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Krock, Jennifer Rose.
Historical morphodynamics of johns pass, west-central florida
h [electronic resource] /
by Jennifer Rose Krock.
[Tampa, Fla.] :
b University of South Florida,
Thesis (M.S.)--University of South Florida, 2005.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
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Mode of access: World Wide Web.
Title from PDF of title page.
Document formatted into pages; contains 112 pages.
ABSTRACT: Johnand#8217;s Pass is a stable mixed-energy inlet located on a microtidal coast in Pinellas County, Florida. It is hydraulically connected to the northern portion of Boca Ciega Bay. Morphological analysis using a time-series of aerial photographs indicated that anthropogenic activities have influenced the evolution of the tidal deltas and adjacent shorelines at Johnand#8217;s Pass. Previous studies have documented the channel dimensions at the location of the existing bridge and calculated the tidal prism. A chronological analysis of these data yielded an increasing trend in the cross-sectional area at Johnand#8217;s Pass from 1873 to 2001. Anthropogenic activities occurring in Boca Ciega Bay impacting this trend begin in the 1920and#8217;s when Indian Pass, approximately 7 km north of Johnand#8217;s Pass, was artificially closed.Other significant events causing an increase or decrease in the cross-sectional area at Johnand#8217;s Pass include dredging and filling in the bay, channel dredging at Johnand#8217;s Pass, and jetty construction. More recent data collected from a simultaneous current meter deployment at Johnand#8217;s Pass and Blind Pass were used to calculate the bay area serviced by each inlet resulting in an area serviced by Johnand#8217;s Pass being 1.8x104 km2 and 0.33x104 km2 serviced by Blind Pass. In comparison, Blind Pass captures 14 percent of the tidal prism that Johnand#8217;s Pass captures and Johnand#8217;s Pass captures 87 percent of the bay prism while Blind Pass captures 13 percent. Using the discharge equation and assuming the channel area was largely constant the tidal prism at Johnand#8217;s Pass was 1.07x107 m3 during the twenty-one day deployment.Based on a historical analysis of the tidal prism this study is within 40 percent of the tidal prism calculated by Mehta (1976) and Becker and Ross (2001) and within 20 percent of the tidal prism calculated by Jarrett (1976) and Davis and Gibeaut (1990). An analysis of the current meter time-series indicated that flood velocities in the channel were influenced by a frontal system passing through the study area during the deployment increasing the amount of potential sediment being deposited in the channel thalweg. The maximum ebb and flood tidal velocities during the deployment were 143 cm/s and 115 cm/s, respectively. Morphological analysis of cross-sectional data from 1995 to 2004 indicated that sediment tends to accumulate along the northern portion of the channel. The channel thalweg tends to accumulate more sediment east of the bridge where wave energy is lower and currents are not as strong. An average net accumulation of 0.5 m per year was estimated along all seven cross-sections. Given the length and width of the surveyed channel, 610 m by approximately 150 m, the sediment flux through the inlet is approximately 45,800 m3/yr along the channel thalweg. A small amount of sediment accumulation has occurred southwest of the bridge in response to channelized flood flows along the newly constructed jetty. An annual sediment budget was estimated for the Johnand#8217;s Pass inlet system using the beach profiles and inlet bathymetry data between 2000 and 2001. Overall, the inlet system has accumulated more sediment than it has lost during this time period.
Adviser: Dr. Ping Wang.
t USF Electronic Theses and Dissertations.
Historical Morphodynamics of John s Pass, West-Central Florida by Jennifer Rose Krock 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: Ping Wang, Ph.D. Richard A. Davis, Jr., Ph.D. Mark A. Ross, Ph.D. Date of Approval: November 18, 2005 Keywords: Tidal Inlets, Inlet Stabilit y, Coastal Geomorphology, Inlet Modeling, West-Central Florida Copyright 2005, Jennifer Rose Krock
ACKNOWLEDGEMENTS I would like to extend many thanks to my advisor Dr. Ping Wang for his guidance and expertise during my studies here at USF. Many thanks to my committee members Dr. Richard A. Davis, Jr. and Dr. Mark A. Ross for their support and advice. Dr. Davis provided insight on the historical details about Johns Pass and helped me focus my thoughts. Dr. Ross provided a unique opportunity to study coastal inlet processes with engineering applic ations using the CMHAS numerical model. Patrick Tara helped imme nsely with the numerical model details. Dr. Rom Goel provided addi tional bridge details from FDOT survey data. Bill Mihalik for provid ing the Army Corps of Engineers channel surveys. A sincere thanks to Bill Seabergh and Dr. Nicholas Kraus at ERDC in Vicksburg for their guidance while working in the physical model. Thank you to the U.S. Army Corps of Engineers for funding this research project. In addition, I would like to thank my family and friends for their support: Mom, Dad, and Matt; my fellow Coastal Research weenies Rip, Marianne, Mark, Dave, Jack, and Alyssa; the Jennifers in Vicksburg; the geology department faculty and staff. Lastly and most importantly, thanks to my fianc Derek Wilson for his patience, love, and support.
i TABLE OF CONTENTS List of Tables iv List of Figures v ABSTRACT xi Introduction 1 OBJECTIVES 3 STUDY AREA 3 Coastal Processes Along the West-Central Florida Coast 5 WEATHER 7 TIDES 7 WAVE CLIMATE 8 LITTORAL DRIFT 9 Johns Pass Inlet System 9 MORPHODYNAMIC CLASSIFICATION OF TIDAL INLETS IN WEST-CENTRAL FLORIDA 9 JOHNS PASS EBB-TIDAL DELTA 11 JOHNS PASS FLOOD-TIDAL DELTA 13 CHANNEL 13
ii Previous Studies 14 MORPHODYNAMIC CLASSIFICATION OF BARRIER-INLET SYSTEMS 14 TIDAL PRISM AND CROSS-SECTIONAL AREA RELATIONSHIPS 15 HYDRAULIC CHARACTERISTICS OF THE JOHNS PASSBLIND PASS SYSTEM 18 HISTORICAL MORPHODYNAMICS OF JOHNS PASS 21 CMHAS HYDRAULIC MODEL 23 Historical Events at Johns Pass 26 HISTORICAL EVOLUTION DEPICTED FROM TIME-SERIES AERIAL PHOTOS 26 NATURAL HISTORY 26 ANTHROPOGENIC INFLUENCES 27 Methodology and Data Base 39 TIDAL CURRENTS 41 WEATHER DATA 42 BATHYMETRIC SURVEYS 42 BEACH PROFILES 43 JOHNS PASS PHYSICAL MODEL STUDY 44 Results and Discussion 47 LONG-TERM TREND IN CROSS-SECTIONAL AREA 47 HYDRODYNAMICS OF JOHNS PASS 49 CHANNEL EVOLUTION 65 SEDIMENT BUDGET 82 SEDIMENT SOURCES AND SINKS 83 ESTIMATION OF TERMS 85
iii Conclusions 87 References 90
iv List of Tables Table 1 Historical review of Johns Pass morphologic dimensions. 21 Table 2 Data sets compiled in this study including the date, location, and sour ce of information. 40 Table 3 A comparison of historical tidal prism calculations to the spring tidal prism at Johns Pass calculated in this study. 51 Table 4 Shoreline change from TG survey one to survey line R127. 71
v List of Figures Figure 1. Lower left: A general location map of the study area in Pinellas County, Florida. Upper right: Image of Boca Ciega Bay (1.Northern Boca Ciega Bay; 2.Southern Boca Ciega Bay) 4 Figure 2. General coastal classification based on an energy relationship between mean wave height and tidal range from Davis and Hayes (1984). Inlets along the west-central coast of Florida fall within the red circle. 6 Figure 3. Classification of tidal inlets along the westcentral coast of Florida (from Davis and Gibeaut, 1990). 11 Figure 4. An oblique aerial photo of Johns Pass taken in 1989 showing the extent of the ebb and flood deltas. 12 Figure 5. Escoffier curve showing the maximum channel velocity for a given cross-sectional area (from Dean and Dalrymple, 2002). 16 Figure 6. University of Florida map of Johns Pass showing the location of the channel gorge by a red line (from Mehta et al., 1975). 19 Figure 7. University of Florida map of Blind Pass showing the location of the channel gorge by a red line (from Mehta et al., 1975). 20
vi Figure 8. Changes in the cr oss-sectional area of Johns Pass and Blind Pass, a multi-inlet bay system, since the 1800s (Adapted from Mehta et al., 1976). 20 Figure 9. Diagrams showing the evolution of Johns Pass from 1883 to 2000 (from Davis and Vinther, 2002). 22 Figure 10. Aerial Photograph of Johns Pass taken in 1926 showing the Madeira Causeway to the north. 27 Figure 11. Aerial Photo of Johns Pass taken in 1951 showing the original bridge across the inlet. 28 Figure 12. Aerial Photo of Johns Pass taken in 1957 showing erosion along Madeira Beach north of the inlet. 29 Figure 13. Aerial photo of Johns Pass taken in 1960 showing finger channels in the bay area. 30 Figure 14. Aerial Photos of Johns Pass taken in 1970 showing OBriens lagoon on the northern tip of Treasure Island. 31 Figure 15. Aerial photo of Johns Pass taken in 1973 showing accretion on Treasure Island. 32 Figure 16. Aerial photo of Johns Pass taken in 1976 showing the ebb-tidal delta outlined by breaking waves. 33 Figure 17. Aerial photo of Johns Pass taken in 1989 showing the erosion along northern Treasure Island and the structure indicated by a red arrow. 34 Figure 18. Aerial photo of Johns Pass taken in 1993 showing the sediment plume on the northern side of the inlet. 35
vii Figure 19. Oblique aerial photo of Johns Pass taken in 1994 showing the outline of the ebb-tidal delta. 35 Figure 20. Aerial photo of Johns Pass taken in 1997 showing relatively stable beaches adjacent the inlet. 36 Figure 21. Aerial photo of Johns Pass taken in 2000 showing erosion along northern Treasure Island before the completion of the south jetty. 37 Figure 22. Aerial photo of Johns Pass taken in 2002 showing the south jetty. 38 Figure 23. Flow patterns and magnitude plot of a 15 second period and 10 foot high waves approaching the jetty. The length of the arrows denotes the magnitude of the currents generated by waves in cm/s. 45 Figure 24. Contour plot of flow patterns where red is a strong current and blue indicates a weak current. 46 Figure 25. Vector plot of the magnitude and direction of currents produced by an 8 second 5 foot wave (from Seabergh and Krock, 2003). 47 Figure 26. Linear trend indicating changes in the crosssectional area of Johns Pass due to significant natural and anthropogenic events. 48 Figure 27. A bar graph showing the changes in the crosssectional area of Johns Pass and the percent change from the prior year indicated on the x-axis. 49 Figure 28. Johns Pass winter 2001 ADP deployment time-series tidal velocities. 53 Figure 29. Blind Pass winter 2001 ADP deployment time-series tidal velocities. 53 Figure 30. Pressure data from NOAA meteorological station located in Clearwater. 55
viii Figure 31. Frequency of wind directions occurring from 1/11/2001 to 1/31/2001 obtained from the NOAA meteorological station in Clearwater, FL. 56 Figure 32. Average daily wind speed and direction data recorded by the NOAA Clearwater meteorological station from 1/11/01 to 2/2/01. 56 Figure 33. Frequency of wind speeds, measured in m/s, that occurred from 1/11/2001 to 1/31/2001 from the NOAA meteorological station in Clearwater, FL. 57 Figure 34. Significant wave height (Hmo) and wave period (Tp) data collected at Johns Pass showing the time when two cold fronts passed through the study area. 58 Figure 35. Johns Pass velocity profiles showing one spring tidal cycle measured at 20 minute intervals. 59 Figure 36. Johns Pass velocity profiles showing a portion of the neap-tidal cycle occurring on 1/16/2001 measured at 20 minute intervals. 60 Figure 37. Johns Pass velocity profiles showing one neap-tidal cycle occurring on 1/31/2001 measured at 20 minute intervals. 61 Figure 38. Johns Pass velocity profiles of one average tidal cycle measured at 20 minute intervals. 62 Figure 39. Linear relationship between the tidal prism and tidal range at Johns Pass. 63 Figure 40. Ebb-tidal prism versus the tidal range at Johns Pass during the study time period. 64 Figure 41. Flood-tidal prism versus the tidal range at Johns Pass during the study time period. 64
ix Figure 42. Channel cross-section at the Johns Pass bridge in 1968. Distance along the x-axis was measured from north to the southern channel bank (CTC, 1993). 66 Figure 43. Three cross-sections of Johns Pass extracted from a bathymetric co ntour map from 1992. A: located at the bridge; B: seaward of the bridge; C: bayward of the bridge (CTC, 1993). 67 Figure 44. GIS map showing the locations of inlet crosssections and beach profiles. 68 Figure 45. USACE cro ss-section one (1995-2004) west of the bridge. 70 Figure 46. TG-02 beach profil e showing net accumulation along the south jetty during construction (2000 01). 72 Figure 47. USACE cro ss-section two (1995-2004) west of the bridge. 73 Figure 48. UASCE crosssection three (1995-2004) west of the bridge. 74 Figure 49. USACE crosssection four (1995-2004) east of the bridge. 75 Figure 50. USACE cross-section five (1995-2004) east of the bridge. 76 Figure 51. USACE cro ss-section six (1995-2004) east of the bridge. 77 Figure 52. USACE cross-section seven (1995-2004) east of the bridge. 78 Figure 53. FDOT survey lin e one located 15 m from the bridge. 79
x Figure 54. FDOT survey line two located 46m from the bridge. 80 Figure 55. FDOT survey line three located 59m from the bridge. 81 Figure 56. Plan view of the Johns Pass inlet system with sediment transport pathways and littoral cells defined. 84 Figure 57. Annual (2000-01) sediment budget diagram for the Johns Pass inlet system. 86
xi Historical Morphodynamics of Jo hns Pass, West-Central Florida Jennifer Rose Krock ABSTRACT Johns Pass is a stable mixed-energy inlet located on a microtidal coast in Pinellas County, Florida. It is hydraulically connected to the northern portion of Boca Ciega Bay Morphological analysis using a time-series of aerial photograph s indicated that anthropogenic activities have influenced the evolut ion of the tidal deltas and adjacent shorelines at Johns Pass. Previous studies have documented the channel dimensions at the location of the existing bridge and calculated the tidal prism. A chro nological analysis of these data yielded an increasing trend in the cross-sectional area at Johns Pass from 1873 to 2001. Anthropoge nic activities occurring in Boca Ciega Bay impacting this trend begin in the 1920s when Indian Pass, approximately 7 km north of Johns Pa ss, was artificially closed. Other significant events causing an incr ease or decrease in the crosssectional area at Johns Pass include dredging and filling in the bay, channel dredging at Johns Pass, and jetty construction. More recent data collected from a simultaneous current meter deployment at Johns Pass and Blind Pass were used to calculate the bay area serviced by each inlet resu lting in an area serviced by Johns Pass being 1.8x10 4 km 2 and 0.33x10 4 km 2 serviced by Blind Pass. In comparison, Blind Pass captures 14 percent of the tidal prism that
xii Johns Pass captures and Johns Pass captures 87 percent of the bay prism while Blind Pass captures 13 percent. Using the discharge equation and assuming the channel ar ea was largely constant the tidal prism at Johns Pass was 1.07x10 7 m 3 during the twenty-one day deployment. Based on a historical analysis of the tidal prism this study is within 40 percent of the tidal pr ism calculated by Mehta (1976) and Becker and Ross (2001) and within 20 percent of the tidal prism calculated by Jarrett (1976) and Davis and Gibeaut (1990). An analysis of the current meter time-ser ies indicated that flood velocities in the channel were influenced by a frontal system passing through the study area during the deployment increasing the amount of potential sediment being deposited in the channel thalweg. The maximum ebb and flood-tidal velocities during the deployment were 143 cm/s and 115 cm/s, respectively. Morphological analysis of cross-sectional data from 1995 to 2004 indicated that sediment tends to accumulate along the northern portion of the channel. The channel thalweg tends to accumulate more sediment east of the bridge where wave energy is lower and currents are not as strong. An average net a ccumulation of 0.5 m per year was estimated along all seven cross-sectio ns. Given the length and width of the surveyed channel, 610 m by ap proximately 150 m, the sediment flux through the inlet is approximately 45,800 m 3 /yr along the channel thalweg. A small amount of sedi ment accumulation has occurred southwest of the bridge in response to channelized flood flows along the newly constructed jetty. An annu al sediment budget was estimated for the Johns Pass inlet system us ing the beach profiles and inlet bathymetry data between 2000 and 2001. Overall, the inlet system has accumulated more sediment than it has lost during this time period.
