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Wilhoit, Jack C.
Morphodynamics of Bunces Pass, Florida
h [electronic resource] /
by Jack C. Wilhoit II.
[Tampa, Fla.] :
University of South Florida,
Thesis (M.S.)--University of South Florida, 2004.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
J. C. Wilhoit?
e 123 leaves : ill. (some col.), maps (some col.) ; 28 cm.
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ABSTRACT: Bunces Pass is an unstructured tide-dominated inlet just north of the main entrance to Tampa Bay, Florida. The inlet has been stable for at least 130 years, as the size, shape, and orientation have remained unchanged. The morphological evolution of the Bunces Pass ebb-tidal delta is influenced by adjacent inlets. Historically, the ebb tidal delta was extremely large, due to the presence of the south channel of Pass-A-Grille Pass. As the tidal prism decreased through the south channel, the sheltering effect produced by the large ebb tidal delta diminished, and large volumes of sand began migrating shoreward. Sediment from the ebb tidal delta accreted along "the Reefs", formed both North Bunces Key and South Bunces Key, and accreted on Mullet Key south of the inlet. Tidal currents at Bunces Pass are primarily ebb-dominant during both summer and winter seasons, though there is flood dominance for several days during neap tides.The ebb dominance is primarily due to the large back-barrier embayment of Tampa Bay, which results in a spring ebb tidal prism of 2.02 x 10p7.The present situation at Bunces Pass shows a stabilized, tide-dominated inlet with a large, elongate ebb delta that is unlikely to change significantly in the future if present conditions are maintained. The prevalent ebb-dominance suggests that the inlet is hydraulically connected to the adjacent and much larger Egmont Channel inlet system, which also serves Tampa Bay. Strong ebb-tidal currents have kept Bunces Pass in dynamic equilibrium with its surrounding environment. The large ebb tidal prism is responsible for explaining how a tide-dominated inlet is maintained in a microtidal environment.
Bunces Pass (Fla.)
t USF Electronic Theses and Dissertations.
Morphodyn amics of Bunces Pass, Florida by Jack C. Wilhoit II A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology College of Arts and Sciences University of South Florida Major Professor: Richard A. Davis, Jr. Ph.D. Peter A. Howd, Ph.D. Eric A. Oches, Ph.D. Ping Wang, Ph.D. Date of Approval: November 18, 200 4 Keywords: ebb tidal delta, hydrodyna mics, microtidal sedimentology Copyright 2004 Jack C. Wilhoit I I
2 ACKNOWLEDGEMENTS I would like to extend my deepest gratitude to Dr. Richard A. Davis Jr. for his guidance, persistence, expertise, and enduring support throughout this project. I would like to extend thanks to Dr. Ping Wang for his advice, motivation, and expertise needed to succeed in the study. Additionally, a sincere thank you to Dr. Peter A. How d, whose support and encouragement helped maintain my motivation throughout the project. I would also like to thank Dr. Eric A. Oches for his support and help with maintaining the big picture. A thanks goes out to the Army Corps of Engineers, Coastal I nlets Research Program, who provided the initial funding for this project. I also would like to thank Captain Rich Young of the USGS and Nicole Elko, formerly with the USGS, for their help deploying and retrieving equipment used in the study. I am extreme ly grateful for the assistance of numerous people involved with the Coastal Research Laboratory at the University of South Florida: Dave Tidwell, for his dedication, motivation, and thought provoking discussions; Alyssa St. John and Tiffany Dawson, for dev oting numerous hours helping sieve sand; Dr. Daidu Fan, for his GIS wizardry, and Dr. Niels Vinther, for his expertise, motivation, and consistent support. Thanks goes out to my parents, grandparents, and brothers and sisters for their love and support, gi ving me the tools necessary to accomplish big things, and for always believing in me. I want to thank Tatiana and Sam for th eir patience, love, and encouragement Lastly, thanks to my wife Catherine, whose love and support kept me strong, whose patience kept me vigil, and for encouraging me to see this project through to completion.
i TABLE OF CONTENTS List of Tables iii List of Figures iv Abstract vi Introduction 1 Objectives 2 Geographic Set tings and Description of Study 5 Geologic History 7 Hydrographic Regime 1 1 Previous Work 14 Tidal Inlet Hydrodynamics 14 Tidal Inlet Morphodynamics 20 West Central Florida Coast 25 Procedures 27 Aerial Phot ography Analysis 28 Bathymetric Survey 29 Tidal Currents and Wave Data 30 Surface Se diment Collection 34 Historical Morphodynamics 36 Sequential Analysis from 1873 to 1997 36 1873 to 1895 36 1904 to 1945 38 1950 to 1963 40 1970s 43 1980s 43 1990s 50 Recent Bathymetry 5 2 Summary of Historical Morphodynamics 5 4 Tidal Pr ocesses at Bunces Pass 5 8 Tides 58 Tidal Cur rents Within the Inlet Throat 6 1
ii Tidal Currents Gulfward of the Inlet Throat 7 3 Tidal Prism 75 Seasonal Vari ations in Tidal Currents 8 2 Factor s Influencing Tidal Flux 8 3 Summary of Tidal Currents 8 9 Sediment Characteristics o f Bunces Pass Ebb Tidal Delta 91 Sediment Characteristics of the Ebb Delta Complex 9 3 Sediment Distribution 101 Conclusions 10 4 References 10 7 Appe ndix 1 20
iii LIST OF TABLES Table 1 Sta bility criterion for sandy coast inlets related to ebb delta size and bypassing (Bruun, 1977). 17 Table 2 Field data sets u tilized in the present study. 27 Table 3 Number of sediment samples collected within the ebb tidal delta sub environments. 34 Table 4 Mean tida l range data from Bunces Pass 60 Table 5 Spring and Neap Tidal Data at Bunces Pass duri ng summer and winter seasons. 61 Table 6 Sediment Characteristics of Bunces Pass Ebb Tidal Delta 92
iv LIST OF FIGURES Figure 1. Bunces Pass, Florida April 2001. 1 Figure 2. Location of the study area, a map of West Central Florida Coast. 2 Figure 3. Bunces Pass ebb tidal delta is super imposed on the larger Egmont Channel ebb tidal delta (Crowe, 1983). 6 Figure 4. Classif ication scheme for inlets in the west central Florida barrier chain (Davis and Gibeaut, 1990) 6 Figure 5. A stratigraphic column for central and south Florida indicates Miocene Tampa Limestone served as the foundation upon which the present barrier is land system developed (Duncan et al., 2003) 8 Figure 6. A structure contour map of Egmont Channel and Bunces Pass showing the ex tent of a paleochannel system. 10 Figure 7. General relationships between tidal range and wave height as it relates to coasta l morpho logy (Davis and Hayes, 1984). 12 Figure 8. Location of ADPs during 2000 s ummer and winter deployments. 31 Figure 9. Bunce s Pass 1873 nautical chart. 37 Figure 10. Bunces P ass 1945 aerial photograph. 39 Figure 11. Bunces Pass 1951 aerial p hotograp h. 40 Figure 12. Bunces P ass 1957 aerial photograph. 41 Figure 13. Bunces P ass 1963 aerial photograph. 42 Figure 14. Bunces P ass 1973 aerial photograph. 44 Figure 15. Bunces P ass 1976 aerial photograph. 45 Figure 16. Bunces Pass 1980 aerial photograph. 46
v Figure 17. Bunces P ass 1986 aerial photograph. 47 Figure 18. Bunces P ass 1991 aerial photograph. 49 Figure 19. Bunces P ass 1997 aerial photograph. 50 Figure 20. Composite bathymetry of Bunces Pass from 2002 and 2003. 51 Figure 21. Bunces Pass morphology from 1873 to 1997. 53 Figure 22. Historic variations in the channel width at Bunces Pass. 54 Figure 23. Bunces P ass 2003 aerial photograph. 55 Figure 24. The orientation and width of Bunces Pass has not changed signif icantly within at least the past 131 years. 56 Figure 25. Plots of tidal ranges along the coast of the Gulf peninsula of Florida 58 Figure 26. Tidal range data was collected at Bunces Pass. 59 Figure 27. Average current velocity data were collected at Bunces Pass to represent both sea sonal and spatial differences 62 Figure 28. During summer neap tides, maximum flood current velocities and durations were greater than those of ebb at Bunces Pass 64 Figure 29. Bunces Pass was ebb dominant during s umm er spring tidal conditions on August 27, 2000. 65 Figure 30. Tidal currents from within the inlet channel during the winter exhibited a similar pattern of flood dominated neap tides. 66 Figure 31. Spring tides during the winter season were completely e bb dominant. 68 Figure 32 Vertical velocity profiles were plotted from data collected within the inlet throat during the summer deployment. 69 Figure 33 Vertical velocity profiles were plotted from data collected within the inlet throat during the wi nter deployment. 71 Figure 3 4 Time velocity data from currents Gulfward of the inlet throat during neap tides were plotted. 73
vi Figure 3 5 Tidal currents Gulfward of the inlet throat were ebb dominant during spring tidal conditions. 74 Figure 3 6 Vert ical velocity profiles from tidal current data Gulfward of the inlet throat were plotted. 76 Figu re 37 Tidal prisms at Bunces Pass were plotted for currents within the inlet during the summer and winter deployments, as well as for currents Gulfward of the inlet during the winter. 7 8 Figure 38 Barometric pressure and peak wind gust data from St. Petersburg from August through December 2000 were plotted. 75 Figure 39. Hourly barometric pressure (blue) and wind direction (red) data from St. Petersbur g, Florida from November 17, 2000 through December 23, 2000 were plotted. 83 Figure 40 A minor front passed on November 18, 2000, causing prolonged ebb duration. 84 Figure 41 Meteorological data from Anna Maria Island, located approximately 12 mil es south of Bunces Pass, were plotted from August 22, 2000 to August 29, 2000 85 Figure 42 The bimodality of the beach/nearshore zone along the west central Florida coast reflects the composition with little overlap between the shell gravel and the q uartz sand (Crowe, 1983). 91 Figure 43 Surface sediment sample locations, October 2002 are representative of five sub environments. 93 Figure 44 Mean grain size data from the various sub environments at Bunces Pass were plotted. 94 Figure 45 The perc entage of carbonate composition in sediments within the various sub environments at Bunces Pass were plotted. 95 Figure 46 The percentage of gravel composition in sediments within the various sub environments at Bunces Pass were plotted. 96 Figure 4 7 Percentages of carbonate and gravel abundance from sediment samples within t he main channel were plotted. 97 Figure 48 Sediment transport patterns on the Bunces Pass ebb tidal delta. 100
vii Morphodyn amics of Bunces Pass, Florida Jack C. Wilhoit II ABSTR ACT Bunces Pass is an unstructured tide dominated inlet just north of the main entrance to Tampa Bay, Florida. The inlet has been stable for at least 130 years as the size, shape, and orientation have remained unchanged. T he morphological evolution of the Bunces Pass ebb tidal delta is influenced by adjacent inlets. Historically, the ebb tidal delta was extremely large, due to the presence of the south channel of Pass A Grille Pass. As the tidal prism decreased through the south channel, the sheltering effect produced by the large ebb tidal delta diminished and large volumes of sand began migrating shoreward. Sediment from the ebb tidal delta accreted along the Reefs, form ed both North Bunces Key and South Bunces Key, and accreted on Mullet Key sout h of the inlet. Tidal currents at Bunces Pass are primarily ebb dominant during both summer and winter seasons though there is flood dominance for several days during neap tides The ebb dominance is primarily due to the large back barrier embayment of T ampa Bay, which results in a spring ebb tidal prism of 2.02 x 10 7 m 3 This tidal prism is more than 400 times the co rresponding littoral drift. It is primarily responsible for maintaining the inlets stability, as well as the development of its large ebb tidal delta. Sediments from the ebb tidal delta at Bunces Pass reflect different degrees of wave versus tidal energy. The s t rong est tidal currents present throughout the entire ebb tidal delta complex mechanically weather shell gravel in the main channe l producing a shelly,
viii fine quartz sand with relatively high amounts of shell gravel and carbonate sand. This sub facies is also present on the north channel margin linear bar, due to the interaction of waves, tidal currents, and a southerly littoral drif t along this coastal reach. Fine quart z sand dominates the off shore and swash platform environments The present situation at Bunces Pass shows a stabilized, tide dominated inlet with a large, elongate ebb delta that is unlikely to change significantly in the future if present conditions are maintained. The prevalent ebb dominance suggests that the inlet is hydraulically connected to the adjacent and much larger Egmont Channel inlet system, which also serves Tampa Bay. Strong ebb tidal currents have ke pt Bunces Pass in dynamic equilibrium with its surrounding environment. The large ebb tidal prism is responsible for explaining how a tide dominated inlet is maintained in a microtidal environment.
1 INTRODUCTION The West Central Florida coast con tains one of the most morphologically diverse barrier/inlet systems in the world. Located on the Florida Peninsula of the Gulf of Mexico, this coast extends nearly 300 km south from Anclote Key to Cape Romano. Barrier islands and inlets of all types and sizes exist along this low energy coast (Davis, 1987) Inlets along this section of Floridas coast vary in size and stability due to the complex interaction between tidal and wave energy. Tide wave dominated, and mixed morphologies are present. This low energy coast provides an opportunity to investigate the cause and effect relationships of tidal energy, wave energy, and sediment supply with regard to inlet morphodynamics. This study examined these factors at Bunces Pass ( Figs. 1 and 2 ), in an effo rt to determine their relative effects on one another. This information will help explain how a pristine, tide dominated inlet has maintained itself in a microtidal environment. Bunces Pass is an unstructured, tide dominated tidal inlet located along the coast of Pinellas County, Florida ( Fig. 2 ). The time of formation is unknown, predating 18 th century Spanish charts. Romans (1775 ) cited evidence that the Bunces Pass ebb tidal delta existed in 1775 when he charted the west coast of Florida. The inlet i s primarily used for recreational purposes and is not artificially maintained. Bunces Pass is characterized by a deep and wide channel thalweg that shallows and widens Gulfward.
2 The inlet shares a portion of Tampa Bays tidal prism with the adjacent and significantly larger Egmont Channel, located south of Bunces Pass ( Fig. 2 ). Objectives and Significance Tidal inlets serve as conduits between bays and open water bodies. It is important to understand the processes responsible for inlet maintenance, in an effort to improve coastal management and engineering practices. This study aids in developing a more comprehensive understanding of tidal inlet morphodynamics on a microtidal coast. The results of this investigation reveal the historic and modern m orphodynamics of Bunces Pass. Processes responsible for the inlets maintenance and long term stability were examined incorporating observations of inlet morphodynamics, hydrodynamics, and Figure 1 Bunces Pass, Florida is a tide dominated inlet with an elongate ebb delta. The inlet is flanked by North Bunces Key to the north and Mullet Key to the south.
