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International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
47
Original Article
Standardized Stress Model for Design of Torrential
Barriers under Impact by Debris Flow (According to
Austrian Standard Regulation 24801)
Johannes HUEBL1, Georg NAGL
1, Jürgen SUDA2 and Florian RUDOLF-MIKLAU3
1 Institute of Mountain Risk Engineering, University of Natural Resources and Life Sciences, Vienna
(Peter Jordanstrasse 82, 1190 Vienna, Austria) 2 alpinfra engineering + consulting GmbH, Lützowgasse 14/1.; 1140 Vienna (Austria)
3 Federal Ministry for Agriculture, Forestry, Environment and Water Management, Vienna (Austria)
Torrential checkdams with energy dissipating, filtering or deflecting function for debris flows are expected
to be subject to extreme dynamic stress that requires the application of high safety standard for design,
construction and maintenance. The standardized procedure for checkdam design has been developed from
comparative calculations of common debris flow models from engineering practice and calibrated by
impact measurements of debris flow laboratory experiments and data available from literature. The model
is based on a combined static- dynamic stress approach, additional impacts by single objects like boulders
are included. The dynamic area of the dynamic component is derived from a characteristic wetted cross
section area of debris flow corresponding to the design event, which can be found at a characteristic cross
section upstream of the checkdam. The static load is based on a hydrostatic fluid assumption and calculated
analogously to water pressure (with debris flow density) and acts upon total construction height. The
dynamic component is calculated according to the momentum equation and acts uniformly distributed on
the dynamic impact area right below the overflow section up to a height of maximum 4 meters. The
maximum local impact of a single boulder is defined to act on an area of 0.7 x 0.7 meter with 1MN. This
new regulation shall guide practitioners for more objectives and save design of checkdams impacted by
debris flows.
Key words: debris flow, impact force, Austrian standard
1. INTRODUCTION
The design of structural mitigation measures
against debris flows is a challenging task for
engineers. In a first step the relevant mass wasting
process has itself to be determined. Subsequently
process based parameters like density, flow velocity
and depth have to be transferred into a model that
represents loading conditions. Additionally the
granular composition of debris flows varies over
time. Nevertheless, it is essential to develop a
practical standardized stress model for the design of
torrential barriers under the impact of debris flows.
The design of the barriers has to follow its
function (ONR24800) that may affect the initiation,
the transport or deposition of debris flows. The
interaction with the structures can be described by
following functional types:
Stabilization and consolidation
Energy dissipation
Deflection
Usually structures of the above mentioned functions
are combined in a so called functional chain [Kettl,
1984].
To harmonize the design of technical structures
the so-called Eurocode was launched. Based on this
concept the ONR Series 2480xx was established,
encompassing torrential processes, snow avalanches
and rock fall. Within this series two regulations
(ONR24801 & ONR24802) specifies the load model
for debris flows based on results of field
experiments, miniaturized flume experiments and
back-analysis
2. EXPERIMENTAL MEASUREMENTS
AND FIELD OBSERVATION
International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
48
Table 1 Empirical coefficient for impact models, comparison of different k factor for selected impact models
Hydraulic Model Study k Notes
Hydrostatic Model
ghkp
density of debris flow
³m
kg
h flow depth m
g acceleration of gravity
²s
m
k empirical factor /
[Lichtenhahn, 1973] k value between 7-10
2.8-4.4
Based on recalculations
[Armanini, 1997] k= 5
5
Theoretical analysis of dynamic impact by
experimental investigations made by
[Armanini and Scotton, 1993]
[Scotton and Deganutti, 1997] k=2.5-7.5
2.5-7.5
Small scale debris flow model test
Modified hydrodynamic
Model
density of debris flow
³m
kg
g acceleration of gravity
²s
m
v velocity
s
m
[Hübl and Holzinger, 2003]
6.0)(8.05.4 ghvp
7.5
Evaluation of model test to the
dependence of Froude number and
pressure.
