Bedrock river bathymetry is dynamic, with incision rates dependent on sediment cover, supply, and mobility in the channel. However, the scale and fluctuation of this dynamic sediment storage is not well understood, particularly in large bedrock rivers where the bed is not visible at low flows. We used repeat, high resolution, multibeam bathymetric surveys from 2021-2023 to characterize bed and bank topography in nine bedrock canyons that are representative of a wide range of width, depth, slope, and velocity observed through the 375 km long Fraser Canyon in British Columbia. Change in elevation as high as 15 m is identified between surveys. We characterize patches of contiguous change to measure changes in sediment storage volume. Our observations reveal that channel morphology determines where storage occurs. We find that sediment is "staged" through canyons, initially being deposited in a canyon near a sediment supply site, then moving downstream as the initial deposit declines. Substantial changes in storage volume occur without substantial changes in patch footprint. These findings provide key context for interpreting the reach-scale structure of bedrock erosion, the long-term evolution of mountain river networks, and the moderation of sediment delivery to lowland environments.
Landslides in glacial and periglacial environments are increasingly affected by climate change, with sudden failures reported in high mountain regions and the Arctic. The complex mechanisms behind these events are often poorly understood due to a lack of dense in situ data. We investigate two slow-moving landslides in Arctic Norway (70° N), the Jettan and Gámanjunni landslides, located approximately 10 km apart: Jettan, a complex slide in micaschist and calcite marble situated below the permafrost boundary, and Gámanjunni, a rotational slide in micaschist situated above the permafrost boundary. Using over a decade of multi-physics observations, including geodetic, borehole, seismic, and hydrological data, we examine surface and subsurface deformation. Both landslides display similar seasonal surface velocity patterns, with peaks in spring and autumn, likely influenced by pore-water infiltration. At Jettan, twelve years of inclinometer data in boreholes reveal a transition from steady state to seasonal deformation in two shear zones. Since 2020, spring accelerations have intensified in years coinciding with deeper snowpacks and associated melt. These observations, together with statistical modeling, suggest that the shear-zones are becoming increasingly localized and sensitive to pore-water pressure. Conversely, autumn acceleration is not seen in localized shear zones but manifests as distributed volumetric deformation. Seismic velocity variations within the landslide body also exhibit seasonal patterns that correspond with geodetic velocity, interpreted as changes in landslide rigidity due to water infiltration. This integrated analysis of surface and subsurface data offers new insights into the evolving deformation of Arctic landslides, emphasizing the influence of hydrological forcings on both seasonal and long-term deformation processes.
Abstract. Quantifying erosion across spatial and temporal scales is essential for assessing different controlling mechanisms and their contribution to long-term sediment production. However, the episodic supply of material through landsliding complicates quantifying the impact of the individual erosional mechanisms at the catchment scale. To address this, we combine the results of geomorphic mapping with measurements of cosmogenic 10Be, 26Al, and 14C concentrations in detrital quartz. The sediments were collected in a dense network of nested sub-catchments within the 12 km2-large Gürbe basin that is situated at the northern margin of the Central European Alps of Switzerland. The goal is to quantify the denudation rates, disentangle the contributions of the different erosional mechanisms (landsliding versus overland flow erosion) to the sedimentary budget of the study basin, and to trace the sedimentary material from source to sink. In the Gürbe basin, spatial erosion patterns derived from 10Be and 26Al concentrations indicate two distinct zones: the headwater zone with moderately steep hillslopes dominated by overland flow erosion, with high nuclide concentrations and low denudation rates (∼ 0.1 mm yr−1), and the steeper lower zone shaped by deep-seated landslides. Here lower concentrations correspond to higher denudation rates (up to 0.3 mm yr−1). In addition, 26Al / 10Be ratios in the upper zone align with the surface production ratio of these isotopes (6.75), which is consistent with sediment production through overland flow erosion. In the lower zone, higher 26Al / 10Be ratios of up to 8.8 point towards sediment contribution from greater depths, which characterises the landslide signal. The presence of a knickzone in the river channel at the border between the two zones points to the occurrence of a headward migrating erosional front and supports the interpretation that the basin is undergoing a long-term transient response to post-glacial topographic changes. In this context, erosion rates inferred from 10Be and 26Al isotopes are consistent, suggesting a near-steady, possibly self-organised sediment production regime over the past several thousand years. In such a regime, individual and stochastically operating landslides result in the generation of an aggregated signal that is recorded as a higher average denudation rate by the cosmogenic isotopes. Although in-situ 14C measurements were also conducted, the resulting concentrations are difficult to interpret as soil mixing (due to landsliding), sediment storage or an increase in erosion rates might influence the 14C concentration pattern in a yet non-predictable way.
