Shorelines

1. Shorelines

2. Importance of Shorelines • Half the world’s population lives within 60 km (37.5 mi) of a coast. • Shorelines provide: • food • transportation • recreation • housing (prime real estate) Fig. 19.24

3. Dynamic Shorelines • Shorelines are continually changing. • Any structure built close to a shoreline should be considered temporary. erosion deposition Fig. 19.27

4. Dynamic Shorelines • The appearance of shorelines controlled by: • rock erodibility • river sed. supply • sed. reworking by waves • sea level rise/fall • crustal subsidence and uplift Fig. 19.2

5. Wave Formation • Atmospheric circulation (wind) generate waves. • energy transferred from the sunto the land surfaceto the atmosphereto the oceansto shorelines • Variations in wind speed, duration, contact time with the water produce differences in wave size.

6. Wave Motion • Wave height – vertical distance from trough to crest. • Wavelength – distance between two wave crests (or troughs). Fig. 19.3

7. Wave Motion • As a wave passes a point, water molecule moves in a circular motion. • Amount of motion decreases with depth. • Wave base – depth where no motion occurs. Fig. 19.4

8. Wave Motion • As a wave approaches a shore, the wave base contacts the sea floor. • wavelength decreases, wave height increases, velocity decreases Fig. 19.5

9. Wave Motion • Tsunamis – as wave approaches shore, very long wavelength converted to very high wave height. • water height remains elevated for up to an hour Fig. 19.12

10. Wave Motion

11. Wave Motion • Breakers – friction with sea floor causes the wave base to slow more than wave crest. • crest outruns the base, wave collapses • energy of wave transferred to land surface Fig. 19.13

12. Sediment Transport • Each time a wave breaks: • sediment lifted and transported towards shore. • backwash carries sediment away from shore Fig. 19.15

13. Sediment Transport • Wave motion can generate mineral deposits. • breakers move all sediments towards shore • backwash carries less dense sediments offshore • dense minerals concentrated on beach Fig. 19.16

14. Sediment Transport • Longshore currents transport sediments parallel to the shoreline (beach drift). • waves strike shoreline at an angle • backwash flows down beach slope, perpendicular to shoreline Fig. 19.8

15. Sediment Transport • Each wave transports sediment in a zig-zag pattern towards then away from the beach. • The net result is that sediment moves parallel to the shore. Fig. 19.8

16. Longshore Current and Beach Drift

17. Groins – walls built perpendicular to beach in order to stabilize beach. Fig. 19.24 Human Influence on Sediment Transport • Shorelines are continually changing, and we want stabile beaches.

18. Human Influence on Sediment Transport Fig. 19.24 • Groins disrupt beach drift sediment transport. • sediment transport blocked on upstream side, deposition occurs • downstream side sediment starved, erosion occurs

19. Disruption of Longshore Transport

20. erosion, downgradient deposition, upgradient longshore current • Which direction is longshore current transporting sediment? • upper left to lower right • or lower right to upper left