Fingering in unsaturated zone flow: a qualitative review with laboratory experiments on heterogeneous systems

Abstract

Unstable unsaturated zone flow (fingering) is a potentially important process in recharge, pollution, and surface water/ground water body interactions. Extending previous workers ' studies on homogeneous systems, sand tank experiments have been carried out on heterogeneous systems. The experiments on initially dry silica sands suggest that (1) stratification will tend to enhance rather than dissipate fingering; (2) in discontinuously layered systems, funnelling influences the location of fingers; (3) in multilayered systems, lateral flow on top of fine-grained layers promotes greater flux (and more fingers) in the down-dip direction; (4) in systems where a top fine-grained layer has a variable thickness, finger frequency and, hence, amount of flow will be greatest where the fine-grained layer is thinnest; (5) surface depressions in an upper fine-grained layer will concentrate flow, with fingers forming below such areas; and (6) in systems where an upper fine-grained layer has macro pores, the latter will concentrate water flow and fingers will form directly below these zones. The experiments also confirmed that fingers can persist in the same locations from one recharge event to another, and that in initially moist sands, fingers are widened. It is clear that a complex interplay between fingering and funnelling processes can occur and that finger behaviour is sensitive to heterogeneity.