A giant debris flow occurred in Zhouqu County, Gannan Tibetan Autonomous Prefecture, Gunsu
Province, in the evening of 7 August 2010, causing 1765 deaths and missing, with enormous property
losses. It ruined 4321 houses and caused 22,667 homeless. The stricken area at Sanyanyu debris flow
was 50.0 hm2
including 3 hm2 urban area and 47 hm2
farmland. A dammed lake 2 km in length was
formed in Bailongjiang River by the debris flow deposit with 8~10 m high, which blocked the river. The
main urban area of Zhouqu city was inundated for one month. This tragic catastrophe raises a topic that
how a giant debris flow develops from a relatively small original one in source area and what
methodology can be used to identify whether a building is in danger or not. In order to understand this
issue, a detailed field survey had been carried out in catchments of Sanyanyu and Loujiayu. The field
survey revealed that flood in upstream eroded the debris barriers and unconsolidated soil bed in channel
and developed into debris flow. The laboratory physical experiments indicated that the major
mechanism of giant natural debris flows formation is scale amplification caused by cascading landslide
dam failures. Another process of scale amplification is that debris flow schleps sediment from erodible
channel bed. At last, a numerical technique will be developed to simulate danger area and momentum of
debris flow. Based on the results of dynamic simulation, a method of hazard assessment will be
established for identifying dangerous area. Hope this methodology can serve for urban management in
mountainous villages and townships.
Debris flows constitute a major natural hazard in mountainous regions. The main elements required for a
practical hazard assessment include the following steps: (i) estimation of potential initiation zones and
sediment sources, (ii) establishment of a relation between the magnitude and frequency of expected future
debris-flow events, and (iii) assessment of the flow behavior and delineation of areas potentially
endangered by flowing debris. A general overview is presented of the main triggering conditions and
initiation mechanisms for debris-flow formation. A brief summary is given of methods to establish a
magnitude-frequency relation and to estimate the total volume of sediments transported to the fan during
so-called “design” events. To assess the runout distance of debris flows and potentially affected areas,
either simple empirical approaches or more physically based numerical simulation models may be used. An
example application for a Swiss debris fan illustrates the variability of the results when using three different
debris-flow simulation models, even though all three models were first calibrated based on the observed
deposition areas of a past event.
Although it is important to understand the behavior of debris flows in the initiation zone for the development of mitigative measures, data are scarce due to difficulties in field monitoring. To clarify debris flow behavior within the initiation zone, we established a monitoring system in the upper Ichinosawa catchment within the Ohya landslide, central Japan. In the Ohya landslide, loose sediments, previously deposited on steep channel bed, is the main source of debris flow material. Video image analysis of six debris flows revealed that the largest boulders in the debris flows were usually smaller than those in the channel deposits. Thus, debris flows appear to facilitate the selective transport of channel deposits in the upper Ichinosawa catchment. Flows that occur during debris flow surges can be classified as either i) flows comprising mainly cobbles and boulders, or ii) flows comprising mainly muddy water. The duration of each flow type is different amongst debris flow events. Flows mainly composed of cobbles and boulders accounted for most of the surges when channel deposits, which were the main source of debris flow material, were abundant. In contrast, flows were mainly composed of muddy water in surges when channel deposits were scarce. The particle size of the boulders had no clear relationship with flow height, with the size of the largest boulders generally ranging from 15 to 40 cm regardless of flow heights (ranging 0-5 m). The particle size of the material entrained by the debris flow differed among debris flow events. Coarse particles were frequently found on the flow surface when the particle size of the channel deposits was larger. Therefore, the characteristics of boulders in debris flows within the debris-flow initiation zone were affected by the volume and the size of sediment at the source of the debris flow material.
This paper presents preliminary experimental results concerning the internal dynamics of a free surface viscoplastic flow down an inclined channel. Experiments are conducted in an inclined channel whose bottom is constituted of an upward-moving conveyor belt with controlled velocity. Carbopol microgel was used as a homogeneous transparent viscoplastic fluid. This experimental setup allows generating and observing stationary gravity-driven surges in the laboratory frame. We used PIV technique (Particle Image Velocimetry) to obtain velocity fields both in the uniform zone and within the front zone where flow thickness is variable and where recirculation takes place. Experimental velocity profiles and determination of plug position are presented and compared to theoretical predictions based on the lubrication approximation.
