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Estuarine Circulation
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The competing effects of density and tidally-driven circulation in estuaries |
Analyses of data in several of Georgia's cloastal plain estuaries have provided information on the response of the density structure to forcing due to wind on the continental shelf. A particular focus deals with secondary circulation in curving channels. |
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The Salinity Response of a Shallow Coastal Plain Estuary to River Discharge , the Spring-Neap Cycle and Coastal Winds |
Trent Moore, Jack Blanton, Susan Elston, Cheryl Burden and Julie Amft |
Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411 |
| Introduction |
We examine the salinity response of the Satilla River estuary to river discharge, neap and spring tides, and coastal winds. The Satilla is a coastal plain estuary in southern Georgia (Fig.1). Freshwater inflow is small, averaging 70 m3s-1, but often rises above 100 m3s-1 during rainy periods and falls to nearly zero during severe droughts. |
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| The semidiurnal tidal range varies from 2 to 3 meters at neap and spring tide, and tidal influence extends 106 km from the mouth. Tidal excursions range between 12 and 15 km in the lower 50 km, and stratification varies from partially mixed at neap tide to well mixed at spring. Our study covers the lower 30 km of the estuary where the channel width is about 500 m to 2000 m at low water with an average low-water depth of 4 m.
A typical axial salinity section taken during the study period shows the tidal excursion of the salinity field and the degree of stratification near the times of slack water (Fig. 2). |
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Data Used
Salinity was measured at 5 stations with salinity and pressure sensors mounted on the bottom (Table 1). These instruments were SeaBird Electronics (SBE) SeaCats or MicroCats sampling at an interval of 0.1 hr. There were also InterOcean S-4 current meters mounted 1 meter from bottom at three stations and RDI 600 KHz acoustic Doppler current meters (ADCP) at two stations. |
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All subsurface pressure data were converted to water level above the instrument using standard UNESCO algorithms. Tidal fluctuations in pressure and salinity were removed with a 40-hr Butterworth low-pass filter.
Wind speed and direction were measured at NDBC Buoy Number 41008 located in Grays Reef National Marine Sanctuary (latitude 31.40°N; longitude 80.87°W). The buoy is located about 70 km northeast of the mouth of the Satilla River estuary. These data, after converting to along-shelf and cross-shelf wind stress components, are used to represent wind forcing on the inner continental shelf. Wind stress data were lightly smoothed with a 3hr Butterworth low-pass filter. Positive along-shelf stress is poleward and positive cross-shelf stress is offshore.
Subtidal (40hlp) fluxes were determined from the ADCP at Station 4. Tidal current amplitude was estimated by cubing the velocity at the 5.76 m bin, then subjecting the result to the 40hlp filter then taking the cube root. The result is proportional to the power expended by the tidal current (Griffin and LeBlond, 1990).
Salinity Response to River Discharge
Bottom salinity throughout a 26 km distance along the channel is compared with river discharge (Fig. 3). The salinity throughout the estuary decreased at the moment discharge began to increase. Salinity of 0 PSU reached its farthest downstream position about 3-4 days before maximum discharge. After than, the salinity regime as a whole remained relatively steady for the next 30 days, after which the regime began to move inland. |
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Throughout the period from maximum discharge to end of the record, a zone of maximum gradient in bottom salinity remained in place at a distance of 14 km inland. The strength varied over time between 1.5-2.5 x 10-3 PSU m -1 . Note the small fluctuations in bottom salinity and its gradient superimposed in the overall trend. We will concentrate our attention on bottom salinity, water level and current fluctuation around Station 4 which is located within the region of maximum bottom salinity gradient.
Salinity Response to Tidal Current Strength |
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| The effect of the spring-neap cycle is noticeable in the subtidal flux of water at Station 4 (Fig. 4a). During spring tides, there was weak to strong outflow at the surface which diminished in strength with depth. Note that, while the 1 February event raised water level by 0.5 m, there was a large efflux of water at the surface associated with the particularly large spring tide.
Large influx and outflux events appear to be correlated with the strength of the tidal current ((Fig. 4b) with an obvious variation with the neap-spring tidal cycle. Note that the strongest tidal current at the first spring tide (S1) is stronger than the other springs (S2 and S3). Also, there are two weak neaps (N2 and N4) that are separated by a stronger neap (N3). At the strongest spring, export of water was seen near the low-tide surface which became weaker at depth. At the two weak neaps, water was imported near the bottom. During the N2 neap tide, the residual circulation throughout the water column appeared to pulse in step with the tidal current strength.
To more clearly see how tidal current strength is related to salinity, we have replotted the tidal current speed and salinity at Station 4 on the same graph (Fig. 5). In general, during weak neap tidal currents (N2 and N4), bottom salinity increased. During spring tides, salinity usually decreased but the timing was not consistent. To examine possible causes of this inconsistency, we examine the influence of coastal winds on water level and associated salinity fluctuations. |
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| Wind stress versus estuarine water level
Coastal wind stress and associated Ekman transport affects water level inside an estuary. Equatorward/poleward stress parallel to the coast raises/lowers sea-level accompanied by subtidal influxes/outfluxes of mass (Fig. 6). Estuarine exchange with the ocean is maximized under these conditions (Klinck, O'Brien and Svensen, 1983). On the other hand, wind stress along the axis of the estuary theoretically causes relatively little exchange with the ocean but can effect transport within the estuary. |
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| Note the visual correspondence of along-shelf wind stress with sub-tidal water level. The downwelling-favorable stress centered on 1 February causes water level to increase 0.5 m. Similar downwelling events on 15 February and 8 March caused water level increases of 0.3-0.4 m. Upwelling wind stress caused the opposite effect. On 1 March and 4 March, water level decreased about 0.4 m.
