A MODIFIED COASTAL VULNERABILITY INDEX (CVI) FOR SMALL ASSOCIATED ISLANDS OF PUERTO RICO AND THE U. S. VIRGIN ISLANDS

The Coastal Vulnerability Index (CVI) is a relative measure that incorporates key physical and geomorphic parameters to determine vulnerability to coastal hazards. In this report, a brief overview of coastal vulnerability to sea level rise is presented for the region of Puerto Rico and the U.S. Virgin Islands (USVI). Small islands (>2 km in length/width) associated with Puerto Rico and the USVI were chosen as a case study. Islands with significant cultural, historic, recreational, or natural resource value were chosen. A USGS CVI analysis of the U. S. Virgin Islands National Park provided methodological basis for our investigation.

The suite of variables outlined by the USGS in their CVI model was modified to reflect the processes which mainly influence coastal vulnerability to sea level rise in the region. These processes are interpreted to be dominantly related to winds/waves/currents, tectonic activity, and regular climatic events such as tropical cyclones. The six "original" variables in the USGS format are geomorphology, shoreline erosion and accretion rates (m/yr), coastal slope (percent), rate of relative sea-level rise (mm/yr), mean tidal range (m), and mean wave height (m). However, the regional tectonic and climatic setting of Puerto Rico and the USVI is not reflected in this list. Therefore, we developed and applied an index which accounts for vegetation, storm frequency, rates of tectonic subsidence/uplift, shoreline erosion and accretion rates (m/yr), coastal slope (percent), rate of relative sea-level rise (mm/yr), and significant wave height. CVI values were computed twice: once in the original USGS format, and again in the newly modified format. Detailed field mapping and ground validation of two islands in addition to literature and aerial photo review contributed to the modification of the index.

It was generally observed that the modified index produced a slightly higher vulnerability value. This checklist is preliminary and must undergo further ground validation and modeling before it can be suitable for vulnerability assessments on the larger islands. Small islands proved to be a useful case study by providing a discrete study area largely isolated from anthropogenic disturbance, and a small enough surface area to minimize travel time and field work expenses.