Open channel flow within and above a layered vegetation: Experiments and first-order closure modeling

Abstract

Flow within vegetation characterized by non-uniform roughness density is drawing significant research attention given its relevance to a plethora of applications in eco-hydraulics including river restoration, and flow in wetland and marshes. The focus here is on flume experiments and modeling of the mean longitudinal velocity profile in a two layered cylindrical vegetation system. Layer 1 represents the region close to the channel bottom where the flow experiences maximum drag due to the densely placed vegetation, while layer 2 represents the flow region above the short vegetation characterized by a smaller vegetation density. Considering the aforementioned arrangements, a new analytical model based on Reynolds-averaged closure principles is proposed to describe the vertical distribution of mean streamwise velocity in an open channel with two different vegetation densities. In the proposed model, the one-dimensional steady and planar-homogeneous momentum equation is used where the turbulent eddy viscosity is assumed to be linearly related to the local mean velocity. The proposed analytical model has been calibrated using experiments reported here in which vegetation is represented by using circular plastic cylinders of two different heights. The proposed model is further tested against published experiments with similar arrangements. In total, 22 different experimental conditions with distinct densities, rigidity, and flow depths have been analyzed. The Root Mean Square Error (RMSE) of the velocity comparisons is found to be less than 0.0342 m/s, which is acceptable.