Journal of Geophysical Research: Earth Surface最新文献

Rockfall and rock avalanches are common in steep terrain on Earth and potentially on other planetary bodies such as the Moon and Mars. Since impacting rocks can damage exposed bedrock as they roll and bounce downhill, rockfall might be an important erosive agent in steep landscapes, even in the absence of water. We developed a new theory for rockfall-driven bedrock abrasion using the ballistic trajectories of rocks transported under gravity. We calibrated this theory using laboratory experiments of rockfall over an inclined bedrock simulant. Both the experiments and the model demonstrate that bedrock hillslopes can be abraded by dry rockfall, even at gradients below the angle of repose, depending on the bedrock roughness. Feedback between abrasion and topographic steering of rockfall can produce channel-like forms, such as bedrock chutes, in the absence of water. Particle size has a dominant influence on abrasion rates and runout distances, while the hillslope angle has a comparatively minor influence. Rockfall transport is sensitive to bedrock roughness; terrain with high friction angles can trap rocks creating patches of rock cover that affect subsequent rockfall pathways. Our results suggest that dry rockfall can play an important role in eroding and channelizing steep, rocky terrain on Earth and other planets, such as crater degradation on the Moon and Mars.

Deltaic channel networks are important conduits for water and material supplies to the fluvial and coastal communities. However, increasing human interventions in river deltas have altered the topology and geometry of channel networks as well as their long-term evolution. While the morphological evolution of a single channel has received extensive studies, the system-wide morphological responses of channel networks to local disturbances remain largely unclear. Here we investigate the morphological responses of a bifurcating channel network subject to local disturbance of channel deepening due to dredging and sand mining through idealized simulations, and further compare the results with the reference scenarios of a single channel and theoretical analysis of the phase plane. The results show that the infilling of the local deepening is associated with the erosion of the entire branch, which also causes system-wide effects on the siltation of the other branch. The morphological responses of the bifurcating channel network consist of a relatively short stage for the infilling of the local deepening followed by a relatively long stage for recovering the equilibrium configuration of the river bifurcation. The system-wide effects of the local disturbance arise from the altered water surface slope and water partitioning downstream of the bifurcation due to the local deepening. Also, the prolonged recovery of the equilibrium configuration is consistent with theoretical analysis, which reveals a slow evolution of the bifurcation when approaching the equilibrium. Our results can help understand the long-term morphological responses of large-scale complex channel networks and inform water managements under increasing human interventions.

Coastal marsh survival may be compromised by sea-level rise, limited sediment supply, and subsidence. Storms represent a fundamental forcing for sediment accumulation in starving marshes because they resuspend bottom material in channels and tidal flats and transport it to the marsh surface. However, it is unrealistic to simulate at high resolution all storms that occurred in the past decades to obtain reliable sediment accumulation rates. Similarly, it is difficult to cover all possible combinations of water levels and wind conditions in fictional scenarios. Thus, we developed a new method that derives long-term deposition rates from short-term deposition generated by a finite number of storms. Twelve storms with different intensity and frequency were selected in Terrebonne Bay, Louisiana, USA and simulated with the 2D Delft3D-FLOW model coupled with the Simulating Waves Nearshore (SWAN) module. Storm impact was analyzed in terms of geomorphic work, namely the product of deposition and frequency. To derive the long-term inorganic mass accumulation rates, the new method generates every possible combination of the 12 chosen storms and uses a linear model to fit modeled inorganic deposition with measured inorganic mass accumulation rates. The linear model with the best fit (highest R²) was used to derive a map of inorganic mass accumulation rates. Results show that a storm with 1.7 ± 1.6 years return period provides the largest geomorphic work, suggesting that the most impactful storms are those that balance intensity with frequency. Model results show higher accumulation rates in marshes facing open areas where waves can develop and resuspend sediments. This method has the advantage of considering only a few real scenarios and can be applied in any marsh-bay system.

