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Tidwell, David. K.
Sedimentation patterns and hydrodynamics of a wave-dominated tidal inlet
h [electronic resource] :
b Blind Pass, Florida /
by David K. Tidwell.
[Tampa, Fla.] :
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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ABSTRACT: Blind Pass, a heavily structured wave-dominated tidal inlet on the west central coast of Florida, has undergone substantial morphologic changes in the past 150 years. Initially Blind Pass was a mixed-energy inlet. In 1848 a hurricane opened a new inlet to the north called Johns Pass, which captured a large portion of the tidal prism of Blind Pass. Since then Blind Pass migrated southward until it was structurally stabilized in 1937. The decreasing tidal prism resulted in significant inlet channel filling. The channel has been dredged 12 times since 1937. The present inlet is stabilized by two jetties and a series of seawalls. Detailed time-series field measurements of bathymetry and tidal flows were conducted between 2001 and 2004, after the last channel dredging in the summer of 2000. The measured depositional rate in the inlet channel approximately equals the net southward longshore transport rate.This suggests that the inlet has served as a trap for the southward longshore transport allowing negligible bypassing to the eroding downdrift beach. Most of the active sedimentation occurs on the northern side of the inlet. The sediment in the thalweg is largely coarse shell lag, indicating adequate sediment flushing by the ebbing tide. The cross-channel flow measurements revealed that ebb flow was approximately twice as high in the channel thalweg as compared with the rest of the channel. The flood flow was largely uniform across the entire inlet and dominated over the northern portion of the inlet due to the weak ebb flow there. This cross-channel flow pattern is crucial to the understanding of the sedimentation patterns in the Blind Pass channel. Two years after the last dredging the mouth has become shallow enough to induce wave breaking across the shoal area.
Adviser: Ping Wang Ph.D.
t USF Electronic Theses and Dissertations.
Sedimentation Patterns and Hydrodynamics of a Wave Dominated Tidal Inlet: Blind Pass, Florida by David K. Tidwell A thesis submitted in partial fulfillment of the requirements for the degree of Master of Geology Department of Geology Colleg e of Arts & Sciences University of South Florida Major Professor: Ping Wang, Ph.D. Richard A. Davis, Ph.D. Mark Rains, Ph.D. Date of Approval: April 12, 2005 Keywords: hydrodynamics, progradation, microtidal, morphodynamics, tidal inlets Copyr ight 2005, David K. Tidwell
Acknowledgments I would li ke to extend my sincerest appreciation to Dr. Ping Wang for h is advising, guidance and financial support which made this project possible. I would also like to thank Dr. Richard A Davis Jr. for serving on my committee and providing useful comments. I would like to thank Dr. Mark Rains for serving on my committee. I would also like to thank the Army Corps of Engineers for funding my research. I am deeply indebted to all the people in the Geology department that aided in my resear ch. Roy Price for his assistance in deploying equipment in the field underwater and hunting equipment in rough conditions. Jeremy for help in doing level and transit survey being the tall rod man and for his assistance in deploying equipment. L ee Florea for helping in the field deploying equipment underwa ter and the rest for help at one point or another, including Jack W ilhoit and Alyssa Saint John. Finally, a special t hanks goes out to my mother for always encouraging me and standing behind me one hundred percent. Without her I would not be the person I am today. Thanks also goes out to my fianc Teri for encouraging me and standing behind me.
i Table of Contents List of Figures ii i Abstract v i Introduction 1 Objectives 3 Study Area 5 Geologic setting 5 The Blind Pass John Pass system 7 Previous Research 15 Inlet hydrodynamics 16 Inlet morphodynamics 18 Tide dominate d inlets 19 Wave dominated inlets 20 Mixed energy inlets 21 Methodology 25 Me asuring sedimentation patters 26 Ti me series bathymetric surveys 26 Level and transit surveys 28 Hydrodynamic measurements 29 Vertical current profile measurements 3 0 Cross channel current profile measurements 32 Results 33 Sediment Characteristics 33 Wind and wave climate 34 Hydrodynamics of Blind Pass 41 C ross channel current profiles 42 Vertical current profiles 47 Sedimentation patterns 53 Sediment ation ov er the entire inl et 53 Detailed sedimentation p atterns of the northern shoal 70
ii Summary 81 Conclusions 8 3 References 8 5
iii List of Figures Figure 1 Blind Pass inlet located in Pinellas County, Florida. 2 Figure 2 Boca Ciega Bay s howing the three inlets, Johns Pass, Blind Pass, and Pass a Grille, that serves it 4 Figure 3 Blind Pass in 1926 with a well developed ebb tidal delta. 9 Figure 4 Time series photos of the Blind Pass evolution from a mixed energy inlet to a wave dominated inlet. 11 Figure 5 Time series of maps showing the migration and eve ntual structural stabilization of Blind Pass. 12 Figure 6 Classification of coastal morphodynamics 14 Figure 7 Inlet morphologies for the west central Florida b arrier chain. 19 Figure 8 Cross se ction of a tide dominated inlet, Bunces Pass, Florida. 20 Figure 9 Cross se ction of a wave dominated inlet, Blind Pass, Florida 2 1 Figure 10 Cross section of a mixed energy straight inlet Stump Pass Florida 24 Figure 11 Aerial photo of Blind Pass showing the shoal area and lines surveyed. 29 Figure 12 Date and location of the U ADP and S ADP deployments. 31 Figure 13 Sediment types in Blind Pass. 34 Figure 14 W ind direc tion and veloci ty for the year of September 2002 through August 2003 36 Figure 15 W ind direc tion and velocity for November 2002 through March 2003 37
iv Figure 16 W ind direction and velocity for April through September 2003 38 Figure 17 W inter front al system in December 2002 39 Figure 18 Six yearly averages of wave heights 41 Figure 19 An example of a c ross channel profile during a spring tide d uring November 2002 deployment. 43 Figure 20 An example of a c ross channel profile during a neap tide during November 2002 deployment. 44 Figure 21 An example of a c ross channel profile during an average tidal phase during November2002 deployment. 45 Figure 22 An example of a c ross channel profile during a spring tidal cycle during April 20 03 deployment at the bend of the inlet. 46 Figure 23 Cross channel flow velocities as related to the tid al water level fluctuations during November 2002 deployment. 47 Figure 24 An example of the vertical current profile in the channel during spring phase during August 2002 deployment. 48 Figure 25 An example of the vertical current profile in the channel during neap phase during August 2002 deployment. 49 Figure 26 Vertical current profile in the channel during an average tidal phase during A ugust 2002 deployment. 50 Figure 27 Vertical current flow velocities as related to the tid al water level fluctuations during August 2002 deployment 51 Figure 28 Depth average velocity profile of the channel during August 2002. 52 Figure 29 Surv ey conducted in the summer of 2000 after dredging of Blind Pass, COE. 54 Figure 30 Bathymetric map of Blind Pass in August 2002. 55 Figure 31 Difference map of sedimentation between July 2000 and August 2002. 56
v Figure 32 Bathymetric map of Blin d Pass in December 2002. 57 Figure 33 Difference map of sedimentation between August and December 2002. 58 Figure 34 Bathymetric map of Blind Pass in May 2003. 59 Figure 35 Difference map of sedimentation between December 2002 and May 2003. 60 Figure 36 Bathymetric map of Blind Pass in August 2003. 61 Figure 37 Difference map of sedimentation between May and August 2003. 62 Figure 38 Bathymetric map of Blind Pass in November 2003. 63 Figure 39 Difference map of sedimentation bet ween August and November 2003. 64 Figure 40 Bathymetric map of Blind Pass in February 2004. 65 Figure 41 Difference map of sedimentation between November 2003 and February 2004. 65 Figure 42 Bathymetric map of Blind Pass in April 2004. 66 Figure 43 Difference map of sedimentation between February and April 2004. 67 Figure 44 Bathymetric map of Blind Pass in June 2004. 68 Figure 45 Difference map of sedimentation between April and June 2004. 68 Figure 46 Volume changes in Blind Pass between August 2002 and October 2004. 7 0 Figure 47 Level and t ransit survey of profiles 1 & 2. 7 2 Figure 48 Level and transit survey of profiles 3 & 4. 74 Figure 49 Level and t ransit survey of profiles 5 & 6. 76 Figure 50 Level and transit surveys of profiles 7 9. 79
vi Sedimentation Patterns and Hydrodynamics of a Wave Domina ted Tidal Inlet: Blind Pass, Florida David K. Tidwell ABSTRACT Blind Pass, a h eavily structured wave dominated tidal inlet on the west central coast of Florida, has undergone substantial morphologic change s in the past 1 50 years. Initially Blind Pass was a mixed energy inlet In 1848 a hurricane opened a new inlet to the north called Johns Pass which captur ed a large p ortion of the tidal prism of Blin d Pass. Since then Blind Pass migrated southward until it was structurally st abilized in 1937. The decreasing tidal prism resulted in significant inlet channel filling. The channel has been dredged 12 times since 1937. The present inlet is stabilized b y two jetties and a series of seawalls. Detailed time series field measurements of bathymetry and tidal flows were conducted between 2001 and 2004, after the last channel dredging in the summer of 2000. The measured depositional rate in the inlet channel approximately equals the net southward longshore transport rate. This suggests that the inlet has served as a trap for the southward longshore transport allowing negligible bypassing to the eroding downdrift beach. Most of the active
vii sedimentation occurs on the northern side of the inlet. The sediment in the thalweg is largely coarse shell lag, indicating adequate sediment flushing by the ebbing tide. The cross channel flow measurements revealed that ebb flow was approximately twice as high in the chann el thalweg as compared with the rest of the channel. The flood flow was largely uniform across the entire inlet and dominated over the northern portion of the inlet due to the weak ebb flow there. This cross channel flow pattern is crucial to the underst anding of the sedimentation patterns in the Blind Pass channel. Two years after the last dredging the mouth has become shallow enough to induce wave breaking across the shoal area. Distinctive seasonal patterns of sedimentation were measured thereafter i n the inlet channel, influenced by seasonal wave climate. The sedimentation is event driven from passage of cold fronts bringing elevated wave energy that accelerates the southward longshore transport. During normal conditions the sediment deposited in t he mouth area is redistributed further into the inlet by the flood current combined with wave driven current.
