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Gravel transport in subaerial environments occurs through different flows that are conveniently classified as debris flows, debris floods and water flows based on their distinct morpho‐sedimentary dynamics and different implications for geomorphic hazard. Because distinctive features allowing gravelly sedimentary bodies to be ascribed to related genetic process are still a matter of discussion, this study aims to establish whether imbrication fabric represents a sedimentological fingerprint potentially applicable towards a more robust genetic classification of gravels. We analysed the fabric of 1007 imbricated clasts from modern and ancient deposits. Our results highlight statistically significant differences between imbrication fabrics in gravels deposited by different flows. Particles imbricated by water flows are typified by low imbrication angles (median of 35°) and elongated clasts oriented perpendicular to the flow. In contrast, debris‐flow gravels exhibit high imbrication angles (median of 65°) and elongated clasts oriented parallel to the flow. Debris‐flood deposits display elongated clasts both parallel and transverse to the main flow and intermediate values of imbrication angle (median of 47°). We propose that imbrication angles result from the combination of stability‐driven selection—a process acting under tractional transport and promoting the remobilization of high‐angle imbrication fabrics—and shear‐stress‐driven overriding—a mechanism leading to the formation of the higher imbrication angles—with the first dominating in water flows and the latter being effective in mass transport processes. The progressive change in imbrication fabrics from fluid‐gravity to sediment‐gravity flow deposits offers easily quantifiable sedimentological evidence to help in distinguishing genetic processes that contribute to the accumulation of gravels in alluvial and colluvial settings. Analysis of imbrication fabric can add valuable information, particularly as regards the classification of (1) coarse deposits in stratigraphic records and (2) modern debris flood deposits. [ABSTRACT FROM AUTHOR]

Severe fire on steep slopes increases stormwater runoff and the occurrence of runoff‐initiated debris flows. Predicting locations of debris flows and their downstream effects on trunk streams requires watershed‐scale high‐resolution topographic data. Intense precipitation in July and September 2013 following the June 2011 Las Conchas Fire in the Jemez Mountains, New Mexico, led to widespread debris flows in the watershed of Rito de los Frijoles. We differenced lidar Digital Elevation Models (DEMs) collected in 2010 and 2016 to map subwatersheds experiencing debris flows and changes in elevation of the trunk stream. Debris flow occurrence was well predicted by previous assessments of debris‐flow hazard; debris flows occurred in 7 of 9 sub‐basins where the debris‐flow hazard was above 60% for the 25‐year rainfall event, and in 0 of 21 basins where debris flow hazard was less than 60%. Debris flows resulted in fan deposition at the confluence with the trunk stream followed by transport during three documented floods. The bed of the 22 km trunk stream increased in elevation by a mean of 0.29 m, but the local change in thalweg elevation was controlled by inputs of water and sediment and longitudinal variation in gradient. Downstream of the mouths of tributaries with debris flows, the thalweg of the trunk stream rose as much as 2 m. Downstream of the mouths of tributaries without debris flows the thalweg of the main stem degraded by as much as 2 m, mobilizing sediment that was then deposited further downstream where the gradient of the trunk stream decreases. In conclusion, the transport of sediment generated by debris flows was predictably related to spatial variation in sediment supply, discharge and gradient. [ABSTRACT FROM AUTHOR]

The evolution of riverbeds is greatly impacted by changes in the water–sediment relationship resulting from the movement of debris flows. However, little is known about the influence of management in chronic sediment source areas on the evolution of main‐stem riverbed morphology. In this study, we used multi‐source remote sensing images covering the past 50 years along with water–sediment and topographic data from a river basin, and sediment transport data from debris flows, to analyze the evolutionary processes of the middle and lower reaches of the Daying River (DYR) at the China‐Myanmar border. Our results showed that (1) management of the sediment source area shifted the riverbed morphology from aggradation to erosion, transforming the river surface from wide and shallow to narrow and deep, and the channel from multi‐threaded to single‐threaded; (2) the active alluvial area, marginal bar area, and average river width exhibited an inverted V‐shaped trend, increasing by at least 23% and then decreasing by more than 59%; and (3) the evolution of riverbed morphology contributed to the stability of downstream riverbeds, improved flood control, and enhanced utilization of the floodplains. [ABSTRACT FROM AUTHOR]

Controlling sediment to reduce debris-flow hazard is generally approached using retention basins that can be closed or have an outlet structure, generally an open check dam. They are usually placed in mild slope zones that allow minimal works for the excavation and the foundation of the outlet structure if present. Recently, it has been shown that the detention of sediments can also be achieved in the high-sloping reaches of debris-flow channels using deposition areas, basins that are open on the downstream side. In this work, we propose an approach for controlling the sediment volume transported by debris flows consisting of a cascade of deposition areas and retention basins. We also include a framework for planning, sizing, and checking the works. Two scenarios are considered, corresponding to the maximum values of the debris-flow peak discharge and volume, respectively. Moreover, the presence or absence of boulders is also considered. For this purpose, a method that evaluates the clogging of a single open check dam as a function of the coarse fraction of the sediment volume is simply extended to the case of multiple dams and implemented in a routing model. The proposed approach is applied along Ru Secco Creek in northeast Italy to defend a resort area and a village hit by a high-magnitude debris flow in 2015. After a careful survey and study, a solution with a combination of deposition areas and retention basins is planned and sized. The validity and performance of the proposed solution are analyzed using debris-flow modeling for two scenarios, considering both the absence and presence of boulders. Most of the sediment volume transported by debris flows is trapped, and a small solid discharge flows downstream of the works. [ABSTRACT FROM AUTHOR]

The ratio of channel width to channel depth (α) is widely used to quantify the valley geometry of natural alluvial river channels. However, rare studies discussed the feasibility of α to constrain the changes of tectonic and climatic forcing in landscape evolution. To reveal the implication of alluvial valley α values on the effect of rock uplift, we selected an ideal site of the Rumei catchment (RMC) in the mountains of southeast Tibet, where is crossed by the active Lancang River Fault (LCRF) with different spatial characteristics. We used remote sensing, topographic analysis, and grain-size data to analyze channel valley geometry (e.g. valley width, depth, gradient, and α), valley geometry relations and grain-size distribution. Based on the present study, we found that there are three definite groups of α values (α ≤ 4, 4 < α ≤ 6, and α > 6) among the 13 channels in the catchment. The low α group corresponds to alluvial channels at the downstream end of the catchment, where the channels are steep and are controlled by rock uplift driven by thrusting. The medium α group includes channels in the headwaters of the catchment of RMC. High α channels are found in the mid-catchment location of RMC. The time of sediment transport in the mid-catchment has been constrained using radiocarbon ages on organic sediments in alluvial terrace deposits, whose results indicate that the high α channels there were disturbed by a major sediment transport event (debris flows or flood deposits) sometime between 630 and 1991 years ago. We observe that the grain size of deposits is not well related with variation of α values in the study area. We interpreted that the spatial difference of α values is dominantly controlled by the thrusting of LCRF. Hence, we concluded that α is a good indicator of the effect of rock uplift on channel morphology, which could be used to constrain the changes of tectonics in tectonically active mountains. [ABSTRACT FROM AUTHOR]

Mount Meager is a deeply eroded quaternary volcanic complex located in southwestern British Columbia (BC) and is known for its frequent large landslides. In 2010, the south face of Mount Meager collapsed, generating a long-runout debris avalanche that was one of the largest landslides (50 × 106 m3) in Canadian history. Over the past 14 years, the landslide deposit has been reworked by stream action, delivering large amounts of sediment to Lillooet River, just downstream. In this study, we investigate 10 years of geomorphic evolution of the landslide deposit using orthophotos and digital elevation models (DEMs) generated using Structure from Motion (SfM) photogrammetry on aerial photographs acquired during unmanned aerial vehicle (UAV) and Global Navigation Satellite System (GNSS) surveys. The SfM products were used to produce a series of precise maps that highlight the geomorphological changes along the lower Meager Creek within the runout area of the landslide. Comparison of DEMs produced from 2010, 2012, 2015, and 2019 imagery allowed us to calculate deposit volume changes related to erosion, transport, and redeposition of landslide material. We estimate that about 1.1 × 106 m3 of sediment was eroded from the landslide deposit over the period 2015–2019. About 5.2 × 105 m3 of that sediment was redeposited inside the study area. About 5.8 × 105 m3 of sediment, mainly sand, silt, and clay, were exported from the study area and are being carried by Lillooet River towards Pemberton, 40 km from Mount Meager, and farther downstream. These remobilized sediments likely reduce the Lillooet River channel capacity and thus increase flood hazards to the communities of Pemberton and Mount Currie. Our study indicates a landslide persistence in the landscape, with an estimated 47-year half-life decay, suggesting that higher flood hazard conditions related to increased sediment supply may last longer than previously estimated. This study shows the value of using SfM in tandem with historic aerial photographs, UAV photos, and high-resolution satellite imagery for determining sediment budgets in fluvial systems. [ABSTRACT FROM AUTHOR]

