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Curvature Effects on Water Exchange at the Entrance to a Tropical Estuary.

Permanent Link: http://ufdc.ufl.edu/UFE0044696/00001

Material Information

Title: Curvature Effects on Water Exchange at the Entrance to a Tropical Estuary.
Physical Description: 1 online resource (49 p.)
Language: english
Creator: Guerra, Gisselle Esther
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: coriolis -- curvature -- estuary -- flow -- residual
Civil and Coastal Engineering -- Dissertations, Academic -- UF
Genre: Coastal and Oceanographic Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: With the purpose of studying the effects of coastline curvature on tidal and subtidal momentum balances, a transect in a tropical estuary was sampled during four semidiurnal surveys. The 12-km long transect was located off a prominent headland, Point San Jose, at the entrance to Estero Real, which is an estuary surrounded by Nicaragua and Honduras within the Gulf of Fonseca. Underway current velocity profiles and surface hydrographic variables were recorded in combination with 3 stations of conductivity-temperature-depth profiles. Measurements were obtained during spring and neap tides in the dry season (March) and wet season (June) of 2001, resulting in a total of 4 tidal cycle surveys. Results indicate that the residual flows near Point San Jose are affected by the curvature effects, as lateral circulation was directed away from the headland at the surface and toward the headland at depth. During the wet season, residual secondary flows near Point San Jose were weaker because of buoyancy effects suppressing them. Residual flows were strongest at spring tides of the dry season because of increased tidal current speeds. The residual flows observed result mostly from a dynamic balance among pressure gradient, Coriolis acceleration, and centrifugal acceleration.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Gisselle Esther Guerra.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Valle-Levinson, Arnoldo.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044696:00001

Permanent Link: http://ufdc.ufl.edu/UFE0044696/00001

Material Information

Title: Curvature Effects on Water Exchange at the Entrance to a Tropical Estuary.
Physical Description: 1 online resource (49 p.)
Language: english
Creator: Guerra, Gisselle Esther
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: coriolis -- curvature -- estuary -- flow -- residual
Civil and Coastal Engineering -- Dissertations, Academic -- UF
Genre: Coastal and Oceanographic Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: With the purpose of studying the effects of coastline curvature on tidal and subtidal momentum balances, a transect in a tropical estuary was sampled during four semidiurnal surveys. The 12-km long transect was located off a prominent headland, Point San Jose, at the entrance to Estero Real, which is an estuary surrounded by Nicaragua and Honduras within the Gulf of Fonseca. Underway current velocity profiles and surface hydrographic variables were recorded in combination with 3 stations of conductivity-temperature-depth profiles. Measurements were obtained during spring and neap tides in the dry season (March) and wet season (June) of 2001, resulting in a total of 4 tidal cycle surveys. Results indicate that the residual flows near Point San Jose are affected by the curvature effects, as lateral circulation was directed away from the headland at the surface and toward the headland at depth. During the wet season, residual secondary flows near Point San Jose were weaker because of buoyancy effects suppressing them. Residual flows were strongest at spring tides of the dry season because of increased tidal current speeds. The residual flows observed result mostly from a dynamic balance among pressure gradient, Coriolis acceleration, and centrifugal acceleration.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Gisselle Esther Guerra.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Valle-Levinson, Arnoldo.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044696:00001


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1 CURVATURE EFFECTS ON WATER EXCHANGE AT THE ENTRANCE TO A TROPICAL ESTUARY By GISSELLE ESTHER GUERRA SAVAL A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Gisselle Esther Guerra Saval

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3

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4 ACKNOWLEDGMENTS I thank my family for always supporting me through all my education and their endless love and encouragement even in the distance. I thank my adviser and chair, Dr. Arnoldo Valle Levinson for his guidance and patience provided through my years at the University of Florida. I al so thank Dr. Robert Thieke for being member of my committee

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF FIGURES ................................ ................................ ................................ .......... 6 LIST OF ABBREVIATIONS ................................ ................................ ............................. 7 ABSTRACT ................................ ................................ ................................ ..................... 8 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ ...... 9 Motiva tion ................................ ................................ ................................ ................. 9 Estuarine Definition ................................ ................................ ................................ ... 9 Theory Background ................................ ................................ ................................ 12 2 METHODS ................................ ................................ ................................ .............. 14 Study Area ................................ ................................ ................................ .............. 14 Data Collection ................................ ................................ ................................ ....... 15 Data Processing ................................ ................................ ................................ ..... 16 Tidal Variab ility ................................ ................................ ................................ 16 Subtidal variability ................................ ................................ ............................ 18 3 RESULTS ................................ ................................ ................................ ............... 21 Tidal variability ................................ ................................ ................................ ........ 21 Subtidal Variability ................................ ................................ ................................ .. 23 4 DISCUSSION ................................ ................................ ................................ ......... 42 5 CONCLUSION ................................ ................................ ................................ ........ 46 LIST OF REFERENCES ................................ ................................ ............................... 47 BIOGRAPHIC AL SKETCH ................................ ................................ ............................ 49

