Multiple frazil ice blockages at a water intake in the St. Lawrence River

This paper presents the results of a field study of multiple frazil ice blockages of a municipal water intake located on the north shore of the St. Lawrence River at Québec City, Canada. Instrumentation used includes a weather station, a digital camera with a telephoto lens, four water temperature probes, an Optical Backscatter System, a pressure sensor and an acoustic sensor (Ice Profiling System). Background ice data is provided by bi-weekly helicopter observations and local water velocity was obtained by analysing surface ice floes movement on digital photos and by performing 1-D and 3-D numerical modeling. Data on flow rates and head losses through the intake were provided by the City and analysed by the authors.The site is unique because the river is artificially kept open all winter by commercial shipping. It is also unique because the intake is located in a tidal reach and is therefore subject to a wide range of upstream and downstream flow velocities. Due to its location, the intake is, at times, sheltered or partially sheltered by border ice. With each change in the semi-diurnal tide, totally new water and ice reach the site.Detailed data is available for about 100 active frazil and anchor ice events leading to 35 blockage events at the intake. After presenting a global summary of the winter's environmental conditions and after presenting summary statistics on blockage events; four selected events are presented in detail. Items discussed are critical temperatures for the formation of frazil ice and the blockage of the intake as a function of time of day, tidal condition and surface ice type. Blockage events are also characterized in terms of the time required for them to develop, their severity and their duration.