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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]

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]

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]

• 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]