E3S Web of Conferences 415, 01032 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501032

Impact force of post-fire debris flows over erodible beds
Min Zheng1*, Haiming Liu1, Sunil Poudyal1 and Charles W. W. Ng1
1Department of Civil and Environmental Engineering of The Hong Kong University of Science and Technology, Hong Kong SAR,China

Abstract. After wildfire events, water repellent soil is often found in the subsurface layer of channel bed
in the burnt area. Debris flows generated from burnt basins and ensuing entrainment of the channel bed poseimminent threat to infrastructure and human lives. However, the fundamental interaction mechanisms ofdebris flow overriding water repellent bed and resulting impact force on debris-resisting barriers have yet tobe elucidated. In this study, physical flume experiments are conducted to simulate post-fire debris flows
overriding and entraining a sand bed with varied wettability. Compared to a wettable bed, water repellent
sediment exhibits a tremendous increase in the erosion depth and subsequent impact force on the barrier.
The test results demonstrate that debris flows overriding water repellent sediment can be particularly
hazardous and the effects of water repellency need to be captured by the design criteria of debris resisting
barriers in burnt basins.

1 Introduction
Over the past few decades, with increases in average
global temperature and extreme climatic events, theprobability of wildfire and burnt area have increased
markedly worldwide [1]. Wildfire can remove
vegetation, lead the soil in the subsurface layer ofhillslopes to be water repellent (i.e., hydrophobic), andcause dry ravels on these burnt hillslopes, deliveringsignificant amounts of hydrophobic sediment to stream
channels [2-4]. With a contact angle (CA) [5] larger than90°, hydrophobic soil inhibits infiltration and leads toincrease in surface runoff, preferential flows, and soil
erosion during rainfall [6]. Post-fire debris flows thatoriginate in the recently burnt hillslopes or catchments
entrain a large amount of the water repellent bed
sediment, exponentially increasing the destructive
potential of the flow [7]. As one of the most importantmomentum exchange processes, debris flow
entrainment has been extensively explored in fieldstudies and laboratory flume tests in the literature toinvestigate the effects of water content of erodible bed[8-9], inclination and thickness of bed sediment [10],and flow composition [11]. Whilst water repellency has
profound influence on the soil hydraulic and mechanicalproperties, the wettability of soil is commonly neglectedin assessing bed entrainment [6], leading to uncertainties
about the characteristics of post-fire debris flows and theefficiency of mitigating barriers in burnt areas. The
specific objective of this study is to conduct a series offlume experiments to simulate the erosion of water
repellent soil by debris flows. Erosion depths of bed
sediment with varied wettability are obtained by using
erosion columns to investigate the characteristics ofentrainment of hydrophobic soil. With the installed rigid

barrier, impact force from flows after entrainment isobtained to assess the mitigation system.

2 Methods
2.1 Flume modelling
In this study, a series of experiments are conducted byusing a 3000 mm-long rectangular flume model (in Fig.
1). The flume has channel width of 200 mm and isinclined to 30° during tests. The upper 1500 mm lengthof the channel has a rigid bed and the lower 1000 mm
has an erodible bed with a thickness of 45 mm
constructed using loose sand sediment. A rigid barrier
equipped with a load cell is installed at the end of thechannel to measure the impact force of flows afterentrainment.

Fig. 1. Flume model and instrumentation.

* Corresponding author: mzhengaj@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, 01032 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501032

Ultrasonic sensors are installed along the centrelineof the flume at the start, middle and the end of theerodible bed to capture the flow thickness during theerosion process. A high-speed camera is mounted on theside to capture the movement of sediment. Erosion
columns [12-13] buried in the bed sediment are used tomeasure the spatial distribution of entrainment depth asshown in Fig. 2(a). As the bed is scoured by the debris
flow, nuts on the columns (in Fig. 2(b)) are removed
together with entrained materials. Intact nuts remaining
in the initial locations indicate undisturbed bed in theexperiments. Only the final erosion depth is measured
based on the numbers of removed nuts [4] while theinstantaneous erosion time could not be measured using
the current setup.

University of Science and Technology. A summary of
the test program is given in Table 1. Firstly, a control
test (test ID: C) without erodible bed is conducted toinvestigate the flow kinematics. The remaining tests are
setup with erodible bed of different contact angles (CA= 50°, 95°, 126°) and volumetric water content values(θw = 0, 20%, 30%) to study the effects of water
repellency and water content of bed sediment on debris
flows entrainment.
Table 1. Test programme.
TestIDC
W_0
W_20
W_30
S_0
S_20
S_30
R_0
R_20
R_30

--

Contactangle CA (°)--

Wettable

50 ± 2.8

Subcriticalwater
repellent

94 ± 5.7

Water
repellent

126 ± 3.4

WettabilityVolumetric watercontent θw (%)-020300203002030

3 Results

Fig. 2. (a) Plan layout of erosion columns; (b) Illustration oferosion columns.

