- Email: [email protected]
Tidal and residual circulation in the St. Andrew Bay system, Florida
ARTICLE IN PRESS Continental Shelf Research 28 (2008) 2678–2688
Contents lists available at ScienceDirect
Continental Shelf Research journal homepage: www.elsevier.com/locate/csr
Tidal and residual circulation in the St. Andrew Bay system, Florida Patrick L. Murphy a,, Arnoldo Valle-Levinson b a b
National Marine Fisheries Service, Southeast Science Center, 3500 Delwood Beach Road, Panama City, FL 32408, USA Department of Civil and Coastal Engineering, University of Florida, 365 Weil Hall, P.O. Box 116580, Gainesville, FL 32611, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 18 December 2007 Received in revised form 21 July 2008 Accepted 10 September 2008 Available online 25 September 2008
Two 24-h surveys were conducted in St. Andrew Bay, Florida, during spring and neap tides to describe the tidal and non-tidal circulation patterns and to determine the factors that affect these patterns. In particular, the effect of tidal forcing in modulating such circulation patterns was explored. Observed velocities were fitted to diurnal and semidiurnal harmonics separating tidal motions from sub-tidal motions. Residual flows were compared with an analytic model that allowed variations in the relative contributions from Coriolis acceleration and friction using the Ekman number. A solution with an Ekman number of 0.04 resembled the observations best and indicated that the hydrodynamics were governed by pressure gradient, Coriolis and friction. Locally, advective accelerations became important around headlands in sub-estuaries in the system. The consistency of the mean pattern from October to March suggests that tides play a minor role in modulating the exchange flow. Deviations from the longterm mean are mainly caused by wind-driven coastal setup and setdown. Published by Elsevier Ltd.
Keywords: Estuarine dynamics Tidal dynamics Residual circulation Buoyancy forcing St. Andrew Bay Florida
1. Introduction The northeastern Gulf of Mexico coast is experiencing rapid urban and suburban development that intensifies pressure on regional estuaries. Determining the effects of coastal development on various biological and chemical processes in these estuaries (Lindall and Saloman, 1977) depends on understanding the underlying physical framework, namely estuarine circulation. Most studies of estuarine circulation have taken place at temperate latitudes where tidal strength is an important agent in modulating the net circulation. For instance, asymmetries in the flood and ebb tide due to bottom friction and non-uniform bathymetry can induce residual flow (Stommel and Farmer, 1952; Fisher et al., 1979; Robinson, 1981; Geyer and Signell, 1990). Modulation of tides by the spring/neap cycle generates a fortnightly frequency on tidal pumping of salinity up an estuary (Jay and Smith, 1990; Hibiya and LeBlond, 1993). Net exchange flows tend to be strongest at neap tides, when tidally induced mixing is the weakest (Nunes and Lennon, 1987). Tides in subtropical systems along the Gulf of Mexico exhibit small ranges (o0.5 m) compared to temperate estuaries (41 m). Effects other than tidal modulation and asymmetries could play a larger role in altering the residual circulation. Many subtropical
Corresponding author. Tel.: +1757 620 1740; fax: +1 850 235 3559.
E-mail address: [email protected] (P.L. Murphy). 0278-4343/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.csr.2008.09.003
systems show seasonal fluctuations in river runoff that impose an annual cycle to their stratification (Schroeder et al., 1990) and hence its circulation (Valle-Levinson et al., 2001). Other studies of subtropical systems along the Gulf of Mexico have found 3–5 day coherence between increased sub-tidal exchange and meteorological forcing associated with passing fronts (Smith, 1977; Swenson and Chuang, 1983). Transient fronts have a twofold effect. First, water exchange between a lagoon and the adjacent ocean may be influenced by the inverse barometer effect (Liu, 1992). In addition, direct sea level set up by the Ekman transport (Wong, 1987) or advection by onshore winds (Smith, 1979) may drive water into and out of these lagoons at volumes larger than the tidal prism. Pritchard’s (1955) two-layer model, resulting from river input at the head of an estuary, is not applicable to many subtropical lagoons. Systems with low freshwater forcing, ubiquitous along the Gulf of Mexico, have weak baroclinic flows easily modified by external forces. The amount of freshwater, the meteorological forcing and the strength of the tidal currents are crucial in determining whether the system is driven by density gradients, tides, or winds. The objective of this study is to determine the spatial structure of the circulation in a subtropical estuary with quasi-steady freshwater input. In particular, this investigation seeks to assess the role of tidal forcing in modifying such a spatial structure. This objective is addressed with data collected at St. Andrew Bay, Florida, during spring and neap tides at different times of the year and by comparing the results to a theoretical model.
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2679
Fig. 1. Map of the St. Andrew Bay system and study area. Transect 1 ¼ P1–P2. Transect 2 ¼ P2–P3. Transect 3 ¼ P3–P4. Transect 4 ¼ P4–P1. ST 2, ST 3, ST 4 ¼ CTD Station.
2. Study area The St. Andrew Bay system in northwest Florida (Fig. 1) is an ecologically important estuary surrounded by rapidly growing communities. The bay is home to one of the most diverse marine populations of any estuary in the northern Gulf of Mexico coast (Ogren and Brusher, 1977). This area hosts a large variety of fauna because of, among other reasons, its low freshwater inflow, low turbidity, extensive sand flats, widespread submerged aquatic vegetation (SAV), and a deep basin with both coarse and fine sediments (Brusher and Ogren, 1976). The St. Andrew Bay system is a drowned river valley, coastal plain estuary that formed 5000 years ago during the Holocene transgression (Salsman et al., 1966). The system consists of four sub-estuaries: North Bay, West Bay, East Bay, and St. Andrew Bay (Fig. 1). The latter is the only sub-estuary with a direct connection to the Gulf of Mexico. The US Army Corps of Engineers maintains a 150 m wide channel at the Gulf of Mexico connection, known locally as ‘‘The Pass,’’ which is armored with jetties on either side.
A minimum 9 m channel depth is maintained from the Pass to the Port of Panama City (St. Andrew Bay) and a paper mill (East Bay), although the channel depth usually exceeds 15 m. Channels in all of the sub-estuaries are generally parallel to the shoreline and are maintained at 4 m. West Bay and East Bay connect to the Gulf Intracoastal Waterway, which allows sheltered navigation from Choctawhatchee Bay to Lake Wimico. The surface area of the bay system is 243 km2. The watershed is approximately 2800 km2 (USEPA, 1999) and is located entirely within Florida (NWFWMD, 2001). Besides non-point runoff and assorted small creeks, the largest freshwater source draining into the estuary is the spillover from the Deer Point Reservoir (Fig. 1). This human-made lake is located at the head of North Bay and is formed as a result of a dam constructed downriver from the confluence of several spring-fed creeks, primarily Ecofina Creek. Daily flow rates are calculated by the Bay County Water Division. Tides in the system are typically diurnal with a mean range of 0.5 m, with a longer ebb flow than flood flow (Ichiye and Jones,
ARTICLE IN PRESS 2680
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
1961; McNulty et al., 1972). Tidal amplitudes of the semidiurnal M2 and S2 harmonics are 5 times smaller than the K1 and O1 diurnal constituents (www.tidesandcurrents.noaa.gov, Station #8729108). An apparent spring/neap cycle in the tides is a fortnightly modulation due to the beating of the diurnal constituents (Kantha, 2005). During a neap tide, the tidal regime is semidiurnal for several cycles, as the diurnal constituents cancel each other. Tidal ranges during these periods are typically 5–20 cm, corresponding to the amplitudes of the M2 and S2 constituents of the overall tide. Information on tidal current spatial variations in the estuary has been absent until now.
3. Data collection and processing Two surveys were carried out to document the spatial patterns of tidal and sub-tidal flows in the St. Andrew Bay sub-estuary during spring and neap tides. One survey took place during a neap tide (October 19–20, 2006), whereas the other survey was during a spring tide (March 22–23, 2007). A 1200 kHz broad-band acoustic Doppler current profiler (ADCP; RD Instruments, Poway, CA) mounted on a 1.2 m sled, pointing downward, was towed around a closed trajectory consisting of four transects across the major channels of St. Andrew Bay (Fig. 1). The ADCP was towed at 2 m/s for an entire diurnal tidal cycle so that intratidal spatial flow changes could be observed. Only minor interruptions in towing occurred (due to vessel repairs and personnel exchanges), and such breaks caused a negligible effect on data coverage. The ADCP collected 1 s pings, which were averaged over 10 ensembles. This yielded a horizontal resolution of 20 m. The vertical resolution of the data was given by a bin size of 0.5 m, and the closest bin to the surface was 1.5 m. Data in the lowest 15% of the water column were discarded due to interference from sidelobe effects (RD Instruments, 1996). Data were compass-calibrated and corrected by the method of Joyce (1989) using a Global Positioning System. A total of 13 and 15 trajectory repetitions were carried out in the neap and spring tide surveys, respectively. Conductivity–temperature–depth (CTD) profiles were recorded at three stations (Fig. 1) during every other repetition of the circuit using a SBE-19 CTD. These stations were chosen to profile the density variability in the middle of the main channels of St. Andrew Bay, and leading into East Bay and West Bay. Data were processed using Sea Bird Instruments software and binned to 1.0 m vertical bins. The degree of water-column stratification was quantified by calculating the potential energy anomaly j of the water column for each CTD cast, following the approach of Simpson et al. (1990): Z Z 1 1 0 ðr¯ rÞgz dz; where r¯ ¼ j¼ rðzÞ dz, h h h where j (J/m3) is the amount of work necessary to completely mix the water column, h the total depth, r the density, and g the acceleration due to gravity (9.8 m/s2). Wind and other meteorological data were collected at Panama City-Bay County International Airport (PFN), located 7 km north of the study area. Water height measurements from NOAA tide station #8729108, Panama City, Florida located adjacent to the study area, were also used to identify sub-tidal forcing effects. Mean daily water flow from the Deer Point Reservoir was calculated from a daily water level measurement at the Watson Bayou Pumping Station and a table lookup interpolation method using values measured by the Bay County Water Division. Calculation of an average flow rate from historical data can be misleading. Only precipitation events inside the watershed will change the daily flow over the dam and annual events such as
snow melt runoff do not influence the spring-fed creeks. The increased flow returns to normal values several days after a precipitation event. This makes computing an average flow from a time series of daily measurements misleading and would inflate the usual 22 m3/s daily flow over the dam (Bay County Water Division, pers. comm.). Such a nuance has not been taken into account by previous studies. For instance, Musgrove et al. (1965) found that the long-term average flow is 28.5 m3/s while Blumberg and Kim (2000) calculated the daily flow from March 1993 to March 1994 to be 38 m3/s. For each transect repetition, the eastward u and northward v components of the ADCP velocity data were interpolated onto a regular grid of distance versus depth values (e.g. Valle-Levinson and Matsuno, 2003). The grid had a resolution of 50 m in the horizontal and 0.5 m in the vertical. The time-series observations (u, v) at each grid point of the 13–15 gridded transects were fitted to semidiurnal (M2 ¼ 12.42 h) and diurnal (K1 ¼ 23.92 h) harmonics using a least-squares method (Lwiza et al., 1991): ðu; vÞ ¼ ðu0 ; v0 Þ þ
m X ðuAj ; vAj Þ sinðoj t þ uyj ; vyj Þ, j¼1
where t is the time; u0 and v0 are the residual components of the velocity; uAj and vAj the amplitudes of the harmonic j of frequency oj; and uyj and vyj the phases, with respect to midnight of the day when the survey began, for each harmonic. Up to ‘m’ harmonics may be fitted to the data. The duration of the surveys was too short to resolve the contribution from all diurnal or semidiurnal constituents. Fitted single diurnal and single semidiurnal amplitudes and phases were expected to be representative of all the constituents interacting during each survey day. Measured residual circulation in Transect 3 was compared to an analytical solution described in Valle-Levinson et al. (2003) to investigate forces governing long-term circulation of St. Andrew Bay. Their analytic model includes pressure gradient, friction, and Earth’s rotation to solve the along-estuary and transverse structure of the flow. Only Transect 3 was compared because boundary conditions of the analytical model require the ocean to be at the down-estuary boundary. The model is adjusted by the dimensionless Ekman number: E¼
Az fH2max
,
where Az is the kinematic eddy viscosity (friction), f the Coriolis force, and Hmax the depth.
