Journal of Geophysical Research: Earth Surface最新文献

Managing and living with geohazards is especially challenging in mountain landscapes and requires an understanding of catchment-scale sediment dynamics and internal system functioning. While sediment budgeting is a valuable framework, challenges remain including partitioning sediment yield by source and grain size and addressing scale issues. This study advances our understanding of bed material dynamics in glacierized mountain catchments by applying a range of complementary techniques to measure sediment transfers in the Fitzsimmons Creek watershed. First, we measured the historical bed material yield using field surveys and historical air photo analysis, revealing an average specific sediment yield of 210 Mg km⁻² yr⁻¹, that varied by a factor of 17 over the 76-year record. Hydro-meteorological and historical analyses suggest that gravel extraction had the largest impact over the past three decades, while an extreme landslide and flood event produced the highest recorded sediment yield. Second, we constructed a detailed sediment budget along the river system using high-resolution, multi-temporal lidar and geomorphic mapping data. Sediment source partitioning indicates that landslide, active channel, and floodplain sources each contributed about one-third of the total sediment supply. Net degradation occurred along the valley bottom upstream of the fan-delta, resulting in steadily increasing downstream sediment yield. This trend is punctuated by chronic landsliding near the outlet, driven by postglacial incision through glaciogenic sediments at a hanging valley step. Contemporary glacial and proglacial sources were not measured directly but surprisingly contributed minimally. These findings provide insight into the sediment dynamics of glacierized mountain catchments and their potential controls.

The survival of salt marshes, especially facing future sea-level rise, requires sediment supply. Sediment can be supplied to salt marshes via two routes: through marsh creeks and over marsh edges. However, the conditions of tides and waves that facilitate sediment import through these two routes remain unclear. To understand when and how sediment is imported into salt marshes, 2-month measurements were conducted to monitor tides, waves, and suspended sediment concentration (SSC) in Paulina Saltmarsh, a meso-macrotidal system. The results show that the marsh creek tends to import sediment during neap tides with waves. A tidal cycle with a small tidal range result in weaker flow in the marsh creek during ebb tides, reducing the export of sediment. Waves enhance sediment supply to the marsh creek by eroding mudflats. However, strong waves can directly resuspend sediment in marsh creeks during spring tides when the water level is above the marsh canopy, enhancing sediment export through creeks. Net sediment import over marsh edges requires the opposite tidal and wave conditions: spring tides with weak waves. Spring tides provide stronger hydrodynamics, facilitating sediment import over the marsh edge. Increased SSC during the ebb phase can occur with strong waves over the marsh edge, resulting in net sediment export. Therefore, the net import or export of sediment, through the creek and over the marsh edge, depends on the combination of tidal and wave conditions. These conditions can vary between estuaries and even individual marshes. Understanding these conditions is crucial for better management of salt marshes.

Shoreline position (e.g., beach width) is a critical component of flooding and overtopping forecasts but difficult to predict accurately. We model beach width changes with a supervised machine learning (ML) approach informed by equilibrium principles. The time history of wave energy anomalies that force equilibrium models is used as an ML input feature. The sweeping simplifying equilibrium model assumptions relating beach width change to anomalies are replaced with data-based ML results. Supervised learning regression methods including linear, support vector, decision trees, and ensemble regressors are tested. Observations for model training and testing includes weekly to quarterly beach elevation surveys spanning approximately 500 m alongshore and 8 years at two beaches, each supplemented with several months of ∼100 sub-weekly surveys. These beaches, with different sediment types (sand vs. sand-cobble mix), both widen in summer in response to the seasonal wave climate, in agreement with a generic equilibrium model. Differences in backshore erodability contribute to differing beach responses in the stormiest (El Niño) year that are reproduced by a simple extra trees regression model but not by the equilibrium model. With sufficiently extensive training data, the ML model outperforms equilibrium by providing flexibility and complexity in the response to wave forcing. The present ML and equilibrium models both fail to simulate a uniquely stunted beach recovery unlike other recoveries in the training data.

