Coastal engineering study of proposed Navarre Pass

COASTAL ENGINEERING STUDY

OF

PROPOSED NAVARRE PASS

73 o0

Sponsor:

Santa Rosa County Beach Administration

Submitted by:

Coastal and Oceanographic Engineering Laboratory
Florida Engineering and Industrial Experiment Station
University of Florida
Gainesville, Florida
February, 1973

L

ABSTRACT

This report describes the results of a coastal

engineering field and numerical study of the proposed

Navarre Pass. The field measurements are based on three

field trips during which bathymetry and tides and currents

were measured. The numerical model simulates the tides

and flows in Santa Rosa Sound and is capable of including

the effects of Navarre Pass. Littoral drift direction

and magnitude are of considerable importance in planning

the inlet; calculations were carried out using shore-based

observations obtained in a program of the Coastal Engi-

neering Research Center.

The results of the study indicate that:

1. Navarre Pass would only reduce slightly the
equilibrium cross-sectional flow areas into
Pensacola (1.8%) and Choctawhatchee (0.1%)
Bays.

2. The velocities through Navarre Pass would
be well within the limits considered safe
for small craft navigation.

3. Planning for artificial transfer of sand
should be based on an annual rate of
200,000 cubic yards to the west. Initial
disposition of dredged material should be
as a feeder beach on the west side of the
Pass.

4. The Pass would cause a localized moderation
of Sound salinities in the vicinity of the Pass.

5. The tide and geometric characteristics are
such that Navarre Pass will always tend to
close unless maintained open by jetties.

Based on these results, it is concluded that if

proper financial provision is made for construction and

maintenance of the inlet, there should be no significant

adverse effects to the stability of the Santa Rosa Island

Beaches, nor to the adjacent waters.

iii

TABLE OF CONTENTS

Page

ABSTRACT ................ ........................... ii

LIST OF TABLES ..... .............................. vii

LIST OF FIGURES ................................. viii

LIST OF SYMBOLS .................................... xii

ACKNOWLEDGEMENTS .................................. .. xiv

I. INTRODUCTION ................................ 1

II. PURPOSES OF STUDY .......................... 3

Impact of Pass on Natural
Processes .................... ..... 3
Conceptual Design Features
of Navarre Pass ................... 4

III. BRIEF HISTORY OF NAVARRE PASS .............. 5

IV. METEOROLOGY AND HYDROGRAPHY OF AREA ........ 12

General Description ..................... 12
Winds ................................ 13
Sea ........... ...................... 13
Swell .... ............................ 17
Tides ................................ .. 17
Offshore Currents ............... ..... 19
Santa Rosa Sound Currents ............ 22

V. FIELD STUDIES AND RESULTS .................. 25

Field Trip No. 1,
May 13-18, 1970 ...... ............. 25
Field Trip No. 2,
December 7-12, 1970 ............... 32
Field Trip No. 3,
July 19-23, 1971 .................. 38

TABLE OF CONTENTS-Continued

Page

VI. LITTORAL DRIFT ............................. 42

Introduction ........................... 42
Littoral Drift Estimates ............. 43
Experimental Groin at Navarre ........ 51
Summary and Recommendations ............. 61

VII. SUMMARY OF NUMERICAL MODEL
CALCULATIONS ............................ 62

Introduction ............................ 62
Results Obtained Using the
Numerical Model ...................... 63
Effect of Navarre Pass on Entrances
to Pensacola and Choctawhatchee
Bays .............................. 63
Maximum Velocities Through
Navarre Pass ...................... 64
Relative Stability of
Navarre Pass ..................... 64

VIII. RECOMMENDED DESIGN OF NAVARRE
PASS INLET ............................. 65

Functional Design ................... 65
Recommended Layout of Navarre Pass ... 65
Disposition of Initially
Dredged Material .................. 73
Alternate Designs ................... 74

IX. SUMMARY AND CONCLUSION ..................... 75

Summary ................................ 75
Conclusion .............................. 75

X. REFERENCES .............. .................. 76

APPENDIX

I. NUMERICAL MODEL OF THE BAY SYSTEM
AFFECTING NAVARRE PASS .................. 78

Introduction ............................ 78
Derivation of the Numerical Model ....... 79

