Jerolmack, Douglas J
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Publication The Pulse of Calm Fan Deltas(2008-07-01) Kim, Wonsuck; Jerolmack, Douglas JAt 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 JUnderstanding 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 Dynamics of River Mouth Deposits(2015-09-01) Fagherazzi, Sergio; Edmonds, Douglas A; Nardin, William; Leonardi, Nicoletta; Canestrelli, Alberto; Falcini, Federico; Jerolmack, Douglas J; Mariotti, Giulio; Rowland, Joel C; Slingerland, Rudy LBars and subaqueous levees often form at river mouths due to high sediment availability. Once these deposits emerge and develop into islands, they become important elements of the coastal landscape, hosting rich ecosystems. Sea level rise and sediment starvation are jeopardizing these landforms, motivating a thorough analysis of the mechanisms responsible for their formation and evolution. Here we present recent studies on the dynamics of mouth bars and subaqueous levees. The review encompasses both hydrodynamic and morphological results. We first analyze the hydrodynamics of the water jet exiting a river mouth. We then show how this dynamics coupled to sediment transport leads to the formation of mouth bars and levees. Specifically, we discuss the role of sediment eddy diffusivity and potential vorticity on sediment redistribution and related deposits. The effect of waves, tides, sediment characteristics, and vegetation on river mouth deposits is included in our analysis, thus accounting for the inherent complexity of the coastal environment where these landforms are common. Based on the results presented herein, we discuss in detail how river mouth deposits can be used to build new land or restore deltaic shorelines threatened by erosion.Publication Robotic Measurement of Aeolian Processes(2015-01-01) Jerolmack, Douglas J; Roberts, Sonia; Reverdy, Paul B; Lancaster, Nick; Nikolich, George; Shipley, Thomas F; Koditschek, Daniel E.; van Pelt, Scott; Zobeck, TedMeasurements used to study wind shear stress and turbulence, surface roughness, sand flux, and dust emissions are typically obtained from stationary instrumentation, and are thus limited spatially. They are also dependent on deployment of instrumentation for specific events and thus the are limited temporally. We have been adapting a rough-terrain legged robot capable of rapidly traversing desert terrain to serve as a semi-autonomous, reactive mobile sensory platform (RHex [1]), which would not share these limitations. We report on early trials of the robotic platform at the Jornada LTER and White Sands National Monument to test the feasibility of gathering measurements of airflow and rates of particle transport on a dune, assessing the role of roughness elements such as vegetation in modifying the wind shear stresses incident on the surface, and estimating erosion susceptibility in an arid soil. The robot not only serves as a mobile platform for science instruments; it can also perform controlled “kick tests” to locally examine soil strength. We outline a strategy for mapping soil erodibility and its controlling parameters using the unique capabilities of RHex, and the implications for understanding erosion and dust emission from complex terrain.Publication Impulse Framework for Unsteady Flows Reveals Superdiffusive Bed Load Transport(2013-04-16) Phillips, Colin B; Martin, Raleigh L; Jerolmack, Douglas JSediment transport is an intrinsically stochastic process, and measurement of bed load in the environment is further complicated by the unsteady nature of river flooding. Here we present a methodology for analyzing sediment tracer data with unsteady forcing. We define a dimensionless impulse by integrating the cumulative excess shear velocity for the duration of measurement, normalized by grain size. We analyze the dispersion of a plume of cobble tracers in a very flashy stream over two years. The mean and variance of transport distance collapse onto well-defined linear and power-law relations, respectively, when plotted against cumulative dimensionless impulse. Data suggest that the asymptotic limit of bed load tracer dispersion is superdiffusive, in line with a broad class of geophysical flows exhibiting strong directional asymmetry (advection), thin-tailed step lengths and heavy-tailed waiting times. The impulse framework justifies the use of quasi-steady flow approximations for long-term river evolution modeling.Publication Hydrogeomorphology of the Hyporheic Zone: Stream Solute and Fine Particle Interactions With a Dynamic Streambed(2012-12-01) Harvey, Judson W; Drummond, Jennifer D; Martin, Raleigh L; McPhillips, Lauren E; Packman, Aaron I; Jerolmack, Douglas J; Stonedahl, Susa H; Aubeneau, Antoine F; Sawyer, Audrey H; Larsen, Laurel G; Tobias, Craig RHyporheic flow in streams has typically been studied separately from geomorphic processes. We investigated interactions between bed mobility and dynamic hyporheic storage of solutes and fine particles in a sand-bed stream before, during, and after a flood. A conservatively transported solute tracer (bromide) and a fine particles tracer (5 μm latex particles), a surrogate for fine particulate organic matter, were co-injected during base flow. The tracers were differentially stored, with fine particles penetrating more shallowly in hyporheic flow and retained more efficiently due to the high rate of particle filtration in bed sediment compared to solute. Tracer injections lasted 3.5 h after which we released a small flood from an upstream dam one hour later. Due to shallower storage in the bed, fine particles were rapidly entrained during the rising limb of the flood hydrograph. Rather than being flushed by the flood, we observed that solutes were stored longer due to expansion of hyporheic flow paths beneath the temporarily enlarged bedforms. Three important timescales determined the fate of solutes and fine particles: (1) flood duration, (2) relaxation time of flood-enlarged bedforms back to base flow dimensions, and (3) resulting adjustments and lag times of hyporheic flow. Recurrent transitions between these timescales explain why we observed a peak accumulation of natural particulate organic matter between 2 and 4 cm deep in the bed, i.e., below the scour layer of mobile bedforms but above the maximum depth of particle filtration in hyporheic flow paths. Thus, physical interactions between bed mobility and hyporheic transport influence how organic matter is stored in the bed and how long it is retained, which affects decomposition rate and metabolism of this southeastern Coastal Plain stream. In summary we found that dynamic interactions between hyporheic flow, bed mobility, and flow variation had strong but differential influences