Gordon G.D. Zhou

Dimensional analysis of natural debris flows

Debris flow is generally composed of a wide range of solid particles and viscous pore fluid . I t flows at a high traveling velocity down a slope channel. Interactions between solid and fluid phases affected by multiple parameters govern the rheological properties of debris flows. A dimensional analysis for a systematic study of the governing parameters is presented. Multiple dimensionless numbers with clear physical meanings are critically reviewed. The applications of field data for studying natural debris flows are demonstrated. Specific values of dimensionless numbers for classifying flowing regimes of natural debris flows on the large scales are obtained. Compared to previous physical model tests on small scales, this study shows that the contact friction between particles dominates in natural debris flows. In addition, the solid inertial stress due to particle collisions and the pore fluid viscous shear stress play key roles in governing the dynamic properties of debris flows. The channel width as a confinement to the flows can affect the solids discharge per unit width significantly. A dimensionless number related to pore fluid pressure dissipation is also found for distinguishing surge flows and continuous flows in field satisfactorily.

Experimental study on cascading landslide dam failures by upstream flows

Landslide dams in mountainous areas are quite common. Typically, intense rainfalls can induce upstream flows along the sloping channel, which greatly affects the stability and failure modes of landslide dams. If a series of landslide dams are sequentially collapsed by an incoming mountain torrent (induced by intense rainfall), large debris flows can be formed in a short period of time. This also amplifies the magnitude of the debris flows along the flow direction. The catastrophic debris flows, which occurred in Zhouqu, China on August 8, 2010, were indeed caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies. Experimental tests were conducted in a sloping channel to understand the dynamic process of cascading landslide dam failures and their effect on flow scale amplification. Similar to the Zhouqu conditions, the modeled landslide dams were distributed along a sloping channel and breached by different upstream flows. For each experiment, the front flows were sampled, the entrained grain sizes were analyzed, and the front discharge along the channel was measured. The results of these experiments show that landslide dams occurring along the channel can be destroyed by both high and low discharge flows, although the mechanisms are quite different for the two flow types. Regardless of flow type, the magnitude of the flows significantly increases after a cascading failure of landslide dams, resulting in an increase in both the diameter and the entrained coarse particles percentage.

Laboratory experiments of water pressure loads acting on a downstream dam caused by ice avalanches

A worldwide decline of mountain glaciers is occurring due to the impacts from climate warming. The retreat of mountain glaciers often leads to different kinds of geo-hazards. Serious surges triggered by glacier avalanches often pose a potential threat to the stability of dams. In this article, four different types of blocks with a constant density of about 900 kg/m3 were used to simulate the glacier avalanches in natural conditions. By considering the raw material properties of the plate and blocks themselves, the plunging velocity of a block was calculated by a theoretical method instead of by video cameras. The effect of the slope angle, distance between the sliding block and the water surface, initial water depth, slide Froude number, geometry, and distance between the plunging point of the sliding blocks and the downstream dam was considered to study the characteristics of the pressure loads acting on the moraine dam. In addition, an empirical equation was obtained to predict the maximum pressure load acting on the dam. Pressure load on the glacier dam is only one of the crucial factors for dam safety analyses. The failure process of a moraine dam, the probable maximum discharge of outburst floods, and the transportation of sediments along the downstream valley should also be considered in future studies.

A new theoretical method for analyzing confined dry granular flows

A granular body may deform in a continuous fashion such that the solid particles remain in close contact. Previous research works have always used the frictional Coulomb-like continuum treatment for analyzing granular bodies. However, this approach is only applicable for quasi-static conditions and cannot capture the complicated granular contact behavior of solid particles inside a failing granular body. This paper applies a revised Savage–Hutter equation to model granular flows moving down a confined, sloping channel. The Coulomb contact friction law is modified to consider the effect of the shear rate inside a granular body. This new method also considers the confinement effect of a sloping channel on granular flow mobility. The derived depth-averaged equations of motion bear a resemblance to nonlinear shallow-water wave equations. Results computed using the derived equations are compared with measurements from flume model tests, and consistency is found between the two.

Experimental study of debris flow caused by domino failures of landslide dams

The formation of landslide dams is often induced by earthquakes in mountainous areas. The failure of a landslide dam typically results in catastrophic flash floods or debris flows downstream. Significant attention has been given to the processes and mechanisms involved in the failure of individual landslide dams. However, the processes leading to domino failures of multiple landslide dams remain unclear. In this study, experimental tests were carried out to investigate the domino failure of landslide dams and the consequent enlargement of downstream debris flows. Different blockage conditions were considered, including complete blockage, partial blockage and erodible bed (no blockage). The mean velocity of the flow front was estimated by videos. Total stress transducers (TSTs) and Laser range finders (LRFs) were employed to measure the total stress and the depth of the flow front, respectively. Under a complete blockage pattern, a portion of the debris flow was trapped in front of each retained landslide dam before the latter collapsed completely. This was accompanied by a dramatic decrease in the mean velocity of the flow front. Conversely, under both partial blockage and erodible bed conditions, the mean velocity of the flow front increased gradually downward along the sloping channel. Domino failures of the landslide dams were triggered when a series of dams (complete blockage and partial blockage) were distributed along the flume. However, not all of these domino failures led to enlarged debris flows. The modes of dam failures have significant impacts on the enlargement of debris flows. Therefore, further research is necessary to understand the mechanisms of domino failures of landslide dams and their effects on the enlargement of debris flows.

