Characteristics of Small-scale Slope Failure Occurrence Depending on Rainfall Pattern
Risa TAKAHASHI1, Takashi YAMADA1*
1 Graduate School of Bioresource, Mie University
*Corresponding author. E-mail: t-yamada@bio.mie-u.ac.jp
INTRODUCTION
In 2011, the Kii Peninsula was affected by a large and widespread sediment-related disaster triggered by the season’s 12th typhoon, which was named “Talas”. Talas was accompanied by continuous rainfall, and 1808.5 mm of rainfall was recorded in Kamikitayama Village of the Nara Prefecture. Although previous studies have considered that shallow landslides are due to local heavey rain, this study aims to illustrate the occurrence characteristics of shallow landslides by focusing on differences in the type (heavy or continuous) of rainfall.
Mitsue Village in the Nara Prefecture and Odai Town were selected as the study sites. Both are rainy areas with an annual precipitation from 1700 mm to 3100 mm. In 2011, Talas brought continuous rainfall that recorded about two-thirds of the annual precipitation.
METHOD
The distribution of shallow landslides at the study sites was determined by aerial photo analysis. Then the landslide depths were measured at the actual places using 2 m poles and laser ranging. Data collected included the width, length, and depth of each landslide. This data was then compared to the shallow landslides caused by local heavy rain due to the 21st typhoon of 2004. Data from 2004 and 2011, 2011 was provided by the Miyagawa Dam (Odai Town) and Tsuchiyahara Observing Station (Mitsue respectively). The times of the landslides for Odai Town were estimated from a hearing survey, while those for Mitsue Village were provided by previous research of the Nara Prefecture Government.
RESULT
Table 1 shows the results. In 2004, local severe rain caused 1060 shallow landslides, which resulted in an average of 360 m2 collapsed land. Although Talas (2011) resulted in fewer landslides, the average collapsed area was two or three times larger. Additionally, the hearing survey showed that the shallow landslides due to the 21st typhoon in 2004 occurred near the rainfall peak, whereas those from Talas in 2011 were initiated several hours after the peak rainfall. (The arrows in Fig.1 denote the times of the landslides.)
CONCLUSIONS
The above results reveal the following characteristics:
(1) A shallow landslide due to local heavy rainfall occurs at the peak of the rain, while that caused by continuous rain occurs after the peak.
(2) A shallow landslide caused by continuous rainfall results in a larger area of collapsed land than that caused by local heavy rainfall.
These characteristics may occur as follow. Slope soil layers might require much longer time for saturation resulting in deep and large slope failure in case of continuous and low intensity rainfall, because rainfall infiltration into slope soil layer would be slightly greater than slope drainage capacity.
Keywords: 2011 typhoon No. 12, shallow landslide, continuous rainfall, aerial photo analysis, hearing survey
0 200 400 600 800 1000 1200 1400 1600
0 20 40 60 80 100 120
cumurative rainfall(mm)
precipitation (mm/h)
lapse time from rain(h) Miyagawa dam(2011)
0 200 400 600 800 1000 1200 1400 1600
0 20 40 60 80 100 120
1 6 11 16 21 26 31 36
cumurative rainfalll(mm)
precipitation (mm/h)
lapsed time from rain(h) Miyagawa dam(2004)
0 200 400 600 800 1000 1200 1400 1600
0 20 40 60 80 100 120
cumurative rainfall(mm)
precipitation (mm/h)
lapsed time from rain(h) Tsuchiyahara(2011)
Fig.1 the times of the landslides and rainfall (The arrows denote the times of the landslides occurred)
Tab.1 dimension of shallow land slides
Odai(2004) Odai(2011) Nara(2011)
2 13
983 1715 1 23
average depth(m) average wigth(m)
1060 360 no data no data
number average collapse area(m^2)
48 1047
