Chapter 14

1. The Dynamic Ocean Chapter 14

2. Agenda 1. Stabilizing the Shore 2. Coastal Classification 3. Tides

3. Stabilizing the Shore • Hard Stabilization • Alternatives to Hard Stabilization • Erosion Problems Along U.S. Coast

4. Hard Stabilization • Hard Stabilization is any form of artificial structure built to protect a coast or to prevent the movement of sand along a beach. Examples: 1. Groins 2. Jetties 3. Breakwaters 4. Seawalls

5. Groinsare structures away from a harbor entrance built straight out from the beach. Both jetties and groins serve to stop the flow of the long-shore current. On the up-current side of the groin sand is dropped and the beach builds out. On the down current side the current is renewed, but now is not carrying any sand, so it erodes the beach on the down-current side. Property owners down-current from such a structure commonly build another groin to protect their beach, and soon a whole set of them, called a groin field, is the result. Back This figure is from Dean, C, 1999, Against the Tide: the battle for America's beaches, Columbia Univ Press, NY, NY

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8. Seawalls are continuous barricades built at the shore line in an attempt to prevent any erosion along the whole area. Instead, the wave energy erodes the sand stored in front of the sea wall. This will eventually undermine the seawall and cause it to collapse and fail. *Back

9. Relocation Relocation deals with relocation of “storm-damaged” or “at-risk” buildings and letting nature reclaim the beach. - This is a very controversial proposal. People/businesses/corporations spend billions each year trying to attain a view of any coastal area. The problem with this is that many times, they are much too close for comfort, and nature can not exploit itself in all its naturalistic ways. Nature knows no other boundary, and we end up with billions and billions of dollars in damage, not to mention many times, death! Picture

10. Alternatives to Hard Stabilization Armoring the coast with hard stabilization has several potential drawbacks, including the cost of the structure and the loss of the sand on the beach. Alternatives to hard stabilization include: • Beach Nourishment • Relocation

11. This here is a picture of Miami Beach. Many scientists want most of these huge buildings to relocate, so nature can take its toll and regain one of its beaches. Back

12. Beach Nourishment Beach Nourishment is the process by which large quantities of sand are added to the beach systems to offset losses caused by wave erosion. * DOWNFALLS * • Beach nourishment, however, is not a permanent solution to the problem of shrinking beaches because the same process that removed the sand in the first place will eventually remove the replacement sand as well. • It can also be very expensive too… imagine having to transport huge amounts of sand to a beach from off shore areas nearby. • Unwanted environmental effects (Hawaii’s sand for instance) Back

13. On East Coast barrier islands, ocean tidal inlets migrate naturally with the longshore current. A jetty system will permanently disrupt the equilibrium of the beach. This may seriously affect the tidal circulation and the health of the wetlands between the barrier islands and the mainland. Inlets with short jetties that don't quite reach the surf will clog up with sand. The sand must be dredged on a regular basis. A "sand by-passing" system may be built to pump sand around the jetties. The sand pumping may come from within the inlet or from the updrift beach. These methods are expensive and must be maintained indefinitely. Next

14. Erosion Problems Along U.S. Coast Atlantic and Gulf Coasts Much of the coastal development along the Atlantic and Gulf coasts has occurred on barrier islands. Barrier islands consist of wide beaches that are backed by dunes and separated from mainland’s by marshy lagoons. When a storm occurs, the barriers absorb the energy of the waves primarily through the movement of sand. Pacific Coast The Pacific Coast is characterized by relatively narrow beaches that are backed by steep cliffs and mountain ranges. A major problem facing the pacific shorelines is the significant narrowing of many of many beaches. This is caused by the man made damns built for irrigation and flood control. Picture Picture

15. Picture of a Man Made Dam Back

16. What causes tides? Tides are daily changes in the elevation of the ocean surface. Other than waves, they are the easiest ocean movements to observe. Tides are created by the imbalance between two forces: • Gravitational force of the Moon and Sun on Earth • If mass increases (), then gravitational force increases () • If distance increases (), then gravitational force greatly decreases () • Centripetal (center-seeking) force required to keep bodies in nearly circular orbits Next

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18. Coastal Classification • To understand any coastal area, many factors must be considered, including rock types, size and direction waves, frequency of storms, tidal range, and offshore topography. • Also Tectonics that “uplift” or “downdrop” the land. • All coastal areas were affected by the melting of the ice age glaciers. Next

