Environmental Science and Engineering Seminar

Cold climates offer river phenomena that are not seen in warmer climates. One such phenomenon is the hanging ice dam. Consider the case of a steep, bedrock reach of a river that gives way downstream to a purely alluvial sand-bed river with a significantly lower slope. Under atmospheric conditions that are well below freezing, a solid ice cover forms over the flow in the low-slope reach. In some cases, however, the strong turbulence in the steeper, upstream reach prevents the formation of an ice cover. Instead, the water supercools and generates copious amounts of frazil ice. These ice particles act like sus-pended sediment, but with a fall velocity directed upward vertically. The open channel flow upstream of the slope break is thus laden with both suspended sand and suspended frazil ice. As this flow dives under the ice cover at the slope break, frazil ice tends to deposit and accumulate on the bottom side of the ice cover. This forces the flow downward, so creating a scour hole in the sand bed. At the height of winter, such scour holes have been measured to have depths of up to 2 m, and depths of as much as 10 m have been inferred. This scour poses risks to infrastructure such as bridge piers and buried pipelines. As summer approaches, however, the scour hole can disappear without a trace. Here we develop a simple morphodynamic model of hanging ice dams involving a) depth-integrated flow relations with and without an ice cover and b) transport, deposition and erosion of frazil ice and sediment. We use this model to gain insight into the scour produced by hanging ice dams, and the morphodynamic response after the solid ice cover breaks up in spring.