Stabilité hydro-mécanique d'un couvert de glace encore solide

Abstract

An experimental study has been made on a scale model of a rectangular canal in the hydraulics laboratory of Laval University in order to simulate the destruction of a solid continuous ice cover under the effect of the tangential forces created by the flow.The canal was 6 ft (1.8 m) wide and 46 ft (14 m) long. At the lower end, an ice retention structure of reticular form was built. The ice was simulated with a wax poured in a continuous and uniform layer; whose properties correspond to that of natural ice at an average scale of 1/25. Ice covers corresponding to thicknesses of 8 to 20 in. (20.3 to 50.8 cm) were simulated at that scale.Tests consisted essentially in increasing progressively the discharge feeding the canal until complete failure of the ice cover and accumulation of the pieces in front of the grid retaining structure.Three important phenomena have been observed and measured. By chronological order the first one is the submersion of the frontal edge of the cover. This happens at a relatively constant Froude number but much higher than the one characterizing the stability of unconsolidated ice jams.An increase in discharge makes the ice cover unstable as it oscillates in the flow. This state is followed closely by the failure of the cover, the movement of the pieces with the flow and their accumulation in front of the retaining structure where they form a dry jam. This phenomenon of failure of the ice cover could be interpreted in the first approximation with two dimensionless numbers one being the Froude number and the other characterizing the ratio of failure forces to gravity forces. We have found that an exponential relationship relates these two numbers and the correlation coefficient for the 25 tests is satisfactory.Finally, we have measured the thrust exerted by the dry jams on the ice retaining structure. There is a very direct hydrostatic relationship between the thrust and flow conditions with a dry jam.These laboratory results are interesting to assess the discharges and water levels required in rivers to break-up ice covers of various resistances. One can estimate these conditions for the case of weakened ice covers before spring break-up. However, more importantly, it is possible to approximate the worst conditions of flooding with an early break-up towards the end of the winter when the ice still has its maximum strength.