14.7: Deposition by Waves
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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 can be seen in Figure 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 (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. Pictured below are examples of these landforms (Figure below); also, an example of all the different landforms waves create (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.
- 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.
- What processes cause spits and barrier islands to form?
- What is longshore drift?
- Describe how sediment moves along the shore.
- What function does a barrier island in its natural state serve?