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6 LIST OF FIGURES Figure page 2 1 Study Area. Gulf of Fonseca ................................ ................................ ............... 20 3 1 Density anomaly (kg/ m 3 ) for Time versus Distance across the transect ............ 27 3 2 Surface streamwise flow gradients (1/s) across the transect with surface flow vectors on top (m/s) ................................ ................................ ............................ 28 3 3 Depth averaged streamwise flow gradients (1/s) across the transect with depth averaged flow vectors on top (m/s) ................................ ........................... 29 3 4 Rossby number using surface flows for time versus distance across the transect ................................ ................................ ................................ ............... 30 3 5 Rossby number using depth average flows for time versus distance across the transect and mean tidal flows (m/s) ................................ .............................. 31 3 6 Tidal Current Amplitude (cm/s) for streamwise flow ................................ ........... 32 3 7 Tidal Current Phase (degrees) for streamwise flow ................................ ............ 33 3 8 Tidally averaged density anomaly (kg/m 3 ) for depth versus distance across the transect ................................ ................................ ................................ ......... 34 3 9 Tidally averaged baroclinic pressure gradient (m/s 2 ) for depth versus distance across the transect ................................ ................................ ............... 35 3 10 Tidally averaged Baroclinic pressure gradient profiles (m/s 2 ) between CTD Stations for depth versus distance across the transect ................................ ...... 36 3 11 Residual streamwise (contours) and stream normal (vectors) flows (cm/s), as calculated using least squares fit to semidiurnal tidal cycle ................................ 37 3 12 Streamwise flows from the Semianalytical solutions, using three Ek and Ke numbers for the study area bathymetry ................................ .............................. 38 3 13 Deviations of the streamwise flow from its vertical mean (cm/s) ......................... 39 3 14 Tidally av eraged Centrifugal acceleration of the streamwise flows across the transect (m/s 2 ) ................................ ................................ ................................ .... 40 3 15 Tidally averaged Coriolis acceleration of the streamwise flows across the transect (m/s 2 ) ................................ ................................ ................................ .... 41

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7 LIST OF ABBREVIATION S S treamwise coordinate S tream normal coordinate Vertical coordinate Local acceleration Velocity gradient Radius of curvature Centrifugal acceleration Coriolis parameter Gravity acceleration ( ) Water surface gradient Vertical eddy viscosity Shear stress (normal direction) Density ( ) H Water depth Vertical mean of velocity Ro Rossby number Ke Kelvin number Ek Ekman number Ri Internal Rossby radius

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8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science CURVATURE EFFECTS ON WATER EXCHANGE AT THE ENTRANCE TO A TROPICAL ESTUARY By Gisselle Esthe r Guerra Saval August 2012 Chair: Arnoldo Valle Levinson Major: Coastal and Oceanographic Engineering With the purpose of studying the effects of coastline curvature on tidal and subtidal momentum balances, a transect in a tropical estuary was sampled d uring four semidiurnal surveys. The 1 2 km long transect was located off a prominent headlan d, Point San Jose at the entrance to Estero Real, which is an estuary surrounded by Nicaragua and Honduras within th e Gulf of Fonseca. Underway current velocity pro files and surface hydrographic variables were recorded in combination with 3 stations of conductivit y temperature depth profiles. Measurements were obtained during spring and neap tides in the dry season (March) and wet season (June) of 2001, resulting in a total of 4 tidal cycle surveys. Results indicate that the residual flows near Point San Jose are affected by the curvature effects, as lateral circulation was directed away from the headland at the surface and toward the headland at depth. During the wet season, residual se condary flows near Point San Jose were weaker because of buoyancy effects suppressing them. Residual flows were strongest at spring tides of the dry season because of increased tidal current speeds. The residual flows observed result mo stly from a dynamic balance among pressure gradient, Coriolis acceleration, and centrifugal acceleration.

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9 CHAPTER 1 INTRODUCTION Motivation Secondary flows are described as the flow that is normal to the main along channel flow. In some cases, they are also referred as the differences between the main along channel flow and the depth averaged flow, yielding to the secondary flow component, as described by Kalkwij and Booij (1986). This secondary flow component typically rises to the helical flow phenomenon. Geyer (1993) reconsidered this concept by applying it to field measurements of tidal flow around a headland. He found out that the seconda ry flow was following the helical phenomenon (low layer flow directed toward the headland and the upper layer flow directed away from it) but the secondary flows were twice as large as predicted by Kalkwij and Booij (1986). This disagreement is caused by t he effects of stratification. Based on current profile field measurements Chant (2002) concluded that the strength and structure of secondary flows in a region of flow curvature will linearly increase with tidal current speed and will shut down at high riv er discharges, due to buoyancy effects. Mostly, all the mentioned field measurements had been done at temperate regions. Therefore, the purpose of this study is to describe the curvature effects at a tropical estuary, as is Estero Real, and compared them w i th the above mentioned studies. Estuarine Definition Estuaries are defined as a semi enclosed basin where the freshwater derived from land and the salt water from the ocean interacts, this definition was proposed by Cameron and Pritchard (1963). Commonly, estuaries are classified based on their water balance, vertical structure of sali nity, and their geomorphology. Based on their water

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10 balance they are subdivided in three categories: positive, inverse and low inflow. Positive estuaries denoted by a strong er outflow at the sur face than a near bottom inflow. Inverse estuaries, as it names says, will behave opposite to a positive estuary, with inflow at the surface and outflow at the near bottom. Finally, low inflow estuaries are caused by a low influence of river discharge with a high evaporation rate. This hi gh evaporation rate will create a positive behavior shoreward and an inverse behavior seaward (Valle Levinson, 2010) Water balance classification denotes the residual exchange flow that remains after the tidal effects are r emoved. Density driven exchange flows can be characterized as vertically and horizontally sheared flows. Vertically sheared flows are defined as outflow of the less dense water at the surface and inflow of dense water at the bottom. Horizontally sheared fl ows are described with inflow at the channel and outflow at the shoals. Valle Levinson (2008) described this density driven exchange flows in function of the Ekman (Ek) and Kelvin (Ke) numbers. ( 1 1 ) ( 1 2 ) Eq uation 1 1 indicates a relationship of the vertical eddy viscosity ( ) the Coriolis parameter ( ) and the water depth ( ) The Ek number compares frictional to Coriolis effects. O n Equation 1 2 is the internal Rossby radius. is the ratio between the buoyant part of the density driven flow over rotation. This semi analytical solution solves for the residual flows in the along and across channel coordinates. The flows are produced by a dynam ics between pressure

