Funneled flow mechanisms in a sloping layered soil: Laboratory investigation

Artificial capillary barriers are being used to divert water away from sens itive underground regions. Conversely, funneled flow over natural capillary barriers may increase the danger of groundwater contamination by decreasin g the travel time and contact area. There have been relatively few experime ntal studies of capillary barrier flow patterns. In this study, water was a pplied uniformly across the top surface of a backlit tilting chamber, 1 cm thick, 110 cm high, and 180 cm long, in which a coarse sand layer was imbed ded in a fine sand. Bedding slope and water application rates were varied b etween 0 degrees and 12 degrees and 1 and 3 cm h(-1), respectively. After a ttaining steady state, matric potential was measured along the textural int erface, and photos of dye traces were taken in order to visualize streamlin es. The funneled flow was characterized by three discrete regions: an initi al capillary diversion, a breakthrough region, and a toe diversion. The bre akthrough region consisted of a significant zone of partial breakthrough wh ere the vertical flux into the coarse layer was less than the water applica tion rate. The lateral distance of the capillary diversion was explained we ll by previously published relationships when the water entry value at the textural interface was replaced by lower, observed matric potential at whic h breakthrough occurred at the most upslope point. The length of the capill ary diversion was overpredicted using the air entry value. Finally, the toe of the coarse layer had significant, observed effects on funneled flow pat terns, which have previously received little, if any, attention. The result s of this study imply that the slope of the coarse layer and infiltration r ate will largely govern the effectiveness of capillary barriers and that ca pillary barriers are less effective than previously assumed.