Relief Threshold for Eolian Sand Transport on Alluvial Fans

Many arid alluvial-fan terraces downwind from eolian sand sources exhibit an abrupt increase in Av-horizon thickness and sand content below a critical elevation which varies from fan to fan. Above this elevation, sand accumulates locally and is not transported across the fan. Below this elevation both coarse and fine eolian sand from nearby playa and channel sources is readily transported across the distal fan. Here we test the hypothesis that these distal-fan eolian “causeways” are controlled by a threshold fansurface relief. We propose that when along-strike relief falls below a critical threshold value, an eolian surface-of-transportation, or “causeway,” develops. To test this hypothesis, we measured multiple along-strike topographic profiles and Av-horizon textures on two piedmonts in the Ivanpah Valley and adjacent Hidden Valley in Clark County, Nevada. Both piedmonts are located near sand-dominated playas and exhibit clear evidence of eolian transport across their distal regions. The near-surface boundarylayer flow above each topographic profile was quantified using the Multi-Spectral Finite Difference (MSFD) numerical model. This model predicts the surface shear stress above complex terrain for any wind condition assuming neutrally-stable flow. The minimum shear stress calculated by MSFD for each profile during extreme wind conditions was compared to the shear-velocity threshold necessary to initiate saltation. Model results showed that in proximal fan areas, along-strike relief was large enough to prevent eolian transport of all but fine sand particles. In distal fan profiles within the eolian causeway, model results predict shear stresses everywhere above threshold for both fine and coarse sand.