Migration of parabolic dunes at Aberffraw, Anglesey, north Wales

Abstract

Aberffraw is a 1-km-wide and 3-km-long transgressive dunefield that extends inland along a northeast–southwest-trending valley from a southwest-facing beach, Traeth Mawr. The prevailing wind is from the southwest, and both the parabolic dunes and the valley within which they lie are sub-parallel to the prevailing wind. The dunefield at Aberffraw includes two foredune ridges and three rows of active compound parabolic dunes. At the landward end is a lake, Llyn Coron, which has been formed by dunes migrating up the valley and damming the river, Afon Ffraw. Between the parabolic dunes are gently sloping interdune areas with a close cropped vegetation. The parabolic dunes at Aberffraw have been migrating inland across the interdune areas. Rates of parabolic dune migration are derived from three sets of aerial photographs taken in 1940, 1982 and 1993. The aerial photographs have been scanned and manipulated in ArcView GIS software. Registration of the aerial photograph to an Ordnance Survey (OS) map was performed using ground control points (GCPs), common fixed features that are identifiable on both the aerial photographs and the baseline map. Attempts to correct for the inherent distortions of aerial photography were made during registration. Standardising the projection of the photographs to a common baseline allows meaningful spatial analysis, and the dune ridges, trailing edges and areas of bare sand were mapped from each photograph as a series of overlays. Rates of dune migration are calculated from the spatial distance between linear trend lines, parallel to the dune crests and perpendicular to the dune migration orientation, applied to sections of dune ridges for 1940 and 1993. Trend lines were only fitted to sections where continuity of dune form was maintained over the given period. The method provides an improved representation of the actual migration rate as it incorporates the whole of the parabolic dune form, and the whole of the compound dune ridge form into the calculation. It effectively measures the centre point or line of a dune or dune ridge as opposed to the variable positions and orientations of the dune crest noses, which represent maximum migration, rather than the mean. Rates of parabolic dune migration range from a minimum of 0 m year −1 to a maximum of 3.6 m year −1, with an average migration rate of 1 m year −1.