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Hydraulics and stability of multiple inlet-bay systems

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Hydraulics and stability of multiple inlet-bay systems
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Hydraulics and stability of multiple inlet-bay systems
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Jain, Mamta
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Gainesville, Fla.
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Coastal & Oceanographic Engineering Dept. of Civil & Coastal Engineering, University of Florida
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English

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UFL/COEL-2002/014

HYDRAULICS AND STABILITY OF MULTIPLE INLET-BAY SYSTEMS: ST. ANDREW BAY, FLORIDA by
Mamta Jain Thesis

2002




HYDRAULICS AND STABILITY OF MULITPLE INLET-BAY SYSTEMS:
ST. ANDREW BAY, FLORIDA
By
MAMTA JAIN

A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA

2002




Copyright 2002
by
Mamta Jain




ACKNOWLEDGMENTS
The author would like to express her deepest and heartiest thanks to her advisor and chairman of the supervisory committee, Dr. Ashish Mehta, for his assistance, encouragement, moral support, guidance and patience throughout this study. Special thanks go to committee member Dr. Robert Dean for his help and advice in solving the hydraulic model equations. Gratitude and thanks are also extended to the other members of the committee, Dr. Robert Thieke and Dr. Andrew Kennedy, for their guidance and assistance. Thanks go to Dr. J. van de Kreeke for his help in solving the linearized lumped parameter model for the stability of inlets.
Assistance provided by Michael Dombrowski of Coastal Tech, for whom the hydrographic surveys were carried out, is sincerely acknowledged. Thanks go to Sidney Schofield and Vic Adams, for carrying out the fieldwork.
The author wishes to acknowledge the assistance of Kim Hunt. Becky Hudson.
and the entire Coastal and Oceanographic Engineering Program faculty and staff for their encouragement and emotional support.
The author would like to thank her husband, Parag Singal, for his love,
encouragement and support, and her parents and family for providing her with mind, body and soul.
Last, but not least, the author would like to thank the eternal and undying Almighty who provides the basis for everything and makes everything possible.




TABLE OF CONTENTS
p42e
A C K N O W L E D G M E N T S ................................................................................................. iii
L IS T O F T A B L E S ............................................................................................................ v ii
L IS T O F F IG U R E S ........................................................................................................... ix
L IS T O F S Y M B O L S ......................................................................................................... x i
A B S T R A C T ..................................................................................................................... x iv
CHAPTER
I IN T R O D U C T IO N ........................................................................................................... I
1. 1 P rob lem D efi n itio n .................................................................................................... I
1. 2 O bjectiv e an d T ask s .................................................................................................. 4
1 .3 T h e sis O u tlin e ........................................................................................................... 4
2 HYDRAULICS OF A MULTIPLE INLET BAY SYSTEM .......................................... 5
2.1 Governing Equations of an Inlet-Bay System .......................................................... 5
2 1.1 Sy stem D efi n itio n ........................................................................................... 5
2 .1 .2 E n erg y B alan ce ............................................................................................... 6
2 .1.3 C ontinu ity E qu ation ........................................................................................ 7
2 .2 T h e L in earized M eth od ............................................................................................. 9
2 .3 M ultiple Inlet-B ay Sy stem ...................................................................................... 11
2.3.1 Two Inlets and Two Bays with One Inlet Connected to Ocean .................... 11
2.3.2 Three Inlets and Two Bays with Two Inlets Connected to Ocean ................ 16
2.3.3 Three Inlets and Three Bays with One Inlet connected to Ocean ................. 19
2.3.4 Four Inlets and Three Bays with Two Inlets Connected to Ocean .............. 24
3 STABILITY OF MULTIPLE INLET-BAY SYSTEMS ............................................... 29
3.1 Stability P roblem D efi nition ................................................................................... 29
3 .2 S tab ility C riteria ...................................................................................................... 2 9
3.2.1 Stability Analysis for One-Inlet Bay System ................................................ 30
3.2.2 Stability of Tw o Inlets in a B ay .................................................................... 32
3.3 Stability Analysis with the Linearized Model ........................................................ 34
3.3.1 Linearized lumped parameter model for N Inlets in a Bay ........................... 35




3.4 A pplication to St. A ndrew B ay System .................................................................. 40
4 APPLICATION TO ST. ANDREW BAY COMPLEX AND ENTRANCES .............. 42
4.1 D escription of Study A rea ...................................................................................... 42
4.2 Sum m ary of Field D ata ........................................................................................... 44
4.2.1 Bathym etry .................................................................................................... 46
4 .2 .2 T id e s .............................................................................................................. 4 8
4.2.3 Current and D ischarge ................................................................................... 51
4 .3 T id a l P rism .............................................................................................................. 5 2
5 RESULTS AN D D ISCU SSION .................................................................................... 54
5 .1 In tro d u ctio n ............................................................................................................. 5 4
5.2 H ydraulics of St. A ndrew B ay ................................................................................ 54
5.2.1 Solution of Equations .................................................................................... 55
5.2. 1. 1 One-inlet one-bay system .................................................................... 55
5.2.1.2 Three inlets and three bays with one inlet connected to ocean ............ 56
5.2.1.3 Three inlets and three bays with two inlets connected to ocean .......... 57
5.2.2 Input Param eters ............................................................................................ 59
5.2.3 M odel Results and Com parison w ith D ata ................................................... 60
5.3 Stability A nalysis .................................................................................................... 62
5.3.1 Input Param eters ............................................................................................ 62
5.3.2 Results and D iscussion .................................................................................. 63
6 CON CLU SION S ............................................................................................................ 73
6 .1 S u m m a ry ................................................................................................................. 7 3
6 .2 C o n c lu sio n s ............................................................................................................. 7 4
6.3 Recom m endations for Further W ork ...................................................................... 74
APPENDIX
A ALGORITHMS FOR MULTIPLE INLET-BAY HYDRAULICS .............................. 76
A I In tro d u c tio n ............................................................................................................ 7 6
A 2 P ro g ra m I ............................................................................................................... 7 6
A .3 P ro g ra m -2 ............................................................................................................... 7 7
B INLET HYDRAULICS RELATED DERIVATIONS ................................................. 80
B I Linearization of D am ping Term ............................................................................. 80
B .2 Shear Stress D ependence on A rea .......................................................................... 81
B .3 G eneral Equation for hydraulic radius .................................................................. 82
B .3.1 Rectangular ................................................................................................... 83
B .3 .2 T ria n g u la r ..................................................................................................... 8 3
B .4 H ydraulic Radius for Triangular Cross-Section ..................................................... 83
v




C CALCULATION OF BAY TIDE AND LINEAR DISCHARGE COEFFICIENTS ..85
D CALCULATIONS FOR STABILITY ANALYSIS ...........................89
D 1 Introduction .......................................................89
D.2 Calculations .............................................................................. 89
D.2.1 Equilibrium velocity.............................................................. 89
D.2.2 Constant for Triangular schemnatization......................................... 89
D.3 Relationship between Flow Curves and Stability of Two Inlets ................... 90
D.4 Matlab Programs ........................................................................ 91
D.4.1 Program-i ......................................................................... 91
D.4.2 Program-2 ......................................................................... 93
LIST OF REFERENCES ....................................................................... 96
BIOGRAPHICAL SKETCH ................................................................... 98




LIST OF TABLES

Table 4e
1. 1 Cross-sectional areas of Johns Pass and Blind Pass in Boca Ciega Bay................ 3
1.2 Cross-sectional areas of St. Andrew Bay Entrance and East Pass....................... 3
1.3 Cross-sectional areas of Pass Cavallo and Matagorda Inlet............................... 3
4.1 Locations of St. Andrew Bay channel cross-sections .................................... 45
4.2 Locations of East Pass channel cross-sections ............................................ 45
4.3 Cross-section area, mean depths and width ............................................... 46
4.4 Tidal ranges in September 2001, December 2001 and March 2002 ................... 51
4.5 Phase lags between the stations and the ocean tide....................................... 51
4.6 Characteristic peak velocity and discharge values........................................ 52
4.7 Flood and ebb tidal prisms.................................................................. 53
5.1 List of input and output parameters for one-inlet one-bay model ...................... 55
5.2 List of input and output parameters for the three inlets and three bays model........ 56 5.3 List of Input and Output Parameters for the four inlets and three bays model........ 58 5.4 Input parameters for the hydraulic model ................................................. 59
5.5 Model results and measurements ........................................................... 60
5.6 Input parameters for stability analysis ..................................................... 63
5.7 Effect of change in bay area and length of East Pass..................................... 65
5.8 Stability observations for St. Andrew Bay Entrance and East Pass.................... 72
C. 1 Weighted-average bay tide ranges and phase differences ............................... 85
C.2 Calculation of (qo 77B1)ma, ('Wi, 17B2)max, and (qBi -17B3)m....................... 87




D 1 Calculation of equilibrium velocity ....................... ..................................................89
D.2 Calculation of a, .................................... ........... 89
viii




LIST OF FIGURES

Figure pg
2.1 One bay and one inlet system................................................................ 5
2.2 Two bays and two inlets with one inlet connected to ocean ............................ 12
2.3 Two bays and three inlets, two inlets are connected to ocean .......................... 16
2.4 Three bays and three inlets with one inlet connecting to the ocean.................... 21
2.5 Three bays and four inlets, two inlets connect to ocean.................................. 25
3.1 Closure curves ............................................................................... 31
3.2 Escoffier diagram............................................................................ 31
3.3 Closure surfaces.............................................................................. 33
3.4 Equilibrium flow curve for Inlet 2.......................................................... 33
3.5 Possible configurations of equilibrium flow curves for a two-inlet bay system ....34 3.6 Equilibrium flow curves for two inlets in a bay........................................... 41
4.1 Map showing the three bays and two inlets and bathymetry of the study area ....... 43 4.2 Aerial view of St. Andrew Bay Entrance in 1993. Jetties are -430 m apart........... 43
4.3 East Pass channel before it's opening in December 2001 ............................... 44
4.4 St. Andrew Bay Entrance bathymetry and current measurement cross-sections....46 4.5 Cross-section A in St. Andrew Bay Entrance............................................. 47
4.6 Cross-section F in East Pass measured by ADCP........................................ 47
4.7 Measured tide in Grand Lagoon on Septemberl 8-19, 2001..............................49
4.8 NOS predicted tide at St. Andrew Bay Entrance on Septemberl8-19, 2001.......... 49
4.9 NOS predicted tide in St. Andrew Bay Entrance on December 18-19, 2001 ......... 50




4. 10 Tide at all selected N O S stations in M arch 2002 ....................................................... 50
5.1 Equilibrium flow curves for rectangular cross-sections, Run No. I ............................ 66
5.2 Equilibrium flow curves for rectangular cross-sections, Run No. 2 ............................ 66
5.3 Equilibrium flow curves for rectangular cross-sections, Run No. 3 ............................ 67
5.4 Equilibrium flow curves for rectangular cross-sections, Run No. 4 ............................ 67
5.5 Equilibrium flow curves for rectangular cross-sections, Run No. 5 ............................ 68
5.6 Equilibrium flow curves for rectangular cross-sections, Run No. 6 ............................ 68
5.7 Equilibrium flow curves for triangular cross-sections, Run No. 7 ............................. 69
5.8 Equilibrium flow curves for triangular cross-sections, Run No. 8 ............................. 69
5.9 Equilibrium flow curves for triangular cross-sections, Run No. 9 ............................. 70
5. 10 Equilibrium flow curves for triangular cross-sections, Run No. 10 .......................... 70
5. 11 Equilibrium flow curves for triangular cross-sections, Run No. I I ......................... 71
5.12 Equilibrium flow curves for triangular cross-sections, Run No. 12 ......................... 71
B I T rapezoidal C ross-section ........................................................................................... 83
B .2 T riangu lar cro ss-section ............................................................................................. 84
C. I Head difference between ocean (Gulf) and bay I ...................................................... 88
C .2 H ead difference betw een bay I and bay 2 .................................................................... 88
D. I General configuration of equilibrium flow curve ...................................................... 90
D.2 General configuration of equilibrium flow curve ...................................................... 90




LIST OF SYMBOLS

Symbols
AB, AB1, AB2, AB3 Ac Ac, Ac2, Ac3, Ac4 ao
aB, aB1, aB2, aB3 ^ B, cBi, B2 B3

ai
a, b, c, A, B Bi
C, C1, C2, C3, C4 CD, CDL1, CDL2, CDL3, CDL4 CK
f
F
g hk
i
K
k

bay water surface areas at MSL flow cross-sectional areas of inlets ocean (Gulf) tide amplitude bay tide amplitudes dimensionless bay tide amplitudes

constant that relates hydraulic radius with area of triangular cross-section constants defined to solve system of equations dimensionless resistance factor coefficients in linear relations of inlet hydraulics linear discharge coefficients prism correction coefficient of Keulegan Darcy-Weisbach friction factor friction coefficient acceleration due to gravity kinetic head
subscript specifying the inlet under consideration Keulegan coefficient of filling or repletion bottom roughness




ken
kex L,, L1, L2, L3, L4
m
P Q, Q1, Q2, Q3, Q4
Qm R, R1, R2, R3, R4 Rt Ro
ri, r2, r3
T
t
11
uB
uc, 11ucl, 11uc2, 1c3, 11c4 11eqi 1umaxl, 1umax2, 1max3, 1max4 11o
X
ao, aB EB1, 8B2, 8B3
-v1, 8v2, v3, 8v4

entrance loss coefficient exit loss coefficient channel lengths sum of entrance and exit losses. tidal prism discharges through inlets peak discharge hydraulic radii bay tide range ocean (Gulf) tide range polar representation of the bay tides tidal period time
velocity
bay current velocity velocities through inlets equilibrium velocity of inlet maximum velocities through inlets ocean (Gulf) current velocity distance between UF and NOS tide stations velocity coefficients high water (HW) or low water (LW) lags inlet velocity lags




Y
0 77o 77B, 77M1, 77B2, 77B3 7BI B1' 7B2' 7B3
eq

specific time when sea is at MSL dimensionless time water elevation
ocean (Gulf) tide elevation with respect to MSL bay tide elevations with respect to MSL dimensionless bay tide elevations maximum bottom shear stress equilibrium shear stress




Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
HYDRAULICS AND STABILITY OF MULTIPLE-INLET BAY SYSTEMS: ST. ANDREW BAY, FLORIDA
By
Mamta Jain
December 2002
Chairman: Ashish J. Mehta
Major Department: Civil and Coastal Engineering
Tidal inlets on sandy coasts are subject to the continuous changes in their geometry and as a result influence shorelines in the vicinity. Since engineering modifications carried out at one inlet can affect the long-term stability of others in the vicinity of the modified inlet, it is important to understand the stability of all inlets connecting a bay to the ocean. Inlet stability is related to the equilibrium between the inlet cross-sectional area and the hydraulic environment.
St. Andrew Bay on the Gulf of Mexico coast of Florida's panhandle is part of a three-bay and two- ("ocean") inlet complex. One of these inlets is St. Andrew Bay Entrance and the other is East Pass, both of which are connected to St. Andrew Bay on one side and the Gulf on the other. Historically, East Pass was the natural connection between the bay and the Gulf. In 1934, St. Andrew Bay Entrance was constructed I I km west of East Pass to provide a direct access between the Gulf and Panama City. Due to the long-term effect of this opening of St. Andrew Bay Entrance, East Pass closed




naturally in 1998. A new East Pass was dredged open in December 2001, and the objective of the present study was to examine the hydraulics and stability of this system of two sandy ocean inlets connected to interconnected bays.
To study the system as a whole, a linearized hydraulic model was developed for a three-bay and four-inlet (two ocean and the other two connecting the bays) system and applied to the St. Andrew Bay system. To investigate the stability of the ocean inlets, the hydraulic stability criterion was extended to the two-ocean inlets and one (composite) bay system using the linearized lumped parameter model. The following conclusions are drawn from this analysis.
The linearized hydraulics model is shown to give good results--the amplitudes of velocities and bay tides are within 5%. The percent error for St. Andrew Bay is almost zero, and for the other bays it is within 20%.
The stability model gives the qualitative results. The bay area has a significant
2
effect on the stability of the two inlets. At a bay area of 74 kM (the actual area of the composite bay), both inlets are shown to be unstable. Increasing the area by 22% to 90
stabilizes St. Andrew Bay Entrance, and by 42% to 105 kM stabilizes East Pass as
2
well. Keeping the bay area at 105 kM and increasing the length of East Pass from 500 m to 2000 m destabilizes this inlet because as the length increases the dissipation in the channel increases as well.




CHAPTER I
INTRODUCTION
1.1 Problem Derinition
Tidal inlets are the relative short and narrow connections between bays or lagoons and the ocean or sea. Inlets on sandy coasts are subject to the continuous changes in their geometry. Predicting the adjustment of the inlet morphology after a storm event in particular, i.e., whether the inlet will close or will remain open, requires knowledge of the hydraulic and sedimentary processes in the vicinity of the inlet. These processes are governed by complex interactions of the tidal currents, waves, and sediment. In spite of recent advances in the description of flow field near the inlet and our understanding of sediment transport by waves and currents (Aubrey and Weishar 1988), it is still not possible to accurately predict the morphologic adjustment of the inlet to hydrodynamic forcing.
Inlet stability is dependent upon the cumulative result of the actions of two
opposing factors, namely, a) the near-shore wave climate and associated littoral drift, and b) the flow regime through the inlet. Depending on the wave climate and the range of the tide, one of these two factors may dominate and cause either erosion or accumulation of the sand in the inlet. However, on a long-term basis, a stable inlet can be maintained only if the flow through the inlet has enough scouring capacity to encounter the obstruction against the flow due to sand accumulation, and to maintain the channel in the state of non-silting, non-scouring equilibrium. If such is not the case and waves dominate, then the accumulated sand will begin to constrict the inlet throat, thereby reducing the tidal




prism. The resulting unstable inlet may migrate or orient itself at an angle with the shoreline depending on the predominant direction of the littoral drift; the channel may elongate, thereby increasing the frictional resistance to the flow, and finally, a stage may be reached when perhaps a single storm could close the inlet in a matter of hours.
Stability criteria based on inlet hydraulics and sediment transport for single inlets have been proposed by, among others, O'Brien (193 1), Escoffier (1940), O'Brien and Dean (1972), Bruun (1978) and Escoffier and Walton (1979). All criteria assume that sufficient sand is available to change the inlet channel geometry in response to the prevailing hydrodynamic conditions. These investigators found various stability parameters to describe the stability of the inlet. It should be noted, however, that while it is relatively easy to deal with the stability of single inlets, the problem becomes complex when, as is commonly the case, more than one inlet connect the ocean to a single bay or more than one interconnected bays. Some examples of such systems are as follows.
Three cases of the history of two inlets in a bay are worthy of citation. One case is that of Boca Ciega Bay on the Gulf coast of Florida, where the co-dependency of two inlets, Blind Pass and Johns Pass, appears to be reflected in the history of their crosssectional areas. While Blind Pass has historically been narrowing due to shoaling, John's Pass has been increasing in size, as shown in Table 1. 1. As a result, Blind Pass now requires regular dredging for its maintenance while severe bed erosion has occurred at John's Pass (Mehta, 1975; Becker and Ross, 2001).
Another example is that of St. Andrew Bay Entrance and the East Pass. As mentioned previously, East Pass used to be a large inlet and was the only natural connection between the Gulf of Mexico and the St. Andrew Bay. In 1934, St. Andrew




Bay entrance was constructed 11I km west of East Pass through the barrier island by the
federal government to provide a direct access between the Gulf and Panama City. Table
1.2 gives the cross-sectional area of each inlet over time.
Table 1. 1 Cross-sectional areas of Johns Pass and Blind Pass in Boca Ciega Bay Year Area (M) Hydraulic Radius (in)
John's Pass Blind Pass John's Pass Blind Pass
1873 474 538 3.7 3.5
1883 432 496 3.8 3.0
1926 531 209 3.9 1.9
1941 636 225 4.1 1.4
1952 849 157 4.6 2.7
1974 883 411 4.9 1.6
1998 950 230 53d0.9d
dEstimated by assuming no change in channel width since 1974.
Table 1.2 Cross-sectional areas of St. Andrew Bay Entrance and East Pass Year Area(i)
St. Andrew Bay Entrance East Pass 1934 1,835 3,400
1946 3,530 2,146
1983 3,943 1,392
1988 Closed
2001 5,210 Reopened
The third example is that of Pass Cavallo and Matagorda Inlet connecting
Matagorda Bay, Texas, to the Gulf Stability analysis carried out by van de Kreeke
(1985) on this system showed that Pass Cavallo is an unstable inlet, which is decreasing
in cross-section, whereas Matagorda Inlet is increasing in size. The areas of crosssections of the two inlets are listed in Table 1.3.
Table 1.3 Cross-sectional areas of Pass Cavallo and Matagorda Inlet Year Area (in)
Pass Cavallo Matagorda Inlet
1959 8,000 Closed
1970 7,500 3,600
The above sets of complex problems are dealt with in this study in a simplified
manner, with the following objective and associated tasks.




1.2 Objective and Tasks
The main objective of this study is to examine the hydraulics and thence the stability of a system of two sandy ocean inlets connected to interconnected bays. The sequence of tasks carried out to achieve this goal is as follows:
1 Deriving the basic hydraulic equations using the linearized approach for a
complex four inlets and three bays system.
2 Solving these equations, applying them to the St. Andrew Bay system, and
comparing the results with those obtained from the hydrographic surveys.
3 Developing stability criteria using the basic Escoffier (1940) model for one inlet
and one bay and then extending this model to the two inlets and a bay.
4 Carrying out stability analysis for N inlets and a bay using the linearized lumped
parameter model of van de Kreeke (1990), and then applying it to the St. Andrew
Bay system.
1.3 Thesis Outline
Chapter 2 describes the hydraulics of the multiple inlet-bay system. It progresses from the basic theory to the development of linearized models for simple and complex systems. Chapter 3 describes the stability of the system, including an approximate method to examine multiple inlets in a bay. Chapter 4 includes details of hydrographic surveys and summarizes the data. Chapter 5 discusses the input and output parameters required for the calculation. It also presents the results. All calculations are given in the appendices. Conclusions are made in Chapter 6, followed by a bibliography and a biographical sketch of the author.




CHAPTER 2
HYDRAULICS OF A MULTIPLE INLET BAY SYSTEM
2.1 Governing Equations of an Inlet-Bay System
2.1.1 System Definition
The governing equations for a simple inlet-bay system may be derived by considering the inlet connecting the ocean and the bay as shown in Figure 2. 1.
A ABB
Q Bay
L
110 Ocean
Figure 2.1 One bay and one inlet system These equations are derived subjected to the following assumptions. I The inlet and bay banks are vertical.
2 The range of tide is small as compare to the depth of water everywhere.
3 The bay surface remains horizontal at all times, i.e., the tide is "in phase" across
the bay. That means the longest dimension of the bay be small compared to the
travel time of tide through the bay.
4 The mean water level in the bay equals that in the ocean.
5 The acceleration of mass of water in the channel is negligible.
6 There is no fresh water inflow into the bay.




7 There is no flow stratification due to salinity.
8 Ocean tides are represented by a periodical function.
2.1.2 Energy Balance
Applying the energy balance between ocean and bay one gets
2 -
77 +a, 7B + +aB uB +c-Ah (2.1)
02g 2g
where
r7o = Ocean tide elevation with respect to mean sea level,
S7B= Bay tide elevation with respect to mean sea level,
u, = Ocean current velocity,
U B= Bay current velocity,
o, and o = Coefficients greater than one which depend on the spatial distribution
of u, and uB, respectively,
XAh = Total head loss between the ocean and the bay, and
g = acceleration due to gravity.
It is also assumed that ocean and bay are relatively deep; thus U, and uB are small enough to be neglected. Then Eq. (2.1) becomes
Ah = r7, -riB (2.2)
There are generally two types of head losses. One includes concentrated or
"minor losses" due to convergence and divergence of streamlines in the channel. The second type is gradual loss due to bottom friction in the channel. The entrance and exit
//2
losses may be written in terms of the velocity head - in the channel, with the entrance 2g

loss coefficient ken and the exit loss coefficient kex, i.e.,




/2
Entrance loss= ken (2.3)
2g
2
Exit loss= kex U (2.4)
2g
where uc is the velocity through the inlet. Gradual energy losses per unit length depend on the channel roughness and are given in form of Darcy-Weisbach friction factor U u2
Gradual loss= C (2.5)
4R 2g
where
f= Darcy-Weisbach friction coefficient,
R = hydraulic radius of channel, and
L = Length of channel.
Substitution of Eqs. (2.3), (2.4) and (2.5) into (2.2) gives
u2(
7o 7 -- k + k x + L (2.6)
2g 4R
or
g = 7 o |.sign(Go Bn) (2.7)
ke +k +
4R
The sign(/o-j7B) term must be included since the current reverses in direction every half tidal cycle.
2.1.3 Continuity Equation
The equation of continuity, which relates the inlet flow discharge to the rate of rise and fall of bay water level, is given as




d
uA = u dA, (AB) (2.8)
dt
where
Q = flow rate through the inlet,
Ac = Inlet flow cross-sectional area, and
AB = bay surface area. Therefore Eq. (2.8) becomes
=AB drlB
u _- d (2.9)
C dt
Eliminating uc between Eq. (2.7) and (2.9) leads to
dB A 2g 1o-B I.sgn(ro-7 ) (2.10)
dt A )
ke + kex +=4R
Next, we introduce the dimensionless quantities
10-o = o _= ; 0-= 2t = -t (2.11)
a a T
0 0
where ao = ocean tide amplitude (one-half the ocean tidal range), T= tidal period and c = tidal (angular) frequency. Substitution into Eq. (2.10) gives
di= K |Go iO .sign(O- -GB) (2.12)
dO
where
K T A, 2ga (2.13)
K = go (.3
2za,- AB ken +k 4R




in which K is referred to as the "coefficient of filling or repletion" (Keulegan, 1967). Keulegan solved the first order differential equation, Eq. (2.12), for bB in terms of the repletion coefficient K and dimensionless time 0 using numerical integration.
2.2 The Linearized Method
A linear method was suggested by Dean (1983) for solving Eq. (2.12). For this approach it was assumed that the velocity u, in Eq. (2.7), is proportional to the head difference ()7, -)7B) rather than the square root of the head difference, according to
u, = gc. (lo-77B) (2.14)
a0
where CDL "linear discharge coefficient." This coefficient is defined as
CL = 1 a (2.15)
kenke f+ __ 7o -I7B )max
4R
where (7o -)7B)max is the maximum head difference across the inlet. Now, combining Eqs. (2.14), (2.9) and (2.11), Eq. (2.12) can be written in terms of the linear relationship as
r o c -d = (2.16)
CL dO]
where
C A (2.17)
C C AR a0
Under assumption (8) the ocean tide is assumed to be periodic. Because of the linear assumption the bay tide is also periodic, it can be written as
,o = cosO (2.18)

(2.19)

r7 = aRl cos(0 E)




10
where a aB= one-half the bay tide range (i.e., bay tide amplitude) and a = lag
a0
between high water (HW) or low water (LW) in the ocean and the corresponding HW or LW in the bay.
Eq. (2.18) and Eq. (2.19) are next substituted into Eq. (2.16) and the following complex number technique is used to solve for aB and EB:
1 Define the following constants:
C= a, o = Re eo)
C
2 Let the following variables be represented in the polar form:
B1 = Re 3,le" rB
3 Therefore
deB
dB= 171
dO
4 So the equations are reduced to
1= (1 + ai)rl (2.20)
1 1 -a
r = Re(rl) Im(r)
1+ai 1+a 1+a2
where
Re(ri) = is the real part of the solution, and
Im(ri) = is the imaginary part of the solution.
The magnitude of ri represents aB1 and the phase lag EaB is represented by the angle of ri:
1
aB (2.21)
aB1 l+ a2

(2.22)

