Influence of averaging interval on shear velocity estimates for aeolian transport modeling |
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| Authors: | Steven L. Namikas Bernard O. Bauer Douglas J. Sherman |
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| Affiliation: | a Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 70803, USA;b Department of Geography, University of Southern California, Los Angeles, CA 90089, USA;c Department of Geography, Texas A&M University, College Station, TX 77843, USA |
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| Abstract: | Recent investigations of aeolian transport have focused on increasingly short time scales because of growing recognition that wind unsteadiness is a major factor in the dynamics of sediment transport. However, the statistical reliability of shear velocity (u*) estimates becomes increasingly uncertain as averaging interval is decreased. This study provides an empirical assessment of the influence of averaging interval on the reliability of u* estimates. The data consist of 15-min wind-speed profiles (1 Hz sampling) collected at four coastal sites. Each profile was subdivided into progressively shorter fixed-length time intervals, and estimates of u* and the 95% confidence interval for u* were determined for each time-block using standard statistical techniques.The logarithmic model accurately represents the measured wind-speed profiles, even with relatively brief averaging intervals. Mean r2 values remain robust down to block lengths as short as 10–20 s, typically retaining better than 98% of the r2 value found for the full-length data sets. Fewer than 2% of the individual 10-s blocks had r2 values less than 0.9. However, mean confidence intervals typically expanded by 70–80% of the full-record value as block length decreased from 900 to 10 s. For highly log-linear profiles, this amounted to an absolute increase from about ±8% to only ±14% of u*, so that the additional information gained through the use of shorter averaging intervals may outweigh the increase in statistical uncertainty. Nevertheless, given that rates of aeolian transport are generally modeled as a function of u*3, this increase in uncertainty may be significant for transport modeling. Thus, very short averaging intervals should be used with caution when predicting aeolian sediment flux. It is proposed that transport modeling should incorporate the shear velocity confidence interval as an indicator of the potential error associated with this source of uncertainty. |
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| Keywords: | Aeolian sand transport Wind-speed profile Logarithmic model Statistical uncertainty |
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