Funnel flow occurs when sloping geological layers cause pore water to flow laterally, accumulating at a low region. If the underlying region is coarser, finger flow may also occur.

The way soil is layered makes a big difference in how water and solutes find paths down to ground water. The way these flow paths merge depends on inhomogeneities in the soil, and sloping structural interfaces have a considerable effect on the degree of merging and rate of flow. Soil deposited by ocean currents, rivers, lakes, or glaciers consist of strata laid down at different times and under different circumstances, and they differ in composition and permeability. Various experiments have shown that at low flow rates, coarse layers act as funnels, collecting water from a broad area and channeling it through a small number of drainage fingers. At higher flow rates, these coarse layers begin to leak, and the funnel effect become less significant (Figure 1).

Figure 1. The funnel effect in sandy soil. With a relatively low flow rate (a), a fast-moving spout forms beneath the lower end of an inclined layer. Such inequalities in penetration are minimized when the flow is greater (b).

Wetting front instabilities can also occur when there is a distinct difference in soil layers. Sloping interfaces of texturally different layers may act as funnels to concentrate flow in a prescribed direction. This phenomenon may also be observed in a laboratory using video-digitization techniques and computer-enhanced coloring (Figure 2). For example, in Figure 2, a uniform layer of coarse sand is surrounded by fine sand in a thin soil column encased between two glass plates. The soil column is backlit and a video camera records the image. Since the course sand has a large pore-size, it transmits more light. As moisture is applied to the surface, the wetter soil transmits even more light.

Figure 2. Textural interfaces and sub-surface layers may cause moisture and solutes to preferentially flow in a prescribed direction.

One might expect that soil water in contact with the coarse layer might travel straight through the layer. Surprisingly however, the movie segment illustrates its quite different behavior. Under unsaturated flow conditions, greater pressure is required to push moisture into a large pore from the small pores. Therefore, moisture tends to flow over the layer rather than through it. At low flow rates, the horizontal component exceeds the vertical component until the edge of the layer is reached where moisture then funnels through. Since moisture is forced to take this narrow route, the concentration of moisture and solutes may once again bypass much of the soil matrix and be directed to the groundwater. The diversion of flow caused by layering is significant because even in areas where groundwater monitoring is conducted routinely; pollution may be missed entirely if the well is placed in the wrong location. In order to prevent groundwater contamination it is therefore desirable to know where the layers and funnels are.

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