SPATIAL-TEMPORAL MORPHODYNAMICS AND EVOLUTION OF COASTAL BLOWOUT DUNES

Blowouts are widespread in coastal and continental dune landscapes and form as depressions, bowls, or troughs in pre-existing sand deposits. Although formed by wind erosion, blowouts also have both erosional and depositional features and their development is facilitated by dominant wind speed and direction, topographic forcing of surface winds, sand supply, vegetation cover and type, climatic variability, water and wave erosion, and anthropogenic activities. The main controlling force for blowout size, shape, and direction of expansion, however, is the wind regime and complex flow dynamics promote and maintain erosion.

Despite their widespread presence and linkage to important land use/cover changes, few studies have explored blowout morphodynamics and evolution. This paper examines the initiation and historical evolution of blowouts at Cape Cod National Seashore (CCNS), Massachusetts, USA, which hosts one of the world’s highest densities of bowl blowouts. Morphological evolution and volumetric changes are examined from historic aerial photography and LiDAR using a the Spatial-Temporal Analysis of Moving Polygons (STAMP) method and statistical change detection of DEMs. Two-dimensional (areal) patterns of erosional basins and depositional lobes are analyzed to identify common movement and morphometric responses. Three-dimensional (volumetric) change analysis is used to quantify sediment budget and morphodynamic responses. Results from sub-populations of blowouts are then compared to landscape-scale changes in land cover, land use, and geomorphology.

Results from both methods combined reveal several common morphodynamic responses that inform a multi-stage model of blowout evolution with 7 key phases: 1) generation; 2) extension (areal increase); 3) deepening (volumetric increase, minimal areal change); 4) expansion (areal + volumetric increases); 5) amalgamation (union with nearby features) or division (into smaller features); 6) stabilization (by vegetation); and 7) re-activation (by disturbance to stabilized surfaces). When compared to landscape patterns and processes, the evolution of blowouts is also affected by land use changes, notably vegetation replanting and ORV traffic, and the occurrence of hurricanes.