1 I ntroduction An inlet is a short, narrow waterway that connects a bay, a lagoon, or an estuary to a larger body of water, generally a sea (Escoffier, 1977). Individual inlets have been studied extensively because of their importance in bay flushing a nd sediment transport. Tidal inlets play an important role in shaping coastlines ( FitzGerald, 1988 ) Based on numerous studies, Dean ( 1988 ) concluded that more than 80% of the shoreline erosion problem along the e ast coast of Florida can be directly link ed to tidal inlets. Blind Pass, with the severe downdrift erosion, serves as an excellent example of inlet induced erosion on the west coast of Florida (Figure s 1 & 2 ) W ith the dramatic growth in coastal development and demand for navigable passages, at tempts to characterize the size and configuration of inlets and their influence on adjacent shoreline have led to new and innovative w ays to evaluate the stability of tidal inlets ( Dean and Dalrymple 2002 )
2 Figure 1 Blind Pass inlet located in Pinella s County, Florida. Blind Pass is one of three inlets that connects Boca Ciega Bay to the Gulf of Mexico ( Becker, 1999 )
3 The stability of an inlet describes the degree to which inlet geometry, including the cross sectional area, location, pla n form and shape are maintained with time (Bruun, 1978). An inlet is determined to be stable when after a small change, the cross sectional area returns to its equilibrium value (van de Kreeke, 1984, 1989). Modulations around an average equilibrium flow area occur with changes in wave conditions and tidal height variations but recovery from extreme events may take o nly days (Byrne et al., 1974). This study examines the sedimentation pattern s of Blind Pass in Pinellas County, Florida (Tidwell et al., 2003; Tidwell an d Wang, in press). The sedimentation in the Blind Pass channel also has direct influence on the erosion and accretion of the adjacent beach. Time series bathymetry surveys were conducted to quantify the pattern and volume of channel filling. Flow patter ns in the inlet were measured and analyzed to explain the sedimentation pattern. Blind Pass serves as an excellent example of a wave dominated, often heavily structured, tidal inlet. The findings of this study should be applicable to other wave dominated inlets. Objectives The general goal of this research is to understand how and when sedimentation occurs in inlet channels and to understand the interaction s among longshore sediment transport, tidal flow patterns and sedimentation patterns in a wave dom inated inlet The specific objectives of this research are 1) to quantify
4 the sedimentation /erosion pattern in the inlet channel 2) to investigate regional weather wave conditions the tidal flow v elocities in the inlet channel, and 3) to examine the rela tionship be tween sedimentation and the driving forces. Figure 2. Boca Ciega Bay showing the three inlets, Johns Pass, Blind Pass, and Pass a Grille, that serves it. Note the intensive dredge and fill in the bay, especially near Blind Pass. Pass A Grill Blind Boca Ciega Bay Johns
5 Study Area Geologic Setting West central Florida rests on a large, stable carbonate platform. This platform was formed during a period lasting from the Mesozoic until the later stages of the Paleogene. During this time, the Suwann ee Straits/Gulf Trough seaways served as a dynamic barrier that blocked terrigenous sediment influx from the southern Appalachians from entering the carbonate producing environments to the south ( Chen, 1965; McKinney, 1984, Pinet and Popenoe, 1985 ). Once the straits were filled with sediment at the end of the Paleogene, the carbonate producing environments were largely snuffed out by the subsequent distribution of dominantly quartz sand by a combination of fluvial processes, longshore transport, and sea le vel fluctuations (Chen, 1965; McKinney, 1984; Pinet and Popenoe, 1985). The west central Florida barrier chain began forming when sea level ri se slowed between 3.5 and 3.0 thousand years ago (ka.) The decrease in rate of sea level rise halted the rapid migration of small barriers and initiated a progradational phase, which was further reinforced by the interception of relict sediment source s (Davis and Kuhn, 1985; Evans et al., 1985). Barrier island formation is dependent on the coincident decline in th e rate of sea level rise and
6 a sufficient sediment supply. Although research indicates that a progradational phase initiated the upward s hoaling and stabilization of the barrier system (Hine et al., 1987), modern analogues dictate that there must have bee n over washing and landward migration during the early stages of barrier development (Davis and Hine, 1989; Davis et al., 1992). Sediment in the study area and adjacent to the study ar ea on the beaches remains largely uniform in its character. The sedime nt is bimodal, consisting primarily of fine to very fine, well sorted an d rounded quartz sand, and sand and gravel sized shells or shell fragments (Davis et al., 1982). The shell is derived locally, while the quartz sand is derived from reworked Neogene t o Quaternary coastal and terrace deposits of the later stage of the Holocene transgression. At present, the study area is sediment starved, with no significant influx of terrigenous sediment to the coast (Davis, 1989 a ). Small amounts of mud, most of whic h is biogenic in origin, are also present (Evans et al., 1985). The bedrock in the region consists of Miocene limestone, which is part of a large, tectonically stable Florida Platform. The elevation of the bedrock surface along the coast ranges widely, v arying between 0 and 20 m below sea level. The shallowest bedrock under the barriers is near Anclote Key in northern Pinellas County and near Venice Inlet in Sarasota County (USACOE, 1962; Davis and Kuhn, 1985). The rock surface along the coast dips gent ly to the west, with a break in slope at the present location of the west central barrier chain and is believed to be responsible for the location of the barrier islands (Evans et al., 1985). Shallow bedrock lends stability to several inlets ( e.g. Tampa Bay Egmont
7 channel and Southwest channel Redfish Pass Venice Inlet, and Boca Grande Pass ) which have become incised into the resistant limestone, and the bedrock is also a barrier to inlet migration where tidal currents are not powerful enough to cut a c hannel into the rock (Davis, 1989 a ). Atop an unconformable contact with Miocene bedrock, Holocene barrier and related sediments lie on a thin, discontinuous layer of Pleistocene sands or clays, typically less than 1 m thick (Brame, 1976; Davis and Kuhn, 1 985; Davis et al., 1989; Gibbs, 1991). Where Pleistocene sediments are absent, Holocene deposits lie directly on the Miocene rocks (Cuffe, 1991). The only exception is in southern Pinellas County. The Blind Pass Johns Pass System The west central Gulf coast of Florida is characterized by a chain of barrier isla nds separated by tidal inlets. Shoreline configurations are influenced largely by the distribution of sediment as well as tidal and wave action (Bruun, 1978). The Boca Ciega Bay region (Figure 2 ) includes inlets with distinct differences in size and flow characteristics. Blind Pass and the adjacent Johns Pass serve a large portion of the Boca Ciega Bay region, with Pass a Grille inlet capturing most of the rest. The connection and interaction between Johns Blind Pass system and Pass a Grille are limited by the Corey C auseway ( Figure 1 ). Blind Pass is a wave dominated inlet an d Johns Pass is a asymmetrical tide dominated inlet (Davis and Gibeaut, 1990). Blind Pass and Johns Pass are separat ed by the Tr easure Island Causeway (Figure 1 ). Blind Pass was a mixed
8 energy offset inlet in the 1800s before Johns Pass was cut by the hurricane of 1848. Johns Pass then started capturing the tidal prism from Blind Pass. At that time Blind Pass had a large well developed terminal lobe at the end of the ebb tidal delta (Figure 3 ). The pass had a large back bay region that it served. Once Johns Pass captured most of the tidal prism of Boca Ciega Bay Blind Pass became a wave dominated inlet and star ted migrating southward. This migration continued until 1937 when a hard structure on the south side was constructed at Blind Pass to stop the southward migration. In the 1950s the Treasure Island Causeway was constructed and limited the interaction bet ween Johns Pass and Blind Pass. The causeway also reduced the back bay area that Blind Pass served. Further fill operations in the 1950s greatly reduced the back bay area that Blind Pass serves and therefore the tidal prism The magnitude and pattern of longshore transport significantly influe nces the inlets configuration. Typical of a wave dominated inlet, Blind Pass is heavily structured The larger coast region is of low, mixed energy (Davis and Hayes, 1984) with a mean annual wave height of abou t 30 cm (Tanner, 1960) and mean tidal range of approximately 75 80 cm (Davis 1989a). Blind Pass separates Treasure Island, to the north, fro m Long Key, to the south (Figure 2) It is a typical wave dominated tidal inlet (Davis, 1997; Davis and Hayes, 19 84) and has been migrating rapidly southward driven by the regional southwar d longshore sediment transport before structurally stabilized in 1937.
9 Figure 3. Blind Pass in 1926 with a well developed ebb tidal delta Figure 4 show s the evolution of Blind Pass from 1926 to the present. Being a wave dominated inlet Blind Pass shows the typical meandering channel that connects the ocean to the back bay ( F igure 2 ) In 1926 Blind Pass still had a large ebb tidal delta (Figure 4 1 926). Before that time, Blind Pass migra ted southward for over 1,850 m (F igure 5 ) Historical aerial photos indicate that the pass moved approximately 670 m between 1926 and 1937 (Tidwell et al., 2003) By 1942 a large portion of the ebb tidal delta had diminished after the stabilization in 1937. The north jetty was constructed in 1962 to keep sediment from infilling the channel, and was extended in 1976 when the original jetty was unsuccessful in its goal (CPE, 1992). At that time the ebb tidal delta had disappeared complet ely. By 1984 all the hard engineering structures were in place and Blind Pass had become a heavily structurally stabilized inlet. Alth ough 1926
10 the inlet position was stabilized, the channel filling has been extremely active (Tidwell an d Wang, in press ) The channel has been dredged 12 times since 1937. The sand dredged from Blind Pass has been used to nourish the updrift Treasure Island to the north and the down drift Upham Beach to the south. The net rate of the southward longshore t ransport was estimated to be approximately 36,000 m 3 per year (Walton, 197 3; Wang et al., 1998a, 1998b). Hydraulic and morphological characteristics of Blind Pass and the adjacent Johns Pass have been affected by several factors including : changes in the configuration of Boca Ciega Bay dredging and development acti vities at individual inlets, and changes in the size of adjacent passes just to name a few While the cross section of Blind Pass has historically been reducing in size due to shoaling, Johns Pass has been increasing in size ( Metha et al., 1976 ). As a result, severe local bed erosion has occurred at Johns Pass (Vincent, 1992). Dredging or development at one inlet can affect flow conditions throughout the bay. In analysis of inlet stability, factors affecting not only the individual inlet, but also the hydraulic system as a whole, must be considered (Dean and Dalrymple, 2002)
11 Figure 4 Time series photos of the Blind Pass evolution from a mixed energy inlet to a wave dominated inlet 1942 1957 1962 1969 1926 1977 1994 2000 1984 1991
12 Figure 5 Time series of maps showing the migration and eve ntual structural stabilization of Blind Pass ( from Barnard, 1998 ).
