Earth Surface Processes and Landforms最新文献

Channel incision and narrowing have occurred in the 20th century in most Alpine rivers. However, the causal links between sediment-related human engineering and exploitation and morphological changes in rivers are mostly unclear. This study presents an analysis of the evolutionary trajectories of the main active channels in the upper Etsch/Adige River basin (Eastern European Alps) coupled with their modifications in terms of coarse sediment transport. Channel planform variations were quantified in 15 rivers (total length of 630 km) using multi-temporal analysis of historical maps and orthophotos. Sediment volumes excavated from river channels or trapped by hydraulic structures (dams and retention basins) were retrieved from historical records, along with geospatial information regarding the presence of lateral and longitudinal consolidation works and land use variations. Results indicate that most rivers underwent slight narrowing and some of them experienced widening, from the mid-19th century to the 1950s. From the 1950s to the late 1990s, severe variations in terms of narrowing and morphological simplification took place in all rivers. The analysis of channel changes in relation to human activities shows that gravel mining carried out in the period 1970s–1990s appears to have been the main cause of sediment imbalance in the rivers which narrowed the most. Since the 2000s, when gravel mining was banned by law, channel adjustments have become negligible throughout study area. Nevertheless, the trapping of a large share of coarse sediment fluxes—at the river basin scale—by retention check dams and hydropower dams has impeded rivers from recovering to their original conditions.

The Golan Heights plateau in northern Israel is underlaid by volcanic rocks ranging in age from ~5.5 to 0.1 Ma. Throughout the Golan Heights, these rocks are covered by shallow soils that rarely exceed 0.5 m in depth. The soils are generally assumed to form a chronosequence, in which their ages correspond to those of the basalts they cover. Such age correspondence would imply that the soils have been slowly accumulating over hundreds of thousands to a few million years, suggesting a generally stable system. The ages of these soils and their temporal correlation to the basalts, however, have never been determined or tested.

Here, we present age constraints for the soils of the Golan Heights. Soils were surveyed and sampled with their corresponding basalt bedrock. Mass balance calculations based on conservative immobile elements, coupled with ³⁶Cl-based basalt denudation rates, suggest that the soil ages are decoupled from the ages of the underlying basalts, and represent up to a few thousand years of soil production, at most. This time frame is orders of magnitude shorter than the basalt age, challenging the prevalent assumption that the soils form a chronosequence.

Our findings strongly suggest that erosion is a significant factor controlling soil formation and accumulation on the plateau, despite the generally flat morphology of the Golan Heights. The erosion is associated with tectonic activity along the Dead Sea transform, with the development of the Kinarot and Hula valleys, and with the consequential development of drainage systems of various sizes on the plateau. Throughout the Golan Heights, the timing of volcanic activity in relation to the development of the valleys and drainage systems also appears to strongly affect soil development and accumulation.

Landslides triggered by rainstorms and earthquakes are prominent geological hazards that exhibit distinctive spatial and morphological characteristics due to diverse instability mechanisms. However, studies on differences between the two types of landslides remain limited. In this study, we explored differences in location and geometric properties between rainstorm-induced landslides in Qinzhou, Longchuan and Fukuoka and earthquake-induced landslides in Lushan, Iburi and Kaikōura. We normalized the location of landslides across the slope and quantified the landslide polygons using four geometric properties. Findings revealed that both location and geometric properties are specific to landslide type and differ between them. Earthquake-induced landslides are more common near the ridge of a slope, while rainstorm-induced landslides are more frequent in the valley or near streams. The quantitative analysis of geometric properties showed that earthquake-induced landslides are generally larger and have a more compact, rounded and less complex shape. The two landslide types present different hazards, particularly in their runout zones, where dispersion of materials occurs. Insights from our quantitative approach serve as a critical foundation for informed decision-making in emergency scenarios and contribute to enhancing landslide hazard management.

