Resistance coefficients from velocity profiles in ice-covered shallow streams

Abstract

Techniques for analyzing flow velocity profiles for streams where the flow depth is less than 1 m should be employed with caution. The two procedures for calculating flow resistance based on the logarithmic law, (1) mean and maximum velocity determinations and (2) intereept evaluation of log depth – velocity plots, should yield the same results for the various resistance coefficients and shear stress values, but they do not. The former procedure generally predicts higher values than the latter and is recommended for shallow streams where the field data were collected. For a 3.8 cm diameter flow sensor, the minimum distance from a boundary to the position of maximum velocity for a good velocity profile appears to be roughly 15–20 cm.The fluid shear stress values for ice covers on this shallow stream range from 0.1 to 17 Pa, depending upon ice type and duration of the cover. The shear stress under frazil ice slush showed a drop of about one tenth the initial value over a 2-week period at one location. The measured division of flow velocities about the maximum velocity associated with the bed and ice boundaries indicated that the mean velocities were not equal but often differed by as much as 25–30%, generally being higher in the section associated with ice, i.e., a rougher ice boundary.The most consistent method of determining the resistance coefficients was to define the cross-sectional shape of the river and ice cover accurately, subdivide the flow areas, use the mean velocity data calculated from the velocity profiles, and then compute the resistance values accordingly. The sensitivity of the roughness coefficient is discussed in terms of its impact on determining the total stage of an ice-covered river.