TABLE OF CONTENTS-Continued

Page

Governing Differential Equations ..... 79
Finite Difference Equations .......... 82
Boundary Conditions ................. 83
Application of the Numerical
Model ................................ 86
Assessment/Calibration of the
Numerical Model .................. 87
Use of Numerical Model to Evaluate
Effect of Navarre Pass ............ 98
Maximum Velocities Through
Navarre Pass ...................... 101

II. STABILITY OF NAVARRE PASS .................. 103

Introduction ........................... 103
Method .................................. 103
Numerical Model ...................... 105
Sedimentary Stability ................ 105
Equilibrium Cross-Sectional
Area .............................. 109
Application of Stability Equilibrium
Concepts to Navarre and
Rollover Passes ..................... 112
Navarre Pass ......................... 115
Rollover Pass ........................ 123
Conclusion Regarding Relative
Stability of Navarre and
Rollover Passes ...................... 123

III. GLOSSARY OF TERMS .......................... 130

Introduction ........................... 130

LIST OF TABLES

Table Page

I Summary of Field Trip Activities
and Information Obtained ................... 26

II Summary of Median Diameters of
Sand Samples Analyzed (Field
Trip of May 1970) .......................... 31

III Summary of Calculated Littoral
Drift Using LEO Data ....................... 50

I-1 Characteristics of Schematized
Bay/Inlet System ........................... 88

I-2 Summary of Measured Tidal
Characteristics ............................ 89

I-3 Dimensions Used in Flow Calculations
at Navarre Bridge .......................... 94

I-4 Predicted Effect of Navarre Pass on Flows
in and out of Pensacola and
Choctawhatchee Bays ........................ 99

I-5 Calculated Maximum Discharges and
Velocities Through Navarre Pass
for Various Tidal Ranges .................. 102

II-1 Ratio R, of Maximum to Minimum
Tidal Ranges During the Period
January 1 to February 9, 1970 .............. 116

vii

LIST OF FIGURES

Figure Page

1 Location Map of Santa Rosa
Island Region ............................... 2

2 Aerial Photograph Prior to Navarre
Pass Cut (Date of Photograph:
February 14, 1963) ......................... 6

3 Oblique View of Gulf Terminus of
Navarre Pass (Date: Unknown,
but Probably August, 1965) ................. 7

4 Aerial Photograph of Navarre Pass
Showing Effect of Westerly Littoral
Drift (Date of Photograph: On or
About September 1, 1965) ................... 8

5 Oblique View of Navarre Pass Shortly
After Closure (Date of Photograph:
September 1965) ....................... ... 9

6 Aerial Photograph Showing Filling of
Pass by Air and Water Transported
Sand (Date of Photograph: June 1970) ...... 11

7 Data Squares in Gulf of Mexico and
Caribbean Sea .......... .................... 14

8 Monthly Wind Roses at Data Square
Off Navarre Pass Area ..................... 15

9 Monthly "Sea" Roses at Data Square
Off Navarre Pass Area ..................... 16

10 Monthly Swell Roses at Data Square
Off Navarre Pass Area ...................... 18

11 Predicted Tides at Galveston, Pensacola and
Miami Harbor Entrances ..................... 20

viii

LIST OF FIGURES-Continued

Figure Page

12 Example of Measured Tides in Gulf
of Mexico and Santa Rosa Sound ............. 21

13 Measured Currents Offshore Navarre
Beach, Florida. May 15-16, 1970 ........... 23

14 Currents Measured at Navarre
Bridge, December 9-11, 1970 ............... 24

15 Measured Tides in Gulf of Mexico
and in Santa Rosa Sound During
Field Trip of May 13-18, 1970 .............. 27

16 Locations of Principal Measurements
Conducted During Field Trips .............. 28

17 Location of Tidal Division Line
at 1545 on May 16, 1970 ............. ..... 30

18 Beach Profiles, December 9, 1970 ........... 33

19 Measured Tides in Gulf of Mexico and
in Santa Rosa Sound During Field Trip
of December 7-12, 1970 .................... 37

