Causes of flooding -
Ice Jams

Ice jams are a major cause of flooding in Canada. In fact, for most Canadian rivers, the annual peak water levels are due to ice jams. These jams result from the accumulation of ice fragments that build up to restrict the flow of water and then act as a temporary obstruction. Jams form during both the freeze-up and breakup periods, but it is usually the breakup jams that have the greater flooding potential. While national figures are not available, on a provincial scale, data for the Saint John River basin in Atlantic Canada indicate that over two thirds of total provincial flood damage costs are due to ice-related events. The section on "Flood events in Canada" contains descriptions of ice jam flooding of the Saint John, St. Lawrence and Winisk rivers.

During the freeze-up period, ice forms on the river surface beginning at the banks. Ice crystals may also develop within the river as frazil ice, in open turbulent water under supercooled conditions, that is, the temperature of the water is slightly below 0°C. It is very common in rapids. The ice crystals tend to coalesce and accumulate, and may become attached to the underside of the ice cover or to the river bed as anchor ice.

The coalescing frazil can also form flocs, which float to the surface when stream turbulence is reduced or when the buoyancy of a growing mass exceeds downward turbulence. The flocs continue to agglomerate and produce pans, which, in turn, may freeze together creating ice floes. Frazil pans and flocs are major components in the formation of a river's initial ice cover. In tranquil reaches, this cover is a mere surface layer of ice floes and pans, but elsewhere it can be several layers thick.

Ice jams during the freeze-up period usually form where floating ice slush or blocks encounter a stable ice cover. The beginning of the ice jam is referred to as the toe and the upstream end as the head. The stable ice is frozen to the banks or is restricted from moving by the channel configuration. Generally, incoming ice fragments either submerge and deposit under the stable ice cover or pile up behind it, or both. There are, however, certain features that, in conjunction with ice cover, or on infrequent occasions, alone, enhance the probability of ice jam formation: bridge piers, islands, bends, shallows, slope reductions, and constrictions.

During breakup in the spring, or during winter thaws, an ice jam results from the accumulation of ice from the breakup of the upstream ice cover. A rise in water levels may result from the spring snowmelt, or common to the southern and Atlantic parts of Canada, a sudden midwinter thaw. Midwinter thaws are often accompanied by substantial rainfall, resulting in a rapid increase in water levels and severe ice jams.

The ice cover may be broken in a major fashion due to uplift pressures exerted by rapidly rising water levels. This causes "hinge cracks," or longitudinal fracture lines parallel to the shores. The cover breaks down into smaller pieces after being set in motion by the flow and impacting with channel boundaries or other sections of ice cover.

Ice jams cause flooding because of two main features. First, ice jam thickness can be considerable, amounting to several metres. Secondly, the underside of the ice cover is usually very rough. Under open water conditions, the only frictional resistance retarding the flow of the water is the streambed. The rougher the streambed, the greater the depth required to pass a given discharge. With an ice jam in place, the additional ice and very rough lower surface retard flow. Therefore, the flow depth has to be much greater than for open water.

If the depth of water necessary to float the jam is added to the depth required to maintain the discharge, it can be seen that extremely high water levels can occur, even at relatively small discharges.

Another potentially damaging condition closely related to ice jam flooding is the surging flow produced by the sudden release of a major ice jam. Flow surges can move as fast as 10 metres per second, or 36 kilometres per hour, which implies that the water level could rise rapidly, allowing little time for emergency measures. Moreover, the velocity of water and floating ice can also be very high, more than 5 metres per second, or 18 kilometres per hour, with the potential for serious erosion or ice impact damages.

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