Chapter 11 An Ice Jam Primer 11-1. Review of Ice Types.pdf

EM1110-2-1612 30 Oct 02 Chapter 11 An Ice Jam Primer 11-1. Review of Ice Types Ice forms in freshwater bodies whenever the surface water cools to 0oC (32oF) or a fraction of a degree lower. There are many types of ice, depending on the precise mode of formation and evolution (Ashton 1986). See Chapter 2 for a thorough review. a. Sheet ice. The ice that forms in calm water, such as lakes or reservoirs, or in slow-moving river reaches where the flow velocity is less than 0.5 m/s (1.5 ft/s), is termed sheet ice. Ice crys- tals formed at the water surface freeze together into skim ice that gradually thickens downward as heat is transferred from the water to the air through the ice layer. Sheet ice usually originates first along the banks and expands toward the center of the water body. In slow rivers, the sheet ice cover may also be created by the juxtaposition of incoming frazil pans generated in faster reaches upstream. Sheet ice that grows statically in place is often called black ice because of its appearance. An ice cover may also thicken at the top surface when water-soaked snow freezes to form snow ice that has a milky white appearance because of small air bubbles. b. Frazil ice. Frazil ice (Figure 11-1) consists of small particles of ice formed in highly turbulent, supercooled water, such as river rapids or riffles, during cold, clear winter nights when the heat loss from the water to the atmosphere is very high. As the frazil particles are transported downstream, they join together to form flocs that eventually rise to the surface where they form frazil pans or floes. Frazil is often described as slush ice because of its appearance. Figure 11-1. Frazil ice and frazil pans, Salmon River, Idaho 11-1 EM1110-2-1612 30 Oct 02 c. Fragmented ice. This type of ice is made up of ice pieces that originated as consolidated frazil ice pans or from the breakup of sheet ice growing at the surface of slow-moving wat