Field reconnaissance has difficulty in providing information on topographic features of debris flow in vegetation-covered mountain regions. The study used drone-captured multispectral imaging, digital terrain models, and geomorphic indices to investigate the sources and erosion patterns of gravelly debris flows in mountainous areas. Geomorphic indices, including the topographic wetness index (TWI), stream power index (SPI), sediment transport index, terrain ruggedness index (TRI), and the vegetation index normal differential water index (NDWI), were used to identify key topographic and hydraulic characteristics that contribute to the initiation of gravelly debris flows. The analysis revealed that the debris flows originated from the softening of yellow soil and the dislodging of gravel in loess areas, both induced by surficial water infiltration. Areas with high TWI and SPI values were vulnerable to erosion from surficial water, leading to the formation of erosion gullies. Likewise, regions with high TWI and NDWI values were likely starting points for upstream landslides. Conversely, areas characterized by low TWI and TRI values but high NDWI were prone to downstream sediment deposition. Drone-based multispectral imaging, augmented by geomorphic and vegetation indices, effectively captures the characteristics of vegetation-covered areas, thereby enhancing debris flow field investigations in an inaccessible mountain area. [ABSTRACT FROM AUTHOR]

The critical rainfall of runoff-initiated debris flows is utmost importance for local early hazard forecasting. This paper presents research on the critical rainfall of runoff-initiated debris flows through comparisons between slope gradients and three key factors, including topographic contributing area, dimensionless discharge, and Shields stress. The rainfall amount was estimated by utilizing in-situ rainfall records and a slope-dependent Shields stress model was created. The created model can predict critical Shields stress more accurately than the other two models. Furthermore, a new dimensionless discharge equation was proposed based on the corresponding discharge-gradient datasets. The new equation, along with factors such as contributing area above bed failure sites, channel width, and mean diameter of debris flow deposits, predicts a smaller rainfall amount than the in-situ measured records. Although the slope-dependent Shields stress model performs well and the estimated rainfall amount is lower than the in-situ records, the sediment initiation in the experiments falls within sheet flow regime due to a large Shields stress. Therefore, further sediment initiation experiments at a steeper slope range are expected in the future to ensure that the sediment transport belongs to mass failure regime characterized by a low level of Shields stress. Finally, a more accurate hazard forecast on the runoff-initiated debris flow holds promise when the corresponding critical slope-dependent dimensionless discharge of no motion, fluvial sediment transport, mass flow regime, and sheet flow regime are considered. [ABSTRACT FROM AUTHOR]

Sediments in steep channels can be mobilized to form stream flows, hyperconcentrated flows and debris flows, which can cause damage to downstream communities. However, the understanding of the sediment-transport mechanisms that control these processes remains incomplete due to the lack of effective monitoring methods. In this study, we utilize seismic data captured during these sediment-laden flows through field experiments and in situ monitoring to offer insights into flow mechanics and sediment transport mechanisms. Results show that sediment transport in stream flows and hyperconcentrated flows is primarily supported by viscous shear and turbulent stresses, whereas grain collisional stresses play a significant role in debris-flow dynamics. By characterizing impact rates, basal impulses and flow discharge, seismic monitoring can reveal the internal flow dynamics and bulk flow characteristics as well as the characteristics of sediment transport. Increasing solid concentrations can elicit positive nonlinearities in the frequency-based scaling relationships between seismic power and hydrographs, indicating transitions in the seismic signal from turbulence-bedload-dominated to bedload-dominated, and grain collisional-dominated regimes. By introducing the ratio of the real shear stress to the critical shear stress, we refined the phase space for sediment stability. Combining this criterion with the absolute seismic power enables us to establish ground-motion thresholds for distinguishing different flow types. Our results highlight opportunities to use seismic data for the quantitative inversion of these fluvial processes and debris flows as well as early warning strategies. Plain Language Summary In steep channels, runoff from intense rainfall transports mud and rocks downstream, resulting in the formation of a dynamic mixture of sediment and water. They not only alter the landscape but also present a significant threat to human life. Understanding the mechanisms of sediment initiation and transport is crucial for the accurately predicting and effectively mitigating these processes. In this work, we employ seismic equipment to quantify ground vibrations generated by diverse channel activities, which provide insights into the internal flow dynamics that are difficult to assess via traditional methods. Based on the ground motion caused by the moving solid-liquid mixtures, we classify sediment-laden flows into three types, each exhibiting distinct sediment transport mechanisms. This classification aligns well with the three field experiments and 11 in situ observations as well as previous observations, and can explain the nonlinear relationship between the seismic energy and flow discharges. Our findings demonstrate the potential of seismic monitoring in acquiring precise data on various channel activities as well as providing early warning signals. [ABSTRACT FROM AUTHOR]

The 2021 Navalacruz wildfire occurred in a mountainous area in the Sistema Central (Spain). Despite having an average low severity index (dNBR), the loss of vegetation cover associated with the fire was responsible for a high rate of sedimentation in the rivers and streams. Additionally, the burned area affected up to 60 cultural heritage sites, including archaeological and ethnological sites, and damage ranged from burnt pieces of wood to the burial of archaeological sites. In the present work, we document and analyze the post-fire evolution in several rivers and streams. This is based on a field survey of infiltration rates, hydrodynamic modeling, and the study of channel morphological changes. Our analysis revealed how the first post-fire rains caused the mobilization and transport of ashes. This created hydrophobicity in the soils, resulting in large amounts of materials being transported to rivers and streams by subsequent medium- and low-magnitude storms. A hydrological and hydraulic model of the study catchments under pre- and post-fire conditions suggests that these trends are a consequence of a post-fire increase in flow rates for similar rainfall scenarios. In this respect, our estimates point at a significant increase in sediment transport capacities associated with this post-fire increase in flow rates. The combination of locally steep slopes with high-severity fire patches, and a considerable regolith (derived from pre-fire weathering), resulted in a series of cascading responses, such as an exacerbated supply of sand to the drainage network and the triggering of debris flows, followed by erosion and entrenchment. [ABSTRACT FROM AUTHOR]

Summary: The main approach to managing the volume of sediment transported by stony debris flows routing along channels is through retention basins and open check dams, usually built in the lower reach just upstream of inhabited areas where slopes are gentler. In some cases, these measures are not sufficient to retain all the volume of sediment transported by debris flows. Works for trapping the sediments should also be placed in the upper reach of debris‐flow channels. In this area, where the channel bed is characterized by very high slopes and vertical variability over time, constructing and maintaining transversal embankments for sediment retention is difficult. In addition, they could also be susceptible to failure, potentially increasing the magnitude of the phenomenon instead of mitigating it. On the other hand, other types of works, such as solid‐body check dams or net barriers, have a decreasing efficiency over time in reducing the volume of sediment transported by debris flows. An alternative solution could be a retention basin open on the downstream side, that is, without the ending transversal embankment or berm. Therefore, it can be designated as a deposition area because the retention effect of the downstream embankment is missing, and sediment deposition only occurs due to the flatness of the basin. For sizing purposes, here, we derive two relationships between the deposition area and the volume of the deposit. These two relationships are derived from a physically based geometrical approach and empirical approach, respectively. After analyzing the morphology of the depositional processes that occurred in an existing deposition area during debris‐flow events, we test the validity of the two relationships. Overall, these can be used independently or in combination for designing a deposition area. At last, we introduce the necessary geomorphological conditions for the construction of the deposition area and guidance for its placement. [ABSTRACT FROM AUTHOR]

In mountain regions, the impact of areas on the sediment conveyance can not only be described by their susceptibility to debris flow release, but also by their structural connectivity to the rivers. This generates the need to combine susceptibility and connectivity for accurate analyses of sediment transport. Our study exploits an approach developed by [Steger, er al. 2022; https://doi.org/10.1002/esp.5421] and upscales it to the South Tyrolean Dolomites region. The approach comprised the modeling of debris flow release susceptibility using an interpretable machine learning algorithm, the training of a logistic regression model, and the combination of the resultant classified maps to create a joint susceptibility-connectivity map. The results show the quantitative thresholds for the susceptibility probability and the Index of Connectivity (IC) that allow to discriminate between susceptible and not susceptible, as well as connected and disconnected areas, which are represented via a variety of maps. [ABSTRACT FROM AUTHOR]

