Measuring the critical turning moment of the Japanese cedar (Cryptomeria japonica) in situ

Trees planted during a long period of afforestation have grown considerably in size and were fully ready for harvest. Recently, there has been a focus on landslide damage related to driftwood in Japan. Driftwood is produced when a landslide occurs in a forest covering a slope, where the slope fails or there is landslide movement that engulfs trees along a path downslope. In situ tree-pulling experiments on the Japanese cedar (Cryptomeria japonica) were carried out to evaluate the characteristics related to resistance against a flowing landslide. Japanese cedar trees were towed at a height of 1.1 m in the downslope direction using a construction vehicle. The traction force was measured using a load cell, and the inclination of a tree in rotational motion was captured using a digital video camera. The video was used to calculate the depth at the centre point of rotation from the ground surface. In 2017, the experiments were carried out in three national forests which had differences in the tree age, the diameter at breast height, the slope, and the surface geology: 1) Takanosu national forest in Gunma prefecture; 2) Amabiki national forest in Gunma prefecture; and 3) Takinoiri national forest in Fukushima prefecture. The relationship between the measured depths at the centre point of rotation and the DBH was evaluated using linear regression. The depths were approximately twice the DBH. A linear relationship between the natural logarithm of the critical turning moment and the natural logarithm of the DBH was also observed. The regression was proven statistically significant, and this equation on the Japanese cedar was thus proposed as a practical estimator for the resistance against debris and driftwood in landslides. Future studies using physical or numerical models can explore the optimal combination of the critical turning moment and the distance between standing trees to mitigate disasters effectively. The findings of our study on the critical turning moment estimated using the DBH is indispensable and plays a crucial role in parameter setting in these studies.