1992 Volume 45 Issue 3 Pages 18-23
A large quantity of trees were fallen down by '91-19 typhoon at the north-western district of Oita -prefecture. The fallen trees were investigated from the view-point of prevention of secondary disaster by debris flow, slope failure and floodwood.
The followings are found through the research.
(1) The fallen trees were occured almost in artificial forests. The greater part of the forest is associated with Japanese Cedar, and some part of the forest is associated with Japanese cypress. In this district, there is few forest except for a coniferous forest, though the age of fallen trees is about between 20 and 50 years. Younger tress are seldom damaged by the storm.
(2) Most fallen trees are fallen from their roots. Some of them are broken at their trunks.
(3) The fallen trees were occured at the slopes where the wind was probably accelerated by the topography.
(4) The depth and diameter of holes turned up by the roots of fallen trees are 0. 5 to lm and 1. 5 to 2. 5m respectively. And then, it seems that the surface soil in fallen areas is disturbed widely in the form of sheet by the fallen trees.
(5) Some slopes failed when the fallen trees occured. The histgrams of the gradient of the slopes which have fallen trees and the one of the slopes which failed. The distribution of former is flat, but the latter one is irregularly peaked at over 33 degrees and the distributions over 33 degrees are simular. The fact suggests that the slopes which have many fallen trees and where gradients are more than 33 degrees are unstable.