The effect of infiltration flux on air counterflow in a 2-D confined sand chamber

The phenomenon of air counterflow during intense rainfall is vital to understanding and modeling the infiltration processes, but up to now, the effect of infiltration flux on air counterflow pattern has been seldom studied. In this study, experiments were carried out to investigate the effect of infiltration flux on air counterflow in a 2-dimensional (2-D) confined sand chamber. The air saturation distribution was monitored using light transmission and the air pressure was measured by manometers. During the experiments, as infiltration flux increased to 1000 mL/min and 1100 mL/min, significant increase in air pressure was observed. Due to the increase of air pressure, water infiltration was retarded, leading to a significant decrease in air outflow rate at high infiltration flux. When the infiltration flux was larger than 800 mL/min, the air flowed out of the sand chamber in the form of air "fingers". The underlying process of the finger-like air outflow pattern was analyzed quantitatively, and the critical infiltration flux leading to air "fingers" was estimated based on the relative air/water permeability of the working sand. The Green-Ampt model showed that the infiltration process was retarded by air counterflow when the air pressure head was comparable to the depth of the wetting front at high infiltration flux.