BEACH EROSION

Examination of the coast by aerial reconnaissance, beach surveys, and comparative analysis of aerial photographs shows that there is erosion on all coasts. The data available for the Caribbean, Central America, Mexico, and the southern part of the United States indicates that erosion is the dominant activity in the beach and coastal areas. There was a history of aggradation beginning roughly 5,000 years ago and then a shift from deposition to erosion during the past 500 years. Although the rate of beach erosion fluctuates, there is presently a general state of recession of Caribbean beaches and shorelines.

The causes of this erosion may be:

These factors are not isolated, but are complexly inter-related. Although the rapid rise of sea level ended some 5,000 years ago, measurements lead some geologists to believe that there has been a slow rise of several centimeters during the last fifty years. In an area of limited tidal range such as Puerto Rico, a rise of several centimeters would lead to changes in the beach equilibrium. However, the change from aggradation to erosion throughout the middle America region is not synchronous as would be expected if the controlling mechanism were continued sea level rise.

Diastrophism, producing changes in the height of the land by vertical rise or fall, also affects the stability of the shoreline. If the land has dropped relative to sea level, there would be increased erosion. These movements and the accompanying effect may be local in nature.

The eolianites and beachrock along the north coast of Puerto Rico form a relatively continuous barrier which protects the shore. Behind this protection, lagoons, tombolos, and other depositional features have formed. This protection is being breached and removed by the natural force of the waves. Where the eolianite is breached, erosion has cut into the land to form lunate bays. Because eolianites on the north coast are loosely cemented, they are being reduced in height and are being destroyed, allowing waves to penetrate to the beaches with greater force. This has upset the established equilibrium and caused retreat of the shore and erosion of the beaches. Beachrock is separated from the coast by tens of meters in many places, indicating recent erosion.

Many beaches are protected by fringing and offshore coral reefs. Increased sedimentation and other factors have led to the diminished ability of many of these reefs to survive, with a resulting increase in coastal erosion as they cease to be a barrier to wave energy. The development of agriculture on the.~ Island and later heavy urbanization and industrialization have been major factors contributing to the loss of reefs.

Human activity is important locally. The activities of man have been varied and striking in contributing to the loss of coastal areas. In some cases the natural transfer of sand from one area to another has been blocked by the construction of new structures such as the causeway between Punta Tocones and Isla de Cabras . This has not only cut off a source of sand nourishment, but has altered the prior circulation patterns, and tombolo of Punta Salinas is now being eroded.

Removal of sand from river mouths includes the collection of littoral drift sand and results in erosion such as that seen near the mouths of the Añasco and Loiza rivers. Removal of sand dunes and beach sand for use as fill and aggregate have certainly contributed to the depletion of many beach systems. In areas where sand dunes have been removed, leaving only a small barrier, waves have started to break over the barrier and are carrying sand away.

Construction activities have crowded close to the shoreline because of limited land areas and reduced construction costs. This has not only aggravated the erosional process, but has put valuable new property in areas of natural erosion. This has in turn created the need to institute urgent and expensive protective techniques to protect the investment. These remedies may have effects not immediately recognized. Construction close to the beach south of Mayagüez Harbor was being rapidly endangered by erosion~ Riprap was emplaced to protect this property, which then cut off a source of sand -- natural coastal erosion -- from the area to the south. The next step was riprap protection for houses to the south which were being threatened by the erosion generated by the riprap. As the problem moved south the entire beach was eventually replaced with riprap. The coast is now stabilized, with a basic change in coastal classification from sandy beach to rocky shoreline, man-made.

It has been suggested that the broad change from aggradation of beach and coastal land to the present widespread state of erosion is part of maturing of the system. The entire process may be an evolving adjustment to the present sea level. There are many areas in Puerto Rico such as the accretionary beach ridges east of Ponce, the Humacao coastal region, and the many tombolos that show past periods aggradation. There is also evidence of a shift to erosion in these same areas.

Beaches are fragile and transitory geological features and require, care and consideration if we are to transmit to our children the joys of a day at the beach.