Cross spectral analyses of wind and subtidal water level (Fig. 7) revealed energy peaks between 0.07 and 0.17 cycle per day (6-14 days). Along-shelf stress had higher energy than cross-shelf stress and was significantly coherent with water level over a large frequency band. A phase lag of nearly 180 degrees indicates that negative stress (downwelling favorable) was associated with higher than average water level. Cross-shelf winds are coherent with water level over a narrower band. Winds blowing away from the estuary are associated with depressed water level while winds blowing into the estuary raise water level. Thus there is a clear link of coastal wind stress and water level inside the estuary. |
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Response of salinity to water level
We examined the possibility that there was also a link between water level and salinity. We found that the two well-defined peaks in the pressure spectra (Fig. 8a) were not evident in the salinity spectra which had a single peak at around 0.05 cpd (20 day period). The salinity spectrum does not reflect the entire variance of the record because the higher salinity present at the beginning of the record never returned until several weeks after the record ended. Coherence between salinity and water level occurred in a narrow band with a period of about 10 days (Fig. 8b). Phase lag was approximately zero (Fig. 8c). Thus, rising salinity occured simultaneously with rising water level. |
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Discussion and Summary
The major event that altered the bottom salinity regime was the single peak in discharge observed in early February (Fig. 3). Salinity decreased throughout the estuary to a minimum about 20 days after the initial increase in discharge. Thereafter, salinity slowly increased but with temporal fluctuations in bottom salinity superposed. These higher frequency fluctuations appeared to be related to either wind-induced fluctuations in water level or fluctuations in tidal mixing that either augumented or erased the gravitational (density-induced) circulation.
See for example the 1-2 February event. Bottom salinity fluctuations appear to be correlated with the strength of the tidal current. However, the downwelling event that occurred at the same time (Fig. 6) coupled with the fact that river discharge was increasing significantly (Fig. 3) induced a complex salinity response. Bottom salinity first increased as water level increased by 0.5 m followed by a large decrease as the effect of rising discharge overcame the effect of the wind. Thus it appears that large increases in water level can transfer large quantities of high salinity water landward, but its timing relative to discharge changes and the spring-neap cycle can make interpretation difficult.
Changes in the estuarine circulation over the spring-neap cycle affect import and export of bottom salinity. We expect large landward tidal fluxes in the intertidal zone to reverse the gravitational circulation (Li and O'Donnell, 1997). The relatively weak landward tidal fluxes of N2 and N4 were not strong enough to reverse the gravitational circulation. However, the salinity response to a change in a circulation mode from gravitational to tidal can be influenced, as well, by water level changes induced by coastal wind stress.
The salinity response to neap tides was clear. Bottom salinity increased directly out of phase with decreases in tidal current strength (Fig. 5). The opposite occurred during spring tide - i.e. bottom salinity dipped to lower levels during spring tide but the timing of the response was complicated by other factors.
Acknowledgments
We gratefully acknowledge the following agencies who supported the work described here: the Georgia Coastal Zone Management Program (Grant No. RR100-279/9262764), National Science Foundation (LMER Grant No. DEB-9412089 and LTER Grant No. OCE-9982133), Georgia Sea Grant Program (Grant No. R/EA-15), and the NOAA Coastal Ocean Program (Grant to South Carolina SeaGrant Consortium entitled "Tidal Circulation and Salt Transport in a Tidal Creek-Salt Marsh Complex."
We appreciate the work of the crew of the R/V Blue Fin which made this field program possible.
References
Griffin, D.A. and P.H. LeBlond. 1990 . Estuary/ocean exchange controlled by neap-spring tidal mixing. Estuarine, Coastal and Shelf Science 30 : 275-297.
Klinck, J., J. O'Brien and H. Svendsen. 1981 . A simple model of fjiord and coastal circulation interaction. Journal of Physical Oceanography 11 : 1612-1626.
Li, Chunyan and James O'Donnell 1997 . Tidally driven residual circulation in shallow estuaries with lateral variation. Journal of Geophysical Research 102 : 27,915-27,929.
Sponsors: NSF LTER
NOAA CZM
| Other references: |
Blanton, J.O., H. Seim, C. Alexander, J. Amft and G. Kineke. 2003. Transport of salt and suspended sediments in a curving channel of a coastal plain estuary: Satilla River, GA. Estuarine, Coastal and Shelf Science 57: 993-1006.
Seim, H.E., J.O. Blanton, and T.F. Gross. 2002. Direct stress measurements in a shallow, sinuous estuary. Continental Shelf Research 22: 1565-1578.
Blanton, J.O., F.A. Andrade, and M.A. Ferreira. 2000. Effect of a broad shallow sill on tidal circulation and salt transport in the entrance to a coastal plain estuary (Mira B. Vila Nova de Milfontes, Portugal). Estuaries 23: 293-304. |
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