Journal of Geophysical Research: Earth Surface最新文献

River fluxes to the Arctic Ocean impact sea ice extent, nutrient availability, and coastal ecosystems. Arctic river deltas modulate fluxes of water, sediment, and nutrients reaching the Arctic Ocean. Many large rivers have estimates or measurements of discharge and sediment concentration upstream of the delta apex, but the magnitude, timing, and spatial distribution of sediment fluxes to the Arctic coast are unknown. We developed a novel reduced-complexity model of suspended sediment transport in Arctic deltas to address this knowledge gap. The model estimates suspended sediment delivery to the coast based on a computed channel network and sediment transport rules. We applied this model to six high-latitude deltas during their open water seasons with different boundary conditions to account for their differences in morphology, seasonality, and hydrology. Flux distributions at the coast are found to be more uneven in larger deltas due to uneven channel spacing and larger variability in channel widths compared with smaller deltas. Given typical active season conditions, the deltas exhibit periods of deposition and erosion but are net depositional overall. Net sediment trapping during the active season ranges from 10% to 70%. Our results suggest that larger, more complex deltas with higher sediment supply and less flashy hydrographs store the most sediment and may therefore be more resilient to land loss. The sediment flux distribution can be used in future studies of coastal biogeochemistry and geomorphology and in regional models to capture the impacts of fluxes on turbidity, marine primary productivity, and Arctic warming.

In this work, we propose a comprehensive two-layer depth-averaged model to study the dynamic behavior of grain-fluid mixtures, which considers the granular dilatancy effects and the different frictional rheologies of grains in different states. Unlike single-phase flows, not only the interaction between granular and fluid phases significantly influence the dynamics of mixtures, but also the phase separation, so that different flow regimes can occur. These include five different possible regimes: two-layer regimes of (a) under-saturated mixture and (b) over-saturated mixture as well as single-layer regimes of (c) saturated mixture, (d) pure grains and (e) pure fluid. Most depth-averaged models in previous studies have considered only one of these flow regimes. The present model is an improved and integrated version of these depth-averaged models. Taking into account that the pure grains and pure fluid in the upper layer, which occur in the regimes of the under-saturated and over-saturated grain-fluid mixtures, respectively, exhibit different flow features than in the lower layer of the saturated mixture, we use a two-phase two-layer depth-averaged model to describe these regimes. This proposed model is possibly the first to employ a two-layer structure to describe all possible different flow regimes simultaneously. The proposed model is then solved numerically using a high-resolution central-upwind scheme and shows its ability to handle different flow regimes. To demonstrate the robustness of the numerical implementation and to evaluate the performance of the model, the numerical results are compared with several experiments reported in the literature, showing a certain qualitative agreement.

Recovering the patterns of glacial erosion over time is key to understanding feedbacks between climate and tectonic processes. Glacial erosion rates have been shown to systematically increase worldwide toward the present since the late Cenozoic, a behavior interpreted as the response of glaciers to a cooling and increasingly variable climate. However, the validity of this signal has been questioned, and suggested to be affected by the incompleteness of the sedimentary record, which can introduce a time dependent bias in the time averaged rates. In this study, we present new glacial erosion rates estimated from sediment accumulations in Lago Argentino, Patagonia, a proglacial basin with a nearly complete preserved sedimentary record. The erosion rates are estimated through the past 20,000 years and averaged over time intervals ranging from sub-decadal to millennial, allowing us to explore erosion rate variability through time and within a glacial cycle. The data show that erosion rates have varied substantially, from 0.43 $��\pm �$ 0.12 to 82.38 $��\pm �$ 17.58 mm/yr, with no systematic increase (or decrease) through time. Rather, erosion occurs during discrete, intense events separated by times of quiescence. In addition, we find that glacial erosion rates have comparable magnitudes when averaged over similar time intervals. Our data show a power-law increase in glacial erosion rates with decreasing averaging time interval, consistent with other observations globally. Given our observed intermittent character of glacial erosion, we attribute this increase to a time averaging bias, rather than to an escalation in magnitude of erosional pulses toward the present.

