The movement of sand by wind is the primary mechanism responsible for delivering sand from beach to dune. As such, it represents a critical linkage in the suite of interactions that connect these places and that control coastal dune development. A key uncertainty lies in the identification of the threshold condition, that is, the wind energy level at which transport begins. Without accurate representation of this threshold, it is not possible to determine whether a given wind event will generate transport. This situation hampers intermediate-scale modeling of aeolian sediment budgets, as it is not currently possible to reliably predict the occurrence of transport events. The goal of this doctoral dissertation research project is to validate and calibrate a new model of the threshold condition for windblown transport on beaches. This effort differs from previous approaches in the inclusion of a new term that accounts for the range of grain sizes present in the sediment bed. The new model provides significantly improved predictive capabilities, based on examination of previously published data. Further, the model also encapsulates the effect of surface moisture on the threshold condition, a key complicating factor on many beaches. Field experiments will be conducted at two locations (Padre Island, TX, and Cape San Blas, FL) to verify the model. Wind velocity and surface moisture contents will be monitored leading up to the start of transport events. Mobile stations, each comprised of a 3-D sonic anemometer and an array of microphone-based saltation detectors, will be installed on the beach at locations of various moisture contents. The anemometers and saltation detectors will be logged in real time, and surface moisture measurements will be conducted using a handheld optical device designed for this purpose. The data recorded during the field study will allow validation and calibration of the model.
This study represents a significant departure from the current theoretical framework that is used to explain and model windblown sand transport. The results will provide improved insight into the transport of sand on beaches, and if the proposed model is successfully validated this study will significantly advance predictive capabilities. This will in turn improve calculations of larger-scale sediment budgets by providing researchers and coastal managers with more accurate estimates regarding how much sand is transported from beach to dune, and when and under what conditions such events can occur. This will contribute to modeling of post-storm dune recovery, for example, and for predicting the time required for dune re-establishment and reconstruction programs. Finally, this study will address one of the most problematic factors complicating wind blown sand transport on beaches: the frequent presence of surface moisture which can inhibit or even completely prevent the movement of sand by wind. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.
