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Using remotely sensed data to detect changes of riverbank in Mekong River, Vietnam
- Discussion
In Sa Dec-My Thuan area, a trend of growing a new meander could be resulted from 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 period 1966 - 1990, changes were not actually analyzed because there were not available images, especially in 1975 - 1988. Thus, the time at which change commenced was not recognized well. It is of considerable importance to study for example, when abnormal changes started, and this could impact other changes on the whole river, particularly in the upper part.
Resolution of the available satellite images also affects the detailed analysis. Recent changes of riverbanks with low intensity (say, less than 30 m-the pixel size of used Landsat images) were not able to detect. Optical images were mis-interpreted in lower river where there are wetland and marsh objects presented. There, shorelines were difficult to identify and it would be more complex when images were covered with cloud as acquired. Radarsat data showed relatively clear shorelines as its texture analyzed whilst ERS 2 satisfied just 60-70%. This could be due to weather conditions (rains, winds), which made the water surface become rougher; consequently noise of backscatter radiation was encountered. In both types of data, identifying shorelines in river mouth area and coastal lines were inevitably difficult. A possible reason is that high amplitude of tides in the Eastern Sea and the time data acquisition does not correspond to the tide extremes.
Fig.3 Change in Tan Chau and Sa Dec-My Thuan area
Fig.4 Change in Long Xuyen area
Conclusion
Results derived from satellite data analysis were compared to previous traditional studies and this showed a similar result. Hot spots in Tien River as Tan Chau, Hong Ngu, Sa Dec-My Thuan and in Hau River as Chau Phu, Long Xuyen, Can Tho were identified. River bank change is mainly happened 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 water flow.
Use of time-series data will show a continuous change, which is 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 gives an objective view when they are applied at large scale. It allows a synoptic viewing to predict changes in large region. In addition, if there is a combination between traditional methods and this approach, a detailed prediction for local scale will be 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 change in conditions of Vietnam. This would help to plan proper uses of land resources for long term and to prevent could-be-avoided damages in short term.
References
- Southern Institute of Water Resource Research - River Training Center: Study on predictive shoreline erosion of Mekong river, 2001. (in Vietnamese: Nghien cuu du bao phong chong xoi lo bo song Cuu Long).
- Remote sensing and Geomatic Center: Study of channel migration in Thu Bon River using remotely sensed data, Ha Noi, 1999. (In Vietnamese: Danh gia tinh hinh bien dong long dan song Thu Bon qua cac tu lieu vien tham-giaidoan 1965-1996).
- Yang, X., Damen M.C.J. and Zuidam R.A.van: Satellite remote sensing and GIS for the analysis of chanel migration changes in the active Yellow river Delta, China, Int 'l. J. of Applied Earth Observation and Geoinformation, Vol. 1, Issue 2, pp. 146-157, 1999.
- Lap, V.T.: Physical Geography of Vietnam, Vol. 1, Education Publisher, Hanoi,Vietnam, 1978. (In Vietnamese: Dia ly tu nhien Viet Nam).
- 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).
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