3 sedimentation patterns. Various tidal inlets along this section of the west central Florida coastline have shown a wide variability in their size, shape, location, and orientation throughout time. These changes are primarily due to the wave dominated nature of inlets in this microtidal environment. However, the fact t hat Bunces Pass has been in existence since at least the late 1700s, suggests that the inlet is relatively stable. Several possibilities exist as to why the inlet has remained stable, which include: 1) the presence of antecedent topography acting as a hea dlands between which tidal flow occur s ; 2) the nature of the tidal Egmont Ch annel South Channel Bunces Pass Pass A Grille Pass Blind Pass Johns Pass TAMPA BAY Pinellas Hillsborough Manatee Figure 2 Map of the Tampa Bay area on the west central Florida coast. Bunces Pass is located between Pass A Grille Pass and Egmon t Channel.
4 hydrodynamics; 3) the location of Bunces Pass relative to its proximity to adjacent inlets; 4) the distribution of sediment along this section of the coastal reach, or 5) the influence of weather related events, from the small scale effects of sea breeze and land breeze to much larger events, such as tropical depressions and hurricanes. In order to assess the morphological changes to Bunces Pass, it was necessary to document the inlets ov erall orientation, shape, and volumetric changes through time. This study, using aerial photographs and maps, chronicled the recorded history of Bunces Pass, with emphasis placed on the inlet channel and its ebb tidal delta. A bathymetric survey enabled a comparison of changes in the inlet with the current bathymetry. This survey also permitted distinguishing the ebb tidal deltaic features for a sedimentological characterization of the current ebb tidal delta. Variable waves, currents, and winds continua lly rearrange sediments of the ebb tidal delta and the adjacent inlet channel. Current measurements were recorded over several spring and neap tidal cycles in an effort to understand inlet hydrodynamics. Surface sediment samples from the ebb delta were a lso collected to characterize the inlets depositional environments. These data enabled the development of a morphodynamic model of the inlet. Specific objectives of this study include addressing the: 1) history of the ebb delta, 2) influence of adjacent inlets on Bunces Pass, 3) influence of inlet hydrodynamics on delta morphology, and 4) maintenance of tide dominated morphology in a microtidal regime, specifically, why Bunces Pass has maintained itself over the period of record. In general, what does tid al current data, historical morphodynamics, and sedimentology on ebb tidal delta reveal about the stability of Bunces Pass ? These points were addressed
5 through a combination of detailed current measurements through the inlet channel, descriptive analyses of time series aerial photography, inlet and ebb delta bathymetry, and the characterization of sediments on the present ebb tidal delta. Geographic Settings and Description of Study Area The Florida Peninsula represents a wide range of coastal morphologie s and can be subdivided into five geomorphic regimes: 1) the east coast barrier system, 2) a limestone arc, the Florida Keys, 3) the mangrove coast of southwest Florida, 4) the central Gulf barrier system, and 5) the marsh coast of the Big Bend area (Davis et al., 1992). The West Central Barrier Chain, located along Floridas west coast, consists of 29 barrier islands and 30 inlets, extending nearly 300 km. Bunces Pass is located near the southern terminus of a 70 km long chain of barrier islands in Pinel las County. This area is a low, mixed energy coast with mean annual wave heights of 25 to 30 cm and tidal range less than a meter. Dominant longshore transport is to the south with several local reversals due to wave refraction and shoreline orientation (Davis, 1999). The inlets in the study area have been classified as tide dominated, wave dominated, or mixed energy due to the relative influences of wave and tidal energy. The primary controlling factor on inlet channel morphology is the magnitude of the tidal prism relative to wave energy (Davis and Gibeaut, 1990). Tidal prisms along this section of the coast range over four orders of magnitude (Davis, 1994), however, tidal range remains fairly consistent throughout the reach. Differences in tidal pris ms are due to differences in the size of back barrier embayments that they serve. There are no major
6 sources of freshwater discharge in the area; therefore, inlet morphodynamics are controlled by inlet system relationships (Davis and Gibeaut, 1990). Bunce s Pass is a distinctly tide dominated, natural inlet characterized by a main channel that is approximately 400 m wide at the throat and 700 m at its Gulfward terminus. The channel has a maximum depth of 9.3 m in the inlet throat, and extends westerly appr oximately 1 km into the Gulf of Mexico. Bunces Pass has an ebb tidal delta that is superimposed on the larger Egmont Channel ebb tide delta ( Fig. 3 ). Bunces Pass is elongate shaped and roughly symmetrical, which is typical of a tide dominated inlet (Davi s and Gibeaut, 1990) ( Fig. 4 ). Figure 3 Bunces Pass ebb delta is superimposed on the larger Egmont Channel ebb delta (Crowe, 1983).
7 Geologic History The Florida Peninsula rests on a large, stable carbonate platform that was isolated from its initial inception in the Mesozoic until the late Paleogene when the Suwannee Straits/Gulf Trough seaways filled (McKinney, 1984). The seaway had prevented sediments from the southern Appalachian Mountains from inundating and smothering the carbonate producing environment to the south. Eventually, the seaways were in filled and peninsular Florida was covered by si liclastics from the north via fluvial and longshore transport processes. The distribution of the quartz sand cover is a result of the combination of antecedent topography and multiple sea level fluctuations (Davis et al. 1992). The geologic history of Pin ellas County indicates that Tampa Bay is situated in the middle of the Neogene carbonate Florida Platform (Duncan et al. 2003) ( Fig. 5 ). This limestone platform, which is characterized by a gentle, Gulfward slope of approximately Figure 4 Classification scheme for inlets in the west central Florida barrier chain. The ocean is to the lef t, the bay to the right. Net littoral drift moves from top to bottom except in the wave dominated case (Davis and Gibeaut, 1990).
8 1m/1km, became exposed and eroded at the end of the Miocene. Karstification of the limestone r esulted in a shelf valley system at the mouth of Tampa Bay that formed during a late Miocene sea level lowstand (Her bert, 1985). The Miocene Limestone, which varies between 0 to 30 m below sea level along this section of Floridas coast, crops out at sea level on the mainland adjacent to Anclote Key and Sand Key (Barnard, 1998). Dramatic sea level fluctuations persisted through the Pliocene and Pleistocene, due to the expansion and con traction of massive ice sheets. As a result, late Miocene Figure 5 A stratigraphic column for central and south Florida (Duncan et al. 2003).
9 carbonate siliclastics became partially covered by Plio Pliestocene terrigenous sediments that are very fossiliferous locally. A dramatic rise in sea level occurred through the Holocene until appr oximately 3000 years before present, when the rate of sea level rise abruptly decreased, setting the stage for the development of the present barrier island system. In northern Pinellas County, a veneer of Holocene sediments thins Gulfward to depths of ap proximately 6 m, where Miocene Limestone is exposed (Evans et al., 1985). T he presence of both fluvial and tidal processes resulted in the general absence of a post Miocene, pre Holocene stratigraphic record along parts of this reach (Ferguson and Davis, 2003) Bunces Pass is situated within Holocene sand that is underlain by Plio Pleistocene sediment. This sediment is further underlain by the Miocene Hawthorne Formation. Well log data from Fort Desoto, located approximately 1 km south of Bunces Pass, in dicate numerous lithofacies are present which represent several different pre Holocene depositional environments (Ferguson and Davis, 2003). Holocene sand extends to approximately 11m below present sea level. This sand rests unconformably over 3.5 m of Pleistocene sand, which rest unconformably over 10 meters of additional Pleistocene sand. Pliocene sediments are an additional 10 meters thick and rest over the blue green clay, which caps the Miocene limestone. The upper surface of the blue green clay, known as the Hawthorne Group, is 38 m below present sea level (Ferguson and Davis, 2003). Bunces Pass is not structurally controlled by underlying bedrock surface, as is common along the barrier system in northern Pinellas County (Ferguson and Davis, 2003 ). However, recent high resolution, seismic data collected at the mouth of Tampa,
10 suggest that the location of Bunces Pass coincides with a huge paleochannel system connected the larger Egmont Channel ( Fig. 6 ) (Duncan et al. 2003). These authors further present evidence that supports a Miocene origin for the shelf valley system underlying the mouth of Tampa Bay. Figure 6 A structure contour map of Egmont Channel and Bunces Pass showing the extent of a paleochannel system. The paleochannel is located approximately below Egmont Channel, although it remains deep to the north towards Bunces Pass (Duncan et al. 2003)
11 Hydrographic Regime The Florida Peninsula occupies a section of a subtropical climate belt that has distinct seasonal weather patterns. During the spring and summer months (March to September), the area is dominated by the western portion of the Bermuda high that produces a clockwise atmospheric circulation. The result is southeasterly prevailing winds. The fall and winter months (October to M arch) are subject to cold fronts that move south into the Gulf of Mexico from Texas and proceed eastward at intervals of approximately 5 8 days (Fox and Davis, 1976). The anti cyclonic systems produce wind direction from the northwest to north. Consequen tly, wind strength is fairly weak during the summer and strong during frontal passage in the winter (Davis and Andronoco 1987). Local and severe thunderstorms are common in late afternoon and evening during the summer (Henry et al., 1994), but they have no significant influence on coastal processes (Davis and Barnard, 2003). The coast is subject to tropi cal storms and hurricanes that originate in the equatorial Atlantic Ocean and move northwest through the Caribbean. However, the hurricane of 1921 was th e only hurricane that has come ashore in the vicinity of Bunces Pass during the past century. Hurricane Pass, located in northern Pinellas County, was formed by the breaching of Hog Island in 1921 as a result (Cuffe, 1991). Hurricanes are frequent in the Gulf of Mexico, but are not common on the west central Florida coast (Davis and Barnard, 2003). Therefore, hurricanes do not play a significant role in local coastal morphodynamics along this section of coastal reach The Gulf of Mexico is a fetch limite d Mediterranean, situated on a gentle sloping continental shelf. The shelf, approximately 200 km wide with a gradient of 1:1300 along
12 this section of Floridas coast (Tanner, 1960), provides protection by dissipating waves. Consequently, wave energy is r elatively low. As previously mentioned, this section of Floridas coast is characterized by nearshore wave heights of 25 to 30 cm (Tanner, 1960; Davis and Andronaco, 1987) and a wave period of 3 to 4 seconds. Offshore data show that from October to April 65% of waves are less than 1 m, whereas from May to September it is 90% (U. S. Weather Command, 1975). The most important reasons for the low wave climate are the limited fetch of the Gulf, the nature and distribution of wind energy, and the very gentle gradient of the inner continental shelf (Davis, 1995). This section of Floridas coast is microtidal, with mixed tides. Semi diurnal cycles of unequal height occur during most of the lunar month, and diurnal tides the remainder of the time (Dept. of Comm erce, NOAA, 1981). Neap and spring tides range between 0.65m and 0.90m, respectively (Davis, 1988). Tidal current velocities vary, with local maximum values ranging from 100 cm/sec at Blind Pass (Tidwell and Wang, 2004) to 160 cm/sec at Bunces Pass (Wilh oit, et al. 2003). The relatively low wave climate coupled with the microtidal environment categorized the inlet as a mixed energy, tide dominated inlet, according the relationship presented by Davis and Hayes (1984) ( Fig. 7 ). Longshore currents along thi s section of Floridas coast flow to the north during the summer due to prevailing winds that vary between the southeast and southwest. The dominant annual longshore transport, however, is to the south due to the energy associated with the passage of wint er cold fronts. These cold fronts are the dominant agent of coastal change. Numerous local reversals of longshore transport exist, due to wave refraction and shoreline orientation Rates of transport range widely, though, the annual net littoral transpo rt rate is less than 50,000 m 3 per year (Davis 1999 ).
13 Figure 7 General relationships b etween tidal range and wave height as it relates to coastal morphology. Bunces Pass (star) is located near the bottom left corner of the chart (after Davis and Hayes, 1984).
14 PREVIOUS WORK Geologists and engineers have approached the study of tidal inlets independently, with the focus of each individually reflecting the objective of the study. Early observations of tidal inlets led to the development of initial theories about inlet mechanics (LeConte, 1905; Gilbert, 1914; Johnson, 1919; and Lucke, 1934). LeConte (1905) studied harbors along the Pacific coast, comparing inlet channel cross sectional areas with tidal prisms. Johnson (1919) analyzed the relationship between the lateral migration of tidal inlets and the shoreward displacement of barrier islands. Lucke (1934) described active processes and surficial sediments at Barnegat Inlet, New Jersey, illustrating how tidal delta deposits can be analyzed to infer an inlets historical activity. Tidal Inlet Hydrodynamics Pioneering studies regarding inlet hydraulics resulted in presumptive analytical solutions and empirical constants, serving as the foundation for f urther research. Initial inlet studies used one dimensional approaches to understand inlet currents and bay tid al responses. Brown (1928 ) used a simple analytical approach at Absecon Inlet, New Jersey to show that inlet hydraulics were strongly influence d by inlet and bay geometries, and ocean tidal range. In 1951, Keulegan solved the one dimensional, depth averaged shallow water wave equation for inlet flow. Later, Keulegan (1967) defined a hydraulic solution for an inlet throat, providing a simple sol ution to inlet hydraulics. He defined
15 the inlet system as a channel (with length L and cross section A c ) connecting a bay (with surface area A b ) to the open ocean. King (1974) solved Keulegans (1967) steady state hydraulics equation, though additionally included the effect of inertia. After inlet hydraulics were described, attempts were made to incorporate sediment transport models. However, stability analysis was hampered by inadequate sediment transport theories. Sager and Hollyfield (1974 ) pioneer ed the development of an actual scale model of Masonboro Inlet, North Carolina. Zarillo and Park (1987) successfully combined the inlet model of Seelig et al (1977) with five sediment transport equations to describe sedimentation at Stony Brook Harbor, w hich is located on the north shore of Long Island, New York. However, these models did not account for major storm events, thus limiting their practicality for long term use. Kraus (1998) later developed mathematical and conceptual models of inlet hydrod ynamic processes. Other researchers approached inlet dynamics by focusing their studies on empirical methods to arrive at fundamental relationships (OBrien 1931, 1969; Escoffier, 1940; Bruun and Gerritsen, 1959, 1960; OBrien and Dean 1972; Mehta et al. 1975; and Jarrett, 1976). OBrien (1931) elaborated on Lecontes (1905) work on Pacific coast inlets comparing inlet size and tidal prism. OBrien observed that inlet size is dependent on the tidal prism. He developed the following fundamental relations hip between inlet throat cross sectional area (A c ) and tidal prism (O), A c = 4.69 x 10 4 O 0.85 (1) where cross sectional area (A c ) is measured in ft 2 and tidal prism (O) is in ft 3 Escoffier (1940) found that a relationship exists between the maximum flow velocity through the throat of the inlet and t he throat cross sectional area to determine if
16 the inlet is stable or unstable. He found that when increasing the cross sectional area, maximum velocity reaches a peak value for some intermediate stage, then decreases for a larger cross sectional area. E scoffier suggested there was a stable maximum velocity through the inlet that would scour any excess sand brought to the inlet by wind or waves. He assumed this maximum velocity to be 3 ft / s based on grain sizes in inlets he studied. Bruun and Gerritsen ( 1959) related the formation, size, and maintenance ability of inlets to a ratio of tidal energy to longshore transport. They further established a stability criterion in which the condition for an inlet to remain open was considered dependent on the abili ty of the channels current to remove littoral drift deposited on the ebb delta (Bruun and Gerritsen, 1960). This measure, S r equals the ratio of the tidal prism passing through the inlet during a one half tidal cycle to the average annual littoral drift reaching the inlet, or P / M The inlet will maintain itself if the tidal prism is sufficiently larger than its corresponding littoral drift. On the other hand, if the stability ratio decreases, the inlet is subject to closure by being overwhelmed by the littoral drift. Bruun (1977) later quantified this relationship ( P / M ) for a sandy coast inlet ( Table 1 ). In light of more data that had been collected since his initial study, OBrien (1969) reviewed his earlier work (OBrien, 1931) regarding the relation ship of tidal prism to cross sectional area. The review included inlets on the Atlantic, Pacific, and Gulf coasts with his previous data. OBrien concluded that his original relationship agreed closely for inlets with two jetties. However, he determined that inlets without jetties were better represented by the linear relationship, A c = 2.0 x 10 5 O. (2)
17 Table 1 Stability criterion for sandy coast inlets related to ebb delta size and bypassing. (Bruun, 1977) Ratio Range, S r Inlet conditions with respect to navigability and stability S r > 150 Conditions are relatively stable and good, litt le ebb delta formation and good flushing 100 < S r < 150 Conditions become less satisfactory, and ebb delta formation becomes more pronounced 50 < S r < 100 The ebb delta may be rather large, but they can usually still be navigated by shallow draft vessels 20 < S r < 50 all inlets are typical "delta bypasses". For navigation, they present "wild cases", unreliable and dangerous S r < 20 entrances appear unstable "over flow channels" rather than permanent entrances The relationship between tidal pris m and inlet stability was fundamentally improved upon by Jarrett (1976). By modifying constants used in OBriens (1931, 1969) equations, and based on current calculated tidal prisms in a variety of coastal settings, Jarrett presented an estimate of the t idal prism serving an inlet without gathering tidal current data. He summarized the following relationship for all coasts as follows: A c = 5.74 x 10 5 O (3) Furthermore, for natural inlets, Jarrett determined that the following relationship exists for stable cross sections: A c = 1.04 x 10 5 O 1.03 (4) It should be mentioned that, while both OBriens (1969) and Jarretts (1976) relationships are appl icable, they were mere approximations with modest accuracy. OBrien formula, converted to metric, yields A = 66 x 10 6 P, and is only 25 percent accurate. Jarretts formula, A = 16 x 10 5 P 0.95 is only fifty percent accurate ( van Rijn, 1998).