Combined Models
c = volume weight of
concentration
Q = Discharge
[Kherkheulidze, 1967]
²)5(1.0 vhcP 1
Total pressure in tons/square meter.
averaged over the depth; water load
momentum and static pressure being taken
into account
[Arattano and Franzi, 2003]
²2
1ghQvF
1.4-1.7
with reference to [Armanini and Scotton,
1993]
There are only a few data published on full size
debris flows impact loads. In Japan observations at
Mt. Yakedake showed a dynamic pressure of 6
tons, estimated from pressure marks on a steel plate
of a size of 15 x 15 cm [Suwa et al., 1973]. At the
Jiangjia River, China, monitoring activities started
in 1973 [Hu et al., 2006; Cui et al., 2005]. Hu et al.
[2011] recorded impact loads of 38 surges with a
flow depth of less than 2 meters and mean
velocities up to 12 m/s. The pressure was then
divided in a fluid and grain impact part. Fluid
pressure attained values up to 100 kN/m², the
estimated grain impact loads reached values an
order of magnitude higher. In Switzerland real scale
tests were performed on a steep slope with a release
volume of 50 m³ [Bugnion et al., 2012 a, b]. Two
sensors of different size measured the impact force.
Additionally velocity, density and flow depth were
recorded. Full size debris flow tests were carried
out in Austria in order to estimate impact forces on
obstacles [Hübl et al., 2009; König, 2006]. To gain
an insight in grain impact loads, additional free-fall
studies with different sized cubes were conducted.
Concrete cubes of 100 and 300 kg weight were
released from 1 to 6 m height. To measure the
impact force, 15 load cells got fixed on a tread bar,
which were arranged grid wise. Another method to
estimate impact forces is the back analysis of
events, if adequate data are available. Based on
back calculation of debris flow volume, peak
discharge and mobility values for impact pressure
were assessed, using the hydrostatic and
hydrodynamic pressure equation. Hungr et al.
[1984] calculated the forces on a destroyed
pre-stressed bridge beam by the November 1983
Charles Creek event. The size of the design boulder
for impact calculations should be equal to the
corresponding flow depth. In Italy, the Sarno event
(May 5-6, 1998) was investigated by [Zanchetta et
al., 2004]. A common method to study the behavior
and the physical properties of debris flows are
miniaturized flume experiments. The most
significant drawback of this kind of tests are
possible scaling errors [Yu, 1992; Armanini and
Scotton, 1993; Scotton and Trivellato, 1995;
Armanini, 1997; Iverson, 1997; Arattano and
Franzi, 2003; Hübl and Holzinger, 2003; Huang et
al., 2007; Tiberghien et al., 2007; Iverson et al.,
2010; Scheidl et al., 2013].
3. HYDRAULIC MODELS FOR IMPACT
CALCUALTION
Several formulas for impact forces calculation
can be found in literature. Hydraulic and solid
collision models are used to represent the
interaction of debris flows with control structures
International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
49
Table 2 Hydrodynamic models Hydraulic Model Study k Notes
Hydrodynamic Model 2vap
density of debris flow
³m
kg
g acceleration of gravity
2s
m
v velocity
s
m
A Area ²m
[Watanabe and Ikeya, 1981] 2a for laminar and fine grained
4
Field measurements of Mt.
