The more prominent the swell size and direction, and the longer and straighter the beach is, the more powerful and swift the long-shore current will be. 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. Unit 9. Unit Learning Objectives. Review Quiz. Key Term Flash Cards. Animations and Animation Quizzes. Beach Drift and Longshore Currents. Contact Your Sales Rep. Higher Education Comment Card. The step-like ridge on the beach, which is developed by waves, is termed the:.
The overall flow of water parallel to the coast and just below the wave breaking zone is termed:. Then the water runs back down the beach, which is called the backwash. With a constructive wave, the swash is stronger than the backwash. Longshore drift is the movement of material along the shore by wave action.
Longshore drift happens when waves moves towards the coast at an angle. Longshore drift provides a link between erosion and deposition. Material in one place is eroded, transported then deposited elsewhere. 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.
Figure The backwash of water rushes down the shoreface perpendicular to the shoreline or a slight downcoast angle, thus creating a zigzag movement of sand. This zigzag motion effectively results in a current parallel to the shoreline. Longshore drift from longshore current is a geological process that consists of the transportation of sediments clay, silt, pebbles, sand, shingle along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction.
This current and sediment movement occur within the surf zone. Even though they bend and become nearly parallel to shore, most waves still reach the shore at a small angle , and as each one arrives, it pushes water along the shore, creating what is known as a longshore current within the surf zone the areas where waves are breaking Figure Where does most of the sand involved in longshore transport in beach compartments eventually end up?
Longshore transport moves sediment upcoast or downcoast along the beach. Longshore transport can change direction depending on which direction the waves approach the beach.
Why shoaling happens: waves get slower, shorter and higher In deep water, a tsunami moves very fast and has a long wavelength and a small amplitude. As it enters shallower water, it slows down and the wavelength decreases.
This causes the wave to become much taller. Longshore currents occur along a coastline. They are caused in the following way: waves strike the shore at an angle and the water accompanying the waves tries to rush back out to sea.
Both temporal and geographic variations may occur in each of these coastal types. Erosional coasts typically exhibit high relief and rugged topography. They tend to occur on the leading edge of lithospheric plates , the west coasts of both North and South America being excellent examples. Groynes control beach material and prevent undermining of the promenade seawall.
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