An intensive frazil ice field sampling campaign was undertaken at the Port of Quebec on the St. Lawrence River from February-March 2009. Two underwater acoustic instruments set at different frequencies of 420 and 1228.8 kHz were used to detect frazil ice in the water column. In this paper, frequency inversion methods are presented and subsequently applied to the observations to estimate frazil ice characteristics and concentration. Using inversion methods, most of the detected ice crystals had estimated radii of 0.06-0.18 mm. This range compares favourably to the estimated value of 0.20 mm obtained by analysing the Rouse number related to the vertical distribution of the frazil crystals. The results were in contrast to a previous study of frazil ice at another site in the St. Lawrence, which reported radii tens of times larger . The tiny crystals observed here were of similar size to those observed in laboratories (e.g. 0.09 mm), suggesting that the particles formed locally. Frequency analyses were also used to estimate the volumetric suspended frazil concentration, which appeared to be on the order of 6 ppm. Based on evidence suggested by the data and the volume backscattering coefficient at the two frequencies, this study also presents the complex sequence of processes that occurs during a typical supercooling frazil event. This paper concludes with future directions for research using acoustic instrumentation for further understanding of frazil ice dynamics.

Winter navigation requires regular and informative ice observations in order to provide safe and effective waterways. Acquiring field data is a costly, risky and difficult operation that can be greatly aided by airborne remote sensing. This paper reports on a field campaign designed to assess the utility of infrared thermography as a means of river ice monitoring. For this study, data were acquired on the St. Lawrence River. In March 2008, airborne infrared georeferenced images were acquired between Montreal and Quebec City, Canada. Following appropriate corrections and calibration, each image provided direct information on surface ice characteristics including superficial concentration and temperature. Ice floe thickness could also be deduced through the numerical analysis of ice surface temperature and near surface air temperature.Taken as a whole, the set of images reveals phenomena and patterns in the observed ice floes. These include: surface median temperature (from À2 to 08C); median thickness (from 1 to 3 cm); concentration (highly variable along successive reaches, varying from 0 to 100%); shape of floes (typically more oblong than circular); mean floe area (up to 5 ha) and number of floes per hectare (typically from 5 to 50). Along successive reaches, sectorial trends are observed for differing hydraulic conditions. Taken together, theses statistics bring out the variability inherent in the ice floes developed along the river, and lead to the conclusion that infrared thermography is a unique and effective tool for river ice monitoring, as it provides both rich visual and quantitative information.

No abstract