Turbulence structure in an ice-covered, sand-bed river

Abstract

Although many high-latitude rivers are characterized by seasonal ice cover, few empirical studies have examined the effect of surface ice on the turbulent properties of the underlying flowing water. This study presents three-dimensional turbulence measurements obtained with an acoustic doppler velocimeter (ADV) in a straight reach of an ice-covered, sand-bed river. The statistical properties of three-dimensional turbulence were analyzed. It is shown that the spatial structure of the flow was nonuniform due to nonuniformity of the ice cover in the cross-stream direction and due to the riverbed geometry. These factors are responsible for the complex spatial structure of the flow field which limits the predictability of available physical models. Observed distributions over depth of Reynolds stresses and mean streamwise velocities generally conform with distributions derived from piecewise solutions of a two-dimensional flow model at least in the central part of the flow, but observed and expected values of some parameters differ substantially. We have hypothetically addressed these differences to the effect of flow stratification, and the values of a stratification parameter have been estimated. The velocity measurements allowed to analyze the coherent structures near the bed and beneath the ice cover. Their spatial scales were evaluated to be as large as the river depth for ejections and 0.7-0.8 times the river depth for sweeps, whereas low-frequency variations of the streamwise velocity were observed to have scales of two to four river depths.