Energy dissipation of debris flow through pile group obstructions

Since the devastating Sichuan Earthquake on May 12, 2008, large-scale landslides and debris flows are predicted to occur in these populous areas over the next 10-30 years. In order to prevent and mitigate the geological disasters, it is of great importance to better understand the mechanism of granular flows and to predict their temporal and spatial scales in an efficient way. In this work, we develop a Roe-type finite volume model of the Savage-Hutter equations. Unstructured grid of either triangular or quadrilateral cells is used to match natural topography wells. After appropriately selecting bed and internal friction coefficients, we conduct a series of numerical flume experiments to simulate debris flow passing through pile group obstruction, which is commonly used in damping the kinetic energy of debris flows. Pile group of different spatial patterns are decorated in the downstream of the flume, and the influences of the spatial distributions of pile group obstruction to their damping effect are investigated.

Air concentration and velocity downstream of an expanding chute aerator

ABSTRACT The mechanisms of air entrainment and velocity distribution at the upper, lower and side air–water interfaces downstream of chute aerators remain unclear. This study reports a series of experiments involving various approach-flow features and geometric parameters of aerators. Analytical solutions are developed for the air concentration profiles in the air–water layers, and it is shown that the solutions agree well with the experiments. Empirical equations are established for the turbulent diffusion of bubbles and the diffusion rate of the aeration layer. The turbulent diffusion coefficient of the lower nappe is found to be approximately 10 times and 1.2–1.8 times larger than those of the upper and side nappes, respectively. The diffusion rate of the aeration layer is the highest at the lower nappe and lowest at the upper nappe, gradually decreasing along the cavity. The velocity distribution indices are found to be different for the upper, lower and side nappes.

A new approach to assessing vulnerability of mountain highways subject to debris flows in China

Mountain highways in China are located in various natural geographical areas with intense tectonic activity, steep topography and a high frequency of extreme precipitation events. These conditions make the highways vulnerable to the occurrence of multiple large debris flows simultaneously during heavy rainfall. To manage this hazard risk, a broader understanding of the hazard effects of debris flows and the vulnerability of highways is needed to reduce the losses resulting from these hazardous events. Accordingly, we analysed the effects of debris flow hazards on mountain highways and established an updated systematic indicator system to describe the vulnerability of highway infrastructure and movable hazard-affected objects. Next, we proposed a new integrated model of highway vulnerability based on the environmental sensibility, structural properties and functional effects of the highway infrastructure and on the exposure probability and quantity of movable hazard-affected objects. By analysing the characteristics of elements affected by debris flows, we developed a systematic and quantitative method of vulnerability assessment for mountain highways. Finally, this implemented method was applied to a case study in the Xiqu section of the Sichuan-Tibet Highway, an area seriously affected by debris flows during each rainy season. The hazard characteristics of disasters were analysed, and the affected highway sections were divided into four vulnerability levels. The analysis of the results indicated that the calculated vulnerability coincides with the actual effects of the disaster, which strongly suggests that the vulnerability assessment generated by the proposed method can serve as a pertinent guide for route selection, road rehabilitation and hazard mitigation of highways affected by debris flows in mountainous regions.

Correction to: Utilizing crowdsourcing to enhance the mitigation and management of landslides

The published version of this article, unfortunately, contained error. The author realized an important change of the disclaimer section, which needs to qualify that only Figure 2 requires the permission from the GEO and not the entire content of the manuscript

Experimental study on the regulation function of slit dam against debris flows

Slit dams are open-type structures used to mitigate debris-flow hazards by constricting the flow and attenuating the kinetic energy. However, slit dams are often filled up as they are designed to impede debris volume instead of reducing kinetic energy of debris flows. To better understand the regulation function of slit dams against debris flows, physical model tests were carried out using a 7-m-long flume. The water content and relative post spacing were varied to discern their influence on the regulation function. Results reveal that the velocity attenuation and trapping efficiency is strongly controlled by water content and relative post spacing. Water content fundamentally reflects the degree of liquefaction (effetive grain-contact stress) and capacity of energy dissipation of debris flows. When water content < 26%, relative post spacing has a noticeable effect on velocity attenuation, trapping efficiency, and run-out distance. In contrast, when water content ≥ 26%, the influence relative post spacing is negligible. Furthermore, a new relationship between velocity attenuation and trapping efficiency for the design of slit dams is proposed to avoid the slit dam being easily filled up by sediments contained in debris flows.