Aggregation of data collected in 28 controlled experiments reveals reproducible debris‐flow behavior that provides a clear target for model tests. In each experiment ∼10 m3 of unsorted, ...water‐saturated sediment composed mostly of sand and gravel discharged from behind a gate, descended a steep, 95‐m flume, and formed a deposit on a nearly horizontal runout surface. Experiment subsets were distinguished by differing basal boundary conditions (1 versus 16 mm roughness heights) and sediment mud contents (1 versus 7 percent dry weight). Sensor measurements of evolving flow thicknesses, basal normal stresses, and basal pore fluid pressures demonstrate that debris flows in all subsets developed dilated, coarse‐grained, high‐friction snouts, followed by bodies of nearly liquefied, finer‐grained debris. Mud enhanced flow mobility by maintaining high pore pressures in flow bodies, and bed roughness reduced flow speeds but not distances of flow runout. Roughness had these effects because it promoted debris agitation and grain‐size segregation, and thereby aided growth of lateral levees that channelized flow. Grain‐size segregation also contributed to development of ubiquitous roll waves, which had diverse amplitudes exhibiting fractal number‐size distributions. Despite the influence of these waves and other sources of dispersion, the aggregated data have well‐defined patterns that help constrain individual terms in a depth‐averaged debris‐flow model. The patterns imply that local flow resistance evolved together with global flow dynamics, contradicting the hypothesis that any consistent rheology applied. We infer that new evolution equations, not new rheologies, are needed to explain how characteristic debris‐flow behavior emerges from the interactions of debris constituents.
Debris flows often occur in burned steeplands of southern California, sometimes causing property damage and loss of life. In an effort to better understand the hydrologic controls on post‐fire ...debris‐flow initiation, timing and magnitude, we measured the flow stage, rainfall, channel bed pore fluid pressure and hillslope soil‐moisture accompanying 24 debris flows recorded in five different watersheds burned in the 2009 Station and Jesusita Fires (San Gabriel and Santa Ynez Mountains). The measurements show substantial differences in debris‐flow dynamics between sites and between sequential events at the same site. Despite these differences, the timing and magnitude of all events were consistently associated with local peaks in short duration (< = 30 min) rainfall intensity. Overall, debris‐flow stage was best cross‐correlated with time series of 5‐min rainfall intensity, and lagged the rainfall by an average of just 5 min. An index of debris‐flow volume was also best correlated with short‐duration rainfall intensity, but found to be poorly correlated with storm cumulative rainfall and hillslope soil water content. Post‐event observations of erosion and slope stability modeling suggest that the debris flows initiated primarily by processes related to surface water runoff, rather than shallow landslides. By identifying the storm characteristics most closely associated with post‐fire debris flows, these measurements provide valuable guidance for warning operations and important constraints for developing and testing models of post‐fire debris flows.
Key Points
We present the first known in situ measurements of post‐fire debris flow
Debris‐flow timing tightly correlated with short duration rainfall intensity
Debris‐flow magnitude not correlated with cum. rainfall or soil water content
Debris flows can be described as rapid gravity-induced mass movements controlled by topography that are usually triggered as a consequence of storm rainfalls. One of the problems when dealing with ...debris flow recognition is that the eroded surface is usually very shallow and it can be masked by vegetation or fast weathering as early as one-two years after a landslide has occurred. For this reason, even areas that are highly susceptible to debris flow might suffer of a lack of reliable landslide inventories. However, these inventories are necessary for susceptibility assessment. Model transferability, which is based on calibrating a susceptibility model in a training area in order to predict the distribution of debris flows in a target area, might provide an efficient solution to dealing with this limit. However, when applying a transferability procedure, a key point is the optimal selection of the predictors to be included for calibrating the model in the source area. In this paper, the issue of optimal factor selection is analysed by comparing the predictive performances obtained following three different factor selection criteria. The study includes: i) a test of the similarity between the source and the target areas; ii) the calibration of the susceptibility model in the (training) source area, using different criteria for the selection of the predictors; iii) the validation of the models, both at the source (self-validation, through random partition) and at the target (transferring, through spatial partition) areas. The debris flow susceptibility is evaluated here using binary logistic regression through a R-scripted based procedure.
