Implications of bank failures and fluvial erosion for gully development: Field observations and modeling

[1] Gully erosion is most commonly triggered by fluvial erosion following natural and anthropogenic disturbances or as a response to changes in climate and tectonic forcing and base level drop. Field observations attribute the headward growth and widening of many gully systems to gravitational mass-wasting processes of oversteepened sidewalls. Soil saturation, groundwater sapping, and tension crack development contribute to the instability. Recent landscape evolution models treat such mass failures as slope-dependent continuous sediment transport processes, sometimes conditioned on a slope threshold or with nonlinear dependence on slope gradient. In this study we first present an explicit physically based theory for the stability analysis of gully heads and walls. The theory is based on the force balance equation of an assumed planar failure geometry of a steep gully wall, with a potential failure plane dipping into the incised gully bed and tension cracks developing behind the scarp face. Then, we test the theory against field data collected in our field site in Colorado and against other published data. Second, the theory is implemented in a one-dimensional hillslope profile development model and the three-dimensional channel-hillslope integrated landscape development (CHILD) to study the effects of soil cohesion, erosion thresholds, and stochastic climate on the tempo of gully development and morphology. Preliminary results indicate that wider and shallower gullies develop and integrate, forming wide valleys, when soil cohesion is small. As soil cohesion increases, erosion slows down, gullies become deeper with vertical walls, and episodic mass failures occur. Differences in storm intensity-duration characteristics and erosion thresholds are predicted to have a significant impact on gully development. Vertical gully walls develop rapidly, and gullies enlarge by slab failures in a climate characterized by high-intensity, short-duration storm pulses. However, under low-intensity, long-duration storms, gullies quickly stabilize, and vertical walls are eliminated and rounded, forming diffusion-dominated hilltops. Erosion thresholds have a similar impact on the tempo of gully erosion but in the opposite direction. Lowering the erosion threshold enhances gully widening by slab failures. Gully walls stabilize when the erosion threshold is high due to a reduction in the erosion of the failure material on the toe of gully walls.

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