1 Introduction Tidal inlets are narrow openings that maintain connection between the open ocean and a bay or lagoon by permitting flows in and out (Johnson, 1919). They are dy namic features of a coastline that exhibit a variety of morpholo gies in response to changing sediment supply, wave climate, tidal regime, and anthropogenic activities. Barrier islands adjacent to an inlet are affected by changes to these sedimentary features near an inlet: ebb-tidal deltas, inlet channel, and flood-tidal deltas (D avis and Gibeaut, 1990). The primary source of sediment supplying these fe atures is the littoral drift along a coast. The stability of tidal inlets is of major concern to coastal engineers and managers. Inlet stability depends on the balance between the amount of water flowing through an inlet, the tidal prism, and the amount of littoral material transported by wave-driven currents and tidal flows across and into the inlet (Bruun, 1966). Many numerical and conceptual models have been developed to try to explain the complex relation ships that exist between barrierinlet and inlet-bay systems. Vinc ent and Corson (1980) summarized a variety of existing models, and classified them as geomorphic models and empirical models of inlet hydraulics. Hayes et al (1970) developed a general classification of barrier islands, including: those dominated by wave action, elongate spitlike barriers, and mixed-energy drumstick barriers (Hayes, 1979). Morphodynamics is a term used to describe the co-adjustment between form and physical processes (Wright and Short, 1984). The oscillatory nature of tidal current s and seasonal variability of waves
that move sediment into a channel are time-dependent forces in which more than one stable equilibrium co ndition may exist (Van de Kreeke, 1984;1989). Assessing an inlets overa ll stability has been done in the past using a variety of methods: incl uding analysis of historical data (e.g., Davis and Vinther, 2002), and a pplication of conceptual models and empirical relationships (e .g., Jarrett, 1976). The above two methods are used in this study. Other methods include Bruun and Gerritsens (1960) tidal prism to littoral drift ratio as well as Walton and Adams (1976) ebb-tidal delta vo lume to littoral drift relation. Comparisons of each method can be made by estimating the tidal prism during average conditions with time (Hume and Herdendorf, 1987). The conservation of mass equation is typically used to relate the volume of water, i.e., the tidal pris m, that enters the bay through an inlet channel during one flood or ebb tidal phase (Keulegan, 1967). 0 T bi A HAVd t ( 1) where, A b is the area of the bay, H is the tidal range in the bay, A i is the cross-sectional area of the inlet, V is the tidal velocity through the inlet, t is time, and T is one complete ebb or flood cycle. Escoffier (1940) presented a simplified version of inlet stability analysis where a bay is connected to the ocean by one inlet. Johns Pass is part of a multi-inlet bay system that requires an analysis of the equilibrium flow areas for multiple inlets. Blind Pass, approximately 5 km to the south, serves the same bay. Stable inlets may experience dynamic fluctuations about an equilibrium ti dal prism and cross sectional area resulting in more than one equilib rium flow area (Van de Kreeke, 1989). 2
3 OBJECTIVES The overall objective of this st udy is to interpret the channel sedimentation and scour patterns in re lation to the tidal flow and tidal prism at Johns Pass. Specific objectives and tasks include: 1) Examining morphological change s by comparing time-series aerial photos; 2) analyzing changes in the cross-sectional area of Johns Pass to depict both short-term and long-term trends; 3) analyzing the tidal velocities at Johns Pass concurrent ly with the meteorological data collected during the same time pe riod and relating to sediment transport; 4) quantifying the tida l prism from the twenty-one day winter deployment in the channel at Johns Pass; 5) using Van de Kreekes (1989) relation for two inlets connected to a bay to calculate the area of Boca Ciega Bay serviced by each inlet; 6) combining all of the data and analysis to provide a sediment budget for the Johns Pass inlet system. STUDY AREA The Pinellas County barrier island chain begins on the north with Anclote Key and extends south to the entrance to Tampa Bay. The origin of evolutionary island develo pment along this coast began in the late Holocene (Evans et al., 1985). Sand Key, a barrier island approximately 25 km long, is the long est island in the chain located on a broad headland. The headland sepa rates the northern and southern portions of the island at Indian Sh ores. This study fo cuses on the area south of Indian Shores and north of the entrance to Tampa Bay (Figure 1). Johns Pass is a structured inlet, located south of the headland at Indian Shores, connecting the Gulf of Mexico to Boca Ciega Bay This
multi-inlet bay system extends south to the entrance of Tampa Bay (Figure 1). Boca Ciega Bay services Johns Pass, Blind Pass, Pass-AGrille, and Bunces Pass inlets from no rth to south. Blind Pass is located approximately 5 km south of Johns Pass at the southern terminus of Treasure Island. Although Pass-A-Gr ille and Bunces Pass are part of Boca Ciega Bay they are not hydraulically connected to Johns Pass and Blind Pass (Davis and Barnard, 2003). Figure 1. Lower left: A general location map of the study area in Pinellas County, Florida. Upper right: Image of Boca Ciega Bay (1.Northern Boca Ciega Bay; 2.Southern Boca Ciega Bay ) 4
5 The southern part of the barrier island chain along Pinellas County, oriented northwest to southeast, includes southern Sand Key, Treasure Island, and Long Key (F igure 1). This area is heavily populated with many condos, hotels, and restaurants in close proximity. Madeira Beach makes up the southern tip of Sand Key directly north of Johns Pass. Treasure Island is the barrier island separating Johns Pass and Blind Pass and is 5 km long (Figure 1); Sunshine Beach is located on the no rthern tip while Sunset Beach is located on the southern end of Tr easure Island. St. Pete Beach is located on Long Key, the last barrier island in the chain just north of the entrance to Tampa Bay. The hydraulic boundaries of Boca Ciega Bay are strongly influenced by the locations of dams and causeways that separate flows into and out of the multi-inlet ba y system (Mehta et al., 1976). The northern boundary is located at th e Narrows near Conch Key and along the Long Bayou Dam in the northeas t. The St. Pete Beach Causeway defines the southern end of the inlet-bay system (Figure 1). Davis and Barnard (2000) indicated that flow between the northern and southern reaches of the bay were segregated after the construction of the causeways and dredged islands. In this study and based on the above study, only the northern portion of Boca Ciega Bay was considered; the northern extent begins in the narrows at Conch Key and the Long Bayou Dam and extends south to the Corey Causeway (Figure 1). Coastal Processes Along the West-Central Florida Coast The eastern Gulf of Mexico is a low wave energy coastal system with a small tidal range. The broad, gently sloping continental shelf
restricts the size of the waves that develop. Episodic tropical storms, hurricanes, and winter cold fronts generate higher energy conditions and are largely responsible for re distributing sediments along the broad inner shelf (Davis and Barn ard, 2000). The orientation of the coast and low relief create comple x reflection patterns (Davis and Gibeaut, 1990). Inlet morphodynamics are controlle d by the interaction of wave and tidal processes. West-central Florida exhibits a broad range of inlet morphodynamics due to the delicate balance between wave and tidal dominance (Davis and Hayes, 1984). These authors developed a morphodynamic classification of coasts based on the mean annual wave height versus the mean tidal range (Figure 2). The studied coast falls into the lower left corner (the circled area) and includes a variety of different types of coasts ranging from tide-dominated to wavedominated (Figure 2). Figure 2. General coastal classification based on an energy relationship between mean wave he ight and tidal range from Davis and Hayes (1984). Inlets along the we st-central coast of Florida fall within the red circle. 6
7 WEATHER The Bermuda high, a clockwise rotating atmospheric pressure cell, dominates weather patterns in this region. The dominant wind direction is from the east and th e prevailing wind direction varies seasonally and is generally out of the southeast during the summer and from the northeast during the wi nter (Henry et al., 1994). During the winter months, November to Ma rch, frontal systems pass through the area from the northwest. These systems affect coastal processes by setting up a regional net southerly littoral drift from the headland at Indian Shores south to Pass-A-Gr ille (Davis and Andronaco, 1987). Hurricane season begins in early June and ends in early November. Typically, hurricanes that enter the Gu lf of Mexico cross the east coast of the Florida peninsula or the straight of Florida and continue on a northwest trajectory. Hurricanes rarely make landfall along the westcentral Florida coastline, however the hurricane of 1848 opened Johns Pass (Davis and Barnard, 2000). TIDES Tides along this coast are mixed semi-diurnal with a spring tidal range of approximately 0.7 m (Mehta et al., 1976). This places westcentral Florida in the microtidal range of 0-2 m (Hayes, 1979). In comparison, tides along the Georgia Bight from Cape Hatteras, North Carolina to Cape Canaveral, Florida range up to mesotidal with spring tidal range exceeding 2 m (Hayes, 1979). At Johns Pass, the ocean tidal range is approximately 0.8 m and the bay range is 0.7 m (Davis and Gibeaut, 1990). Slack water at Johns Pass is approximately forty minutes between flood and ebb tide (NOAA, 2004). Time-velocity asymmetries can exist due to phase differences in the semi-diurnal
8 tidal constituents resulting in ebb or flood-tidal dominance in terms of velocity (Militello and Hughes, 2000). WAVE CLIMATE Offshore wave approach is se asonal along the Pinellas County coast with swell propagating towards the east to southeast in the summer and east to northeast in the winter (CTC, 1993). The Coastal Data Network, operated by the Un iversity of Florida from 1984 to 1989, collected monthly wave data along Clearwater Beach. Average wave heights in the summer were 19 cm with an average period of 4.63 s while during the winter aver age wave heights were 32 cm with an average period of 5.64 s (Wang et al., 1990). Davis and Fox (1977) conducted a field study at Treasure Island just south of Johns Pass in which they measured average wave periods of 5.3 s and average breaking wave heights of 35 cm in January. They indicated that waves are affected by tidal currents and longshore currents. The wave data collected along Treasure Island, so uth of the inlet entrance, were influenced mostly by flood-tidal cu rrents into the inlet. The southerly longshore current influenced wave measurements near the inlet entrance during ebb tide. A study by Davis and Andronaco (1987) observed wave heights of 60-70 cm during the passage of one winter cold front near Clearwater Beach. Average annual si gnificant wave height at this location was 37 cm with average pe riod 5.8 s (Davis and Andronaco, 1987). From the present study, based on a twenty-one day ADP deployment in January 2001, the av erage significant wave height was 13 cm and average wave period 4.2 s in the Johns Pass channel. The highest significant wave height was nearly 50 cm with a peak wave
9 period of 3.5 s. These high waves in the inlet channel were related to the passage of a strong winter cold front. A detailed discussion about the frontal system can be found in the section entitled Hydrodynamics of Johns Pass. LITTORAL DRIFT Northerly winds prevail during the winter generating oblique breaking waves that drive longshore currents. Littoral drift refers to the volume of sediment transported along the coast each year (Komar, 1998). Sediment transport along the Pinellas County coast is generally toward the south from the headland at Indian Shores. Local beach and nearshore topography may complic ate the transport magnitude and direction. The volume of sediment intercepted by an inlet is typically stored in ebb and flood-tidal deltas allowing reasonable estimates of the net littoral drift to be made using the ebb delta volume (Walton and Adams, 1976). Davis and Gibeaut (1990) found a net littoral drift of 38,200 m 3 /yr at Johns Pass. Tidwell ( 2005) found a net littoral drift of 35,000 m 3 /yr in the vicinity of Blind Pass. Local reversals near the entrance to tide-dominated (or mixed-energy) inlets with large ebbtidal deltas are common along this coast (Hine et al., 1986). Johns Pass Inlet System MORPHODYNAMIC CLASSIFICATION OF TIDAL INLETS IN WESTCENTRAL FLORIDA Previous studies on the morpho logy and hydrodynamics of tidal inlets along the west-Florida coast include Mehta et al., 1976; Lynch-
10 Blosse and Davis, 1977; Davis and Hayes, 1984; Cuffe and Davis, 1993; Dean and OBrien, 1987; Davi s and Gibeaut, 1990; Wilhoit et al., 2003; and Tidwell and Wang, 2004. Mehta et al., (1976) studied Johns Pass and Blind Pass intensivel y and is discussed in detail in a later section. Lynch-Blosse and Davis (1977) examined the sedimentation patterns at Dunedin and Hurricane Passes in northern Pinellas County. Dean and OBrien (1987) compiled information on 37 of the west coast inlets in an effo rt to describe the long-term stability of inlets along this coast. Davis and Hayes (1984) compared numerous inlets along the west-central coast of Florida in an effort to define the dominant factor (tides and waves) determining inlet morphology. From this Davis and Gibeaut (1990) developed a morphodynamic classification of tidal inlets (Figure 3) that included four types. The wave-dominated inlets tend to have shallow unstable channels and poorly developed ebb-tidal deltas. Conversely, tide-dominated in lets have narrow, deeply incised channels with well-defined ebb-tida l deltas. Mixed-energy inlets are controlled by both waves and tides. According to Davis and Gibeaut (1990), Johns Pass is a mixed-energy straight inlet. The ebb and flood-tidal deltas along the west-centr al Florida coast is influenced by inequalities in ebb and flood-tidal velocities, one being much higher than the other (Caldw ell,1955; Hubbard, 1977).
