1. Effects of Density and Bathymetry on Salinity Transport in the Savannah Estuary: Port Wentworth to I-95
Author: Bryan N. Riggs
B.A., History, Eastern Kentucky University, 2007
M.S., Geological Engineering, Missouri University of Science and Technology, 2013
Co-Author: Dr. Francisco Cubas
Associate Professor, Georgia Southern University,
Civil Engineering
April 7, 2017

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Hypothesis
The presence of deep channel geometries in the Savannah Estuary upstream of Port Wentworth causes denser salt water to flow to the bottom of the channels, removing it from the mixing process and therefore prevents the transport of salinity further upstream in the estuary.
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Agenda
Hypothesis
Background
Savannah Estuary
Savannah Harbor Expansion Project
Floridan Aquifer Overpumping
Salinity Intrusion into the Savannah Estuary
Possible Impacts on Raw Water Impoundment
Estuary types
Problem Statement
Criteria For Success
Methodology Overview
Methodology: USGS Gauging Station Study
Methodology: Field Study
Results
USGS Gauging Station Study
Field Study Salinity Profiles
High Tide/ Low Tide density comparison
Discussion
Discussion: Data Collection
Discussion: Field Study
Conclusions
References
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4. Background: Savannah Estuary
Termination point of the Savannah River
Part of a Deltaic System
Lower Delta contains Savannah Harbor
Upper Delta contains “head-of-tides” located at I-95
Figure 1, created by the author, is taken from Michels description of the Savannah Delta system
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5. Background: Savannah Harbor Expansion Project (SHEP)
Panama Canal has been expanded
Larger ships need to find a freshwater port in USA
Savannah Harbor will be deepened from 42’-48’
Figure 2 is a side-by-side comparison of two container ships, one of larger TEU capacity than the other, the draft required for each vessel. The larger vessels is a cross section of the sort of cargo ships that will be entering the Savannah Harbor. (photo courtesy of
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6. Background: Floridan Aquifer Overpumping
Floridan primary source of ground water in coastal Georgia
Beufort Arch (Miocene era Ocala limestone) prevents seawater intrusion into surficial layer
Overpumping by Savannah will lead to sea water intrusion
City must move to surface water to provide city/industry with water
Figure 3 is an infographic of pumping levels as of 2014 in Savannah and surrounding areas which rely on Floridan aquifer. Taken from
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7. Background: Salinity Intrusion in the Savannah Harbor
Historic trend for salinity intrusion
Intrusion follows Savannah Harbor Deepening
Current intrusion is well into Savannah National Wildlife Refuge
Figures 4-7 are documented increases in salinity in the
Savannah Harbor as the channel has been deepened.
E.A. Paul A. Conrads 2006, 6)
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8. Background: Estuary types
Salt-Wedge Estuary
Strong tidal forces meets strong river flow creating a wedge of salinity
Partially Mixed Estuary
Strong tides encounter a weaker river force and less dense freshwater causes the salt water to push down and forces some up in a mixing motion
Well Mixed Estuary
Weak river forces create vertical salinity columns of uniform salinity
Fjord
Retreating glaciers form sills which prevent the mixing of denser salt water with less dense fresh water
Figures 9-12 shows all the different types of estuaries.
All representations taken from
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Problem Statement
Increased volume of salt water threatens the raw water intake at Abercorn Creek
However, salinity levels at I-95 remain near constant
Salinity levels in the downstream estuary vary with tides
Therefore something is preventing the advection of salinity further north to I-95
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Criteria for Success
This study is successful if:
A channel 3’-5’ deeper than ambient channel depth is located
Salinity downstream of the channel has salinity levels .25 ppt higher at the bottom than at the top (partially mixed estuary)
Salinity in the upstream portion has uniform levels (>.1 ppt) from surface to bottom (well mixed estuary)
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Methodology Overview
This study will be conducted in two parts:
1) Data collection from stationary USGS gauging stations along the Savannah Estuary from Ft. Pulaski up to I-95
2) Field study of the area between Port Wentworth and I-95 to collect data on depth, temperature, and density
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12. Methodology: USGS Gauging Station Study
110 days of data will be collected from USGS gauging stations
Data collected will include tide data, salinity, and discharge
Mean high tide data will be collected to see how salinity decreases through the estuary
Table 1 is the list of USGS Gauging Stations
Analysis of Bathymetry data
Soundings collected by USACE in the hydro-survey will be mapped in ARC-GIS
Figure 13 is a map of the data collection study area. The callout boxes name the location of the USGS gauging stations data was collected from. Map data taken from USACE REDI.
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13. Methodology: Field Study
Seven waypoints investigated
Raw USACE data highlights varied geometries
Data included temperature (c), salinity (µS/cm), and depth (ft)
Samples taken at 2’, 4’, and then 4’ increments
Experiment will target 1 low tide, 1 mid tide, and 2 high tide events
Figure 14 is a google maps image of the study area showing the 7 selected waypoints (WP’s).
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14. Results: USGS Gauging Station Study: 110 Day Trend AUG-DEC 2016
The figure reads from left-to-right and is the high tide data from the USGS Gauging Station study. All data was collected during high tide
Salinity levels in the lower estuary remained two orders of magnitude above salinity levels at I-95. The data is presented in a logarithmic scale to show the disparity in salinity levels measured in µS/cm
The drastic drop in salinity from 7-9 October was due to post-Hurricane Matthew runoff and increased freshwater flow from further up river.
