Longshore currents are a common and influential natural phenomenon found along coastlines worldwide. These currents involve the movement of water parallel to the shoreline, primarily within the area where waves break. They are a significant force in shaping the sandy beaches and coastlines we observe, constantly influencing the distribution of sand and other sediments.
Understanding Longshore Currents
Longshore currents flow parallel to the coastline within the surf zone, the area where waves consistently break. The formation of these currents begins with waves approaching the shore at an angle, rather than directly perpendicular to it. As these angled waves enter shallower water, the part of the wave closer to the shore begins to slow down first, while the part further out continues at its original speed.
This difference in speed causes the wave to bend, or refract, and creates a component of water movement that runs along the shore. The continuous arrival of these angled waves pushes water along the beach, generating the longshore current. The strength and speed of a longshore current can vary, influenced by factors such as wave height, the angle at which waves approach the shore, and the slope of the beach. While often slow, moving at speeds typically between 0.1 to 0.53 meters per second, they can reach up to 4 kilometers per hour and even exceed 2.5 meters per second during storm conditions.
Role in Coastal Dynamics
Longshore currents play a substantial role in the continuous reshaping of coastal environments. They are the primary driver of a process known as longshore drift, which is the movement of sand and sediment along the coastline. As the longshore current flows parallel to the shore, it carries suspended sediment particles within the water. This constant transportation of material results in the gradual shifting of beaches over time.
The combined effect of water moving along the shore and waves washing sediment up and down the beach in a zigzag pattern contributes to this significant transport. This process can lead to the accumulation of sand in some areas, forming or enlarging features like beaches, spits, and sandbars. Conversely, it can also cause erosion in other areas, as sand is continuously removed.
Safety and Interaction with People
While less hazardous than rip currents, longshore currents can still affect beachgoers. They are characterized by their parallel movement along the shore, which distinguishes them from rip currents that flow directly away from the shore. If caught in a longshore current, swimmers might find themselves gradually carried down the beach from their original entry point.
The main concern with longshore currents for swimmers is fatigue if they attempt to fight against the current to stay in place. Instead, if you find yourself being carried by a longshore current, it is advisable to swim parallel to the shore, in the same direction as the current, until you are out of its main flow. Once clear, swim towards the shore. Staying calm and conserving energy are important actions. Always swimming near lifeguard-patrolled areas provides added safety and guidance.