EB1 = tan 1 a




The velocity uoj through inlet 1 is therefore given by
Uci = UmaxI Cos (0 Evl) (2.23)
where Um, is the maximum velocity through inlet 1, Ev1 is the phase lag between the velocity in inlet 1 and HW or LW in the ocean.
Substituting for and r/Bl from Eq. (2.18) and Eq. (2.19) in Eq. (2.14) and
combining Eqs. (2.23) and (2.14) we get the required expression for Umaxl. It should be noted that velocity is out of phase with respect to displacement by r/2. Therefore, Ev=
2.3 Multiple Inlet-Bay System.
2.3.1 Two Inlets and Two Bays with One Inlet Connected to Ocean
In the case of two bays with one inlet connecting to the ocean and the second
connecting the bays as shown in Figure 2.2, the eight assumptions mentioned in section
2.1.1 and the linear relationship both hold. In a manner similar to that employed for a single inlet-bay case, the velocity relationship and the equation of continuity for two-bay system may be written with reference to the notation of Figure 2.2.
Thus the following relationships are obtained:
Uc1 -CDL (r/o qB") (2.24)
a0
ducl duBo
Qd u1Ac= ABl + AR B2 (2.25)
Q1= olol= dt+ B dt
UC2 = V zr-17B2 (2.26)
Q2 = u2Ac2 = AB2 dr/B2 (2.27)
dt




77o Ocean
Figure 2.2 Two bays and two inlets with one inlet connected to ocean. where
uc1, uc2 = velocities through the inlets 1 and 2,
Qi, Q2 = discharges through inlets 1 and 2,
A,,, Ac2 = inlet flow cross-sectional areas, and
AB1, AB2 = bay water surface areas.
CDL1 a (2.28)
en1 o B1)max
+k + B
e ex 4R
C 1 al
CDL2 a (2.29)
k +k + A2 B1 '7B2)max(2
4en exR2
where
L1, L2 = inlet lengths, and
R1, R2 = hydraulic radii of the channels.
Eliminating ucl between Eq. (2.24) and Eq. (2.25) gives
7 1 I d1d + AB2 dB2] (2.30)
C, dt AL- dt

where




A a
CI =CDL1A c2Al (2.31)
Combining Eq. (2.26) and Eq. (2.27) yields
BI- 17B2 = L dB2 (2.32)
where
C2 DL2 2g (2.33)
AB2 V aBI
The dimensionless ocean tide is given by Eq. (2.18), and the dimensionless tides in bays 1 and 2 now become
B1 = aB1 cos(O- -B1) (2.34)
iB2 = aB2 cos(O- B2) (2.35)
where
aB
aBl
a
0
aBl = one-half the tide range (i.e., amplitude) in bay 1.
aB2 B2
a
a0
aB2 = one-half the tide range (i.e., amplitude) in bay 2.
B1 = lag between high water (HW) or low water (LW) in the ocean and
corresponding HW or LW in the bay 1.
EB2 = lag between high water (HW) or low water (LW) in the ocean and
corresponding HW or LW in the bay 2.
Eq. (2.30) and Eq. (2.32) can be expressed in the dimensionless form as




a doR AR2 dqR
9 -q i = + AB2 dB2 (2.36)
' Cl dO ABI dO
B B2 =- [-(2.37)
C2 dO
The above equations are solved by the matrix method assuming the variables to be complex numbers. The solution is obtained as follows:
1 Define the following constants
-=a, = b, A2 = A, o =Re (e')
C C2 A
2 Let
d^-l B i11 d^B2
71 = Re(aBle 1=)r, qB2 =Re(aB2e'(' B2))r2 dO dO 2
Ri ) i,7R2dO ~'dO
3 So the equations are reduced to
1= (ai + 1)r, + aAir2 (2.38)
0 = -r, + (bi + 1)r2 (2.39)
4 In the matrix form they become
1J rjJ ai+l bif (2.40)
0 r2 1 bi+ 1
5 The solution is
-i(b -i)
r = (2.41)
X
-1
r2 = (2.42)
X
where

X =(ab- 1)-i(aA+a+b); X = (ab- 1)+i(aA+a+b)




XX = (ab 1)2 +(aA+a+b)2
Re()b(b+a+aA)-(ab-1) m()= -b(ab-1)-(a+b+aA)
Re(r1) =; Im(r})=
Re(r2) _-(ab 1). m(r2) = -(a + b + aA)
Re(r; ) =, ;Jm(r;)=
AT7 AT7
The amplitudes (aB1 andaB2 ) of bays 1 and 2 are the magnitudes of the complex numbers r, and r2 and the corresponding phase lags are the angles of the complex numbers:
aB1 = Re(t)2 + Im()2 (2.43)
Im (J;r)
BI = -tan (2.44)
Re(rj))
2 = Re(r2)2 + Im(r2)2 (2.45)
Im (J;r)
-B2 -tan1 'm(r2) (2.46)
Re(r2))
The velocities uc and Uc2 through inlets 1 and 2, respectively, are therefore given by
UC1 = Umaxi cos(0-- c) (2.47)
Uc2 = Umax2 cos (0 Ev2,) (2.48)
where Umaxl and Umax2 are the maximum velocities through inlets 1 and 2, respectively, Ev and 8v2 are the phase lags between the velocity in inlet 1 and HW or LW in the ocean, and in inlet 2 and HW or LW in the ocean.
Substituting for qo and r7, from Eqs. (2.18) and (2.34) in Eq. (2.24) and
combining Eqs. (2.47) and (2.24) we get the required expression for Umaxl. Similarly we




can obtain the expression for Umax2. It should be noted that velocity is out of phase with respect to displacement by zr/2. Therefore, Ev1 = EB1-zr2 and. Ev2 = EB2-2Z2. 2.3.2 Three Inlets and Two Bays with Two Inlets Connected to Ocean
The inlet bay system is defined in Figure 2.3. In this system two bays are connected to each other with inlets 2 and inlet 3 and 1 connects bay 1 to the ocean.
L, L3
Figure 2.3 Two bays and three inlets, two inlets are connected to ocean.
The velocity in inlets 1 and 2 is given by Eq. (2.24) and Eq. (2.26) respectively. The velocity in inlet 3 is given by Eq. (2.49):
UC3 = 2g(. (r/o -77'.) (2.49)
where uc3 = velocity through the inlet 3 and
DL- a (2.50)
ken +k +L3 (7o -17B1)max
,c4R3
where
L3 = inlet 3 length, and
R3 = hydraulic radius of inlet 3 channel. The governing equations of continuity are




Q1 +Q3 ucAc, +uc3Ac3 = AB d- +AB2 dB2 (2.51)
dt dt
Q2 = Uc2Ac2 =AB2 dB2 (2.52)
dt
where
Qi, Q2, Q3 = discharges through inlets 1, 2 and 3,
Ac1, Ac2, Ac3 = flow cross-sectional areas at inlets 1, 2 and 3, and
AB1, AB2 = bay water surface areas.
Substituting for the velocity expressions in the above equations we obtain
1 do L AB2 d]B2 (2.53)
r7o riBI =- dr7I1 + A 2d72(2.53)
CI +C3 dt AB, dt
7B- IB2 = dR2 (2.54)
where C1 and C2 are expressed by Eqs. (2.31) and (2.33), and
C3 DL3A 2g (2.55)
Stating Eqs. (2.53) and (2.54) in the dimensionless form we obtain
o = g dB + AB2 dB2 (2.56)
C, + 3 dO ABI dO
B B2 = 7 [ 2L (2.57)
C2 dO
where o, O'1 and 'B2 are defined in Eqs. (2.18), (2.34) and (2.35), respectively. The solution of the system of Eqs. (2.56) and (2.57) is given below.




S Define the following constants
C C AB2
=a,- = b A, o=Re(e0)
C2 +C3 C2 ABI
2 Let
== Re e ReeEBB d r d=2 _r,2
?B1 ) = r? B2 = Re (e'(O -B2)) =r2; dO I dO
3 So the equations are reduced to
1= (ai + 1)r, + aAir2 (2.58)
0 = + (bi + 1)r2 (2.59)
4 Solve these equations by the matrix method.
J r2 \l bi + ) (2.60)
0 r2 -1 bi+1
5 Solving the above equations yields
-i(b-i)
= (2.61)
X
-1
r2 = (2.62)
X
X= (ab-1)-i(aA+a+b); X= (ab-1)+i(aA+a+b)
XX = (ab -1)2 + (aA + a + b)2
Re(r) b(b+a+aA)-(ab-1) m(r)= -b(ab-1)-(a+b+aA)
Re(r2) -(ab 1) m(r2) = -(a + b + aA)
Re(r,) =, Im(r,)=
The amplitudes (aB1 andaB2 ) of bays 1 and 2 are the magnitudes of the complex numbers r, and r2 and the phase lags are the corresponding angles:




B1 = Re(t2) + Im() (2.63)
'B1 = tan 1Km(rj) (2.64)
stRe(ri)
cB2 = Re(r2)2 + Im(r2)2 (2.65)
B2 tan 1 m(r2) (2.66)
Re(r2))
The velocities uc and Uc2 through inlets 1 and 2, respectively, are given by Eqs. (2.47) and (2.48), and uc3 through inlet 3 is obtained from
"c3 = 2max3 COS (0 v,3) (2.67)
where Umax3 is the maximum velocity through inlet 3 and Ev3 is the phase lag between velocity in inlet 3 and HW or LW in the ocean. Substituting for q, and Wi from Eqs. (2.18) and (2.34) into Eq. (2.49) and combining Eqs. (2.49) and (2.67) we get the required expression for Umax3. Then the phase lag Ev3 8 -Z12.
2.3.3 Three Inlets and Three Bays with One Inlet connected to Ocean.
This inlet bay system as defined in Figure.2.4 has three interconnected bays with inlets 2 and 4, while inlet 1 connects bay 1 to the ocean. The velocities in inlets 1 and 2 are given by Eqs. (2.24) and (2.26), respectively. The velocity in inlet 4 is given by Eq. (2.68):
24 = 1DL4 (B17 B3) (2.68)
where u4 = velocity through the inlet 4 and




1aBl
CDL4
+ken +k L4 (B1 -B3)max 4R4
with L4 = inlet length and R4 = hydraulic radius of inlet 4 channel.
The governing continuity equations are
d]2 druB3
Q1 = uc1Ac,1 = AB1 +AB2B2 +AB3 dt (2.69)
cit cit
Q2 = uz2A2 = AB2 dB2 (2.70)
dt
Q4 = uc4Ac4 = AB3 d1B3 (2.71)
dt
Q1, Q2, Q4 = discharges through inlets 1, 2 and 4
Acl, Ac2, Ac4 = flow cross-sectional areas at inlets 1, 2 and 4.
AB1, AB2, AB3= bay water surface areas.
Substituting the velocity expressions in the above equations we obtain
7o 1 dI+ AB2 dB2 A B3dB3 (2.72)
SnI C, dt A, dt A, dt
B 7B drB2 (2.73)
C2 L dt ]
B 1 -3 IdB3J (2.74)
C4 dt
where C1 and C2 are as expressed by Eqs. (2.31) and (2.33), and
4 CDL4A 2 (2.75)
AB3 BI




770
Figure 2.4 Three bays and three inlets with one inlet connecting to the ocean. Stating the above equations in the dimensionless form the desired solution is obtained by solving the following three equations:
i 7 dBl + AB2 d B2 + AB3 d B3 (2.76)
1lCL- Ri Roi1d
CL dO ABI dO ABI dO
ud3
B1 B2 7 (2.77)
C2 dO
B1-B3 B d (2.78)
C4 dO
where uo, 'B1 and 7B2 are defined by Eqs. (2.18), (2.34) and (2.35), respectively, and
B3 is
OB3 = B3 cos(O CB3) (2.79)
As before the above equations are solved by using complex numbers as follows:




S Define the following constants
C C C AR2 A
-=a, b, =c, AB2 -A, =B, =Re(e)
C C2 C4 ABI A BI
2 Let
3B, = Re (e( "B1) = r, 32 = Re(e'( B2)) = r2, 17B3 = (e i( B3 3
dym -r, d r2 d 3 =
dO dO dO
3 So the equations are reduced to
1= (ai +1)r + aAir2 +aBir3 (2.80)
0 -r, + (bi + 1)r2 + Or3 (2.81)
0 -r, + Or2 + (ci + 1)r3 (2.82)
4 Solving the equations by matrix method:
1 'r "ai +1 aAi aBi
= r2 bi+1 0 (2.83)
0 r3 -1 0 ci + 1)
yields
(c -i)(b -i)
r = 0(2.84)
X
-i c-i
r2 = (2.85)
X
-i(b i
r3 ) (2.86)
X
X = (ac+ab+bc+aBb+aAc- 1)+i(-aA+abc-a-b-c-aB) = (ac+ab+bc+aBb+aAc- 1) -i(-aA+abc-a-b-c-aB)
XX =(ac+ab+bc+aBb+aAc -1)2 +(-aA+abc-a-b-c-aB)2




b (1 + aBc+c2)+c2 (1+aAb)+a(bA+Bc)+1
Re(1) =
XX
-a(1+A+B+b2 +c2 + b2c2 +Bbc+Ac2 +Bb2 -bcB)
Im(r}) =
Re(r2) -ab aBb + aBc +1+c2 -abc2 AT
Re (r2) = X2
im(r2) -(a+b+aA+aB+aBbc + ac2 +bc2 +aAc2 ) AT
Re(r-) ac aAc + abA ab2c + b2 +1
Re (r;) =
AT
im(r) -(a+c+aA+aB+abcA+ab2 + cb2 + aBb2)
The amplitudes (aB1, aB2 and aB3) ofbayl, bays 2 and 3 are the magnitudes of the complex numbers rl, r2 and r3, and the corresponding phase lags are the angles of the complex numbers:
aB1 = Re()2 + Im()2 (2.87)
BI = tan Im(rj) (2.88)
sRe(ri)
aB2 = FRe(r2)2 + Im(r2)2 (2.89)
Im (J;r)
B2 -tan1 I1(r2 (2.90)
Re(r2)
aB3 = Re(r3)2 + Im(r3)2 (2.91)
-B3 -tan 1 (r3 (2.92)
Re(r3)