20 Measured Tides in Gulf of Mexico and
in Santa Rosa Sound During Field Trip
of July 19-23, 1971 ........................ 39

21 Experimental Groin Under Construction ...... 41

22 Coastal Sector Method Used by Coastal
Engineering Research Center in
Reporting Wave Direction .................. 48

23 Locations of CERC LEO Data Used in
Littoral Drift Calculations. Drift
Directions and Net Annual Rates
Also Shown ................................. 49

24 Photographic History of Navarre Experimental
Groin .......................................... 52-59

LIST OF FIGURES-Continued

Figure Page

25 Weir Jetty System at Hillsboro
Inlet, Florida ............................... 68

26 St. George Island Cut. Note Erosion
Where Bank Protection is Not
Provided .......... ......................... 71

I-1 Bay System Represented in
Numerical Model ............................ 80

I-2 Illustration of Bay Segment
Representation ............................. 84

I-3 Schematization of Pensacola Bay/
Choctawhatchee Bay/Santa Rosa Sound/
Gulf of Mexico System ...................... 85

I-4 Comparison of Measured and Calculated
Ratios of Sound to Gulf Tidal Ranges
Versus Gulf Tidal Range ................... 91

I-5 Comparison of Measured and Calculated
Phase Lags Between Gulf and Sound
Tidal Extremes ............................. 92

I-6 Comparison of Measured and Computed
Santa Rosa Sound Tides and
Discharges .......... .... ................... 95

II-1 Schematic Illustrating Stability Analysis
for Single and Multiple Inlets ............. 104

II-2 Illustration of Escoffier's Stability
Concept .................................... 107

II-3 Equilibrium Cross-Sectional Area and
Tidal Prism Relationship (From O'Brien) .... 111

II-4 Variation of Maximum Inlet Velocity with
Cross-Sectional Area for Equilibrium
Conditions ................................. 113

II-5 Variation of Maximum Velocity with Inlet
Cross-Sectional Area and Tidal Range.
Navarre Pass, Florida ..................... 114

LIST OF FIGURES-Continued

Figure Page

II-6 Predicted Tides at Galveston, Pensacola
and Miami Harbor Entrances. Note Differ-
ences in Tidal Range Variations ............ 117

II-7 Cumulative Probability Distributions
for Predicted Gulf Tidal Ranges at
Navarre and Rollover Passes ................ 119

II-8 Auxiliary Diagram for Determination
of Tidal Range Corresponding to
Sedimentary Equilibrium (Example
Shown for AC = 5000 ft2) ................... 120

II-9 Stability Analysis for Navarre
Pass, Florida .............................. 122

II-10 Area Map Showing Location of
Rollover Fish Pass ......................... 124

II-11 Numerical Model Representation of
Galveston Bay .............................. 125

II-12 Variation of Maximum Velocity with
Inlet Cross-Sectional Area and Tidal
Range Rollover Fish Pass, Texas .......... 126

II-13 Stability Analysis for Rollover
Fish Pass, Texas .......................... 127

LIST OF SYMBOLS

Symbol Description

A,, A2 Flow areas through Navarre Bridge

Ac Cross-sectional flow area of inlet

ACE Equilibrium cross-sectional flow area

A* Critical cross-sectional flow area
C
A Plan area of bay segment
P
Cf Wind stress coefficient

D Total depth = h + n

f Darcy-Weisbach friction factor

g Gravitational constant

-G Subscript referring to "Gulf" variable

h Depth below mean sea level

K Entrance loss coefficient
en
K Exit loss coefficient
ex
z Length of bay segment or inlet

n Exponent of velocity in sediment transport
relationship

n Subscript referring to nth bay segment
-n

p Tidal prism

P Probability in percent

q Discharge per unit width in the x-direction

qR Runoff in cubic ft/sec per foot of bay
length

xii

LIST OF SYMBOLS-Continued

Symbol Description

Q Total discharge across bay segment or through
inlet

R Ratio of maximum to minimum tidal ranges

R Hydraulic radius, also tide range

-S Subscript referring to "Sound" variable or
"Spring" tidal range

t Time

T Tidal period

U Wind speed at 30 ft reference elevation

V Water velocity, in bay segment or through
inlet

w Width of bay segment considered

x Horizontal distance coordinate aligned with
bay axis

y Horizontal distance coordinate perpendicular
to bay axis

B Angle of wind vector relative to bay axis

n Water surface displacement from mean sea
level, positive upwards

7T Numerical constant, 3.14159 .

