Forms of Coastal Erosion

Episodic vs. Continuous Coastal Erosion

• Coastal erosion can occur gradually over time, or it may occur in dramatic episodes, due to such factors as hurricanes or tsunamis. The varying wave patterns of El Nino can also cause regular, episodic erosion of coastal areas. The amount of erosion due to a specific event can vary significantly. Some coastal areas are subject to continuous erosion, such as the Mississippi River delta. Rather than increasing in size as sediment is deposited, the levees built along the Mississippi River have blocked sediment flow into the delta. The lack of river sediment deposition into the delta combined with the water's eroding effects on the delta mean that this area is being continuously washed away without being replenished by rich soil from the river.

Types of Coastal Erosion

• Coastal erosion can affect various parts of the coastal environment. Beach erosion occurs when sand and rock from beaches are removed by wind or water. Sand moves offshore and the coastal land area shrinks. Dune erosion, also known as "scarping," occurs when sand dunes are washed away by large waves or blown away by strong winds. Scarping usually occurs during storms or severe weather. Overwash occurs when sediment is washed inland during a storm, past the point where normal waves would be able to reach. This means that sediment is lost, since it is deposited so far inland that it cannot be washed out toward the coast again. Bluffs, a type of broad-faced cliff that usually rise over 25 feet above surrounding water and land, are also at risk for erosion. They can be eroded by sea waves, wind and groundwater runoff.

Mechanisms of Coastal Erosion: Hydraulic Action and Corrosion/Solution

• One major mechanism of coastal erosion is hydraulic action. Hydraulic action is due to the weight of the water crashing against rocks on the shore. Rocks contain tiny pores filled with air, and these imperfections can cause the rock to crack or splinter when it is repeatedly battered by seawater. These cracks and splinters can lead to caves, arches and even "stacks," which are free-standing rock pillars that were once connected to nearby land before the connection was eroded away. Corrosion, also known as solution, occurs when rock and land are eroded due to a chemical reaction between the land and the water. For example, the acidic pH of seawater reacts with the basic pH of limestone, causing the limestone to dissolve and wear away.

Mechanisms of Coastal Erosion: Corrasion and Attrition

• Corrasion occurs when rocks, boulders and pebbles tumble against each other in the seawater. These rocks break apart over time, becoming rounder and smoother, due to the action of the water and interaction with other rocks. Eventually, corrasion will grind rocks down to small particles of sand. This process of grinding down is called "attrition."