The velocities uc1 and uc2 through inlets 1 and 2, respectively, are given by Eqs. (2.47) and (2.48), and uc4 through inlet 4 is given by
Uc4 = Umax4 COs ( ,4 ) (2.93)
where Umax4 is the maximum velocity through inlet 4 and Ev4 is the corresponding phase lags between this velocity and HW or LW in the ocean. Substituting for qiB and q7B3 from Eqs. (2.34) and (2.79)into Eq. (2.68) and combining Eqs. (2.93) and (2.68) we get the desired expression for Umax4. Phase lag Ev4 = EB3-7Z2.
2.3.4 Four Inlets and Three Bays with Two Inlets Connected to Ocean.
This system as defined in Figure 2.5 has three interconnected bays with inlets 2 and 4, while and inlets 1 and 3 connect bay 1 to the ocean. The velocities in inlets 1, 2, 3 and 4 are given by Eqs. (2.24), (2.26), (2.49) and (2.68), respectively.
The governing continuity equations are written as follows.
Q1 +Q3 = 1clAc + U3A3 = AB dRB + AB2 dB2 d+ A B3 (2.94)
dt dt dt
Q2 = uzA2 = AB2 dB2 (2.95)
dt
Q4 = u4dA4 = AB3 drB3 (2.96)
dt
Next, substituting the velocity expressions in the above equations yields
7o BI = 1 [ d AB2 dB2 AB3 dB3 (2.97)
C1 +C3 dt AB1 dt AB dt
B1 7- B2 = Ic dB2 (2.98)
CI dt I




L1 3 77o
Figure 2.5 Three bays and four inlets, two inlets connect to ocean.
1 Idyls
7BI 7B3 = 'LBJ (2.99)
C4dt
where C1, C2, C3 and C4 are as expressed by Eqs. (2.31), (2.33), (2.55) and (2.75), respectively.
Now we may state the above equations in the dimensionless form as
o = c [ d + AB2 dB2 AB3 dB3 (
-o -s =+ (2.100)
C, +C3 dO ABI dO ABI dO
61 B2 7 d (2.101)
C2 dO
Bl On3 = (2.102)
C4 dO
where Go, O1, B2 and B3 are defined by Eqs. (2.18), (2.34), (2.35) and (2.79), respectively. These equations are solved as follows:




S Define the following constants
C C A A3
=a, = b, = c A, B3 = B, = Re(e)
CC +C3 2 4 A Ai B
2 Let
,B, = Re (e '(OB1) ) 1 B2 = Re (e B2 2 1 B3 = (e i( B3 3 ,
d i =r d i2 d ir3
dO 'dO dO 3
3 So the equations are reduced to
1= (ai +1)r +aAir2 +aBir3 (2.103)
0 = -r, + (bi + 1)r2 + Or3 (2.104)
0 =-r, +0r2 +(ci+)r3 (2.105)
4 Solve these equations by matrix method:
1 r "ai +1 aAi aBi
= r2 bi+1 0 (2.106)
0 r3 1 0 ci + 1)
5 Thus we obtain
(c- i)(b -)
r = ((2.107)
X
-i(c-i)
r2 = (2.108)
X
-i(b -i)
r3 = (2.109)
X
X = (ac+ab+bc+aBb+aAc- 1)+i(-aA+abc-a-b-c-aB) X= (ac+ab+bc+aBb+aAc- 1)-i(-aA+abc-a-b-c-aB)




XX = (ac + ab + bc + aBb + aAc -1)2 +(-aA+abc-a-b-c-aB)2
b2(1 +aBc +c2 + c2 (1+aAb)+a(bA+Bc)+1
Re(rj) =
XX
-a(i +A+B+b2 +c2 + bc2 +Bbc + Ac2 +Bb2 bcB)
Im(r}) =
Re(r2) -ab aBb + aBc +1+c2 -abc2
Re (r2) = 7
im(r2) -(a + b + aA + aB + aBbc + ac2 + bc2 + aAc2 ) AT
Re(r-) ac aAc + abA abc + b2 +1
Re (r;) =
AT
-(a+c+aA+aB+abcA+ab2 +cb2 +aBb2)
Im(r)) = 7
The amplitudes (aB1, aB2 and aB3) ofbaysl, 2 and 3 are the magnitudes of the complex numbers rl, r2 and r3, and the corresponding phase lags are the angles of the complex numbers:
aB1 = Re()2 + Im()2 (2.110)
'B1 = tan lIm(Q) (2.111)
sRe(zi))
aB2 = Re(r2 ) + Im(r)2 (2.112)
B2 tan 1 (m(r2- (2.113)
Re (r2)
aB3 = 4Re(r)2 + Im(r)2 (2.114)




28
Im (3 )
-B3 -tan 1 (2.115)
Re(r3) Then the velocities ucl, uc2, uc3 and uc4 are given by Eqs. (2.47), (2.48), (2.67) and (2.93), respectively.




CHAPTER 3
STABILITY OF MULTIPLE INLET-BAY SYSTEMS
3.1 Stability Problem Definition
An inlet is considered stable when after a small change the cross-sectional area
returns to its equilibrium value. Each inlet is subject to two opposing forces, the waves on one hand, which tend to push sand into the inlet, and the tidal current on the other hand, which tries to carry sand out of the channel back to the sea or the bay. The size of the inlet and its stability are determined by the relative strengths of these two opposing forces.
3.2 Stability Criteria
Inlet stability as considered here basically deals with the equilibrium between the inlet cross-section area and the hydraulic environment. The pertinent parameters are the actual tide-maximum bottom shear stress i and the equilibrium shear stress i eq. The equilibrium shear stress is defined as the bottom stress induced by the tidal current required to flush-out sediment carried into the inlet. When i equals i eq the inlet is considered to be in equilibrium. When i is larger than i eq the inlet is in the scouring mode, and when i is smaller i eq the inlet is in the shoaling mode. The value of equilibrium shear stress depends on the waves and associated littoral drift and sediment. Considering inlets at equilibrium on various coasts, Bruun (1978) found the value of equilibrium stress in fairly narrow range:
3.5Pa < q < 5.5Pa




The value of actual shear stress is obtained from
Z PF~max IUmax 1(3.1) where F is the friction coefficient, a function of bottom roughness, k, u,,, is the maximum tidal velocity in the inlet, a function of area and length of the inlet, as discussed in Chapter 2 and p is the fluid density. Therefore, i can be written as a function of following form
where m is the sum of entrance and exit losses. The plotted function i()is called a closure curve, as shown in Figure 3. 1. It is clear from the calculation shown in the Appendix B that i is a strong function of A and a weak function of L, m, k. The strong dependence of i on A explains why inlets adjust to changes in the hydraulic environment primarily via a change in the cross-sectional area.
3.2.1 Stability Analysis for One-Inlet Bay System
Making use of the Escoffier (1940) diagram, Figure 3.2, one can study the
response of the inlet to change in area. In the Figure, A, and A,, both represent equilibrium flow areas, with A, representing unstable equilibrium and A,, representing stable equilibrium. If the inlet cross-sectional area A were reduced but remained larger than A,, the actual shear stress would be larger than the equilibrium shear stress and A would return to the value A,,. If the cross-sectional area were reduced below A,, the shear stress would become lower than its equilibrium value and the inlet would close. If A became larger than A,,, the actual shear stress would become larger than equilibrium value and A would return to A,,. Note that the equilibrium condition only exists if the line z^ z^eg

intersects the closure curve i=i()




7->
A
Figure 3.1 Closure curves (source: van de Kreeke, 1985)

A, A,
Unstable Stable A
equilibrium equilibrium
Figure 3.2 Escoffier diagram (source: van de Kreeke, 1985)




The equilibrium interval for the stable cross-section, AH, ranges from A, to infinity.
3.2.2 Stability of Two Inlets in a Bay
Similar to a single inlet, it can be shown that shear stresses ii and 2 for two
inlets in a bay strongly depend on A, and A2 and are weak functions of (L1, kj, in1, L2, kc2, Mn2). The functions il (A1, 4,) and i2 (A1, 4,) are referred to as a closure surfaces. The shape of i2 (A,A,) is qualitatively illustrated in Figure 3.3. For a constant A,, the curve il (A,) is similar to the closure curve shown in Figure 3. 1. The value of i2 decreases with increasing A,.
With the help of a closure surface in Figure 3.3, the loci of (A,, A2) for which
'2 ', Ii2te'q+1 1t2 = 'q 1 are plotted in Figure 3.4. The locus oft2 = 'q is
referred to as the equilibrium flow curve for Inlet 2. Using the same reasoning as for a single inlet and assuming that the cross-sectional area of Inlet 1 is constant, it follows that if A2 =A,, Inlet 2 will shoal and close; if A2 =AH, Inlet 2 will scour until the crosssectional area attains a value A5, and if A2 =A,,,, Inlet 2 will shoal until the cross-sectional area attains the value A,.
The locus of (A,, A2) for which Inlet 2 has a stable equilibrium flow area is the enhanced (by a thicker line) part of the equilibrium flow curve for Inlet 2. Similarly, the locus of (A,, A2) for which Inlet 1 has a stable equilibrium flow area is the enhanced part of the equilibrium flow curve for Inlet 1. The condition for the existence of stable equilibrium flow areas for both Inlet 1 and Inlet 2 is that the enhanced parts of the equilibrium flow curves intersect. The common equilibrium interval of the two is




,T2 =eg

Figure 3.3 Closure surfaces (source: van de Kreeke, 1985) A1 A,, A, A,,, A2
t II
i o / Te /
%It 7-eq
A,/
Figure 3.4 Equilibrium flow curve for Inlet 2 (source: van de Kreeke, 1985)




A2 A2
2
2
(a) (b)
A2 A2
2 2
c (d)
Figure 3.5 Possible configurations of equilibrium flow curves for a two-inlet bay system.
Stable equilibrium flow area is represented by 0 and unstable equilibrium is represented by Q The hatched area in (a) represents the domain of the stable
equilibrium flow area (source: van de Kreeke, 1990)
represented by the hatched rectangle in Figure 3.5 (a). The general shapes of the equilibrium flow curves and their relative positions in the (A,, A2) plane are presented in Figure 3.5. The detailed explanations to the Figure 3.5 are given in Appendix D.
3.3 Stability Analysis with the Linearized Model Due to the complex nature of sediment transport by waves and currents it is difficult to carry out an accurate analysis of the stability of single or multiple inlet




systems. We will therefore attempt to carry out an approximate analysis based on the van de Kreeke (1990) linearized lumped parameter model.
The justification for use of simple model is that for purpose of this study the stability analysis serves to illustrate a concept rather than to provide exact numerical results. Accurate numerical values can only be obtained by using a full-fledged twodimension tidal model to describe the hydrodynamics of the bay.
3.3.1 Linearized lumped parameter model for N Inlets in a Bay
The basic assumptions of the Linearized lumped parameter model are as follows: I The linearized model assumes that the ocean tide and the velocity are simple
harmonic functions.
2 The water level in the bay fluctuates uniformly and the bay surface area remains
constant.
3 Hydrostatic pressure, and shear stress distribution along the wetted perimeter of
the inlet cross-section is uniform.
4 For a given bay area and inlet characteristics, the tidal amplitude and/or tidal
frequency must be sufficiently large for equilibrium to exist. Similarly, larger the
littoral drift due to waves, larger the equilibrium shear stress required to balance it
and therefore the equilibrium velocity, the larger the required bay surface area,
tidal amplitude and the tidal frequency or, in other words, Eq. (3.17) and Eq.
(3.19) must be satisfied for the existence of equilibrium areas.
5 There is no fresh water discharge in the bays.
6 In a shallow bay the effect of dissipation of tidal energy cannot be ignored,
especially if the bay is large.
Inlet flow dynamics of the flow in the inlets are governed by the longitudinal pressure gradient and the bottom shear stress, van de Kreeke (1967),
0 1 OP Z- (3.2)
p cx p h
in which is the pressure, p is the water density, h is the depth and Z- is the bottom shear stress. This stress is related to the depth mean velocity u




r =pFulIu (3.3)
where F fI8, is the friction coefficient. Integration of Eq. (3.2) (with respect to the longitudinal x-coordinate) between the ocean and the bay yields (van de Kreeke 1988).
Ui IUi 2gR (q, q") (3.4)
mR + 2FLj,
In Eq. (3.4), ui refers to the cross-sectional mean velocity of the ith inlet, g is the acceleration due to gravity, m, is the sum of exit and entrance losses, R, is the hydraulic radius of the inlet, Li is the length of the inlet, q, is the ocean tide, and B is the bay tide. The velocity u, is positive when going from ocean to bay.
Assuming the bay surface area to fluctuate uniformly, flow continuity can be expressed as
N u = AB dB (3.5)
1 1 d t
in which Ai is the cross-sectional area, AR is the bay surface area and t is time.
Considering ui to be a simple harmonic function of t, Eq. (3.4) is linearized as shown in Appendix B to yield
8 -- max2gi = (17, U17B) (3.6)
3;r m,] + 2_1
in which Umaxi is the amplitude of the current velocity in the ith inlet. It follows from Eq. (3.5) and Eq. (3.6) that for a simple harmonic ocean tide (in complex notation)
r7o (t) = aoe j' (3.7)
and assuming A, and AB to be constant, we obtain
Ui = UmaxIej(Jt+ ) (3.8)




where the phase angle E, is considered to be the same for all inlets. Differentiating Eq. (3.6) with respect to t, eliminating dBl/dt between Eq. (3.5) and Eq. (3.6), and making use of the expressions for u, and r7o yields an equation for ui and E,
N 1 8
"maiA ABBu Zmauij = ABaojue J (3.9)
2g 3ff
in which the dimensionless resistance factor Bi is defined as
A2=[ (3.10)
where B, is the function of A,. Now, equating the real and imaginary parts of Eq. (3.9) and eliminating the phase angle c yields the equation for Umaxi
L 2 L+ [ABCl2 B 4xi =[ABaoC]I L umaxAd (3.1 1)
For equilibrium flow i, = Teq,. Using linearized version in Eq. (3.6) and Eq. (3.3), the equilibrium velocity can be written as
e req(3.12)
8/ 3ffp17
where the approximate value of seqi can be taken from Mehta and Christensen (1983). For equilibrium flow areas lmaxi = .maxeqiI substituting this value Eq. (3.11) becomes:
L "2 L+ [B']2B4iUmax eqi "[BaOJ] L max qi~i (3.13)
When the maximum tidal velocity in all the inlets equals the corresponding equilibrium value, i.e., Umax, = "max eq.i for i 1,2 ........... N, the difference between the bay and the
ocean tides becomes constant. So from Eq. (3.4) it follows that




