Hong-wei Zhou

A Mathematical Model for Forecasting the Dam-Break Flood Routing Process of a Landslide Dam

Once a landslide dam bursts, its reservoir discharges quickly in a flood which will cause catastrophic damage to life and property downstream. For a specific landslide dam, the peak flow rate and the evolution of downstream flood are influenced by the shape and size of the dike breach when dam-break occurs. According to the general nature of landslide dams and field observations of dike-breach development patterns, a dike-breach propagation mode has been determined. By combining an improved empirical equation with knowledge of the dike-breach propagation mode, a mathematical model for forecasting dam-break flood routing has been developed and is presented here. Sensitivity analysis was then carried out based on the computed results for peak flow rate and the flood evolution curve under different parameters. The computed results showed that the width coefficient and the depth coefficient had similar effects on the dam-break flood but that the impact of the depth coefficient was more significant than that of the width coefficient. Finally, the proposed model was used to calculate the flood evolution for the Tangjiashan landslide dam. The computed results showed that the error between the simulated result and the measured data was less than 5%.

MICROMECHANICS DAMAGE MODELING OF BRITTLE ROCK FAILURE PROCESSES UNDER COMPRESSION

This paper presents a numerical simulation method for the brittle rock failure process under compression, by combining the finite element method with micromechanics damage theory. When considering the rock as a homogeneous material, the initial elastic constant of each computational element is the same, but the microcrack distribution in the rock follows a statistical distribution. Consequently, in the loading process, microcrack propagation in each element is different, leading to an inhomogeneous distribution of changes in elastic constant. Under increased loading, this distribution will ultimately be reflected in the macro-failure mode of the rock. To investigate the macromechanics of the rock failure process, the damage variables and effective elastic constants are used to reflect the propagation of microcracks, thus coupling the micromechanics and macromechanics of the rock failure process. Finally, the paper demonstrates the numerical simulation method by simulating the failure of sandstone; these computational results show that the method performs well in simulating the mechanical characteristics of the brittle rock failure process.

Effects of a flexible net barrier on the dynamic behaviours and interception of debris flows in mountainous areas

Flexible net barriers are a new type of effective mitigation measure against debris flows in valleys and can affect the kinematic energy and mass of debris flows. Here, ten flume tests were performed to study the dynamic behaviours of debris flows with differences in volumes, concentrations (solid volume fraction), and travel distances after interception by a uniform flexible net barrier. A high-speed camera was used to monitor the whole test process, and their dynamic behaviours were recorded. A preliminary computational framework on energy conversion is proposed according to the deposition mechanisms and outflow of debris flow under the effects of the flexible net barrier. The experimental results show that the dynamic interaction process between a debris flow and the flexible net barrier can be divided into two stages: (a) the two-phase impact of the leading edge of the debris flow with the net and (b) collision and friction between the body of the debris flow and intercepted debris material. The approach velocity of a debris flow decreases sharply (a maximum of 63%) after the interception by the net barrier, and the mass ratio of the debris material being intercepted and the kinetic energy ratio of the debris material being absorbed by the net barrier are close due to the limited interception efficiency of the flexible net barrier, which is believed to be related to the flexibility. The energy ratio of outflow is relative small despite the large permeability of the flexible net barrier.

A Plasticity Criterion for Mixed Mode Fracture Initiation with Crack Surface Frictional Characteristics Under Compression

A new plasticity criterion for mixed mode crack initiation angles under compression is presented, based on the characteristics of the plastic core region surrounding the crack tip with crack surface friction. The shape and size of the plastic core region were thoroughly analyzed under compression and the non-dimensional variable radius of the plastic core region for mixed mode compressive fracture was considered. Obviously, the mixed mode fracture initiation under compression is influenced by the frictional characteristics of the crack surface and the external compressive loading state. The loading ratio, Poissons ratio and the influence of the friction coefficient on the crack tip plastic radius were analyzed. The proposed criterion shows that the crack extends in the direction of the global minimum of the plastic core region boundary. The presented criterion was simulated for various compressive loads and was compared, with other available criteria, against the laboratory experimental data. It can be seen that the presented criterion provides a better agreement with the experimental data.