18 OBrien an d Dean (1972) calculated the stability of an inlet affected by deposition, combining Keulegans (1967) hydraulic solution for an inlet throat, OBriens (1931) equilibrium relationship relating cross sectional area to tidal prism, and Escoffiers (1940) in let stability concept to define a stability index. OBrien and Dean (1972) converted OBriens stability relationships into a maximum tidal velocity requirement to improve upon Escoffiers assumed 3 ft / s The investigators determined that for natural inle ts, the maximum velocity (U max ) is given as follows: U max = 2.13 X 10 4 A c 0.3 (5) I n a study comparing Blind Pass, Florida and Johns Pass, Florida, Metha et al. (1975) proposed a calculation that related mean tidal prism ( P m ) to the spring prism ( P s ). They suggested the following relationship exists: P m = 0.85 P s (6) Walton and Adams (1976) found a good correlation between spring tidal prism and ebb tidal delta size, rather than the tidal range entirely. This work was based on the method for calcula ting ebb delta volume p roposed by Dean and Walton (1975 ) which determined the following: V = (10.7 x 10 5 ) P 1.23 (7) (Note: volume is in yds 3 prism is in ft 3 as measured during spring tide conditions.) Continued research on inlet hydraulics resulted in the characterization of additional inlet parameters, including basin frictional effects. Vincent and Corson (1980) developed a geometric measurement scheme for ebb tidal deltas and main inlet channels. They defined and measured 13 parameters describing tidal inlet geometry for 67 US inlets on the Atlantic, Gulf, and Pacific Coasts, showing ebb delta geometry to be
19 systematic. Boon and Byrne (1981) discussed tidal current data recorded in 1978 at Wachapreague Inlet, Virginia, that showed ebb dominance. These authors showed that major reductions in the cross sectional area of the ebb dominated inlet throat resulted in a transition from ebb to flood dominance, with respect to peak current velocities. This occurred once inlet hydraulics became more influen ced by frictional effects than basin hypsometry. Additional tidal inlet research included the recognition of flow patterns and channeling in and around tidal inlets (Price, 1963; Postma, 1961, 1967; Wright and Sonu, 1975; Boothroyd, 1985; and Ozsoy, 1986). Postma (1961, 1967) emphasized the importance of time velocity asymmetry. Time asymmetry occurs when the maximum flow velocities occur not at mid tide, but at some other stage of the tidal cycle. For example, the maximum flood velocity may occur after the time of mid flood in the marginal flood channels, where as the maximum ebb velocity occurs near the time of low tide in the main ebb channel (Boothroyd, 1985). Velocity asymmetry means that the maximum flood and ebb currents are not the same. Meanwhi le, Price (1963) described a flaring tidal jet, created by tidal currents issuing through an inlet. Concurrently, other studies examined inlet dynamics by compari ng wave forces. OBrien ( 1976) suggested that inlet closure depends on relative strengths of wave and tidal forces. He suggested that a ratio of normal incident wave energy over one tidal period to the tidal energy through the inlet over a tidal period can be used to evaluate the stability of an inlet. Hubbard et al (1979) later showed that this ratio could be used to differentiate morphological types of inlets. Mehta and Hou (1974) studied the amount of
20 work being done by tidal currents. They defined a ratio, C, of longshore wave power over the amount of work done by tidal currents during one half of the tidal cycle. Bruun (1986) later reviewed Mehta and Hous (1974) stability index. He recognized that it was similar to his own, but that it is better to express wave power in terms of its sedimentary effect, hence the use of total littoral drift rate. Bruun further suggests that these inlets bypass sediment through bar migration around the channel. It is important to note that Bruun implicitly included processes occurring on ebb tidal deltas in his stability index by using the total litto ral drift rate. Van de Kreeke (1992) pointed out that stable inlet z sections in nature vary with time, and a stable cross section can lie between two equilibrium points A c1
21 Early morphodynamic studies included the work of Pierce (1 970) who reviewed the origin of inlets from a morphological perspective. Hayes, et al. (1970) demonstrated that wave activity mobilizes sediment from the distal platform of the ebb tidal delta, which is then welded to the down drift side. Galvin (1971) s tudied the regional morphology of inlets and found that, regardless of size, inlets have four characteristic planforms: over lapping offset, updrift offset, downdrift offset, and negligible offset. Boothroyd and Hubbard (1974) mapped sediment transport pat terns and delineated flood and ebb dominant areas occurring in a mesotidal inlet at Parker and Essex Estuaries in Massachusetts. They correlated bedforms with current velocities, depths, and the amount of tidal asymmetry. Later work by Hine (1975), Hubba rd (1975), and FitzGerald (1976) demonstrated the usefulness of analyzing bedforms and sediment bodies to deduce hydraulics and sedimentation dynamics on ebb tidal deltas and in main channels. Hayes (1975) expanded upon earlier work by Davies (1964) and de vised a shoreline classification scheme using tidal ranges that develop similar morphologies throughout various wave climates. Hubbard (1975) studied the effect of dominant wave approach direction as waves move around the margin of a delta, noting a rever sal in the direction of sediment transport. Meanwhile, Oertel (1975) presented a model for the shapes of ebb deltas of non stratified Georgia estuaries that could be used for mesotidal inlets in general. Hayes and Kana (1976) proposed a standard model o f ebb delta morphology, while reinforcing the importance of time velocity asymmetry and the horizontal segregation of flow. Concurrently, Hubbard and Barwis (1976) proposed a general
22 model for mesotidal ebb deltas that displayed tidal current flow pattern s. They re emphasized the importance of time velocity asymmetry for creating and maintaining inlet morphology, specifically ebb tidal deltas. Oertel (1977) observed that the main factors influencing patterns of cyclic development are tidal currents, wave currents, bathymetric shielding, and channelization. Nummedal and Fisher (1978) pointed out that relative magnitudes, as opposed to absolute magnitudes, of tide and wave forces are the important factors concerning morphology and stability. Hubbard et al (1979) stated that morphological variability was largely explained by waves and tides, noting that other factors such as tidal prism, inlet cross section area, and shape, the nature of the back barrier bay, the degree of flood or ebb dominance, fresh wate r input, relative changes in sea level, and sediment supply have a lesser control on morphological variability. Hayes (1979) developed a classification for inlets based on ebb tidal delta morphology and tidal range. This classification scheme encompassed five coastal environments, including: microtidal, low mesotidal, high mesotidal, low macrotidal, and macrotidal. Hayes (1980) further reviewed the general morphology and sedimentation patterns in tidal inlets, suggesting that inlet morphology is variable and depends on the ratio of wave energy to tidal current energy, the volume of tidal prism, the nature and size of back barrier area, and the time velocity asymmetry of tidal currents. Hayes also suggested that inlets in areas of large tides and small w ave heights have large, well developed ebb tidal deltas and small to non existent flood tidal deltas. Davis and Fox (1981) demonstrated the interplay between wave induced longshore currents and tidal currents on ebb tidal deltas. They showed that longshor e
23 currents might reinforce or suppress tidal currents, particularly in lateral flood channels. Davis and Fox further suggested that changing weather patterns may control sediment bypassing characteristics and the configuration of ebb tidal deltas, because longshore currents are caused by waves. Davis and Hayes (1984) compared tidal prisms with tidal ranges from select inlets in the Gulf of Mexico, the Atlantic Ocean, and the Pacific Ocean and found no correlation. They concluded that regardless of absolut e tidal ranges, coastal morphology is primarily dependent upon the relative influence of tides and waves. Furthermore, they devised a relationship between tidal range and wave height as it relates to coastal morphology ( Fig. 7 ). Some coastal geologists ha ve attempted to describe the sediment bypassing processes described by Bruun (1978, 1986). FitzGerald, et al. (1976) provided evidence that sand bypassing is episodic along the South Carolina coast. FitzGerald (1982) observed that a continuous migration of sand around the swash platform periphery occurs at wave domin ated inlets. Conversely, bar w elding may occur on both sides of the main channel at tide dominated inlets. He suggested the importance of inlet size and main channel orientation in controlli ng location along the shoreline at which bypassed sand was deposited. FitzGerald and Nummedal (1983) later determined that delta morphology and inlet flow resulted from increased sediment input at Price Inlet, South Carolina. FitzGerald (1984) further doc umented that a close relationship exists between ebb tidal delta processes and inlet shoreline changes. FitzGerald (1988) later stated that stable inlet processes along mixed energy coasts result in the intermittent bypassing of discrete packets of sedime nt. He suggested that sand bypassing occurs by means of large
24 bar complexes that form on the ebb tidal delta, migrate landward, and weld to the downdrift shoreline. Oertel (1988) developed a conceptual model for ebb jet flow. He considered the ebb jet to be divided into two zones. Oertel suggested that material is deposited over an area just beyond the distal end of the near field jet as the flow velocity decreases below a certain critical value. This deposition area is known as the far field, and t ends to be fan shaped due to the lateral spreading of the jet in the presence of landward approaching waves. Davis and Gibeaut (1990) further developed Hayes (1979) morphodynamic classification of inlets in order to put inlets in perspective regarding the ir behavior and effect on adjacent shorelines. They created a classification based on morphology and described how the morphological variations are due to relative magnitudes of wave and tidal energy ( Fig. 4 ). Specifically, their classification was prima rily based upon the seaward portion of the inlet including the channel, the shoreline, and the ebb tidal delta morphology. They did not address flood deltas. Davis and Gibeaut (1990) categorized inlets into four basic types, including: tide dominated, wa ve dominated, mixed energy straight, and mixed energy offset. Gibeaut and Davis (1991) later presented a computer model simulation of ebb tidal deltas. They prepared quantitative simulation models of delta planforms, based on their previous qualitative s tudy (Gibeaut and Davis, 1988). Recent morphodynamic studies have encompassed both geologic and engineering disciplines, while focusing on ebb delta deposition for navigation concerns. Mehta, et al. (1996) developed a numerical model for calculating the g rowth of an ebb shoal. They computed the amount of sediment deposition brought by the ebb jet on a fixed, planar
25 morphological feature. Kraus (1998) developed a model from the minimum inlet channel cross section based on the balance between longshore tra nsport driven sand and the scouring action of the tide. Gaudiano and Kana (2001) further quantified inlet bypassing along the South Carolina coast, suggesting that inlets tend to move sand around them in discrete packets, rather than continuously, as prev iously noted by FitzGerald ( 1988). Kraus (2002 ) recently proposed a reservoir model for calculating natural sand bypassing. West Central Florida Coast Previous inlet studies along the west Florida coast have dealt with inlet hydraulics and sedimentation patterns (Mehta et al., 1976; Lynch Blosse and Davis, 1977; Davis and Hayes, 1984; and Cuffe, 1993). Mehta et al., (1976) studied Johns and Blind Pass in an effort understand inlet hydrodynamics. Tidal current measurements were collected over several sp ring and neap tidal cycles and tidal prisms were calculated for each inlet. Lynch Blosse and Davis (1977) studied inlet sedimentation and the stability of Dunedin and Hurricane Passes, also located within Pinellas County. Davis and Hayes (1984) describe d the relationship between wave and tide domination along the west central Florida coast by comparing numerous inlets of various sizes, as previously mentioned. They stressed the relative influence of wave and tide domination is the primary factor in det ermining coastal morphology, specifically with regards to wave and tide dominated inlets. Davis et al. (1987) documented the effects of washover fans on north Caladesi Island following Hurricane Elena in 1985. Cuffe (1993) studied the development and s tratigraphy of the ebb and flood deltas at Hurricane Pass. She developed a stratigraphic facies model of microtidal tidal deltas.
26 While some research has studied inlet processes, recent studies have focused on inlet responses. Hine et al. (1986) studied the impact of Gulf Coast inlets on coastal sand budgets. Dean and OBrien (1987) reviewed 37 of Floridas west coast inlets. They discussed the long term equilibrium of tidal inlets and made recommendations for management practices where appropriate. This study included the state of Pass A Grille and Bunces Passes. Davis et al (1987) described sequences of inlet formation and closure for inlets along this section of Floridas coast. Gibeaut and Davis (19 88) devised a qualitative morphodynamic classifi cation of tidal inlets from examples on the west Floridas coast. The classification was based on the two dimensional configurations of ebb deltas, main channels, and adjacent shorelines. They considered inlet morphologies in four categories based on re lative magnitudes of wave and tidal energy : tide dominated; wave dominated; mixed energy straight; and mixed energy offset. Davis and Gibeaut (1990) later gave a more detailed sedimentological history for all inlets along the west central Florida coas t. Davis and Barnard (2000) documented how anthropogenic factors in back barrier areas influence tidal inlet stability along this section of Florida s coast.
