Accretion was measured for 1971 to 1977. The added sand was from the major flood during tropical storm Eloisa in 1975. These accretions can been seen on the comparisons of the 1973 shoreline to the 1975 shoreline.
Accretion continued from 1977 to 1993, but severe erosion occurred on the south beach close to Rio Grande de Añasco from 1987 to 1993. Loss of sand was caused by floods during the period and the construction of beach facilities on and near the beach and related activities.
Certain patterns and common elements can be seen in the erosion of the shoreline in Puerto Rico. At many locations, the initiation of rapid erosion commences suddenly, after a period of very little or no erosion. After a period of erosion, a shoal (usually rock underlying relatively thin beach sands) may become exposed in front of the eroding beach, resulting in a great reduction in the rate of erosion, as seen at north shore, beachrock and eolianite ridges may contribute to beach protection. In some areas, the shoreline has adjusted to the altered wave and current regime and the reduced sediment supply from rivers, and a new equilibrium has been established.
The shorelines at Isla Cabras and Punta Iglesia have both shown a great increase in erosion since 1971. The eastern shoreline of Isla Cabras was rebuilt to restore the prior shoreline position. Erosion has slowed drastically at Levittown with the development of a protective shoal (beachrock ridges) in front of the beach. At Arecibo, the beach system had established a new equilibrium, which was changed by shoreline construction. At Punta Salinas, the beach may have been stabilized with construction of a jetty on the west shore and emplacement of riprap on the east shore. But, this riprap has resulted in problems at the Levittown beach.
Erosion is fairly widespread in Puerto Rico (Morelock and Trumbull, 1985). Changes in the wave regime of the winter storms, erosion of natural barriers, loss of reef and mangrove protection, and reduced sediment input from the rivers (all rivers in Puerto Rico now have dams) may be contributing factors to the prevalance of erosion. There is evidence that the shoreline may establish a new equilibrium after a period of erosion. The shoreline of Puerto Rico that shows no erosion or even accretion is generally fronted by mangrove forest and extensive reef tracts on the shelf.
Changes in the beach sands to higher terrigenous content have occurred in the last ten years (Barreto, pers. obs.). This probably resulted from urban expansion south and west of the San Juan Metropolitan Area. The vegetated mountain areas have been populated along the river basins resulting in increased land erosion and sediment production.
Barreto, Maritza. 1997. "Shoreline Changes in Puerto Rico (1936-1993)." Ph.D., University of Puerto Rico.
Morelock, Jack. 1984. Coastal Erosion in Puerto Rico. Shore and Beach: 18-27.
Morelock, Jack, and J. Trumbull. 1985. Puerto Rico Coastline. The World's Coastlines. Ed. Bird. New York: Van Nostrand Reinhold.