Yong Li

Debris Flow Warning Threshold Based on Antecedent Rainfall：a Case Study in Jiangjia Ravine, Yunnan, China

Debris flows in Jiangjia Ravine in Yunnan province, China are not only triggered by intense storms but also by short-duration and low-intensity rainfalls. This reflects the significance of antecedent rainfall. This paper tries to find the debris flow-triggering threshold by considering antecedent rainfall through a case study in Jiangjia Ravine. From 23 debris flow events, the I-D (Intensity-Duration) threshold was found, which is very close to the line of 95th percentile regression line of rainfall events, representing that 95% of rainfalls can potentially induce debris flows and reflects the limitation of I-D threshold application in this area. Taking into account the effect of antecedent rainfall, the debris flow-triggering threshold for rainfall quantity and intensity is statistically and empirically derived. The relationships can be used in debris flow warning system as key thresholds. Coupling with the rainfall characteristics in this area, new thresholds are proposed as triggering and warning thresholds.

The formation and development of debris flows in large watersheds after the 2008 Wenchuan Earthquake

The Wenchuan earthquake has caused abundance of loose materials supplies for debris flows. Many debris flows have occurred in watersheds in area beyond 20xa0km2, presenting characteristics differing from those in small watersheds. The debris flows yearly frequency decreases exponentially, and the average debris flow magnitude increases linearly with watershed size. The rainfall thresholds for debris flows in large watersheds were expressed as Iu2009=u200914.7 D−0.79 (2xa0hu2009<u2009Du2009<u200956xa0h), which is considerably higher than those in small watersheds as Iu2009=u20094.4 D−0.70 (2xa0hu2009<u2009Du2009<u200937xa0h). A case study is conducted in Ergou, 39.4xa0km2 in area, to illustrate the formation and development processes of debris flows in large watersheds. A debris flow develops in a large watershed only when the rainfall was high enough to trigger the wide-spread failures and erosions on slope and realize the confluence in the watershed. The debris flow was supplied by the widely distributed failures dominated by rill erosions (14 in 22 sources in this case). The intermittent supplying increased the size and duration of debris flow. While the landslide dam failures provided most amounts for debris flows (57xa0% of the total amount), and amplified the discharge suddenly. During these processes, the debris flow velocity and density increased as well. The similar processes were observed in other large watersheds, indicating this case is representative.

Variation in grain size distribution in debris flow

Grain composition of debris flow varies considerably from fluid to deposit, making it uncertain to estimate flow properties (e.g., density, velocity and discharge) using deposit as done in practice. Tracing the variation of grain composition is thus more important than estimating some certain properties of flow because every debris flow event consists of a series of surges that are distinct in properties and flow regimes. We find that the materials of debris flows, both the fluid and the source soils, satisfy a universal grain size distribution (GSD) in a form of P (D) = CD - μexp( -D/Dc), where the parameters C, μ and Dc are determined by fitting the function to the grain size frequency. A small µ implies a small porosity and possible high excess pore pressure in flow; and a large Dc means a wide range of grain composition and hence a high sediment concentration. Flow density increases as µ decreases or Dc increases, in a power law form. A debris flow always achieves a state of certain mobility and density that can be well described by the coupling of µ and Dc, which imposes a constraint on the fluctuations of flow surges. The GSD also describes the changes in grain composition in that it is always satisfied during the course of debris flow developing. Numerical simulation using the GSD can well illustrate the variation of µ and Dc from source soils to deposits.

Activity and distribution of geohazards induced by the Lushan earthquake, April 20, 2013

An Ms7.0 earthquake, focal depth 13xa0km, struck Lushan on April 20, 2013, caused 196 deaths and 21 missing, 13,484 injuries, and affected more than two million people. A field investigation was taken immediately after the quake, and the induced hazards were analyzed in comparison with the Wenchuan earthquake. We have identified 1,460 landslides and avalanches and four dammed lakes, which were generally small and concentrated on high elevation. Avalanches and rockfalls developed in cliffs and steep slopes of hard rocks, including Jinjixia of Baosheng Town and Dayanxia of Shuangshi Town, Lushan, and the K317 section the Xiaoguanzi section north to Lingguan Town along the provincial highway S210. Landslides were relatively less, mainly in moderate and small scales, developing in sandstone, shale, and loose colluviums. Only one single large landslide was observed to turn into debris slide-flow. Dammed lakes were formed by avalanches and landslides, all in small size and of low danger degree. The earthquake-induced hazards distributed in belt on the hanging wall along the faults, and their major controlling factors include tectonics, lithology, structure surface, and landform. More than 99xa0% landslides were within 30xa0km to the epicenter, and 678 within 10xa0km, accounting for 46xa0% of the total; about 50xa0% landslides were distributed on slopes between 35° and 55°, and 11xa0% on slope exceeding 75°; 60xa0% on slopes at the altitudes between 1,000 and 1,500xa0m, 77xa0% on slopes between 900 and 1,500xa0m; and 24 and 62xa0% in hard rocks and section between hard and soft rocks, respectively. Compared with the case of Wenchuan earthquake, both the number and extension of landslides and avalanches in Lushan earthquake-affected area are much smaller, only 5.53xa0% in number and 0.57xa0% in area. The earthquake has increased the instability of slope and potentiality of landslide and debris flow. Accordingly, the active period is expected to be relatively short comparing with that in Wenchuan earthquake-hit area. However, the insidious and concealed hazards bring difficulty for risk investigation.

