Increased Bed Erosion Due to Ice

Abstract

Excessive sediment is the number-one cause for listing impaired or threatened rivers on the U.S. Environmental Protection Agency's 303(d) list of impaired waters. Bed erosion under ice covers can be a significant cause of sediment in rivers and can destabilize contaminated sediments. Real-time measurements of scour under ice using Time Domain Reflectometry (TDR) scour monitors at three sites indicated that ice covers could significantly accelerate bed erosion in rivers, especially when the ice cover is restrained from responding vertically to changes in discharge. Three surface conditions were simulated in a recent laboratory investigation: open water, a floating ice cover, and a restrained ice cover with hydrostatic head conditions simulating increased discharge. The underside of the simulated ice cover was either smooth or rough to simulate thermally grown ice or consolidated brash ice, respectively. The laboratory study produced similar results to the field observations and concluded that increases in hydrostatic head increased the maximum velocity beneath the ice for the restrained cover case. The velocity profile shifts toward the smoother boundary, which is usually the bed for a rough ice cover case, thereby increasing the shear stress on the bed and subsequently increasing bed erosion. This paper is an overview of the field observations and laboratory tests and summarizes the effects an ice cover has on the scour process.