<p>The aeolian saltation cloud is controlled by the rebound and splash of particles upon impact with the bed. The vertical particle concentration profile and the subsequent reduction in near-bed fluid velocity are intricately linked. However, conceptual and numerical models of the fundamental interactions between the impacting and rebounding particles are often difficult to validate. Currently, sensor capabilities are limited in measuring particle-bed interactions directly. We present a series of wind tunnel experiments using Particle Tracking and Imaging Velocimetry (PTV/PIV) to overcome these measurement limitations by unobtrusively measuring particles in transport under various flow and particle concentration regimes.</p><p>Two synchronized high-speed video cameras captured the sand grains in motion. A 2 mm sheet of light from a 7-watt laser diode and an array of high-powered LEDs illuminated the particles. From the PTV data, we calculated the splash event impacts and ejections and trajectory characteristics of the particles in transport over flat and rippled beds. Additionally, a laser particle counter and sediment traps estimated sediment flux, while a pitot tube and sonic anemometer measured flow regimes. A TLS measured ripple dimensions.</p><p>We report the results from a set of wind tunnel experiments over flat and rippled beds that includes the direct observations of (1) the splash events across the stoss and lee slope, (2) the spatial variability of the vertical concentration profiles of particles in transport, (3) the impact, rebound, and ejection angles and velocities of splash events during low, moderate and high transport rates. We find the splash events change with transport rate. We find the splash event characteristics change with transport rate. We propose future models to include the transition of particle-to-bed interactions with sediment transport flux.</p>
We formulate soil grain transport by rain splash as a stochastic advection‐dispersion process. By taking into account the intermittency of grain motions activated by raindrop impacts, the formulation indicates that gradients in raindrop intensity, and thus grain activity (the volume of grains in motion per unit area) can be as important as gradients in grain concentration and surface slope in effecting transport. This idea is confirmed by rain splash experiments and manifest in topographic roughening via mound growth beneath desert shrubs. The formulation provides a framework for describing transport and dispersal of any soil material moveable by rain splash, including soil grains, soil‐borne pathogens and nutrients, seeds, or debitage. As such it shows how classic models of topographic “diffusion” reflect effects of slope‐dependent grain drift, not diffusion, and it highlights the role of rain splash in the ecological behavior of desert shrubs as “resource islands.” Specifically, the growth of mounds beneath shrub canopies, where differential rain splash initially causes more grains to be splashed inward beneath the protective canopy than outward, involves the “harvesting” of nearby soil material, including nutrients. Mounds thus represent temporary storage of soil derived from areas surrounding the shrubs. As the inward grain flux associated with differential rain splash is sustained over the shrub lifetime, mound material is effectively sequestered from erosional processes that might otherwise move this material downslope. With shrub death and loss of the protective canopy, differential rain splash vanishes and the mound material is dispersed to the surrounding area, again subject to downslope movement.
Ephemeral channels incise into the piedmonts (both alluvial fans and pediments) of the northeastern Sonoran Desert, USA. Located around metropolitan Phoenix, this tectonically quiescent region experienced only aggradation in endorheic structural basins throughout the Pliocene. A wave of aggradation then followed Salt and Gila river integration at the start of the Pleistocene. Aggradation of piedmont base levels continued throughout the rest of the Quaternary. This paper explores two hypotheses to explain piedmont incision despite rising base levels. The classic explanation is that incision is part of the evolution of desert mountain ranges as they decrease in size. A new alternative we propose here involves a lateral shift in base level from Pliocene endorheic basin playas to positions kilometers closer to range fronts in response to river integration. We present a thought exercise of modeling a pediment longitudinal profile as a 1D diffusive system, and we also analyze incision into alluvial fans of the Sierra Estrella range. While our 1D modeling results for pediments are consistent with both explanations for range-front incision, Sierra Estrella bajada incision is best explained by the sudden relocation of the base level to the toe of desert piedmonts.