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11 gradient, frictional and Coriolis influences, not considering advective effects (Valle Levinson, 2008) Estuaries are frequently described in terms of their along estuary and across estuary momentum balan ces. The along axis will be where the main flow occurs and the across axis will be the perpendicular t o the a long axis. Equations 1 3 and 1 4 will describe all the terms in the along and across momentum balances respectively. ( 1 3 ) Equation 1 3 describes the along estuary momentum balance. The left hand side (lhs) includes four terms, being the first one the local accelerations and the three remaining terms represent advection of the along estuary flow. The right hand side (rhs) includes three t erms, being the first one the Coriolis acceleration, followed by pressure gradient (which includes the contributions of the barotropic and baroclinic pressure gradients) and finally the frictional term. Now, the across estuary momentum balance will be desc ribed. ( 1 4 ) The lhs of the Equation 1 4 includes four terms, the first one defines the local accelerations, and the remaining terms represent the advection. The rhs first term is the Coriolis acceleration, the second one represents the pressure gradient and the last one describes the frictional terms. The following section will describe the applied theory used for this analysis, which includes a momentum balance in stream wise and stream normal coordinates, in order to conveniently represent and identify the curvature effects caused by Coriolis and centrifugal acceleration.

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12 Theory Background For this analysis a curvilinear system is adopted, where will be the dire ction of the vertically averaged flow, will be the cross stream of that flow and will be the vertical direction. The momentum balance that was explained on the previous section will be transformed in this section into the curvilinear system, similar variables will be listed. First, some assumptions will be made, (except at the center of the headland eddy and close t o slack waters), and neglecting vertical advection. The stream normal momentum balance equation, which is the one used in this s tudy, will be approximated by: ( 1 5 ) Where is the local radius of curvature, the Coriolis acceleration, is the water level and is th e vertical eddy viscosity Geyer (1993) assumed a uniform density for his study, but for our study the baroclinic pressure term will be included and evaluated (Geyer, 1993). Now, the vertical mean of Equation 1 6 is calculated, later these two equations will be subtracted one from the other to have an expression for secondary flows. The stream normal momentum equation in terms of the vertical mean would be the following ( 1 6 ) Where is the across bottom shear stress, is the water depth, the first term is considered small and can be neglected. When s ubtracting Equation 1 6 from Equation 1 5 the follow ing equation is obtained ( 1 7 )

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13 Equation 1 7 contains the following terms, on the left hand side (lhs) (in order of appearance) the time var iation, streamwise advectio n, frictional dissipation of the streamwise flow bottom friction and the baroclinic pressure gradient. On the right hand side (rhs) appears first the centrifugal acceleration and the Coriolis acceleration due to the streamwise flow deviations around its v ertical mean. Those two terms in the rhs would be solved for this study. In order to compare the advection over rotation, th e Rossby number was calculated. The Rossby number is a ratio between advection and rotation. In this case, the relative importance o f centrifugal acceleration over Coriolis acceleration will be employed for the Rossby Number. ( 1 8 ) The following chapter will explain in detail the methods used for the analysis of the curvature effects due to centrifugal and Coriolis acceleration.

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14 CHAPTER 2 METHODS Study Area The Gulf of Fonseca is located on the Pacific side of Central America. It ha s a surface area of ~ 1600 km 2 a mean depth of ~15 m and a volume of ~2.4 x10 10 m 3 (Valle Levinson and Bosley, 2003). The waters of th e G ulf are shared by the countries of El Salvador, Honduras and Nicaragua. The complex morphology of the Gulf includes four tributaries branching, sudden bathymetry changes, headlands and several islands. The principal rivers ar e the Choluteca in Honduras, the Goascoran, which sep arates Honduras and El Salvador, an d the Estero Real in Nicaragua (Vi llela, 2001) Annual precipitation is ~2400 mm, producing a regular river discharge through the year. Wet season spans from May to October with northeasterly to easterly moist winds, linked to the Caribbean Trades. Dry Season spans from November to April with southwest erly winds wi th intermittent north erly winds (Valle Levinson and Bosley, 2003 ) The Gulf is dominated by semidiurnal tides (12.42 hr) and presents an average tidal range of 1.8 m. Spring tides are 3.5 m over the datum, while neap tides are 0.2 m over the d atum Choluteca River represents 49% of the fresh water supply to the Gulf, leaving the remaining 41% shared between Goascaran, Estero Real and land run off (Ward, 2000) Gulf of Fonseca is a flooded coastal indentation formed b y faulting and volcanism and is connected to the Pacific Ocean by a 30 km width entrance and 20 m depth. It presents tidal flats, tidally flushed mangroves, and submerged pinnacles rocks in the open water. A study realized during 1994, showed that salinity ranges from 36.2 36.9 % during the time of the study (Currie, 1994) and it also showed that evaporation