Across varied environments, meandering channels evolve through a common morphodynamic feedback: the sinuous channel shape causes spatial variations in boundary shear stress, which cause lateral migration rates to vary along a meander bend and change the shape of the channel. This feedback is embedded in all conceptual models of meandering channel migration, and in numerical models, it occurs over an explicit timescale (i.e., the model time step). However, the sensitivity of modeled channel trajectory to the time step is unknown. In numerical experiments using a curvature-driven model of channel migration, we find that channel trajectories are consistent over time if the channel migrates ≤10% of the channel width over the feedback timescale. In contrast, channel trajectories diverge if the time step causes migration to exceed this threshold, due to the instability in the co-evolution of channel curvature and migration rate. The divergence of channel trajectories accumulates with the total run time. Application to hindcasting of channel migration for 10 natural rivers from the continental US and the Amazon River basin shows that the sensitivity of modeled channel trajectories to the time step is greatest at low (near-unity) channel sinuosity. A time step exceeding the criterion causes over-prediction of the width of the channel belt developed over millennial timescales. These findings establish a geometric constraint for predicting channel migration in landscape evolution models for lowland alluvial rivers, upland channels coupled to hillslopes and submarine channels shaped by turbidity currents, over timescales from years to millennia.

The Coast of Louisiana is affected by accelerating sea level rise compounded by land subsidence, leading to land loss. Vertical crustal motions in the region are caused by natural and anthropogenic processes that vary temporally and spatially across the Gulf of Mexico. We investigate the role of growth faulting contributions to subsidence in a case study of Baton Rouge, where two E-W striking, down-to-the-south normal faults, the Denham Springs and Baton Rouge faults, cut compacted Pleistocene strata, and where sediment compaction should be minimal. We used InSAR time series and LiDAR differencing data spanning 1999–2020 to quantify modern vertical and horizontal displacements. After calibration with GNSS data, both methods reveal similar spatial patterns in ground motion, with the faults delimiting areas with different absolute rates. On average the area north of the Baton Rouge fault is subsiding faster than the south, opposite to the long-term sense of fault slip. LiDAR mean vertical rates range between −5 to −11 mm/y and −2.4 to −7 mm/y. InSAR time-series mean rates in the LOS direction range between −10.9 to −13.6 mm/y and −8 to −10.6 mm/y, respectively, for the north and south areas. Subsidence in the northern area likely is controlled by groundwater level changes caused by pumping as indicated by groundwater extraction models. The southern area average is likely influenced by the injection of fluids. Our results suggest volumetric changes caused by fluid extraction and injection in regions separated by growth faults that are creeping to accommodate the spatial variations in subsidence.

The two-way interactions between biological and physical processes, bio-geomorphic feedback, play a vital role in landscape formation and evolution in salt marshes. Patchy vegetation represents a typical form of scale-dependent feedback in salt marshes and is primarily responsible for the formation of efficient drainage networks. The intuitive manifestation of scale-dependent feedback is the heterogeneity of flow and landscape. Process-based modeling is an essential tool for exploring flow heterogeneity, but calculations for small spatial scales and over long time frames can be prohibitively costly. In this study, we proposed a deep learning model architecture, UNet-Flow, based on convolutional neural networks (CNNs), which is used to build a surrogate model to simulate a flow field induced by salt marsh patchy vegetation. After optimizing and evaluating the model, we discovered that UNet-Flow exhibits a speed improvement of over four orders of magnitude compared to single-process simulations using the free surface flow model TELEMAC-2D, with acceptable levels of error. Furthermore, we proposed an approach that combines the process-based model SISYPHE with the deep learning method to model geomorphic heterogeneity. After numerous simulations of flow heterogeneity modeling using UNet-Flow, we obtained a significant logarithmic relationship between scale-dependent feedback strength and vegetation stem density, and an ascending-descending trend in feedback strength was observed as the number or surface area of vegetation patches increased. Finally, we investigated the relationship between geomorphic heterogeneity and vegetation-related variables. This study represents a noteworthy effort to study bio-geomorphology using deep learning methods.

Glacial debris flows occurring on the Tibetan Plateau consistently result in significant property damage and loss of human life. A comprehensive field investigation was conducted in Tianmo valley along the Sichuan-Tibet Highway to reveal the dynamics of a debris flow that occurred on 11 July 2018. Furthermore, a depth-averaged multiphase debris flow model was proposed and employed to reconstruct the characteristics of the debris flow. The model derivation, implementation, evaluation, and application were presented to demonstrate its performance. The Voellmy model was chosen because it adequately accounts for both basal frictional effects and the entrainment phenomenon. The entrainment processes, the ice melting, and the lubrication effect, were also taken into consideration. Based on the numerical results combined with field investigation data, the kinetic characteristics of the glacial debris flow were analyzed. The Tianmo valley has a small area, but the volume and erosion rate of debris flows were much larger than that of two-phase debris flows in the same location due to ice melting. The simulation results demonstrated that the glacial debris reached a peak velocity of 20 m/s. Additionally, the volume of the debris flow increased by 50% due to the erosion over a short runout distance of approximately 4,000 m. This increase was a result of the high velocity and abundant entrainment sources on the slope. This study aims to improve understanding of the high velocity and destructive potential of debris flows in the Tianmo valley.

