On the Destructive Force of Avalanches

On the Destructive Force of Avalanches - Page 60

of avalanche snow according to Equation VI can no longer inundate the protected object. The protective action against powder avalanches is to be surveyed tak­ing into consideration the lateral spreading velocity and the mechanics of the trajectory of free streams. At times powder avalanches deposit much snow im­mediately behind protective structures by eddy action without a large pressure effect. The wing walls of the wedge- shaped bulk head must be sufficiently long to correspond to the lateral spreading velocity of the avalanche given by Equa­tions ( 1) and ( 2). Such wedge- shaped bulk heads have stood the test of exist­ing experience as long as they were built high enough [ l, 3, 5]. The wedge-shaped barriers can be built smaller if their protective action is confined to the ground floor, whose roof must then hold out against the possibility of being overrun by an avalanche. For the protection of persons on public thoroughfares, mountainside protective niches located sufficiently nearby are advisable. D. Diversion walls and deflectors of satisfactory height, arranged at a considerable distance from the objects to be defended, protect against ground and surface avalanches only to a limited degree; after snow fills in behind these defense structures their effectiveness in causing a loss in velocity is greatly diminished, as the catastrophe at Airolo has shown. Nevertheless the striking of a free stream upon a horizontal surface always causes a distribu­tion of the vertical velocity component and the plastic compressibility of specific kinds of snow causes, with each damming or deflection of the avalanche, significant losses in energy, which can be estimated with the aid of Equations ( I) and ( II). The kinetic energy of avalanche snow is equal to the damming pressure ps = Y v2/ 2g kg- m/ m3; with a deflection of the avalanche over the angle p the pressure occurring there is p ~ p s sinp. If the snow had a density Yo before the deflection then this is increased by p to the density Yd* which is maintained by a sufficiently long action of p. The permanent compression work amounts to: p dV „ p ( 1 - Y0/ Yd ) in kg- m/ m3 and the decrease in the kinetic energy per cubic meter of snow is p dV. The velocity v^ after the deflection is: YVl 2/ 2g = ps - p dV Dynamic experiments are necessary to check the time dependent compression law. Oriented experimental tests have already indicated that considerable energy los­ses occur when avalanches overrun terraces of sufficient size and number ( see also Equation VI). Experience confirms the universal postulate of the momentum theorem: No energy loss occurs without the absorption of force- and conversely. The deflection of an avalanche by diversion walls is more reliable the small­er the angle between them and the flow direction of the avalanche. Short, stag­gered, buffer walls arranged in series are more effective than a wall of great length built at right angles to the avalanche path, since uniform flow movement of the avalanche is prevented in this way. In so doing, the total length of the structure can be reduced and this saving is usefully transferred into 59