MONITORING OF RIVER-BED CHANGE BASED ON EROSION AND SEDIMENTATION IN THE DRAINAGE BASIN

Monitoring of erosion and sedimentation in river-beds is important for detecting potential flood hazard and to adequately plan fluvial channel management. Erosion potential of river subbasins can be monitored to quantify potential sediment flux. The Universal Soil Loss Equation (USLE), aided by a Geographic Information System (GIS), was used to compute soil erosion rates in several drainage sub-basins. River-bed profiles were monitored by aerial photography, annual satellite images, echo-sounding of bottom profiles, and geodetic measurement of levees and floodplains. The Keum River basin of Korea was used as a pilot study for monitoring of river-bed change. The USLE-based erosion potential, indicates a mean soil erosion rate of 1.8 kg/m/y, a rate high enough to cause river-bed rise or fall. Detection of river-bed change was performed by using LANDSAT TM images from 1982 to 2000. Geometric correction, image stretching, and image classification were employed to improve image quality. Image processing shows that the Exposed Area of Sediment Distribution (EASD) decreased from 1991 to 1995, then increased after 1995. Change mainly occurred downstream of the confluence of tributaries with the main channel. River-bed profiles were made from mid-stream toward downstream. Based on geodetic data, 3D-geomorphological maps were made for the pilot area in order to trace any change in river-bed geomorphology. The comparison of 3D-geomorphological maps between 1988 and 1999 shows a net increase of a river-bed change at a rate of +5 cm/y in the downstream area.

Natural river-bed change in the pilot study area is interpreted to be caused by high rates of soil erosion in the drainage sub-basins of the main river, particularly near tributary confluences.  However, artificial removal of fluvial sand in the mid-stream areas for aggregate resulted in the decrease of the EASD between 1991 and 1995. After 1995, the EASD conspicuously enlarged synchronously with decreased extraction of sand aggregate. In addition, construction of underwater structures including bridges, reclamation of sand bars for rice fields, and dike construction along the river and estuarine mouth resulted in a net increase of the EASD.