50- AND 100-YEAR SHORELINE CHANGE PREDICTIONS FOR SELECT SMALL ISLANDS OF PUERTO RICO AND THE UNITED STATES VIRGIN ISLANDS

Predicting how shorelines will respond to rising sea level is critical for developing sound coastal management and land use planning guidelines. However, projections of shoreline position 50 or 100 years into the future can be dubious at best depending upon the input data. The greater the number of generations of data used, the higher the confidence that the historical behavior of the shorelines has been captured, however, past behavior is not necessarily indicative of future behavior. Add in the uncertainty about sea-level rise rate, and projected shoreline positions may be nothing more than conjecture.

An attempt was made to refine the shoreline position projection methodology by combining historical behavior of the shoreline with composition of the material into which the shoreline will be eroding. Thus, the input data were historical shoreline change, projected sea-level rise, shoreline type, and inland substrate. Evaluations were made along virtual transects at 10-meter spacing, giving highly-detailed output of projected shoreline position. The study was based on a coastal vulnerability analysis of 20 small islands around Puerto Rico and the U. S. Virgin Islands. Islands ranged in size from approximately 0.5 – 366 acres. Shoreline and island interior compositions are quite varied and include igneous, metamorphic, and sedimentary rock; fine, medium, and coarse sand; gravel; mangroves; and engineered structures. Land cover and land use also vary highly from island to island.

Results were mixed, with some shorelines projecting to highly irregular positions. Insufficient input data and resultant limits in trend analysis are probably the main controlling factors. It is likely that simple straight-line trend is not an accurate predictor of shoreline behavior. However, it is a place to start when attempting to make management plans for several decades into the future.