4. Results and discussion 4.1. Forcing agents Southerly winds for 2 days prior to the October 2006 survey raised sea levels in the bay (Fig. 2). As the wind relaxed, sea level returned to predicted values. The March 2007 wind record indicated a diurnal sea breeze pattern that rotated clockwise, reaching a daily maximum of southerly velocity just after sunset and a minimum just before sunrise (Fig. 2). Water levels measured at the Deer Point Reservoir were 1.59 and 1.60 m during the October and March surveys, respectively. These measurements represent a primarily spring-fed freshwater flow of 34 m3/s into North Bay during both surveys. 4.2. Hydrography Water temperature in the St. Andrew Bay system exhibited a weak gradient with depth but salinity controlled density (Fig. 3).
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2681
Fig. 2. Tidal height (upper) and wind velocity (lower) for October 2006 neap tide (left) and March 2007 spring tide (right). (-) is the measured tidal height and (- -) is the predicted tidal height. Vertical lines indicate beginning and end of the surveys. Wind vectors follow the oceanographic convention. Only sporadic wind measurements were available during the 24 h preceding the October 2006 survey.
The density change was much larger during spring tides than during neap tides, although small isopycnal changes are apparent in the neap CTD casts. It is noted that although the tide is half as high during a neap tide (semidiurnal), it has half as much time as a spring tide (diurnal) to enter and leave the bay. Consequently, typical tidal currents along Transect 3 are comparable during neap and spring tides. In October, the potential energy anomaly j of all three stations showed a semidiurnal frequency consistent with the dominant tidal amplitude (Fig. 4). The small difference in j between high and low tide indicated small intratidal variations of stratification. This is supported by weak tidal variations (of neap tides) and by the observed sea level, which was nearly twice that predicted because of persistent southerly winds forcing Gulf of Mexico water into the estuary. During the March survey, the tidal regime imposed a diurnal signal on j at all three stations (Fig. 4). At the end of flood, normally the period of lowest stratification, the j values (40–60 J/m3) of the three stations were similar to the lowest j values calculated in October. This suggests flood tide inundates St. Andrew Bay with Gulf of Mexico water regardless of spring or neap tides. By the end of ebb, j increased by 2–3 times the highest October values. Higher values of j during spring tides than during neap tides suggest effects of advection by the tidal currents. In temperate estuaries, spring tidal currents typically increase mixing and decrease j (Simpson et al., 1990). The influence of Gulf of Mexico waters on stratification and the exchange between East Bay and West Bay were observed by plotting surface salinity data measured every 10 s during the
March survey with an SBE-37 (Fig. 5). No surface salinity data were available for the October survey. The March survey began just before the end of flood tide when all the transects had salinities near 32–33, associated with Gulf of Mexico waters. As the tide fell, the lower salinity water from up the estuary traveled across Transects 2 and 4 into the survey area while the higher salinities were advected seaward across Transect 3. During ebb, the lowest salinity water of the survey (27) appeared from East Bay across the northern part of Transect 2. It was then advected along Transect 1 and crossed the northern part of Transect 4 into West Bay. This up-estuary flow occurred while the ebbing tide was advecting high salinity water out of West Bay over the southern section of the same Transect 4. At the end of ebb tide, salinities along the transects were uniformly 29, then increased as flood tide began again. The southern portions of Transects 2, 3, and 4 showed earlier increases in salinity during flood tide compared to the northern portions due to their proximity to the Gulf of Mexico. These measurements show a daily inter-bay circulation and advection of salt that is more complex than longterm averages, which have a net outflow of water from all the estuaries into the Gulf of Mexico (Blumberg and Kim, 2000).
4.3. Tidal circulation Tidal ellipses along all four transects were calculated for surveys following Souza and Simpson (1996). Changes in ellipticity (rotation) or changes in orientation of adjacent ellipses are interpreted as regions of flow convergence
both tidal tidal and
ARTICLE IN PRESS 2682
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
Fig. 3. CTD density (kg/m3) time-series data by month and station. Arrows and vertical lines represent time of individual casts.
Fig. 4. Potential energy anomaly j (upper) for each CTD station. Cross-sectional average of Transect 3 velocity (lower) with the corresponding tidal height. Negative is out of the estuary. (J) is at the median time of each transect.
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2683
Fig. 5. Surface salinity contours for March 22–23, 2007. Transects were measured clockwise around a four-cornered circuit starting at the beginning of Transect 1.
divergence (Li, 2002). Convergences aggregate flotsam and prey while divergences are associated with dispersion. A convergence of tidal flow will typically become a divergence when the tide reverses. In October, the diurnal tidal signal was suppressed by an interaction between the K1 and O1 constituents, resulting in small diurnal ellipses and large semidiurnal ellipses (Fig. 6). Smaller semidiurnal ellipses during March were more representative of the usual diurnal tide (Fig. 7). Tidal currents at Transect 1 were similar between surveys, but currents were stronger in March at the other transects. Strongest currents were found at the surface and over deep water, indicative of bottom friction effects on tidal flow (Valle-Levinson and Lwiza, 1995). Transect 1 tidal ellipses showed marked bifurcation of tidal flow before entering West Bay and East Bay in both October and March (Figs. 6 and 7). During flood, currents from the Gulf of Mexico meet the northern boundary of St. Andrew Bay and are forced toward either East Bay or West Bay. Transect 2 had ellipses parallel to the coast that slowly rotated 901 before getting oriented perpendicular to East Bay. March surface diurnal flows on Transect 2 increased from 13 cm/s at the southern end of the transect to 30 cm/s in the northern segment of the transect. This was due to a constriction at the entrance to East Bay that accelerated flow and caused convergence of flood flows and divergence of ebb flows. October semidiurnal surface speeds on Transect 2 ranged from 7 cm/s at the southern end to 20 cm/s at the constriction. The same pattern of convergence due to bathymetry was observed by Sepulveda et al. (2004) in the Rio de la Plata Estuary, Argentina. Webb et al. (2007) concluded
that regions of tidal convergence caused observed axial fronts aligned with the bathymetry in St. Augustine Inlet, Florida. Transect 2 tidal ellipses from both surveys suggest that conditions exist for axial fronts to form regardless of spring or neap conditions in St. Andrew Bay. In fact, the largest horizontal salinity gradients were observed in Transect 2 (Fig. 5). Rectilinear tidal ellipses in Transect 3 resulted from the tidal flow through the Pass (Figs. 6 and 7). Ellipses displayed an increasing ellipticity from the western end of the transect toward the middle of the channel. This tidal jet that enters the bay during flood is weakened at the edges by friction along the shoreline. In October, tidal flow reached 20 cm/s while in March the diurnal amplitude reached 40 cm/s. Semidiurnal and diurnal tidal ellipses on the east side of Transect 3 have equivalent amplitudes during the October survey. In March, diurnal amplitude in this area was 2–3 times stronger than semidiurnal amplitudes corresponding to strengthening of the tide. During both surveys, Transect 4 tidal ellipses were alongchannel in the northern part of the section and rotated 601 in the south. Highly rectilinear ellipses near the western end of Transect 3 were of comparable magnitude to ellipses near the southern end of Transect 4 during both surveys. In October, semidiurnal tidal ellipses at 5 m were similar in orientation to surface ellipses while ellipses at 9 m were oriented along the channel. In March, both semidiurnal and diurnal tidal ellipses at 5 and 9 m were oriented along the channel. There is a prominent shoal at the southern end of Transect 4 corresponding to the largest amplitude tidal ellipses (Figs. 6 and 7). The shoal is a common feature formed by asymmetries in
ARTICLE IN PRESS 2684
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
Fig. 6. Tidal ellipses for October 19–20, 2006. Black ellipses are counter-clockwise and gray ellipses are clockwise.
Fig. 7. Tidal ellipses for March 22–23, 2007. Black ellipses are counter-clockwise and gray ellipses are clockwise.