3.1 Debris flow entrainment
Figure 3 shows the final erosion depth profile at each
erosion column along the centreline of bed sedimentwith 30% volumetric water content and three different
contact angles. The duration of entrainment process ineach test ranges from 0.32 s to 0.35 s. The maximum
erosion depth in wettable bed (test ID: W_30) is about
12 mm, while maximum erosion depth in the subcriticalwater repellent bed (test ID: S_30) and water repellent
bed (test ID: R_30) reaches 39 mm and 42 mm,
respectively. Under similar-sized debris flows, a highvariability of erosion depths as well as a high variabilityin spatial patterns of erosion is observed along the bed
profile. The largest erosion depths are mostly found in
the initial 200 mm length of the bed sediment. Further
downstream, the erosion gradually decreases along thebed. Additional erosion peaks are always observed inhydrophobic sediment. The spatial variability in erosion
depth, i.e., peaks and valleys may be controlled by theflow dynamics as described in [4; 11]. After reaching
the erodible bed, the overriding flow imposes shearing
forces to mobilize a large volume of bed material as thefirst erosion peak. The eroded bed then may deflect theoverriding flow and decrease shearing forces, leading tothe observed erosion valley. This modified trajectory
causes the flow to land and impact further downstream
along the bed, making another erosion peak. Comparedto two peaks in the subcritical water repellent bed (testID: S_30), a third erosion peak close to the barrier
location is observed in water repellent bed (test ID:R_30). The total integrated amount of erosion depth in
R_30 is twice than that in S_30, indicating water
repellent soil is more easily removed by overriding
flows.

2.2 Material
Toyoura sand, a wettable quartz sand with a mean
particle diameter of 0.2 mm and an initial contact angle
around 50°, is adopted to construct the erodible beds.
Toyoura sand is treated by adding either stearic acid ordimethyldichlorosilane (DMDCS) to change the contact
angles, which are commonly used for generating
synthetic water repellent soils [5, 14-15]. Stearic acidwith a mass ratio of 0.2g/kg is applied to get a contact
angle around 90° as subcritical water repellent sand.
DMDCS with a mass ratio of 0.05g/kg is added toachieve severe water repellency with maximum contact
angle of around 130°.
A volume of 0.027 m3 debris-flow mixture with thesame composition is used in all tests. Debris flow wasinitiated by releasing the debris mixture to simulate adam break. The debris-flow mixture consists of solid
volumetric fraction of 55% and fluid fraction of 45%,
which is comparable with the mixture in [11]. The debris
solid volume fraction constitutes 8% Kaolinite clay and
92% Leighton Buzzard sand fraction C. Water is used as
fluid phase in the debris mixture. This solid fraction iscomparable with the mixtures used in USGS flume tests[16]. To assess the significance of different stresscomponents, dimensionless numbers [16] of flow
mixtures including Savage number (NSav) and Friction
number (NFric) are calculated. Based on NFric ~ 103 andNSav < 0.1, the frictional stresses dominate the behaviourof debris flow used in this study.
2.3 Test programme
In total, ten laboratory experiments are performed using
the flume model (in Fig. 1) at The Hong Kong2

E3S Web of Conferences 415, 01032 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501032

demonstrates that debris flows overriding water
repellent sediment are particularly hazardous in terms of
the increased impact force on rigid barrier compared tounburnt conditions.

Fig. 3. Erosion depth along centreline of erodible bed for threeexperiments with 30% volumetric water content.

3.2 Impact force

Fig. 4. The plot of effects of bed water content and wettability
on hydrodynamic coefficient α which characterises thenormalised peak impact force of debris flow.

The severity in downstream hazard after entrainment oferodible bed is evaluated using the impact force
measured on a rigid barrier installed at the end of theflume. The hydrodynamic model [17] is used to quantify
debris flow impact force, as follows:
𝐹 = 𝛼𝜌𝑣 ℎ𝑤(2)where α denotes hydrodynamic pressure coefficient, vrepresents the frontal flow velocity, ρ, h and w are thedensity, thickness, and width of debris flows,respectively. The hydrodynamic coefficient α indifferent erodible bed sediment cases is back calculatedwith input values of the measured peak impact force
value and the impact flow velocity and depth of control
test. The hydrodynamic pressure coefficient is adimensionless measure of impact force on the barrier.
The higher the value of 𝛼 compared to 1.0, the larger theimpact force on the barrier.
Figure 4 shows the variation of hydrodynamic
coefficient (α) with soil moisture content in variedly
wetted erodible beds. When increasing the volumetricwater content of erodible bed from 0 to 30%, the impact
force increases, irrespective of the wettability ofsediment. The decrease of α in wettable bed with 20%
volumetric water content (test ID: W_20) corresponds
to the minimum erosion depth as matric suction within
unsaturated wettable bed improves the sediment
strength, comparable with the findings in [12]. For water
repellent soil with contact angle of 126°, the impact
force against a rigid barrier increases by 80%, revealingthat unsaturated hydrophobic sediment during rainfalls
leads to a more critical situation compared to the drycondition. Furthermore, debris flows overriding bed
sediment with high contact angles show a tendency toinduce higher impact forces. The higher impact forces
are consistent with the larger erosion depth inhydrophobic soil (in Fig. 3), as a large amount ofhydrophobic bed sediment is entrained, moving together
with the overriding flows to impact against the rigidbarrier downstream. For a water repellent bed (CA =126°) with 30% volumetric water content, thehydrodynamic coefficient reaches 1.8, which is higher
than the value recommended by the design guideline [18]applied to rigid barriers in Hong Kong region. It

