Year of Publication: 1996
Abstract:
This report has reviewed the processes by which an ice cover forms on large regulated and non- regulated rivers. Explicit equations and algorithms have been presented that quantify these processes. Work that had been undertaken previously on the Peace River was also described to provide a framework for the calibration of these algorithms for the Peace River in both its regulated and non-regulated condition. The significant theoretical advances that were made include the development of a procedure to forecast freeze-up on a non-regulated river and the derivation of a stability relationship that uses both air temperature and discharge to determine whether a juxtaposed or consolidated ice cover will form. The latter development is important to characterize the type of ice cover that will occur on the Peace River under regulated conditions.
In addition, the hydraulic characteristics of the Peace River were evaluated for six distinct reaches between the Slave River and Taylor using the existing data base. The climatological characteristics of the basin were summarized, along with a description of the spatial and temporal variation in the flows for the periods before and after regulation.
Prior to regulation, at flows of less than 1000 m3/s, the river cooled from a maximum annual water temperature of about 22°C to 0°C at the same rate as the declining air temperature Ice began to form in early November in most years, and an ice cover formed by multiple lodgements when the surface ice concentration neared 100% and the discharge decreased sufficiently to reduce the width of the flow by about 10%. A stable ice cover usually formed in early November at Peace Point and in late November or early December at Peace River. There is no data for Taylor, although the freeze-up probably occurred in early December. The ice thickness associated with this type of freeze-up generally ranged from 0.5 to 1.0 m. The stage increase was typically between 1.0 to 2.0 m. In some cases, due to declining flows during and following the formation of the ice cover, the stage decreased after the ice cover was established.
Since regulation, the discharges are, on the average, about two to three times greater than those prior to regulation. This high discharge of relatively warm water from upstream of the has delayed the time of freeze-up and shortened ice duration of the ice cover significantly in the reaches upstream of Fort Vermilion. At Taylor, and upstream of the BC/Alberta border, an ice cover is an exception rather than a rule. At Peace River, and downstream to Fort Vermilion, the freeze-up date has been delayed by as much as one to two months. Only minor effects due to regulation are evident on the freeze-up ice regime downstream of the Vermilion Chutes and at Peace Point.
After regulation, the ice cover downstream the Notikewin River generally forms by juxtaposition due to the very mild slopes. The ice cover thickness in these two reaches is only about 0.5 m thick, immediately after freeze-up and the stage increase associated with freeze-up is only about 1 to 2 m. The increase in stage is due mostly to the additional flow resistance of the ice cover. In the reaches between the Notikewin River and Dunvegan, where higher slopes are evident, either a juxtaposed or consolidated ice cover can form. For typical post-regulation discharges, the air temperature must be at least -30°C for a juxtaposed cover to form. To ensure that a juxtaposed ice cover forms, regardless of the air temperatures expected, the discharge should be less than 800 to 1000 m3/s. The stage increase under a juxtaposed ice cover is less than 2 m, but for a consolidatedice cover the stage increase can be as great as 5 ra, with an ice thickness of about 4 m. Between Hudson Hope and Dunvegan, the steeper river slopes prevent the formation of a juxtaposed ice cover for any reasonable combination of discharge and air temperature. Although the development of an ice cover in these two reaches is infrequent and when it does occur its duration is short lived, the formation thickness can approach 5 m and the increase in the stage can be up to 6 m.
The main physical impacts on the environment relate primarily to (1) the existence of high water levels for long periods of time in areas where a consolidated ice cover has developed, (2) the losses in up to 30% of the flow into channel storage as the ice cover advances, (3) the potential unstable water levels and ice thicknesses that are evident within 100 km of the advancing ice cover, (4) the reduction in the duration of an ice cover for most of the length of the Peace River, and (5) dramatically thicker deposits of frazil in low velocity areas of the river upstream of the Vermilion Chutes.
Although algorithms have been developed for many of the process identified on the Peace River, additional work is required to improve the modelling capabilities. Additional observations and measurements need to be carried out downstream of the Vermilion Chutes to characterize better the freeze-up process in that reach. It is also suggested that bench marks established around the Vermilion Chutes as part of this study be referenced to a common datum. This will improve the understanding of the hydraulics of the Chutes. From a modelling point of view, more work is required to verify the stability criteria used in determining the dominant mode of cover formation. An important component of this work will be the unsteady simulation of a consolidation event. Also, some effort must be expended to explicitly model the formation of frazil ice floes.