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2685
Fig. 8. Mean cross-channel gradient of (–K–) salinity and (–&–) velocity. Measurements are scaled by the largest value in each time series.
sediment transport by energetic reversing tidal flows around a headland (Berthot and Pattiaratchi, 2006). Centrifugal force adds momentum to the flow as it curves around the headland. A momentum balance between centrifugal force, Coriolis, and friction is kept by increasing the friction, i.e. accelerating the flow. Similar observations of this curvature-induced acceleration have occurred at Sandy Hook, NJ (Doyle and Wilson, 1978), Gay Head, MA (Geyer, 1993), and Point Tortuga, Chile (Valle-Levinson et al., 2000). Increase in tidal current amplitude from east to west along Transect 3 and decrease in tidal current amplitude from south to north along Transect 4 are conducive to the formation of significant shear fronts due to the large acrosschannel velocity gradients. Nunes and Simpson (1985) and Turrell et al. (1996) found that lateral shear of an axial current imposed on an axial salinity gradient in the Conwy estuary forced a lateral density gradient that induced secondary circulation. A secondary circulation works to redistribute the density field, causing axial aggregation of particulates, prey, foam, and surface drifters. For each transect, a time series of the mean hourly lateral salinity gradient produced by a shear current and the mean hourly lateral gradient of the along-channel velocity was calculated from surface salinity and ADCP data collected during March (Fig. 8). Gradients are small at the beginning of ebb tide and reach a maximum during largest tidal flows associated with differential seaward advection of lower salinity water. When the ebb tide current approaches slack water, a secondary flow due to the cross-channel baroclinic pressure gradient redistributes the density field, which results in a zero lateral salinity gradient in Transects 2, 3, and 4 by the end of ebb tide (08:00 GMT).
4.4. Mean flow 4.4.1. Observed residual currents Residual currents for each survey are presented in Figs. 9 and 10. Residuals indicated typical estuarine circulation with outflow at the surface and inflow at depth, except along Transect 2 during October. Overall, the magnitudes of the outflow velocities were slightly larger (o5 cm/s) in the March survey relative to the October survey. However, net inflows were larger in the October survey (o3 m/s). Transect 4 had inflow over the entire shipping channel and outflow over the shoal at the southern end. The residual current of Transect 1 during both surveys indicated a strictly westward flow from East Bay to West Bay. This pattern does not follow Stommel and Farmer’s (1952) presumed pressure gradient in their source/sink analogy of water entering and leaving an estuary through an inlet. Rather, it more closely resembles the gyre measured by Webb et al. (2007) at a trifurcation of St. Augustine Inlet, Florida. A surface residual cyclonic gyre in St. Andrew Bay has been modeled, but these are the first direct measurements (Rodriguez and Wu, 1990).
4.4.2. Comparison of residual currents to the analytical model Results of the analytic model, which finds the long-term circulation by adjusting the dimensionless Ekman number (E), are presented in Fig. 11 with observed along-estuary residual flow at Transect 3 during both surveys. Transect 3 velocity measurements were rotated 301 to better align the along-estuary flow with the thalweg. A solution with E ¼ 0.04, which is a low-to-moderate frictional influence, matches well with the observed residual flow. This corresponds to values found for the James River and Chesapeake Bay mouth. A sensitivity analysis shows that the
ARTICLE IN PRESS 2686
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
Fig. 9. Sub-tidal circulation along transects resulting from removal of diurnal and semidiurnal tidal signals during the October 19–20, 2006 survey.
Fig. 10. Sub-tidal circulation along transects resulting from removal of diurnal and semidiurnal tidal signals during the March 22–23, 2007 survey.
solution remains fundamentally the same in terms of its spatial structure within a range of low to intermediate Ekman numbers (E ¼ 0.005–0.075). In this range, the exchange pattern is unaltered as the interface between inflows and outflows changes very little. These results indicate that the overall residual dynamics are governed by pressure gradient, Coriolis, and friction. Advective accelerations may become relevant locally, for example around the cape on the western end of Transect 3 and southern end of Transect 4. The centripetal acceleration (a), given by a ¼ v2 =R where the velocity of the flow v ¼ 0.15 m/s and the radius of the headland R ¼ 400 m, has values of 105 m/s2. This locally dominates the momentum balance as seen by the 0 isotach sloping upward from the right to the left instead of the quasigeostropic slope indicated by the analytic solution. It is possible that advective terms related to lateral shears in the tidal flow, wðqu=qzÞ þ vðqu=qyÞ, play an essential role in the dynamics (Lerczak and Geyer, 2004). The data collected during the two surveys do not include vertical velocities. If w ¼ 0, calculating the contribution of lateral shear to the momentum produces a lateral structure unlike the structure predicted by Lerczak and Geyer (2004).
Columbia River (Jay and Smith, 1990) and the Fraser River (Geyer and Farmer, 1989). Gradient Richardson numbers, Ri, calculated at the time of every CTD cast illuminate the effect of increased tidal currents in St. Andrew Bay. The gradient Richardson number is a measure of vertical stability based on the ratio of the local density gradient, which dampens turbulence, to velocity shear, which generates turbulence. The effects of stratification are negligible until Ri40.03, and mixing is completely suppressed when Ri40.25. During both surveys Ri never fell below 1.5, showing that mixing was suppressed during both neap and spring tide, even though the tidal velocities in March were 50% larger than October. The increased stratification in March was most likely caused by the larger tidal advection of lower salinity water from the head of the bay and was not mixed into the water column. This indicated that tidal mixing plays a negligible role in modulating the non-tidal exchange in the system. The small tidal prism observed in the St. Andrew Bay system could allow wind forcing or freshwater input to dominate exchange with the Gulf of Mexico. Occasional large overflows from Deer Point Reservoir could significantly increase stratification and drive a temporary salt-wedge circulation similar to the response of Charlotte Harbor, Florida, to increased river flow (Miller and McPherson, 1991). Patos Lagoon, Brazil, is a shallow subtropical estuary connected to the ocean through a narrow inlet much like the Pass into St. Andrew Bay. The vertical salinity structure in Patos Lagoon has been documented to change among the salt wedge, partially stratified, and well mixed depending on relative river discharge and wind action (Mo¨ller et al., 2001).
4.4.3. Short-term deviations from long-term mean Subtropical microtidal estuaries with limited freshwater input, like the St. Andrew Bay system, may exhibit transitional deviations from the long-term dynamics suggested by the analytical model. The spring/neap modulation of the tides has been observed to impress a fortnightly signal on circulation in the
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2687
Fig. 11. Along-estuary residual current (m/s). + values are out of the estuary.
Blumberg and Kim (2000) found that atmospheric forcing on the Gulf of Mexico affects sea level at the Pass, thereby changing the height of the water throughout the bay system. This response of the estuary to coastal setup is consistent with observations in other Florida estuaries such as Sebastian Inlet (Liu, 1992), Indian River Lagoon (Smith, 1993), and Florida Bay (Wang et al., 1994). The role of atmospheric forcing is investigated by estimating the volume flux (QR) as Q R ¼ AðqZ=qtÞ where the surface of the bay A ¼ 243 m3, qZ=qt is the rate of sub-tidal sea level change with time (Goodrich, 1988). The mean flux from remote forcing during the October survey is 326.25 m3/s directed out of the estuary. This is a rebound from coastal setup by several days of strong southerly winds that raised sub-tidal sea level in the bay by 0.15 m. The October 0 m/s isotach is higher in the water column than predicted by the analytical model because of this coastal setup prior to the survey (Fig. 11). The March mean flux from remote forcing was 10.80 m3/s into the bay, reflecting a temporary rebound from sea level 0.05 m lower than predicted. This lower sea level is reflected in a lower than predicted 0 m/s isotach. Observed residuals between the two surveys are different from the expected analytical solution due to remote forcing and not to a spring/neap tidal cycle.
4.4.4. Residual baroclinic flow versus residual barotropic flow By far the largest residual flow during both surveys was measured at the southern end of Transect 4. These large velocities are not only due to centrifugal force but also due to the baroclinic pressure gradient set up by the Deer Point Reservoir spillover. Li et al. (1998) separated baroclinic from barotropic residual flow in the James River, VA, over a transect similar in bathymetry to Transect 4 (i.e. a deep channel flanked by a large shoal). Analogous to Transect 4, they also measured a strong down-estuary residual
over the shoal with a weaker up-estuary residual at the channel edge furthest from the shoal. Their measurements occurred during neap tides when the tidal forcing was at a minimum. Tidal forcing in the St. Andrew Bay system is always less than or equal to that of the James River neap tides. This allows baroclinic residual flow to overwhelm tidally induced residual flow at the southern end of Transect 4.
5. Conclusion This study provides the most comprehensive observational analysis of the St. Andrew Bay system. Hydrographic measurements indicate that the estuary will switch from diurnal tides to semidiurnal tides due to the beating of the K1 and O1 frequencies. The diurnal tides produce tidal straining through the advection of low salinity surface water not mixed by the tidal currents. Currents during the semidiurnal regime are not strong enough to mix the less-stratified water column. The tidal circulation of the bay is dominated by a jet that flows in from the Gulf of Mexico, encounters a land barrier, and bifurcates toward East Bay and West Bay. The ebb tide does not follow a traditional source/sink model of tidal circulation because part of the outflow from East Bay flows into West Bay. Tidal circulation out of West Bay is accelerated by a large shoal at the southern end of Transect 4. Comparison of an analytic model to the observed residual currents indicates that the basic hydrodynamics are controlled by pressure gradient, friction, and Earth’s rotation. The consistency of the mean pattern from October to March suggests that tidal forcing plays a minor role in modulating the exchange flow. Shortterm deviations from the long-term mean are mainly caused by coastal setup and setdown.
ARTICLE IN PRESS 2688
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
The seasonal variability of meteorological forcing should substantially affect the annual sub-tidal exchange. Particle trapping from this exchange (Brown et al., 2005; Chant and Stoner, 2001; Scully and Friedrichs, 2007) would significantly affect material transport in and out of the estuary (e.g. pollutants, harmful algal blooms, and planktonic organisms). The influence of coastal setup and setdown on material transport warrants further investigation.
