Classification of microtopographical features along the shore of Balkhash Lake by ALOS/PRISM DSM

Yasunori Nakayama1, Yoichi Hara2, and Kunihiko Endo1 1 Dept. of Geosystem Sciences, Nihon University

nakayama@chs.nihon-u.ac.jp 2 Urayasu Junior High School

Abstract

The purpose of this study is to analyze the

distribution and the classification of gravel bar along the shore of Balkhash Lake by using the high precision DSM newly produced from the stereo pair data of PRISM carried in ALOS, and to consider the age and environment of formation of every classified gravel bar, and the long-term environmental change based on water level fluctuation of the lake. The classification of gravel bar distribution pattern based on the characteristics such as height, number and direction of the ridge line was carried out through extraction and comparative analysis of the DSM data and ALOS pan-sharpened image in eighteen areas along the shoreline.

The result of this study showed that the distribution pattern of the gravel bars along the shore was classified into three typical groups. The difference in distribution of three groups is based on the influence of the waves with the strong prevailing wind. Moreover, according to the feature of the classified group, the water level change situation of Balkhash Lake was divided into three stages in the past about 26,000 years,, and the tendency of drawdown was shown as the whole. 1. Introduction

The authors have been investigating the climate change of the arid region of the Asian Continent in recent years (last several decades) in comparison with the climate change tendency in the last several hundreds or thousands years by analyzing water level fluctuation of closed Balkhash Lake in Central Asia, Kazakhstan as an index of the change. The

investigation object is the distribution pattern of a few lines of small ridges, microtopographical features which consists of the sand or sand and gravel (gravel bar) as shown in Figure 1 along the Balkhash Lake shore formed by the waves and wind accompanying the water level fluctuation from the past about 26,000 to recent years. Since the distribution pattern of the gravel bars is reflecting the water level based on the past climate change, the possibility of restoration of the past climatic environment situation of Balkhash Lake by the analysis was reported [1]. Moreover, since the high precision DSM (Digital Surface Model) based on satellite data compared with the existing DEM was required for the extraction of these gravel bars, development and its verification of the new algorithm for producing the DSM from Advanced Land Observing Satellite (ALOS) Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) data have been carried out [2].

Figure １. Gravel bars in the north shore

This study aimed at the classification based on the distribution situation of the gravel bar by DSM analysis for the consideration of the formation environment and the age, and the change in a long period of time.

Specifically, the extraction of the gravel bar as microtopographical feature based on the analysis of precise DSM derived from ALOS/PRISM stereo pair data was firstly carried out in plural areas where the distribution was observed by the interpretation of ALOS pan-sharpened image along the shoreline of Balkhash Lake. Next, based on the classification result of the extracted gravel bars due to the distribution pattern, the estimation of the formative environment of the microtopographical feature and the possibility of the climate change restoration for past about 26,000 years were considered. 2. Methodology 2.1. Study area and used data Balkhash Lake is a closed lake located in eastern Kazakhstan of Central Asia, and has the water area of approximately 18.2 ×103 km2 and the average water depth of approximately 6 m in the existing statistics [3]. It is an important lake for monitoring of the climate fluctuation in the Asian Continent due to the location of Central Asia. It was reported that the water level of this lake has repeated change by the about 3 m range of fluctuation in past about 100 years [4]. According to the integrated investigation with the analysis result of the recent change by satellite data and the existing data, it was shown that there were 2 periods indicating about 2.5 m and about 1.5 m higher water level compared with the present within the past 100 years by the end of the 1980s. Present water level change is showing a little upward tendency. Moreover, distribution of plural lines of gravel bars showing a few meter heights corresponding to recent change and the higher ones was observed along the shore by the field survey [5]. The data used for interpretation of micro-topographical features and DSM production was observed by AVNIR-2 and PRISM installed in the ALOS satellite. Especially, based on the measurement of the parallax to the difference height of topographical feature by using PRISM stereo pair data, high precise DSM which each mesh is indicated with 2.5m size and the height in every meter was produced. 2.2. Extraction and classification of gravel bars According to the interpretation of gravel bar distribution using the pan-sharpened image derived from Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) and PRISM data, eighteen object areas along the most eastern and south-eastern shore of Balkhash Lake where the distribution was clear were

firstly selected as shown with A to R in Figure 2. High precision DSM image was produced using PRISM data in every area. Next, extraction analysis of ridge lines of gravel bar and creation of about 50 transverse topographic section showing the topographical feature height which is extend towards inland from the shore were carried out by the analysis of DSM data in each area. Based on the analyzed image of ridge line of gravel bar and the relative height transverse cross section, the distribution characteristics of height, number and ridge line direction of gravel bar was made into a table. Thus, the classification of the distribution group of gravel bar was carried out by dividing with the application of some threshold values to the relative height and the number of the ridge line. Finally, the formative age and environment of gravel bar in the classified group, and environmental change over the past about 26,000 years of this area were described.

