Estimation of Mean Flow Velocity in Ice‐Covered Channels

Abstract

Point‐measurement methods for estimating the mean velocity of vertical distributions of streamwise velocity in ice‐covered channels are evaluated in this paper. The evaluation uses profiles generated numerically, based on a two‐power law description of vertical distribution of streamwise velocity. The two‐power law simplifies into the well‐known power‐law expression for open‐water velocity profiles. Its validity was verified using measured velocity profiles obtained from rivers and a laboratory flume. The profiles are representative of flows subject to various combinations of bed and ice‐cover conditions. Values of estimation bias (the percent error in estimating average velocity from a few point measurements instead of integrating over the entire vertical velocity profile) were determined for several point‐measurement methods, including those currently used by the United States Geological Survey (USGS) and the Water Survey of Canada, and proposed new methods. The systematic variation of bias values with ice‐cover and bed‐resistance conditions was also determined. The method with the least overall bias is the so‐called usual two‐point method, in which velocity measurements taken at 0.2 and 0.8 of flow depth are averaged to obtain an estimate of mean velocity. This method produces a remarkably small bias; its maximum value is close to 2%. Estimation accuracy is enhanced if a coefficient of 0.98 is applied to the two‐point method.