Shoreline Protection

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 upriver 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.