27 PROCEDURES This study of Bunces Pass consists of historical research and field work. A review of aerial photographs and maps dating back to the late 19 th century was conducted to evaluate the long term morphologic changes to the inlet. Field data collection included inlet current measurements, channel and ebb delta bathymetry, and analysis of sed iment samples from various sub environments of the tidal delta complex. Samples were collected using a boat deployed grab sampler and were analyzed to represent sediment distribution. Previously collected o ffshore wave data (Davis and Andronaco, 1987) we re obtained to characteri ze the wave climate within this coastal reach. Additionally, historic weather and tidal records were reviewed to compare with morphological changes to the inlet and its ebb de lta. The data ( Table 2 ) were used to determine minimu m and maximum values of the inlets cross sectional area, width, and current velocities, as well Table 2 Field data sets utilized in the study. Data Set Date Nautical charts 1873, 1904 Aerial photography 1945, 1957, 1963, 1976, 1980, 1991, 1997, 1999, 2000, 2001, 2003, 2004 Bathymetry 2/1/03, 3/26/04 8/17/00 9/11/00 Tidal currents and wave data 11/18/00 12/21/00 Sediment samples 10/20/2002
28 as tidal prism, and ebb/flood asymmetry. Inferences regarding sediment transport patterns were availa ble by reviewing historical and current sediment distribution throughout the entire ebb delta complex. Results were compared with past data sets to help determine the processes involved in shaping present inlet morphology. Aerial photography A collecti on of old maps, nautical charts, and aerial photos was used to depict a historic representation of inlet morphogenesis. Eleven aerial photograph sets and two nautical charts, 1873 and 1904, were scanned to create digital images and registered to a U.S. Ge ological topographic map using MapInfo software. For display purposes, only nine images were chosen to represent coastline changes. Coastline and ebb tidal delta features were digitized at a 1:20,000 scale. Channel width, channel orientation, and ebb ti dal delta area were computed for each image. Inlet channel and ebb delta features were defined based on changes in bathymetry in the 1873 and 1904 maps. However, the aerial photographs required a different interpretation because they have a different data source. The ebb delta was easily defined on most photos. Ti dal delta sediment bodies are light in contrast to darker and deeper channels. Ebb delta features were outlined based on general morphology and water color contrast. However, when images were o bscured, deltaic features were outlined based on wave refraction patterns. It should be noted that there is an inherent amount of error produced when utilizing this interpretation method. These errors include: perspective, weather conditions, time of day, and reflection. All may obscure an image. Therefore, the
29 calculations produced from these images are approximations, and should be treated accordingly. The lack of photographs covering the offshore portion of the inlet further limited the availability of ebb delta information. In general, this problem increases as the scale of the imagery increases. Barnard (1998) previously calculated historic channel widths and cross sectional areas at Bunces Pass. Inlet features were digitized using AutoCad. Cross sectional areas were determined by two methods for accuracy. The first involved using the area function feature of AutoCad. This value was double checked using Microsoft Excel. In only one case did the two values differ than more than 0.2% (Barnard, 199 8). These data were further confirmed using MapInfo, in which inlet features were digitized as well (Wilhoit, et al., 2003). Bathymetric survey The Coastal Research lab conducted a bathymetric survey of Bunces Pass and its associated ebb delta in March 2003 and March 2004. Prior to the survey, a recent aerial photo (2001) was consulted to determine the extent of bathymetry required. The survey was conducted from a 7m Sea Hawk boat. To determine lo cation throughout the survey, an Ashtech Zextreme glob al positioning system (GPS) was used in conjunction with a narrow beam echo sounder that was placed on nearby Mullet Key. Meanwhile, a 500 kHz SonTek Argonaut SL current meter was moored offshore to gauge tidal fluctuation. In an effort to characterize t he submarine topography of the inlet and ebb delta, parallel lines were traversed from the shoreline to 0.5 km off shore. Calm weather con ditions persisted throughout both survey s
30 The bathymetric data were recorded with the Ashtech Zextreme software, as tidal pressure data wee recorded using SonTek ADP software version 2.1. All data were processed at the Coastal Research Lab using Microsoft Excel. Bathymetric data were put into an X, Y, and Z format to represent longitude, latitude, and depth, respecti vely. Tidal fluctuation data were later compared with verified water levels from Mullet Key to calibrate bathymetric data with an accurate vertical tidal datum. The modified bathymetric data were displayed in a Geographic Information System (GIS) using A rcGIS TM 8.3. Areas between data points were interpolated by nearest neighbor analysis. Cross sections of the inlet channel at the specific ADP locations were generated using ArcGIS TM 8.3 in conjunction with Microsoft Excel. These allowed for further an alyses of cross sectional areas and tidal prisms, and are discussed later. Tidal currents and wave data The Coastal Research Lab, with the assistance of the United States Geological Survey (USGS), deployed Acoustic Doppler Profilers (ADPs) at Bunces Pass ( Fig. 8 ). The investigations were conducted in August 2000 and November 2000, in an effort to represent seasonal current variations within the inlet. The summer deployment lasted for 22 days, from August 17, 2000 to September 9, 2000. The winter deploym ent lasted 35 days, from November 16, 2000 to December 21, 2000. Two ADPs were submersed during each period; one located directly in the main channel in the inlet throat at 6 m depth, and a second located westward toward the Gulf of Mexico at 7.3m depth.
31 Barometric pressure, tidal current velocities, significant wave height, and various other parameters were continuously recorded for two minutes every 20 minutes at 9 hertz. The data were recorded in bins at 0.5 m intervals in the water column, beginni ng at 0.9m above the bottom. The ADP transmits sound bursts into the water column, which are scattered back to the instrument by particulate matter suspended in the flowing water. A sensor listens for the return signal and assigns depth and velocity to t he received signal Figure 8 Location of ADPs during 2000 summer and winter deployments. Note the proximal and distal locations of the tripod locations during each deployment. Photograph taken in 2004. Distal Proximal
32 based on return time and the change in the frequency caused by the moving particles, respectively. This change in frequency is referred to as the Doppler shift. An internal microprocessor calculates velocity vectors (Pratt, et al., 199 9). On August 17, 2000 two ADPs were deployed at Bunces Pass, which were retrieved on September 9, 2000. Data from the distal tripod was unrecoverable. However, data from the proximal tripod were successfully retrieved and downloaded at the USGS office in St. Petersburg, Florida. On November 16, 2000, the USGS redeployed the ADPs in an effort to characterize seasonal differences at the inlet The same protocol was used. Both tripods were retrieved on December 21, 2000 and had a 100% data recovery over t he time of deployment. All tidal current data were processed and analyzed at the Coastal Research Lab utilizing Sontek ViewADP software and Microsoft Excel In an effort to minimize cross shore interference, the data were manipulated to align the instrume ntation with north. Additionally, tidal data ele vations were calibrated with instrument depth s to give accurate water levels These data were not referenced to a vertical datum, and should be treated as relative values. All tidal data were exported to a Microsoft Excel spreadsheet, however only 12 of the 20 cells from each 20 minute data interval were used for analyses This was done to eliminate any erroneous data which may have been recorded out of the water due to changes in tidal range or wave swel ls. Next, the data were manipulated using Microsoft Excel. All data sets were further combined into one spreadsheet for vector analysis of th e tidal currents. Specific to neap and spring tidal conditions, individual flood and ebb flows were compared to o bserve differences between of peak current velocities, mean current
33 velocities, flow durations, and tidal prisms. This allowed for the characterization of each neap or spring tide cycle as either flood or ebb dominant. However, the presence of semi diur nal tidal inequalities at Bunces Pass did not allow for a synoptic analysis of every neap or spring tide. In other words, during a given neap or spring tide, the beginning of an ebb flow might start 12 hours after the end of a given flood flow, because s emi diurnal inequalities present during that 12 hour time period were not representative of the given neap or spring tide. However, other neap or spring tides did allow for a synoptic analysis, whereby the peak flood currents were immediately followed by peak ebb currents for the given cycle. For this reason, tidal velocities presented herein are displayed with respect to their time duration, and not according to specific dates and times. Further implications of tidal inequalities at Bunces Pass are disc ussed later. Tidal prism data from both within and Gulfward of the inlet were further calculated from tidal current velocity data and cross sectional area data acquired during the 2002 bathymetric survey. These allowed for comparisons of water volumes fl owing through the inlet at each respective location during summer and winter conditions in 2000. Given the historic stability of the inlet, which is discussed later, it was assumed that changes in inlet geometry from 2000 to 2002 were insignificant. Wav e measurements were continuously recorded at the Egmont Channel buoy gauge located off the Gulf coast of St. Petersburg, Florida from August through December 2000. The data were available from the Tampa Bay Physiographic Oceanic Real Time System (PORTS) o n line.
34 Surface sediment collection and analysis In order to characterize the sediment distribution on the ebb delta and channel, eighty six surface sediment samples were collected from the ebb delta complex on October 20, 2002. This was to geographical ly represent sediment distribution. Samples were obtained by deploying a clamshell grab with a 15 x 15 cm opening at predetermined locations. These locations, based on 1997 and 2001 aerial photos, were representative of five sub environments, including: the main channel, channel margin linear bars, marginal flood channels, the swash platform, and offshore. Depth and time were recorded, as well GPS coordinates. A handheld Garmin GPS unit monitored the position Only the top 5 centimeters of sediment wer e placed in a plastic bag and labeled. The numbers of samples collected within each sub environment are listed in Table 3 Table 3 Number of sediment samples collected within each sub environment All samples were later analyzed at the Coastal Research Lab using the Folk (1974) method. Analysis included mean grain size, shell and quartz fractions, as well as Sub Environment Samples collected Main Channel 12 Chan nel Margin Linear Bars North 10 South 10 Marginal Flood Channels North 8 South 9 Swash Platform 31 Offshore 6
35 percentages of the sand, gravel, and mud. In the lab, samples were removed from bags and placed in beakers. Each sample was originally rinsed three times with distilled water to remove salt residue. After a sample was dry, it was split into a 30 to 50 gram parcel for analysis. The excess sample was stored and cataloged. The initial sample was sieved using a Rotap, separating gravel, sand, and mud, according to the Wentworth classification scheme. The sand portion was sieved a second time at 0.5 phi intervals. The sample mass in each sieve was recorded and entered into a Microsoft Excel spreadsheet. The samples were then acidified with diluted HCl to determine the amount of calcium carbonate in ea ch sample. Upon complete dissolution, the remaining quartz fraction was rinsed, dried, and weighed. The initial gravel weight was added to the CaCO 3 weight, as the gravels were assumed to be 100% calcium carbonate. There was a trace of mud observed in a ll samples. Results were averaged for specific environments, permitting for a comparison between the different sub environments. A specific sedimentological review of all samples is discussed later.
36 HISTORICAL MORPHODYNAMICS A review of aerial photos to chronicle the history of Bunces Pass permits a characterization of morphodynamic changes that have occurred at the inlet. While sand bodies can change from year to year, major trends in morphology last over decades. The current investigation document ed changes at Bunces Pass from 1873 to 1997. Both natural and anthropogenic changes in the littoral system north of the inlet had a dramatic effect on the ebb delta complex at Bunces Pass. Meanwhile the main inlet channel experienced minor changes throug hout the same time period. Sequential Analysis from 1873 to 1997 1873 to 1895 The Bunces Pass ebb tidal delta was mapped when t he first reliable chart of the area was surveyed in 1873 74 ( Fig. 9 ) The Bunces Pass and south channel of Pass A Grille Pass ebb tidal delta were combined at this time, and remained so until the early 1970s. The Reefs, an inappropriately named, extensive mangrove community that grew due to accretion, were present at this time and defined the shoreward boundary of the ebb delta In 1873, the delta extended westward into the Gulf of Mexico approximately 1.6 km west of the inlet throat. This early depiction shows an ebb delta area of 2.6 km 2 T he
37 south channel of Pass A Grille Pass was large at this time, in comparison to the n orth channel of Pass A Grille Pass.