The Alpine cryosphere changes at unprecedented speed, affecting the thermal, hydrological, and mechanical state and behaviour of rock slopes. While numerous studies investigated singular drivers for progressive rock slope failures, the knowledge of hydro-thermo-mechanically coupled processes remains scarce. In this paper, we investigate the 2024 permafrost rock slope failure at Platteikogel with a volume of 50 000 m3 (3395 m a.s.l., above Vernagtferner, Austria). We aim to assess how observed ice apron loss and related permafrost warming promote the release mechanism. We reconstructed multidecadal thermal evolution accounting for the thermal impact of ice apron loss. Based on field observations, we derived a conceptual model on how ice apron loss potentially affects rock slope destabilization. Integrating the outcome of the preceding steps, we performed a mechanical stability analysis assuming that the rock slope failed along ice-filled discontinuities. The mechanical model indicates that the failure can not be solely explained by a warming-driven decrease in shear strength of ice-filled discontinuities, suggesting that other failure processes superimpose or even dominate. The implemented system feedback related to ice apron loss suggests that hydrostatic pressure buildup due to water infiltration and rockfall-induced unloading thereby promoted the Platteikogel rock slope failure. In summary, we demonstrate that ice apron loss not only leads to increased rockfall activity but also accelerates progressive failure, promoting the detachment event. In upcoming decades, ice aprons on steep rock slopes above 3000 m in the European Alps are expected to experience drastic area loss, exposing potential source zones for future rock slope failures.
Geomorphology as a discipline is defined by the use of topographic form to understand surface processes on Earth and other planets. In practice this requires drawing connections between quantitative metrics of surface geometry and rates of erosion and deformation, to understand the spatial partitioning of different erosion processes and the feedback between them. Curvature, perhaps the most fundamental way to measure and categorize surfaces of any kind, also appears explicitly in many erosion models and is therefore of significance to geomorphology. However, there is ambiguity in how curvature of discretely sampled topographic surfaces such as digital elevation models is defined and calculated. In this study we use a formal surface theory approach to compute intrinsic and extrinsic curvature metrics, and associated shape-class distributions, of approximate steady-state fluvial topography of the Oregon Coast Range, USA. We develop a workflow, including careful spectral filtering to isolate wavelengths of interest, that provides a nuanced view of landscape geometry that is consistent and accurate across steep landscape regions. Two invariants of the curvature tensor - the mean and Gaussian curvatures - reveal systematic structure of topographic geometry in channel and ridge networks that captures transitions between hillslope, debris flow, and fluvial process regimes. Mean curvature and associated shape classes are equipartitioned between concave-down and concave-up elements, forming complementary branching structures that span the landscape. These results suggest that formal surface theory approaches could prove valuable in improving process regime identification from digital elevation data in fluvial landscapes.