Purpose: Mountain rivers differ from alluvial rivers due to their morphology and hydraulic characteristics. Within various types of mountain rivers, the canyon rivers are those under the direct or indirect influence of canyon landscapes. Studies involving canyons are extensive in the literature. However, there is a lack of knowledge to establish a relationship between morphology, topography, local sediments, and river channel characteristics. This study aims to understand the relationship between morphology, topography, and local sediments in a canyon river in Southern Brazil. Methods: Topobathymetric data were obtained during a field survey using a total station and an RTK-GPS device for three reaches (canyon landscape (CL), transition landscape (TL), and floodplain landscape (PL)) in the canyon river. Features such as bankfull width, depth, channel slope, and floodplain width were obtained. Geomorphic patterns of the reaches were evaluated in terms of their classification and similarity level through cluster analysis. Sediment size was surveyed using the zig-zag pebble-count procedure. Results: Results suggest that sediment supply is an essential characteristic of the canyon's influence on river morphology. This condition improves the heterogeneity observed in the Geomorphic Unit Tool and topographical results. The thalweg lines are more complex in CL than the TL and PL reaches. Three grain-size-distribution curves presented similar sediment size and mean diameters. Sediments larger than d80 remain immobile or, at least, need fractioning for transporting or depositing somewhere. Alternatively, these sediments may be transported during debris flow events or discharges with a high return period. Conclusion: The influence of the canyon on river characteristics weaken from upstream, close to the gorge, to downstream toward the floodplain. The valley shape can affect sediment transport since the entrenchment caused by the canyon walls influences the water level and the hydraulic conditions for incipient motion conditions. [ABSTRACT FROM AUTHOR]

Post-wildfire floods are receiving greater attention as wildland-urban interfaces become more common and as catastrophic wildfires have increased in frequency. Sediment sourcing, transport, and deposition in the post-fire environment receive attention due to the severity of risk caused by debris flows and concentrated sediment flood flows. This study provides a series of sediment model predictions based on MUSLE and the WARSSS suite of models that included: ERMIT, BANCS, and FLOWSED/POWERSED for the 2019 Museum Fire (809 Ha of steep slope Pinus ponderosa forest in the Spruce Wash watershed). A comparison is provided for the internet-based WEPPcloud post-fire sediment model. Empirical evidence from four floods in 2021 indicated 9,900 Mg of sediment yield to city of Flagstaff neighborhoods, the WEPP model estimated 3870 Mg/year, MUSLE predicted 4860 Mg/year (based on the four events), and the WARSSS suite of models predicted 4630 Mg/year. Both WEPP and WARSSS estimated more sediment yield from channels than hillslope (51 %/49 % and 60 %/40 % respectively) though the spatial patterns differ between the models. Sediment mitigation structures, or "work areas", are discussed as real-world applications of sediment forecasting for reducing downstream impacts. Continued revisions of sediment forecasts, based on case studies such as this one, can provide managers and policy makers with tools for risk mitigation and emergency management. [ABSTRACT FROM AUTHOR]

Post-wildfire floods are receiving greater attention as wildland-urban interfaces become more common and as catastrophic wildfires have increased in frequency. Sediment sourcing, transport, and deposition in the post-fire environment receive attention due to the severity of risk caused by debris flows and concentrated sediment flood flows. This study provides a series of sediment model predictions based on MUSLE and the WARSSS suite of models that included: ERMIT, BANCS, and FLOWSED/POWERSED for the 2019 Museum Fire (809 Ha of steep slope Pinus ponderosa forest in the Spruce Wash watershed). A comparison is provided for the internet-based WEPPcloud post-fire sediment model. Empirical evidence from four floods in 2021 indicated 9,900 Mg of sediment yield to city of Flagstaff neighborhoods, the WEPP model estimated 3870 Mg/year, MUSLE predicted 4860 Mg/year (based on the four events), and the WARSSS suite of models predicted 4630 Mg/year. Both WEPP and WARSSS estimated more sediment yield from channels than hillslope (51%/49% and 60%/40% respectively) though the spatial patterns differ between the models. Sediment mitigation structures, or "work areas", are discussed as real-world applications of sediment forecasting for reducing downstream impacts. Continued revisions of sediment forecasts, based on case studies such as this one, can provide managers and policy makers with tools for risk mitigation and emergency management. [ABSTRACT FROM AUTHOR]

Intense rainfall events in mountain regions result in pronounced geomorphic activity including landslides, debris flows, sediment transport, and large woody debris. This study investigated the factors that affect wood deposition patterns during debris flows and the influence of wood on sediment deposition. We conducted flume experiments to simulate debris flow fans with driftwood and divided the experiments into plain and confluence sets. The plain set considered the condition where the debris flow tributary met a mild terrain, whereas the confluence set considered where the tributary met a mainstream. The results revealed that wood with roots was liable to form a log jam on the tributary, and small logs could travel long distances. In the confluence set, the mainstream flow caused asymmetric expansion of the debris flow fan and drove the small wood logs drifting downstream. In addition, wood logs may affect the sediment morphology, particularly in the plain set, which has no mainstream effect. In the plain set, driftwood constricted the development of fans in both volume and area. The wood also deflected the path and deposition of debris flows, causing an uneven fan surface. Our experiment results were consistent with the Japanese historical events, including the asymmetrical fans expansion in the confluence environment and the correlation between the travel distance and the size of the driftwood. This study provides valuable insights into the deposition patterns of debris flows with driftwood in mountain regions, filling the gap left by previous researches in driftwood-influenced debris flow deposition on fans. [ABSTRACT FROM AUTHOR]

The Upper Precambrian section in the Sandalash, Talas, and Chatkal ranges are studied to refine the age and genesis of Neoproterozoic diamictites of the Middle Tianshan. The results of detrital zircon dating show that diamictites were deposited in the Cryogenian and Ediacaran. The diamictites in the lower part of the Shorashu Formation comprise tillites, which were likely deposited at the end of the Cryogenian during the Marino glaciation (~640–635 Ma). In the upper part of the Shorashu Formation, the diamictites represent debris flows and grain flows associated with turbidites and, apparently, have nonglacial origin; their age is no older than the Early Ediacaran 616 ± 7 Ma. The pinching out of the Shorashu Formation to the north and the predominantly southerly directions of sediment transport in turbidites indicate that a non-depositional uplift located north of the basin. The distribution of detrital zircon ages in the studied samples are characterized by the main peaks within the ranges of 700–800 and 1800–1900 Ma and subordinate peaks at 2400–2500 Ma and are almost identical to the distributions of ages in the Proterozoic sedimentary rocks of the Tarim Craton. The comparison of the studied sequences with the sections of the Kuruktag Range in the northeastern Tarim Craton suggests that the diamictites of the lower part of the Shorashu Formation are analogs of the Tereeken tillite, which deposited at the end of the Cryogenian, whereas the slope facies of the upper part of the section are correlated with turbidites of the Zhamokti and Yukkengol formations of the early Ediacaran. [ABSTRACT FROM AUTHOR]

Liang, Z.; Ji, D.; Zhang, X.; Lingyun, W.; Zhang, T.; Li, S.; Yong, J.; Luo, J.; Liu, Y., and Zeng, J., 2023. Carboniferous oceanic basin sediment-gravity flows in the margin of the Junggar Basin, NW China: Characterisation and implications for hydrocarbon potential and evolution pacing. Journal of Coastal Research, 39(5), 890–906. Charlotte (North Carolina), ISSN 0749-0208. Several Carboniferous deep-marine-sandstone reservoirs have recently been discovered in the Junggar Basin, NW China. These reservoir sandstones were deposited by sediment-gravity flows that occurred on the continental slope of the Junggar Basin, and they have high hydrocarbon exploration potential. In this study, careful observations and petrological and geochemical analyses of samples from outcrop sections of Carboniferous rocks from the Junggar Basin were conducted to establish the deep-marine sedimentary characteristics, depositional environments, and mechanism of these gravity-flow deposits, as well as a sedimentary model for their occurrence. The deep-marine, gravity-flow deposit types include those of turbidity currents, debris flows, and slumps. The debris-flow deposits can be divided into pebbly debris-flow deposits and sandy debris-flow deposits. Results allow the relationship between the type and nature of gravity flows and ocean basin evolution to be established. During the early Carboniferous, the ocean basin had sufficient space to accommodate incoming sediment, and the sediment transport distance was long. The gravity-flow sediments of this period are dominated by sandy debris-flow deposits and turbidity-flow deposits characterised by thin individual layers but thick accumulations of multiple deposits. With continuous narrowing of the oceanic basin during the late Carboniferous, the basin had a smaller accommodation space. The main types of gravity-flow deposition during this period were pebbly and sandy debris flows, with rare thin turbidity-flow deposits. These deposits were coarser grained than their early Carboniferous equivalents, and their cumulative thicknesses were smaller. The results of the study provide insights into the prediction of deep-marine reservoir sandstones and the distribution of hydrocarbons in the Junggar Basin. [ABSTRACT FROM AUTHOR]