Figure 3. Classification of tidal in lets along the west-central coast of Florida (from Davis and Gibeaut, 1990). More recently, Cuffe and Davi s (1993) studied ebb and floodtidal delta development and stratigraphy at Hurricane Pass, a mixedenergy inlet. Wilhoit et al., (2003) described the morphodynamics of Bunces Pass, a stable tide-dominated inlet, in southern Pinellas County while Tidwell and Wang (2004) conduc ted a field study of Blind Pass, a wave-dominated inlet, located north of Bunces Pass and south of Johns Pass. JOHNS PASS EBB-TIDAL DELTA Ebb-tidal deltas are ephemera l depositional features of a coastline. The Johns Pass ebb-tidal delta is located seaward of the entrance (Figure 4) extending to approximately 1600 m offshore (Davis and Gibeaut, 1990). The natura l ability of this inlet to bypass sediment from the ebb-tidal delta to the adjacent shoreline has been 11
disrupted several times since the 1960s due to dredging (CTC, 1993). In 1991 the USACE dredged an estimated 43,000 m 3 from the channel seaward to the ebb delta and the sediment was placed along Treasure Island and Long Key (Elko and Davis, 2000). The ebb-tidal delta was then separated into two tabular sand bodies with the north side forming a long channel-margin linear bar perpendicular to the shoreline trend (Davis, 1995). The southern portion was smaller and was in a general crescent shape. A portion of the channel seaward of the bridge was dredged again in 2000 while the southern extension jetty was being constructed (Davis and Wang, 2002). In principle, jetties are designed to confine tida l flows to the navigable portion of the channel to reduce shoaling (Seabergh and Krock, 2003). This may enhance ebb-tidal delt a growth (USACE, 1995). Figure 4. An oblique aerial phot o of Johns Pass taken in 1989 showing the extent of the ebb and flood deltas. 12 The sedimentary characteristics of the ebb delta were described from seven core borings that were collected and analyzed by Williams Earth Sciences, Inc. (1999). Thei r analysis indicated a higher
13 percentage of finer sediment on th e seaward portion as compared to more shell gravel content on the inle t side. The core boring located on the seaward end of the ebb delta wa s driven 3.5 m into the sediment. It was composed of 96 percent fine sand, 3 percent mud, and 1 percent gravel. In contrast, the bori ngs located on the inlet side were driven 1.5 m deep and were composed of 78 percent sand, 20 percent shell gravel, and 2 percent mu d (Williams Earth Sciences, 1999). JOHNS PASS FLOOD-TIDAL DELTA The flood-tidal delta at Johns Pass is a storm-generated deposit formed during the hurricane of 1848 and consists of four welldeveloped sedimentary lobes separated by two small, deeply incised channels. One of the lobes is intert idal and all are orientated in a northeast to southwest direction along the channel. These stable features consist of algal mats and mangrove roots. Five sedimentary facies were characterized based on sediment composition, thickness, and depositional environment (Shock, 1994). The bay area was described as a low wave-energy en vironment comprised of a modern upper layer composed of quartz sand located in the inter-tidal zone, a muddy quartz sand layer often capped by a peat layer associated with mangroves or marsh grasses, and a bottom layer with an increased amount of broken shell. CHANNEL The Johns Pass channel is aligned in a northeast to southwest trend. In general, the navigable portion of an inlet channel is known as the gorge or thalweg (USACE, 1995). The channel at Johns Pass has
14 been federally maintained (by th e USACE) since 1964 in accordance with the Rivers and Harbors Act. The time interval between dredging is about five years in which the channe l is dredged to 3 m deep by 46 m wide (USACE, 2004). The channel length is 1.2 km from the ebb-tidal delta to the flood-tidal delta. Since the 1960s the thalweg has migrated southward 100 m in response to shoreline hardening on the south side (Mehta et al., 1976). The cross-sectional area of the inlet channel also increased during this time in response to dredging activities that began in the 1960s (Davis and Gibeaut, 1990). Previous Studies MORPHODYNAMIC CLASSIFICATION OF BARRIER-INLET SYSTEMS Many barrier island-tidal inlet re lations have been explored to try and explain the controls of this complex system along various coastlines (FitzGerald et al., 1978; Oertel, 1979; 1988; FitzGerald and Hayes, 1980). They include inle t hydrodynamics and sediment transport mechanisms (Hubbard 1975; Hubbard et al., 1979), shoreline configuration (Galvin, 1971), tidal range and barrier island morphology (Hayes, 1975; 1979), ti dal inlet sedimentation patterns and associated tidal deltas (Hayes et al., 1970), ebb-tidal delta volume and tidal prism (Walton and Adams, 1976), ebb-tidal delta morphology and sediment bar bypassing (Sex ton and Hayes, 1982; FitzGerald, 1982; 1984), bay area and inlet geom etry (FitzGerald et al., 1984) as well as a classification of inlet type determined by an energy regime affecting inlet configuration (Dav is and Hayes, 1984; Davis and Gibeaut, 1990). The reservoir model (Kraus, 2002) is a recent
development in numerical model simulation of inlet morphological evolution. A general classification of inlets was based on a relative energy regime, t he geomorphology of ebb and flood-tidal deltas, and barrierinlet orientation (Hayes, 1970; 1979; Davis and Gibeaut, 1990; 1991). A morphodynamic classification of tidal inlets including wavedominated, tide-dominated, and mixe d-energy (straight and offset) is discussed in the previous section (Figure 3). TIDAL PRISM AND CROSS-SECTIONAL AREA RELATIONSHIPS OBrien (1931) conceived an empirical relation on tidal prism and cross-se ctional area taking the general form: ( 2) nCPA where, A is the inlet minimum cross-sectional area, P is the equilibrium tidal prism, and C and n are empirically defined constants. OBrien established a relationship among inlets on the Pacific coast 40854.6910c A xP ( 3) Escoffier (1940) developed a hydraulic relation based on the maximum channel velocities a nd cross-sectional area of an inlet (Figure 5). The inverse relation between cross-sectional area and channel velocity was then used to find an equilibrium velocity from which the critical velocity necessary to maintain a stable inlet can be found. OBrien (1931) found a general critical velocity of 1 m/s by relating the tidal prism to cross sectional area of an inlet, ( 4) cAxP4105 The Escoffier curve (Seabergh an d Kraus, 1997) indicates that as the cross-sectional area of an inlet increases the maximum velocity in 15
the channel increases until an equilibrium condition is reached. The graph is interpreted by noting the inte rsection of two points of critical areas, A c1 and A c2 where points between A c1 and A c2 are stable inlet conditions maintaining the cha nnel thalweg area (Figure 5). Bruun (1966) developed a stability criteria for inlets using the tidal prism to littoral drift ratio. He subdivided inlets into three categories based on this relation: 300 M P denotes good conditions, 150 M P fair conditions, and 100 M P denotes poor conditions, where is the tidal prism and P M is the littoral drift. The term conditions here is used to describe the flushing capability of an inlet where good conditions are satisfied when the ti dal prism is large enough to move sediment through the channel and p oor conditions being an unstable inlet that may close. Figure 5. Escoffier curve showing the maximum channel velocity for a given cross-sectional area (from Dean and Dalrymple, 2002). 16
Keulegan (1967) summarized the effects of flow through an open channel into a bay having certain dimensions, channel roughness, and variability in the tidal range. He developed a coefficient, the coefficient of repletion or filling K that related the channel length and width to the ti dal range thereby quantifying the effects of the channel on the fl ow (Keulegan, 1967). Jarrett (1976) improved upon OBriens tidal prism to equilibrium area relationship by including more inlets on the Atlantic Gulf, and Pacific coasts. He found that there was a correlation among th ose inlets that had one, two and no jetties. Previous studies (e.g., Seabergh, 1998) on Johns Pass have used Jarretts equation for al l inlets on the Gulf Coast: ( 5) 84041002.5 PxAc Jarrett (1976) reported the tidal prism at Johns Pass as 1.4x107 m3 using channel dimensio ns from the 1951 U.S. Coast and Geodetic Survey nautical chart. For Gulf Coast inlets without jetties, the following relationship was developed: ( 6) 86041051.3 PxAc At that time, insufficient data exis ted to calculate a relation among dual-jettied inlets on the Gulf Coas t. The data used to calculate tidal prism from Equation 5 exhibited more scatter than the data Jarrett (1976) used to calculate this relati on on other coasts. He concluded that this equation was therefore not as reliable due to tidal variations among Gulf Coast inlets (Jarrett, 1976). In the Gulf of Mexico the tide varies with the lunar phases sw itching between diurnal and semidiurnal. When the moon is near th e equator the tides in the eastern Gulf of Mexico are semi-diurnal and small (Jarrett, 1976). The tidal prism is usually calculated during the diurnal spring tidal phase in order to get the maximum prism through an inlet. Meteorological 17
conditions can have significant effects on tides possibly causing conditions when the tidal prism can be larger during the semi-diurnal phase than the diurnal (Jarrett, 1976). HYDRAULIC CHARACTERISTICS OF THE JOHNS PASS-BLIND PASS SYSTEM Johns Pass and Blind Pass make up a hydraulic system that was studied by the University of Florid a in 1974. The purpose of the study was to collect tidal current data and sediment samples at each inlet to describe the initiation of sediment movement near the bed. An analysis of inlet hydrodynamics with the associated frictional resistance to flow revealed the quadratic relationship between bed shear stress ( 0 ) and the depth-averaged velocity, u (Mehta et al., 1975): 242 0uf ( 7) where f is a friction coefficient. In op en channel hydraulics, the vertical velocity profile is typically described by a logarithmic curve where u is the velocity, u is the friction (shear) velocity, is the Van Karman constant, z is the bed elevation, and z 0 (often referred to as bottom roughness) is the origin of the logarithmic velocity profile: 0 *ln 1 z z u u ( 8) The velocity profiles were used to calculate hydraulic parameters, i.e., u* and z o associated with the initiation of sediment movement in the channel. Figures 6 and 7 outline the thalweg at Johns Pass and Blind Pass, indicated by a red line on each map. To summarize their results 18
at Johns Pass and Blind Pass, the spring tidal prism at each location was determined to be 6.0x106 m3 and 0.8x106 m3 while the crosssectional area was 883 m2 and 41 m2, respectively (Mehta et al., 1975). The tidal prism to cross-sectio nal area ratio given by Johnson (1972), 1003105 Px A Pc (in English units), was used to show the relation between tidal prism and cross-sectional area. This equation was developed for inlets along the west coast and was disregarded after Jarrett (1976) found an empirical relation among inlets in the Gulf of Mexico. Figure 6. University of Florida ma p of Johns Pass showing the location of the channel thalweg by a red line (from Mehta et al., 1975). Mehta et al., (1976) were the first to show that the crosssectional area of Johns Pass can be rel ated to the cross-sectional area of Blind Pass (Figure 8). They pointed out that the inlets have coevolved through their connection to Boca Ciega Bay Once Indian Pass 19
had closed to the north in 1926 the cross-sectional area of Johns Pass began to increase while Blind Pass decreased. Figure 7. University of Flor ida map of Blind Pass showing the location of the channel thalweg by a red line (from Mehta et al., 1975). 0 200 400 600 800 1000 18731883192619371952197419832001YearCross-sectional Area, m22.00E+07 2.20E+07 2.40E+07 2.60E+07 2.80E+07 3.00E+07Bay Area, m2 John's Pass Blind Pass Boca Ciega Bay Figure 8. Changes in the cross-se ctional area of Jo hns Pass and Blind Pass, a multi-inlet bay system, si nce the 1800s (Updated from Mehta et al., 1976). 20
21 HISTORICAL MORPHODYNAMICS OF JOHNS PASS Hine et al., (1986) and Davis and Gibeaut (1990) studied the historical morphodynamics of inlets along the west-central coast of Florida and documented changes that have occurred at Johns Pass. They reported channel dimensions, from field measurements, aerial photographs, and nautical char ts (Table 1). The morphologic classification of Johns Pass from 1885 to 1962 was a mixed-energy offset inlet (Davis and Gibeaut, 1990) that was later, in 1973, classified as a tide-dominated inle t based on morphological evidence and channel geometry. Table 1. Historical review of Johns Pass morphologic dimensions (from Davis and Gibeaut, 1990). Date Area (m 2 ) Width (m) Depth (m) Ebb-Tidal Delta 1x10 6 m 3 Flood-Tidal Delta 1x10 6 m 3 1873 474 130 1883 431 113 1926 531 136 8.5mean 1941 636 155 1949 782 190 4mean 1952 849 183 7mean 4.817 1957 150 1962 170 1966 180 6dredge depth 1973 190 1974 882 180 11max 1976 200 1980 190 1984 180 3.838 0.382 1992 5.5mean
Davis and Vinther (2002) conducted a similar study on the morphodynamics of tidal inlets using time-series aerial photographs to examine the shape and locations of th e ebb and flood-tidal deltas at Johns Pass (Davis and Vi nther, 2002) (Figure 9). Figure 9. Diagrams showing the evolution of Johns Pass and associated tidal deltas from 1883 to 2000 (from Davis and Vinther, 2002). This schematic representation of the ebb delta at Johns Pass shows complete formation by 1945. Shortly thereafter dredging and 22
filling in the bay area began in th e 1950s shown by a decrease in the size of the ebb-tidal delta. During the 1960s the USACE began dredging the channel essentially cr eating two channel margin linear bars. From 1990 to 2000 the northern portion of the ebb delta increased while the southern half did not change significantly. CMHAS HYDRAULIC MODEL The Center for Hydrologic and Aq uatic Modeling at the University of South Florida developed a hydrau lic model to simulate scour and deposition in a multi-inlet bay sy stem (Ross et al., 1999). The model was employed in a study of localized scour near a bridge pier at Johns Pass (Vincent et al., 2000). Several different construction and seasonal scenarios were tested. Model details are user defined input parameters that include: a bathymetric grid, subgrid features, tides, winds, waves, sediments, and the output time step The model uses the equations of motion and continuity (9, 10, and 11) derived from the Navier-Stokes equation for an incompressible vi scous flow and depth-averaged transport, 2 21 d UQf X x H gd x P dV y d UV x d U t U ( 9) 2 21 d VQf Y y H gd x P dU x d UV y d V t V ( 10) R y V x U t H ( 11) where, t is time; x and are horizontal coordinates; Uand Vare depth averaged velocities; y H is the water level; R is the source and sink 23
