Transverse mixing in an ice‐covered river

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Abstract
The effect of an ice cover on transverse mixing in the Lesser Slave River (Alberta) is investigated on the basis of data from two tracer tests. The tests took place under similar conditions of flow depth, one under open water conditions and the other under ice. Average transverse exchange (or mixing) coefficients for the reach Ez are computed by using a curvilinear coordinate system and the moments technique of Aris. The normalized exchange coefficient k = Ez/u* R for ice‐covered conditions is found to be approximately 50% smaller than the corresponding coefficient for open water conditions. Since an ice cover reduces the product u*R (friction velocity times hydraulic radius) by a factor of approximately 2(2)1/2, if depth remains constant, the results indicate that an ice cover can reduce the mixing capacity of a river substantially. The importance of accounting for depth and width variations in computing mixing coefficients is discussed and illustrated. Bend‐induced spiral motions appear to delay vertical mixing until transverse mixing is completed.

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