Oceans and Coasts. Chapter 18

Transcription

1 Oceans and Coasts Chapter 18

2 Exploring the oceans

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4 The ocean floor

5 Sediments thicken and the age of the seafloor increases from ridge to shore

6 The continental shelf off the northeast United States

7 Constituent make-up of sea water

8 Coriolis effect

9 World ocean currents

10 The layered ocean

11 The long road home: the ocean conveyer belt

12 The Gulf Stream

13 Tides Daily changes in the elevation of the ocean surface Causes of tides Tidal bulges are caused by the gravitational forces of the Moon, and to a lesser extent the Sun

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15 Tides Tides and Earth s rotation Tidal friction against the ocean floor acts as a weak brake that is steadily slowing Earth s rotation The day is increasing by seconds per century This small effect becomes very large over millions of years Length of each day must have been shorter in the geologic past

16 Tides Other factors that influence tides Shape of the coastline Configuration of the ocean basin Tidal currents Horizontal flow of water accompanying the rise an fall of the tide

17 Tides Spring tides Occur during new and full moons Gravitational forces of the Moon and Sun are added together Especially high and low tides Large daily tidal range

18 Tides Neap tides Occur during the first and third quarters of the moon Gravitational forces of the Moon and Sun are offset Daily tidal range is least

19 Tidal range The height difference between mean low water and mean high water

20 Tidal currents Tides Types of tidal currents Flood current advances into the coastal zone as the tide rises Ebb current seaward-moving water as the tide falls Areas affected by the tidal currents are called tidal flats Occasionally form tidal deltas

21 Tidal deltas

22 The shoreline: A dynamic interface The shoreline is a dynamic interface (common boundary) between air, land, and the ocean The shoreline is constantly being modified by waves Today the coastal zone is experiencing intense human activity

23 Waves Wind-generated waves provide most of the energy that shapes and modifies shorelines Characteristics of Waves Surface waves derive their energy and motion from the wind

24 Characteristics of Waves Parts of a wave Crest top of the wave Trough low area between waves Measurements of a wave Wave height the distance between a trough and a crest Wavelength the horizontal distance between crests Waveheight

25 Waves Characteristics of Waves Measurements of a wave Wave period the time interval between the passage of two successive crests Height, length, and period of a wave depend on Wind speed Length of time wind has blown Fetch the distance that the wind has traveled across open water

26 Wave of oscillation Wave energy moves forward, not the water itself Occur in the open sea in deep water

27 Changes that occur when a wave moves onto shore

28 Wave of translation Breakers Begins to form in shallower water when the water-depth is about one-half the wavelength and the wave begins to feel bottom Types of Waves

29 Mainly along the Atlantic and Gulf coasts Low ridges of sand that parallel the coast 3 to 30 kilometers offshore Probably form in several ways Barrier Islands

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32 Profile of a typical barrier island Winter beach Shorter berm Off-shore bars Summer beach Wider berm Ridge and runnel

33 Foreshore: Between high and low tide lines. Characterized by: low angle bedding Evidence of wave action (symmetrical ripples) Incorporation of marine skeletal remains i.e. seashells. swash backwash

34 Backshore: Between normal high tide and margin of beach. Characterized by berms, shelflike storm deposits

35 Backbeach: Lagoon Side Tobay Beach

36 Sand Bars Winter Summer

37 Ridge and Runnel

38 Winter: Small berms, large offshore sand bars. Coarse sediments in beach deposits. Any large storm can produce a "winter beach" profile. Summer: Large berms, small offshore sand bars. Fine sediments in beach deposits. /lectures/30.html

39 Wave erosion Breaking waves exert a great force Wave erosion is caused by Wave impact and pressure Abrasion by rock fragments

40 Wave refraction and longshore Wave refraction transport Moving sand along the beach Oblique waves also produce longshore currents» Currents in the surf zone» Flow parallel to the coast» Easily moves fine suspended sand and rolls larger sand and gravel along the bottom

41 Sediment transport by longshore curren Waves approach beach at an angle Waves touch bottom at <1/2 wavelength Leading edge slows down Refracts the wave towards shore Sand moves up the beach at an angle Straight down the beach Back up in the next wave

42 Rip Currents After a storm water piles up on the beach The backwash finds a topographic low and rips back out to sea

43 Erosional and Depositional Shoreline Features Features vary depending on several factors including The rocks along the shore Currents Wave intensity Whether the coast is stable, sinking, or rising

