Abstract: The accurate evaluation of water-gas transport is important for pollution control in landfill earthen covers. With poor sealing effects, most of the existing model test instruments cannot accurately control the air pressure and simulate the boundary conditions of landfills. To solve this problem, a large-scale flume system with controllable/measurable boundary conditions is developed. The pore water pressure, pore air pressure and water content in the inclined cover layer are monitored to calculate the percolation, surface runoff and lateral drainage. Each side of the flume is sealed by squeezing the O-ring through the sleeve and inner/outer sleeve plates. After checking, the sealing devices ensure the effectiveness of the test system in evaluating the cover performances to minimize infiltration and gas emissions. The flume system is used to investigate the short-term performance of an inclined capillary barrier cover with Bermuda grass in coupled water-gas migrations under heavy rainfall. It is found that the final cumulative percolation through the cover accounts for 2.4% of the total rainfall depth. The suction in the top sand layer is higher than the breakthrough value during rainfall, indicating the effectiveness of capillary barrier. Also, the lateral drainage capacity is greatly underestimated, without considering the pore air pressure.