Open Channel Flows over Gravel and Vegetation Roughness Elements

Professor LI Chi-wai (PolyU)

Abstract
Open-channels of simple geometry are desirable for efficient water conveyance. Recently, large and flexible roughness elements, such as gravels and vegetation, are commonly deployed into artificial open-channels to stabilize the channel sectional shapes and to maintain the ecological balance there. Consequently, the hydrodynamic behavior of flows in these channels will be significantly affected. The determination of the velocity and turbulence intensity profiles, as well as the hydraulic resistance, is of practical importance in the engineering design of these channels. This study aims to quantify the gravel and vegetation induced roughness effects on flows and mixings in open-channels. Experiments on flows over gravels and simulated vegetation have been conducted. A numerical model incorporating the Drag Force Method and a modified Spalart-Allmaras turbulence closure is proposed. The computed velocity profiles and hydraulic resistance agree well with the corresponding measured results. Particularly, the model reproduces the S-shape velocity profile for sparsely distributed large size roughness elements. The simulation methodology is robust and can be easily integrated into the existing numerical models.

About the Speaker
Professor C. W. LI earned his PhD from the University of Hong Kong in 1987 and is a Professor in the Department of Civil and Environmental Engineering of the Hong Kong Polytechnic University (PolyU). His research interests include numerical wave prediction; wave-current-structure interaction; 3D modeling of flow, water qualities and near field dispersion in coastal and estuarine waters; modeling of wind flow around structures. He has published over 150 papers in International Journals, Conference Proceedings and Book Chapters. He is an associate editor of two international Journals. He is also a specialized consultant in the areas of hydraulic, coastal and wind engineering.