Prepared for County of Sarasota Coastal

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FINAL REPORT LITTLE SARASOTA BAY CIRCULATION STUDY

Prepared for

County of Sarasota Coastal Zone Management Division Environmental Services Department 9250-110-RT

Contract No. C82-66

Prepared by

Stergios A. Dendrou Charles I. Moore Raymond Walton

CAMP DRESSER & MCKEE 7630 Little River Turnpike Annandale, Virginia 22003

August 1983

Suggested reference Camp Dresser & McKee and

Mote Marine Laboratory. 1983. Little Sarasota Bay circulation study. Sarasota County. Contract no C82-66. Mote Marine Laboratory Technical Report no 57. 175 p. Available from: Mote Marine Laboratory Library. TABLE OF CONTENTS

Section Page No.

LIST OF FIGURES ...... iii

LIST OF TABLES ...... ix

I INTRODUCTION ...... I-1 DESCRIPTION OF THE STUDY AREA ...... I-1 SCOPE OF WORK ...... I-3 REPORT OUTLINE ...... I-4

II RESULTS AND CONCLUSIONS ...... II-1

III LITTLE SARASOTA BAY MODEL ...... III-1 DYNAMIC ESTUARY MODEL ...... III-1 MODEL THEORY ...... III-1 Basic Hydrodynamic Equations ...... III-4 Numerical Solution-Stability ...... III-5 Boundary Conditions ...... III-6 LITTLE SARASOTA BAY GRID NETWORK ...... III-7 Geometric Input Data ...... III-7

IV PRELIMINARY ANALYSIS ...... IV-1 TIDES AND TIDAL PHASING ...... IV-1 WIND EFFECTS ...... IV-16 FRESHWATER INFLOWS ...... IV-17

V FIELD PROGRAM ...... V-1 INTRODUCTION ...... V-1 BATHYMETRIC SURVEY ...... V-1 TIDE GAGES ...... V-2 CURRENT VELOCITY STUDIES ...... V-5 DYE AND DROGUE STUDIES ...... V-9

VI MODEL CALIBRATION ...... VI-1 CALIBRATION PROCEDURE ...... VI-1 CALIBRATION RESULTS ...... VI-2 July Simulation Period ...... VI-3 April Simulation Period ...... VI-9 DISCUSSION ...... VI-9

VII MODEL VERIFICATION ...... VII-1 VERIFICATION PROCEDURE ...... VII-1 NOVEMBER SIMULATION RESULTS ...... VII-1 DISCUSSION ...... VII-19 TABLE OF CONTENTS (Continued)

Section Page No.

VIII MODEL APPLICATIONS ...... VIII-1 ANALYSIS OF BAY CIRCULATION ...... VIII-1 "NO-NAME" STORM ANALYSIS ...... VIII-18 Introduction ...... VIII-18 Simulation of Storm ...... VIII-23 Simulation Results ...... VIII-25 IMPACTS OF INTRACOASTAL WATERWAY ON BAY CIRCULATION .... VIII-43 CONSEQUENCES OF MIDNIGHT PASS DREDGING ...... VIII-46 CONVECTION AND DISPERSION CHARACTERISTICS AROUND BIRD ISLAND - DYE STUDY ...... VIII-62

IX CONCLUSIONS AND RECOMMENDATIONS ...... IX-1

REFERENCES

APPENDICES (Separate Volume)

A Preliminary Model Inputs B Bathymetric Survey Data C Reduced Tide Data for Four Study Periods D Field Data E Final Model Inputs F Velocity Vector Plots for April and July Periods G Program Listing

ii LIST OF FIGURES

Figure No. Page No.

I-1 Little Sarasota Bay ...... I-2

III-1 Definition Sketch - Node-Link Estuary Model ...... III-2

III-2 Little Sarasota Bay DEM Link Node Network ...... III-8

IV-1 Predicted High and Low Tides at St. Petersburg, April 1982 ...... IV-7

IV-2 Diurnal Tide at all Boundaries ...... IV-8

IV-3 Semi-Diurnal Tide at all Boundaries ...... IV-10

IV-4 Semi-Diurnal Tide in Phase at Stickney and Blackburn Points ...... IV-12

IV-5 Semi-Diurnal Tide with Stickney Point Lagged One Half- Hour from Blackburn Point ...... IV-13

IV-6 Semi-Diurnal Tide with Stickney Point Lagged Two Hours from Blackburn Point ...... IV-14

IV-7 Effect of 50 mph Wind from Stickney Point ...... IV-18

IV-8 Little Sarasota Bay Drainage Subbasins ...... IV-19

V-1 Bathymetry Transects ...... V-3

V-2 Location of Tidal Gages and Velocity Measurement Stations ...... V-4

VI-1 Observed vs. Simulated Elevations at Gage No. 2, July 13-14, 1982 ...... VI-5

VI-2 Observed vs. Simulated Elevations at Gage No. 3, July 13-14, 1982 ...... VI-6

VI-3 Observed vs. Simulated Elevations at Gage No. 4, July 13-14, 1982 ...... VI-7

VI-4 Observed vs. Simulated Elevations at Gage No. 5, July 13-14, 1982 ...... VI-8

VI-5 Observed vs. Simulated Velocity at Station No. 1, July 13-14, 1982 ...... VI-10

VI-6 Observed vs. Simulated Velocity at Station No. 2, July 13-14, 1982 ...... VI-11

iii LIST OF FIGURES (Continued)

Figure No. Page No.

VI-7 Observed vs. Simulated Velocity at Station No. 3, Magnitude and Direction, July 13-14, 1982 ...... VI-12

VI-8 Observed vs. Simulated Velocity at Station No. 4, Magnitude and Direction, July 13-14, 1982 ...... VI-13

VI-9 Observed vs. Simulated Velocity at Station No. 5, July 13-14, 1982 ...... VI-14

VI -10 Observed vs. Simulated Velocity at Station No. 6, July 13-14, 1982 ...... VI-15

VI -11 Observed vs. Simulated Velocity at Station No. 7, July 13-14, 1982 ...... VI-16

VI-12 Observed vs. Simulated Velocity at Station No. 8, July 13-14, 1982 ...... VI-17

VI-13 Observed vs. Simulated Velocity at Station No. 9, July 13-14, 1982 ...... VI-18

VI-14 Observed vs. Simulated Velocity at Station No. 10, Magnitude and Direction, July 13-14, 1982 ...... VI-19

VI-15 Observed vs. Simulated Velocity at Station No. 11, Magnitude and Direction, July 13-14, 1982 ...... VI-20

VI-16 Observed vs. Simulated Velocity at Station No. 12, July 13-14, 1982 ...... VI-21

VI-17 Observed vs. Simulated Elevation at Gage No. 2, April 28- 29, 1982 ...... VI-22

VI-18 Observed vs. Simulated Elevation at Gage No. 3, April 28- 29, 1982 ...... VI-23

VI-19 Observed vs. Simulated Elevation at Gage No. 4, April 28- 29, 1982 ...... VI-24

VI-20 Observed vs. Simulated Elevation at Gage No. 5, April 28- 29, 1982 ...... VI-25

VI-21 Observed vs. Simulated Velocity at Station No. 1, April 28-29, 1982 ...... VI-26

VI-22 Observed vs. Simulated Velocity at Station No. 2, April 28-29, 1982 ...... VI-27

iv LIST OF FIGURES (Continued)

Figure No. Page No.

VI-23 Observed vs. Simulated Velocity at Station No. 3, Magnitude and Direction, April 28-29, 1982 ...... VI-28

VI-24 Observed vs. Simulated Velocity at Station No. 4, Magnitude and Direction, April 28-29, 1982 ...... VI-29

VI-25 Observed vs. Simulated Velocity at Station No. 5, April 28-29, 1982 ...... VI-30

VI-26 Observed vs. Simulated Velocity at Station No. 6, April 28-29, 1982 ...... VI-31

VI-27 Observed vs. Simulated Velocity at Station No. 7, April 28-29, 1982 ...... VI-32

VI-28 Observed vs. Simulated Velocity at Station No. 8, April 28-29, 1982 ...... VI-33

VI-29 Observed vs. Simulated Velocity at Station No. 9, April 28-29, 1982 ...... VI-34

VI-30 Observed vs. Simulated Velocity at Station No. 10, Magnitude and Direction, April 28-29, 1982 ...... VI-35

VI -31 Observed vs. Simulated Velocity at Station No. 11, Magnitude and Direction, April 28-29, 1982 ...... VI-36

VI-32 Observed vs. Simulated Velocity at Station No. 12, April 28-29, 1982 ...... VI-37

VI-33 Velocity and Vorticity at the Centroid of a Grid Element VI-40

VI-34 Vorticity Calculations at Station 4 ...... VI-41

VI-35 Vorticity Field and Sub-Grid Scale Vorticity ...... VI-44

VII-1 Observed vs. Simulated Elevation at Gage No. 2, November 10-11, 1982 ...... VII-3

VII-2 Observed vs. Simulated Elevation at Gage No. 3, November 10-11, 1982 ...... VII-4

VII-3 Observed vs. Simulated Elevation at Gage No. 4, November 10-11, 1982 ...... VII-5

VII-4 Observed vs. Simulated Elevation at Gage No. 5, November 10-11, 1982 ...... VII-6

V LIST OF FIGURES (Continued)

Figure No. Page No.

VII-5 Observed vs. Simulated Velocity at Station No. 1, November 10-11, 1982 ...... VII-7

VII-6 Observed vs. Simulated Velocity at Station No. 2, November 10-11, 1982 ...... VII-8

VII-7 Observed vs. Simulated Velocity at Station No. 3, Magnitude and Direction, November 10-11, 1982 ...... VII-9

VII-8 Observed vs. Simulated Velocity at Station No. 4, Magnitude and Direction, November 10-11, 1982 ...... VII-10

VII-9 Observed vs. Simulated Velocity at Station No. 5, November 10-11, 1982 ...... VII-11

VII-10 Observed vs. Simulated Velocity at Station No. 6, November 10-11, 1982 ...... VII-12

VII-11 Observed vs. Simulated Velocity at Station No. 7, November 10-11, 1982 ...... VII-13

VII-12 Observed vs. Simulated Velocity at Station No. 8, November 10-11, 1982 ...... VII-14

VII-13 Observed vs. Simulated Velocity at Station No. 9, November 10-11, 1982 ...... VII-15

VII-14 Observed vs. Simulated Velocity at Station No. 10, Magnitude and Direction, November 10-11, 1982 ...... VII-16

VII-15 Observed vs. Simulated Velocity at Station No. 11, Magnitude and Direction, November 10-11, 1982 ...... VII-17

VII-16 Observed vs. Simulated Velocity at Station No. 12, November 10-11, 1982 ...... VII-18

VIII-1 Simulated Boundary Flows, April 28-29, 1982 ...... VIII-8

VIII-2 Simulated Boundary Flows, July 13-14, 1982 ...... VIII-9

VIII-3 Simulated Boundary Flows, November 10-11, 1982 ...... VIII-10

VIII-4 Simulated Velocity Vectors at Full Flood Flow, July 14, 1982 ...... VIII-12

VIII-5 Simulated Velocity Vectors at Flood Flow Near High Tide, July 14, 1982 ...... VIII-13

vi LIST OF FIGURES (Continued)

Figure No. Page No.

VIII-6 Simulated Velocity Vectors at High Tide, July 14, 1982. VIII-14

VIII-7 Simulated Velocity Vectors at Ebb Tide Near High Tide, July 14, 1982 ...... VIII-15

VIII-8 Simulated Velocity Vectors at Full Ebb Tide, July 14, 1982 ...... VIII-16

VIII-9 Observed Barometric Pressure, Wind Velocity, Magnitude and Direction for "No-Name" Storm ...... VIII-21

VIII-10 Measured Marigrams for "No-Name" Storm ...... VIII-22

VIII-11 "No-Name" Storm Tide, Surge and Predicted Astronomic Tide at Gage 5 ...... VIII-24

VIII-12 Observed vs. Simulated Tides at Gage 2 for "No-Name" Storm ...... VIII-26

VIII-13 Observed vs. Simulated Tides at Gage 3 for "No-Name" Storm ...... VIII-27

VIII-14 Observed vs. Simulated Tides at Gage 4 for "No-Name" Storm ...... VIII-28

VIII-15 Observed vs. Simulated Tides at Gage 5 for "No-Name" Storm ...... VIII-29

VIII-16 Interaction of Embayments During High Wind Conditions. VIII-31

VIII-17 Simulated Boundary Flows for "No-Name" Storm ...... VIII-32

VIII-18 Simulated Velocity Vectors at 1:00 AM for "No-Name" Storm ...... VIII-33

VIII-19 Simulated Velocity Vectors at 2:00 AM for "No-Name" Storm ...... VIII-34

VIII-20 Simulated Velocity Vectors at 3:00 AM for "No-Name" Storm ...... VIII-35

VIII-21 Simulated Velocity Vectors at 4:00 AM for "No-Name" Storm ...... VIII-36

VIII-22 Simulated Velocity Vectors at 5:OO AM for "No-Name" Storm ...... VIII-37

VIII-23 Simulated Velocity Vectors at 6:00 AM for "No-Name" Storm ...... VIII-38

vii LIST OF FIGURES (Continued)

Figure No. Page No.

VIII-24 Simulated Velocity Vectors at 7:00 AM for "No-Name" Storm ...... VIII-39

VIII-25 Simulated Velocity Vectors at 8:00 AM for "No-Name" Storm ...... VIII-40

VIII-26 Simulated Velocity Vectors at 9:00 AM for "No-Name" Storm ...... VIII-41

VIII-27 Simulated Velocity Vectors at 10:00 AM for "No-Name" Storm ...... VIII-42

VIII-28 Scenario 1 Simulated Velocity Vectors at Full Flood Flow, July 14, 1982 ...... VIII-52

VIII-29 Scenario 1 Simulated Velocity Vectors at Flood Flow Near High Tide, July 14, 1982 ...... VIII-53

VIII-30 Scenario 1 Simulated Velocity Vectors at High Tide, July 14, 1982 ...... VIII-54

VIII-31 Scenario 1 Simulated Velocity Vectors at Ebb Flow near High Tide, July 14, 1982 ...... VIII-55

VIII-32 Scenario 1 Simulated Velocity Vectors at Full Ebb Flow, July 14, 1982 ...... VIII-56

VIII-33 Scenario 2 Simulated Velocity Vectors at Full Flood Flow, July 14, 1982 ...... VIII-57

VIII-34 Scenario 2 Simulated Velocity Vectors at Flood Flow near High Tide, July 14, 1982 ...... VIII-58

VIII-35 Scenario 2 Simulated Velocity Vectors at High Tide, July 14, 1982 ...... VIII-59

VIII-36 Scenario 2 Simulated Velocity Vectors at Ebb Flow near High Tide, July 14, 1982 ...... VIII-60

VIII-37 Scenario 2 Simulated Velocity Vectors at Full Ebb Flow, July 14, 1982 ...... VIII-61

VIII-38 Dye Study at Midnight Pass ...... VIII-64

VIII-39 Dye Study at Midnight Pass ...... VIII-65

VIII-40 Dye Study at Blackburn Point ...... VIII-66

VIII-41 Dye Study at Blackburn Point ...... VIII-67

viii LIST OF FIGURES (Continued)

Figure No. Page No.

VIII-42 Dye Study - Predicted and Observed Plume after Four Hours ...... VIII-69

VIII-43 Dye Study - Concentration vs Time at Various Nodes . . . . . VIII-70

ix LIST OF TABLES

Table No. Page No.

