Author(s): Terry D. Prowse
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Keywords: No Keywords
Abstract: Porosity is a variable frequently used in the analysis of river ice jams, such as in the direct calculation of jam volume, thickness, compressive strength, the coefficient of internal friction and ultimately in the prediction of water levels. Although a porosity of 0.4 is commonly assumed in the case of breakup jams, no field verification of this value has been made. This paper reports on the first calculation of ice jam porosity based on direct field measurements of the energy and mass balance of an ablating ice jam. On April 29,1983, an ice jam developed at the confluence of the Liard and Mackenzie Rivers and increased to a maximum length of approximately 22.8 km by May 4. After this time, discharge lowered, the toe of the jam remained stationary, and the jam decreased in length to only 6.3 km on May 9, before finally releasing into the Mackenzie River. Since flow velocities were too low to permit entrainment of ice beneath the jam, shrinkage of the accumulation was considered to be due to melt processes. Estimates of surface melt from atmospheric sources were made using known meteorologic and jam surface conditions. Notably, atmospheric heat inputs were less than 3% of those due to heat transferred by the incoming flow. Total jam volume was calculated using observed areal dimensions of the ice surface and an estimate of thickness obtained from application of the theory of floating, equilibrium jams. Ice jam porosity was then calculated by comparing the volume of ice melted within the jam to measured changes in the total (ice and water) jam volume.
Year: 1988