13 The tidal prism through Blind Pass has been decreasing steadily since the late 1800s. Most of the t idal prism has been captured by Johns Pass to the north, which was cut by the hurricane of 1848 (Davis and Barnard, 2000). The decreasing tidal prism contributed significantly to the decreasing ability of sediment flushing at Blind Pass. Before the 1950 s, an ebb tidal shoal existed, as evident from the aerial photos. Loosing the tidal prism has altered Blind Pass from a mixed energy inlet to a wave dominated inlet ( Figure 6 ) Wave sheltering, sand bypassing, and onshore migration of the ebb shoal caus ed considerable accretion at the Upham Beach directly downdrift of the inlet (Mehta et al., 1976). The ebb shoal started to diminish in the late 1950s due to further decrease of the tidal prism, partially accelerated by the causeway construction and the dredge and fill operations in Boca Ciega Bay ( Figure 2 ) Losing the wave sheltering and sand bypassing, Upham Beach has experienced chronic shoreline erosion since the 1970s. Frequent beach nourishment has been necessary to maintain the beach Upham Be ach has become a persistent erosional area among the generally successful beach nourishment projects in Florida (Davis et al., 2000 ; Elko et al., in press ). Interaction between tidal flows and longshore transport is important in controlling sedimentation i n the channel and sediment bypassing around the inlet Tidal flow patterns and magnitudes are str ongly influenced by tidal prism: t herefore, tidal prism greatly influences the deposition of sediment in inlet area. For i nlets that have the channel in the middle the possibility of sediment deposition is relatively low (Mehta et al. 1976). I nlets with large ebb shoals have
14 a bypassing mechanism where sediment is able to bypass the channel thalweg and continue moving alongshore Inlets that have complex channels tend to be sediment sink s regardless of tidal flow velocities (FitzGerald and Hayes, 1979 ). Blind Pass is one example of a complex channel inlet where the inlet has a sharp bend and a long path to the back bay (Mehta, 1976). Blind Pass provides a n excellent example of a wave dominated inlet which has active sedimentation in the channel. This sedimentation may also affect the tidal flow velocities in the inlet by reducing the channel cross section Figure 6 Classification of coastal morphodynam ics (from Davis and Hayes, 1984) The study area falls near the lower left corner
15 Previous Research Tidal inlets are an integral part of barrier island and back barrier systems. Inlets exist in dynamic equilibrium with littoral drift, which tends to move sediment into the inlets, and tidal currents, which flush sediment from the inlets (Bruun and Gerritsen, 1960; Jarrett, 1976; OBrien and Dean, 1972). The equilibrium of inlets is maintained when tidal currents flush littoral drift derive d sedime nt from the inlet at a similar rate as it is introduced. The interactions between tide and wave generated sediment transport determine the morphology of a tidal inlet (FitzGerald, 1984). Inlets serve as passageways for tidal currents that transfer water between coasta l bays and the open sea. They also serve as navigational passages for vessels to th e open ocean from the back bay region. Inlet throats are the narrow channels which separate adjacent barrier islands and thus carry the entire tidal flux in and out of the inlet (Kumar and Sanders, 1974). In lets originate in two ways. The first is by breaching of a barrier island during a major event, such as a storm. Large waves combined with a significant storm surge and strong, elevated tidal currents cu t the inlet. If the cut is fairly deep and there is enough tidal flux to offset the longshore sediment transport, then the inlet may remain open. The second manner is from spit growth across an embayment. By closing off the embayment only a small tidal channel remains
16 to flush the bay with tidal currents. The pathway is maintained by strong tidal currents (Kumar and Sanders, 1974). In the documented past, barrier island breaching is the only manner of inlet origin on the West Central coast of Florida (K umar and Sanders, 1974). This is not surprising because barrier extension is a gradual process that requires headlands which are largely non existent along the west central coast of Florida. Inlet hydrodynamics The morphodynamics of tidal inlets are co ntrolled by the hydrodynamics and sediment supply, especially the flow velocity magnitude and directional pattern through the inlet channel. Boon and Byrne (1981) showed that major reductions in the cross sectional area of an ebb dominated inlet throat re sult in a transition from ebb to flood dominance, with respect to peak current velocities, once inlet hydraulics become more influenced by frictional effects than basin hypsometry. Frictional effects increase with an increase in the ratio of wetted channe l perimeter to cross sectional area. The ebb tide is more strongly influenced by frictional effects than the flood tide because the ebb tide occurs during higher water than flood tide The ebb tide duration increase and the corresponding shorter flood t ide duration require higher peak flood velocities to move the same volume of water. Inlet hydraulics responds primarily to variations in cross sectional area and ocean tidal ra nge (Smith and Zarillo, 1988).
17 FitzGerald (1976, 1984) and FitzGerald et al. ( 1984a) illustrated that waves play a major role in shoal development at stable inlets, whereas tidal currents tend to scour and maintain the inlet channel. Bruun and Gerritsen (1960) p redicted t he inlet will close when tidal flushing is overwhelmed by wa ve generated sand transport and shoaling. Van de Kreeke (1984, 1989) showed that when actual shear stress equals the equilibrium shear stress, the inlet is in equilibrium with the hydraulic environment. When the actual shear stress is larger than the eq uilibrium shear stress, the inlet is in a scouring mode; when the actual is smaller than the equilibrium shear stress, the inlet is in a shoaling mode. The inlet is determined to be stable if after a small change, the inlet cross sectional area unconditio nally returns to its equilibrium value. Van de Kreeke (1988) also showed that because of the nonlinear nature of the dynamics, inlets do not merely transmit the ocean tide harmonics but i n addition act as filters. Inlets transfe r energy to higher harmoni cs. W hen bays are connected to the ocean by more than one inlet, residual currents are generated. Higher harmonics and residual currents are important when dealing with transport processes. Bruun and Gerritsen (1960) suggested a W /M ratio for inlet stabi lity of a semi diurnal inlet that when W /M < 100 = poor stability 200 > W /M > 100 = fair stability W /M > 200 = good stability
18 where W is tidal prism for spring tides and M is littoral drift from the up drift direction. They also suggested that this W /M ra tio should change for a diurnal inlet to W /2M because the length of the tide is twice as long for diurnal tide then the semi diurnal tide. They also concluded that the shape of the cross section and the stability shear stress are important factors for gor ge stability. van de Kreeke (1984) concluded that for an inlet to be stable the hydraulic radius must be proportional to the cross sectional area. He also concluded that for a two inlet system, such as the Johns Pass Blind Pass system, both of the in let ratios must intersect for both inlets to be stable. If this is not me t one or both of the inlets may shoal and close. Inlet morphodynamics Davis and Gibeaut (1990) identified four general m orphologies for tidal inlets based on the relative influenc es of wave and tidal energy (Figure 7 ) Wave energy affects an inlet through shoaling processes, erosion of a djacent beach shorelines, and gen erating longshore currents that transport sediment in to the inlet channel. The main significance of tidal energy is in the generation of tidal currents, which scour the inlet channel and deposi t sediment primarily on the ebb and flood tidal deltas (Kumar and Sanders, 1974). The dynamic balance between wave and tide energies determines the morphodynamics and therefo re the stability of tidal inlets.
19 Figure 7 Inlet morphologies for the west central Florida barrier chain. Hatching indicates areas along the shoreline that are most affected by tidal inlet dynamics. The ocean is to the left and the bays are to the r ight (Davis and Gibeaut, 1990). Tide dominated Inlets Tide dominated inlets are characterized by a short, stable deeply incised inlet throat, a long, shore normal channel, and a large, seaward prograding ebb tidal delta (Figure 7 ) The inlet cross sec tional bathymetric profiles are deep and symmetrical reflecting tide domination (Figure 8 ) These inlets contain larger ebb tidal deltas than other types of inlets because wave action cannot off set the tide dominated flushing and erosion Tide dominated inlets typically do not have a terminal lobe associated with them. The ebb tidal deltas may extend several kilometers shielding the downdrift beach from wave action. If refraction occurs a
20 depositional zone along the downdrift beach may develop The la rge, stable ebb tidal deltas associated with tide dominated inlets can be an effective barrier to littoral drift, resulting in downdrift e rosion (Hayes, 1979). Figure 8 Cross se ction of a tide dominated inlet, Bunces Pass, Florida (from Barnard 1998) Wave dominate Inlets Wave dominated inlets are typically smaller, shallower, and much less stable than the other types of inlets (Figure 7 ) With poorly developed to non existent ebb tidal deltas and small tidal prisms, these inlets migrate downdrift und er the dominating influence of littoral drift. The presence of significant longshore currents and sediment transport across the inlet mouth leads to a shallow and strongly asymmetric bathymetric profile (Figure 9 ) The inlet channel trends in a direction approaching shore parallel when fully developed with a sharp turn at the entrance to the ocean, such as the case of Blind Pass As the inlet migrates, the adjacent barrier beaches are affected by rapid expansion of
21 the updrift beach and erosion at the d owndrift beach As the channel is lengthened it becomes less efficient for tidal exchange (Hayes, 1979). The instability and migration potential of wave dominated inlets makes them a potential navigational hazard for developed coasts. Figure 9 Cross se ction of a wave dominated inlet, Blind Pass, Florida (from Barnard 1998) Mixed energy Inlets Mixed energy inlets have intermediate morphologies between tide dominated and wave dominated inlets, containing geomorphologic features indicative of both wav e and tidal energy. There are two types of mixed energy inlets: mixed energy straight inlet and mixed energy offset inlet (Figure s 7 & 10 ). The most common tidally influenced features are a fairly stable inlet throat and a relatively large, well develop ed, ebb tidal delta. The main ebb channel is deflected in the direction of the dominant littoral drift. This trend can be
22 interrupted if changes in littoral drift magnitude and direction shift the orientation of the channel, or the ebb tidal delta is bre ached and a more efficient channel forms, usually in a spill over lobe channel (FitzGerald, 1988 ). Inlet channel cross section bathymetric profiles show an asymmetry with greater depths located in the downdrift direction. Ebb tidal deltas are also asymme tric as a result of wave influence. Inlet throat position is typically rather stable similar to the tide dominated inlet but relatively slow tidal currents and increased littoral processes may result in considerable sedimentation in the inlet mouth. Cha nging coastal conditions, such as tidal prism or wave energy, can result in the reworking of inlet sediment bodies, or it can cause the inlet to evolve into another morphology altogether (Davis and Gibeaut, 1990). Sediment bypassing at mixed energy inlets occurs through two primary ways : spit/ebb tidal delta breaching; and the construction, migration, and shoreline attachment of bar complexes. T he result of both processes is swash bar migration and welding to the downdrift beach. Spit/ebb tidal delta bre aching is more common in mixed energy offset inlets, while the migration of bar complexes around the terminal lobe of the ebb tidal delta is more common in mixed energy straight inlets (FitzGerald, 1988). Mixed energy straight inlets show sediment bypassin g primarily via bar migration and attachment. These inlets have limited offset between adjacent shorelines, a well defined terminal lobe due to greater wave influence, and a main ebb channel directed toward the dominant littoral drift. A stable throat an d
23 a relatively large, well developed ebb tidal delta differentiate it from a wave dominated inlet. The formation of a terminal lobe differentiate s it from a tide dominated inlet (Davi s and Gibeaut, 1990). Mixed energy offset inlets produce s horeline offs et when swash bar migration occurs in the wave shadow of the ebb tidal delta (Hayes et al., 1970; 1974). A nodal zone develops coincident with continued ebb tidal delta development such that waves are refracted to such a degree as to create a reversal of littoral drift allowing the downdrift shoreline to accrete and limit sediment supply to the further reaches of the barrier island. If the depositional zone continues on the downdrift beach a drumstick barrier island will form. A drumstick barrier is the most indicative morphologic feature of a mixed energy barrier coast. By limiting the amount of sediment transported to downdrift beaches, the updrift shoreline consists of a narrow, transgressing, recurved spit, susceptible to breaching (Hayes et al., 19 70; 1974; Hayes, 1975). Over time, the nodal zone migrates downdrift with growth and migration of the ebb tidal delta, shifting the location of the local reversal, and changing the area of the drumstick that experiences accretion (Reynolds, 1988). Sedime nt is able to bypass the inlet mainly by breaching of the spit/ebb tidal delta if the ebb tidal delta is asymmetrical Spill over lobe channels are common features in mixed energy offset inlets, cutting through the more exposed portions of the ebb tidal d eltas. Better developed channel margin linear bars and more digitate terminal lobes are signs of greater tide influence for this type of
24 mixed energy inlet (FitzGerald, 1988). Stump Pass is an example of such type of inlet (Figure 10 ) The West Central F lorida coast has all four types of tidal inlet (Davis and Hayes 1984). For example, Stump Pass is a mixed energy inlet, Bunces Pass is a tide dominated inlet and Blind Pass is a wave dominated inlet. This study focuses on the sedimentation pattern in th e wave dominated Blind Pass. A recent channel dredging in the summer of 2000 provided an excellent opportunity to monitor the sedimen tation and to quantify the rate and pattern of sediment accumulation. Figure 10 Cross section of a mixed energy straig ht inlet Stump Pass, Florida (from Bar nard 1998)
25 M ethodology The present study examines inlet sedimentation through an intensive field investigation A large amount of field data was collected using various methods. Time series bathymetric su rveys were conducted to quantify the magnitude and pattern of sedimentation. Tidal flows were measured using two different acoustic doppler current profilers. The field measurements were also designed to quantify possible seasonal variations in sedimentat ion patterns. I nnovative field methodologies were developed during this study. The main focus of this study is on the sedimentation/erosion patterns in the inlet channel. Sedimentation patterns were measured in two different ways. The first is through vessel based bathymetric surveys that were conducted quarterly for a year and a half. The second was via profiles surveys ( level and transit surveys ) that were conducted monthly for a year. The tidal flow velocities were collected four times during the 1 8 month study period The equipment was deployed for roughly one month during each of the four deployments. Because the main focus of this research is on sedimentation patterns, the tidal flows were analyzed to relate to the sedimentation patterns. The results will be discussed separa tely in the following sections and synthesized together at the end. Field observations indicate that wind patterns and wind driven waves have significant
26 influence on the Blind Pass inlet processes and sedimentation. The w ind data at Clearwater station, which is approximately 15 miles north of the study site, were obtained from the NOAA website. Wave conditions were measured approximately 500 m offshore the study area in 4 m water depth W ind characteristics and tidal flo w patterns across the inlet and throughout the water column are discussed first, followed by s edimentation patterns Measuring s edimentation patterns Time series b athymetric surveys Bathymetric surveys were conducted roughly quarterly beginning in Augu st 2002 and continuing to June 2004 to capture temporal changes in sedimentation. The goal was to determine the time series sedimentation patterns and possible seasonal trends Quarterly surveys provide detailed measurements on morphological changes Th e bathymetric surveys were conducted using a combination of an echo sounder for depth and a synchronized Global Positioning System ( GPS ) for horizontal position ( Figure 12 ) In situ t ide induced water level variations were measured to correct the tidal inf luence on the float platform survey. A n underwater benchmark was established to relate the water depth to NGVD 29. NGVD zero equals roughly to 0.15 m below mean sea level in the study area. The surveys were conducted using a 22 ft C Hawk, owned by the C oastal Research Lab at the University of South Florida The echo
27 sounder was mounted below the steps on the back of the boat 24 cm below water level. This has been determined to be the most stable and most accessible because the mounting is not permanent. The manufacture specified accuracy for the echo sounder is 1 cm. Repeated field measurements indicate that this accuracy can be achieved over flat water, e.g. at the boat dock. A limitation of the echo sounder is that it can not operate stably in wat er shallower than 30 cm. The GPS is a real time kinematics system made by As h tech with a manufacture specified accuracy of 2.5 cm for this type of operation Having locations of accurate control points, an accuracy of 1 cm can be achieved. For the pre sent study, the GPS was sampled at 2 H z and then aver aged to 1 H z to match the echo sounder data. The GPS system used in the survey is a two part system including a base station and a rover unit. The use of this system is to give a precise location each survey point The base station is typically (but not necessarily) placed on top of a bench mark, either R 142 or R 143. The reason for placing the base unit on top of the bench mark is to ensure the accuracy of the position, because each bench mark has a n exact location. The rover unit is mounted on the boat. The antenna for the rover unit is mounted directly above the echo sounder. This mounting gives the exact position for each water depth recorded.
