earth, 10e - chapter 20 -...
TRANSCRIPT
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ShorelinesEarth, 10e - Chapter 20
Stan Hatfield Southwestern Illinois College
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The Shoreline—A Dynamic Interface
• The shoreline is a dynamic interface (common boundary) among air, land, and the ocean.
• The shoreline is constantly being modified by waves.
• Today, the coastal zone is experiencing intense human activity.
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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.
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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.• The coastline marks the coast’s seaward edge.
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The Coastal Zone
• Shore is divided into the following areas: • Foreshore is the 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.
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The Coastal Zone
• A beach is 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.
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The Coastal Zone
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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.
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Waves• Characteristics of waves
• Parts of a wave–Crest is the top of the wave.–Trough is the low area between waves.
• Measurements of a wave–Wave height is the distance between a trough and a
crest.–Wavelength is the horizontal distance between
crests.
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Waves• Characteristics of waves
• Measurements of a wave–Wave period is 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 is the distance that the wind has traveled
across open water.
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Basic Parts of a Wave
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Waves• Types of waves
• Wave of oscillation–Wave energy moves forward, not the water itself.–Occurs in the open sea in deep water
• Wave of translation–Begins to form in shallower water when the water
depth is about one-half of the wavelength and the wave begins to “feel bottom.”
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Waves• Types of waves
• Wave of translation–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.
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A Wave Movingonto the Shore
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Wave Erosion• Breaking waves exert a great force.• Wave erosion is caused by:
• Wave impact and pressure• Abrasion by rock fragments
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Breaking Waves
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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.
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Sand Movement on the Beach
• Wave refraction• Bending of a wave• Causes waves to arrive nearly parallel to the shore• Consequences of wave refraction–Wave energy is concentrated against the sides and
ends of headlands.
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Refraction of Waves
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Sand Movement on the Beach
• Wave refraction• Consequences of wave refraction–Wave energy is spread out in bays and wave
attack is weakened.–Over time, wave erosion straightens an
irregular shoreline.• 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.
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Sand Movement on the Beach
• Wave refraction• 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
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Movement of Sand by Longshore Current
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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
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Shoreline Features• Features caused by wave erosion
• Wave-cut cliffs• Wave-cut platforms• Features associated with headlands–Sea arch–Sea stack
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Wave-Cut Platform and Marine Terrace
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Sea Stack and Sea Arch
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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.
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Shoreline Features• Features related to beach drift and longshore
currents• Baymouth bar is a sand bar that completely crosses
a bay.• Tombolo is a ridge of sand that connects an island
to the mainland or another island.
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Depositional Features of Coastal Massachusetts
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Shoreline Features• Barrier islands
• 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
• If the shoreline remains stable, shoreline erosion eventually produces a straighter coast.
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Stabilizing the Shore• 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
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Stabilizing the Shore• Three basic responses to erosion problems
1. Building structures– Jetties
» Usually built in pairs to develop and maintain harbors
» Extend into the ocean at the entrances to rivers and harbors
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Jetties Are Built to Prevent Deposition
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A Breakwater at Santa Monica, California
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Stabilizing the Shore• Three basic responses to erosion problems
1. Building structures– Groins» Built to maintain or widen beaches» Constructed at a right angle to the beach to
trap sand
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A Series of Groins Along the Coast of England
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Stabilizing the Shore• Three basic responses to erosion problems
1. Building structures– Breakwaters» Barriers built offshore and parallel to the
coast» Protect boats from the force of large
breaking waves
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A Breakwater at Santa Monica, California
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Stabilizing the Shore• Three basic responses to erosion problems
1. Building structures– Seawalls
» Barriers parallel to shore and close to the beach to protect property
» Stop waves from reaching the beach areas behind the wall
– Often, the building of structures is not an effective means of protection.
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Stabilizing the Shore• Three basic responses to erosion problems
2. Beach nourishment– The addition of large quantities of sand to the
beach system– Only an economically viable long-range
solution in a few areas3. Abandonment and relocation of buildings away
from the beach
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Stabilizing the Shore• Contrasting the Atlantic and Pacific Coasts
• Shoreline erosion problems are different along the opposite coasts.• Atlantic and Gulf Coasts–Broad, gently sloping coastal plains–Tectonically quiet regions
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Hurricanes• Hurricanes
• Most violent storms on Earth• The following are needed to be considered a
hurricane:–Wind speed of more than 74 miles per hour–Rotary cyclonic circulation
• Profile–Form between latitudes of 5 and 20 degrees
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Global Occurrence of Hurricanes
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Hurricanes• Hurricanes
• Profile–Typhoons in the western Pacific–Cyclones in the Indian Ocean
• Parts of a hurricane–Eyewall near the center–Rising air– Intense convergence
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Cross-Section of a Hurricane
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Hurricanes• Hurricanes
• Profile–Wind speeds reach 300 kilometers per hour.–Generate 50 foot waves at sea
• Hurricane formation and decay–Form in all tropical waters except the South
Atlantic and eastern South Pacific–Energy comes from condensing water vapor.
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Hurricanes• Hurricanes
• Hurricane formation and decay –Develop most often in late summer–Diminish in intensity when they move over cooler
ocean water, land, or an unfavorable large-scale flow
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Hurricanes• Hurricanes
• Destruction from a hurricane depends on:–Strength of storm–Size and population density of the area affected–Shape of ocean bottom near the shore
• Categories of hurricane destruction–Storm surge—large dome of water that sweeps
across the coast when a hurricane makes landfall–Wind damage– Inland flooding from torrential rains
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Hurricane Winds in the Northern Hemisphere
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Coastal Classification• 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
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Coastal Classification• 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
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Chesapeake and Delaware Bays Are Examples of Estuaries
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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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Tides Are Caused by the Gravity of the Moon
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Tides• Monthly tidal cycle
• Spring tides–Occur during new and full moons–Gravitational forces of the Moon and Sun are
added together.–Produce especially high and low tides–Large daily tidal range
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Tides• Monthly tidal cycle
• 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
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Tides• Other factors that influence tides
• Shape of the coastline• Configuration of the ocean basin
• Tidal patterns• Diurnal tidal pattern = a single high tide and a
single low tide each tidal day.• Semidiurnal tidal pattern = two high tides and two
low tides each tidal day.
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Tides• Tidal patterns
• 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
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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
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Tidal Delta and Tidal Flats
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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 0.002 seconds per century.–This small effect becomes very large over millions of
years.• The length of each day must have been shorter in
the geologic past.
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End of Chapter 20