Figure ２. Analysis areas along shore of Balkhash Lake 3. Analysis results 3.1. Classification of gravel bar distribution

The result of making a list of the characteristics of the gravel bar based on the comparison of extraction and profile images is shown in Table 1. The distribution pattern of the gravel bar can be classified into three typical groups. The distribution feature and schematic image of three kinds of classification groups are shown as (1) to (3) in Table 2 and Figure 3, respectively, and the classification group number of each object area is also described in Table 1. ALOS pan-sharpened and PRISM DSM image covering the typical area (area N, K and Q) classified into each group are shown in Figure 4 - 6. The area of gravel bar distribution is shown by the dashed line in each pan-sharpened image.

The characteristic of heights of gravel bars classified into three groups shows that the lower ranges 1 to 3 m, middle ranges 4 to 7 m, and higher ranges 8 to 10 m. Since the freshness of the gravel bars of the

lower group has been investigated by field survey, it was presumed that these ones had been formed within past about 100 years.

Table 1. Characteristics and classified groups

of gravel bar distribution

Area and line

Shore direction

Number of gravel bar Group Lower

(1-3m) Middle

(4-7m)

Higher (8-10m)

A East 3 (1) B South 3 (1) C East 3 (1) D West 1 1 (1) E South 1 1 (2) F South 1 1 (1) G East 2 (1)

H-1 West 3 1 (2)

H-2 South-west 3 2 (3)

J East 1 3 1 (2)

K-1 South-west 2 1 (2)

K-2 East 1 1 3 (2)

L-1 North-east 2 2 (2)

L-2 South-east 2 (1)

M East 1 4 (2) N-1 South 3 3 (2)

N-2 South-west 1 1 1 (1)

O North-east 1 1 (2)

P North-east 1 (1)

Q West 4 3 (3) R West 3 (1)

Table 2. Characteristic of gravel bar distribution for every classification group

Group Characteristics

(1) Distribution of several lower gravel bars and a few higher ones without middle one

(2) Middle and higher gravel bars are distributed in the stepwise from the lower one along the shoreline toward inland.

(3) Distribution of the higher gravel bar heighboring on the shoreline, and distribution of several lower gravel bars by the side of the inland

The distribution feature of the classification group

of gravel bar depends on the difference in the direction which the shore faces. That is, most lake shore areas which faced the south of the central part from the

western part along the north side of Balkhash Lake had group (1). On the other hand, it was shown that most lake shore areas facing west had group (3), and most areas facing the east had group (2) in the eastern part of the lake. Moreover, the gravel bar along the shore faced west in the northern shore of the eastern part of the lake has a tendency of higher with steep slope, and the one along the shoreline faced east in the same shore has a tendency of lower and gentle slope. 3.2. Formative environment of gravel bar It was considered that the difference in distribution feature of gravel bars was caused by the excellence direction of the wind and the waves rather than due to the topographical features such as slop and roughness before the lake shore formation. That is, the strong prevailing wind from the direction of northeast and southwest with northwest blows in the region of Balkhash Lake [6]. It can be presumed that the gravel bar in recent years was formed along the western and central shore due to a little steep slopes or terraces at the back of the north side in Balkhash Lake. Since the shore which faced east in the eastern part of the north side of the lake is hardly eroded by waves because of the high probability of the shore distribution behind windward north or northwest land topography, it was presumed that several gravel bars from lower to higher were left in turn. It was also presumed that only higher gravel bar remained without lower gravel bar remaining by the erosion based on the waves by strong westerly wind in the shore which faced west in the eastern part of the north and south side of the lake. According to the report that the water level of about 26,000 years ago was higher than the present one about 10 to 13 m and the change in the past 120 years was within about 3 m, it was considered that the gravel bar of 1 to 3 m height was formed within the past 120 years, and the gravel bar of 5 to 7 m height was based on the water level change during past thousands to hundreds years. Furthermore, this study result suggested that the water level of Balkhash Lake was in a drawdown tendency, with repeating the rise and decline, in the past about 26,000 years 4. Conclusion

The conclusion of this study is summarized as

follows. 1) By the extraction analysis of the gravel bar using DSM produced from ALOS/PRISM stereo pair data, the distribution pattern of the gravel bars along the Balkhash Lake shore was classified into three groups.

The waves by the strong prevailing wind which had affected the distribution pattern of the several directions of ridge lines of gravel bar were presumed. 2) According to the height of the gravel bar for every classified group, the past water level fluctuation of Balkhash Lake was divided into three stages, such as the lower level 1 to 3m, the middle level 4 to 7m and the higher level 8 to 10m. It were presumed that lower one was newly formed in the period from about 1910 to around 1970, and the water level showed a drawdown tendency over the past about 26,000 years.

Group (1)

Group (2) Group (3) Figure 3. Schematic images of three groups of gravel bar distribution

Figure 4. Pan-sharpen and DSM images of area N in group (1)

Figure 5. Pan-sharpen and DSM images of area K in group (2)

Figure 6. Pan-sharpen and DSM images of area Q in group (3)

Acknowledgment

This study was carried out a part of the project “Historical Interactions between Multi-cultural Societies and the Natural Environment in a Semi-arid Region in Central Eurasia” of the Research Institute for Humanity and Nature. Authors express their great gratitude to Prof. Jumpei Kubota.

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