Several pipe exits were discovered in the torrent bed material in debris flow generation areas of the Nishinokaito River, Mount Fujiwara, Mie prefecture, Japan. These exits were in a sand and gravel layer between 1.0 and 1.5 m below the surface of the torrent bed material. Gravel supported the internal walls of the pipe exits. Subsurface flows spouted when the rainfall intensity exceeded 5 to 7 mm in 10 minutes and the soil water index exceeded about 110 mm. Due to hydrological processes, subsurface flows spouting at the new pipe exits during heavy rainfall may lead to debris flow generation. We studied the critical rainfall thresholds using the radial basis function network (RBFN) method.
In this article, experimental tests were conducted to study the characteristics of the surge wave and its effects on the failure process of a glacier dam. The results indicated that two obvious surges were monitored by pore pressure transducers (PPTs) when the blockage slid into water. The surge wave attenuated exponentially near the plunging location and then attenuated slowly when it propagated downstream. When the surge wave reached the glacier dam it climbed up along the dam and flew over. Sequential propagating surges exerting on the dam were one of the main factors causing the failure of the glacier dam. The failure mechanism of glacier dam triggered by the surge waves under different initial water supply conditions was primarily analyzed in this article.
Discussions on pulsation in debris flows that rush down from Kamikamihorizawa torrent of Mount Yakedake, Japan, Jiang-jia torrent in Yunnan Province, China, and Aa torrent in Karakorum, Pakistan suggest that 1) periodic pulsation at Kamikamihorizawa is caused by flow instability, and non periodic one by response to rainfall intensity; 2) periodic pulsation at Jiang-jia is produced by both or one of two mechanisms: discrete mobilization of visco-plastic slurry deposits and instability in the flows; 3) pulsation in the flows at Aa torrent is brought about by drainage instability of melt water from glaciers. Its periodicity is brought about by both daily variation in solar radiation and storage capacity of melt water to a threshold for a drainage instability; 4) Non periodic pulsation is also brought about in a situation that multiple landslides are induced in succession to generate debris flows. The 20 August 2014 Hiroshima debris-flow disaster seems to have been enlarged by such a pulsation.
Since 2004, debris-flow evacuation implemented based on critical rainfall intensity for warnings of debris-flow occurrences has been a regulation in Taiwan, and its many implementations in recent years have proven successful. When deciding on the rainfall intensity associated with evacuation decisions, the social cost of debris-flow disasters should be taken into account in addition to the vulnerability of the area to such disasters. However, in current practical debris-flow evacuation processes, that cost is not considered. As such, the aim of this paper is to explore how rainfall intensity and social cost relate to the decision to evacuate ahead of a debris-flow. In doing so, decision-making and cost analysis methods commonly used in management science were employed to model a rationally simplified hypothesis and develop a systematic decision-making process regarding debris-flow evacuation. This process is influenced by uncertain parameters, such as typhoon-rainfall levels and debris-flow occurrences, and determined through statistical analysis; further, the value parameters of costs in terms of deaths and evacuation payments are also considered. Finally, the model thus developed was applied to a severe debris flow disaster simulation that occurred in Taiwan in 1990 for empirical testing. Results of this study suggest that the rainfall intensity and social costs are highly pertinent to decisions regarding debris-flow evacuation when considered from a financial perspective. Moreover, by quantifying the social costs of debris-flow evacuation through decision analysis, this study may help explain the relationship between the vulnerability of an area to disasters, the social costs involved, and the decision-making criteria. It may also provide scholars a better understanding of optimal strategies for the advancement of debris-flow disaster prevention.
