GL 107 Earth Systems Science

K. Hannula

Discussion questions for 11/9/07

1.  Describe the behavior of waves:

2.  What is longshore drift? 

See Fig. 15.14, p. 418 in textbook for a sketch.

Longshore drift is an ocean (or large lake) current that forms parallel to the shore as a result of obliquely breaking waves.  When waves break, they carry water and sand up the beach at an angle, but when they return to the ocean, they slide back perpendicular to the beach.  The movement of many waves, both on the beach and in the surf zone, results in sand being carried along the shore.

How does longshore drift change the shape of beaches when breakwaters, groins, and other structures are built in order to try to stabilize beaches?

See Fig. 15.21 and 15.22, p. 422-423 in text, for some examples.

Wherever wave action is slowed (such as behind the breakwater in Santa Monica, California), sand is deposited.  Where the longshore drift picks up again, the current erodes the existing beach.

3.  Where is wave erosion most intense? Why?

Wave erosion is most intense on headlands: areas that stick out into the oceans.  Because waves are bent toward parallelism with the beach as they come in, wave energy is concentrated on headlands and lessened in bays.  The result is that headlands tend to be eroded and bays tend to fill in; if there were no other factors other than coastal erosion, eventually all coasts would erode to straight beaches.