E3S Web of Conferences 415, 05031 (2023)
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https://doi.org/10.1051/e3sconf/202341505031

Bridge obstruction caused by debris flow: a practical procedurefor its management in debris-flow simulations
Daniel Zugliani1*, Atousa Ataieyan1, Raffaele Rocco2, Nathalie Betemps2, Paolo Ropele2 and Giorgio Rosatti1
1Department of Civil, Environmental and Mechanical Engineering, University of Trento, via Mesiano, 77 38123 Trento, Italy
2Dipartimento programmazione, risorse idriche e territorio, via Promis, 2/A 11100 Aosta, Italy

Abstract. Bridge clogging due to a debris flow is a phenomenon scarcely studied but critical in hazard
mapping in the mountain area. Since a rational and systematic approach is still missing, we propose apractical method to deal with this phenomenon in a numerical framework. We tested this methodology byusing, as a numerical model, the two-phase, mobile-bed model TRENT2D and, as a site test case, the villageof Voueces in the north-west part of the Italian Alps. The application shows reasonable results and highlights
the importance of a mobile-bed approach.

1 The role of bridges in determiningdebris flow hazard in mountain rivers
Mountain valleys are usually carved by streams that, incase of heavy rainfall phenomena, can be affected bydebris flows or hyper-concentrated flows. In
anthropized areas, roads often cross the rivers withbridges which, in case of extreme events, can induce
large deposits of sediments. These phenomena produce
a rising in the bed level and the stony (and possiblywood) debris can clog the bridge section with aconsequent massive overflow. This situation is very
dangerous and, in terms of territorial hazard, potentiallyeven worse than a destroyed bridge.
In Aosta Valley, a region located in north-west sideof the Italian Alps, the problem of bridge occlusion due
to debris flow is quite common and causes large
damages to infrastructures and, if the bridge occlusion
occurs in the villages, also to buildings and cultivatedareas (Figure 1).

Fig. 1. Consequences of a bridge clogging in the village ofVoueces, Ollomont municipality (AO, Italy), following thedebris flow event of August 2017.

While the clogging caused by wood debris is a topicrecently tackled in the scientific literature [1-3], theclogging caused by stony debris flows is far less studied
and the mechanism and the conditions of occurrence arestill largely unknown. Due to this lack of knowledge,
this scenario is rarely considered for the design andverification of the hydraulic section of a bridge, leading
to a potential underestimation of the hazard induced bysuch structures when a debris flow occurs. In fact, themaximum level of the flow in the section of a bridge wascommonly calculated (and is still calculated) assuming
a fixed bed condition and a water-only discharge equal
to the peak discharge of the relevant debris flow.
The fixed-bed approach is largely insufficient anddoes not guarantee a good level of safety at least for thefollowing reasons:• The flow resistance of a debris flow is far larger
than the resistance of clear water therefore, at thesame flow rate, debris flow depths are larger thanpure water depths.
• Clogging often occurs after significant sedimentdeposition in the bridge section but a fixed-bed
approach is unable to describe this phenomenon.
• In a water-only condition, when the flow levelexceeds the lower level of the deck of a bridge,
water can flow in a pressure regime. On thecontrary, in the debris-flow case, a pressure
regime is highly unlikely (though not impossible
since field evidence shows that, for a smallperiod, it could happen) while a clogging could
likely happen.
Both the fixed-bed water-only scenario and nonobstruction one can produce a potential underestimationof the debris flow hazard maps near mountain streams
crossed by bridges. Despite the lack of a rational theory,a reasonable practical approach for simulating theeffects of bridge obstruction in debris flow simulations
is highly advisable.

* Corresponding author: daniel.zugliani@unitn.it© 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/).

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2 A practical methodology for thesimulation of a bridge clogging
scenario

4. The second simulation is then prepared, in which
the initial elevation of the computational domain
is set equal to the elevation of the bottom reachedat the to instant in the previous simulation. Theelevation, where the bridge is located, is set equal
to the upper level of the deck.
5. If in the mobile-bed simulation there are zones
with limited erodible thicknesses, the initialvalues for this second simulation are set equal to
the sum of the erodible thicknesses of the mobilebed simulation and the values of excavations anddeposits that occurred up to the to instant. A nullerodible thickness is imposed on the bridge area,making it non-erodible.
6. The hydrograph of the second simulation iscreated starting from the one related to themobile-bed simulation and deleting all theinstants before to (i.e., taking only the instants
between to and the end).
The overall simulation is then given by the successionof the mobile-bed simulation, between the initial time ofthe hydrograph and the instant to, and the simulation
with obstruction, between to and the final instant of thehydrograph.

The identification of clogging conditions on a bridge israther difficult, as it depends on many factors that canhardly be defined a priori.
To tackle the problem within the framework of adebris-flow numerical simulation, in this study weassume that clogging occurs when the flow reaches alevel equal to or slightly above the lower level of thebridge for a reasonable period (about ten minutes) andthat the deposition under the bridge is, at the same
period, such that the free span has the same order ofmagnitude of the biggest sediments transported by theflow (about three times the quantile 90% of the channel
grain size). We call the time at which this situation
occurs the conventional time of possible occlusion anddenote it by the symbol to. From this time onward, since
the bridge deck consists of concrete or another hardsurface that commonly is not eroded (bridge collapse iscommonly linked to other mechanisms), the bridge's
upper surface can be considered as a non-erodible bed
level having an elevation equal to the elevation of theroad. Furthermore, when a bridge is clogged by a debris
flow, the observations suggest that the obstruction can
only be removed mechanically (e.g., with an excavator
as in Figure 2). This means that, once the clogging isformed, the mixture will flow over the deck of the bridge
for the remaining instants of the event.