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15 (~2800 mm/yr) is sometimes higher than precipitation ( ~2400 mm ) causing higher salinity values. This investigation was conducted along an 11 km transect between Nicaragua and Honduras. This transect was at the entrance of Estero Real, an embayment within the Gulf o f Fonseca. At the southern end of the transect, Point San Jose headland extended into the Gulf being the area of interest of this study because of its possible influence on secondary flows. The cross sectional bathymetry is characterized by a 22 m deep channel (South) followed by a 10 m shoal ( North), looking seaward. The stud y area is governed by semidiurnal tides, with an average tidal range of 1.2 m/s. The vertical water column is well mixed. Data Collection Sampling of underway currents and hydrographic variables took place along the transect at spring and neap tides during wet and dry seasons. Underway currents were collected with a 600 kHz acoustic Doppler current profiler (ADCP). Data collection took place during March 12 13 for spring tides and March 19 20, 2001 for neap tides, dry season; and for the wet season it took place June 23 24 for spri ng tides and June 17 18, 2001. The ADCP recorded the ave rage of ~10 pings distributed over sampling intervals of 30 s for the dry season and 45 s for the w et season at 0.5 m bins size. The instrument was mounted on a 1.2 m long cat amara n towed from the starboard side at speeds of ~2.5 m/s. Navigation data was collected from a Gar min GPSMAP 185 GPS onboard. A SBE 19 Conductivity Temperature and Depth Profiler ( CTD ) was used to collect profiles of salinity, temperature and density a t three stations within the transect.

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16 Stations were located at both ends and at the deepest part of the transect, of each ADCP transect. Also, underway Conductivity Temperature (CT) data was collected through the ADCP transects. The instrument was towed from the shipboard, allowing a continuous data collection. Data Processing Data collected from the ADCP and the CTD were converted to .txt format and analyzed in Matlab. Velocity profiles from the ADCP were arranged first in regular matrices and then corre cted considering the After the correction, flows were separated by transect repetitions and then arranged into a regular grid. The CTD data were imported into Matlab for further analysis. Tidal Variability Density anomalies v ariations at the surface over the tidal cycl e were analyzed for the three available data sets (spring and neap tides for wet season and spring tides for dry season) Contours from the density anomalies variations were gene rated and plotted for analysis. The ADCP arranged the veloc ity currents in E W and N S component with the built in c ompass. C urrents were rotated to the principal axis of maximum variance (Emery and Thomson, 1998). This procedure was done first by plotting the E W and N S, along the x ax is and y axis, respectively in a scatter plot The second step was to determine a trend line and the angle between the trend line and the x axis and finally, using this angle to rotate to the stream wise and stream normal velocity components. The vertical spacing of 0.25 m and horizontal spacing of 15 m was used for the dry season, and for the wet season the vertical spacing was 0.5 m and the horizontal was 20 m. Each velocity profile starts at a depth of 0.8 m for the dry season and at 0.6 for

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17 the wet seas on, both extending to 85% of the water depth. A least square s fit with a semidiurnal (12.42hr) harmonic was applied to the flows ( 1 9 ) ( 1 10 ) On Equation 1 9, the first term of the rhs is the residual f low in the streamwise component would be the amplitude, the semidiurnal harmonic (12.42 hr) and would be the phase. Equation 1 10 would be the same as Equation 1 9 for the stream normal component. Using the harmonic cons tants from the least square fit analysis (residual flow, amplitude and phase) a reconstructed velocity was calculated in a uniform grid. The uniform grid was generated using the distance of the transect and the depth of it, then the velocity was interpolat ed over it. The amplitude and phase of the tidal flow was plotter over bathymetry, with th e purpose to describe the flow. In order to evaluate the change of the streamwise velocity across the transect, gradients of it were calculated with the surface and depth averaged flows and plotted against time. Then vectors of the streamwise and stream normal were plotted on top of the gradient versus time plots. This representation was done to evaluate w here the stronger flows w here locater and to relate the direction and strength o f the flows with its gradients. Since the purpose of this study is to evaluate the importance of advection, the Rossby number wa s calculated for two data sets. Both Rossby numbers were calculated using Equation 1 11 the difference between then is the choice of streamwise

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18 flow ; one was performed using surface flow s whereas the o ther was using depth averaged flow s. ( 1 11 ) Depth average and surface streamwise flows were similar in structure but different in magnitude, so two Rossby numbers were calculated to show this similitude. Subtidal variability In order to study the tidally averaged stratification across the estuary, the density anomaly obtained from the CT was evaluated o section Density anomaly was also evaluated from the data of the CTD at the three stations. Baroclinic pressure gradient (Equation 1 12 ) was calculated for the CTD and the CT data sets. With the CTD data baroclinic pressure gradien ts between stations were calculated and plotted over depth. While with the CT data, the baroclinic pre ssure gradients were tidally averaged and then represented in a plot distance versus depth. ( 1 1 2 ) Residual flows, or mea n flows, were obtained from the least square fit analysis. Stream wise flows were pl otted in contours of it with stream normal flow vectors on top of it. In order to know the vertical deviations of the stream wise flow around its vertical mean, the term was calculated. For this calculation, the verti cal average of the stream wise flow component was computed and then extracted from the reconstructed s treamwise flow component, and later was averaged over time. Contours of the deviations of the strea m wise velocity component along its vertical mean were also generated. Residuals flows were then compared to the semi analytical solution of the density driven flow, based on different Ek and Ke