I-1 Little Sarasota Bay Characteristics ...... I-4

IV-1 Tidal Differences Between Venice Inlet and Sarasota, and St. Petersburg ...... IV-2

IV-2 Predicted Tides at St. Petersburg, Florida, 1982 ...... IV-3

IV-3 Rainfall Duration-Frequency ...... IV-17

V-1 Tide Gage Location ...... V-6

V-2 Current Stations Location ...... V-8

VI-1 Standard and Maximum Errors Between Simulated and Observed Hydrographic Data - July and April Periods ...... VI-4

VI-2 Vorticity Calculations at Station 4, July 14, 1982 . . . . . VI-42

VII-1 Standard Error and Maximum Errors Between Simulated and Observed Hydrographic Data - November Period ..... VII-2

VIII-1 Time and Elevation of Observed High and Low Tides ...... VIII-3

VIII-2 Simulated Boundary Flood and Ebb Flows ...... VIII-6

VIII-3 Simulated Bay Volumes ...... VIII-7

VIII-4 Net Simulated Flow Through Boundaries ...... VIII-17

VIII-5 Simulated Bay Volumes, "No-name" Storm ...... VIII-43

VIII-6 Pre- and Post-Intracoastal Waterway Average Maximum Flood and Ebb Velocities through Passes ...... VIII-45

VIII-7 Simulated Flows and Maximum Velocities for Dredging Scenarios at Stickney Point ...... VIII-49

VIII-8 Simulated Flows and Maximum Velocities for Dredging Scenarios at Midnight Pass ...... VIII-50

VIII-9 Simulated Flows and Maximum Velocities for Dredging Scenarios at Blackburn Point ...... VIII-51 I. INTRODUCTION

This report presents the results of the Little Sarasota Bay Circulation Study, performed for the County of Sarasota, Coastal Zone Management Division, Environmental Services Department. This study encompassed the development of a numerical computer model of the Bay circulation and the design of the field program to provide in situ data for the calibration and verification of the model. The data and the calibrated model were used to analyze the Bay circulation and the impacts of various past and possible future developments on the circulation.

DESCRIPTION OF THE STUDY AREA

Little Sarasota Bay is one of many interconnected embayments that extend along the Gulf Coast of Southwest Florida. Like many others of these embayments, Little Sarasota Bay has been greatly impacted by the changes imposed by man. These changes are mostly associated with the urbanization and development of this area of the State and include: the dredging of channels, the most important being the Intracoastal Waterway maintained by the U.S. Army Corps of Engi- neers; the stabilization of the shorelines; and the dredging and stabilization of the passes to the open Gulf.

Little Sarasota Bay is a relatively small body of water. At its northern end, Stickney Point, the bay is connected to the larger Sarasota Bay and Big Sara- sota Pass through Roberts Bay. South of Little Sarasota Bay is Dryman Bay, Blackburn Bay and the Venice Inlet. Blackburn Point forms the southernmost point of Little Sarasota Bay. Midnight Pass is the only direct Pass between Little Sarasota Bay and the Gulf of Mexico. The Bay and the Gulf are separated by Siesta Key, north of Midnight Pass, and by Casey Key south of the Pass. A map of the Bay is presented in Figure I-1.

Midnight Pass is a natural pass that has not been physically controlled or sta- bilized by man. Like all natural passes, its location, size, and shape are the

I-1 result of the interaction of the Bay and the Gulf through the tidal oscillations and currents, and wave forces predominant in the Gulf of Mexico. Evident- ly, the various forces that act on the Pass are not in a state of stable equi- librium since its location, depth, and other characteristics are continuously changing over the period of weeks and months. The shallowness of this pass and the constantly moving channels and sand bars make the Pass unnavigable by all but small boats. There is recurring pressure to dredge and stabilize this entrance to allow the passage of larger vessels. One objective of this study is to assess the effect that dredging Midnight Pass would have on the circulation within the Bay.

The Intracoastal Waterway traverses the length of the Bay. This channel has a project depth of nine feet Mean Low Water (MLW) and width of 100 feet. The Waterway plays an important role in the circulation of the Bay. The Intra- coastal Waterway also forms the connection to Big Sarasota Pass to the north, and Venice Inlet to the south. It is conjectured that the Intracoastal Water- way with the associated enhanced connection to Venice Inlet and Big Sarasota Pass may have contributed to the destabilization of Midnight Pass by changing the mode of oscillation of the Bay and reducing the velocities and flows through the Pass. The impact of this past dredging is also analyzed.

The water in Little Sarasota Bay is shallow. Neglecting the Intracoastal Water- way, the deepest point is six feet MLW. The average depth of the Bay is 3.8 feet MLW. Numerous shallow areas, peninsulas, mangrove islands and oyster bars effectively divide the Bay into several smaller embayments. Some characteristics of the Bay are shown in Table I-1.

SCOPE OF WORK

The objectives of this study are to analyze of the circulation in Little Sara- sota Bay and to develop a numerical predictive model of the most important processes governing the Bay circulation. This model will then be used for plan- ning, assessing and designing management alternatives affecting commercial, recreational and other uses of the Bay. In developing this model, the following tasks were performed: (1) a preliminary analysis for the determination of the most important processes governing the Bay circulation; (2) design of a

I-3 Table I-1 LITTLE SARASOTA BAY CHARACTERISTICS

Length of Intracoastal Waterway 5.00 N. Mi. Average Width 0.75 N. Mi. Surface Area 102,000,000 Sq. Ft. 2,350 Acres 3.67 Sq. Mi. 2.77 Sq. N. Mi. Volume* 517,000,000 Cu. Ft. Average Depth* 5.0 Feet Maximum Depth** 7.2 Feet Area of Land Surface Draining to Bay 15.0 Sq. Mi.

* Assumed water surface 1.2 feet above Mean Low Water (1954 survey), which is approximate Mean High Water (1960-1978 tidal epoch). ** Depth at approximate Mean High Water (19604978 tidal epoch) neglecting Intracoastal Waterway. Project Depth of Intracoastal Waterway = 9.0 feet MLW. field program; (3) model set-up, calibration, verification; and (4) sample model applications.

REPORT OUTLINE

Section III of this report summarizes the theory of the Dynamic Estuary Model (DEM) and the network development for Little Sarasota Bay. Section IV presents a preliminary analysis of Bay circulation, on the basis of which the field program was designed. The design of the field program is presented in Section V. Sections VI and VII present the model calibration and verification respectively. Section VIII illustrates four applications of the Model in analyzing simplified scenarios of Bay management. Finally, Sections IX and II present extended and abbreviated conclusions of this study and recommendations for the

I-4 use and the further development of the results of this study. A separate volume of appendices contains a compilation of all relevant data assembled for the study.

I-5 II. RESULTS AND CONCLUSIONS

The study of the Little Sarasota Bay circulation included the development of a computer model of the Bay hydraulics, the collection of field data by the in- stallation of gages and by field surveys, the calibration and verification of the model, and four model applications. The main results, conclusions and recommendations are summarized below:

The main tidal forcing occurs at the north and south passes, Stickney Point and Blackburn Point respectively; Midnight Pass provides minor tidal exchange with the Gulf of Mexico.

All gages are tidally in phase within 10-20 minutes; the Bay experiences a unimodal oscillation, rising and falling as one body of water.

This results in, and explains the shallows and bars in the mid-section of the Bay.

Flows in the Bay are predominantly affected by the presence of the Intracoastal Waterway; flows and velocities occur mainly along the Intracoastal and other channels.

Persistent vorticity is developed during ebbing tide in the tidal flats of the northern Bay; its effect on the Bay mixing properties may prove significant.

Little Sarasota Bay Circulation is intimately associated with conditions in the Gulf, and Sarasota Bay and Blackburn Bay; changes and modifications in one will affect the others.

I I-l The DEM model accurately simulates processes affecting Bay circulation; in future scenario studies attention should be placed on address- ing all important factors, especially those omitted in the present version of the model.

Some of these omitted factors are: interaction with other bays; calibrated wind stress term for storm conditions; systematic interpreta- tion of sub-grid scale vorticity for resultant velocity at any point within the Bay.

Recommendations for future work are as follows:

Inclusion in present model of above mentioned missing factors.

Application of a mass transport model to Little Sarasota Bay to include: salinity effects, the simulation of conservative and non- conservative constituents, and the analysis of vorticity and its effect on Bay mixing and dispersion properties.

Analysis of Little Sarasota Bay interaction with neighboring bays and embayments.

In-depth analysis of Midnight Pass dredging and stabilization including: impacts on circulation in Little Sarasota and neighboring bays, the cause of recent pass movement, future pass stability, and impacts on shoaling within the Bay.

Adaptation of the DEM model and/or plotting capabilities to a microcomputer for in-house use.

II-2 III. LITTLE SARASOTA BAY MODEL

DYNAMIC ESTUARY MODEL (DEM)

DEM is a two-dimensional, vertically integrated, hydrodynamic model based on a link-node discretization of the computational domain (3). The model used was originally developed by Water Resources Engineers, (later incorporated with CDM), as modified for the New York Flood Insurance Study (4). The modification that is important to this study is the improvement of the treatment of the boundary conditions to obtain accurate initial transient simulations from a flat sea cold start. Several modifications were made to the program output to provide the time history of the Bay volume and the time and magnitude of the maximum and minimum volumes, as well as the maximum and minimum tidal elevations at all nodes. Additional changes include the option of making a line printer map of the link node network and the plotting of time histories for selected nodes.

MODEL THEORY

The hydrodynamic behavior of a water body influenced by external forces is governed by two fundamental equations, namely, the momentum equation and the continuity equation. A complete description of the movement of water requires a simultaneous solution of the two equations. In order to solve these two coupled equations, the program utilizes a link and node discretization of the prototype water body, Figure 111-1 A reach or link is defined as a channel where water flows from one end to the other. The ends of the reach are further defined as nodes.

If the heads at the end nodes at any given time are known, the hydraulic gradient along the channel and thus the flow rate may be calculated. By the trans- fer of water from one end to the other according to the computed flow, the head at the node will subsequently be altered, defining a stepwise hydraulic computation on the discretized system representing the water body.

III-1 Figure III-1 Definition Sketch - Node-Link Estuary Model.

III-2 The nodes are defined by the surface area, depth and starting water surface elevations. The nodal surface area is defined as the area enclosed by the perpendicular bisectors of its connecting links and the coastline for nodes near the shoreline. To assure mass conservation, the sum of the surface areas for all nodes must equal the total surface area of the water body. For areas in which the surface area varies greatly with water elevation, the program has an option to vary the surface area with depth. The depth of the node is defined as the average depth of the area represented by the node. This depth multi- plied by the surface area represents the nodal volume, and the total Bay volume is computed as the sum of all nodal volumes. Depths on charts are referenced to some datum, usually Mean Low Water (MLW). A correction factor is included to correct this to other data. It is advantageous to reference all elevations to the National Geodetic Vertical Datum of 1929 (NGVD) since this is the one standard datum that does not change with time or space. Unless special conditions warrant the use of other starting elevations, the starting water surface is specified at Mean Sea Level (MSL). The nodes are also identi- fied by an X and Y coordinate. Although not strictly needed by the computational scheme, this information is used to compute and plot link information if desired and to compute the wind effects.

The hydraulic computation requires the following information for links: the width, length, depth or hydraulic radius and friction coefficient (Manning's n). Values for these parameters are selected to represent the conveyance characteristics of the water body between the nodes. For a two-dimensional water body, the appropriate width is often difficult to assess. For consistency, the width is defined by the length of the perpendicular bisector of the channels between their intersections with adjacent bisectors. The depth of the link represents the controlling depth for the link or channel. Often this is the average of the depths of the two end nodes and DEM will compute it if unspecified. The length of the link is also computed from the X and Y coordinates of the end nodes if it is unspecified. The Manning's n friction coefficient is the only calibration parameter of the model assuming a correct spatial geometric-discretization of the water body. The range of hydraulically plausible values for Manning's n is between 0.015 and 0.08. Finally, identification of the nodes at the ends of the link are also needed in order to define the connectivity of the network that is used to model the Bay.

III-3 Basic Hydrodynamic Equations

The one-dimensional momentum equation is written as follows:

V = Velocity t = Time x = Distance H = Water surface elevation from the datum plane g = Gravitational acceleration Sf = Energy gradient W = Wind stress

The energy gradient S of turbulent flow is proportional to the square of the mean velocity according to Manning's equation;

(III-2) where;

n = Friction coefficient (Manning's n) R = Hydraulic radius of the channel

The wind stress term is given by:

(III-3)

The second equation necessary to complete the mathematical formulation of the problem is furnished by the continuity equation, which states that the net

III-4 effect of water flowing into a node through links or importation, is to raise the water surface elevation at the node, i.e.,

(III-4) where;

Asj = Surface area associated with the junction j Qi = Flow of a connecting channnel Qj = Water importation rate to the junction k = number of links entering junction j

Numerical Solution-Stability

Numerical solution of Equations III-1 and III-4 entails a rewriting of both equations in finite difference form. Two types of integration are necessary for their solution, namely, a space integration and a time integration. A stepwise procedure is used for the spatial integration of Equations III-1 and 111-4, whereby the momentum equation is used to solve for link flow, and the continuity equation is used to solve for the nodal head. Both equations are explicitly solved for the unknown variables. The hydraulic computations within a given time interval proceed as follows:

1. Compute the flow rate in each link according to the hydraulic gradient and other hydraulic conditions existing at the beginning of a time interval.

2. Compute the rise or fall of the water surface (head) at each node based on the link flow and the importation or withdrawal of water at the node.

3. Update the geometric and hydraulic conditions for the computation of the next time interval.

Time integration is performed numerically by using a modified Euler technique. Each computational time-step is subdivided into two parts, whereby the time

III-5 rate of change of a variable calculated at the half time-step is used to project the variable from one time-step to the next full time-step. The intermedi- ate time-step computations improve the stability and accuracy of the model. Stability of the scheme for each link is governed by the Courant-Friederichs- Lewy condition:

(111-5)

= Computation time-step = Length of link = Gravitational acceleration = Depth of water = Celerity of the free gravity wave

An efficient discretization has an equal maximum allowable (stable) time-step for all links. Therefore, shallow links should have shorter lengths and deep areas should have longer links. The stability of the entire model is governed by the time-step of the most critical link. The above stability condition provides a guide to the link lengths to be used but may often be exceeded without producing stability problems in the DEM model.

Boundary Conditions

Two types of boundary conditions exist in the DEM model, namely flow boundary conditions (known discharge) and tidal boundary conditions (known water elevations). Flow boundary conditions are used for the head-waters of rivers, whereas tidal boundary conditions are used at the mouth of estuaries. Astrono- mic tidal boundary conditions can be developed by the program by fitting a least squares curve to the observed highs and lows of a given tidal cycle. The program has also the option of reading actual measured marigrams and linearly interpolating between the appropriate values corresponding to the integration time-steps. The option is also available to switch from a fitted astronomic tide to a measured marigram at any given day of the simulation. All boundaries at which the tidal elevations are not specified react as reflecting boundaries (i.e. as a vertical wall).

III-6 LITTLE SARASOTA BAY GRID NETWORK

The input data to the link node model consist of detailed geometric data of the spatial discretization of the study area and hydrodynamic variables which characterize the conveyance properties of the network. In addition, the input data stream controls the computational characteristics of the model (time- step, boundary conditions, etc.) and the extent of the printed output. A listing of the complete input data set of this preliminary Little Sarasota Bay Grid Network is included in Appendix A.

Geometric Input Data

The most significant task in developing the data base for the Link-Node Model is the establishment of the physical network of channels and junctions. This configuration requires, among other things, the careful consideration of the bathymetry and coastline of the area, the required degree of resolution, and the inherent limitations of the model. In this last category, the maximum time- step, computer memory requirements, and the corresponding cost per run are important considerations.

