<p>Vacuum-aided vertical drain is an established dewatering method. The vacuum pressure exerts a force inside the drain, lifting the groundwater and lowering the groundwater table. Along with the inflowing groundwater, a volume of air often gathers towards the drain and forms an air–water mixture transported in the drainpipe. The presence of air complicates the pressures in the conduit and therefore affects the dewatering efficiency. This paper presented a numerical model for orifice-induced air–water flow in vertical drains. Using the mass conservation, the model can calculate pipe pressure distributions and water discharge capacity. The model was validated against the results obtained from the computational fluid dynamics calculations. The proposed model was applied to the example problems to examine the effects of orifices on the pipe pressure distributions, water discharge rates, and lift depths. Nonlinear pressure distributions occurred in the drainpipe, and pressure dropped more noticeably in the presence of air. More water was discharged when the orifice was located closer to the lower end than to the upper end. Under one standard atmosphere vacuum pressure the lift depth was approximately 6.3–7.5 m depending on the orifice size.</p>