Sakurajima
[Egli, 2000] 42a
4-8
after [GEO, 2000]; with density
1800-2200
³m
kg
[Zhang, 1993] 53a
6-10 after [Zhang and Yuan, 1985;Zhang
et al.,1990]
[Wendeler, 2008] 27.0 a
1.4-4.0 Full scale tests at the Illgraben
Switzerland
[Bugnion et al., 2012a,b] 8.04.0 a with max of 2
0.8-1.6
Real scale tests with a release
volume of 50 m³;velocity vary
between 2 and 13 m/s
[Hungr et al., 1984]
1a 1.5 A
3
Load area should be range over an
area as wide the design debris flow
reach, but 1.5 greater height
[Mizuyama, 1979] 1a
2
Separated the debris flow pressure
into fluid and boulder impact force
[Armanini and Scotton, 1993] 2.245.0 a
0.9-4.4
[VanDine, 1996] 1.5 A
1.5A Recommend the momentum
equation after [Hungr et al.,1984]
and Hertz equations
[Tiberghien et al., 2007]
13.5
[Lo, 2000] 3a
6
[Du et al., 1987] 3a
6
[Hübl et al., 2009]. Furthermore the hydraulic
models can be distinguished between hydrostatic,
hydrodynamic and combined models (Table 1).
Transferring the hydrodynamic to hydrostatic
models can be carried out by the application of the
Bernoulli energy line. This results in an empirical
factor k, assuming Froude number is 1,
)1(2
1 2vahgk
where is density, g is gravitational acceleration, v
is mean flow velocity, h is flow depth, and a and k
are coefficients (see Table 1 and 2).
4. COLLISON MODELS
The point impact load due to boulders carried in
the flow may be more important for the design of
certain structural elements [Hungr et al., 1984].
Most collision models for boulder impacts are based
on the Hertz equation [Hertz, 1881]. Timoshenko
and Goodier, [1970] propose this approach
especially for inflexible structures. VanDine [1996]
and Hungr et al. [1984] recommended the Hertz
equation too, but due to deformation and associated
energy loss Hungr et al. [1984] proposed a reduction
factor of 0.1. Lo [2000] and Sun et al. [2005] pursue
this proposition.
4.1 Hertz equation The Hertz equation was reformulated by [Zhang
et al., 1996] for the calculation of boulder impact
load FHertz:
)2(5,1 nKFCHertz
Kc is a dimensionless reduction factor, α and n can
be calculated by Eqs. (3) and (6), respectively
)3(4
54,0
2
n
vmbb
where, mb is boulder mass [kg] and vb the impact
velocity in [m/s];
International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
50
Fig. 1 Comparison of Impact force by Hertz to the force of stiffness method from different length of a simple supported beam
Table 3 Geometric and material data
Spherical intender
Diameter 0.5 m
Material Concrete
Concrete strength Characteristic cube
strength 30 N/mm²
E Modulus 31,000 N/mm²
Density 2500 kg/m³
Beam 0.3 x 0.2m and
0.5 x 0.5m
Length 3-6m
Cross section 0.3m x 0.2m and
0.5m x 0.5m
Concrete strength Characteristic cube
strength 30 N/mm²
E Modulus 31,000 N/mm²
Density 2500 kg/m³
)4(1
B
B
B
Ek
)5(1 2
b
b
b
Ek
where bis the poisson ratio for the boulder,Bis
the poisson ratio for the barrier both equal to 0.2, Eb
is the modulus of elasticity for the boulder, and EB
for the barrier (both equal to 31,000 [N/mm²].
)6(
3
4 5,0
Bb
b
kk
rn
with br as the radius of the elastic sphere.
4.2 Flexible stiffness method
For flexible elements, Hungr et al. [1984] and
also the Chinese standard [MLR, 2006] recommend
to calculate the impact force by the stiffness
method. For a simple beam [Kramer, 2007] the
force to reach the equivalent maximum static
deflection Fstiff can be calculated by Eq. (7).
)7(gmVgmFStiff
Where m is the mass [kg], g gravitational
acceleration [m/s²], l length of beam [m],
)8(2
1stat
s
hV
h is the fall height [m] and sstat is the deflection
from dead load [m].
)9(48
³
IE
lgms
stat
Eq. (9) describes the stiffness of a simple supported
beam with E modulus of elasticity [N/mm²] and I
area moment of inertia [m4].
Figure 1 shows dimensionless reduction factor
of stiff members by comparing the impact force of
Hertz to the stiffness method. The parameters for
comparison of the two different methods are
described in Table 3.