The network for Little Sarasota Bay is shown in Figure 111-2. The three boundaries with outside tidal influence are located at Stickney Point, Midnight Pass and Blackburn Point. The location of the link representing Midnight Pass reflects the “present" (May, 1982) location of the inlet. In the preliminary model the network does not include the links and nodes representing the Gulf of Mexico, Blind Pass, and the rivers and canal inflow points shown on Figure 111-2, with the Bay being discretized by 67 nodes and 141 links. These additional 19 nodes and 45 links were added to the final Little Sarasota Bay Circulation Model as reported in Section VI. In this preliminary model the geometry and bathymetry are based on the 1954 hydrographic survey by the U.S. Coast and Geodetic Survey with the Intracoastal Waterway deepened to its present depth. Neglecting the Midnight Pass area, this data accurately reflects the modern bathymetry. The hydrodynamic parameters associated with the links and nodes, reported in Appendix A, represent the average or controlling value for the area of the Bay that the link or node represents. The Manning's n roughness coefficient was assigned a value of 0.03 for all

III-7 Figure III-2

III-8 links, a number that reflects the roughness effects of shallow waters. This model was used to make several sensitivity runs in analyses that are described in Section IV. The model was subsequently modified upon completion of the bathymetric survey to more accurately represent existing conditions. The finalized model also includes the extension of the network into the Gulf, canals, and rivers, and a variable roughness coefficient as described in Section VI.

III-9 IV. PRELIMINARY ANALYSIS

The previously described model of Little Sarasota Bay was used in several preliminary simulation runs. These runs allowed a preliminary assessment of the sensitivity of the circulation to tidal height and phase at the boundaries of the Bay, to wind forcing, and to freshwater inflows. This preliminary phase of the project was designed to:

0 Develop and test the numerical grid. O Prepare the hydrodynamic model for its application to Little Sarasota Bay. O Perform sensitivity tests to gain an understanding of the Bay circulation and its sensitivity to various factors.

These results are qualitative in nature but are very important to designing an efficient field program.

TIDES AND TIDAL PHASE

The tides of Little Sarasota Bay, which are directly influenced by the tides of the Gulf of Mexico, are very complex. The tide predictions provided by the National Ocean Survey (1) were the sole source of tide data available at the time of the preliminary analysis. Predictions for Venice Inlet and Sarasota are available based on the predicted tides at St. Petersburg. Time and height difference for these two locations to St. Petersburg are presented in Table IV-1. Using this table, predictions of the time of occurrence and height of the high and low tides at these locations can be obtained. However, the correc- tions to predict the tides at the tidal boundaries of Little Sarasota Bay, namely Stickney Point, Midnight Pass and Blackburn Point, are unknown. Such prediction could be derived from data collected during this study.

IV-1 TABLE IV-1. TIDAL DIFFERENCES BETWEEN VENICE INLET AND SARASOTA, AND ST. PETERSBURG (FROM REF. 1)

Differences Location Time Height Ratio High water Low Water High Water Low Water (Hrs) (Hrs)

Venice Inlet (inside) -2.03 -1.63 0.91 0.91 Sarasota, Sarasota Bay -1.63 -0.97 0.91 0.91

The predicted tides for 1982 for St. Petersburg is given in Table IV-2 The tides for April 1982 at St. Petersburg are plotted in Figure IV-1. This figure represents a typical lunar cycle. The tide along the Gulf Coast of Florida shows a mixture of both diurnal and semi-diurnal modes. The tide is semi- diurnal when the moon is on the equator (1). Note that the diurnal and semi- diurnal tides alternate twice each lunar cycle. Also, the tidal range during the diurnal tides tends to be larger than the semi-diurnal.

A simple way to analyze the effects of the tidal range is by considering the tidal volume exchange during a tidal cycle. The average depth of the Bay is five feet. Thus, during an average tidal excursion of 1.5 feet, approximately 30 percent of the total Bay volume is exchanged. In the semi-diurnal mode, the exchange takes place in approximately one half the duration of the diurnal mode, and consequently, the flood and ebb tide velocities are proportionally increased, assuming an equal tidal excursion.

Simulation runs were made to compare the circulation of the Bay under diurnal and semi-diurnal conditions with the forcing at all three boundaries in phase. The diurnal tide induced at the boundaries shown in Figure IV-2 has a tidal excursion of 2.7 feet, and produces a net intertidal volume exchange of 2.667 x 108 cubic feet, approximately 51 percent of the approximate Mean High Water Bay volume as reported on Table I-1. The total Bay volume variation with time is also shown in Figure IV-2. Since the tides are included at all three boundaries in phase, the flow rates through them are approximately equal. The peak

IV-2 Table IV-2 Predicted Tides at St. Petersburg, FLA., 1982 (1)

Time meridian 75° W. 0000 is Midnight. 1200 is noon. Heights are referred to "mean lower low water (called Gulf Coast LOW Water Datum) which is the chart datum of soundings.

IV-3 Table IV-2 Continued

Time meridian 75° W. 0000 is midnight. 1200 is noon. Heights are referred to mean lower low water (called Gulf Coast Low Water Datum) which iS the chart datum of soundings.

IV-4 Table IV-2 Continued

Time meridian 75° W. 0000 is midnight. 1200 is noon. Heights are referred to mean lower low water (Called Gulf Coast Low Water Datum) which is the chart datum of soundings.

IV-5 Table IV-2 Continued

Time meridian 75° W. 0000 iS midnight. 1200 is noon. Heights are referred to mean lower low water (called Gulf Coast Low Water Datum) which is the chart datum of soundings.

IV-6 IV-7 Figure IV-2 Diurnal Tide at all Boundaries

IV-8 velocities through Stickney Point, Midnight Pass and Blackburn Point are 1.31, 1.64 and 1.69 feet per second respectively.

The semi-diurnal tide, shown in Figure IV-3, has a tidal excursion of 1.79 feet. This tide produces a net intertidal volume of 1.791 x 108 cubic feet or 35 percent of the approximate Mean High Water volume. Again, the flow rates through the passes are approximately equal. The variation of the rates of inflow with time is also shown in Figure IV-3 as well as the Bay volume history. The peak velocities at Stickney Point, Midnight Pass and Blackburn Point are 1.56, 2.05 and 2.04 feet per second respectively. Although the tidal excursion, and hence the intertidal volume for the semi-diurnal tide is smaller than that of the diurnal tide, the velocities and flow rates at the passes are larger due to the shorter period of the tidal cycle.

The role of the three passes in influencing the tidal and other circulation characteristics of Little Sarasota Bay is very important in attempting to analyze the impacts of various future scenarios. Two additional simulations were made using the semi-diurnal tide of Figure IV-3. First, the tide was induced at Midnight Pass only, with the other two boundaries left free to oscillate. In a second run, this tide was induced in phase at Blackburn and Stickney Points while Midnight Pass was left free to oscillate. One of the expected results was that the intertidal volumes for both these simulations were smaller because of the reduced inlet capacity from the case where tidal forcing was imposed on all three passes. The run with the forcing at Blackburn Point and Stickney Point produced an intertidal volume of 1.787 x 108 cubic feet, which corresponds to a 0.22 percent decrease from the previous base run. In comparison, the run with the forcing at Midnight Pass produced an intertidal volume of 1.453 x 108 cubic feet, that is, an 18 percent decrease from the base run. This analysis shows that while all three cases are equally plausible from a hydraulic standpoint, it is more likely that all three boundaries play an important role, although Midnight Pass may not be as important as the other two Passes.

The phase difference between the tides at the three boundaries is a matter of conjecture at the present time. However, it can be safely said that high tide outside of Midnight Pass, which is located between Venice Inlet and Sarasota,

IV-9 Figure IV-3 Semi-Diurnal Tide at all Boundaries IV-10 would occur sometime between the high tides at those locations. The tide at Blackburn Point is very likely propagated from Venice Inlet. Since the general direction of tide propagation in the Gulf of Mexico is from South to North, it is likely that the tides at Blackburn Point and Midnight Pass would occur approximately in phase.

Stickney Point, on the other hand, is probably affected by Big Sarasota Pass near Sarasota. The tide at Sarasota occurs 0.4 hours after the tide at Venice Inlet. This delay, plus the time required for the tide to propagate through Roberts Bay to Stickney Point, could result in a time lag of up to one hour with respect to the tide at Midnight Pass and Blackburn Point. Of course, all above arguments are approximate, since the interactions at the three boundaries have not been considered, and the travel times are based on the celerity of a free gravity wave.

Several runs were made to investigate the effect of phase differences between the tides at Stickney Point and Blackburn Point. The results of three runs with the tides induced at Blackburn and Stickney Points with Midnight Pass un- forced are shown in Figures IV-4, IV-5 and IV-6. Figure IV-4 shows the base run where the boundaries are in phase. Note that the flow rates at the boundaries are nearly equal. Figure IV-5 shows the effect of the tide at Stickney Point being lagged by one half hour. The tidal exchange volume is nearly equal to the case where the tides are in phase. The flows in the leading boundary (Blackburn Point) are increased and those in the other boundary are decreased. The results of the run where the tide at Stickney Point lags two hours behind that of Blackburn Point are shown in Figure IV-6. In this case, the tidal exchange volume is slightly increased. The boundary with the leading tide controls the circulation while the other boundary merely reacts to the induced elevations in the Bay.

Some preliminary conclusions are that the tides at the boundaries are very dependent on each other. Also, the capacity of the boundaries to transmit water is large compared to the total Bay volume. In addition, the differences in phase between the tide at the two ends of the Bay cannot exceed the travel time of long gravity waves to propagate the length of the Bay. As anticipated, the travel time is very sensitive to the depth of the Bay. Simulations show

IV-11 Figure IV-4 Semi-Diurnal Tide in Phase at Stickney Point and Blackburn Point

IV-12 Figure IV-5 Semi-Diurnal Tide with Stickney Point Lagged one-half hour from Blackburn Point

IV-13 Figure IV-6 Semi-Diurnal Tide with Stickney Point Lagged two hours from Blackburn Point IV-14 that, with the old Intracoastal Waterway geometry with a depth of 5 feet, the travel time is 1.55 hours. The deepening of the Intracoastal waterway to 9 feet drastically reduces the simulated travel time to 0.56 hours. It can be safely said at the present time that the Intracoastal waterway plays a rather important role in the stability of Midnight Pass, as well as in the circulation patterns of the Bay.

WIND EFFECTS

The effect of the wind on Little Sarasota Bay proper is relatively small. Several test runs were made blowing a 50 MPH uniform wind both on a flat sea and superimposed on an astronomic tide. In the first run the boundaries were left free to oscillate (reflecting boundaries), so as to study the effect of the wind on the Bay acting as an isolated body of water, with no flow occurring through any of the Passes. A 50 MPH wind blowing from Stickney Point along the axis of the Bay produces a steady state wind set-down of 1.81 feet at Stickney Point and a set-up of 1.59 feet at Blackburn Point. A wind of 10 MPH, much closer to the normal range, was imposed on the model blowing along the long axis from Blackburn Point to Stickney Point. This wind produced a steady state set-up of 0.026 feet at Stickney Point and a set-down of 0.0203 feet at Blackburn Point, roughly proportional to the square of the wind speed reduction, since the wind stress is proportional to the square of the wind speed.

One interesting effect of the wind on the Bay detected by the DEM model was that it set up eddy currents within the Bay. This phenomenon is evidently caused by the fact that the wind shear stress is inversely proportional to the depth. The shallow links near the shore respond more strongly to the wind stress than the deeper Intracoastal links, producing a net moment. This produces a net flow along the shore in the direction of the wind, with a return flow along the Intracoastal. Such eddy currents, or vorticities, may also be tidally induced.

The second run imposed a 50 MPH wind on the Bay from Stickney Point with Blackburn and Stickney Points tidally forced. The forcing at these boundaries was the pure astronomic tide shown in Figure IV-4 The net effect of the wind is twofold. The first effect is to increase or decrease the average level of

IV-15 the tide. This is shown on the top plate of Figure IV-7 for a node near Stick- ney Point. The second effect is to cause a net flow through the Bay in the direction of the wind. This effect at Stickney Point is also shown in Figure IV-7. This second effect is probably exaggerated by the fact that the boundaries do not have the effect of the wind on the surrounding water bodies.

In summary, high winds can have a significant effect on the Bay circulation. However, under normal wind conditions, the effect on the Bay proper is not large. It can be concluded that in combination with the effects produced on the boundary elevations, the wind should be considered a very important factor in the Bay circulation.

FRESHWATER INFLOWS

The volume of runoff produced by rain-storms over the watershed draining into Little Sarasota Bay has a significant effect on the quality of the waters of the Bay. A total land surface area of 15.0 square miles drains directly into the Bay. Approximately 30 percent of this area is developed with a great poten- tial for the development of the rest of the area in the near future. The drainage subbasins to the Bay are delineated in Figure IV-8.

Analysis shows that 14.8 inches of rainfall excess or runoff over this area is equivalent to the entire volume of the Bay. The rainfall frequency duration analysis shown in Table IV-3, extracted from TP-40 (2) shows that rainfall of one half of this volume occurs frequently. These amounts can have a significant effect on the salinity and, if these waters carry a high pollutant load, the water quality of the Bay. The effect on the elevations within the Bay, however, are small because of the large flow capacities of the Passes to the Gulf of Mexico.

IV-16 Table IV-3 RAINFALL DURATION - FREQUENCY

Inches of Rainfall

30 minutes 1 hour 2 hours 3 hours 6 hours 12 hours 24 hours

1 year 1.6 2.0 2.4 2.6 3.2 3.7 4.5 2 years 1.8 2.2 2.8 3.0 3.7 4.5 5.3 5 years 2.2 2.8 3.5 4.0 4.7 5.8 7.0 10 years 2.4 3.0 4.0 4.5 5.7 7.0 8.5 25 years 2.8 3.4 4.5 5.7 6.5 8.1 10.0 50 years 3.0 3.8 5.0 5.8 7.3 9.2 11.9 100 years 3.5 4.0 5.5 6.3 8.2 10.5 12.2

IV-17 Figure IV-7 Effect of 50 MPH Wind from Stickney Point Figure IV-8 Little Sarasota Bay Drainage Subbasins

IV-19 V. FIELD PROGRAM

INTRODUCTION

An extensive field program was performed for Little Sarasota Bay to provide needed data to set up, calibrate, and verify the circulation model. This program focused on the following elements:

0 bathymetric survey 0 tidal gages O current velocity studies O dye and drogue studies.

These surveys and the required data reduction are described below.

BATHYMETRIC SURVEY

The most recent bathymetric survey of Little Sarasota Bay was performed in 1954 by the U.S. Army Corps of Engineers. The depths from this survey were used in the preliminary model analysis. The character of the Bay has been drastically changed since this survey by the dredging of the Intracoastal Waterway and disposal of the spoil, shoaling along the length of the bay and particularly around the Bird Keys area, and the progressive shallowing and movement northward of Midnight Pass. These changes made it necessary to perform a bathymetric survey to reflect present day hydrographic conditions.

A two day survey was performed over April 26-27, 1982. The survey was performed with a boat equipped with a SITEX fathometer. Transects were surveyed by locating a known starting position on one shore and navigating the survey boat along a straight line to the opposite shore using a hand-held compass and a fixed throttle setting. A graduated staff was used to measure the depth at the beginning and end of each transect and in shallows where the boat was unable to navigate. Twenty-six such transects were made across

V-l the bay at approximately 400 yard intervals. In addition, many other transects were surveyed, particularly around Bird Keys, to provide needed extra coverage and detail. These transects are shown in Figure V-1.

The resulting strip charts were digitized and the elevations corrected to NGVD from concurrent staff gage observations. The survey data and plotted transects are reported in Appendix B. These depths were used to update the Bay circulation model to accurately represent the present Bay circulation.

TIDE GAGES

Little Sarasota Bay has three tidal entrances; Stickney Point, Midnight Pass, and Blackburn Point. To correctly simulate the Bay circulation, tidal information at these boundaries is needed. In addition, elevations within the Bay are needed to provide data with which to calibrate and verify the model. A total of six recording gages were placed in the Bay, one at each of the inlets and an additional three interspersed around the Bay. These locations are shown on Figure V-2.