We developed a new rule-based, cellular-automaton algorithm for predicting the hazard extent, sediment transport and topographic change associated with the runout of a landslide. This algorithm, which we call MassWastingRunout (MWR), is coded in Python and implemented as a component for the package Landlab. Given the location and geometry of an initial landslide body (i.e., landslide polygon), MWR models the downslope progression of the runout process and evolves the underlying terrain. Runout behavior is controlled by mass continuity, topography, and rules for erosion and deposition, which can be informed from field observations. MWR includes a calibration utility that uses a Markov Chain Monte Carlo algorithm to sample model parameter space and tune the model to match observed patterns of landslide runout extent, deposition and erosion. Output from the calibration utility informs probabilistic implementation of MWR. Here we demonstrate calibrated model performance relative to a range of observed runout phenomena and terrain, including debris flows in channelized, low-energy-dissipation terrains and debris avalanches on open-slope, moderate, to high-energy-dissipation terrains. We test model ability to predict runout probability at a case study site using parameters that were determined through calibration to a different site. Finally, we show how to use a calibrated MWR model to determine runout-probability from an expert-defined, potentially unstable slope and a landslide hazard map. [ABSTRACT FROM AUTHOR]

Steep landscapes evolve largely by debris flows, in addition to fluvial and hillslope processes. Abundant field observations document that debris flows incise valley bottoms and transport substantial sediment volumes, yet their contributions to steepland morphology remain uncertain. This has, in turn, limited the development of debris‐flow incision rate formulations that produce morphology consistent with natural landscapes. In many landscapes, including the San Gabriel Mountains (SGM), California, steady‐state fluvial channel longitudinal profiles are concave‐up and exhibit a power‐law relationship between channel slope and drainage area. At low drainage areas, however, valley slopes become nearly constant. These topographic forms result in a characteristically curved slope‐area signature in log‐log space. Here, we use a one‐dimensional landform evolution model that incorporates debris‐flow erosion to reproduce the relationship between this curved slope‐area signature and erosion rate in the SGM. Topographic analysis indicates that the drainage area at which steepland valleys transition to fluvial channels correlates with measured erosion rates in the SGM, and our model results reproduce these relationships. Further, the model only produces realistic valley profiles when parameters that dictate the relationship between debris‐flow erosion, valley‐bottom slope, and debris‐flow depth are within a narrow range. This result helps place constraints on the mathematical form of a debris‐flow incision law. Finally, modeled fluvial incision outpaces debris‐flow erosion at drainage areas less than those at which valleys morphologically transition from near‐invariant slopes to concave profiles. This result emphasizes the critical role of debris‐flow incision for setting steepland form, even as fluvial incision becomes the dominant incisional process. Plain Language Summary: Steep landscapes evolve due to erosion by debris flows—sediment‐rich slurries that transport abundant wood and other debris—in addition to erosion by rivers and sediment transport on hillslopes. Although debris flows have long been recognized for their ability to erode channels and transport sediment, quantifying how they set the form of steeplands has remained a challenge. We use a landform evolution model that includes debris‐flow bedrock erosion to produce synthetic landscapes that exhibit similar patterns in valley‐bottom slope as those observed in the San Gabriel Mountains, California. Notably, we observe that the drainage area where simulated erosion by rivers begins to exceed that by debris flows does not correspond with the drainage area where the form of valley bottoms exhibits a transition. Rather, river incision outpaces incision by debris flows at drainage areas smaller than the morphologic transition point, implying that even as debris‐flow incision decreases in magnitude relative to river erosion, debris flows still have an outsized ability to dictate the morphology of steep landscapes. Key Points: Steepland morphology in the San Gabriel Mountains, California, corresponds with catchment‐averaged erosion ratesA landform evolution model with an incorporated debris‐flow process law reproduces relationships between morphology and erosion rateModeled fluvial erosion outpaces debris‐flow incision upstream from the transition between colluvial valleys and concave channels [ABSTRACT FROM AUTHOR]

Debris flows move as surge waves and they transport volumes of sediment of various magnitudes at irregular time intervals. As exemplified by Jiangjia Gully (JJG; Yunnan Province, China), the sediment transport of such surges can fluctuate by up to four orders of magnitude within a single debris flow event. This study investigated the variation of the sediment transported by a series of surges based on observations of 3000 surges in JJG. It was found that the fluctuation in sediment transport evolved with surge progress and then finally decayed in a power law form with a decay coefficient of 0.1–0.5, which can be considered a representative of the underlying dynamics. Moreover, the cumulative sediment yield followed a unified sediment size distribution (SSD) that can be expressed as P (S) = C S - β exp (- S / S c) , with parameters well associated with the material composition of the surges. Moreover, this distribution also applies to transport processes in large-scale rivers, which suggests that the SSD could be used to predict the scale of sediment transport in rivers. [ABSTRACT FROM AUTHOR]

Driftwood is one of the important physical components in mountainous rivers which causes severe hazards due to the clogging of bridges, culverts, and narrow sections during floods. Therefore, the understanding of driftwood dynamics and mitigation measures are crucial for managing wood in rivers. Open check dams are the most commonly used engineering measure for preventing driftwood from reaching downstream areas. Nevertheless, these open check dams frequently lose their sediment transport function when they are blocked by sediment and driftwood, especially during major flood events. This paper proposes a new type of open check dam for preventing from clogging. Thus, flume experiments were conducted to examine the influence of different types of open check dams on the characteristics of driftwood deposition. For the model with wood length (LWD)=16.5 cm, wood diameter (D) =15 mm, and wood number (N)=172, the highest trapping efficiency was observed with 90.1% and 87.2% retention rates for the classical debris flow breaker and curved footed open check dams, respectively. Laboratory tests showed that through this proposed design, woody debris blockage in a very short time was prevented from the accumulation of woods beside the dam. In addition to this, most of the sediment passed through the check dam and most of the driftwood got trapped. It can be briefly stated that the geometrical design of the structure plays an important role and can be chosen carefully to optimize trapping efficiency. By designing this type of open check dams in mountain river basins, it may provide a better understanding of the driftwood accumulation and basis for the optimal design of these structures. Further development of the solution proposed in this work can pave the way for designing different types of open check dams for effective flood management. [ABSTRACT FROM AUTHOR]

The 2008 Wenchuan earthquake triggered rapid local geomorphic changes, shifting abundant material through exogenic processes and generating vast amounts of loose material. The substantial material movement increased the geohazard (flash floods, landslides and debris flows) risks induced by extreme precipitation in the area. Intervention measures such as check dams, levees and vegetated slopes have been constructed in specific locations to reduce sediment transport and thereby mitigate the impact of ensuing geohazards. This study assessed the short–medium-term effects of interventions, including multiple control measures, in a post-earthquake mountainous region. Taking the Xingping valley as an example, we used CAESAR-Lisflood, a two-dimensional landscape evolution model, to simulate three scenarios, unprotected landscape, present protected landscape and enhanced protected landscape, between 2011 and 2013. We defined two indices to assess the intervention effects of the three scenarios by comparing the geomorphic changes and sediment yields. The results show that the mitigation measures are effective, especially the geotechnical engineering efforts in combination with ecological engineering in the upstream area. The spatial patterns of erosion and deposition change considerably due to the intervention measures. Additionally, the effectiveness of each intervention scenario shows a gradual decline over time, mainly due to the reduction in the reservoir storage capacity. The enhanced scenario performs better than the present one, with a more gradual downward trend of effectiveness. The simulation results evaluated the ability and effectiveness of comprehensive control measures and will support optimal mitigation strategies. [ABSTRACT FROM AUTHOR]

Extreme rainfall events in mountainous environments usually induce significant sediment runoff or mass movements—debris flows, hyperconcentrated flows and stream flows—that pose substantial threats to human life and infrastructure. However, understanding of the sediment transport mechanisms that control these torrent processes remains incomplete due to the lack of comprehensive field data. This study uses a unique field data set to investigate the characteristics of the transport mechanisms of different channelized sediment‐laden flows. Results confirm that sediments in hyperconcentrated flows and stream flows are mainly supported by viscous shear and turbulent stresses, while grain collisional stresses dominate debris‐flow dynamics. Lahars, a unique sediment transport process in volcanic environments, exhibit a wide range of transport mechanisms similar to those in the three different flow types. Furthermore, the Einstein number (dimensionless sediment flux) exhibits a power‐law relationship with the dimensionless flow discharge. Machine learning is then used to draw boundaries in the Einstein number‐dimensionless discharge scheme to classify one flow from the other and thereby aid in developing appropriate hazard assessments for torrential processes in mountainous and volcanic environments based on measurable hydrologic and geomorphic parameters. The proposed scheme provides a universal criterion that improves existing classification methods that depend solely on the sediment concentration for quantifying the runoff‐to‐debris flow transition relevant to landscape evolution studies and hazard assessments. Key Points: Dimensionless analyses reveal distinct transport mechanisms of debris, hyperconcentrated, and stream flowsBoundaries drawn by Support Vector Machines distinguish different flows in the Einstein number and dimensionless flow discharge phase diagramLahars exhibit a wide range of flow dynamics and sediment transport mechanisms from stream flow to debris flow [ABSTRACT FROM AUTHOR]

