E3S Web of Conferences 415, 05002 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341505002

Debris flow hazard assessment: laboratory experiences andnumerical modelling
Francesco Castelli1, Valentina Lentini2*, and Alessandra Di Venti3
1Faculty of Engineering and Architecture, University Kore of Enna, 94100 Enna, Italy

Abstract. In the last years, the study of debris flow has become very important in the research activity andin the engineering practice. The use of numerical models is able for the study of debris flow’s propagation.
The study area is located in the north-east Sicilian coast and in particular in the municipality of Gioiosa
Marea, severely affected by landslides occurred from 2000 to 2013. In this case study, the FLO-2D code has
been used. The soil parameters has been determined by means of in situ investigations and laboratory tests.
The knowledge of the physical and mechanical properties is very important to define the input parametersof the model, including the volumetric concentration associated with the soil friction angle.

1 Introduction
Debris flows are a moving mass of loose soil particles
ranging from clays to rocks with high water content,
driven by gravity [1]. The speed and volume of debris
flows make them very dangerous.
The use of numerical models capable of simulating
the propagation of a debris flow are a suitable support
tool for the drafting of hazard maps [2-5]. Overall, adebris flow event can be modelled [6] through three data
sets: digital terrain model, hydrological data andrheological properties of the water and sedimentmixture.
To define the input parameters of the model thatdefine the rheology of the soil, a physical andmechanical characterization is required, in order todetermine the volumetric concentration associated withthe friction angle of the soil.
Often due to extreme meteoric events, the process of
meteoric infiltration results in an increase in the degreeof saturation that involves the increase of the weight unit
volume and therefore of the unstable actions and thereduction of the soil strength which can cause thecollapse.
Although the soil has good strength characteristics
on high slopes, the soil stability is linked to thebeneficial effect that partial saturation conditions giveon the soil shear strength. Due to intense extreme
meteoric events, increments of degree of saturation andthe consequent suction’s decrease lead to reductions inshear strength that are incompatible with the stability
condition.
To analyze this aspect and evaluate the soil failure
parameters, some controlled suction triaxial tests wereperformed. At the aim to obtain the friction angle ofunsatured soil to link with numerical modelling of debris
flows through the volumetric concentration, the resultsof suction controlled triaxial tests were interpreted.

2 The case study of Capo Calavà
The study of interaction between debris flows and linear
infrastructures is part of the research project TEMIMIRATI - Technologies and Innovative Models for
Risk Mitigation in Critical Infrastructures - funded by
MUR (Italian Ministry of University and Research) at
the University Kore of Enna with reference to the Sicily.
From 2003 to 2013, several districts of Gioiosa
Marea in the north-east Sicilian coast, such as CapoCalavà, Contrada Ridente and Scoglio Nero, wereaffected by several landslides that involved large
volumes of soil, by including the highway SS113,
known as “Settentrionale Sicula” (Figure 1).
2.1 Soil geotechnical characterization
A detailed geotechnical investigation including insitu and laboratory tests for the evaluation of thephysical and mechanical properties of the soil involvedin the debris flows has been performed in the study area.
In particular, n.3 boreholes have been carried out of
which S1 and S2 at a depth of up to 10 m and S3 at a
depth of up to 15 m (Figure 2).
To determine physical and mechanical parameters,
standard laboratory tests have been performed on somesamples retrieved by the boreholes. The results ofstandard classification tests are reported in Table 1 interms of unit weight γ, grains specific density γs, naturalwater content wn and liquid limit wl for each testedsample.
The mechanical properties in terms of friction angle
have been evaluated for the sample S2C2 by means of
direct shear test. The shear strength parameters [7] forthe tested sample are ’ = 26° and c’ = 0 kPa.

© 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, 05002 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341505002

Fig. 1. Debris flow in SS 113 km 84+800 in the municipalityof Gioiosa Marea (ME) in December 2011.

Fig. 3. Triaxial testing machine for unsatured soil.

