Shoreline Protection

Shoreline protection

Shoreline protection is the engineering effort designed to lessen or eliminate coastal erosion. Because sea level is rising and we have chosen to develop coastal areas, shoreline erosion has become a common and urgent problem for many communities. In essence, shoreline protection consists of engineered structures or other solutions meant to slow erosion by rising sea levels and storm wave action.

The shoreline is the area located between the low tide mark and the highest point on land that storm waves impact. They are dynamic features in that they move landward or seaward depending on rise or fall of sea level and the amount of uplift or subsidence (sinking) of the area. Currently sea level is rising-in the past century it has risen more than 4.5 in (12 cm) globally. Two-thirds of the world's people currently live near shorelines. New York, Los Angeles, Tokyo, London, and Rio de Janeiro are just a few of the major cities built near the sea.

In the past, shoreline protection was considered a local project. A single landowner or community designed a site-specific defense against erosion. While this effort might solve their erosion situation, the problem with this approach is that it often results in erosion on adjacent or nearby stretches of coast. Then the adjacent or nearby communities must also take defensive action. Unfortunately many coastal dwellers still tend to defend shorelines in this manner. However, other residents are finally beginning to grasp the concept that the shoreline environment is a system in its entirety, with many processes at work within it. If you make changes to any part of the system, a natural response, however unexpected, is likely to occur.

Types of shorelines

Not every shoreline is identical. Those located where mountain building processes, such as uplift and folding and faulting are active, consist of rough, steep cliffs and rocky stretches reaching out into the sea, as well as beaches. These coastlines tend to be irregular, jutting in and out along their length. Shorelines found where these processes are not active tend to have long, wide beaches and often are characterized by islands located seaward of the shoreline, known as barrier islands. Both of these shoreline environments face unique erosion problems.

Crashing waves exert tremendous erosional power on rocky cliffs and so present serious problems to communities and homeowners that build roads and other structures upon these cliffs. Lateral erosion rates from constant wave action are as much as 6 ft (2 m) per year in some areas of the world. To slow the undercutting of cliffs, concrete structures or large boulders are often placed at the water's edge to absorb the force of breaking waves. However, minimizing the effect of urbanization on the cliffs is at least as important to slowing the rate of erosion. Constructing roads, homes, and other structures on sea cliffs increases the load on a cliff face and tends to weaken it, increasing the likelihood of slumping or landsliding. Storm water runoff from urban areas can also quickly weaken or erode cliffs. Taking measures to restrict these practices is a practical and effective approach to slowing coastal erosion.

Beaches, whether they are nestled in bays between rocky protrusions or stretch for hundreds of miles uninterrupted, are also subjected to powerful erosional forces. Rivers are the main source of sediment for many of our beaches. Once the sediment is deposited on the beach, currents transport it along the shoreline, in a process known as longshore drift. Eventually some of the sediment is lost in deep trenches or canyons, often called sinks, on the sea floor. The system made up of these combined processes is called a littoral cell.

Our shorelines consist of numerous littoral cells providing beaches with their allotment or "budget" of sediment. Each beach has a unique sediment budget. If more sediment is brought in than is lost, the budget is positive and the beach grows, but if the opposite occurs, beach erosion takes place. The dams we construct up-river can limit the amount of sediment initially reaching the beach. The hard structures placed along our coasts for shoreline protection further rob beaches of sediment by keeping it from being transported downcurrent. In addition, waves, their height, how fast they follow one another, and their direction, directly affect the amount of sediment on shore. Storm waves are particularly damaging to sandy beaches. Human activity also plays a substantial part in causing erosion of our beaches. Structures designed to stabilize or add sediment on one beach often deplete sediment on beaches downcurrent. Perhaps the most significant threat to beaches, however, is rising sea level along coasts where buildings are at risk from beach erosion.

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