Gage Number 1-Stickney Point was in place for a study being performed in Sarasota Bay to the north. The location of this gage is extremely suitable for use as a forcing for the model at the Stickney Point entrance. Gages 2 and 3 were located within the northern Bay between Stickney Point and the Bird Keys. The location of these gages was selected to serve two purposes: first, to provide calibration points in this area, and second to observe and document cross-bay water surface gradients. These gages were delibera- tely located on either side of the Bay for this purpose. Gage number 4 - Midnight Pass was located as close to the Pass as possible so as to obtain an exact boundary forcing in this critical area, yet to ensure that the gage location was safely removed from the erosional movement of the Pass. Gage 4 is located on a dock near the mouth of Blind Pass, several hundred yards north of the Pass. In retrospect, a gage located in the Gulf outside Midnight Pass would have been much more useful in studying this area. How- ever, the cost for such a gage was considered prohibitive.

V-2 V-3 V-4 Gage number 5 - Osprey Fishing Pier was located on the east side of the southern Bay between the Bird Keys and Blackburn Point. Finally, Gage number 6 was located at Blackburn Point to provide the model boundary condition at this location.

These gages were placed during April 1982 and will be in place for approximately one year. A detailed description of their location is given in Table V-1. At each location a staff gage and a Leopold-Stevens model 7001 contin- uous punched tape recorder were placed. This recorder records elevations at six-minute intervals. A stilling well was used at each location to dampen out short-period oscillations. These gages were fitted with fabricated metal covers and locked to minimize vandalism damage. The staffs and gages were subsequently leveled into the National Geodetic Vertical Datum of 1929 (NGVD). The relationship of the staff zero to NGVD are also reported in Table V-1. All elevations in this report are referenced to the NGVD datum. The gages were maintained to NOAA standards with visits to the gages twice each week. This, and the fact that the gages were new when placed, provided consistent and accurate tidal elevation data throughout the duration of the study.

The paper punch tapes from Gages 2 through 6 were converted to magnetic tape and corrected to NGVD by Mote Marine Laboratory for several periods. The tapes from Gage 1 were read by the USGS in Tampa. The raw data were reduced for four simulation periods including the three field trips described in the next section and the "No-Name" storm of June 17-18, 1982 described in Chapter VIII. The data for these periods are listed in Appen- dix C.

CURRENT VELOCITY STUDIES

Current velocity data are necessary to ensure that the model accurately simulates the existing water circulation conditions. The velocity measurements were designed to be collected in a series of three 24-hour field surveys. The first trip, April 28-29, 1982, was designed to occur during a fully diurnal tide, and the second trip, July 13-14, 1982, monitored a

V-5 TABLE V-1 TIDE GAGE LOCATION

Elevation of Gage Staff Zero Number Name & Location (ft)

1 STICKNEY POINT - on dock approximately 1/8 -5.8663 mile south of canal mouth at Siesta Key Marina. Operated by Mote Marine Lab for Sarasota Bay 201 Project (1/82-12/82)

HOLIDAY HARBOR - on private dock, northside -2.5970 of canal approximately 150 feet in from canal mouth.

WEST BAY - on private dock, westside of -2.3923 Little Sarasota Bay approximately 1.6 miles south of Stickney Point bridge.

MIDNIGHT PASS - on boat dock near mouth -3.3020 of Blind Pass at old Mote Marine Laboratory, southern end of Siesta Key. Vicinity of NOS Station

OSPREY FISHING PIER - on County maintained -3.4285 fishing pier and boat ramp at end of Main Street in the town of Osprey, Florida.

BLACKBURN POINT - on fender beneath center -2.8685 pivot of swinging bridge, west side of Intra- coastal Waterway channel. Site of NOS Tide Station.

V-6 semi-diurnal tide so as to cover the full range of astronomic tide conditions in the Bay. The third trip, November 10-11, 1982 was selected to cover the Bay under wind storm conditions. During late fall-early winter, a predictable succession of low atmospheric pressure systems pass over this area of Florida. These storms produce steady winds which range between 10 and 25 miles per hour. Unfortunately, the data collection effort caught just the end of one of these storms.

The surveys were performed using two boats equipped with ENDECO 105 current meters. Stations were set up in the Bay at which current velocities and directions were recorded at the surface, middle and bottom depths (surface and bottom velocities only were measured at shallow stations). In addition, water depth, boat compass heading, wind speed and direction, air tempera- ture and cloud cover observations were made at each station. Salinity and dissolved oxygen were also measured in situ and control samples were taken. Six stations were assigned to each boat based on an estimated two hours to cover all stations.

The twelve current velocity station locations are shown in Figure V-2 and are described in Table V-2. Stations 1, 2, 5, 6, 7, 8, 9, and 12 are located at narrows and other control stations where currents are strong and inherently bi-directional providing a direct indication of flow volume. The remaining four stations are located in areas of 2-dimensional flow, two in the northern Bay and two in the south.

The data from these field trips are compiled in Appendix D. Since the numerical circulation model is vertically integrated, the observations at the surface, mid-depth, and bottom must be integrated to provide the depth- averaged velocity before comparisons between the observed and simulated velocities can be made. Analysis of the observed record shows that in many instances the flow at various depths is in different, and at times opposite, directions at many of the stations. Careful consideration of each observation was required to determine the average station velocity and direction that represents the current field and to identify measurements that were in error or affected by the hull of the boat or the propeller. Also,

V-7 TABLE V-2 CURRENT STATION LOCATIONS

Station Depth Number (ft.) Description

North Bay

12 Stickney Point Bridge 12 red marker 54 (4 sec. flasher) 6 red marker 2 - entrance to Holiday Harbor 4 pile SW of green marker 49A 12 red marker 48 9 in channel between Bird Key & Siesta Key

South Bay

7 12 green marker 43 8 12 green marker 41 (4 sec flasher) 9 8 channel between Bird Key & Casey Key 10 4 SW of green marker 39 11 4 east of green marker 37 12 12 Blackburn Point Bridge

V- 8 the small observed velocities (below 0.10 knots), particularly at the stations of 2-dimensional flow, are near the lower limit of detection (thres- hold) of the current meters used. The integrated velocities, magnitude, and direction, used for comparison with the simulation results are also reported in Appendix D.

DYE AND DROGUE STUDIES

The movement of surface and near surface markers such as dyes and drogues give very useful information about circulation and dispersion characteristics of the Bay. The design of a field program requires knowledge of existing flow patterns. Poor design often will lead to expensive dye being washed to the ocean or giving other poor-return information. The dye studies performed in July were preceded by various drogue studies using num- bered grapefruit.

The dye studies were performed on July 14, 1982 at both Midnight Pass and Stickney Point. Twenty-five pounds of 20 percent Rhodamine WT dye were released at both Midnight Pass and Stickney Point. The movement of the dye was monitored by overflight photography and a boat equipped with a fluorometer to provide in situ concentration measurements. The dye was released at flood tide at Midnight Pass and Blackburn Point and followed as it moved into the Bay. The data from these studies as well as a concurrent drogue study as reported by Mote Marine Laboratory are included in Appendix D.

v-9 VI. MODEL CALIBRATION

The Link-Node Model network node locations and link definition used in the preliminary analysis was found to adequately represent the circulation of Little Sarasota Bay. In the final model the nodal and channel depths were adjusted using the results of the bathymetric survey to accurately represent the existing Bay configuration. The model was then calibrated using the field velocity measurements at current stations and water levels at gages from both the April and July surveys. The July survey occurred during a semi-diurnal tide, including two full oscillations per day. Therefore it was used as the primary calibration period. The April period involved a diurnal tide. Both periods were used for calibration because Gage 1 was not operating during the July period, and to ensure that the model accurately simulates the full range of tidal conditions.

Calibration sets were set-up wherein the measured tides at Gages 1, 4 and 6 were used to force the boundaries at Stickney Point, Midnight Pass and Blackburn Point respectively. Since Gage 1 was not operating during the July period, the recorded elevations from Gage 2 were transposed to represent the boundary conditions at Stickney Point. The simulation periods included the day prior to the first day of the velocity measurements to provide more than adequate model start-up time.

CALIBRATION PROCEDURE

The model calibration was performed primarily for the July period. The April period was used for minor adjustments and verification. Thus, the final model was calibrated to accurately represent the circulation produced by both modes of tidal forcing, diurnal and semi-diurnal, that occur in Little Sarasota Bay.

The preliminary runs were made using a uniform Manning's n equal to 0.03 and no wind forcing. Subsequent simulation runs were improved by simulating

VI-1 the wind stress with the wind as measured at the Mote Marine Weather Sta- tion located in Sarasota Bay 5 miles northwest of Stickney Point for both the July and April periods. Manning's n was also varied within the Bay as follows: the 12 foot deep Intracoastal Waterway was given a value of 0.03; the shallow portions of the Bay with a depth less than 3 feet were given a value of 0.06, and the remaining areas were given a value of 0.04. These changes produced improved velocities - magnitude, direction and phasing - at Blackburn and Stickney Points and other areas of the Bay. However, the simulated circulation around Midnight Pass and the north and south passes around Bird Island was entirely inaccurate. Analysis showed that this was caused by an inaccurate boundary condition imposed at Midnight Pass. Gage 4, being located inside the Bay, is highly affected by the tidal forcing propagating through Stickney Point and Blackburn Point. Accurate representation of the flow through Midnight Pass requires a boundary point located outside the Bay to represent the tidal conditions in the Gulf of Mexico. Therefore, the model was extended into the Gulf by adding 9 nodes and 17 links. Successful simulation of the flow reversals at Midnight Pass was accomplished by forcing the Gulf boundary nodes with the observed elevations at Gage 4 moved forward in time 18 minutes. This procedure is used in all subsequent runs. Final calibration of the model was achieved by minor adjustments to link widths and Manning's n within the channels north and south of Bird Keys and Midnight Pass. A listing of the final model input stream is included in Appendix E.

CALIBRATION RESULTS

The results from the final calibrated model from the July and April simulation periods are presented below. The July period is presented first since it was used as the primary calibration period.

For both simulation periods the following information is provided. Compari- son plots of the observed and simulated tides at Gages 2, 3, 4 and 5 are presented. Comparisons at Gages 1 and 6 are not shown since these are used to force the model at the boundary gage location. The simulated velocities are compared with the depth-averaged observations made during the field

VI-2 data collection trips at the twelve velocity measurement stations. For the predominantly one-dimensional locations, Stations numbers 1, 2, 5, 6, 7, 8, 9 and 12, the sign convention is positive velocities in the flood direction. The flood velocities are positive towards model node number 34, located just northeast of Bird Island. Therefore, the positive flows are toward the south at Velocity Stations 1, 2 and 5 and to the north at Stations 7, 8 and 12. Stations 6 and 9, located in the north and south passes around Bird Keys, flood into the Bay. For the two-dimensional Stations, numbers 3, 4, 10 and 11, the velocity is shown both in magnitude and direction of flow in degrees measured clockwise from north. The simulated velocity at these locations is calculated as a distance-weighted vectorial composition of the velocities in the three linearly dependent directions of the links surrounding the station. All times are referenced to Eastern Standard Time.

The standard error and maximum error between the simulated and observed velocities, directions, and elevations at all locations and gages are reported in Table VI-1 for both periods. The standard error between the observed and simulated values, respectively Yo and Ye, for N observations is computed as follows:

(VI-l)

The standard error has the same units as the compared simulated and observed quantities. The standard and maximum errors represent a numerical indication of the accuracy of the simulation and are reported for complete- ness, as should be done in all modeling. The significance of these values should not be overestimated. They are only indicators of the overall model performance.

July Simulation Period

The calibrated model simulation results for the July 13-14, 1982 measurements are presented in this section. Figures VI-1 through VI-4 show the simulated tides compared with the measured tide at Gages 2, 3, 4 and 5. The

VI-3 TABLE VI-1 STANDARD AND MAXIMUM ERRORS BETWEEN SIMULATED AND OBSERVED HYDROGRAPHIC DATA - JULY AND APRIL PERIODS LEVELS IN FEET - NGVD VELOCITIES IN FEET PER SECOND DIRECT ION IN DEGREES

July Period April Period

Standard Maximum Standard Maximum Location Error Error Error Error

Gage 2 0.025 0.07 0.047 0.08 Gage 3 0.03 0.07 0.02 0.06 Gage 4 0.05 0.10 0.03 0.07 Gage 5 0.03 0.07 0.04 0.08 Station 1 0.27 0.51 0.40 0.86 Station 2 0.35 0.76 0.30 0.51 Station 3 Velocity 0.26 0.62 0.33 0.78 Direction 97 173 83 153 Station 4 Velocity 0.15 0.49 0.17 0.35 Direction 97 157 76 155 Station 5 0.20 0.40 0.23 0.38 Station 6 0.30 0.54 0.13 0.39 Station 7 0.22 0.30 0.40 0.91 Station 8 0.07 0.16 0.22 0.36 Station 9 0.44 0.98 0.58 1.53 Station 10 Velocity 0.05 0.07 0.08 0.17 Direction 70 139 48 63 Station 11 Velocity 0.05 0.17 0.06 0.16 Direction 102 162 91 158 Station 12 0.26 0.49 0.55 1.24

VI-4 Figure VI-1. Observed vs. Simulated Elevation at Gage No.2, July 13-14, 1982

VI-5 Figure VI-2. Observed vs. Simulated Elevation at Gage No.3, July 13-14, 1982

VI-6 Figure VI-3. Observed vs. Simulated Elevation at Gage No.4, July 13-14, 1982

VI-7 Figure VI-4. Observed vs. Simulated Elevation at Gage No.5, July 13-14, 1983

VI-8 results at Gages 2, 3 and 5 are in excellent agreement with the corresponding measured tidal elevations. The simulated elevations at Gage 4 over- predict the ebb tide. The ebb tide phase at Gage 4 is mostly affected by the Gulf of Mexico outside of Midnight Pass, where the boundary conditions are extrapolated, i.e. not actually measured. This error, in turn, also explains some of the error in the simulated velocities around Bird Keys, especially near low tides. The standard error at all gages is less than 0.05 feet with a maximum error of 0.10 feet as reported in Table VI-1.

The depth-averaged velocities observed at the twelve stations during the field survey are compared with the simulation results in Figures VI-5 through VI-16 The standard error at these locations is also presented in Table VI-1. The results at some stations are excellent. In-depth discussion of the velocity results and alternate interpretations follow the presenta- tion of the April period simulation.

April Simulation Period

Figures VI-17 through VI-20 show the simulated elevations compared with the measured tide at Gages 2, 3, 4 and 5, for the April 28-29, 1982 field measurement period. These results show an excellent simulation of the tidal elevations within the Bay. The standard error at these gages as reported in Table VI-1 is less than 0.05 feet with a maximum error of 0.08 feet.

The depth-averaged velocities observed at the twelve stations during the field survey are compared with the simulated velocities in Figures VI-21 through VI-32. The standard error at these locations is presented in Table VI-l. General discussions of the calibration results follow in the next section.

DISCUSSION

The Little Sarasota Bay Circulation Model accurately simulates the tidal elevations within the Bay. This is to be expected because of the small size of the Bay and the fact that the elevations are imposed at Midnight Pass and Stickney and Blackburn Points.

