E3S Web of Conferences 415, 06018 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506018

Physical model investigation of the transition of a debris flowfrom the aerial to the water phase
Christian Tognacca1* , Enea Toschini1, Mattia Benagli1, and Gioele Maddalena1
1

Laboratorium3D Sagl, Via Prada 6, 6710 Biasca, Switzerland

Abstract. In order to study the phenomena of debris flow motion and deposition at the transition betweena channel and an underwater storage area, experiments were carried out on a physical model. The current
situation and the planned works were reproduced in two idealised 1:100 scale models. The experiments
show that the flows experience an abrupt transition into water with the creation of a water wave and enter
the lake undergoing a decrease in front velocity, but without creating upstream stagnation or local deposits
due to the sudden phase change. Instead, the debris flows move to the lower part of the model, where they
are deposited in the horizontal plane and in the lower parts of the conoid. The construction of a retention
basin in the lake does not decisively influence the dynamics of the transition between the aerial andunderwater phases. The results of the physical modelling provide indispensable data for the validation of
the 3D numerical codes currently being tested.

1 Situation

3 Physical model

The Steinlaui torrent in the municipality of Lungern,canton Obwalden (Switzerland), is known for its
significant debris flow phenomena. These pose aconsiderable risk to the road infrastructure runningalong the eastern side of the Lungeren lake (KantonObwalden, 2016). In order to reduce the risk of debris
flows, the construction of a new section of the nationalroad and the associated bridge over the Steinlaui torrent
includes the construction of a retention basin
downstream of the paved channel that runs under thebridge. The basin is planned within the lake (ASTRA,2017).

The experiments are performed in an idealised 1:100
scale model that reproduces approximately 70 metres ofchannel and the storage area within the lake over alength of approximately 100 meters. The dimensions andgeometry of the model are presented in the figure below.
The paved channel has a slope of 24%, a riverbed
width of 12 meters and banks with a slope of 1:1. In
contrast, the underwater cone has a slope of 33%. The
planned retention basin for the debris flows is createdwith a 6-meter-deep ditch that extends from the edge ofthe lake for approximately 60 metres, and thenreconnects with a horizontal plane to the current surfaceof the underwater conoid.

2 Aim of the work
The aim of the work is the analysis of the feasibility of
the retention basin for debris flows within the lake. In
particular, the flow and deposition phenomena of thedebris flows entering the lake are to be studied. Amongthe issues raised with the project is the possible
influence of the deposits in the basin on the upstream
flow conditions, which could lead to a reduction of thehydraulic capacity near the bridges of thecommunication routes along the lake.
The experiments will provide a series of qualitative
observations as well as quantitative data that will be
used for the development and initial validation of the2D and multiphase 3D numerical codes.

*Fig. 1. Idealised physical model, scale 1:100, for the actualsituation.

The flow is generated by suddenly releasing themixture placed in a vertical cylindrical tank with acircular outlet by opening the gate valve at the outlet.
For the generation of the debris mixture with the desired
dynamic characteristics (in particular peak flow, total
volume and front propagation velocity), the outlet
opening and the height of the mixture in the verticalcylinder are adjusted.

Corresponding author: christian.tognacca@laboratorium3d.ch© 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, 06018 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506018

The debris mixture consists of fine mineral powder,sand, gravel and water. The mean grain size of thematerial used in the physical model is 3 mm, while themaximum grain size is 8 mm. The water content isvaried between 20 and 30% to achieve the desired flow
conditions.

The spectrum of flow front velocities analysed in thelaboratory varies between 10 and 18 m/s, whichcorresponds to the velocities that can be expected fordebris flows in steeply sloping channels such as theSteinlaui (J = 24%). The analysed runoff depths vary
between 4 and 5 metres.

4 Instrumentation

6 Observations and results

In the physical model, the debris flows entering thechannel are measured by detecting the decrease in thelevel of the mixture in the vertical tank with a sonarsensor, the flow depths are measured over two sectionsof the channel by means of sonar sensors, and thevelocity of the flow front along the entire model ismeasured by analysing the frames of the video footage(taken with a GoPro-Hero8® at a rate of 25 fps). Also
the water wave developping on the basin surface after
the impact of the debris flow is measured by a sonarsensor. An example of the measurements is shown infigure the following figure. All sonar sensors are oftype BAUMER UNAM 18U6903/S14 with a sonicfrequency of 380 kHz and a response time <10 ms.

The experiments carried out show that the flows enter
the lake with an abrupt transition generating a water
wave on the lake surface and experiencing a decrease infront velocity, despite the increase in gradient from 24%
to 33% between the channel and the subaqueous cone,
without causing stagnation upstream. Both in the canalsection and on the first part of the cone, velocities arehigh and range between 10 and 18 m/s in the canal anddecrease to around 6-8 m/s on the underwater cone.
However, no deposits are observed at the phasetransition.
Most of the deposits are observed in the lower part ofthe model at the bottom of the lake as well as at thebottom of the cone.
The construction of a retention basin in the upper partof the underwater cone does not significantly condition
the flow and deposit dynamics in the transition zone
between the aerial and underwater phases of the debrisflow.

7 Conclusions
The experiments on the physical model carried out
proved to be extremely important for understanding thephenomena accompanying the transition of a debris flowfrom the air phase to the underwater phase, which had
not previously been the subject of this type of research.
The idealised 1:100 scale model and the measuring
instruments used also make it possible to generate
fundamental quantitative results for the validation andcalibration of the two- and three-dimensional numericalmodels currently being developed in our laboratory.
The assessment of the scale effects and thepossibilities of the scalability of the data collected on thephysical model at a scale of 1:100 is currently being
researched through the study of the transport and thedeposition phenomena of debris flow using a family ofmodels (at scales of 1:100, 1:50 and 1:25).

Fig. 2. Example of measurements during an experiment. The
zigzag line is the discharge of the debris flow entering in themodel (with a maximum of approx. 7 l/s), the continous lines
with maximal values around 55 mm are the flow depths
measurend in the channel, and the wavy line represents thedepth of the water wave developping in the basin. All results
are in model scale units.

5 Experiments
The experiments were carried out for the current
geometry and for the design situation (with containment
basin in the lake) by varying the volume of the melt, themaximum outflow and the speed of the melt front. A
total of 42 experiments were carried out.

References
12Fig. 3. Debris flow enters into water creating a wave on thelake surface for the project situation (with underwater
retention basin).

2Kanton Obwalden, Integralprojekt Naturgefahren,
N8 Giswil Süd – Lungern Nord, Fachdossier
wasserbauliche Massnahmen, Beilage R4,
Gefahrenbeurteilung Wasserprozesse, belop/Geotest(2016)
Schweizerische Nationalstrassen (ASTRA), Brücke
Steinlaui, N8 / Lungern Nord-Giswil Süd.
Ausführungsprojekt, IG Kaiserstuhl (2017)