SECN Inventory [Complete]: Back Barrier Erosion at Cumberland Island National Seashore

Starkey, Eric; Cooper, Christopher (Stephen)

On May 22 and 23, 2019, the Southeast Coast Network (SECN) conducted an island margin survey of five sites; Cumberland Wharf (CW), Brickhill Bluff (BB), Plum Orchard (PO), Dungeness Wharf (DW), and Raccoon Key (RK). These five sites had previously been monitored by the USGS from December 2011 to February 2013 to determine the average rate of island margin retreat at these pre-identified erosional hotspots (Calhoun and Riley 2016). These sites were resurveyed in 2017 and 2018 to determine if the rates of island margin retreat from 2013 to 2017 and 2017 to 2018 (which include the effects of Hurricane Matthew [Oct. 7–8, 2016] and Hurricane Irma [Sept. 10–12, 2017]) were consistent with the rates observed from 2011 to 2013 (McDonald and Gregory 2019). McDonald and Gregory (2019) found a similar pattern of gradual erosion at Raccoon Key and larger areas of change at the other sites with little change between. In addition to the areas of gradual change at Raccoon Key, large areas of change were also identified, suggesting punctuated erosion events are also influencing areas of the island with similar margin morphology. The purpose of the 2019 resurvey was to determine the rate(s) at which the island margin retreats during a non-hurricane year. It was hypothesized that the amount and rate of change at the sites would be more similar to the results of Calhoun and Riley (2016) than to the results of McDonald and Gregory (2019). The 2019 resurvey found that the percent of retreat at all sites was much lower than during all previous years. The amount of island margin change was most similar to the change detected between 2011 and 2013 by Calhoun and Riley (2016). The relatively high percent of the bank that did not change coupled with the fact that no major hurricane affected the island between the 2018 and 2019 surveys suggests that the large areas of change (observed during previous surveys) may be extreme event driven. If no major hurricanes affect the island during the 2019 hurricane season, an additional survey would help determine average non-hurricane year rate(s) of change. Similar processes of margin toe erosion, undercutting, and eventual bank failure are affecting all of the sites. As the island margin becomes undercut, a threshold will be reached and an overhang or block of bank will detach from the margin, causing the island margin to retreat. The main difference between the study sites (which creates the differences in the rate[s] of retreat) are the height and steepness of their banks, bank sediment composition, and type and density of vegetation cover. The main report provides some background on each of the study sites, a general overview of the methods used to collect and process the data, and a discussion of the results and processes causing these changes.

SECN Inventory [Complete]: Back Barrier Erosion at Cumberland Island National Seashore

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