The development of dune blowouts, as measured with erosion pins and sequential air photos

Abstract

The development of blowouts in dune terrains where no stabilization measures are taken requires constant watch to detect unwanted developments at an early stage. Two methods for investigating blowout development are in use. Since 1983, blowouts in a 40 ha section of Meijendel, a coastal dune area near The Hague have been twice yearly measured with erosion pins. Most of the 32 to 35 blowouts grew in length against the prevailing SW wind. The total blowout deflation area increased from 2.8 to 3.3% of the terrain in 4 years time. Air photos of 1958, 1967, 1977, 1984 and 1986 were used to study blowouts in a nearby area, De Blink north of Noordwijk, one of the most severely eroded dune terrains along the Dutch mainland coast. Total deflation area here increased from 0.8% in 1958 to 3.7% in 1986. There is a definite modal length class but the size is site-specific: 25 to 30 m in Meijendel, 15 to 20 m in De Blink. Many blowouts disappear shortly after they are formed. From the results it appears that the processes involved in the formation of blowouts can best be investigated with erosion pins whereas aerial photographs are preferable when changes in the spatial and temporal pattern of blowouts and the contributing factors are of interest.