The compression phases were carried out for threedifferent suction matrix values (100 kPa, 150 kPa and200 kPa) for the mean stress value (p-ua) equal to 120
kPa, 240 kPa and 360 kPa.
Constant water (CW) content conditions tests wereperformed at strain rates of 0.1 mm/min. As an example,
in Figure 4 the stress-strain curves for 3 specimens withmean stress value equal to 360 kPa are compared (Figure4). As the suction (ua-uw) increases, both the values ofthe deviator and the peak axial deformation increase andthe specimens show a tendency towards a more fragile
failure, with deviatoric stress values that degrade more
quickly in the post-peak phase.
Applying the extend Mohr-Coulomb failureenvelope for unsaturated soils proposed by [8], it waspossible to calculate the shear stress at failure τf forsuction s values equal to 100 kPa, 150 kPa and 200 kPa:

Fig. 2. Location of boreholes within the study area.

(1) f  c '  (  ua )tan '  (ua  uw )tanb

Table 1. Physical parameters of the soil
Depthγγswnwl[m][kN/m3][kN/m3]

(%)

(%)

S1C1

2.70 - 3.00

18.76

24.49

5.30

-

S1C2

5.20 - 5.60

-

26.18

6.62

-

S2C1

4.00 - 4.40

-

25.50

5.53

-

S2C2

8.00 - 8.40

17.99

27.18

8.11

16.42

 f  c  (  ua )tan '(2)-c  c  (ua  u w )tanb(3)SampleS3C1

1.70 - 2.00

-24.82

9.23

where c’ and φ’ are the effective cohesion and theeffective friction angle obtained from saturated test, (σua ) is stress at failure, (ua- uw) is suction matrix at failureand φb is angle describing increase in shear strength withmatric suction. Equation (1) can be written as:'1200

2.2 Controlled suction triaxial tests

1000
800
q [kPa]Controlled suction triaxial tests were carried out
applying the axis translation technique on unsaturated
reconstituted specimens with 50 mm in diameter and
100 mm in height by means of the triaxial testingmachine for unsaturated soils available at the Soil
Dynamics and Geotechnical Engineering Laboratory of
the University of Enna (Figure 3).
The value of matrix suction s = ua - uw was between
100 and 200 kPa, being ua air and uw pore-waterpressures. The specimens were prepared with a watercontent above the liquid limit and consolidated under thepre-consolidation pressure. Air and water pressure wereapplied to the initially saturated specimens, dependingon the aimed suction. Then, the specimens reached tofailure, keeping the matrix suction constant.

600400ua - uw = 100 kPa
ua - uw = 150 kPa

200ua - uw = 200 kpa

004

812Axial strain  [%]

1620Fig. 4. Stress-strain curves obtained by CW test.

The effective shear strength parameters for saturated
soils c’ = 0 kPa and φ’ = 26° were obtained by direct
shear test. The extended Mohr-Coulomb failureenvelope for CW tests was plotted using Equation 2.

2E3S Web of Conferences 415, 05002 (2023)
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Peak deviator stress was used to plot the Mohr
circles to obtain the extended failure envelopes forunsaturated soils (Figure 5). The values of total cohesionintercept increase with matric suctions. The φb angle for
CW tests [9] was obtained by plotting the total cohesionintercept with matric suction (Figure 6).

cc  0, 9cb

s = 200 kPa
s = 150 kPa
s = 100 kPa
s = 0 kPas = 0 kPa' = 26 °c' = 0 kPa

600500
 [ kPa]

for α ≤ 21°(5)for α ≥ 21°(6)In the simulations with the FLO-2D model, thevalues of the empirical parameters 1 = 0.006032, 1 =19.9, 2 = 0.000707 and 1 = 29.8 have been chosenfrom literature [10] by selecting those that have similargeomorphological and lithological characteristics withthe studied area.

