|Van Pelt, Robert - Scott|
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 7/30/2004
Publication Date: 9/14/2004
Citation: Van Pelt, R.S., Zobeck, T.M., Popham, T.W. 2004. Factors influencing the stochastics of near-surface wind speeds in wind storms [abstract]. International Symposium on Sand and Dust Storm, September 12-14, 2004, Beijing, China.
Interpretive Summary: Wind erosion events vary greatly in intensity from incidental saltation due to occasional gust to large, intense storms resulting in considerable soil loss and thick clouds of fugitive dust that reduce visibility to less than a quarter mile and obliterate the sun. While these more intense events are relatively uncommon, their effects can be devastating and extremely high mean wind speeds appear to be a contributing factor. We instrumented several different crop field surface types with anemometers to measure the wind speeds at 1, 50, 100, and 200 cm above the surface and recorded wind speeds at these heights at intervals of 1 sec anytime the average 200 cm wind speed was above 3.5 m/s. We found that for 200 cm average wind speeds at and below about 13 m/s (30 mph), the wind speed profiles conformed to the expected situation where the wind speed decreased logarithmically to a very near surface wind speed approaching zero. Above that threshold, however, the variability of near surface wind speed increased markedly and incident gusts could exceed the 200 cm average. We also found that increasing surface roughness with closely spaced tillage elements or by leaving crop residues in place ameliorated this phenomenon.
Technical Abstract: Wind erosion events vary greatly in intensity from the incidental saltation during afternoon wind gusts to very intense events that result in large amounts of soil loss and fugitive dust. From analysis of 172 recorded wind erosion events at Big Spring, Texas, we found that the measured soil loss spanned four orders of magnitude, storm intensity was log normally distributed, 7 % of the total soil loss occurred during a single event, and half of the total soil loss occurred during the most intense 10 % of the events. We investigated the wind speed profiles at 1 sec intervals over several field surfaces using a mast of cup anemometers at 0.5, 1.0, and 2.0 m and a hot wire anemometer at 0.01 m. We found that for bare, tilled, and sparsely vegetated surfaces, the profiles and near-surface (0.01 m) wind speeds were consistent with the Prandtl equation for 2 m average wind speeds up to about 13 m/s. For 2 m average wind speeds above about 13 m/s, the wind speed profiles were less correlated with the Prandtl equation, the near surface wind speeds became much more erratic, and, for relatively bare flat surfaces, the 90th percentile near-surface wind speed often exceeded the 2m mean. In general, the effects noted on near-surface wind speed intensity and variability decreased with increasing surface roughness from closely spaced fresh tillage elements or standing crop residue.