Effect of Density of Trees on Drag Exerted on Trees in River Channels

Recently, riparian forests have attracted attention as they are effective for ecological preservation and landscape enhancement. Uses of such forests in flood prevention, sediment control, and erosion control works have been actively promoted. This study aims to clarify the effect of density of riparian trees on drag exerted on trees along river channels under the regime of bed load transport. Hydraulic model experiments were performed using a straight channel, and the methods to calculate the drag coefficient necessary for deriving drag exerted on trees were studied. Previous studies have stated that the drag coefficient of a cylinder is fairly constant when Reynolds number,Re, is between 103 and 105 (e.g., Schlichting, 1979). This study clarified, however, that in cases of relatively dense arrangements of model trees (cylinders), the drag coefficient varies greatly with the density of the trees. As such, correlation between the drag coefficient and Reynolds number was found to be slight. Test results indicated that the drag coefficient of trees,Cd, correlates strongly with the coefficient of velocity,U′/U*′, friction factor of the channel bed,f′, and roughness concentration of trees × flow depth,aH′ or ratio of the area occupied by trees,λ. Furthermore, the drag coefficient,Cd, was also found to be slightly correlated with the gradient of the channel bed,I. Equations using either of the following parameters were obtained for deriving the drag coefficient,Cd, based on the experiment results:aH′; λ; U′/U*′; and Froude numberFr. These equations allow fairly accurate calculation of drag exerted on trees.