Explain how longshore currents are formed

The net movement of sediment due to longshore transport is to the south along both coasts of the continental United States, because the storms and high winds that originally create the swell tend to occur at higher latitudes and move to the south. Rip currents often occur where there is a channel between sandbars that makes it easier for the retreating water to escape.

The currents lose strength quickly just outside of the surf zone, but they can be dangerous to swimmers who get caught in them and are pulled away from shore. Swimmers caught in a rip current should not try to swim directly back to shore, as it is difficult to fight the current and the swimmer can quickly tire. Explain the origins of a wave-cut platform.

Previous: Next: Chapter 18 Geology of the Oceans. Share This Book Share on Twitter. Waves form when wind blows over water. The size of the waves depends on the wind speed, the area over which it is blowing, and time.

The important parameters of a wave are its amplitude, wavelength, and speed. The water beneath a wave is disturbed to a depth of one-half the wavelength, and a wave is slowed when it approaches shallow water.

A longshore current develops where waves approach the shore at an angle, and swash and backwash on a beach move sediment along the shore.

The combined effect of these two processes is sediment transport by longshore drift. Coasts that have experienced uplift within the past several million years tend to have irregular shapes and are dominated by erosional processes.

Wave paths are bent where the coast is irregular and wave energy is focused on headlands. Rocky headlands are eroded into sea caves, arches, stacks, and sea cliffs, and the areas around these features are eroded into wave-cut platforms.

Over the long term millions of years , irregular coasts are straightened. They are responsible for many rescues along the coast by sweeping swimmers and surfers down the beach into a variety of hazards. They also have a large inpact on the shoreline. A shoreline is not static. Importantly, the longshore current not only moves water in the surf zon. Figure 1 illustrates that the direction of longshore current is a function of the angle of wave appoach.

For example, if the waves appoach the shoreline from the south, longshore current moves from south to north. But longshore current and the resulting transport of sediment is only one of the dynamic processes that constantly alter the shoreline.

As waves repeatedly hit the shore, water moves onto the beach and then retreats in a continuous cycle. However, the waves are not all that moves on the shoreline.

In fact, the sediment on the shore is also always on the move. Great energy is expended on the beach as waves crash against the shoreline. This energy allows the water to transport sediment.

The grains are lifted as the waves in the swash zone move onto the beach, and then the grains are deposited again as the water retreats. In this case, no real net movement of sand occurs in the swash zone.