The longshore transport is the process responsible for the movement of sand and sediment along the coastline. Sand and other particles are transported with the help of the longshore current and beach drift. Learn how these factors shape shorelines.
Let’s say you and some friends were playing volleyball on the beach and the ball got knocked out of the court and landed in the swash zone, which is the area at the water’s edge where waves lap up onto the beach.
If you watched the ball for a few minutes, you would probably notice that it not only moved in and out with the waves, but it also got carried down the shoreline.The volleyball moving down the beach represents part of something called the longshore transport. Longshore transport is defined as the movement of sand and sediment parallel to the coastline. In this lesson, you will learn how processes such as beach drift and the longshore current contribute to longshore transport, and how these processes shape the shorelines of the world.
The shoreline is constantly changing as sand and sediment is picked up and deposited along the shore by waves. Waves tend to come ashore on an angle as opposed to straight on because they’re pushed and bent by influences such as the wind and the shallow waters near the shore. As these angled waves come ashore and break, they create the longshore current, which is defined as the ocean current that travels parallel to the shore.
You might see the longshore current referred to as the ‘littoral current.’ The word ‘littoral’ comes from the Latin language and means ‘shore.’ So whether it is called the longshore or littoral current, it is the current that flows along with the shore. As the angled waves reach the shoreline, they push water, sand and sediment in one direction, down the length of the beach.
Longshore currents occur along any beach that is exposed to breaking waves, and the currents vary in strength depending on the shape and characteristics of the beach. For example, on a long, straight beach with prominent waves and swells, the longshore current can be very strong and pose a threat to swimmers and surfers. A strong longshore current can sweep a person down the coast and make it difficult for that person to make it back to shore. This is one reason it’s a good idea to swim at beaches manned by lifeguards.
Beach Drift and Swash
The longshore current is part of the longshore transport that moves sand and sediment down the coastline, but there is also another process that contributes to the longshore transport called beach drift, or longshore drift.Beach drift is defined as the progressive movement of sand and sediment along the beach. This is an easy term to recall if you remember that beach drift is what causes sand and other particles to ‘drift’ down the beach.
In fact, beach drift is what carried our volleyball down the beach at the beginning of our lesson. As the water waves continually lap up on the swash zone of the shore, they carry with them stirred-up grains of sand and other particles. The water that moves onshore from the waves is referred to as swash.
This is one of those fun terms to learn because you can think of it as if the ocean is taking a bucket of water from its vast supply and ‘splashing’ or ‘washing’ it onto the beach. In other words, ‘swashing’ it ashore.
Swash comes up on shore at an angle because the waves that carry the water strike the shore at an angle. So we see that the sand and other particles caught up in the wave get carried ashore in the same direction as the wave’s motion.
But the swash is pulled back into the ocean under the force of gravity, so it returns to the ocean in a straight path that is perpendicular to the shoreline. The receding water moving off of the beach and washing back into the ocean is referred to as backwash.So we see that the swash flows onto the beach at an angle carried by the waves. But the water returns to the ocean in a direction that’s perpendicular to the shore. Because of this, the sand and sediment is continually deposited in a zigzag pattern down the beach. This is the beach drift in action.
And because the waves that wash up on shore are driven by the longshore current, we see that beach drift and longshore current work together to create the longshore transport, resulting in a net overall transport of sand and sediment down current along the coast.
Let’s review. Waves tend to come ashore on an angle, and this helps create the longshore current, or ‘littoral current,’ which is the ocean current that travels parallel to the shore. The swash zone is the area at the water’s edge where waves lap up onto the beach, and the term swash is used to describe the water that moves onshore from the waves. Swash comes ashore at an angle equal to the angle of the waves.
However, backwash, which is the receding water moving off of the beach and washing back into the ocean, returns to the ocean in a straight path that is perpendicular to the shore. This creates the zigzagged pattern of beach drift, or longshore drift, which is the progressive movement of sand and sediment along the beach. Beach drift and the longshore current contribute to the longshore transport, which is the movement of sand and sediment parallel to the coastline.
When this lesson is done, you should be able to:
- Describe the longshore current
- Define swash and backwash
- Explain what causes the zigzag pattern of beach drift
- Recognize what creates the longshore transport