E3S Web of Conferences 415, 06012 (2023)
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https://doi.org/10.1051/e3sconf/202341506012

Flow-type landslides Impacting V-shaped Diversions: PhysicalModelling
Ruoying Li1, and Clarence E. Choi1*,
1The Department of Civil Engineering, The University of Hong Kong, HKSAR China

Abstract. In rural and sparsely populated areas, government issued pamphlets often recommend theconstruction of V-shaped diversions to protect personal property from flow-type landslides hazards,
including debris flows. V-shaped diversions are advantageous because they attract low impact forces andrunup heights due to their oblique impact angle. However, current design approaches are empirical, so it isunclear what resisting forces and wall heights are required. In this extended abstract, details of a newexperimental setup and some preliminary results are presented. It is envisioned that findings from this study
will help to shed light on scientific-based recommendations to design V-shaped diversions to enhance theresiliency of mountain communities globally.

1 Introduction
Flow-type landslides, including debris flows, snow
avalanches, and granular flows, surge downslope under
the influence of gravity and have been reported to cause
fatalities and damage to infrastructure globally [1, 2, 3].
In rural areas, government-issued pamphlets
recommend property owners to construct V-shaped
diversions as personal protection against flow-typelandslide hazards [3, 4]. Owing to an oblique impact
orientation between a V-shaped diversion and a flow,
the resisting force and runup heights are lower compared
to an orthogonal impact orientation. This means that Vshaped diversions do not require designs that are bulky.
Despite the high engineering value of V-shaped
diversions, they are designed empirically. For example,Fig. 1. shows a conceptual schematic diagram of a Vshaped diversion [5]. It is recommended that thedeflector angle should be “such that the area enclosedby the retaining walls is greater than the desired levelarea”, and “grade beam should extend two feet below
the slope surface and be provided with three feet offreeboard”. Besides these recommendations, it isrecommended that the design of diversions should
follow the procedures for other retaining structures [5].
Evidently, conventional retaining structures and Vshaped diversions subjected to oblique dynamic loading
are fundamentally different [6]. However, there is noguidance on the design wall height against runup and an
optimised diversion angle to reduce the impact force [7,8].
In this study, a new experimental setup, along with
its instrumentation and modelling procedures ispresented. Then, some preliminary results are shown. It
is expected that the findings from this study will help toprogress towards scientific-based design guidance for
V-shaped diversions to empower vulnerable

communities in mountainous regions to protect
themselves from debris flow hazards.

Fig. 1. Schematic diagram of a typical V-shaped diversion[5].

2 Methodology
Figure 2 shows the new experimental model developed
for this study. It consists of a flume, which has a lengthof 1.5 m, a width of 0.2 m, and a depth of 0.3 m. The
flume consists of a 0.3 m long container for storing thegeomaterial initially at the upstream end. The inclinationof the flume is selected to be 𝜁 = 35° to generate highenergy flows on obstacles [9, 10].
The diversions are designed with three different
diversion angles 𝛼 (i.e., 30°, 60°, 120°) while keeping
the distance d between the two side walls constant. AFroude number Fr of 6 is selected to model supercriticalflow [11, 12].

* Corresponding author: cechoi@hku.hk© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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https://doi.org/10.1051/e3sconf/202341506012

3 Preliminary results
Figure 3 shows the impact behaviour as captured by thetop camera. Grids on the board are spaced at an intervalof 100 mm. The apex of the model diversion is placed
200 mm downstream from the mouth of the flume. The
flow impacts the diversion (i.e., 𝛼 = 30°, and d = 100mm).
It can be observed that oblique shocks form when
the granular flow impacts the diversion. The shock angle
𝛽 at quasi-steady state is approximately 80°. At the apexof the diversion wall, the granular material is observed
to jump and form a stagnation point. The observed
impact can be used to evaluate analytical formulations
for predicting the runup height and impact force.

Fig. 2. A new experimental model (a) Oblique schematicwith a slope angle ζ for both the flume and the clear board;(b) Diversion details.

2.1 Instrumentation
Top- and side-view cameras (i.e., 240 frames per second
at a resolution of 1920×108) are mounted around themodel to capture the flow and impact behaviour. A loadcell is sandwiched between a fixed joint and a frontal
diversion wall. Laser cartography is adopted to measure
the spatiotemporal changes in flow height. This method
makes use of refracted laser beams and the shadow bar
technique to capture dynamic changes in the flowheight. The laser beams pass through vertically placed
cylindrical lens and are then refracted to a surface [13].
The height difference is proportional to the difference
between the original reference line and the distorted
line, which needs to be calibrated using a standard object
with a known height beforehand. The contour of thethickness can then be deduced from the captured images[14]. This method has been successfully evaluated by
Mcdonald and Anderson in 1996 [15].

Fig. 3. Oblique shock evolution with time from (a) initial
impact to (d) steady state flow.

4 Summary
Diversion structures are more compact alternatives
compared to traditional rigid barriers because they
deflect the flow material to reduce the impact force andrunup height. The described experimental model setup
in this extended abstract will be used to optimise the thediversion angle, impact force, and wall height for a wide
range of flow types. It is envisioned that findings fromthis study can be used to rationalise the design of Vshaped diversions for vulnerable communities globally.
The work described in this extended abstract is supported by
the HKU Presidential PhD Scholar Programme (HKU-PS).
The authors are grateful for the financial sponsorship from The
University of Hong Kong.

2.2 Modelling procedures
In each model test, the model diversion is installed onan unchannelised board with dimensions of 1.0 m by 1.0m to enable the impact behaviour to be studied. The
diversion is affixed orthogonally to an inclined clear
board with a fixed joint behind the V-shaped walls.
High-speed cameras are then mounted around themodel. Geomaterial is prepared in the storage container
at the upstream end of the flume. The material is retainedby a gate, which is released by lifting it vertically. The
granular material used in the test is the Toyoura sand. A
total mass of 10 kg is prepared, and then the flume isinclined to the target angle. Afterwards, the granularmaterial is released from the container. The flow
accelerates down the flume under the influence ofgravity and impacts the model diversion.

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