Hydrometeorological Triggering of Periglacial Debris Flows Using a Bayesian Approach: A Case Study of the Hailuogou Gully Region, China

Mount Gonggais located in the east of the Qinghai–Tibet Plateau; many debris flows have occurred in small basins with a small glacier cover or snow cover in this area. The hydrometeorological conditions that caused debris flows in this region are complex, making forecasting and early warning difficult. Previous studies for these small-glacial-covered basins have primarily considered rainfall as the only inducing factor of debris flows, and often the effects of temperature are neglected. Thus, we carried out a probabilistic analysis of variables derived from hydrometeorological factors for the Mount Gongga region, Sichuan, China, where debris flows were recorded on 14 days between 1988 and 2019. By analyzing hydrological characteristics when debris flows occurred, three distinct dominant trigger types could be identified. The results show that 7 (50%) of the observed debris flow events during the study period, high-intensity rainfall was the dominant trigger, snowmelt by high temperature was identified as the dominant trigger for 2 (14%). Furthermore, 5 (36%) debris flow events could be attributed to the combined effects of long-lasting (or short-medium) rainfall and sustained higher temperatures. We find that the differences between the trigger types are statistically significant, and a susceptibility prediction differentiating between trigger types can outperform simple rainfall-only situations. This study contributes to an improved understanding of the hydrometeorological impact on debris flow initiation in high elevation watersheds.

Rainfall Monitoring to Support Temporal and Spatial Information of Debris Flow Initiation in Merapi Volcano

Debris flow frequently attacks rivers on slopes of Merapi Volcano and causes fatalities and damage of infrastructures. To reduce the risk of debris flow, a warning system has been developed by Sabo Office Center. Critical line and snake line graph are applied in Merapi Volcano to monitor characteristics of rainfall in the upland river basin. However, this warning system cannot predict the arrival time and location of the debris flow occurrence. Numerical simulation seems to be a good tool to improve its performance. This research proposed an idea to combine rainfall-based warning system with the numerical simulation model. This model used slope stability theory to identify debris flow initiation. Results of this research showed that fluctuation of rainfall intensity reflects changes of debris flow initiation area. The more severe rainfall intensity, the larger volume of surface flow, and thus the greater debris flow initiation takes place. When the rainfall monitoring is combined with the debris flow simulation results, there is a tendency of the enlargement of the debris flow area to follow the growth of the hourly rainfall.

Research Progress of Initial Mechanism on Debris Flow and Related Discrimination Methods: A Review

The initial of debris flow can be classified into two types based on their triggering positions, that is, debris flow from slope and debris flow from gully or channel. For the former, great progress has been achieved on the mechanisms of soil failure and liquefaction. The framework established by a series of theories or laws, such as the Mohr–Coulomb criteria, the unsaturated soil mechanics, and the critical state of soil mass, has been used widely in industry and research. However, the details and discrimination basis for the transformation process from landslide into debris flow still need to be further clarified. Relatively, debris flow from gully or channel is more complex due to its various mass sources and the diversity of processes. Nevertheless, through a great number of case studies and experimental statistics, people have gradually recognized the influential rule and critical condition of factors from landform, hydrology, and other aspects on debris flow initiation. Furthermore, based on the theories of granular flow, continuum mechanics, and rheological law, some typical event-based scenarios can also be reproduced by different single-/two-phase depth integral/average numerical models. However, some key knowledge on mechanism and application level is still insufficient, such as the erosion and entrainment mechanism of materials from different sources, the boundary tractions and materials exchange, as well as the selection of prediction indicators. Three current discriminated methodologies for debris flow initiation, that is, the safety factor method, the rainfall indicator method, and the comprehensive assessment method, were summarized in this article. Considering the technical limitation of each methodology, it is believed that the establishment or improvement of a unified, stable, and open-access database system for event registration and query, as well as the development of large-scale and high-precision rainfall monitoring, is still regarded as the important aspect of debris flow prevention in the future. In addition, as an economic and efficiency means for obtaining information on potential threats and real-time hazard messages, the multielement method for debris flow is recommended as a long-term reference.

Rainfall Warning Model for Rainfall-Triggered Channelized Debris Flow Based on Physical Model Test—A Case Study of Laomao Mountain Debris Flow in Dalian City

Debris flows are among the most frequent and hazardous disasters worldwide. Debris flow hazard prediction is an important and effective means of engineering disaster mitigation, and rainfall threshold is the core issue in debris flow prediction. This study selected the Laomao Mountain debris flow in Dalian as the research object and explored the relationship among the percentage of coarse sand content of soil, rainfall conditions and the critical rainfall values that induce debris flows on the basis of field investigation data, combined with the results of a flume test, soil suction measurement and geomechanical analysis. The new multi-parameter debris flow initiation warning models were obtained through the mathematical regression analysis method. The critical rainfall values of debris flows in this area were calculated by the previous research on the mechanism of hydraulic debris flow initiation (HIMM). Lastly, the multi-parameter debris flow initiation warning models were compared and analyzed with the critical rainfall values obtained using the HIMM method and the rainfall information available in historical rainfall data, and the reliability of the models was verified. The comparison results showed that the new multi-parameter debris flow initiation warning models can effectively modify the traditional intensity–duration model and have certain reliability and practical values. They can provide an effectual scientific basis for future work on the monitoring and prediction of debris flow disasters.

