PowerPoint Presentation - Geophysics 189 Natural Hazards

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Coastal Processes and Hazards
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Outline

• Why is this important?
• Definitions
• How waves work
• Interaction at shoreline
• Importance of beaches
• Human impacts on beaches

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Why is this important?

• 70 of Earths surface is water
• Means a lot of coastlines
• In U.S. - 30 coastal states
• Projection by 2010 75 of population will live
  within 75 km of coastline
• 1,358 people/coastal mile
• High concentration of people and property!

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Possible Hazards

• Already covered
• Hurricanes, tsunami, noreasters
• Discuss today
• Waves, tides, erosion, sea level rise

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Coastlines

• Regions where land meets sea
• Can be of different forms
• Long sandy beaches
• Rocky cliffs
• Coral reefs

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Sandy N. Carolina coastline
Rocky coastline of Maine
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Waves and Tides

• Key forces that act to alter coastlines
• Important for erosion, moving material along coast

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How waves work

• Caused by wind blowing over water surface
• Transfer energy from air to water
• 5-20 km/hr breeze small (lt 1cm high) ripples
• 30 km/hr full size waves

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Wave height

• Depends on
• Wind speed
• Direction of wind blowing
• Length of water over which wind is blowing
• Consistency of wind direction

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What is Water Doing?

• Particle of water rotates in place with circular
  orbit
• Orbit decreases in size with depth

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Orbital Motion
You probably have felt the same motion in the
waves!
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Wave Description

• Wavelength (L)
• Wave Height
• Period (T)
• Related to velocity of the wave
• VL/T
• Typical T of few-20 sec, L of 6-600 m means V of
  3-30 m/s

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Swells

• Interference of many sets of waves
• Usually related to storms, multiple storms
• Occasionally constructive interference occurs
• Produce very large waves (rouge waves)
• Can sink ships, may impact shorelines

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Waves Near Coastlines

• 1st orbital motion changes to elliptical when
  depth is lt 1/2 L
• Why? Friction with bottom

shallow-water wave
deep-water wave
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Waves Near Coastlines

• 2nd wave slows down, L gets smaller
• Leads to more water, energy in shorter length
  taller waves

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Waves Near Coastlines

• 3rd at certain height (17 height to
  wavelength), wave is too steep and breaks
• Topples forward, forms the bubbly, foamy stuff

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Slope of Near-shore

• Impacts wave breaks
• If gently sloping bottom, waves break farther
  from shore
• If steeply sloping bottom, waves break closer to
  shore
• Rocky cliffs break directly on rocks with large
  force

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Wave Refraction

• As waves get closer to shore, they bend to a
  direction roughly parallel to shore
• Wave refraction, similar to light
• Important for areas with bays and headlands
• Headlands water depth shallows quickly, waves
  slow and converge at this point
• Bays water in center is deeper, area is more
  protected

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Bending of wave crests due to refraction as waves
slow down in progressively more shallow water
depths
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Wave refraction concentrates energy at headlands,
thereby causing increased erosion
Wave refraction decreases energy at bays, thereby
causing increased deposition
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Longshore Drift

• Waves arrive at small angle to shore, go up on
  beach at an angle
• Moves sand grains (and people) at an angle
• Very efficient at transporting sand to/from
  beaches

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Beach

• Shoreline made of sand/pebbles
• Important for recreation, housing
• Also as a natural barrier to absorb energy in
  breaking waves
• Various beach processes affect how much beach is
  present during the year

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General Beach Cycle

• Summer
• Generally fewer storms, lower wind speeds, waves
  with shorter L and height
• Act to push offshore sand onshore, build wide,
  sandy beaches

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• Summer beach near San Diego, CA

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• Winter beach (same one) near San Diego, CA

Note sandy beach is gone, due to large storm waves
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General Beach Cycle

• Winter
• More energetic storms, waves erode beach sand,
  carry offshore
• With less sand, energetic waves can attack
  coastal features such as roads, houses

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Human Impacts

• Humans like to live near the beach!
• Nice climate
• Great views
• Additional food sources
• Want to minimize risk from big waves, hurricanes,
  erosion of cliffs and beaches

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Human Impacts

• In order to mitigate hazards, we build
• Seawalls
• Dams
• Groins
• Jetties
• Structures have multiple impacts

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Dams

• Dam rivers that add water, sand into ocean
• Many built to provide freshwater reservoirs for
  coastal communities
• Problem sand in rivers adds to beach
  development. By cutting off this supply, adds to
  problem of shrinking beaches

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Seawalls/Cliff Protection

• Build structure for protection of beach or cliff
• Changes beach dynamics
• What happens?
• Ocean waves break on wall because beach narrows
• Steepens slope offshore, leads to larger waves
• Can over time erode seawall or undercut base

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Seawalls can cause beaches to disappear,
construction of new seawalls over time Also,
reduce attractiveness of coastline, property
values
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Seawall, coastal GA
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Groins and Jetties

• Elongate mass (usually rock or concrete) built
  perpendicular to shoreline
• Purpose keep sand on the beach
• Problem longshore drift still occurs
• Leads to deposition on 1 side, erosion on other
  side

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Jetty in NJ - note longshore drift is from left
to right here
Groin leads to deposition updrift, erosion
downdrift
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Beach Replenishment

• Actively transporting sand onto a beach
• Usually pump it from offshore
• Can be very expensive (millions/mile of beach)
• May have to be repeated every year
• Examples Waikiki, HI Miami, FL

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1981 Miami Beach
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Next Time

• Stream/River Processes