44 Depositional features related to beach drift and longshore currents Spits Elongated ridges of sand extending from the land into the mouth of an adjacent bay Baymouth bar Often the end of a spit hooks landward in response to wave-generated currents a sand bar that completely crosses a bay

45 Spits and Baymouth Bars

46 Depositional features along the coast of Massachusetts

47 Shoreline Erosion

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49 Shoreline erosion problems Shoreline erosion is influenced by several local factors including Proximity to sediment-laden rivers Degree of tectonic activity Topography and composition of the land Prevailing wind and weather patterns Configuration of the coastline and nearshore areas

50 Mitigating shoreline erosion problems Three basic responses to erosion problems Building structures Groins»Built to maintain or widen beaches»constructed at a right angle to the beach to trap sand

51 Groins Deposition on updrift (upcurrent) side Erosion on downdrift (downcurrent) side

52 Mitigating shoreline erosion problems Three basic responses to erosion problems Building structures Jetties»Usually built in pairs to develop and maintain harbors»extend into the ocean at the entrances to rivers and harbors

53 Jetties are built to prevent deposition in navigation channels

54 Erosion from a jetty

55 Mitigating shoreline erosional problems Three basic responses to erosion problems Building structures Breakwater»Barrier built offshore and parallel to the coast»protects boats from the force of large breaking waves

56 Mitigating shoreline erosional problems: Breakwaters

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58 Mitigating shoreline erosional problems: Seawalls Three basic responses to erosion problems Building structures Seawall»Barrier parallel to shore and close to the beach to protect property»stops waves form reaching the beach areas behind the wall Often the building of structures is not an effective means of protection

59 Seawalls

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63 Mitigating shoreline erosion problems Three basic responses to erosion problems Beach nourishment The addition of large quantities of sand to the beach system Only an economically viable long-range solution if a few areas Abandonment and relocation of buildings away from the beach

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66 Shorelines Shorelines are dynamic environments If a shoreline remains stable,the result of shoreline erosion and deposition is to eventually produce a straighter coast

67 Headlands

68 Wave refraction and longshore transport Wave refraction Bending of a wave along a headland Causes waves to arrive nearly parallel to the shore Consequences of wave refraction Wave energy is concentrated against the sides and ends of headlands

69 Wave refraction

70 Wave refraction and longshore transport Wave refraction Consequences of wave refraction Wave energy is spread out in bays and wave attack is weakened Over time, eave erosion straightens an irregular shoreline

71 Shoreline features Features caused by wave erosion Wave-cut cliffs Wave-cut bench (platform) Features associated with headlands Tombolo a ridge of sand that connects an island to the mainland or another island Sea arch Sea stack

72 Wave cut cliff

73 Undercutting

74 Formation of a wave cut bench

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76 Erosional features

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80 Shorelines Contrasting the Atlantic and Pacific Coasts Pacific Coast Relatively narrow beaches backed by steep cliffs and mountain ranges A major problem is a significant narrowing of many beaches Shoreline erosion varies considerably from one year to the next largely because of the sporadic occurrence of storms

81 Emergent and submergent coasts Emergent coasts Develop because of uplift of an area or a drop in sea level Features of an emergent coast Wave-cut cliffs Wave-cut platforms

82 Example of an emergent coast Tectonically active margins

83 Deltas are also emergent coasts Sediment continues to build the shoreline both up and out

84 The Mississippi Delta

85 Shorelines Contrasting the Atlantic and Pacific Coasts Atlantic and Gulf coast Tectonically passive margin Development occurs mainly on the barrier islands (also called barrier beaches or coastal barriers)»barrier island face the open ocean»the receive the full force of storms Wide beaches Easily effected by Storms Rising sea level

86 Emergent and submergent coasts Submergent coast Caused by subsidence of land adjacent to the sea or a rise in sea level Features of a submergent coast Highly irregular shoreline Estuaries drown river mouths

87 Development of a submerged coast

88 Chesapeake Bay Estuary is a good example of a submergent coastline The Hudson River Estuary

89 Atlantic vs. Pacific coasts

90 Changing Sea Level

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92 Sea level changes

93 A delicate balance

94 Coastal Landforms

95 Rocky headlands

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102 Coral Reefs

103 Fringing Reefs - Atolls