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19 bathymetry was plugged into the mod el, along with three Ek numbers (0.01, 0.05, 0.1), and three Ke numbers (0.2, 1.0, 2.0). The centrifugal acceleration term from the stream wise momentum balance term was calculated using a radius of curvature ( ) of 5 km. Tidally averaged c ontours of centrifugal acceleration we re plotted over bathymetry. Coriolis acceleration, also from the stream wise momentum balance was calculated using since the study area is close to the Equator is a small number. Equation 1 1 3 was used for the calculation, where is the angular velocity and is the latitude of the study area. ( 1 13 )

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20 Figure 2 1 Study Area. Gulf of Fonseca

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21 CHAPTER 3 RESULTS The results of this investigation are presented in terms of tidal and subtidal variabili ty In formation on the tidal variability includes: surface density anomalies ; gradients ( ) for the surface and depth averaged streamwise flow components, the Rossby number and its a mplitude and phase The latter described the structure of the tidal flow and were obtained from the least square fit The following section, subtidal variability encompasses the dens ity anomaly across the transect and the baroclinic pressure gradient s both between the CTD stations an d across the transect (using CT data) Residual streamwise a nd stream normal flows were obtained across the transect D eviations of the streamwise flow from its vertical mean were utilized in the analysis of centrifugal and rotational accelerations Residual flows of the streamwise component were then compared with the solutions obtained at different nine combinations of Ek and Ke numbers using the bathymetry from the study site. Tidal variability Density anomaly values at t he surface were obtained for both spring and neap tides during the dry season but only for spring tides during the wet season. The CT was malfunctioning during the neap tides of the wet season, causing a loss of data during that period. Through out the dry season, both tides had higher density anomaly values of approximate 22 21 kg/m 3 as compared to the wet season, which had a value of only 20 20.5 kg/m 3 Lateral gradients of surface flows were stronger closer to the headland an d in the shallowest section after the channel during spring tides and during neap tides the strongest section was also closer to the bend and in the shallowest

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22 section but smaller compared to the spring tides. A s expec ted th e velocity vectors were strongest at spring tides and weake st during neap tides. Positive gradients illustrated flows moving seaward while negative gradients depicted shoreward flows ( Figure 3 2 ). Lateral gradients of de pth averaged streamwise flow were similar in structure as the one showed with the lateral gradients of the surface streamwise flow with stronger gradients closer to the bend and in the shallowest section of the transect ( Figure 3 3 ). Depth averaged flows were illustrated by vectors on top of the gradients. Circulation cells were observed closer to the bend, then inflow at the shallowest section after the channel and at the end of the transect outflow ( Figure 3 3 A, B & D). But during the neap tides of the dry season ( Figure 3 3 A ) depth averaged flows were flowing shoreward until the end of the channel and then changed to outflow, for the first 5 hours of measurements. The remaining p ortion of the period, the flows showed a convergence at the shallowest section (after the channel) ( Figure 3 3 C). Where stronger streamwise lateral variability was p resent, Rossby number was greater than the unity ( >1) as expected ( Figure 3 4 & Figure 3 5 ). The higher Rossby number was found at the shallowest sections of the transect, being closer to the bend and at the end of the channel. The amplitude and phase for the stream wise flow for each data set were obtained fro m the least square fit analysis Weaker flows around ~50 cm/s were observed at the neap tides for both seasons. M eanwhile the stronger flows were present at the spring tides during both seasons, with the highest values at the channel (~100 cm/s) ( Figure 3 6 ). The phase indicates how fast a flow will change of direction, lower values will denote a flow that will change first than a flow with a higher phase value. Neap tide, Dry season

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23 h ad smaller values at the bottom as the spring tide, wet season Figure 3 7 A & D. While during spring tide, dry season flows at the area closer to the headland (West) l ags behind the remainder of the transect ( Figure 3 7 B). Flows during the wet season, neap tide changed direction first at the channel and at the shoal, than the sect ion closer to the bend ( Figure 3 7 C). Subtidal V ariability Tidally averaged d ensity anomal ies for spring tides ( dry season ) and for n eap tides ( both seasons ) increased with depth, indicating a stable water column The isopycnals were tilted down toward the right ( looking into the estuary ) as a result of the effects of curvature, which can be Coriolis, Centrifugal acceleration or a combination of both As expect ed, the tidally averaged density anomalies were greatest during the dry season, with values around ~22 kg/m 3 and lower during the wet season with both values around ~20 kg/m 3 for both tides ( Figure 3 8 ). The baroclinic pressure gradient ( Figure 3 9 ) indicates little to no variation at the top of the water column, while from the mid depth to the bottom of channel the b aroclinic pressure gradient revealed some variation. Data was available for the neap tides at the dry season and for the spring ti des at both seasons. Using only surface density values from the CT data the same behavior was obtained as using the tidally averaged value s between stations ( Figure 3 10 ) The magnitude of the baroclinic pressure gradient was f or the contours and for the profiles ( Figure 3 9 Figure 3 10 ). Residual stre amwise subtidal flows were similar in st ruc ture for both spring and neap tides during the dry season During dry season Figure 3 11 (A B) depict an inverse circulation with inflow (+) at the surface a nd outflow ( ) at the bottom. Small