The ability to understand and predict coarse sediment transport in torrent catchments is a key element for the protection and prevention against the associated hazards. In this study, we collected data describing sediment supply at 99 torrential catchments in the Northern French Alps. The sample covers a wide range of geomorphic activity: from torrents experiencing debris flows every few years to fully forested catchments exporting small bedload volumes every decade. These catchments have long records of past events and sediment supply to debris basins. The mean annual, the 10-year return period and the reference volume (i.e. the 100-year return level or the largest observed volume) of sediment supply were derived for studied torrents. We examined the relationships between specific sediment supply volumes and many explanatory variables using linear regression and random forest approaches. Results showed that the ratio of sediment contributing area (bare soil) to catchment area is the most important predictor of the sediment production specific volumes (m3/km2). Others variables such as the Metlon index or the indices of sediment connectivity have also an influence. Several predictive models were developed in order to estimate the sediment supply in torrents that are not equipped with debris basins. [ABSTRACT FROM AUTHOR]

Drainage channels with step-pool configurations, one type of debris-flow drainage system used in gullies with steep gradients, have good application potential because of their high-performance control effect and engineering value. In this study, a pool depth design method is proposed based on the hydraulic jump theory. The dynamic behavior of debris flows in drainage channels with step-pool configurations is explored via a series of specially made flume experiments, where the debris-flow bulk density and the scale and depth of the step-pool configuration are the main controlling factors. The debris-flow pattern and flow depth are investigated along with the material-interchange phenomenon. There is a dramatic difference in the dynamic behaviors of different types of debris flows. Because of the associated intense turbulence and particle collision, the observed material-interchange phenomenon was greatest when the debris flow with a bulk density of 1700 kg/m3 passed through the drainage channel. The analysis indicated that, when a debris flow passes through a step-pool configuration, the flow depth increases significantly as a result of the hydraulic jump. However, a step-pool configuration with an overly large depth traps sediment, which is detrimental to transporting sediment. In addition, modified coefficients of the proposed design method were determined based on an analysis of the experimental results. The analysis further indicated that the coefficients significantly depend on the debris-flow viscosity and turbulence. Our results can be used to improve understanding of step-pool configurations and provide a reference for practical engineering design. [ABSTRACT FROM AUTHOR]

Knowledge of how debris flows result in the fan-shaped morphology around a channel outlet is crucial for mitigation of debris-flow-related disasters and investigation of previous sediment transport from the upper channel. Therefore, using a flume connected to a deposition area (inundation plane), this study conducted fan-morphology experiments to assess the effects of differences in grain-size distribution within debris flows on changes in fan morphology. Two types of debris-flow material, i.e., monogranular particles comprising monodispersed sediment particles and multigranular particles comprising polydispersed sediment particles, were used to generate monogranular and multigranular experimental debris flows, respectively. By adjusting the average grain size coincident between the monogranular and multigranular flows, we generated two types of debris flow with similar debris mixture hydrographs but different grain-size distributions in the flume. Although the flow depths were mostly similar between the monogranular and multigranular flows before the start of the debris-flow runout at the deposition area, the runout distances of the front of the multigranular flows were shorter than those of the monogranular flows. The difference in runout distance was responsible for the variations in the extent to and location in which the debris flows changed their direction of descent, resulting in the different shapes and morphologies of the fans in response to grain-size distribution. Although the direction of descent of the flows changed repeatedly, the extent of morphological symmetry of the debris-flow fans increased at a similar time during fan formation irrespective of the grain-size distribution. In contrast to this similarity in the rate of change in fan symmetry, the shift of the multigranular flow directions eventually increased the extent of asymmetry in fan morphology and expanded the scale of deviations in fan morphology between experimental test runs. Therefore, wide-ranging grain-size distributions within debris flows likely result in complex fan morphology with a high degree of asymmetry. [ABSTRACT FROM AUTHOR]

The 2008 Wenchuan earthquake triggered local geomorphic changes rapidly and gradually and produced abundant materials through external processes. The abundant materials increased the risks of geomorphic hazards (flash floods, landslides, and debris flows) induced by extreme precipitation in the area. To reduce sediment transport present in geomorphic hazards, intervention measures such as dams, levees, and vegetation revetments have been constructed in specified sites. This study concentrated on the assessment of intervention effects incorporated with various facilities on post-earthquake fragile mountains in the short-medium term. Take the Xingping valley as an example, we used the CAESAR-Lisflood landscape evolution model to simulate three different scenarios including unprotected landscapes, present protected landscapes, and enhanced protected landscapes in 20112013. We compared the geomorphic changes and defined two indicators to assess the intervention effects. The results showed that the mitigation facilities were effective, especially engineering measures that cooperated with vegetation revetments in the upstream area, and the present mitigation measures were inadequate to stop materials loss and prevent hazards from the upstream area. Moreover, the effectiveness reduced gradually caused by the storage capacity of dams decreased. The simulation methods assessed the ability and effectiveness of cooperated control measures and could support optimum mitigation strategies. [ABSTRACT FROM AUTHOR]

The rheological behavior of soil–water mixtures can affect the flow dynamics of natural processes such as flow-type landslides and sediment transport. In this study, an experimental investigation was carried out on soil–water mixtures containing particles up to 0.425 mm with volumetric solid concentrations of 0.05≤CV≤0.35 using a wide-gap rotational viscometer, and the contributions of particle–fluid, collisional, and colloidal interactions to their rheological behavior were evaluated thoroughly. The torque-scaling method was applied to identify the flow regime and extract the effective rheological parameters from turbulent flows. A transition between dilute and concentrated mixtures can be identified based on their sediment type, CV , and the predominance of particle–fluid and interparticle interactions. For dilute mixtures (CV<0.25–0.28) the particle-fluid interaction is dominant. The colloidal interactions induce another force scale that is independent of the shear rate. Thus, the Bingham model can be applied to such mixtures (debris or mud flood). For concentrated mixtures (CV>0.25–0.28), the shear-thickening rheology can be applied for granular mixtures (debris flows), because the collisional interactions are dominant, whereas shear-thinning rheology can be applied for fine-grained mixtures (mudflows), because colloidal interactions are dominant. [ABSTRACT FROM AUTHOR]

We conducted experiments using an experimental flume with two variable streambed gradients in the upstream and downstream parts with various debris flows, composition sizes, and supply flow rates. We investigated the transition processes of sediment transport modes along the longitudinal distances from the gradient change point using the transition mode indices, I C s ¯ x , I h ¯ x , and I U ¯ x ; these indices were calculated based on measurements of sediment transport concentrations, flow depths, and gravel migration velocities in the debris flow's front in the downstream part. Using these indices, we postulated that after the debris flow passed the gradient change point, the transition of the sediment transport modes progressed by changing the measured parameters to those in the steady-state condition on the gradient of the downstream parts. In addition, these indices suggested that the gravel migration velocities in the flow front interior changed most rapidly after passing the gradient change point, and that flow depths tended to change most slowly. Finally, the indices suggested that as the debris flow material became finer and the supplied flow rates became larger, the longitudinal transition sections tended to be longer because the momentum needed to transport the material was less than the total debris flow momentum. [ABSTRACT FROM AUTHOR]

Rainfall-induced shallow landslides and debris flows are one of the most common erosion process and primary channel initiation mechanisms in many steep landscapes. Their initiation conditions are physically controlled by the soil properties and topographically influenced by the competition between area- (A) and slope-dependent (S) sediment transport process. In this work, the A-S relationship of landslides in two forests was compared with respect to the physical properties of soil and plant roots. The results reveal that landslides in the Pinus tabuleaformis forest commonly have larger surface- and contributing area, deeper failure plane and gentler slope gradient than those in the Larix Kaemphferi forest. The saturated hydraulic conductivity in the Pinus tabuleaformis forest is higher and strongly correlates to plant root biomass. The effective cohesion and inner frictional angle of soil mass in the two forests are similar. Faster saturated hydraulic conductivity may lead to the higher upslope contributing area of landslides in the the Pinus tabuleaformis forest. A combination of finite-slope model and precipitation interception model reveals that landslides in the Pinus tabuleaformis forest require higher rainfall amount that those in the Larix Kaemphferi forest. Last but not least, this work provides a clue that strong root network and high saturated hydraulic conductivity may promote the A-S condition. [ABSTRACT FROM AUTHOR]

Debris-flow modeling is a great challenge due to its complex physical mechanism that remains poorly understood. The present research incorporates the effect of rheological features of the non-Newtonian fluid into the complete quasi-single-phase mixture model, which explicitly accommodates the interactions between flow, non-uniform sediment transport, and bed evolution. The effect of rheological features is estimated by Hersch–Bulkley–Papanastasiou model that can be simplified to Bingham or Newtonian models with specific coefficients. The governing equations are solved by a fully conservative numerical algorithm, using an explicit finite volume discretization with well-balanced slope-limited centered scheme combined with an implicit discretization method. One set of large-scaled U.S. Geological Survey debris-flow experiments is applied to investigate the influence of the non-Newtonian fluid on debris-flow modeling. It is found that the present model closed by Hersch–Bulkley–Papanastasiou model performs better than the models without considering effect of rheological features, which may facilitate the development of quasi-single-phase mixture modeling for debris flows. [ABSTRACT FROM AUTHOR]

