ESYS 150 LECTURE 11 COASTAL PROCESSES

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ESYS 150LECTURE 11COASTAL PROCESSES

• Coastline
• Waves in general
• Waves on the coastline
• Human effects on the Coast

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COASTLINECOASTAL USRates of erosion on US
beaches

• Severely eroding (red) Most people want to
  live on
• Moderating eroding (orange) beach but it is
  disappearing
• Reasonably stable (purple) from most places.

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WAVESENERGY PULSEParticles rotate in place

• Particles rotate in the direction of the wave
  front.
• At the surface, the diameter of particle motion
  H
• Orbit decreases with depth 0 when z L/2
• Waves formed by frictional drag of wind on water
  surface.
• Wave height a function of wind velocity,
  consistency, duration and fetch.
• Can travel thousands of miles from where formed.

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WAVESENERGY PULSEParticles rotate in circles

• Involve both longitudinal and transverse motion.
  As wave travels through water the particles
  rotate in a clockwise fashion.
• Depth of the water is greater than the height of
  the waves.
• Blue particles show that each particle travels in
  a circle.

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WAVESSWELLS AND ROGUE WAVE

• Usually many different storms, producing waves of
  different characteristics. They interfere with
  each other to produce small swell.
• b) When they become synchronized they can produce
  huge waves that can overwhelm small boats. Waves
  can have amplitudes of between 5 and 15 m.

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WAVES ON COASTLINEWHY WAVES BREAKWave moves
into shallow water

• At water depths less than 1/2 the wavelength
  waves change shape.
• Friction on the ocean floor causes particle
  motion to go elliptical and the waves get larger.
• Thus H increases and L decreases.

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WAVES ON COASTLINEWHEN WAVES BREAKSimple theory
(Miche 1944)

• In deep water when d/L gt 0.5 or d gt L/2 then only
  the wavelength is important.
• Breaking occurs when H/L 0.14 or (HL
  17)
• In shallow water, when d/L lt 0.05 or d lt L/20
  then only the depth of the water is important.
• Breaking occurs when H 0.8d or (d 1.3H)
• In practice H/d at breaking depends also on both
  the wave steepness and the beach slope.

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WAVES ON THE COASTLINEZONES ON THE
BEACHFormation of actual breakers

• Schematic cross section showing deep-water waves
  entering shallow water.
• Wavelength decreases as wave height increases and
  the wave breaks.
• Note the transition, breaker, surf and swash
  zones.

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WAVES ON COASTLINEWHY WAVES CURL

• In a breaking wave, circular rotating water
  slowed at base.
• Rolls forward at the top. Creates a curl.

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WAVES ON THE COASTLINESUMMER/ WINTER
BEACHESSummer

• Summer conditions at Boomer Beach, La Jolla.
• Summer storms less active, waves short
  wavelength, small height.
• Abundant small waves push off-shore sand up the
  beach.

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WAVES ON COASTLINESUMMER/WINTER BEACHESWinter

• Active winter storms, long wavelength waves with
  abundant energy.
• Large breakers, large backwash, pulls the sand
  off the beach.

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WAVES ON COASTLINEREAFRACTION OF WAVES AROUND A
POINTFalse Point, La Jolla

• View north of wave refraction around False Point.
  
• Wave refraction around a headland.
• Wave first entering shallow water slows down and
  converges on
• headland. Wave portion still in deep water races
  ahead and stretches
• out creating a bending process known as wave
  refraction.

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WAVES ON COAST LINEREFRACTION OF WAVESDirection
of prevalent swell and Scripps Canyon control
structure of the waves at Blacks Beach.
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WAVES ON COAST LINEREFRACTION OF WAVESCreates
high wave heights north of canyon head
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WAVES ON COASTLINELONGSHORE DRIFTRiver of sand

• Schematic map of longshore drift. Waves run up
  the beach at an angle but return to sea as a
  perpendicular backwash.
• Creates a longshore transport of water and sand
  along the coast.
• A river of sand.

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WAVES ON COASTLINESUBMARINE CANYONSCoast of San
Diego County.

• Two different cells.
• Silver Strand cell from Tijuana river sands
  driven north by waves from summer hurricanes off
  Mexico.
• Oceanside cell is beach and sand moving south
  pushed by waves from north Pacific winter storms.
• The sand flow ends when it pours into La Jolla
  Canyon - a submarine canyon.

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HUMAN EFFECTS ON COASTDAMSMalibu Canyon Dam

• Dam built in 1925 was filled by sediment 13 years
  later.
• Coastal zone short of fresh water. Build dams
  across rivers.
• Dams also catch sand, cuts off beach supply,
  reduces protection against erosion. Torrey Pines
  beach losing sand. Eroding faster.

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HUMAN EFFECTS ON COASTCLIFF PROTECTIONRetreating
cliffs in Solana Beach

• Condos have wonderful views but beach retreating
  at 10 ft per 100 years.
• Want to protect the beach from erosion. How?

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HUMAN EFFECTS ON COASTCLIFF PROTECTIONConcrete
wall and rocks

• Sunset Cliffs San Diego. Protected by a cement
  wall and riprap.
• Waves erode underneath and around the structure.
• Also wall reflects waves hitting them. Rebounding
  water surges power fully erode the beach.
• Riprap reduces reflection but eventually waves
  work way under the riprap.
• Eventually have very serious problem

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HUMAN EFFECTS ON COASTSEA WALLBuilt on beach to
provide protection

• A before the wall,
• B build he first wall - waves bounce off wall
  erode the beach
• C 2 - 40 years - all sand removed by waves which
  work to undermine wall
• D Later 10 - 60 years - build bigger seawall,
  increases reflection, beach disappears again and
  ocean much deeper.
• Longshore drift can offset this

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HUMAN EFFECTS ON THE COASTGROINSShip Bottom New
Jersey

• Groins - short elongate mass perpendicular to
  coast line to trap sand.
• Interfere with longshore drift with deposition on
  upside and erosion on downside.
• What is the direction of longshore drift?

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HUMAN EFFECTS ON COASTBREAKWATER Built parallel
to coast to provide protection

• Stops waves from hitting the beach. Sand is
  deposited behind and on the upside of the
  breakwater.
• Needs permanent dredging to keep the harbor or
  sheltered area open.

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HUMAN EFFECTS ON COASTJETTIESCreate harbor and
channels for boat passage.

• Need to be built out just the right length to
  create a large enough tidal prism (in-and-out
  volume) of seawater to keep them clear.
• Extend well beyond the breaker zone and interfere
  with longshore drift.
• Have deposition on the upside and erosion on the
  down-drift side.