Earth Surface Processes and Landforms最新文献

With the intensification of global warming, the frequency and intensity of extreme rainfall events continue to rise, especially unstable precipitation, which poses a significant challenge to the stability of grottoes. The variations in water pressure and humidity induced by rainfall exacerbate the propagation of fissures in the roof plate, further compromising the structural integrity of the grotto's roof. This study takes the Dazu Rock Carvings in China as a case study, utilising Rayleigh wave imaging monitoring, theoretical analysis, numerical simulation, and on-site multi-dimensional monitoring methods to reveal the mechanisms behind the formation of fissures in the grotto roof and the deformation behaviour under both natural and rainfall conditions. Based on these findings, corresponding support strategies are proposed. The results show that the fissures in the grotto roof are primarily caused by the settlement deformation and tensile-shear failure of the surrounding rock. Rainfall increases the tensile stress on the roof plate, further exacerbating the fissure propagation. In addition, the formation of a cavity on the southern side accelerates the instability of the roof. The concentration of tensile and shear stresses causes instability in the grotto sidewalls near the cavity, and rainfall further intensifies this trend. To mitigate the expansion of fissures in the roof plate, the proposed reinforcement strategy utilises the bearing capacity of the overlying rock layers to support the roof while minimising interference with the sculptures inside the grotto. This study not only helps clarify the evolution of the roof damage process but also provides theoretical guidance for formulating appropriate support strategies.

Side channel restoration—including creation, rehabilitation and enhancement—is a common strategy to mitigate habitat degradation in regulated rivers. While short-term ecological benefits are well documented, the longer-term geomorphic evolution of restored side channels remains less understood. In natural systems, side channels typically occur at dynamic bifurcations influenced by slope and sediment supply, whereas restoration efforts in regulated rivers often prioritize static design targets (e.g., a fixed inundation area at a given flow). We monitored two restored side channels along a regulated river in California over a five-year period to investigate how geomorphic and habitat conditions evolve post-restoration. Our objectives were to (1) document geomorphic and habitat changes and (2) assess how excess shear stress and relative sediment supply influence channel evolution. We tracked changes in sediment and large wood budgets, bed profiles, grain size distributions, bar formation, inundation patterns and tracer rock displacement and interpreted these in the context of reach-scale and geomorphic-scale shear stress. Results show that the steeper, upstream site experienced greater erosion and a loss of low-flow inundation area due to higher flow energy and excess shear stress, while the downstream site remained relatively stable and gained inundated habitat. Importantly, reach-scale excess shear stress served as an effective proxy for relative sediment supply, explaining observed differences in geomorphic response between sites. Both channels lost more large wood than they recruited, highlighting the need for integrated sediment and wood management. Normalized rates of geomorphic change declined over time, suggesting that the primary morphological adjustments occurred shortly after construction. These findings underscore the importance of reach-scale context in designing and evaluating side channel restoration and demonstrate how multi-scalar monitoring—particularly incorporating reach-scale excess shear stress—can improve understanding of post-restoration dynamics.

River bifurcations are the fundamental building blocks of a variety of fluvial environments such as braiding and anastomosed rivers, alluvial fans, and river deltas. Their long-term equilibrium configurations have been widely explored, together with the influence of several external forcing factors, whereas less attention has been devoted to investigate the characteristic timescale with which bifurcations evolve over time. In this work, we address this issue by combining the results of a 1-D numerical model with those obtained through a linear stability analysis that accounts for the length of bifurcates. Numerical results show that the timescale of the adaptation of water and sediment partition at the bifurcation node is much shorter than the time required to achieve the long-term equilibrium of the bifurcates. We find that the nodal point evolution becomes faster as the value of the width-to-depth ratio increases above the critical threshold for the bifurcation stability, while it gets slower as the length of the bifurcates increases. The timescale becomes independent of the branch length when this length exceeds a threshold value above which the effect of the downstream boundary condition no longer affects the evolution of the bifurcation node. The analysis of a large dataset of gravel-bed bifurcations reveals that the evolutionary timescale of most of them is larger than that of natural flow variations. Moreover, the rate at which the water and sediment partitioning at bifurcations changes over time is generally smaller than the fluctuation rate of sediment transport caused by the migration of bars in the upstream channel, especially for bifurcations with long branches.