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24 discrepancies in location of the greatest flows were found for spring and neap tides. For neap tides, the s tronger flows (~ 15 c m/s) at the surface were limited to the channel. In contrast, during spring tides, the stronger surface flows extended over the s hoal ( ~ 15 c m/s). During the dry season, outflow was primarily restricted to the region closer to the b ottom (>8 m depth) with val ues ranging from 15 0 c m/s Streamwise flows were vertically and horizontally sheared for the neap and spring tides of the dr y season. A complete different structure was observed during the wet season (Figure 3 11 C & D). In the neap and spring tides a combination of outflows and inflows at the surface were observed (Figure 3 11 C & D ) Streamwise flows were vertically and horizontally sheared during the neap tides, while during spring tides streamwise flows were only horizontally sheared ( Figure 3 11 C & D) Stream normal flows were direc ted toward the bend (Westward) at the bottom, while at the surface were directed away from the bend caused by Point San Jose ( Eastward). Stronger stream normal flows moving in the helical form as a result of channel curvature were ob served during the dry season (both tides) ( Figure 3 11 A & B ) Stream normal flows at neap tides of the wet season, displayed a weak helical form closer to the bend, but also exhibited divergence toward the bottom of the channel and convergence at the end of the channel ( Figure 3 11 C ) Results from the semianalytical solution for the streamwise flows are showed on Figure 3 12, were positive flows indicate inflow and negative flows indicate outflows. This model was applied to the ba thymetry for the study area with nine combinations of Ekman and Kelvin Number s Streamwi se flows were vertically a nd horizontally sheared for all the combinations of Ek and Ke Inflows were observed at the surface whil e

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25 outflows at the bottom, characteristic for an inverse circulation. Black lines on each gr aph within Figure 3 12 indicate the 0 m/s velocity line. When the exchange flow was tilted toward the left (looking seaward) and being more prominent at low Ekman numbers Stronger deviations of the streamwise flow around its vertical mean were obs erved during the d ry season P ositive deviations were found at the surface while negative deviations were found at the bottom. Such deviations were weaker during the wet season Neap tides showed negative deviations at the bottom of the channel, and weaker positive deviatio ns at the west of the transect and at the shoal. In contrast, spring tides only had weaker positive variations at the surface in the transition between the channel and the shoal ( Figure 3 13 ) As expected, stronger centrifugal acce lerations were surface during spring tides for both s eason s Conversely, during neap tides the centrifugal acceleration was at i ts lowest with values around ( Figure 3 14 ) Neap tides of the wet season had a complete different structure than the other three dat a sets. The tidal ly averaged Coriolis accelerations ( Figure 3 14 ) were similar in structure to deviations of the s treamwise flows around the vertical mean ( Figure 3 13 ) Tidally averaged Coriolis accelerations were on the order of S trong est accelerations were found during the dry season, with at th e surface and at the bottom. Simila r structure was observed at spring tides in the wet season, but with weaker va lues compared to the dry season ( at the surface and at the bottom). During spring tides the strongest accelerations were

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26 obs erved in the transition bet ween channel to shoal in both seasons. A unique structure was observed during neap tides of the wet season. Stronger positive values were foun d at the cha nnel and in the shoal, while stronger negative values were located at the bottom of the channel ( Figure 3 15 ) On average, the Coriolis accelerat ion s ( Figure 3 15 ) w ere one order of magnitude smalle r than the c entrifugal acceleration s ( Figure 3 14 )

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27 Figure 3 1 Density anomaly (kg/ m 3 ) for T ime versus Distance across the transect. A) Spr ing tide, Dry Season. B) Spring tide, Wet Season. C) Neap tide, Dry Season

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28 Figure 3 2 Surface streamwise flow gradients (1/s) across the transect with surface flow vectors on top (m/s). A) Neap tide, Dry Season. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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29 Figure 3 3 Depth averaged streamwise flow gradients (1/s) across the transect with depth averaged flow vectors on top (m/s). A) Neap tide, Dry Seaso n. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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30 Figure 3 4 Rossby number using surface flows for time versus distance across the transect. A) Neap tide, Dry Season. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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31 Figure 3 5 R ossby number using depth average flows for time versus distance across the transect and mean tidal flows (m/s). A) Neap tide, Dry Season. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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32 Figure 3 6 Tidal Current Amplitude ( c m/s) for streamwise flow. A) Neap tide, Dry Seaso n. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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33 Figure 3 7 Tidal Current Phase (degree s) for streamwise flow. A) Neap tide, Dry Seas on. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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34 Figure 3 8 Tidally averaged d ensity anomaly (kg/m 3 ) for depth versus distance a cross the transect. The magenta ( ) symbol indicates the three CTD stations. A) Neap tide, Dry Seas on. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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35 Figure 3 9 Tidally averaged baroclinic pressure gradient (m/s 2 ) for depth versus distance across the transect. A) Spring tide, Dry Season. B) Spring tide, Wet Season. C) Neap tide, Wet Season

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36 Figure 3 10 Tidally averaged Baroclinic pressure gradient profiles (m /s 2 ) between CTD Stations for depth versus distance across the transect. A) Spring tide, Dry Season. B) Spring tide, Wet Season. C) Neap tide, Wet Season

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37 Figure 3 11 Residual streamwise (contours) and stream normal (vectors) flows (cm/s), as calculated using least squares fit to semidiurnal tidal cycle. A) Neap tide, Dry Seaso n. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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38 Figure 3 12 Streamwise flow s from the Semianalytical solutions, using three Ek and Ke numbers for the study area bathymetry

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39 Figure 3 13 Deviations of the streamwise flow from its vertical mean (cm/s). A) Neap tide, Dry Seaso n. B) Spring tide, Dry Seas on. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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40 Figure 3 14 Tidally averaged Centrifugal acceleration of the streamwise flows across the transect (m/s 2 ). A) Neap tide, Dry Seas on. B) Spring tide, Dry Season. C) Neap tide, Wet Season. D) Spring tide, Wet Season

