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

STATES VIRGIN ISLANDSDavid M. Bush1, Chester W. Jackson, Jr.2, William J. Neal3, and Pablo A. Llerandi-Román3, (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)*

% Erosio

n

Mean Erosion

Rate (m/yr)

Mean Accretion

Rate (m/yr)

Number of Transects

① Isla de los Palomas -0.16 95 -0.18 0.09 80

② Isla de Cabras -0.35 75 -0.50 0.11 363

③ Palominos -0.04 63 -0.14 0.12 362

④ Palominitos -0.69 75 -0.96 0.13 44

⑤ Cayo Icacos -0.30 93 -0.37 0.58 415

⑥ Cayo Diablo -0.41 76 -0.57 0.10 165

⑦ Cayo Ratón 0.02 42 -0.07 0.08 38

⑧ Isla Culebrita -0.09 67 -0.18 0.10 811

⑨ Cayo Algodones -0.05 63 -0.15 0.12 90

⑩ Cayo Santiago 0.02 55 -0.20 0.25 254

⑪ Cayo Batata -0.05 45 -0.20 0.08 33

⑫ Cayos de Ratones -0.04 64 -0.10 0.07 339

⑬ Isla Caja de Muertos -0.10 85 -0.13 0.07 705

⑭ Isla Morrillito -0.09 95 -0.09 0.03 96

⑮ Isla de Cardona 0.05 60 -0.23 0.49 88

⑯ Cayo Río -0.08 65 -0.17 0.08 170

⑰ Isla Magueyes 0.05 51 -0.11 0.22 171

⑱ Isla Matei -0.01 56 -0.09 0.08 537

⑲ Cayo Ratones -0.83 95 -0.87 0.16 43

⑳ Buck Island -0.12 56 -0.35 0.17 430

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.

A B

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 Ratones 951 Isla de los Palomas 9514 Isla Morrillito 955 Cayo Icacos 9313 Isla Caja de Muertos 856 Cayo Diablo 764 Palominitos 752 Isla de Cabras 758 Isla Culebrita 6716 Cayo Río 6512 Cayos de Ratones 649 Cayo Algodones 633 Palominos 6315 Isla de Cardona 6018 Isla Matei 5620 Buck 5610 Cayo Santiago 5517 Isla Magueyes 5111 Cayo Batata 457 Cayo Ratón 42

Table 2. Islands sorted by mean shoreline change rate.

Island/Cay(with map number)

Mean Shoreline Change Rate

(m/yr)19 Cayo Ratones -0.834 Palominitos -0.696 Cayo Diablo -0.412 Isla de Cabras -0.355 Cayo Icacos -0.301 Isla de los Palomas -0.1620 Buck Island -0.1213 Isla Caja de Muertos -0.108 Isla Culebrita -0.0914 Isla Morrillito -0.0916 Cayo Río -0.0811 Cayo Batata -0.059 Cayo Algodones -0.053 Palominos -0.0412 Cayos de Ratones -0.0418 Isla Matei -0.017 Cayo Ratón 0.0210 Cayo Santiago 0.0217 Isla Magueyes 0.0515 Isla de Cardona 0.05

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 year Low rate Moderate rate High rate

2060 0.07 m (0.20 ft) 0.6 m (1.97 ft) 0.57 m (1.87 ft)

2110 0.14 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.