Gully erosion is a complex form of erosion with a large environmental and economic impact, causing loss of fertile soil, siltation of rivers, and changes in the landscape and drainage channels, which have impacted the environmental balance of the Pampa biome in Southern Brazil. The complexity of this process leads to the formation of different shapes of gullies. Studying the morphology of gullies and the main erosion mechanisms acting on them is important to understand the erosion process. With high-resolution data from UAV survey and data from fieldwork, the morphometry and the main erosive drivers of three spatially close gullies with different shapes and intensities in the Pampa biome region were analysed. The V1 gully is the largest of the three studied, showing variation in depth and active erosive mechanisms; V2 has the shortest length, the widest cross-sections and the most significant average depths, with mass movements along almost its entire length. V3 has the greatest length, lowest depth and eroded volume because of less intense mass movements and more stable side walls. Even though various factors interfere with erosion mechanisms and the shape of gullies, the type of soil material proved to be a determining factor in the erosion mechanisms. Sandy materials proved to be more susceptible to mass movements, vertical development, and the concentration of faults and fractures, while predominantly clayey materials proved to be more cohesive and resistant to erosion, especially subsurface erosion, resulting in less occurrence of mass movements. The results presented in this work help to understand the accelerated erosion processes in the Pampa biome in Southern Brazil, elucidating the main factors that influence the shape and expansion of gullies, which is extremely important for determining effective conservation practices.

This study evaluates the reliability of the ORSA2D_WT model, a Eulerian–Lagrangian model, in simulating large wood (LW) transport in the Tagliamento River. The model implements a literature strategy to account for sliding and rolling entrainment modes, besides floating. Overall, the model demonstrated an acceptable level of accuracy in replicating LW entrainment with a successful prediction of the behaviour of 8 out of 11 entrained logs, and 29 out of 37 stable logs were observed during field surveys. The findings are based on a limited number of comparisons, including 36 logs in the Cornino reach and 12 in the Flagogna reach, with only 2 GPS trajectories available, emphasising the exploratory nature of the study and the preliminary validation of the model. While the model effectively predicted LW dynamics under simplified conditions, discrepancies in trajectories near islands and areas of complex flow dynamics highlighted challenges in capturing intricate LW transport. Sensitivity analysis revealed the significant influence of wood density on LW transport, with wet density (WD) conditions showing notable deviations from the observed data. These findings emphasise the complex interplay among density, buoyancy, and hydrodynamic forces, underscoring the need for precise density estimates in LW transport modelling. Additionally, the initial orientation of logs was found to significantly affect transport dynamics, with logs aligned parallel to the flow experiencing longer displacements, while perpendicular or oblique orientations increased hydrodynamic forces, anticipating entrainment, and also fostered early deposition because of the higher interaction with the riverbanks. The model displayed an overestimation of LW mobility, compared to field surveys observations. This limitation highlights the need for a more realistic representation of log interactions, partial burial and structural features such as root wads and branches. To enhance the model's accuracy and reliability, future improvements should focus on better representing wood accumulations and partial burial, as well as optimising computational efficiency. These advancements will enable more comprehensive analyses and improve the model's applicability and robustness for real-world scenarios.

While the influence of climate on landscapes is conceptually intuitive, quantifying it remains challenging due to the myriad of ways earth surface processes respond to climate. In this study, we investigate if and how climate impacts fluvial relief and hillslope morphology by examining the relationship between basin average cosmogenic ¹⁰Be-derived erosion rates and basin average topographic metrics across climatic gradients in the Bhutan Himalaya. We selected this region because it is an ideal natural laboratory for assessing climate controls on landscapes, with large precipitation variations, minimal lithologic differences and extensive existing datasets. Our findings suggest that increasing precipitation may drive several trends: (1) nonlinearity between erosion rate and fluvial metrics (k_snQ) increases, (2) the threshold hillslope gradient declines and (3) the characteristic hillslope length increases. Although these trends are weak and subject to considerable uncertainty, the subtle variations still conform with a conceptual model where wetter climates promote mass movement, extend hillslope length and reduce mountain relief as indicated by elevated nonlinearity between fluvial relief and erosion rate in wetter regions. This consistency suggests that more carefully crafted data or experimental designs offer the hope of quantifying climate's role in landscape form. Our study provides valuable insights for future research on sampling strategies and data analysis aimed at extracting climatic signals from observational datasets.

Aerial photography and satellite imagery can be used to characterize landscape change over time and help to understand how these changes are related to climate and hydrology. Publicly available optical imagery from sources such as the United States National Agricultural Imagery Program (NAIP) is particularly valuable in this context due to its high temporal and spatial resolution. However, the exact time an image was acquired is often unknown, which complicates, if not precludes, linking images with other types of high temporal resolution data, such as streamflow records. In this letter, we propose a ‘sundial method’ to infer image acquisition time from shadow orientation. This approach involves measuring the direction of a shadow on the image and using solar geometry calculated for the known image date and location to infer the former sun position. Time estimates for 16 Worldview satellite and six NAIP aerial images based on 407 independent measurements of shadow orientation demonstrate the sundial method had an error of 2.1 ± 3.4 min, indicating that image acquisition times can be inferred with a high degree of accuracy and precision. Sensitivity analyses confirm the robustness of the method across different object types, shadow lengths, and solar zenith angles, while also providing practical guidelines regarding the number of measurements required and errors associated with uncertainty in the image date.

