1. 50- AND 100-Y EAR S HORELINE C HANGE P REDICTIONS FOR S ELECT S MALL I SLANDS OF P UERTO R ICO AND THE U NITED S TATES V IRGIN I SLANDS David M. Bush 1, Chester W. Jackson, Jr. 2, William J. Neal 3, and Pablo A. Llerandi-Román 3, (1) Department of Geosciences, University of West Georgia, Carrollton, GA 30118, dbush@westga.edu, (2) Department of Geology and Geography, Georgia Southern University, Statesboro, GA 30460, (3) Department of Geology, Grand Valley State University, Allendale, MI 49401 ABSTRACT 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. STUDY AREA Twenty islands were selected for inclusion in the study because they exhibit either historical, cultural, recreational, or scientific importance, or some combination of factors. Table 1. Mean shoreline change data for the twenty study area islands. Island/Cay (with map number) Mean Shoreline Change Rate (m/yr)* % Erosion Mean Erosion Rate (m/yr) Mean Accretion Rate (m/yr) Number of Transects ① Isla de los Palomas -0.1695-0.180.0980 ② Isla de Cabras -0.3575-0.500.11363 ③ Palominos -0.0463-0.140.12362 ④ Palominitos -0.6975-0.960.1344 ⑤ Cayo Icacos -0.3093-0.370.58415 ⑥ Cayo Diablo -0.4176-0.570.10165 ⑦ Cayo Ratón 0.0242-0.070.0838 ⑧ Isla Culebrita -0.0967-0.180.10811 ⑨ Cayo Algodones -0.0563-0.150.1290 ⑩ Cayo Santiago 0.0255-0.200.25254 ⑪ Cayo Batata -0.0545-0.200.0833 ⑫ Cayos de Ratones -0.0464-0.100.07339 ⑬ Isla Caja de Muertos -0.1085-0.130.07705 ⑭ Isla Morrillito -0.0995-0.090.0396 ⑮ Isla de Cardona 0.0560-0.230.4988 ⑯ Cayo Río -0.0865-0.170.08170 ⑰ Isla Magueyes 0.0551-0.110.22171 ⑱ Isla Matei -0.0156-0.090.08537 ⑲ Cayo Ratones -0.8395-0.870.1643 ⑳ Buck Island -0.1256-0.350.17430 SHORELINE CHANGE Shoreline position rate-of-change was calculated using end point rate in the AMBUR program (Jackson et al., 2012) Historical shoreline position data digitized from NOAA t-sheets and aerial photographs Historical shorelines were digitized as continuous lines Virtual transects were cast by AMBUR every 10 meters A total of 5,234 transects were generated among all the study area islands Length of all shorelines digitized totaled 106 km Total area of all the islands in the study was 609.97 hectares, just over 6 square kilometers Shoreline Change Analysis Statistics calculated using AMBUR (v.1.03-19) available at https://r-forge.r-project.org/R/?group_id=476A total of 5,234 transects were generated among all the study area islands AMBUR was then used to project the 50- and 100-year shoreline positions Palominos, PR Isla Palominos The “fill-the-bathtub” or linear superposition model. Superimpose a given amount of sea-level rise on the modern topography. Shown is Isla Palominos: (A) the current shoreline, and (B) the shoreline with +5 meter sea-level rise. From NOAA Sea Level Rise Viewer. The NOAA Sea Level Rise and Coastal Flooding Impacts viewer can be accessed at http://coast.noaa.gov/digitalcoast/tools/slr. AB The simplest way to project shoreline position. Table 3. Islands sorted by percent of island shoreline eroding. Island/Cay (with map number) % Erosion 19 Cayo Ratones95 1 Isla de los Palomas95 14 Isla Morrillito95 5 Cayo Icacos93 13 Isla Caja de Muertos85 6 Cayo Diablo76 4 Palominitos75 2 Isla de Cabras75 8 Isla Culebrita67 16 Cayo Río65 12 Cayos de Ratones64 9 Cayo Algodones63 3 Palominos63 15 Isla de Cardona60 18 Isla Matei56 20 Buck56 10 Cayo Santiago55 17 Isla Magueyes51 11 Cayo Batata45 7 Cayo Ratón42 Table 2. Islands sorted by mean shoreline change rate. Island/Cay (with map number) Mean Shoreline Change Rate (m/yr) 19 Cayo Ratones-0.83 4 Palominitos-0.69 6 Cayo Diablo-0.41 2 Isla de Cabras-0.35 5 Cayo Icacos-0.30 1 Isla de los Palomas-0.16 20 Buck Island-0.12 13 Isla Caja de Muertos-0.10 8 Isla Culebrita-0.09 14 Isla Morrillito-0.09 16 Cayo Río-0.08 11 Cayo Batata-0.05 9 Cayo Algodones-0.05 3 Palominos-0.04 12 Cayos de Ratones-0.04 18 Isla Matei-0.01 7 Cayo Ratón0.02 10 Cayo Santiago0.02 17 Isla Magueyes0.05 15 Isla de Cardona0.05 Projected sea-level positions assuming a low, moderate, and high rate of sea-level rise over the next 50 and 100 years. The moderate-rate values were used in the current shoreline position projections. Numbers are relative to current sea level. Data from PRCCC (2013). Planning yearLow rateModerate rateHigh rate 20600.07 m (0.20 ft)0.6 m (1.97 ft)0.57 m (1.87 ft) 21100.14 m (0.40 ft)1.9 m (6.2 ft)1.70 m (5.59 ft) Sea-Level Projections Table 2 Negative = erosion, positive = accretion Greatest rate is -0.83 m/yr (Cayo Ratones) Most island shorelines are very nearly stable, four are accreting Most rates are within the statistical margin of error of +/- 0.2 m/yr Table 3 All but two islands have at least 50% of their shoreline experiencing erosion. Four islands have 95% of their shoreline eroding. Eight islands have at least 75% of their shoreline eroding.