VI-9 Figure VI-5 Observed vs. Simulated Velocity at Station 1, July 13-14, 1982

VI-10 Figure VI-6. Observed vs. Simulated Velocity at Station 2, July 13-14, 1982

VI-11 OBSERVED VS SIMULATED VELOCITY AT STATION 3, JULY 13-14,1982

Figure VI-7. Observed vs. Simulated Velocity at Station 3, Magnitude and Direction, July 13-14, 1982 VI-12 OBSERVED VS SIMULATED VELOCITY AT STATION 4, JULY 13-14,1982

Figure VI-8. Observed vs. Simulated Velocity at Station 4, Magnitude and Direction, July 13-14, 1982

VI-13 Figure VI-10. Observed vs. Simulated Velocity at Station 6, July 13-14, 1982

VI-15 Figure VI-11. Observed vs. Simulated Velocity at Station 7, July 13-14, 1982, Positive Flows Toward the North

VI-16 Figure VI-12. Observed vs. Simulated Velocity at Station 8, July 13-14, 1982

VI-17 Figure VI-13. Observed vs. Simulated Velocity at Station 9, July 13-14, 1982

VI-18 OBSERVED VS SIMULATED DIRECTION AT STATION 10, JULY 13-14,1982

Figure VI-14. Observed vs. Simulated Velocity at Station 10, Magnitude and Direction, July 13-14, 1982 VI-19 OBSERVED VS SIMULATED VELOCITY AT STATION 11, JULY 13-14,1982

JULY 13 JULY 14

OBSERVED VS SIMULATED DIRECTION AT STATION 11, JULY 13-14, 1982

JULY 13 JULY 14

Figure VI-15 Observed vs. Simulated Velocity at Station 11, Magnitude and Direction, July 13-14, 1982 VI-20 Figure VI-16. Observed vs. Simulated Velocity at Station 12, July 13-14, 1982

VI-21 Figure VI-17. Observed vs. Simulated Elevation at Gage No.2, April 28-29, 1982

VI-22 Figure VI-18. Observed vs. Simulated Elevation at Gage No.3, April 28-29, 1982

VI-23 Figure VI-19. Observed vs. Simulated Elevation at Gage No.4, April 28-29, 1982

VI-24 Figure VI-20. Observed vs. Simulated Elevation at Gage No.5, April 28-29, 1982

VI-25 Figure VI-21. Observed vs. Simulated Velocity at Station 1, April 28-29, 1982

VI-26 Figure VI-22. Observed vs. Simulated Velocity at Station 2, April 28-29, 1982

VI-27 Figure VI-23. Observed vs. Simulated Velocity at Station 3, Magnitude and Direction, April 28-29, 1982 VI-28 Figure VI-24. Observed vs. Simulated Velocity at Station 4, Magnitude and Direction, April 28-29, 1982 VI-29 Figure VI-25. Observed vs. Simulated Velocity at Station 5, April 28-29, 1982, Positive Flows Toward the South.

VI-30 Figure VI-26. Observed vs. Simulated Velocity at Station 6, April 28-29, 1982

VI-31 Figure VI-27. Observed vs. Simulated Velocity at Station 7, April 28-29, 1982, Positive Flows Toward the North

VI-32 Figure VI-28. Observed vs. Simulated Velocity at Station 8, April 28-29, 1982

VI-33

Figure VI-30. Observed vs. Simulated Velocity at Station 10, Magnitude and Direction, April 28-29, 1982 VI-35 Figure VI-31. Observed vs. Simulated Velocity at Station 11, Magnitude and Direction, April 28-29, 1982 VI-36 Figure VI-32 Observed VS. Simulated Velocity at Station 12, April 28-29, 1982

VI-37 The model produces accurate simulations of the time of flow reversals at all one-dimensional velocity stations. These are Stations 1, 2, 5, 6, 7, 8, 9 and 12 (Figure V-3). The stations located at the boundaries, i.e. Sta- tions 1, 6, 9 and 12, and those located at narrow passages within the Bay, namely Stations 2 and 8, are important control stations where the velocities are indicative of the volumes exchanged during the tidal cycle between the various sections of Little Sarasota Bay. At these stations the observed velocities exhibit a stable tidal variation with flows reversing between flood tide and ebb tide. The model accurately simulates the velocities at these stations for both the April and July periods, and therefore, accurately simulates the flushing properties of the Bay, volume and period, between major portions of the Bay.

The observed velocities at the two-dimensional areas of the Bay, i.e. Sta- tions 3 and 4 in the upper portion of the Bay, and Stations 10 and 11 in the lower Bay, show little or no activity most of the time (less than 0.02 fps). This is compatible with the shallowness of these areas and the distance away from the intracoastal waterway where most of the flow occurs. The simulated results at these stations for the July period, Figures VI-7, VI-8, VI-14, and VI-15 do represent the general observed trends, especially the flow direction. However, Stations 3 and 4 in the upper Bay show short- lived spurs of activity on July 14, 1982 (Figures VI-7 and VI-8) and also on April 28, 1982 (Figures VI-23 and VI-24) of the order of 0.4 to 0.6 fps which are not picked-up by the simulation model. Interestingly, Stations 10 and 11 in the lower Bay also show some similar activity for both periods, but of a much lesser intensity. It is important therefore to identify the processes associated with these two-dimensional current observations; to explain them; and to draw conclusions about their importance on the general Bay circulation and in particular on the water quality in the Bay.

First, the possibility of measurement error was investigated. The consistency of the observations at Stations 3 and 4 for both calibration periods (and later for the November, 1982 verification period as reported in Chap- ter VII) precluded the possibility of an accidental measurement error, such as secondary current induced by the wake of the propeller of the measurement boat or other Bay traffic. A source of a systematic error was then

VI-38 looked into, such as the presence of a channel in the vicinity of Stations 3 and 4 or the effect of a river discharge or an underground outlet in the vicinity of these stations. No such features were encountered in the neigh- borhood of Station 3, even though the similarity of the effects observed at both Stations 3 and 4 rendered this scenario an improbable one. Therefore, it was concluded that the observed current measurements were real. Subse- quent analysis revealed a probable physical explanation for the existence of these currents, an explanation of the reasons that they are not simulated by the model, and a procedure to estimate the magnitude of these currents.

The Dynamic Estuary Model (DEM) simulates long period waves. By all accounts, the observed current at Stations 3 and 4 show a six-hour period- icity which is in the detectable range of the DEM model. Moreover, the consistency of the observed current oscillations at Stations 3 and 4 for all three periods of observation (April, July and November) reduces the plausi- bility of these currents being caused by localized, variable (gusty) winds. Therefore, it is believed that the observed currents are of tidal origin - indeed the peak current measurements coincide with ebbing tides near the time of slack high water - but they have a sub-grid scale. They are caused by tidally induced vorticity of a scale smaller than the tri- angles of the DEM network. Figure VI-33 illustrates this point. The total velocity at a point inside a grid element (triangle), say the centroid, is calculated as the weighted vectorial sum of the velocities along the linearly dependent directions of the sides of the triangle (Figure VI-33a). If all three velocities turn in the same direction, and are of the same magnitude, the resultant velocity vanishes (Figure VI-33b). However, their effect can be measured in terms of the vorticity which, in this case represents a sub-grid closed circulation, and which is measured as the rate of change of the velocity profile in the transverse direction. Conversely, if the vorticity is known, and the characteristic size of the vortex d is known, the velocity vector magnitude V in the periphery of the vortex can be estimated as V = (Figure VI-33b). These calculations were performed for Station 4 as shown in Table VI-2 and resulted in a much closer fit with the observed current measurement, Figure VI-34.

VI-39 Figure VI-33. Velocity and Vorticity at the Centroid of a Grid Element

VI-43 Figure VI-34. Vorticity Calculations at Station 4

VI-41 TABLE VI-2 VORTICITY CALCULATIONS AT STATION 4, JULY 14, 1982.

7 -0.06 0.02 -0.00 0.12

8 -0.06 0.00 -0.00 0.18

9 -0.05 -0.02 -0.01 0.24

10 -0.05 -0.02 -0.01 0.24

11 -0.06 -0.03 0.00 0.27

12 -0.05 -0.02 0.01 0.18

13 -0.02 -0.02 0.00 0.12

14 0.06 -0.03 -0.02 0.02

15 0.11 -0.02 -0.04 0.14

16 0.06 0.01 -0.01 0.18

17 0.12 0.00 -0.03 0.26

18 0.15 -0.00 -0.04 0.32

VI-42 A physical explanation of the presence of these currents is in order so as to ascertain that the above calculations and predictive improvements are not fortuitous. The starting point in this physical explanation is the observation that the initiation of thesecurrents coincides with the Bay ebb tide. The bulk of the Bay mass moves northward towards Stickney Point along the Intracoastal Waterway which provides by far the largest cross-sectional flow area. It is a well understood phenomenon that tidal flats do not drain directly towards the outlet of the Bay but rather drain first into the main channel. It is believed that this two stage process, and the significant shear force that it encompasses between main channel and tidal flats, causes the generation of vorticity, Figure VI-33c. The size and magnitude of this vorticity depends primarily on the size and geometry of the Bay. Indeed, a similar effect is observed in the lower Bay, at Stations 10 and 11, Figures VI-14, VI-15, and VI-30, VI-31, but of a much smaller intensity. This may be due to an inappropriate location for Stations 10 and 11 or more likely to the narrower and more uniform geometry of the lower Bay.

In terms of modeling requirements, the following observation is made. All distributed parameter models (finite difference, finite elements, or DEM) do not account explicitly for vorticity, namely its generation and conservation. A usual way around this difficulty with distributed parameter models is to deploy so many grid elements as to capture the general vortex circulation pattern, as illustrated in Figure VI-35a. Such an approach for Little Sarasota Bay would be prohibitively costly and impractical. On the other hand, this vorticity is seen to be of purely tidal origin and is not specifically related to non-linear effects. Such effects are observable over longer time periods, weeks or months. Even though not very significant in terms of the overall Bay circulation, this vorticity is important in estimating and evaluating the dispersion and mixing properties of the Bay. Therefore, it is recommended that the vorticity evaluation be added as a post-processor to the present version of the DEM model, as illustrated in Figure VI-35b. Incidentally, it is entirely possible that the vorticity activity was enhanced by the dredging of the Intracoastal Waterway.

VI-43 Figure VI-35. Vorticity Field and Sub-grid Scale Vorticity

VI-44 In summary, the Little Sarasota Bay model accurately simulates the circulation produced by tidal forcing and local wind. The observed data at many locations within the Bay show large velocities that are due to an ebb tide associated vorticity. The model does simulate the net flows through major portions of the Bay, and can be extended to simulate these sub-grid scale vorticities. More importantly, pertinent selection of tide gages and current stations for the field data, together with a judicious selection of runs of the simulation model permitted us to address and understand all physical processes that govern the circulation in Little Sarasota Bay and that may affect the dispersion and mixing properties of the Bay.

VI-45 VII. MODEL VERIFICATION

VERIFICATION PROCEDURE

The verification of the calibrated Little Sarasota Bay Circulation Model was performed by simulating a separate period from those used to calibrate the model with no modification made to the calibration parameters. The third 24-hour field data collection period took place during November 10-11, 1982. This trip was designed to make current velocity measurements and other observations during conditions of frontal system passage with relatively high winds. This period provided the data against which the model was verified. The wind measured at the Mote Marine Meteorologic Sta- tion was input to the model. The measured winds reached speeds of approximately five miles per hour. These observations show that the wind was from the northeast and swung around to the southeast at about 10:00 AM EST on November 11. The tide during this period was semi-diurnal but exhibited the influence of the passage of the frontal system on the Gulf of Mexico. As shown on Figure VII-1 this effect is observable by an increasing average sea level over the period.

NOVEMBER SIMULATION RESULTS

The simulated tides at Gages 2, 3, 4 and 5 are compared with the observed elevation in Figures VII-1 through VII-4. Effective simulation starts at 4:OO EST with the first 4 hours of simulation being used for model start- up. The model accurately simulates the tidal elevations with a maximum standard error (equation VI-1) of 0.04 feet and a maximum error of 0.10 feet as reported in Table VII-1.

The depth-averaged velocities observed at the twelve velocity measurement stations during the field survey are compared with the simulation results in Figures VII-5 through VII-16. For the one-dimensional stations, positive

VII-1 TABLE VII-1 STANDARD ERROR AND MAXIMUM ERRORS BETWEEN SIMULATED AND AS OBSERVED HYDROGRAPHIC DATA - NOVEMBER PERIOD. LEVELS IN FT - NGVD, VELOCITIES IN FPS, DIRECTION IN DEGREES.

Standard Maximum Location Error Error

Gage 2 0.03 0.07 Gage 3 0.04 0.08 Gage 4 0.04 0.10 Gage 5 0.04 0.09 Station 1 0.45 0.74 Station 2 0.85 1.45 Station 3 Velocity 0.21 0.57 Direction 102 166 Station 4 Velocity 0.26 0.62 Direction 125 169 Station 5 0.33 0.78 Station 6 0.31 0.75 Station 7 0.28 0.44 Station 8 0.17 0.24 Station 9 0.38 0.66 Station 10 Velocity 0.07 0.11 Direction 105 162 Station 11 Velocity 0.08 0.14 Direction 94 171 Station 12 0.52 0.94

VII-2 Figure VII-1. Observed vs. Simulated Elevation at Gage No.2, November 10-11, 1982

VII-3 Figure VII-2. Observed vs. Simulated Elevation at Gage No.3, November 10-11, 1982

VII-4 Figure VII-3. Observed vs. Simulated Elevation at Gage No.4, November 10-11, 1982

VII-5 Figure VII-4. Observed vs. Simulated Elevation at Gage No.5, November 10-11, 1982

VII-6 Figure VII-5. Observed vs. Simulated Velocity at Station 1, November 10-11, 1982

VII-7 Figure VII-6. Observed vs. Simulated Velocity at Station 2, November 10-11, 1982

VII-8 OBSERVED VS SIMULATED VELOCITY AT STATION 3, NOVEMBER 10-11,1982

OBSERVED VS SIMULATED DIRECTION AT STATION 3, NOVEMBER 10-11, 1982

Figure VII-7. Observed vs. Simulated Velocity at Station 3, Magnitude and Direction, November 10-11, 1982 VII-9 OBSERVED VS SIMULATED VELOCITY AT STATION 4, NOVEMBER 10-11, 1982

OBSERVED VS SIMULATED DIRECTION AT STATION 4, NOVEMBER 10-11, 1982

Figure VII-8. Observed vs. Simulated Velocity at Station 4, Magnitude and Direction, November 10-11, 1982

VII-10 Figure VII-9 Observed vs Simulated Velocity at Station 5 November 10-11, 1982, Positive Flows Toward the South

VII-11 Figure VII-10. Observed vs. Simulated velocity at Station 6, November 10-11, 1982

VII-12 Figure VII-11. Observed vs. Simulated Velocity at Station 7, November 10-11, 1982, Positive Flows Toward the North

VII-13 Figure VII-12. Observed vs. Simulated Velocity at Station 8, November 10-11, 1982

VII-14 Figure VII-13. Observed vs. Simulated Velocity at Station 9, November 10-11, 1982

VII-15 OBSERVED VS SIMULATED VELOCITY AT STATION 10, NOVEMBER 10-11, 1982

NOVEMBER 10 NOVEMBER 11

OBSERVED VS SIMULATED DIRECTION AT STATION 10, NOVEMBER 10-11, 1982

NOVEMBER 10 NOVEMBER 11

Figure VII-14. Observed vs. Simulated Velocity at Station 10, Magnitude and Direction, November 10-11, 1982

VII-16 OBSERVED VS SIMULATED VELOCITY AT STATION 11, NOVEMBER 10-11, 1982

OBSERVED VS SIMULATED DIRECTION AT STATION 11, NOVEMBER 10-11, 1982

Figure VII-15. Observed vs. Simulated Velocity at Station 11, Magnitude and Direction, November 10-11, 1982

VII-17 Figure VII-16. Observed vs. Simulated Velocity at Station 12, November 10-11, 1982

VII-18 flows denote the flood direction and negative flows denote the ebb direction of flow. Therefore, positive flows are toward the south at Velocity Stations 1, 2 and 5 and to the north at Stations 7, 8 and 12 with Stations 6 and 9 flooding into the Bay. Both the velocity magnitude and the direction are plotted at the two-dimensional Stations 3, 4, 10 and 11. For these stations the plotted direction represents the direction in which the water flows, in degrees measured clockwise from north. The simulated velocities at these locations represent a distance weighted vectorial average of the velocities in the three linearly dependent directions of the links surrounding the station. All times are referenced to Eastern Standard Time.

