Geology Reference
In-Depth Information
Table 10.2.
Ice-jam damage on Yukon and Mackenzie Rivers, Canada, 1960-1985.
Ye a r
C o s t of fl ooding (Can. $)
Yukon
Yukon River, Dawson City
1979
2 910 136.00
Yukon River, Dawson City
1967
182 912.00
Yukon River, Dawson City
1965
158 573.00
Yukon R iver, Dawson City
1963
27 994.0 0
Stewart River, Mayo
1973
23 041.00
Stewart River, Mayo
1972
14 890.00
Stewart River, Mayo
1965
103 833.00
Ross River, Ross River
1973
45 824.00
Northwest Territories
Mackenzie River System:
Hay River, Hay River
1985
695 736.00
Hay River, Hay River
1963
3 953 374.00
Liard River, Fort Simpson
1963
3 294 479.00
Source: Van der Vinne et al. (1991).
channels intensifi es freeze-up and break-up. The broader issues of groundwater hydrology
in permafrost regions have been discussed in Chapter 6.
The process of river-ice formation is described in the northern hydrological and geotech-
nical literature (Gerard, 1990) and is illustrated in Figure 10.2A. In the initial stages in
autumn, turbulence generated by stream fl ow is suffi cient to produce supercooled water.
Hence, ice forms fi rst over quiet waters along the banks. This ice is termed “sheet” ice.
Elsewhere, fl oating ice crystals (“frazil” ice) form in deeper water. Where this comes in
c ont ac t w it h t he bed , a s i n sha l low reaches, it for m s “a nchor” ic e. A s f reez i ng c ont i nues, t he
frazil particles form slush, which in turn agglomerates into frazil pans. At the same time, ice
grows outwards from the river bank and eventually frazil pans lodge against the bank-fast
ice. As more frazil pans arrive from upstream, the initial pack grows in the upstream direc-
tion. At this point, water level begins to rise in response to increasing resistance to fl ow
beneath the ice and the necessity for the channel to carry the discharge with increased
resistance. The depth increase is typically greater than 30% of the mean depth of the open-
water situation. It is further enhanced by the fact that the ice cover, or pack, fl oats with more
than 90% of its thickness submerged; this necessitates an additional rise in water level.
River geometry and weather conditions then infl uence the fi nal stages of freeze-up. If dis-
charge is slow and air temperatures are low during pack formation, the pack will be thin and
frazil production will be high. In this case, the pack forms as one pan and progression
upstream is rapid. If discharge is high, and air temperatures are mild during pack forma-
tion, the pack will be thick and the rate of frazil-pan production will be slow. In this case,
the increase in water level will be large but pack progression upstream will be slow.
River icings are further phenomena which may occur during freeze-up and in winter
(Figure 10.2B). They should be distinguished from the groundwater icings described in
Chapter 6. River icings are attributed either to a reduction in the cross-sectional area of
an ice-covered channel as freezing advances or to an increase in snow load on an initial
ice cover thus raising the hydrostatic head beneath the ice to an elevation higher than the
ice surface (Wankiewicz, 1984). Because the water cannot escape from the banks due to
freezing of the active layer, fractures in the ice allow water to escape over the ice cover,
to subsequently freeze as an icing. In shallow braided streams, small icing mounds, 1-3 m
high, may develop in response to localized restrictions of fl ow by ice freezing to the bed