Temporal differentiation of rainfall thresholds for debris flows in Wenchuan earthquake-affected areas

AbstractnThe Ms 8.0 Wenchuan earthquake greatly altered the threshold for rainfall-triggered debris flows in the affected areas. It is of both scientific and practical significance to determine the rainfall thresholds. This study examines one of the regions most prone to debris flows to analyze the characteristics of rainfall that caused debris flows, and to explore local rainfall thresholds. We applied the relation between rainfall intensity and duration, peak intensity and event amount, and other single factor approaches. Comparison of effectiveness and accuracy indicates that the event rainfall is the most sensitive factor for forecasting. Analysis of the annual rainfall thresholds showed that the rainfall conditions required for debris flows have increased on the continent during the past 6xa0years. Besides the rainfall fluctuations over the past few years, material changes were the primary reason for threshold variability. Recovery of vegetation plays an important role in reducing potential loose material that supplies volume for debris flows. Natural solidification, decrease of the potential erosion depth, and surface coarsening make it more difficult to initiate a debris flow, and ultimately increased rainfall conditions required. The change in rainfall thresholds can be predicted and verified for the entire earthquake-affected region.

Influence of Flow Width on Mean Velocity of Debris Flows in Wide Open Channel

Debris flow in a wide open downstream channel has a significant transverse velocity component that strongly influences its mean longitudinal velocity. Investigation of observation data of debris-flow surges at Jiangjia Ravine in China shows the dependency of Manning resistance of debris flows on the ratio of flow width to depth. Regression fit reveals a power function relationship between the Manning resistance coefficient and the width-to-depth ratio. This derives a new formula of mean velocity incorporating the influence of flow width. The result indicates that the width-to-depth ratio can be viewed as a kind of shape roughness analogous to grain roughness in the Darcy-Weisbach resistance coefficient expression. DOI: 10.1061/(ASCE)HY.1943-7900.0000648

Distribution characteristics of Geo-hazards in Ganxi Valley after the Wenchuan earthquake

A devastating earthquake of Ms 8.0 struck the Wenchuan area on May 12, 2008. The quake triggered numerous landslides and avalanches in Ganxi Valley. Aerial photo interpretation indicates that the landslides and avalanches, presently in a stable state, are concentrated in 91 subregions of the valley, occupying 10.42xa0km2, about 29% of the valley area. Some laws have been found on the gradient, aspect, elevation, distance of rupture, and formation lithology of geo-hazards in Ganxi Valley: (1) They are mainly situated on slopes between 30° and 50° and increase in number with increasing slope; (2) Failures are mainly in a southerly direction; and most occurred between 1,000 and 1,600xa0m, especially between 1,000 and 1,400xa0m; (3) Most are within 2,000xa0m of the Beichuan fault or within 1,000xa0m of other major faults; (4) most failures occurred from the T1fxa0+xa0t formation in the study area. The geo-hazards status quo of Ganxi Valley provides foundation for the scheme of hazard reducing in the future.

Debris flow density determined by grain composition

Density is one of the most important parameters of debris flows. Because observing an active debris flow is very difficult, finding a method to estimate debris flow density is urgently needed for disaster mitigation engineering. This paper proposes an effective empirical equation in terms of grain size distribution (GSD) parameters based on observations in Jiangjia Gully, Yunnan Province, China. We found that the GSD follows P(D)xa0=xa0KD-μexp(−xa0D/Dc), with μ and Dc representing the fine and coarse grains, respectively. In particular, μ is associated with some characteristic porosity of soil in the natural state and increases with increased porosity. Dc characterizes the grain size range of the flow and increases with the grain concentration. Studies show that flow density is related to both parameters in power law. Here, we propose an empirical equation for estimating flow density: ρxa0=xa01.26μ-0.132xa0+xa00.049Dc0.443, which provides not only an estimation of the density for a flow, but also describes the variation in density with the GSD of material composition; this provides important information related to the design of debris flow engineering structures.

Fractal Structure of Debris Flow

One of the most remarkable characteristics of debris flow is the competence for supporting boulders on the surface of flow, which strongly suggests that there should be some structure in the fluid body. This paper analyzed the grain compositions from various samples of debris flows and then revealed the fractal structure. Specifically, the fractality holds in three domains that can be respectively identified as the slurry, matrix, and the coarse content. Furthermore, the matrix fractal, which distinguishes debris flow from other kinds of flows, involves a hierarchical structure in the sense that it might contain ever increasing grains while the total range of grain size increases. It provides a possible mechanism for the boulder suspension.