Using Wavelet Decomposition to Assess the Development of Padre Island National Seashore, Texas, USA

Abstract:

This study examines the relationship between modern barrier island morphology and offshore bathymetry to better understand the development history of Padre Island National Seashore (PAIS), Texas, USA. The widely accepted hypothesis for the development of PAIS is that the dunes on the mainland were partially submerged during the Holocene transgression, resulting in distinct islands that eventually coalesced by sediment transported alongshore; however, this hypothesis does not follow the development history of other barrier islands throughout the Gulf of Mexico. This study uses electromagnetic inductance (EMI) surveys, bathymetric contours, and island morphometrics to better understand the development of PAIS. A 100km long EMI survey was conducted to aid in identifying infilled subsurface paleochannels that dissect the island. Bathymetric contours were extracted from NOAA digital elevation models (DEMs) up to 7km offshore by offsetting the shoreline east in 1km intervals. Island width and volume, beach width and volume, and dune height and volume were extracted from a 100 km LiDAR-derived digital elevation model (DEM) using an automated approach. Apparent conductivity at all three frequencies (from the EMI survey), bathymetric contours, and island morphometrics were subsequently decomposed using a continuous wavelet transformation (CWT). Beach width, dune height, and dune volume exhibit a very similar waveform that coincides spatially with an inflection point in the waveform for the offshore bathymetric contours. Portions of the island proximal to the infilled paleochannels tend to have a wider beach and taller/higher volume dunes. The high degree of similarity between the subsurface paleochannels, offshore bathymetry, and island morphometrics support a new theory of development for PAIS. We argue that PAIS is not a series of partially submerged relict dunes, as previously proposed, rather that framework geology is an important factor affecting modern island morphology; and that the island transgressed landward during the Holocene similar to other barrier islands throughout the Gulf or Mexico.