Acknowledgments Both surveys were accomplished with the collaboration of University of Florida students and NMFS Panama City Laboratory personnel. The scientific support of Christopher Devick, Ilgar Safak, Hande Caliskan, Marta Ribera, Sangdon So, and Chloe Winant is appreciated. Boats were safely operated with the expertise of John Brusher, Andrew David, Doug Devries, Michelle Duncan, and Christopher Palmer. AVL acknowledges support from NSF project OCE-0551923. Pete Sheridan, Stacy Harter, and two anonymous reviewers provided valuable comments to the original manuscript. Special thanks to Christopher Gardner for his assistance in the middle of the night to save the October survey. References Berthot, A., Pattiaratchi, C., 2006. Mechanisms for the formation of headlandassociated linear sandbanks. Continental Shelf Research 26, 987–1004. Blumberg, A.F., Kim, B.N., 2000. Flow balances in St Andrew Bay revealed through hydrodynamic simulations. Estuaries 23, 21–33. Brown, C.A., Jackson, G.A., Holt, S.A., Holt, G.J., 2005. Spatial and temporal patterns in modeled particle transport to estuarine habitat with comparisons to larval fish settlement patterns. Estuarine Coastal and Shelf Science 64, 33–46. Brusher, H.A., Ogren, L.H., 1976. Distribution, abundance and size of penaeid shrimps in the St. Andrew Bay system, Florida. Fishery Bulletin US 74, 158–166. Chant, R.J., Stoner, A.W., 2001. Particle trapping in a stratified flood-dominated estuary. Journal of Marine Research 59, 29–51. Doyle, B.E., Wilson, R.E., 1978. Lateral dynamic balance in the sandy hook to rockaway point transect. Estuarine and Coastal Marine Science 6, 165–174. Fisher, H.B., List, E.J., Koh, R.C.Y., Imberger, J., Brooks, N.H., 1979. Mixing in Inland and Coastal Waters. Academic Press, New York, 483pp. Geyer, W.R., 1993. Three dimensional flow around a headland. Journal of Geophysical Research 98, 955–966. Geyer, W.R., Farmer, D.M., 1989. Tide-induced variation of the dynamics of a salt wedge estuary. Journal of Physical Oceanography 19, 1060–1072. Geyer, R.W., Signell, R., 1990. Measurements of tidal flow around a headland with a shipboard acoustic Doppler current profiler. Journal of Geophysical Research 95, 3189–3197. Goodrich, D.M., 1988. On meteorologically induced flushing in three US East Coast Estuaries. Estuarine, Coastal, and Shelf Science 26, 111–121. Hibiya, T., LeBlond, P.H., 1993. The control of fjord circulation by fortnightly modulation of tidal mixing processes. Journal of Physical Oceanography 23, 2042–2052. Ichiye, T., Jones, M.L., 1961. On the hydrography of the St. Andrew Bay system, Florida. Limnology and Oceanography 6, 302–311. Joyce, T.M., 1989. On in situ calibration of shipboard ADCPs. Journal of Atmospheric and Oceanic Technology 6, 169–172. Jay, D.A., Smith, J.D., 1990. Residual circulation in shallow estuaries. 1. Highly stratified, narrow estuaries. Journal of Geophysical Research 95, 711–731. Kantha, L., 2005. Barotropic tides in the Gulf of Mexico. In: Sturges, W., Fernandez, A.L. (Eds.), Geophysical Monograph Series. Circulation in the Gulf of Mexico, Vol. 161. American Geophysical Union, Washington, DC, pp. 159–163. Lerczak, J.A., Geyer, R.W., 2004. Modeling the lateral circulation in straight, stratified estuaries. Journal of Physical Oceanography 34, 1410–1428. Li, C., 2002. Axial convergence fronts in a barotropic tidal inlet—Sand Shoal Inlet, VA. Continental Shelf Research 22, 2633–2653. Li, C., Valle-Levinson, A., Wong, K.C., Lwiza, K.M.M., 1998. Separating baroclinic flow from tidally induced flow in estuaries. Journal of Geophysical Research 103 (C5), 10405–10417. Lindall Jr., W.N., Saloman, C.H., 1977. Alteration and destruction of estuaries affecting fishery resources of the Gulf of Mexico. Marine Fisheries Review 39 (9), 1–7. Liu, J.T., 1992. The influence of episodic weather events on tidal residual currents: a case study at Sebastian Inlet, Florida. Estuaries 15, 109–121. Lwiza, K.M.M., Bowers, D.G., Simpson, J.H., 1991. Residual and tidal flow at the tidal mixing front in the North Sea. Continental Shelf Research 11, 1379–1395.
McNulty, J.K., Lindall Jr., W.N., Sykes, J.E., 1971. Cooperative Gulf of Mexico Estuarine Inventory and Study: Florida. Phase I: Area Description. NOAA Technical Report NMFS CIRC 368, 126pp. Miller, R.L., McPherson, B.F., 1991. Estimating estuarine flushing and residence times in Charlotte Harbor, Florida, via salt balance and a box model. Limnology and Oceanography 36, 602–612. Mo¨ller, O.O., Castaing, P., Saloman, J.C., Lazure, P., 2001. The influence of local and non-local forcing effects on the subtidal circulation of Patos Lagoon. Estuaries 24, 297–311. Musgrove, R.J., Foster, J.B., Toler, L.G., 1964. Water Resources of the Ecofina Creek basin area in northwestern Florida. Florida Geological Survey Report Investigation, vol. 41, 51pp. Nunes, R.A., Lennon, G.W., 1987. Eposodic stratification and gravity currents in a marine environment of modulated turbulence. Journal of Geophysical Research 92, 5465–5480. Nunes, R.A., Simpson, J.H., 1985. Axial convergence in a well-mixed estuary. Estuarine Coastal Shelf Science 20, 637–649. NWFWMD, 2001. The Big Picture: St. Andrew Bay Watershed. Public Information Bulletin 01-4. Department of Environmental Protection, Northwest Florida Water Management District, Surface Water Improvement and Management Program. 6pp. Ogren, L.H., Brusher, H.A., 1977. The distribution and abundance of fishes caught with a trawl in the St. Andrew Bay system, Florida. Northeast Gulf Science 1, 83–105. Pritchard, D.W., 1955. Estuarine circulation patterns. Proceedings of the American Society of Civil Engineers 81, 717/1–717/11. RD Instruments, 1996. Acoustic Doppler Current Profiler Principles of Operation: A Practical Primer, second ed. RD Instruments, Poway, CA, 51pp. Robinson, I.S., 1981. Tidal vorticity and residual circulation. Deep Sea Research 28A, 195–212. Rodriguez, P.E., Wu, T.S., 1990. Initial analysis of the circulation and flushing characteristics of the St. Andrew Bay system. Northwest Florida Water Management District Water Resources Special Report 90-1, 103pp. Salsman, G.G., Tolbert, W.H., Villars, R.G., 1966. Sand-ridge migration in St. Andrew Bay, Florida. Marine Geology 4, 11–19. Schroeder, W.W., Dinnel, S.P., Wiseman, W.J., 1990. Salinity stratification in a riverdominated estuary. Estuaries 13, 145–154. Scully, M.E., Friedrichs, C.T., 2007. Sediment pumping by tidal asymmetry in a partially mixed estuary. Journal of Geophysical Research 112 (C7), C07028. ˜ an, M.B., 2004. Observations of subtidal Sepulveda, H.H., Valle-Levinson, A., Framin and tidal flow in the Rı´o de la Plata Estuary. Continental Shelf Research 24, 509–525. Simpson, J.H., Brown, J., Matthews, J., Allen, G., 1990. Tidal straining, density currents, and stirring in the control of estuarine stratification. Estuaries 13, 125–132. Smith, N.P., 1977. Meteorological and tidal exchanges between Corpus Christi Bay, Texas, and the northwestern Gulf of Mexico. Estuarine and Coastal Marine Science 5, 511–520. Smith, N.P., 1979. Tidal dynamics and low frequency exchanges in the Aransas Pass, Texas. Estuaries 2, 218–227. Smith, N.P., 1993. Tidal and nontidal flushing of Florida’s Indian River Lagoon. Estuaries 16, 739–746. Souza, A.J., Simpson, J.H., 1996. The modification of tidal ellipses by stratification in the Rhine ROFI. Continental Shelf Research 16, 997–1007. Stommel, H., Farmer, H.G., 1952. Abrupt change in width in two layer open channel flow. Journal of Marine Research 11, 205–214. Swenson, E.M., Chuang, W., 1983. Tidal and subtidal water volume exchange in an estuarine system. Estuarine Coastal and Shelf Science 16, 229–240. Turrell, W.R., Brown, J., Simpson, J.H., 1996. Salt intrusion and secondary flow in a shallow, well-mixed estuary. Estuarine Coastal and Shelf Science 42, 153–169. USEPA, 1999. Ecological condition of estuaries in the Gulf of Mexico. EPA 620-R-98004. US Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Gulf Ecology Division, Gulf Breeze, Florida. Valle-Levinson, A., Lwiza, K.M.M., 1995. Effects of channels and shoals on the exchange between the lower Chesapeake Bay and the adjacent ocean. Journal of Geophysical Research 99 (C3), 5149–5169. Valle-Levinson, A., Matsuno, T., 2003. Tidal and subtidal flow along a cross-shelf transect on the East China Sea. Journal of Oceanography 59, 573–584. Valle-Levinson, A., Moraga, J., Olivares, J., Blanco, J.L., 2000. Tidal and residual circulation in a semi-arid bay: Coquimbo Bay, Chile. Continental Shelf Research 20, 2009–2028. Valle-Levinson, A., Delgado, J.A., Atkinson, L.P., 2001. Reversing water exchange patterns at the entrance to a semiarid coastal lagoon. Estuarine Coastal and Shelf Science 53, 825–838. Valle-Levinson, A., Reyes, C., Sanay, R., 2003. Effects of bathymetry, friction, and Earth’s rotation on estuary/ocean exchange. Journal of Physical Oceanography 33, 2375–2393. Wang, J.D., van der Kreeke, J., Krishnan, N., Smith, D., 1994. Wind and tide response in Florida Bay. Bulletin of Marine Science 54, 579–601. Webb, B.M., King, J.N., Tutak, B., Valle-Levinson, A., 2007. Flow structure at a trifurcation near a North Florida inlet. Continental Shelf Research 27, 1528–1547. Wong, K.C., 1987. Tidal and sub-tidal variability in Delaware’s inland bays. Journal of Physical Oceanography 17, 413–422.
