Societal risks associated with debris flow hazards are significant and likely to escalate due to global population growth trends and the compounding effects of climate change. Quantitative risk ...assessment methods (QRA) provide a means of anticipating the likely impacts and consequences of settlement in areas susceptible to landslide activity and are increasingly being used to inform land use decisions that seek to increase disaster resilience through mitigation and/or adaptation. Current QRA methods for debris flow hazards are based primarily on empirical vulnerability functions that relate hazard intensity (depth, velocity, etc.) to expected levels of loss for a given asset of concern, i.e. most of current methods are dedicated to loss-intensity relations. Though grounded in observed cause-effect relationships, empirical vulnerability functions are not designed to predict the capacity of a building to withstand the physical impacts of a debris flow event, or the related uncertainties associated with modelling building performance as a function of variable debris flow parameters. This paper describes a methodology for developing functions that relate hazard intensity to probability of structural damage, i.e., fragility functions, rather than vulnerability functions, based on the combined hydrodynamic forces of a debris flow event (hazard level) and the inherent structural resistance of building typologies that are common in rural mountainous settings (building performance).
Hazard level includes a hydrodynamic force variable (FDF), which accounts for the combined effects of debris flow depth and velocity, i.e. momentum flux (hv2), material density (ρ) and related flow characteristics including drag (Cd) and impact coefficient (Kd). Building performance is measured in terms of yield strength (Ay), ultimate lateral capacity (AU) and weight to breadth ratios (W/B) defined for a portfolio building types that are common in mountain settlements. Collectively, these model parameters are combined using probabilistic methods to produce building-specific fragility functions that describe the probability of reaching or exceeding successive thresholds of structural damage over a range of hazard intensity values, expressed in terms of momentum flux. Validation of the proposed fragility model is based on a comparison between model outputs and observed cause-effect relationships for recent debris flow events in South Korea and in Colombia. Debris flow impact momentum fluxes, capable of resulting in complete damage to unreinforced masonry buildings (URM) in those regions are estimated to be on the order of 24 m3/s2, consistent with field-based observations. Results of our study offer additional capabilities for assessing risks associated with urban growth and development in areas exposed to debris flow hazards.
•Method introduced is simple, consistent, and useful for quantitative risk analyses.•Models used for obtaining tsunami fragility curves are also useful for debris flows.•Method presented requires only 6 geological and structural parameters.•Complete damage momentum flux threshold in URML buildings in some zones is 24 m3/s2.•Debris flow fragility curve uncertainties are similar to those for other hazards.
Runoff‐generated debris flows are a potentially destructive and deadly response to wildfire until sufficient vegetation and soil‐hydraulic recovery have reduced susceptibility to the hazard. Elevated ...debris‐flow susceptibility may persist for several years, but the controls on the timespan of the susceptible period are poorly understood. To evaluate the connection between vegetation recovery and debris‐flow occurrence, we calculated recovery for 25 fires in the western United States using satellite‐derived leaf area index (LAI) and compared recovery estimates to the timing of 536 debris flows from the same fires. We found that the majority (>98%) of flows occurred when LAI was less than 2/3 of typical prefire values. Our results show that total vegetation recovery is not necessary to inhibit runoff‐generated flows in a wide variety of regions in the western United States. Satellite‐derived vegetation data show promise for estimating the timespan of debris‐flow susceptibility.
Plain Language Summary
Debris flows caused by excessive surface‐water runoff during intense rainfall can be a deadly and destructive hazard in mountainous areas after wildfire. In some cases, debris flows have only occurred in the burned area in the weeks to months after the fire, while, in other cases, debris flows occurred over several years. Though the recovery of vegetation is important for stabilizing sediment and reducing debris‐flow likelihood, uncertainty remains about how much recovery is needed to inhibit debris flows and about how much time is needed to reach this level of recovery. Knowing for how long debris flows are likely to be a hazard is important for managing risks to residents and infrastructure. To investigate this issue, we assembled a data set of 536 debris flows from the western United States and used satellite‐derived vegetation data to calculate the recovery condition of the burned area when each debris flow occurred. We found that the vast majority of the debris flows initiated when the burned area had not yet reached two‐thirds of its prefire vegetation condition. Burned areas that were slower to recover tended to experience debris flows over more protracted timescales.
Key Points
Majority (>98%) of western United States postfire debris flows occurred when leaf area index was less than 2/3 of typical prefire values
Total recovery of vegetation not necessary to inhibit debris flows
Remotely sensed postfire vegetation state useful to evaluate elevated debris‐flow susceptibility with time
Alluvial fans develop their semi‐conical shape by quasi‐cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris‐flow fans, these quasi‐cyclic avulsions are poorly ...understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris‐flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris‐flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run‐out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris‐flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low‐gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in‐channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris‐flow avulsion cycles are governed by the same large‐scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris‐flow fans. This novel result provides a basis for modelling of debris‐flow fans with applications in hazards and stratigraphy.
