Coastal Deposition

Parts of a beach

Coastal Transport

• The water that hurtles up the beach as a wave breaks is called swash.

• The water that returns to the sea is called backwash.

• Swash pushes sand up the shoreline at an angle.

• Backwash pulls the sand back to sea perpendicular to the coast.

Longshore Drift

Longshore Current

• The zig-zag motion of swash and backwash is called a longshore current (or drift).

• It transports eroded material along the shore.

Longshore Drift

Products of Coastal Deposition

• Spits – finger-like ridges of sand at a bay entrance

• Hooks – spits that become curved

• Baymouth bars – spits that block a bay entrance

Hook and Spit

Cape Cod, MA

Baymouth Bar

Human Interference with Erosion/Deposition

• Groins – walls that jut out perpendicular to the shore that trap sand thus increasing deposition. Ex. Miami Beach

• Jetties – pairs of walls that extend the banks of a river channel so sediment is carried farther offshore

• Beach nourishment – sand is imported to restore beaches that have lost sediment and shrunk

Groins

Groins off Cape May, NJ

Jetties

Jetties at Miami Beach, FL

Coastlines and Plate Tectonics

• Divergent plate boundaries typically have broad, gently sloping continental shelves which reduces the energy of incoming waves and usually results in a depositional coastline, i.e. sandy beaches

Typical Coastline on East Coast

Cape Hatteras, NC

Coastlines and Plate Tectonics

• Convergent plate boundaries typically have narrow, steep continental shelves which increases the energy of incoming waves, allowing them to strike the shore directly and usually results in an erosional coastline, i.e. rocky shores with wave-cut notches, sea arches and sea stacks.

Typical Coastline on West Coast

Crescent Beach, OR