Influence of a spatially complex framework geology on barrier island geomorphology

Abstract

Barrier island response and recovery to storms, and island transgression with relative sea level rise, can be influenced by the framework geology. The influence of framework geology on barrier island geomorphology has previously been examined in areas where the framework is rhythmic alongshore or consists of an isolated paleo-channel or headland. The purpose of this paper is to examine the influence of framework geology on beach and dune geomorphology at Padre Island National Seashore (PAIS), Texas, USA, where the framework geology is variable alongshore. Alongshore beach and dune morphometrics and offshore bathymetric profiles were extracted from a combined topography and bathymetry digital elevation model (DEM) using an automated approach along the ~100 km study area, and an electromagnetic induction (EMI) survey was used to map the subsurface framework geology. Wavelet decomposition, Global Wavelet (GW), and bicoherence analyses were used to test for spatial relationships between and within the extracted alongshore metrics. GW trendlines demonstrate that beach and dune morphometrics are structurally controlled. Hotspots in wavelet coherence plots between framework geology and alongshore island morphometrics indicate that the paleo-channels dissecting the island influence beach and dune morphology, with large dunes found in the area directly landward of the paleochannels. Bicoherence analysis of alongshore beach and dune morphometrics indicates that low-frequency oscillations due to framework geology interact with higher-frequency oscillations, with greater small-scale variability in the dune line directly landward of the paleo-channels. These results suggest that the paleo-channels of PAIS non-linearly influence beach and dune morphology, which in turn alters the response of the island to storms and sea level rise. It is argued that an understanding of the framework geology is key to predicting island response to sea level rise and framework geology needs to be included in barrier island models. This paper demonstrates that an irregular framework geology influences small-scale coastal processes, and creates interactions across scales that influence beach and dune morphology and affects barrier island response to storms and sea level rise.