28 Level and transit surveys During the study, the north ern side of the inlet had become too shallow to survey with the boat except during spring high tides Traditional l evel and transit procedures with an automated electronic total station and a scaled rod were used to survey the northern shoal These data were combined with the boat survey data to create the bathymetric map of the entire inlet channel. Nine lines were surve yed across the northern shoal The first line start ed at the tip of the northern jetty and extend ed to approximately 1.5 m water depth The lines were approximat ely equal spaced from the mouth of the inlet channel to the bend of the inlet channel resulting in a total of 9 lines (Figure 11) The level and transit surveys were conducted monthly for 1 year with the goal of quantify ing the detailed changes in the shoaling area The survey data were processed in the Geographic Information System (ARCGIS 8.3). Contour maps of the inlet were created using both GIS and Surfer software Time series bathymetric comparison s and calculati on s of sedimentation/erosion volumes were conducted using Surfer. The hydrodynamic data and wind data w ere processed using Excel Level and transit data was also processed using Excel and further analyses were conducted using BMap developed by the US Arm y Engineer Research and Development Center
29 Figure 1 1 Aerial photo of Blind Pass showing the shoal area and lines surveyed. Hydrodynamic measurement s The hydrodynamics, especially the tidal flow patterns through the inlet c hannel were measured in detail, both temporally and spatially The goal of t his aspect of the study was to examine the relationship between tidal flow patterns and the sedimentation patterns. Four one month long deployments were cond ucted at two location s ( F igure 12 ) Two types of current meters were used: an upward looking acoustic Doppler current profiler (U ADP) measuring current profile through the water column and a side looking acoustic Doppler current profiler (S ADP) measuring flow across the in let channel. Both these units are capable of measuring tidal water changes along with velocity measurements and waves Line 1 Line 2 Line 3 Line 5 Line 6 Line 7 Line 8 Line 9 Line 4
30 Vertical current profile measurements The U ADP looks up through the water column measuring velocities and current direction. The U ADP is capable of collect ing up to 25 bins through the water column with a resolution of 25 cm. In this study twenty measurements were collected with the resolution of 25 cm. There is a blanking distance of approximately 1 m so the near bottom velocities could not be measured. Each measurement was averaged over a 120 s period with an interval of 1200 s between measurements. The U ADP was deployed using a platform designed and built in the Coastal Research Laboratory at the University of South Florida. T his platform is 1 m tall and was driven into the sediment to a depth of approximately 80 cm. The height remaining is for mounting the U ADP on top of the platform. Typical deployment uses a tripod and heavy weights for anchoring. The platform was driven in to the subsurface using a 4.5 kg sledge hammer. The advantage of this deployment technique is that stability of the platform is created resulting in no movement as bottom eleva tion changes The disadvantage is that as the bed level changes the measured velocities represent different levels in the water column.
31 Figure 12 Date and location of the U ADP and S ADP deployments The measured velocity profiles can be used in two aspects of the study. The first is to examine the magnitude of tidal flow in the inlet channel. Studying the peak velocities will help to determine if the flows are sufficiently st rong to maintain the channel or if the channel will tend to scour or fill. Secondly, the measured profiles allow better quan tification of discharge through the channel, which in turn allows more accurate calculations of tidal prism. The current profile data, also allows the calculation of bottom friction. The present study focuses mostly on the first aspect of the analysis. S ADP U ADP 82902,110102 82902,110102 40903, 72903 40903, 72903
32 Cross channel current profile measurements The Sontek S ADP measures current distribution across the inlet channel ( F igure 12 ) The S ADP can collect up to 5 measurements with a bin size as small as 10 cm. A limitation of the S ADP is that it can only sample up to 102 m. Because the inlet is greater than 150 m wide the flows across the entire inlet channel c ould not be measured. In this study five measurements with a bin size of 20 m i.e. averaging over a 20 m distance, were conducted This averag e interval is greater than desired but it give s an adequate measure of the flow across most of the inlet given that cross channel variation is not too abrupt The S ADP was mounted on an 8 cm diameter aluminum pipe that was driven into the sediment with a vibracore device The S ADP was deployed at the same time as the U ADP The cross channel flow pattern provide s valuable information for sedimentation pattern analysis These data along with the U ADP profile measurement s provide a quasi 3 D quantific ation of the tidal flows in the inlet. Because the north ern shoal area is away from the channel where the vertical velocity profile was measured, the S ADP data are important in determining areas of weak flow during flood and ebb tides.
33 R esults Sed iment Characteristics At present time, s ediment i n the Blind Pass inlet varies greatly from the channel thalweg to the shoal area. The main composition of the sediment in the channel thalweg is shell lag. The mean grain size a t t he mouth area in the tha lweg averaged 0.4 phi and is bimodal with the coarse peak mostly shell debris (Figure 1 3 BPI 1) This grain size is due to the flow velocities in the channel thalweg. The bimodal sediment is from cohesion between the fine sediment and the coarse shell l ag. T he mean grain size a long the slope of the main channel averaged 1.5 phi with considerable amount of shell debris The coarser material is the result of flow velocities of lesser magnitude than in the channel thalweg. T he mean gr ain size i n the sho al are a averages 2.5 phi dominated by the fine quartz sand (Figure 1 3 BPI 9) This portion of the inlet receives velocities of 40 cm/s or less. These velocities are not great enough to move fine large amounts of fine sediment. The mean grain size close to the north jetty is larger than over the majority of the shoal area with a large amount of coarse shell debris The mean grain size at the north jetty averaged 0.1 phi and is poorly sorted This is caused by the selective sediment transport by the b reak ing waves at the north jetty similar to the surf zone
34 Figure 13. Sediment types in Blind Pass. Wind and Wave Climate Wind is caused by differences in atmospheric pressure between areas. One of the main causes of pressur e variations is temperature (Henry et al., 1994). The latitude of the West Central coast of Florida is within a subtropical belt in which there is a distinct seasonal cha nge in weather conditions W ith the exception of the northern half of Florida during the winter, regional scale winds over the state usually come from the Atlantic Ocean o ut of the Bermuda Azores high. The high pressure cell migrates north in the winter and south in the summer, the direction from which the air reaches peninsular Florida differs slightly in those two seasons. In the spring and summer these winds prev ail from channel thalweg BPI-1 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 Size Fraction (phi) Weight Percentage slope of the channel BPI-5 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 Siz e Fraction (phi) Weight Percentage shoal area BPI-9 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 Siz e Fraction (phi) Weight Percentage north jetty BPI-15 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 Size Fraction (phi) Weight Percentage
35 the southeast quadrant (Winsberg, 1990). During the fall and winter the winds are from the northe ast quadrant Local and severe thunderstorms are common in late aft ernoon and evening during the summer (Henry et al., 1994) but they have no significant influence on coastal processes F rom September 2002 to August 2003 the first 12 months of the study period, the strongest average wind s were 3.8 m/s and were from the southwest between 180 225 degrees ( F igure 14 ). The dominant wind (18% of time) wa s from the southeast between 1 3 5 180 degrees The yearly average helps to describe the wind climate for the West Coast of Florida. This yearly average removes any storm win ds or summer sea breezes that occur during the year. The average does tell the dominant wind pattern in terms of direction over the area. The prevailing winds are from the east with the velocities lower than the predominant winds. The predominant winds are produced from cold fronts syste ms that pass through the area The average wind direction for the year was 163 degrees with an average wind velocity of 2.7 m/s.
36 September 2002 August 2003 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 0-45 45-90 90-135 135-180 180-225 225-270 270-315 315-360 % of time 0 1 2 3 4 5 6 7 8 9 m/s wind direction wind velocity shoreline orientation 320 degrees Figure14 W ind direc tion and velocity for the year of September 20 02 through August 20 03 (St. Petersburg Florida weather station ). During the winter months (November 2002 March 2003) the greatest wind velocities we re from the south southwest (Figure 15 ) with an average velocity of 4.1 m/s The dominant wind pattern was from the northeas t with a lower veloci ty During the winter months cold fronts pass through the area with intense winds. T he pattern of wind change becomes regular with the passage of frontal systems. The average wind direction for this period of time was approximately 161 degrees with an average velocity of 3.0 m/s. The wind direction for the winter was approximately equal to the yearly average of 163 degrees and the wind velocity was slightly higher than the yearly average of 2.7 m/s.
