Abstract
Drag coefficients of a real tree trunk and a branch and sheltering effects of upper trunk or branch on downstream one were investigated in detail. Willow and Robinia pseudoacacia were selected for the real trees. Drag coefficient starts to decrease from relatively low Reynolds number and is smaller than that of a smooth circular cylinder in whole Reynolds number range investigated. Leaves increases the drag coefficient around 40 to 100 % with increasing the ratio of leaves in comparison to that with only branch cases. Sheltering effects becomes larger with the existence of leaves. The drag coefficient of downstream branch approaches around 95% of that of a single branch with increasing Reynolds number notwithstanding the existence of leaves. The moment by drag force of a whole tree was calculated for the strong wind event that overtuned trees in the Arakawa river. The calculated value was compared with the experimental threshold values, and was validated within a reasonable limit.
