GeoJourna138.3: 273-278. © 1996 (March) Kluwer Academic Publishers. Printed in the Netherlands.

Geomorphological zoning for flood inundation using satellite data

Haruyama, Shigeko, Dr., Waseda University, Faculty of Education, Department of Geography 1-6-1, Nishiwaseda, Shinjuku-ku, Tokyo, Japan

Ohokura, Hiroshi, Mr., The National Research Institute for Earth Science and Disaster Prevention, 1-3, Tennodai, Tukuba city, Ibaragi, Japan

Simking, Tongchai and Ramphin, Mr. & Mrs., National Remote Sensing Center of Thailand, 196 Phaholyothin Road, Bangkok, Thailand

ABSTRACT: The authors investigated geomorphological features on the central plain of Thailand utilizing satellite remote sensing data and made geomorphological land classifica- tion map showing flood-stricken area. Land classification maps showing flood-striken area tell us former flood inundation area, such as inundation depth, inundation width, flood flow course and flood direction, as well as estimating of the features of flooding. Thus map is useful for planning of flood control works.

We classified land form units in the central plain of Thailand as following; delta, tidal flat, lagoon, mud spit, back marsh, natural levee, fan and former river course and so on. After that, the principal component analysis is applied to Landsat TM data and gives good results for photo interpretation of land form units and we transfer geomorphological land classification map to make zoning map of flood risk for the purpose of evaluating the flood damages.

1. Introduction

When we want to develop the area, we must know the natural environment of the area in detail and we must pay attention to prevent natural disasters. The relationship between land forms, hydrology and human activities is close. Because the geomor- phology was formed by the crustal movemen t and deposit ion or erosion by fluvial or marine action. This action appears at the destruction of the existing topography, for example, vertical or horizontal faults caused by earthquakes, land collapse caused by torrential rainfall and deposition of sand and gravel caused by flooding. The geomorphologica l land classification map shows the history of natural dis- asters in the area in the past.

M. Oya (1956) made the first geomorphological land classification map of Nobi Plain and demon- strated the validity of the map at the Ise Bay Typhoon. The ministory of construction of Japan asked him to prepare geomorphological land classi- fication maps after the typhoon and Haruyama, one of the authors, have jointed to some projects making

the maps since 1977. In the case of Japan, we can use aerial photographs and detail topographical maps for geomorphological survey, but such a data for inves- tigation of geomorphology is very scarce in the case of another Asian countries.

We have tried to use satellite remote sensing data for making method of geomorphological land clas- sification map. Two geomorphological land classifi- cation maps in the central plain of Thailand are made based on satellite remote sensing data (Ohokura et al. 1989; Ohokura et al. 1992). These maps are derived from Landsat image interpretation, ground truth and field survey.

2. Geomorphological land classification map in the central plain of Thailand

We have made a geomorphological survey map of the central plain of Thailand showing classification of flood-inundation areas using the following procedure. (1) We have made a base map Utilizing the map made by the Royal Thai Survey, Scale is 1/250,000. The

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limit of the area is 13°8 , N ~ 15025 , N and 91030 , E ~ 101°10 ' E. (2) We have used the Landsat images which were taken by Landsat 2, 4 and 5. The dates of the images, which were used mainly, are 3 January, 1979, 2 February, 1985, 12 April, 1984, i.e., the dry season, 12 May, 1986, 20 July, 1985 and 4 November, 1986, i.e., the raining season. (3) We have classified the geomorphology utilizing the above mentioned images and have made a provi- sional geomorphologic map. (4) In December 1987, we conducted research including a brief measurement by hand level, a surface geological survey by boring stick and by collecting information of past flooding in the plain. Thus the map was to be put into final form by checking with the results of the field survey of the area.

The geomorphological units in the central plain of Thailand are classified as follows; hill, terrace, valley plain, fans (fan I, fan II and fan III), natural levees (higher and lower), back marshes (back marsh I, back marsh II, back marsh III), deltas (delta I, delta II, delta III, delta IV), mud spits (higher and lower), lagoons (higher and lower), tidal flat (higher and lower) and former river courses and so on.