37 November 2002 March 2003 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 0-45 45-90 90-135 135-180 180-225 225-270 270-315 315-360 % of time 0 1 2 3 4 5 6 7 8 9 m/s wind direction wind speed Figure 15 W ind direc tion and velocity for November 2002 through March 2003 ( St. Petersburg, Florida weather station ). During the summer months (April September 2003) the most intense wind s we re from the south (F igure 16 ) with an average velocity of 3.3 m/s The dominant wind patte rn was from the south where velocities are greatest The average wind direction for the summer of 2003 was 167 degrees with an average wind velocity of 2.7 m/s. The wind direction for the summer of 2003 was approximately the same as the yearly average of 162 degrees and the wind velocity was 0 .4 m/s slower than the yearly average of 2.7 m/s.
38 April September 2003 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 0-45 45-90 90-135 135-180 180-225 225-270 270-315 315-360 % of time 0 1 2 3 4 5 6 7 8 9 m/s wind direc t ion wind v elocity Figure 16 W ind direction and velocity for April through September 2003 (St. Petersburg, Florida weather station ). Passages of cold fronts have significant influ ence on the coastal behavior in west central Florida ( Henry, 1994 ). Figure 24 shows a typical winter frontal system that passes through the study area. T he blue lines represent wind direction and the length of the line is proportional to the magnitude of the wind speed The wind direction changes systematically and in a clockwise manner with the passage of each extra tropical system during the winter months. The wind speed varies greatly with the passage of an extra tropical system. Twenty four to fort y eight hours before the system arrives the winds are typically out of the southeast and are low to variable, 2 3 m/s. Within 24 hours before the system arrives the wind changes direction and is from the southwest. The wind speed increases dramatically a nd the strongest winds occur assoc iated with the system (Figure 17 ). As the frontal system passes the wind changes direction
39 clockwise and is from the northwest. The winds subside some, but remain considerably greater than the winds from the southeast. Twenty four hours after the system passes the winds have changed and are from the northeast. The wind speed then subsides Figure 17 W inter frontal system in December 2002 (NOAA archives). The wind patterns described above represent winds affecting the study area which is orientated toward the southwest (Figure 1) On average, t he wind condition is largely the same for the two years (August 2002 through July 2004) in terms of seasonal distribution. The wind velocity for t he year averages approximately 3 m/s and a note worth adding each seasonal average is also approximately 3 m/s. T he dominant wind direction is largely out of the east. The average data also suggest that the stronger winds on average are out of the south west to north west 2 m/s
40 The West Central coast of Florida is of low, mixed energy (Davis and Hayes, 1984) with a mean annual wave height of about 30 cm (Tanner, 1960) and mean tidal range is approximately 75 80 cm (Davis 1989 a ). The area does not experience wave heights above 60 cm except during stronger storm conditions. These s torms bring heightened wave energy but for a limited time typically 2 5 days. These waves are not sustained fo r a significant period of time, typically less than 48 hours (Figure 17 ). The wave direction is derived directly from the wind direction. As the wind is chang ing directions the wave height is reduced by wind blowing over the water from a different direction. Wave growth may take a certain amount of time Thus, for faster moving frontal systems the wind generated wave and therefore, sediment transport may not be significant Larger systems or systems that stall for several days are able to move up to large amount s of sediment. Figure 18 illustrates six years of yearly aver age wave heights. The most common wave height is 0.0 0.3 m, which is to be expected for this generally low energy coast (Figure 14 ). W ave height and therefore wave energy, is largely controlled by regional wind speed and duration. Therefore, the strong wind accompanying the passages of winter cold fronts may generate relatively large waves coming from the north (Figure 17 ). Distance swells have little contribution to the wave climate in the study area, unless during very intense storms such as hurrican es. For example, Hurricane Ivan passing through the center of the Gulf in 2004 generated large swells.
41 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 0.0 -0.3 0 .3-0.6 0.6-0.9 0.9-1.2 1.2-1.5 1.5-1.8 1 .8-2.1 2.1-2.4 2.4-2.7 wa ve height (m) % o f t im e annually 1984 1985 1986 1987 1988 1989 Figure 18 Six yearl y averages of wave heights (Wang, 1991 ). Hydrodynamics of Blind Pass Stable inlets are characteri zed by a non migrating inlet throat and a stable main ebb channel position through the ebb tidal delta (FitzGerald, 1982). Blind Pass being a heavily structured wave dominated tidal inlet does not illustrate this stability. Its stability was achieved by hard engineering structure s. The goal of the hydrodynamic study is to link the present sedimentation patterns in the inlet channel with tidal flow conditions
42 Cross channel current profiles The S ADP measured the cross chan nel distribution of tidal flows (Figures 19 23 ) The x axis represent s the mid point distance from the instrument to each measurement bin The first measurement is at 12 m and the last measurement is 92 m from the instrument. It is worth noting that each measurement is an average of a 20 m bin The po sitive velocity represents flood flow and the negative velocity represents ebb flow. Each line represents one 2 min average measure ment every 20 min. Each graph (Fig ure 19 23 ) illustrates the flow pattern over one tidal cycle. During spring tides, ebb flow velocities are almost twice as great in the channel thalweg as flood flows. F lood flow is largely unif orm across the entire channel while ebb flow is much greater in the channel thalweg (Figure 19 location 72 m) Over the north ern shoal the above velocit y pattern is reversed with flood flow being nearly twice as great as ebb flow A close examination of the flow profiles reveals that while ebb directed flow was measured in the channel thalweg flood directed flow was sometimes measured over the n or thern shoal (Figure 19 location 12m). F lood directed flow was also measured when ebb flow is greatest in the channel Field observations indicated that a large eddy sometimes formed and is likely responsible for the flood directed flow over the shoal
43 11/05/02 -100 -80 -60 -40 -20 0 20 40 60 80 100 12 32 52 72 92 distance from s-adp (m) velocity (cm/s) 11/4/02 23:20 11/5/02 1:00 11/5/02 1:40 11/5/02 2:00 11/5/02 2:20 11/5/02 2:40 11/5/02 3:00 11/5/02 3:20 11/5/02 3:40 11/5/02 4:40 11/5/02 5:20 11/5/02 5:40 11/5/02 6:20 11/5/02 8:20 11/5/02 9:00 11/5/02 9:20 11/5/02 10:00 11/5/02 10:20 11/5/02 10:40 11/5/02 11:40 11/5/02 16:00 11/5/02 20:00 Figure 19 A n example of a c ross channel profile during a spring tide during November 2002 deployment. During neap tides, the overall ebb flow velocities a re approximately the same as flood flow velocities contrary to the sharp differences observed du ring the sprin g tides ( F igure 20 ) Si milar to the spring tide case, flood flow is unif orm across the channel while ebb flow is much greater in the channel thalweg (Figure 20 locations 52m & 72m) F lood and ebb flows in the channel average d about 40 cm/s Over the northern shoal area flood flo w reaches more than 40 cm/s while ebb flow only reaches 10 cm/s. In the channel the ebb and flood flow velocities are almost a mirror im age, while in the shoal area flood flows are u p to four times greater than ebb flows. In the shoal area a flood directed flow was sometimes measured during the ebbing tide, similar to the spring tide case.
44 11/01-11/02/02 -50 -40 -30 -20 -10 0 10 20 30 40 50 12 32 52 72 92 distance from s-adp (m) ve locity (cm/s) 11/1/02 22:20 11/1/02 22:40 11/1/02 23:00 11/1/02 23:20 11/1/02 23:40 11/2/02 0:00 11/2/02 0:20 11/2/02 0:40 11/2/02 1:00 11/2/02 1:20 11/2/02 1:40 11/2/02 2:00 11/2/02 2:20 11/2/02 2:40 11/2/02 3:00 11/2/02 3:20 11/2/02 3:40 11/2/02 4:00 11/2/02 4:20 11/2/02 4:40 11/2/02 5:00 11/2/02 5:20 11/2/02 5:40 11/2/02 6:00 11/2/02 6:20 11/2/02 6:40 11/2/02 7:00 11/2/02 7:20 11/2/02 7:40 11/2/02 8:00 11/2/02 8:20 11/2/02 8:40 11/2/02 9:00 11/2/02 9:20 11/2/02 9:40 11/2/02 10:00 11/2/02 10:20 Figure 20 An example of a c ross channel profile during a neap tide during November 2002 deployment. During a mean ( 0.7m ) tide the ebb tidal flow s a re almost double flood flows (F igure 21 ) Similar to the spring and neap tide cases, flood flow is largely unif orm across the channel while ebb flow is muc h greater in the channel. D uring ebb tide the flow in the channel is fo ur times greater than in the shoal area. In the shoal area a flood directed flow was sometimes measured during ebbing tide, similar to spring and neap tide cases
45 11/03 11/04/02 -100 -80 -60 -40 -20 0 20 40 60 80 100 12 32 52 72 92 distance from s-adp (m) ve locity (cm/s) 11/3/02 21:40 11/3/02 22:00 11/3/02 22:20 11/3/02 22:40 11/3/02 23:00 11/3/02 23:20 11/3/02 23:40 11/4/02 0:00 11/4/02 0:20 11/4/02 0:40 11/4/02 1:00 11/4/02 1:20 11/4/02 1:40 11/4/02 2:00 11/4/02 2:20 11/4/02 2:40 11/4/02 3:00 11/4/02 3:20 11/4/02 3:40 11/4/02 4:00 11/4/02 4:20 11/4/02 4:40 11/4/02 5:00 11/4/02 5:20 11/4/02 5:40 11/4/02 6:00 11/4/02 6:20 11/4/02 6:40 11/4/02 7:00 11/4/02 7:20 11/4/02 7:40 11/4/02 8:00 11/4/02 8:20 11/4/02 8:40 11/4/02 9:00 11/4/02 9:20 11/4/02 9:40 11/4/02 10:00 11/4/02 10:20 11/4/02 10:40 11/4/02 11:00 11/4/02 11:20 11/4/02 11:40 Figure 21 An example of a c ross channel profile during an average tidal phase during N ovember2002 deployment. Tidal flow at the bend in the inlet channel is considerably greater than at the mouth area for both flood and ebb tides During spring tides ebb tidal flow is almost double the flood flow (Figure 22 ) D uring ebb tide the flow in the channel is much greater than in the southeast corner of the bend In the shoal area, during spring tides, weak flows were measured during the flooding and ebbing tide, similar to the normal and neap tide cases ( Figures 19 & 20 ) The velocities in the bend area are much greater than at the mouth of the inlet, due to change in channel configuration and narrowing of the channel around the bend. T he channel thalweg is the deepest in the bend area due to increased ebb flow
46 4/18/03 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 0 10 20 30 40 50 60 70 80 90 100 distance from s-adp (m) velocity (cm/s) 0:00 0:20 0:40 1:00 1:20 1:40 2:00 2:20 2:40 3:00 3:20 3:40 4:00 4:20 4:40 5:00 5:20 5:40 6:00 6:20 6:40 7:00 7:20 7:40 8:00 8:20 8:40 9:00 9:20 9:40 10:00 10:20 10:40 11:00 11:20 11:40 12:00 12:20 12:40 13:00 13:20 13:40 14:00 14:20 14:40 15:00 15:20 15:40 16:00 16:20 16:40 17:00 17:20 17:40 18:00 18:20 18:40 19:00 19:20 19:40 20:00 20:20 20:40 21:00 21:20 21:40 22:00 22:20 22:40 23:00 23:20 23:40 Figure 22 An example of a c ross channel profile during a spring tidal cycle during April 2003 deployment at the bend of the inlet (see Figure 20 ). Figure 23 illustrates the relationship between f low velocities at different locations across the channel and the tidal phase. The darker colors represent the flow velocities in the shoal area and the lighter colors repre sent the flow velocities in the channel. The maximum ebb flows reach ed 1 m /s during spring ebb tide O verall, t he flow velocities correspond well to the tidal phas e and th e strongest flow occur s in the latter portion of the flood ing and ebb ing tide s, as expected As found by Hayes (1979), in m ost estuaries the maximum ebb currents occur late in the tidal cycle. The flood directed flow over the shoal area (the blue line) is apparent during the ebb phase.
