E3S Web of Conferences 415, 06013 (2023)
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https://doi.org/10.1051/e3sconf/202341506013

Multiple debris-resisting barriers with basal clearance: a study
on impact force
Haiming Liu1*, Clarence E. Choi2 and Charles W. W. Ng1
1Department of Civil and Environmental Engineering of The Hong Kong University of Science and Technology, Hong Kong SAR,

China
2Department of Civil Engineering of The University of Hong Kong, Hong Kong SAR, China

Abstract. Optimising debris-resisting barriers is of paramount importance on constructing cost-effectiveand eco-friendly mitigation works. Multiple barriers with basal clearance can potentially serve as an optimal
approach because they can facilitate flow energy dissipation, reduce the impact force on each barrier, easethe maintenance and resist a large flow volume. However, the design impact force of multiple barriers withbasal clearance remains empirical. In this study, physical model tests were carried out to investigate theimpact force of idealised dry granular flow against dual rigid barriers with basal clearance using a 5-m-long
flume model. Measured impact forces show that basal clearance attenuates the impact force exerted on thesecond barrier by apportioning the impact forces from basal discharge and overflow. Basal discharge
dissipates the kinetic energy of landing flow and reduces the impact force of overflow. A rational design ofbasal clearance serves as an efficient measure for optimising the design of multiple barriers.

1 Introduction
Multiple barriers are advantageous on retaining large
volumes of debris avalanches and debris flows
compared with a single barrier because they canprogressively arrest the flow and minimise the flow
acceleration by facilitating the energy dissipation,
thereby reducing barrier sizes. For closed-type multiplebarriers [1], even flow with a small volume can beresisted by upstream barriers, resulting in an increase ofmaintenance work or a decrease of total retention
capacity for next flow event. To optimise the design ofmultiple barriers, basal clearance [2–4] is usuallyconstructed beneath the barriers to allow for dischargeof flow (Fig. 1). Moreover, a basal clearance also serves
to reduce the impact forces on barriers [5]. Despite thevalue of basal clearance, the design remains empirical.
In this study, impact dynamics of debris flow against
multiple barriers have been investigated by conducting
laboratory-scale flume model tests. The debris flows andmultiple barriers were simplified as monodisperse dry
granular flow and dual rigid barriers. The effects ofbasal clearance height on the impact dynamics areexamined.

2 Flume modelling
The experiments were carried out in a rectangular flume
model. The flume had a total length of 5 m, a width of
0.2 m and a depth of 0.5 m. A storage container at theupstream of the flume had a length of 0.5 m to retain thetest material with a gate. A pneumatic device wasinstalled on the flume to control the gate. The pneumaticdevice was activated during the test to lift the gate andrelease all the source material down the flume.

Fig. 1. Schematic of the interaction between debris flow anddual rigid barriers with a basal clearance beneath the firstbarrier.

Figure 2 shows a schematic side view of the test setup and instrumentation. The flume was inclined at 25º.
The first and the second rigid barriers were installed
1100 and 2500 mm away from the storage container,
respectively. A load cell was installed on the second
barrier to measure the flow impact force. High-speed
cameras served to measure the flow impact velocity,
flow depth and impact kinematics. A video camera(model no.: GoPro Hero7) served to provide an overall
view of the impact kinematics in a qualitative manner.
Ultrasonic sensors were mounted to measure the flow
depth.
A total mass of 50 kg of monodisperse glass beads
with a diameter of 3 mm were adopted for each test.
Before carrying out tests for flow–barrier interaction, acontrol test without installing any barrier was conducted* Corresponding author: hliubc@connect.ust.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/).

E3S Web of Conferences 415, 06013 (2023)
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https://doi.org/10.1051/e3sconf/202341506013

to obtain the flow velocity and flow depth along theflume using high-speed camera. Then, the location ofthe first barrier was determined by targeting the Froudenumber of the flow as 3. The first barrier had a barrierheight of 2h0 to allow for both overflow and basal
discharge. Basal clearance height Hc beneath the firstbarrier was varied from 0 to 0.8h0. Details of the test setup, instrumentation and plan were reported by [6].

density of the flow; v1 is the impact velocity at the firstbarrier; h0 is the impact flow depth at first barrier; w is
the flow width. The normalised impact force indicatesthe ratio of impact force between the two barriers. Withthe presence of basal clearance, the impact force on thesecond barrier is influenced by both the basal discharge
and overflow from the first barrier. As a result, the time
history of the impact force shows two peaks during theimpact, where the first peak is caused by the basal
discharge and the second peak is caused by theoverflow. With the increase of basal clearance beneaththe first barrier, the impact force from basal discharge
increases while the impact force from overflow
decreases.

Fig. 2. Schematic of the test set-up and instrumentation [6].

