Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p21b..02l&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P21B-02
Physics
5415 Erosion And Weathering, 5470 Surface Materials And Properties, 5494 Instruments And Techniques, 1625 Geomorphology And Weathering (1824, 1886)
Scientific paper
Although the physics of the movement of sand-sized particles by wind has been extensively studied, significant uncertainties remain in our understanding of the effects of surface roughness on aeolian transport processes. Accounting for the effects of non-erodible, isolated roughness elements on sediment transport by wind is necessary for the development of models that realistically predict rates of transport for complex surface types on Earth and Mars. Many surfaces in the McMurdo Dry Valleys of Antarctica exhibit a striking resemblance to rocky surfaces on Mars, making this area ideal as a Mars analog site in addition to providing fundamental information on sand transport processes in a cold arid environment. We conducted studies of sand transport rates (using both sand traps and Sensit saltation sensors), boundary layer winds, and surface shear stress at a site in the eastern part of the Victoria Valley for a 3-week period in January 2003, and generated a unique data set on temporal and spatial variations in boundary layer winds, surface shear stress, and sediment transport by wind in a natural setting. The surface in this area consists of a gently undulating sand sheet (median grain size 300 µm) with scattered clusters of poorly sorted angular boulder- and cobble-sized clasts, interspersed with patches of angular medium gravel- to cobble-sized sub-angular rock fragments. Overall roughness density for the area is 0.0039, with an average aerodynamic roughness of 0.0013 m. Surface shear stress was measured using Irwin Sensors with the total shear stress being derived from wind profile parameters. Approximately 20 percent of the regional wind shear stress interacts with the ground surface, indicating a significant partitioning of shear stress, even with a rather sparse roughness element density. Data were obtained for seven sand transport events ranging in duration from 271 to 1451 minutes. The threshold wind shear velocity for sand transport was determined via the time-fraction equivalence method of Stout and Zobeck (1997) and ranged between 0.30 and 0.35 m/s, equivalent to a wind speed at 6 m of 6.2 to 7.4 m/s. This compares to threshold wind shear velocity of 0.29 m/s calculated using the Bagnold formula. Sand transport intermittency as defined by Stout and Zobeck (1997) varied from 0.03 to 0.90, indicating considerable variations between events in the intensity of saltation and in the intermittency function. These variations can be characterized by: (1) a "saltation duration" curve - the cumulative percentage of the event at which different levels of intermittency occur and (2) the percentage of the event during which saltation is continuous (transport intermittency = 1). In turn, these parameters correlate with the overall wind conditions for the event as characterized by the ratio between wind shear velocity and threshold wind shear velocity and the percentage of the time wind shear velocity exceeds the threshold wind shear velocity. The saltation duration curve and percent of time during which saltation is continuous provide means to quantify aeolian activity at a site and can be used to compare levels of aeolian activity among different aeolian environments. Research supported by NSF OPP-0088136.
Cupp K.
Gillies J. A.
Lancaster Nick
Nickling William G.
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