terms; is the density of water; P is atmospheric pressure; is the gravitational acceleration; g is the Coriolis factor; is the bottom friction factor and Qis the resultant transport (Vincent et al., 2000). f The combined wave and current fric tion factor is calculated using the Engelund and Hansen equation (12) (Vincent et al., 2000) that is applied to the sediment transport equation for bed load sediment transport. 22 cwcw cwuf (12) where, is the friction factor due to currents and waves and is the shear stress due to currents and wave s. The condition of erosion or deposition is made by applying the Hjulstrom curve at each grid cell. Finally, the continuity equation relate s the volumetric rate of transport to the change in depth over time. Simulated output consists of updated bathymetry. In this capaci ty the study found that the scour magnitude changed relative to se asonal and annual variability with definite long-term trends toward increasing depths next to the southern pier. Scour depths of se veral feet per year under normal conditions were observed in the field in 1998 (Pitman-Hartenstein & Assoc., 1998) and also simulated by the model. Armoring the inlet beneath the bridge did not appear to be a feasible solution (Vincent, 1992). cwf cwu 24 The hydraulic model of Boca Ciega Bay was calibrated from bathymetry and tidal velocity data collected in 1998 and used to demonstrate its applicability to inlet stability analysis by calculating the tidal prism from simulation result s (Becker and Ross, 2001). Becker (1999) concluded that Johns Pass and Blind Pass are relatively isolated from southern Boca Ciega Bay by causeways and bridges citing the relation between the cro ss-sectional area of Johns Pass that
25 was documented by Mehta et al., (1976). The tidal prism was calculated using simulated velocities of 1.0 and 0.7 m/s in the main channel for peak spring and neap flood velocities, respectively. The spring velocity is lower while the ne ap velocity is higher compared to the findings of this study discussed in the results section. This would affect tidal prism calculations in using the discharge equation, Q=VA, where V is the flow velocity in the channel and A is the cross-sectional area of the inlet channel. The average spring tidal prism, Q, was then determined to be 1.98x10 7 m 3 at Johns Pass (Becker and Ross, 2001). The tidal prism calculated from data collected in 1998 at Johns Pass by Becker and Ross (2001) was over three times higher than the 1974 prism of 0.6x10 7 m 3 obtained by Mehta et al., (1976). The Becker and Ross (2001) prism was also over 30 percent higher than the 1951 prism of 1.4x10 7 m 3 reported by Jarrett (1976). The reason for the larger tidal prism, calculated by Becker and Ross (2001), compared to Mehtas (1976) lower prism is due to the different methods used in the calculations. Mehta used an average tide condition while the others used a spring tide. Also, Mehta used field measurements from a current meter deployment and Jarretts 1951 prism was based on an empirically defined relationship be tween the cross-sectional area and the tidal prism for Gulf Coast inlets The location of the current meter in the channel is important because calculations are based on the cross-sectional area at the instrument location and assuming that the measured velocity represents the average velocity over the entire cross-sectional area.
26 Historical Events at Johns Pass HISTORICAL EVOLUTION DEPICTED FROM TIME-SERIES AERIAL PHOTOS The available aerial photographs span from 1926 to 2002. Timeseries morphological changes, more specifically, visible changes in shoreline position adjacent to Johns Pass were examined. In addition, the construction of jetties, groins beach nourishment activities, and their effects on the barrier-inlet morphology at Johns Pass were discussed based on the aerial views. NATURAL HISTORY Johns Pass was opened in 1848 duri ng an intense hurricane that breached the barrier island chai n along the Pinellas County coast (Mehta et al., 1976). Many inlets connected to Boca Ciega Bay have opened and closed over the course of the bay evolution, including Indian Pass located 7 km to the no rth of Johns Pass. This inlet was artificially closed in 1924 due to in stability (Davis and Barnard, 2000). They indicated that the natural history of this inlet ends in the 1920s, when causeway construction across Boca Ciega Bay began. 1926 The barrier islands adjacent the inlet have not been significantly affected by anthropoge nic activities in this photo as indicated by the lack of buildings. The Bay Pines causeway, visible in the upper right corner of the ph otograph, was constructed across Boca Ciega Bay during the 1920s The Bay Pines and Corey Causeways were located north and south of Johns Pass, respectively.
Figure 10. Aerial photograph of Johns Pass taken in 1926 showing the Madeira Causeway to the north. The first bridge across Johns Pa ss connecting Madeira Beach and Treasure Island was built in 1926. By 1937 the Treasure Island Causeway had been constructed so uth of Johns Pass and north of Blind Pass (Davis and Barnard, 2000). Johns Pass was in a juvenile stage of development as channel dime nsions were increasing, the tidal prism increased. Barrier island migration was affecting the channel location at Blind Pass as Treasure Island migrated 3 km to the south (Mehta et al., 1976). Prior to this shift, Blind Pass had a larger crosssectional area and captured more of the tidal prism in Boca Ciega Bay than Johns Pass (Figure 8). ANTHROPOGENIC INFLUENCES Since the 1920s the bay area has been reduced approximately 28 percent by dredging and filling for causeway construction and finger channels, thereby diminishing the ba y tidal prism. Sediment bypassing 27
from the ebb-tidal delta to the adjacent beach at Johns Pass was disrupted by dredging and jetty construction on the north side. Placement of sand on the adjacent beaches for nourishment caused disturbances in the sediment budget as the beach to the north, Madeira Beach, began to erode severely while Treasure Island on the south side did not change considerably. 1951 Inlet width had increased, as compared to the 1926 photo (Figure 11). The adjacent barrier islands were slightly offset from one another as Treasure Island was protruding further seaward. The ebb-tidal delta has formed an asymmetric lobe with two shore perpendicular sand bodies along the channel margins. Sediment bypassing is visible near the centra l portion of Treasure Island; this region had a much wider beach than the northern portion. Johns Pass now had a larger prism than Blind Pa ss. The original bridge across the inlet appears in this photograph. Ov erall, the morphological features corresponded to the balance betw een the tidal flushing and the longshore sediment transport to the south. Figure 11. Aerial photo of Johns Pass taken in 1951 showing the original bridge across the inlet. 28
1957 A severe erosion problem no rth of the inlet intensified and the city of Madeira Beach installe d a system of thirty-seven groins to capture the sediment transported with the littoral drift moving south (Figure 12), (Eldred, 1976). Finger channels constructed from dredged material appear in the bay area as dredging and filling activities increased dramatically. This was a common practice at the time. In 1959, fifty-six groins were installed on southern portion of Treasure Island to mitigate shoreline erosion (Elko and Davis, 2000). Figure 12. Aerial photo of Johns Pass taken in 1957 showing the erosion along Madeira Beac h north of the inlet. 1960 The USACE began dredging the navigation channel in the bay area as a result of the Rivers and Harbors Act. The navigation channel has been maintained to a depth and width of 3 m and 46 m, respectively (USACE, 2004); total dept h and width of this inlet exceeds these values as discussed in the section on channel evolution, however the USACE is only responsible for this section of the channel. The channel at Johns Pass was initially dredged by the USACE in 1961. 29
Shortly thereafter the north jetty was constructed and 280 m of the shoreline along the south side of the inlet channel was hardened in 1966. The dredged material was deposited offshore (Davis and Barnard, 2000). The Madeira Beach groin system was effective and the beach was accumulating sediment (Eldre d, 1976). Initial channel dredging began in 1961 with 131,500 m 3 from Johns Pass and Blind Pass. The sand was placed 600 m offshore and 600 m along Treasure Island. Later, in the early 1970s, this sediment migrated toward shore forming the OBriens lagoon. The lag oon was artificially filled in the late 1970s (Elko and Davis, 2000). The main channel began migrating 91 m south and eroded the southern shoreline (Elko and Davis, 2000). The flood-tidal delta in this photog raph has not changed while the ebbtidal delta has a more asymmetric geometry with visible southward bypassing. Figure 13. Aerial photo of Johns Pass taken in 1960 showing the finger channels in the bay area. 30
1970 Erosion on northern Treasure Island causes local concern (Figure 14). An attempt was made to reverse this trend by placing a sediment pile along the coast in th e nearshore zone allowing waves to push the sand onto the beach. The trend of onshore migration of the sand body is apparent in this phot o. Compared to the previous photo (1960) Madeira Beach has widened and extends to the tip of the northern jetty. The original bridge at Johns Pass was removed and a new bridge was constructed seaward of the old one by 1971 at the location of the minimum channel width. Figure 14. Aerial photos of Johns Pass taken in 1970 showing OBriens lagoon on the northe rn tip of Treasure Island. 1973 Shoreline hardening along the south side of the channel was done to stabilize the new bri dge and control erosion near the southern pier (Figure 15). Continue d onshore movement of the sand pile resulted in its attachment to the northern tip of Treasure Island nourishing the beach. The channel thalweg is visible along the south side by the darker gray color. Made ira Beach and the flood-tidal delta 31
are relatively stable compared to the south side of the inlet. The barrier islands and the finger channels have become densely populated. Figure 15. Aerial photo of Johns Pass taken in 1973 showing accretion on the northern tip of Treasure Island. 1976 The OBriens lagoon was ar tificially filled by the USACE shortly after this photograph was taken (Figure 16). The northern end of Treasure Island had widened due to the attachment of the offshore sand body. The white caps in the ph oto outline the location of the ebb delta; it has an asymmetric lobe shape skewed to the south in the direction of the longshore transport. Localized scour near one of the bridge piers on the south bank de veloped (UF, 1969) and it was shown that the erosion in this area wa s due to flood flows that were concentrated on the south side. The cross-sectional area of Johns Pass was increasing while at the same time the cross-sectional area of Blind Pass was decreasing (Mehta et al., 1976). 32
Figure 16. Aerial photo of Johns Pass taken in 1976 showing the ebbtidal delta outlined by breaking waves. 1989 The northern jetty was extended and the beach accumulated sediment to the tip of the extension (Figure 17). The shoreline was hardened with rock rubble to mitigate the erosion along the southern side of the inlet. The northern tip of Treasure Island has eroded and a structure was installed to try to control the erosion there. Channel dredging at Johns Pass and Blind Pass (250,000 m 3 ) took place in 1991 and the sediment was pl aced on Treasure Island. Details of the bridge and boat docks are visible in this photo. 33
Figure 17. Aerial photo of Johns Pass taken in 1989 showing the erosion along northern Treasure Isla nd and the structure indicated by a red arrow. 1993 This photo includes only the channel area. A sediment plume is visible in the northeastern portion of the inlet near Hubbards boat dock (Figure 18). A small amount of sediment is visible in the lower left corner of the photo indi cating sediment encroachment into the navigation channel. Details of the bridge are visible in this photo. Bascule bridges are common where large vessels enter and exit an inlet because they can be raised an d lowered. Johns Pass is frequently used by local fishing charter boats and recreational watercraft (CTC, 1993). The number of marinas, boat traffic, and vehicles has increased since the 1970s prompting the need fo r a twin span Bascule bridge to replace the older fixed bridge. 34
Figure 18. Aerial photo of Johns Pass taken in 1993 showing the 1994 The ebb-tidal delta is visible in this oblique aerial photo ed sediment plume on the nort hern side of the inlet. by breaking waves (Figure 19). Th e oblique photo shows the curved shoreline with the point in the no rth being the headland at Indian Shores. The attachment point of the southward sand bypassing is visible. The erosion at the northern tip of Treasure Island has stopp as indicated by the relatively wide beach there compared to the 1989 photo. Figure 19. Oblique aerial photo of Johns Pass taken in 1994 showing the outline of the ebb-tidal delta. 35
1997 The ebb-tidal delta has tw o visible channel margin lobes 36 ssing into ng (Figure 20). A sediment plume is visi ble in the lower right corner near the northern tip of Treasure Island. The northern jetty has accumulated sediment to the tip resulting in sediment bypa the inlet channel. A considerable amount of sand has accumulated along the inside of the southern channel next to the rock rubble structure. This accumulation led to the construction of a jetty alo the south side. Figure 20. Aerial photo of Johns Pass taken in 1997 showing relatively stable beaches adjacent the inlet. 2000 The dredged channel is visi ble in this color photo as a 2004). Followed by maintenance dredging of the channel seaward of the darker blue color that extends thro ugh the ebb-tidal delta where the passing boats are located (Figure 21) In 1999, construction began on a 125-m long terminal groin on the so uth side of the inlet to prevent further channel shoaling from the so uth. This photo was taken before the jetty was completed. Rip-rap was placed along the bottom northeastern portion of the inlet (Pitman-Hartenstein & Assoc.,
37 and bridge February to August (Becke r and Ross, 2001). The ebb-tidal delta is very well defined in this photo. The northern half is larger slightly offset towards the south. The southern ebb delta is nearly perpendicular to the shoreline. Figure 21. Aerial photo of Johns Pass taken in 2000 showing erosion along northern Treasure Island be fore the completion of the south jetty. 2002 Construction of the curved jetty on the south side was ompleted and northern Treasure Is land has accumulated sediment to c the tip of the jetty (Figure 22). Some sediment can be seen on the inside of the channel banks seaward of the bridge. The southern end of Treasure Island was re-nourished in August 2004. Two category 3 hurricanes, Frances and Jeanne, crossed the Florida Peninsula on a northwest trajectory causing considerable changes along the westcentral Florida coast (Elko, in press). The impacts of the 2004 hurricanes to Johns Pass are beyo nd the scope of this study.
