●A Case of Utilizing Results

Case of early clarification of wide area sedimentdisasters using satellite images
MIZUNO Masaki, Senior Researcher
HAYASHI Shinichiro, Researcher
SATO Takumi, Guest Research Engineer
Research Center for Disaster Risk Management, Erosion and Sediment Control
Division(Key words) Satellite image, interpretation, river course blockage, natural dam
1. Introduction
When an earthquake or torrential rainfall etc. causesa landslide forming a large natural dam etc., there is adanger of a secondary disaster such as an inundation
caused by the collapse of the natural dam, so when theGreat East Japan Earthquake and typhoon No. 12 of2011 caused disasters, wide area landslide groundinterpretation was performed using satellite images,
quickly clarifying the landslide ground which hadappeared.
2. Interpreting landslide ground over a wide area
after Great East Japan Earthquake
During the Great East Japan Earthquake, strong
seismic intensity was observed over a wide region, soin order to confirm and clarify the state of theoccurrence of large-scale landslide ground, over awide area without omission, optical satellite images
were used to visually interpret the landslide ground.
The regions which were the object of the landslide
ground interpretation were “regions encompassing
districts where the seismic intensity was weak 6 ormore, and districts where the seismic intensity wasstrong 5 or more” (see Fig. 1), based on the estimatedseismic intensity distribution announced by theMeteorological Agency. The regions whose images
were used for the interpretation were, in order to moreaccurately discriminate landslide regions, allotted andinterpreted beginning with images with high resolution
selected from among obtainable images in thesequence: Google Earth images, ALOS stereopia
images, ALOS pan-sharpened images, andALOS-AVNIR-2 images.
The results of the interpretation of landslide groundusing satellite images showed that there wereapproximately 200 landslide locations covering a totalof about 300,000 m2 of ground. It also confirmed that
there were no landslides large enough to have formed
a natural dam.

2011/04/10 [AVNIR2]
2011/04/10 [PRISM]
2011/03/12 [AVNIR2/PRISM]
2011/03/12–04/06 (Google Earth )ALOS PRISM AVNIR-2
ステレオペア画像
Stereopiaimages
ALOS パンシャープン画像
ALOS
Pan-sharpened images
ALOS画像images
ALOSAVNIR-2
AVNIR-2

Estimatedseismic
推計震度5 強以上
intensity of 5 or more

Figure 1. Region with Seismic Intensity of Strong 5or Higher and Region Interpreted
3. Searching for river course blockages (naturaldams) caused by Typhoon No. 12 of 2011
Typhoon No. 12 of 2011 brought prolonged
intensive rainfall ending September 4, causing severe
damage on the Kii Peninsula. So on September 5, ahelicopter was used to conduct a survey to check forthe formation of natural dams. The results confirmed
large-scale natural dams at two places (Nagadono,Kumano), but most of the region was cloud-covered,so it could not be surveyed by helicopter. So satellite
SAR images, which are obtained by wide area
photography (30km×50km) were used to search forimage patterns similar to those of the natural dam
formed at Nagadono to interpret landslide ground,
abstracting unconfirmed river course blockages at 8locations including Akatani and Kurihira (see Fig. 2●A Case of Utilizing Resultsfor example at Akatani). On the following day,
September 6, after the weather had returned to normal,
the river course blockage locations abstracted by thesatellite SAR images were visually confirmed fromhelicopters, and an emergency survey stipulated by theSediment Related Disaster Prevention Law was started
at the sites of large-scale river course blockages at
Nagadono, Akatani, Kurihira, and Kumano.
Aerial photograph taken on
September 10, 2011

FloodedgroundLanslideground2011.9.5 観測
TerraSAR-X@PASCO

Figure 2. River Course Blockage at Akatani
Discovered by the Satellite SAR
In the past it was impossible to perform surveys toconfirm the formation of natural dams at night andduring bad weather, but interpreting landslide groundusing wide-area satellite SAR images successfully
sped up the search for previously unconfirmed natural
dams in mountains even during bad weather,accelerating the start of emergency surveys andevacuation of downstream residents as required by theSediment Related Disaster Prevention Law when alarge-scale sediment disaster is imminent.