Journal
SEDIMENTOLOGY
Volume 51, Issue 1, Pages 95-108Publisher
BLACKWELL PUBLISHING LTD
DOI: 10.1046/j.1365-3091.2003.00613.x
Keywords
critical threshold; grain entrainment; sand transport; shear velocity; time fraction equivalence method; wind speed
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This paper assesses the practical use and applicability of the time fraction equivalence method (TFEM; Stout & Zobeck, 1996) of calculating a wind speed threshold for sand grain entrainment in field situations. A modification of the original method is used and is applied to 1 Hz measurements of wind speed and sand transport on a beach surface. Calculated grain entrainment thresholds are tested in terms of the percentage of sand transport events that they explain. It was found that the calculated thresholds offered a poor representation of the occurrence of saltation activity, explaining only about 50% of the measured transport events. Results are discussed in terms of system response time, wind speed measurement height, undetected events and sampling period. A shear velocity threshold for grain entrainment was also calculated, but this also failed to explain a high proportion of the sand transport events. The best results (67-91% of transport events explained) were found by calculating a threshold based on time-averaged (approximate to 40 s) wind velocity measurements. The applicability of a single threshold to a natural grain population is discussed. A natural surface is likely to possess a range of thresholds varying over short time scales in response to parameters such as grain rearrangement and changes in moisture conditions. The results show that calculated thresholds based on 40 s time-averaged data consistently explain a high proportion of the recorded sand transport events. This is because such a time-averaged approach accounts for higher frequency variability inherent in the sand transport system.
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