DISCUSSION

The results of the simulation of the November period show the same overall level of accuracy as for the two calibration periods. What is more important, however, is that this level of accuracy was achieved with the same set of model parameters set for the calibration periods. This indicates that the model is stably tuned to represent the vertically averaged flow circulation in Little Sarasota Bay, and that it is not biased to imitate only one set of particular conditions. The flow in Little Sarasota Bay is, strictly speaking, three-dimensional. However, simulations of the three data periods of April, July, and November, 1982, amply show that the horizontal two-dimensional Dynamic Estuary Model does capture the essence of the important physical processes occurring in the Bay. Interestingly, the observed velocities at the two-dimensional Stations 3 and 4 (Figures VII-7 and VII-8) show the same current activity that was present in the April and July period. This consistency adds credence to the arguments about vorticity associated with ebb flow that were presented in the discussion of the previous section.

The peculiarity of the wind condition for the November period should not go unnoticed. While the simulated currents at all one-dimensional stations seem to be close to the observed curves, the simulation at Stations 1, 2 and particularly at Station 12 (Figures VII-5, VII-6 and VII-16 respectively) seems to be off target. This is due to the fact that the boundary tidal signal at Stations 1 and 12, respectively, Stickney Point and Blackburn

VII-19 Point, is affected by the wind conditions in the neighboring Roberts and Blackburn Bays. That is, these boundary nodes transmit momentum, as well as water elevations, which becomes important under wind conditions. Ideally, radiation boundary conditions should be imposed at these boundaries. These are not always readily known and this requirement would significantly reduce the flexibility of application of the model to analyze future scenarios for an effective Little Sarasota Bay management program. A more feasible solution would be to extend the model network to include the bays north and south of Little Sarasota Bay. This point is further elaborated on in the next section, on the simulation of the "No-Name" tropical storm of June 18, 1982.

VII-20 VIII. MODEL APPLICATIONS

The main goal of the present study was to develop a predictive model of the hydrodynamic circulation of Little Sarasota Bay. This was accomplished through the following steps. A preliminary analysis was used to guide the design of a field data collection program. The field data was then used in conjunction with the development of the numerical model to understand the physical processes controlling the Bay circulation, and to properly incorporate them in the model. This model will be used as a predictive tool to aid in the analysis of various scenarios for the systematic management of Little Sarasota Bay. The present chapter gives a series of examples of model application by way of five sample cases that were analyzed to gain further insight in the processes affecting the Bay, and to illustrate the usefulness of the model. Although several generalizations can be drawn from these sample cases, they are only presented for illustrative purposes and should not be construed as full-scale scenario analyses.

Once calibrated and verified, the circulation model of Little Sarasota Bay was subsequently used to study: the present mode of Bay circulation; the Bay response to a storm event such as the June 18, 1982 “No-name” storm; the impacts of the past dredging of the Intracoastal Waterway on the Bay circulation; and the possible impacts of future dredging activities, such as the dredging of Midnight Pass, on Bay circulation and pass stability. A future application of the circulation model involves its interface with a water quality model for analyzing the water quality of Little Sarasota Bay. An example of such an application of the model is also used in this chapter to illustrate the July dye study around Midnight Pass.

ANALYSIS OF BAY CIRCULATION

Analysis of the simulation results and the observed data for the three simulation periods provides an understanding of the mode of circulation within

VIII-1 Little Sarasota Bay. While the previous sections focused on model calibration and verification exclusively, this section provides an in-depth analysis of the data generated by these model runs. This is very useful because the understanding of the present Bay conditions is essential to estimating the impacts and effects of future as well as past changes brought about on the Bay.

The tides on the Gulf Coast of Florida are mixed diurnal and semi-diurnal. The April simulation period covered a fully diurnal tide, while the July and November periods covered a semi-diurnal tide. The observed high and low tides for these periods are summarized in Table VIII-1. The tides are nearly equal everywhere within the Little Sarasota Bay. The high and low tides occur first at Blackburn Point (Gage 6). The tide at Stickney Point (Gage 1) occurs next, on the average six minutes later. The high and low tides at the remaining gages occur almost simultaneously with differences in timing mostly obscured by the coarseness of the time increment, i.e. by the fact that the observations are reported at six minute intervals. High tide reaches these gages approximately 16 minutes after Blackburn Point, with the high and low tides at the Midnight Pass gage being the latest.

The predicted tides at St. Petersburg, Florida, published annually by The National Ocean Survey, are also shown in Table VIII-1. In these tide tables, St. Petersburg is the reference gage for this section of the Gulf Coast of Florida. For the six days of the studied data periods, the high and low tides at Blackburn Point occur on the average 57 minutes before the predicted tides at St. Petersburg, with no observable difference between high and low tides. Comparison with the published tidal differences between St. Petersburg and Venice Inlet to the south and Sarasota Bay to the north, reported in Table IV-1, show that the tides within Little Sarasota Bay occur much later than at either of these two locations. Since the predicted tidal elevations are related to the local Mean Low Water, no comparison of elevations can be made.

The above observations indicate that Little Sarasota Bay is primarily forced through Blackburn Point and Stickney Point. The tide propagates quasi-simultaneously northward from Venice Inlet and southward from Sarasota Bay.

VIII-2 TABLE VIII-1 TIME AND ELEVATION OF HIGH AND LOW TIDES

Times are Eastern Standard Time Reported as Decimal Hours Elevations for Gages are Feet NGVD

Gage 1 Gage 2 Gage 3 Gage 4 Gage 5 Gage 6 St. Petersburg*

April 28

Time 0.3 0.65 0.60 0.50 0.60 23.90 23.85 Elevation -0.53 -0.51 -0.53 -0.51 -0.47 -0.46 -0.40

Time 5.85 5.9 5.9 5.95 5.95 5.80 Elevation 0.32 0.34 0.33 0.34 0.33 0.37

Time 7.20 7.25 7.35 7.50 7.30 7.00 Elevation 0.25 0.28 0.26 0.28 0.30 0.32

Time 15.35 15.50 15.60 15.60 15.80 15.65 16.17 Elevation 1.49 1.53 1.49 1.49 1.52 1.51 2.40

April 29

Time 1.70 2.10 2.05 2.05 1.90 1.45 1.05 Elevation -0.72 -0.70 -0.71 -0.69 -0.67 -0.67 -0.30

Time 16.05 16.25 16.30 16.50 16.60 16.40 17.20 Elevation 1.37 1.40 1.36 1.35 1.39 1.39 2.30

July 13

Time 0.35 0.45 0.70 0.50 0.30 0.70 Elevation -0.05 -0.05 0.0 -0.04 -0.03 0.50

Time 6.80 6.90 6.90 6.80 6.65 7.43 Elevation 1.09 1.09 1.10 1.10 1.09 2.00

Time 12.40 12.45 12.50 12.20 12.00 13.25 Elevation 0.37 0.36 0.40 0.40 0.39 1.10

Time 17.60 17.55 17.45 17.40 17.15 18.87 Elevation 0.97 0.97 1.00 0.95 0.94 1.90

(Continued on next page)

* Predicted elevations for St. Petersburg are referenced to Mean LOW Water (MLW). 1 Missing observed data.

VIII-3 TABLE VIII-1 TIME AND ELEVATION OF HIGH AND LOW TIDES (Continued)

Times are Eastern Standard Time (EST), Reported as Decimal Hours Elevations for Gages are Feet NGVD

Gage 1 Gage 2 Gage 3 Gage 4 Gage 5 Gage 6 St. Petersburg*

July 14

Time 0.45 0.50 0.50 0.15 0.14 1.30 Elevation 0.10 0.10 0.14 0.13 0.14 0.80

Time 6.95 7.15 7.20 7.10 6.95 8.05 Elevation 1.25 1.24 1.27 1.24 1.23 2.10

Time 14.20 14.35 14.20 14.05 13.80 14.73 Elevation 0.18 0.18 0.24 0.21 0.21 0.90

Time 19.30 19.35 19.40 19.15 19.20 20.50 Elevation 0.84 0.82 0.86 0.79 0.78 1.60

November 10

Time 2.45 2.55 2.65 2.75 2.60 2.30 4.28 Elevation 0.34 0.36 0.38 0.40 0.41 0.40 1.20

Time 7.80 8.15 8.25 8.15 8.0 9.68 Elevation 1.11 1.16 1.16 1.18 1.19 1.80

Time 16.05 16.30 16.35 16.35 16.40 16.00 16.63 Elevation -0.25 -0.23 -0.19 -0.15 -0.15 -0.14 0.40

Time 22.65 22.60 22.50 22.70 22.75 22.70 23.37 Elevation 0.93 0.98 1.00 0.99 0.99 0.98 1.80

November 11

Time 4.25 4.40 4.20 4.10 4.15 3.70 5.40 Elevation 0.14 0.17 0.20 0.23 0.23 0.23 0.80

Time 10.20 10.60 10.65 10.55 10.45 10.35 11.17 Elevation 1.28 1.32 1.33 1.32 1.32 1.33 1.70

Time 16.35 16.60 16.75 16.60 16.50 16.25 17.33 Elevation 0.25 0.27 0.28 0.31 0.32 0.32 0.60

Time 22.60 22.75 22.85 22.80 23.00 22.85 23.70 Elevation 1.64 1.66 1.68 1.67 1.67 1.66 1.90

* Predicted elevations for St. Petersburg are referenced to Mean Low Water (MLW) 1 Missing observed data.

VIII-4 Forcing through Midnight Pass is seen to be minor. The elevations measured at Gage 4, which is located just inside the pass, are predominantly influenced by the forcing through the northern and southern tidal boundaries. This is further substantiated by analysis of the simulated boundary flows for the April, July, and November, 1982 periods. The net ebb and flood tidal flows and the times of flow reversal through Stickney Point, Midnight Pass and Blackburn Point are reported in Table VIII-2. The corresponding flows are also plotted in Figures VIII-1 through VIII-3 for these periods. The flows through Stickney Point and Blackburn Point tend to be equal with each contributing an average of approximately 42 percent of the total Bay volume exchange during a tidal cycle. Midnight pass contributes the remaining 16 percent to the net tidal exchange. The actual flow through each of these boundaries for any period is dependent on the wind conditions and other Bay interactions. Analysis shows that the flow through Midnight Pass is nearly equally distributed between the north and south passes around Bird Keys with an average of 44 and 56 percent of the flow going to the north and south passes respectively. Simulated Bay volumes for the April, July, and November periods are summarized in Table VIII-3. One set of volumes is given for every slack water to slack water tidal excursion within each simulation period. The intertidal volumes are comparable to the last column in Table VIII-2.

The time of flow reversal at the boundaries, as reported in Table VIII-2, varies between high and low tides. The flow reversed from flood to ebb first at Stickney Point in seven out of eleven reversals. Midnight Pass and Blackburn Point reversed on the average 20 and 29 minutes later, respectively. However, during reversal from ebb to flood flows at low tide, Midnight Pass is seen to reverse first. Blackburn Point reversed on the average 54 minutes later and Stickney Point reversed one hour and 36 minutes after Midnight Pass. This is indicative of the fact that, at ebb tide, Midnight Pass is affected by high waters in the Bay, in response to low levels in the Gulf (passive flow conditions). However, at flood tide Midnight Pass is more affected by the Gulf (active flow conditions). This behavior is produced by the complex nature of the forcing through the three tidal boundaries. What can be concluded is that there is greater irregularity associated with time of reversal when flow is from flood to

VIII-5 TABLE VIII-2 SIMULATED BOUNDARY FLOOD AND EBB FLOWS

* Times are presented in decimal hours Eastern Standard Time. ** Flood flow is positive. + Before simulation period. a Flow did not reverse at Blackburn Point.

VIII-6 TABLE VIII-3 SIMULATED BAY VOLUMES

April 2.0 493.0 303.9 189.1 2.2 492.0 280.5 212.5 2.07 480.3 280.5 199.8

July 1.13 460.0 369.3 90.6 0.72 460.0 381.9 78.1 0.60 444.0 381.9 62.1 0.88 440.0 360.0 80.0 1.15 473.8 360.0 113.8 1.05 473.8 367.5 106.3 0.63 424.3 367.5 56.8

November 1.36 462.9 332.0 130.9 1.18 444.0 332.0 112.0 0.79 444.0 366.5 77.5 1.13 476.7 366.5 110.2 1.01 476.7 378.8 97.9 1.38 513.4 378.8 134.6

VIII-7 Figure VIII-1. Simulated Boundary Flows, April 28-29, 1982

VIII-8 Figure VIII-2. Simulated Boundary Flows, July 13-14, 1982

VIII-9 Figure VIII-3. Simulated Boundary Flows, November 10-11, 1982

VIII-10 ebb, since it is controlled by the timing of the level decrease in the Gulf. The Gulf has a much larger mass and inertia as compared to the Bay and therefore controls this process. Since these results are based on only six days of simulation, caution should be used in generalizing these results to other periods.

Two-dimensional velocity plots of the simulation results for the April and July period were made to provide a visual representation of the vector velocity fields within Little Sarasota Bay. Plots for one full day of simulation at one hour intervals are included in Appendix F. Examples from a portion of the July simulation are presented in Figures VIII-4 through VIII-8. These figures show the progressive velocity fields at full flood tide, near slack tide, slack tide at high water, starting ebb tide, and full ebb flow respectively. These plots show that the tidal flows from Blackburn Point and Stickney Point meet at a point north of Bird Keys which is characterized by small velocities throughout the period. This part of the Bay experiences essentially only minor vertical water movement. The resulting shoaling produced by these small velocities is evidenced by the shallow bars and oyster beds that characterize this area of the Bay.

The plots also show that the flow within the Bay is dominated by the Intracoastal Waterway which runs along its axis. Velocities within the waterway are much larger than those in the two-dimensional areas of the Bay on either side. Inclusion of the vorticity effect in these shallow areas will only slightly modify these vector velocity plots.

The question of net flow through the Bay boundaries is an important consideration in evaluating the flushing properties of Little Sarasota Bay. Table VIII-4 shows the simulated net flows through Stickney Point, Midnight Pass and Blackburn Point, (positive inflow, negative outflow) during the simulated period (high to high), for each of the April, July and November observation periods. These net flows are adjusted for the change in Bay volume, which is due to the difference in elevation between the starting and end points of the period over which the flows are computed. During the April and November periods, there is net inflow through Blackburn Point and Midnight Pass, and net outflow through Stickney Point. During the July

VIII-11 Figure VIII-4. Simulated Velocity Vectors at Full Flood Flow, July 14, 1982

VIII-12 Figure VIII-5. Simulated Velocity Vectors at Flood Flow Near High Tide, July 14, 1982

VIII-13 Figure VIII-6. Simulated Velocity Vectors at High Tide, July 14, 1982

VIII-14 Figure VIII-7. Simulated Velocity Vectors at Ebb Tide Near High Tide, July 14, 1982

VIII-15 Figure VIII-8. Simulated Velocity Vectors at Full Ebb Flow, July 14, 1982

VIII-16 TABLE VIII-4 NET SIMULATED FLOW THROUGH BOUNDARIES

Period Stickney Point Midnight Pass Blackburn Point

106ft3 106ft3 106ft3

April -27.51* 7.12 20.38

July 0.97 32.41 -33.39

November -135.71 25.45 110.26

* Flood flow is positive

VIII-17 period, there is net inflow through Midnight Pass and Stickney Point, and net outflow through Blackburn Point. These net flows can be explained by the transition from semi-diurnal to diurnal tide conditions (April) or vice versa and the effect of wind. Theoretically, over a complete tidal cycle, i.e., when tidal conditions return to the starting conditions, there should be no net flow through the Bay. However, there may be net flow through the Bay during portions of the tidal cycle. In addition, wind conditions induce flow through the Bay. No generalization can be made from the three simulation periods. However, the model could be used to detect trends in inflow and outflow during a complete tidal epoch.

