Collaborative Research: Aeolian Grain Entrainment Over Flexible Vegetation Canopies: Theoretical Models, Laboratory Experiments and Fieldwork

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Vegetation plays a significant role in modulating the entrainment of sediment by wind. Once entrained sand and dust particles have a crucial influence on weather and climate, air quality, landforms, and other components in the land-atmosphere system. Vast regions across the planet in areas susceptible to wind erosion have vegetation covers composed of grasses and herbs with flexible stems and blades. This type of vegetation deforms under sufficiently-high wind speeds, which alters wind-particle interactions. However, current understanding of the complex interactions remains limited, and predictive models of grain entrainment that account for the impact of canopy morphing are lacking. This project aims to elucidate the underlying physics of wind-vegetation-particle interactions for flexible canopies and develop predictive equations for aeolian grain entrainment. The model developed and validated in this research will open a pathway for understanding and predicting aeolian entrainment across a wide range of vegetation cover amounts, wind conditions, and grain properties. Results from the research endeavors will be integrated into educational activities involving K-12 education, undergraduate research, and graduate research training. The outreach activities including hands-on experiments and field trips will provide unique opportunities for attracting students, especially young women in STEM career-paths.The primary objective of this project is to combine theoretical modeling, laboratory experiments and field tests to elucidate the underlying physics of grain entrainment for flexible vegetation canopies that will aid in improving prediction of particle entrainment thresholds, concentrations and flux rates. A reduced-order model based on the balance between wind load on vegetation and blade restoring forces will be developed and validated by wind tunnel experiments to predict vegetation postures and velocity statistics within the canopy. This understanding will then be applied to determine the aerodynamic loads on aeolian particles, where an advection-dispersion equation will be used to predict vertical profiles of particle mass concentration. Finally, results from the predictive models and laboratory experiments will be further evaluated by field tests at the Oceano Dunes State Vehicular Recreation Area, Pismo Beach, CA using recently developed sand transport instruments for measuring vertical profiles of particle flux for different flexible and rigid vegetation densities. Altogether, this project will deepen fundamental understanding of the mechanisms through which flexible vegetation covers modulate particle entrainment by wind, leading to improved predictive threshold and transport rate models. The research results can also lead to developing more effective control strategies for reducing wind erosion and dust emissions using vegetative covers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.