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Last modified
7/14/2009 5:02:34 PM
Creation date
6/1/2009 12:42:10 PM
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UCREFRP
UCREFRP Catalog Number
8270
Author
Hayse, J. W., S. F. Daly, A. Tuthill, R. A. Valdez, B. Cowdell and G. Burton.
Title
Effect of Daily Fluctuations from Flaming Gorge Dam on Ice Processes in the Green River.
USFW Year
2000.
USFW - Doc Type
ANL/EA/RP-102041,
Copyright Material
NO
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not form in an area once a solid ice cover has been established because the ice cover insulates the <br />underlying water from sub-zero air temperatures and prevents supercooling. Soon after they are <br />formed, frazil ice crystals are a few tenths of a millimeter in diameter. Typically, frazil ice <br />crystals will be approximately uniformly distributed throughout the depth of the flow, especially <br />in highly turbulent, shallow reaches. The many rapids located in the high-gradient reach through <br />Split Mountain Canyon, just upstream of the study area, are particularly favorable for Brazil ice <br />production during cold weather. Following formation, Brazil ice is transported downstream by <br />the river current and evolves in form as it is transported (for a more complete description of this <br />process, see Daly 1994). Individual frazil crystals gather into larger and larger masses of ice <br />known as flocs. In areas with higher water velocities and turbulent flows, frazil ice can remain <br />mixed throughout much of the water column. Under less turbulent conditions, frazil flocs rise to <br />the water surface. Frazil slush is the collection of frazil flocs and individual Brazil crystals on the <br />water's surface in a distinct layer. Frazil slush at the water surface has a marked tendency to <br />clump together. The initial clumps, if they remain at the surface long enough, can further clump <br />together and form pans, or small floes. These pans often grind against one another, causing them <br />to become roughly circular in shape and gain upturned edges. At this point they are known as <br />pancake ice (photographs and additional descriptions of pancake ice in rivers can be seen in <br />Ashton [1986] and Beltaos [1995]). Frazil slush and floes can accumulate along or abrade the <br />edge of border ice, which forms along the banks of the river channel. It is common to see <br />parallel lines of raised frazil slush along the inside edge of border ice, marking periods when <br />Brazil ice accumulated along the border ice. <br />In slower moving areas of the river, such as regions behind islands or in the lee of <br />sandbars, where there is very little or no mixing due to the locally reduced velocity, the surface <br />of the water can cool sufficiently for ice crystals to form directly on the water surface. Ice that <br />forms directly on the water surface in areas with little or no flow velocity is said to result from <br />static ice formation. This type of ice is also formed on lakes and ponds during periods of low <br />winds. Generally the surface flow velocity must be approximately 1 foot per second or less for <br />static ice to form. Static ice formation starts in a very thin layer of supercooled water at the <br />water surface, and is probably initiated by the introduction of seed ice crystals from the air. Ice <br />thickens as a result of continued ice formation at the ice/water interface as heat is transferred <br />from the ice/water interface through the ice and into the atmosphere. <br />The formation of a stable riverine ice cover results from the interaction between the <br />transported ice pieces and the flowing water. In this case the cover is said to form dynamically. <br />Ice covers that form dynamically progress in an upstream direction from an initiation point as ice <br />is transported to and deposited at the leading edge (upstream edge) of the ice cover by the flow <br />of the river. The actual process that occurs at the leading edge depends on the hydraulic flow <br />conditions and the form of the arriving ice. The processes at the leading edge are described in <br />general below in an order which reflects the relative flow velocity at which they occur, from the <br />lowest flow velocity to the highest. However, it is more common to refer to the non-dimensional <br />-3-
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