Beach, Dune, and Nearshore Analysis of Southern Texas Gulf Coast Using Chiroptera LIDAR and Imaging System

摘要

LIDAR data and color infrared aerial imagery were acquired for southern Padre Island and Brazos Island, Texas, in 2013 to calculate rates of shoreline change; analyze beach-dune system volume; and test bathymetric LIDAR capabilities along the Texas Gulf of Mexico shoreline. Data were acquired using a Chiroptera airborne system, which simultaneously collects topographic and bathymetric LIDAR and high-resolution imagery. Shoreline position was extracted from LIDAR digital elevation models (DEMs) to compare with historical shoreline positions for shoreline change analyses. Long-term rates (1937-2013) of gulf shoreline change for southern Padre Island and Brazos Island averaged 2.2 m/y of retreat, with 86% of sites retreating. Retreat rates decreased over the last decade (2000-13) to 1.1 m/y (76% of sites retreating). The trend changed between 2010 and 2013: 64% of monitoring sites advanced at an average distance of 4.9 m. Beach and dune volumes above threshold elevations (1 to 6 m above mean sea level) were extracted from DEMs to assess geographic and temporal patterns of sand storage. The undeveloped area of southern Padre Island had 2 to 4 times the volume of sand at lower threshold elevations and 7 times the volume at higher elevation thresholds than did the heavily developed southernmost section of the island. A constant trend across the study area is that volume reduced by approximately half with each 1 m increase in threshold elevation. Beach and dune system volume in the study area increased steadily since 2000, mirroring the decreased retreat rates observed in the shoreline movement analysis. Possible causes of the decreased shoreline retreat rates and increased subaerial sand storage are a lack of tropical cyclone impacts to the study area during the most recent periods, as well as sediment contribution from beach nourishment activities. Bathymetric LIDAR detected the seafloor in low-turbidity areas less than 4 m deep.