LOCAL FLOW AND SCOURING PHENOMENA AROUND A COLONY MODELLED BY CYLINDERS WITH DIFFERENT POROSITY

Abstract

Local flow phenomena and scouring around a modeled emergent macrophytes' colony, were investigated by water-flume experiment. The characteristics of the colony of Phragmites australis, Typha angustifolia and Zizania latifolia were decided as a fuction of the colony's porosity. The porosity of the colony was represented by seven cylinders' diameter and their distances. In case that the distances were two times of the diameter (D), horseshoe vortex were formed in front of each cylinder, however in case that the distance was less than the diameter, the horseshoe vortex around each cylinder was not formed, in stead, large horseshoe vortex was formed around the modeled colony (7 cylinders). Local scour phenomena were changed according to the flow phenomena and the scour depth in front of the cylinder became larger with decreasing porosity. With same porosity, the sour depth became larger with increasing velocity under clear-water scour condition. The scour depth can be evaluated by the single cylinder's formula and the porosity effect.