38 In 1873, the Bunces Pass channel was approximately 500 meters wide with a maximum channel depth of 6.5 meters. The main channel configuration was a curved shape with an orientation that changed as the inlet exited the c oast. Bunces Pass entered the Gulf of Mexico at an azimuth of 240 degrees, turned north to 310 degrees for 0.5 km, then proceeded west with an orientation of 270 degrees. Figure 9 The Bunces Pass ebb delta in 1873. The south channel of Pass A Grille Pass was very large and served as the northern ex tent of the Bunces Pass ebb delta complex. Depths are shown in feet. 0.5 km 1873 North channel of Pass A Grille Pass South channel of Pass A Grille Pass The Reefs BUNCES PASS Mullet Key
39 The channel width decreased steadily during the next decade as its maximum chann el depth shallowed as well. By 1883, the Bunces Pass channel was approximately 40 0 meters wide with a similar maximum depth of 6.3 meters. Historical documents further reveal that the channel width did not change through 1895 (Barnard, 1998) 1904 to 1 945 There are no data available from 1895 to 1903. However, a 1904 nautical chart displayed an ebb delta at Bunces Pass with an area of 4.3 km 2 While the ebb delta experienced a dramatic increase in the size over 31 years, the orientation of the main ch annel did not change. Historical information is scarce for Bunces Pass between 1904 and 1926, when the first aerial photos are available. However, i n 1921, a hurricane struck the northern Pinellas County coast with a storm surge of 2.9 meters. Historica l documents reveal another hurricane passed through Tampa Bay in 1926 as well, with an even greater storm surge of 3.7 meters. Unfortunately, the perspective of the 1926 photos is extremely poor Therefore, t he storm effects at Bunces Pass were not docum ented, but probably had an effect on the ebb delta morphology. T he first decent quality aerial photo of Bunces Pass was taken in 1945. In 1945, the cross sectional area of the inlet throat measured 1452 m 2 (Barnard, 1998) at MLLW, though the channel wid th remained constant at 440 m ( Fig. 10 ). Bunces Pass continued to enter the Gulf of Mexico at 240 degrees orientation before turning to 310 degrees for 0.5 km. However, the channel bifurcation no longer existed, as the channel orientation now rotated to 285 degrees at its terminus. Compared to earlier
40 nautical charts, the south channel of Pass A Grille Pass had decreased in size, while the north channel of Pass A Grille Pass began to enlarge. By 1945, the Bunces Pass ebb delta was reduced to an area o f 3.7 km 2 For the first time, two ebb deltas were distinguishable. The ebb delta of the south channel of Figure 10 In 1945, the channel width was 400 m. The ebb delta was reduced to an area of 3.7 km 2 Note the emergence of a swash bar on the southwestern portion of the swash platform. 0.5 km 1945 South channel of Pass A Grille Pass The Reefs BUNCES PASS Mullet Key
41 Pass A Grille Pass was clearly superimposed on the larger Bunces ebb delta. Therefore, the northern edge of the south channels delta was included in the analysis. 1950 to 1963 In 1950, a hurricane produced the highest tides since the 1926 hurricane, which were over 3 m above normal. The effects at Bunces Pass were not specifically documented, however by 1951, the main channel width had slightly d ecreased to 390 meters ( Fig. 11 ). The main channel orientation remained unchanged. However, the ebb 0.5 km 195 South channel of Pass A BUNCES PASS Mullet Figure 11 By 1951 the main c hannel width had slightly decreased to 390 meters. The main channel orientation remained unchanged. Note the development of a large swash bar on the southwestern portion of the swash
42 delta had increased in area to 4.6 km 2 It appears that the south channel of Pass A Grille Passs efficiency continued to diminish. Sand bodies conti nued to coalesce on the periphery of the swash platform, developing into swash bars. A large and distinctive swash bar emerged south of the main channel. Marginal flood channels were easily distinguishable south of the swash platform, though are less obv ious to the north. The delta continued to grow westward, and had an area of 4.7 km 2 in 1957 ( Fig. 12 ). The channel width had also resumed its historic maximum of 500 meters as well. Figure 12 In 1957, the ebb delta continued to grow westward, and had an increased area of 4.7 km 2 The channel width had also resumed its historic maximum of 500 m as well. At this time, the ebb delta of the south channel of Pass A Grille Pass was superimposed on the larger Bunces Pass ebb delta. 0.5 km 1957 South channel of Pass A Grille Pass BUNCES PASS Mullet Key
43 Small bars developed on the northern periphery of the swash platfor m. The terminal lobe of the south channel ended abruptly as it intersected Bunces Pass. Small marginal flood channels were distinguishable on the north swash platform. Sedimentation within the south marginal flood channel of Bunces Pass allowed the swas h bar on the southern platform to begin welding to Mullet Key. By 1963, the Bunces Pass ebb delta had only slightly decreased in area to 4.45 km 2 ( Fig. 13 ). At this time, the swash bar first identified in 1951 had welded to Mullet Key. The development o f new marginal flood channels resulted around the periphery. A new swash bar, oriented southeast to northwest, emerged Gulfward of Mullet Key. Figure 13 Bunces Pass in 1963. The south channel of Pass A Grille Pass is narrow compared to the 1957 image. North Bunces Key is present. Bunces Pass orientation has remained unchanged. 0.5 km 1963 South channel of Pass A Grille Pass North Bunces Key BUNCES PASS Mullet Key
44 1970s There are very few photos from 1963 to 1973. Several hurricanes affected the west central Florida duri ng this time period, including Hurricane Alma in 1964 and Hurricane Agnes in 1971. North Bunces Key was breached and severely eroded by Hurricane Agnes, allowing tidal flow again through the south channel of Pass A Grille Pass. This storm also caused ext ensive erosion of the ebb delta, decreasing to an area of 2.8 km 2 by 1973 ( Fig. 14 ). Meanwhile, t he tidal prism flowing through the south channel of Pass A Key soon sealed the breach of 1971, as a new swash bar developed west of the key in response to th e diminished flow from the south channel of Pass A Grille Pass ( Fig. 9 ). Accretion continued on the swash bar located on the southern swash platform. Meanwhile, the main channel of Bunces Pass remained unchanged, as the channel width slightly decreased t o 450 meters. Channel margin linear bars were not well defined. By 1975, the south channel of Pass A Grille Pass had closed again due the enlargement of North Bunces Key. The ebb delta continued to grow, as South Bunces Key first emerged, a nd increased in area to 3.6 km 2 by 1976 ( Fig. 15 ). A large swash bar developed south of the main channel, extending 500 meters Gulfward. 1980s There are little data between 1976 and 1980. However, by 1980, the ebb tidal delta decreased in area to 2.6 km 2 ( Fig. 16 ). The relic ebb delta of the southern channel continued to erode. The large swash bar present in 1976 had developed into South Bunces Key. N ew flood channels emerged along North Bunces Key, as channel margin linear bars continued to extend Gu lfward on both sides of the main channel
45 In 1982, North Bunces Key was breached and overwashed significantly during the passage of winter frontal systems. The breach re opened the south channel of Pass A Grille Pass. Winter storms continued to plague the coast for the Figure 14 Bunces Pass in 19 73 Note the development of North Bunces Key, which coincides with the decrease in the tidal prism and effectiveness of south ch annel of Pass A Grille Pass. 0.5 km 1973 North Bunces Key BUNCES PASS Mullet Key South Channel of Pass A Grille Pass
46 next few years. By 1984, the main channel width of Bunces Pass had increased to 490 meters, as the inlet continued to flush a portion of Tampa Bay tidal prism. In 1985, both North and South Bunces Key were overwashed significa ntly b y Hurricane Elena. North Bunces Key was transported about 100m landward and storm passes bisected both islands. Figure 15 Bunces Pass in 1976. Accretion continued along North Bunces Key, further stabilizing its position. A large swash bar developed Gulfward of Mullet Key. Large swash bar 0.5 km 1976 North Bunces Key BUNCES PASS Mullet Key
47 Figure 16 Bunces Pass in 1980. The large swash bar present in 1976 had developed into South Bunces Key. Vegetation on the island further stabilized its location. 0.5 km 1980 South Bunces Key BUNCES PASS Mullet Key North Bunces Key
48 By 1986, the tidal delta had decreased to 1.5 km 2 ( Fig. 17 ). Marginal flood channels had developed around South Bunces Key, which had begun to weld to the channels had developed around South Bunces Key, which had begun to weld to the Figure 17 Bunces Pass in 1986. The ebb tidal delta had decreased to an area of 1.5 km 2 after the passage of Hurricane Elena in 1985. North Bunces Key was transported approximately 100 m landward. Storm passes bisected both North and South Bunces Keys. By 1986, South Bunces Key had effectively welded to Mullet Key. 0.5 km 1986 North Bunces Key BUNCES PASS Mullet Key South Channel of Pass A Gri lle Pass
49 adjacent Mullet Key. The main channel orientation had not changed. By 1987, the channel width had decreased to 422 meters, with a cros s sectional a rea of 1317 meters (Barnard, 1998). 1990s The ebb delta continued to erode during the next five years, and was reduced to an area of 1.3 km 2 by 1991 ( Fig. 18 ). The south channel of Pass A Grille Pass continued to close due to longshore drift, coupled wit h an insufficient tidal prism. Channel margin linear bars further stabilized the position of the ebb jet, thus reinforcing the position of the ebb delta. In 1995, the south channel of Pass A Grille Pass was completely closed by North Bunces Key. At the t ime, the storm pass on South Bunces Key was finally closed as well. By 1997, the ebb tidal delta had resumed growth to an area of 1.5 km 2 and the channel was 390 meters wide ( Fig. 19 ) (Barnard and Davis, 1999). The channel width has been decreasing stead ily due to accretion on the north side of the inlet. The present situation is a wide tidal channel with a large ebb tidal delta. The ebb delta is elongate, nearly perpendicular to the coastal trend at this location. It has a shape that is typical of a t ide dominated ebb delta (Davis and Gibeaut, 1990).
50 Figure 18 Bunces Pass in 1991. The south channel of Pass A Grille Pass continued to close. Channel margin linear bars f urther stabilized the position of the ebb jet, thus reinforcing the position of the ebb delta. The relic South Bunces Key, which was now part of Mullet Key, was still bisected by storm passes. 0.5 km 1991 North Bunces Key BUNCES PASS Mullet Key
51 Recent Bathymetry Bathymetric surveys of the main channel and ebb delta comp lex were conducted in March 2003 and March 2004 Changes to large scale features between the survey dates were minimal Combined, these data provided a detailed map of the bathymetry of the inlet system ( Fig. 20 ). The map shows a main channel with a maximum depth of 9.3 m located directly within the inlet throat. The main channel is oriented approximately Figure 19 Bunces Pass ebb delta in 1997. The delta is elongate, with well developed channel margin linear bars along the main channel. The south channel of Pass A Grille Pass is inactive. The channel orientation of Bunces Pass has remained unchanged for over 127 years. 0.5 km 1997
52 F igure 20 Composite bathymetry of Bunces Pass from 2003 and 2004
53 310 degr ees, then bends to the left to 290 degrees. This channel is flanked by shore normal margin linear bars which protrude westward approximately 1.5 km. The entire swash platform is relatively shallow and asymmetrical. The north portion is narrow, while the south portion of the swash platform is very broad. The entire ebb delta currently extends westward approximately 2.5 km. Summary of Historical Morphodynamics T he ebb delta complex at Bunces Pass experienced significant changes from 1873 to 1997 ( Fig. 21 ). Th e decay of the south channel of Pass A Grille Pass over time significantly reduced the size of the Bunces Pass ebb delta, changing its shape from a broad lobe to a narrow feature. The terminal lobe of the Bunces Pass ebb delta, which is practically non existent due to tidal action, extended Gulfward 1 km during the same time period. A relationship exists between the historic channel width and the ebb tidal delta area, suggesting that Bunces Pass is in dynamic equilibrium with its surrounding environm ent ( Fig 22 ). During this time period, the main chan nel orientation and location remained constant. Seasonal northward longshore transport, coupled with wave sheltering effects and an abundance of sediment resulted in the development of swash bars on t he southern portion of the swash platform. These swash bars periodically traversed across the swash platform, eventually welding to the adjacent Mullet Key. This pattern was evident in both 1951 and 1980 images. The south channel of Pass A Grille Pass is a tidal channel with an ebb delta that was historically superimposed on the larger Bunces Pass ebb delta. The south channel,
54 Figure 21 a i Bunces Pass morphology from 1873 to 1997. Inlet channels are light blu e, land is dark brown, a migrating swash bar is dashed brown, and the ebb delta is sand brown. Note the formation of North Bunces Key (1963). Additionally, note the migration and emergence of South Bunces Key (1976) (from Wilhoit et al. 2003 ).
55 which had a significantly smaller tidal prism in comparison to Bunces Pass, was more susceptible to outside wave action. Th e decrease in the south channels tidal prism resulted in the extinction of the channel and the eventual destruction its ebb delta in the 1970s The present situation shows a stabilized, tide dominated inlet with a large, elongate ebb delta ( Fig. 23 ) t hat is unlikely to change significantly in the future if present conditions are maintained. The tidal inlet is very wide with a deep channel thalw eg that sufficiently carries an enormous tidal prism. The main channel width did not ch ange more than 20% fr om 1873 to 1997 ( Fig. 24 ), while the corresponding ebb delta decreased C C h h a a n n n n e e l l w w i i d d t t h h ( ( m m ) ) A A r r e e a a ( ( k k m m 2 2 ) ) N N O O D D A A T T A A A A V V A A I I L L A A B B L L E E Figure 22 Historic variations in the channel width and ebb tidal delta area at Bunces Pass reveal that a relationship exists between each. Bunces Pass appears to be in dynamic equilibrium with its surrounding environment.
56 Figure 23 Bunces Pass, Florida in 2003. The present situation shows a stabilized, tide dominated inlet with a large, elongate ebb delta that is unlikely to change significantly in the future.
57 over 50% during the same time period This shows that the inlet has remained in dynamic equilibrium with its surrounding environment for at least 130 years. 1997 channel 1873 channel 2004 channel Mullet Key 1873 Ebb Tidal Delta Figur e 24 The position, orientation, and width of Bunces Pass has not changed significantly in over 13 0 years.
58 TIDAL PROCESSES AT BUNCES PASS Tidal range and current velocity data were recorded at Bunces Pass in August, September, November, and December 2000. Tidal currents were continuously monitored from within the inlet throat during the summer and winter seasons, in an effort to characterize seasonal variations. Tidal current data collected during the 22 day summer deployment included one spring and two neap tidal cycles. The winter deployment collected data for 35 days and included two spring and three neap tides. Tidal cu rrents were additionally monitored Gulfward of the inlet throat during the winter deployment, in an effort to characterize spatial variations within the inlet ( Fig. 8 ). The analysis of these data permits an evaluation of inlet conditions responsible for t he hydrodynamic behavior of Bunces Pass. Tides Tides throughout the Gulf of Mexico are mixed and microtidal. In general, this section of Floridas coastline is characterized by tide domination and relatively high tidal ranges at both ends of the reach (D avis, 1988), with wave domination and lower tidal ranges in the center ( Fig. 25 ). The highest spring ranges are 1.4 m in the Ten Thousand Islands area south of Cape Romano. There is a general decrease toward the north with the central part of the peninsu la having spring ranges of less than 1 m Proceeding north, maximum spring range increases to 1.1 m. Neap and spring tidal ranges vary between
59 0.65 m and less than 1 m, specifically along the west central Florida coast. As previously mentioned, this c orresponds to a mean tidal range of 0.75 m to 0.85 m ( Fig. 7 ) (Davis, 1989). In general, tidal range data show that Bunces Pass is characterized by unequal semi diurnal tides, which periodically transition to diurnal during neap tide s, as occurred on Sep tember 5, 2000 ( Fig. 26 ). The computation of mean tidal range for each data set was obtained by subtracting the lowest tidal range value from the highest per each successive 12 hour tidal period then averaging the differences However, the presence of s emi diurnal inequalities obscured mean tidal range calculations, as low inequality Figure 25 Plots of tidal ranges along the coast of the Gulf peninsula of Florida. There is an increase in tidal range at the ends of t he reach, with lower ranges in the center. The west central coast of Florida is well within the microtidal range. (After Davis, 1988)
60 Figure 26 Tidal range data were collected at Bunces Pass to represent both seasonal and spatial differences as indicated by A) tidal range with in the inlet throat during the Summer; B) tidal range within the inlet throat during the Winter; and C) tidal range Gulfward the inlet throat during the Winter. Note the semi diurnal inequalities which are characteristic of mixed tidal systems. The tidal regime is predominantly semi diurnal, with a transition to diurnal during the summer neap tide on September 5, 2000.