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41 Figure 3 15 Tidally averaged Coriolis acceleration of the streamwise flows across the transect (m/s 2 ). A) Neap tide, Dry Season B) Spring tide, Dry Season C) Neap tide, Wet Season. D) Spring tide, Wet Season

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42 CHAPTER 4 DISCUSSION The purpose of this study was to investigate curvature effects i n the tidal and subtidal stream wise momentum balance around a prominent headland. It was required to find observational evidence where along the transect advection ( in this case centrifugal acceleration) was strongest. The streamwise tidal flows and their variations along the transect were investigat ed Surface streamwise flows during spring tides were the strongest among the four data sets. Positive lateral s treamwi s e gradients indicated seaward surface flows, while negative values de s i gnated shoreward flows When the slack waters where caused by the ch ange from ebb to flood convergence was observed. Conversely, divergence occurred during the slack waters from flood to ebb. Depth averaged streamwise flows were consistent during the dry season, with inflows through the first 6 km of the transect and outflows through the remaining of it. At the wet season depth averaged flows were changing from inflow to outflow through the whole section. Lateral gradients of depth averaged streamwise flows also showed stronger gradients at the shallowest sections of the transect (closer to the bend and after the channel), same pattern observed with the lateral surface streamwise gradients. T idal current amplitude and phase were computed to get an understandi ng of the tidal flow structure. As expected, larger amplitudes ( ~100 cm/s) were at spring tides and weaker ( 5 5 0 cm/s ) at neap tides. Phase structure was similar d uring the dry season on both tides; the only variation was negative values closer to the head land (West) This variation indicates a fast changing flow i n this area, while a smaller value indicates an slower change. During the wet season, the neap and spring tides exhibited marked

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43 differences in phase behavior At neap tides, higher values of the phase where closer to the headland (West) and during spring tides the higher phase values were closer to the headland (120 degrees). Tidal d ensity was calculated to examine freshwater intrusion. The ava ilable data sets during included the neap an d spring t ides of the dry season and the spring tides of the wet season. A nearly homogeneous density distribution was observed at the dry season density values were similar between the tides, and at the wet season density was ~2 units less from the dry season. T o verify where the centrifugal acceleration was dominating over Coriolis the Rossby number was computed. S urface Rossby number s suggested centrifugal dominance (>1) near the bend and after the channel at the shallowest part the transect Coriolis was strongest in the shoal S harp changes in Rossby number were mostly observed during spring tides. Likewise, using the depth averaged of streamwise flows Rossby number showed higher value s closer to t he ben d and in the shallowest immediately part after the channel, and smaller values at the shoal. The subtidal flow structure was examined through calculations of density, baroclinic pressure gradient using surface and depth averaged value s residual flows, ver tical variations of the streamwise flow around its vertical mean, Coriolis and c ent rifugal acceleration Density contours of the three available data sets illustrated a stable water column tilted to the right looking into the estuary. The tilting of the is opycnals indicate d the influence of rotation due to centrifugal or Coriolis accelerations which is consistent with findings of Doyle et al., 1976. Baroclinic pressure gradient

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44 computed at the surface and with depth averaged values showed a homogeneous we ll mixed water column with some variations located at the bottom of the channel. R esidual flow obtained from the least squares fit analysis, simulated inverse circul ation during the dry season and a complete different structure during the wet season (vertically and horizontally sheared flows) (Valle Levinson, 2003 ). S tream wise residual flow s were shoreward at the surface and seaward at the bottom during the dry season. D uring the wet season surface flows were directed seaward with the exception of shoreward flows in the transition between the channel and the shoal (Neap tide) During spring tide, residual flows were mostly shoreward at the shoal and in the center of the channel. S tream normal flows were moving away from the bend at the surface and t oward the bend at the bottom. Thi s pattern is consistent with results by curvature effects caused by a headland (Kalkwijk and Booij, 1986; Geyer, W. 1993; Chant and Wilson, 1997; Chant, R. 2002; Georgas and Blumberg, 2003; Alaee et al. 2004) Streamwise fl ows obtained from the least square fit were then compared with the semianalytical solution. Since the balance of the model is between pressure gradient, friction and C oriolis it cannot replicate the outflow that is observed closer to the bend during the d ry season. But it can resemble how the exchange flow w ill be horizontally and vertically sheared. Deviations of the streamwise flow around its vertical mean were calculated to analyze flow curvature (Kalkwijk and Booij 1986; Geyer, W. 1993) During spring tides, higher tidal amplitude caused higher c entrifugal accelerations Coriolis accelerations followed the same structure of the deviations of the streamwise flow around its vertical mean, but were one order of magnitude smaller

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45 Results herein support the consideration of advection in the dynamics of curv ed flows. Near the headland, where advection in the form of c entrifugal acceleration was its strongest, it was one order of magnitude higher than Coriolis acceleration. However, Coriolis acceleration could not be neglected since it also pla yed a role in the dynamics. Baroclinic p ressure gradient wa s al so of comparable magnitude to centrifugal and Coriolis accelerations. T he resulting momentum balance was between pressure gradient, Coriolis and advection.