Supercritical flows in mountain rivers create complex flow-obstacle interactions that govern infrastructure vulnerability and channel morphodynamics, yet current understanding remains focused mostly on smooth-bed assumptions that poorly represent natural gravel-bed channels, where grain-scale roughness fundamentally alters flow physics near the bed and around obstacles such as bridge piers and in-stream vegetation. This study quantifies how bed surface characteristics control velocity fields, turbulent structures, and bed stress patterns around obstacles in supercritical flow through high-resolution detached eddy simulations coupled with volume-of-fluid free surface tracking. We examined three morphodynamic states representative of natural channel evolution: smooth beds analogous to bedrock channels, rough flat beds representing post-flood recovery conditions where sediment has been deposited as relatively uniform gravel sheets, and equilibrium scoured beds representing quasi-steady morphodynamic states. Digital representation of detailed bed surface elevation, including individual sediment grains, was considered using Structure-from-Motion photogrammetry. Numerical simulations reproduced characteristic supercritical flow structures including wall-jet formations, horseshoe vortex systems, and reverse spillage phenomena across all bed configurations. We observed that grain-scale roughness completely transforms flow organization from coherent, predictable vortical structures to chaotic flow fields dominated by grain-roughness effects. While smooth beds exhibit symmetric stress distributions with organized patterns, rough beds generate highly skewed distributions with extreme spatial variability, where coefficient of variation increases from 37 % to 115 %. Individual grains work as micro-obstacles, creating localized stress concentrations exceeding smooth-bed conditions by factors of 2-3, which can fundamentally alter sediment transport mechanisms. An equilibrium scour hole creates hierarchical flow disturbances where large-scale topographic modifications interact with grain-scale disruptions to produce the most complex stress fields observed. These findings demonstrate that engineering design standards based on smooth-bed assumptions can significantly underestimate the spatial heterogeneity and peak stress magnitudes characteristic of natural rough-bed conditions. The transition from organized stress patterns in smooth beds to grain-scale dominated physics in rough beds necessitates fundamentally different approaches to flow prediction, infrastructure design, and morphodynamic modelling in steep channel environments.
Azerbaijan hosts the world's highest concentration of mud volcanoes (MVs) on Earth, including some of the largest edifices that produce kilometre-scale mud breccia flows. Traditionally, such flows have been attributed to major eruptive events; however, recent study suggests that many of these flows form through prolonged creeping processes, analogous to warm-based glacier motion. To assess the prevalence and characteristics of this phenomenon, we analysed historical satellite imagery from Google Earth for several dozen mud volcanoes across the Caspian Basin, complemented by field observations at selected sites. Our analysis reveals that 19 mud volcanoes exhibit measurable creeping surface displacement of preexisting mud flows, with rates ranging from a few metres to tens of metres per decade. While some volcanoes, such as Goturdag, display continuous deformation across entire flow lengths, most of them exhibit only episodic motion in connection to mud volcano eruptions. While most creeping flows occur outside inhabited areas, landforms such as Bozdaq Gobu represent a potential hazard to settlements and infrastructure. These findings highlight the importance of post-eruptive creep in mud volcano evolution and emphasise the need for ongoing monitoring and hazard awareness.
Understanding the spatio-temporal dynamics of suspended sediment source activation is essential for effective ecological management, risk assessment, and infrastructure planning. Provenance analysis, which traces sediment origins, plays a crucial role in these applications, but is often based on costly fingerprinting methods. In this study, we validate a time- and cost-effective fingerprinting approach based on X-ray diffraction (XRD) data. We implement and compare two non-linear inversion schemes (steepest descent and Quasi-Newtonian) applied to binned XRD data and spatial information on potential source areas, in order to invert detrital mineralogical data into erosion rate maps while quantifying posterior uncertainty and error propagation. Forward-inverse tests with synthetic data demonstrate consistent convergence of the posterior solution and reveal the influence of geological complexity, tracer selection, and signal blending on inversion performance. The application to real-world datasets from the Gornergletscher catchment further validates the practical utility and robustness of the model.
