Pham Bach Viet
Information and Remote Sensing Division,
Institute of Physics, Hochiminh City, Vietnam
Lam Dao Nguyen
Information and Remote Sensing Division,
Institute of Physics, Hochiminh City, Vietnam
Ho Dinh Duan
Asian Institute of Technology, Thailand
Traditional methodologies in the study of riverbank change require conventional surveys, repeated measurements to identify and to evaluate changes. Hydrology, geomorphology and geology make use of data obtained from their surveys as input in their mathematical modeling. Recent studies on Mekong River have been focused on erosion processes of shorelines at hot spots1). A common feature of all these studies is that they are localized in extent. Remote sensing techniques offer another approach to this issue – the use of satellite imagery combined with other digital data to extract information and derive certain measurements, as in an assessment of channel migration of Thu Bon River using scanned data- aerial photos and satellite imagery2). A typical study of channel migration of Yellow River (China) made use both analog and digital data with a time sequential imageries of 19 dates from 1976 to 19943).
This paper presents an application of time-series satellite digital data of different sources composed of optical and radar imageries in shoreline change detection and to demonstrate a capability of remotely sensed data with digital processing and GIS analysis for river studies in a large area.
Mekong River Circumstances
Derived from Tibet, Mekong River reaches Vietnam-Cambodia border at its last lower part and passes through 250 km of the territory to end at the Eastern Sea with its two primary branches, Tien and Hau river (fig.1). Mekong River plays a significant role in this area on domestic water supply, transportation, irrigation, drainage, aquatic resources and others. Many villages and towns are located along its streams and about 50% of the Mekong delta population lives totally on Mekong River.
Geologically, Mekong delta is of a typical formation of Quaternary and recent sediments, especially there was a hidden fault at Mekong river bank under the delta from Tonle Sap lake to river mouth area; this fault represents the border area between the ancient orogenenic stages of Tectonic phases4). These conditions make the river flow in a northwest – southeast direction from the mainland to the sea and the soil texture of the riverbank is unstable. Average discharge of Mekong River is 15,000 m3/s (at Kratie station), maximum discharge can reach over 60,000 m3/s during periods of flood while during dry season it is about 2,000 m3/s5).
A system of canals was constructed for the primary goal of navigation. Since the late 1980s and middle 1990s, a network of canals has been created for both
navigation and irrigation, which divert the Mekong water for cultivation in Vietnam as well in the adjoining area of Cambodia. Accompanying the network, a system of dikes and irrigation gates were also constructed for preventing saltwater intrusion in estuarine area and along the coastal zone. Under these circumstances, and together with the high fluctuation range of river discharge, erosions and accretions along the Mekong River have occurred rather frequently.
There were 10 digital images of two types – optical and radar, consisting of 7 dates during the period 1989 – 1999. Optical images include Messr (MOS-1b, Japan), Landsat TM, both with high quality (low cloud). Messr images with 4 bands cover only 80 km x 80 km, thus the two adjoining scenes of 1989 and 1990 were used as one date. Radar images of ERS-2 in September and October 1999 were also analyzed as one date because its swath width is just 100 km x 100 km. Landsat TM (180km) and Radarsat (170 km in extended mode) images cover almost the whole Mekong River (of Vietnam territory). (Table 1)
Table 1 List of data used
|1||04.12.1989||Optic||Digital||Messr (MOS-1b)||4||50 x 50||1 date|
|2||24.01.1990||Optic||—||Messr (MOS-1b)||4||50 x 50|
|3||13.03.1995||Radar||—||Radarsat||1||25 x 25||C – 5.6cm|
|4||21.02.1996||Optic||—||Landsat TM||3||30 x 30|
|5||01.10.1999||Radar||—||ERS-2||1||30 x 30||C – 5.7cm1 date|
|6||12.09.1999||Radar||—||ERS-2||30 x 30|
|8||12.04.2000||Radar||—||Radarsat||1||25 x 25||1 date|
|9||24.11.2000||Radar||—||Radarsat||1||25 x 25|
|10||06.09.2001||Optic||—||Landsat 7-ETM||Pan||15 x 15|
Topographic maps constructed in the period 1966 – 1968 from aerial photos and geometric measurement, in the scale of 1: 50,000 and projection UTM, were used as a baseline data source in this study because of their precision. Shorelines of these maps were taken to be an initial datum for change detection analysis.
All images were geo-rectified to topographic maps of UTM before processing, interpreting and analyzing. After Landsat image had been registered to UTM projection, Messr images were resized accordingly for geo-rectification.
In order to extract shorelines from images, each type of imageries was processed in different methods based on the level of distinction between water – land and soil – vegetation that the image can reveal. In order to distinguish shoreline or riverbank objects on optical images, NDVI (normalized difference vegetation index) was computed, taken from the formula of [Infra Red – Red] / [Infra Red + Red]. For Landsat images, spectral bands 4 and 3 are used, and for Messr they are band 4 and band 2. Radar images were analyzed on their grayscale texture to create a set of descriptor images. Shoreline is the border between two objects say land and water, the difference in them makes a slight change of backscatter reflectance of radar signals. This also was the process of discriminating land from water, wet land with or without vegetation cover.
Changes in riverbank were directly traced out by comparing the two or three images in pairs. Results of interpretation were transformed into GIS layers by years, in vector format. Shorelines were re-corrected at segments, as they have been mis-interpreted because wetland areas are located next to the bank or optical images are cloudy or due to speckle noise of radar images.
Change of river segments was detected by superimposing data layers together by the order of raster – vector or vector – vector. Erosion and accretion on river were located and an estimation was made with the aid of GIS.