61 values of tidal range skewed mean values negatively. Therefore, mean tidal range was calculated two ways; one which included these semi diurnal inequalities, and another in which these inequalities were removed ( Table 4 ). The removal of these inequalities provided a mean tidal range data set, which more accurately represented tidal conditions at Bunces Pass. Following, this discussion will compare mean tidal ranges valu es which exclude all semi diurnal inequalities. Tidal range data from Bunces Pass during the summer had a mean tidal range of 0.61 m, with a mean spring tidal range of 0.98 m. This was similar from the same location during the winter, which had a mean tidal range of 0.63 m, and a spring range of 0.99 m ( Fig. 26 ). These da ta further coincide with mean tidal range values of 0.63 m from nearby Mullet Key (PORTS, NOAA 2004). Tidal currents within the inlet throat Time velocity data show that Bunces Pass was ebb dominant overall, though intermittent periods of flood dominanc e occurred during neap tides ( Table 5 Fig. 27 ). Summer Proximal All data 43 cm Minus inequalities 61 cm Winter Proximal All data 41 cm Minus inequalities 63 cm Winter Distal All data 38 cm Minus inequalities 60 cm Table 4 Mean tidal range data from Bunces Pass
62 Date Location relative to Channel Throat Tidal Conditions Tidal Cycle Tidal Prism (10 6 m 3 ) Duration (min) Vmax (m/s) Vavg (m/s) Flood 6.65 400 0.50 0.26 8.23.00 Proximal NEAP Ebb 4.38 280 0.48 0.26 Flood 11.43 400 0.83 0.42 8.27.00 Proximal SPRING Ebb 23.38 460 1.43 0.84 Flood 5.34 340 0.43 0.26 9.5.00 Proximal NEAP Ebb 2.72 280 0.31 0.16 Flood 3.21 320 0.29 0.18 11.18.00 Proximal NEAP Ebb 5.82 360 0.49 0.29 F lood 9.44 400 0.77 0.43 11.25.00 Proximal SPRING Ebb 21.86 460 1.36 0.90 Flood 3.22 320 0.34 0.18 12.2.00 Proximal NEAP Ebb 2.42 300 0.29 0.14 Flood 12.40 420 0.85 0.54 12.11.00 Proximal SPRING Ebb 23.99 460 1.54 1.01 Flood 5.46 380 0.46 0.26 12.18.00 Proximal NEAP Ebb 5.95 340 0.45 0.33 Flood 1.11 300 0.21 0.09 11.18.00 Distal NEAP Ebb 4.22 420 0.35 0.22 Flood 2.39 340 0.48 0.17 11.25.00 Distal SPRING Ebb 15.37 460 1.14 0.77 Flood 1.10 240 0.20 0.10 12.2.00 Distal NEAP Ebb 2.07 320 0.26 0.14 Flood 1.98 320 0.34 0.14 12.1 1.00 Distal SPRING Ebb 15.62 460 1.21 0.80 Flood 1.89 320 0.25 0.13 12.18.00 Distal NEAP Ebb 2.83 320 0.28 0.20 Table 5 Spring and Neap Tidal Data at Bunces Pass during summer and winter seasons
63 Figure 27 Average current velocity data were collected at Bunces Pass to represent both seasonal and spatial differences as indicated by A) average velocity within the inlet throat during the Summer; B) average velocity within the inlet throat during the Winter; and C) average velocity Gulf ward the inlet throat during the Winter.
64 This pattern was evident within the inlet throat, though it was absent Gulfward of the inlet due to the pr esence of m arginal flood channels, which are discussed later. Bunces Pass was characterized by flood dominant neap tides and ebb dominant spring tides during the summer of 2000. For example, during the neap tides of August 23, 2000 and September 5, 2000, maximum flood current velocities were greater than maximum ebb velocities ( Table 5 Fig. 28 ) Similarly, overall flood durations were greater than ebb durations Peak flood current velocities were only slightly greater than maximum ebb velocities on Aug ust 23, 2000 Average current velocities were identical for both the ebb and flood flow. However, on September 5, 2000, maximum flood current velocities were 138% greater than ebb. Similarly, the average flood velocity on this date was 163% greater tha n the average ebb velocity. Though the ebb duration remained consistent during both dates, the flood duration on August 23, 2000, was 60 minutes longer than on September 5, 2000. A transition from a semi diurnal to diurnal tidal regime on September 1, 20 00 may account for the increased flood velocities during this neap tide, as well as the difference in flood durations. Conversely, the spring tide on August 27, 2000 was completely ebb dominant ( Fig. 29 ). Peak ebb current velocities were almost d ouble the corresponding flood, as were the average ebb current velocities double those of flood. Additionally, the ebb flow duration was 6 0 minutes longer than the flood duration ( Table 5 ). This ebb duration was 142% longer than the ebb durations during neap con ditions. The velocity and duration these ebb tidal currents resulted in a tremendous volume of water, or tidal prism, exiting Gulfward through Bunces Pass during the summer spring tide.
65 Figure 28 During summer neap tides, maximum flood current velocities and durations were greater than those of ebb at Bunces Pass. A) On August 23, 2000, maximum flood velocities were only slightly greater than maximum ebb velocities. B) On September 2, 2000, maximum flood current velocities were 138% greater than ebb. Though the ebb duration remained consistent for both dates, the flood duration on August 23, 2000, was 60 minutes longer than on September 5, 2000.
66 Tidal currents measured from the same location during the winter exhibited a similar pattern of flood dominated neap tides and ebb dominated spring tides. Though present during the winter this pattern was not consistent. In general, neap tides had flood current velocities which were minimally greater than those of e bb. However, the durations of ebb flow were only marginally smaller than those of flood; a subtle difference in inlets hydrodynamic behavior ( Table 5 ) Additionally, average flood current velocities were not consistently greater than average ebb velocit ies, as were tidal currents during the summer neap conditions. For example, on December 2, 2000, and December 18, 2000, maximum flood current velocities were slightly greater than those of Figure 29 Bunces Pass was ebb dominant during summer spring tidal conditions on August 22, 2000. Peak ebb current velocities were double those of flood. The ebb duration was longer than the flood duration as well.
67 ebb Similarly, f lood durations minimall y exceeded ebb duration s by 20 minutes on December 2, 2000 and 40 minutes on December 18, 2000 ( Table 5 Fig. 30 ). However, the average flood velocity was 25% less than the average ebb velocity on December 18, 2000 This resulted in more water cumulatively e bbing from the inl et during the flood dominant neap tide. Spring tidal currents, on the other hand, were consistently ebb dominant during the winter ( Table 5 Fig. 31 ). For example, on November 25, 2000, peak ebb current velocities of 1.36 m/s were almost double peak flood velocities of 0.77 m/s. The ebb flow duration was 60 minutes longer than the flood as well. Similarly, on December 11, 2000, the highest ebb velocities encountered throughout the entire study were observed at 1.54 m/s. Flood velocities only reached 0.8 5 m/s. The 460 minute ebb flow was identical to during both winter spring tides, which was consistent with the ebb flow duration observed during the summer spring tide. These data suggest that a consistent ebb flow continually exits the inlet during spri ng tides, regardless of the season. This is due to the inlets hydraulic connection with other inlets that serve Tampa Bay, which include, Passage Inlet, Egmont Channel, and the Southwest Channel. Vertical profiles of horizontal velocity were plotted fr om data within the inlet throat ( Fig. 32 ). They indicate that maximum currents do not occur at the same position in the water column. In a typical open flow channel, the bottom portion of the water column has the lowest velocity due to friction from bott om of the channel. Theoretically m aximum velocities occur at the top of the water column, due to the Law of the Wall Howe ver, maximum current velocities occur in the middle of the water column when weather is a factor The upper portion of the water c olumn is typically obscured by
68 Figure 30 Tidal currents measured from within the inlet throat du ring the winter exhibited a similar pattern of flood dominated neap tides. For example, A) on December 2, 2000 and B) December 18, 2000, maximum flood current velocities and durations were slightly greater than those of ebb. However, the durations of ebb flow were only marginally smaller than those of flood; a subtle difference in inlets hydrodynamic behavior.
69 Figure 31 Spring tides during the winter deployment were completely ebb dominant. A) On November 25, 2000, peak ebb current velocities of 1.36 m/s were almost double peak flood velocities of 0.77 m/s. B) On December 11, 2000, the highest ebb velocities encountered throughout the entire study were observed at 1.54 m/s. Flood velocities only reached 0.85 m/s. The ebb flow duration was consis tently 460 minutes on both dates.
70 0 1 2 3 4 5 6 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 Velocity (cm/s) Elevation above bed (m) ebb flood August 23, 2000 A) 0 1 2 3 4 5 6 -50 -40 -30 -20 -10 0 10 20 30 40 50 Velocity (cm/s) Elevation above bed (m) ebb flood September 6, 2000 C) 0 1 2 3 4 5 6 -200 -150 -100 -50 0 50 100 150 200 Velocity (cm/s) Elevation above bed (m) ebb flood B) August 27, 2000 Figure 32 Vertical p rofiles of horizontal velocities during A) neap tide on August 23, 2000; B) spring tide on August 27, 2000; and C) neap tide on September 6, 2000. Data from the upper portion of the water column were removed to reduce obscuring interpretations. Profiles are relatively uniform throughout the water column and display asymmetry with respect to flood or ebb dominance.
71 variations in local wind and surface waves (Smith and Zarillo, 1988). Therefore, data from this upper most portion of the water column is typically removed during data analysis, as is previously described. Bunces Pas s exhibits the characteristics of an open flow channel, which are more obvious during Spring tides. Vertical profiles of horizontal velocity were relatively homogenous throughout most of the water column During the summer neap tide on August 23, 2000 th e nearly symmetrical profiles indicate similar peak velocities during the ebb and flood flow. However, during neap tidal conditions o n September 6 2000, greater flood velocities were apparent. On both dates, m aximum current velocit ies minimally increase d with elevation above the bed. However, an upward increasing trend in the ebb current velocities, which is typical of most open flow channels, was easily identified during spring tidal conditions on August 27, 2000 (Smith and Zarillo, 1988) ( Fig. 32 ). These data indicate that, in general, bottom friction had a minor influence on the overall shape of the current profiles. However, ebb currents in the bottom portion of the water column were more strongly affected by bottom friction during spring tides, t han were flood currents. This occurs because greater turbulence is generated by strong ebbing spring tidal currents than is created by weaker neap tidal currents as they interact with the channel floor deposits. Variations between neap and spring veloci ty profiles were consistent throughout both the summer and winter seasons, though slightly greater ebb velocities were present during the winter deployment ( Fig. 33 ).
72 December 2, 2000 December 11, 2000 December 18, 2000 A) B ) C ) Figure 33 Vertical profiles of horizontal velocity were plotted from data on A) December 2, 2000, B) December 11, 2000, and C) December 18, 2000. These profiles are indicative of open channel flow at Bunces, and are consistent with horizontal velocit ies during summer conditions.
73 Tidal currents Gulfward of the inlet throat Tidal currents Gulfward of the inlet throa t were completely ebb dominant, due to the lack of influence from marginal flood channels. Both neap and spring tidal data were observed during the same time as the proximal tripod data. However, ebb and flood cycle lengths were systematically lengthened by 40 minutes at this location. Differences between the onset and duration of ebb and flood flows were possibly the result of momentum generated by ebb cu rrents, and is discussed later. Time velocity data indicate tidal currents were ebb dominant through out both neap and spring tidal cycles at this location. During neap tides, maximum ebb velocities were on average 0.0 5 m/s greater than flood velocities. This was true with the exception of the November 18, 2000 neap tide, when peak ebb velocities of 0.3 5 m/s were almost double t he peak flood velocities of 0.21 m/s. On December 2, 2000 ebb flow lasted 80 minutes longer than flood ( Fig. 3 4 Table 5 ). However, the flood and ebb durations were equal on December 18, 2000. The cause for this extended flood duration is due to the influence of weather, and it is consistent with currents measured within the inlet throat. T idal currents Gulfward of the inlet throat during spring tide conditions were characterized by peak ebb velocities and durations that signifi cantly exceed those of the corresponding flood ( Fig. 3 5 Table 5 ) For example, on November 27 2000, maximum ebb and flood current velocities of 1.14 m/s and 0.20 m/s, were observed during the 460 minute and 340 minute cyc les, respectively. Similarly, o n December 11, 2000, p eak ebb and flood velocities of 1.21 and 0.34 m/s lasted for 460 minute and 3 2 0 minute s respectively
74 Figure 34 Time velocity data from currents Gulfward of the inlet t hroat during neap tides were plotted. Data from this location show that neap tides are ebb dominant. A) On December 2, 2000, peak ebb velocities were marginally greater than flood, while the ebb duration was 80 minutes longer as well. B) On December 18, 2000, peak ebb velocities were minimally greater than flood. The ebb duration was identical to the flood duration, which was 80 minutes longer on this date.
75 Figure 35 Time velocity data from currents Gulfward of the inlet throat were plotted, indicating complete ebb dominance during spring tidal conditions. A) On November 25, 2000, and B) December 11, 2000 peak ebb velocities were significantly greater than flood, as were overall ebb durations.
76 E bb and flood durations respectively, were consistent during spring tide conditions Gulfward of the inlet. The extended du ration of the ebb flow may be attributed to the momentum generated by the ebb current velocities. The tremendous volume of water exiting the inlet at such high velocities, coupled by the absence of retarding effects from marginal flood currents, result in a longer period of time required to slow down the ebb momentum so that the corresponding flood flow may resume. Vertical profiles of horizontal velocity Gulfward of the inlet were consistent wi th profiles within the inlet throat, which were relatively hom ogenous throughout most of the water column ( Fig. 3 6 ). A n upward increasing trend in the current velocities was also obvious, indicating the occurrence of flow channelization at this location. Similarly, it is also apparent that horizonatal velocities we re greatest during the ebb flow, further indicating ebb dominance. Tidal prism Tidal prisms at Bunces Pass reflect cu mulative ebb dominance both within and Gulfward of the inlet throat. T idal prisms within the inlet throat exhibit a pattern of flood domi nated neap tides and ebb dominated spring tides. During summer conditions, the average neap flood prism of 6.00 x 10 6 m 3 was 74% larger than ebb. However, an enormous ebb prism characterized spring tides. For example, on August 27, 2000, the spring ebb prism of 23.38 x 10 6 m 3 was 1 05% larger than the flood ( Table 5 ). D uring summer spring tides, a much greater volume of water is flushed Gulfward by tremendously strong ebb currents. Bunces Pass was cumu latively ebb dominant from August 17 to September 9, 2000.