47 11/2-11/5/02 S-ADP -100 -80 -60 -40 -20 0 20 40 60 11/2/02 0:00 11/2/02 12:00 11/3/02 0:00 11/3/02 12:00 11/4/02 0:00 11/4/02 12:00 11/5/02 0:00 11/5/02 12:00 11/6/02 0:00 velocity (cm/s) 0.6 0.8 1.0 1.2 1.4 1.6 1.8 water depth (m) 12 32 52 72 92 tide Figur e 23 Cross channel flow velocities as related to the tid al water level fluctuations during November 2002 deployment. Vertical current profiles The U ADP measures current profiles through the water column. It provides an average velocity over a 0 .3 m bin The U ADP was also deployed four times (08/2002, 11/2002, 04/2003, 07/2003) each deployment roughly one month in duration, during the research simultaneously with the S ADP. Similar to the S ADP the f irst two deployments were toward the mouth of the inlet and the last two were deployed at the bend (see Figure 12 ) The U ADP is deployed in the channel at both locations Inlet flow measurements are typically conduct ed in the channel thalweg (Metha et a l. 1976 ), probably because the greatest tidal flow velocities usually occur there The U ADP cannot measure flow within 1 m from the bed t h erefore, it is not appropriate for shallow deployments in the shoal area
48 V elocity profiles are largely uniform t hroughout the water column 1 m above the bed. During the spring tide ebb velocities are more than twice flood v elocities with peak velocities approximately 115 cm/s for ebb and 50 cm/s for flood (Figure 24 ) The peak flow velocities for the ebb tide excee ding 1 m/s only occur for a limited time of less than 1 hour F lood flow velocities remained at approximately 30 cm/s or above for an extended period of time of wel l over an hour. T wo red lines are drawn at 40 cm/s and 40 cm/s to show the clear ebb domi nance in terms of flow velocities. Figure 24 illustrates the velocity patterns over one spring tidal cycle. 9/06/02 t idal ve locities 0 100 200 300 400 500 600 700 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 velocities (cm/s) e levation above bed (cm) 0:00 0:20 0:40 1:00 1:20 1:40 2:00 2:20 2:40 3:00 3:20 3:40 4:00 4:20 4:40 5:00 5:20 5:40 6:00 6:20 6:40 7:00 7:20 7:40 8:00 8:20 8:40 9:00 9:20 9:40 10:00 10:20 10:40 11:00 11:20 11:40 12:00 12:20 12:40 13:00 13:20 13:40 14:00 14:20 14:40 15:00 15:20 15:40 16:00 16:20 16:40 17:00 17:20 17:40 18:00 18:20 18:40 19:00 19:20 19:40 20:00 20:20 20:40 21:00 21:20 21:40 22:00 22:20 22:40 23:00 23:20 23:40 Figure 24 An example of the vertical current profile in the channel during spring phase during August 2002 deployment
49 During a neap tide e bb velocities are also twice the flood v elocities with peak velocities approximately 70 cm/s for ebb flow and 30 cm/s for flood flow ( Figure 25 ) The peak flow velocities for the ebb tide exceeding 60 c m/s only occur for a limited time of less than 1 hour F lood flow velocities remained at approximately 30 cm/s or above for an extended period of time of wel l over an hour. The uniform velocity profile and ebb dominance are also obvious. 8/29/02 0 100 200 300 400 500 600 700 -80 -60 -40 -20 0 20 40 60 velocities in cm/s e levation above bed (cm) 0:00 0:20 0:40 1:00 1:20 1:40 2:00 2:20 2:40 3:00 3:20 3:40 4:00 4:20 4:40 5:00 5:20 5:40 6:00 6:20 6:40 7:00 7:20 7:40 8:00 8:20 8:40 9:00 9:20 9:40 10:00 10:20 10:40 11:00 11:20 11:40 12:00 12:20 12:40 13:00 13:20 13:40 14:00 14:20 14:40 15:00 15:20 15:40 16:00 16:20 16:40 17:00 17:20 17:40 18:00 18:20 18:40 19:00 19:20 19:40 20:00 20:20 20:40 21:00 21:20 21:40 22:00 22:20 22:40 23:00 23:20 23:40 Figure 25 An example of the vertical current profile in the channe l during neap phase during August 2002 deployment. During a mean tide condition, t he ebb flow is considerably greater than the flood flow in respect to velocity ( Figure 26 ) Peak ebb and flood flows are approximately 90 cm/s and 50 cm/s respectively. The peak flood and ebb flows
50 are limited in time typically less than an hour. While the peak ebb flows are less than double the flood flows the ebb velocity does remain over 60 cm/s for well over an hour. The flood flow is greater than 40 cm/s but onl y for an hour during the flood tide. 8/24/02 0 100 200 300 400 500 600 700 -100 -80 -60 -40 -20 0 20 40 60 80 velocity in cm/s height above bed (cm) 0:00 0:20 0 :40 1:00 1 :20 1:40 2 :00 2:20 2 :40 3:00 3 :20 3:40 4:00 4:20 4 :40 5:00 5 :20 5:40 6 :00 6:20 6 :40 7:00 7 :20 7:40 8:00 8:20 8 :40 9:00 9 :20 9:40 10:00 10:20 10:40 11:00 11:20 11:40 12:00 12:20 12:40 13:00 13:20 13:40 14:00 14:20 14:40 15:00 15:20 15:40 16:00 16:20 16:40 17:00 17:20 17:40 18:00 18:20 18:40 19:00 19:20 19:40 20:00 20:20 20:40 21:00 21:20 21:40 22:00 22:20 22:40 23:00 23:20 23:40 Figure 26 Vertical current profile in the channel during an average tidal phase during August 2002 deployment. F igure 27 illustrate s the relationship between the ph ase of tidal water level change and that of the velocity. The uniform velocity profiles are shown by the overlapping the velocity curves similar to the S ADP case, t he peak flow velocities are measured in the middle of the flood and ebb tide Blind Pass is clearly in a mixed diurnal tidal zone with the diurnal tide being much less frequent than the semi diurnal tide The tidal range averages between 0.4 m during neap tidal phases to 0.9 m during spring tidal phases.
51 August 2002 deploy ment -120 -100 -80 -60 -40 -20 0 20 40 60 80 8/22/02 0:00 8/25/02 0:00 8/28/02 0:00 8/31/02 0:00 9/3/02 0:00 9/6/02 0:00 velocity (cm/s) 5.5 5.7 5.9 6.1 6.3 6.5 6.7 water depth (m) 0.4 1 2.2 3.1 4.3 5.2 6.1 tide Figure 27 Vertical current flow velocities as related to the tid al water level fluctuations during August 2002 deployment A significant cross channel velocity variation was measured during the ebb tide near the mouth of the inlet The vel ocity is much greater over the channel thalweg than over the northern shoal. A weak ebb flow was measured at the southeast corner at the bend of the inlet. F lood flow, on the other hand, is large ly uniform across the entire channel. T herefore, flood flow is relatively stronger over the northern shoal d ue to the much weaker ebb flow. E bb flow d emonstrate d clear dominance over flood flow in the channel Comparing the deployment at the mouth to the deployment at the bend, the bend velociti es are 50 % greater than at the mouth in terms of peak velocities for flood and ebb flow This accelerated ve locity in the bend is due to change in flow direction. Field observation s suggest that the velocities in the channel landward of the bend leading up to Boca Ciega Bay are proportional to the velocities at the mouth to slightly slower.
52 The velocity profil e s throughout the water column are largely unifo rm all the time 1 m above bed. Peak ebb velocity is typically twice the peak flood velocity in the channel thalweg However, the peak ebb flow tends to last for only a short period of time, less than one ho ur, before a rapid decrease. Flood flow s on the other hand, tend to maintain a certain level approximately 40 cm/s, for an extended period of time of well over one hour Ebb dominance is clear ly demonstrated by the velocity profile me asurements in the channel. The measured tides show Blind Pass is a mixed diurnal tidal inlet where th e diurnal tide occurs slightly more frequently than the semi diurnal tide. Ebb flows peak at approximately 1 m/s, but do not remain great enough for an extended length of time to flush adequate sediment out of the inlet (Figure 28 ) August 2002 deployment -120 -100 -80 -60 -40 -20 0 20 40 60 80 8/22/02 0:00 8 /25/02 0:00 8 /28/02 0:00 8/31/02 0:00 9/3/02 0:00 9 /6/02 0:00 velocity (cm/s) 5.5 5.7 5.9 6.1 6.3 6.5 6.7 water depth (m) dept h average velocity (cm/s) water dept h (m) ebb flow flood flow Figure 28 Depth average velocity profile of the channel during August 2002.
53 Sedimentation patterns FitzGerald (1976, 1984), FitzGerald et al. (1984a), Gallivan and Davis (1981), an d Hubba rd et al. (1979) concluded that waves transported sediment into tidal inlets and were the primary mechanism of shoal growth and migration. They noted that tidal currents removed sand from the inlet channel and ge nerally deposited it on the sub merged port ions of shoals. Bruun and Gerritsen (1960) observed that tidal inlets are subject to closure when wave generated sediment transport over whelms tid al currents flushing the inlet. S edimentation over the entire channel S ix bathymetric surveys were conduc ted in Blind Pass during the study period One survey was conducted by the US Army Corps of Engineers (COE) in July 2000, shortly after the last channel dredging in the summer of 2000. The quarterly b athymetric surveys allow detailed examination of sedim entation and erosion in the inlet channel and calculation of time series volume change The s ame color scheme is used for all the bathymetric maps with blue color representing deep er water and red color representing shallowe r water (Figure 29 ) The firs t bathymetric survey conducted by the COE show ed the deep and wide channel that resulted from the dredging ( F igure 29 ) The entire inlet
54 was dredged including around the bend and just offshore of the inlet. Before this survey the inlet was much like it was in June of 2004 as shown by the aerial photo taken just before the dredging ( see Figure 12 ) The dredged material was used for nourishment of the downdrift chronically eroding Upham Beach. Th e entire inlet extended a short distance from the north a nd south jetties and was dredged to approximately 6.5 m deep. 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0 50 100 150 200 Easting (m) N o r t h i n g ( m ) Figure 29 Survey conducted in the summer of 2000 after dredging of Blind Pass, COE. Comparing the August 2002 bathymetry to the post dredging bathymetry, there is a large shallow (red) a rea near the inlet mouth and the channel has narrowed two year s after the dredging (F igure s 29 and 3 0 ) The sedimentation over a 2 year period in this area is approximately 3 m thick (Figure 3 1 ) The sedimentation is thicker closer to the north jetty and thins towards the south jetty 2002 Considerable s edimentation was also measured at the bend of the inlet
55 along the south jetty. Th e sedimentation along the northern side of the inlet and also at the corner is consistent with the measured weak ebb flow (Figure 2 1 ) The channel appears to be slightly deeper as compared to post dredging bathymetry T his indicates that ebb flow tends to concentrate along the southern side This is consistent with the cross shore flow measurement conducted after this peri od (Fig ure 21 ) Comparing the two surveys, a net sedimentation of approximately 70,000 m 3 was measured during the two years. Because the 2000 survey was conducted with a substantial different survey grid, as compared to the later surveys, this volum e cal culation may involve some uncertainty The area of sedimentation coincides with the area with weak ebb tidal flow, such as along the northern side and at the southeast corner. 0 50 100 150 200 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 Easting (m) N o r t h i n g ( m ) Figure 30 Bathymetric map of Blind Pass in August 2002.