Debris flow is one of the most dangerous natural processes in mountain regions and it occur in a wide variety of environments throughout the world. In the Italian Alps, some tens of thousands of damaging debris flow and, in general, torrential floods associated to intense sediment transport in secondary catchments have been documented in the last 300 years. These have caused socio-economic damage, damage to anthropogenic structures or infrastructures and in many cases casualties. Often, in the same basins, the occurrence of debris-flow processes recurs many years later. Prediction can often be spatial and based on the magnitude of the largest known process, while the temporal forecast is the most uncertain. It is also possible to increase the resilience of the population and of the territory. The present study aims at investigating different levels of debris-flow hazard in urban areas on Alpine alluvial fans and proposes a strategy for debris-flow prevention based on historical research and on a simplified analytical approach, methods that also involve relatively low costs. For such analysis, Ischiator stream catchment (ca. 20 km2) and its alluvial fan (NW Italy) were selected. This area was partly affected by historical torrential flood associated to intense sediment transport and debris-flow processes. Present-day instability conditions along the slope and the stream network were detected and synthesized through surveys and aerial photo interpretation integrated by satellite images (period 1954–2021). An estimation of the potential amount of moving detritus, referred to as debris flow, was carried out regarding the June 1957 debris-flow event, based on the predictive models. The individual hazard index value was estimated based on different methods. The results indicate that 56% of the area is exposed to flood associated to intense sediment transport hazard, which fluctuates from high to very high levels; such results are supported by debris-flow historical records. Since today almost half of the settlement (Bagni di Vinadio) is located on potentially risk-exposed areas, the urban evolution policy adopted after the 1957 event failed to manage the risk connection to debris-flow activity. [ABSTRACT FROM AUTHOR]

The occurrence of extreme rainfall events over mountain catchments is likely to trigger mass failure processes on hillslopes and sediment transport within the channel network. Understanding the degree of linkage between the sediment sources and transfer pathways downstream is essential in those catchments where settlements are affected by hydro-geomorphic processes, in order to plan suitable interventions for risk mitigation. A recent storm (October 2020) named "Alex", estimated to be an extratropical cyclone with a more than 100-year return period, hit the Mediterranean Alps at the border between Italy and France, triggering deadly landslides and flooding processes. This work focuses on two adjacent mountain catchments located in the Liguria region (Italy) which share the same outlet, where a small village was damaged by high-magnitude sediment transport and deposition processes during the storm. In order to analyze sediment dynamics at the catchment scale leading to severe damage to the urban settlements at the outlet, an integrated approach was used. Very high resolution (0.5 m) satellite imagery was employed to map the mass failure processes on the hillslopes with a visual interpretation. This dataset was then compared with a map of the Index of Connectivity created using the SedInConnect 2.3 software to characterize the pre-event structural connectivity in both catchments. The integration of these two datasets permitted understanding that debris slides evolving into debris flows downstream supplied most of the sediments to the channel network. These processes concentrated mainly in a specific catchment where hillslopes have the highest connectivity with the valley bottom. This outcome provided a clear evidence that inventory maps developed soon after an extreme event, coupled with sediment connectivity datasets, can provide relevant information for future land management and risk mitigation strategies. These can be exploited to prioritize structural interventions in specific areas, to reduce the connectivity or disconnect inhabited areas from the hydro-geomorphic systems and avoid future damage. • We analyzed the geomorphic impact of a storm on two adjacent mountain catchments. • Debris slides turned into debris flows represented the principal sediment sources. • The Index of Connectivity allowed knowing the degree of linkage with valley bottoms. • Part of the mobilized sediments travelled up to the outlet damaging an urban center. • Inventory maps coupled with sediment connectivity represent relevant potentialities. [ABSTRACT FROM AUTHOR]

Large earthquakes rapidly denude hillslopes by triggering thousands of coseismic landslides. The sediment produced by these landslides is initially quickly mobilised from the landscape by an interconnected cascade of processes. This cascade can dramatically but briefly enhance local erosion rates. Hillslope and channel processes, such as landsliding and debris flows, interact to influence the total mass, caliber, and rate of sediment transport through catchments. Calculating the sediment budget of an earthquake lends insight into the nature of these interactions. Using satellite imagery derived landslide inventories, channel surveys and a literature review combined with a Monte Carlo simulation approach we present a constrained sediment budget of the first decade after the 2008 Mw7.9 Wenchuan earthquake. With this sediment budget we demonstrate that debris flows are dominant process for delivering sediment into channels and that large volumes of sediment remain in the landscape. In our study area over 88% (469.7 Mega tonnes) of the coseismically generated sediment remains on the hillslopes in 2018. Of the 12% of the sediment that was mobilised, 67% (45.2 ± 22 Mt) was mobilised by debris flows. Despite the large proportion of sediment remaining on the hillslope, the frequency of debris flows declined significantly over our observation period. The reduction in debris‐flow frequency is not correlated to reductions in the frequency of triggering storms, suggesting changes in the mechanical properties of hillslope sediment may drive this observation. The stabilization of coseismically generated sediment greatly extends its residence time and may influence catchment sediment yields for centuries or millennia. Plain Language Summary: Earthquakes produce large volumes of sediment by triggering landslides in mountain ranges. After many earthquakes there is an order‐of‐magnitude increase in erosion rates, however this period of enhanced erosion is short lived. Understanding the processes which control the timespan of the elevated erosion rates and the rates at which they move sediment is vital for determining the continuing impact the earthquake has on a landscape. Using satellite imagery to map and track the movement of sediment after the 2008 Wenchuan earthquake we show that more than 88% (469.7 mega tonnes) of the sediment produced by the earthquake remains on the hillslope after a decade. Debris flows initiating in the landslide deposits are responsible for most of the erosion during this time. The frequency of these flows decreases rapidly after the earthquake indicating the sediment can stabilize quickly. The stabilized sediment could reside in the mountain range for hundreds or thousands of years indicating that it could have a significant impact on erosion rates and landscape evolution. Key Points: More than 88% (469.7 Mt) of sediment produced by the 2008 Mw7.9 Wenchuan earthquake remains on the hillslope 10 years after the eventDebris flows rather than fluvially driven erosion are the key process in transporting sediment from the hillslope into the main riverEvacuation of landslide deposits is highly stochastic indicating the need for long observation periods to estimate residence time [ABSTRACT FROM AUTHOR]

Traditionally, interactions between tributary alluvial fans and the main river have been studied in the field and in the laboratory, giving rise to different conceptual models that explain their role in the sediment cascade. On the other hand, numerical modeling of these complex interactions is still limited because the broad debris flow transport regimes are associated with different sediment transport models. Even though sophisticated models capable of simulating many transport mechanisms simultaneously exist, they are restricted to research purposes due to their high computational cost. In this article, we propose a workflow to model the response of the Crucecita Alta alluvial fan in the Huasco Valley, located in the Atacama Desert, Chile, during an extreme storm event. Five different deposits were identified and associated with four debris flow surges for this alluvial fan. Using a commercial software application, our workflow concatenates these surges into one model. This study depicts the significance of the mechanical classification of debris flows to reproduce how an alluvial fan controls the tributary–river junction connectivity. Once our model is calibrated, we use our workflow to test if a channel is large enough to mitigate the impacts of these flows and the effects on the tributary–river junction connectivity. [ABSTRACT FROM AUTHOR]

Summary: The retreat of alpine glaciers since the mid‐19th century has triggered rapid landscape adjustments in many headwater basins. However, the degree to which decadal‐scale glacier retreat is associated with systematic or substantial changes in overall coarse sediment export, with the potential to impact downstream river dynamics, remains poorly understood. Here, we use repeat topographic surveys to assess geomorphic change in four partly glaciated basins on a stratovolcano (Mount Rainier) in Washington State at roughly decadal intervals from 1960 to 2017. The proglacial extents of the four basins show distinct geomorphic trajectories, ranging from substantial and sustained net erosion to relatively inactive with net deposition. These different trajectories correspond to differences in initial (1960) valley floor gradients, and can be effectively understood as valley floor grade adjustments. Significant erosion was most often accomplished by debris flows triggered by extreme rainfall or glacial outburst floods, though a single rockfall mobilized more material than all other events combined. Year‐to‐year runoff events had little measurable geomorphic impact. Exported material tended to accumulate in broad deposits within several kilometers of source areas and largely remained there through the end of the study period. Over 10‐ to 100‐year timescales, we find that the impact of glacier retreat on coarse sediment yield may then vary substantially according to the geometry and storage trends of the newly exposed valley floor; the timing of that response may also be dictated, and potentially obscured, by stochastic and/or extreme events. [ABSTRACT FROM AUTHOR]

