Research Trends and Results
Urgent Determinant Investigation of Landslide Dams with SAR SatelliteImages
MIZUNO Masaki, Senior Researcher; KAMIYAMA Joko, Researcher;
EKAWA Masafumi, Guest Research Engineer; KANBARA Jun-ichi, HeadErosion and Sediment Control Division, Research Center for Disaster Management
Keywords: SAR image, landslide dam, interpretation
1. Introduction
If abnormal phenomenon such as a large-scale collapsethat may lead to the formation of a landslide dam is likely
to occur in a wide area due to heavy rain, etc., it isnecessary to prevent the occurrence of secondary
disasters due to breaches or otherwise by quickly
detecting the formation, position, and size of the landslide
dam. This paper introduces a method of urgent
determinant investigation of landslide dams using images
obtained from synthetic aperture radar ("SAR"). Its aim is
to investigate ways to detect landslide dams even at night
or in bad weather where visual investigation using ahelicopter is difficult.
2. Characteristics of SAR images
SAR is an activity sensor usually installed in artificial
satellites and airplanes. It observes backscatteredmicrowaves that are originally irradiated obliquely to theearth’s surface.
SAR images have unidentifiable
elements due to shadows, reversing, or tilting (Fig. 1)
since microwaves are irradiated obliquely downward, But,
SAR has an advantage it that observations can be madewithout sunlight, i.e., observation at night or in bad
weather, since microwaves penetrate clouds.
In addition, unidentifiable parts can be reduced using
images observed from multiple directions, such as
SAR Satellite

UnidentifiableUnidentifiable

manually in accordance with the below check list byenlarging images. They are otherwise read inconsideration of geomorphic characteristics of landslidedams (Fig. 2) such as dammed lakes, collapsed areas, andcolluvial sediment blocking a river channel, and inconsideration of the aforementioned geometriccharacteristics of SAR images.
Collapse

AkadaniDammedlakeCollapse

CFigure 2: Example for SAR Image of Landslide Dam (Kii
Peninsula in 2011)
Table: Check List for Dual-Polarization SAR Image Interpretation
Check areas

Check items

Scale of collapse

Shadow formed(Radar shadow)

ofthePositionon theScar of debris
flow

Scar of flow down
Debris flow deposit

imageearth

Inside the collapsedland
Colluvial sediment(Blocked riverchannel)

ED
angle

E Tilt foreward(Foreshortening)

Local terrain

Slope gradient

Relative
positional
relation

Relation of upper andlower, etc.

Figure 1: Geometric Characteristics of SAR Images(reversal of image, tilt)

River channel

Dammed lake

BAC

Reversal of upper and lower (Layover)D

observation from ascending (northward) / descending(southward) orbit.
3. Interpretation of SAR images
When using dual-polarization SAR images forinterpretation, it is desirable to use wide-area images thathave a resolution higher than HH + HV, i.e., 8 m (of thecombinations of transmitted / received horizontally
polarized wave H and vertically polarized wave V) andare observed at the incidence angle of approx. 35-50.
Objects that can be identified with dual-polarization SAR
images are mainly collapsed land with a plane projection
area of about 1 ha or more. SAR images are readColluvial
sediment

RADARSAT-2 Data and Products c MacDONALD, DETTWILER AND ASSOCIATES LTD. (2012) - All Rights
Reserved RADARSAT is an official mark of the Canadian Space Agency

Main scarp

A

Collapse

FigureExample for SAR Image of Natural Dam (Kii
River2:channel
blocked by colluvial
Peninsulain 2011)sediment

Collapsed
landSurface

Ui

Nagatono

Off-nadir angle

Incidence[Dual polarization]
Color composite image
R: HH polarized wave
G: HV polarized wave
B: HH polarized wave

Collapse

Bare ground

B

Dammedlake
Landslidedam

Evaluation
Decision criteria- Is red-purple bare ground in the dual-polarization image?
- Located on a slope? (Colony or river channel on flat land or gentle slope?)- Are any main scarps seen around dammed lakes?
- Are any shadows / layovers due to level difference seen around the main scarp?
- Is the shape of the main scarp circular to the direction of the slope?
- Are any collapsed shapes seen under the main scarp?
- Is the collapsed shape consistent with the direction of the slope?
- Are colluvial deposits seen from inside the collapsed land to the lower area?
Evaluation index- Is the colluvial deposit tongue-shaped?
○: Shape can be identified- Is the extent of the colluvial deposit consistent with the topographic shape?
△: Not clear but shape can be- Is the blocked river channel shaped to aggrade the valley?
identified
X: No shape can be identified- Has a dammed lake formed upstream of the blocked river channel?
-: Impossible to identify- Is the shape of fallen trees, etc. seen on the colluvial deposit?
- Is the scale of collapse large enough to block the river channel?
- Is the transport area seen from the lower part of the collapsed land?
- Is there scar of flow down of debris due to widening of the river channel, outflow of vegetation,etc.?
- Is there debris flow deposition (debris flow terrace, alluvial cone) seen?
- Are there any slopes near the dammed lake?
- Is the neighboring slope so steep it may collapse?
- Are there any abnormalities in the positional relations of the main scarp, colluvial deposit, anddammed lake?
- Is the extent of colluvial deposit consistent with the topographic shape?
- Is a dark area suspicious of a dammed lake seen?
- Is the width abnormal compared with the water-route width upstream and downstream?
- Is there flooding because of artificial structures, such as a dam or intake weir?
Collapse scar / Bare rock area

Surface failure
Judgment

New collapse

Large-scale collapse

Deep-seated landslideRiver channel blockage

Scar of debris flow

4. Conclusions
Since engineers have started using SAR images indisaster response, as was the case following Typhoon No.
12 TALAS in 2011, we will continue to upgrade theinvestigation method into their practical use.