An experimental study of gully sidewall expansion

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 selfsimilar 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.