In this study, beach cusp characteristics were explored using 15 months of 3D lidar scans collected hourly at the Field Research Facility in Duck, NC. Fourier analyses were performed on lidar-derived beach elevation contours to generate spatial cusp spectra. Active cusp events were identified on the basis of the location and magnitude of each spectrum's peak and used to evaluate conditions during cusp formation and evolution. Cusps primarily developed during times with normally-incident, long period, low energy wave conditions with low frequency spread, and reflective beach conditions. The upper and lower beaches often exhibited different behavior and morphologies, with persistent upper-beach cusps lasting days to months and dynamic lower-beach cusps evolving over individual tidal cycles. At times, beaches exhibiting multiple cusp systems reverted to a single cusp system extending over the entire beach when the high-tide waterline reached the upper-beach cusps, with the location and spacing of the resulting lower-beach cusps controlled by the upper-beach cusps. These observations are consistent with a “morphological coupling” hypothesis proposing that hydrodynamic-morphodynamic feedbacks between the swash and upper-beach cusps can result in the formation of lower-beach cusps with a related wavelength as the tide falls. However, there were also times when the high-tide waterline reached the upper-beach cusps that did not result in a unified beach state. These results suggest that while morphological coupling is often an important factor in controlling the development of new lower-beach cusps, this coupling cannot initiate cusp formation in hydrodynamic conditions outside those favorable for cusp activity.

Hillslope topographic change in response to climate and climate change is a key aspect of landscape evolution. The impact of short-duration rainstorms on hillslope evolution in arid regions is persistently questioned but often not directly examined in landscape evolution studies, which are commonly based on mean climate proxies. This study focuses on hillslope surface processes responding to rainstorms in the driest regions of Earth. We present a numerical model for arid, rocky hillslopes with lithology of a softer rock layer capped by a cliff-forming resistant layer. By representing the combined action of bedrock and clast weathering, cliff-debris ravel, and runoff-driven erosion, the model can reproduce commonly observed cliff-profile morphology. Numerical experiments with a fixed base level were used to test hillslope response to cliff-debris grain size, rainstorm intensities, and alternation between rainstorm patterns. The persistence of vertical cliffs and the pattern of sediment sorting depend on rainstorm intensities and the size of cliff debris. Numerical experiments confirm that these two variables could have driven the landscape in the Negev Desert (Israel) toward an observed spatial contrast in topographic form over the past 10⁵–10⁶ years. For a given total storm rain depth, short-duration higher-intensity rainstorms are more erosive, resulting in greater cliff retreat distances relative to longer, low-intensity storms. Temporal alternation between rainstorm regimes produces hillslope profiles similar to those previously attributed to Quaternary oscillations in the mean climate. We suggest that arid hillslopes may undergo considerable geomorphic transitions solely by alternating intra-storm patterns regardless of rainfall amounts.

Heavy-mineral suites are used widely in sandstone provenance and are key when connecting source and sink. When characterizing provenance related signatures, it is essential to understand the different factors that may influence a particular heavy-mineral assemblage for example, chemical weathering or diagenetic processes. Hydrodynamics, causing size-density sorting, exert major control on the distribution of heavy minerals. Here, we highlight the effect of grain-size inheritance, essentially the absence of certain grain sizes within a specific heavy-mineral species, on two distinct types of sediments. Modern deposits from a high-energy beach in NW Denmark give an analog for heavily reworked sediment, primarily controlled by hydrodynamic processes. In contrast, three Palaeogene turbidite successions in the Eastern Alps were sampled, presenting a more complex history that includes diagenesis. All samples were processed for their heavy-mineral compositions using Raman spectroscopy, and several techniques applied to determine the effect of grain-size inheritance. Results show that (a) even within the hydrodynamically well-sorted beach and placer deposits, evidence of grain-size inheritance is apparent, and (b) turbidites of variable heavy-mineral composition show strong effects of grain-size inheritance for several mineral species. Moreover, considerable intersample contrasts within single turbidite beds are observed. We enforce the importance of understanding grain-size inheritance, as well as other processes effecting size-density relations in clastic sediment that go well beyond purely hydrodynamic control of intrasample heavy-mineral variability.