3 Results3.1 Observed impact kinematics
Figure 3 shows the observed impact kinematics capturedby the video camera for test where the first barrier had aheight of HB1 = 2h0 and basal clearance of Hc = 0.6h0. A
wide field of view was adopted to enable capturing theimpact kinematics on both barriers. At t = 0.5 s, the basal
discharge was approaching to the second barrier and theoverflow occurred on first barrier. At t = 1.2 s, the basal
discharge impacted on the second barrier and theoverflow from the first barrier landed on the basal
discharge. The landed flow interacted with the basal
discharge and flowed downstream to the second barrier(Fig. 3c). The initially arrested basal discharge by thesecond barrier formed a deposition and cushioned thesubsequent impact. As the impact process continued, thedeposited material behind the second barrier continued
to accumulate and pileup to the upstream direction (Fig.
3d). At the end of the impact, the deposited material
reached the basal clearance of the first barrier andblocked the discharge (Fig. 3e).
The observed impact kinematics indicate that thepresence of a basal clearance can apportion theimpacting flow against the first barrier from basal
discharge and overflow. The basal discharge served as asoft loose material for the overflow to land and attenuate
the kinetic energy. The basal discharge deposited on thesecond barrier also acted as a cushion layer for thesubsequent overflow impact.

Fig. 3. Observed impact kinematics [6] by video camera oftest with first barrier height HB1 = 2h0 and Hc = 0.6h0: (a) t =0.5 s; (b) t = 1.2 s; (c) t = 1.4 s; (d) t = 2.4 s; (e) t = 6.1 s.
Note that the kinematics captured by the video camera is not
for quantitative analysis due to the fish-eye effect.

3.2 Measured impact force

Fig. 4. Time history of impact force on the second barrier fordifferent basal clearance heights beneath the first barrier: (a)Hc = 0.2h0; (b) Hc = 0.4h0; (c) Hc = 0.6h0; (d) Hc = 0.8h0.

Figure 4 shows the time histories of measured impact
force on the second barrier for basal clearance heights
beneath the first barrier ranging from 0.2h0 to 0.8h0. The
impact force is normalised by the theoreticalhydrodynamic impact force F = αρv12h0w, where α is theimpact coefficient and is selected as unity, ρ is the bulk

The maximum impact force on the second barrier
decreases when Hc ranges from 0.0 to 0.6h0 due to the
reduced overflow and the cushioning effect from thedeposition of the basal discharge (Fig. 3b). When Hc =2

E3S Web of Conferences 415, 06013 (2023)
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https://doi.org/10.1051/e3sconf/202341506013

References

0.8h0, the impact force on the second barrier increases
because the impact force by basal discharge surpassed
the impact force by overflow. This implies that theimpact force on the second barrier for Hc/h0 ≤ 0.6 andHc/h0 > 0.6 can be estimated from the impact forces ofoverflow and basal discharge, respectively.
To characterise the governed impact force on thesecond barrier, Ng et al. [6] proposed an analytical
approach to estimating the impact forces of overflowand basal discharge considering basal clearance heightbeneath the first barrier. The maximum impact force on
the second barrier was considered as the maximum
estimated impact force from overflow and basaldischarge. By comparing with the measured maximum
impact forces in this study as shown in Fig. 5, this
proposed method performs reasonably well. More
importantly, the proposed method captures theminimum impact force at Hc/h0 = 0.6, indicating thepotential for optimising the design of multiple barriers
with an optimal basal clearance.

1.

2.

3.
4.

5.
6.

Fig. 5. Impact force on the second barrier for different basal
clearance heights beneath the first barrier.

4 Conclusions
In this study, physical flume experiments were carriedout to study the effects of basal clearance beneath thefirst barrier on the impact dynamics against dual rigid
barriers. The basal clearance of the first barrier canregulate the impact force exerted on the second barrier
by dissipating the kinetic energy of landing flow andapportioning the load contributions from basal discharge
and overflow. The basal discharge governs the impact
force when Hc/h0 ≥ 0.8, whereas the overflow governs
the impact force when Hc/h0 ≤ 0.6. These two featuresindicate a minimum impact force on the second barrier
when Hc/h0 = 0.6. The two features also have been well
captured by an analytical approach, which can serve tooptimise the impact force on multiple barriersconsidering basal clearance height.
The work described in this paper was supported by theResearch Grants Council of the Hong Kong Special
Administrative Region, China (Areas of Excellence Scheme:AoE/E-603/18; Early Career Scheme Fund: 27205320).

3

C. W. W. Ng, C. E. Choi, H. Liu, S. Poudyal, J. S.
H. Kwan, Design recommendations for single anddual debris flow barriers with and without basal
clearance, in Proceedings of Workshop on World
Landslide Forum (pp. 33-53). Springer, Cham(2020)
C. Morstabilini, I. Boschini, F. Zambrini, G.
Menduni, M. L. Deana, N. Zorzi, Application of
an innovative, low-maintenance weir to protect
against debris flows and floods in Ottone, Italy, in
Debris-Flow Hazards Mitigation: Mechanics,Monitoring, Modeling, and Assessment Proceedings of the 7th International Conference on
Debris-Flow Hazards Mitigation, Colorado, USA,pp. 996–1003 (2019)
G. Piton, A. Recking. J. Hydraul. Eng. 142, 2(2015)
C. Wendeler, A. Volkwein, A. Roth, B. Herzog, N.
Hahlen, M. Wenger, M. Hazard prevention using
flexible multi-level debris flow barrier, inInterpraevent (Vol. 1, pp. 547–554) (2008)
C. E. Choi, C. W. W. Ng, H. Liu, Y. Wang. Can.
Geotech. J. 57, 2 (2020)
C. W. W. Ng, H. Liu, C. E. Choi, A. Bhatta, M.
Zheng. Can. Geotech. J. 59, 7 (2022)