Patterns of bar development and sand flat initiation in the South Saskatchewan River

Abstract

This is a study of the development of bars and the initiation of sand flats in a braided sand-bed river. The changes in morphology, flow direction and hydraulic character on the surface of bars are documented. This information is used to assess conditions on the surface of the bar as it migrates downstream and prior to the initiation of a sand flat. The relationship between channel topography and bar development is also considered. Several bars were monitored in the sandy braided South Saskatchewan river from May to August, 1988. Measurements included flow direction, depth, velocity, bedload transport and topographic surveys of the channel reach. Diagrams illustrating channel topography, bar morphology, flow direction and hydraulic character of the surface are presented. These diagrams are used to identify changes in the spatial pattern of the variables as the bar features migrate downstream, and to identify changes in channel topography. The data suggest that bar development and sand flat development are associated with the channel topography and morphology, particularly with the location and alignment of deep troughs such as the thalweg. The troughs tend to draw flow over the surface of the bars creating asymmetric hydraulic conditions, thus affecting the downstream migration of the bar. The flow and sediment are directed away from the downstream edge of the bars resulting in some of them diminishing into the channel bed. In some cases, the asymmetric hydraulic conditions over the surface of the bars resulted in the feature changing from one descriptive bar type to another. The exposure of sand flats was only monitored at one study site, however observations were made in other parts of the river. At the site which was monitored, flow directed toward a channel trough created a shallow sluice which locally lowered the water surface elevation, thus contributing to the exposure of the sand flat. The channel morphology was also a strong influence on the development of the sand flat at this site. In other parts of the river, shoaling of the channel thalweg and general aggradation of the channel preceeded the initiation of sand flats.