令和6年度砂防学会研究発表会概要集

R3-13

SHALLOW LANDSLIDE OCCURRENCE RELATIVE TO FOREST MANAGEMENT INCLUDING
CLEARCUTTING, FOREST THINNING
SOWMYA NAGARAJ1, FUMITOSHI IMAIZUMI2
1 Graduate School of Integrated Science and Technology Shizuoka University
2 Faculty of Agriculture, Shizuoka University

1. Introduction
Landslides triggered by hydro-geomorphic factors result in sedimentdisasters. Forest tree root cohesion provides tensile strength to the soil

梅ヶ島that increase the shear resistance of soil, contributing stability ofhillslopes (Imaizumi et al., 2008; Imaizumi and Sidle, 2021). Forestharvesting sometimes increases the landslide frequency due to root

有東木

静岡県

井川

decay of harvested trees. Effects of forest harvesting on occurrence oflandslides are possibly different among the harvesting methods. To
examine the effects of forest managements i.e., clearcutting and forest
thinning in the mountain region, occurrence of shallow landslides was

大山

鍵穴

investigated in Abe River watershed, Shizuoka prefecture.
2. Study site

Fig. 1 Study site in Shizuoka prefecture,

Occurrence of shallow landslides and forest management histories
were investigated in Abe River watershed, Shizuoka prefecture

central part of Japan (GSI map).

(Fig.1). Total area of the study site is approximately 542 km2. Geology
in the study site is mainly composed of Paleogene sandstone and mud stone, named Setogawa group. The study site consists of
6 meteorological stations marking an annual precipitation at AMeDAS of 1740 mm in total of the 6 sites.
3. Methodology
New occurrence of shallow landslides in Abe River basin was investigated using Google Earth Pro images in the period from2004 to 2021. Timing and location of clearcutting was also assessed using Google Earth images. Forest thinning histories wasinvestigated using Forest Cloud Open System, Shizuoka prefecture. Clearcutting is removal standing trees, while thinning.
removal of specified ratio of standing trees (35-40% in study area). Forests that experienced clearcutting and thinning wereclassified into 4 elapsed time classes: 0-5, 6-10, 11-15, and 16-20 years. Landslide frequency and landslide area ratio in each
elapsed time class were assessed using QGIS (Evans et al., 2020). The landslide frequency and area ratio were also calculated
in the areas without recent forest management as control areas.
4. Results and Discussion
As a result, total 802 landslides, including 105 landslides in clearcutting and 16 landslides in thinning sites, were mapped in thestudy site. After clearcutting, temporal change in the area ratio peaked during 1-5 years (7.8 x 10-4 m2 m-2 yr-1; Fig. 2a). Landslide
frequency was also higher in the clearcutting site during 1-5 years (2.25 km-2 yr-1; Fig. 2b). High landslide area ratio andfrequency were affected by decay of roots that results instability in the shallow soil of the hillslope initiating landslides (Imaizumiand Sidle, 2021). Both of the area ratio and frequency decreased gradually after 15 years of clearcutting due to the influence of- 149 -(a)Temporal changes in the landslide frequency

after forest management(b) Temporal changes in the landslide frequency
after forest management

Fig. 2 Temporal changes in clearcutting and thinning

root regeneration after replantation. Besides, thinning exhibits similar trend in landslide area ratio and frequency as theclearcutting (Fig. 2). Landslide area ratio in thinning site was high during 1-5 years (1.4 x 10-4 m2 m-2 yr-1). Landslide frequency
in the thinning site was also high in the same period (0.106 km-2 yr-1). Due to decay of roots in the site, it results in instability ofthe soil initiating landslides. Forest thinning landslide area ratio and frequency progressively decreased between 10-15 years andhad no mark of landslide occurrence after 15 years of harvesting. Decreasing in the landslide area ratio and frequency were likely
affected by root growth of non-harvested trees in thinning areas (Abe et al., 2004). The study clearly compares the effects ofdifferent forest management activity in the mountain region evaluating the area ratio and the frequency in the elapsed time period.
Comparing clearcutting and thinning, the clearcutting resulted in higher landslide area ratio and frequency. Furthermore, the root
degradation and regeneration explain the root reinforcement that provides stability to the shallow soil mantle indicating theimportance of forest management in the initiation of shallow landslides.
5. Conclusions
Forest harvesting impact was different between the type of forest management (clearcutting and thinning) on the occurrence oflandslides due to root degradation that initiates shear failure of the shallow soil in the hillslope. Based on the slope stability
related to root decay and recovery in the forest harvested areas explains the root cohesion and reinforcement that provides tensile
strength to the shallow soil in the hillslope (Imaizumi and Sidle., 2021).

REFERENCE:
Abe, K., Kurokawa, U., Takeguchi, Y. (2004) Journal of the Japan Landslide Society, 41(3), 225-235.
Evans, J. E., Imaizumi, F., Ohsaka, O., Ogawa, S. (2020) Earth Surface Processes and Landforms 45, 3280-3292.
Imaizumi, F., Sidle, R. C. (2021) Geografiska Annaler: Series A, Physical Geography103, 1-14.
Imaizumi, F., Sidle, R. C., Kamei, R. (2008) Earth Surface Processes and Landforms, 33, 827-840.

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