Figure 22. Aerial photo of Johns Pass taken in 2002 showing the south jetty. Bridge improvements were conducted in 1998 as a counter easure to the localized scour near the southern pier. Fortifications ne n nt amic ed immediately updrift and llation of ent, m included crutch bents that were added underneath the pier (Pitma Hartenstein & Assoc., 1998). The southern jetty was constructed in 1999 to help stabilize and nourish the beach on Treasure Island. Following this in 2000, the navigati on channel seaward of the bridge was dredged and the fill was placed on the northern tip of Treasur Island to help build up the beach in front of the jetty. The northeaster section of the channel was lined with rip-rap by the Florida Departme of Transportation (FDOT) as a countermeasure to structural degradation of the northern pier. In summary, the Johns Pass system has been very dyn over the years. Erosion has o ccurr downdrift of the inlet at differe nt times. Intensive engineering activities, including construction and extension of jetties, insta groin fields, shoreline armoring, shoreface and beach nourishm 38
39 in he ous Methodology and Data Base There is a wealth of information on Johns Pass available from ederal agencies, State departments, Pinellas County, and the on tal ith and channel dredging, imposed signif icant and variable influences on the morphodynamics. In addition, the bridge, which is typically built the narrow section of the inlet, has suffered problems due to pier scour. Overall, no clear trend in morphodynamic evolution can be identified from the time-series aeri al photos, probably because of t complicated interactions between th e natural processes and numer engineering efforts. F University of South Florida Coasta l Research Lab. A summary table (Table 2) has been compiled to give the details of the informati including the type, dates, location, and source in which the following types of data were compiled: aerial photos, flow measurements at Johns Pass and Blind Pass, bathymetric surveys, and beach profiles. Two physical model studies were al so conducted at the USACE Coas and Hydraulics Laboratory (CHL). The first study was designed to model Johns Pass inlet in 1998 prior to the construction of the south jetty. The second was a more general model of an idealized inlet w dual jetties in 2003.
40 able 2. Data sets compiled in this study including the date, location, nd source of information. The hydrodynamic and bathymetri c data were analyzed in the rder specified below. An analysis of each data set (Table 2) included the T a Description Dates Location Source Bathymetric Surveys Annually 1995-2000, 2002-2004 Johns Pass USACE 1968 Jacksonville District UF Inlet Crosssections Quarterl-2004 1992 Johns Pass bridge y 2001 FDOT CTC Beach Profis Treasure Is. Mah USF Coastal Research Lab le Quarterly 2000-2002 deira Bc Acoustic Doppler Series 21 days, 1/11/20012/2/2001 Johns Pass USF Coastal Re d Profiler Time Blind Pass search Lab an USGS Meteorological ata: Winds and D Pressure 8726724 1/11/2001-2/2/2001 Clearwater Beach Station No. NOAA NDBC Aerial hotographs P 1873, 1926, 1942, 1945, 1951, 1957, 1960, 1962, 1970, 1973, 1975, 1976, 1979, 1980, 1984, 1988, 1989, 1990, 1992, 1993, Johns Pass USF Coastal Research Lab 2001, 2002 Inlet and vicinity Images 2000, 2001, 2002 Pinellas County USGS and Pinellas County Nautical Charts 1873, 187 1952, 1956, 1992, 2000 Boca Ciega Ba s US Coast and Ge y 9, 1895, 1909, 1967, 1972, 1974, y Pinella County odetic Surve NOAA Office of Coast Survey o the peak spring and neap tidal velocities at Johns Pass; tidal prism calculations based on flow and cross-section measurements; the amount of accretion or erosion from a time sequence of inlet crosssections at Johns Pass; and an asse ssment of inlet stability using Jarrett (1976) and Van de Kreeke (1989) methods.
41 illet after the les g IDAL CURRENTS Water-level fluctuations, tidal currents, and waves at Johns Pass e measured simu ltaneously using a SonTek, Inc. th nde ght (0.5 m) from the sensor to the water An effort was also made to qu antify the volume of sediment entering the channel by monitoring the growth of a f construction of the south jetty from 2000 to 2001. Five beach profi were surveyed after construction and the amount of accretion alon the shoreline near the structure was calculated. T and Blind Pass wer upward looking Acoustic Doppler Pr ofiler (U-ADP). The sensors were located in the gorge of each inle t on the seaward side. The sensors recorded data from January 11 to February 2 2001 in twentyminute intervals, sampling for two minutes and recording the averag during those two minutes. Vertically throughout the wate r column, the velocity data are sampled in bins of equal hei surface. Ten bins were used in th e flow analysis of Johns Pass and Blind Pass. The data included the time-averaged velocity in each bin and the pressure (water depth) above the sensor. The raw data was given as the northing and easting component velocities that were added in an Excel spreadsheet to gi ve the vector-sum velocity along the channel. The pressure data we re converted from decibars to meters in order to get the water level above the sensor. A wave analysis of the pressure data was done using the SonTek software giving the peak wave period and significant wave height.
42 EATHER DATA Meteorological data such as barometric pressure, wind speed nd direction were acquired online at the NOAA National Ocean Service data were collected from an automated sage of The USACE bathymetric surveys from 1995 to 2004 included the aintained portion of the channel plus 61 m along both sides toward under used for the 2000, 2002 and 2003 am ard W a web address. The meteorological station in Clearwater the closest station to the study area. This data was compared to the wave data to examine the relationship between weather conditions, especially the pas frontal systems, and hydrodynamic conditions. BATHYMETRIC SURVEYS m the shoreline. The echoso surveys (by a USACE survey crew ) was a Ross Smart Sounder. The 2004 survey was done by a private contractor for the USACE using a Hydrotrac depth recorder. In all case s, the surveys used a single be echosounder. The vertical datum used was NAVD88. A software program, Corpscon, developed by the USACE was used to convert older horizontal coordinates referenced to NAD27 into NAD83. The survey lines were closely spaced along the channel from the seaw entrance to the edge of the adjace nt barrier islands in the bay area Only those time-series survey lines that overlapped spatially were used in this analysis. Seven lines were selected from the bathymetry surveys to examine the cross-sectio nal changes. The cross-sections are labeled one through seven with one being the seaward-most extending survey across the tip of the south jetty and seven being the eastern-most. Cross-section four is located at the bridge where the inlet is the narrowest.
43 he the northe rn shore to the channel centerline. All de n Level and transit surveys were conducted on northern Treasure land prior to and after the construction of the southern jetty. Five ly 61 m apart, we re surveyed for a period of 10 as tion one nd Rdeira Beach and are used Quarterly profile surveys were conducted by FDOT along the northeastern section of the channel at 15 m, 46 m, and 59 m from t base of a bridge pier on pth measurements were refere nced to the base of the pier. These surveys were used in this st udy to supplement the USACE data. All the survey data were mapped in ArcGIS, the Geographic Information Systems software versio n 9.0, and georeferenced to a aerial photo of Johns Pass in 2000. Seven cross-sections were extracted from the attributes table in ArcGIS and imported into an Excel spreadsheet fo r further analysis. BEACH PROFILES Is lines, approximate months using an electronic total station and a survey rod. The benchmark was located at R-127 and the vertical datum was referenced to NGVD29. The beac h volume and shoreline change between the time-series profiles was calculated. This data set w included directly following the anal ysis of the USACE cross-sec because the southern end of th e channel survey extends to the shoreline where the jetty is now located. Beach profile surveys have been done quarterly by he USF Coastal Research Lab along Madeira Beach. Monuments R-123 a 122 are located north of Johns Pass on Ma approximately 300 m apart. An an alysis of the volume change between these monuments was done to quantify the volume of sediment that the beach receives annually. This information was
44 s inlet UDY A physical model study of Johns Pass was conducted by the SACE in 1998 prior to construction of the south jetty (Seabergh, hrough the inlet from the outh e s n d the channel entrance. Flood tty t to determine an appropriate sedime nt budget for the Johns Pas system between 2000 and 2001. JOHNS PASS PHYSICAL MODEL ST U 1998). Seabergh investigated the flows t s side to determine if flow patterns and velocities would change significantly due to the addition of a jetty. Currents and waves wer generated based on the Wave Information Study (WIS) data. It wa determined that average wave conditions at Johns Pass are 4 to 6 s i period and 0.45 m in height. Currents and waves were measured in the model at selected locations al ong the channel using 2D SonTek Acoustic Doppler Velocimeters (ADV) and capacitance wave gages. Dye tracer tests were also conduc ted to indicate changes in flow patterns with and without a jetty. An analysis of the data indicated that there was a maximum increase of 5 percent during ebb and floo flows due to the addition of a jetty. These results indicated that a jetty would not have a significant impact on the flow and therefore it was reasonable to assume that significant amounts of sediment would not be permitted to flow into the channel. Dye tracers were used to simulate the patterns of sediment transport around the jetty into the channel. The desired affect of the jetty was to deflect the flow away from flows were added to enhance wave-generated currents along the je to get the maximum potential flow into the channel. The study noted that only under extreme wave condit ions would more active sedimen
45 ized h and Krock, 2003). Once again t he c transport occur. At Johns Pass this most often occurs during the passage of strong winter cold fronts. Following this physical model wa s a similar study of an ideal inlet physical model in 2003 (Seabe rg the flow near a structured inlet entrance was investigated, however the experimental design and condit ions were not the same. Several different types of structures were co nstructed on one side of the inle entrance with varying lengths and angles to the shoreline. They included a spur jetty having differe nt lengths and angles as well as a weir jetty. The purpose was to st udy the current patterns along t structure. Spectral wave conditions were generated in the offshore region to setup flows near the stru cture. The figure (Figure 23) below demonstrates a typical output that was produced from hydrodynami data collection efforts near the jetty. Figure 23. Flow patterns and magnitude plot of a 15 second period and 10 foot high proto-type waves a pproaching the jetty. The length of the arrows denotes the magnitude of the currents generated by waves in cm/s.
46 The waves are approaching from an angle towards the shoreline reating a strong current along the length of the jetty. In the absence o The 4 and c of a flood current the fl ow is directed away fr om the inlet entrance towards the offshore region. Adding a flood current to a wave condition caused an increase in the flow from the tip of the jetty int the inlet, therefore a spur was a dded along the tip of the jetty. spur effectively redirected the flow away from the inlet entrance and back to the adjacent beach. The figures below demonstrate this behavior, indicated in green and ye llow, where the currents were the strongest and the direction to which they were flowing (Figures 2 25). Figure 24. Contour plot of flow pa tterns where red is a strong current and blue indicates a weak current.
