Seawalls and Beach Erosion

Seawalls and beach erosion

Beaches are one of the most important economic and environmental zones along coasts everywhere. In the United States alone, there are over 19,000 miles of beaches, 500 miles of which are within designated National Parks. Beaches are the boundaries between land and sea. A large part of the human population lives within driving distance of a beach. One of the recent environmental problems of importance is the increasing loss of beaches due to erosion of the shorelines.

Sediments, transported from inland resources, stock the rich supply of sands on beaches. Rivers drain inland valleys and plains and carry eroded particles from their source to their final deposition at sea. An enormous amount of sand and mud is carried by rivers such as the Nile and Mississippi. These rivers nourish farmers' lands and provide transport for economic goods. Because of their value, they are not often dammed, and their flow remains relatively unimpeded. However, unlike their famous counterparts, other rivers that feed coastal areas have been dammed in more than one place. The dams can provide everything from hydroelectric power to reservoirs for recreational and agricultural use.

The sediment load effect of large dams is often not observable until the river reaches the shore. The dams act as brakes for flowing water . When velocities are reduced, so is the carrying capacity of the river. Sediment loads are dumped on the upriver side of the dam. For local operators, the problem is one of continuously having to dredge the dams so the water can run through the turbines with the same force. For beaches, it is the reduction and even loss of sediment supply. The usual supply of sand is drastically reduced. Sand starvation occurs near the river mouths.

Another contributing factor to beach erosion is the loss of vegetation. The roots of plants keep sands in place. Shoreline plants are unique in that they have adapted to saltier conditions than their freshwater counterparts. When homes and business encroach on this delicate habitat the function they serve is lost. With increasing urbanization, more and more beach ecosystems are destroyed.

The Atlantic seaboard is an excellent example of a region that is battling beach erosion. Along the coast, a long-shore current displaces the sand from the river source and down the coast. This current is referred to as littoral drift. Waves refract at often-steep angles off the shore and move sand from its original place to one a bit farther down the coast. Over time, the transport of sediment moves sand far away from the river mouths. It is deposited on beaches where it builds up into the shoreline. When sand supplies are decreased and the sediment loads are reduced, the waves continue to work away at the beach. They move sand in a daily cycle of littoral drift. Unfortunately, when the sand is reduced, the shorelines are eaten away by the longshore transport. For many this means the loss of sandy beaches and even the loss of homes.

Many types of remedies are attempted to reduce beach erosion. Some have had limited success while others appear to slow the erosion of sand. Some of the more long-lasting remedies are sea walls and jetties. Sea walls were designed to slow or reduce the impact of waves on the beach. They are built on top of and parallel to the beach. As waves come toward shore they first strike the sea wall and their energy is dissipated. When the waves finally reach the shore they are so weak that they carry hardly any sand away.

Another structure, the jetty, is built perpendicular to the beach. Jetties are walls of rocks or cement that jut out into the water. They are often constructed in groups. Their purpose is to control the flow of water along the shore and to sometimes block the movement of sand and sediment. Technically, a jetty keeps sand from moving to a certain location, such as a harbor, while a groin is a jetty that simply keeps sand from moving down the shore. When constructed in groups, groins are called groin fields. The fields buffer sand removal along the beach.

While certainly effective, none of these structures will ultimately end beach erosion. Increased awareness of the problem has teamed geologists with environmental scientists. Many alternate plans for saving beaches are being considered. Resuming the sediment flow is the ultimate solution, and remains an economic and technological challenge.

See also Beach and shoreline dynamics; Ocean circulation and currents; Sedimentation; Wave motions