Two separate study areas were selected in the Messina province (southern Italy) in its Ionian (Itala catchment) and Tyrrhenian sides (Saponara catchment), each hit by a severe debris flow event (in 2009 and 2011, respectively).
The investigation attested that the best fitting model in the calibration areas resulted poorly performing in predicting the landslides of the test target area. At the same time, the susceptibility models calibrated with an optimal set of covariates in the source area allowed us to produce a robust and accurate prediction image for the debris flows activated in the Saponara catchment in 2011, exploiting only the data known after the Itala-2009 event.
Glacial debris flows are a common catastrophic hazard in alpine regions due to glacier recession and permafrost thawing. They are frequently triggered by heavy rainfall and outburst floods. To date, ...studies on debris flows originated from debris-ice mixture slope failures triggered by glacier melting in low-intensity rainfall conditions are still limited. In this study, a typical glacial debris flow that occurred in September 2020 on the west side of Namcha Barwa in southeast Tibet, China, is taken as a benchmark to reveal the initiation mechanisms and dynamics of debris-ice landslide-debris flow hazard chains. The flow discharge, erosion, entrainment, deposition, and dynamic process of the debris flow are evidenced by both field investigations and numerical simulations with a single-phase cell-based simulation program, Erosion–Deposition Debris Flow Analysis (EDDA) (Chen and Zhang, 2015) and a multi-phase flow model r.avaflow (Pudasaini and Mergili, 2019). This hazard chain was originated from a debris-ice mixture slope failure with a volume of 1.14× 106 m3 in a temperate area under little rainfall. The mobile mixture entrained saturated moraines and water in the ravine channel and rapidly transformed into a viscous debris flow, traveling about 9.78 km and entraining 50% more materials along the path. The peak discharge and frontal flow velocity at the outlet of the ravine reached 4700 m3/s and 11.4 m/s, respectively. About 3.75× 105 m3 of debris deposited on the riverbed and partially blocked the Yarlung Tsangpo, causing severe damage to local infrastructures. This study provides a scientific basis for understanding the evolution process of a debris-ice landslide-debris flow hazard chain and for minimizing its adverse impact on key infrastructures in the region.
•We report a debris-ice mixture slope failure that travelled about 10 km under little precipitation.•Parameters of the slope failure and subsequent debris flow were determined by combining multi-source data.•The hazard chain is reconstructed via single- and multi-phase modelling and evidenced by field observations.•Phase transformation from ice to water plays a key role in the mobility of the long-run debris-ice flow.•The volume amplification, flow depth and velocity, and erosion depth were simulated properly.
This study aimed to establish a simple approach to evaluate the performance of an intercept dam during a debris-flow event. We applied the two-phase debris-flow model proposed by Pudasaini (2012) and ...combined it with the dam moment-balance equation, to describe the state of the debris flow and the structural integrity and functions of such dams. To delineate dam performance, we introduced two new variables, representing reservoir capacity and anti-overturning stability of the dam, respectively. In addition, we developed a grid optimization technique to improve calculation efficiency. The feasibility of the proposed model was validated by simulating a debris-flow event in the Hongchun catchment, Sichuan, China, where a system of structures had been constructed to prevent debris flows, or to mitigate the possible hazard. Our results suggested that the process of debris-flow propagation and the variation in dam performance caused by the interaction between the debris motion and the dam were appropriately described. Further, we have presented a discussion on dam collapse and solid–fluid separation to clarify their effects on debris-flow mitigation.
•A two-phase model is combined with the dam moment-balance theory to describe the conditions of debris flow and the dam•Two new variables are introduced to describe dam performance•New grid-optimization technique is developed to improve calculation efficiency•Outburst flow after dam collapse can cause severe erosion and violent collision with downstream dam
PDF