ByU2 maxeql B2 U2 maxeq2 U 2 max Uqi B N U2 maxeqN (3.14) Eq. (3.13) and Eq. (3.14) constitute a set of N simultaneous equations with Nunknowns [A1, A2...,AN]. In general, more than one set of equilibrium flow areas [A1, A2... ,AN] will satisfy these equations. Since the dimensionless resistance factor Bi is a function of A. Therefore, whether for a given ocean tide (a, c-) and bay surface area (AB), Eq. (3.13) and Eq. (3.14) yield sets of solutions [A1, A2 .... AN] that are real and positive depends on the particular form of R, f(A).
The function R, f(Ad plays an important role in the hydrodynamic efficiency of an inlet. For a given head difference, exit and entrance loss coefficients, friction factor and inlet length, the maximum tidal velocity increases with the increasing value of R, see Eq. (3.4). Therefore, larger the value of R, for a given value of A, larger the discharge. For a rectangular channel, =- and for triangular channel R = a1< (See Appendix B).
Analytical solutions to equation Eq. (3.13) and Eq. (3.14) can be found by restricting attention to the friction-dominated flow in the inlets, i.e. m=0
From Eq. (3.10) with m = 0, we obtain
B 2I Li (3.15)
For rectangular inlets, substituting R in Eq. (3.15) and then in Eq. (3.13) JWj

and Eq. (3.14) we get




A 2
(LRu2eqi2
(ABao) ABao)4 ~ 2 I2g (ABg)2 (FLu3eq1 ...NLNWNu3N)2 (3.16)
2 uleq N... F LeqN)2
When any A, (from Eq. (3.16)) is known, the cross-sectional areas of the other inlets follow from Eq. (3.14), with B, given by Eq. (3.15), provided that
ABpa2o> 2 I L u3eq1 -... FL 3eqN ) (3.17)
4 >2( ) )FIII gq U
This is a quadratic equation in A,2 for which we have two sets of real and positive roots and two sets of complex roots.
For the triangular cross-section, R = af substituting this in Eq. (3.13) and Eq. (3.14) we get,
!ureql 2 + ....+ N A3 (ABaoC) A +
(3.18)
8~ (Ap)2 [L 4 = 0
3[_ C g eq,
in which sets of A, are given by Eq. (3.18) (as we have two real and positive solution for A;). When any A, is known, the cross-sectional areas of the other (N-1) inlets follow from Eq. (3.14) with B given by Eq. (3.15). One root of Eq. (3.18) is always negative. The other two are real and positive roots provided that
Aa3 3 82 5 FlL 2 5 FLN 2
A a3 > ueq +.N. Ne (3.19)
2 3 l a2ga ag




The above stability concept, when applied to a multiple-bay inlet system,
becomes complicated because the loci of the set of the values [A,, A2.... .AN] for which the tidal maximum of the bottom shear stress equals the equilibrium stress, are rather complicated surfaces and make it difficult to determine whether inlets are in a scouring mode or shoaling mode. With some simplifying assumptions, the stability analysis for a multiple-inlet system can be reduced to that for a two-inlet system. This is considered next in the context of the St. Andrew Bay system.
3.4 Application to St. Andrew Bay System In the above model if N=2, the model can be applied to the two inlet system. The equilibrium flow curves for Inlet 1 and Inlet 2 are calculated from Eq. (3.11) with u 11eq. The equilibrium flow areas are given by the solution of Eq. (3.16) for rectangular inlet and Eq. (3.18) for triangular cross-section. Figure 3.6 illustrates the equilibrium flow curve. A line can be drawn passing from the intersection of two equilibrium flow areas. Above the line B1 >B2 and therefore U1 112. Figure 3.6 can be elaborated as follows:
1 When the point defined by the actual cross-sectional areas [A,, A2] is located in
the vertically hatched zone or anywhere outside the curves, (Zone-i), both inlets
close.
2 When the point is located in the crosshatched zone, (Zone-2), Inlet 1 will remain
open and Inlet 2 will close.
3 When the point is located in the diagonally hatched zone, (Zone-3), Inlet 1 will
close and Inlet 2 will remain open.
4 Finally, when the point is located in the blank zone, (Zone-4), one inlet will close
and the other will remain open. However, in this case which one closes depends
on the relative rates of scouring and or/shoaling.




The St. Andrew Bay system is similar to the case of two inlets in a bay. In reality there are three interconnected bays, but only one is connected with the Gulf So there is no forcing due to ocean tide from the other two bays. Thus, all the bays collectively behave as if there is only one bay connected by two inlets. So the linear model for N inlets can be applied to the St. Andrew system, where N = 2. The development of equilibrium curves for this case is discussed in Chapter 5.
Zone-4
F b ofInlet
Zone- 1<--- A,1
Figure 3.6 Equilibrium flow curves for two inlets in a bay (source: van de Kreeke, 1990)




CHAPTER 4
APPLICATION TO ST. ANDREW BAY COMPLEX AND ENTRANCES
4.1 Description of Study Area
St. Andrew Bay is located in Bay County on the Gulf of Mexico coast of Florida's panhandle. It is part of a three-bay and two-inlet complex. One of these inlets is St. Andrew Bay Entrance and the other is East Pass, which are connected to St. Andrew Bay on one side and the Gulf on the other. The other two bays are West Bay and the East Bay, which connect to St. Andrew Bay, as shown in the Figure 4.1 Note that West Bay as shown also includes a portion called North Bay. Prior to 1934, East Pass was the natural connection between St. Andrew Bay and the Gulf. In 1934, St. Andrew Bay Entrance (Figure. 4.2) was constructed I I km west of East Pass through the barrier island by the federal government to provide a direct access between the Gulf and Panama City. The entrance has since been maintained by the U. S Army Corps of Engineers (USACE), Mobile District. The St. Andrew Bay State Recreational Area is located on both sides of this entrance, which has two jetties 430 m apart to prevent the closure of the inlet.
The interior shoreline of the entrance has continually eroded since it's opening. An environmentally sensitive fresh water lake located in the St. Andrew Bay State Recreational Area is vulnerable to the shoreline erosion and USACE has placed dredged soil to mitigate shoreline erosion.
East Pass finally closed in the 1998, due to the long-term effect of the opening of St. Andrew Entrance. In December 2001, a new East Pass was opened (Figure 4.3), and the effect of this new inlet is presently being monitored over the entire system.




Figure 4.1 Map showing the three bays and two inlets and bathymetry of the study area.
Dots show location of tide stations.

Figure 4.2 Aerial view of St. Andrew Bay Entrance in 1993. Jetties are -430 m apart.




Figure 4.3 East Pass channel before it's opening in December 2001. Plan view (preconstruction) design geometry and then anticipated current measurement transects are shown. The dots show the new cross-section (source: Jain et al.,
2002)
4.2 Summary of Field Data
Three hydrographic surveys were done by the University of Florida's Department of Civil and Coastal Engineering in the years 2001 and 2002. Figure 4.4 shows the bathymetry of St. Andrew Bay Entrance and the different cross-sections measured during the surveys. Cross-sections A-1, A-2 and B-i, B-2 were measured in September 2001, A'-1, A'-2, B'-1, B'-2, C'-1, C'-2, in December 2001, and D-1, D-2 in March 2002. Flow discharges, vertical velocity profiles and tide were also recorded. The tide gage (in the September 2001 survey only) was located in waters (Grand Lagoon) close to the entrance channel. The discharge and velocity data was measured with a vessel-mounted Acoustic Doppler Current Profiler, or ADCP (Workhorse 1200 kHz, RD Instruments, San Diego, CA), and the tide with an ultrasonic recorder (Model #220, Infinities USA, Daytona Beach, FL). The coordinates of the cross-section end-points are given in Table 4.1.




Table 4.1 Locations of St. Andrew Bay channel cross-sections Section Side Latitude Longitude Northing Easting Date
A A-i 3007.70 -8543.36 412452.62 1613441.90 09/18/01
A A-2 30.07.44 -85 43.28 410875.80 1613857.60 09/18/01
B B-i 3007.35 -8543.91 410315.83 1610524.00 09/18/01
B B-2 3007.17 -8543.71 409240.00 1611584.60 09/18/01
A' A'- 1 3007.18 -8543.72 409256.63 1611563.75 12/18/01
A' A'-2 3007.40 -8543.91 410626.10 1610534.09 12/18/01
B' B'- 1 3007.43 -8543.30 410766.60 1613757.91 12/18/01
B' B'-2 3007.68 -8543.44 412309.71 1613034.11 12/18/01
C' C'- 1 3007.06 -8543.90 408542.02 1610606.43 12/18/01
C' C'-2 3007.27 -8544.01 409822.96 1610030.59 12/18/01
D D-1 3007.42 -8543 .32 410714.20 1613635.15 03/28/02
D D-2 3007.65 -8543. 58 412134.85 1612294.58 03/28/02
Measurements were also taken at the new East Pass after it's reopening in
December 2001. The locations of the East Pass cross-section coordinate end points are
given in Table 4.2. Flow cross-section and vertical velocity profiles were measured along
cross-section E in December 2001 and F in March 2002.
Table 4.2 Locations of East Pass channel cross-sections ______Section Side Latitude Longitude Northing Easting Date
E E-i 3003.78 -85 37.07 388325.56 1646376.03 12/19/01
E E-2 3003.79 -85 37.12 388371.27 1646103.36 12/19/01
F F-i 3003.78 -85 3707 388325.55 1646376.03 03/27/02
F F-2 3003 79 -85 37 12 388371.26 1646103.35 03/27/02




102000
411000.008'
A D-1
410000.00 cA
409000.004 A8
C.'
1610000.00 1611000.00 161200000 1613000,00 16140000 1615000.00 Figure 4.4 St. Andrew Bay Entrance bathymetry and current measurement cross-sections.
Depths are in feet below MLLW (source: Jain et al. 2002)
4.2.1 Bathymetry
The bathymetry of the study area is shown in Figure 4.1. During the hydrographic surveys the bottom depth was measured by the ADCP at all cross-sections shown in Figure 4.4. These have been compared with a bathymetric survey of 2000. Figures. 4.5 and 4.6 are example of measurements along cross-sections A and F, respectively. The trends in the two sets of depths are qualitatively (although not entirely) comparable. Areas, mean depths and widths are summarized in Table 4.3. Table 4.3 Cross-section area, mean depths and width Section Cross-section Area (m2) Width (m) Mean Depth (m)
A 6250 493 11.0
B 6600 457 10.6
A' 5210 525 10.0
B' 5640 544 11.0
C' 5220 425 11.5
D 5970 528 11.9
E 255 109 3.0
F 300 85 2.5




47
Batymetry side-A Side-A-1 Side- A-2
0
0 100 200 300 400 500 600
-2
-4
-6
-8
a
S-10
-12 -14 -16 -18
S-4--ADCP ---Bathymetry chart Distance (m)
Figure 4.5 Cross-section A in St. Andrew Bay Entrance measured and compared with
2000 bathymetry. Distance is measured from point A-1. The datum is mean
tide level (source: Jain and Mehta, 2001)
Bottom Contour F-1 F-2
0
-0.5 0 3.5 11 18.2 24 31.5 37 46.6 59 72.6 75 84.5
-1
E -1.5
-
S-2.5
-3
-3.5
-4
Distance (m) from F-1
-4- ADCP
Figure 4.6 Cross-section F in East Pass measured by ADCP. Distance is measured from
point F-1. The datum is mean tide level (source: Jain and Mehta, 2002)




4.2.2 Tides
As noted, tide was measured in September 2001 in Grand Lagoon close to the entrance channel, at Lat: 30 07.9667, Long: -85 43.6667. Tide variation in the channel was compared with the predicted National Ocean Service (NOS) tide at St. Andrew Bay channel with reference station at Pensacola after applying the correction factors for the range and the lag. The measured tide is shown in Figure 4.7 and the corresponding NOS tide in Figure 4.8. Both show general similarities, although the measured one should be deemed more accurate. The data indicate a weak semi-diurnal signature with a range variation of 0. 11 to 0. 18 m. In the month of December and March no tides were measured, only the NOS tides were reported using the tide at Pensacola; see Figure 4.9 and Figure 4. 10.
For East Pass the same tide was assumed as for St. Andrew Bay Entrance. Five other NOS stations are also located in the study area as shown in Figure 4. 1. The ranges of tides for September 200 1, December 2001 and March 2002 at these stations are given in the Table 4.4. These tides were found by applying correction factors for the range and for the lag (see Appendix Q. The Gulf tidal range, 2a, was obtained by applying an amplitude correction factor to the tide measured at the Grand Lagoon gauge (see calculations in Appendix Q. Semi-diurnal tides were reported in September 2001 with the tidal period of 12.42 h. The tides in December 2001 were of mixed nature with a period of approximately 18 h. In contrast, diurnal tides were reported in March 2002 with the period of 25.82 h. The approximate tide level in each bay was then found by weighted-averaging the tide over the number of stations in that bay. The phase lag between the tides of all the stations were calculated by plotting all the tides in Figure
4. 10, and the results are summarized in Table 4.5.




49
Tide at St Andrew Bay Entrance
-Tide at St Andrew Bay Entrance
0.6 0.5
- 0.4
3 0.3 0.2
0.1
0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O o 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Time (hrs)
0 -~ -~ -~ -~ N~ 0 0~ 0~ 0
09/18/01 Time (hrs) 09/19/01
Y V
Figure 4.7 Measured tide in Grand Lagoon on Septemberl8-19, 2001. The datum is
MLLW (source: Jain and Mehta, 2001)
NOS Tides Tides
0.45 0.4
E 0.35
0.3
> 0.25
0.2
S0.15
0.1 0.05
0
Time (hr:min) K 09/18/01 09/19/01
Y Y
Figure 4.8 NOS predicted tide at St. Andrew Bay Entrance on Septemberl8-19, 2001;
reference station is Pensacola. The datum is MLLW.




