Continental Shelf Processes

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Continental Shelf Processes
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Areas on the shelf

• Inner shelf 0 - 20m depth
• Middle shelf 20 - 40m depth
• Outer shelf 40m Shelfbreak (100m)
• US East Coast Wide, gently sloped shelves.
  Varies from in width from 30 miles at Cape
  Hatteras to 100 miles wide in Georgia.
• US West Coast Narrow, steep shelves.
• Width is approx. 25 30 miles.

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Physical Processes
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Sources of Currents on Continental Shelves

• Waves
• High frequency oscillatory currents (T 3-30
  seconds)
• Infragravity waves - wave groups (T 30 5
  minutes)
• Tides
• Inlet Mouth Currents
• Rotary Continental Shelf Flow
• Wind
• Ekman Transport
• Wind-driven along-shelf currents
• Permanent (Recurring) Currents
• Gulf Stream Intrusions

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Atlantic Coast - wave and tidal climate
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Gulf of Mexico wave and tidal climate
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Pacific Coast wave and tidal climate
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WINDS

• SEASONAL VARIABILITY
• Summers Bermuda High over Atlantic resulting in
  dominant southerly winds
• Winter dry, cold air mass over N. American
  continent northerly winds with strong southerly
  winds due to advancing frontal systems
• Spring and fall transitional months
  intermittent strong northerly winds
• SYNOPTIC VARIABILTY
• Frontal systems 2 7 days (winter)

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Wind climatology
Spring
Summer
Winter
Fall
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Wind-driven Currents

• Winds typically need to have a duration of over
  36 hours to generate wind-driven currents on the
  shelf.
• Low-frequency currents that can overwhelm tidal
  cycle (period gt24 hours) .
• Across-shelf currents are due to wind set-up due
  to on-shore winds.
• Along-shelf currents move parallel to the coast

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Tides on Continental Shelves

• Usually A Progressive Wave Along the Coast
• Amplified Height in Continental Embayments
• Georgia Embayment 8, Cape Hatteras 3.6
• Generates Rotary Water Motion - Tidal Current
• Like Elliptical Water Motion in a Shallow Water
  Wave
• Continues as a Wave Up Estuaries and Rivers
• Conway is 8 Hours After Ocean

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South Atlantic Bight
(Chen)
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South Atlantic Bight Tidal Currents

• Typically range from 0 20 cm/s on the
  continental shelf
• Increase closer to the coast
• Rotate clockwise in an elliptical shape on the
  shelf (tidal ellipse)

(Chen)
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Waves on Continental Shelves

• Transitional waves
• 1/20 lt D/L lt ½
• Shallow water waves
• D/L lt 1/20

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Wave DissipationWave heights decrease as waves
traverse the shelf towards the coast.
d 30m
d 15m
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Wave Energy Dissipation

• Energy of waves is dissipated due to bottom
  friction from the seafloor as waves traverse the
  continental shelf.
• E1/8(density)(g)H2
• Shallow wide shelf more energy dissipated
  smaller waves along the coast

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How do we measure physical processes on the
continental shelf?

• NOAA Integrated Ocean Observing System
• In-situ measurements from buoys and mooring
  stations.
• Sampling cruises

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Physical oceanographic instrumentation
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Acoustic Doppler Current Profiler
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Waves across the shelf

• Waves create high shear on the seafloor and
  result in sediment resuspension and bio erosion
  of hardbottom habitats.
• As the depth of the shelf decreases, waves effect
  the seabed more creating larger shear stresses.
• More sediment transport on inner-shelf and
  nearshore regions
• Hardbottom habitats can be broken down and
  transported during storms

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Hydrodynamics in the bottom boundary layer
2 bottom boundary layers

• Slowly varying currents develop a thick bbl
  where shear stresses are relatively small
• Thin wave bbl creates high shear stresses
• These two interact non-linearly creating high
  combined w-c shear stresses

Current
1-10 m
wave
mm
Suspended sediment transport Bedload sediment
transport
Seabed
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Bottom Boundary Layer Processes
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Bottom boundary layer processes - Why is the
bottom boundary layer of the ocean important?
The oceanographic processes that occur within a
few meters of the bed can have a wide-scale
impact on processes that occur throughout the
continental shelf. This is primarily due to two
main factors 1) the bottom boundary layer
serves as a transition region for the exchange of
nutrients, chemicals, and biota between the
overlying water column and the seabed 2) the
bottom boundary layer is a source of turbulent
kinetic energy and strong dissipative shear
stresses.
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River Plume Processes

• Nutrients
• Turbidity
• Freshwater

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Model of River Runoff
Which way is the water flowing after leaving the
mouth?
(Chen)
Buoyant Coastal Current
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Gulf Stream Effects

• Gulf Stream runs along edge of continental shelf
• Topographical upwelling
• Charleston Bump
• Eddies and filaments intrude onto the shelf