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The Restless Ocean. The Shoreline: A Dynamic Boundary The shoreline is a dynamic boundary between air, land, and the ocean. The shoreline is constantly.

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Presentation on theme: "The Restless Ocean. The Shoreline: A Dynamic Boundary The shoreline is a dynamic boundary between air, land, and the ocean. The shoreline is constantly."— Presentation transcript:

1 The Restless Ocean

2 The Shoreline: A Dynamic Boundary The shoreline is a dynamic boundary between air, land, and the ocean. The shoreline is constantly changing. The coastal zone sees intense human activity.

3 Ocean Water Composition – Marine salinity is –3.5 % (35 ppt, 35,000 ppm) –Mostly Cl and Na –Greater at tropics; less at poles Times Atlas of the World W. W. Norton

4 Ocean Water Temperature –Average 17 °C (63 °F), colder at poles, warmer at tropics –Water moderates temperature fluctuations –Bottom waters are < 4 °C (39 °F) Times Atlas of the World W. W. Norton

5 Ocean water movements  Surface circulation Ocean currents are masses of water that flow from one place to another Surface currents develop from friction between the ocean and the wind that blows across the surface Huge, slowly moving gyres

6 Ocean water movements  Surface circulation Five main gyres North Pacific Gyre South Pacific Gyre North Atlantic Gyre South Atlantic Gyre Indian Ocean Gyre Related to atmospheric circulation

7 Average ocean surface currents in February–March

8 Ocean water movements  Surface circulation Deflected by the Coriolis effect To the right in the Northern Hemisphere To the left in the Southern Hemisphere Four main currents generally exist within each gyre Importance of surface currents Climate Currents from low latitudes into higher latitudes (warm currents) transfer heat from warmer to cooler areas

9 Ocean water movements  Surface circulation Importance of surface currents Climate Influence of cold currents is most pronounced in the tropics or during the summer months in the middle latitudes Upwelling The rising of cold water from deeper layers Most characteristic along west coasts of continents Brings greater concentrations of dissolved nutrients to the ocean surface

10 Ocean water movements  Deep-ocean circulation A response to density differences Factors creating a dense mass of water Temperature – cold water is dense Salinity – density increases with increasing salinity Called thermohaline circulation

11 Ocean water movements  Deep-ocean circulation Most water involved in deep-ocean currents begins in high latitudes at the surface A simplified model of ocean circulation is similar to a conveyor belt that travels from the Atlantic Ocean, through the Indian and Pacific Oceans, and back again

12 Idealized “conveyor belt” model of ocean circulation

13 The coastal zone  The land–sea boundary Shoreline – contact between land and sea Shore – area between lowest tidal level and highest areas affected by storm waves Coastline – the seaward edge of the coast Beach – accumulation of sediment along the landward margin of the ocean

14 The coastal zone

15 Ocean water movements  Waves Energy traveling along the interface between ocean and atmosphere Derive their energy and motion from wind Parts Crest Trough

16 Ocean water movements  Waves Measurements of a wave Wave height – the distance between a trough and a crest Wavelength – the horizontal distance between successive crests (or troughs) Wave period – the time interval for one full wave to pass a fixed position

17 Characteristics and movement of a wave

18 Ocean water movements  Waves Wave height, length, and period depend on Wind speed Length of time the wind blows Fetch – the distance that the wind travels As the wave travels, the water passes energy along by moving in a circle Waveform moves forward At a depth of about one-half the wavelength, the movement of water particles becomes negligible (the wave base)

19 Changes that occur when a wave moves onto shore

20 Wave erosion  Wave erosion Caused by Wave impact and pressure Breaks down rock material and supplies sand to beaches Abrasion – sawing and grinding action of water armed with rock fragments

21 Sand movement on the beach  Beaches are composed of whatever material is available Some beaches have a significant biological component Material does not stay in one place Wave energy moves large quantities of sand parallel and perpendicular to the shoreline

22 Beaches and shoreline processes  Wave refraction Bending of a wave Wave arrives parallel to shore Results Wave energy is concentrated against the sides and ends of headland Wave erosion straightens an irregular shoreline

23 Wave refraction along an irregular coastline

24 Shorelines are continually modified by erosional and depositional processes. –Over time, a geologically stable coastline will become less jagged. Headlands erode… Creating sediments… Deposited in embayments… Shoreline Evolution

25 Beaches and shoreline processes  Longshore transport Beach drift – sediment moves in a zigzag pattern along the beach face Longshore current Current in surf zone Flows parallel to the shore Moves substantially more sediment than beach drift

26 Beach drift and longshore currents

27 Shoreline features  Erosional features Wave-cut cliff Wave-cut platform Marine terraces Associated with headlands Sea arch Sea stack

28 Wave Cut Platform A flat bench-like surface carved by wave attack on receding sea cliffs.

29 Sea arch

30 A sea stack and a sea arch

31 Sea Stacks Isolated remnants of headland rock formed when sea arches collapse

32 Shoreline features  Depositional features Spit – a ridge of sand extending from the land into the mouth of an adjacent bay with an end that often hooks landward Baymouth bar – a sand bar that completely crosses a bay Tombolo – a ridge of sand that connects an island to the mainland