Debris-flow monitoring sites provide many important inputs on their mechanics and strongly improved the understanding on this hazardous process. Monitoring data are the basis for future early warning systems (EWS) and alarm systems (AS). In this study, results from the Rebaixader monitoring are presented and evaluated for the implementation in an EWS and AS at catchment scale. The key parameters are the rainfall thresholds for the warning and the ground vibration produced by the moving debris flow for the alarm emission. At regional scale, a preliminary EWS for a test area in the Central-Eastern Pyrenees is evaluated. The EWS is based on quantitative precipitation estimates obtained from the weather radars and a simple susceptibility model, which is applied in each basin of the test area. The experiences gathered in the Pyrenees show that the knowledge on initiation and flow behaviour of debris flows has strongly advanced and facilitate the set-up of operational EWS or AS. However, there are still remaining various uncertainties (especially related to the adequate definition of thresholds), which must be evaluated and continuously eliminated.
The output of the seismic devices commonly employed for the monitoring of debris flows, such as geophones and seismometers, is a voltage that is directly proportional to the ground vibration velocity. The output signal in analogical form is usually digitalized at a fixed sampling frequency to be opportunely processed. The processing is performed to both reduce the amount of data to be stored in a data-logger and to reveal the main features of the phenomenon that are not immediately detectable in the raw signal, such as its main front, eventual subsequent surges, the wave form and so on. The processing also allows a better and sounder development of algorithms, when seismic devices are employed for warning purposes. However, the processing of the raw signal alters in different ways the original raw data, depending on the processing method adopted. This may consequently limit or reduce the efficacy of the warning. Different methods of data processing can be found in literature, each with its own advantages and shortcomings. In this paper we will explore and discuss the effects of some of these latter on the efficacy of the algorithms employed for warning, applying them to the seismic recordings obtained in the instrumented basins of Gadria (Italy), Rebaixader (Spain) and Illgraben (Switzerland).
Rock particles in debris flows and other geophysical granular flows are reduced in size through abrasion and
fragmentation. A better understanding of the controls on particle wear in geophysical granular flows is needed
for inferring flow conditions from flow deposits, estimating the initial size of sediments entrained in the flow,
modeling flow dynamics, and mapping hazards. We used three rotating drums to create laboratory granular
flows. Drum diameters range from 0.2 to 4.0 m, with the largest drum able to accommodate up to 2 Mg of
debris, including boulders. We began the experiments with well-sorted, angular coarse particles in clear water.
After each 0.25 km of travel distance, we quantified the particle size distribution. Rates of coarse particle
wear, and production of fragments and fine particles scaled with the rate of energy expenditure per unit bed
area, or unit drum power. We used this power scaling to estimate the rate of particle breakdown in a debrisflow
dominated catchment in the Sierra Nevada, California.
Engineering simulation tools for predicting the flow and deposition behavior of debris flows make use of
simple rheologic flow laws describing flow resistance. In this contribution we test the possibility to
parameterize simple flow models by laboratory investigations. We estimate parameters for the Bingham
model from a suite of laboratory experiments in different setups. Material samples were taken from fresh
deposits of a muddy debris flow and analyzed over a range of volumetric sediment concentrations and
maximum grain sizes. Our results are relatively consistent between most setups. Estimated rheologic
parameters show an exponential dependence on volumetric sediment concentration and a systematic
variation for mixtures of different maximum grain sizes. Though a rheologic interpretation of bulk flow
behavior seems feasible at the laboratory scale, extrapolation of rheologic parameters to prototype flow
situation for direct use in numerical simulation tools is not recommended.
This paper presents the findings of an investigation on the prevention and mitigation of debris flow hazards by using steel open-type dams. First, the actual cases of trapping hazardous debris flow by steel open-type dams were surveyed. Through a field survey of actual cases, we classified them into four distinct scenarios based on the trapping type of debris flow: Scenario A (wooden debris + rocks + sediment), Scenario B (wooden debris + sediment), Scenario C (rocks + sediment) and Scenario D (wooden debris only). Second, recent trapping cases on protection and mitigation by various steel open dams were introduced. Third, trapping scenarios A, B, C and D were confirmed by performing physical model tests. Finally, a safety check of a steel open dam against a large rock was verified by two impact analyses, the finite element method (FEM) impact analysis using ANSYS Autodyn software, and the three dimensional (3-D) impact frame analysis.