During the last decade, meter-resolution topo-bathymetric digital elevation models (DEMs) have become increasingly utilized within fluvial geomorphology, but most meter-scale geomorphic analyses are done on just one to a few rivers. While such analyses have contributed greatly to our collective understanding of river discharge-topography interactions, which is applicable in both river restoration design and environmental flow regulation contexts, their generalizability across a range of river types remains largely unevaluated. This study assessed the dominance of a single hydro-morphodynamic mechanism, flow convergence routing, in 35 ephemeral rivers divided among five river types in California's South Coast region by answering five questions. Geomorphic covariance structure (GCS) analysis was performed on longitudinal standardized width and standardized, detrended bed elevation spatial series from meter-resolution DEMs. All river types had coherent, multi-scalar structures of longitudinal fluvial topography, implicating a process-morphology link. GCS metrics revealed that landform patterning was consistent with the requirements of the morphodynamic mechanism of flow convergence routing. Thus, that process was found to be a broadly relevant channel altering mechanism among sites, but its relationship with water stage differed between river types. Specifically, river types in unconfined valleys exhibited a strong bankfull width control over base flow bed undulations, with no obvious flood-stage control over bankfull landform patterning. River types in partially confined valleys also exhibited strong bankfull width control over base flow bed undulations, but their bankfull landform patterns appear to have coalesced with coherent width and bed elevation undulations during flood flows. Finally, metrics for confined river types showed that it takes higher magnitude, less frequent floods to set their coherent width and bed elevation undulations, but even these channels do exhibit flow convergence routing when given enough discharge for sufficient duration.

Gully erosion, considered as a type of intensive erosion, is the dominant source of sediment at small watershed scales in certain environments. Variation of vegetation may result in the changes in near soil surface characteristics, which likely further affect the resistance of gully systems to erosion. However, the potential effects of near soil surface characteristics of plant communities on resistance to erosion are still unclear in gully systems. This study was performed to investigate the variations in resistance of gully systems to erosion under different plant communities and to identify the dominant influencing factors leading to these variations on the Loess Plateau. Five typical plant communities (two grasses, two shrubs and one forest) that distributed on different gully systems were selected. Six hundred undisturbed soil samples collected from different sites of gully systems were subjected to detach by overland flow under six different shear stresses (6.66 to 15.02 Pa). The results showed that the mean soil detachment capacity of gully systems covered by grass and shrub communities was 0.15 and 0.37 times to that of gully system covered by forest. Compared to forest gully systems, rill erodibility reduced by 29.8% to 85.6% for the other four plant communities. The relative rill erodibility of different vegetation communities generally increased from grass to shrub and forest communities. The critical shear stress of forest gully system was 58.7% and 63.8% of gully systems covered by grass (6.22 Pa) and shrub (5.73 Pa) communities. Bulk density, soil cohesion, water stable aggregate, root mass density, and the thickness of biological soil crust were the dominant factors affecting the resistance of gully systems to soil erosion. Rill erodibility decreased logarithmically with increasing soil cohesion, water stable aggregate and root mass density. Critical shear stress increased with the increase of soil cohesion and root mass density as a power function, and linearly with the increase of water stable aggregate. These results show how vegetation can mitigate against the erosion induced by concentrated flow in relatively stable gully systems.

Gully erosion is the main source of sediment in watersheds, and the assessment of soil erosion and deposition in gully systems is very important for land utilization and watershed management. Soil erosion and sediment transport are multiscale, and the digital elevation model (DEM) is one of the most important means to study the scale effect of soil erosion and deposition. To clarify the effect of DEM cell size on soil erosion and deposition in gullies, a series of DEMs with cell sizes of 0.5, 0.7, 1, 1.5, 2, 2.5, 3, 4 and 5 m were generated based on detailed field measurements in dry-hot valley region. Meanwhile, the unit stream power erosion and deposition (USPED) model was chosen to simulate soil erosion and deposition in this study. The results showed that cell size can greatly influence the average slope and aspect of the DEMs, while the accuracy and average elevation of the DEMs were only affected intensively when the cell size exceeds 3 m. As for the spatial distribution pattern of soil erosion and deposition, the increase of the DEM cell size could cause a concentration of soil erosion and deposition in main channels while weakening the occurrence on hillslopes, and the spatial proximity of soil erosion and deposition would not change with the DEM cell size. The highly positive correlation in simulation results for soil erosion and deposition was primarily observed in DEM cell sizes range of 0.5 to 2 m and from 3 to 5 m. Meanwhile, a slight increase in DEM cell size would significantly affect the simulated results of soil erosion and deposition based on the USPED model when DEM cell size ranged from 0.5 to 0.7 m, whereas the change in DEM cell size would not affect the simulation results when DEM cell size was greater than 0.7 m.

