Soil freeze-thaw-induced changes to a simulated rill: potential impacts on soil erosion

Flows in natural rills on hillslopes transport significantly more sediment down slope than overland flows, which makes rills geomorphically significant landscape features. The erosivity, e.g., sediment transport capacity, of the flows in rills is partially a function of the cross-sectional geometry of the rill which determines flow velocity and depth. Conversely, rill geometry is determined by flow erosivity and by soil processes that affect the erodibility and stability of soil along the rill. Thus, complex feedbacks exist in the mechanics of rill erosion. The objective of this study was to measure the effects of one of the soil processes that affect rill geometry, soil freeze-thaw (FT) cycling. An unvegetated rectangular rill was subjected to two FT cycles, each lasting several days, in a controlled laboratory setting. The FP cycling increased the water content and reduced the cohesion in the surface soil along the rill sufficiently to induce soil slumps and mud flows along the sidewalls of the rill. These mass failures changed the rectangular rill to a triangular one, which reduced the hydraulic radius of the rill by 32%. Using Manning's equation, it was estimated that this new geometry could reduce the velocity of the flow in this altered rill by about 25%. The persistence of the new rill shape and, thus, that of the slower flow would depend on the complex interactions between flow velocity and the resistance of the slumped sediment to those flows. That persistence was not investigated in this experiment. These results can be used in parameterizing models of rill evolution that incorporate widening of rills by mass failures along the rill sidewalls. (C) 2000 Elsevier Science B.V. All rights reserved.