3 Application to the Voueces village
study caseAs a test case, we used the village of Voueces (AO,Italy) which was subject to a debris flow event in August
2017 (Figure 1). Since the purpose is not to perform aback analysis of the event that occurred, but to verify if
and how the presence of a bridge clogging modifies theflooding area, as forcing conditions we used a mixturehydrograph associated with a return period of 100 years.
We performed three different types of simulation:• Fixed-bed simulation where the mixture wasconsidered as pure water flowing over a fixed
bed condition. In this case, the presence of thebridge deck was not considered.
• Mobile-bed simulation where the mixture wasconsidered as composed of water and sedimentflowing over a mobile bed condition. Again, thepresence of the bridge was not considered.
• Obstruction scenario simulation where theprocedure described in section 2 was carried out.
The first results analysis we performed was thecomparison between the fixed-bed simulation and themobile-bed simulation. These results are mapped inFigure 3 where the extensions of the two inundated areas
are drawn in different colours. The area obtained withthe mobile-bed simulation is around 50000 m2 while, asexpected, the results obtained with the pure-liquid,
fixed-bed assumption are considerably different since,in this case, the discharge overtops the banks only in avery limited area and with limited depths, producing a
40% less inundated area (about 30000 m2).
The second analysis we performed was to test
whether the results obtained with the obstruction
scenario simulation can be reasonably employed inhazard mapping work.

Fig. 2. Mechanical removal of the clogging occurred during
the debris flow event of August 2018 in Val Ferret, near
Courmayeur (AO, Italy).

We have implemented this procedure by using theTRENT2D model [4] that, in its more recent version [57], can deal with both mobile bed conditions and nonerodible bed conditions. Since the model does not
automatically set up this type of scenario, it is necessary
to produce it “manually”, dividing the overall event
simulation into two separate simulations, carried out incascade as follows:
1. A whole mobile-bed simulation is performed inwhich the bridge deck is not considered.
2. The time evolution of the free surface and bottom
elevation near the bridge cross-section, whereclogging may occur, is analysed.
3. If the conventional conditions for occlusion,
described above, are reached, then the to instantcan be identified.

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bridge area caused by a significant decrease in bed
slope.

Fig. 3. Comparison of maximum flooding area obtained withthe fixed-bed simulation and the mobile-bed simulation. The
circle highlights the position of the bridge. Please refer to thetext for the meaning of the simulations.

Fig. 4. Comparison of maximum flooding area obtained withthe “standard” simulation and the obstruction scenario. The
circle highlights the position of the bridge. For the meaningof the simulations refers to the text.

For this purpose, a comparison between the mobilebed simulation and the obstruction scenario was carried
on. These results are shown in Figure 4 where the twomaximum inundated areas are drawn in different
colours. The effect of the bridge clogging leads to alarger inundated area of about 62000 m2 than themobile-bed case even if, in this specific case, it is notstriking (24% increase). This is due to the large
deposition phenomenon occurring, anyway, near theThe comparison between the fixed-bed andobstruction scenario is presented in Figure 5, wherelongitudinal profiles of the river near the bridge areplotted at different instants and for different conditions.

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Fig. 5. Longitudinal profile of the river near the bridge. The green line represents the maximum free surface (η) for the fixed-bed
simulation; the black line is the bed elevation at the beginning of the simulation; dashed lines represent the bed (red) and free surface(blue) elevations at time to for the mobile-bed simulation; the solid red line represents the bed elevation, while the blue is free surface
elevations for a time greater than to in the case of the obstruction scenario simulation. The black and grey areas indicate the zone thatis filled for the second simulation of the obstruction scenario simulation.

It is evident that with the fixed-bed simulation themixture flows under the bridge without interferences(green line). Using a mobile-bed approach, instead, thebed elevation starts to increase causing, at t=to, theinterference between the free surface and the lower sideof the bridge deck (blue dashed line for the free surface
and red dashed line for bottom elevation). When thishappens, the bridge section (the grey and the black area)
is closed and the mixture must flow over the deck (bluesolid line) producing a conspicuous increase ofdeposition (solid red line) in the upstream zone and alarge overflow (not reported here).
Finally, a comparison between the results of theobstruction scenario with the inundated area that
occurred during the August 2017 events (not reportedfor brevity) was carried on. The results show that eventhough the hydrological forcing of this event is ratherdifferent from that of the synthetic hydrograph with areturn period equal to 100 years and the DTM used in
the simulations (current state) is somewhat different
from the pre-event condition, the extension of theinundated area is reasonably similar.

have proposed for simulating the bridge clogging
scenario seems to be a reasonable and effectiveapproach.
3. The proposed procedure is general, and it can beused also with other numerical models able totackle the mobile-bed condition and the nonerodible zone.
This research has been partially developed with the frameworkof the research project “Progetto WEEZARD: un sistema
integrato di modellazione matematica a servizio dellasicurezza nei confronti di pericoli idrogeologici in ambiente
montano” funded by Fondazione CARITRO - Cassa diRisparmio di Trento e Rovereto.

4 Conclusions

4.

References
1.
2.
3.

Bridge clogging due to debris-flow events is an
important phenomenon that affects mountain areas.
Even if preliminary, the results of this work suggest that:
1. The widely used approach of designing andverifying bridge sections by using a fixed-bed
water flow model leads to a significantunderestimation of the inundated area. Thismeans that the hazard associated with debris
flows phenomena is drastically underestimatedand a false sense of security can arise among thepopulation.
2. Lacking a rational and systematic theory fordebris clogging and pending the development ofa better methodology, the simple procedure we

5.
6.

7.

4

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