E3S Web of Conferences 415, 01005 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501005

Effects of flow-bed interactions on barrier impact
Weerakonda Arachchige Roanga K. De Silva1*, Haiming Liu1, Clarence Edward Choi2, and Charles Wang Wai Ng1
1The Hong Kong University of Science and Technology (HKUST), Hong Kong SAR, China
2The University of Hong Kong (HKU), Hong Kong SAR, China

Abstract. Debris flow mobility is governed by complex interactions at the flow-bed interface. Theseinteractions may cause flow bulking and increase in momentum. Both factors need to be considered for thedesign of barriers installed along the flow path. In this study, channelized debris flow over a wet erodible
bed impacting a terminal flexible barrier is modelled in a 28-m-long and 2-m-wide flume facility with 20oslope inclination in Hong Kong. Tests with flow volumes of 6 m3 and 9 m3 overriding both erodible andnon-erodible beds are conducted. The change in normalised flow energy over the erodible bed section, thechange in flow momentum and the impact dynamics on the terminal barrier is assessed.

1 Introduction
Debris flows occur in mountainous regions when massesof saturated sediment and water surge downslope andcause fatalities and damage to infrastructure [1]. The
most distinct feature of debris flows compared to otherlandslide types is its ability to erode bed material [2,3].
The spatial-temporal patterns of erosion and deposition
strongly influences the final debris flow volume andmobility of debris flow [4,5]. Evidently, a robust barrier
design must cater for any change in volume andmomentum of a debris flow before impacting it.
In this study, debris flows with volume 6 m3 and 9 m3
are tested in a 28-m-long flume model, with and without
an erodible bed to investigate the effects of erosion on theflow kinematics and impact mechanisms on a flexiblebarrier.

length (Le) of 6 m and thickness (te) of 120 mm. The V9E
test consists of an erodible section with Le of 8.5 m and teof 200 mm (Figure 1). The debris flow material consistsof gravel 36%, sand 61%, clay 3% by mass with a 30%
volumetric water content. The erodible bed material
consists of fine coarse sand 33%, sand 63% and fines 4%
by mass. The details of preparation of erodible bed arepresented in [7] and a summary of the test program isgiven in Table 1.

2 Physical flume modelling
The 28-m-long flume facility at the Kadoorie Centre in
Hong Kong is used in this study to conduct a series ofphysical experiments. A terminal flexible barrier with aheight of 1.5 m and a width of 4 m is installed at a distanceof 4.4 m from the outlet of the inclined section of theflume in the horizontal runout zone. The barrier has threeload bearing cables holding up a ring net panel. Load cells
are attached to each cable to measure the cable impactforces. Figure 1 shows the flume model with an erodible
bed and the instrumentation setup. Details of theinstrumentation can be found in [7].
Four tests were conducted. The volumes were varied
as 6 m3 (V6) and 9 m3 (V9). For each flow volume acontrol test was conducted where the entire flume lengthis non-erodible (denoted as S). For the erodible bed tests,
the flume base was modified to include wet soil bedmaterial. V6E test consists of an erodible section with aFig. 1. 28-m-long flume modified to include the erodible bed inthe V9E test (HSC denotes high-speed camera, GP denotes
GoPro camera and black boxes marked on the flume base shows
the locations of the instrumentation boxes and flow heightsensors).

Table 1. Test Program
Test ID
V6S
V6E
V9S
V9E

Flow Volume(m3)669
9Bed Setup
Non-erodible bed
Erodible bed (Le = 6 m)
Non-erodible bed
Erodible bed (Le = 8.5 m)* Corresponding author: warkdesilva@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, 01005 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501005

3 Interpretation of test results(x/L = 1), the energy of the flow is further dissipated in all
tests, before reaching the barrier.
The normalised flow momentum change ( 𝛤n =𝑉n × 𝑆n ) prior to the impact on barrier can be calculated
relative to the control tests. The normalised flow frontal
velocity prior to the barrier location (at Box-5) (𝑆n ) andthe normalised frontal flow volume at impact (𝑉n ) are used
for the calculation (Table 2) [2]. Furthermore, the peak
impact force of the terminal flexible barrier can also be
normalised with corresponding control tests. It isobserved that at the impact location, the flow frontal
velocity is reduced, and the flow volume is slightly
increased due to the presence of the erodible bed. A valueof 𝛤n < 1 indicates that flow momentum is lost in theerodible bed tests compared to the control tests. Amaximum momentum loss of ~22% is observed for theV6E test where higher erosion led to a higher reduction in
the normalised flow energy compared to V9E test (Figure2). Moreover, the normalised peak impact force (Fn)against the flexible barrier conforms with the observed 𝛤n ,which shows a higher momentum loss and thus a lower
measured peak impact force at terminal barrier.

