Abstract

Portions of the back-island dune fields on Padre Island, Texas, undergo seasonal destructive and constructive phases in which they are reduced to a nearly planar surface during the winter and then reform during the spring and summer. Initial depositional sites for dry sand that we observed were associated with roughness elements that caused a lowering of the transport capacity of the flow. Bedforms develop through a series of morphologic and dynamic stages: 1) irregular patches of dry sand, 2) wind-ripple protodunes with leeward flow expansion, 3) grainfall protodunes with a range from leeward flow expansion to flow separation, 4) barchan dunes with grainflow, and 5) crescentic ridges that characterize the mature field. The survival and further development of protodunes and dunes is favored by increased size, a more advanced stage of development, unidirectional and moderate wind, and a plentiful sand supply. These factors can be viewed as the result of the interrelationship of aspects of eolian transport, the nature of leeward secondary airflow patterns, and sand supply. Dune-field ordering occurs by repeated merging, splitting, cannibalization, and lateral linking of dunes. On Padre Island and other areas of dunes that we studied, flow conditions across portions of interdune flats favor the transport of dry, loose sand from interdune flats onto the dunes, so that dunes grow at the expense of interdune flats. Dry-surface deposits are, therefore, unlikely to accumulate on interdune flats, but rather to characterize interdune depressions that may represent the minimum distance to which an interdune flat can be closed. Otherwise, accumulations on interdune flats most commonly should be formed on wet or damp surfaces or occur associated with some mechanism that protects the deposits from deflation. Padre Island dune fields represent a type of eolian system where accumulations of dunes and interdune flats occur during periods of a water-table rise, with bypassing and deflation occurring as the water table becomes static or falls, respectively.

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