50
NOS Tides

0.5 0.4
E 0.3 > 0.2
a1)
w
-j
S0.1
-0.1
-0.2
- Tides

CD) L0 0 t 0) CD '.O O0 0 Cc 0) D.0
o i to C~ o CN 0 uNLOOCD o C4 Co)
0 ,N "0 C0D N4 t) NO 0N
Time (hr:min) 12/19/01
Figure .9 NOSprediced8tid inS.2nre/a1EtaneonDcmbr181,101

Figure 4.9 NOS predicted tide in St. Andrew Bay Entrance on December 18-19, 2001;
reference station is Pensacola. The datum is MLLW.
Tides in all the Stations

045 --- Gulf and Channel Entrance --- Laird Bayou
-X- Parker -*- Lynn Haven

-0-- Panama City
---West Bay Creek

0.35
S03 S0.25 ,
, 0.2 0.15
01 0.05

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
M D D D D 0 D 00 M D2 0 M 0
Time (hrs)
Figure 4.10 Tide at all selected NOS stations in March 2002.




Table 4.4 Tidal ranges in September 2001, December 2001 and March 2002. so Station Name September December March
No Range (in) Range (in) Rane()
1 Gulf of Mexico ("Ocean" tide) 0.216 0.572 0.425
2 Laird Bayou, East Bay 0.236 0.624 0.465
3 Parker, East Bay 0.236 0.624 0.465
4 Lynn Haven North Bay 0.236 0.624 0.465
5 Panama City, St. Andrew Bay 0.203 0.53 5 0.3 97
6 Channel Entrance, St. Andrew Bay 0.197 0.520 0.386
7 West Bay Creek 0.23 6 0.624 0.465
Table 4.5 Phase lags between the stations and the ocean tide. S No IStations Time Lag
1 Gulf of Mexico ("Ocean" tide) 0 h
2 Laird Bayou, East Bay + 2h
3 Parker, East Bay +2 h
4 Lynn Haven North Bay +2 h
5 Panama City, St. Andrew Bay +1 h
6 Channel Entrance, St. Andrew Bay +1 min
7 WestfBay Creek + 3h
4.2.3 Current and Discharge
Currents and discharges were measured with the ADCP at all the six crosssections in St. Andrew Bay Entrance (Figure 4.4) and at two cross-sections in East Pass
(Figure.4.3). The detailed velocity and discharge curves are shown in Jain and Mehta
(2001), Jain et al. (2002) and Jain and Mehta (2002). The measurements are summarized
in the Table 4.6.
From Table 4.6 it is observed that the average peak velocity in St. Andrew Bay
channel was approximately 0.63 m/s (at or close to the throat section) and at East Pass it
was approximately 0.50 m/s. The peak discharge value at St. Andrew was 4200 M /s and
at East Pass it was 139 M3/S.




Table 4.6 Characteristic peak velocity and discharge values Quantity Velocity (m/s) Discharge (m3/s)
Cross-section Peak Flood Peak Ebb Peak Flood Peak Ebb
A 0.63 -0.62 4200 3620
B 0.45 -0.34 2980 2250
A' 0.68 -0.69 3620 3920
B' 0.69 -0.66 4061 3876
C' 0.67 -0.77 3480 3750
D 0.42 -0.49 2509 2777
E 0.51 -0.49 139 165
F 0.43 -0.38 114 101
4.3 Tidal Prism
Tidal prism is the volume of water that enters the bay during flood flow. Tidal prism for St. Andrew Bay system was calculated using the approximate formula
P QmT (4.1)
where Qm is the peak discharge (Table 4.6), Tis the tidal period (12.42 hrs for September 2001, 18 hrs for December 2001 and 25.82 hrs for March 2002) and the coefficient CK =
0.86 (Keulegan, 1967). This tidal prism was compared with the O'Brien (1969) relationship of Eq. (4.2), where A, is the throat area, P the tidal prism on the spring range for sandy inlets in equilibrium, and a and b are the constants:
A, = a Pb (4.2)
For inlets with two jetties, a = 7.49x10-4 and b = 0.86 (Jarrett, 1976). And for
inlets without jetty (East Pass), a = 3.83x105 and b = 1.03. The values of the tidal prism are summarized in Table 4.7. Spring ranges are reported in Table 4.4.
It should be noted that the prism values from the O'Brien relationship are mere estimates.




Table 4.7 Flood and ebb tidal prisms
Quantity Prism (m3) from peak Prism (m3) from O'Brien
discharge
Cross-sections Flood Ebb Peak Flood Peak Ebb
A 7.0x1 07 6.0x1 07 11.4x107 10.3x1 07
A' 8.6x107 9.4x107 09.0x107 10.4x107
D 8.6x107 9.4x107 10.0x107 09.7x107
E 3.3x106 3.9x106 03.8x106 04.6x106
F 3.9x106 3.5x106 03.6x106 03.6x106




CHAPTER 5
RESULTS AND DISCUSSION
5.1 Introduction
There are two aspects of this chapter, one dealing with the hydraulics of the St.
Andrew Bay system and the other with its stability. The linearized approach developed in Chapter 2 is used to examine the hydraulics of St. Andrew Bay under different conditions. The model is run as one-inlet/one-bay system for both September 2001 and March 2002. It is also run as a three-inlets/three-bays system in September 2001 when East Pass was closed, and as a three-bays/four-inlets system when East Pass was open in March 2002. Hydraulic parameters related to tides and currents thus obtained are then compared with values from the hydrographic surveys done in September 2001 and March 2002.
In contrast to hydraulics, the (linearized lumped paramter model) inlet stability model developed in Chapter 3 is applied only to St. Andrew Bay. A qualitative approach is developed to discuss the results and graphs have been plotted to show stability variation.
5.2 Hydraulics of St. Andrew Bay
The solution of equations for the linear model, derived in Chapter 2, forms the basis of calculation of the hydraulic parameters characterizing the system. One begins with the basic model of one-inlet (St. Andrew Bay Entrance) and one-bay (St. Andrew Bay) system, when East Pass was closed. As noted the model is then extended to the complete system of three bays (St. Andrew Bay, East Bay and North + West Bays) and




three inlets when East Pass was closed in September 2001, and finally as three bays and
four inlets when East Pass was open in March 2002.
5.2.1 Solution of Equations
The solutions of the relevant hydraulic equations are given in Chapter 2. A Matlab
program (see Appendix A) was developed to solve the one-inlet bay system as well as the
multiple-inlet bay system. The input and output parameters for each system are listed in
the tabular form.
5.2.1.1 One-inlet one-bay system
The one-inlet one-bay system is based on solving Eq. (5.1):
B 7 I (5.1)
CL dO]
The required input and output parameters for this case are given in Table 5.1.
Table 5.1 List of input and output parameters for one-inlet one-bay model. Input Parameters
a, Ocean tide amplitude (Gulf of Mexico)
T Time period of tide
aBl Bay 1 tide amplitude (St. Andrew Bay)
ABI Bay 1 surface area
L, Length of inlet 1 (St. Andrew Bay Entrance)
R, Hydraulic radius of inlet 1
A,, Inlet 1 cross-section area
k Entrance and exit losses
,f Friction factor
(/o 17)max Maximum ocean-bay tide difference
Output Parameters
r/1 Bay 1 tide
aBl Bay 1 tide amplitude
B1 Phase difference between bay 1 and ocean tides
Umaxl Maximum velocity through Inlet 1
Evi Phase difference between velocity in Inletl and ocean tide
(/o IJ/B)max Maximum ocean-bay tide difference




5.2.1.2 Three inlets and three bays with one inlet connected to ocean
This system is based on solving Eq. (5.2), Eq. (5.3) and Eq. (5.4):
-i= [d Bl ,ARB2 dB2 ARB3 d B37
C1 L A RI dO ABI dOJ

(5.2) (5.3)

' M 72 = c [d OB a
2 dW-

7B1 3 c4L d3 (5.4)
The required input and output parameters for this case are given in Table 5.2 Table 5.2 List of input and output parameters for the three inlets and three bays model.

Input Parameters
a, Ocean Tide amplitude (Gulf of Mexico)
T Time period of the tide
aB1 Bay 1 tide amplitude (St. Andrew Bay)
aB2 Bay 2 tide amplitude (East Bay)
aB3 Bay 3 tide amplitude (West Bay)
AB1 Bay 1 surface area
AB2 Bay 2 surface area
AB3 Bay 3 surface area
L, Length of inlet 1 (St. Andrew Bay Entrance)
R, Hydraulic radius of inlet 1
A,, Inlet 1 cross-section area
L2 Length of inlet 2 (connecting East Bay and St. Andrew Bay)
R2 Hydraulic radius of inlet 2
A,2 Inlet 2 cross-section area
L4 Length of inlet 4 (connecting West Bay and St. Andrew Bay)
R4 Hydraulic radius of inlet 4
A4 Inlet 4 cross-section area
k Entrance and exit losses
,f Friction factor
(77o -U 1) Maximum ocean-bay tide difference
(17B1 -17B2) max Maximum Bay 1 and Bay 2 tide difference (17B1 -173) Maximum Bay 1 and Bay 3 tide difference

)




Table 5.2 (continued)
Output Parameters
77B1 Bay 1 tide
aBl Bay 1 tide amplitude
EB1 Phase lag between bay 1 and ocean tide
77B2 Bay 2 tide
aB2 Bay 2 tide amplitude
EB2 Phase lag between bay 2 and ocean tide
77B3 Bay 3 tide
aB3 Bay 3 tide amplitude
EB3 Phase lag between bay 3 and ocean tide
UMax1 Maximum velocity through Inlet 1
Evl Phase difference between velocity of Inlet 1 and the ocean tide
Umx2 Maximum velocity through Inlet 2
Ev2 Phase difference between velocity of Inlet 2 and the ocean tide
Uma4 Maximum velocity through inlet 4
Ev4 Phase difference between velocity of Inlet 4 and the ocean tide
(7o -17M) max Maximum ocean-bay tide difference
(17B1 -17B2) max Maximum Bay 1 and Bay 2 tide difference (17B1 -17B3) m. Maximum Bay 1 and Bay 3 tide difference

5.2.1.3 Three inlets and three bays with two inlets connected to ocean
This system is based on solving Eq. (5.5), Eq. (5.6) and Eq. (5.7):
B = c [ + d Bl + AB2 dB2 + A B3 dB3
C, +c L, dO ABI dO ABI dO]

o- Bd B27B1UB2 c2LdO
- Bd -B3 B1- U3 =C4 dO

(5.5)
(5.6) (5.7)

The required input and output parameters for this case are given in Table 5.3.




Table 5.3 List of Input and Output Parameters for the four inlets and three bays model.
Input Parameters
a, Ocean Tide Amplitude (Gulf of Mexico)
T Time period of the tide
aBl Bay 1 tide amplitude (St. Andrew Bay)
aB2 Bay 2 tide amplitude (East Bay)
aB3 Bay 3 tide amplitude (West Bay)
AB1 Bay 1 surface area
AB2 Bay 2 surface area
AB3 Bay 3 surface area
L, Length of inlet 1 (St. Andrew Bay Entrance)
R, Radius of inlet 1
A,, Inlet 1 cross-section area
L2 Length of inlet 2 (connecting East Bay and St. Andrew Bay)
R2 Radius of inlet 2
A,2 Inlet 2 cross-section area
L3 Length of inlet 3 (East Pass)
R3 Radius of inlet 3
A,3 Inlet 3 cross-section area
L4 Length of inlet 4 (connecting West Bay and St. Andrew Bay)
R4 Radius of inlet 4
A4 Inlet 4 cross-section area
k Entrance and exit losses
.f Friction factor
(77o -U7l) max Maximum ocean-bay tide difference
(17M -17B2) max Maximum Bay 1 and Bay 2 tide difference (17M -17B3) max Maximum Bay 1 and Bay 3 tide difference Output Parameters
r7i Bay 1 tide
aBl Bay 1 tide amplitude
EB1 Phase lag between bay 1 and ocean tide
17B2 Bay 2 tide
aB2 Bay 2 tide amplitude
EB2 Phase lag between bay 2 and ocean tide
17B3 Bay 3 tide
aB3 Bay3 tide amplitude
8B3 Phase lag between bay 3 and ocean tide
UMaX1 Maximum velocity through Inlet 1
Ev1 Phase difference between velocity of Inlet 1 and the ocean tide
Umax2 Maximum velocity through Inlet 2




Table 5.3 Continued)
Output Parameters
8v2 Phase difference between velocity of Inlet 2 and the ocean tide
"lmax3 Maximum velocity through Inlet 3
8v3 Phase difference between velocity of Inlet 3 and the ocean tide
llmax4 Maximum velocity through Inlet 4
8v4 Phase difference between velocity of Inlet 4 and the ocean tide
(7o -17B1) Maximum ocean-bay tide difference
(u7B1 -17B2) max Maximum Bay 1 and Bay 2 tide difference (17B1 -17B3) m. Maximum Bay 1 and Bay 3 tide difference
5.2.2 Input Parameters
Table 5.4 provides the input values for all the three cases of the model as
described in Section 5.2.
The amplitude in each bay is found by applying a weighting factor proportional to
the tide station contribution to the total bay area.
2 Initial values are assumed for (qm -r/7j) m, (1iB1 -7B2) m, (17B1 -/7B3) m, for the
initial calculation. The September 2001 tide showed a semidiurnal signal, with a
period of 12.42 h. The tide in March 2002 showed diurnal signature with a period of 25.82 h. The model was run three times for three different cases as described in
Section 5.2. Details regarding all input parameters are found in Jain and Mehta
(2002), and are also summarized in Chapter 4. Table 5.4 gives values of all input
parameters required for the model.
Table 5.4 Input )arameters for the hydraulic model. Input Values Remarks
Parameters Sept2001 March 2002 a, 0.109 m 0.212 m Calculated from UF tide gauge data,
calculations shown in Appendix C. T 12.42 h 25.82 h NOS Tides Tables.
a~l 0.103 m 0.201 m
aB2 0. 115 m 0.226 m Calculated in proportion to the
aB3 0. 118 m 0.233 m contributing tide at station.
aB3 0. 118 m 0.233 m
AB1 74 km
AB2 54 km2 From the USGS topographic maps.
AB3 155 km2