Volcanoes worldwide have undergone cyclic destruction of their edifices, generating catastrophic volcanic debris avalanches. Augustine Volcano in Alaska, USA, has a history of debris avalanches, causing cyclic destruction of the edifice and cascading hazards. These collapses, together with eruption-related changes in the edifice structure, change the slope stability hazard of the volcano over time. This study aims to develop a current view of the slope stability hazard at Augustine Volcano by (a) characterizing collapse-prone source areas on the edifice under various scenarios typical of dynamic volcanic environments and (b) identifying the controlling factors that underlie the slope stability hazard. Scenario-based slope stability assessment was conducted using a quasi-3D limit equilibrium method to test for the effect of various factors that drive or resist failure, including topography, shallow edifice structure, strength of edifice-forming materials, pore fluid pressure distribution, and local and regional seismicity. Results show that in all scenarios assessed, the slopes of Augustine Volcano are stable with a factor of safety (FOS) greater than 1. The FOS, however, decreases with decreasing strength of edifice-forming materials, pore fluid pressurization, and earthquake loading. The location of the relatively less stable slope, changes to the southwestern flank when accounting for subsurface heterogeneities derived from geophysical observations. Subsurface heterogeneity is thus a key underlying factor, along with steep topography, in controlling where collapse-prone source areas occur, and it should be accounted for in volcanic slope stability hazard assessments.

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.

Splash functions delineate the intricate interaction between wind-driven particles and the bed. However, due to limitations in measurement methods, achieving a comprehensive understanding of splash functions remains challenging. In this study, we utilized a high-speed system and particle trajectory tracking algorithm to reconstruct 857 collision events involving natural sand particles obtained from field samples and artificial glass microspheres interacting with a bed composed of analogous particles in a wind tunnel. During the experiments, the ratio of the wind shear velocity (u_*) to the impact threshold (u_*ti) consistently ranged between approximately 1.22 and 1.79. Our findings indicate that the impact angle remains independent of both impact velocity and particle size, maintaining an approximate value of 10.5 ± 6.5°. The evaluation of splash functions depends on the criterion used to define ejection, that is, the ratio of the centroid's height of the liftoff particles to their particle size (H/d). Additionally, at the same critical height (H/d = 1.5), our splash functions show differences of varying degrees from previous experiments and theoretical studies performed under no wind conditions. We believe that the main reason for these differences may be that the energy exchange between the bed surface and the airflow increases the looseness of the large-particle (>0.1 mm) bed surface. These findings hold significant implications for accurately modeling sand-bed collisions in natural environments.

Understanding particle fragmentation and its resulting particle-size distribution is essential for comprehending shear zone formation, structure, and frictional behavior in faults and landslides, particularly at high normal stresses. 3-D fractal dimension (D₃) is used as a measure of particle-size distribution, and for the potential self-similarity physics. Previous research suggests D₃ – 2.58 based on the “constrained comminution” model, or D3 = 3.00 considering large shear displacement. However, field data from rock avalanches reveal scattered D₃ that deviate from these predictions, possibly due to the neglection of the underlying fragmented physics, such as the particle-size-dependent fragmentation probability. Herein, we conducted rotary shear experiments to investigate the evolution of D₃ under varying normal stresses, velocities, and mineral compositions. Experimental results demonstrate that D₃ monotonically increases with shear displacement and converges to an ultimate value, significantly influenced by mineral composition but less affected by shear velocity and confining stress within the experimental conditions. A modified large-strain model that considered size-dependent grain-breakage probability was proposed, which may explain the observed divergence of D₃ from previous predictions. This model highlights the complex mechanisms involved in particle breakage within dense grain-flows, resulting in the high but scattered D₃ observed in natural shear zones. Furthermore, we recognize that additional mechanisms, such as abrasion and grinding, can contribute to the particle size reduction and influence the ultimate fractal dimension. This study provides valuable insights into the dynamics of particle fragmentation in shear zones and has implications for understanding various geological processes.