Contents lists available at ScienceDirect
Continental Shelf Research journal homepage: www.elsevier.com/locate/csr
Tidal and residual circulation in the St. Andrew Bay system, Florida Patrick L. Murphy a,, Arnoldo Valle-Levinson b a b
National Marine Fisheries Service, Southeast Science Center, 3500 Delwood Beach Road, Panama City, FL 32408, USA Department of Civil and Coastal Engineering, University of Florida, 365 Weil Hall, P.O. Box 116580, Gainesville, FL 32611, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 18 December 2007 Received in revised form 21 July 2008 Accepted 10 September 2008 Available online 25 September 2008
Two 24-h surveys were conducted in St. Andrew Bay, Florida, during spring and neap tides to describe the tidal and non-tidal circulation patterns and to determine the factors that affect these patterns. In particular, the effect of tidal forcing in modulating such circulation patterns was explored. Observed velocities were fitted to diurnal and semidiurnal harmonics separating tidal motions from sub-tidal motions. Residual flows were compared with an analytic model that allowed variations in the relative contributions from Coriolis acceleration and friction using the Ekman number. A solution with an Ekman number of 0.04 resembled the observations best and indicated that the hydrodynamics were governed by pressure gradient, Coriolis and friction. Locally, advective accelerations became important around headlands in sub-estuaries in the system. The consistency of the mean pattern from October to March suggests that tides play a minor role in modulating the exchange flow. Deviations from the longterm mean are mainly caused by wind-driven coastal setup and setdown. Published by Elsevier Ltd.
Keywords: Estuarine dynamics Tidal dynamics Residual circulation Buoyancy forcing St. Andrew Bay Florida
1. Introduction The northeastern Gulf of Mexico coast is experiencing rapid urban and suburban development that intensifies pressure on regional estuaries. Determining the effects of coastal development on various biological and chemical processes in these estuaries (Lindall and Saloman, 1977) depends on understanding the underlying physical framework, namely estuarine circulation. Most studies of estuarine circulation have taken place at temperate latitudes where tidal strength is an important agent in modulating the net circulation. For instance, asymmetries in the flood and ebb tide due to bottom friction and non-uniform bathymetry can induce residual flow (Stommel and Farmer, 1952; Fisher et al., 1979; Robinson, 1981; Geyer and Signell, 1990). Modulation of tides by the spring/neap cycle generates a fortnightly frequency on tidal pumping of salinity up an estuary (Jay and Smith, 1990; Hibiya and LeBlond, 1993). Net exchange flows tend to be strongest at neap tides, when tidally induced mixing is the weakest (Nunes and Lennon, 1987). Tides in subtropical systems along the Gulf of Mexico exhibit small ranges (o0.5 m) compared to temperate estuaries (41 m). Effects other than tidal modulation and asymmetries could play a larger role in altering the residual circulation. Many subtropical
Corresponding author. Tel.: +1757 620 1740; fax: +1 850 235 3559.
E-mail address: [email protected] (P.L. Murphy). 0278-4343/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.csr.2008.09.003
systems show seasonal fluctuations in river runoff that impose an annual cycle to their stratification (Schroeder et al., 1990) and hence its circulation (Valle-Levinson et al., 2001). Other studies of subtropical systems along the Gulf of Mexico have found 3–5 day coherence between increased sub-tidal exchange and meteorological forcing associated with passing fronts (Smith, 1977; Swenson and Chuang, 1983). Transient fronts have a twofold effect. First, water exchange between a lagoon and the adjacent ocean may be influenced by the inverse barometer effect (Liu, 1992). In addition, direct sea level set up by the Ekman transport (Wong, 1987) or advection by onshore winds (Smith, 1979) may drive water into and out of these lagoons at volumes larger than the tidal prism. Pritchard’s (1955) two-layer model, resulting from river input at the head of an estuary, is not applicable to many subtropical lagoons. Systems with low freshwater forcing, ubiquitous along the Gulf of Mexico, have weak baroclinic flows easily modified by external forces. The amount of freshwater, the meteorological forcing and the strength of the tidal currents are crucial in determining whether the system is driven by density gradients, tides, or winds. The objective of this study is to determine the spatial structure of the circulation in a subtropical estuary with quasi-steady freshwater input. In particular, this investigation seeks to assess the role of tidal forcing in modifying such a spatial structure. This objective is addressed with data collected at St. Andrew Bay, Florida, during spring and neap tides at different times of the year and by comparing the results to a theoretical model.
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2679
Fig. 1. Map of the St. Andrew Bay system and study area. Transect 1 ¼ P1–P2. Transect 2 ¼ P2–P3. Transect 3 ¼ P3–P4. Transect 4 ¼ P4–P1. ST 2, ST 3, ST 4 ¼ CTD Station.
2. Study area The St. Andrew Bay system in northwest Florida (Fig. 1) is an ecologically important estuary surrounded by rapidly growing communities. The bay is home to one of the most diverse marine populations of any estuary in the northern Gulf of Mexico coast (Ogren and Brusher, 1977). This area hosts a large variety of fauna because of, among other reasons, its low freshwater inflow, low turbidity, extensive sand flats, widespread submerged aquatic vegetation (SAV), and a deep basin with both coarse and fine sediments (Brusher and Ogren, 1976). The St. Andrew Bay system is a drowned river valley, coastal plain estuary that formed 5000 years ago during the Holocene transgression (Salsman et al., 1966). The system consists of four sub-estuaries: North Bay, West Bay, East Bay, and St. Andrew Bay (Fig. 1). The latter is the only sub-estuary with a direct connection to the Gulf of Mexico. The US Army Corps of Engineers maintains a 150 m wide channel at the Gulf of Mexico connection, known locally as ‘‘The Pass,’’ which is armored with jetties on either side.
A minimum 9 m channel depth is maintained from the Pass to the Port of Panama City (St. Andrew Bay) and a paper mill (East Bay), although the channel depth usually exceeds 15 m. Channels in all of the sub-estuaries are generally parallel to the shoreline and are maintained at 4 m. West Bay and East Bay connect to the Gulf Intracoastal Waterway, which allows sheltered navigation from Choctawhatchee Bay to Lake Wimico. The surface area of the bay system is 243 km2. The watershed is approximately 2800 km2 (USEPA, 1999) and is located entirely within Florida (NWFWMD, 2001). Besides non-point runoff and assorted small creeks, the largest freshwater source draining into the estuary is the spillover from the Deer Point Reservoir (Fig. 1). This human-made lake is located at the head of North Bay and is formed as a result of a dam constructed downriver from the confluence of several spring-fed creeks, primarily Ecofina Creek. Daily flow rates are calculated by the Bay County Water Division. Tides in the system are typically diurnal with a mean range of 0.5 m, with a longer ebb flow than flood flow (Ichiye and Jones,
ARTICLE IN PRESS 2680
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
1961; McNulty et al., 1972). Tidal amplitudes of the semidiurnal M2 and S2 harmonics are 5 times smaller than the K1 and O1 diurnal constituents (www.tidesandcurrents.noaa.gov, Station #8729108). An apparent spring/neap cycle in the tides is a fortnightly modulation due to the beating of the diurnal constituents (Kantha, 2005). During a neap tide, the tidal regime is semidiurnal for several cycles, as the diurnal constituents cancel each other. Tidal ranges during these periods are typically 5–20 cm, corresponding to the amplitudes of the M2 and S2 constituents of the overall tide. Information on tidal current spatial variations in the estuary has been absent until now.
3. Data collection and processing Two surveys were carried out to document the spatial patterns of tidal and sub-tidal flows in the St. Andrew Bay sub-estuary during spring and neap tides. One survey took place during a neap tide (October 19–20, 2006), whereas the other survey was during a spring tide (March 22–23, 2007). A 1200 kHz broad-band acoustic Doppler current profiler (ADCP; RD Instruments, Poway, CA) mounted on a 1.2 m sled, pointing downward, was towed around a closed trajectory consisting of four transects across the major channels of St. Andrew Bay (Fig. 1). The ADCP was towed at 2 m/s for an entire diurnal tidal cycle so that intratidal spatial flow changes could be observed. Only minor interruptions in towing occurred (due to vessel repairs and personnel exchanges), and such breaks caused a negligible effect on data coverage. The ADCP collected 1 s pings, which were averaged over 10 ensembles. This yielded a horizontal resolution of 20 m. The vertical resolution of the data was given by a bin size of 0.5 m, and the closest bin to the surface was 1.5 m. Data in the lowest 15% of the water column were discarded due to interference from sidelobe effects (RD Instruments, 1996). Data were compass-calibrated and corrected by the method of Joyce (1989) using a Global Positioning System. A total of 13 and 15 trajectory repetitions were carried out in the neap and spring tide surveys, respectively. Conductivity–temperature–depth (CTD) profiles were recorded at three stations (Fig. 1) during every other repetition of the circuit using a SBE-19 CTD. These stations were chosen to profile the density variability in the middle of the main channels of St. Andrew Bay, and leading into East Bay and West Bay. Data were processed using Sea Bird Instruments software and binned to 1.0 m vertical bins. The degree of water-column stratification was quantified by calculating the potential energy anomaly j of the water column for each CTD cast, following the approach of Simpson et al. (1990): Z Z 1 1 0 ðr¯ rÞgz dz; where r¯ ¼ j¼ rðzÞ dz, h h h where j (J/m3) is the amount of work necessary to completely mix the water column, h the total depth, r the density, and g the acceleration due to gravity (9.8 m/s2). Wind and other meteorological data were collected at Panama City-Bay County International Airport (PFN), located 7 km north of the study area. Water height measurements from NOAA tide station #8729108, Panama City, Florida located adjacent to the study area, were also used to identify sub-tidal forcing effects. Mean daily water flow from the Deer Point Reservoir was calculated from a daily water level measurement at the Watson Bayou Pumping Station and a table lookup interpolation method using values measured by the Bay County Water Division. Calculation of an average flow rate from historical data can be misleading. Only precipitation events inside the watershed will change the daily flow over the dam and annual events such as
snow melt runoff do not influence the spring-fed creeks. The increased flow returns to normal values several days after a precipitation event. This makes computing an average flow from a time series of daily measurements misleading and would inflate the usual 22 m3/s daily flow over the dam (Bay County Water Division, pers. comm.). Such a nuance has not been taken into account by previous studies. For instance, Musgrove et al. (1965) found that the long-term average flow is 28.5 m3/s while Blumberg and Kim (2000) calculated the daily flow from March 1993 to March 1994 to be 38 m3/s. For each transect repetition, the eastward u and northward v components of the ADCP velocity data were interpolated onto a regular grid of distance versus depth values (e.g. Valle-Levinson and Matsuno, 2003). The grid had a resolution of 50 m in the horizontal and 0.5 m in the vertical. The time-series observations (u, v) at each grid point of the 13–15 gridded transects were fitted to semidiurnal (M2 ¼ 12.42 h) and diurnal (K1 ¼ 23.92 h) harmonics using a least-squares method (Lwiza et al., 1991): ðu; vÞ ¼ ðu0 ; v0 Þ þ
m X ðuAj ; vAj Þ sinðoj t þ uyj ; vyj Þ, j¼1
where t is the time; u0 and v0 are the residual components of the velocity; uAj and vAj the amplitudes of the harmonic j of frequency oj; and uyj and vyj the phases, with respect to midnight of the day when the survey began, for each harmonic. Up to ‘m’ harmonics may be fitted to the data. The duration of the surveys was too short to resolve the contribution from all diurnal or semidiurnal constituents. Fitted single diurnal and single semidiurnal amplitudes and phases were expected to be representative of all the constituents interacting during each survey day. Measured residual circulation in Transect 3 was compared to an analytical solution described in Valle-Levinson et al. (2003) to investigate forces governing long-term circulation of St. Andrew Bay. Their analytic model includes pressure gradient, friction, and Earth’s rotation to solve the along-estuary and transverse structure of the flow. Only Transect 3 was compared because boundary conditions of the analytical model require the ocean to be at the down-estuary boundary. The model is adjusted by the dimensionless Ekman number: E¼
Az fH2max
,
where Az is the kinematic eddy viscosity (friction), f the Coriolis force, and Hmax the depth.
