Mean flow and turbulence structure of open-channel flow through non-emergent vegetation

The ability of turbulence models, based on two equation closure schemes (th e k-epsilon and the k-omega formulations) to compute the mean flow and turb ulence structure in open channels with rigid, nonemergent vegetation is ana lyzed. The procedure, developed by Raupach and Shaw (1982), for atmospheric flows over plant canopies is used to transform the 3D problem into a more tractable 1D framework by averaging the conservation laws over space and ti me. With this methodology, form/drag related terms arise as a consequence o f the averaging procedure, and do not need to be introduced artificially in the governing equations. This approach resolves the apparent ambiguity in previously reported values of the drag-related weighting coefficients in th e equations for the turbulent kinetic energy and dissipation rates. The wor king hypothesis for the numerical models is that the flux gradient approxim ation applies to spatial/temporal averaged conservation laws, so that the e ddy viscosity concept can be used. Numerical results are compared against e xperimental observations, including mean velocities, turbulence intensities , Reynolds stresses, and different terms in the turbulent kinetic energy bu dget. The models are used to further estimate vegetation-induced flow resis tance. In agreement with field observations, Manning's coefficient is almos t uniform for some critical plant density and then increases linearly.