Understanding river incision model is crucial for predicting long-term landscape evolution. For the bedrock channel incision model (detachment-limited (DL) model: erosion rate E=KAmSn where A is drainage area, S is channel gradient), parameters (K, m, and n) can be estimated via slope-area analysis if E is known. Using 10Be denudation rate, previous studies globally compiled the parameter values for variable lithology. However, limited data availability for soft sedimentary rock restricts the applicability of global compilation. In addition, measuring the 10Be concentration in sedimentary rock is challenging in humid and tectonically active regions. To address this, slope-area analysis was conducted in the Kamikita Coastal Plain, Japan, where lithology (Miocene to Pleistocene sedimentary rocks) and uplift rate (similar to 0.2 mm yr-1 for the past 300 ka) are assumed to be uniform. River incision rates were derived approximately from widely distributed marine terraces (MIS 5e-11). For six target rivers, DL-like behaviour was confirmed in the limited areas located upstream of the alluvium distribution. The reference concavity m/n was 0.44 +/- 0.10, typical for steady-state channels. Across the m/n range of 0.4-0.6, the exponent n consistently exhibited nonlinearity ranging between 1.14 to 1.34, which is consistent with the previous global compilations. This observed nonlinearity likely reflects transient landscape responses to past sea-level changes, which generated slope-break knickpoints at similar elevations. Finally, the estimated erosion coefficient K (10-5-10-6) agreed with the global relationship with unconfined compressive strength qu (K proportional to 1/qu2), supporting the significant influences of bedrock lithology on K.
Sediment characteristics and grain-size distribution are crucial for understanding natural hazards, hydrologic conditions, and ecosystems. However, traditional methods for collecting this information are costly, labor-intensive, and time-consuming. To address this, we present OrthoSAM, a workflow leveraging the Segment Anything Model (SAM) for automated delineation of densely packed pebbles in high-resolution orthomosaics. Our framework consists of a tiling scheme, improved seed (input) point generation, and a multi-scale resampling scheme. Validation using synthetic images shows high precision close to 1, a recall above 0.9, with a mean IoU above 0.9. Using a large synthetic dataset, the two-sample Kolmogorov-Smirnov test confirms that there is no significant difference between the predicted and the ground-truth grain size distributions. We identified a size detection limit of 30 pixels; pebbles with a diameter below this limit are not reliably detected. Applying OrthoSAM to orthomosaics from the Ravi River in India, we delineated 6087 pebbles with high precision (0.93) and recall (0.94), based on manual verification of each predicted mask. The resulting grain statistics include area, axis lengths, perimeter, RGB statistics, and smoothness measurements, providing valuable insights for further analysis in geomorphology and ecosystem studies.
Reconstructing the transport histories and provenances of glacial sediments and ice-contact deposits (e.g. tills, moraines) in formerly glaciated regions remains a major challenge, particularly at icefield- to ice-sheet scales and over multi-millennial timescales. Yet such reconstructions are central to key questions in Quaternary science, including estimates of past glacial erosion rates and sediment fluxes, the role of subglacial sediment storage in erosion reduction, or the reconstruction of past ice-flow dynamics, ice divides, and transfluences. While numerical modelling can enable one to reproduce past glacial sediment transport via coupling glacier models with particle tracking, this becomes computationally unfeasible over large spatial domains and paleo timescales using traditional computing. As a result, no study to date has simulated glacial sediment transport using large particle numbers (tens of millions) across continental-scale icefields such as the one occupying the European Alps during the Last Glacial Maximum (LGM). Here, we overcome this limitation using the Instructed Glacier Model (IGM), which allows the coupling of 3D Lagrangian particle tracking with high-resolution glacier simulations, both accelerated on Graphics Processing Units (GPU). This unlocks the modelling of ice advection of millions of particles at minimal additional computational cost, allowing simulations of glacial sediment transport across the European Alps over multi-millennial timescales (40-18 ka) and at the unprecedented spatial resolution of 300 m. We achieve similar to 50 & times; faster computation tracking 20 million particles across the Alps using a single GPU instead of 60 CPU threads. In doing so, we produce the first Alps-wide modelling reconstruction of glacial sediment transport during the LGM, using process-based particle seeding schemes to represent both subglacial (e.g. abrasion, plucking) and supraglacial (e.g. rockfall, landslides) sediment sourcing. Results are analysed through complementary "sink-to-source" (deposit provenance) and "source-to-sink" (potential depositional pathways) analyses, enabling us to reconstruct the LGM glacial transport of numerous ice-contact deposits and surface lithologies across the Alps. We find that supraglacially sourced glacial sediments are typically eroded earlier, experience longer glacier residence times, and undergo greater cumulative ice-free exposure than those of subglacial origin, with implications for the interpretation of cosmogenic nuclide inheritance in glacial deposits. Our new coupled glacier-particle modelling framework opens avenues for quantitative model-data comparisons using glacial geomorphology and provides a powerful tool for reconstructing paleo ice dynamics, sediment provenance, and Quaternary glacial landscape evolution.