The effect of tetrahedron framed permeable weirs on river bed stability in a mountainous area under clear water conditions

A flexible riverbed protection called tetrahedron framed permeable weirs (TFPW) is proposed to protect riverbeds in mountainous areas from scouring. Under clear water conditions, a series of laboratory flume experiments were performed to study the effects of TFPW with different layout types on the stability of riverbeds. The objectives of this paper were to advance understanding of the role that TFPW play in the erosion process of river beds and to optimize the TFPW design for reducing velocity, promoting sediment deposition and good structural stability. Data on velocity distribution and variation, equilibrium bathymetry, flow resistance, bed form characteristics and structural stability were collected and analyzed. The results indicate that (1) with good structural stability, all the TFPW with different layout types had significant effects on the stabilization of the riverbed by reducing velocity, raising the water level, increasing the roughness coefficient, protecting the riverbed from degradation and promoting deposition; and (2) the random Double TFPW with large rates of deceleration, large deposition ranges, and good structural stability, and the paved Single TFPW with small rates of deceleration but large deposition ranges and perfect structural stability, were suitable and optimal for riverbed protection in a clear water channel.

Dynamical evolution properties of debris flows controlled by different mesh-sized flexible net barriers

Because the flexible net barrier is a gradually developed open-type debris-flow counter-measure, there are still uncertainties in its design criterion. By using several small-scale experimental flume model tests, the dynamical evolution properties of debris flows controlled by large and small mesh-sized (equal to D90 and D50, respectively) flexible net barriers are studied, including the debris flow behaviors, segregation, and permeability of sediments, as well as the energy absorption rates and potential overtopping occurring when debris flows impact the small mesh-sized one. Experimental results reveal that (a) two sediment deposition patterns are observed depending on variations in debris flow textures and mesh sizes; (b) the aggregation against flexible net barriers is dominated by flow dynamics; (c) the segregation and permeable functions of the barrier are determined by the mesh size, concentration, and flow dynamics; and (d) the smaller mesh-sized flexible net barrier tends to be more efficient in restraining more turbulent debris flows and can absorb greater rate of kinematic energy, and finally, the great kinematic energy dissipation that occurs when secondary debris flows interact with the post-deposits in front of the small mesh-sized flexible net barrier is believed to cause the failure of overtopping phenomenon. The mesh size is concluded to be the decisive parameter that should be associated with debris flow textures to design the control functions of flexible net barriers.

An experimental study on controlling post-earthquake debris flows using slit dams

AbstractPost-earthquake debris flows often break out in groups frequently, which are usually caused by the abundant loose solid materials that produced by earthquake. Slit dams represent a practical and effective kind of countermeasure for controlling the post-earthquake debris flow. Flume experiments were carried out to study the interaction mechanism and the effect of slit dams on the post-earthquake debris flows. The results showed that affected by the slit dams, some certain types of deposits formed on the upstream. The steeper the flume slope, as well as the greater the width and the density of slits, the easier the lateral deposit became “V” shaped. Otherwise, the lateral deposit was more likely to be “–‒” shaped. When the flume slopes were 12°, 16° and 20°, the profiles of the deposits would be long-shallow type, short-thick type and short-shallow type, respectively. The slope of the deposition first decreases and then increases with the flume slope increasing within a certain range. The slit dam can trap the coarse sand and discharge the fine sand. The maximum attenuation rate can reach 44.4%. The effect of this capacity gradually weakens as the flume slope is increased. When the width or the density of slits is smaller, the greater the rate of decrease in peak sand discharge and the greater the effect of peak cutting will be. The reduction in the sediment storage rate is likely due to the increase in the width and the density of the slits. With the increase in flume slope, the sediment storage rate first increases, then decreases, which reaches a maximum value when the flume slope is 16°.

MULTI-LEVEL, MULTI-FACTOR OPTIMIZATION IN DECISION MAKING FOR EMERGENCY TREATMENT OF QUAKE LAKES

One hundred and thirty three quake lakes were formed in Wenchuan Earthquake of May 12. There may be several or even a dozen quake lakes in the same drainage basin. During the emergency treatment of quake lakes, due to constraints of resources and time, it is often imperative to select one or a few quake lakes of high priority in the same drainage basin for treatment in order for the entire drainage basin to safely pass the flood season. There are many factors involved in the optimization of emergency treatment project of quake lakes and these factors are usually of different nature. This article proposes to use a multi-level, multi-factor optimization method in decision making of emergency treatment of quake lakes based on the concept and operation properties of fuzzy-consistent matrix. This method is convenient and practical when many qualitative factors are involved and the determination of membership function is difficult. A case study was conducted for the 16 quake lakes on the Mianyuan River by using the proposed method. Three quake lakes selected according to case study results went through emergency treatment before the 2009 flood season, helping the entire Mianyuan River safely pass the 2009 flood season.