Using remotely sensed data to detect changes of riverbank in Mekong River, Vietnam

Pham Bach Viet, Lam Dao Nguyen and Ho Dinh Duan
Information and Remote Sensing Division -
Institute of Physics, Hochiminh City
Email: vientham@hcm.vnn.vn

Introduction
Traditional methodologies in 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). The common feature to 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 in 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 250 km of the territory to end at the Eastern Sea with the two primary branches, Tien and Hau river (fig.1). Mekong River keeps a significant role in this area on domestic water supply, transportation, irrigation, drainage, aquatic resources and others. Many villages and towns are located along the 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 sediment, especially there was a hidden fault at Mekong river sank under the delta from Tonle Sap lake to river mouth area; this fault also is 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 soil texture of 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 in flood period while during dry season it is about 2,000 m³/s⁵).

A system of canals constructed for the primary goal of navigation. By time, till late 1980s and middle 1990s, a network of canals has been created for both navigation and irrigation, which divert the Mekong water for cultivation, located both in Vietnam as well in adjoining area in Cambodia. Accompanying the network, a system of dikes and irrigation gates also were 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 more frequently.


Fig.1: Mekong River
Method

1. Data used
   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 image 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)
   
   Topographic maps constructed in period 1966 - 1968 from aerial photos and geometric measurement, in 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.


Table 1 List of data used


No	Date	Type	Format	Images	Bands	Resolution (m)	Notes
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
11	1966-1968	Vector	--	Topographic maps		1/50,000	UTM



2. Data processing
   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 method 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 land - water, difference of those make 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.



3. Change identification
   Changes of 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 cloudy, 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

1. Results
   Results of analysis and interpretation of time series data were compared to each other indicating spatial changes of shorelines. There are two main parts of changes as the following.
   
   Shoreline erosion mainly occurred in Tien River from Tan Chau area to lower stream My Thuan, while in Hau river the phenomenon was less severe from Can Tho upwards Chau Doc.
   
   
   1. Tien River
      Great changes are distributed in Tien river where there are many meander bends making river flow direction changed continuously (figure2), extended from 4 to 10 km in length, 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
      	Hong Ngu	8	100
      	An Phong	4	120
      	Tan Thanh	4	130
      	My Xuong	9	250
      	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
      Sa Dec	10	1,100
      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 period 1966 - 1990, in Thuong Thoi Tien - Long Son area (Phu Chau - Tan Chau), erosion at left bank and accretion at right bank made the river channel almost shifted leftward and narrowed. Flow direction 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 Dec in about 20 km. The river was narrower and new "bottle necks" appeared in Sa Dec, Tan Qui Dong, My Thuan (fig. 3). From 1995 to 1999, the situation was more complex. Bottle-necks changed in both their upper and lower parts, while relatively stable banks in previous period began to be affected by the changes.
   
   
   
   Fig.2 Flow directions of Tien river
   
   2. Hau River
      Here 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. 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, stream in An Chau area became more straight because of erosion and accretion in both sides of riverbank, making the channel sifted leftward. At contiguous lower stream, the bend at Ong Ho islet (figure 3) was expanded as deposition at the slipoff slope side.
   
   
   
   3. Aits (islet)
      Aits on Mekong River almost are natural bars or dunes. Naturally, their occurrence and distribution depend on river discharge and transported matter in stream (suspended load). Analyzing images also indicated spatial change of aits in shape resulting from erosion and accretion at both tips of them. This made a shift almost down stream. In some areas, these changes made streams narrower.


2. 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

1. 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).



2. 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).



3. 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.



4. Lap, V.T.: Physical Geography of Vietnam, Vol. 1, Education Publisher, Hanoi,Vietnam, 1978. (In Vietnamese: Dia ly tu nhien Viet Nam).



5. 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).