A COASTAL VULNERABILITY ANALYSIS OF SELECT SMALL ISLANDS OF PUERTO RICO AND THE UNITED STATES VIRGIN ISLANDS

Coastal vulnerability was analyzed for 20 small islands around Puerto Rico and the U. S. Virgin Islands. A coastal vulnerability index (CVI) was utilized based on shoreline composition, sea-level change, offshore slope, and shoreline change rate. Shoreline position rate of change and CVI were calculated using AMBUR and AMBUR-HVA (Jackson et al., 2012). The 20 islands encompass an area of 609.97 hectares. Islands ranged in size from 147.93 - 0.21 ha. The area of the islands based on oldest shoreline position data used is 630.29 ha. The area of modern shorelines is 609.97 ha. The total measured land loss is 20.32 ha over the approximately 50-year period of study.

A total of 5,234 virtual transects along the total of 106 km of shoreline analyzed. Of these transects, 3608 exhibit erosion. The maximum erosion along a single transect is ‑132.35 m, the mean for all transects exhibiting erosion is ‑9.34 m; the maximum rate is -2.97 m/yr, with a mean erosion rate of -0.24 m/yr. For those transects exhibiting positive shoreline change (accretion), the maximum along a single transect is 69.42 m, the mean is 5.69 m; the maximum accretion rate is 1.8 m/yr, the mean is 0.14 m/yr. The overall mean shoreline change is -4.7 m (erosion) and the overall mean rate is -0.12 m/yr (erosion). For an individual island, the greatest mean shoreline change rate is -0.83 meters/year, exhibited by Cayo Ratones. Many of the islands are very nearly stable. Three islands have positive shoreline change rates. Many of the rates are within the statistical margin of error of +/- 0.2 m/yr. The largest percentage of an individual shoreline’s shoreline exhibiting erosion is 95%, found on three islands. The smallest percentage of an island’s shoreline exhibiting erosion is 42%. CVI is calculated for each transect, and a mean CVI is calculated for each island. CVI values are relative numbers based on the number of parameters employed in any given study. In this study, utilizing four input parameters, the theoretical range of CVI values is 1.00 -12.50, with larger numbers reflecting greater vulnerability. Actual values for individual transects range from 1.22 to 7.91, for island mean the range is 2.65 – 6.95. Based on this study, a series of management recommendations were developed to aid in developing land use and management plans.