HILLSLOPES, LANDSLIDES, AND DRY RED KIDNEY BEANS

We report a series of analog experiments designed to investigate the temporal characteristics of landslides and the concomitant development of hillslope morphology in response to base-level fall. The experimental apparatus is a rectangular box measuring in cm 100 (ht) x 80 x 10. The 10 cm vertical panel is able to slide and simulate a falling base-level for material within the box, which consists of dry red kidney beans. Dry red kidney beans possess a crudely ellipsoidal shape, allowing a degree of interlocking between beans and thus simulating cohesion between jointed and fractured rock mass. A width of 10 cm prevents side-wall friction from affecting bean slides (Aalto and others, 1997).

Our investigation stems from the original analysis of Densmore and others (1997) and from their result that inner gorges, commonly observed in a variety of actively deforming landscapes, can be part of the natural hillslope development process rather than a signal of recent incision or rock uplift. We add to earlier models by testing the rate of base-level fall, lowering the side panel by increments of 0.5 and 1.0 cm/time-steps, the magnitude dictated by the size of the kidney bean (i.e., jointed rock particle).

A relatively low rate of base-level fall resulted in more frequent small bean-slides, the common generation of inner-gorges of up to 4 bean-heights, and in a characteristic pitted morphology of the hillslope. In contrast, a base-level fall of 1 cm/time-step (i.e. slightly larger than the typical kidney bean) resulted in largely slope-clearing bean-slides and inner gorges were only rarely developed. Our results indicate that the morphology of hillslopes generated by climate-driven landslides should reflect the combination of the rate of base-level fall and the characteristic length-scale of the bulk rock strength, a result that may be tested in actively deforming regions with high-resolution topography and bedrock geology.

Refs: Aalto, R. and five others, Science, 277, 1909-1914, 1997; Densmore, A. L. and three others, Science, 275, 369-372, 1997.