77 0 1 2 3 4 5 6 7 -40 -30 -20 -10 0 10 20 30 40 Velocity (cm/s) Elevation above bed (m) ebb flood December 2, 2000 0 1 2 3 4 5 6 7 -150 -100 -50 0 50 100 150 Velocity (cm/s) Elevation above bed (m) ebb flood December 11, 2000 0 1 2 3 4 5 6 7 -40 -30 -20 -10 0 10 20 30 40 Velocity (cm/s) Elevation above bed (m) ebb flood December 18, 2000 Figure 36 Vertical profiles of horizontal velocities from tidal currents Gulfward of the inlet were plotted for A) December 2, 2000; B) December 11, 2000; and C) December 18, 2000. A) B ) C )
78 Tidal prism data from currents within the inlet throat during the winter further indicate cumulative ebb dominance. Neap flood tidal prisms were similarly greater than ebb, though this pattern was inconsistent. For example, on December 2, 200 0, the flood tidal prism of 3.22 x 10 6 m 3 was 33% greater than the ebb of 2.42 x 10 6 m 3 ( Table 5 ) However, p eak flood velocities exceeded peak ebb velocities on December 18, 2000. T he overall flood duration was 40 minutes longer than the ebb on this date as well. One would expect a larger flood prism due to the greater flood velocities and durations. However, the average flood velocity was only 0.26 m/s, compared to the average ebb velocity of 0.33 m/s, resulting in an ebb prism that was 8% greater tha n the flood prism ( Table 5 ). While peak current velocities reflect hydrodynamic thresholds acting on morphodynamic processes, average current velocities better reflect the overall volume of water passing through the inlet at Bunces Pass thus characterizi ng the inlet s hydraulic processes. There were significant increases in both flood and ebb current velocities and tidal prisms from December 2, 2000 to December 18, 2000. The primary factor responsible for the increase in these data was the influence of weather on tidal currents on this date, and is discussed later. Spring ebb tidal prisms during the winter were comparable to summer conditions in that they were significantly greater than those of the corresponding flood ( Fig. 3 7 ) For example, o n Novemb er 25, 2000 the ebb prism was 1 32% greater than the flood. Similarly, the spring ebb prism on December 11 2000 was 93% greater than the flood
79 Figure 37 Tidal prisms at Bun ces Pass were plotted for currents A) within the inlet during the August to September 2000 deployment, B) within the inlet during the November to December 2000 deployment, and C) Gulfward of the inlet during the November to December 2000 deployment. Ebb tidal prisms during spring tides were tremendously greater than flood prisms, which remained fairly constant throughout. During neap tides, flood prisms were greater than ebb for only one or two days. A) B ) C )
80 prism ( Table 5 ) These data suggest that more water was consistently exiting the inlet than was entering du ring the winter. Therefore, Bunces Pass was predominantly an outlet during the winter season, continuously emptying the Tampa Bay estuary. This further implies that Bunces Pass is part of a multi inlet system serving Tampa Bay. Tidal prisms were also cal culated for currents Gulfward of the inlet throat. During neap tides, the average ebb prism was 69% greater than the flood again, with the exception of November 18, 2000. For example, o n December 2, 2000, the ebb prism of 2.07 x 10 6 m 3 was 88% greater t han the flood prism due to greater peak ebb velocities and overall duration. Similarly, on December 18, 2000, the ebb prism of 2.83 x 10 6 m 3 was 50% greater than the flood ( Table 5 ) Ebb prisms Gulfward of the inlet throat were significantly larger than those of the corresponding flood during spring tides ( Fig 3 7 ) For example, o n November 25, 2000, the ebb prism of 15. 37 x 10 6 m 3 was 5 43 % grea ter than the flood prism of 2.39 x 10 6 m 3 The s e corresponded with data from December 11, 2000, when the ebb p rism of 15.62 x 10 6 m 3 was overwhelming 6 88 % grea ter than the flood prism of 1.98 x 10 6 m 3 ( Table 5 ). The average ebb duration was 23 0 minutes longer than the corresponding flood during spring tides at this location As previously mentioned, the extended duration of the ebb flow may be attributed to the tremendous momentum generated by the ebb current velocities as well as head differences between Tampa Bay and the Gulf of Mexico The enormous volume of water exiting Bunces Pass at such high velocities, coupled by the absence of retarding effects from marginal flood currents Gulfward of the inlet, result s in longer period s of time required to slow down the ebb momentum so that the
81 corresponding flood flow can resume. In general the magnitude of differe nce between flood and ebb tidal prisms Gulfward of the inlet was most dramatic during spring tides. The nature of flood dominated neap tides at Bunces Pass is not clearly understood. However, their occurrence may be due to the fact that there is a distinc t difference in head between the ocean and the bay between spring and neap tides. During spring tides, a large head exists due to the increase in tidal range, specifically during ebbing conditions. The large volume of water exiting the inlet results in t he tidal currents within the inlet continuing to ebb as flooding commences. Flooding tides struggle against a large volume of ebbing water, requiring a long time to slow down and overcome. Conversely, during neap tides, the head difference is less. The inlet is still ebbing as flooding commences. However, flooding tides struggle against a much smaller volume of ebbing water, allowing flooding currents to dominate for one or two days during neap tidal conditions The data acquired from Bunces Pass were co mpared to published equations relating inlet cross sectional area with the tidal prism. Utilizing a cross section obtained from 2003 bathymetry data, a mean tidal prism was calculated at Bunces Pass. Following Jarrett ( A c = 5.74 X 10 5 O ), a cross sectional area of 1379 m 2 equates to a spring tidal prism of 2.03 x 10 7 m 3 Mehta et al. (1975) proposed a conversion, relating spring and mean tidal prism, whereby P m = 0.85 P s Following, this equate s to a mean tidal prism of 1.72 x 10 7 m 3 at Bunces Pass. These values are slightly greater than though remain in agreement with tidal prism data compiled by Barnard (1998), who reported a cross sectional area of 1379 m 2 with of tidal prism of 1.2 x 10 7 m 3 The difference may be accounted fo r by the location and precision of the ADPs used during the 2000
82 deployment. Their location within the inlet was precise whereas the location of sensor equipment during the 1997 study was restricted to surface conditions during the time of deployment. Additionally, peak ebb current velocities collected during the 1997 study were approximately half of those collected in 2000, further suggesting the most recent data more accurately represents conditions within Bunces Pass. The tidal prism data from the inlet during both deployments in 2000 ( Table 5 ) were quite comparable with spring prism results acquired from Jarretts equation. Seasonal variations in tidal currents Minor seasonal variations in tidal currents were observed between the summer and winte r deployments. These variations include a slight decrease of 0.02 m in mean tidal range from August September to November December, 2000 ( Fig. 26 ). The minimal difference which is partially attributed to seasonal ch anges in seawater temperatures known as thermal expansion, is negligible Time velocity data indica ted seasonal differences Neap tides during the summer were characterized by peak flood velocities and durations that exceeded those of ebb. This behavior contrasts with winter neap conditions when ebb durations were longer. In general, neap flood durations during the summer were longer compared to the winter. The reason for the seasonal difference may be attributed to an increase in northeasterly winds blowing to the southwest during the wi nter deployment. The presence of these winds might have a negative impact on flood tidal currents by diminishing their ability to flood the inlet efficiently Further research is necessary to quantify this hypothesis.
83 Fac tors influencing tidal flux Sev eral key factors responsible for controlling velocity asymmetry at tidal inlets include: climatic conditions; forcing tides due to tidal harmonic constituents; friction; and changes in inlet channel geometry (Walton, 2002). Tidal currents throughout the e ntire deployment were subject to minimal at mosphe ric influences, though wind was predominantly from the northeast during the winter. Barometric pressure and wind gust data from St. Petersburg, Florida (NOAA, PORTS, 2000) indicate that few major fronts pas sed within 10 km of Bunces Pass during the August September and November December deployments ( Fig. 3 8 ). Figure 38 Barometric p ressure (blue) and peak wind gust (pink) data from St. Petersburg from August through December 2000 were plotted. Dates during which ADP tripods were deployed are bracketed in red. Data acquired from NOAA, PORTS, 200 0
84 Several low pressure systems were present during both deployments, a greater extent during the winter than the summer ( Fig. 3 9 ). The passage of the se systems had a n effect on tidal current velocities and durations. For example, a minor front passed near Bunces Pass during the week of November 16, 2000 ( Fig. 40 ). O n November 18, 2000 the inlet was not temporarily flood dominant, as was common of ot her neap tidal condition. On this date, the ebb duration was exceedingly long. As low pressure systems approach the coast, southwesterly winds enhance flooding tides following the observed minimum in atmospheric pressure The p assage of such systems pro duced easterly winds which enhance ebb tides. As this system approached, easterly gusting winds produced a 360 minute ebb flow within the inlet throat, which was unusually longer than the flood duration of 320 minutes. Maximum ebb velocities exceeded flo od velocities as well. In short, Bunces Pass was overwhelming ebb observed dominant during this neap cycle due to the minor influence of weather. This event was in tidal currents both Gulfward and within the inlet throat. Additi onal meteorological effects include sea breeze and land breeze, which are due to temperature differences in land and water surface temperatures. Sea breeze occurs in the morning as land surface temperatures increase and may magnify the effect of flooding tides Land breeze occurs later in the day and may magnify ebbing tides. The occurrence of these phenomena was observed at Anna Maria Island, located approximately 12 miles south of Bunces Pass during the week of August 22 to August 29, 2000 (NOAA, PORTS, 2000) ( Fig. 41 ). T hese phenomena are prevalent during the summer, and occur to a lesser extent during the winter along this section of Floridas
85 Figure 39 Hourly barometric pressure (blue) and wind direction (red) data from St. Petersburg, Florida were plotted from A) August 17 through September 9, 2000, and B) November 17, 2000 through December 23, 2000. While no major storms occurred during the summer deployment, several low pressure systems were present during the winter deployment. As these fronts approached, the wind was from the southwest, which enhanced flood tides. The passage of each system was identified by the lowest value for barometric pressure within a 24 hour period (circles). After the fronts passed, north easterly winds were produced, which enhanced ebb tides. A) B )
86 Figure 40 The effects of low pres sure systems on tidal currents at Bunces Pass were observed by A) the passage of a low pressure system on November 19, 2000, and B) the net effect on tidal currents Gulfward of the inlet (dates outlined in red). November 19, 2000. A) B )
87 coast Tidal currents at Bunces Pass may be enhanced by these phenomena, however, the extent of the ir influence in unclear. Any influence, however, would be considered relatively insignificant compared to other d ominant factors that control tidal asymmetry. The presence of tidal harmonics is the primary mechanism for asymmetry in tidal inlets (Boone an d Byrne 1981; Aubrey and Speer 1985; DiLorenzo 1988). These higher harmonic tidal components are often referred to as overtides and are often created by non linear distortions of the tidal wave as is propagates from deep into shallow water. Figure 41 Meteorological data from Anna Maria Island, located approximately 12 miles south of Bunces Pass, were plotted from August 22, 2000 to August 29, 2000. A comparison of air temperature versus wind direction show that both sea breeze and land breeze occur locally along this coast. These phenomena are prevalent during the summer, though occur inconsistently during the winter, and may enhance tidal currents. Data acquired from NOAA, PORTS, 2000
88 DiLorenzo (1 988) found that flood or ebb velocity dominance was controlled by the phasing between the M 2 constituent, which has a period of 12.4 hours, and its first harmonic, the M 4 constituent In brief the phase angle relationship between these two constituents r esults in flood dominance, n o dominance, or ebb dominance. The amplitude phase angle is the amplitude phase difference between the M 2 and M 4 constituents of tide, where bay tide can be related to channel velocity through the inlet continuity equation. Whe n the phase angle is between 0 to 180 degrees, the relationship is c haracterized as out of phase This results in an eb b dominant behavior, with faster peak ebb currents. The impact of overtides at Bunces Pass in uncl ear, though may be considered an additional factor that controls tidal asymmetry. For a further review of tidal harmonics, the reader is referred to Boone and Byrne (1981), DiLorenzo (1988), and Walton (2002). Another cause of tidal velocity asymmetry is due to friction. Mota Oliveria (1970) concluded that head losses associated with higher friction in the inlet channel should bring about a decrease in bay tidal prism and consequently decrease in natural flushing capacity. He postulated that greater friction moves an inlet system towa rd flood dominant behavior. Selig and Sorenson (1978) found the same via numerical modeling, and agreed that greater friction lead s to increasing flood dominance in the inlet system. Speer and Aubrey (1985) also found via numerical modeling a trend tow ard flood dominance in shallow channels where friction increases as a function of decreasing water depth. This further implied that shallower channels are more flood dominant than deeper channels. In accordance with these findings, it is acceptable to sa y that inlets with deeper channels would be more prone to ebb dominance, while shallower channels with higher
89 friction would be more prone to flood dominance. Regardless, frictional effects at Bunces Pass are considered negligible due to an average depth of 7 m. The extent of bed friction at Bunces Pass is unclear due to the nature of which current velocity data was collected. The ADP sensor collects measurements in 0.5 m bins, or intervals starting at 0.9 m above the ocean floor ( Figs. 32 and 33 ) Ther efore, the bottom data set is collected 1.4 m above the ocean floor, and does not record specific current velocities on the bottom boundary. A n upward trend in horizontal velocities was observed during spring ebb flows, which was indicative of an open flo w channel This further implied that bed friction wa s present However, vertical profiles of horizontal velocity show that the influence of bed friction on overall tidal current velocities is minimal. Changes in channel geometry with respect to time can also influence tidal asymmetry. However, a historical analysis of Bunces Pass reveals that minimal change to the inlet geometry has occurred during the past 130 years. Therefore, this factor does not appear to influence tidal asymmetry at Bunces Pass. S ummary of Tidal Currents A review of tidal current data from Bunces Pass demonstrates that the inlet is primarily ebb dominant, both with respect to peak velocity and duration. Spring tidal conditions were overwhelming ebb dominant during both seasons, wh ile neap tides exhibited inconsistent patterns of flood dominance which lasted for one or two days This intermittent flood dominance may be the result of a variety of factors that influ ence tidal asymmetry, including climatic conditions and overtides I t is difficult to document
90 local sea breeze at Bunces Pass, therefore further research is needed to quantify this effect. S ea breeze does not significantly affect inlet processes at Bunces Pass. E bb current velocity data were greater during the winter se ason. Spatial variations in inlet currents are present at Bunces Pass. W ell developed marginal flood channels transport a significant por tion of the flood tidal prism. This results in flood durations and peak flood velocities that are greater within the inlet throat, compared to the Gulfward lo cation, because marginal flood channels are absent Gulfward. Generally speaking, more water exits Bunces Pass main channel than enters. This is probably due to the inlets connection with adjacent inlets. Bunces P ass shares a portion of Tampa Bays tidal prism with Egmont Channel and its associated South Channel. Apparently, the volume of water entering Tampa Bay from these inlets is not redistributed proportionally. If more water exits Bunces Pass than enters, t hen this would imply that less water exits Egmont Channel, the South Channel, and Passage Channel than enters thereby accounting for the difference. Previous research has determined that Egmont Channel is ebb dominant, while the South Channel and Passage Channel are both flood dominant (Berman 2002). Combined, the data further suggest that more water is stored in the back barrier embayment of Tampa Bay during summer flood tides than during winter. This water accumulates during neap floods, and is flush ed Gulfward during spring ebb flows. The large ebb tidal prism is primarily responsible for the stability of Bunces Pass and its tide dominated morphology.