Table 5.4 (continued)
Input Values Remarks
Parameters Sept 2001 March 2002 L, 1340 m
R 10 m Measured in survey
Ac1 6300 m2
L2 1000 m
R2 9 m
Ac2 1.9x104 m2 From the USGS topographic maps. Ac2,
L4 1000 m A,3, Ac4 are zero for one inlet bay case
R4 12 m
Ac4 9.7x103 M2
L3 400 m.
400 m Measured in survey. Ac3 is zero for threeR3 3 m
A3 255 m2 bays and three-inlets case.
k 1.05
f 0.025
(qo -r/l),,,, 0.037 0.036 Assumed initial values. Calculations are (N -1B2) 0.060 0.063
max 0.06shown in appendix C (7B1 -17B3) max 0.099 0.998
5.2.3 Model Results and Comparison with Data
Model results are given in Table 5.5.
Table 5.5 Model results and measurements. One Inlet One Bay System, September 2001 Output parameters Model Measurement %error
aBl 0.10 m 0.10 m 0%
EB1 0.36 rad 0.34 rad 6%
Ucl max 0.65 m/s 0.63 m/s 3%
8 v -1.20 rad -1.22 rad 2%
(r7o -7B1) max 0.038 0.036 6%
Three Bay Three Inlets System, September 2001 aBl 0.10 m 0.10 m 0%
EB1 0.34 rad 0.34 rad 0%
aB2 0.10 m 0.11 m 9%
EB2 0.37 rad 0.91 rad 59%
aB3 0.10 m 0.12 m 17%
CB3 0.54 rad 1.26 rad 57%




Table 5.5 (continued)
Three Bay Three Inlets System, September 2001 Output parameters Model Measurement %error
rcl max 0.62 m/s 0.63 m/s 2%
Sv1 -1.11 rad -1.20 rad 7%
Uc2 max 0.04 m/s Not measured
Sv2 -1.21 rad Not measured
c4 max 0.20 m/s Not measured
Sv4 -1.04 rad Not measured
('7o -B1) max 0.037 0.037 0%
(7B1 -7B2) max 0.003 0.060 95%
(7B1 -7B3) max 0.020 0.098 80%
One Inlet One Bay S stem, March 2002 aB1l 0.20 m 0.20 m 0%
EB1 0.17 rad 0.17 rad 0%
Uecl max 0.63 m/s 0.65 m/s 3%
vi1 -1.40 rad -1.40 rad 0%
(17o -IB1) max 0.036 0.036 0%
Three Bay Four Inlets System, March 2002 aB1 0.21 m 0.20 m 5%
EB1 0.16 rad 0.16 rad 0%
aB2 0.21 m 0.22 m 5%
CB2 0. 18 rad 0.44 rad 59%
aB3 0.21 m 0.23 m 9%
CB3 0.26 rad 0.60 rad 57%
zCl max 0.60 m/s 0.65 m/s 8%
Sv1 -1.35 rad -1.40 rad 4%
Uc2 max 0.04 m/s Not measured
Ev2 -1.40 rad Not measured
1c3 max 0.60 m/s 0.55 m/s 9%
6,3 -1.35 rad -1.40 rad 4%
c4 max 0.22 m/s Not measured
Sv4 -1.31 rad Not measured
(7o -B1) max 0.035 0.035 0%
(1 -17B2) max 0.003 0.063 95%
(71 -1B3) max 0.012 0.010 20%
It is evident from Table 5.5 that the linear model gives good results. The percent
error decreases if the system is modeled as a three-bay system, which is actually the case.
Velocity and tide amplitudes are within reasonably small error limits. The phase
differences between ocean (Gulf) and bay tides from data are very approximate as they




are calculated based on weighted-average tides at selected stations. Moreover, there are very few stations to yield a good value of tide for a bay. Note that the input values for (17,
-I77M) max, (77M -17M) ma, (17M -17B3)m isx' also approximate. Sample calculation for (U7, -7MB) max, (77B1 -177M) max (77M -173) max is given in Appendix C.
5.3 Stability Analysis
The stability analysis developed in Chapter 3 is now applied to St. Andrew Bay system. This analysis is done for a two-inlet bay system using van de Kreeke's (1990) linearized lumped parameter model. The two inlets, to which the model is applied, are St. Andrew Bay Entrance and the new East Pass opened in December 2002. Calculations related to stability are given in Appendix D. A Matlab program (Appendix D) has also been developed for doing the analysis and generating equilibrium flow curves for the two inlets. There are two programs, one for rectangular channel cross-section and another for triangular channel cross-sections.
5.3.1 Input Parameters
Input parameters required for the Matlab program (Appendix D) are listed in
Table 5.6. Since the objective was to study the effect of bay area on the stability because the results are sensitive to it, it is held constant for a particular set of calculation, but is varied for generating different sets of equilibrium flow curves. Similarly the length of East Pass, believed to have an uncertain value due to the complex bay shoreline and bathymetry in that region is also varied to study its effect on the system.




Table 5.6 Input parameters for stability analysis.
Input Parameters for December 2001
a, 0.26 m Amplitude of ocean tide
T 18.0 hrs Time period of tide
AB 74-105 km 2Area of bay (St. Andrew Bay (varied from 74 to 105 km2 Inlet 1
Ue7l 10.40 m/s Equilibrium velocity for Inlet 1 (see Appendix D)
W,1 525 m Width of InletlI
L, 1340 m Length of Inlet 1
a1 0.138 Constant for triangular cross-section for Inlet 1 (see Appendices
C and D)
F, 0.004 Friction coefficient for Inlet 1
Inlet 2
Ueq2 10.45 m/s Equilibrium velocity for Inlet 2 (see Appendix D)
WV2 300 m Width of Inlet 2
L2 500-2000 m Length of Inlet 2 (East Pass) (varied from 500 m to 2000 m) a2 0.187 Constant for triangular cross-section for Inlet 2 (see Appendices
C and D)
F2 0.004 Friction coefficient for Inlet 2
5.3.2 Results and Discussion
As noted, it is found that two inlets can never be unconditionally stable
simultaneously in one bay. The bay area has a large effect on the stability of the inlets.
Table 5.7 summarizes this effect. It is clear that with a small increase in bay area the
inlets become stable. This is also demonstrated with the help of equilibrium flow curve in
the Figure 5. 1, Figure 5.2 and Figure 5.3 for rectangular cross-section and Figure 5.7 and
Figure 5.9 for triangular cross-section. The cross-sectional area pair during December
2001 (Table 4.3) [5210, 255] is shown by the dot. Figure 5.1 and Figure 5.7 have small
bay areas, and the dot lies outside the equilibrium flow curve indicating that both inlets
are unstable. As the bay area increases St. Andrew becomes stable (Figure 5.2 and Figure
5.7), and a further increase in bay area also stabilizes East Pass (Figure 5.3 and Figure
5.9). However, in reality we cannot increase the bay area beyond a reasonable limit,
because then the basic assumption of bay tide fluctuating evenly in the bay does not hold.




Moreover, in a shallow bay the effect of dissipation of tidal energy cannot be ignored, especially if the bay is large. Also as per Figure 3.5 two inlets are not stable simultaneously.
An increase in the length of East Pass has a destabilizing effect on that inlet as shown in the Table 5.7. Note also that for a rectangular cross-section (Figure 5.3) with the length of East Pass of 500m, this inlet is stable, whereas with a length of 2000 m (Figure 5.6) the inlet is instable. This is because as the length increases the dissipation increases. Friction dominated losses, (F 0.004, R 3m (2FL IR)) for East Pass with 500 m length is 1.33, where as that for 2000 m length it is 5.33. The same cases occur in Figure 5.9 and Figure 5.12.
The other effects on the stability model are the approximation in the cross-section of the inlet. It is clear that triangular cross-section is a better approximation than rectangular section, because with the same parameters for rectangular cross-section in Figure 5.6, East Pass is predicted to be unstable whereas in Figure 5.12 for triangular cross-section, East Pass is stable even though barely, which is not believed to be the case for this newly opened inlet.
Table 5.8 gives the qualitative indication of the stability. The various zones
mentioned in the Table 5.8 are described in Section 3.4 and Figure 3.6. It is clear from these results that St. Andrew is a stable inlet (for a realistic bay area) as opposed to East Pass. This is also evident from the Figure 3.5, which shows that two inlets cannot be stable simultaneously, because we for unconditional stability, need four real points of intersection of equilibrium flow curve and none of the solutions (neither rectangular cross-section nor triangular cross-section) gives four real solution.




The model does not yield an analytic solution for a more realistic parabolic crosssection. Another weakness is due to the assumptions made in Chapter 3 including a bay
area in which the tide is spatially always in-phase, and simple a harmonic function for
tide. These assumptions are not always satisfied.
Table 5.7 Effect of change in bay area and length of East Pass.
Rec nglarcross-section
Run Bay East Pass Result
2
No. area (kin) Length
1 74 500 Both inlets unstable (Figure 5. 1)
2 90 500 St. Andrew becomes stable (Figure 5.2)
3 105 500 St. Andrew stable, East Pass barely stable (Figure
4 74 2000 Both inlets unstable (Figure 5.4)
5 90 2000 St. Andrew barely stable (Figure 5.5)
6 105 2000 St. Andrew stable, East Pass unstable (Figure 5.6)
Triangular cross-section
7 74 500 Both inlets unstable Figure (5.7)
8 90 500 St. Andrew becomes stable (Figure 5.8)
9 105 500 Both inlets stable (Figure5.)
10 74 2000 Both inlets unstable (Figure 5. 10)
11 90 2000 St. Andrew stable (Figure 5.11)
12 105 2000 St. Andrew stable, East Pass just stable (Figure 5.12)*
Two inlets cannot be simultaneously stable, because according to Figure 3.5, for unconditional stability we need four real points of intersection of equilibrium flow curve, which is not possible in either rectangular cross-section solution nor triangular crosssection solution.




66
In let Stability -1 (rectangular section)

4500 4000 3500 3000 S2500
-~2000 LU 15001000 500
0

2000

3000

Al, St Andrew (m2)

- East Pass St Andrew

Figure 5.1 Equilibrium flow curves for rectangular cross-sections, Run No. 1.
Inlet Stability 2 (rectangular section)

6000

50-
NO
1000-U

0 1000

2000

3000
Al, St Andrew (m2)

5000

6000

- East Pass St Andrew

Figure 5.2 Equilibrium flow curves for rectangular cross-sections, Run No. 2.

5000

.......... .....




67
Inlet Stability 3 (rectangular section)

6000
500 -E "..-4000"
3000 "-100
. .. ...........
0 1000 2000 3000 4000 5000 6000 7000
Al, St Andrew (m2)
- East Pass St Andrew
Figure 5.3 Equilibrium flow curves for rectangular cross-sections, Run No. 3.
Inlet Stability -4(rectangular section)

4500 4000 3500
E
3000 2500 2000 L 1500
1000 500
0
0

2000

3000

4000

5
5000

Al, St Andrew (m2)

- East Pass St Andrew

Figure 5.4 Equilibrium flow curves for rectangular cross-sections, Run No. 4.

X.iiiiiiiiir~ ii __




Inlet Stability 5(rectangular section)
6000
5000
.. -___E 4000 ---2000 i
UU 2000
13000 U
0 --- --- I--- -- ----- -- -- --
0 1000 2000 3000 4000 5000 6000
Al, St Andrew (m2)
-East Pass St Andrew
Figure 5.5 Equilibrium flow curves for rectangular cross-sections, Run No. 5.
Inlet Stability 6 (rectangular section)
6000
- U i~
5000 -----4000
30 100 3.0 06
Al, St Andrew (m2)
- East Pass St Andrew
Figure 5.5 Equilibrium flow curves for rectangular cross-sections, Run No. 5.
6000 |"-50 I .-i-.m
100300000000000 60 70
- atPs St ndre
Figure 5.6 Equilibrium flow curves for rectangular cross-sections, Run No. 6.




69
Inlet Stability -7 (triangular section)
5000
4500
R 4000
3500 3000 2500
2000 1500 1000 500
01
0 1000 2000 3000 4000 5000
Al, St Andrew (m2)
East Pass St Andrew
Figure 5.7 Equilibrium flow curves for triangular cross-sections, Run No. 7.
Inlet Stability 8 (triangular section)
6000 5000
E
4000 3000
LJ_ 2000
1000
0
0 1000 2000 3000 4000 5000 6000
Al, St Andrew (m2)
East Pass St Andrew
Figure 5.8 Equilibrium flow curves for triangular cross-sections, Run No. 8.




70
Inlet Stability 9 (triangular section)

7000 6000
S4000 S3000 LI 20001000
0
0 1000 2000 3000 4000
Al, St Andrew (m2)

5000 6000 7000

- East Pass St Andrew
Figure 5.9 Equilibrium flow curves for triangular cross-sections, Run No. 9.
I nlet Stability 10 (triangular section)

1000

2000 3000
Al, St Andrew (m2)

4000

5000

- East Pass St Andrew

Figure 5. 10 Equilibrium flow curves for triangular cross-sections, Run No. 10

5000
45001 i'40001 ..3500 S3000 a25001
S20001 LU1500 10001 5001
0~




Inlet Stability 11 (triangular section)

6000 5000
E
4000 3000
Lu 2000
1000
0

2000

3000
Al, St Andrew (m2)

4000

5000

6000

- East Pass St Andrew Figure 5.11 Equilibrium flow curves for triangular cross-sections, Run No. 11.
Inlet Stability 12 (triangular section)

7000 6000 5000
4000 3000
LU
2000 1000
0
0 1000 2000 3000 4000
Al, St Andrew (m2)

5000 6000 7000

- East Pass St Andrew

Figure 5.12 Equilibrium flow curves for triangular cross-sections, Run No. 12.




Table 5.8 Stability observations for St. Andrew Bay Entrance and East Pass. Figure Placement of cross-sectional Observations
area pair [A1, A2], (black dot)
Figure 5.1 Zone-i Both inlets are unstable
Figure 5.2 Zone-2 St. Andrew Bay Entrance is stable
Figure 5.3 Zone-4 Only one is stable i.e. St. Andrewa
Figure 5.4 Zone-i Both inlets are unstable
Figure 5.5 Zone-2 St. Andrew Bay Entrance is stable
Figure 5.6 Zone-2 St. Andrew Bay Entrance is stable
Figure 5.7 Zone-i Both inlets are unstable
Figure 5.8 Zone-2 St. Andrew Bay Entrance is stable
Figure 5.9 Zone-4 Only one is stable i.e. St. Andrewa
Figure 5.10 Zone-i Both inlets are unstable
Figure 5.11 Zone-2 St. Andrew Bay Entrance is stable
Figure 5.12 Zone-4 Only one is stable i.e. St. Andrewa
a As per Figure 3.6, it is clear that even in Zone-4 only one inlet is stable, this is further clarified from Figure 3.5, which shows that only one inlet can be stable at one time.