In summary, the important features of the Little Sarasota Bay circulation are outlined below. The Bay receives its tidal forcing through Midnight Pass, Blackburn Point, and Stickney Point. Midnight Pass is small and narrow and also unstable. Its location is moving constantly with the present trend being northward. The Pass tends to be short period surface wave dominated and the net tidal volume exchange through the Pass accounts for only 16 percent of the total Bay exchange even with its direct connection with the Gulf. Blackburn Point and Stickney Point are equally important in controlling the tidal circulation of Little Sarasota Bay. The many bays along this coast of Florida are strongly connected by the Intracoastal Waterway. The bays to the north and south of the study area form a complex connection of the Bay to the passes to the Gulf of Mexico to the north and south. Net flow through the Bay, North or South, occurs within the simulated tidal cycles (including passage from diurnal to semi-diurnal tide), and is strongly affected by sustained wind conditions.

"NO-NAME" STORM ANALYSIS

Introduction

A minor tropical depression occurred during the year of operation of the tide gages in Little Sarasota Bay. This storm was quite unique in that it developed rapidly over the Gulf of Mexico and struck the Gulf Coast of Florida with little or no forewarning. This tropical storm, locally known

VIII-18 as the "No-name" storm, struck the Little Sarasota Bay area on June 18, 1982. The storm, although weak in comparison with a hurricane, produced considerable damage and erosion along the Florida Gulf Coast.

A study was made of the effects of the storm surge and high winds on the circulation within Little Sarasota Bay. It is important to realize at the outset that the effect of the storm on the Gulf of Mexico, which is reflec- ted in the measured tides at the boundaries of Little Sarasota Bay, pre- dominates over the effect of the storm on the Bay itself. When the storm is over deep water, the low atmospheric pressure associated with the storm's center, or eye, causes a local rise in the water surface known as the in- verted barometer effect. This perturbation, in turn, propagates as a long- period gravity wave and combines with the prevailing astronomic tide to produce a rise in the water surface elevation above the normal astronomic tide level. The timing of the storm surge with the normal tide is very important in determining the maximum observed total water elevation known as the storm tide.

While the atmospheric depression causes the generation of surges in deep water, wind is important locally near shore and within shallow embayments. The wind stress on the water surface is inversely proportional to the depth and proportional to the wind velocity squared. Therefore, in the shallower waters near the shore and within the embayments, the effect of the wind is much more pronounced, and produces wave set-up associated with the piling up of water as the wind blows on shore, and wave run-up associated with the wind induced surface waves. Within the shallow, interconnected embayments along the Gulf Coast the wind effect is compounded by the interaction of the various embayments. High winds produce a tilting of the water surface in the direction of the wind within a water body and seiching to restore the original water surface configuration after the storm has passed. When the wind is blowing along the longitudinal axis, the set-up at one end of the Bay is associated with a set-down at the corresponding end of the neighboring bay which causes an increased net flow in or out of the Bay. These effects were observed during the "No-name" storm in Little Sarasota Bay and quantified with the DEM model.

VIII-19 Barometric and wind observations during the "No-name" storm were obtained from the Mote Marine Laboratory meteorologic station located on City Island. The observed barometric pressure in millibars (MB) along with the average wind speed and direction are presented in Figure VIII-9. The wind speed is measured in miles per hour (MPH) in the direction that the wind is blowing, measured clockwise from north. Throughout this report time is referenced to Eastern Standard Time (EST).

Although no synoptic weather map analysis of the storm was made, the above observations show that the storm's center passed west of Little Sarasota Bay, passing closest to the study area around 4:00 EST on June 18. The winds of over 25 MPH started abruptly at midnight and ended almost as abruptly at 6:00 AM EST. Since the storm passed west of the Bay, the winds were blowing from the south until the time that the barometric low passed the area, and then swung around to the west as the storm moved away.

The storm produced a peak storm tide of approximately 4.2 feet NGVD within Little Sarasota Bay. The observed tidal marigram has a relatively flat peak with the maximum tide occurring over a period from approximately 7:12 until 8:24 EST on June 18. Unfortunately, three of the six gages, numbers 1, 2 and 3 topped out during the storm. That is, the water surface during the storm exceeded the maximum measurable elevation at the gage. Also, the dock on which Gage 2 is attached came loose and floated during the storm. There is also evidence that Gage 5 topped out at 4.24 feet which is near the peak tide at this location. The measured elevations during the storm are shown in Figure VIII-10. Time zero on this plot corresponds to midnight (EST) on June 17. Finally, the data for Gage 1 are not available for this period due to a gage malfunction. In spite of the poor available data, a number of useful observations were made.

Several important phenomena can be observed from the above data when compared with the measured wind speed and barometric pressure of Figure VIII-9. The peak surge occurs four hours after the storm passes the area. This time lag is related to the time of propagation of the storm surge from

VIII-20 BAROMETRIC PRESSURE

JUNE 18 JUNE 17 TIME HOURS

AVERAGE WIND SPEED

JUNE 18 JUNE 17 TIME HOURS

WIND DIRECTION

JUNE18 JUNE 17 TIME HOURS

Figure VIII-9. Observed Barometric Pressure, Wind Velocity, Magnitude and Direction for “No-name” Storm

VIII-21 VIII-22 its point of origin in the Gulf of Mexico to the Bay. The strong winds blowing from the south produced a longitudinal set-up within the Bay of approximately 0.5 feet during the time of maximum southerly winds. This set-up is observable as the difference between the observed elevations at Gages 2 and 6 located in the northern and southern Bay respectively. The longitudinal bay set up diminished as the wind shifted to the west.

The storm produced some minor overtopping at Casey Key during the maximum water surface elevation into Blind Pass and Heron Lagoon. These small inflows are not important as far as the Bay hydrodynamics is concerned.

The tidal elevations observed during the storm incorporate the increase in water surface produced by the storm superimposed on the normal astronomic tide. In the open ocean both long period waves, namely surges and astronomic tides, do not interact. That is, the presence of one has no effect on the other, and they can be studied independently. This is not the case near shore, and especially within embayments where these waves interact and where their separate effects are not superimposable. This is often referred to as the non-linearity of surges and tides. Figure VIII-11 shows the measured storm tide at Gage 5; the normal astronomic tide during this period, predicted from the observed tide prior to and after the storm; and the difference between the predicted and observed tide, that is the storm produced surge. It is interesting to notice that the peak storm surge of 3.38 feet coincided with a low astronomic tide. Had the storm occurred six hours later, the peak storm tide would have been at least one foot higher than that actually produced.

Simulation of Storm

The calibrated Little Sarasota Bay Model was used to simulate the circulation within the Bay during the "No-name" storm. The results of the simulation are mostly qualitative in nature for several reasons. First, the topping out of the gages used for boundary conditions renders the results uncertain. Second, simulation of a high wind event may require adjustment (further calibration) of the wind-stress model to account for the stronger wind forcing conditions, 25 MPH as compared with the less than 6 MPH winds

VIII-23 VIII-24 during the calibration and verification periods. Most importantly, the Little Sarasota Bay Model in its present form does not account for the interaction with the neighboring Bays. Nevertheless, this exercise is useful in providing an understanding of the reaction of the Bay to storm conditions and in identifying the level of interaction between various Bays and ways to augment the present model to account for these effects.

The storm was simulated using the calibrated Little Sarasota Bay model. The model requires water level boundary conditions (i.e. measured water surface elevations) at Stickney and Blackburn Points, and outside of Midnight Pass. Data for Gage 1, located at Stickney Point was not available due to a gage malfunction. Thus, the measured elevations at Gage 2 were used for this boundary as was done for the July calibration. In addition, the dock which is used to anchor Gage 2 floated during the storm and was repaired as the tide receded. An estimation was made of the time and magnitude of the floa- tation of the dock with which the "observed" elevation at this gage was reconstructed and used at the Stickney Point boundary. This limits somewhat our ability to accurately analyze the interaction of Little Sarasota with the Bay to the north. Furthermore, for the Midnight Pass boundary condition, the observed elevations at Gage 4 were transposed to the ocean boundary nodes by moving the observations forward 18 minutes in time. Additional uncertainty was introduced by the gage overtopping which filtered out the tide peak at that location. The marigram was reconstructed from the observed elevations at Gages 3 and 5. The wind field shown in Figure VIII-9 provided the meteorologic forcing for this simulation. Although the model has the ability to simulate the atmospheric pressure gradient effect on the water, this effect is negligible in a water body as small as Little Sara- sota Bay. The simulation was made for the 2-day period, June 17-18, 1982, starting at midnight (EST) on the 17th. The time on all subsequent plots relates to hours after this initial time of simulation. The plots begin at 5:00 EST in order to account for model start-up time.

Simulation Results

The simulated elevations are compared with the observed elevations at Gages 2, 3, 4 and 5 in Figures VIII-12 through VIII-15. The Root Mean Square

VIII-25 VIII-26 VIII-27 VIII-28 VIII-29 (RMS) error, which is equivalent to the standard error calculated by Equa- tion VI-l, and the maximum error are also reported on these figures. The model simulates the "observed" elevations very well, which provides some degree of confidence in the reconstructed marigrams imposed at the three boundaries.

Initial runs of the model produced net flows in the Bay in the opposite direction of the prevailing wind. This is explained as follows: the boundary conditions imposed at the Stickney Point and Blackburn Point incorporate the wind set-up along the axis of the Bay. Thus, the water profile is tilted towards Stickney Point producing a net hydraulic gradient pointing towards Blackburn Point. That is, the Bay experiences a net set-up at Stickney Point, and a net set-down at Blackburn Point. This gradient produces a net flow opposite to the wind direction. However, a similar set-up and set-down in the neighboring bays would tend to reverse the gradients at Stickney Point and Blackburn Point thereby reducing the net flow through the Bay. This point is illustrated in Figure VIII-16. Accurate simulation of the set-up and set-down in the Little Sarasota Bay, and the boundary conditions imposed by the neighboring bays at Stickney Point and Blackburn Point, require the extension of the network into the neighboring bays. Furthermore, such an approach will also provide the possibility to calibrate the wind friction term in the model for storm wind-speed conditions.

As a first approximation in the present example of model application, the wind stress term was increased to balance the hydraulic gradient term, thus producing a more realistic flow pattern. The resulting simulated flows through the boundaries are reported in Figure VIII-17. Velocity vector plots showing the simulated internal circulation patterns of the Little Sarasota Bay during the storm are presented in Figures VIII-18 through VIII-27. The storm produced maximum velocities of 2.72 and 2.56 feet per second at Stickney Point and Midnight Pass respectively, much increased from the normal tide conditions. The plots also show increased velocities everywhere within the Bay which are wind induced. It is interesting to note that the effect of the wind blowing along the longitudinal axis of the Bay, which prevails up to 4:00 EST, is to produce a net flow in the direction of the wind in the shallow portions of the Bay, and a flow opposed to the wind

VIII-30

Figure VIII-17. Simulated Boundary Flows for "No-name" Storm

VIII-32 Figure VIII-18. Simulated Velocity Vectors at 1:00 a.m. for "No-Name" Storm

VIII-33 Figure VIII-19. Simulated Velocity Vectors at 2:00 a.m. for "No-Name" Storm

VIII-34 Figure VIII-20. Simulated Velocity Vectors at 3:00 a.m. for "No-Name" Storm

VIII-35 Figure VIII-21. Simulated Velocity Vectors at 4:00 a.m. for "No-Name" Storm

VIII-36 Figure VIII-22. Simulated Velocity Vectors at 5:00 a.m. for "No-Name" Storm

VIII-37 Figure VIII-23. Simulated Velocity Vectors at 6:00 a.m. for "No-Name" Storm

VIII-38 Figure VIII-24. Simulated Velocity Vectors at 7:00 a.m. for "No-Name" Storm

VIII-39 Figure VIII-25. Simulated Velocity Vectors at 8:00 a.m. for "No-Name" Storm

VIII-40 Figure VIII-26. Simulated Velocity Vectors at 9:00 a.m. for "No-Name" Storm

VIII-41 Figure VIII-27. Simulated Velocity Vectors at 1O:OO a.m. for "No-Name" Storm

VIII-42 in the deeper Intracoastal Waterway. The storm tide is also shown to reverse first at Blackburn Point due to the wind blowing from the west.

Bay volumes at high water and low water, and intertidal volumes are shown in Table VIII-5. The tidal excursion is seen to have increased drastically during the "No-Name" storm as compared to all previous conditions, April, July and November, from between 1 and 2 ft to 3.50 ft. Accordingly, the intertidal volume also increased dramatically to 350 million cubic feet. Therefore, storm wind conditions can be expected to provide a major flushing of Little Sarasota Bay.

TABLE VIII-5 SIMULATED BAY VOLUMES - "NO-NAME" STORM

Tidal Bay Volumes Intertidal Excursion High Water Low Water Volume

Ft 106 ft3 106 ft3 106 ft3

1.06 530.0 393.7 137.0 1.03 530.0 420.4 109.6 3.50 767.7 420.4 347.3 3.51 767.7 418.1 349.1

IMPACTS OF INTRACOASTAL WATERWAY ON BAY CIRCULATION

The present circulation of Little Sarasota Bay is dominated by the Intra- coastal Waterway. The Waterway extends along the Gulf Coast of Florida running through the many embayments that lie behind the seaward barrier islands or keys. The project depth of the waterway is nine feet referenced to mean low water with a 100 foot bottom width. The present Waterway was dredged in late 1963 through 1964 and is maintained by the U.S. Army Corps of Engineers. Prior to this time the Waterway was approximately five feet deep. Earlier dredging was initially performed in the 1920's.

One can speculate that, prior to any dredging activity, the separate embayments were more or less independent with only shallow connections between

VIII-43 the bays and with primary forcing directly from the Gulf of Mexico through passes in the barrier islands. The construction of the Intracoastal Water- way has since drastically changed this mode of exchange.

The National Ocean Survey (NOS) performed a tidal current study in the area during 1955, prior to the dredging of the present Intracoastal Waterway. This survey shows that the tide at that time was flooding through Midnight Pass into Blackburn Bay through Blackburn Point. The flow directions reported at Blackburn Point are opposite to those observed in the field surveys for the present study. The 1955 survey also reports zero velocities through Stickney Point. Interestingly, the 1955 survey is used as the basis for the Tidal Current Tables published annually by the NOS. The Current tables show Blackburn Point (Little Sarasota Bay, south end, bridge) flooding at a direction of 167° into Blackburn Bay in the 1983 edition (5).

The present configuration average maximum ebb and flood velocities extracted from the calibration and verification simulations are compared with the pre-Intracoastal velocities in Table VIII-6. Although the NOS data probably represent spring tides, while our results are more representative of average tidal conditions, the differences in velocities are significant. In addition, the changes in exchange flow volumes through the passes produced by the Intracoastal are even greater than indicated by the velocities, considering the approximately 80 percent increase in flow area through Black- burn Point and Stickney Point resulting from the Intracoastal Waterway dredging, and the subsequent narrowing and shallowing observed at Midnight Pass. Lack of accurate tidal boundary elevations for the pre-Intracoastal conditions precluded a simulation of the pre-Intracoastal Bay circulation.

VIII-44 Table VIII-6 PRE- AND POST-INTRACOASTAL WATERWAY AVERAGE MAXIMUM FLOOD AND EBB VELOCITIES THROUGH PASSES (fps) (1 fps = 1.7 knots)

Present Pre-Intracoastal Location Average Maximums (fps) Average Maximums (fps) Flood Ebb Flood Ebb

Stickney Point 0.98 -0.95 0.0 0.0 Midnight Pass 0.93 -0.70 3.17 -2.36 Blackburn Point 0.74 -0.68 -2.36 1.18

Note: Positive flows are into Little Sarasota Bay.

Midnight Pass has a long documented history of instability. In fact, remnants of the past locations of the Pass to Little Sarasota Bay are evident, present- ly known as Blind Pass and Heron Lagoon on Siesta Key. There is evidence that the Intracoastal Waterway has accelerated this natural process. A pre- Intracoastal bathymetric survey (1954-1955) shows the Pass to be twelve feet deep (MLW) and 170 feet wide. At that time the Pass was 1,000 feet south of its present location. The Pass configuration and velocity observations show that the Pass was tidally dominated and fairly stable. After the dredging of the Intracoastal the Pass became shallow. The present Pass is less than five feet deep (MLW) and much narrower than the 1954 configuration. The configuration of the Pass changes drastically over periods of a few weeks, and its direction of movement seems to be controlled by the average direction of the prevailing wind.

