The Role of Diffusive Hillslopes in Landscape Evolution: An Experimental Investigation

Abstract:

The competition between diffusive sediment transport on hillslopes and advective transport in valleys sets fundamental spatial and temporal landscape scales, including ridge-valley spacing and landscape response time. However, the interactions between hillslopes and channels are difficult to measure in natural landscapes, due to long timescales and confounding climatic and lithologic factors. Laboratory experiments allow us to observe complex sediment dynamics in a controlled setting, but past work on erosive landscapes does not include diffusive hillslopes and hence cannot provide insight into channel-hillslope interactions. Here, we present the first results from a novel laboratory experiment combining hillslope diffusion and valley advection. Our experimental apparatus, the eXperimental Landscape Modeling (XLM) facility at the St. Anthony Falls Laboratory, consists of a 0.5 m x 0.5 m test flume filled with crystalline silica (D₅₀ = 30μ) mixed with water, a high-resolution laser scanner to measure topography, and a series of load cells to measure sediment flux. Baselevel lowering is simulated by dropping two motorized weirs. During each run, we alternated between: (1) advective transport induced by a series of misting nozzles, where drops are not large enough to disturb sediment on impact, and (2) diffusive rainsplash transport driven by a constant head drip tray. We report a series of experiments where the relative strength of advection and diffusion are varied systematically between runs, testing theoretical predictions that dominantly diffusive landscapes will have larger ridge-valley spacing and respond more slowly to perturbations in baselevel. Our work provides an invaluable dataset for both testing numerical models of landscape evolution and guiding field investigations of channel-hillslope interactions.