Evolution of a dune from crescentic to parabolic form in response to short-term climatic changes: Rabjerg mile, Skagen Odde, Denmark

Rsbjerg Mile, a complex parabolic dune, reaches a height of 20 m above the deflation plain. The width of the dune is 900 m and the length of the whole dune is 1500 m. The dune is covered by 0.5-1.5 m high transverse dune ridges. The dune has migrated approximately 1 km since 1887 A.D., accompanied by a change in the migration direction from ESE (100 degrees) to ENE (68 degrees). The morphology of Rabjerg Mile through the last 100 years is illustrated by a geographical information system (Arc/Info) applied to a series of high-quality topographical maps. Based on the digitized contour lines, the evolution of Rabjerg Mile can be followed from an initial crescentic form in 1887 A.D. to a complex parabolic dune in this century. Dune dynamics and sites of erosion and deposition are evaluated from the digital terrain models. Changes in vegetation cover, documented from the maps and air photos, indicate that vegetation in the study area steadily increased with time. Calculations of resultant drift directions and gross bedform-normal transport are based on climatic data from the Skagen lighthouse (14 km NE of Rabjerg Mile). The calculated directions of the resultant drift indicate that the Rabjerg Mile changed from an almost perfect transverse bedform (1887-1924) to an oblique imperfectly transverse bedform (1966 to present). The bedform alignment followed the gross bedform-normal transport rule in most years. The evolution of a parabolic dune after 1924 is related to a period of reduced wind energy and increased vegetation cover. This parabolic dune was maintained after 1960 when wind energy increased and the wind regime was more complex. At the same time the dune became markedly asymmetric because of the frequent occurrence of strong southerly and easterly winds.