Gravel beach systems provide vital protection from coastal flooding and erosion. They are highly dynamic and exhibit complex responses to hydrodynamic forcing over a range of temporal (hourly centennial) and spatial scales (m to km). Yet gravel beach evolution, particularly at interannual to decadal scales, across the spectrum of coastal settings, remains poorly understood. We use four decades of Satellite-Derived Shoreline (SDS) data to explore the morphodynamic behaviour of 45 selected gravel beach systems around the United Kingdom and Ireland. We apply a site-specific SDS extraction methodology and derive shoreline trends along 1554 shore-normal transects. Our findings indicate significant variability in decadal trends between sites ranging from 0.60 m/year retreat to 2.24 m/year progradation, with 36% of sites showing significant long-term trends over the study period. Nesses and spits were by far the most dynamic systems exhibiting the largest changes at transect level (from 4.73 m/year retreat to 10.5 m/year progradation), and the most significant changes in planform shape, while most constrained and unconstrained sites remained stable. We classify the observed behaviours, providing a first inventory of morphodynamic behaviours across different gravel beach systems in the United Kingdom and Ireland. We find that leading regional winter-averaged atmospheric indices provide some insight into planform behaviour over the entire domain, with 16 sites (35.6%) showing at least moderate (R ≥ 0.4) statistically significant correlations (p ≤ 0.05). Our results provide a deeper understanding of the long-term behaviour of gravel beach systems that can inform more effective coastal management strategies.

Remote sensing has emerged as an effective tool for characterizing river systems, and machine learning (ML) techniques could make this approach even more powerful. To explore this possibility, we developed an ML-based workflow for hyperspectral imaging of river bathymetry using an ensemble of regression trees (HIRBERT). This approach involves using paired observations of depth and reflectance to select wavelength bands as predictors and then train a depth retrieval model; applying the model to the image yields a spatially continuous bathymetric map. We used data from five rivers with diverse morphologies and optical characteristics to assess whether HIRBERT can (1) provide more accurate depth estimates than a band ratio-based algorithm and (2) extend the range of depths detectable via remote sensing. Relative to single band combinations identified via optimal band ratio analysis (OBRA), regression tree ensembles improved depth retrieval performance, with observed versus predicted (OP) regression $��R^2�$ values increasing for all five sites. Similarly, HIRBERT provided more reliable depth estimates than OBRA over the full range of depths present along each river. These results suggest that by incorporating additional spectral information from multiple wavelength bands, ML could enhance bathymetric mapping across a range of river environments. In addition, we show how graphical tools can facilitate interpretation of ML-based depth retrieval models and yield insight regarding relationships between depth and reflectance. The HIRBERT workflow is packaged in free, standalone software developed to support applications in river research and management. Although ML can enhance remote sensing of river bathymetry, the limitations of this approach must also be acknowledged: Field measurements of water depth are required to train a depth retrieval model and the resulting model should only be applied to the image from which the training data were derived. The inherently image-specific nature of this approach implies that developing generalized regression tree ensembles that could be applied at larger scales would require additional research.

Recent releases of high-resolution elevation data in British Columbia have revealed several previously unidentified meltwater-derived landforms associated with the last Cordilleran Ice Sheet (CIS). Prominent among these are small subglacial meltwater corridors (sSMCs): morphologically distinct channels up to 10s of km long that are eroded into glacial sediments, with varying intra-corridor landform assemblages. Intra-corridor landforms identified include curvilinear ridges, eskers and multi-ridged fans. The sSMCs are one or more orders of magnitude smaller than the previously described Chasm and Green Lake meltwater corridors, the latter two formed from an extremely large supraglacial lake outburst flood. Results from geomorphological, sedimentological and near-surface geophysical methods show that an erosional unconformity cut into the regional till sheet forms the base of each sSMC. This unconformity extends from the channel boundary and includes intra-corridor curvilinear ridges and erosional remnants, consistently topped by a 0.2–0.3 m thick concentration of cobbles. A discontinuous channel-wide fill (5–15 m thick) buries some curvilinear ridges and is sometimes capped by an intra-corridor esker. Multi-ridged fans occurring downflow from braided outwash plains postdate the sSMCs and all other intra-corridor landforms. We interpret the sSMCs as palimpsest landforms that developed over several melt seasons through the continuous reoccupation by subglacial meltwater flow (and later proglacial streams). Each trough found lateral to a curvilinear ridge represents an individual subglacial meltwater channel incised over one or more melt seasons. The broad corridor fill records higher-than-average melt season discharge within portions of the sSMC, burying or further eroding curvilinear ridges and potentially forming eskers during late-stage waning flows. Results from this study indicate that the last CIS had an active and dynamic subglacial drainage system during deglaciation.