Frazil-ice entrainment of sediment: large-tank laboratory experiments

Laboratory experiments that simulate natural ice-formation processes and sediment entrainment in shallow water are presented. A 10-30 cm s(-1) current was forced with impellers in a 20 m long, I in deep indoor tank. Turbulence in the flow maintained a suspension of sediments at concentrations of 10-20 mg L-1 at 0.5 in depth. Low air temperatures (similar to -15degreesC) and 5 in s(-1) winds resulted in total upward heat fluxes in the range 140-260 W m(-2). The cooling produced frazil-ice crystals up to 2 cm in diameter with concentrations up to 4.5 g L-1 at 0.5 in depth. Considerable temporal variability with time-scales of < 1 min was documented. A close to constant portion of the smaller frazil crystals remained in suspension. After some hours the larger crystals, which made up most of the ice volume, accumulated as slush at the surface. Current measurements were used to calculate the turbulent dissipation rate, and estimates of vertical diffusion were derived. After 5-8 hours, sediment concentrations in the surface slush were normally close to those of the water. After 24 hours, however, they were 2-4 times higher. Data indicate that sediment entrainment depends on high heat fluxes and correspondingly high frazil-ice production rates, as well as sufficiently strong turbulence. Waves do not seem to increase sediment entrainment significantly.