Abstract
The efficiency of high-resolution Light Detection and Ranging (Lidar) analyzed under the framework of Geographic Information Systems (GIS) aids in evaluating morphodynamic evolution of pristine and developed barrier islands along Florida’s southwest coast. Three islands were studied: two pristine, Keewaydin and Cayo Costa, and one developed, Captiva. Lidar data collected from NOAA Digital Coast were used to create Digital Elevation Models (DEMs). The DEMs, for various years, were overlaid with coastal survey reference stations, R-monuments, established by the Florida Department of Environmental Protection to monitor beach profiles. The profile data was processed using EXCEL and SANDs for volumetric comparison of barrier island erosion and accretion scenarios, and then the volume change data was compared to the available FDEP’s Critically Eroded Beaches in Florida reports. The R-monuments for Keewaydin are R 90 through R 131, with two additional monuments R 132 and R 133 that were created for this study to collect data from the elongated tip from 2004 to 2017. Keewaydin experienced the greatest erosion from the northern section, R 90 to R 99 with an average of -11.74%, and Section 5, R 130 to R 133, experienced the greatest accretion with an average volume increase of 27.25%. This accretion took place because of the longshore current that transports sediments from the northern end to the southernmost tip.
The R monuments for Cayo Costa are R 27 through R 65, and they were analyzed from 1998 to 2015. However, the data from 1998 are of poor resolution was omitted from beach volume change studies. Section 2, R 027 to R 033, had the greatest accretion with an average of 10.10%. The dunes were impacted by various hurricanes, but most of sediments settled in the lagoon/bay. The southernmost section, R 057 to R065, had the highest overall average erosion with 23.37% loss of sediments that most took place along the dune structures. The ebb tidal delta at the southern end of Cayo Costa (Captiva Pass) retained most of the sediments flushed out of the bay.
Captiva Island is highly developed and relies on nourishment projects every couple of years due to extreme erosion. The data analyzed was from 1998 to 2015, but 1998 could not be analyzed in detail for its non-classification. Monuments R 083 through R 102 are all deemed critically eroded. Although the overall sediment budget over the eleven years show a positive accretion rate, it is all due to the frequent nourishment projects, and the overall trends are major erosion on the dune structures.
Developed islands are not able to undergo natural processes that usually allow one portion of the barrier island to accrete while another erodes to maintain balance. Hurricanes also play a critical role in morphological evolution of barrier islands, especially in the deflation of dunes and overwash. Studying the sand volume changes of beach dunes and frontal beach on pristine islands over various years through SANDs Asset Management system can better predict long-term evolution of barrier islands including highly developed islands. Beach volume analysis using SANDs analysis was comparable to the FDEP’s Critically Eroded Beaches in Florida reports for both pristine islands Keewaydin Island and Cayo Costa Island, but not for the developed island Captiva due to the frequent infusion of sediments from the restoration projects. The health of barrier islands is critical in protecting the mainland from various climate change impacts.