4. Results and discussion 4.1. Forcing agents Southerly winds for 2 days prior to the October 2006 survey raised sea levels in the bay (Fig. 2). As the wind relaxed, sea level returned to predicted values. The March 2007 wind record indicated a diurnal sea breeze pattern that rotated clockwise, reaching a daily maximum of southerly velocity just after sunset and a minimum just before sunrise (Fig. 2). Water levels measured at the Deer Point Reservoir were 1.59 and 1.60 m during the October and March surveys, respectively. These measurements represent a primarily spring-fed freshwater flow of 34 m3/s into North Bay during both surveys. 4.2. Hydrography Water temperature in the St. Andrew Bay system exhibited a weak gradient with depth but salinity controlled density (Fig. 3).
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2681
Fig. 2. Tidal height (upper) and wind velocity (lower) for October 2006 neap tide (left) and March 2007 spring tide (right). (-) is the measured tidal height and (- -) is the predicted tidal height. Vertical lines indicate beginning and end of the surveys. Wind vectors follow the oceanographic convention. Only sporadic wind measurements were available during the 24 h preceding the October 2006 survey.
The density change was much larger during spring tides than during neap tides, although small isopycnal changes are apparent in the neap CTD casts. It is noted that although the tide is half as high during a neap tide (semidiurnal), it has half as much time as a spring tide (diurnal) to enter and leave the bay. Consequently, typical tidal currents along Transect 3 are comparable during neap and spring tides. In October, the potential energy anomaly j of all three stations showed a semidiurnal frequency consistent with the dominant tidal amplitude (Fig. 4). The small difference in j between high and low tide indicated small intratidal variations of stratification. This is supported by weak tidal variations (of neap tides) and by the observed sea level, which was nearly twice that predicted because of persistent southerly winds forcing Gulf of Mexico water into the estuary. During the March survey, the tidal regime imposed a diurnal signal on j at all three stations (Fig. 4). At the end of flood, normally the period of lowest stratification, the j values (40–60 J/m3) of the three stations were similar to the lowest j values calculated in October. This suggests flood tide inundates St. Andrew Bay with Gulf of Mexico water regardless of spring or neap tides. By the end of ebb, j increased by 2–3 times the highest October values. Higher values of j during spring tides than during neap tides suggest effects of advection by the tidal currents. In temperate estuaries, spring tidal currents typically increase mixing and decrease j (Simpson et al., 1990). The influence of Gulf of Mexico waters on stratification and the exchange between East Bay and West Bay were observed by plotting surface salinity data measured every 10 s during the
March survey with an SBE-37 (Fig. 5). No surface salinity data were available for the October survey. The March survey began just before the end of flood tide when all the transects had salinities near 32–33, associated with Gulf of Mexico waters. As the tide fell, the lower salinity water from up the estuary traveled across Transects 2 and 4 into the survey area while the higher salinities were advected seaward across Transect 3. During ebb, the lowest salinity water of the survey (27) appeared from East Bay across the northern part of Transect 2. It was then advected along Transect 1 and crossed the northern part of Transect 4 into West Bay. This up-estuary flow occurred while the ebbing tide was advecting high salinity water out of West Bay over the southern section of the same Transect 4. At the end of ebb tide, salinities along the transects were uniformly 29, then increased as flood tide began again. The southern portions of Transects 2, 3, and 4 showed earlier increases in salinity during flood tide compared to the northern portions due to their proximity to the Gulf of Mexico. These measurements show a daily inter-bay circulation and advection of salt that is more complex than longterm averages, which have a net outflow of water from all the estuaries into the Gulf of Mexico (Blumberg and Kim, 2000).
4.3. Tidal circulation Tidal ellipses along all four transects were calculated for surveys following Souza and Simpson (1996). Changes in ellipticity (rotation) or changes in orientation of adjacent ellipses are interpreted as regions of flow convergence
both tidal tidal and
ARTICLE IN PRESS 2682
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
Fig. 3. CTD density (kg/m3) time-series data by month and station. Arrows and vertical lines represent time of individual casts.
Fig. 4. Potential energy anomaly j (upper) for each CTD station. Cross-sectional average of Transect 3 velocity (lower) with the corresponding tidal height. Negative is out of the estuary. (J) is at the median time of each transect.
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2683
Fig. 5. Surface salinity contours for March 22–23, 2007. Transects were measured clockwise around a four-cornered circuit starting at the beginning of Transect 1.
divergence (Li, 2002). Convergences aggregate flotsam and prey while divergences are associated with dispersion. A convergence of tidal flow will typically become a divergence when the tide reverses. In October, the diurnal tidal signal was suppressed by an interaction between the K1 and O1 constituents, resulting in small diurnal ellipses and large semidiurnal ellipses (Fig. 6). Smaller semidiurnal ellipses during March were more representative of the usual diurnal tide (Fig. 7). Tidal currents at Transect 1 were similar between surveys, but currents were stronger in March at the other transects. Strongest currents were found at the surface and over deep water, indicative of bottom friction effects on tidal flow (Valle-Levinson and Lwiza, 1995). Transect 1 tidal ellipses showed marked bifurcation of tidal flow before entering West Bay and East Bay in both October and March (Figs. 6 and 7). During flood, currents from the Gulf of Mexico meet the northern boundary of St. Andrew Bay and are forced toward either East Bay or West Bay. Transect 2 had ellipses parallel to the coast that slowly rotated 901 before getting oriented perpendicular to East Bay. March surface diurnal flows on Transect 2 increased from 13 cm/s at the southern end of the transect to 30 cm/s in the northern segment of the transect. This was due to a constriction at the entrance to East Bay that accelerated flow and caused convergence of flood flows and divergence of ebb flows. October semidiurnal surface speeds on Transect 2 ranged from 7 cm/s at the southern end to 20 cm/s at the constriction. The same pattern of convergence due to bathymetry was observed by Sepulveda et al. (2004) in the Rio de la Plata Estuary, Argentina. Webb et al. (2007) concluded
that regions of tidal convergence caused observed axial fronts aligned with the bathymetry in St. Augustine Inlet, Florida. Transect 2 tidal ellipses from both surveys suggest that conditions exist for axial fronts to form regardless of spring or neap conditions in St. Andrew Bay. In fact, the largest horizontal salinity gradients were observed in Transect 2 (Fig. 5). Rectilinear tidal ellipses in Transect 3 resulted from the tidal flow through the Pass (Figs. 6 and 7). Ellipses displayed an increasing ellipticity from the western end of the transect toward the middle of the channel. This tidal jet that enters the bay during flood is weakened at the edges by friction along the shoreline. In October, tidal flow reached 20 cm/s while in March the diurnal amplitude reached 40 cm/s. Semidiurnal and diurnal tidal ellipses on the east side of Transect 3 have equivalent amplitudes during the October survey. In March, diurnal amplitude in this area was 2–3 times stronger than semidiurnal amplitudes corresponding to strengthening of the tide. During both surveys, Transect 4 tidal ellipses were alongchannel in the northern part of the section and rotated 601 in the south. Highly rectilinear ellipses near the western end of Transect 3 were of comparable magnitude to ellipses near the southern end of Transect 4 during both surveys. In October, semidiurnal tidal ellipses at 5 m were similar in orientation to surface ellipses while ellipses at 9 m were oriented along the channel. In March, both semidiurnal and diurnal tidal ellipses at 5 and 9 m were oriented along the channel. There is a prominent shoal at the southern end of Transect 4 corresponding to the largest amplitude tidal ellipses (Figs. 6 and 7). The shoal is a common feature formed by asymmetries in
ARTICLE IN PRESS 2684
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
Fig. 6. Tidal ellipses for October 19–20, 2006. Black ellipses are counter-clockwise and gray ellipses are clockwise.
Fig. 7. Tidal ellipses for March 22–23, 2007. Black ellipses are counter-clockwise and gray ellipses are clockwise.