The spike in salinity at I-95 (blue area on bottom ) was due to low flow periods from further north in the watershed
Elba Islands data was unavailable from the beginning of the study until mid-September
Estuary entrance
Direction of Tidal Movement
Figure 15 is a logarithmic chart displaying the massive disparity in salinity from Fort Pulaski to I Salinity level variations were due to storm runoff and lunar effects
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15. Results: Mean High Tide Slope Analysis
Direction of Tidal Movement
Approximately 6.5 miles between each gauging station along the Savannah Estuary from Ft. Pulaski to I-95
Salinity is measured in µS/cm and the disparity in mean tide data is shown with the drastic drop from Port Wentworth to I-95
The slope of decrease from Ft. Pulaski to Elba Island is µS/cm over 6.5 miles
The slope from Elba Island to USACE Boat dock is µS/cm over 6.5 miles and the slope from USACE Boat Dock to Port Wentworth is 7915 µS/cm over 6.5 miles. So far the decrease between decreases has been over 2000 µS/cm and a gradual decrease is observed
The slope from Port Wentworth to I-95 is µS/cm over 6.5 miles. The gradual decrease shown further down is now drastically different
R2=.9924
Figure 16 is slope analysis of the mean levels of salinity dilution moving through the channel. The linear trend is disrupted by the time measurements are taken at I-95 with a two-fold decrease in salinity levels not seen elsewhere in the channel.
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16. Results: Field Study Salinity Profiles
Direction of Tidal Movement
Direction of Tidal Movement
Figure 17 is the salinity profile of low tide data
The near vertical lines indicate a well mixed estuary
during low tide
Salinity levels were only high at the start of the study
area at Houlihan Boat Dock (HBD) and low up to I-95
Figure 18 is the salinity profile of high tide data
Moving from right to left is HBD, WP 3-12, and I-95
Notice the salinity bulge or plume which is characteristic in
a partially mixed estuary. However, past WP 6, the deepest part,
salinity values become more uniform like a well mixed estuary
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17. Results: Density and Salinity Comparison
Direction of Tidal Movement
Direction of Tidal Movement
Figure 20 During high tide salinity and density decrease gradually from HBD
to WP 3
From WP 3 to WP 6 salinity levels out but density decreases due
to increase in freshwater mixing
From WP 6 to WP 8 salinity continues to decrease but salinity
levels off
WP level off in salinity and density due to increase in freshwater
and failure of salinity to continue to move upstream past the deepest part
of the study area, WP 6
Figure 19 Low tide density super-imposed over salinity
Salinity decreases in a mostly linear fashion as the study
area behaves as a well mixed estuary during low tide periods
Density values vary due to temperature variations between
the first day samples were collected in late November and the
next samples collected in early December
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18. Discussion: Data Collection
Linear decrease in salinity suddenly becomes non-linear (2 fold decrease)
Same trend noticed by Dial Cordy, Paul Conrads (2 separate studies), and Tetratech
Lower estuary’s maintenance dredging provides uniform channel bottom
Varied channel geometry upstream of Port Wentworth must cause decrease in salinity
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19. Discussion: Field Study
This trend has been noticed by other authors (Paul Conrads, Dr. Alexander, Tetra Tech, Dial Cordy)
Density provided the “trail” to follow the path of the salt water
Deep channels acting as a slope retard salinity, forming a “sill” similar to fjords
Poggioli postulated slope affecting predicted position of salinity
I95
WP12
WP10
WP 8
WP 6
WP 3
HBD
Figure 21, created by David Penn, is a bathymetric map of the upper estuary created using raw bathymetric data collected by USACE Savannah District during the 2014 hydrologic study. Darker blue areas near WP’s 3-8 indicate very deep channels (in excess of 30 feet!).
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20. Discussion: Field Study
Figure 22 created by the author to show the movement of high tide water as it encounters the slightly stronger freshwater of the Savannah River. The denser water moves toward the bottom of the deep channel at WP 6 and is removed from the mixing process. This accounts for the sudden drop in salinity and density between WP’s 6 and 8. Further effects of geometry retard the salinities movement.
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Recommendations
Collecting data during summer months to look for seasonal variability
Comparison with EFDC and WASP data
USACE conduct feasibility study on digging channels to prevent advection further upstream
Dissemination as a possible best practice to USACE
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Conclusions
Horizontal channels do exist between Port Wentworth and I-95
Creates trap for salinity similar to the sills in a fjord which prevents freshwater from mixing with deeper, denser salt water
These traps prevent transport of salinity upstream in the estuary
Scientific contribution is the use of density to demonstrate movement of denser salt water into channels which creates the observed behavior
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Acknowledgements
Dr. Francisco Cubas
Dr. John Dryden
USACE Savannah District, specifically;
Traci Hendren, P.E.
Bryan Robinson, P.E.
Beth Williams, P.E.
United States Army for funding my degree
My loving wife, Andrea Riggs, for supporting me through all this research and being my best, most prompt, proofreader
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References
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