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USGS CMG InfoBank: Source of Sand

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Comment: 09:09 - 10:43 (01:34)
Source: Annenberg/CPB Resources - Earth Revealed 24, Waves, Beaches and Coasts
Keywords: "Thomas Hartnett", erosion, headland, sand, beach, sediment, ocean, river, stream, wave, coastline, surf, "swash zone", gravity, "long-shore current"

Our transcription: The erosion of coastal headlands is by no means the only source of sand.
Most beach sand comes from sediment that is brought down to the ocean by rivers and streams.
Once the sand reaches the ocean, the waves distribute it along the coastline.
This occurs as a result of wave movement up on to the sloping part of the beach, then back down again.
Each cycle of wave movement carries particles up and down the beach slope.
Because waves usually break at a slight angle to the shore, the grains of sand in this cycle are gradually worked along the shoreline in a zigzagged path.
Sand gets moved along the beach face by the waves approaching the coastline at an angle, and when the waves break they have the momentum from their falling forward at that angle.
So the waves rush up the beach face in the swash zone at an angle.
But then gravity's going to pull that water straight back down the beach face.
So what you and I see is kind of an arc shape of water swashing up and then going straight back down.
The result is that as this occurs thousands of times a day, the sand moves and has a zigzag motion up and down the beach face.
The yellow dye shows this movement.
This flow of water along the shoreline is known as the "long-shore current."

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Page Last Modified: Thu Oct 31 04:27:54 PDT 2013  (chd)