experimental study to evaluate the effectiveness of the variation in crown portion of a tree on the flow properties considering the finite length forest

This study clarified the effects of changing the ratio of the tree crown part to tree height in an inland finite length coastal forest under fixed bed conditions with special emphasis on the reduction of energy, velocity, and fluid force behind the forest and the difference in velocity between the forest and an adjacent gap region. a series of tests were conducted under sub-critical flow circumstances while altering different variables such as the crown height ratio (ch′=0.2-0.5 where ch′=chth in which ch is the crown distance from the ground surface and th is the tree height), forest width-to-length ratio (wc/ ls= 1 , 1.5 where wc and ls is cross-stream length (width) and stream-wise length of a forest, named fm-1 and fm-2, respectively), and the initial froude number condition (fro) , which ranged between 0.67 and 0.76. the results showed that an fm-2 case with the lowest tree crown height (ch′=0.2) created a significant backwater rise, accompanied by a higher flow resistance than an fm-1 configuration. the maximum energy loss, around 67%, was observed with the lowest tree crown in an fm-2 case as compared to no-crown and high-crown models. this forest configuration reduced the flow velocity and fluid force behind the forest by around 50% and 59%, respectively. on the other hand, flow velocity in fm-1 with lower crown height ratios increased only approximately (3–4%) compared to the no-crown (nc) case. however, the energy loss was only 50% in the fm-1 case with the lowest crown height ratio, whereas fm-2 had a negative impact on the gap area due to a linear increase in flow velocity, and velocity increased from (ch′=0.4-0.2), around 44–69% as compared to the nc case (18%). the best reductions of the oncoming current energy, velocity, and fluid force behind the forest were seen when the fm crown height ratio was reduced, and its width-to-length ratio was increased. this design of an inland forest as a natural bioshield would reduce the potential damage by future tsunami disasters