56 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 50 100 150 200 Easting (m) N o r t h i n g ( m ) Figure 31 Diff erence map of sedimentation between July 2000 and August 2002 More sedimentation occurred between August and December 2002 (F igures 3 0 and 32 ). During this time t he shoal area at the mouth of the inlet became wider and extended further into the Gulf. The surface of the northern shoal undulated considerably These undulations are the result of sediment being deposited during cold fronts. Some irregularities in the sedimentation pattern, e.g., the two shallow areas along the edge of the deep channel, were measured. The sedimentation continued its trend of prograding landward and southward (F igure 33 ). The shoal area extended more than half of the distance of the north jetty landward of the mouth. The sedimentation at the bend of the inlet did not ex tend southward as much as it did in the mouth area. However, the water depth decreased in this area. The channel maintained similar
57 configuration, but was slightly shallower than in August A net accumulation of approximately 25 000 m 3 occurred during t hese four months. This accumulation of sediment is likely the result of four winter frontal systems that had passed through the area. The northern shoal area has become so shallow that wave breaking was induced. The r edistribution of the sediment in the shoal area was mostly driven by landward flows generated by wave breaking and flood tidal flow processes 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 Easting (m) N o r t h i n g ( m ) 0 50 100 150 200 Figure 32 Bathymetric map of Blind Pass in December 2002.
58 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 50 100 150 200 N o r t h i n g ( m ) Easting (m) Figure 33 Difference map of sedimentation between August and December 2002. S edim entation continued in the shoal area between December 2002 and May 20 03 (F igure s 32 and 34 ). More sedimentation occurred in th e mouth area, h owever, the shallow shoal did not prograde further south ward into the channel. A considerable amount of sedimenta tion occurred in the middle of the shoal area. A major trend observed from the May 2003 survey is that the shallow shoal has prograded so uthward considerably toward the channel, resulting in a steep slope into the channel at the bend The shoal area at t he bend accreted at a greater rate than in the middle and near the inlet mouth areas ( F igure 35 ). This accretion is the result of bypassing that occurred seaward of the bend area. A spit had started to grow toward the channel at the bend. Another spit i n the middle of the shoal was also identified. Both spit features could be observed in the field during
59 spring low tides. The bend area had the most active sedimentation during the six months between December 2002 and May 2003. The southeastern corner o f the inlet continued to accrete, but at a slower rate. The channel has largely kept its configuration and its depth. A net accretion of 2 800 m 3 occurred during these five months 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 Easting (m) -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 N o r t h i n g ( m ) 0 50 100 150 200 Figure 34 Bathymetric map of Blind Pass in May 2003.
60 0 50 100 150 200 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 N o r t h i n g ( m ) Easting (m) Figure 35 Di fference map of sedimentation between December 2002 and May 2003. The shoal area in August 2003 was slightly smaller and deeper, as compared with the bathymetry in May 2003 (F igure s 34 & 36 ). This reduction in the size of the shoal is related to the su mmer wave climate. The stronger winds out of the south southwest during the summer months bring wave energy, perpendicular to the coast, directly into the inlet (Figure 2 ). These waves are not large, but with the lack of sediment input from the north to replace the sediment being redistributed further in to the inlet, they change the inlet into a scour mode. The shoal area at the mouth of the inlet had been eroded both horizontally and vertically (F igure 37 ). Some erosion was also measured in the middle of the shoal area, but to a lesser extent than at the mouth of the inlet. A considerable
61 amount of erosion was measured at the bend of the inlet. The shoal retreated toward the northern jetty and the spit was largely removed. The southeastern corner of t he inlet has continued to accrete slowly and prog rade toward the channel. The channel had largely kept its configuration and overall depth throughout the summer. A net erosion of 11 000 m 3 of sand was measured during these three summer months The erosio n resulted from the lack of sediment input into the inlet channel during the summer month, in combination with possible sediment flushing by ebb tide and further redistribution landward of the bend. 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 N o r t h i n g ( m ) Easting (m) 0 50 100 150 200 Figure 36 Bathymetric map of Blind Pass in August 200 3.
62 0 50 100 150 200 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 N o r t h i n g ( m ) Easting (m) Figure 37 Difference map of sedimentation between May and August 2003. Sedimentation resumed in the shoa l area between August 2003 and November 2003 (F igure s 36 and 38 ). The mouth area was slightly shallower than in August, resulting from the ac cretion (F igure 39 ). The middle of the shoal area had also accreted upward and prograded toward the south jetty. The bathymetry over the shoal is smoother than from the previous surveys. This is the result of wave breaking over the accreting shoal and s moothing of the shoal area. Significant southward progr adation and upward accretion had occurred near the bend of the inlet. The entire shoal area dipped gently toward the south and broke dramatically near the channel, approaching the angle of repose of roughly 28 degrees at the bend of the inlet. The channel became narrower, but
63 remains at a similar depth as before. The southeastern cor ner of the inlet at the bend remained stable during this period. A net sand accumulation of 24 000 m 3 occurred in the se three months This accumulation is likely resulted from the renewed sediment supply from the north induced by the cold fronts at the beginning of the winter season. 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 Easting (m) N o r t h i n g ( m ) 0 50 100 150 200 Figure 38 Bathymetric map of Blind Pass in November 2003.
64 0 50 100 150 200 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 N o r t h i n g ( m ) Easting (m) Figure 39 Difference map of sedimentation between August and November 2003. The shoal area was largely stable between November 2003 and February 2004, with some local accumulation (F igure s 38 & 4 0 ). A large portion of the shoal area near the northern jetty was very shallow and became exposed during low tide. The level of sedimentation in the inlet is the result of continuous frontal passages on a 5 7 day interval. This allowed sediment to replace sediment eroded by normal processes. The greatest sedimentation was measure d in the middle of the shoal area (Figure 41 ) A net erosion of 4 000 m 3 was measured during these three months The small net volume change indicate that the sand was mostly redistributed during this three winter months.
65 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0 50 100 150 200 Easting (m) N o r t h i n g ( m ) Figure 40 Bathymetric map of Blind Pass in February 2004. 0 50 100 150 200 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 N o r t h i n g ( m ) Easting (m) Figure 41 Difference map of sedimentation between November 2003 and February 2004.
66 A considerable amount of sedimentation occurred over a large portion of the shoal area between February 2004 and April 2004 (F igure s 4 0 and 42 ). Substantial accumulation occurred at the bend of the inlet (Figure 43 ) The channel largely maintained its configuration. H owever, a considerable amount of accumulation occurred along the northern slope, resulting in a narrower channel. Duri ng these months frontal passages were still passing through the area and sediment continued to be transported into the inlet. The reason for accumulation near the channel is because the shoal had no more accommodation space. A net sand accumulation of 18 000 m 3 occurred during these three months This net accumulation is related to the last few cold fronts passing through the study area at the end of the winter season. 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0 50 100 150 200 Easting (m) N o r t h i n g ( m ) Figure 42 Bathymetric map of Blind Pass in April 2004.
67 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 N o r t h i n g ( m ) Easting (m) 0 50 100 150 200 Figure 43 Difference map of sedimentation between February and April 2004. Significant erosion in the shoal area occurred between April and June 2004 approximately 0.5 m (Figure s 42 & 44 ). A c onsiderable amount of erosion was measured at t he mouth of the inlet (Figure 45 ) The middle of the shoal was stable during this period of time. At the bend of the inlet, slight erosion was measured. This erosion is a result of normal processes in the inlet. The frontal systems have stopped passing through the area and the summer weather climate is taking over. Neglible sediment is entering the inlet and erosion is greater than accumulation. A net erosion of 12 000 m 3 was measu red during these three months This net erosion resulted from the lack of sediment supply from the nort h during the summer months, combined with the ebb flushing and further sediment redistribution into the back bay.
68 0 50 100 150 200 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 Easting (m) N o r t h i n g ( m ) Figure 44 Bathymetric map of Blind Pass in June 2004. 125350 125400 125450 125500 125550 125600 125650 125700 377250 377300 377350 377400 377450 377500 377550 377600 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 N o r t h i n g ( m ) Easting (m) 0 50 100 150 200 Figure 45 Difference map of sedimentation between April and June 2004.
69 The s ediment budget for Blind Pass demonstrated a continued trend of accretion, although several erosi onal periods particularly during the summer, occurred (Figure 46 ). The accumulation trend can be modeled reasonably well with a linear curve with a correlati on coefficient, R 2 of 0.87 : V= 3503 T where V is net volume change and T is time in number of months. The time series volume change reveals that the sedimentation is occurring during the winter months with the frequent passage of cold fronts. The inlet i s in scour mode during the summer months due to the lack of sediment supply There is one exception and that is between November 2003 and February 2004. T his linear trend indicates that the overall rate of sedimentation has not slowed down toward the end of the study period. However, because the inlet channel is largely full at the end of the study period it is anticipated that the rate of accretion will r educe and bypassing will increase. Some bypassing was observed near the end of this study. Th e observed trend was not a southerly bypassing to the downdrift eroding Upham Beach, but a bypassing around the bend and further into the channel that leads to Boca Ciega Bay
70 Volume changes in Blind Pass R 2 = 0.87 0 20000 40000 60000 80000 100000 120000 0 5 10 15 20 25 30 month s s ince Augu s t 2002 cubic meters of sediment Dece mber 2002 August 2003 May 2003 No v ember 2003 Februar y 2004 A pril 2004 June 2004 Septe mber 2004 October 2004 Figure 46 Volume changes in Blind Pass between August 2002 and October 2004 De tailed sedimentation patterns over the northern shoal N ine profiles over the northern shoal were surveyed monthly following traditional level and transit procedures M onthly surveys are examined here to depict detailed movement of the sand in the nort hern portion of the inlet. Four areas with different patterns of accumulation and erosion in the shoal area can be distinguished from the nine profiles: the first area were in the vicinity of profiles 1 and 2, the second were at profiles 3 and 4, the thi rd were at profiles 5 and 6 and the fourth were at profiles 7 through 9 (Figures 47 50 ) A similar color scheme is used in the following discussion emphasizing seasonal trends. Not every monthly survey is included in the figures. The February 20 03 pro file serves as the baseline for comparison The lighter colors represent the summer season and the darker colors represent the winter season
71 Profiles 1 and 2 show ed similar general trend of sedim entation and erosion. Profile 1 is at the end of the jetty at the mouth of the inlet ( see Figure 12 ) Between February and May 2003 there was some accretion at the end of profile 1. This accretion is due to the passage of frontal systems during this period. The northern portion of the profile is sheltered from the wave by the jetty. The jetty is acting as a break water and refracting the waves around into the inlet. The first area to accumulate is at profile 2 and then progrades back towards the direction of the incoming wave energy. During the summer the ar ea experienced erosion due to wave breaking at the beginning of the shoal area. During the winter months accretion and progradation occurred near profile 1. There is significant accumulation at the onset of winter, but towards the end of winter slight er osion occurred (Figure 4 7 A) At the beginning of the winter season there was accommodation space at profile 1 but after the area filled then bypassing and slight erosion occurred. A net accretion of 0.2 m and progradation of 20 m occurred over the entire year at profile 1 Slight ly different sedimentation/erosion pattern s were measured at profile 2 (Figure 47 B). During the summer months erosion was measured at profile 2. The erosion is similar to profile 1, but of a lesser degree. Accretion during the winter months was almost identical to profile 1. Profile 2 is slightly landward of profile 1 (see Figure 12 ) The wave breaking that occurred at profile 1 also occurred at profile 2 to a slightly lesser extent. A net accretion of 0 .3 m and progradation of 60 m occurred over the entire year at profile 2.
