Seepage of bank slope due to water level drawdown is a process of water-air two-phase seepage; in which the saturated soil transforms unsaturated soil. At present

in the relevant studies the air phase is considered as a passive phase

the influence of pore air is ignored. In this paper

a water-air two-phase flow model is proposed based on the water

air mass balance equations

Darcy’s law and multiphase flow theory. Using the efficient integrated finite difference method

a fully implicit simultaneous computation code is developed. By switching primary variables according to different phase states

a set of phase states (single phase and two-phase) which is variable with space and time are taken into account. The water-air two-phase flow model is verified by simulating water through an earth dam with vertical faces known as Muskat problem. The unsteady seepage problem of bank slope due to water level drawdown is simulated numerically. The results indicate that above the free table the value of capillary pressure is much smaller than the value of pore water pressure; there is a saturated capillary water belt with relative large area

in which the pore of soil is filled with water and the matric suction is zero. So the saturated capillary water belt may endanger slope stability; and in refined analysis of slope stability

the influence of pore air pressure should be taken into account.