This work presents an analysis of a debris flow deposit below Earth’s surface in the Mammoth Cave System in Kentucky, USA, and is the first study to characterize an in-cave debris flow to this level of detail. The deposit, named Mt. Ararat by cavers, has a maximum thickness of 7 m, a head-to-tail length of 75 m, and a total volume of about 3400 m3, as determined by terrestrial LiDAR and electrical resistivity surveys. The deposit is chaotic, angular, matrix-supported, and roughly inversely graded, with grain sizes, quantified through various grain-size distribution measuring techniques, ranging from clay through boulders larger than 1 m. The clasts are predominantly Mississippian Big Clifty sandstone, which is allochthonous in this part of the cave. The angularity of the blocks in the deposit indicate that they had not experienced significant erosion; and therefore, are determined to have been transported only a relatively short distance over a short time. The deposit profile is compound in appearance with two heads. We thus interpret this as a debris flow deposit resulting from two distinct flow events, and present a chronology of events leading to the present-day Mt. Ararat in Mammoth Cave. The findings of this work will inform further studies of karst-related erosional events, sediment transport, and deposition at different scales in karst aquifers, as well as the ways in which surface and subsurface processes interact to contribute to karst landscape evolution. [ABSTRACT FROM AUTHOR]

Lake Chalco lies south of the Basin of Mexico and has been the subject of studies related to Late Quaternary climate variability. In 2016, the International Continental Scientific Drilling Program "MexiDrill Project" recovered a 520-m sediment record from Lake Chalco. Magnetic susceptibility measurements revealed substantial changes in sediment physical properties between 343 and 285 m depth, suggesting changes in composition associated with fluctuations in the depositional environment. We targeted sediments in the 343–285 m interval for high-resolution facies analysis, to develop a model of Lake Chalco formation. We identified three facies associations, using sediment composition, texture, mineralogy and micro-morphological characteristics: (1) detrital facies, consisting of laminated silt, massive sand, stratified silty sand, clast-supported gravel and matrix-supported gravel; (2) biogenic facies, which include diatom ooze and bivalve coquina; and (3) volcaniclastic facies, represented by clast-supported pumice deposits. We propose that formation of Lake Chalco occurred in four stages, which we identified by changes in sediment characteristics. The first stage was an alluvial delta environment, in which debris and hyper-concentrated flows were the main sediment transport agents. The second was characterized by turbulent flows in a fluvial deltaic environment, which alternated with laminar flows associated with floodplains. The third stage was a time of fluvio-lacustrine transition in the basin, with formation of the previously identified Paleo-Chalco-I Lake, in response to wet conditions. During the fourth stage, a deep eutrophic lake formed (Paleo-Chalco-II), with an origin that appears to have been related to regional volcanism. Our working age-depth model indicates establishment of the lake at ca. 400 ± 46 ka. This paper presents the only available record of the transition from alluvial to lacustrine sedimentation of Lake Chalco. Our results allow us to establish (1) how the lake was formed and what the phases of its development were, (2) how a major volcanic event altered and transformed the lacustrine sedimentation, and (3) which climatic conditions dominated during the lake formation. The age for the onset of the lacustrine sedimentation in Chalco is for the first time constrained to around 400 ka. This enables to expand our knowledge of the climate for a time for which there is no information from terrestrial records of tropical North America. [ABSTRACT FROM AUTHOR]

Traditionally, interactions between tributary alluvial fans and the main river have been studied on the field and in the laboratory, giving rise to different conceptual models explaining its role in the sediment cascade. On the other hand, numerical modeling of these complex interactions is still limited because the broad debris flow transport regimes are associated with different sediment transport models. Even though sophisticated models capable of simulating many transport mechanisms simultaneously exist, they are restricted to research purposes due to their high computational cost. In this article, we propose a workflow to model the response of an alluvial fan in the Huasco Valley, located in the Atacama Desert, during an extreme storm event. For the Crucecita Alta alluvial fan, five different deposits were identified and associated with different debris flow surges. Using a commercial software, our workflow concatenates these surges into one model. This study depicts the significance of the mechanical classification of debris flows to reproduce how an alluvial fan controls the tributary-river junction connectivity. Once our model is calibrated, we use our workflow to test if a channel is enough to mitigate the impacts of these flows and the effects on the tributary-river junction connectivity. [ABSTRACT FROM AUTHOR]

Wildfire risk has increased in recent decades over many regions, due to warming climate and other factors. Increased sediment export from recently burned landscapes can jeopardize downstream infrastructure and water resources, but physical landscape response to fire has not been quantified for some at‐risk areas, including much of northern California, USA. We measured sediment yield from three watersheds (13–29 km2) that drain to Whiskeytown Lake, California, within the area burned by the 2018 Carr Fire. Structure‐from‐Motion photogrammetry on aerial images combined with sonar bathymetric mapping of submerged areas indicated first‐year post‐fire sediment yields of 4,080 ± 598 t/km2 (Brandy Creek), 2,700 ± 527 t/km2 (Boulder Creek), and 305 ± 58.0 t/km2 (Whiskey Creek)—some of the first post‐fire yields measured in northern California and 64, 42, and 4.8 times greater than pre‐fire yields, respectively. These were measured during a wet year and resulted largely from rilling erosion and fluvial sediment transport, without post‐fire debris flows. Rilling preferentially developed in contact with dirt roads, aided by thin soils and exposed bedrock, and on slopes vegetated by chaparral pre‐fire. The second post‐fire year (a dry year) was characterized by fluvial reworking and delta progradation of the first‐year deposits and relatively little new sediment export. First‐year sedimentation of 111,000 m3 represented minor loss of storage capacity in Whiskeytown Lake but would be detrimental to smaller reservoirs; in general, increased sediment yields from western US watersheds as fire and extreme rainfall increase will likely pose risks to water quality and storage. Plain Language Summary: Climate change is increasing wildfires across many regions, including California. Burned landscapes typically produce large amounts of sediment after a fire, which reduces water quality, decreases space for water storage in reservoirs, and sometimes produces hazardous debris flows. We investigated landscape response after the 2018 Carr Fire in northern California. Sediment yield measured in the first year after the fire (a wet year) varied among three study watersheds, ranging from one to two orders of magnitude greater than before the fire. The primary means of sediment mobilization was by water flowing over the land surface, instead of by landslides. These results will aid prediction of future fire response elsewhere in northern California, a growing research need given the increasing tendency for large fires to affect northern California. Key Points: Aerial photogrammetric and sonar bathymetric mapping quantify sediment volume change for 2 years after the major 2018 Carr Fire, CaliforniaFirst‐year post‐fire sediment yields in three watersheds were 64, 42, and 4.8 times greater than long‐term yields, some of the first post‐fire data from northern CaliforniaHillslope erosion was dominated by rilling rather than landslides; rilling commonly occurred downslope of dirt roads [ABSTRACT FROM AUTHOR]

A mega-earthquake in a mountainous region can trigger thousands of landslides. Subsequently, these loose co-seismic materials will experience the erosion-transport-sediment process. This process is closely associated with a series of cascading hazards, such as landslides, debris flows and floods. The disturbance and legacy effects of earthquakes have become the focus of attention for post-earthquake reconstruction and disaster mitigation. By combining the modeling of seismic-induced landslide magnitude with an analysis of the coseismic material sediment connectivity and transfer processes, we assessed the impacts of the 2008 Wenchuan earthquake on the mass balance and subsequent geohazard activities. The 2008 Wenchuan earthquake produced 22,785 Mt of deposit storage, 37.9 % of which (∼8636 Mt) is connected to the channels and prone to erosion and transfer by hydrodynamic forces, and the remaining 62.1 % (∼14,149 Mt) was deposited on hillslopes far from the channel network and contributed little to the sediment dynamics. Long-term field observations found a causal relationship between decreasing surface erosion rate and gradual healing of co-seismic deposits due to coarsening by progressive removal of fine grains. We evaluated the residence time of fine-grained (<0.25 mm) landslide materials using the post-earthquake sediment evacuation rate and identified three stages regarding the enhanced erosion and surface mass wasting after the 2008 Wenchuan earthquake, namely, the high activity period (within 40.6 ± 5.4 years), the low activity period (40.6 ± 5.4–107.1 ± 14.2 years), and the steady period (after 107.1 ± 14.2 years). This has important implications for the hazard assessment of landslides and debris flows in understanding and modeling the post-seismic geohazard evolution over multiple timescales. • We explored the control of geological and environmental factors on the magnitude of co-seismic landslides. • A modelling of capturing seismic-shaking patterns derived from earthquake-induced landslides was developed. • We explained how the landslide-channel connectivity regulates post-seismic sediment transport. • we quantified the timescale of the enhanced surface mass wasting related to the 2008 Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

