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<br />." <br /> <br />nOBSl <br /> <br />2.3 Frazi1 Ice Production <br />Where turbulent water surfaces are exposed to extremely cold air, the water <br />rapidly loses heat and becomes super-cooled. Turbulent flowing rivers in <br />this condition then produce a sort of slush-ice that is termed "frazil" <br />ice. The frazi1 ice produced in rivers is not just confined to small bits <br />of ice crystals floating on the surface, but can occur at considerable depth <br />and, under certain ideal conditions, the frazi1 ice volume becomes a <br />substantial percentage of the total flow. <br /> <br />At certain stages in the production process, the frazil ice has a strong <br />tendency to stick to any object placed in the flowing water. Frazil ice is <br />thus a tremendous problem at water works intakes, where it readily clings to <br />trashracks and inlet structures and clogs them. In the case of long <br />stretches of a natural river, the frazi1 will cling to the edges, especially <br />where there is a partial ice cover along the banks. <br /> <br />The physical conditions that cause frazil ice to form have generally been <br />determined with regard to the relationship between heat loss and time. For <br />any particular river, turbulence, air temperature, and wind velocity are all <br />factors that can result in the necessary heat loss and consequently initiate <br />and sustain frazil ice production. Measurements in eastern Canada indicate <br />that frazi1 production in moderately turbulent flow will start when <br />temperatures reach 150 to OOF. <br /> <br />The frazil ice formation in the super-cooled water of a river will occur <br />throughout the entire cross section of flow. The individual porous f1ake- <br />1 i ke crys ta 1 s then ri se toward the surf ace and st i ck together, formi ng a <br />floating slush. The frazi1 accumulations exposed to the surface then freeze <br />more solidly and form a near-circular shape with a surface ice crust. All <br />of this action is extremely variable. The amount of frazi1 produced is not <br />just a function of temperature, but also varies with tile degree of river <br />turbu1 ence avail ab 1 e to keep the fraz il mixed. Where stretches of more <br />tranquil water follow a turbulent frazi1-producing section, the <br />agglomeration of the ice to the surface becomes much more pronounced. <br /> <br />-6- <br />