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 Coastal Physics 


Accounting for Circulation Complexities in Georgia Estuaries:
A seminar delivered to staff of Environmental Protection Division of Ga DNR
 

Jack Blanton, Skidaway Institute of Oceanography
In collaboration with
Al Garrett, Savannah River National Laboratory
17 July 2009

 
Objectives
 
  • Describe factors that make Georgia estuaries complicated when compared to “text-book” estuaries
  • Demonstrate the complexities using historical studies and a state-of-the-art numerical hydrodynamic model
  • Comments on assumptions that should be incorporated into models
 
Laurel View River: a typical estuary
 
Large intertidal areas perforated by numerous tidal creeks
 
Typical hypsometric curves in Georgia
 
So what’s the problem?
Large intertidal areas store and release large volumes of water every tidal cycle and are able to trap tracers (“tidal trapping”)

Residence times are governed by tidal prism (large intertidal areas), low-water volumes (small) and the fraction (between 0 and 100%) of water returned during each tidal cycle

Abrupt changes in depth and channel curvature distort tidal currents from their sinusoidal shape causing the average flow over days to weeks to change from seaward (in deeper parts) to landward (along channel flanks)

 
Plots of water level versus water velocity in channels illustrate that slack water seldom occurs at the times of low and high water
 
 
Some Extremes

Factors increasing tidal current distortion

  • Large tidal range and small water depth
  • Strong changes in channel curvature and cross section
  • Increased friction
 
Marshes store and release water over several days (poorly flushed)
 
HTO tidal pumping into marshes
 
 
Background circulation (averaged after many tidal cycles) inhibits efficient transport of water seaward
 
Flow direction reverses across the channel due to changing water depth
 
Recirculating eddies develop due to curving flow thus reducing flushing efficiency
 
Groves Creek simulation combines and illustrates all these effects
 
ALGE: a 3-D finite-difference hydrodynamic model
Developed by:
* Dr. Alfred Garrett
* Savannah River National Laboratory, Aiken SC
* Calculates transport and mixing of conservative and non-conservative tracers
Groves Creek design:
* 69,825 nodes
* Node size = 4 m
* Vertical resolution: 1 m
* Time step: < 1 sec
 
Dye spreads out of channel and into marsh and, while diluted, tends to remain there
Dye Release

Colorbar of each frame normalized to frame's maximum dye concentration. Red dots are reference points.
 

 
Dye is retained in model domain where it decays exponentially
Dye decay in Groves Creek model domain
 
Key management issues
  • Storage and release of water from marshes increases residence time of water
  • Overtopping of water out of the channels causes marsh areas to be chronically “dosed” in areas close to outfalls leading to gradual degradation of water quality over time
 
Management model should be designed to incorporate these features
  • Must allow for overtopping of channels
  • Must allow for fact that flow along channels is not uniformly seaward
  • Must be able to assess wind effects
 
ACKNOWLEDGMENTS
  • US Department of Energy; National Science Foundation; Georgia Coastal Zone Management Program
  • Savannah River National Laboratory (SRNL) for model development and applications
  • Dr. Al Garrett (SRNL Project manager and model developer)
  • Colleagues at SRNL (J. Bollinger, D. Hayes, L. Koffman) and SkIO (D. Savidge, C. Alexander, J. Amft, T. Moore, M. Robinson, C. Venherm)
 


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