800
700

1tan  tan   tan 

400

3.1 Hydrological study

300200A preliminary hydrological study was carried out inorder to characterize morphologically the hydrographicbasin. The flood hydrograms were obtained for thereturn times taken into consideration (T=5, 10, 20, 50,100, 300 years). These hydrograms constitute the input
data to be assigned to the inflow cell within the FLO-2Dcalculation code.

10000200

400

600

800

1000

1200

14001600

 [ kPa]

Fig. 5. Mohr circles as a function of suction.

300

3.2 Input data
I ntercept cohesion c [kPa]250

For the purposes of debris flow modelling with theFLO-2D, a discretized DTM was used with grid of 2 mx 2 m squares. A Manning coefficient of 0.02 and 0.03
[s/m1/3] was assigned taking into account the road, somebuildings and wooded patches. Subsequently, theoutflow data (cells in which the water leaves the system)
and inflow data (flood hydrograms) were attributed tothe cells. The inlet flow is calculated according toEquation (4) as a function of the volumetricconcentration and dependent on the value of the internal
friction angle of the material, obtained for both saturated
and unsaturated soil.
In this study, several simulations were carried out,by varying the volumetric concentration (cv = 0.436 forsaturated soil and cv = 0.569 for partially saturated soil)
and for different return times.

200150100500050100150200250

Suction s [kPa]Fig. 6. Intercept cohesion versus suction.

3 Numerical Modelling
The numerical models are useful to simulate thepropagation of debris flow. In this case-study, the FLO2D code was used. FLO-2D [6] is a two-dimensional
hydraulic model which, starting from one or more floodhydrograms as input and using a completely dynamicapproach, predicts the flood area, the speed and depth of
the water or debris flow for each cell in which thetopography has been discretized.
A debris flow event can be modelled through three
data sets: digital terrain model (DTM), hydrological
data, and rheological properties of the water andsediment mixture.
The discharge rate value of the debris flows for thebasin has been calculated by the Equation:
𝑄𝑄

3.3 Numerical results
The results obtained by numerical simulations arepresented in terms of maximum depth (Figure 7 and 8)
and maximum flow velocity (Figure 9 and 10).
The simulations carried out by attributing higher
concentration values to the input hydrogram return
greater deposit depths and lesser extent-flooded areas
than those with a lower concentration; on the contrary,as the concentration value decreases, there is an increasein the debris area with a global decrease in depths,
attributable to a greater fluidity of the mixture.
The increase in flow depths is recorded above all incorrespondence of the inhabited centre where thehydrogeological risk is greater due to the huge damage
that a debris flow can cause to structures andinfrastructures and to the loss of human life.
The results are in good agreement with previously
recorded real events, by confirming the possibility ofusing input parameters available in literature formaterials with similar geotechnical characteristics.

(4)

where Qdf is the discharge of debris flow, Ql is the liquid
discharge rate (given by the hydrograph), c and cb are
the concentration of the solid phase in the debris flow
and into the soil, respectively. The debris flow
concentration is calculated, according to the following
expressions, where  if the slope angle in the upstream
section of the model:
3E3S Web of Conferences 415, 05002 (2023)
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a prediction methodology, capable of providing useful
information on the dynamics of debris flows.
The laboratory tests were essential to understand thebehaviour of unsaturated soil and to derive theparameters to be implemented in the calculation code toachieve debris flows numerical modelling. Triaxial tests
with controlled suction were indispensable to estimate
the value of the friction angle.
The numerical results, obtained by varying the return
time and the volumetric concentration, are in good
agreement with the real events and confirming thepossibility to use input parameters available in literature
involving materials with similar geotechnical properties.

Fig. 7. Maximum flow depths for cv = 0.436 and Tr = 50years.

References
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Fig. 8. Maximum flow depths for cv = 0.569 and Tr = 50
years.

Fig. 9. Maximum flow velocities for cv = 0.436 and Tr = 50years.

Fig. 10. Maximum flow velocities for cv = 0.569 and Tr = 50years.

4 Conclusions
The present study concerns the assessment hazard oflandslides with rapid kinematics, connecting laboratory
experiences and numerical modelling, in order to define4