p Mass density of water

Pa Mass density of air
a Angular frequency of tide

T Wind stress on water surface

Tb Frictional stress on bottom of water column

xiii

ACKNOWLEDGEMENTS

Many individuals have contributed in a variety of

ways to the study reported herein. The efforts of the

Staff of the Department of Coastal and Oceanographic

Engineering who participated in the field program are

appreciated.

The cooperation of the Santa Rosa County Beach

Administration was most helpful, and the interaction and

discussions with Messrs. Baskerville and Escoffier of

Baskerville-Donovan Engineers, Inc., contributed to the

final design presented in this report. Captain R. W. Slye

kindly photographed the experimental groin and provided

comments regarding its performance. Mr. W. J. Wells was

instrumental in implementing this study and maintained an

interest throughout the investigation.

Mr. Walter Burdin of the Mobile District of the

U. S. Army Corps of Engineers provided several aerial

photographs and a continuing interest in this project.

The Coastal Engineering Research Center willingly

provided their Littoral Environmental Observation (LEO)

data which included observations of wave height, period

and direction. Mr. Curtis Baskette, a Graduate Student,

became interested in and developed a computer program to

compute littoral drift from the LEO data.

xiv

The study was under the general direction of

R. G. Dean, Professor of Coastal and Oceanographic

Engineering.

I. INTRODUCTION

In May, 1970, the Santa Rosa County Beach

Administration contracted with the Coastal and Oceanographic

Engineering (COE) Department of the University of Florida

to carry out a coastal engineering study of the proposed

Navarre Pass through Santa Rosa Island. Santa Rosa Island

is a narrow barrier island with an east-west axis paralleling

the mainland; the island is separated from the mainland

by Santa Rosa Sound, see Figure 1. The proposed site for

Navarre Pass is approximately at the mid-point of the

Island and several thousand feet east of the Navarre

Bridge; the approximate latitude and longitude of the

Pass Site are: 30023' N and 86051'10" W, respectively.

The Pass was first cut through the Island in

July, 1965, however by September, 1965, the cut had

widened and shoaled and was impassable to small craft.

Since closing, the Pass reportedly has been reopened at

least twice by a hurricane (Camille, 1969) and a severe

winter storm. In December, 1970, the berm elevation

across the original cut had been built up to an elevation

of approximately +6 ft MSL.

- 1 -

Pensacola Bay

Rosa
Island

Boy

'East Bay

Choctawhatchee

'Navarre
Pass
Location

'Destin
(East)
Pass

GULF

OF

MEXICO

MAP OF SANTA ROSA ISLAND REGION

Bay

..,..,
i~~

r. -
'~""tlb~6~
a...

'' '

FIGURE I LOCAT ION

II. PURPOSES OF STUDY

The two primary purposes of the study include:

(1) The impact of the Pass on natural processes, and (2)

Recommendations relative to the conceptual design of the

Pass.

Impact of Pass on Natural Process

The various natural processes of interest in this

study are discussed in the following paragraphs.

1. Beach stability.--The possible effect of the Pass
on the adjacent beaches was the consideration of
greatest concern. Santa Rosa Island beaches are
presently some of the finest in the State, are
unencumbered by groin and seawall structures and
are relatively stable. The deleterious effects
on beach stability of inlet excavation and/or
modification along the Florida East Coast has
justifiably caused concern relative to future
inlet modification. The littoral drift*
characteristics in the area are particularly
relevant to the matter of beach stability. The
quantities and directions of littoral drift are
also of interest in the configuration of the
jetties, design of bypassing features and
financial provision to mechanically transfer
the sand interrupted by the presence of the Pass
and jetties.

2. Stability of neighboring passes.--The passes to
the east and west (East Pass and Pensacola Bay
Entrance, respectively) would be influenced to
some extent by the proposed Navarre Pass. It is
conceivable that the water flowing through the
Pass could "capture" a significant amount of the
flow presently occurring through East Pass and
Pensacola Bay Entrance, thereby causing these
*Glossary of terms is provided as Appendix III.