The fan located in the plain can scarcely be seen along the Chao Phraya river. There are fans and dissected fans which gradient are gentle along the Kraseo and Mac Klong rivers and in the western part of the plain. Natural levees develop along the Mae Nam Chao Phraya river and expand especially on the right side of the bank between Nakhon Sawan and Chainat. At Chainat where is apex of the plain, a delta zone begins and the Chao Phraya river branches out into two distributaries, the SuphanBuri and Noi rivers. These rivers are accompanied with higher natural levees and deeper back marshes lying on the outside of the levees. Former river courses with/ without a natural levee and deep back marshes lie scattered on the back marshes.

The delta is developed around Ayuthaya and located along lower Chao Phraya and SuphanBuri rivers. The coastal topography consists of sandspits, mud spits and lagoonal swamps from the lower delta to the mouths of the rivers. Lagoons were reclaimed during the 19th century for rice paddies (Haruyama 1991).

the plain areas are shown in the table on each the maps.

In the natural levee region, the flood water flows down the mainstream of the Chao Phraya, the Suphanburi and the Noi. The water overflowing from these rivers runs through the abandoned river courses and through back marshes. The type of flooding along the Bang Pakong river is the concentration type. And the flood water coming from the Bang Pakong river pours into the Bang Pakong again. The period of inundation is short and shallow in the former lagoon area. Salt water intrusion is seen in the tidal flat. Recently, Metro-Bangkok has been attacked by frequent flooding. The flooding is caused not only by the geomorphological factor but also human impacts (Haruyama 1991).

We evaluated the flood risk of the land forms in the central plain of Thailand (Figure 1, Table 1).

3. Geomorphological land classification and zoning for flood inundation

A geomorphological land classification map is intended to enable us to estimate the nature and extent of floods. The reason why such survey maps indicate the features of floods is that the relief features of plain have been formed by repeated floods over the affected areas. The relationships between the features of the flooding and the micro-topography in

Figure 1. Geomorphological zoning for flood risk in the central plain of Thailand. 1. Bank collapse; 2. Sheet flood; 3. Flash water route; 4. Deep inundation; 5. Inundation stage 1; 6. Inundation stage 2; 7. Inundation stage 3; 8. Storm surge; 9. Inland water; 10. Spring at the skirt of alluvial fan.

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Table 1. Land form and flood risk in the central plain of Thailand

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Land form Flood risk for probability of exceedance

Storm surge risk for probability of exceedance

Flood risk for probability of 1/10 years

Terrace F F F Alluvial terrace F F F Valley plain C F D Fan E F E Higher natural levee D F F Lower natural levee D D D Back swamp B C B Delta C B B Emergent mud dune D D D Mud dune D D D Emergent lagoon B B C Lagoon B B C Former river course A A A Former pond B C B Peat land B C B Reclaimed land C D D

A: Main flash water route; B: Submerged for long time with deep depth of flood water; C: Flooded by not only over- flowing from the river but also local rainfall; D: This area is submerged in an extraordinary flood time, but the flood water drains off well; E: Sheet flood; F: Never submerged

4. Coloration of land form elements in the TM mosaic image

After we made the geomorphological land classifi- cation maps of the central plain of Thailand, we tried to do supplementary digital analysis in the same area. First, some land form elements in the Landsat images were identified by field surveys and spectrum char- acteristics of the identified elements in the images were extracted; a natural levee, a deep inundating back marsh and tidal flat have fixed spectral charac- teristics but a delta, a lagoon and a mud spit differ in different localities (Ohokura et al. 1993). Second, in order to check and correct the results extracted by the conventional classifier, a post-processing method using the shapes of the geomorphological elements was introduced. This method consists of a fusing process and an examining process. The ratio (so- called the circularity or compactness) of the 'area of the connecting region of the same geomorphological element' to 'the length of its boundary line squared' was adopted as the characteristic shape in the examining process. The mean values of circularity for every geomorphological elements were calculated by using the result of photo interpretation. The values range from 0.012 to 0.134 and circularity can be used as a performance measure to examine the classified result.

After that, the principal component analysis was applied to Landsat-5 TM image and eigenvalues and eigenvectors were calculated from a correlation coef- ficient matrix of 7 bands of the images. From the eigenvectors, it is said that an image of the first principal component roughly corresponds to the sum

of the brightness of all bands. An image of the second principal component express the vigorousness of vegetation because the component in band 4 has a large value of 0.96. The third component has a large value. 0.80, for a thermal infrared band and has a high correlation with the ground surface temperature. If the brightness of the visible bands increases, the gain of the third principal component decreases because the component of the visible bands have negative values from -0.43 to -0.22. If blue, red and green colors are assigned to the first, second and third principal components, respectively, the image shown in Photo 1 has coloration of each geomorphological elements as shown in Table 2 (Photo 1, Table 2). The elements could be classified easily, especially the areas that look similar in color in a false or natural color image (Haruyama 1992).

