Airborne Radar Survey of a Brash Ice Jam in the St. Clair River

A brash ice jam in the South Channel of the St. Clair River was profiled in February 1987 using a helicopter-borne short-pulse radar operating in the UHF band near 500 MHz. During the same time, measurements of the brash ice depth and water temperature were made from a Coast Guard icebreaker. The returned radar pulses consisted of a strong coherent reflection from the water surface, preceded and followed by incoherent returns from the brash ice. The measured waveform time delays were then converted to mean freeboard height of the brash ice pieces above the water surface. Given the mean freeboard height, an estimate of the total brash ice thickness was made. This estimate was greater than the range of the direct shipboard measurements. The difference is believed due to differences between ice porosity above and below the water line, to melting within the ice and to partial submergence of some of the surface pieces. It is concluded that this technique could be used for mapping relative brash ice depth if the complexities of automating waveform analysis could be overcome.