1. Shorelines Chapter 20

2. 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

3. The Outline of Cap Cod Results from the Interplay of Modern Shoreline Processes & Pleistocene Deposition on Soft Sediments
Martha’s Vineyard
Nantucket

4. Unlike the East and Gulf Coasts, the Pacific Coast Erodes Very Slowly Because it is Composed of Resistant Rock, Not Sediment

5. The coastal zone
Clarification of terms used to describe the land-sea boundary
Shoreline is the line that marks the contact between land and sea
The shore is the area that extends between the lowest tide level and the highest elevation on land that is affected by storms

6. The coastal zone
Clarification of terms used to describe the land-sea boundary
The coast extends inland from the shore as far as ocean-related features are found
Coastline marks the coast’s seaward edge

7. The coastal zone Shore is divided into the
Foreshore – area exposed at low tide
Backshore – landward of the high-tide shoreline
The nearshore zone lies between the low tide shoreline and the point where waves break at low tide
Seaward of the nearshore is the offshore zone

8. The coastal zone
Beach – an accumulation of sediment found along the landward margin of the ocean or a lake
The relatively flat platform composed of sand and marked by a change in slope at the seaward edge is a berm
Beach face is the wet sloping surface that extends from the berm to the shoreline

9. The coastal zone consists of several parts
The coastal zone consists of several parts. The beach is an accumulation of sediment on the landward margin of the ocean or of a lake. It can be thought of as material in transit along the shore.

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

11. Waves Characteristics of Waves Parts of a wave Measurements 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
Wave period – the time interval between the passage of two successive crests

12. Wave Characteristics Include Height, Length, & Period
Wave Characteristics Include Height, Length, & Period. Note That Negligible Water Movement Occurs Below ½ of a Given Wavelength

13. Waves Characteristics of Waves
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

14. Waves Types of waves Waves of oscillation
Wave energy moves forward, not the water itself
Occur in the open sea in deep water

15. Movement in a Wave of Oscillation The wave energy advances, but the water itself and anything floating on it, moves very slightly from its original position

16. Waves Types of waves Wave of translation
Begins to form in shallower water when the water-depth is about one-half the wavelength and the wave begins to “feel bottom”
As the speed and length of the wave diminish, the wave grows higher
The steep wave front collapses and the wave breaks along the shore
Turbulent water advances up the shore and forms surf

17. Movement in a Wave of Translation Both the Wave Form & its Energy Advance. As the Wave Form “Feels Bottom”, it Grows Higher and Finally Breaks onto the Shore

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

19. Waves Breaking Against the Shore Can Be Powerful and The Erosive Work that They Accomplish Is Great

20. Undercutting of a Cliff on the Oregon Coast by the Erosive Power of Breaking Waves

21. Abrasion can also be intense in the surf zone, as these rounded boulders along the California coast demonstrate.

22. Sandstone cliff undercut by wave erosion in British Columbia, Canada

23. Sand movement on the beach
Movement perpendicular to the shoreline
Waves seldom approach the shore straight on, but rather at an angle
When waves reach shallow water with a smoothly sloping bottom they are bent and tend to become parallel to the shore

24. Wave refraction and longshore transport
Bending of a wave
Causes waves to arrive nearly parallel to the shore

25. Wave Refraction Causes Waves to Bend Around the End of a Beach at Stinson Beach, California

26. Wave refraction and longshore transport
Consequences of wave refraction
Wave energy is concentrated against the sides and ends of headlands
Wave energy is spread out in bays and wave attack is weakened
Over time, wave erosion straightens an irregular shoreline

27. Because of Wave Refraction, the Greatest Erosive Power is Concentrated on Headlands. In Bays, the Force of Waves is Much Weaker

28. Wave refraction and longshore transport
Moving sand along the beach
Waves that reach the shoreline at an angle cause the sediment to move along a beach in a zigzag pattern called beach drift
Oblique waves also produce longshore currents
Currents in the surf zone
Flow parallel to the coast
Easily move fine suspended sand and roll larger sand and gravel along the bottom

29. Beach Drift and Longshore Currents Move Sediment Obliquely Up the Beach and Directly Down the Beach. These Actions Transport Material Along the Beach.