The flow kinematics can be characterised by the flow
frontal velocity and peak flow depths measured at specificlocations along the flume transportation zone (𝑥 = 0 m to𝑥 = 15 m). By using these two flow parameters, theevolution of the flow energy along the flume length canbe calculated as:
𝐸 = 𝑧 + ℎfe 𝑐𝑜𝑠𝜃 + 𝑣f2 /2𝑔(1)where 𝑧 is the elevation from datum, 𝑣f is the measured
flow frontal velocity, ℎfe is the measured peak flow depth
adjusted to consider the erosion depths, 𝜃 is the flume
inclination of 20o and 𝑔 is the gravitational acceleration.
The 𝑣f is indicative of the changes in flow front
kinematics during the interaction with the erodible bed [2]and governs the peak impact load on the terminal flexible
barrier [8]. The datum is taken at the end of the flumetransportation zone (i.e., 𝑥/𝐿 = 1.00, where 𝑥 is the length
along the flume from gate and 𝐿 is the total length of 15m). The calculated 𝐸 is normalised by the flow energy at𝑥/𝐿 = 0.23 (at Box-1) such that the normalised flowenergy (𝐸n ) is unity at that specific location.
Figure 2 shows the 𝐸n for the four tests along thenormalised flume length. 𝐸n decreases along the flume
length for all tests. Interestingly, both the erodible bed
tests show a higher energy dissipation after the flow
reaches the erodible bed (𝑥/𝐿 = 0.44 for V9E and 𝑥/𝐿 =0.60 for V6E) compared to the corresponding control
tests.
Test V6E with the highest erosion volume shows ahigher dissipation of flow energy, where a maximum of~40% energy is lost compared to the test V6S by the time
the flow reaches the end of the inclined section of theflume. This implies with an increase of eroded bed
material the dissipation of flow energy also increases. Inessence, the increase in potential energy (i.e., flow depth)
from the addition of eroded material is relatively lower
compared to the decrease in flow kinetic energy (i.e., flowvelocity).

Table 2. Normalised flow momentum and peak impact force atthe terminal flexible barrier
TestID

V6E/S
V9E/S

Normalised energy En

0.8

0.6

V6E

0.5

V9S

0.4

V9E

Normalisedflowmomentum(𝜞𝐧 )Normalisedpeak
impactforce (Fn)

0.68
0.94

1.15
1.02

0.78
0.96

0.91
0.98

A series of physical experiments were conducted in astate-of-the-art 28-m-long flume to model the effects offlow-bed interactions on flow kinematics and terminal
barrier impact.
Normalised flow energy reduction of up to ~ 40% isobserved at the end of flume transportation zone for theerodible bed tests compared to the control tests with nonerodible bed. Results show that erosion induced reductionin flow kinetic energy is higher than the increase inpotential energy from the eroded mass, leading to lower
peak impact forces on the terminal barrier. The findings
indicate the need for future research to further verify andevaluate erosion induced changes in flow kinematics andbarrier impact.

0.9

V6S

Normalise-d flow
volume(𝑽n)4 Conclusions

1.00.7

Normalise-d flow
frontalvelocity(𝑺n)~ 40%

Max reduction
in energy

Acknowledgements

0.3

The work in this paper was supported by AoE/E-603/18 grantfrom the Research Grant Council of the Government of Hong
Kong Special Administrative Region, China. W. A. R. K. De
Silva gratefully acknowledges the support of the Hong Kong
PhD Fellowship Scheme (HKPFS) provided by the RGC ofHKSAR, China.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Normalised distance along flume x/LFig. 2. Normalised flow energy along the flume inclined
distance (start of the erodible section shown by vertical lines at
𝑥/𝐿 = 0.44 for V9E and 𝑥/𝐿 = 0.60 for V6E)

The observed changes in flow kinematics and flowenergy over the erodible bed section and its effects on theimpact dynamics are assessed. Due to the abrupt transitionin flume angle from 20o to 0o at the end of erodible bed

References[1]

2M. J. Froude, D. N. Petley, Nat. Hazards Earth Syst.
Sci. 18, 2161 (2018)

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[2]
[3]
[4]
[5]
[6]

[7]
[8]

https://doi.org/10.1051/e3sconf/202341501005

R. M. Iverson, M. E. Reid, M. Logan, R. G. LaHusen,
J. W. Godt, J. P. Griswold, Nat. Geosci. 4, 116 (2011)
P. Song, C. E. Choi, J. Geophys. Res. Earth. Surf. 126,e2020JF005930 (2021)
T. de Haas, W. Nijland, S. M. de Jong, B. W. McArdell,
Sci Rep 10, 14024 (2020)
S. P. Pudasaini, M. Krautblatter, Nat. Commun. 12, 1(2021)
C. W. W. Ng, C. E. Choi, H. Liu, S. Poudyal, J. S. H.
Kwan, Understanding and Reducing Landslide
Disaster Risk: Volume 1 Sendai Landslide Partnerships
and Kyoto Landslide Commitment, WLF 2020
H. Vicari, C. W. W. Ng, S. Nordal, V. Thakur, W. A.
R. K. De Silva, H. Liu, C. E. Choi, Can. Geotech. J. 59,1007 (2022)
H. W. K. Lam, E. H. Y. Sze, E. K. L. Wong, S. Poudyal,C. W. W. Ng, et al., Can. Geotech. J. 59, 12 (2022)

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