Friction factors in tidal channels with differing bed roughness

Abstract

During 1965–1966 a field program was conducted within six tidal channels in Puget Sound, Washington, and the Strait of Juan de Fuca. The purpose of the program was to investigate the logarithmic nature of the velocity distribution within 1.5 m of the bed, and the frictional influence that different bed configurations have upon the adjacent fluid. Measurements were made of the velocity distribution, bed configuration, and sediment textural characteristics in each of the channels. Velocity data were collected continuously for periods of several to 20 h. The bed configuration in the study areas varied from rocks and gravel to small ripples and sand deformed into irregular roughness patterns. Generally the flow within a channel can be divided into two categories conforming to “hydrodynamically rough” and “hydrodynamically transitional” flow conditions. This division is based on a significant decrease in the dispersion of the drag coefficient as the Reynolds number increased. The boundary between transitional and rough flow seems to be qualitatively related to the geometry of the bed, with the more complex beds becoming fully rough at higher Reynolds number than more simple roughness patterns. By grouping all data, a mean and 95% confidence limit was assigned to the drag coefficient, which represents a more or less average value for the shallow marine environment regardless of bed configuration and flow conditions. The grouped data are also divided into two regions (transitional and rough) according to a roughness Reynolds number, with representative values of the drag coefficient assigned to each region.