Author(s): Norio Tanaka; Takeshi Takemura; Yusuke Shirono
Keywords: Colony type grass; Drag coefficient; Eragrostis curvula; Flow structure; Sand deposition
Abstract: Flow structures and sand deposition around a colony-type roughness model, hereafter called the ‘colony model’, in uniform flow were investigated to elucidate the roughness characteristics of Eragrostis curvula, an invasive plant, and Phragmites japonica, a native plant, on a gravel bar in Japan. The flow was visualized, velocity and drag force were measured and the volume of sand deposited behind the colony was measured in experiments and field observations. In the emergent condition, the flow structure around the colony model changed with increasing L/D (L: spacing between each cylinder, D: diameter of a single cylinder). In the range of 0. 25 2. The flow and drag characteristics of the whole colony model changed when the L/D was around 1. The drag coefficient decreased with increasing inclination of the colony, and the drag coefficient in the submerged condition was smaller than that in the emergent condition. The sand deposition capacity decreased drastically with increasing inclination. E. curvula forms a denser colony (L/D =1-2) than P. japonica (L/D >3) does, and the inclination of the E. curvula colony is smaller than that of the P. japonica colony. This caused the large difference in sand accumulation behind the colonies.