The development of blowouts in ‘de blink’, a coastal dune area near Noordwijkerhout, The Netherlands

Abstract

AbstractSurface changes in six blowouts in the coastal dunes near Noordwijkerhout between s‐Gravenhage and Zandvoort were measured weekly over a period of two years at 80 erosion pin sites. Readings were correlated with wind variables derived from nearby Valkenburg airbase for the same weekly periods. The values of the wind variables is determined by the number of hours that specific average wind speeds and directions were recorded.The first part of this paper deals with the process of wind erosion. Special attention is given to the relationship between surface changes and wind speed, wind direction, season and rain. Surface changes are associated with wind speeds as low as 1.25 m/s, but net lowering, i.e. erosion, is best correlated with wind speeds between 8.75 and 10 m/s. At higher wind speed there is a tendency for the blowouts to be filled in. Winds from an easterly direction generally correlate with accumulation, whereas winds from the west correspond with accumulation at low speeds and with erosion at high speeds. Springtime and autumn are the most important seasons for blowout development. The effect of rain seems to be limited: the correlations are not improved by excluding rainy days, perhaps because wet sand will be dried rapidly by strong winds.In the next part of the paper, the response of blowouts to the erosion process is discussed. Wind erosion is dominant at the windward side because contrarily to what is often assumed the erosivity decreases as soon as the air is loaded with sand. As a result, blowouts grow southwestwards against the prevailing wind. Nevertheless, much sand is also shifted by winds from other directions. The forms of the blowouts determine in their turn the efficacy of the wind. The errosivity of the wind is most pronounced in circular blowouts, it decreases as the blowout grows in length.Finally, a model of blowout development is proposed which is controlled by width, depth and length ratios.