ICE FORMATION AND BREAKUP IN STEEP STREAMS - River Ice

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Proceedings oh the 18th IAHR International Symposium on Ice (2006) South West River Stavilla 1007.0 99.4 1006.5 99.2 Elevation (m) 1006.0 1005.5 Elevation (m) 99.0 98.8 1005.0 98.6 1004.5 4830 4840 4850 4860 4870 4880 4890 4900 4910 4920 East (m) 98.4 0 20 40 60 Distance from lower end (m) Jan 23 - with anchor ice dams Jan 25 - ice free conditions Anchor ice dam Water level with ice Water level without ice Position of anchor ice dams Figure 5 Water levels with anchor ice dams Figure 6 shows the velocity profiles together with a control profile at a location upstream of the anchor ice areas which is not much influenced by the dams. Velocity (cm/s) 80 70 60 50 40 30 20 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Position from left bank (m) Velocity - with ice Velocity - no ice 80 70 60 50 40 30 20 10 0 1.5 2.0 2.5 3.0 3.5 4.0 Position from left bank (m) 80 60 40 20 0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Position from left bank (m) Figure 6 Velcoity distribution with and without ice As mentioned, break-up’s were observed during the study period. In Stavilla the break up during winter of 2005/06 created steep wave fronts and periods with local jamming and water level rises. At the present time no measurements on ice breakup have been carried out, but during February 2006 ice breakup in Stavilla measurement equipment located 1.5 meters over normal river level was flooded due to damming caused by the ice jam that formed, so breakups are also very important for the understanding of the small river environment. -122-
Proceedings oh the 18th IAHR International Symposium on Ice (2006) DISCUSSION There are currently not many studies reported in literature on ice formation and breakup in small streams, and results from the studies done does not give clear boundaries for the processes controlling the ice formation. In general the observations in both rivers in this study follow the process of ice formation were retrograde buildup is prevents described by Tesaker (1994). The difference is mainly that South West River is more unstable due to larger temperature fluctuations than Stavilla and therefore showed more frequent anchor ice releases. The time to ice development on stabilized anchor ice dams was thereby prolonged and it took longer time to proceed beyond step 2 in Tesaker’s process description. River slope and temperature is used as a method to differentiate between retrograde and anchor ice controlled ice build up. Both study streams are well within all slope criteria’s as defined by Tesaker (1998) and Kanavin (1970). Hirayama (1986 and 2002) defined an ice formation classification based on temperature and river slope. Both study rivers fall within the type II ice formation classification, which is dominated by border ice formation. This corresponds well with the observations done at the two study sites. Velocities measurements at South West River show a reduction in velocity after the ice dam was in place, and in all cases the velocity was below the 0.6 m/s limit proposed by Tesaker as limit for ice cover formation by accumulation. But both in South West River and in Stavilla it was observed that anchor ice formation was needed to form an ice cover at sites with ice free water velocities below 0.6 m/s, and this indicates that the velocity criteria may not be directly applicable but should more be used as an average over longer river reaches. CONCLUSION Ice formation in steep rivers may significantly alter the physical conditions. During freeze up and break up the conditions may become highly variable due to dynamic ice formation. A combination of discharge and accumulated cooling controls the timing of the anchor ice formation. In this study we found that anchor ice dams were favored on locations with shallow, natural weirs and/or in areas with emergent boulder. During freezing events, the river was transformed into a succession of small pools replacing the original riffle habitat in the reach. There was no observation of flooding outside the channel margins due to the anchor ice dam formation in neither in Stavilla nor in South West river. In the South West River observations of anchor ice dam releases during the study period were mainly controlled by smaller variation in water temperature. In Sokna we saw two ice runs over the period, both related to mild weather conditions with rain and increased discharge. The ice formation in both reaches follows the same pattern regarding positions of dams and ice cover formation each time, but timing of events, duration of freeze up period and ice thickness varies with climate and discharge. More studies are needed to fully understand the growth and break up of ice in small rivers and to develop methods to predict and analyze the ice dynamics and to link this to ecosystem response. ACKNOWLEDGEMENTS The authors wish to thank all the people that participated in the field campaigns. Thanks to Jo Halvard Halleraker at SINTEF Energy Research who provided the Sokna climate data. -123-
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