Asymmetric tidal dunes with intermediate (10-17 degrees) to low-angle slopes (< 10 degrees), usually with an irregularly-shaped lee side, are often found in natural, constrained tidal environments such as tidal rivers, estuaries and tidal channels. However, previous studies on bedform flow dynamics have largely focused on high-angle dunes with a simple (straight) lee side, generally found in flume studies or small rivers. This study provides a detailed characterisation of the flow and turbulence over asymmetric tidal dunes under an idealised tidal flow condition based on laboratory measurements. Specifically, we aim to address how tidal dune shape, especially the lee side geometry, controls the properties of flow separation and resulting turbulence structures. Furthermore, we address how flow bidirectionality changes flow and turbulence over the same tidal dune geometry. To achieve this, we conducted large-scale, high-resolution flume experiments over two idealised dune morphologies which represent natural asymmetric tidal dunes with intermediate- to low-angle slopes. The flow condition was an idealised representation of tidal flow for which the same unidirectional steady currents were imposed first in one direction, then in the opposite direction. Our results show that for the case of an intermediate-angle tidal dune and when the flow was directed from the gentle stoss to the steep lee slope, a downward expanding turbulent wake and a small, near-bed permanent flow separation were detected. A small flow separation was also detected for the case of low-angle tidal dune. When the flow was reversed and directed from the steep stoss to the gentle lee slope, flow direction significantly altered the flow dynamics for both dunes as no permanent flow separation was observed and turbulence structure was similar to that over a flat bed. Interestingly, we demonstrated that a small intermittent flow separation can still form even for tidal dunes with very gentle slope (4 degrees) provided that a short steep portion is present. This implies that low-angle dunes can generate flow resistance and can potentially contribute to sediment mobilisation above low-angle dunes. Overall, our study highlights the significant impact of dune morphology, particularly the lee side slopes, and flow direction on the flow and turbulence dynamics above asymmetric tidal dunes. Our findings can have further implications on the parameterisation of hydraulic roughness, estimation of sediment transport and the resulting morphodynamics in natural shallow water environments.
Bedrock river incision is a fundamental process driving the evolution of mountainous landscapes. Bedrock strength is often considered a primary control on incision rates and river profile morphology, with laboratory experiments showing a strong correlation between erosion rate and tensile strength. However, in natural settings, lithological boundaries frequently do not correspond to changes in the channel gradient. This study addresses this apparent paradox by integrating field observations with numerical experiments in the tributaries of the Abukuma River basin, northeastern Japan. Field surveys were conducted to measure bedrock tensile strength, riverbed gravel grain size, and the spatial distribution of lithologies. Despite more than an order-of-magnitude variation in bedrock tensile strength across the study area, the channel slopes remained nearly uniform. Numerical experiments were performed using three models of bedrock river erosion to investigate the underlying mechanisms. Among them, the sediment-flux-dependent model, which explicitly incorporates sediment cover and tool effects, most accurately reproduced the observed longitudinal profiles. The results reveal that the local lithology does not directly influence channel slope due to a negative feedback between sediment cover and river gradient. Higher bedrock erodibility reduces channel slope and sediment transport capacity, promoting sediment cover. The resulting sediment cover suppresses further erosion and offsets the effect of bedrock strength. These findings highlight the limited role of bedrock strength in controlling channel gradients and underscore the importance of sediment dynamics, particularly sediment supply and grain size, in shaping fluvial topography. Future research should explore how lithology-dependent variations in sediment characteristics influence river profile development.