In July 2012, a landslide dam subsequent to the large-scale collapse of a mountain edifice occurred on Ambon Island in the eastern part of Indonesia. A height of the dam was approximately 140 m and a volume of dammed water was approximately 250 million m3. The dam failed about a year after its formation in July 2013, and a large-scale flood wreaked havoc on the downstream area. In this disaster, the information on the topographic changes before and after the dam failure was obtained from digital maps (DEM) based on satellite images and field measurement. Furthermore, field surveys examined the various conditions of the sites around the landslide dam and a process of the outburst flood was recorded on video. Simulation of a landslide dam outburst flood was conducted using LADOF model that reproduces the process of overflow and erosion of a landslide dam outbreak and the calculation result was verified by field measurement data. The results are as follows. (1)The peak flow rate of the landslide dam outburst flood determined by simulation was approximately 17,000 m3/s. (2)The occurrence and duration of the flood, longitudinal profile of the dam, inundated area and flow depth obtained from the simulation almost agree with the actual results obtained from field measurement. (3)The phenomena whereby the progress of longitudinal erosion caused the collapse of the stream bank and rapidly expanded the river channel width could be confirmed by analysis of the video images.
Intermittent surges of debris flows are frequently observed in mountain regions. This type of flow is considered to be characterized by developing roll waves (surges) due to flow instabilities and by weak sediment concentrations. For an understanding of fluctuation of the flow depth, wave equations are needed. It was presented a wave equation of roll waves based on shallow water momentum equation, and analytical solution on an initial and boundary condition and some numerical solutions were shown in this paper. These results show an improved understanding of the phenomena of intermittent debris flow.
Torrential checkdams with energy dissipating, filtering or deflecting function for debris flows are expected to be subject to extreme dynamic stress that requires the application of high safety standard for design, construction and maintenance. The standardized procedure for checkdam design has been developed from comparative calculations of common debris flow models from engineering practice and calibrated by impact measurements of debris flow laboratory experiments and data available from literature. The model is based on a combined static- dynamic stress approach, additional impacts by single objects like boulders are included. The dynamic area of the dynamic component is derived from a characteristic wetted cross section area of debris flow corresponding to the design event, which can be found at a characteristic cross section upstream of the checkdam. The static load is based on a hydrostatic fluid assumption and calculated analogously to water pressure (with debris flow density) and acts upon total construction height. The dynamic component is calculated according to the momentum equation and acts uniformly distributed on the dynamic impact area right below the overflow section up to a height of maximum 4 meters. The maximum local impact of a single boulder is defined to act on an area of 0.7 x 0.7 meter with 1MN. This new regulation shall guide practitioners for more objectives and save design of checkdams impacted by debris flows.
In this work, we study the impact of a transient free-surface flow of viscoplastic fluid on a rigid obstacle. This study is conducted numerically using the SPH (Smoothed Particle Hydrodynamics) method, and the Herschel-Bulkley rheological model. The SPH code and its specific adaptations to our needs are presented. The capacity of the code to meet the requirements of our objectives is validated on classic benchmarks. The virtual experiment setup is presented. The local characteristics of the flow near the obstacle, the length of the dead-zone of fluid at rest which forms upstream of the obstacle and the shape of the pressure signal applied to the obstacle are analyzed with reference to the inclination angle and Froude number of the incident flow. This analysis highlights the existence of two impact regimes referred to as the dead-zone and jet impact regimes respectively with a transition occurring for values of the Froude number about 1.3 to 1.5. These values are coherent with previous experimental studies.
Debris flows that are confined by canyons generally exhibit distributary behavior once they exit the canyons, usually creating some sort of debris fan. This distributary nature is commonly observed in fluvial processes, as avulsion out of established flow paths allows the system to methodically fill topographic lows and develop regular, fan-shaped deposits. For debris flows, avulsion represents a serious hazard, because future debris flows may occur in areas that have not experienced events in the recent past, and flows may occur at significant distances across the fan away from currently active channels. It is important to be able to identify avulsion-susceptible areas, to quantify the likelihood of avulsion, and to model and mitigate the possibility of avulsion. Map views of several debris fans showing locations of successive events were analyzed to evaluate the degree of avulsion. In addition, cross-fan sections at three locations in Colorado were interpreted stratigraphically and analyzed to calculate a modified compensation index, Kcv, a single number that indicates significant avulsion activity (Kcv near one), or low avulsion activity (Kcv near 0.5). Areas with typical debris-flow characteristics (abundant coarse clasts, thick units, large lobes, high clay content) tended to have higher compensation indices than areas with typical stream-flow characteristics (thinner, with less clay and coarse clasts). Finally, several sites are reviewed where an understanding of avulsion could help anticipate flow behavior and direct mitigation efforts.