Figure 25. Vector plot of the magnitude and direction of currents produced by an 8 second 5 foot wave (from Seabergh and Krock, 2003). Results and Discussion LONG-TERM TREND IN CROSS-SECTIONAL AREA An analysis of the changes th at have occurred in the crosssectional area of Johns Pass was a ccomplished using a time-series of aerial photos in Table 1 above as well as the cross-sectional data (Figures 26 and 27). There are th ree key time periods when the anthropogenic activities had a major influence on the cross-sectional area of Johns Pass. The first was th e construction of the causeways in the 1920s that reduced the bay tidal prism (Davis and Barnard, 2000). The second major influence was dredging and filling in the bay area since the 1950s. The third ev ent was the construction of the northern jetty in 1961 to mitigate shoreline erosion on Madeira Beach. The southern jetty was added in 2000 to control erosion on Treasure Island. 47
200 400 600 800 1000 1873188319261940195219741983199219992001 YearCross-sectional Area, m2 Inlet Opened Indian Pass Closes Causeway Construction Dredging/filling Beach N ourishment Jetty Construction Bridge Improvements Figure 26. Linear trend indicating changes in the cross-sectional area of Johns Pass due to significant natural and anthropogenic events. In Figure 27 below increases and decreases in the inlet area are given as the percent change from the previous year on the graph. Notice the increasing trend in th e cross-sectional area up to 1952 followed by fluctuating changes in th e cross-sectional area within 30 percent. 48
-300 -200 -100 0 100 200 3001873188319261940195219741983199219992001YearChange in Cross-sectional Area, m2-30 -20 -10 0 10 20 30Percent change in the crosssectional area from the prior year Percent Change Change Figure 27. A bar graph showing the changes in the cross-sectional area of Johns Pass and the percent change from the prior year indicated on the x-axis. HYDRODYNAMICS OF JOHNS PASS It is important to examine the tidal flow and tidal prism to understand the sedimentation and erosion patterns in an inlet. Circulation patterns are specific to each inlet (Militello and Hughes, 2000), however some properties ar e common among all inlets. A few of these include: ebb or flood domi nance, channelized flow during ebb and flood tidal cycles, and jetty controls on flow patterns. The tidal prism during every ebb and flood-tidal cycle was calculated from the twenty-one day tidal flow measurement at Johns Pass. For simplicity as well as spatia l and temporal limitations on the field measurements, it is assumed that the velocity measured near the channel centerline represents the flow across the entire channel, i.e. the tidal flow is uniform across the entire channel. The tidal prism was calculated using the following equation: 49
(13) TttAtVP0)()( where, V is the depth-averaged velocity in the channel; A is the inlet channel cross-sectional area; t is the time interval (20 minutes for this case); and T is the tidal cycle (one complete ebb or flood). Because the average tidal range is approximately 0.7 m, or about 10 percent of the water depth, it is re asonable to assume that the crosssectional area does not change signif icantly with tidal fluctuations. In addition, the smaller cross-sectional ar ea that occurs during lower tide compensates for the larger cross-sect ional area during higher tide. The simplified Equation 13 then becomes: T ttV msl AP 0 )( (14) where, A msl is the cross-sectional area at mean sea level. Using the equation above, the tidal prism was calculated by summing the measured velocities and multiplyin g by the cross-sectional area and the sampling time interval. The instrument was deployed in the vicinity of cross-section three (character istics of the cross sections are discussed in detail in the following sections). Because there was no survey data available for 2001 when the instrument was deployed, the average of 2000 and 2002 was used to represent the cross-sectional area of Johns Pass. This averag e cross-sectional area of 680 m 2 was then used in Equation 14. A spring ebb-tidal prism at Johns Pass was thus determined to be 1.07x10 7 m 3 (Table 3). The spring prisms calculated by this study are 40 percent higher than the average tidal prism obtained by Mehta et al., (1976) which was 0.6x10 7 m 3 for Johns Pass. Compared to the spring tidal prism obtained by Becker and Ross (2001) based on model results (1.98x10 7 m 3 ), the prism at Johns Pass obtained by this study is 40 percent lower. This is the 50
result of the methodologies used in calculating the tidal prism. Table 3 lists the tidal prisms obtained by previous studies since 1926. Table 3. A comparison of historic al tidal prism calculations to the spring tidal prism at Johns Pass calculated in this study. Date Tidal Prism (m 3 ) Method Source 1926 1.68x10 7 HAPbay Calculated from historical data in Table 1 1949 1.40x10 7 Spring tidal flows Davis and Gibeaut, 1990 1952 1.42x10 7 Spring tidal flows Jarrett, 1976 1974 6.0x10 6 Average of field measurements Mehta et al., 1976 1999 1.98x10 7 Simulated spring tidal velocities Becker and Ross, 2001 2001 1.07x10 7 Measured spring tidal flows This study Corresponding to the above sp ring ebb-tidal prism, the measured tidal range was 1.07 m at Johns Pass. Tidal prism can also be calculated as: (15) HAPbay therefore, Johns Pass is serving a bay size of 1.6X10 7 m 2 Based on the 2000 NOAA nautical chart (Table 2), the area of Boca Ciega Bay north of the Treasure Island Caus eway and south of Conch Key and the Long Bayou Dam (Figure 1) is 1.8X10 7 m 2 roughly equal to the bay size served by Johns Pass. A si milar exercise was done for Blind Pass. The area south of Treasure Is land causeway and north of Corey Causeway is 0.33 X10 7 m 2 roughly equals the bay size served by Blind Pass. The sum of both these areas is equal to the area of northern Boca Ciega Bay (Figure 1). This suggests that the causeways have 51
52 significantly partitioned Boca Ciega Bay and influenced the tidal prism and tidal circulation at this inlet. Following Van de Kreekes methodology, a range of values could be used to report the tidal prism based on the spring and neap tidal velocities and equilibrium area. T hus for comparison, the neap ebbtidal prism at Johns Pass was 0.63x10 7 m 3 while at Blind Pass the corresponding neap ebb-tidal prism was 0.09x10 7 m 3 Therefore, Blind Pass conveys 14 percent of the tidal prism that Johns Pass conveys. Johns Pass serves approximat ely 87 percent of northern Boca Ciega Bay and Blind Pass serves roughly 13 percent. Figures 28 and 29 summarize the twenty-one day tidal flow measurements. The deployment time-series begins on January 11 th 2001 during a spring ebb-tidal cycle. Spring tide occurred on January 11 th and neap tides occurred on January 16 th and 31 st The large spike that occurs in the middle of both time-series represents a weather system that passed through the study area on January 20 th A lag exists between the tidal cycles at Johns Pass and those at Blind Pass. Johns Pass leads Blind Pass by 20 minutes on average and in some cases as much as 40 minutes.
John's Pass ADP Deployment -150 -100 -50 0 50 1001110000 1122340 1142320 1162300 1182240 1202220 1222200 1242140 1262120 1282100 1302040Time, mmddhhVelocity, cm/s Figure 28. Johns Pass winter 2001 ADP deployment time-series tidal velocities. Blind Pass ADP Deployment -120 -80 -40 0 40 801110000 1122340 1142320 1162300 1182240 1202220 1222200 1242140 1262120 1282100 1302040Time, mmddhhVelocity, cm/s Figure 29. Blind Pass winter 2001 ADP deployment time-series tidal velocities. 53
54 A time-series of depth-averaged velocities were plotted in the figures above (Figures 28 and 29). Notice the magnitude of the maximum ebb velocities at Johns Pass roughly equals the maximum ebb velocities at Blind Pass (Figures 28 and 29), while the maximum flood velocities at Johns Pass were much greater. In Johns Pass (Figure 28) the maximum ebb veloci ty is about 15 to 30 percent greater than the maximum flood velocities, while the ebb-tidal velocities at Blind Pass (Figure 29) were much larger; up to 200 percent more than flood-tidal veloci ties. Similar results at Blind Pass were obtained by Tidwell (2005). The maximum spring ebb-tidal velocity at Johns Pass was 143 cm/s on day one of the deployme nt and the corresponding spring flood-tidal velocity was 115 cm/s in the channel (Figure 28). It is worth noting that the difference be tween maximum ebb velocities is much greater than that between ma ximum flood velocities. This could be the result of the location of the flow meter; based on the findings of Tidwell (2005), the ebb flow at Blind Pass is concentrated in the channel thalweg where the flow me ter was located, while the flood flow is largely uniform across the entire channel. A different flow pattern occurs at Johns Pass as flood -tidal currents are stronger along the channel margins and ebb-tida l currents reach their maximum velocities in the channel thalweg (Mehta et al., 1976). Two neap tidal cycles were reco rded during the ADP deployment; the first occurred on January 16 th and the second on January 31 st In general, neap tidal velocities we re much lower than the spring velocities; the measured neap ebb and flood-tidal velocities on the 16 th were 58 and 50 cm/s, respectively. Th e ebb and flood velocities were 57 and 52 cm/s, respectively for neap tides occurring on the 31 st The
difference between spring ebb and sp ring flood-tidal velocities at Johns Pass is larger than the difference between the neap tidal cycles. Three frontal systems passed thro ugh the study area during the deployment. Sharp increases and decreases in local barometric pressure is a method of detecting storm fronts in addition to local variations in wind speed an d direction. Between January 19 th and the 21 st during the deployment a winter cold front came through the area (Figure 30). The pressure decrease coincides with the large spike in flood-tidal velocities occurring on the 20 th of January at Johns Pass in Figure 28 above. The figures below show the variations in barometric pressure and winds as the system passes through the study area (Figures 30-33). 1010 1015 1020 1025 10301110000 1130000 1150000 1170100 1190100 1210100 1230100 1250100 1270100 1290100 1310100 1330100Time, hourlyPressure, mbars Figure 30. Pressure data from NOAA meteorological station located in Clearwater. The dominant wind direction is from the southeast with a 71 percent occurrence during the period of deployment (Figure 31). Winds from the northwest occurred only 14 percent of the time but 55
correspond to the highest average daily wind speeds that occurred between January 18 th and 24 th (Figure 32). Wind speeds up to 10 m/s were recorded during the passage of one winter cold front (1/23/2001) (Figure 32). The most frequently o ccurring wind speeds ranged from 1 to 6 m/s occurring 85 percent of the time (Figure 33). Frequency of Wind Directions (deg) Occurring During a 21-day Time-period5% 71% 10% 14% 0-89"NE 90-179"SE 180-269"SW 270-359"NW Figure 31. Frequency of wind di rections occurring from 1/11/2001 to 1/31/2001 obtained from the NOAA meteorological station in Clearwater, FL. 0 2 4 6 8 10 12 1/81/131/181/231/282/2 Time, mmddWind Speed, m/s0 60 120 180 240 300 360Wind Direction, deg Average Daily Wind Speed Average Daily Wind Dir Figure 32. Average daily wind speed and direction data recorded by the NOAA Clearwater meteorological station from 1/11/01 to 2/2/01. 56
Frequency of Wind Speeds (m/s) Occurring During a 21-day Time-period5% 10% 29% 32% 24% 9-10 7-8 5-6 3-4 1-2 Figure 33. Frequency of wind speed s, measured in m/s, that occurred from 1/11/2001 to 1/31/2001 from the NOAA meteorological station in Clearwater, FL. The measured wave heights and wave periods in the channel also corresponded to two of the frontal systems that passed through the area with high waves corresponding to the frontal passages (Figure 34). This occurred near the beginning of the deployment, shown on the graph as a peak wave height of almost 50 cm and wave periods of 4 to 6 seconds. The second occu rrence was in the middle of the deployment and lasted a couple of days. It is shown as a large range in wave heights of approximately 5 to 45 cm and wave periods of 4.5 seconds. The variations in wave heights may be caused by wavecurrent interactions. The second fr ontal system lasted more than 2 days (Figure 30). 57
0 10 20 30 40 501110000 1120920 1131840 1150400 1161320 1172240 1190800 1201720 1220240 1231200 1242120 1260640 1271600 1290120 1301040 1312000Time, daysWave Height, cm4 5 6 7 8Wave Period, s Hmo Tp Figure 34. Significant wave height (Hmo) and wave period (Tp) data collected at Johns Pass showing the ti me when two cold fronts passed through the study area. Figures 35 through 38 illustrate vertical current profiles measured at Johns Pass. In these figures, each line represents a profile measurement recorded once every 20 minutes and each figure represents an entire tidal cycle. The vertical axis indicates the distance from the sensor upward. The sens or was mounted roughly 1 m above the bed. The first measurement wa s conducted roughly 0.675 m (0.5 to 0.75 m measurement bin) above th e sensor, or 1.675 m above bed. Positive velocity indicates flood-tidal flow and negative velocity indicates ebb flow. The velocity rang e during spring ebb-tidal flow is larger than those recorded during spring flood-tidal flows (Figure 35). Spring ebb-tidal velocities ranged from 5 to 130 cm/s, while spring flood-tidal velocities were from 25 to 110 cm/s. The velocity profiles in this figure are nearly uniform thro ughout the water column indicating that either bottom friction did not have significant influence on the shape of the profile or vertical mi xing due to waves was significant. 58
Similar profiles were measured by Tidwell (2005) at Blind Pass. It is worth noting that the measurement started at about 1.7 m above bed. Near-bed velocities could not be measured by this instrument. Spring 0 1 2 3 4 5 6 7 8 -150-100-500 50100150Velocity, cm/sdistance above sensor, m Figure 35. Johns Pass velocity profiles showing one spring tidal cycle measured at 20 minute intervals. Neap tide occurred twice during the ADP deployment with drastically different results (Figures 36 and 37). A portion of the neap tidal cycle that was recorded on January 16 th is shown in the figure to accentuate the velocity profile that crosses the y-axis (Figure 36). This may be the result of the first of three frontal systems passing through the study area. Notice that the flood-tidal velocities have a much larger range of values than the ebb-tidal velocities. The velocity profile crossing the line between ebb and fl ood-tidal flows indicates that the upper half of the profile is ebbing while the lower half is flooding. This could be the result of wind shear st resses acting on the surface layers forcing the surface flow to move in the opposite direction of the tidal flow. Also, the neap flood-tidal velo cities are much weaker than the 59
ebb-tidal velocities possibly indicati ng that the flood-tidal flows were suppressed by the weather conditions Figure 36 provides an example of a meteorological tide. Neap 1/16/2001 05:20-08:400 1 2 3 4 5 6 7 8 -60-40-200204060 Velocity, cm/sDepth, m Figure 36. Johns Pass velocity profiles showing a portion of the neap tidal cycle occurring on 1/16/2001 measured at 20 minute intervals. The neap tide that occurred on January 31 st (Figure 37) has a more uniform profile like those in Figure 29. The neap ebb-tidal velocities ranged from 5 to 60 cm/s while neap flood-tidal velocities were from 5 to 50 cm/s (Figure 38) The velocity profiles are probably more representative of an averag e neap-tidal cycle during calmer weather conditions than what occurre d during the previous neap tide. 60
