Transverse Mixing Tests in Natural Streams

Field tests to determine the transverse mixing coefficients of three river reaches, under both open water and ice covered conditions, are described. To interpret the test results, the streamtube method for mixing calculations, developed earlier by others, is used. For evaluating transverse mixing coefficients, the widely used moments method is modified so as to be consistent with the streamtube approach. Application of the modified method to test data gave consistent results, thus reinforcing the streamtube approach. For the tests described here, the transverse mixing coefficient ranges between 0.01 m²/s and 0.09 m²/s; when this coefficient is nondimensionalized with the hydraulic radius and shear velocity, it lies between 0.4 and 2.5. Open water values of the dimensionless transverse mixing coefficient are less than the corresponding values under an ice cover by as much as two and one-half times.