Avalanche catching dams

Avlanche catching dams are constructed in run-out zones of avalanches, where the region below the dam cannot be protected by protection measures such as avalanche fences or protecting forests in the release area which prevent avalanches to start. Avalanche catching dams are designed to totally retain avalanches or at least to slow them down as far as possible so that they do not endanger human settlements or roads. Figure 1 shows two examples of avalanche catching dams.

Figure 1: Avalanche catching dams near Vals, Switzerland

Information on the expected volume and flow velocity of the avalanche are used to estimate the necessary dam height by a rule of thumb. However these estimation requires expert knowledge to judge the influence of factors such as terrain structure, expected flow characteristics of the avalanche and dam geometry in a proper way. For Example, dams tend to be the more effective, the steeper their hillside front is. To date the estimation of the necessary dam height has been of entirely subjective nature. That is, the responsibility for the validity is entirely on the experts in charge of dimensioning the dam.

In a project which has been funded by the Swiss Envirionmental and Forest Acency and the Canton Valais, a guideline was developed which puts essential points of the estimation of the dam height on a more objective base. Scaled experiments on a laboratory chute and on the SLF snow chute at Weissfluhjoch indicated that the snow height behind the dam depends on two dimensionless numbers and on the dam geometry (i.e. on the inclinations of the hillside dam face and of the hillside dam apron).

The dimensionless number characterise the ratio of flow velocity and the velocity of a downflow disturbance travelling against the flow direction (Froude number Fr) and the length of the avalanche expressed in units of ist flow height (finite size number V_adim). These two measures and the dam geometry define whether the avalanche simply runs up the dam face until standstiil or whether it develops a so-called shock wave. The two different impact characteristics required different dam heights to two stop avalanches which have the same velocity. Up to now, these characteristics have not explicitly been included in the dam design procedure.

Figure 2: Evolution of the impact of an avalanche on a dam in a laboratory experiment.

Figure 2 shows the time evolution of the impact of an avalanche on a dam in a laboratory experiment. Three phases of the impact evolve: (i) first front (frames 1,2), (ii) generation of a shock wave (frames 3,4) and (iii) a shockwave travelling upstream at constant height. A video (link) illustrates the avalanche impact from another perspective. Figure 3 givs a raw sketch of the basic theoretical concept which exploits the Froude-analogy between granular flow and hydraulics.

Figure 3: Principle sktech of a shock wave

Starting from the experimental results, a theoretical description of the avalanche-dam interaction was developed, which the new dam design guideline is based on. Generally speaking, the guideline is based on the fact that it depends on the dam geometry and the dimensionless number Fr and V_adim which of the impact phases (i) and (iii) form at all and, if they form, to which extent they dominate the impact process.

Figure 4 is a flow chart of the basic computations and decisions which are involved in the new dam design guideline.

Contact

• Stefan Margreth