Flow Characteristics beneath a Simulated Partial Ice Cover: Effects of Ice and Bed Roughness

Abstract

At the onset of winter in cold regions, border ice may form along the banks of a river and grow outward toward the center of the channel. This type of transient ice process is not well understood, and current empirical models of border-ice growth are not universally applicable from site to site. This may be in part attributable to a lack of understanding of the flow characteristics beneath a partial ice cover, because a very limited number of studies have investigated this phenomenon. To help address this knowledge gap, an experimental program was developed at the Hydraulics Research and Testing Facility at the University of Manitoba, Canada. A 14-m-long, 1.2-m-wide rectangular channel was constructed, and a simulated ice cover was installed on each side wall. A set of four experiments was designed to quantify the effects of channel-bed and border-ice roughness on the flow characteristics within a partially covered rectangular channel. Detailed streamwise velocity measurements were taken using an acoustic Doppler velocimeter beneath the ice, as well as within the open-water portion of the channel. It was found that roughness had a significant impact on flow redistribution beneath the ice cover by reducing flow beneath the cover and increasing flow in the open-water section. A rough surface was shown to decrease water velocity adjacent to the rough boundary while increasing the applied shear stress on the rough boundary. A corresponding increase in water velocity adjacent to the opposite smooth boundary and decrease in shear stress was observed. This flow and shear-stress redistribution may impact border-ice growth rates (both laterally and in terms of thickness) as well as sediment and ice erosion and deposition.