4 Conclusions
In this study, physical flume experiments are carried out
to study the influence of water repellency of erodible
bed on debris flow entrainment and impact against arigid barrier. The amount of basal erosion increases withthe increase in the contact angle of bed sediment. Whenthe volumetric water content of the erodible bed is 30%,
the maximum erosion depth in water repellent bed canreach up to four times that of wettable bed. Similarly,
when contact angle of erodible bed increases to 126°,
impact force increases by 80% compared with the nonerodible bed condition. The larger hydrodynamic
coefficient for a hydrophobic erodible bed highlights theinadequacy of existing guidelines for designingmitigation structures against post-fire debris flows.
More tests would be conducted with varied flow types(e.g., surface runoff and hyper-concentrated flows inburnt areas) and bed condition (e.g., burnt ash layer on
top of hydrophobic soil) to further improve the hazard
assessment of post-fire debris flows.
The work described in this paper was supported by a grantfrom the Research Grants Council of the Hong Kong Special
Administrative Region, China (Project No. AoE/E-603/18). S.
Poudyal gratefully acknowledges the support of Hong Kong
PhD Fellowship Scheme (HKPFS) provided by the RGC ofHKSAR.

References
1.

2.

3

A. Sommerfeld, C. Senf, B. Buma, A. W. D’Amato,T. Després, I. Díaz-Hormazábal… R. Seidl, Nat.
Commun. 9, 1 (2018)
E. J. Gabet, J. Geophys. Res. Solid Earth 108, B1(2003)

E3S Web of Conferences 415, 01032 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501032

3.

M. C. Palucis, T. P. Ulizio, M. P. Lamb, GSA
Bulletin 133, 9-10 (2021)
4. C. W. W. Ng, M. Zheng, H. Liu, S. Poudyal, J.
Geophys. Res. Earth Surface 127, e2022JF006783(2022)
5. S.D.N. Lourenço, Y. Saulick, S. Zheng, H. Kang, D.
Liu, H. Lin, T. Yao, Acta Geotech. 13, 1 (2018)
6. S.D.N. Lourenço, D. Gallipoli, C.E. Augarde, D.G.
Toll, P.C. Fisher, A. Congreve, Géotechnique 62, 3(2021)
7. Y. Cui, D. Cheng, D. Chan, ISPRS Int. J. GeoInformation 8, 1 (2019)
8. R. M. Iverson, M. E. Reid, M. Logan, R. G.
LaHusen, J. W. Godt, J. P. Griswold, Nat. Geosci. 4,2 (2011)
9. M. E. Reid, R. M. Iverson, M. A. T. T. H. E. W.
Logan, R. G. Lahusen, J. W. Godt, J. P. Griswold,Ital. J. Eng. Geol. Environ. 367-374 (2011)
10. A. Mangeney, O. Roche, O. Hungr, N. Mangold, G.
Faccanoni, A. Lucas, J. Geophys. Res. Earth
Surface 115, F3 (2010)
11. T. De Haas, T. V. Woerkom, Earth Surf. Processes.
Landf. 41, 13 (2016)12. P. Song, C.E. Choi, J. Geophys. Res. Earth
Surf. 126, 5 (2021)
13. H. Vicari, C. W.W. Ng, S. Nordal, V. Thakur, W.
R. K. De Silva, H. Liu, C. E. Choi, Can. Geotech.
J. 59, 6 (2022)
14. D.A.L. Leelamanie, J. Karube, J. Soil Sc. Plant
Nut. 55, 4 (2009)15. S. Zheng, S.D.N. Lourenço, P.J. Cleall, T.F.M.
Chui, A.K.Y. Ng, S.W. Millis, J. Hydro. 554, 582599 (2017)
16. R. M. Iverson, Rev. Geophys. 35, 3 (1997)
17. J. Hübl, J. Suda, D. Proske, R. Kaitna, C. Scheidl,
In Proceedings of international symposium onwater management and hydraulic engineering (ed.
C. Popovska), pp. 137–148. Skopje, Macedonia: SsCyril and Methodius University (2009)
18. GEO, Technical Guidance Note No.52 (TGN52)
Enhanced Technical Guidance on Design of Rigid
Debris-resisting Barriers (2020)4