5. Geomorphological aspect on A SAR

JERS-1 carrying a SAR system was launched in February 1992. Although the L-band SAR system had a trouble on its antenna at first, this trouble was overcome and acquisition of the SAR images all over the world with an on-board tape recorder started. So, the general distribution of the images to users does not start yet. NASDA distributed them to the par- ticipants of ' the Verification Program of JERS-1' and the members of 'the Earth Environment Observation Committee.' This report is the analyzed results of the SAR images of the Central Plain of Thailand.

In the Central Plain of Thailand, the typical aspect of the geomorphological elements on the JERS-1

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Photo 1. Landsat TM mosaic image which was processed by principal component analysis (1st component: blue; 2nd: red; 3rd: green).

Table 2. Coloration of geomorphological elements in the TM mosaic image processed by the principal components analysis (1 st component: blue; 2nd component; red; 3rd component; green)

Color Characteristics Geomorphological element

Blue Except for band 4, all bands have high radiance. Medium or low temperature.

Red High plant vigor. Low temperature.

Green Low radiance of visible and intermediate infrared band. Low plant vigor. High temperature.

Magenta Medium plant vigor. High radiance invisible and intermediate infrared. Low temperature.

Cyan Low plant vigor. High radiance in visible and intermediate infrared bands. High temperature.

Yellow High plant vigor. High temperature.

Black Low plant vigor. Low radiance in all bands. Low temperature.

Slightly high area, plateau and alluvial fan with bare surface or covered with low dense vegetation.

(High) mountainous area covered with dense vegetation. River course, abandoned river and back marsh covered with dense vegetation.

Bare soil surface with some humidity.

Delta and valley plain covered with dense vegetation.

Dry slightly high area, plateau, alluvial fan without vegetation.

Natural levee, pediment and slightly high area covered with dense vegetation.

Water surface (river, abandoned river, back marsh).

SAR image taken in the latter half of the rainy season, is as follows.

On the JERS-1 SAR image, the brightness of the geomorphological elements on aquite gently undu- lating lowland reflects landuse conditions.

For example, a city area appears very bright

because a crowd of artificial structures increases back scattering. And some areas covered with dense tall trees also appear bright. Inversely, areas with very smooth surface, such as a back marsh with a water surface, appear very dark or in black.

Serial four JERS-1 SAR images in the western

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part of central plain of Thailand are analyzed with visual photo interpretation. The path and rows of the images are path 128, row 275,276, 277 and 278, and the acquisition date is 22 September 1992, in the latter half of the rainy season. The process level of the images is level 2.1, on which pixel sizes in both range and azimuth directions are corrected to 12.5 m and discrepancy of sensitivity in a range direction is also corrected.

Some portions of the images overlap with the areas of the two maps which we compiled on our Japan-Thailand joint research project from 1986 to 1991. We compared and checked the SAR images with the two geomorphological land classification maps and got the results as following. A terrace has various aspects; A dense forest appears bright and an arable land which is recently cleared appears in gray with white rectangle patches. An alluvial fan is generally used as a sugarcane field and appears in lighter gray or gray. A valley plain used as a paddy appears in black or dark gray. A natural levee which is covered with tall trees and a residential district appears in white or light gray. A inundated back- marsh appears in black. Some deltas used as a paddy appear in black or dark gray, others used as a coconut farm or an orchard appear in white or light gray. A depressed area in an alluvial fan is used as a paddy, and appears in dark gray or black. A former river course has a water surface and appears in black. In a lowland, a village is located on a natural levee and most natural levees are covered with vegetation containing tall trees in the central plain of Thai- land. A back marsh is used as a paddy. A deep back marsh and a former river course are left as it is and these generally have water surfaces because of inun- dation.

However, some places where the geomorpholog- ical maps and Landsat TM images show deep inun- dation do not always appear in black. And because it is supposed that brightness of a back marsh on the SAR images does not directly reflect a ground surface level or a inundating depth of a back marsh, further investigation is required to clarify what the brightness reflects. A natural levee appears in white or light gray on the SAR image because tall trees and houses increase back-scattering. A city area appears in white because a crowd of artificial structures increases back-scattering much more. A valley plain used as a paddy appears darker than a surrounding terrace.