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2685
Fig. 8. Mean cross-channel gradient of (–K–) salinity and (–&–) velocity. Measurements are scaled by the largest value in each time series.
sediment transport by energetic reversing tidal flows around a headland (Berthot and Pattiaratchi, 2006). Centrifugal force adds momentum to the flow as it curves around the headland. A momentum balance between centrifugal force, Coriolis, and friction is kept by increasing the friction, i.e. accelerating the flow. Similar observations of this curvature-induced acceleration have occurred at Sandy Hook, NJ (Doyle and Wilson, 1978), Gay Head, MA (Geyer, 1993), and Point Tortuga, Chile (Valle-Levinson et al., 2000). Increase in tidal current amplitude from east to west along Transect 3 and decrease in tidal current amplitude from south to north along Transect 4 are conducive to the formation of significant shear fronts due to the large acrosschannel velocity gradients. Nunes and Simpson (1985) and Turrell et al. (1996) found that lateral shear of an axial current imposed on an axial salinity gradient in the Conwy estuary forced a lateral density gradient that induced secondary circulation. A secondary circulation works to redistribute the density field, causing axial aggregation of particulates, prey, foam, and surface drifters. For each transect, a time series of the mean hourly lateral salinity gradient produced by a shear current and the mean hourly lateral gradient of the along-channel velocity was calculated from surface salinity and ADCP data collected during March (Fig. 8). Gradients are small at the beginning of ebb tide and reach a maximum during largest tidal flows associated with differential seaward advection of lower salinity water. When the ebb tide current approaches slack water, a secondary flow due to the cross-channel baroclinic pressure gradient redistributes the density field, which results in a zero lateral salinity gradient in Transects 2, 3, and 4 by the end of ebb tide (08:00 GMT).
4.4. Mean flow 4.4.1. Observed residual currents Residual currents for each survey are presented in Figs. 9 and 10. Residuals indicated typical estuarine circulation with outflow at the surface and inflow at depth, except along Transect 2 during October. Overall, the magnitudes of the outflow velocities were slightly larger (o5 cm/s) in the March survey relative to the October survey. However, net inflows were larger in the October survey (o3 m/s). Transect 4 had inflow over the entire shipping channel and outflow over the shoal at the southern end. The residual current of Transect 1 during both surveys indicated a strictly westward flow from East Bay to West Bay. This pattern does not follow Stommel and Farmer’s (1952) presumed pressure gradient in their source/sink analogy of water entering and leaving an estuary through an inlet. Rather, it more closely resembles the gyre measured by Webb et al. (2007) at a trifurcation of St. Augustine Inlet, Florida. A surface residual cyclonic gyre in St. Andrew Bay has been modeled, but these are the first direct measurements (Rodriguez and Wu, 1990).
4.4.2. Comparison of residual currents to the analytical model Results of the analytic model, which finds the long-term circulation by adjusting the dimensionless Ekman number (E), are presented in Fig. 11 with observed along-estuary residual flow at Transect 3 during both surveys. Transect 3 velocity measurements were rotated 301 to better align the along-estuary flow with the thalweg. A solution with E ¼ 0.04, which is a low-to-moderate frictional influence, matches well with the observed residual flow. This corresponds to values found for the James River and Chesapeake Bay mouth. A sensitivity analysis shows that the
ARTICLE IN PRESS 2686
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
Fig. 9. Sub-tidal circulation along transects resulting from removal of diurnal and semidiurnal tidal signals during the October 19–20, 2006 survey.
Fig. 10. Sub-tidal circulation along transects resulting from removal of diurnal and semidiurnal tidal signals during the March 22–23, 2007 survey.
solution remains fundamentally the same in terms of its spatial structure within a range of low to intermediate Ekman numbers (E ¼ 0.005–0.075). In this range, the exchange pattern is unaltered as the interface between inflows and outflows changes very little. These results indicate that the overall residual dynamics are governed by pressure gradient, Coriolis, and friction. Advective accelerations may become relevant locally, for example around the cape on the western end of Transect 3 and southern end of Transect 4. The centripetal acceleration (a), given by a ¼ v2 =R where the velocity of the flow v ¼ 0.15 m/s and the radius of the headland R ¼ 400 m, has values of 105 m/s2. This locally dominates the momentum balance as seen by the 0 isotach sloping upward from the right to the left instead of the quasigeostropic slope indicated by the analytic solution. It is possible that advective terms related to lateral shears in the tidal flow, wðqu=qzÞ þ vðqu=qyÞ, play an essential role in the dynamics (Lerczak and Geyer, 2004). The data collected during the two surveys do not include vertical velocities. If w ¼ 0, calculating the contribution of lateral shear to the momentum produces a lateral structure unlike the structure predicted by Lerczak and Geyer (2004).
Columbia River (Jay and Smith, 1990) and the Fraser River (Geyer and Farmer, 1989). Gradient Richardson numbers, Ri, calculated at the time of every CTD cast illuminate the effect of increased tidal currents in St. Andrew Bay. The gradient Richardson number is a measure of vertical stability based on the ratio of the local density gradient, which dampens turbulence, to velocity shear, which generates turbulence. The effects of stratification are negligible until Ri40.03, and mixing is completely suppressed when Ri40.25. During both surveys Ri never fell below 1.5, showing that mixing was suppressed during both neap and spring tide, even though the tidal velocities in March were 50% larger than October. The increased stratification in March was most likely caused by the larger tidal advection of lower salinity water from the head of the bay and was not mixed into the water column. This indicated that tidal mixing plays a negligible role in modulating the non-tidal exchange in the system. The small tidal prism observed in the St. Andrew Bay system could allow wind forcing or freshwater input to dominate exchange with the Gulf of Mexico. Occasional large overflows from Deer Point Reservoir could significantly increase stratification and drive a temporary salt-wedge circulation similar to the response of Charlotte Harbor, Florida, to increased river flow (Miller and McPherson, 1991). Patos Lagoon, Brazil, is a shallow subtropical estuary connected to the ocean through a narrow inlet much like the Pass into St. Andrew Bay. The vertical salinity structure in Patos Lagoon has been documented to change among the salt wedge, partially stratified, and well mixed depending on relative river discharge and wind action (Mo¨ller et al., 2001).
4.4.3. Short-term deviations from long-term mean Subtropical microtidal estuaries with limited freshwater input, like the St. Andrew Bay system, may exhibit transitional deviations from the long-term dynamics suggested by the analytical model. The spring/neap modulation of the tides has been observed to impress a fortnightly signal on circulation in the
ARTICLE IN PRESS P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
2687
Fig. 11. Along-estuary residual current (m/s). + values are out of the estuary.
Blumberg and Kim (2000) found that atmospheric forcing on the Gulf of Mexico affects sea level at the Pass, thereby changing the height of the water throughout the bay system. This response of the estuary to coastal setup is consistent with observations in other Florida estuaries such as Sebastian Inlet (Liu, 1992), Indian River Lagoon (Smith, 1993), and Florida Bay (Wang et al., 1994). The role of atmospheric forcing is investigated by estimating the volume flux (QR) as Q R ¼ AðqZ=qtÞ where the surface of the bay A ¼ 243 m3, qZ=qt is the rate of sub-tidal sea level change with time (Goodrich, 1988). The mean flux from remote forcing during the October survey is 326.25 m3/s directed out of the estuary. This is a rebound from coastal setup by several days of strong southerly winds that raised sub-tidal sea level in the bay by 0.15 m. The October 0 m/s isotach is higher in the water column than predicted by the analytical model because of this coastal setup prior to the survey (Fig. 11). The March mean flux from remote forcing was 10.80 m3/s into the bay, reflecting a temporary rebound from sea level 0.05 m lower than predicted. This lower sea level is reflected in a lower than predicted 0 m/s isotach. Observed residuals between the two surveys are different from the expected analytical solution due to remote forcing and not to a spring/neap tidal cycle.
4.4.4. Residual baroclinic flow versus residual barotropic flow By far the largest residual flow during both surveys was measured at the southern end of Transect 4. These large velocities are not only due to centrifugal force but also due to the baroclinic pressure gradient set up by the Deer Point Reservoir spillover. Li et al. (1998) separated baroclinic from barotropic residual flow in the James River, VA, over a transect similar in bathymetry to Transect 4 (i.e. a deep channel flanked by a large shoal). Analogous to Transect 4, they also measured a strong down-estuary residual
over the shoal with a weaker up-estuary residual at the channel edge furthest from the shoal. Their measurements occurred during neap tides when the tidal forcing was at a minimum. Tidal forcing in the St. Andrew Bay system is always less than or equal to that of the James River neap tides. This allows baroclinic residual flow to overwhelm tidally induced residual flow at the southern end of Transect 4.
5. Conclusion This study provides the most comprehensive observational analysis of the St. Andrew Bay system. Hydrographic measurements indicate that the estuary will switch from diurnal tides to semidiurnal tides due to the beating of the K1 and O1 frequencies. The diurnal tides produce tidal straining through the advection of low salinity surface water not mixed by the tidal currents. Currents during the semidiurnal regime are not strong enough to mix the less-stratified water column. The tidal circulation of the bay is dominated by a jet that flows in from the Gulf of Mexico, encounters a land barrier, and bifurcates toward East Bay and West Bay. The ebb tide does not follow a traditional source/sink model of tidal circulation because part of the outflow from East Bay flows into West Bay. Tidal circulation out of West Bay is accelerated by a large shoal at the southern end of Transect 4. Comparison of an analytic model to the observed residual currents indicates that the basic hydrodynamics are controlled by pressure gradient, friction, and Earth’s rotation. The consistency of the mean pattern from October to March suggests that tidal forcing plays a minor role in modulating the exchange flow. Shortterm deviations from the long-term mean are mainly caused by coastal setup and setdown.
ARTICLE IN PRESS 2688
P.L. Murphy, A. Valle-Levinson / Continental Shelf Research 28 (2008) 2678–2688
The seasonal variability of meteorological forcing should substantially affect the annual sub-tidal exchange. Particle trapping from this exchange (Brown et al., 2005; Chant and Stoner, 2001; Scully and Friedrichs, 2007) would significantly affect material transport in and out of the estuary (e.g. pollutants, harmful algal blooms, and planktonic organisms). The influence of coastal setup and setdown on material transport warrants further investigation.