CHAPTER 6
CONCLUSIONS
6.1 Summary
St. Andrew Bay, which is a composite of three interconnected bays (St. Andrew Bay proper, West Bay + North Bay and East Bay) is located in Bay County on the Gulf of Mexico coast of Florida's panhandle. It is part of a three-bay and two-inlet complex. One of these inlets is St. Andrew Bay Entrance and the other is East Pass, which are both connected to St. Andrew Bay on one side and the Gulf on the other. Prior to 1934, East Pass was the natural connection between St. Andrew Bay and the Gulf. In 1934, St. Andrew Bay Entrance (Figure 4.2) was constructed 11I km west of East Pass through the barrier island to provide a direct access between the Gulf and Panama City. The interior shoreline of the entrance has continually eroded since it's opening. East Pass was closed in 1998, which is believed to be due to the opening of the St. Andrew Bay Entrance.
In December 2001, a new East Pass was opened (Figure 4.3), and the effect of this new inlet is presently being monitored over the entire system. Accordingly, the objective of the present work was to examine the hydraulics of the newly formed two-("ocean") inlet/three-bay system and its hydraulic stability, especially as it relates to East Pass.
The first aspect of the tasks performed to meet this objective was the development of equations for the linearized hydraulic model for the system of three bays and four inlets (two ocean and two between bays), and solving and applying them to the St. Andrew Bay system. The second aspect was the development of the ocean inlet stability criteria using the Escoffier (1940) model for one inlet and one bay and extending this




model to the two ocean inlets and a bay. Stability analysis for the St. Andrew Bay system was then carried out using the linearized lumped parameter model of van de Kreeke (1990).
6.2 Conclusions
The following are the main conclusions of this study:
1 If the system is modeled as a three-bay system as compare to a one-bay system,
the error in the phase difference, 8B1, decreases from 60o to 00o and in the velocity amplitude from 300 to 20o. Moreover the error in maximum head difference, (Uo,
qWi) ,, also decreases from 600 to 00o.
2 The amplitudes of velocities and bay tides are within 50o, which is a reasonably
small error band. The percent error for St. Andrew Bay is almost 00o, and for the
other bays it is within 2000.
3 The bay area has a significant effect on the stability of the two inlets. At a bay
area of 74 km2 both inlets are unstable. Increasing it by 220o to 90 km2 stabilizes
St. Andrew Bay Entrance, and by 420o to 105 km2 stabilizes East Pass as well.
4 Two inlets can never be simultaneously unconditionally stable.
5 Keeping the bay area at 105 km2 and increasing the length of East Pass from 500
m to 2000 m destabilizes this inlet because as the length increases the dissipation
in the channel increases as well.
6 A triangular channel cross-section is a better approximation than a rectangular
one, because given the same values of all other hydraulic parameters, St. Andrew
Bay Entrance with a rectangular cross-section is found to be barely stable,
whereas with a triangular cross-section it is found to be stable, as is the case.
6.3 Recommendations for Further Work
Accurate numerical values required for the stability analysis of a complex inletbay system can only be obtained by using a two- (or three)-dimensional tidal model to describe the hydrodynamics of the bay.
Freshwater discharges from the rivers into the bay should be incorporated through

numerical modeling.




75
Including a more realistic assumption for the channel cross-section can improve the stability analysis.




APPENDIX A
ALGORITHMS FOR MULTIPLE INLET-BAY HYDRAULICS
A.1 Introduction
The linearized approach described in Chapter 2 has been used to evaluate the
hydraulic parameters of the multiple inlet bay system. The differential equations,
developed by this approach (Chapter 2), Eq. (2.100), Eq. (2.101) and Eqs (2.102), are
solved in Matlab Program-1 (given below). These are the general equations for four inlets
and three bays system. These equations can be used to solve from one bay system to the
complex three bays system. Note that for solving Program-i, the Matlab version should
have a symbolic toolbox. The present program is solved in Matlab release 6.1. The
solution from Program-1 is used as input to Program-2 (given below). The required input
parameters and output for Program-2 are listed in Table 5.3 of Chapter 5.
A.2 Program-1
%UNIVERSITY OF FLORIDA %CIVIL AND COASTAL ENGINEERING DEPARTMENT %PROGRAM FOR SOLVING THE EQS 2.100, 2.101, 2.102 % ALL CONSTANTS DEFINED IN CHAPTER 2
clear all
syms ab cAB
tl=sym('thetal')
t2=sym('theta2')
t3=sym('theta3')
rl= sym('al*exp(-i*tl)') r2=sym('a2*exp(-i*t2)') r3=sym('a3*exp(-i*t3)') C=[a*i+1 a*A*i a*B*i;-1 b*i+1 0;-1 0 c*i+1] D=[1;0;0]
%END




A.3 Program-2
%UNIVERSITY OF FLORIDA %CIVIL AND COASTAL ENGINEERING DEPARTMENT %PROGRAM FOR CALCULATION OF MULTIPLE INLET-BAY HYDRUALICS %FOR ONE -INLET BAY CASE, FOR Ac2, Ac3, Ac4 EQUAL TO ZERO %INLET 1 AND INLET 3 CONNECTS BAY1 TO THE OCEAN
clear all
g=9.81;
ao=0.212;%ocean tide amplitude theta=0;%ocean tide phase etao=ao*cos(theta);%ocean tide T=25.82;%time period q=2*pi/(T*3600)%sigma k=1.05;% entrance and exit loss f=0.025;%friction factor aB 1 =0.201 ;%approximate amplitude of bays aB2=0.226;
aB3=0.2325;
%ml=max(eta0-etab 1l),m2=max(etab 1l-etab2),m3=max(etab 1l-etab3) ml=0.023;
m2=0.0527;
m3=0.123;
%Inlet 1
L1=1340;%Length of inlet R1i=10;%hydraulic radius Ac l=6300;%CROSS-SECTION AREA of the inlet Fl=k+(f*L1)/(4*R1);%friction factor F includes ken kex fL/4R
%Inlet 2
L2=1000;%Length of inlet R2=9;%hydraulic radius Ac2=1.9* 10A4;%CROSS-SECTION AREA of the inlet, it is zero for one inlet bay case F2=k+(f*L2)/(4*R2);%friction factor F includes ken kex fL/4R
%Inlet 3
L3=400;%Length of inlet R3=3;%hydraulic radius Ac3 =255;%CRO S S-SECTION AREA of the inlet F3=k+(f*L3)/(4*R3);%friction factor F includes ken kex fL/4R
%Inlet 4
L4=1000;%Length of inlet R4=12;%hydraulic radius Ac4=9.7* 10A3;%CROSS-SECTION AREA of the inlet F4=k+(f*L4)/(4*R4);%friction factor F includes ken kex fL/4R




%bayl area ABI=74*10A6; %bay2 area AB2=54*10A6;
%bay3 area AB3=155*10A6;
%calculations CDL 1 =sqrt(ao/(m 1 *F 1)) CDL2=sqrt(aB 1/(m2*F2)) CDL3 =sqrt(ao/(m *F3)) CDL4=sqrt(aB 1/(m3 *F4))
C 1 =CDL1*Acl /AB 1 *sqrt(2*g/ao) C2=CDL2*Ac2/AB2*sqrt(2*g/aB 1) C3 =CDL3 *Ac3/AB 1 *sqrt(2*g/ao) C4=CDL4*Ac4/AB3*sqrt(2*g/aB 1)
%ALL THE CONSTANTS ARE DEFINED IN THE THESIS a=q/(C1+C3) if Ac2==0 b=0
else b=q/C2 end
if Ac4==0 c=0
else
c=q/C4
end
A=AB2/AB1 B=AB3/AB1 rl=(c-i)*(b-i)/(-i*a*A+i*a*c*b+a*c-i*c-i*b-1+c*b-i*a*B+a*bi*a+a*B*b+a*A*c)%SOLUTIONS ARE OBTAINED FROM ANOTHER r2=-i*(c-i)/(-i*a*A+i*a*c*b+a*c-i*c-i*b-l1+c*b-i*a*B+a*bi*a+a*B*b+a*A*c)%MATLAB PROGRAM WHICH HAS SYMBOLLIC TOOLBOX. r3=-i*(b-i)/(-i*a*A+i*a*c*b+a*c-i*c-i*b-l+c*b-i*a*B+a*b-i*a+a*B*b+a*A*c) aB l=abs(rl)*ao eB l=-angle(rl) aB2=abs(r2)*ao eB2=-angle(r2) aB3=abs(r3)*ao eB3=-angle(r3) etaB l=aB l*cos(theta-eB 1) etaB2=aB2*cos(theta-eB2)




etaB3=aB3*cos(theta-eB3) CDL11 sqrt(ao/(max(etao-etaB 1)*F 1)) CDL22=sqrt(aB 1/(max(etaBl 1-etaB2)*F2)) CDL33 =sqrt(ao/(max(etao-etaB 1 )*F3)) CDL44=sqrt(aB 1/(max(etaB 1 -etaB3)*F4))
C 11 =CDL1*Acl/AB 1 *sqrt(2*g/ao) C22=CDL2*Ac2/AB2*sqrt(2*g/aB 1) C33 CDL3*Ac3/AB1 *sqrt(2*g/ao) C44=CDL4*Ac4/AB3*sqrt(2*g/aB 1)
%velocity in the inlet uc l=sqrt(2*g/ao)*CDL1*(etao-ao*rl) uclmax=abs(ucl)
evl=-angle(ucl)
uc2=sqrt(2*g/aB 1)*CDL2*(ao*rl-ao*r2) uc2max=abs(uc2)
ev2=-angle(uc2)
uc3=sqrt(2*g/ao)*CDL3*(etao-ao*rl) uc3max=abs(ucl)
ev3=-angle(ucl)
uc4=sqrt(2*g/aB 1)*CDL4*(ao*rl-ao*r3) uc4max=abs(uc4)
ev4=-angle(uc4)
%END




APPENDIX B
INLET HYDRAULICS RELATED DERIVATIONS B.1 Linearization of Damping Term
The linearization of the damping term in Eq. (3.6) is done as given in Bruun (1978). The bay tide response is represented by
aB aBsin(O -B) (B.1)
where
0 = -t, dimensionless time.
T
aB one-half the tide range (i.e., amplitude) in the bay, and EB lag between high water (HW) or low water (LW) in the ocean and corresponding HW or LW in the bay. Also,
7o = ao sin(O) (B.2)
from the continuity equation we further have
Au = AB duB (B.3)
dt
where A, is the area of cross-section of the inlet and AB is the surface area of the bay.
The time of HW or LW in the bay, i.e., when drB = 0, coincides with time of dt
slack water, i.e., u = 0, so that EB is also the lag of slack water after HW or LW in the ocean. Thus it can be written as
dr7Bd r7B 2 aB2 COS(0 -B)Icos(O- B) (B.4)
cit it (a o( B




or in terms of Fourier series Eq. (B.4) can be written as
8 s in
dt, dt cr2 aBn 22) cosn(O-EB) (B.5)
dt dt n, n(4 n2
where n takes only odd integral values. For linearization purposes n=l, so that Eq. (B.5) becomes
duB dB 2 aB2 8 cos n (0- E) (B.6)
dt dt 3;
The amplitude of the tidal velocity is given by
ascrAB
umax = (B.7)
A
Therefore, it can be written as
8
u = 8 "u=maxu (B.8)
3;r
where umax is the amplitude of the u.
B.2 Shear Stress Dependence on Area For each inlet discharge is defined as a time varying function:
Qi (t) AB dqB (B.9)
4 dt
Q,(t) =-A, (B.10)
The expression for maximum tidal velocity can be obtained by the solution of the above equations with the simplifying assumptions mentioned in Chapter 2.
umax = C(K)sin7 2rABaO K (B. 11)
AT K
where K is the coefficient of repletion,




T A4 2gRa
K= A (B.12)
2z-a, A V mR, + 2FL, and
K K= K (B.13)
is summation is over all the inlets. The function C(K) sin 7 is a monotonically increasing function with C 0 for K 0 and C 1 for K o, y is a specific time when sea is at MSL, as defined by Kuelegan (1951)
It is seen below that the bottom shear stress, r, varies strongly with the crosssectional area. This can be shown with the help of approximate analytical solution carried out by Keulegan (1951). Substituting the value of u from Eq. (B. 11) in Eq. (3.1), and taking C(K) sin 7 ; 1 and F = 0.003:
" ;p 2xaA, 1
A; pF2 AB 2 (B.14)
It is clear from the above equation that i has a strong dependence on A.
B.3 General Equation for hydraulic radius.
Consider the general trapezoidal cross-section:
1 1 (Bo
Area, A= -(B+Bo)h= -Bh 1+- B
2 2 B
Wetted perimeter, P = Bo + 2 (B Bo )2 + h2 = B + 1- + 4
4 B
- h 1+ B
Hydraulic radius, R -A 2
P Bo BO 2 h 2
B+ 1 +4
B B




83
< B
h
Bo
Figure B. 1 Trapezoidal Cross-section
Now consider two cases: 1) Rectangular cross-section, i.e., B0 B, and 2) Triangular cross-section, i.e., B0 = 0. B.3.1 Rectangular
B B0, Therefore hydraulic radius for a rectangle is
A h
B.3.2 Triangular
For triangular section, B0 0 1h
A 2
PR 1+4 h
B.4 Hydraulic Radius for Triangular Cross-Section
For a triangular cross-section the hydraulic radius is related as a square root of the area, as shown below:




84
Figure B.2 is a triangular cross-section where /is the angle with the horizontal on both the sides:
1
Area A = h2h tan /
2

Wetted perimeter P Hydraulic radius R-

2h
cos/7

1 -A = avA
2 sin/ cos/Y

Figure B.2 Triangular cross-section.

(B.15)