A model study to investigate the sediment transport and change in surface sediments at the seabed is conducted for marine sand mining operation in Kyunggi Bay, Korea. A new methodology established through field experiment is proposed and applied in numerical simulation to predict the redistributionof sediments deposited away from the dredging site. Differentiation of sediment composition into each grain size group yields the different transport behavior in nature and in numerical simulation. A three dimensional numerical model, ROMS (Regional Ocean Modeling System) is used for sediment transport modeling using realistic surface and tidal forcing. The marine sand mining intensity is imposed on the modeling along with empirical parameters estimated from the field experiment.In the current study, a new procedure of estimating the suspended sediment source from the dredger has been suggested based on numerous field surveys, especially employing discrimination of size class of composed sediments.The present study improves our ability to predict the reworking processes of composed sediments by employing new concept of size-dependent behavior. However it is still open to further work to include the resuspension process for more quantitative prediction of seabed change arising from long-term dredging of marine sand in shallow waters as in Kyunggi Bay.