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
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