Towards real-time global assessment of post-fire debris flow hazards with remotely sensed data

<p>As the risk of wildfires increases worldwide, burned steeplands are vulnerable to the secondary hazard of widespread sediment mobilization through debris flows. Following an initial burn, sediment and soil previously restrained by vegetation are no longer consolidated, allowing for easy mobilization into channels and along steep hillslopes through runoff.  Sufficiently powerful rainfall incorporates entrained material into turbulent flows and serves as the primary trigger for debris flow initiation. There is thus an ongoing need to establish the relationship between rainfall and debris flow initiation based on a variety of spatiotemporal preconditions. Previous work establishes regional and local thresholds to constrain the effect of rainfall in recently burned areas, but no empirical or numerical solution has worldwide application. Building from regionally-based efforts in the U.S., this work considers how remote sensing data can be applied to better approximate the post-fire debris flow hazards worldwide using freely available global datasets and software. Our work assesses the utility of remote sensing resources for analyzing burn characteristics, topography, rainfall intensity/duration, and, thus, debris flow initiation. Early results show that global observations are sufficient to delineate background rainfall rates from storms likely to cause debris flows across a variety of burn severity and topographic conditions. However, the dearth of publicly-available post-fire debris flow inventories globally limit the ability to test how the model framework performs within different climatologic and morphologic areas. This work will present preliminary analysis over the Western United States and demonstrate the feasibility of a global, near-real time model to provide situational awareness of potential hazards within recently burned areas worldwide. Future work will also consider how global or regional precipitation forecasts may increase the lead time for improved early warning of these hazards.</p>

Debris flow initiation from ravel-filled channel bed failure following wildfire in a bedrock landscape with limited sediment supply

Steep, rocky landscapes often produce large sediment yields and debris flows following wildfire. Debris flows can initiate from landsliding or rilling in soil-mantled portions of the landscape, but there have been few direct observations of debris flow initiation in steep, rocky portions of the landscape that lack a thick, continuous soil mantle. We monitored a steep, first-order catchment that burned in the San Gabriel Mountains, California, USA. Following fire, but prior to rainfall, much of the hillslope soil mantle was removed by dry ravel, exposing bedrock and depositing ∼0.5 m of sandy sediment in the channel network. During a one-year recurrence rainstorm, debris flows initiated in the channel network, evacuating the accumulated dry ravel and underlying cobble bed, and scouring the channel to bedrock. The channel abuts a plowed terrace, which allowed a complete sediment budget, confirming that ∼95% of sediment deposited in a debris flow fan matched that evacuated from the channel, with a minor rainfall-driven hillslope contribution. Subsequent larger storms produced debris flows in higher-order channels but not in the first-order channel because of a sediment supply limitation. These observations are consistent with a model for post-fire ravel routing in steep, rocky landscapes where sediment was sourced by incineration of vegetation dams—following ∼30 years of hillslope soil production since the last fire—and transported downslope by dry processes, leading to a hillslope sediment-supply limitation and infilling of low-order channels with relatively fine sediment. Our observations of debris flow initiation are consistent with failure of the channel bed alluvium due to grain size reduction from dry ravel deposits that allowed high Shields numbers and mass failure even for moderate intensity rainstorms.

Change Detection and Feature Extraction of Debris-Flow Initiation by Rock-Slope Failure Using Point Cloud Processing

Our understanding of debris-flow initiation by slope failure is restricted by the challenge of acquiring accurate geomorphic features of debris flows and the structural setting of the rock mass in the remote mountainous terrain. Point cloud data of debris flows in Sabino Canyon, Tucson, Arizona, July 2006, with initiation by joint-controlled rock slope were obtained using multitemporal LiDAR scanning. Topographic changes were detected by comparing historical LiDAR scanning data of this area since 2005 by adopting open-source CloudCompare software. The results showed persistent scour and erosion in the debris flows after 2006. Point cloud data of joint-controlled rock in the initiation zone were generated by the means of photogrammetry using Pix4D software. The joint planes, the dip direction and the dip value of the joint plane, the joint spacing, and the joint roughness were therefore acquired by point cloud processing. Our study contributes a foundation for analyzing the relationship between the rock features, the generation of slope failure, and the initiation of debris flows.