Acknowledgments Both surveys were accomplished with the collaboration of University of Florida students and NMFS Panama City Laboratory personnel. The scientific support of Christopher Devick, Ilgar Safak, Hande Caliskan, Marta Ribera, Sangdon So, and Chloe Winant is appreciated. Boats were safely operated with the expertise of John Brusher, Andrew David, Doug Devries, Michelle Duncan, and Christopher Palmer. AVL acknowledges support from NSF project OCE-0551923. Pete Sheridan, Stacy Harter, and two anonymous reviewers provided valuable comments to the original manuscript. Special thanks to Christopher Gardner for his assistance in the middle of the night to save the October survey. References Berthot, A., Pattiaratchi, C., 2006. Mechanisms for the formation of headlandassociated linear sandbanks. Continental Shelf Research 26, 987–1004. Blumberg, A.F., Kim, B.N., 2000. Flow balances in St Andrew Bay revealed through hydrodynamic simulations. Estuaries 23, 21–33. Brown, C.A., Jackson, G.A., Holt, S.A., Holt, G.J., 2005. Spatial and temporal patterns in modeled particle transport to estuarine habitat with comparisons to larval fish settlement patterns. Estuarine Coastal and Shelf Science 64, 33–46. Brusher, H.A., Ogren, L.H., 1976. Distribution, abundance and size of penaeid shrimps in the St. Andrew Bay system, Florida. Fishery Bulletin US 74, 158–166. Chant, R.J., Stoner, A.W., 2001. Particle trapping in a stratified flood-dominated estuary. Journal of Marine Research 59, 29–51. Doyle, B.E., Wilson, R.E., 1978. Lateral dynamic balance in the sandy hook to rockaway point transect. Estuarine and Coastal Marine Science 6, 165–174. Fisher, H.B., List, E.J., Koh, R.C.Y., Imberger, J., Brooks, N.H., 1979. Mixing in Inland and Coastal Waters. Academic Press, New York, 483pp. Geyer, W.R., 1993. Three dimensional flow around a headland. Journal of Geophysical Research 98, 955–966. Geyer, W.R., Farmer, D.M., 1989. Tide-induced variation of the dynamics of a salt wedge estuary. Journal of Physical Oceanography 19, 1060–1072. Geyer, R.W., Signell, R., 1990. Measurements of tidal flow around a headland with a shipboard acoustic Doppler current profiler. Journal of Geophysical Research 95, 3189–3197. Goodrich, D.M., 1988. On meteorologically induced flushing in three US East Coast Estuaries. Estuarine, Coastal, and Shelf Science 26, 111–121. Hibiya, T., LeBlond, P.H., 1993. The control of fjord circulation by fortnightly modulation of tidal mixing processes. Journal of Physical Oceanography 23, 2042–2052. Ichiye, T., Jones, M.L., 1961. On the hydrography of the St. Andrew Bay system, Florida. Limnology and Oceanography 6, 302–311. Joyce, T.M., 1989. On in situ calibration of shipboard ADCPs. Journal of Atmospheric and Oceanic Technology 6, 169–172. Jay, D.A., Smith, J.D., 1990. Residual circulation in shallow estuaries. 1. Highly stratified, narrow estuaries. Journal of Geophysical Research 95, 711–731. Kantha, L., 2005. Barotropic tides in the Gulf of Mexico. In: Sturges, W., Fernandez, A.L. (Eds.), Geophysical Monograph Series. Circulation in the Gulf of Mexico, Vol. 161. American Geophysical Union, Washington, DC, pp. 159–163. Lerczak, J.A., Geyer, R.W., 2004. Modeling the lateral circulation in straight, stratified estuaries. Journal of Physical Oceanography 34, 1410–1428. Li, C., 2002. Axial convergence fronts in a barotropic tidal inlet—Sand Shoal Inlet, VA. Continental Shelf Research 22, 2633–2653. Li, C., Valle-Levinson, A., Wong, K.C., Lwiza, K.M.M., 1998. Separating baroclinic flow from tidally induced flow in estuaries. Journal of Geophysical Research 103 (C5), 10405–10417. Lindall Jr., W.N., Saloman, C.H., 1977. Alteration and destruction of estuaries affecting fishery resources of the Gulf of Mexico. Marine Fisheries Review 39 (9), 1–7. Liu, J.T., 1992. The influence of episodic weather events on tidal residual currents: a case study at Sebastian Inlet, Florida. Estuaries 15, 109–121. Lwiza, K.M.M., Bowers, D.G., Simpson, J.H., 1991. Residual and tidal flow at the tidal mixing front in the North Sea. Continental Shelf Research 11, 1379–1395.
McNulty, J.K., Lindall Jr., W.N., Sykes, J.E., 1971. Cooperative Gulf of Mexico Estuarine Inventory and Study: Florida. Phase I: Area Description. NOAA Technical Report NMFS CIRC 368, 126pp. Miller, R.L., McPherson, B.F., 1991. Estimating estuarine flushing and residence times in Charlotte Harbor, Florida, via salt balance and a box model. Limnology and Oceanography 36, 602–612. Mo¨ller, O.O., Castaing, P., Saloman, J.C., Lazure, P., 2001. The influence of local and non-local forcing effects on the subtidal circulation of Patos Lagoon. Estuaries 24, 297–311. Musgrove, R.J., Foster, J.B., Toler, L.G., 1964. Water Resources of the Ecofina Creek basin area in northwestern Florida. Florida Geological Survey Report Investigation, vol. 41, 51pp. Nunes, R.A., Lennon, G.W., 1987. Eposodic stratification and gravity currents in a marine environment of modulated turbulence. Journal of Geophysical Research 92, 5465–5480. Nunes, R.A., Simpson, J.H., 1985. Axial convergence in a well-mixed estuary. Estuarine Coastal Shelf Science 20, 637–649. NWFWMD, 2001. The Big Picture: St. Andrew Bay Watershed. Public Information Bulletin 01-4. Department of Environmental Protection, Northwest Florida Water Management District, Surface Water Improvement and Management Program. 6pp. Ogren, L.H., Brusher, H.A., 1977. The distribution and abundance of fishes caught with a trawl in the St. Andrew Bay system, Florida. Northeast Gulf Science 1, 83–105. Pritchard, D.W., 1955. Estuarine circulation patterns. Proceedings of the American Society of Civil Engineers 81, 717/1–717/11. RD Instruments, 1996. Acoustic Doppler Current Profiler Principles of Operation: A Practical Primer, second ed. RD Instruments, Poway, CA, 51pp. Robinson, I.S., 1981. Tidal vorticity and residual circulation. Deep Sea Research 28A, 195–212. Rodriguez, P.E., Wu, T.S., 1990. Initial analysis of the circulation and flushing characteristics of the St. Andrew Bay system. Northwest Florida Water Management District Water Resources Special Report 90-1, 103pp. Salsman, G.G., Tolbert, W.H., Villars, R.G., 1966. Sand-ridge migration in St. Andrew Bay, Florida. Marine Geology 4, 11–19. Schroeder, W.W., Dinnel, S.P., Wiseman, W.J., 1990. Salinity stratification in a riverdominated estuary. Estuaries 13, 145–154. Scully, M.E., Friedrichs, C.T., 2007. Sediment pumping by tidal asymmetry in a partially mixed estuary. Journal of Geophysical Research 112 (C7), C07028. ˜ an, M.B., 2004. Observations of subtidal Sepulveda, H.H., Valle-Levinson, A., Framin and tidal flow in the Rı´o de la Plata Estuary. Continental Shelf Research 24, 509–525. Simpson, J.H., Brown, J., Matthews, J., Allen, G., 1990. Tidal straining, density currents, and stirring in the control of estuarine stratification. Estuaries 13, 125–132. Smith, N.P., 1977. Meteorological and tidal exchanges between Corpus Christi Bay, Texas, and the northwestern Gulf of Mexico. Estuarine and Coastal Marine Science 5, 511–520. Smith, N.P., 1979. Tidal dynamics and low frequency exchanges in the Aransas Pass, Texas. Estuaries 2, 218–227. Smith, N.P., 1993. Tidal and nontidal flushing of Florida’s Indian River Lagoon. Estuaries 16, 739–746. Souza, A.J., Simpson, J.H., 1996. The modification of tidal ellipses by stratification in the Rhine ROFI. Continental Shelf Research 16, 997–1007. Stommel, H., Farmer, H.G., 1952. Abrupt change in width in two layer open channel flow. Journal of Marine Research 11, 205–214. Swenson, E.M., Chuang, W., 1983. Tidal and subtidal water volume exchange in an estuarine system. Estuarine Coastal and Shelf Science 16, 229–240. Turrell, W.R., Brown, J., Simpson, J.H., 1996. Salt intrusion and secondary flow in a shallow, well-mixed estuary. Estuarine Coastal and Shelf Science 42, 153–169. USEPA, 1999. Ecological condition of estuaries in the Gulf of Mexico. EPA 620-R-98004. US Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Gulf Ecology Division, Gulf Breeze, Florida. Valle-Levinson, A., Lwiza, K.M.M., 1995. Effects of channels and shoals on the exchange between the lower Chesapeake Bay and the adjacent ocean. Journal of Geophysical Research 99 (C3), 5149–5169. Valle-Levinson, A., Matsuno, T., 2003. Tidal and subtidal flow along a cross-shelf transect on the East China Sea. Journal of Oceanography 59, 573–584. Valle-Levinson, A., Moraga, J., Olivares, J., Blanco, J.L., 2000. Tidal and residual circulation in a semi-arid bay: Coquimbo Bay, Chile. Continental Shelf Research 20, 2009–2028. Valle-Levinson, A., Delgado, J.A., Atkinson, L.P., 2001. Reversing water exchange patterns at the entrance to a semiarid coastal lagoon. Estuarine Coastal and Shelf Science 53, 825–838. Valle-Levinson, A., Reyes, C., Sanay, R., 2003. Effects of bathymetry, friction, and Earth’s rotation on estuary/ocean exchange. Journal of Physical Oceanography 33, 2375–2393. Wang, J.D., van der Kreeke, J., Krishnan, N., Smith, D., 1994. Wind and tide response in Florida Bay. Bulletin of Marine Science 54, 579–601. Webb, B.M., King, J.N., Tutak, B., Valle-Levinson, A., 2007. Flow structure at a trifurcation near a North Florida inlet. Continental Shelf Research 27, 1528–1547. Wong, K.C., 1987. Tidal and sub-tidal variability in Delaware’s inland bays. Journal of Physical Oceanography 17, 413–422.