The Normalized Difference Vegetation Index (NDVI) can be effectively used for monitoring the spatial and temporal dynamics of riparian vegetation. However, quantitative and efficient evaluations of the links between NDVI and bio-hydromorphological processes remain limited, particularly in river management contexts where dense in-channel vegetation can obstruct flow and reduce conveyance capacity. Using 200 cloud-free Sentinel-2 images (2015-2024) covering a 20-km reach of the Chikuma River (Japan), we evaluated the utility of high temporal resolution NDVI and greenness index (defined as NDVI > 0.2) as quantitative indicators of bio-hydromorphological dynamics and its implications for riverine management. The analysis focused on the relationships between NDVI dynamics, flood magnitude, relative elevation along lateral channel morphology, and seasonal vegetation variability within a frequently disturbed channel. The results show that NDVI fluctuations strongly correspond to flood disturbances at lower relative elevations, whereas vegetation at higher elevations remains relatively stable. The annual maximum greenness ratio was well described by a logistic model along the cross-sectional transects. Annual greenness ratio exhibited clear seasonal patterns, showing a late-summer (August-September) greenness peak. These spatiotemporal and seasonal NDVI characteristics demonstrate the potential of Sentinel-2 imagery to operationalize both the "when" (timing) of vegetation management and the "where" (priority zones defined by relative elevation), providing a transferable, remotely sensed basis for flood-risk mitigation in frequently disturbed riverine environments.
Recent recognition of a new type of fluvial system - fluvial fans - introduces a fan-shaped channel network that appears similar to that of river-dominated deltas. Deltas form where rivers enter lakes and oceans, while fluvial fans are terrestrial landforms. However, fluvial fans can reach the shorelines of oceans or lakes, and in such cases the distinction between fluvial fan and river-dominated delta channel networks becomes ambiguous. We currently lack fundamental understanding of these two landforms' morphometric differences, despite their high socioeconomic significance, vulnerability to natural hazards, and key differences in how these landforms respond to global climate change and urbanization. Here we review the relevant conceptual differences in delta and fluvial fan network morphodynamics, propose a set of quantitative morphometric criteria to distinguish fluvial fan and delta channel networks, and test these criteria on 40 deltas and 40 fluvial fans from across the world. This initial attempt to contrast and distinguish deltas and fluvial fans based on their channel network morphometrics demonstrates that quantifying channel network angles (mean of 74.0 degrees for deltas and 55.0 degrees for fluvial fans) and trends in normalized channel widths and lengths provide efficient criteria, but some ambiguities remain that need to be resolved in future work. This research advances our mechanistic understanding of fluvial fan and delta channel networks and the recognition of modern and ancient landforms on Earth and other planetary bodies, such as Mars and Saturn's moon Titan.
