Modeling flow in an open channel with heterogeneous bed roughness

Physical and numerical simulations of open channel flow over a heterogeneously roughened bed are examined. The velocity field is mapped at four different cross sections by using an acoustic Doppler velocimeter and the boundary shear stress obtained from both the velocity data and application of a Preston tube. These data illustrate that in the current arrangement the roughness elements dominate the physics of the flow and act as a source of vorticity, which is initially manifested as local boundary shear stress and, in turn, affects the secondary flow structures and momentum transfer in the channel. This paper presents the first attempt at using the Shiono-Knight method to model velocity and boundary shear stress distributions in a heterogeneous open channel. Appropriate advice concerning values of calibration parameters and discretization of the cross section is given. The combined physical/numerical approach provides an insight into the changing properties of the flow structure within a nonuniform roughness environment and will be of use to environmental regulators and river managers and academics working in the area.