4.3 Design models of different countries
Kwan [2012] gives an overview of the state of
art knowledge in different countries on the design
of impact loads on torrential check dams. It’s based
on literature review. The summary of original
recommendation is given in Table 4.
5. DESIGN MODEL OF AUSTRIAN
STANDARD ONR 24801
The newly developed Austrian Standard
ONR24801 provides a standardized model for
construction of debris flow barriers, which has been
International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
51
Table 4 Compression of design practice of different countries
[Kwan, 2012]
Design Practice Dynamic Pressure Boulder impact
load
Hongkong
[Lo, 2000]
2vp
3
Hertz equation with
load reduction factor
0.1
British Columbia
[VanDine, 1996]
2vp
1
Hertz equation and
reference to [Hungr
et al. 1984]
China
[MLR, 2006]
2vp
1 for
cirular
structure
33.1 for
rectangular
structure
Flexural Stiffness
equation
Japan
[NILIM, 2007]
2vp
1
Modified Hertz
equation, load
reduction factor less
than 0.1
Taiwan
[SWCB,2005]
2vp
1
Modified Hertz
equation, load
reduction factor of
0.2-0.5. Separate
consideration of
dynamic pressure and
boulder impact
Fig. 2 Distinction between load and process model
developed from field and miniaturized experiments
[Scheidl et al., 2013; Hübl and Holzinger, 2003;
Proske et al., 2008] and also from engineering
practice from the Austrian torrent control service.
The proposed method should enable practitioners to
properly design debris flow countermeasures with
the restriction that usually only a few data are
available. Naturally, simplifications and
assumptions are necessary. Therefore, the process
“debris flow” and the interaction with the structure
itself are divided (Fig. 2). At the interface the
parameters of the process have to be transferred to
impact parameters that act on a specified load area.
Basis of design is the load distribution and the force
size of the stress model.
5.1 Load area
Fig. 3 Cross section area projected to the impact area on the
barrier for calculation
Fig. 4 Cross-section of a wide barrier
After a debris flow event the most obvious
evidences are flow marks on both sides of the
channel. Hence, the cross sectional area of past
debris flows is the only parameter that can be
measured directly in ungauged catchments. This
cross section area stands for the magnitude of
design event. For the calculation of the impact area
on the structure, the cross sectional area of the
design event of this upstream reference
cross-section (AQM) is projected to the barrier
(AQdyn). Both areas have to be the same size, the
projected impact area is localized just below the
discharge section of the barrier (Fig. 3).
The design impact area AQdyn is approximated
by a rectangle, with a height of 2-4 m, depending
on the type of debris flow. For muddy debris flows
the height hdyn assumed to be 2 m, for coarse
grained debris flows 4 m.
)10(dyndynQdyn bhA
with
mhmdyn
42
)11(QMQdyn
AA
If the width of the barrier (bGes) is more than three
times of bdyn, several load cases for variable impact
areas (AQdyn) have to be shifted to more distal parts
of the barrier (Fig. 4).
5.2 Characteristic properties
Relevant process parameters, like velocity and
density, have to be estimated. To overcome
extensive calculations, a range of possible values of
density and velocity are given for different types of
alpine debris flow according to ONR 24800. The
recommended data are derived from measured or
observed alpine debris flows (Table 5).
International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
52
Table 5 Characteristic values for processes of debris flows
Channel
process Debris flood
Debris flow
stony muddy
Density (ρ)
[kg/m³] 1300-1700 1700-2000 2000-2300
Velocity
(v) [m/s] 3-5 3-6 5-10
Fig. 5 Stress combination model for debris flow
5.3 Calculation of debris flow impact
The design model is based on a combined static
and dynamic load, resulting from the momentum
equation. It is supposed that the highest impact
force results from the initial contact of the debris
flow with the barrier structure. Sediment behind the
barrier may dampen the impact of the process and
reduce the dynamic pressure on the structure. The
stress model is composed of the following
components:
Dynamic debris flow pressure (pdyn)
Static debris pressure (pst)
Imposed load (pa)
Equivalent static load for the impact of a
single component (eg. tree trunk, large
boulder), (Fe) (Fig. 5).