Neap 1/31/2001 01:20-08:200 1 2 3 4 5 6 7 8 -65-45-25-5153555 Velocity, cm/sDepth, m Figure 37. Johns Pass velocity profiles showing one neap tidal cycle occurring on 1/31/2001 measured at 20 minute intervals. An average tidal cycle yielded a range of ebb-tidal velocities from 5 to 100 cm/s; similarly, flood-tid al velocities ranged from 10 to 85 cm/s (Figure 38). The velocity profiles were also very uniform throughout the water column. In this case, the number of 20 minute ebb-tidal velocity profiles exc eeds the number of flood profiles indicating that it takes longer for th e bay to empty than it does for it to fill. This slight inequality is likel y the result of other inlets feeding the bay, such as Blind Pass to the south; by the location of the gage, as found by Mehta et al. (1976), the flood flow tends to focus along the margins of the inlet; or this ex ample may also be influenced by the particular characteristics (i.e., longer ebbing than flooding) of this tide. It is important to consider the location of the current meter when looking at each of these figures because the ADP instrument collected data only at this positi on. The ADP was located west of the bridge at approximately 7.5 m wa ter depth, therefore it was not 61
located in the channel thalweg. Channel morphology changes dramatically from the inlet entrance towards the bay area, as discussed in the following section, which may also have a significant impact on the channel velocities. Average0 1 2 3 4 5 6 7 8 -115-75 -35 5 45 85 Velocity, cm/sDepth, m Figure 38. Johns Pass velocity profiles of one average tidal cycle measured at 20 minute intervals. An analysis of the tidal range and respective tidal prism was performed in order to assess the re lationship between tidal range and tidal prism. One would expect there to be a linear relation between the tidal prism and tidal range because the bay area should be fixed in such a short term and there are no significant freshwater inputs into the system. Figure 39 illustrates the relationship between the calculated tidal prism based on equation 14 and the measured tidal range. 62
P = 2E+07R r2 = 0.96 0.0E+00 2.0E+06 4.0E+06 6.0E+06 8.0E+06 1.0E+07 1.2E+07 1.4E+07 1.6E+07 1.8E+07 2.0E+07 00.20.40.60.811.2 Tidal Range, mTidal Prism, m3 Figure 39. Linear relationship between the tidal prism and tidal range at Johns Pass. There is a strong linear relation ship between the tidal prism and tidal range, with a correlation coefficient, r 2 =0.96 (Figure 39), as expected. The linear relationship and similar ratio for both ebb and flood tides (Figures 40 and 41) indicate the ocean water that enter s Johns Pass is exiting from Johns Pa ss. It does not mean that ocean water is preferentially entering Johns Pass and exiting through Blind Pass, or vice versa. The linear relationship should not be as strong and the ratio should not be the same if this were the case. Further analysis of the ebb and flood-tidal prisms vers us the tidal range at this location revealed that the ebb-tidal prism corresponds very well with the tidal range, with an R 2 value of 0.98 (Figure 40). The departure from the linear trend in Figure 38 is caused by several flood tides. The R 2 value for the flood tides was 0.83 (Figur e 41), much less than the 0.98 for the ebb tides and the overall 0.96. 63
Ebb R2 = 0.980.0E+00 4.0E+06 8.0E+06 1.2E+07 1.6E+07 2.0E+07 00.20.40.60.811.2Tidal Range, mTidal Prism, m3 Figure 40. Ebb-tidal prism versus the tidal range at Johns Pass during the study time period. FloodR2 = 0.83 0.0E+00 2.0E+06 4.0E+06 6.0E+06 8.0E+06 1.0E+07 1.2E+07 1.4E+07 00.20.40.60.8 Tidal Range, mTidal Prism, m3 Figure 41. Flood-tidal prism versus the flood tidal range at Johns Pass during the study time period. The R 2 values may also indicate that flood tides are more influenced by this particular frontdriven weather conditions than the ebb tides. Due to the channel alignment, northeast to southwest, winter cold fronts that pass thro ugh the study area from the north 64
may enhance flood tides by increasi ng the wind and therefore flood velocity near the inlet entrance forcing more water into the Johns Pass. Because of the shape and orientation of Boca Ciega Bay, the northerly cold front winds tend to drive water out of Blind Pass, and therefore, offset the linear relation ship between tidal range and tidal prism. This may lower the R 2 value causing the relationship between the tidal prism and tidal range to be weaker. The northwest winds influence flood-tidal currents at Johns Pass by increasing the velocity into the channel on a flood tide, by examining the data points in Figure 41 that lie above the best-fit line it was noted that flood-tidal velocities were enhanced by winds that occurred from 1/20 to 1/21 during a frontal system (Figures 28 and 41). At Johns Pass ebb and flood-tidal velocities were 93 and 109 cm/s on January 20 th (Figure 28). The simple relationship illustrated in Figure 38 can be used to calculate the tidal prism for Johns Pass based on tidal ranges. It is much easier to use the measured ti dal range and the tidal range data are available from numerous sources than data on tidal currents and prism. Based on this study, the fo llowing equation is suggested for calculating the tidal prism at Johns Pass: (16) R P 7102 CHANNEL EVOLUTION The channel morphology at Johns Pass was mapped by the University of Florida (1968) prior to the construction of the bridge in 1972. The channel cross-section shown in the figure below (Figure 42) indicates that the deepest portion of the channel is located on the south side from the channel centerlin e. The cross-section is read from 65
north to south or from left to ri ght. The figure indicates that the channel was 183 m wide with a maximum depth of 10 m. 1968 Cross-section-12 -10 -8 -6 -4 -2 0 06984122152183 Distance, mDepth, m Figure 42. Channel cross-section at the Johns Pass bridge in 1968. Distance along the x-axis was meas ured from north to the southern channel bank (CTC, 1993). Channel surveys have also been done by private engineering companies such as the one below (Figure 43) showing three crosssections from a 1992 bathymetric survey of Johns Pass (CTC, 1993). The cross-sections compare reason ably well with the later surveys (1995-2004). Cross-section A in Figure 43 located at the bridge indicates that the channel thalweg wa s 3.7 m deeper than the previous survey in 1968 and at least 3 m deep er than the late r surveys. The easternmost cross-section (A) shows the location of the main channel to the north, in contrast to the ma ximum depth on the seaward side to the south (C). These show that the channel shifts from the south near the Gulf entrance to the north on the bayside. 66
Figure 43. Three cross-sections of Johns Pass extracted from a bathymetric contour map from 1992. A: located at the bridge; B: seaward of the bridge; C: bayward of the bridge (CTC, 1993). In Figure 44 below the locations of the inlet cross-sections and beach profiles that are used in this study to determine the amount of sediment deposition and erosion ha ve been plotted and numbered for reference. 67
! ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 68Figure 44. GIS map showing the locations of inlet cross-sections and beach profiles. Seven channel cross-sections were chosen here in order to determine the degree in which sedimentation and scouring has occurred along the channel within the last 10 years. Factors considered in the cross-section selection included the scour hole on the bayside of the bridge, the channel offset near the inlet entrance southward, the G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F G F CS 1 CS2 CS3 CS4 CS5 CS6 CS7 TG-2 TG-3 TG-4 TG-5 R127 FDOT B OC A C IE G A BAY GULF OF MEXICO
69 location of the channel thalwe g, and the changes due to the construction of the south jetty. The seven lines are shown in Fi gure 44. Spacing between each cross-section is approximately 90 m totaling 610 m from the bay to the seaward extent of the surveys. Plots of each survey line (CS1-CS7) location matched well from year to year, except in some cases near the ends of the survey lines (Figur e 44). This was most likely due to obstructions such as docks and boat traffic during the survey. Channel orientation is from the northe rn shoreline in each graph. Beginning with the seaward-mo st cross-section one, the maximum depth in 1995 was 5 m (Fig ure 45). The depth decreased in the thalweg between 1995 and 1998 by 0.6 m. The channel width at the 3 m contour decreased between 1995 and 1997 by 2.8 m from the northern slope of the channel bank Sedimentation occurred rather uniformly along the northern slope wh ile the southern half experienced rapid accretion near the jetty. Cross-section one indicates ther e was accumulation in front of the fillet (Figure 45) from 150 to 200 m along the southern portion of the inlet. The channel has a round u-shape. The su rveys extended south to the shoreline prior to je tty construction. The amount of accretion between the 1995 and 1998 pr ofiles was calculated. Starting from the north side of the channel (n orth is to the left in the figure) 137 m to 201 m, the amount of accretion was 41 m 3 /m.
CS1-6 -5 -4 -3 -2 -1 0 0 50 100 150 200 250 Distance, mDepth, m 1995 1997 1998 1999 2000 2002 2003 2004 Figure 45. USACE cross-section one (1995-2004) west of the bridge. On the landward (south) side of the jetty, five beach profiles were surveyed 60 m apart from TG-02 to R127 (Figure 44) on Treasure Island (Figure 46). A la rge amount of sand accumulated rapidly to the south of the jetty on Treasure Island. The amount of shoreline change that occurred in front of the jetty was calculated between May 2000 and February 2001 (Table 4). As expected, there was a trend of rapid shoreline gain from R-127 to TG-02 (Table 4). The shoreline change between each profil e was referenced to the month of May so that each measurement made thereafter is the amount of change since May. The shoreline gained nearly 50 m, the most that was measured, at TG-02. In general, the amount of ac cretion along the shoreline away from the inlet decreased by 5 m giving the shape of a fillet. An examination of cross-section one on the inlet side of the jetty from 2000 to 2002 indicates that there ha s been a trend of erosion along 70
71 the jetty. This suggests that the new jetty concentrated the flow into the inlet by increasing tidal veloci ties along the jetty. The amount of erosion on the inlet side of cr oss-section one corresponds to sedimentation observed at cross se ctions further into the inlet, therefore the sediment was transp orted further into the inlet by increased flow along the jetty. The shoreline gain at TG-02 was (Figure 46) nearly 5 times the amount of gain at other locations (Table 4). From the shape of the fillet, the accumulation was caused by sediment transport from the south side of the inlet, as opposed to regional southward longshore sediment transport. Wave refracti on around the ebb delta may be responsible for this reversal, a ty pical case for drumstick barrier islands (Hayes, 1979). Table 4. Shoreline change from TG-02to survey line R127. The month of May was used as the baseline survey from which the change in volume was calculated. m m M m m Monument TG-2 TG-3 TG-4 TG-5 R127 May 0 0 0 0 0 August 25.54 1.31 4.92 -6.39 -4.23 September 8.62 4.89 2.52 13.41 December 23.13 1.38 2.55 0.68 -4.82 February 0.58 0 0.7 6.68 0 Total Cum. Change 49.25 11.31 13.06 3.49 4.36
TI-G-2 -3.0 -1.5 0.0 1.5 3.0 0.020.040.060.080.0100.0120.0140.0160.0180.0 Distance from monument (m)Elevation (m) May '00 August '00 December '00 February '01 0 NGVD Figure 46. TG-02 beach profile showing net accumulation along the south jetty during construction (2000-01). Cross-section two shows the location of the maximum depth in 1995 was 6 m and decreased 2 m by 1999, followed by erosion of 1 m. The amount of accretion in the thalweg between 1995 and 1999 was less than 1 m (Figure 47). An accretional trend along the northern bank occurred between 1995 and 1999. This section of the channel changed very little from 2000 to 2004. Both accretion and erosion have occurred along the south side at this location. Before the jetty was constructed sediment accumula tion occurred followed by scour along the channel bank after the je tty was constructed. The channel thalweg shifted toward the south, as compared to cross-section one, which is further Gulf ward. 72
CS2-8 -6 -4 -2 0 0 50 100 150 200 Distance, mDepth, m 1995 1997 1998 1999 2000 2002 2003 2004 Figure 47. USACE cross-section two (1995-2004) west of the bridge. Cross-section three (Figure 48) is narrower and deeper than the previous cross-sections located west of the bridge. The same sedimentation and erosion patterns th at occurred at CS2 are occurring at CS3 only to a greater degree. The maximum depth and width in 1995 was 7 m and 125 m, respectively (Figure 48). Between 1995 and 1999 accretion in the channel thalwe g of 1.7 m occurred followed by erosion of 1.6 m (Figure 48). The deep er thalweg that is visible after 2000 is a direct result of channel dredging in 2000 and it has shifted further to the south compared to the previous cross-sections. 73
CS3-12 -10 -8 -6 -4 -2 0 0 50 100 150 200 Distance, mDepth, m 1995 1997 1998 1999 2000 2002 2003 2004 Figure 48. UASCE cross-sectio n three (1995-2004) west of the bridge. Cross-section four (Figure 49) is the closest one to the bridge (i.e., the location of the minimum width). This profile begins at the 2 m depth mark and terminates at the scour hole near the seawall on the south side of the inlet. Between 1995 and 2000 the thalweg accumulated 1.5 m followed by er osion from 2000-2004 of 1 m. The cross-section terminates near the seawall located approximately 122 m from the northern bank. The channel has a well-defined v-shape with the v-notch located on the south side indicating where the highest flows in the channel occur. Erosio n has been occurring here since 2000 when the jetty was constructed on the south side. The southward shifting of the channel thalweg, as observed in the previous profiles continues at this cross section. 74
CS4-12 -10 -8 -6 -4 -2 0 020406080100120140 Distance, mDepth, m 1995 1997 1998 1999 2000 2002 2003 2004 Figure 49. USACE cross-section four (1995-2004) east of the bridge. Cross-section five (Figure 50) is a departure from the previous trend seaward of the bridge where th e thalweg is south of the channel centerline. The channel thalweg is now located to the northern side of the inlet. The maximum depth in 1995 was 7.6 m and has been accumulating sediment up to 2004 (F igure 50). This depositional trend is uniform across the entire cross-se ction with an accumulation of 1 m in the thalweg. The reason for the accumulation along this profile from 2003 to 2004 is likely the result of sediment deposition that was scoured from the newly constructed jetty. Jetties te nd to confine flows, resulting in increased velocity and therefore caus ing scour. The eroded sediment is then deposited along the channel in the less turbulent bay area, such as in CS5. 75
CS5-10 -8 -6 -4 -2 0 0 50 100 150 Distance, mDepth, m 1995 1997 1998 1999 2000 2002 2003 2004 Figure 50. USACE cross-section five (1995-2004) east of the bridge. Cross-section six (Figure 51) shows no indication of any trends and is more sporadic than any of the other cross-sections. The channel thalweg is located further to the no rthern side of the cross-section, showing the northward shift of the ch annel thalweg. The profile begins at the 6 m depth contour along the northern portion of the channel and continues to the south side. Th e reason for this is because the Corps of Engineers maintains the na vigable channel to a certain depth and width, therefore the entire widt h of the inlet cross-section in the bay area was not surveyed as the channel becomes wider and shallower. The maximum depth was 7.6 m in 1995 (Figure 51). In some places along this transect vast erosion has occurred while at others large amounts of deposi tion. Between 1995 and 2004 the north side had accumulated 1 m while the south side eroded 1.7 m from 2002 to 2004 (Figure 50). Local constr uctions of docks and piers have affected the morphology in a spor adic manner at this location. 76
CS6-10 -8 -6 -4 -2 0 020406080100120140160180 Distance, mDepth, m 1995 1997 1998 1999 2000 2002 2003 2004 Figure 51. USACE cross-section si x (1995-2004) east of the bridge. Cross-section seven (Figure 52), located the furthest east, is deeper than cross-section six and the channel thalweg has more of a v-shape. The channel thalweg is along the northern portion of the channel here where it connects to the Intercoastal Waterway in the bay. The maximum depth was 10 m in 1995 and decreased to approximately 9 m in 2004 (Figure 52). Sedimentation has occurred at this cross-section between 75 to 100 m distance along the x-axis. A large amount of accumulation, a la yer of roughly 2 m, has occurred between 1995 and 2004. 77
