Which came first: erosion or deposition?
Both erosion and deposition are seen in this photo. The beach sands were deposited but waves are now eroding them away. At the shore, there's always a battle between the two types of forces. What happens when deposition is greater than erosion? What happens when erosion is greater than deposition?
The transport of sediments by longshore currents is called longshore drift. Longshore drift is created in this way: Sediment is moved up the beach by an incoming wave. The wave approaches at an angle to the shore. Water then moves straight offshore. The sediment moves straight down the beach with it. The sediment is again picked up by a wave that is coming in at an angle. So longshore drift moves sediment along the shore. This zig-zag motion is show in Figure below and at the link below.
Longshore drift carries particles of sand and rock down a coastline.
Landforms Deposited by Waves
Longshore drift continually moves sand along the shore. Deposition occurs where the water motion slows. The smallest particles, such as silt and clay, are deposited away from shore. This is where the water is calmer. Larger particles are deposited onshore. This is where waves and other motions are strongest.
In relatively quiet areas along a shore, waves deposit sand. Sand forms a beach, like the one in Figure below.
Manhattan Beach in Southern California has a pier coming off of a sandy beach.
Waves also move sand from the beaches on shore to bars of sand offshore as the seasons change. In the summer, waves have lower energy so they bring sand up onto the beach. In the winter, higher energy waves bring the sand back offshore.
Examples of features formed by wave-deposited sand.
A spit is a ridge of sand that extends away from the shore. The end of the spit may hook around toward the quieter waters close to shore.
Waves may also deposit sediments to form sandbars and barrier islands. You can see examples of these landforms in Figure below and an example of all the different landforms waves create in the Figure above.
A barrier island is a long strip of sand. The sand naturally moves in the local currents. People try to build on barrier islands.
In its natural state, a barrier island acts as the first line of defense against storms such as hurricanes. A natural barrier island is a vegetated sandy areas in which sand can move. When barrier islands are developed, hurricanes damage houses and businesses. A large hurricane brings massive problems to the urbanized area.
barrier island: Long, narrow island composed of sand; nature’s first line of defense against storms.
beach: The sediments on a shore.
longshore drift: The movement of sand along a shoreline.
spit: Long, narrow bar of sand that forms as waves transport sand along shore.
- The shore may have a lot of sediment washed from land or eroded from cliffs. The sediment is transported by currents.
- Transported sand will eventually be deposited on beaches, spits, or barrier islands.
Use this resource to answer the questions that follow.
Spits & Marshes-Coastal Deposition at http://vimeo.com/346559 (2:28)
- What is longshore drift?
- What is a split?
- How do spits form?
- List three examples of spits.
- What forms behind a spit?
- Where does the sediment come from that is found at the shore?
- What processes cause spits and barrier islands to form?
- What is longshore drift?