The debris flow impact model is based on the
following equations, which represent the
momentum equation for fluvial water pressure.
)12(²vpdyn
)13(hgpst
According to the Eurocode 1990, the characteristic
value of action should correspond to the upper
values with an intended probability of not being
exceeded during some specific reference period in
time. The design value Pd of an action P can be
expressed by
)14(stdynGd
PPP
)15(Qdyn
d
d
A
Pp
With
Pd as the design value of action [kN]
pd as the design value of pressure [kN/m²]
Table 6 Partial factor of safety G, for Limit State of STR
(Internal failure or excessive deformation of the structure or
structural member) for the design situations of BS1 (persistent
conditions), BS2 (transient conditions) and BS3 (accidental
conditions)
Limit
state
Duration Ultimate limit state
BS1 BS2 BS3
STR Permanent unfavourable 1.35 1.20 1.00
favourable 1.00 1.00 1.00
Variable unfavourable 1.50 1.30 1.00
favourable 0 0 0
Table 7 Static equivalent load
Process Static load FE Impact area
[kN] [m]
Coarse grained
debris flow 1000 0.7 x 0.7
Fig. 6 Energy dissipating function type
G is a partial safety factor for the action which
takes account of the possibility of unfavourable
deviations of the values from the representative
values (Table 6).
The partial factor depends on the classification by
their variation of time (design situation):
Persistent design situation (e.g. debris flow)
Transient design situation (e.g. avalanche,
shrinking)
Accidental design situation (e.g. earthquake,
rock fall)
5.4 Single boulder impact
Furthermore, the impact of a single component
has to be taken into account for Ultimate Limit
State assessment. The impact area has to be
localized at an adverse position on the barrier
(Table 7).
5.5 Adaption for compound barriers For compound barriers ONR 24801
recommends an adapted arrangement of the impact
International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
53
Fig. 7 Comparison of published dynamic impact pressures with calculated pressures from ONR 24801 without boulder impact, with
design values (partial factor of safety G = 1.35) and characteristic values.
Fig. 8 Impact force by Hertz equation with reduction factor Kc = 0.1
International Journal of Erosion Control Engineering Vol. 10, No. 1, 2017
54
area. The influence width be has to be adjusted
according to the following rules, where bl is the
width of the slit.
mb
bmb l
el
22
mbmbel
12
6. COMPARISIONS AND CONCLUSIONS
To test the ONR Standard, the results of the
calculations with different debris flow densities
were compared with published data sets [Hu et al.,
2011; Proske et al., 2008]. Most authors only
mention maximum pressure and do not distinguish
between dynamic and grain impact pressure.
Therefore it is difficult to compare the results of the
ONR approach with published data. Only Hu et al.
[2011] divide their data into different impact
components. The calculated characteristic values of
the dynamic pressure based on the ONR method
correspond well with the published data for
dynamic pressure of Hu et al. [2011]. Our proposed
design values represent the uncertainty of the data
and imply the factor of safety for unfavorable
situations (Fig. 7). The maximum reported forces
are covered by considering the single boulder
impact approach.
Applying the Hertz method with a reduction
factor Kc of 0.1 indicates that a boulder impact with
a mass of 1300 kg and an impact velocity of 9 m/s
(or with a mass of 10000 kg and an impact velocity
of 3 m/s) does not exceed the design value
according to ONR24801 (Fig. 8).
The comparison reveals that the results of the
ONR Standard fit well to the published data.
Though our proposed method represents a very
practical approach, it is based on physical principles.
Within the next years adaptions will be applied due
to gained experience and new scientific results.
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Received: 8 July, 2015
Accepted: 2 March, 2016