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

Abstract

[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.

Impacts of Changing Hydrology on Ravine Growth: Experimental Results

“…In one numerical study, Istanbulluoglu et al (2005) tested the effect of changing rain intensity while storm volume was held constant. The modeled results of their study showed an increase in the volume of sediment eroded as intensity increased.…”

“…Slab failure relies on the constant wetting and drying experienced at the headcut between events. Tension cracks form 5 as the soil dries, and during wet times water flows into these cracks dislodging the slab (Dietrich and Dunne 1993;Istanbulluoglu et al, 2005). Another form of slab failure occurs in layered substrate where a lower highly erosive layer is scoured away leaving the upper resistant layer overhanging until it eventually breaks away (Chu-Agor et al, 2008;Holland and Pickup, 1976;Robinson and Hanson, 1994;Stein and La Tray, 2002).…”

“…Ravine initiation and growth have been found to be a function of sediment texture and erodibility, slope, land-use, vegetation 15 cover, soil moisture, headcut size, climate, and hydrology (Ambers et al, 2006;Chiverrell et al, 2007;Flores-Cervantes et al, 2006;Istanbulluoglu et al, 2005;Knapen et al, 2007;Nachtergaele et al, 2001;Poesen et al, 2003Poesen et al, , 2011Stankoviansky, 2003;Vandekerckhove et al, 2000;Vanwalleghem et al, 2003Vanwalleghem et al, , 2005, but understanding the relative importance of each factor remains a challenge. Of the many variables impacting ravine erosion, the two humans most readily impact are hydrology and land cover.…”

“…Another form of slab failure occurs in layered substrate where a lower highly erosive layer is scoured away leaving the upper resistant layer overhanging until it eventually breaks away (Chu-Agor et al, 2008;Holland and Pickup, 1976;Robinson and Hanson, 1994;Stein and La Tray, 2002). In both of these slab failure mechanisms, head cut retreat temporarily ceases after slab failure with the slab protecting the head cut until the slab itself is eroded away (Robinson 10 and Hanson, 1994;Istanbulluoglu et al, 2005). Granular erosion is driven by high shear stress at steep head cut slopes and can be enhanced through focused erosion of plunge pools and impinging jet scour (Alonso et al, 2002;Flores-Cervantes et al, 2006).…”

Analysis of slope movement initiation induced by rainfall using the Elastoplastic Barcelona Basic Model

“…This phenomenon could lead to an increase in permeability and enhance shallow slope instability (Istanbulluoglu, Bras, & Flores-Cervantes, 2005). Taking into account the cracking and the subsequent permeability variation should be a future challenge of this research.…”