Submitted to: International Symposium on Gully Erosion Under Global Change Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2004
Publication Date: 5/1/2004
Citation: Dabney, S.M., Shields Jr, F.D., Langendoen, E.J., Temple, D.M. 2004. Grass hedge effects of gully hydraulics and erosion. International Symposium on Gully Erosion Under Global Change Proceedings. CD-ROM. Interpretive Summary: Gully erosion is a serious problems on sloping land and at the field edges of flat lands next to deep channels and ditches. We conducted an experiment to determine if a series of grass hedges could control concentrated flow erosion on steep slopes and assessed possible mechanisms of failure as a function of soil type, hedge spacing, and flow rate. Our results showed established switchgrass hedges placed with approximately 0.5 m vertical interval at slopes less than or equal to 33% steepness were effective in preventing measurable erosion of concentrated during simulated runoff events, but cumulative erosion during 5 month of natural runoff destroyed the central portions of two grass hedges on an erodible site. Our results are important to farmers and conservation planners because they demonstrate the potential and limitations of using plantings of native vegetation to stop the growth of small gullies.
Technical Abstract: Concentrated flow can cause gully formation on sloping lands and in riparian zones adjacent to incising stream channels. Current practice for riparian gully control involves blocking the gully with a structure comprised of an earthen embankment and a metal or plastic pipe. Measures involving native vegetation would be more attractive for habitat recovery and economic reasons. To test the hypothesis that switchgrass (Panicum virgatum L.) hedges planted at 0.5-m vertical intervals within a gully would control erosion, we established a series of hedges in four concentrated flow channels. Two of the channels were previously eroded trapezoidal channels cut into compacted fill in an outdoor laboratory. The other two channels were natural gullies located at the margin of floodplain fields adjacent to an incised stream. While vegetation was dormant, we created artificial runoff events in the two laboratory gullies and one of the natural gullies using synthetic trapezoidal-shaped hydrographs with peak discharge rates of approximately 0.03, 0.07, and 0.16 m3 s-1. During these tests we monitored flow depth, velocity, turbidity, and soil pore water pressures. The fourth gully was subjected to a series of natural runoff events over a five-month period with peaks up to 0.09 m3 s-1. Flow depths in all tests were generally < 0.3 m, and flow velocities varied spatially and exceeded 2.0 m s-1 at the steepest points of the gullies. Erosion rates were negligible for controlled flow experiments, but natural flows in the fourth gully resulted in 1 m of thalweg degradation, destroying the central portions of the grass hedges, most likely due to the highly erodible nature of the soils at this site. Geotechnical modeling of soil steps reinforced with switchgrass roots showed that factors of safety > 1 for step heights < 0.5 m, but instability was indicated for step heights >1 m, consistent with our observations.