ICE FORMATION AND BREAKUP IN STEEP STREAMS - River Ice

Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
draw sharp boundaries between the processes. Further, Hirayama et al. (2002) found similar
results in which a slope limit of 0.001 defined border ice dominated flow, but with a transition
between retrograde build up and border ice down to about a slope of 0.0003.
In general we would expect ice growth in a steep river in a series of processes that overcomes the
velocity and slope factor that prevents the usual retrograde build up of the ice (Tesaker, 1994).
These can be summarized as follows:
- Border ice formation in slow flowing areas along the bank and in areas with emergent
boulders.
- Anchor ice formation and the accumulation of anchor ice and frazil into anchor ice dams.
This usually occurs in areas with large substrate and boulders, over natural weirs in the
river and in shallow or restricted areas of the stream. With prolonged cold conditions the
anchor ice dams may freeze and stabilize.
- Anchor ice dams create a back water effect and reduce the upstream velocity. This allows
surface ice formation, and also ice cover build up by accumulation of floating ice.
- With time the dams may drain leaving ice covers that span the river width or causing a
cracked and refrozen cover behind the dam. The reach develops a stable ice cover
- A stabilized reach may hold until mild weather and increased discharge break up the ice.
In steep stream ice run caused by collapsed dams is common also in winter with just small
fluctuations in temperature
Over the last years a research program has been undertaken as cooperation between researchers in
Canada and Norway to study winter habitat for juvenile Atlantic salmon. As a part of this, work
has been undertaken to study the ice processes in several small streams and how they impact on
habitat availability. This paper gives an overview of this work and shows ice dynamics of two
streams with comparative hydraulic features.
a)
b)
Figure 1 Study sites
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