Much of process geomorphology is concerned with understanding the development and evolution of landforms like bedforms, channel bars, beaches, and dunes that are comprised of unconsolidated or poorly consolidated sediments. Fundamental to understanding the dynamics of these systems is the ability to model and predict sediment transport rates. Numerous models have been designed to predict aeolian sediment transport rates, almost all of which depend on the third power of shear velocity (a variable representing the magnitude of wind shear on the bed surface). Most estimates of shear velocity are derived as the product of the slope of a measured velocity profile and the von Karman constant (the typical value of von Karman constant is 0.4). The von Karman constant therefore is fundamental for the prediction of aeolian transport, but hydrodynamic research in laboratory flumes indicates that this ""constant"" is not constant in the presence of suspended sediments, because those sediments decrease it significantly (perhaps to less than 0.2) as the sediment concentration increases. Such a change would cause order-of-magnitude errors in sediment transport rate predictions. Surprisingly, there have been no attempts to evaluate the stability of the von Karman constant in aeolian sand transport systems. This doctoral dissertation research project will conduct a field experiment designed to assess the variability of the von Karman constant under a range of sand transport conditions. The results will be compared with general turbulent mixing theories, and they will be compared with results obtained in water. Sand transport will be measured with continuous-weighing traps and with microphone-based saltation sensors and the wind field with thermal and ultrasonic anemometers. Estimates of shear velocity will be derived from direct measurement of vertical and horizontal velocity fluctuations and from velocity profiles. After the field experiment, enough data will be acquired to evaluate and quantify sediment influences on the wind velocity profile as manifested in changes in the von Karman constant.
This project will evaluate and quantify sediment influences on turbulence from perspectives of different physical fluid dynamic theories. Substantial variability in the von Karman constant will cause disproportionate errors in predicted transport rates. This potentially means that all transport studies based on wind profile analysis are flawed to an unknown extent. This project therefore has the potential to fundamentally change the interpretation of sediment-laden wind velocity profiles and to open debate about the role of sand transport in the alteration of its carrier flow. Although this research is concerned with sand transport only, the results also will be applicable to fluvial systems. The project should lead to improved sediment transport models and a sounder basis for wind-blown dust prediction, dune stabilization, desertification assessment, and related environmental planning challenges. As a Doctoral Dissertation Research Improvement award, this project will help launch the research career of a promising doctoral student while also providing a second doctoral student with experience in field experimentation with complex instrumentation systems.