River erosion is a fundamental process that impacts, among others, mountain landscape evolution. Mountain rock lithologies often exhibit bedding, joints, and fractures that are thought to alter the incision efficiency of rivers compared to intact, massive rocks. The presence of close enough planar mechanical discontinuities allows the creation and entrainment of large blocks through plucking, a process that adds to abrasion, and potentially macroabrasion, by the transported sediment. Despite preliminary attempts to include shallow fracturing in theoretical models of bedrock incision and a couple of studies that quantified the relative importance of abrasion and plucking processes in situ, we are still lacking ways to systematically probe the role of fractures on bedrock erosion rates and processes. Due to the complex interactions at play, here we investigate this question via an experimental approach, using a new erosion mill designed to erode a fractured concrete disk with a diameter of 17 cm. We simulate vertical or dipping fractures by embedding a 3D-printed plastic mesh in the concrete, using BVOH - a plastic that softens in cold water - creating mechanical weaknesses with a controlled pattern. We explore 10 different geometries and run 4 additional experiments without fractures for control. We record the topographic evolution every 2 min by photogrammetry and derive erosion maps by measuring elevation changes between successive scans. Our results show that fractures influence the morphodynamical evolution of the disks and the relative contributions of abrasion and plucking. However, abrasion systematically remains the dominant erosion mechanism, with plucking contributing at most to 29 % of the total erosion for vertical fractures spaced by 20 & times;20 mm2, 40 % for one specific dip angle (67 degrees), and less than 10 % for most experiments. Average erosion rates show a modest (20 %) increase with the fraction of plucking, but do not show a clear relationship with fracture density and the presence of fractures. We suggest that the rate of erosion by plucking is limited by the depth and slow rate of horizontal fracture propagation between pre-existing vertical fractures, such that in our experimental setup, abrasion is systematically a dominant component. These findings emphasize the critical role of block preparation and loosening for plucking to be an effective process compared to abrasion. This new setup allows abrasion, macroabrasion, and plucking driven by bedload impacts to be studied in controlled situations, albeit with the well-known limits of abrasion mills and without the variety of natural processes that can drive fracture propagation. Further experiments should expand the parameter space of the erosion efficiency problem (i.e., sediment mass, grain size, flow velocity, intact rock mass strength, 3D fracture patterns) to help in developing mechanistic models applicable in natural environments.
The Outdoor Experimental River Facility (OERF) is a new large-scale, semi-natural research facility designed to study river dynamics at scales that bridge small laboratory models and natural rivers. The facility comprises a 50 m long, 20 m wide floodplain corridor and is designed to sustain discharges up to 800 Ls-1, allowing subcritical, fully rough flow with field-like Reynolds numbers approaching 105 - beyond values typical of small-scale planform experiments constrained by Froude similarity. This paper reports the first of three planned experimental campaigns at the OERF, providing a foundational assessment of facility capabilities and operational constraints to guide subsequent sinuous and vegetated experiments with sediment recirculation. In an initial 338 h (similar to 14 d) straight-channel run without upstream sediment supply, a bi-modal gravel-sand bed (initial median diameter = 10 mm) progressively armoured to similar to 22 mm, and reach-scale planform change remained modest despite a width-to-depth ratio of 12 and near-threshold mobility. A three-phase, mathematically designed inlet bar-pool perturbation increased local velocities by 8 %-27 % and produced limited lateral bank erosion (similar to 2.5-7.5 cm). The results delineate a narrow operational window for sustained bar growth and migration, long adjustment times, practical constraints of outdoor operation, and the moderating role of bank-material strength and toe armouring. Together, these findings show that field-like hydraulics are achievable within the facility while clarifying what limits mobility at this scale, and they motivate future experiments that couple hydrodynamic similarity with controlled sediment recirculation/feed and refined boundary controls to advance understanding of controls on bank erosion and planform evolution.
Duricrusts form near the top of or within the regolith. Once exhumed, they are resistant to erosion and are often observed capping hilltops. Two hypotheses have been proposed to explain their formation. One calls upon seasonal fluctuations in water table height causing cycles of dissolution and precipitation that concentrate hardening species transported from distant sources. The other assumes that hardening is the ultimate phase of laterisation of the regolith by progressive leaching of the soluble elements that leads to in situ concentration of the hardening species. Here we propose a numerical model for the formation of duricrusts following the latter hypothesis, which we will term the in situ or laterisation (LAT) model. In , we developed a similar model representing the other model (named here the transport or Water Table Fluctuation (WTF) model).The LAT model we present here assumes that the rate of hardening is a self-limiting process that takes place at a rate determined by a laterisation time scale, tau l, and is linearly proportional to precipitation rate. Laterisation is accompanied by mass