Influence of outer-bank inclination and roughness on cross-stream cells in open channel bends

摘要

bends is poorly known. Its study is relevant for the design of bank protection schemes in river restoration projects. This paper investigates the influence of outer bank roughness and inclination on the cross-stream circulation cells in a sharp laboratory open-channel bend by means of high-resolution three-dimensional velocity measurements with an Acoustic Doppler Velocity Profiler. Three test conditions were analyzed where only the outer-bank characteristics were varied: 1) vertical outer-bank with smooth PVC; 2) vertical outer-bank with 3-cm stones simulating riprap; 3) 30°-inclined outer bank with 3-cm stones. In all measurements the pattern of cross-stream circulation is characterized by the existence of two cells: center-region cell and outer-bank cell. For rectangular channels, with increasing outer bank roughness the outer bank cell amplifies and widens considerably constraining the center region cell and so increasing the protective effect on the outer bank zone. In trapezoidal channels the outer-bank cell is smaller and weaker than in rectangular experiments regardless the outer-bank roughness, however, still protecting the outer-bank. A term-by-term analysis of the downstream vorticity equation suggests that the centrifugal force and the cross-stream turbulent stresses drive the outer-bank cell for all experiments.