Numerical Study of River Bed Morphology Based on Water Level Management on the Upstream of Dalsung Weir in Nakdong River

Abstract

After Four Major Rivers Restoration Project, several new weirs have been built on the Han, Nakdong, Geum and Yeongsan Rivers. As a result, river environment has been effected. Unfortunately, there are a few studies on bed morphology due to the weir operations. One of the function of multipurpose weirs is to control the water depth and discharge. Bed morphology in rivers is driven by several parameters, such as water depth, velocity, discharge, water surface slope, geometry, and river bed materials, etc., and which are interacted each other. Therefore, different hydraulic conditions will lead to different bed evolutions. The objective of this study is to apply an open source CFD software package, the TELEMAC2D, to simulate hydrodynamics, sediment transport and bed morphology on a stretch of Nakdong River located between two weirs, Gangjeong and Dalsung Weirs, whose length is around 20 km. The real bathymetry of the river stretch have been implemented in the numerical model. Numerical results have been validated against available field observation data, and thereafter the numerical model can be applied to answer the what-if questions following the practical water level management. The critical change in bed morphology occurs at the conjunction of Nakdong and Keumho Rivers, and at the meanders. The impact of water level management to bed evolution is intensively analysed from several practical management scenarios. Higher bed evolution occurs once lower water level scenario is applied to the upstream of Dalsung Weir. The study on the effect of sediment gradation shows that the fine and very fine sands have been easily transferred to the downstream than other grade of the sediments. Consequently, these ranges of sands are accumulated more on the upstream of Dalsung Weir. To operate Dalsung Weir, maintaining water surface elevation at EL. 14 m seems to be effective to prevent significant bed evolution, which has been shown from the simulations of several management scenarios.