loss, at a rate set by a mass loss time scale, tau m, that can potentially be different from tau l and causes lowering of the topographic surface. We also test three laterisation modes, that depend on whether laterisation takes place above the water table only (percolation mode), below the water table (saturated mode) or everywhere (everywhere mode). This model for the formation of duricrusts is embedded in a previously published model for regolith formation .Here we present results obtained from the new LAT model by varying both the model parameters and the external forcing functions, namely, U the uplift rate and P, the precipitation rate. We show that duricrust formation by laterisation is favored by a small uplift rate as well as a strong precipitation rate. The smaller the laterisation time scale and the mass loss time scale, the thicker the duricrust, but if the ratio between the two time scales, tau m / tau l is too small, no duricrust can form or, in the saturated mode, the duricrust is progressively buried during its formation. We also derive a simple analytical expression for the conditions under which a duricrust will form within a regolith. This relationship implies that, as shown in , for regolith to form, the time scale for primary weathering, tau w, that controls the rate of propagation of the weathering front into the bedrock must be smaller than the erosion time scale, tau e, that controls the rate of surface erosion, and for a duricrust to form, the time scale for secondary weathering, or laterisation time scale, tau l, must be smaller than the primary weathering time scale.The model also predicts hardening (or duricrust) age distributions that can be compared to ages obtained by (U-Th) / He dating of goethite in ferricretes for example. We show that these age distributions can be used to differentiate between the different modes of laterisation. We also show how peaks in age distributions appear to correlate very well with climatic events, but not with periods of enhanced uplift (or base level fall). The model also predicts the total mass loss by chemical vs. physical erosion. We show that the ratio between the two is mostly a function of the laterisation time scale and how it varies during climate or tectonic cycles.Finally, we show how the model predictions can be compared to those of the WTF model to help determine by which process a given duricrust formed. We also show, however, that there might be situations where the geometry, thickness and position of the duricrusts may not be unequivocal signatures of a given process.
This study investigates spatial and temporal variability of bedload transport in four Swiss mountain streams using continuous Swiss Plate Geophone (SPG) monitoring. This surrogate measuring system had been calibrated in previous studies to produce reliable estimates of bedload transport rates. The measurements were analysed at two different time scales: short-term transport events typically covering a duration of a few weeks and multi-year annual transport totals. Power-law relations between dimensionless transport intensity and shear stress were derived to evaluate the temporal variability in the steepness of transport relations and in the reference shear stress. Results were compared with predictive equations developed for mountain streams. Findings show substantial variability both within and across sites, likely reflecting the influence of sediment availability, stream slope, streambed texture and flow history. Overall, continuous monitoring highlights the strong role of temporal spatial variability on bedload transport levels, possibly due to changing sediment availability and bed surface composition, and with implications for predictive modelling and river management.
The grain sizes of sediments in channels have been linked to landscape characteristics, such as flow distance from headwaters, topographic relief, lithology and climate, in landscapes with little past or present glacial influence. Few studies have explored the controls on sediment characteristics in formerly glaciated landscapes. In this study, we document river surface grain sizes at 279 localities across Scotland. We collect photographs of gravel bars through a citizen science survey, Scotland's Big Sediment Survey. Grain sizes distributions are extracted from the photographs using both manual and automated techniques. We investigate whether grain sizes can be correlated and predicted from environmental variables (e.g., basin slope, flow distance from headwaters) through Spearman's correlation statistics and random forest regression modelling. In contrast to other studies that have primarily focused on non-glaciated landscapes, we find no apparent controls on surface grain sizes in channels across Scotland. Specifically, we find no significant Spearman's relationships between d84 and environmental variables; the strongest relationship was found between d84 and average basin aridity with a weak r2 value of 0.34. We also find that the predictability of our random forest model is poor and only captures 20 % of the variance of d84. We find no correlation between grain size and flow competence, which suggests that sediment is both transport-limited and supply-limited. We propose that Scotland's post-glacial legacy drives the lack of sedimentological trends documented in this study, and that changes in landscape morphology and sediment sources caused by glacial processes lead to a complete decoupling between fluvial sediment grain size and environmental variables. This interpretation aligns with other studies that have highlighted the ongoing role of the post-glacial legacy on landscape evolution in tectonically quiescent terrains, both in Scotland and globally. Our results suggest that fluvial sediment grain size cannot be predicted by a global model based on environmental variables in post-glacial landscapes.