on base flow retention and flood mobilization of solutes and fine particulates. These hydrogeomorphic relationships have implications for microbial respiration of organic matter, carbon and nutrient cycling, and fate of contaminants in streams.Publication Quantifying the Significance of Abrasion and Selective Transport for Downstream Fluvial Grain Size Evolution(2014-11-01) Miller, Kimberly Litwin; Szabó, Tímea; Jerolmack, Douglas J; Domokos, GaborIt is well known that pebble diameter systematically decreases downstream in rivers. The contribution of abrasion is uncertain, in part because (1) diameter is insufficient to characterize pebble mass loss due to abrasion and (2) abrasion rates measured in laboratory experiments cannot be easily extrapolated to the field. A recent geometric theory describes abrasion as a curvature-dependent process that produces a two-phase evolution: in Phase I, initially blocky pebbles round to smooth, convex shapes with little reduction in axis dimensions; then, in Phase II, smooth, convex pebbles slowly reduce their axis dimensions. Here we provide strong evidence that two-phase abrasion occurs in a natural setting, by examining downstream evolution of shape and size of thousands of pebbles over ~10 km in a tropical montane stream. The geometric theory is verified in this river system using a variety of manual and image-based shape parameters, providing a generalizable method for quantifying the significance of abrasion. Phase I occurs over ~1 km, in upstream bedrock reaches where abrasion is dominant and sediment storage is limited. In downstream alluvial reaches, where Phase II occurs, we observe the expected exponential decline in pebble diameter. Using a discretized abrasion model (the so-called “box equations”) with deposition, we deduce that abrasion removes more than one third of the mass of a pebble but that size-selective sorting dominates downstream changes in pebble diameter. Overall, abrasion is the dominant process in the downstream diminution of pebble mass (but not diameter) in the studied river, with important implications for pebble mobility and the production of fine sediments.Publication Sorting Out Abrasion in a Gypsum Dune Field(2011-06-01) Jerolmack, Douglas J; Reitz, Meredith D; Martin, Raleigh LGrain size distributions in eolian settings are the result of both sorting and abrasion of grains by saltation. The two are tightly coupled because mobility of particles determines abrasion rate, while abrasion affects the mobility of particles by changing their mass and shape; few field studies have examined this quantitatively. We measured grain size and shape over a 9 km transect downwind of a line sediment source at White Sands National Monument, a gypsum dune field. The sediment source is composed of rodlike (elongate), coarse particles whose shapes appear to reflect the crystalline structure of gypsum. Dispersion in grain size decreases rapidly from the source. Coarse particles gradually become less elongate, while an enrichment of smaller, more elongate grains is observed along the transect. Transport calculations confirm that White Sands is a threshold sand sea in which (1) the predominant particle diameter reflects grains transported in saltation under the dune-forming wind velocity and (2) smaller, elongate grains move in suspension under this dominant wind. Size-selective transport explains first-order trends in grain size; however, abrasion changes the shape of saltating grains and produces elongate, smaller grains that are spallation and breaking products of larger particles. Both shape and size changes saturate 5–6 km downwind of the source. As large particles become more equant, abrasion rates slow down because protruding regions have been removed. Such asymptotic behavior of shape and abrasion rate has been observed in theory and experiment and is likely a generic result of the abrasion process in any environment.Publication Multiscale Bed Form Interactions and Their Implications for the Abruptness and Stability of the Downwind Dune Field Margin at White Sands, New Mexico, USA(2014-11-01) Pelletier, Jon D; Jerolmack, Douglas JThe downwind margin of White Sands dune field is an abrupt transition from mobile aeolian dunes to a dune-free vegetated surface. This margin is also relatively stable; over the past 60 years it has migrated several times more slowly than the slowest dunes within the dune field, resulting in a zone of dune coalescence, aggradation, and, along most of the margin, development of a dune complex (i.e., dunes superimposed on draas). Repeat terrestrial laser scanning surveys conducted over a 3 month period demonstrate that sediment fluxes within the dune complex decrease on approach to the margin. Computational fluid dynamics modeling indicates that this decrease is due, in part, to a decrease in mean turbulent bed shear stress on the lee side of the dune complex as a result of flow line divergence or sheltering of the lee-side dunes by the stoss side of the dune complex. Conservation of mass demands that this decrease in bed shear stress causes aggradation. We speculate that aggradation on the lee side of the dune complex further enhances the sheltering effect in a positive feedback, contributing to the growth and/or maintenance of the dune complex and a relatively abrupt and stable dune field margin. Our model and data add to a growing body of evidence that aeolian dune field patterns are influenced by feedbacks that occur at scales larger than individual dunes.Publication Real and Apparent Changes in Sediment Deposition Rates Through Time(2009-09-01) Schumer, Rina; Jerolmack, Douglas JField measurements show that estimated sediment deposition rate decreases as a power law function of the measurement interval. This apparent decrease in sediment deposition has been attributed to completeness of the sedimentary record; the effect arises because of incorporation of longer hiatuses in deposition as averaging time is increased. We demonstrate that a heavy-tailed distribution of periods of nondeposition (hiatuses) produces this phenomenon and that observed accumulation rate decreases as tγ−1, over multiple orders of magnitude, where 0 < γ ≤ 1 is the parameter describing the tail of the distribution of quiescent period length. By using continuous time random walks and limit theory, we can estimate the actual average deposition rate from observations of the surface location over time. If geologic and geometric constraints place an upper limit on the length of hiatuses, then average accumulation rates approach a constant value at very long times. Our model suggests an alternative explanation for the apparent increase in global sediment accumulation rates over the last 5 million years.