Mechanisms of Initiation, Runout, and Rainfall Thresholds of Extreme-Precipitation-Induced Debris Flows

The frequency of unprecedented extreme precipitation events is increasing, and consequently, catastrophic debris flows occur in regions worldwide. Rapid velocity and long-runout distances of debris flow induce massive loss of life and damage to infrastructure. Despite extensive research, understanding the initiation mechanisms and defining early warning thresholds for extreme-precipitation-induced debris flows remain a challenge. Due to the nonavailability of extreme events in the past, statistical models cannot determine thresholds from historical datasets. Here, we develop a numerical model to analyze the initiation and runout of extreme-precipitation-induced runoff-generated debris flows and derive the Intensity-Duration (ID) rainfall threshold. We choose the catastrophic debris flow on 6 August 2020 in Pettimudi, Kerala, India, for our analysis. Our model satisfactorily predicts the accumulation thickness (7 m to 8 m) and occurrence time of debris flow compared to the benchmark. Results reveal that the debris flow was rapid, traveling with a maximum velocity of 9 m/s for more than 9 minutes. The ID rainfall threshold defined for the event suggests earlier thresholds are not valid for debris flow triggered by extreme precipitation. The methodology we develop in this study is helpful to derive ID rainfall thresholds for debris flows without historical data.

Magnitude and Timing of the Tiltill Rockslide in Yosemite National Park, California

ABSTRACT Yosemite National Park, California, is one of the best-documented sites of historical rockfalls and other rock slope failures; however, past work shows that this record does not capture the infrequent largest occurrences, prehistoric events orders of magnitude larger than the largest historic ones. These large prehistoric events are evident as voluminous bouldery landslide deposits, permitting volume and age quantification to better understand local volume–frequency relationships, potential triggering mechanisms, and the hazard such events might pose. The Tiltill rockslide in northern Yosemite is one such example, consisting of 2.1 × 106 m3 ± 1.6 × 106 m3 of talus (1.5 × 106 m3 original volume of rock mass) that slid across the floor of Tiltill Valley, partially damming Tiltill Creek to create a seasonal pond that drains through and around the rockslide mass. This volume and the rockslide's effective coefficient of friction, 0.47, place it near the boundary between long-runout landslides and ordinary Coulomb failure. Although the rockslide superficially appears to consist of two separate lobes, statistically indistinguishable 10Be exposure dates from eight samples indicate a single event that occurred at 13.0 ± 0.8 ka. The age of the Tiltill rockslide and its relatively low elevation compared to equilibrium line altitudes at this place and time make glacial debutressing a highly unlikely triggering mechanism. Seismic shaking associated with fault rupture along the eastern Sierra Nevada is shown to be a plausible but unverified trigger.

Controls on the Effect of Impact Scraping on High-position and Long-runout Landslides

Landslides in mountainous areas act as an important control on morphological landscape evolution and represent a major natural hazard. The dynamic characteristics of a landslide directly relate to the distance it traveled and the scale of the resulting disaster. Based on extensive field investigations, we explored the effect of impact scraping on high-position landslides. During a rapid landslide, impact scraping amplifies the volume of the landslide and the size of the area affected by the landslide. Without acknowledging this effect, it is easy to underestimate the risk presented by a given potential landslide. In this study, we investigate the impact scraping of landslides that travel for significant distances both vertically (high-position) and horizontally (long-runout). There are four developmental stages of high-position, long-runout landslides: high-position shearing, gravitational acceleration, impact scraping, and debris deposition. Impact scraping amplifies the scale of the disaster by drastically increasing the volume of the landslide debris. After accounting for the effect of impact scraping, the total volume of the landslide exhibits a strong correlation with its travel distance. Additionally, the material properties of the erodible layer influence the landslide mobility. High-position and long-runout landslides have multiple scraping modes, including the embedding and excavation mode, the entrainment mode, the pushing and sliding mode, and the impact and splashing mode. In this study, we aim to provide insight that will improve the disaster modeling and risk assessment of high-position landslides, as well as to offer theoretical support for high-position and long-runout landslide dynamics research, disaster prevention and mitigation, and first responder rescue operation planning.

Experimental Preliminary Analysis of the Fluid Drag Effect in Rapid and Long-runout Flowlike Landslides

During a landslide, the multi-phase nature of landslide debris defines its mobility. Eventually, frictional forces cause the slide energy to dissipate, and contact forces transmit the energy into nearby material. To analyze the dynamic characteristics of high-velocity long-runout landslides, we conducted flume model tests to empirically determine the mobility characteristics of flow-like landslides with various slide materials. Our conclusions are as follows: (1) Liquid-phase flow-like landslides are highly mobility and have long runout; solid-phase flow-like landslides are highly destructive because of their higher kinetic energy; and two-phase flow-like landslides are both highly mobility. (2) During a two-phase flow-like landslide, the mobility ability of the liquid-phase material is stronger than that of the solid-phase material; when the liquid slide volume fraction is sufficiently large, the liquid phase exerts a drag force on the solid phase. (3) Various liquids exert different drag effects on the solid; the solid-liquid velocity difference and the liquid viscosity determine the drag intensity and the mobility and depositional characteristics of the landslide.