High‐intensity and short‐duration rainfalls can generate sudden and abundant runoff at the base of rocky cliffs that, entraining sediments, may originate debris flows. Two gauge networks have been ...set up in headwater sites of Dolomites (Northeastern Italian Alps) to monitor rainfall corresponding to the debris‐flow activity occurring there. The rain gauges are positioned both upstream and downstream the initiation areas of debris flows. Other five rain gauges sparse in the area integrate the two networks. In the years 2009–2020, rain gauges recorded rainfalls that triggered 41 debris flows. In most cases, rainfalls show a higher spatial variability along with both distance and altitude. Precipitation data are then compared with rainfalls estimated through a weather radar far about 70 km from there, to verify the possible interchangeability of the two measurement systems for the prediction of debris‐flow occurrence through suitable modeling of triggering discharges. The following results are obtained: (1) raw‐radar images mostly tend to underestimate precipitations recorded by rain gauges; (2) such underestimation entails, on average, a larger one on the simulated discharges and the prediction of debris‐flow occurrences (missed in 65% of the cases). Some methods for the correction on ground truth of raw‐radar images are applied to assess their use for evaluating the triggering discharges. Results show that once corrected using rain gauge data, radar‐derived rainfall estimates produce debris‐flow initiation predictions that more frequently match observations. Therefore, the presence of rain gauges close to the watershed centroids results essential for early warning systems based on triggering discharge modeling.
Key Points
In the last decade several debris flows have been triggered in the highly tourist Boite Valley
Rain gauge and radar recorded rainfalls have been used to simulate debris‐flow triggering discharges
In this area radar needs the presence of rain gauges to be used in early warning systems
Subaqueous sandy mass-transport deposits (SMTDs), which are a type of sedimentary deposit formed in deep water environments, have attracted increased attention in recent years. Based on the analysis ...of outcrops, cores and thin sections using X-ray diffraction and scanning electron microscopy, numerous sandy mass-transport deposits or sandy debrites have been identified in the delta-front and deep lake environments recorded by the Chang 6-7 Member of the Yanchang Formation in the Ordos Basin; thus, a mass-transport model has been established in this paper. These sandy mass-transport deposits are characterized by two distinct features: first, sandstones are characterized by a low matrix content and are defined as massive clean sands; second, massive sandstones are characterized by the widespread development of a rimed chlorite cement with a two-layer structure, which is identical to that present in distributary channel sands of the delta-front environment of the Yanchang Formation. The occurrence of and components of clay minerals indicates that the isopachous rimed cements in the inner layer are related to the original sedimentary environment. Rimed chlorite cements are formed from isopachous clay films that originally adhered to the surfaces of the detrital particles and represent syn-depositional products. Further studies suggest that during the process of retransporting delta-front sediments and forming the sandy debris flows, this material, which was adhered to the surfaces of clastic particles as isopachous clay coatings, together with the small amount of clay-water matrix present in the flow, acted as an “adhesive agent” between particles during the subsequent remobilization of the deposits and the formation of sandy debris flows. The adhesion strength and local matrix support strength created by the clay-water matrix (gels), together with the frictional strength of the sediments, provided support strength for the subaqueous mass transport of the Yanchang Formation. The adhesion formed by the isopachous clay rim or the clay-water matrix is especially dominant and is considered to be the root cause holding the sandy debris flows together during their mass transport. This research may have global implications for better understanding analogous subaqueous mass-transport processes and the distribution of sandstone reservoirs in other locations.
•Numerous sandy mass-transport deposits or sandy debrites are identified in the Ordos lacustrine basin.•The isopachous clay coatings are widely present on the clean sand grains in sandy debrites.•These clay coatings dictated the transport and deposition processes of the sand as it was remobilized down slope from a deltaic setting to a deep-water lake setting.•The clay coatings acted as an adhesive agent that was directly responsible for the type of sandy mass-transport deposit emplaced.
In South Korea, the risk of debris-flow is relatively high due to the country's vast mountainous topographical features and intense continuous rainfall during the summer. Debris-flows can result in ...the loss of human life and severe property damage, which can be made worse due to the poor spatiotemporal predictability of such hazards. Therefore, it is essential to research the preemptive prediction and mitigation of debris-flow hazards. For this purpose, this study developed an ANN model to predict the debris-flow volume based on 63 historical events. By considering the morphology, rainfall, and geology characteristics of the studied area in central South Korea, the data of 15 debris-flow predisposing factors were obtained. Among these data, four predisposing factors (watershed area, channel length, watershed relief, and rainfall data) were selected based on Pearson's correlation analysis to check for significant correlations with the debris-flow volume. To determine the best performing ANN model, a validation testing was carried out involving ten-fold cross-validation with MSE and R2 using both training and validation datasets, which were randomly split into a 7:3 ratio. The model performance validation results showed that an ANN model with two hidden neurons (4×2×1 architecture) had the highest R2 value (0.828) and the lowest MSE (0.022). In addition, in a comparative study with other existing regression models, the ANN model showed better results in terms of adjusted R2 value (0.911) using all datasets. Furthermore, 94% of the observed debris-flow volumes from the ANN model were within 1:2 and 2:1 lines of the predicted volumes. The results of this study have shown the potentiality of the developed ANN model to be a useful resource for decision-making and designing barriers in areas prone to debris-flows in South Korea.
