Clarence Choi

Froude characteristics of both dense granular and water flows in flume modelling

One of the challenges associated with small-scale flume modelling is achieving dynamic similarity. Froude (Fr) scaling is commonly adopted to capture the bulk characteristics of a flowing medium. Although the Fr number cannot capture the micro-interactions of a flow medium, it is commonly adopted by engineers, due to its simplistic nature for characterising mass-wasting processes. Given the prevalence of Fr scaling, an improved understanding of the development of Fr characteristics for channelized surge flows is certainly warranted. A 5-m long rectangular flume model was adopted to carry out experiments using dense uniform dry granular and water flows, separately. Laser and photoconductive sensors, and high speed imagery were used to estimate flow velocity and thickness. Results reveal that the Fr behaviour of uniform dry sand and water flows is dependent on its energy-dissipation mechanism. The initial volume has a greater influence on suppression of Fr conditions compared to shallower channel inclinations for both granular and water flows. The major limitation of small-scale flume modelling lies in limited initial volumes. Limited initial volumes lead to shallow flow depths and results in the flow velocity controlling Fr development of the flow mass with transportation. Frictional materials are not favourable for developing low Fr numbers in flume modelling.

Performance of landslide debris-resisting baffles

Arrays of landslide debris baffles are commonly installed upstream of rigid barriers in Hong Kong to dissipate flow energy impacting a rigid barrier. Currently, baffles are installed using prescriptive and empirical approaches in Hong Kong without design recommendations. Given the engineering value of baffles, an improved understanding of their interaction mechanisms is warranted. Flume modelling and numerical back analysis using the discrete element method (DEM) were adopted to study the influence of landslide debris baffles on impact on a downstream terminal rigid barrier. Froude scaling was used to dynamically characterise the flow. The optimum geometrical configuration was examined. The results revealed that an array of baffles is effective in reducing the peak dynamic impact induced by debris on a rigid barrier and that overflow processes need to be controlled. Baffle heights exceeding 1.5 times the approach flow depth (h) exhibited little incremental influence on reducing the peak dynamic impact forces induced on the rigid barrier. At least two staggered rows are required to effectively intercept discharge from the first row and reduce the frontal debris impact force. The optimum spacing between successive rows (L) and transverse blockage is L/D = 3 (D is the slit opening) and 30%, respectively.

Interaction between dry granular flow and deflectors

The application of seawall deflectors for reflecting inviscid waves into the sea have been well established. Recently, rigid barrier deflectors have been proposed prescriptively for mitigating geophysical landslides, but flow characteristics differ fundamentally from waves and merit investigation. In this study, flume tests were used to calibrate a discrete element model to explore the interaction between dry granular flow and rigid barrier deflectors. The deflector angle and length and the effective height (distance between deflector tip and channel base) were studied and compared to barriers without deflectors. Findings reveal that deflectors initially prevent spilling of vertical runup and reduce flow energy underneath the deflector. However, controlling overflow depends heavily on the deflector angle and length, with the effective height as ultimate governing parameter. The additional height provided by the deflector should therefore be considered as part of the design height rather than a prescriptive add-on. Longer deflector lengths shield deadzones from energy losses through grain shearing, thus resulting in higher peak overflow velocities. It is recommended that deflector lengths should be less than 10% of the expected flow depth to suppress peak overflow velocities. Perpendicular deflectors tend to enhance faster energy dissipation through increased deadzone confining stress.

Geophysical flows impacting a flexible barrier: effects of solid-fluid interaction

Flexible barriers undergo large deformation to extend the impact duration, and thereby reduce the impact load of geophysical flows. The performance of flexible barriers remains a crucial challenge because there currently lacks a comprehensive criterion for estimating impact load. In this study, a series of centrifuge tests were carried out to investigate different geophysical flow types impacting an instrumented flexible barrier. The geophysical flows modelled include covered in this study include flood, hyperconcentrated flow, debris flow, and dry debris avalanche. Results reveal that the relationship between the Froude number, Fr, and the pressure coefficient α strongly depends on the formation of static deposits called dead zones which induce static loads and whether a run-up or pile-up impact mechanism develops. Test results demonstrate that flexible barriers can attenuate peak impact loads of flood, hyperconcentrated flow, and debris flow by up to 50% compared to rigid barriers. Furthermore, flexible barriers attenuate the impact load of dry debris avalanche by enabling the dry debris to reach an active failure state through large deformation. Examination of the state of static debris deposits behind the barriers indicates that hyperconcentrated and debris flows are strongly influenced by whether excessive pore water pressures regulate the depositional process of particles during the impact process. This results in significant particle rearrangement and similar state of static debris behind rigid barrier and the deformed full-retention flexible barrier, and thus the static loads on both barriers converge.