19. EMERGENT COASTS There are 2 basic coastal classifications, and they are based upon changes that have occurred with respect to sea level. Emergent Coasts: These develop either because an area experiences uplift or as a result of drop in sea levels. Submergent Coasts: These are created when sea levels rise, or the land adjacent to the sea subsides

20. Emergent Coast Back

21. Submergent Coast Back

22. The monthly tidal cycle(29½ days) About every 7 days, Earth alternates between: • Spring tide • Alignment of Earth-Moon-Sun system (syzygy) • Lunar and solar bulges constructively interfere • Large tidal range • Neap tide • Earth-Moon-Sun system at right angles (quadrature) • Lunar and solar bulges destructively interfere • Small tidal range Next

23. The lunar day • Tidal bulges follow Moon as it rotates around Earth • Lunar day is 50 minutes longer than a solar day because the Moon is moving in its orbit around Earth Next

24. Relative sizes and distances on Earth, Moon, and Sun • The Sun is much more massive than the Moon but much further away • Solar bulges are 46% the size of lunar bulges Next

25. Earth-Moon-Sun positions and the monthly tidal cycle Next

26. Tidal patterns • Diurnal • One high and one low tide each (lunar) day • Semidiurnal • Two high and two low tides of about the same height daily • Mixed • Characteristics of both diurnal and semidiurnal with successive high and/or low tides having significantly different heights Next

27. Tidal bore = a true tidal wave • Wall of water that moves upriver • Caused by an incoming high tide • Occurs in some low-lying rivers • Can be large enough to surf or raft Next

28. Summary of tides on Earth • Most locations have two high tides and two low tides per lunar day • Neither the two high tides nor the two low tides are of the same height because of the declination of the Moon and the Sun • Yearly and monthly cycles of tidal range are related to the changing distances of the Moon and Sun from Earth • Each week, there would be alternating spring and neap tides Next

29. Gravitational forces on Earth due to the Moon • Force decreases with increasing distance • Force is directed toward the Moon’s center of mass Next

30. Tidal Currents The term Tidal Current is used to describe the horizontal flow of water accompanying the rise and fall of the tides. Tidal Flats: This is the area affected by the advancing and retreating of tidal currents Tidal Deltas: An accumulation of sediment on the shoreward or ocean-ward side of an inlet Next

31. Types of Tidal Deltas • Ebb tidal deltas: • An ebb tidal delta is an accumulation of sand on the seaward side of an inlet formed by the ebb tidal current. • Weak tidal currents relative to wave activity inhibits the formation of ebb tidal deltas along microtidal coasts. Macrotidal coasts typically lack shore-parallel barriers, hence inlets and their deltas. •  Ebb tidal deltas most commonly occur in front of mesotidal (or mixed energy) inlets where they interact with waves and longshore drift. Barriers are often drumstick-shaped with down drift offsets. • The length (degree of protrusion) of an ebb tidal delta reflects the relative strengths of tidal flow and longshore current. The asymmetry, or lean, of the delta is determined by the direction longshore transport. • . Next

32. Types of Tidal Deltas (Cont.) Flood tidal delta: • A flood tidal delta is an accumulation of sand on the shoreward sided of an inlet. These deltas are initially formed during storm surges and maintained by flood currents. • Flood tidal deltas become stabilized when salt marshes establish on them. • Salt-marsh grasses cause the land to be built up to high-tide level and new land is added to the island • In mesotidal systems, flood tidal deltas transform into marshland causing the inlet to migrate to a new position. New tidal deltas are created along the entire area of inlet migration • Widening by inlet migration is an important process on the North Carolina Islands. Next

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34. Types of Tidal Currents Significance/Importance • Inlets offer access to sheltered bays and lagoons. • Inlets enable the exchange of water and nutrients between the ocean and back barrier region required for finfish and shellfish reproduction. • Tidal deltas are major sediment sinks, capturing a large amount of sediment in the littoral system. • Ebb tidal deltas refract ocean waves, thereby altering the local littoral dynamics. • Flood tidal deltas aid in widening the back barrier region allowing the barrier system to migrate landward. • Stabilization of an inlet require an understanding the circulation of sediment in and around an inlet as well as the relationship between inlet throat dimensions and tidal prism and back barrier area. Next

35. THE DYNAMIC OCEAN THE END

36. Centripetal forces on Earth due to the Moon • Force is the same everywhere on Earth • Force is directed perpendicular to Earth’s center everywhere on Earth Next

37. Resultant forces • Resultant forces are: • The difference between gravitational (G) and centripetal (C) forces • Directed away from Moon on the side of Earth opposite Moon • Directed toward Moon on the side of Earth facing Moon Figure 9-4 Next