Based　on　the　explicit　finite　element　schemes　for　analyzing　dynamic　response　of　solid,　porous　media,　and　fluid,　the　effects　of　sediment　on　the　dynamic　pressures　acting　on　rigid　dam　due　to　horizontal　ground　motion　are　studied.　In　this　investigation,　sediment　is　firstly　modeled　as　porous　media　and　the　effects　of　its　saturation,　thickness,　permeability　and　porosity　on　dynamic　pressures　are　analyzed;　and　then　sediment　is　modeled　as　one-phase　solid　and　one-phase　fluid　respectively　and　the　computing　results　are　compared　with　those　obtained　by　modeling　sediment　as　porous　media.