This paper presents the results of a computational fluid dynamics (CFD) model study of the hydraulics of groundwater flow and porous pipe underdrains. The study was conducted using a three-dimensional CFD model built in ANSYS Fluent. The model was validated by replicating previous experimental results of saturated subsurface flow (water surface level above the aggregate) for a 10.2-cm perforated pipe shrouded in loose laid aggregate. The CFD model consistently overpredicted the flow rate for a given head and aggregate depth by an average of 11%. After considering the effect of pipe perforation blockage due to aggregate, the average overprediction decreased to only 6%. The discharge coefficient for the perforated pipe computed using the CFD model was 0.54 compared with 0.49 from experiments. It was also found that the discharge was quite small at the upstream end of the pipe, with the bulk of the water entering the pipe in the vicinity of the outlet. Finally, the computational results showed that, for saturated flow conditions, the flow was predominantly in the vertical direction within the aggregate whereas it was mainly horizontal when the water surface level was below the top of the aggregate level (unsaturated condition). The losses associated with the two flow scenarios were explored. The results obtained have practical applications in the design and analysis of porous pipe underdrains.