[H-RE13] New Developments in Engineering Geology

[HRE13-05] Quantitative Geoecological Assessment of landslides – The Need for an Interdisciplinary Approach to Solving Applied Geological Issues

*Takehiro Ohta¹, Taihei Kunimaru², Takumi Taketa¹, Michisuke Yamamoto³, Takahito Kuroki⁵, Masayuki Imoto^2,8, Takumi Kiyama^2,7, Tomohiro Tsuji¹, Shimpei Miyamoto⁴, Hiroyuki Morikawa⁴, Satoshi Nishiyama⁶ (1.Division of Earth Science, Biology and Chemistry, The Graduate School of Science and Technology for Innovation, Yamaguchi University, 2.Geosphere Sciences, Faculty of Science, Yamaguchi UniversityYamaguchi University, 3.OSA, 4.Chuden Engineering Consultants, 5.Faculty of Letters, Kansai University, 6.Faculty of Environmental, Life, Natural Science and Technology, Okayama University, 7.Tokiwa Consultants, 8.GEO Techno Kansai)

Keywords:Quantitative Geoecology, Assessment of landslides, Interdisciplinary Approach

In addition to topographical and geological conditions, environmental conditions such as groundwater and vegetation are involved in the occurrence of landslides, rockfall, and debris flow. Therefore, to overcome natural disasters or coexist with them and build a sustainable society, comprehensive science that can elucidate the mechanisms of various phenomena and propose solutions to problems is necessary (Mokudai, 2021). Koizumi (1993) stated that a geoecological method is necessary to understand the local natural environment, including geology, topography, soil, and vegetation, as a series of systems, which can be an important method for considering natural disasters. In other words, slope disasters, which are events in which the landform development process influences human activities, also require geoecological considerations. However, most of the previous geoecological studies on slopes have been often based on qualitative survey methods such as aerial photo interpretation. Recent developments in laser scanning technology have made it possible to acquire very detailed data, enabling measurement of ultra-fine-scale topography such as rock block distribution on slopes and even the trunk diameter and branches and leaves of individual trees (Haraguchi et al., 2018).
In this study, we attempted to quantify the relationship between the vegetation conditions and topography by using a laser drone to measure in detail the vegetation conditions, such as tree height and canopy density, etc., as well as the microtopography of the slope in the Iwakuni City area in Yamaguchi Prefecture where many landslides occurred in the July 2008 heavy rainstorm. The quantified vegetation conditions were then used to determine the relationship between vegetation and topography. By comparing the quantified vegetation conditions with landslide occurrence, a method to quantitatively evaluate landslides based on the characteristics of the topography and vegetation was studied. On the other hand, when considering the effect of vegetation on individual landslides, it is necessary to consider the effect of tree root systems on slope stability and the effect of forest meteorology such as rainfall interception by tree canopy. Therefore, we conducted theoretical calculations of slope stability based on observations of root system distribution, measurements of soil properties such as strength and soil moisture content, and observations of meteorological conditions that affect soil moisture content on slopes where laser drone measurements were taken and constructed a mechanical model between root system and soil/rock mass that takes pore water into account.
The results of the study are summarized below.
1) The shallow landslides occurred on the upper valley wall slopes, except for those originating from forest roads.
2) The grain size composition of the sand that makes up the soil layer differs depending on the vegetation.
3) Rainfall conditions in the forest differ depending on vegetation, and the rainfall interception effect of tree branches and leaves is observed but is less pronounced at rainfall intensities of 10 mm/30 min and above.
4) In the soil layer on the upper valley wall slope of the young forest where the landslide occurred, infiltrated precipitation is more likely to be retained as groundwater than in other areas.
5) A method for calculating the safety factor of slopes considering the strength of the root system was established, but the reinforcement effect of the root system is small in this studied area.
Based on the above results, the mechanism of shallow landslide during July 2008 heavy rainfall in the target area was estimated. Moreover, a slope hazard assessment method using a quantitative geoecological approach was proposed.