A field study of suspended frazil ice particles

Abstract

Abstract This study presents comprehensive hydraulic, meteorological and ice data during a full winter period at a tidal river site situated on the St. Lawrence River at Quebec City, Canada. After briefly presenting surface ice floe and anchor ice characteristics, the article makes in-depth analyses of over 26 470 suspended frazil ice concentration profiles. Three distribution models are evaluated and the model that performed the best in describing the data was the Rouse suspended sediment model (modified for ice particles). It was found that the Rouse number ( Z ) had a mean, a median and a standard deviation of 0.48, 0.39 and 0.36 respectively. The corresponding computed frazil ice particles rise velocity ( w r ) distribution had a mean, a median and a standard deviation of respectively 0.92 cm/s, 0.76 cm/s and 0.73 cm/s. The dependency of w r on local turbulence was quantified. A literature review of rise velocity equations, as a function of ice particle diameter, was carried out and a specific relationship is recommended as the most promising candidate for further study. Based on this relationship, the mean frazil ice particle diameter was found to be 3.15 mm. The paper explores fundamental relationships between air temperature, water temperature including supercooling, wind, water depth and velocity, surface ice floe concentration and thickness, mean suspended frazil concentration and anchor ice thickness. The paper ends with a discussion of the fundamental inter-dependency between the size of frazil ice particles and the Rouse number. This leads to the hypothesis that the dominant Rouse number of most river flows will normally be close to Z ≈ 0.4. Furthermore, it proposes that the size of the frazil ice particles in a water body is controlled, at its lower end, by its basic size distribution during formation and, at the upper end, by the limiting rise velocity corresponding to Z ≈ 0.8. Finally, it suggests that the size distribution of frazil particles situated under the surface floes, and therefore available for entrainment, depends on the time-history of turbulent conditions encountered during the ice floe's voyage.