Characterizing suspended frazil ice in rivers using upward looking sonars

This paper describes frazil ice events detected using upward looking sonars, one high (546 kHz) and one low (235 kHz) frequency, during the 2009/2010 freeze-up field deployment on the North Saskatchewan River, Edmonton, AB, Canada. The field acoustic data have been processed to compute time series of the depth averaged volume backscatter strength, Svd (dB). A review of three scattering models: the sphere, prolate spheroid, and disk models, together with their formulation, assumptions and limitations are presented. Estimation methods for the concentrations and sizes from Svd (dB) using scattering models are discussed. The applicability of the three scattering models to measuring suspended frazil ice was investigated using laboratory data reported by Ghobrial et al. (2012a). It was found that the laboratory measured concentrations were in a good agreement with concentrations estimated with these scattering models. It was also found that the disk model is the most applicable because it provides more realistic particle sizes. In total, eight frazil events were detected with the sonars during the field deployment. Preliminary linkages between the meteorological (air and water temperatures) and surface ice conditions measured at the site, and the duration and magnitude of the detected frazil events are presented. Both the laboratory results and the disk scattering model were used to estimate frazil ice concentration and sizes from the field data. Concentrations ranging between 0.01 and 0.05% and disk radii between 0.13 and 0.21 mm have been estimated.