DUNE MORPHOLOGY AT PISMO BEACH, CALIFORNIA

The Pismo Dunes are located approximately 250 km northwest of Los Angeles and consist of 90 km² of transverse, parabolic and paleodunes. They are one of the largest dune complexes on the west coast and are the largest remaining south of San Francisco Bay, but have garnered the attention of relatively few process morphology studies. The dune field includes 12 km²of actively migrating transverse dune ridges advancing onshore in three distinct phases separated by small depressions readily visible with a LiDAR-derived elevation model. A field investigation by Tchakerian (1986) revealed uniform increase in slip face height and crestline wavelength inland with no apparent change in grain size. Measurement of recent aerial imagery shows variable migration rates throughout the dunes and wavelengths between 30 m closest to the beach and up to 250 m further inland. These values are significantly higher than those observed three decades ago. During El Niño and La Niña periods, westerly winds advance onshore nearly perpendicular to the crests. It is hypothesized that particularly strong ENSO periods may have led to the development of distinct dune phases with depressions and the development of defects along the dune crest. Defects associated with the wakes of incipient vegetation and inter-dune depressions are conspicuous and widespread, though localized and variable through time and space. Aerial imagery taken in September 1994 shows a more even distribution of defects across the dune field than currently visible.

The patterns are complicated by the closure of the dune field to over-sand vehicles in 1982. The closure of much of the complex to vehicular traffic in 1982 may play a role, as Tchakerian’s crestline wavelength measurements were far smaller than those obtained for this study while maintaining likewise increases between phases. At a decadal scale, excessive vehicular traffic may have impeded the transition of emergent, defect-ridden dune forms into mature transverse ridges. Despite the astounding lack to studies focusing on the Pismo Dunes, the complex presents multiple opportunities for inquiry regarding climatic control on dune field evolution, defect law and complex landform pattern development, and long-term anthropogenic alteration of coastal process morphology.