Shoreline Protection

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Shoreline Protection

Shoreline protection is intended to decrease or eliminate coastal erosion. Because sea level is rising and many coastal areas have become highly developed, shoreline erosion is an issue for many communities. In essence, shoreline protection involves the construction of engineered structures or other techniques to slow erosion.

The shoreline is the area located between the low tide mark and the highest point on land affected by waves during storms. Shorelines are dynamic features in that they move landward or seaward depending on the rise or fall of sea levels 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. Many coastal dwellers still protect shorelines in this manner. However, others are beginning to understand that the shoreline environment is a system in its entirety, with many processes at work within it. If any part is changed, a natural response, however unexpected, is likely to occur.

Types of shorelines

Shorelines located where mountain building processes such as uplift, 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. Shorelines in less active areas 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 erode rocky cliffs and present problems to communities and homeowners that build roads and other structures along the 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 can 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 via 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.

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 that are constructed upriver can limit the amount of sediment initially reaching the beach. The hard structures placed along coasts for shoreline protection further rob beaches of sediment by keeping it from being transported downcurrent. In addition, waves 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 beach erosion. 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 levels along coasts where buildings are at risk from beach erosion.

Types of shoreline protection

Historically, the structures developed for shoreline protection were constructed of durable materials such as rock and reinforced concrete. They were designed to withstand the force of wave action. Such hard stabilization methods are still in use today and include seawalls, revetments, breakwaters, impermeable groins, and jetties.

Seawalls are structures built at the water’s edge of concrete or large stone known as riprap. Their purpose is to bear the full force of the wave action, thereby protecting the cliff face. However, they also encourage the beach in front of them to decrease in width and are considered an eyesore by many people.

Revetments of broken concrete or riprap are powerful devices for reducing the energy from wave action, and they are repaired inexpensively. Their irregular surface offers protection from wave run-up, or the movement of breaking waves up the shore. Revetments often limit access to the beach and, as with seawalls, can be unsightly.

Groins are sediment traps. They protrude at right angles from the shore and trap sediment carried by longshore drift on their upcurrent sides. However, this sediment never reaches the downcurrent side of the groin, so the beach narrows. For this reason multiple groins are usually constructed in an area.

Breakwaters may be connected to the shoreline at one end or completely separate from it. Their purpose is to bear the brunt of the waves, producing calmer water shoreward of the structure. Jetties are used to keep a channel open and are placed one on each side of the channel’s outlet. Both of these structures impact littoral longshore transport causing beach buildup on their updrift sides and erosion downdrift. Dredging is often required to keep them functioning.

While hard structures continue to be used for shoreline defense, soft stabilization methods are becoming more prevalent in coastal areas, either as the sole method of protection or in conjunction with hard stabilization practices. The most utilized form of soft shoreline protection is beach nourishment, or the replenishing of sands on an eroding or retreating beach. Its greatest advantage is that nourishment extends the time until erosion undermines the structures behind the beach. Beach nourishment also allows for a wider, more usable beach, which provides better recreational areas and economic revenue for those living near it. But it also has disadvantages. It is extremely costly, and nourishment must be performed every few years to keep beaches from retreating after storms. In addition, impacts to the beach ecosystem often occur during, or as a result of, the nourishment. If excessively muddy sands are used, organisms may be smothered, and building beaches steeper than their original profile may limit their use by various forms of marine life.

Recently, new types of soft stabilization have been introduced. Wave screens, submerged breakwaters, active submerged breakwaters, and floating breakwaters do not disturb or change current flow, but rather allow water and fish to pass through their partially transparent structure. Improved physical structures that aid in shoreline protection are not the only ideas under consideration for the future. Enhancement of the environment

KEY TERMS

Barrier island —An island separated from the mainland by a lagoon. They are formed from deposition of sediment during shoreline processes.

Beach nourishment —The artificial process of adding sediment to a beach to improve recreation and appearance and to provide a buffer to coastal erosion.

Littoral cell —The system of sediment movement that delivers sediment to the shoreline, transports it along the shoreline, and may eventually result in its loss in deeper water away from the shore.

Longshore drift —Transport of sediments by currents flowing parallel to the beach.

through vegetation of the shores and an understanding of how each inhabitant of the shore environment contributes to the health and well being of the coast must play an active part in coastal planning.

For example, recent research has shown the eggs of the Loggerhead turtle provide much-needed nutrients to beach areas where they nest. These nutrients ensure healthy stands of coastal vegetation, which help keep the beach in place. In an effort to protect the threatened turtles, their nests are often relocated, depriving the original nesting sites of these nutrients. Taking into account such nuances when considering the type of shoreline protection to use will allow for a more complete and natural form of shoreline protection.

Past trends in shoreline protection have emphasized expensive engineered defenses. The realization that shorelines are dynamic and erosion is a natural and inevitable process has led to revolutionary and controversial ideas about shoreline protection. Sea levels are expected to continue rising. Should they increase dramatically, expensive engineered structures and replenished beaches may become ineffective. Some communities have considered the idea of relocating buildings. Along very densely populated coastlines this is not a feasible alternative, but the idea of restricting coastal development is gaining support. North Carolina has strict regulations governing the types and sizes of structures that can be built on its shoreline.