To consider effective measures using water duct for reducing anoxia in coastal dreaged trenches, we first developed a water quality model including dissolved oxygen and sufide processes copuled with hydrodynamic model using MIKE3 and ECO Lab (DHI). The model was applied to reproduce water quality processes in dredged trenches with one-way nesting approach and verified through comparison with measured data. Peforming numerical experiments installing water duct and transporting oxygen rich water, we showed that transport of surface water with less dense is effective and more practical for the present purpose.