Semi-arid environments are characterized by infrequent large magnitude rainfalls that produce flash flood events with high sediment concentration. Control structures such as check dams are widely used in this environment for mitigation. However, their impact on the overall sediment balance of watersheds, particularly those severely affected by anthropogenic activity, is sparsely documented. This study used topographic measurements, sediment analysis, and fallout isotope techniques to assess the effectiveness and service life of 18 rock and masonry check dams that were constructed in the 1930s for controlling sediment fluxes in an 11 ha mountainous watershed in southern Arizona. All of the dams are currently filled with sediment resulting in reduction of local channel gradients by 35 to 71% to between 0.04 and 0.28 depending on location within the reach. Sedimentation occurred over multiple decades at a relatively slow average rate of 0.59 t ha⁻¹ y⁻¹ indicating low instantaneous retention efficiency. The smaller headwater dams were filled soon after construction; however, their share of the overall storage capacity was minor. Although evaluation of ¹³⁷Cs was an effective method for dating sediment, the ²¹⁰Pb dating method was not satisfactory because of sediment sorting effects and complex deposition patterns. The abundance of similar control structures in the region points to the opportunity to better understand the process impacts of check dams over multiple decades to inform planning and design of their use in future mitigation projects.

Disasters occurring at loess slopes in seasonal frozen regions are closely related to changes in the thermo-hydro-mechanical (THM) state in loess by freeze–thaw (FT) action. Current research on FT-induced soil slope failure focuses on frozen stagnant water effects, while the intrinsic connection between the FT-induced stagnant water effect and soil strength deterioration remains unclear. In this study, by taking the FT-induced loess slope failure as an example, field surveys, boreholes, exploratory wells, and 3D topographic mapping were used to reveal the landslide features and stratigraphic information; Furthermore, the temporal and spatial variation of water and heat in loess slope was revealed by on-site monitoring data; A THM coupled model of frozen soil was established using COMSOL Multiphysics simulation software to reconstruct the frozen stagnant water process of shallow loess slope, as well as the influence of THM field on loess landslide. The results show that the effects of FT in the seasonally frozen region occurred in the shallow layer of the loess slope. The water-ice phase transition during FT process broke the phase equilibrium of loess. Numerical calculations and field monitoring indicated a continuous migration of water to the freezing front, creating a water-enriched zone inside the loess. Both the impact of the frozen stagnant water and changes in the stress field led to the degradation of loess structure and reduced the strength properties, thus threatening the stability of the loess slope. The study results can contribute to an in-depth understanding of the mechanism underlying FT loess landslides in seasonal frozen regions, and provide a scientific basis for the evaluation and prevention of FT landslides.

Soil moisture patterns are crucial factors in gully bank erosion and may significantly vary depending on the water conditions before winter freezing. Rainfall events can evoke different moisture responses throughout the soil profile of gullies. However, the relative importance of prewinter conditions and rainfall events to the soil moisture responses of gully banks in seasonally frozen regions remains unclear. In this study, a field infiltration experiment of a gully bank with four prewinter soil moisture regimes and three replicates was conducted in October 2017 in the Mollisol region of China. We aimed to elucidate the temporal variations in moisture responses under different prewinter water treatments at various distances to the gully bank after three representative rainfall events in 2018. The response magnitude (∆s) varied widely and was largest in the intermediate layers (30–50 cm) for wetter prewinter regimes (i.e., HI and MED) but was larger in the upper soil layers (0–30 cm) for drier prewinter regimes (i.e., CK and LO). In contrast to the findings of other studies, we found that the relative influences of prewinter conditions and rainfall events on the soil moisture responses varied with soil depth and distance to the gully edge. The soil moisture response increased with rainfall depth, and the effect of prewinter conditions dominated until the end of the rainy season (August). The most important explanatory variables for the response magnitude (∆s) were rainfall events (46%), soil depth (10%), and prewinter conditions (7%). This study increases the understanding of the heterogeneity in soil moisture responses and the relationship to rainfall events under different prewinter conditions on gully banks in the Mollisol region. Understanding subsurface hydrological processes and identifying the relative importance of prewinter conditions and rainfall are beneficial for building gully bank erosion models for seasonally frozen regions.