were found to have a strong influence on turbulence and therefore sediment transport. Comparison with field studies suggests that the mechanisms described herein are active in natural systems.
  • Publication
    Sedimentary Bed Evolution as a Mean-Reverting Random Walk: Implications for Tracer Statistics
    (2014-09-16) Purohit, Prashant K; Martin, Raleigh L; Jerolmack, Douglas J
    Sediment tracers are increasingly employed to estimate bed load transport and landscape evolution rates. Tracer trajectories are dominated by periods of immobility (“waiting times”) as they are buried and reexcavated in the stochastically evolving river bed. Here we model bed evolution as a random walk with mean-reverting tendency (Ornstein-Uhlenbeck process) originating from the restoring effect of erosion and deposition. The Ornstein-Uhlenbeck model contains two parameters, a and b, related to the particle feed rate and range of bed elevation fluctuations, respectively. Observations of bed evolution in flume experiments agree with model predictions; in particular, the model reproduces the asymptotic t−1 tail in the tracer waiting time exceedance probability distribution. This waiting time distribution is similar to that inferred for tracers in natural gravel streams and avalanching rice piles, indicating applicability of the Ornstein-Uhlenbeck mean-reverting model to many disordered transport systems with tracer burial and excavation.
  • Publication
    Shredding of Environmental Signals by Sediment Transport
    (2010-10-01) Jerolmack, Douglas J; Paola, Chris
    Landscapes respond to climate, tectonic motions and sea level, but this response is mediated by sediment transport. Understanding transmission of environmental signals is crucial for predicting landscape response to climate change, and interpreting paleo-climate and tectonics from stratigraphy. Here we propose that sediment transport can act as a nonlinear filter that completely destroys (“shreds”) environmental signals. This results from ubiquitous thresholds in sediment transport systems; e.g., landsliding, bed load transport, and river avulsion. This “morphodynamic turbulence” is analogous to turbulence in fluid flows, where energy injected at one frequency is smeared across a range of scales. We show with a numerical model that external signals are shredded when their time and amplitude scales fall within the ranges of morphodynamic turbulence. As signal frequency increases, signal preservation becomes the exception rather than the rule, suggesting a critical re-examination of purported sedimentary signals of external forcing.
  • Publication
    Barchan-Parabolic Dune Pattern Transition From Vegetation Stability Threshold
    (2010-10-01) Reitz, Meredith D; Jerolmack, Douglas J; Ewing, Ryan C; Martin, Raleigh L
    Many dune fields exhibit a downwind transition from forward-pointing barchan dunes to stabilized, backward-pointing parabolic dunes, accompanied by an increase in vegetation. A recent model predicts this pattern transition occurs when dune surface erosion/deposition rates decrease below a threshold of half the vegetation growth rate. We provide a direct test using a unique data set of repeat topographic surveys across White Sands Dune Field and find strong quantitative support for the model threshold. We also show the threshold hypothesis applied to a barchan dune results naturally in its curvature inversion, as the point of threshold crossing progresses from the horns to the crest. This simple, general threshold framework can be an extremely useful tool for predicting the response of dune landscapes to changes in wind speed, sediment supply, or vegetation growth rate. Near the threshold, a small environmental change could result in a drastic change in dune pattern and activity.
  • Publication
    Diffusive Evolution of Experimental Braided Rivers
    (2014-05-01) Reitz, Meredith D; Jerolmack, Douglas J; Lajeunesse, Eric; Limare, Angela; Devauchelle, Olivier; Métivier, François
    Water flowing over a loose granular bed organizes into a braided river, a network of ephemeral and interacting channels. The temporal and spatial evolution of this network of braided channels is not yet quantitatively understood. In ∼1 m-scale experiments, we found that individual channels exhibit a self-similar geometry and near-threshold transport conditions. Measurements of the rate of growth of topographic correlation length scales, the time scale of system-slope establishment, and the random spatial decorrelation of channel locations indicate together that the evolution of the braided river system may be diffusive in nature. This diffusion is due to the separation of scales between channel formation and network evolution, and the random motion of interacting channels when viewed at a coarse-grained scale.
  • Publication
    Spatial Grain Size Sorting in Eolian Ripples and Estimation of Wind Conditions on Planetary Surfaces: Application to Meridiani Planum, Mars
    (2006-12-01) Jerolmack, Douglas J; Mohrig, David; Grotzinger, John P; Fike, David A; Watters, Wesley A
    The landscape seen by the Mars Exploration Rover (MER) Opportunity at Meridiani Planum is dominated by eolian (wind-blown) ripples with concentrated surface lags of hematitic spherules and fragments. These ripples exhibit profound spatial grain size sorting, with well-sorted coarse-grained crests and poorly sorted, generally finer-grained troughs. These ripples were the most common bed form encountered by Opportunity in its traverse from Eagle Crater to Endurance Crater. Field measurements from White Sands National Monument, New Mexico, show that such coarse-grained ripples form by the different transport modes of coarse- and fine-grain fractions. On the basis of our field study, and simple theoretical and experimental considerations, we show how surface deposits of coarse-grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces. Activation of Meridiani Planum coarse-grained ripples requires a wind velocity of 70 m/s (at a reference elevation of 1 m above the bed). From images by the Mars Orbiter Camera (MOC) of reversing dust streaks, we estimate that modern surface winds reach a velocity of at least 40 m/s and hence may occasionally activate these ripples. The presence of hematite at Meridiani Planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments; however, the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site.