The role of wind in sediment removal from potholes in semiarid environments

Potholes, or rock basins, are depressions that have been carved out of bedrock. While there has been speculation in the literature regarding the processes involved in the addition or removal of sediment in existing potholes, the processes have not been systematically documented. This study attempts to fill this knowledge gap through data collection on 50 potholes in the Navajo Sandstone located on the top of Rock Window Mesa in Chinle Valley, AZ. For spatial analysis, we obtained a high-resolution digital elevation model and detailed orthophoto of the mesa. These data allowed us to precisely measure many aspects of the potholes, which we then compared to wind data we collected to draw conclusions on how pothole morphology, catchment size, and water retention capacity affect the sediment removal processes. Wind measurements were done on 11 potholes with a range of sizes representative of the 50 potholes on the mesa. Wind speed across the top of the mesa is almost always greater than wind speed in the pothole at any given moment, and there is a robust linear relationship between the wind speed above the pothole and the wind speed in the pothole for all shapes and sizes of potholes tested. In dry conditions, the wind speed threshold for sand removal from the potholes is about 50 km/h, independent of pothole size. We observed the formation of wind eddies in the potholes which formed vortices several meters in height, and these eddies, traveling upward with a helical motion, removed the sand. We conclude that wind is an effective agent for the removal of sediment from the potholes, preventing their infill via the accumulation of sand debris brought in by the wind and spalled from the pothole sides.