Vegetation plays a crucial role in coastal dune building. Species-specific plant characteristics can modulate sediment transport and dune shape, but this factor is absent in most dune building numerical models. Here, we develop a new approach to implement species-specific vegetation characteristics into a process-based aeolian sediment transport model. Using a three-step approach, we incorporated the morphological differences of three dune grass species dominant in the US Pacific Northwest coast (European beachgrass Ammophila arenaria, American beachgrass A. breviligulata, and American dune grass Leymus mollis) into the model AeoLiS. First, we projected the tiller frontal area of each grass species onto a high resolution grid and then re-scaled the grid to account for the associated vegetation cover for each species. Next, we calibrated the bed shear stress in the numerical model to replicate the actual sand capture efficiency of each species, as measured in a previously published wind tunnel experiment. Simulations were then performed to model sand bedform development within the grass canopies with the same shoot densities for all species and with more realistic average field densities. The species-specific model shows a significant improvement over the standard model by (a) accurately simulating the sand capture efficiency from the wind tunnel experiment for the grass species and (b) simulating bedform morphology representative of each species' characteristic bedform morphology using realistic field vegetation density. This novel approach to dune modeling will improve spatial and temporal predictions of dune morphologic development and coastal vulnerability under local vegetation conditions and variations in sand delivery.

Debris flows are solid-liquid mixtures originating in the upper part of mountain basins and routing downstream along incised channels. When the channel incises an open fan, the debris flow leaves the active channel and propagates downstream along a new pathway. This phenomenon is called an avulsion. We retrieve the most probable avulsion pathways leveraging a Monte Carlo approach based on using Digital Elevation Models (DEMs). Starting from LiDAR-based DEMs, we build an ensemble of synthetic DEMs using a local Gaussian probability density function of local elevation values and obtain an ensemble of drainage networks using a gravity-driven routing algorithm. The ensemble of drainage networks was used to obtain the most probable pathways of avulsions. We applied our methodology to a real monitored fan in the Dolomites (Northeastern Italian Alps) subjected to debris-flow activity with avulsions. Our approach allows us to verify the consistency between the occurrence probability of a synthetic pathway and those that historically occurred. Furthermore, our approach can be used to predict future debris-flow avulsions, assuming relevance in debris-flow risk assessment and planning of debris-flow control works.

Gravel bed rivers often display pool-riffle morphology. Downstream changes in channel width are often correlated with pool-riffle topography. Considering self-formed, alluvial, straight and single-thread gravel-bed rivers, here we provide an analytical solution for their equilibrium pool-riffle morphology in the presence of spatially varying widths. Our analytical model, based on mass and energy conservation for water flow, shows how bed level perturbations are closely linked to downstream width variations under the influence of water discharge. The analytical model is validated against experimental data and applied to reconstruct the natural pool-riffle bed profile. The validated model shows how increasing water discharge (i.e., water depth and flow velocity) enhances the amplitude of pool-riffle sequences. The proposed model can provide valuable guidance for river restoration projects.

Post-fire debris flows alter impacted fluvial systems, but few studies quantify the magnitude and timing of reach-scale channel response to these events. In August 2020, the Big Creek watershed along California's central coast burned in the Dolan Fire; in January 2021, an atmospheric river event triggered post-fire debris flows in steep tributaries to the Big Creek. Here, we characterize the evolution of fluvial morphology and grain size in Big Creek, a cascade and step-pool channel downstream of tributaries in which post-fire debris flows initiated, using pre- and post-fire structure from motion (SfM) and airborne lidar surveys. We also make comparisons to Devil's Creek, an adjacent basin which burned but did not experience post-fire debris flows. We observe grain size fining following debris flows in Big Creek, but the coarsest 40% of the grain size distribution remained essentially unchanged despite reorganization of channel structure. Changes in grain size and elevated post-fire peak flows account for approximately equal portions of a substantial increase in modeled bedload transport capacity one year post-fire. In Big Creek, geomorphic recovery is well underway just two years post-fire. A valley-spanning log jam, which formed during debris flows, acts as a sediment trap upstream of our Big Creek study reach, and is partially responsible for accelerating recovery processes. In contrast, Devil's Creek exhibited little change in morphology or grain size despite elevated post-fire peak flows. This period of geomorphic dynamism following the Dolan Fire has complex ecological impacts, notably for the threatened anadromous salmonid spawning habitat in Big Creek.