Results and Discussions
Results of analysis and interpretation of time series data were compared to each other indicating spatial changes in shorelines. There are two main parts of changes:
(i) shoreline erosion mainly occurred in Tien River from Tan Chau area to lower stream My Thuan, (ii) and in Hau river the phenomenon was less severe from Can Tho upwards Chau Doc.
Fig.2: Flow direction of Tien River
Great changes are distributed in Tien river where there are many meandering bends making river flow in continuously changed directions (figure2), extending from 4 to 10 km in length. It has eroded into land from 100 to 1,000 meters. The eroded areas are presented in table 2.
Table 2 Location of erosion areas in Mekong River within period 1966 – 1999
|River branch||Areas||Length (km)||Width (m)|
|Tien river- Left bank||Thuong Phuoc-Thuong Thoi Tien||6||1,000|
|Chau Thanh-Sa Dec-My Thuan||6||100-350|
|Cho Lach-Ben Tre||3.5-4.5||250-400|
|– Right bank||My Luong-Long Dien||4||120|
|Hau river- Left bank||Nhon Hoa-An Chau||4.5||200-800|
|– Right bank||An Chau-Long Xuyen||2.6||100|
|Binh Thuy-Can Tho||2.8||150|
Within the period 1966 – 1990, in Thuong Thoi Tien – Long Son area (Phu Chau – Tan Chau), erosion at left bank and accretion at right bank have made the river channel almost to shift leftward and it has also narrowed. Flow direction has also changed. Gentle bends tend to be more meandered before flowing into Tan Chau area. Flow in Lower Tan Chau area (Thuong Thoi Tien – Hong Ngu) kept changing from 1990 to 1995 and 1999. Similarly, in Sa Dec area this phenomenon also occurred and spread from My Thuan upward Tan Qui Dong-Sa December for about 20 km. The river was narrower and new “bottlenecks” appeared in Sa December, Tan Qui Dong, My Thuan (fig. 3). From 1995 to 1999, the situation was more complex. Bottlenecks changed in both their upper and lower parts, while banks which were relatively stable in the previous period began to be affected by the changes.
In Hau river changed areas are not as large as in Tien River. They extend from 2.5 to 4.5 km and 100 to 1,000 meters in length and width respectively. Narrow river segments were eroded in upper part and accreted in lower part as in Long Xuyen and Can Tho area (Binh Thuy – Phu An). Especially, the stream in An Chau area became more straight because of erosion and accretion in both sides of riverbank, making the channel shift leftward. At contiguous lower stream, the bend at Ong Ho islet (figure 3) was expanded as deposition at the slipoff slope side.
Fig. 3 Change in Tan Chau and Sa Dec-My Thuan area
Fig. 4 Change in Long Xuyen area
Aits on Mekong River are natural bars or dunes. Naturally, their occurrence and distribution depend on river discharge and transported matter in stream (suspended load). Analysis of images also indicated spatial change of aits in shape resulting from erosion and accretion at both of their tips. This almost made a shift down stream. In some areas, these changes made the streams narrower.
In Sa Dec-My Thuan area, the trend of growing new meanders could be the result of erosion on the undercut bank of Sa Dec and accretion on the slipoff slope bank of Dong Thap (right and left bank of Mekong River, respectively). A reverse process happened in lower part of My Thuan area.
Within the period 1966 – 1990, changes were not actually analyzed because images were not available images, especially for 1975 – 1988. Thus, the time of commencement of could not be well-recognized. It is of considerable importance to the study, for example, the time when abnormal changes started, and this could impact other changes on the entire river, particularly its upper part.
Resolution of the available satellite images also affects detailed analysis. Recent changes of riverbanks with low intensity (say, less than 30 m-the pixel size of used Landsat images) were not detectable. Optical images were mis-interpreted in lower river where wetlands and marsh objects are present. There, shorelines were difficult to identify and it would be more complex when acquired images were covered with cloud. Radarsat data showed relatively clear shorelines in terms of the texture analyzed whilst ERS 2 satisfied just 60-70%. This could be due to weather conditions (rains, winds), which made the water surface rougher and consequently noise of backscatter radiation was encountered. In both types of data, identifying shorelines in river mouth area and coastal lines was inevitably difficult. A possible reason was that high amplitude of tides in the Eastern Sea and the time of data acquisition does not correspond to the tide extremes.
Results derived from satellite data analysis were compared to previous traditional studies and the results were similar. Hot spots in Tien River namely Tan Chau, Hong Ngu, Sa Dec-My Thuan and in Hau River namely Chau Phu, Long Xuyen, Can Tho, were identified. River bank change has mainly taken place on Tien River. Meander bends in both Tien and Hau rivers suffer from the process of erosion and accretion, which resulted from the shape of the channel and the intensive flow of water.
Use of time-series data showed a continuous change, which was relevant to other factors, such as hydrological regime, weather conditions, watershed management and infrastructures along the riverbank.
Remote sensing techniques provide a useful tool and satellite data give an objective view when they are applied on a large scale. It allows a synoptic viewing for predicting changes in a large region. In addition, if there is a combination between traditional methods and this approach, a detailed prediction for local scale can made available. Moreover, high and very high resolution data and more frequent dates of data acquisition, which are quite feasible today, would remarkably support this approach for monitoring and prediction river-bank changes in conditions obtaning in Vietnam. This would help to plan proper uses of land resources for long term and to prevent could-be-avoided damages in short-term.
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- Southern Sub-Institute of Irrigation Survey and Planning,: Building baseline data for flood modeling of Mekong delta. State project. Hochiminh city, Vietnam, 1999. (In Vietnamese: Nghien Cuu Khoa Hoc Cap Nha Nuoc, Xay Dung Co So So Lieu Thong Nhat Cho Mo Hinh Toan Tinh Lu Dong Bang Song Cuu Long, Bao Cao Tong Ket).