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<br />.' <br /> <br />ODHS5 <br /> <br />The rate of the ice jam movement is qufte directly related to the volume <br />rate of frazf1 ice production upstream. The first deep cold period of the <br />winter usually results in the greatest rate of frazil ice production, since <br />there is lfttle ice cover formed in from the river banks and a larger <br />surface area is exposed to the air temperatures. Later in the winter <br />season, colder temperatures than those experienced in the early season are <br />often required to produce the same volume of frazil. <br /> <br />Where a high volume of frazf1 is being produced and collected at the head <br />end of the jam, the jam front can move upstream at rates up to 500 feet per <br />hour. While the water does continue to flow underneath the jam, the packing <br />at the face and weight of thickened slabs often causes additional <br />constriction of the flow area. If this occurs over long distances, the flow <br />system acts similarly to a pipe or closed conduit flow. At the head end of <br />the jam, the river flow depth must then increase in order to build pressure <br />to force the flow under the ice jam surface. If the river banks are not <br />very high, this rise in water level will cause a breakout into the <br />surrounding flood plain and the flow wf11 seek a new overland surface <br />channe 1. <br /> <br />There are also situations where the pressure under the previously jammed <br />areas breaks through the ice covering and again permits flow outside the <br />channel. The potent i a 1 for caus ing the out-of-bank flooding is extremely <br />erratic and depends on such factors as where the ice jam head pauses, the <br />bank depth, the river channel shape, and the thickness of the jam. <br /> <br />The upstream movement of a jam wf11 slow as it approaches stretches of the <br />river where a natural ice cover has insulated the water from cold ambient <br />conditions. Likewise, if the cover is completely over the river, the <br />advancement of the ice jam will be halted. <br /> <br />The foregoing described actions of ice jam movements have been observed in <br />the Gunnison River. The site-specific elements of frazil production, ice <br />jam movement, and break-out flooding will be analyzed and presented in <br />following sections. <br /> <br />-10- <br />