Estuarine

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Estuarine Coastal Modelling Course
MODULE II ESTUARINE CIRCULATION
Stephen Nash, Environmental Change Institute, NUI
Galway
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ESTUARINE CIRCULATION CONTENT- I Tidal
Currents II Bathymetry III Coriolis
force IV Meteorological Effects V Freshwater
Inflows VI Density VII Advection, Diffusion and
Dispersion
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I TIDAL CURRENTS
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I TIDAL CURRENTS Currents

• May be tidal or non-tidal
• Tidal due to tidal oscillations
• Non-tidal
• Bathymetry
• Coriolis force
• Wind
• Rivers
• Density
• Tidal non-tidal currents hydrodynamic
  circulation

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I TIDAL CURRENTS Tidal Wave

• Shallow-water wave
• Tidal pattern
• Low tide
• Flood tide
• High tide
• Ebb tide
• Current varies with tide

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I TIDAL CURRENTS Open Sea Tidal Currents

• Progressive wave
• Current directly proportional to
• tidal range
• Currents typically weak
• 5 10 cm/s

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I TIDAL CURRENTS Estuarine Tidal Currents

• Coastline shape affects wave behaviour
• progressive wave ? standing wave
• Elements of both waveforms
• Tidal reversal at high/low water
• No flow at high/low (slack) water
• Max. currents at mid-tide
• Lag in tides along estuary

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I TIDAL CURRENTS Tidal Wave Distortion

• Caused by
• Channel shape
• Bathymetry
• Extreme distortion
• ? Tidal bores

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I TIDAL CURRENTS Current Velocity

• Vector quantity ? speed and direction
• Temporal variation

• Spatial variation flow field

• Variation with depth

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I TIDAL CURRENTS Current Velocity Display

• Time-series
• Current ellipse / current rose (temporal
  variation)
• Stick plot
• Vector snapshot (spatial variation)
• Identification of hydrodynamic features gyres,
  jets, etc.

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I TIDAL CURRENTS Current Velocity Display

• As a time Series
• Plots the variation of current magnitude and
  direction with time
• Can be displayed as total magnitude or as ebb
  (negative) and flood (positive)

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I TIDAL CURRENTS Current Velocity Display

• As a Current Ellipse (or Rotary Current)
• Current velocities at different hrs represented
  by vectors with common origin
• Open Sea elliptical
• Estuaries predominantly in two directions (ebb
  and flood)

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I TIDAL CURRENTS Current Velocity Display

• As a Vector Stick Plot
• Hourly current vectors plotted on a horizontal
  time axis

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I TIDAL CURRENTS Current Velocity Display

• As a Vector Snapshot
• Plot of current vectors at an instant in time ?
  flow field

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I TIDAL CURRENTS Residual Currents

• Tidal effects only
• Flood transport ebb transport ? zero net
  transport
• Interaction of tides with other forces
  bathymetry
• Flood transport ? ebb transport ? some net
  transport
• Residual current algebraic sum of velocity over
  a tidal cycle
• Residual circulation IS estuarine circulation

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I TIDAL CURRENTS Residual Currents

• Residual current vector snapshots

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II BATHYMETRY
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II BATHYMETRY Definition

• Bathymetry is... underwater topography
• 2D, 3D

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II BATHYMETRY Significance

• Interaction with tides ? current velocities
• Bathymetric steering

• Tidal pumping

• Tidal trapping

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II BATHYMETRY Jets and Gyres

• Phenomena caused by interaction between
  bathymetry, tidal wave and sometimes wind
• Jet
• A high velocity fluid stream caused when water is
  forced through a narrow/constricted opening into
  a wider estuarine basin
• Gyre
• Circular movements of water about a particular
  point (the eye), may be clockwise or
  anticlockwise
• Current velocity at/near the eye is close to zero
• Usually accompany jet formations

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II BATHYMETRY
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III CORIOLIS FORCE
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III CORIOLIS FORCE

• Apparent force to allow for earths rotation
• Northern hemisphere ? flows deflected to right
• Southern hemisphere ? flows deflected to left
• Dependent on latitude
• Importance ? Rossby Number, RO

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IV METEOROLOGICAL EFFECTS
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IV METEOROLOGICAL EFFECTS Wind

• Impact on Currents
• Wave generation
• Large waves can affect circulation patterns
• Promotes vertical mixing
• Surface currents
• Drag force on water surface ? acceleration of
  particles
• Fetch, wind velocity, duration
• Wind effect ? water depth, stratification
• Timescales

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IV METEOROLOGICAL EFFECTS Wind

• Surge/swell sea level fluctuations caused by
  strong winds
• High winds cause water to pile-up
• Much higher tides than normal
• Devastating effects when combined with high
  spring tides
• Atmospheric pressure
• Low pressure ? higher sea level
• High pressure ? lower sea level
• Combine with strong winds
• Meteorological tides

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IV METEOROLOGICAL EFFECTS Storm Surge
Hurricane Katrina On the Gulf Coast
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V FRESHWATER INFLOWS
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V FRESHWATER INFLOWS

• Significant impact on hydrodynamics mixing
• Stratification and associated circulation
  patterns
• Mixing effects from variations in flow
• Outfall location
• Momentum effects on local currents
• Duration of flood and ebb currents

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VI WATER DENSITY
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VI WATER DENSITY Density

• Pure water ? 1,000 kg/m3
• Ocean water at sea surface ? 1,027 kg/m3

• Factors Affecting Density
• Temperature temp ? ? density ?
• Salinity salinity ? ? density ?
• Temp has greater effect than salinity
• Thermohaline circulation

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VII ADVECTION, DIFFUSION AND DISPERSION
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VII ADVECTION, DIFFUSION AND DISPERSION

• Advection
• Transport due to bulk movement of water
• Moves matter from one position to another

• Diffusion
• Transport due to random movement of water
• Molecular diffusion from Brownian motion of
  water particles
• Turbulent diffusion from eddies
• Migration of a solute due to concentration
  gradients

• Advection
• Mixing due to non-uniformities in velocities

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ESTUARINE CIRCULATION CONTENT- I Tidal
Currents II Bathymetry III Coriolis
force IV Meteorological Effects V Freshwater
Inflows VI Density VII Advection, Diffusion and
Dispersion
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Estuarine Coastal Modelling Course
MODULE II ESTUARINE CIRCULATION
Stephen Nash, Environmental Change Institute, NUI
Galway