CS7-12 -10 -8 -6 -4 -2 0 0 50 100 150 200 Distance, mDepth, m 1995 1997 1998 2000 2002 2003 2004 Figure 52. USACE cross-section se ven (1995-2004) east of the bridge. The seven survey lines cover an area of roughly 610 m long and 150 m wide. Based on the above analysis, a net accumulation of approximately 0.5 m along the channel per year can be generalized. This will yield a yearly average sediment accumulation of 45,800 m 3 /yr along the studied section of the Johns Pass channel. A larger portion of this sediment accumulation occu rred on the bay side from CS5 to CS7, while a smaller portion accumulated between CS1 and CS3. Although the average accumulation is determined to be 0.5 m thick, cross-section four shows the reverse trend due to scouring near the pier pilings, particularly on the south side of the inlet. A small section in the northeastern portion of the channel has been surveyed by the FDOT. Localized deposition has been occurring possibly due to the installation of countermeasures in 2000 to reduce the potential for scour undermining bridge pier pilings. Mr. Hubbard, the owner of Hubbards Marina, voic ed concerns about sedimentation 78
occurring adjacent to the marina cl aiming the rate of deposition had increased due to riprap installati on (Pitman-Hartenstein & Assoc., 2004). Three FDOT survey lines were chosen, out of the nine from 2001 to 2004, that clearly show an appreciable amount of erosion or accretion. The quarterly survey lin es are vastly different due to seasonal variations in deposition and erosion as well as localized dredging near Hubbards boat do ck (Pitman-Hartenstein & Assoc., 2004). Three lines were surveyed at 15 m, 46 m, and 59 m from the bridge along the northeast channel bank (Figure 44). Survey line one (Figure 53) is the furthest from th e boat dock and the closest to the bridge and therefore exhibits relati vely natural sedimentation patterns. The upper half of the profile accumulated 0.6 m from November 2001 to April of 2002 (Figure 53). The lower half eroded 1.5 m during this time. Figure 53. FDOT survey line one located 15 m from the bridge. 79
In contrast, survey lines two and three (Figures 54 and 55) were more influenced by activities occurring near the dock. Survey line two (Figure 54) shows accretion from th e 3 m contour line down to 7.5 m depth between November 2001 and March 2002. The April 2003 profile shows significant scouring compared to the March 2002 survey. Anthropogenic affects are more noticeable in FDOT survey line three between November 2001 and April 2002 (Figure 55) where scour from dredging activities occurred near the boat dock. Sedimentation occurred along survey lin e three in a thickness of up to 0.9 m between April 2002 and April 2003 (Figure 55). Figure 54. FDOT survey line two located 46 m east of the bridge. 80
Figure 55. FDOT survey line three located 59 m east of the bridge. Sediment being transported ar ound the northern jetty is accumulating along the northeastern po rtion of the channel. This is the result of the channel orientation an d flood-tidal circulation patterns on this side of the inlet. During flood-t idal cycles sediment moves into the channel margins and as flood-tidal velocities decrease more sediment drops out of suspension during slack tide followed by a turning of the tide (ebb tide) when circulation pattern s create eddies near bridge pier pilings on the northeast side of the channel (Pitman-Hartenstein & Assoc., 2004). In summary, the results presente d here are comparable to the channel condition report by the USACE (USACE, 2004). Sedimentation is occurring along the northern ha lf of the channel seaward of the bridge as well as north and east of the bridge where the channel widens (USACE, 2004). This depositional trend along the northeastern portion of the channel has been occurring since the early 1990s 81
82 (Pitman-Hartenstein & Assoc., 2004). The USACE cross-sections from 1995 to 2004 show accretion along th e southern half of the channel east and west of the bridge and erosion at the bridge indicating dynamic bottom fluctuations in response to tidal flow intensities. Flood-tidal flows are increased near jetties moving more sediment into the inlet and allowing it to accumulate in the bay area. Near the entrance to a jettied inlet, flood-t idal currents are channelized along the margins while ebb flows are more uniformly distributed throughout the channel (Kieslich, 1981). This was also observed in physical model tests by Seabergh and Krock (2003). Before the construction of the south jetty, sedimentation occurred in that area due to the reversed longshore sediment transport. SEDIMENT BUDGET A sediment budget is a quantitative method of equating distribution and transport associated with sources, sinks, and storages (Komar, 1996). Examples of sediment budgets that were used as a framework for the Johns Pass inlet system include Hand, 1998; Elko, 1999; Rosati and Kraus, 1999. The analyses done by Hand and Elko are similar in that they both exam ined rates of shoreline change for long and short-term time periods in order to determine sediment transport rates and volumes along a particular section of the Florida coast. Rosati and Kraus developed a software program known as the Sediment Budget Analysis System (S BAS) that uses the mass balance equation to determine sediment transport rates from user input volume changes. The purpose of doing a sediment budget for the Johns Pass inlet system is to co mbine the analysis of the beach profiles and inlet bathymetry in order to calculate the annual sediment
83 transport rates in the vicinity of the inlet. This was accomplished by first determining the littoral cell boundaries. SEDIMENT SOURCES AND SINKS An inlet sediment budget consists of three key sedimentary bodies: the adjacent barrier islands, the ebb tidal delta, and the flood tidal delta. The primary source of se diment into the inlet system is due to the longshore current that tran sports sediment south along the coast. Onshore and offshore winds transport sand from beach dunes into the backbay and nearshore areas. Sand is transported onshore by waves and tides from nearshore bars Barrier islands are both sources and sinks for sediment. Periodic be ach renourishment is a mechanical means of placing sediment into a li ttoral cell representing a gain or accumulation along the beach. Sediment movement in the offshore direction due to beach erosion by severe storms results in a loss to a littoral cell. Inlets are generally considered sediment sinks, however structures, such as jetties, that are intended to confine tidal flows in the channel also reduce the sediment flux in the inlet. Jetties interrupt sediment movement along the updrif t side by trapping sand moving south with the longshore current, however at Johns Pass there is a reverse current along the south side of the inlet that transports sediment north into the inlet, while a portion of this sediment also accumulates along the south jetty. The ebb and flood tidal deltas associated with an inlet are both so urces and sinks. As the tidal deltas reach an equilibrium state the volu me of sand in storage remains constant. A stable ebb tidal delta will bypass sand around the inlet entrance to the downdrift beach. Se diment transport out of the inlet system to the south is considered a loss.
In the diagram below (Figure 56) the potential sediment transport pathways are denoted by ar rows from the source to the sink. The littoral cells are represented by green rectangles and include: the nearshore sand bar off of Madeira Beach (dV1), the ebb tidal delta (dV2), the flood tidal delta (dV3), and the nearshore sand bar off of Treasure Island (dV4). The annual vo lume change (dV) in each littoral cell as well as the annual sediment transport rate (Q) are discussed in the next figure (Figure 57). Figure 56. Plan view of the John s Pass inlet system with sediment transport pathways and littoral cells defined. Some assumptions were made in the sediment budget analysis based upon a review of the aerial photos and profile data. It is 84
85 assumed that the offshore movement of sediment from the adjacent beaches is negligible as well as the movement of sand from sand dunes by aeolian transport. The most recent ebb and flood tidal delta volumes from 1984 are given in Table 1. The flood tidal delta is assumed to be more stable than the ebb tidal delta based on the aerial photo analysis. The volume change and rate of sediment transport from the ebb tidal delta to the northern tip of Treasure Island is based on the beach profile analysis. Usin g the terminal groin surveys that were done by the USF Coastal Research lab between May 2000 and February 2001 the sediment transport into the inlet from the south was estimated. Sediment transport into the inlet around the north jetty is done using the FDOT inlet profiles indicating the amount of accretion along the northeastern portio n of the inlet. Lastly, in order to balance the sediment budget the inlet system is considered a closed system. ESTIMATION OF TERMS Sediment transport to or from a littoral cell is denoted by a Q (Figure 56) and given in cubic meters per year. The sediment flux in the inlet is based upon the dimensio ns of the inlet and the inlet crosssectional area analysis. The dimens ions are 330 m long, 167 m wide at the bridge to 183 m wide on the oc ean and bay sides of the inlet. Sediment transport between littoral cells is given by a blue arrow and red text denoting the transport variable (Q1, Q2, Q3, and Q4) in m 3 /yr. The annual volume change within each littoral cell is estimated in cubic meters (dV1, dV2, dV3, dV4) from the beach profile analysis. In the diagram below (Figure 57) dV1, north of the inlet, represents a net gain into the littoral cell. Beach profiles were
examined along Madeira Beach between R-122 and R-123 below the waterline (-1.5 m NGVD) extending 100 m in the cross-shore direction and 300 m along the beach. The FDOT profiles of the northeastern portion of the inlet provide a method of calculating the net transport into the inlet around the north jetty. In similar fashion, the net gain into the littoral cell south of the inle t, dV4, is defined by the northern portion of Treasure Island between TG-02 and R-127. A loss within a littoral cell is denoted by a negati ve number in black text and a blue arrow representing onshore sedime nt transport. The ebb and flood tidal deltas are represented by dV2 and dV3, respectively. Figure 57. Annual (2000-01) sedime nt budget diagram for the Johns Pass inlet system. 86
87 The longshore sediment transpor t south, Qs, is approximately 60,000 m 3 /yr and the net volume of se diment gained in dV1 is approximately 6,200 m 3 /yr with approximately 3,800 m 3 /yr being deposited on the beach by waves and 8,000 m 3 /yr moving into the inlet around the north jetty. The sediment transport rate between dV1 and dV2, Q1, is approximately 45,800 m 3 /yr. The sediment flux in the inlet is approximately 19,700 m 3 /yr and ,300 m 3 /yr, Q2 and Q3 respectively. The flood tidal delta, dV3, is accumulating approximately 500 m 3 /yr, while the ebb tidal delta, dV2, is receiving approximately 15,800 m 3 /yr. The northern tip of Trea sure Island is accumulating sediment along the south jetty. Approximately 34,500 m 3 /yr is being transported to the beach on Trea sure Island, while approximately 2,300 m 3 /yr is accumulating in fr ont of the jetty and 3,100 m 3 /yr is being transported back into the in let during flood tides. The net accumulation on the northern po rtion of Treasure Island is approximately 26,700 m 3 /yr. Conclusions Johns Pass is a stable mixed-energy straight inlet that is located on a microtidal coast. The inlet is hydraulically connected to the northern half of Boca Ciega Bay Morphological analysis using a time series of aerial photographs indica ted that anthropogenic activities have had substantial influences on the evolution of the tidal deltas and shoreline boundaries adjacent to Johns Pass. The long-term trend shows an increase in the cross-sectio nal area of this inlet since 1873. Major changes occurring in the vicini ty of Johns Pass that caused an increase or decrease in the channel area were found to be associated
88 with the construction of the causeways across Boca Ciega Bay dredging and filling in the bay, and je tty construction on the south side of the inlet. A historical review of the ebb-tidal delta geometry indicated that dredging has caused this sediment body to become separated into two channel margin linear bars. Time series analysis of tidal velocities and meteorological data during the study time period indica te that three frontal systems passed through the study area enhancing flood-tidal velocities into the Johns Pass inlet. The northerly wind acco mpanying the frontal passage tends to enhance ebb flow out of Blind Pa ss. The small departures from the linear trend in the flood-tidal prism to tidal range relation, given by the lower r 2 value (0.83), coincide with these meteorological events more so than the departures in ebb-tidal prism to range relation having an r 2 value of 0.98. Overall, the spring velocity pr ofiles indicated that the magnitude of the ebb-tidal velocities were high er than the flood velocities. Using equation 14 and the peak spring e bb-tidal velocity at Johns Pass the tidal prism was calculated as 1.07x10 7 m 3 from an ADP deployment in the channel. The tidal prism at Blind Pass was found to be 14 percent that at Johns Pass. From this calculation the bay area serviced by each inlet was found to be 1.8x10 7 m 2 and 0.33x10 7 m 2 respectively. Therefore, Johns Pass serves roughly 87 percent of the bay and Blind Pass serves 13 percent. Morphologic analysis of the cross-sections at Johns Pass from 1995 to 2004 indicated the net sedime nt flux through the inlet is approximately 45,8000 m 3 /yr on average. The ch annel area west of the bridge has a u-shaped. At the br idge the cross-section is more vshaped from scouring near the southe rn bridge pier. The area east of the bridge is wider than the seawar d entrance. The northern half of
89 the channel appears to be accumulati ng sediment on the bay side of the bridge due to increased flood flows along the newly constructed jetty in 2000. Currents in this area of the channel are also affected by bridge piers, boat docks, and local activities. The sediment budget diagram for the Johns Pass inlet system indicates that the inlet and th e adjacent barrier islands are accumulating more sediment than th ey are losing resulting in a net sediment gain into the system. This short-term analysis (annual) of the profile data indicates that the inlet cross-sectional area is decreasing, however the long-term trend indicates that the crosssectional area is increasing. The inlet must try to maintain an equilibrium cross-sectional area by reaching a critical maximum velocity in the channel to be stable. Anthropogenic activities at Johns Pass, such as dredging and jetty construction, have caused short-term variability in inlet stability.
90 References Becker, M. L., 1999. Interaction of tida l inlets on a microtidal coast: A study of Boca Ciega Bay, Johns Pass, and Blind Pass. Tampa, FL: University of South Florida, unpublished Masters Thesis, p. 13-18. Becker, M. L. and Ross, M. A., 2001. Interaction of tidal inlets in a multi-inlet bay system: A case study along the central Gulf Coast of Florida. J. Coastal Research, 17: p. 836-849. Bruun, P., 1966. Tidal inlets and littoral drift. Universitetforlaget, Oslo, Norway, p. 121-124. Bruun, P. and Gerritsen, F., 1960. Stability of Coastal Inlets Amsterdam, 123p. Caldwell, J. M., 1955. Tidal currents at inlets in the United States: Proc. Amer. Soc. Civil Eng. p. 81. Coastal Technology Corporatio n (CTC), 1993. Johns Pass inlet management plan. CTC, Vero Beach, FL, p. 28. Cuffe, C. K., and Davis, R. A. Jr ., 1993. Origin, development, and stratigraphy of tidal deltas at Hurricane Pass, Pinellas County, Florida an example of modern tidal delta architecture from a microtidal coast. Coastal Zone Amer. Soc. Civil Eng ., p. 2570-2584. Davis, R. A. Jr., 1995. Holocene geology and morphodynamics of the barrier island/tidal inlet system: West-Central Florida. 1 st Congress, SEPM Sedimentary Geology, St. Pete Beach, FL, p. 18. Davis, R. A. Jr. and Fox, W. T., 1977. Interaction between wave and tide generated processes adjacent to inlets: A feasibility study. Technical Report, Office of Naval Research No. 388, p.136.
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