Observations of wind‐blown sand under various meteorological conditions at a beach

Abstract

Field observations were conducted on a natural, open ocean beach system in Japan to investigate characteristics of wind‐blown sand transport under various weather conditions including a storm event. Data sets over periods of several hours included blown sand impact counts, three‐dimensional wind conditions, hourly precipitation, and moisture content of the sediment surface. The intermittent blown sand impact data shifted by 1 s ahead of the wind velocity correlated with the wind velocity during a no‐rainfall period (for an assumed dry surface) and in the longshore wind direction (for sufficiently long fetch length). The 5‐min mean wind velocity/impact counts relationship was well constrained by both second‐ and third‐order polynomial fitting of velocity under similar weather conditions. During a no‐rainfall period and in the longshore wind direction, the relationship between the wind velocity and sand flux estimated from the counts coincides with existing studies in wind tunnel experiments. The sand flux, however, decreased by 1 order of magnitude because of a change in the wind direction from longshore to cross‐shore and then by more than 1 order of magnitude because of an increase in the moisture content. Threshold wind velocity calculated using the time fraction equivalence method with the impact counts and the horizontal wind velocity in 5‐min sampling periods was approximately equal to the value predicted using Bagnold's equation during the no‐rainfall period and increased significantly during the rainfall phase. The sand flux sensor has several limitations for complex conditions in the field; however, it provides a number of characteristics of sand transport under various meteorological conditions.