30. 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

31. Shoreline features Features caused by wave erosion Wave-cut cliffs
Wave-cut platform
Features associated with headlands
Sea arch
Sea stack

32. An Elevated Wave-Cut Platform on the California Coast
An Elevated Wave-Cut Platform on the California Coast. A New Platform is Forming at the Base of the Cliff

33. A Sea Arch & Sea Stack Produced by Concentrated Wave Attack on a Headland

34. A Sea Arch & Sea Stack on Mexico’s Baja Peninsula Produced by Concentrated Wave Attack on a Headland

35. Shoreline features
Features related to beach drift and longshore currents
Spits
Elongated ridges of sand extending from the land into the mouth of an adjacent bay
Often the end of a spit hooks landward in response to wave-generated currents

36. Shoreline features
Features related to beach drift and longshore currents
Baymouth bar – a sand bar that completely crosses a bay
Tombolo – a ridge of sand that connects an island to the mainland or another island

37. Spit & Baymouth Bar on Coast of Martha’s Vineyard

38. Tombolo – a Ridge of Sand that Connects an Island to the Mainland

39. Shoreline features Barrier islands
Formed 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
Modified Spits
Storm Generated Sand Ridges
Submerged Sand Dunes from Glacial Lowstand

40. Barrier Islands Are Present Along Most of the Texas Gulf Coast

41. Barrier islands are also present along much of the Atlantic Coast

42. Shoreline features
If the shoreline remains stable, the result of shoreline erosion and deposition is to eventually produce a straighter coastline

43. How a Stable Coastline is Straightened

44. 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

45. Wave erosion caused by strong storms forced abandonment of this highly developed shoreline on Long Island, New York. Coastal areas are dynamic places that can change rapidly in response to natural forces.

46. 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

47. Jetties Protect the Harbor from Erosion or Sediment Deposition, But They Create Problems of their Own

48. Jetties at the Harbor Mouth Protect the Harbor from Erosion
Jetties at the Harbor Mouth Protect the Harbor from Erosion. In this Case, They also Create a Shelter for Boats
Note Sediment Buildup

49. 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

50. Groins Are Built Perpendicular to the Beach to Slow the Erosion of Sand by Longshore Currents. Unfortunately they Often Cause Additional Current Erosion on their Leeward Sides

51. Shoreline erosion 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

52. Breakwater at Santa Monica, California
Breakwater at Santa Monica, California. It Provides a Sheltered Harbor for Boats, But Also Traps Sand from Longshore Currents, Creating Beach Growth

53. Shoreline erosion problems
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

54. Sea Wall Built to Protect Homes from Storm Waves Along the Beach
Sea Wall Built to Protect Homes from Storm Waves Along the Beach. After its Construction, the Beach Narrowed Dramatically, Destroying the Very Tourist Attraction That Brought People to this Location

55. 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 for a few areas with dense population, abundant sand & low wave energy
Abandonment and relocation of buildings away from the beach

56. The Results of Beach Nourishment in Miami
The Results of Beach Nourishment in Miami. As a Short Term Solution, Nourishment Clearly Works, But Must Be Repeated Frequently

57. Shoreline erosion problems
Contrasting the Atlantic and Pacific Coasts
Shoreline erosion problems are different along the opposite coasts
Atlantic and Gulf coast
Broad, gently sloping coastal plains
Tectonically quiet regions

58. Shoreline erosion problems
Contrasting the Atlantic and Pacific Coasts
Because the Coastlines Dip Gently & Consist of Loose Sediment, any Shift in Topography or Sea Level Causes Large Changes
An Example is the Vanishing Wetlands of Louisiana

59. Coastal Wetlands Provide Nesting for Birds & Homes for Wildlife and Fish. They Depend on Sediment & Fresh Water From the Mississippi & Storm Protection by Barrier Islands. When these Factors Change, Wetlands are Lost

60. Shoreline erosion problems
Contrasting the Atlantic and Pacific Coasts
Atlantic and Gulf coast
Development occurs mainly on the barrier islands (also called barrier beaches or coastal barriers)
Barrier islands face the open ocean
They receive the full force of storms

61. Shoreline erosion problems
Contrasting the Atlantic and Pacific Coasts
Hurricanes also have a major impact on the eastern & southern shores of the U.S.