The Effect of Climate Change on Alpine Mountain Hazards Chain: A Case Study in Tianmo Ravine, Tibet, China

Mountain hazards behave a relatively high incidence under regional climate change condition. This compound hazard is often initiated by environment and climate change. A combination of glacial melting and rainfall-induced compound disaster event happened in Tianmo Ravine, Bomi County, Tibet, China on 25 July and 4 September 2010, separately. The debris flow with 450,000 m3 volume transported along gully and then flushed into Parlung Tsangpo River in the deposition zone, resulting in a destruction of 400 m length section of the G318 highway from Bomi County to Lhasa. A detailed interpretation of this disaster was conducted using unique and high-resolution images obtained through remote sensing. The source material, terrain condition, and climate condition were analyzed to have a comprehensive understanding of environmental background for hazard initiation. It is concluded that the occurring of this disaster event is the comprehensive result of multiple factors led by the climate change. Abundant antecedent precipitation, melting of the glacier, and the instability of soil caused by wetting-drying cycles, led to the occurrence of the Tianmo Ravine disaster. Based on taking various factors into account synthetically, the comprehensive hazards pattern is named using “Mountain hazards pattern in glacial alpine region under climate change (MH-GA-CC)”. The climate is the special factor feature that should be taken into account emphatically. The study here provides a scientific base pattern for the future risk assessment of glacial alpine areas under regional climate change background.

Surge impact behavior of granular flows: effects of water content

Understanding the fundamental dynamics of interaction between multi-phase geophysical flows and engineering structures is crucial for mitigating geophysical hazards. Specifically, liquid phase between particles induces matric suction which could play a significant part in regulating flow dynamics and warrants further consideration. In this study, flume model tests were conducted to investigate the effects of water content (0–30%) on the impact behavior of granular flows. The particle image velocimetry technique was adopted to visualize the impact kinematics and the impact force was measured through a model barrier system. Results revealed that, besides geometric effects (kinetic sieving), mechanical effects (shearing and collision) are also vital for the mechanism of reverse segregation. At higher water contents, 20 and 30% in this study, discrete-surge impact, rather than a progressive impact process, was observed. The discrete surges induce impulses on the barrier. The discrete surges result from self-organization of unsaturated granular flows to overcome the enhanced shear strength induced by matric suction. Finally, a dimensionless index, namely the suction number, is used to quantify the effect of suction on the dynamic behavior of granular flows. Even for large-scale geophysical flows, if the content of fine particles is high, effect of suction should not be neglected.

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

Utilizing crowdsourcing to enhance the mitigation and management of landslides

Landslides are mainly triggered by earthquakes and rainfall and have poor temporal predictability. Landslides pose significant threats to settlements and infrastructure in mountainous regions around the world. To mitigate this natural hazard, a new paradigm of landslide mitigation and management is required. Increasing smartphone ownership around the world, especially in developing countries, offers scientists an opportunity to embrace crowdsourcing so as to improve landslide research. This paper presents a new landslide information system (LIS) comprising a smartphone app and an administrative interface and database. The mobile app has been published for both iPhone and Android platforms. The interface of the smartphone app is powered by the highly-customizable Google Maps platform, which is overlaid with real-time landslide data. Users can choose between visualizing “known sites” and “contribution” of landslide data. The visualization option shows published landslides and areas that are susceptible. Users can contribute their GPS coordinates and multimedia to enhance landslide reports. A comparison with similar systems, potential applications, and challenges of using smartphone technology for mitigating landslides are also discussed.