Title: An experimental study of gully sidewall expansion Authors
Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: July 15, 2011
Publication Date: September 18, 2011
Citation: Wells, R.R., Momm, H.G., Bennett, S.J., Bingner, R.L., Dabney, S.M. 2011. An experimental study of gully sidewall expansion. Proceedings of the International Symposium on Erosion and Landscape Evolution, 18-21 Sept 2011. Anchorage, AK. pp. 333-341. Interpretive Summary: Soil erosion devastates arable land and infrastructure and strains the balance between economic stability and viability. Recent studies indicate that ephemeral gully erosion (small channels eroded by concentrated runoff that can be filled by normal tillage) may be a significant form of erosion and source of sediment on cropland in the U.S. (averaging around 40% of the sediment delivered to the edge of the field in some documented studies). The current study was designed to isolate one aspect of the ephemeral gully erosion problem: channel widening. Experiments on channel widening rates, quantified under controlled experimental conditions, revealed that discharges of ~50 L min-1 in the lower sloped beds (1-4%) and ~30 L min-1 in the steeper slope beds (8%) were responsible for excessive widening. Lower flow rates produced small amounts of widening and larger flow rates merely flushed sediment, no significant widening took place. Both water surface and sediment discharge records show good agreement with expectations; i.e. increased flow rates bring about an increase in the occurrence of bank failures and the flow changes accordingly. Ephemeral gully channel widening is not currently well understood. This research program will provide much needed predictive equations for gully channel width.
Technical Abstract: Soil erosion, in its myriad forms, devastates arable land and infrastructure and strains the balance between economic stability and viability. Gullies may form in existing channels or where no previous channel drainage existed. Typically, gullies are a result of a disequilibrium between the eroding force exerted by concentrated flowing water and the resistance of the earth materials in which it is flowing, caused by either an increase in erosional forces (related to a concentration of flow, constriction of flow, an increase in discharge, or decrease in sediment load) or decreased erosional resistance (related to a decrease in cover or some surface disturbance causing decreased cohesion). A gully is a complicated system as its evolution is controlled by water erosion at the gully head and bed, which triggers gravitational mass-movement on gully sidewalls. Gully erosion usually, but not always, includes one or more headcuts that migrate upslope over time. These are step changes in bed surface elevation where intense, localized erosion takes place, and thus are commonly associated with significant increases in sediment load. Reported experimental data shows that actively migrating gully headcuts display a self-similar organization with migration rates dependent on upstream flow depth and discharge, tailwater depth, and soil properties. The depth of gullies is often limited by the presence of a non-erodible or impervious soil layer. When erosion reaches such a layer, the gully typically widens, creating a wide shallow cross section. Once a gully is initiated, transport and deposition of the eroded soil and widening of the gully channel, further govern its evolution. Our knowledge of these processes in shallow concentrated flows within agricultural soils, however, is still quite limited and largely scaled down from river hydraulics. Experiments were conducted to examine channel sidewall expansion due to overland flow discharge. Packed soil beds were subjected to simulated rainstorms and clear-water overland flow. During overland flow, the flow rate was systematically increased to induce widening within the channel. Within these channels, equilibrium must be maintained between potential scour depth and potential channel width. Channel expansion and peaks in sediment discharge occurred episodically, linked directly to increases in upstream discharge.