Erosion Processes in Gullies Modified by Establishing Grass Hedges

Concentrated flow can cause gully formation on sloping lands and in riparian zones of floodplains adjacent toincising stream channels. Current practice for riparian gully control involves blocking the gully with an earthen embankmentand installing a pipe outlet. Measures involving native vegetation would be more attractive for habitat recovery and economicreasons. To test the hypothesis that switchgrass (Panicum virgatum L.) hedges planted at 0.5 m vertical intervals within a gullywould control erosion, we established a series of hedges in several concentrated flow channels. Two of the channels werepreviously eroded trapezoidal channels cut into compacted fill in an outdoor laboratory. The other channels were locatedat the margin of floodplain fields adjacent to an incised stream channel (Little Topashaw Creek) in Chickasaw County,Mississippi. While vegetation was dormant following two growing seasons, we created artificial runoff events in our testgullies using synthetic trapezoidal-shaped hydrographs with peak discharge rates of approximately 0.03, 0.07, and 0.16 m3s-1, flow rates similar to those observed during natural runoff events in gullies at Topashaw. During these tests, we monitoredflow depth, velocity, turbidity, and soil pore water pressures. Flow depths were generally <0.3 m, and flow velocities variedspatially and exceeded 2.0 m s-1 at the steepest points in some tests. Erosion rates remained modest for the conditions tested,as long as slopes were less than 3 horizontal to 1 vertical (33%) and step height between hedges was less than 0.5 m. Stabilitymodeling of soil steps reinforced with switchgrass roots showed that cohesive forces were 3 times greater than shearing forcesfor 0.5 m step heights, and that therefore mass failure was unlikely even with the surcharge weight of a 0.2 m depth of pondedwater. For step heights greater than 1 m, however, mass failure was observed and predicted to be the dominant erosionmechanism.