33 Spit Elongated ridges of sand extending from the land into the mouth of an adjacent bay.

34 Aerial view of a spit and baymouth bar along the Massachusetts coastline

35 Tombolo

36 Shoreline features  Depositional features Barrier islands Mainly along the Atlantic and Gulf Coastal Plains Parallel the coast Originate in several ways

37 Barrier Islands Coast parallel sand ridges 3 to 30 km offshore. Protect a quiet lagoon. Heavily utilized.

38 Because shorelines are continually being modified, they are notoriously unstable in human terms. This creates problems because of intense development in the coastal zone. These problems result from erosion during… –Hurricanes –Normal wave attack Shoreline Evolution

39 Because shorelines are continually being modified, they are notoriously unstable in human terms. This creates problems because of intense development in the coastal zone. These problems result from erosion during… –Hurricanes –Normal wave attack Shoreline Evolution

40 Shoreline Erosion  Shoreline erosion is influenced by the local factors Proximity to sediment-laden rivers Degree of tectonic activity Topography and composition of the land Prevailing wind and weather patterns Configuration of the coastline

41 3 Fundamental Responses to Erosion Problems –Build control structures. –Beach nourishment. –Abandonment and relocation. Responses to Shoreline Erosion

42 Stabilizing the shore  Hard stabilization –Responses to erosion problems Types of structures Groins – barriers built at a right angle to the beach that are designed to trap sand Breakwaters – barriers built offshore and parallel to the coast to protect boats from breaking waves Seawalls – Armors the coast against the force of breaking waves Often these structures are not effective

43

44 Groins –Built to maintain or widen beaches. –Constructed at a right angle to the beach to trap sand. –Create erosion problems “downdrift.” Responses to Shoreline Erosion

45 Seawall –Barrier parallel to shore and close to the beach to protect property. –Stops waves from reaching the beach behind the wall. –Results in… Destruction of beaches Enhanced damage when seawall is undermined and removed. Accelerating expenditures to rebuild bigger seawalls. Responses to Shoreline Erosion

46 Stabilizing the shore  Responses to erosion problems Alternatives to hard stabilization Beach nourishment by adding sand to the beach system Relocating buildings away from beach  Erosion problems along U.S. coasts Shoreline erosion problems are different along the opposite coasts

47 Miami Beach before beach nourishment

48 Miami Beach after beach nourishment

49 Stabilizing the shore  Erosion problems along U.S. coasts Atlantic and Gulf Coasts Development occurs mainly on barrier islands Face open ocean Receive full force of storms Development has taken place more rapidly than our understanding of barrier island dynamics

50 Stabilizing the shore  Erosion problems along U.S. coasts Pacific Coast Characterized by relatively narrow beaches backed by steep cliffs and mountain ranges Major problem is the narrowing of the beaches Sediment for beaches is interrupted by dams and reservoirs Rapid erosion occurs along the beaches

51 Coastal classification  Shoreline classification is difficult  Classification based on changes with respect to sea level Emergent coast Caused by Uplift of the land, or A drop in sea level

52 Coastal classification  Classification based on changes with respect to sea level Emergent coast Features of an emergent coast Wave-cut cliffs Marine terraces

53 Coastal classification  Classification based on changes with respect to sea level Submergent coast Caused by Land adjacent to sea subsides, or Sea level rises Features of a submergent coast Highly irregular shoreline Estuaries – drowned river mouths

54 Major estuaries along the East Coast of the United States

55 Emergent Coasts Develop due to land uplift or sea level fall –Emergent coast features Wave-cut cliffs Wave-cut platforms

56 Tides  Changes in elevation of the ocean surface  Caused by the gravitational forces exerted upon the Earth by the Moon, and to a lesser extent by the Sun

57 Idealized tidal bulges on Earth Idealized tidal bulges on Earth

58 Tides  Monthly tidal cycle Spring tide During new and full moons Gravitational forces added together Especially high and low tides Large daily tidal range

59 Earth–Moon–Sun positions during the Spring tide

60 Earth–Moon–Sun positions during the Neap tide

61 Tides  Monthly tidal cycle Neap tide First and third quarters of the Moon Gravitational forces are offset Daily tidal range is least  Tidal patterns Many factors influence the tides Shape of the coastline Configuration of the ocean basin Water depth

62 High tide in the Bay of Fundy along the Nova Scotia coast

63 Low tide in the Bay of Fundy along the Nova Scotia coast

64 Tides  Tidal patterns Main tidal patterns Diurnal tidal pattern A single high and low tide each tidal day Occurs along the northern shore of the Gulf of Mexico Semidiurnal tidal pattern Two high and two low tides each tidal day Little difference in the high and low water heights Common along the Atlantic Coast of the U.S.

65 Tides  Tidal patterns Main tidal patterns Mixed tidal pattern Two high and two low waters each day Large inequality in high-water heights, low- water heights, or both Prevalent along the Pacific Coast of the U.S.

66 Tides  Tidal patterns Tidal currents Horizontal flow accompanying the rise and fall of tides Types of tidal currents Flood current – advances into the coastal zone Ebb current – seaward-moving water Sometimes tidal deltas are created by tidal currents

67 Features associated with tidal currents

68 End of Chapter 13


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