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46 CHA PTER 5 CONCLUSION The observ ed s tream normal circulation was due a direct result of the curvature effects caused by th e Point San Jose headland. C urvature effects were identified through lower layer flow directed toward the bend and the upper layer flow directed away from the bend, as described by Rozovski i 1957. Other observational evidence included the tilt ed isopycnals suggesting the influence of rotation (Coriolis) i n the flow dynamics. N o stratification exi sted in the evaluated data sets, and the water column was stable with higher values of density anomaly at the bottom. S treamwise gradients calculated with the surface and depth averaged streamwise flows, showed higher values and sharp variations closer to the headland (West) and at the shallowest section after the channel Rossby numbers indicated the dominance of advection ( in thi s case cen trifugal acceleration) over rotation (Coriolis). Th e baroclinic pressure gradient wa s included in the dynamics since its order of magnitude wa s comparable to the other terms In conclusio ns, the momentum balance was between the pressure gradient, advectio n and rotation.

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47 LIST OF REFERENCES Alaee, M. J., Ivey, G., & Pattiaratchi, C. (2004). Secondary circulation induced by flow curvature and Coriolis effects around headlands and islands. Ocean Dynamics (54) 27 38. doi:10.1007/s10236 003 0058 3 Chant, R. J. (1997). Secondary circulation in a highly stratified estuary. Journal of Geophysical Research 102 (C10) 23207 23215. Chant, R. J. (2002). Secondary circulation in a region of flow curvature: Relationship with t idal forcing and river discharge. Journal of Geophysical Research 107, (C9), 3131 141 1411. doi:10.1029/2001JC001082 Chen, Q., Wang, L., & Tawes, R. (2008). Hydrodynamic Response of Northeastern Gulf of Mexico to Hurricanes. Estuaries and Coasts 1098 1116. Currie, D. (1994). Ordenamiento de Camaronicultura Estero Real, Nicaragua. Panama: PRADEPESCA. Doyle, B. E., & Wilson, R. (1978). Lateral dynamic balance in the Sandy Hook to Rockawat Point transect. Estuarine and Coastal Marine Science 6 165 174. Emery, W. J., & Thomson, R. E. (2004). Data Analysis Methods in Physical Oceanography. Amsterdam: second ed. Elsevier. Georgas, N., & Blumberg, A. F. (2003). The influence of centrifugal and Coriolis in a curving estuary. Estuarine and Coastal mode ling (pp. 541 558). Monterey, California: American Society of Civil Engineers. Geyer, W. R. (1993). Three Dimensional tidal fow aund headlands. Journal of Geophysical Research 98 955 966. Huijts K., Schuttelaars, H. M., de Swart, H. E., & Friedrichs, C T. (2009). Analytical study of the transverse distribution of along channel and transverse residual flows in tidal estuaries. Continental Shelf Research 29 89 100. doi:10.1016/j.csr.2007.09.007 Joyce T. M. (1989). In situ "Calibration" of Ship Boards ADCPs. Journal of Atmospheric and Oceanic Technology 169 172. Kalkwijk, J., & Booij, R. (1986). Adaptations of secondary flow in nearly horizontal flow. Journal of Hydraulic Research 24 (1) 19 37. Mellor, G. L. (1996). Introduction to Physical Oceanography. Baltimore: Springer.

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48 Nidzieko, N. J., Hench, J. L., & Monishmith, S. G. (2009). Lateral circulation in a well mixed and stratified estuarine flows with curvature. Journal of Physical Oceanography 39 (4) 831 851. doi:10.1175/2008JPO4017.1 R ozovskii, I. L. (1957). Flow of Water in Bends of Open Channels. Israel Program for Scientific Translation Simpson, J. H., Brown, J., Matthews, J., & Allen, G. (1990). Tidal straining, density currents, and stirring in the control of estuarine stratifica tion. Estuaries 13 125 132. Simpson, J., Sharples, J., & Rippeth, T. P. (1991). A prescriptive model of stratification induced by freshwater runoff. Estuarine, Coastal and Shelf Science 33 23 35. Valle Levinson, A. (2008). Density driven exchange flow in terms of the Kelvin and Ekman numbers. Journal of Geophysical Research 113 C04001. doi:10.1029/2007LC004144 Valle Levinson, A. (2010). Contemporary Issues in Estuarine Physics. United Kingdom: University Press, Cambridge. Valle Levinson, A., & Reyes, C. (2003). Effects of bathymetry, friction, and rotation on estuary ocean exchange. Journal of Physical Oceanography 33 2375 2393. Valle Levinson, A., Wong, K. C., & Lwiza K. M. (2000). Forthnightly variability in the transverse dynamics of a c oastal plain estuary. Journal of Geophysical Research 105 3413 3424. Villela, L. (2001). Golfo de Fonseca. Conociendo las Areas Nucleo del Corredor Biologico. Guatemala: PROARCA/COSTAS Ward, G. (2000). Evaluation of Shrimp Farming Impacts in Golfo de F onseca Region, Honduras. Oregon State University. Corvallis, Oregon: Pond Dynamics/ Aquaculture CRSP. Waterhouse, A. F., & Valle Levinson, A. (2010). Transverse structure of subtidal flow in a weakly stratified subtroppical tidal inlet. Continental Shelf Research 30 281 292.

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49 BIOGRAPHICAL SKETCH Gisselle earned her Bachelor of Science in Environmental Engineering at the Technological University of Panama in 2009. Later in 2010 s he was awarded with a Fulb right Laspau scholarship for her m aster studies. Due to the importance of c oasts and o ceans at her country, Panama, she decided to pursue a m in c oastal an d oceanographic e ngineering at the University of Florida