72 Figure 47 Level and t ransit survey of profiles 1 & 2. Profiles 3 and 4 show ed similar general trend of sedimen tation and erosion. During the summer months slight erosion occurred at profile 3 (Figure 48 A). The area experienced less erosion than profiles 1 &2. This is due to the wave energy had decreased in intensi ty when it reached this area. Between October and December 2003 there was significant accretion that happen ed over the shoal area. This accumulation is a result of frontal passages that brought Profile 1 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation NGVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 A. Profile 2 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation NGVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 B.
73 heightened wave energy in the inlet. The heightened wave energy deposited sediment further in the inlet, landward of profiles 1 & 2. Between December 2003 and Februar y 2004 progradation occurred at profile 3. The progradation is a result of lack of accommodation space in the shoal area around profiles 3 & 4. A net accretion of 0 .4 m and progradation of 3 0 m occurred over the entire year at profile 3 Between February and October 20 03 the area around profile 4 eroded slightly (Figure 48 B) Between October and December 20 03 significant accretion of 0.7 m and progradation of 15 m toward the southern jetty occurred in this area. During this time there were several fron tal passages that passed through the area. This is at the onset of winter and frontal systems that pass often have more energy associated with them. There was also accommodation space generated from the summer months when the inlet was in scour mode. Be tween December 20 03 and February 20 04 little accretion occurred in this area, while significant progradation of 20 m occurred toward the southern jetty Near the north jetty a small channel developed from wave induced currents. This is the result of wave s breaking at the jetty at an oblique angle. A net accretion of 0 .4 m and progradation of 25 m occurred over the entire year at profile 4
74 Figure 48 Level and transit survey of profiles 3 & 4. The area around profil e 5 remained largely stable during the summer months of 20 03 (Figure 49 A). During the onset of winter accretion was not measured at profile 5. This is the result of the area seaward of profile 5 was the area of active sedimentation. There was enough ac c ommodation space at Profile 4 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation NGVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 B. Profile 3 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation NGVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 A.
75 profiles 1 4 to hold all the sediment being deposited in the inlet There was some accretion near the channel as a result of wave breaking. A net accretion of 0 .2 m and progradation of 30 m toward the southern jetty occurred over the en tire year at profile 5 The area at p rofile 6 remained stable near the northern jetty b etween February and December 20 03 (figure 49 B) Profile 6 did not receive any sediment input until after the profiles seaward of it filled with sediment. Between Dece mber 2003 and February 2004 significant accumulation and progradation occurred in this area. The accumulation is a result of bypassing over and around the seaward profiles. The large wave energy transports the sediment over the shallow area and deposits it in the deeper water. A net accretion of 0.6 m and progradation of 40 m toward the south jetty occurred over the entire year at profile 6
76 Figure 49 Level and t ransit survey of profiles 5 & 6. P rofiles 7 through 9 show ed the same general trend of sedimentation and erosion (Figure 50 A) S ignificant vertical erosion of 0 .75 m and progradation of 15 m occurr ed along profile 7 b etween February and May 20 03. This is a result of sediment being redistributed in this are a. The sediment had accreted to near the water level. Wave breaking in this area suspended the sediment and moved it southward and deposited it in deeper water. Because the shoal area was narrow the sediment was not transported far before it was deposit ed in deeper Profile 5 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) e levation NGVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 A. Profile 6 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation NGVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 B.
77 water. After this initial erosion took place the area at profile 7 remained largely stable until February 2004 with the respect to accumulation. At the end of profile 7 progradation did occur. This progradation is the result of sediment bei ng redistributed in the inlet during the summer months from normal wave action. The area at the end of profile 7 does not have significant flow velocities to prohibit the accumulation of sediment. However, there will be a limit to the progradation where the velocities are strong enough to prohibit sedimentation. A net accretion of 0.4 m and progradation of 55 m towards the south jetty occurred over the entire year at profile 7 A general trend of progradation occurred at profile 8 (Figure 50 B) In Febru ary 2003 a small channel was measured near the north jetty. This is the result of wave generated currents mobilizing sediment and caring it towards the bend of the inlet. A spit was also measured during the initial level and transit survey. This spit is consistent with the bathymetric survey that was conducted in May 2003 ( see Figu res 34 & 35 ). During the summer months slight erosion occurred and progradation was the mode at profile 8. The area continued to progradation with some slight accumulation du ring most of the rest of the year and in the beginning of the winter season. However, between December 2003 and February 2004 there was significant accumulation and progradation due to accommodation space being filled at the profiles seaward of profile 7 (Figure 49 A). The progradation is halted at the south side because of the flow in the channel. A net accretion of 0 .35 m and progradation of 60 m towards the south jetty occurred over the entire year at profile 8
78 The area around profile 9 remained the same in terms of accumulation between Febru ary and December 2003 (Figure 50 C) Between December 2003 and February 2004 a significant amount of accumulation occurred at profile 9. This accumulation is also the result of accommodation space being filled up in the profiles seaward of profile 7. The area prograded continuously during the study. The progradation was slow during the summer months and accelerated during the winter months, prograding almost 40 m between October 2003 and February 2004. A net ac cretion of 0 .4 m and progradation of 55 m towards the south jetty occurred over the entire year at prof ile 9 Sedimentation over the entire inlet was measured during the study. The sedimentation is event driven by extra tropical systems that move through the area during the fall and winter months (see Figures 15 & 17 ). Due to the eventual nature of these systems, seasonal averages of wind speed and direction were not significantly influenced. Accumulation typically started at the mouth of the inlet and p rograded into the inlet channel and toward the south jetty. The progradation occurred mainly in the northern portion of the inlet from the mouth toward the bend. The accumulation also prograded south toward the channel thalweg. Another area of accumulat ion occurred in the bend of the inlet at the southeastern corner. The channel has been largely stable in both shape and depth. Field observations indicated the sediment is being moved around the bend and accumulating on the west side of the channel leadi ng up to Boca Ciega Bay
79 Figure 5 0 Level and t ransit surveys of profiles 7 9. Profile 7 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation N GVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 A. Profile 8 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation N GVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 B. Profile 9 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0 20 40 60 80 100 120 distance from north jetty (m) elevation N GVD (m) 2/03 5/03 8/03 10/03 12/03 2/04 C.
80 Detailed profile surveys of the northern shoal area provide d a more in depth understanding of how sediment move s in the shoal area. Accretion ty pically start ed at the north jetty at the mouth area and prograde d landward and also towards the south jetty. The upward a ccretion is limited by wave breaking and the shoal has not completely emerged The breaking wave is capable of suspending a large am ount of sediment. These sediments are transported further into the inlet by flood current and wave driven currents. Progradation landward and toward the south jetty occurred at the same time so the sand body expanded in both directions simultaneously. S ubstantial net s edimentation seemed to occur during storm events and wa s then redistributed in the inlet under normal conditions. This is shown by the time lag between significant sedimentation events measured from the inlet mouth to the bend area.
81 S u mmary S edimentation in Blind Pass is event driven by extra tropical systems. Once the sediment accreted to cause wave breaking at the inlet mouth area, bypassing occurred in the form of sediment being pushed further into the inlet. Active sediment susp ension by breaking waves combined with wave driven current s and flood tidal current s are responsible for this sediment redistribution further into the inlet. Little to no sediment was bypassed downdrift, but rather further into the inlet around the bend. This is evident from recent field observations. A large amount of sediment had accumulated on the seaward side of the inlet prograding toward Boca Ciega Bay The only mechanism that can mitigate this trend is to dredge Blind Pass on a regular basis, dep ending on the severity and number of extra tropical systems during winter months. The sedimentation patterns at Blind Pass are controlled by several factors including availability of sediment, intensity and frequency of high energy events, magnitudes and patterns of tidal velocities, and tidal prism. These factors govern when and how sediment is transported into and out of Blind Pass. These factors will also govern if Blind Pass will remain open or close. The sediment supply is mostly coming from the n orth bypassing the north jetty and is driven largely by the passages of winter cold fronts. Little to no sediment is supplied from the
82 south because of the severely sand depleted Upham Beach contained by the jetty, the relatively weak wave forcing from th e south, and the much stronger tidal flux along the southern side. The number of intense extra tropical systems that pass through the area has significant influence on the magnitude and pattern of sedimentation in Blind Pass. During winter months the pas sages of extra tropical systems overwhelm tidal flow flushing and deposit sediment in the inlet. The ebb flow under normal conditions, peaking at approximately 1 m/s, is capable of flushing sediment out of the channel. Active sedimentation occurred alon g the northern side of the inlet due to the sediment supply from the north and the weak ebb flow there. A linear relationship is found between the amount of sedimentation and time, (V= 3500T). The several minor erosional periods did not change this overa ll trend. As the sedimentation continues, it is reasonable to believe that the sedimentation rate should slow down due to the reduced accommodation space. However, this did not happen during the present study period. The sedimentation/erosion patterns d emonstrate a clear trend of seasona l variation Generally, at the beginning of the winter season, a large amount of sediment was brought into the inlet by southward longshore sediment transport and deposited near the mouth area. The sediment was then red istributed further in the inlet. More sedimentation may occur toward the end of the winter season. During the summer season, due to the lack of sediment input from the north, the inlet is largely in an erosional mode, resulting in net volume loss.
83 C o nclusions The wind direction is largely out of the east on a yearly average W ind velocities associated with cold fronts are significant in transporting sed iment The f low velocities in the channel thalweg reached over 1 m/s during spring ebb tides but typically last less than one hour While f loo d flow velocities maintain ed at 40 cm/s or greater for over one hour. The greatest flow velocities occur during the spring tidal phase and are (on average of 30 cm/s) greater than the neap flows At the bend o f the inlet the flow velocities range from 10% 50% greater than at the mouth. The tidal flows in the bend exhibit the same characteristics as at the mouth in terms of neap and spring tides. The cross channel distribution of tidal flow played a signifi cant role in determining the sedimentation patterns. Ebb flow is much greater in the channel thalweg than along the n orthern shoal, while flood flow is largely uniform across the channel The ebb velocities over the shoal area are gr eatly reduced from th ose in the channel thalweg which agrees well wit h the sedimentation over the northern shoal
84 Sedimentation is significantly influenced by the passage of cold fronts during the winter Upward a ccretion is limited by wave breaking in the shoal area and acti ve sediment bypassing occurs after wave breaking depth is reached Toward the end of the study period, sediment was transported and deposited around the bend and up the channel toward the back bay area. During t he summer the inlet receives little sedimen t influx from the weak south to north longshore transport The lack of sediment input and further sediment redistribution and ebb flushing resulted in net erosion in the inlet channel. During the 20 month study period 73,000 m 3 of sediment has accumulate d in Blind Pass. A linear trend of sediment accumulation occurred in the inlet during the study period. N o signs of slowing down toward the end of the study period can be distinguished The sediment bypassing was landward toward the back bay region and n ot southerly toward the downdrift beach.
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