The implementation of check dams serves as a crucial measure in mitigating debris flow events, effectively stabilizing sediments and diminishing the magnitude of such occurrences. However, less attention has been paid to the fact that intact check dams also promote the release of sediment downstream, and the sediment control mechanism of the check dam during debris flow has not been clearly elucidated. In this study, according to the field survey data of the typical check dam and its surrounding sediment in Chutou Gully, several numerical flume models are built to explore the variation of the fluid kinetic energy and sediment alteration under the different channel gradients and different check dam setting patterns. An improved evaluation index for sediment transport of the check dam, DEI total , is put forward to evaluate the sediment control efficiency of the established flume models. The results show the influence range of the check dam can be generally divided into positive and negative regions. The positive regions effectively intercept sediment, while the negative regions enhance the fluid erosion on sediment and promote the release of sediment. Both the positive and negative regions of the check dam are affected by the channel gradient and dam height. In addition, with the increase in sediment diameter, the resistance of sediment to fluid erosion is significantly improved. • A detail investigation of the typical check dam and the surrounding sediment are presented. • An improved evaluation index is constructed and used to assess the check dam effect on sediment. • The control mechanism of the check dam on sediment during the debris flows were revealed. • We discussed the arrangement suggestion of the check dam resisting debris flows in gullies. [ABSTRACT FROM AUTHOR]

Historical patterns of debris flows have been reconstructed at the town of Forest Falls in the San Bernardino Mountains using a variety of field methods (mapping flow events after occurrence, dendrochronology evidence, soil chronosequences). Large flow events occur when summer thunderstorms produce brief high-intensity rainfall to mobilize debris; however, the geomorphic system exhibits properties of non-linear response rather than being a single-event precipitationdriven process. Previous studies contrasted the relative water content of flows generated by varying-intensity summer thunderstorms to model factors controlling flow velocity and pathway of deposition. We hypothesize that sediment discharge in this geomorphic system exhibits multiple sources of complexity and present evidence of (1) thresholds of sediment delivery from sources at the higher reaches of bedrock canyons, (2) storage effects in sediment transport down the bedrock canyons, and (3) feedbacks in deposition, remobilization, and transport of sediment across the alluvial fan in dynamic channel filling, cutting, and avulsion processes. An example of the first component occurred in March 2017, when snowmelt generated a rapid translational landslide and debris avalanche of about 80,000 m3; this sediment was deposited in the bedrock canyon but moved no farther down gradient. The second component was observed when accumulation of meta-stable sediments in the bedrock canyon remained in place until fluvial erosion and subsequent debris flow provided dynamic instability to remobilize the mass downstream. The third component occurred on the alluvial fan below the bedrock canyon, where low-water-content debris flows deposited sediments that filled the active channel, raising the channel grade level to levee elevation, allowing for subsequent spread of non-channelized flows onto the fan surface and scouring new channel pathways down fan. A conceptual model of spatial and temporal complexities in this debris-flow system is proposed to guide future study for improved risk prediction. [ABSTRACT FROM AUTHOR]

• An improved depth-averaged two-phase continuum flow model is adopted to investigate the effects of pore-fluid pressure and bed sediment conditions on multi-size debris flow. • New mechanisms are proposed to numerically explain the phenomenon of "the larger sediment sizes of bed materials, the higher erosion rate" and front coarsen of debris flows on the erodible bed. • The non-dimension number and energetics of the debris flows are investigated to demonstrate it is necessary to incorporate water-sediment and particle-particle interactions. Debris flows are classical two-phase flows that are greatly affected by bed sediment conditions. However, the underlying mechanism remains uncertain from either a theoretical or numerical perspective. Here an existing depth-averaged two-phase continuum flow model is further improved by incorporating the effects of pore-fluid pressure and bed sediment conditions. By numerical simulation of the USGS experiments, we have numerically reproduced the mechanism that "the coarser the bed sediments and the larger the basal pore pressure, consequently the larger the erosion rates". Moreover, an additional mechanism for this phenomenon has been revealed. Specifically, debris flows on steep slopes are likely to fall into a high shear stress regime, under which conditions the sediment transport capacity always takes a maximum value and is independent of the sediment size. Therefore, the sediment settling velocity that is proportional to the sediment size affects the erosion rate directly. It is also observed that the added potential energy due to a net erosion has positive effects on the kinetic energy of the debris flow. Notably, coarse and fine particles exhibit distinct roles in energetics and the former is characterized by more intensive interactions with the water phase, which should be appropriately considered in the modeling. Finally, particle size coarsening is observed in the head region, which is attributed to the hiding/exposure mechanisms of nonuniform sediments. These findings can enhance the understanding of the mechanism between the bed conditions and debris flow. [ABSTRACT FROM AUTHOR]

Wildfire alters vegetation cover and soil hydrologic properties, substantially increasing the likelihood of debris flows in steep watersheds. Our understanding of initiation mechanisms of postwildfire debris flows is limited, in part, by a lack of direct observations and measurements. In particular, there is a need to understand temporal variations in debris‐flow likelihood following wildfire and how those variations relate to wildfire‐induced hydrologic and geomorphic changes. In this study, we use a combination of in situ measurements, hydrologic monitoring equipment, and numerical modeling to assess the impact of wildfire‐induced hydrologic and geomorphic changes on debris‐flow initiation during seven postwildfire rainstorms. We predict the impact of hillslope erosion on debris‐flow initiation by combining terrestrial laser scanning surveys of a hillslope burned during the 2016 Fish Fire with numerical modeling of sediment transport throughout a 0.12‐km2 basin in southern California. We use measurements of sediment thickness within the channel to constrain numerical experiments and to assess the role of channel sediment supply on debris‐flow initiation. Results demonstrate that debris flows initiated during rainstorms where hillslopes contributed minimally to the event sediment yield and suggest that large inputs of sediment from rill and gully networks are not essential for runoff‐generated debris flows. Simulations suggest that both the gradual entrainment of sediment and the mass failure of channel bed sediment can increase sediment concentration to levels associated with debris flows. Finally, postwildfire debris‐flow initiation appears closely linked to the same rainfall intensity‐duration threshold despite temporal changes in the sediment source, initiation processes, and hydraulic roughness. Key Points: We use a numerical model to fingerprint the sediment that contributed to debris flows during a sequence of postfire rainstormsBoth gradual entrainment and mass failure of the channel bed can increase sediment concentrations to levels associated with debris flowsPostwildfire debris‐flow initiation seems closely linked to rainfall intensity despite temporal changes in sediment source and initiation style [ABSTRACT FROM AUTHOR]

Beam dams are a highly effective and commonly used open-type check dam in debris-flow hazard mitigation. In this study, dimensional analysis was used to obtain empirical equations for quantitatively determining the sediment-trapping and flow-regulating characteristics of a beam dam. To determine the coefficients of the empirical equations, flume experiments were conducted to simulate the trapping and regulating processes. The flow pattern, trapping, and regulating characteristics were investigated when debris flows passed through a beam dam. Debris-flow bulk density and peak discharge, and sediment-trapping ratios, were measured directly and indirectly. The results showed that three blocking actions occurred, and that blockage-breaking considerably influenced the trapping and regulating performance of the beam dam. The relative opening size and the sediment concentration were the two main factors affecting the performance of the beam dam. The ratio of trapping sediment decreased with relative opening, and increased with sediment concentration as well as reducing ratio of bulk density and reducing ratio of peak discharge. The sediment concentration and relative opening were the leading factors influencing the trapping and regulating sediment of a beam dam, followed by flume gradient. The results showed that the calculated values obtained using empirical equations were in good agreement with the values derived from the experiments, and that the deviation was acceptable. Finally, taking Zechawa Gully as an example, using the empirical equations we designed the opening size of a beam dam aimed to trap sediment and regulate peak discharge of debris flow in the main gully. [ABSTRACT FROM AUTHOR]

Sediment connectivity within a catchment depends largely on the morphological complexity of the catchment and is strictly related to the anthropogenic modification of the landscape. In this context, the present research evaluates the role of anthropogenic effects on landscape modifications and the resulting influence on sediment delivery. An assessment of sediment connectivity was carried out for three different human impact scenarios: (i) drainage system density reduction, (ii) road network variation and (iii) land use changes. In addition, shallow landslides were used as sediment source areas to evaluate the potential connection between these sediment sources and downstream areas (e.g. main channels and road network). Two small catchments in the Oltrepò Pavese area (Northern Apennines, Italy), with different size and morphological setting, were analysed: the Rio Frate (1.9 km 2 ) and the Versa (38 km 2 ) catchments. In both areas, several shallow landslides were triggered in 2009 (Rio Frate and Versa) and in 2013 (Versa). Results highlight the role of the landscape complexity in coupling/decoupling upstream sediment sources, such as shallow landslides, from the main channel network and roads. In addition, the analysis identified instability phenomena characterized by high connectivity values, allowing determination of the areas in which mobilized sediment could potentially damage important infrastructures such as the road network or contribute to flooding induced by aggradation or obstruction of the river bed. The proposed approach provides a methodological framework to help improve watershed and land management strategies, especially in shallow landslides prone-areas. [ABSTRACT FROM AUTHOR]