Drag coefficients of a real tree trunk and the sheltering effects of an upstream trunk on a
downstream one in a linear arrangement with different spacings and inclinations were investigated in
detail. In addition, for elucidating the change of drag coefficient for an overturned tree, drag force acting
on a real tree with roots was also measured in this study. For the measurement of drag force with different
inclinations, Terminalia Cattapa and Albizia sp., vegetated in Sri Lanka, were selected in this study. Drag
coefficient of inclined tree trunk has the similar tendency in relation to the Reynolds number with that of
vertical standing tree investigated in Tanaka et al.(2011). For the vertical tree trunk with rough surface,
drag coefficient of rear-side tree trunk was decreased with decreasing L/d (where, L is spacing and d is
the diameter of trunk). In addition, as a result of mutual interference experiment of two inclined tree trunk,
the drag coefficient of rear-side trunk decreased with the increase of the inclination. Under the influence of
the increment of projected area due to existence of roots and shear force acting on tree trunk surface, the
drag coefficient of a tree with roots became similar value (1.0-1.2) comparing with that of a vertical
standing tree
