Persistent turbidity in reservoirs and their downstream after flood events is one of most important environmental issues in Korea. Recently, modification of withdrawal facility and installation of a new selective withdrawal structure (SWS) have been implemented for the mitigation of downstream impact and sediment loading into water treatment facilities. This study was to explore the characteristics of flood density flow induced into Soyang Reservoir and the transport processes of suspended sediments (SS) through application of coupled two-dimensional hydrodynamic and particle dynamic models (TM-1, TM-2 and TM-3). The TM-3 including a turbidity attenuation rate as a lumped parameter showed best performance in reproducing the magnitude and distribution of SS in the reservoir. The validated model was applied to evaluate the effectiveness of SWS, which was designed for the reservoir, with 6 different historical flood events. The magnitude of vertical mixing of the turbidity plume and its persistence within the reservoir were closely correlated to the ratio of the volume of turbidity flow to the total reservoir storage (the θ value). The operation of SWS showed a positive effect as long as θ is between 0.3 and 0.6 but negative when θ = 0.83 for the study reservoir, thus it should be optimized based on the θ value for a better management of the reservoir.

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