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Remotely sensed geohazards for assessing alternative corridors

Jian-King Liu
Research Scientist and Head of
Mineral Resources Department
Energy and Resources Laboratories, ITRI
Hsin-Chu 310, THAIWAN
Tel: +886-3-591-6352; Fax: +886-3-582-0017
E-mail: [email protected]

Abstract
The most difficult part of constructing a sound national highway system in Thailand is to manage the difficulties of crossing the mountainous terrain, where are geologically hazard-prone areas and there are very rare existing geological information. Therefore remotely- sensed data have been employed very extensively since 1990 to give information on geo-hazard. It was also found that by using air-photos taken 40 years ago, abandoned river courses and ancient ponds in the level and hilly terrine, where now are covered by recent deposits and highly cultivated, can be readily identified to give indications of geological sensitive areas. It is, therefore, concluded that when conventional aerial photographs are employed together with satellite images, the best benefit by using remote sensing techniques can be gained. In mountainous terrain, images can be used to locate potential Landslides; in hilly or level terrain, images can be used it identify weak and sensitive geological units. This information is valuable for assessing alternative highway corridors.

Introduction
More than two thirds of Taiwan Island are covered by rugged terrain. The small portion of the relative level areas is over populated, where most of towns are with a density of more than 2000 people per square kilometer. Having a sound highway system is critical to improve the quality of life. The most difficult part of it is to cross the mountainous terrain, where are geologically hazard-prone areas and there are very rare existing geological information.

As imposed by time constraints, insufficiency of geological information, and the low accessibility of the mountainous terrain, the benefits to adopt remote sensing techniques are obvious. Therefore, remotely sensed data have been employed very extensively since 1990 to give information on geo-hazard. It was also found that by using air-photos taken 40 years ago, abandoned river courses and ancient ponds in the level and hilly terrain, where now are covered by recent deposits and highly cultivated, can be readily identified to give indications of geological sensitive areas. As the experience shows, conventional aerial photographs are employed together with satellite images in the cases discussed in this paper. 

Physiographic Background of Taiwan
It is shown in Figure 1 that more than two thirds of Taiwan Island are covered by rugged terrain. The small portion of the relative level areas (less than 30%) are over populated, where most of towns are with a density of more than 2000 people per square kilometer. Due to the rapid economic growth in recent years, an urgent need to have a sound highway system in deemed critical to improve the quality of life. However, there are various types of geological hazards distributed all over the Island. To have an approach for quick assessment for corridor selection in the early stage of highway engineering would contribute to the society significantly. Methodology was developed in the past few year, by adopting all types of airborne and space-borne images in various type or stage of highway planning, as will be discussed in the following paragraphs (Liu, 1983, 1984, 1986, 1987)

Selected Cases Since 1990
As shown in Figure 2, a national highway network has been initiated in Taiwan and various types of remotely-sensed data has been applied for various types of analysis in various engineering stages (Table 1).

A Typical Approach
The approach adopted for the creation of a set of geological hazard maps in the stage of corridor selection in the preliminary planning phase of Southern-cross National Highway (Number 2 in Figure 2) is given here for an example. There are three steps in this project, including 

  1. the creation of criteria and guidelines as the philosophy for the decision making by using results from remotely-sensed data; 
  2. establishing procedures for analyzing images and other ancillary information; and
  3. fusion of all the resultant maps from step2.

The criteria are drafted on basis of five principles, namely 

  1. the geological information required in feasibility study stage, as a contrast to what can be extracted from remote -sensed data; 
  2. adequacy of existing and available information for ancillary information; 
  3. reliability and repeatability of evaluation, meaning that an objective approach is preferable; 
  4. available means; and 
  5. time constraint. 

Guidelines under these criteria include:

  1. information to be extracted from remotely-sensed data is concentrated on major geological constraints; 
  2. themes by remote sensing techniques are : drainage system, lineaments, landslides, major geological structure, lithology, and geothermal sites; 
  3. applying GIS technology for compiling and integrating various sources of information; 
  4. a well-defined criteria with a scale of 5 classes for intra-factors and expert-rated weighing table for inter-factors. 

Thus, the resultant weighted scores of 8 alternatives are as shown in Table 2. In addition to geological considerations, two more areas were considered in the corridor selection processes, namely the engineering considerations and environmental considerations (Figure 2). A combined order of the study is shown in Table 3.

Table 1. Six Selected Cases since 1990
Name of Plan(Study Area Size) Time ConstraintOf Image Study Type of image Data Used Items Processed
Case 1.
2nd Freeway(Taipei-Iiam)(260Km2)
5 months 1. 1974-1975 Airphotos
2. 1990 SPOT image
3. 1990 LANDSAT image
4. 1982 SLAR image
1. drainage system
2. lineaments
3. landslides
4. major structures
5. image tone analysis
Case 2.
Southern-crossNational Highway(360Km2)
10 months 1. 1974-1976 Airphotos
2. 1990 SPOT image
3. 1990 LANDSAT image
4. 1982 SLAR image
1. drainage system
2. lineaments
3. landslides
4. major structures
5. lithology
6. geothermal sites
7. mineral resources
8. DTM
9. GIS ANALYSIS
Case 3.
2nd Fw Suao Extn.Section (360Km2)
4 months 1.1974-1975 Airphotos
2.1993 Airphotos
1.geological hazards
2.landuse change
Case 4.
3rd Fw FongyenSection (36Km2)
1 months 1. 1948-1948 Airphotos
2. 1974-1975 Airphotos
3. 1993 LANDSAT image
1.tracing
2 faults
Case 5.
3rd Fw Nanto Sec.(72Km2)
2 months 1. 1948-1948 Airphotos
2. 1974-1975 Airphotos
3. 1993 LANDSAT image
1. geological hazards
2. major structure
Case 6.
Eastern NationalHighway(3500Km2)
6 months 1. 1991-1992 Airphotos
2. 1988 image
3. 1988 LANDSAT image
4. 1982 SLAR image
1. lithology+weak rocks
2. lineaments
3. landslides
4. major structure
5. geothermal sites
Table 2. Weighted Scores of 8 alternatives
Alternatives Themes 1 2 3 4 5 6A 6 7 8
1.Landslies 58.68 83.24 82.74 74.559 81.00 58.90 886.80 83.37 77.41
2. Streams 77.76 62.62 56.14 52.31 52.59 48.03 67.43 91.97 93.63
3.Sturcture 9.70 7.65 8.30 6.53 5.00 5.15 6.30 5.0 7.15
4.Lithology 9.70 101.85 85.52 96.32 98.55 117.84 97.43 120.05 141.61
5.Lineaments 136.94 26.0 27.00 23.0 17.0 19.0 10.0 24.0 16.0
6.Geothermal 18.4 0.37 0.37 3.90 3.90 3.9 0.59 1.40 1.4
Total Score 1.25 281.74 260.07 256.65 258.04 252.82 268.55 325.61 337.2
Grade NG G B B B B B NG NG
Order 7 6 4 2 3 5 5 8 9
N.B. Better: Total score