Fangqiang Wei

A seismically triggered landslide dam in Honshiyan, Yunnan, China: from emergency management to hydropower potential

At 16:30 (Beijing time) on August 3rd, 2014, an Ms 6.5 earthquake occurred in Ludian County in Yunnan Province, China. The Ludian earthquake triggered a collapse in Chapingzi, right bank of the Niulanjiang River, and a landslide in Hongshiyan at the left bank on the same location. These events occurred 1.5xa0km downstream of the Hongshiyan hydropower station. Debris deposits from the collapse and the landslide blocked the river channel and formed a landslide dam. In this study, the Hongshiyan landslide dam induced by the Ludian earthquake was chosen as the study case to discuss integrated landslide dam management and its potential hydropower uses. After emergency management measures are carried out, further analysis shows the outflow through the Hongshiyan landslide dam is manageable, which indicate safety issues are controllable. Two management schemes are discussed in this paper: one that uses the Hongshiyan landslide dam for power generation and one that removes the debris deposit. The result shows that using the strengthened landslide dam for power generation is feasible and economically sound. This research estimate a capital recovery period of approximately 3 to 4xa0years with an investment-return ration of 1:40, which is considered a significant economic benefit.

Mean Velocity Estimation of Viscous Debris Flows

The mean velocity estimation of debris flows, especially viscous debris flows, is an important part in the debris flow dynamics research and in the design of control structures. In this study, theoretical equations for computing debris flow velocity with the one-phase flow assumption were reviewed and used to analyze field data of viscous debris flows. Results show that the viscous debris flow is difficult to be classified as a Newtonian laminar flow, a Newtonian turbulent flow, a Bingham fluid, or a dilatant fluid in the strict sense. However, we can establish empirical formulas to compute its mean velocity following equations for Newtonian turbulent flows, because most viscous debris flows are turbulent. Factors that potentially influence debris flow velocity were chosen according to two-phase flow theories. Through correlation analysis and data fitting, two empirical formulas were proposed. In the first one, velocity is expressed as a function of clay content, flow depth and channel slope. In the second one, a coefficient representing the grain size nonuniformity is used instead of clay content. Both formulas can give reasonable estimate of the mean velocity of the viscous debris flow.

A Model of Debris Flow Forecast Based on the Water-Soil Coupling Mechanism

Debris flow forecast is an important means of disaster mitigation. However, the accuracy of the statistics-based debris flow forecast is unsatisfied while the mechanism-based forecast is unavailable at the watershed scale because most of existing researches on the initiation mechanism of debris flow took a single slope as the main object. In order to solve this problem, this paper developed a model of debris flow forecast based on the water-soil coupling mechanism at the watershed scale. In this model, the runoff and the instable soil caused by the rainfall in a watershed is estimated by the distributed hydrological model (GBHM) and an instable identification model of the unsaturated soil. Because the debris flow is a special fluid composed of soil and water and has a bigger density, the density estimated by the runoff and instable soil mass in a watershed under the action of a rainfall is employed as a key factor to identify the formation probability of debris flow in the forecast model. The Jiangjia Gulley, a typical debris flow valley with a several debris flow events each year, is selected as a case study watershed to test this forecast model of debris flow. According the observation data of Dongchuan Debris Flow Observation and Research Station, CAS located in Jiangjia Gulley, there were 4 debris flow events in 2006. The test results show that the accuracy of the model is satisfied.

Monitoring and recognition of debris flow infrasonic signals

Low frequency infrasonic waves are emitted during the formation and movement of debris flows, which are detectable in a radius of several kilometers, thereby to serve as the precondition for their remote monitoring. However, false message often arises from the simple mechanics of alarms under the ambient noise interference. To improve the accuracy of infrasound monitoring for early-warning against debris flows, it is necessary to analyze the monitor information to identify in them the infrasonic signals characteristic of debris flows. Therefore, a large amount of debris flow infrasound and ambient noises have been collected from different sources for analysis to sum up their frequency spectra, sound pressures, waveforms, time duration and other correlated characteristics so as to specify the key characteristic parameters for different sound sources in completing the development of the recognition system of debris flow infrasonic signals for identifying their possible existence in the monitor signals. The recognition performance of the system has been verified by simulating tests and long-term in-situ monitoring of debris flows in Jiangjia Gully, Dongchuan, China to be of high accuracy and applicability. The recognition system can provide the local government and residents with accurate precautionary information about debris flows in preparation for disaster mitigation and minimizing the loss of life and property.

A regional-scale method of forecasting debris flow events based on water-soil coupling mechanism

A debris flow forecast model based on a water-soil coupling mechanism that takes the debris-flow watershed as a basic forecast unit was established here for the prediction of disasters at the watershed scale. This was achieved through advances in our understanding of the formation mechanism of debris flow. To expand the applicable spatial scale of this forecasting model, a method of identifying potential debris flow watersheds was used to locate areas vulnerable to debris flow within a forecast region. Using these watersheds as forecasting units and a prediction method based on the water-soil coupling mechanism, a new forecasting method of debris flow at the regional scale was established. In order to test the prediction ability of this new forecasting method, the Sichuan province, China was selected as a study zone and the large-scale debris flow disasters attributable to heavy rainfall in this region on July 9, 2013 were taken as the study case. According to debris flow disaster data on July 9, 2013 which were provided by the geo-environmental monitoring station of Sichuan province, there were 252 watersheds in which debris flow events actually occurred. The current model predicted that 265 watersheds were likely to experience a debris flow event. Among these, 43 towns including 204 debris-flow watersheds were successfully forecasted and 24 towns including 48 watersheds failed. The false prediction rate and failure prediction rate of this forecast model were 23% and 19%, respectively. The results show that this method is more accurate and more applicable than traditional methods.

Measuring Internal Velocity of Debris Flows by Temporally Correlated Shear Forces

Debris flow is a kind of geological hazard occurring in mountain areas. Its velocity is very important for debris flow dynamics research and designing debris flow control works. However, most of past researches focused on surface velocity and mean velocity of debris flow, while few researches involve its internal velocity because there is no available method for measuring the internal velocity for its destructive power. In this paper, a method of temporally correlated shear forces (TCSF) for measuring the internal velocity of debris flows is proposed. The principle of this method is to calculate the internal velocity of a debris flow using the distance between two detecting sections and the time difference between the two waveforms of shear forces measured at both sections. This measuring method has been tested in 14 lab-based flume experiments.

Modeling potential scenarios of the Tangjiashan Lake outburst and risk assessment in the downstream valley

This research is devoted to Tangjiashan Lake, a quake landslide-dammed lake, situated in Sichuan Province, China, which was formed by a landslide triggered by the Wenchuan Earthquake on 12 May 2008. A STREAM_2D two-dimensional hydrodynamic model of Russia was applied to simulate the process of two flood scenarios: 1, lake dam outbreak, and 2, dam overtopping. An artificial dam outbreak was made after the earthquake to lower the water level of the lake in 2008, which led to a great flood with a maximum water discharge of more than 6400 m3/s. The negative impact of the flood was reduced by a timely evacuation of the population. Flood hazards still remain in the event of new landslides into the lake and lake dam overtopping (Scenario 2), in which case a maximum water discharge at the dam crest would reach 5000 m3/s, placing the population of Shabacun and Shilingzi villages in the zone of flood impact.