The effect of the root system on preventing shallow landslides is exerted when roots are pulled out by means of surface soil displacement. Accordingly, resistance may be exerted immediately against the displacement of surface soil or after any significant displacement occurs due to differences in root shape or the hardness of soil in which the roots grow. In addition, the radial growth of roots with the rootstock as a nucleus will result in the roots being pulled out in various directions when a shallow landslide occurs, and that may influence the manner of developing root resistance. That being the case, this study conducted a pull-out test of Cryptomeria japonica roots with the objective of clarifying the effect of differences in the pull-out displacement and pull-out direction (pull-out angle) of roots on the manner of development regarding pull-out resistance and pull-out displacement, and studied a method of evaluating the effect of the root system on preventing shallow landslides in consideration of the pull-out displacement and pull-out angle. In this study, both pull-out displacement and pull-out resistance were measured, with the pull-out angle being varied from 0 degrees to 90 degrees. The study revealed that (1) the maximum pull-out displacement value differs among the respective roots, (2) the pull-out resistance of vertical roots with the same diameter increases in line with a higher pull-out angle, and (3) the maximum pull-out displacement tends to increase in line with a higher pull-out angle. Based on the findings above, we proposed a method of calculating the resultant force of pull-out resistance per the displacement of several roots by calculating the resistance according to the pull-out displacement exerted by one root, at intervals of 1 mm. The use of this method to evaluate the effect of roots on preventing shallow landslides revealed that it is approximately 40% of the effect evaluated by using the conventional method of summing up the maximum pull-out resistance.
There are many cases where debris flows are generated frequently after a volcanic eruption, and the frequency of such occurrences gradually decreases with the laps of time. As a major reason for this, a change in the characteristics of runoff depending on volcanic ash with a low infiltration rate has been pointed out by previous works. However, even now, debris flows have been occurring for about once or twice a year for more than 20 years after the eruption of Mt. Unzen. The temporal change in the characteristics of runoff depending on volcanic ash cannot explain the actual conditions described above. Aiming at interpretation of the actual condition, we researched the topographical and geological characteristics of source areas of debris flow. Firstly, we confirmed that the post-eruption infiltration rate had been recovered to a level similar to that before the eruption through conducting field measurements. Then, after conducting topographical analysis and making field observations of debris flows, we succeeded in pinpointing the location of recent major source area of debris flow. Based on the erosion conditions, topographical and geological characteristics and groundwater discharge at the pinpointed source area, we examined the major factors for the generation of debris flows. As a result, it was suggested that debris flows occurred mainly because of the existence of groundwater and differential erosion, which depended on the discontinuity of new pyroclastic-flow deposits and preeruptive ground surface below. Based on this finding, we proposed an initiation model of debris flow in current situation of the recent major source area of debris flows in Mt. Unzen. We also proposed a schematic diagram showing the temporal change of dominant factors of debris flows after a pyroclastic eruption.
This study presents a novel 2D numerical simulation model for debris flow based on diffusion wave equation considering the continuity and time-step dependency. A congenital problem of the diffusion wave equation is the numerical instability which collapses the continuity law of water and sediment, because the diffusion wave equation does not represent the acceleration and deceleration process of the dynamic flow. To overcome this disadvantage the present study proposes a new numerical scheme called correcting continuity equation (CCE) method that allows for avoiding the continuity issue. The numerical model was verified in a past debris flow event that occurred in Japan. The numerical model well simulated overall pattern of the deposition height and water surface elevation observed in the debris flow event. Furthermore, the CCE approach was found to satisfy the continuity of water and sediment completely. In addition, the numerical result implied importance of stability condition considering both advection and diffusion property of the diffusion wave equation.