Safe navigation in rivers often requires repeated dredging operations. River dunes are causing shallow areas, so a better understanding of dune behavior can aid decision-making on dredging strategies. A new dune-tracking method was developed to analyze dunes after dredging. This knowledge was combined with information on dredging methods and intensity. This enables the analysis of dune development after dredging and the assessment of the impact of dredging methods such as topping (removing sediment from the crest) and swiping (transferring sediment from the crest to the trough). The results show that dredging increases the growth rate of dunes, especially when the discharge increases after dredging. For the studied dredging intensities, topping has a more pronounced effect than swiping on the immediate reduction of dune height. The changes in dune heights due to dredging are within the natural dune height variability. In the long term, topped dunes exhibit larger growth rates than those undergoing swiping. So dredging does have an effect on dune dynamics, in particular on the dune height growth rates.

Waves, rivers, and tides shape delta morphology. Recent studies have enabled predictions of their relative influence on deltas globally, but methods and associated uncertainties remain poorly known. Here, we address that gap and show how to quantify delta morphology within the Galloway ternary diagram of river, wave, and tidal sediment fluxes. We assess delta morphology predictions compared to observations for 31 deltas globally and find a median error of 4% (standard deviation of 11%) in the river, tide, or wave-driven sediment fluxes. Relative uncertainties are greatest for mixed-process deltas (e.g., Sinu, error of 49%) and tend to decrease for end-member morphologies where either wave, tide, or river sediment fluxes dominate (e.g., Fly, error of 0.2%). Prediction uncertainties for delta morphologic metrics are more considerable: the delta shoreline protrusion angles set by wave influence have a median error of 45%, the delta channel widening from tides 25%, and the number of distributary channels 86%. Larger sources of prediction uncertainty are (a) delta morphology data, for example, delta slopes that modulate tidal fluxes, (b) data on river sediment flux distribution between individual delta river outlets, and (c) theoretical basis behind fluvial and tidal dominance. Broadly, these methods will help improve delta morphology predictions and assess how natural and anthropogenic forces affect morphologic change.

Creeping landslides may fail catastrophically, posing significant threats to infrastructure and lives. Landslides weaken over time through rock mass damage processes that may occur by steady-state creep or transient accelerations of slip, called creep bursts. Creep bursts may control landslide stability by inducing short-term damage and strain localization. This study focuses on the Åknes landslide in Norway, which moves up to 6 cm per year and could potentially trigger a large tsunami in the fjord below. An 11-year data set is compiled and analyzed, including kinematic, seismic, and hydrogeological data acquired at the landslide surface and in a series of boreholes. An annual average of two creep bursts with millimeter amplitude has been recorded within the shear zone in each borehole, accounting for approximately 11% of the total displacement. Creep bursts detected simultaneously in multiple boreholes are preceded by increased seismic activity and rising water pressure. However, most creep bursts are observed in only one or a few boreholes. These bursts often happen during seasonal high and low groundwater levels in autumn and spring, respectively, correlating with local peaks in water pressure. No such correlation is observed during summer. We propose that creep bursts can have different causes and hypothesize that rock degradation leads to some creep bursts independent of water pressure variations. In contrast, the largest creep bursts are correlated with variations in absolute water pressure or gradients of water pressure within the shear zone. Our findings emphasize the complexity of a dense data set requiring multiple mechanisms to explain creep burst dynamics.