ICE FORMATION AND BREAKUP IN STEEP STREAMS - River Ice

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Proceedings oh the 18th IAHR International Symposium on Ice (2006) RESULTS Figure 2 shows air temperature and water level fluctuations for Stavilla for the winter 2005/06, and figure 3 shows the air temperature for South West River. 15 10 Air temperature (deg C) 5 0 -5 -10 -15 -20 02.01 09.01 16.01 23.01 30.01 06.02 Figure 3 Temperature South West River 28-2 2-3 3-3 4-3 Figure 4 Growth of anchor ice dams in Stavilla In Stavilla we saw border ice formation during the first cold days in late October (2003) and early November (2005). During the first cold period in both years, five anchor ice dams was established which all except for the lower dam was drained. In addition, anchor ice formed on large boulders and on the river bottom of the lower part of the reach. The edges of the initial anchor ice dams -120-
Proceedings oh the 18th IAHR International Symposium on Ice (2006) froze over, creating a constriction in the river channel which increased the border ice cover and worked as a foundation for repeated anchor ice dam formation over the following days. We observed an initial anchor ice thickness of 58 cm in 2003 and the highest dam reached nearly 80 cm after several repeated events. Eventually, the river froze over. During winter of 2003 the ice cover lasted until spring. In the winter of 2005 the ice was removed from the river at two occasions, the first on the 11 th of December and the second on the 2 nd of February (see Figure 2). This lead to three periods of ice growth in Sokna this winter each produced a complete ice layer following the same pattern as outlined above. The development of anchor ice dams over a period in early March 2005 can be seen on photos in figure 4. During freeze up ice conditions in South West River were highly dynamic causing repeated ice runs several times before it stabilized in mid February. Anchor ice formed during cold nights and during nights with snow fall. On these events, four larger anchor ice dams repeatedly established reaching up to 60 cm in height on the same locations characterized by emergent boulders. During mid-day anchor ice ceased and the dams broke and drained causing a”domino-effect” downstream. During days with air temperature above -15 C the anchor remained in the river. In the beginning of the freeze up surface ice growth occurred along the river margins and around boulders. After the establishment of the anchor ice dams, the surface ice cover further developed on ponds behind each dam. During freeze up water level fluctuated repeatedly in relation to establishment and drainage of anchor ice dam. During the freeze-up period, the water level in both reaches showed a “stair step” effect due to the damming. Figure 5 show a water level measurement for Stavilla with anchor ice dams in place, and a similar graph for South West River measured after one anchor ice event in late January. Upstream anchor dams, velocities were reduced, promoting the development of a stable ice cover. In South West River velocities and depths were measured in two sections at positions where anchor ice dams were formed. Table 1 shows the mean and maximum values measured in each section. Table 1 Measurements at anchor ice dam locations. Velocities (v) in cm/s and depths (d) in cm. With anchor ice dams Without anchor ice dams Mean v / d Max v / d Mean v / d Max v / d 28 / 45 33 / 50 39 / 33 46 / 38 37 / 43 53 / 48 60 / 29 76 / 30 -121-
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ICE FORMATION AND BREAKUP IN STEEP STREAMS - River Ice

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