Application of RS and GIS in natural disaster survey: a case study
of mountainous landslide caused by the herb typhoon
Chinsu Lin *, Yeong-Kuan Chen **, Shoei-Jyi
Wu ***
* Assistant Professor, Dept. of Forestry
National Chiayi Institute of Technology
300 University Road, Chiayi (600)
Tel: (886)-5-2717476 Fax: (886)-5-2788724
E-mail: chinsu@rice.cit.edu.tw
China Taipei
** Professor, Dept. of Forestry
National Taiwan University
1, Sec. 4, Roosevelt Road, Taipei (106)
Tel: (886)-2-23639799 Fax: (886)-2-23639799
E-mail: ykchen@ccms.ntu.edu.tw
China Taipei
*** Researcher, Council of Agriculture
Executive Yuan, Republic of China
37 Nanhai Road, Taipei (100)
Tel: (886)-2-23124081 Fax: (886)-2-23125857
E-mail: sjwu@mail.coa.gov.tw
China Taipe
Keywords: Natural Disaster, Landslide, Herb Typhoon, Remote Sensing, GIS
Abstract Steep terrain relief and unstable river system in Taiwan make landslides and
massflows happen frequently when a storm visits there in the summer. Such natural disasters
usually cause huge property damages and loss of human lives. Understanding the location and
influences of natural disaster is important and helpful for post-disaster financial support and
environmental restoration. It could also be a good reference for natural disaster prevention and
land management planning. The Herb typhoon visited the middle-south part of Taiwan on July
31, 1996, and resulted in serious landslides and massflows. After that event, the eroded mass
settled down and became barelands. In this study, SPOT XS images of new barelands and their
spatial pattern in Chiayi slopeland area, middle-south area of Taiwan was analyzed. The results
showed that area of new barelands from June to December 1996 was 9726.72 ha which were
about 8.69 % of the whole area. New bareland area that happened at each site was more than
1000 ha, and Alishan was the largest one. Twenty-nine percentage of the new bareland patches,
or 2836.77 ha, were nearby rivers or roads, in which 32 % (900.63 ha) were located on the site
where the rivers and roads pass through. This phenomenon demonstrated the importance and
necessity of conservation for the land nearby the stream bank and road. In view of land slopes,
most of the new barelands were distributed on slopeland under 35 degree. The percentage of
both original and newly-happened barelands are decreasing from the 1
st slope class (< 5° ) down
to the 4
th slope class (26° – 35° ) and then increasing, and just look like a parabolic form. The
percentage of newly occurred barelands in developing cover types, e.g. orchard, areca, and tea
field etc. is almost 6.5 to 10.5 times larger than the one in the forested area. It proved that the
storm-damage resistance of forest is the best in all vegetative cover types.
Introduction
The Herb typhoon visited the middle-south part of Taiwan on July 31, 1996. He brought a heavy
rain and resulted in serious landslides and massflows in mountainous area (TFB, 1998; CWG,
1999). Huge properties and lots of human lives were damaged and lost in that disaster. This
study aimed to understand the landslide distribution caused by the storm. Landslide is a process
of mass movement. After that process, the eroded masses settled down and became barelands.
According to such concept, the new barelands occurring after the Herb typhoon were surveyed
by techniques of remote sensing, and the spatial patterns of those barelands were also analyzed
by geographic information systems in this study.
Materials and Methods
The mountainous area of Chiayi County, the middle-south part of Taiwan, was chosen as a study
site. This site has an area of about 112000 ha and well developed streams and road networks.
The terrain relief is steep in its eastern part. Average annual rainfall was more than 4000 mm
(CCG, 1995; Liou et al., 1998).
Satellite Images and Auxiliary Data
Four SPOT XS images (list as table 1) were used to analyze the land cover of study site. Two of
them were scanned before Herb visited day (July 31, 1996) and the others scanned after that day.
Land cover mosaic of per pair image was produced since the study site is located on two
adjacent paths and each path contains only partial area of it. Auxiliary data, such as roads,
streams, cover types in forestland, and digital terrain model were generated from some released
maps. These maps were digital road network map (IOT, 1998), topographic map, forest
resources survey map (TFB, 1995) and photo base map.
Table 1. Scanning parameters of SPOT images used in this study *
|
| Image code ** |
Scanned date | Azimuth |
Elevation | Incidence |
Pixel spacing |
|
| T1R |
1996/6/1 | 161.891° |
41.358° | 27.547° |
12.5 m × 12.5m |
| T1L |
1996/6/2 | 155.652°
| 39.730° |
-12.719° |
12.5 m × 12.5m |
| T2R |
1996/12/21 | 155.646°
| 39.191°
| -22.313° |
12.5 m × 12.5m |
| T2L |
1997/11/20 | 157.552°
| 44.134°
| -12.718° |
12.5 m × 12.5m |
|
*Geocoded level 10 images were supplied by the NCRS, Taoyuan
** Image code T1 andT2 represents the images scanned before and after the Herb typhoon; R and
L stand for right and left part of the study site
Bareland Detection Algorithm ¾ Maximum Likelihood Method
A supervised classification method, maximum likelihood classifier (ERDAS, 1997;
Schowengerdt, 1997), was applied for land cover extraction, in which multiple signatures of
bareland, vegetation, and water classes were selected according to the transformed divergence
(TD) between classes. TD threshold set in signature selection was 1900 (Jensen, 1986).
Classification accuracy was assessed with two indices, the overall accuracy and kappa
coefficient of agreement (Hudson and Ramm, 1987).
Matrix Analysis
This study used matrix analysis (Lin and Chiao, 1997) to derive the change map of land cover in
the study site nearby the Herb typhoon. In Eq. (1), the parameter N is the number of classes, i.e.
3, in T1 and T2 thematic maps, the parameter a and b represents their class code, and 0 stands
for the background.
Mat_change = Either 0 IF (a IS 0 OR b IS 0)
OR [a + N(b-1)] OTHERWISE (1)
