Waves
Waves are the result of energy passing through water, causing water particles at and near the surface to move in a circular (“orbital”) motion. Although the water appears to travel along with the wave, it only moves a small amount as the wave passes through it. Waves are most often generated by wind, so energy is ultimately being transferred from the atmosphere into the ocean.
The wavelength of a wave is the distance from crest to crest or trough to trough (the highest and lowest points of a wave, respectively), and the height of a wave is its vertical difference between its crest and its trough. The depth to which the wave energy reaches is known as the wave base, and this distance is roughly equal to half of the wave’s wavelength.
Below is an example of two different kinds of waves, plunging and spilling, shot from the coastal plain of North Carolina.
Longshore Currents
Longshore currents occur when wind pushes the waves at an angle the shoreline, which ultimately moves sand down the beach in the direction of the current rather than just back and forth with breaking waves.
When longshore currents attack the shoreline, The beach expands outward and sideways due to the sand brought by the waves. This is pictured in the image above. Longshore currents can also contribute to the formation of barrier islands. As the currents bring the sand sideways, sand gets added onto existing sandbars, eventually accumulating into a barrier island. These deposited sands being added to one end of the barrier island is another example of a spit. The effects of longshore currents on erosion prevention will be discussed further in the erosion stabilization section.
Refracting Waves
When a shoreline is straight, the energy of the waves all stop at the same place, keeping the lines of waves parallel to each other (even if they aren’t parallel to the shoreline, like with longshore currents). However, when there is a large protrusion in the coastline, such as a peninsula, the energy of the waves hitting the tip of the peninsula stops sooner than the waves around it, causing the rest of the waves to bend, or refract, around the peninsula. All of this concentrated energy erodes down the peninsula, with the most vulnerable spot being the toward the bottom where it is connected to the mainland. This spot erodes completely first, leaving a small island, and eventually after thousands of years the island erodes away and the coastline is straightened out.
This image shows the parallel lines of wave refraction, and how a jutting out land mass disrupts these refractions. You can also see how the base of the peninsula is being eroded the fastest, and will eventually form a small island before it is completely eroded away.
Active and Passive Margins
Active continental margins are located at convergent plate boundaries, where oceanic crust is being subducted beneath the continental crust. Active margins typically have much narrower continental margins, irregular coastlines with narrow beaches, and coarser sediment than passive margins. These coastlines are less susceptible to sea level rise and risks associated with hurricanes, but the potential for local earthquakes (within the subduction zone) creates a high risk for tsunamis. An example of an active margin would be the Pacific coast.
Passive continental margins are coasts that are not situated at plate boundaries; they occur at the interior of tectonic plates. These margins are typically characterized by a broad continental margin, a regular coastline with wide beaches, and finer sediment. Since areas along these coasts are situated quite close to sea level, they are especially susceptible to sea level rise and hurricanes (in which high winds and storm surge can reach far inland).
Since the east coast of the United States is a passive continental margin, it is particularly vulnerable to those associated hazards.