- 3 -

passes to shoal and a resulting increased
dredging requirement or a decreased equilibrium
cross-sectional area.

Conceptual Design Features of Navarre Pass

Based on the results of the study, recommendations

will be presented relating to the following conceptual

design features of the Pass:

1. Inlet dimensions and layout.--The primary factors
considered in the inlet dimensions and layout
will be: safe navigation, minimum effects on
adjacent beaches, effect on neighboring passes,
and maintenance costs.

2. Sand transfer and disposition of initial
excavation material.--The initial and maintenance
sand disposition including quantities will be
recommended so as to result in a minimum
interruption of the natural sand transport
processes and beach stability.

3. Channel protection.--Unless provided with
adequate protection against erosion, the banks
of the cut and dunes will erode due to water
and wind forces and tend to deposit in the Pass.
The resulting deposition of material can interfere
with navigation and cause an added dredging
cost. Rip-rap or vertical sheet piling will
represent the best form of bank protection in
the cut whereas vegetation, if properly maintained
could provide good protection against erosion
by wind of the dunes and portions of the cut
above water.

- 4 -

III. BRIEF HISTORY OF NAVARRE PASS

Navarre Pass was originally cut in July, 1965, by

a pipe-line dredge at a cost of $30,000. The original

dimensions were 100 ft wide by 9 ft deep. The primary

purposes of the Pass included a more direct access to

Snapper grounds and to provide a general economic

stimulus to this portion of the Santa Rosa Island area.

Figures 2 and 3 are aerial photographs prior to

the Pass dredging and shortly after the dredging,

respectively. The date of the photograph in Figure 3 is

not known, but was probably taken in August, 1965. Note

that some narrowing of the mouth of the Pass has occurred

on the east side indicating the effect of westerly

littoral drift. The photograph presented in Figure 4 was

taken on or about September 1, 1965, and presents a more

advanced case of deposition against the near-Gulf portion

of the east side of the cut. The shoaling is not apparent

from this photograph, but probably has reached an advanced

stage. Figure 5 represents a photograph taken in

September, 1965, after complete closure of the Pass.

Again the effects of the westerly littoral drift in dis-

placing the channel to the west are evident. Hurricane

Betsy occurred during September 8-11, 1965, and presumably

- 5 -

2 AERIAL PHOTOGRAPH
NAVARRE PASS CUT
PHOTOGRAPH: FEBRUARY

-6-

FIGURE

PRIOR
(DATE
14,

TO
OF
1963)

w

4W1C
J.

3>

2 i

L OI
Abr CS
4 -- oro
ilrjlok
~~4L

'o U

FIGURE 3 OBLIQUE VIEW OF

OF NAVARRE PASS

PROBABLY AUGUST,

GULF TERMINUS

(DATE: UNKNOWN,

1965)

-7-

41Wr:

-r

offiTRIM .0 T 1 .

FIGURE 4 AERIAL PHOTOGRAPH OF NAVARRE
PASS SHOWING EFFECT OF WESTERLY
LITTORAL DRIFT (DATE OF
PHOTOGRAPH : ON OR ABOUT
SEPTEMBER 1, 1965)

- 8 -

FIGURE 5 OBLIQUE VIEW OF NAVARRE PASS

SHORTLY AFTER CLOSURE (DATE OF

PHOTOGRAPH : SEPTEMBER 1965)

-9-

L
,~,
f
Ir ~C
-r- ~r- -n.
-e ~c,
-L-
---
-

L;
4

.,~c~--
r~Pyp- c. i -- -~--~

-,

~i~'

was instrumental in the rapid development of the final

closure stages.

As shown in Figure 6, by June, 1970, the Pass

had filled substantially so that the only remnants of the

channel remaining below water are at the Sound side of the

Pass.

According to R. Bruno (1), the Pass has been opened

naturally on at least two occasions since 1965. One of

these occurred during Hurricane Camille in August, 1969

and the other opening resulted from a winter storm. No

information is available concerning the extent of these

openings nor of the magnitudes of the resulting flows

through the Pass Site. Presumably the Pass closed fairly

rapidly after each opening.

