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Large-scale sediment disasters caused byTyphoon No.12 in the Kii Mountains
OKAMOTO Atsushi, HeadUCHIDA Taro, Senior Researcher
Research Center for Disaster Risk Management, Erosion and Sediment ControlDivision(Key words) Deep-seated landslide, river course blockage (natural dam), landslide soil volume
1. Introduction
Typhoon No.12 approached and made landfall on
the Japanese Archipelago in early September 2011,
causingd a severe disaster on the Kii Peninsula. Rainwas long-term relatively intensive rainfall, whichbegan to fall on August 31 and continued for 5 days,
ending on September 4. In the village ofKamikitayama in Nara prefecture, although themaximum hourly rainfall was less than 50 mm/h, thetotal rainfall reached 1,800 mm.
Based on a report by the Erosion and SedimentControl Department, Ministry of Land, Infrastructure,
Transport and Tourism of October 26, the rainfall
resulted in a total of 100 sediment disasters occurredin the three prefectures of Nara, Wakayama, and Mie,
resulting in 56 dead and missing persons.
2. Outline of the sediment disasters
This typhoon caused many deep-seated landslides,
forming river course blockages (landslide dams) at 17locations (Photo 1). At five of these locations wherethe height of landslide dam was 20 m or higher, andwhere there are about 10 or more buildings withhabitable rooms downstream from the landslide dam,
emergency surveys were performed by the Ministry ofLand, Infrastructure, Transport and Tourism under theLaw for Partial Revision of the Act on Sediment
Disaster Countermeasures for Sediment Disaster
Prone Areas, which was enacted on May 1, 20111).

Photo 1. River Course Blockage at Nagadono Valley inTotsukawa Mura in Nara Prefecture
3. Landslide ground distribution
The spatial distribution of the landslide wasTable 1. Results of Interpreting Landslide Ground andEstimating Landslide Soil Volume
Large-scalelandslide*
Number of landslides
3,07776Approx. 10Approx. 52
Landslide areas (m )millionmillionApprox. 100Approx. 80
Landslide volume (m3)millionmillion* Landslide producing sediment volume of approximately
100,000m3 or more
All landslides

surveyed by performing an in-situ survey and using
aerial photographs and satellite images. The surveyed
area was 4,800km2. The landslide volumes at eachlandslide were calculated using an empirical equation
concerning landslide area and landslide volume1).
It was estimated that landslides occurred at 3,077
places in the survey range, the landslide area (onlyscar, excluding runout and deposited area) reachedapproximately 10 million m2, and the landslidevolume was approximately 100 million m3.
The Public Works Research Institute has surveyed
inventories of large-scale deep-seated landslides(volume larger than 100,000m3) caused by torrential
rain or melting snow since the beginning of the Meiji
Period (1867-1912), and confirmed 188 cases. Incontrast, Typhoon No.12 caused 76 large scale
deep-seated landslides with landslide volume of
approximately 100,000m3 or more. This fact showsthat although there were differences in survey methods,
Typhoon No.12 caused more deep-seated landslides
than past disasters. Landslide area shows thatlarge-scale deep-seated landslides accounted for about
50% of total landslide area, and that about 80% oflandslide volume was produced by large-scalelandslides. This suggests that large-scale deep-seatedlandslides have an extremely great impact on sedimentproduction.
[Reference]
1) Civil Engineering Journal, Vol. 53, No. 12, p. 4-7,
2011