91 SEDIMENT CHARACTERISTICS OF BUNCES PASS EBB TIDAL DELTA Sediments along west central Floridas coast are predominantly fine quartz sand mixed with varying amounts of biogenic carbonate sand and gravel. This is the result of the depositional history of siliclastic sediment on the Florida Platform, which had begun by mid Cenozoic time (late Oligocene ). Numerous sea level fluctuations and N S longshore transport deposited siliclastic material as far south as the extreme southern margin of the Florida Platform. However, the lack of an effective transport mechanism along the western portion of the plat form prohibited the transport of this sediment into deeper water. This resulted in quartz rich sediment on the eastern portion of the Florida Platform which transitions into carbonate rich sediment toward the west (Hine et al., 2003). During the past f ew thousand years, sediments have been reworked from previously existing Holocene and older strata and transported landward over the shallow and gently slopin g shoreface by waves (Dean, 198 7 ). As a result, the unconsolidated sediment of the west central F lorida coast represents reworked Plio Pleistocene, highstand quartz sand deposits mixed with carbonate debris ranging from sand to gravel size (Hine et al., 2003). The Holocene sediment represents a veneer (varying from a few centimeters to 3 0 m in thickn ess) which overlays an irregular Miocene Limestone (Duncan et al., 2003). The distribution and thickness of these sediments is the result of antecedent topography, multiple sea level fluctuations, and longshore transport processes.
92 Sediments along this s ection of Floridas coast are dominated by calcium carbonate and quartz with small amounts of clay minerals. The c alcium carbonate is all skeletal material which ranges in size from gravel to sand. However, quartz sand is the primary constituent of thes e sediments. The reworking of these sediment grains numerous times has pro duced a supermature (Folk, 1974 ) mode, with a mean of 2.6 2.7 phi (Davis, 1994). Minor amounts of clay minerals are present, though are restricted to back barrier embayments. The te xtures of sediments along this reach range widely. Mean grain size of surface sediments range from fine sand to coarse gravel, and sorting also displays a broad range (Davis, 1994). The mean grain size reflects the composition. Sediments high in carbona te content are relatively coarse, while those which are predominately quart z are fine sand. In the inlet, t he relationship between grain size, sorting, and physical energy is such that the coarsest sediment s are located within the high est energy environmen t s while the fine st sediment s are deposited in the low est energy environment s Typically, c oarse sediment s are located within the main inl et channel, where tidal currents are the strongest. Fine sediment s are located along the periphery of the swash pla tform, where tidal currents are absent and wave energy is low. The bimodality of the beach/nearshore zone reflects the composition with little overlap between the shell gravel and the quartz sand ( Fig. 4 2 ) (Crowe, 1983).
93 Sediment Characteristics of th e Ebb Delta Complex In October 2002, surface sediment samples were collected from five sub environments throughout the ebb tidal delta complex at Bunces Pass ( Table 6 Fig. 4 3 ). These sub environments include: the main channel, channel margin linear bars, marginal flood channels, and the swash platform. Bunces Pass is a tide dominated inlet, and lacks a well developed terminal lobe. Therefore, terminal lobe surface samples were substituted with offshore samples. All samples were analyzed for percentages of their grain size, shell and quartz fractions, as well as percentages of their sand, gravel, and mud content. In general, mean grain size ranges between 0.23 and 3.47 (phi) ( Fig. 4 4 ). Figure 4 2 The bimodality of the beach/nearshore zone along the west central Flo rida coast reflects the composition with little overlap between the shell gravel and the quartz s and ( Crowe, 1983 ).
94 These data provide a general pattern of sediment distribution within the ebb delta environment ( Table 6 ). Although the west coast of Florida is considered to be sediment starved (Davis et al. 1982), the Egmont Bunces tidal delta complex represents a local sediment sink. The extensive delta complex extends up to 10 km offshore and has an estimated v olume of 336 406 x 10 6 m 3 ( Hine et al., 1986 ), providing abundant sediment for local delta and barrier island morphogenesis. This is evident by the formation of North Bunces Key, located just north of the main channel, as well continued accretion along Mu llet Key to the south. T he mean grain size of the surface sediments range from 2.4 phi (0.19 mm) to 3.2 phi (0.11 mm) although some sediments are both coarser and finer ( Fig. 4 4 ). Most Table 6 Ebb Delta Surface Sediment Composition, October 2002, B unces Pass, Florida Sub Environment Mean Grain Size, (phi) Sand % Gravel % Carbonate % Main Channel Channel Margin Linear Bars North South Swash Platform Marginal Flood Channels North South Offshore 2.12 2.12 2.93 3.05 2.84 3.03 3.14 94.3 3 95.01 99.63 99.22 96.37 98.82 99.33 5. 61 4.92 0.36 0.73 3.60 1.1 4 0 .55 30.21 27.24 9.10 9.36 11.09 7.15 7.88
95 samples fro m north side of Bunces Pass are coarse as is sedimen t from the channel floor. These coarse sediments are the result of both relic and recent shell debris ( Table 6 ). Main Channel Sediments within the main channel represent a very high energy sub environment proximally, with a decrease in current veloci ty G ulfward. They are primarily composed Figure 43 Sur face sediment sample locations from Octo ber 2002 are representative of five sub environments, including: the main channel; channel margin line a r bars; marginal flood channels; the swash platform; and the offshore environment.
96 of fine quartz sand, with shell gravel and carbonate sand. The highest amount of carbonate sediment, specifically carbonate sand and shell gravel is found in the main channel, due to the high energy nature of tidal c urrents ( Fig. 4 5 ). Gravel is present in the channel throat, though does not occur Gulfward of Bunces Pass as carbonate sand becomes more prevalent ( Fig. 4 6 ). The shell gravel is characterized by weathered and fragmented shells within a poorly sorted sand matrix. This sub facies represents channel lag deposits, and is similar to samples taken in several Figure 44 Mean grain size data from the various sub enviro nments at Bunces Pass were plotted. The mean grain size of the surface sediments range from 2.4 phi (0.19 mm) to 3.2 phi (0.11 mm) although some sediments are both coarser and finer.
97 of the present tidal inlets along this coast (Kowalski, 1995; and Barnard, 1998). The high energy currents allowed only large sizes of bivalves to acc umulate, concentrating shell material. The shells beco me imbricated and remain on the channel floor like armour. Ver y fine sand is incorporat ed into the channel floor as sediment i s captured by the shell gravel. The scouring action of the channel has reworked older Holocene sediments, incorporating them into recent channel deposits. The abundance of carbonate sand Figure 45 The percentage of carbonate composition in sediments within the variou s sub environments at Bunces Pass were plotted. The highest amount of carbonate sediment, specifically shell gravel, is found in the main channel, due to the high energy nature of the currents
98 ( Fig. 47 ) further reflects the high energy nature of the sub environment, as they are the direct result of the degradation of older Hol ocene biogenic sediments. However, carbonate sand abundance may additionally be a result of the technique used to acquire the sediment sample. A clam shell grab sampler may not adequately collect a representative sample of the surface within the main cha nnel. The armored nature of this sub environment may prevent adequate collection of shell gravel, which could fall out of the bucket if not close properly upon extraction. Figure 46 The percentage of gravel composition in sediments within the various sub environments at Bunces Pass were plotted. Gravel is abundant in the channel Gulfward of the inlet throat, though decreases as the channel progresses Gulfward.
99 Channel margin linear bars The channel margin linear bars have developed where w ave energy intersects tidal energy, effectively depositing coarse sediments as the wave energy decreases abruptly. Sediments vary on the north and south channel margin linear bars, with coarse, shelly sand to the north. North channel margin linear bar se diments contain a significant amount of carbonate material, as well as the second highest abundance of gravel found throu ghout the entire delta complex. This suggests that north channel margin linear Figure 47 Percentages of carbonate and gravel abundance from sediment samples within the main channel were plotted. Sample MW 5 contained a significant amount of carbonate material, though there was a minor amount of gravel. This indicates that this sample was predominantly carbonate sand, which is indicative of the high tidal energy at this location. Main Channel Sediments 0 10 20 30 40 50 60 70 80 90 100 MC-1 MC-2 MC-3 MC-4 MC-5 MC-6 MC-7 MC-8 MC-9 MC-10 MC-11 MC-12 Location Percent % Gravel % Carbonate
100 deposits represent a high energy sub environment speci fically due to the direct interaction with onshore waves and tidal currents Conversely, the south channel margin linear bar is subject to lower wave and tidal energy. Sediments are composed of quartz sand, mixed with 8% carbonate sand, the lowest found t hroughout the entire delta. Shell gravel constitutes less than 1% of the population of sediments within the south channel margin linear bar. Swash bars and swash platform The swash platform acts like a shie ld, decreasing wave energy as it progresses land ward. Sediment in this sub environment is composed of quartz sand, with less than 1 0 % carbonate material. T he deposition of minor amounts of shell gravel occurs which are concentrated on swash bars and develop o n the southern swash platform. This is due to the shoreward propagation of waves over the swash platform, which effectively dissipates wave energy. Coarse shell gravel account s for the maj ority of the carbonate material in this sub environment; h owever, a modest portion of the carbonate sediment is sand. Marginal Flood Channels The north marginal flood channel contains twice the amount of carbonate sediment and three times more gravel specifically than the south marginal flood channel. Longshore transport deposits coarse carbonate shell gravel north of the main channel. Only fine sand is transported around the periphery of the ebb tidal delta. Therefore, the
101 south marginal flood channel contains an abundance of quartz sand, with minor carbonate sediment and minim al gravel. Offshore Offshore sediments are predominantly comprised of fine sand. This environment is exposed exclusively to direct wave action. Sediments are composed of quartz sand, mixed with 8% carbonate material, the lowest found throughout the entire delta. Shell gravel is min imal, again suggesting that the carbonate material is primarily sand. Sediment Distribution T he present ebb delta at Bunces Pass is the result of sediment distribution in response to the interaction of winds, waves, and extremely strong ebb tidal currents Dominant longshore transport carries littoral sand southward from the vicinity of Indian Rocks Beach. Strong ebb currents from Bunces Pass restrict this southward transport, resulting in sediment deposition as surface waves interact with tidal currents Deposition occurs as critical velocity thresholds required to keep sediment in suspension are abruptly diminished. Coarser sediments, relatively high in carbonate composition, are deposited on the northern portion of the delta, while finer sands are tr ansported around the ebb delta ( Fig. 4 8 ). During flooding tides, flood currents transport a significant volume of water into the inlet, primarily through the marginal flood channels. Some of this volume is transported across the swash platform, bringing w ith it minor amounts of suspended
102 sediment. However, ebb currents with velocities twice as large, flush any sediment accumulations Gulfward, thus resulting in a scoured inlet with a deep gorge. As previously mentioned, Bunces Pass currently appears t o be in a state of dynamic equilibrium. Net sand accumulation is negligent due to efficient flushing mechanisms of the ebb jet. These overwhelmingly ebb dominant currents effectively scour the main channel, resulting in an armoured channel floor composed of large, reworked Holocene carbonate skeletal debris Carbonate skeletal debris is present in marginal flood channels north of the main channel, though is absent to the south. Figure 48 Sediment transport patterns at Bunces Pass ebb tidal delta. Longshore transport carrie s sediment into the system from the north, traveling south. Strong ebb tidal currents interact with waves, depositing coarse shell material, before finer sands migrate around the periphery. During the summer, prevailing southerly winds transport these fi ner sands shoreward, creating swash bars that eventually wield to the mainland. Longshore Transport
103 P redominantly find sand is transported southward around the ebb delta, leavi ng behind coarser sediment. As expected, the coarsest grains and the highest percentage of carbonate material throughout the entire deltaic complex are found within the main channel and the north channel margin linear bar These ref lect the highest energy environment s Conversely, the finest grains with minimal to no carbonate material present are located distally of the main channel, in a low energy, wave dominated environment.
104 CONCLUSIONS Bunces Pass is a tide dominated, natural inlet of unknown or igin. The inlet has maintained its current size, location and orientation fo r at least the past 130 years Sedimentation on the ebb tidal delta at Bunces Pass is strongly influenced by the interaction with adjacent inlets. Historically, the ebb tidal de lta was extremely large, due to the presence of the south channel of Pass A Grille Pass. As the tidal prism diminished through the south channel, the sheltering effect produced by the large ebb tidal delta weakened, and large volumes of sand began migrati ng shoreward. Erosion of the large swash platform resulted in early accretion along the Reefs, and later the formation of both North Bunces Key and South Bunces Key, as well accretion along Mullet Key south of the inlet. The morphology of Bunces Pass ha s remained tide dominated for at least 130 years. A well developed ebb tidal delta is present Gulfward of the inlet due to the accumulation of sediments by both wave and tide dominated processes. However, the ebb tidal delta has changed size and shape d uring the same time period from a broad, lobate shaped delta, due to the diminished effectiveness of wave sheltering. This was a direct result of the extinction of the adjacent south channel of Pass A Grille Pass and the subsequent erosion of its ebb tidal delta, which was superimposed on the larger Bunces Pass ebb delta.
105 Bunces Pass is characterized by ebb tidal currents which overwhelmingly exceed those of the flood for most of the lunar month. However, flood dominant tidal currents irregularly occur dur ing neap tides, when current velocities are lowest. The spring ebb tidal prism of 2.03 x 10 7 m 3 is twice t hat of the flood. This enormous spring ebb tidal prism is over 400 times greater than the corresponding littoral drift rate of 50,000 m 3 per year, a nd is primarily responsible for the inlets stability The influence of weather at Bunces Pass has not affected the inlets self regulating behavior. T he modern eb b tidal delta is composed of two sub facies A fine, quartz sand is produced in the lowe r e nergy, wave dominated environments specifically on the swash platform and offshore environments. A sub facies consisting of shelly, quartz sand with minor amounts of carbonat e sand is present in the higher energy environments. Strong tidal currents in th e main channel cause the mechanical weathering of shell gravel, producing a significant amount of carbonate sand. Wave and tidal interaction north of the main channel deposit coarse carbonate sand and shell gravel on the north channel margin linear bar T he presence of antecedent topography does not account for the stability of Bunces Pass, as the inlet is underlain by over 30 m of unconsolidated Plio Pleistocene sediments. The location of Bunces Pass with regards to its proximity to adjacent inlets, as w ell as the distribution of sediment along this section of the coastal reach, has affected sedimentation on the ebb tidal delta; however, neither has affected the inlets stability. T he influence of weather related events has also affected sedimentation on the ebb tidal delta and its surrounding barrier islands, but has not affected the stability of the inlet. Therefore, the nature of tidal hydrodynamics are primarily responsible for the stability of Bunces Pass.
106 Historical photos show much evidence that barrier islands along this section of the coast have undergone significant changes over the period of record. Similarly, development within the back barrier embayments has restricted tidal flow through numerous inlets. However, there is little evidence t o suggest that changes either Gulfward of Bunces Pass or within Tampa Bay have affected its tidal prism. Therefore, it is assumed that the volume of water flowing though Bunces Pass is similar to what it has been historically. Strong ebb tidal currents ha ve kept Bunces Pass in dynamic equilibrium with its surrounding environment. The large ebb tidal prism solely is responsible for explaining how a tide dominated inlet is maintained in a microtidal environment. The prevalent ebb dominance further suggests that the inlet is hydraulically connected to the adjacent and much larger Egmont Channel inlet system, which also serves Tampa Bay. The present situation at Bunces Pass shows a stabilized, tide dominated inlet with a large, elongate ebb delta that is unl ikely to change significantly in the future if present conditions are maintained.
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