On the SAR image of path-row 128-276, we can see an alluvial fan spreading east in the Kraseo river basin. The skirt of the fan where many springs appears in black or dark gray on the image. The fan is subdivided into three fans from south to north. The southem most is old fan. This fan appears darker than the new fan which is situated in the central part and piles up the old one. However, another northern old fan on the phase of erosion appears almost as bright

as the new one except the portion where the erosion has progressed.

The alluvial fan of the Mae Klong river which has an apex in Tha Muang subdivided into the northern old fan and the southern new fan. Many depressed areas used as a paddy appear in black on the new fan. Another portion of the new fan are mainly used as a sugarcane field and appear in slightly lighter gray than that of the northern old fan. The existence of dark depressed areas and the appearance in lighter gray make it easy to distinguish the new fan from the old one. On the SAR image a oval part about 10 km south of Kanchanaburi is conspicuous for high brightness. Although a terrain is a convex form with a very gentle slope, dense trees on the surface makes it bright.

On the SAR image of path-row 128-278, we can see a bright part around the estuary of the Mae Klong river. This part is a delta and is mainly used as a coconut farm. Coconut trees are planted along ridges. As channels between the ridges are always filled with water for irrigation, this area is not especially dry. However, tall and dense coconut trees increase backscattering.

We can use the SAR Image for land classification and flooding condition on each geomorphological units. Comparing the geomorphological land classi- fication map, flooding condition map and landuse map, we propose policies of the zoning for mitiga- tion of flooding of this area.

6. Conclusions

In the Landsat TM image, the following geomor- phological elements appear in fixed spectral charac- teristics: a natural levee, a valley plain, an alluvial fan, a river surface with muddy water, a deep inun- dating back marsh and a tidal flat. Use of multi- temporal images provides good results for digital classification of geomorphology. Circularity of the geomorphological elements can be introduced into post-processing of digital classification as threshold parameter. Joint use of the circularity and fusing process of expanding and contracting a boundary of the element improves the classified results. The principal component analysis is applied to Landsat TM data and it gives good results for photo inter- pretation.

On the JERS-1 SAR image, the brightness of the geomorphological elements on a quite gently undu- lating lowland reflects landuse conditions. For example, a city area appears very bright because a crowd of artificial structures increases backscattering. And some are as covered with dense tall trees also appear bright. Inversely, areas with very smooth surface such as a back marsh with a water surface appear very dark or in black. We can use the SAR Image for making method of geomorphological land

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classification map and flooding condition on each land form units.

We would like to propose thus technology for discrimination of land form units to put to practical use for disaster prevention works, planning for mitigation of flooding.

References

Oya, M.: Geomorphological land classification map of the Nobi Plain. Ministory of Construction, Japan 1956.

Haruyama, S.: Recent changes in flooding in the central plain of Thailand. Geographical Journal 100, 2 (1991)

Haruyama, S.; Ohkura, H.; Oya, M.: Map making of flood pre- vention in the Kraseo river, western part of the central plain of Thailand utilizing satellite remote sensing data. The map 30, 2 (1992)

Haruyama, S.: Geomorphology of the central plain of Thailand and its relationship with recent flood conditions. GeoJournal 31, 4 (1993)

Ohkura, H.; Haruyama, S.; Oya, M.; Vibulsresth, S.; Simking, R.; Suwanwerakamtorm, R.: A geomorphological land classifi- cation for the flood-inundated area in the central plain of Thailand using satellite remote sensing technology. Research Notes of the National Research Center for Disaster Prevention 83 (1989)

Ohkura, H.; Uehara, S.; Haruyama S.; Oya M.; Vibulsresth, S.; Simking, R.; Slinking T.: A geomorphological land classifi- cation for the Krasieo River basin using satellite remote sensing technology. Research Materials of the National Research Institute for Earth Science and Disaster Prevention 150 (1992)

Ohkura, H.; Simking, R.; Suwanwerakamtorm, R.; Haruyama, S.; Oya, M.: Geomorphological land classification map showing flood inundation area using satellite data. Fifth International Space Conference of Pacific-Basin Societies 1993 Shanghai 1993.