The dredging of the Intracoastal also had an effect on the shoaling processes within the Bay. With primary forcing through Midnight Pass as existed prior to the Intracoastal, flow velocities around Bird Island and in the central portion of the Bay in general were large enough to preclude shoaling in this area

VIII-45 of the Bay. However, simulations show that the construction of the Intra- coastal shifted the primary forcing to Stickney and Blackburn Points. This resulted in a nodal tidal point located north of Bird Keys. Thus, small velocities in this area produced shoaling by deposition of sediments which has greatly increased in recent years.

Finally, it can be conjectured that the dredging of the Intracoastal Waterway was a definite detriment to the water quality of the Bay. Prior to the Intra- coastal, the primary tidal exchange was directly to the relatively clean and well-mixed waters of the Gulf of Mexico. Present circulation patterns show a tidal exchange of water between the interconnected bays to the north and south. Although this effect was not important when the Intracoastal was con- structed, the water quality impacts from these changes may become more important as the region accepts new growth and development.

CONSEQUENCES OF MIDNIGHT PASS DREDGING

Midnight Pass has been variously dredged and blasted over the years, but never maintained in a regular manner. Midnight Pass is one of a few natural uncon- trolled and unstabilized passes along the Gulf Coast of Florida. To many resi- dents the natural beauty of the area, and Midnight Pass in particular, is considered a valuable resource. The Pass has a long history of constant movement. However, as described in the previous section, the dredging of the Intra- coastal Waterway greatly reduced the stability of the Pass. The Pass is pre- sently dominated by the action of surface waves, and the trend of its movement north or south tends to be primarily affected by the passage of severe storms and seasonal prevailing winds. Unfortunately, the recent northward trend of the Pass has placed the Pass the farthest north it has been in over 30 years. The erosion on the south end of Siesta Key has completely undermined the old Mote Marine Laboratory structures located on Casey Key and is causing concern to the neighboring property owners. The recent northward movement of the Pass has renewed interest in dredging and stabilizing the Pass. This action is also supported by property owners around Little Sarasota Bay since, at present, Midnight Pass is unnavigable to all but small boats. The nearest access to the Gulf of Mexico is through the Intracoastal Waterway to either Big Sarasota

VIII-46 Pass to the north or Venice Inlet to the south, both located a distance of four miles from either end of Little Sarasota Bay.

Two dredging scenarios are plausible. Scenario one entails the dredging of a channel through Midnight Pass, and along the south pass, to the Intracoastal Waterway, south of Bird Keys. Scenario two includes, in addition to the above channel, a second channel through the north pass around Bird Keys, to the Intracoastal Waterway. The dredged channels are assumed to have the same di- mensions as the Intracoastal Waterway with a nine foot MLW project depth and a 100 foot bottom width.

The Little Sarasota Bay Circulation Model was used to provide a first cut, rudimentary analysis of the effect of the dredging on the Bay circulation. It must be emphasized that this is by no means a full scale study of the Pass dredging. Such a comprehensive study would have to include consideration of littoral and other Gulf Coast currents, the effect on the neighboring bays, the ensuing sediment movement, and other aspects.

In the present example of model application, the links which represent the passes around Bird Keys were deepened and widened to simulate the dredged channel configuration for the two scenarios. The extended network into the Gulf of Mexico enables the model to accurately simulate the ensuing hydraulic changes around the Pass. However, an increase in the influence of the Gulf on the Bay through the dredged Pass is anticipated to also affect the boundary conditions at Stickney Point and Blackburn Point. In the present configuration of the Bay, Midnight Pass plays a minor role on the elevations and circulation within the Bay. As mentioned earlier, Stickney Point and Blackburn Point provide the main tidal forcing to Little Sarasota Bay, from Sarasota Bay and Venice Inlet respectively. The effect of the Midnight Pass dredging on Stickney Point and Blackburn Point cannot be fully understood with the present model network.

Both the April and July periods were simulated for both scenarios producing a total of four runs. Thus, both diurnal and semi-diurnal tidal effects were analyzed. The simulated results show significant changes produced by the Midnight Pass dredging throughout the Bay. Velocity vector plots for a

VIII-47 flooding through ebbing cycle for the July period are presented in Figures VIII-28 through VIII-32 for scenario one and in Figures VIII-33 through VIII-37 for scenario two. The results can be directly compared with the calibrated model results at the same times shown in Figures VIII-4 through VIII-8. During the flood tide the results show reduced velocities in the Southern Bay with little tidal exchange through Blackburn Point for both scenarios. The flow through Stickney Point is also decreased. The simulated times of reversals and the intertidal flow volumes and maximum velocities through the three tidal boundaries for each scenario are compared with the existing conditions in Tables VIII-7, VIII-8, and VIII-9. Under present conditions, Blackburn and Stickney Points each contribute on average of 42 percent of the tidal exchange over any tidal period with Midnight Pass contributing 16 percent. For scenario one, Midnight Pass contributes 65 percent of the net tidal inflow with Stickney Point contributing 27 percent and Blackburn Point the remaining eight percent. During the flood tide for the two-channel dredging scenario two, Midnight Pass, Stickney Point, and Blackburn Point contribute 81, 15 and four percent of the net tidal inflow respectively. During the ebb tide (Figures VIII-31, VIII-32, VIII-36 and VIII-37) Blackburn Point exhibits increased activity.

All above results do not include the effects of Midnight Pass dredging on Stickney Point and Blackburn Point in terms of water level and flow variation with time. The boundary conditions used in the above simulations of Stickney Point and Blackburn Point are observed conditions under present Midnight Pass conditions. It is likely that the dredging of Midnight Pass will affect the Northern and Southern bay boundaries. This could enhance or reduce the above simulated effects and probably accounts for the differences between flood and ebb tides. The boundary effects can easily be evaluated with the present model by expanding the model network into portions of Sarasota Bay and Blackburn Bay.

These results indicate that the dredging of Midnight Pass would have a definite effect on the circulation of Little Sarasota Bay. Scenario one with one pass to the south of Bird Island results in much reduced velocities in the southern Bay. The main tidal flow proceeds, from the channel, northward along the Intracoastal east of Bird Island to meet the tidal wave propagating south from Stickney Point at a location north of Bird Keys near the present nodal location. Flows in the northern Bay seem to be only slightly affected by this

VIII-48 TABLE VIII-8 SIMULATED FLOWS AND MAXIMUM VELOCITES FOR DREDGING SCENARIOS AT MIDNIGHT PASS

VIII-50 TABLE VIII-9 SIMULATED FLOWS AND MAXIMUM VELOCITES FOR DREDGING SCENARIOS AT BLACKBURN POINT

VIII-51 Figure VIII-28. Scenario 1 Simulated Velocity Vectors at Full Flood Flow, July 14, 1982

VIII-52 Figure VIII-29. Scenario 1 Simulated Velocity Vectors at Flood Flow Near High Tide, July 14, 1982

VIII-53 Figure VIII-30. Scenario 1 Simulated Velocity Vectors at High Tide. July 14, 1982

VIII-54 Figure VIII-31. Scenario 1 Simulated Velocity Vectors at Ebb Flow near High Tide, July 14, 1982

VIII-55 Figure VIII-32. Scenario 1 Simulated Velocity Vectors at Full Ebb Flow, July 14, 1982

VIII-56 Figure VIII-33. Scenario 2 Simulated Velocity Vectors at Full Flood Flow, July 14, 1982

VIII-57 Figure VIII-35. Scenario 2 Simulated Velocity Vectors at High Tide, July 14, 1982

VIII-59 Figure VIII-36. Scenario 2 Simulated Velocity Vectors at Ebb Flow Near High Tide. July 14. 1982

VIII-60 Figure VIII-37. Scenario 2 Simulated Velocity Vectors at Full Ebb Flow, July 14, 1982

VIII-61 scenario. Scenario two with a second channel along the north channel around Bird Island results in a larger effect on the entire Bay circulation. Flows are reduced everywhere within the Bay, except the dredged channels.

It can be concluded that the reduced flows and associated diminished flushing within the Bay for both scenarios would have a definite impact on the water quality of the Bay. However, other aspects are equally or more important in studying the effect of the Midnight Pass dredging on Little Sarasota Bay. These include: Gulf littoral currents and associated sediment transport and sediment migration, pass stability, short-term and long-term shoaling within the Bay and especially the Intracoastal Waterway, and finally the effect of the dredging on neighboring Bays. These effects should be systematically evaluated in a comprehensive study of the impacts of dredging and/or stablilizing Midnight Pass. The Little Sarasota Bay. Dynamic Estuary Model would be an in- valuable tool for such an assessment as illustrated by the above rudimentary application of the model.

CONVECTION AND DISPERSION CHARACTERISTICS AROUND BIRD ISLAND - DYE STUDY

Dye studies were performed in the Midnight Pass - Bird Keys area and at Blackburn Point on July 14, 1982. The results of these field surveys are reported in Appendix D. This discussion is mainly concerned with the Bird Island study. Twenty-five pounds of 20 percent Rhodamine WT dye was released in Midnight Pass during flood tide. The dispersion and movement of the dye was monitored by overflight photography and in situ fluorometer concentration measurements. The dye proceeded up the northern channel of Bird Island. The dye plume extended nearly to the Intracoastal Waterway before reversing and flowing back through the Pass with the ebb tide and dispersing in the Gulf of Mexico. Typical aerial photography snapshots of the plume at Midnight Pass are shown in Figures VIII-38 and VIII-39. Twenty-five pounds of 20 percent Rhodamine WT dye was released at the Blackburn Point bridge during the next flood tide. Photographs of the extent of the plume along the Intracoastal waterway are shown as Figures VIII-40 and VIII-41. Detailed figure captions are on the next page.

VIII-62 The DYNTRAN (DYNamic TRANsport) version of the Dynamic Estuary Model was exer- cised in a test example of the model's ability to simulate the mass transport properties of the Bay using the Midnight Pass dye study. DYNTRAN represents a further development of DEM to include the dynamic mass transport of salt and a second non-conservative constituent. The density of salt is weakly linked to the momentum equation through an equation of state. In this application, salt is not simulated. The transport of the dye through the nodes and links of the north pass of the Bird Keys was simulated by releasing the dye as a point release at the node most closely representing the actual release location (node 65). The observed and simulated dye plumes after four hours are compared in Figure VIII-42 and the simulated dye concentrations at the affected nodes are shown in Figure VIII-43. These results are based on an earlier, crudely calibrated version of the simulated hydraulic flow field.

The above model application represents a crude analysis of the dynamic transport model. However, the DYNTRAN transport model or the DYNQUAL water quality model are directly compatible with the Dynamic Estuary Model as set-up and calibrated for Little Sarasota Bay. These models, along with an adequate field program, are strictly applicable to assessing the present water quality of the Bay and to determining the effects of future development and improvements, such as Midnight Pass dredging or increased urbanization of the surrounding drainage basins on the Bay, as a direct extension of this study.

VIII-68 Figure VIII-42. Dye Study - Predicted and Observed Plume After Four Hours

VIII-69 VIII-70 IX. CONCLUSIONS AND RECOMMENDATIONS

A study of the circulation in Little Sarasota Bay was performed. It encompassed the deployment of a numerical hydrodynamic model of the Bay; the design of a field program for the collection of in situ data; the calibration and verification of the numerical model with the field data; and a series of applications of the model to illustrate its use as a predictive and decision making tool for improved management of the Bay resources. Equally important with the development of the model was the improved understanding of the Bay circulation patterns, and possible adverse effects from navigational improvements and other land use projects. The most important findings are summarized below.

The main tidal forcing for the Bay occurs at the north and south passes of the Bay, Stickney Point and Blackburn Point respectively, with secondary forcing occurring at Midnight Pass. This forcing causes the Bay to rise and fall as one body (unimodal oscillation) with small horizontal velocities occurring at the center section of the Bay just north of Bird Keys. This is the cause of the presence of the many keys and shallows in the mid-section of the Bay. An average tidal excursion for the Bay is 1.5 feet. The excursion varies greatly during the year and between the diurnal and semi-diurnal tides. This excursion causes approximately 30 percent of the total volume of the Bay to be exchanged during the tidal cycle. By assuming a completely mixed system, the detention time is on the average 3.3 tidal cycles. Of course, the detention time in areas of stagnation and incompletely mixed zones is much greater.

The present day circulation patterns in the Bay are seen to be drastically affected by the dredging in the 1960's of the Intracoastal Waterway. The Intra- coastal causes most of the tidal exchange to take place through Stickney Point and Blackburn Point with Sarasota Bay and Blackburn Bay respectively. As a con- sequence, Midnight Pass has atrophied and become unstable, moving continuously around the midsection of the Bay in reponse to the predominant monthly wind directions, Gulf Coast littoral currents and other effects, thereby jeopardiz- ing development on the various keys of Little Sarasota Bay. The stabilization

IX-1 of Midnight Pass should be studied by analyzing all causes of Pass movement carefully, and specifically the currents in the Gulf of Mexico. The decreased Bay response to Midnight Pass does not in itself cause the Pass to move. It only reduces the stabilizing forces that tend to oppose Pass movement.

The Intracoastal Waterway has also significantly altered the flow and velocity field in the Bay; Most of the flow and the highest velocities are observed along the Waterway and other channels with 180° flow reversals between ebb and flood tides. The tidal flats are merely flooding from the Intracoastal and draining into the Intracoastal at ebb tide. The configuration of the tidal flats in the northern part of the Bay however, causes the consistent generation of vortices at ebbing tide. While being local and short-lived phenomena, these vortices may significantly alter the mixing properties of that part of the Bay and should be carefully considered in establishing the dispersion properties for the evaluation of the water quality in the Bay.

Wind conditions also affect the Bay circulation because of its shallowness. Equally important however, wind conditions also affect the shallow Gulf shelf and the neighboring Sarasota and Blackburn Bays. The complex interactions of these embayments need to be further analyzed. Perhaps the most cost-effective way to include these effects in the model circulation is by extending the network to represent portions of the neighboring bays.

In conclusion, valuable insight was gained into the circulation of Little Sarasota Bay. The circulation model proved a valuable tool in gaining this insight and analyzing the impacts of various developments on this circulation. Future work should focus on the following areas:

0 thorough case-studies of growth scenarios and other projects, such as Midnight Pass dredging and stabilization;

0 studying neighboring bay interaction by extending the present network;

IX-2 0 set-up a water quality model of the mass transport of conservative and non-conservative substances using one of several models directly com- patable with the present model. This model would be used to analyze in depth the dispersion and mixing properties of the Bay as they may be affected by dredging and other projects in the Bay; and

0 adaptation of the computer models and/or the associated plotting capa- bility to a microcomputer for in-house use.

IX-3 REFERENCES

(1) U.S. Department of Commerce, "Tide Tables 1982 - High and Low Water Pre- dictions - East Coast of North and South America Including Greenland", National Ocean Survey, National Oceanic and Atmospheric Administration.

(2) Weather Bureau, U.S. Department of Commerce, Technical Paper No. 40, "Rainfall Frequency Atlas of the United States", January 1963.

(3) Water Resources engineers, "Dynamic Estuary Model (DEM)", prepared for Berkeley, Charleston and Dorchester Counties, South Carolina, December 1977.

(4) Camp Dresser & McKee, "New York City Flood Insurance Study, Report NO. 5, Simulation of Hurricanes", pp. VI-1, VI-30, Annandale, VA., November 1980.

(5) U.S. Department of Commerce, "Tidal Current Tables 1982 - Atlantic Coast of North America," National Ocean Survey, National Oceanic and Atmospheric Administration.