62. Hurricanes Are Large Tropical Storms that Form During the Summer and Fall Months. They Are Classified on the Basis of Wind Speed & Barametric Pressure

63. Shoreline erosion problems
Hurricanes Can Cause Severe to Catastrophic Damage to Low Lying Coasts by Three Major Factors
Storm Surge
Wind Damage
Inland Flooding

64. Storm Surge from Hurricane Camille (a Category 5 Storm) Completely Destroyed this Beach Apartment Complex in Mississippi

65. Storm Surge & Wind Do Extensive Damage Near the Coast; Flooding Is Often the Major Problem Inland

66. With Hundreds of Miles of Exposed Coastline, Texas Is A Target For Hurricanes From June to November

67. Paths of Category 3 and Stronger Hurricanes that Struck the Texas Coast A = 1900-1949, B = 1950-1993

68. Major Hurricanes Along the Texas Gulf Coast

69. Homes Closest to the Shoreline Were Reduced to Rubble

70. Lucas Terrace Apts. Along the Beach
Lucas Terrace Apts. Along the Beach. Over 50 People Died Here, But 22 People & 1 Cat Survived

71. Over 6,000 Dead Were Found & Required Burial After The Storm

72. Many Bodies Were Dumped at Sea, But Washed Up Again on The Beach

73. Shoreline erosion problems
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

74. Emergent and submergent coasts
The Atlantic & Gulf Coasts Are Gently Dipping, Primarily Submergent, Coastlines
The Pacific Coast Is a Steeply Dipping, Emergent Coastline

75. The Effects of Sea Level Change on Submergent & Emergent Coastlines

76. Emergent and submergent coasts
Sea Level Fluctuations
Occur Naturally For Various Reasons
Glacial Waxing & Waning
Changes in Ocean Basin Size
Tectonics
Can Also Be Impacted By Manmade Global Warming

77. Sea Level Fluctuations Are Natural Events, But They May Be Enhanced by Global Warming, which Produces Glacial Meting & Ocean Water Heating & Expansion

78. 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

79. An Elevated Wave-Cut Platform on the California Coast
An Elevated Wave-Cut Platform on the California Coast. A New Platform is Forming at the Base of the Cliff

80. Emergent and submergent coasts
Caused by subsidence of land adjacent to the sea or a rise in sea level
Features of a submergent coast
Highly irregular shoreline
Estuaries – drowned river mouths

81. Chesapeake Bay is a Good Example of a Submergent Coastline, with Drowned Rivers Forming Estuaries Along the Coast

82. 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

83. Tidal Bulges Are Caused by the Gravitational Effects of Moon & Sun on the Uniformly Thick Body of Ocean Water

84. Tides Monthly tidal cycle (spring and neap 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

85. Tides Monthly tidal cycle (spring and neap 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

86. The Combined Gravitational Effects of Moon & Sun Can Produce Particularly High Tides (Spring Tides) or Particularly Low Tides (Neap Tides)

87. Tides Other factors that influence tides Shape of the coastline
Configuration of the ocean basin

88. Tidal Fluctuations in the Bay of Fundy, Nova Scotia, Reach 56 Feet
Tidal Fluctuations in the Bay of Fundy, Nova Scotia, Reach 56 Feet. Boats Are Left High and Dry at Low Tide and Extensive Tidal Flats Are Created

89. Tides
Tidal patterns
Diurnal tidal pattern = a single high tide and a single low tide each tidal day
Semidiurnal tidal pattern = 2 high tides and 2 low tides each tidal day

90. Tides Tidal patterns Tidal currents
Mixed tidal pattern = large inequality in high water heights, low water heights or both
Tidal currents
Horizontal flow of water accompanying the rise and fall of the tide

91. Tidal patterns & their occurrence along portions of the coastlines of North & South America. Diurnal tidal patterns shown one high & one low tide each day. Semidiurnal patterns show two highs & two lows of approximately equal height each day. Mixed tidal pattern shows two highs & two lows of unequal height.

92. Tides Tidal currents 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

93. A Flood Tidal Delta Created by Tides Moving into the Bay

94. 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