| Abstract: | Detailed field measurements were made of the degree of surface level change in a blowout, situated in the coastal dune area of Meijendel, The Netherlands. A formula was established to express the erosivity of the wind for the corresponding 34 measurement intervals, over a period of about 3 years. Having established, for 12 wind sectors, the relationship between wind velocity at the nearest standard weather station and at five locations in the blowout, correlations were derived between the deflation rate in the blowout and the wind erosivity using standard hourly wind data. The winter season, although the most windy, is by far the least effective season: the threshold shear wind velocity is disproportionally increased because the moisture content of the surface sand is high. The soil moisture conditions are described as a function of the daily precipitation and evaporation rate. The two most relevant upper and lower threshold shear wind velocities are determined empirically. During the summer season, deflation rate is even higher than indicated by the net surface level change because deflation is partly compensated by deposition of sand transported by rainwash from the blowout margin. With regard to spatial variation in the deflation rate within the blowout, it appears that locations with relatively high velocities coincide with higher deflation rates. However, the spatial variation is commonly less than is expected from the horizontal wind velocity distribution. In view of this the role of wind direction, blowout morphology and algal coverage of the soil in the deflation rate is discussed. |