•A prediction model for debris-flow volume based on an artificial neural network (ANN) is proposed and verified.•The systematic procedure of modeling an ANN model for predicting the debris-flow volume is described.•The developed ANN model was compared to other existing regression models.•The ANN model showed better results in terms of predicting the debris-flow volume than the existing regression models.
To cope with debris‐flow hazards, a common practice is the mapping of threatened areas through routing models. Considering the primary role of topography in affecting the mobility of gravity‐driven ...flows, its proper representation through digital elevation models (DEMs) is a requirement in routing modelling applications. The ‘quality’ of DEMs mainly depends on the quality, resolution and spatial arrangement of the topographic measurements (i.e. on the employed survey technology). Nevertheless, no attempt to systematically evaluate the influence of the topographic data source on the behaviour of routing models has been carried out. To address this, we initially assess the performances of both terrestrial‐ (i.e. global navigation satellite system, GNSS) and airborne‐based (i.e. full‐waveform LiDAR and structure‐from‐motion, SfM digital photogrammetry) survey technologies in characterizing the topography of a debris‐flow channel. Afterwards, we investigate whether the topographic data source can effectively influence the behaviour of a geographic information system (GIS)‐based cell routing model. Regarding the assessment of the survey technologies performances, the ‘standard’ statistic‐based approach indicated that GNSS and full‐waveform LiDAR can provide an accurate digital representation of the gully. However, the analysis of the shapes stressed that the most faithful and finer reproduction of the topographic singularities is yielded by the photogrammetrically reconstructed surface due to the extremely high data source resolution. Furthermore, the pairwise comparison of derived elevation models pointed out that meaningful discrepancies among tested survey technologies can be detected in morphologically complex areas because of the inherent limits of the terrestrial‐based method. Here, this research showed how these discrepancies have the potential to affect simulated flow dynamics, even if not in a meaningful way from a risk planning and management point of view. Overall, it appears that the topographic data source does not truly represent a determining factor in modelling applications of channelized debris‐flow routing.
Overall, despite the observed model sensitivity to the topographic data source, for the analysed channel reach, the recognized differences in the routing‐modelling outcomes can be regarded as negligible from a risk planning and management point of view, thus suggesting that the topographic data source does not truly represent a determining factor in modelling applications of channelized debris‐flow routing.
The run out and destructive potential of gravitational multi-phase flows is largely determined by the mixture composition, the material properties of the solid particles and the fluid. One instrument ...to expand the understanding of the governing processes of flow is laboratory experiments. In this study, we concentrate experimentally on landslide-induced stony debris flows as a particular type of flow-like mass movement. We aim to observe different natural flow types for varying initial and boundary conditions. In a laboratory flume, 12.0 m long, 1.3 m wide and 0.3 m deep, we initiate stony debris flows and measured flow variables such as flow depth, mass, bulk density, front velocity and front shape for varying particle size, solid volume fraction and basal roughness. Our experimental results reveal that flow type and evolution changes significantly for different solid volume fractions, as well as for different basal roughness. The particle size had a noticeable effect on flow velocity and front shape. The smooth surface facilitated rapid, shallow, and turbulent flows. In contrast, experiments with rough beds showed relatively lower velocities and dense flow behaviour. Although the flow parameters covered only a small spectrum of the naturally possible parameter space, flow phenomena such as phase-separation, longitudinal sorting, steep front surges, or front overtopping were observed. We group our observed flows regarding the flow properties and classify them into common flow types: (1) debris flood (hyperconcentrated flow), (2) debris flow, and (3) non-liquefied debris flow. To compare our results with natural events and other experimental results, we analyse the data with several dimensionless numbers. The flows were generally dominated by grain collision on the smooth surface. Naturally, frictional forces gain more influence on the rough surface but did not overrule collisional forces. Viscous forces played only a minor role in our experiments, due to the lack of highly viscous fluid. Overall, we infer, that our well-controlled experiments mimicked natural stony debris flow and give new profound insights into the causal relationship of how the initial and boundary conditions affect the flow evolution.
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•Landslide‐induced stony debris flows were initiated in a 6.7 m long laboratory flume.•Flow type and evolution change significantly for different solid volume fractions.•Particle size had a noticeable effect on flow velocity and front shape.•Experimental flows could be assigned to three natural flow types.•The flows were generally dominated by grain collision.
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