- 10 -

- A ~ -I--

FIGURE 6 AERIAL PHOTOGRAPH SHOWING
FILLING OF PASS BY AIR
AND WATER TRANSPORTED
SAND (DATE OF PHOTOGRAPH:
JUNE 1970)

- 11 -

IV. METEOROLOGY AND HYDROGRAPHY OF AREA

General Description

The general offshore region near Santa Rosa Island

is characterized by prevailing easterly winds with strong

northerly winter winds. The easterly winds result in

predominately southeasterly waves occurring in the near-

shore region. These waves are responsible for the

predominately westerly littoral drift. Tides in this area

are predominately diurnal (i.e., a period of 24 hours) with

the diurnal tidal range at Pensacola listed at 1.3 ft.

During the field trips conducted in conjunction with this

study, Gulf tides were measured from the Navarre Pier with

tidal ranges in excess of 2 ft. Concurrent measurements of

the tides in Santa Rosa Sound demonstrated that the tidal

lag between the Gulf and the Sound generally varies between

2 to 32 hours and there is little if any reduction in

tidal range (at Navarre Bridge, where the Sound tidal

measurements were conducted). The Gulf nearshore currents

were not studied extensively, however during one field trip

an easterly current greater than 1 ft/second was measured

fairly near shore. On later field trips, existing near

shore currents were observed to be much weaker and were

not measured.

- 12 -

Winds

Data representing the offshore winds in the Gulf

of Mexico are available in Reference 2. These data are

the results of observations and measurements obtained

from ships; the data are presented as average monthly

conditions by the 5 degree latitude and longitude data

squares shown in Figure 7. For the square off the Navarre

Pass site, the monthly data are presented in Figure 8.

The most persistent winds are seen to be from the east

(easterly winds), with easterly winds of 11 to 16 knots

occurring 8% of the time and easterly winds of 17-27 knots

occurring 4% of the time. Calms occur about 11% of the time.

With the predominant easterly winds, it is clear that the

resulting predominant waves and littoral drift will be

directed toward the west.

During the period December through June, there are

reasonably strong southeast winds and during October through

March, fairly strong north and northeast winds occur.

Sea

The average distribution of sea (i.e., locally

generated waves, generally of short period) obtained from

Reference 2 are presented in Figure 9. Because sea

results from the local winds, there is a strong resemblance

between the wind roses presented in Figure 8 and the sea

roses. It should be stressed that these sea roses pertain

- 13 -

-4-

- -V

I I
I :I

--1

950 90 850

800

IN GULF OF MEXICO AND CARIBBEAN

a
C:5

750

700

FIGURE 7 DATA SQUARES

SEA

9475

S 10 20 30 40 50 60 70 80 90 100

January

9778

12 ---I--*

0 020 30 40 50 60 TO 80 90 (0

May

February March

11666

19

o 10 2 u0 n 40 50 60 70 80 90 100

June

I .. . .
j ...----- ----------- --~--

July

o 10 o20 30 0 50 6) 7 80 9C, I-C00

April

12412

24

o 10 2o 30 40 0 60 ro 8o 9 o10o

August

11886

II
Se---mber

0 10 20 30 40 50 60 70 80 90 100

September

0 o1 20 l o 40 50 60 70 80 90 00

October

10554

0 10 20 30 40 50 60 70 80 90 100

November

0 10 20 30 40 50 60 7 80 90100

December

FIGURE 8 MONTHLY WIND ROSES

AT DATA SQUARE OFF

NAVARRE PASS AREA

(SEE FIGURE 7)

DATA FROM REFERENCE 2

- 15 -

LEGEND
. NUMBER or -- 3 K 315
OBSERVATIONS KNOTS
A-10 11-16 11727 SB
23 )-cI -
% BEAUFORT
BEAUfORT 0-1 ----' -- PERCENT FREQUENCY -
ROSE SCALE (PERCENT FREQUENCY)
10 0 10 20 30 A40

DOUBLE CIRCLE INDICATES THEORETICAL WIND ROSE.

WIND SPEED SUMMARY (ALL DIRECTIONS)
BEAUFORT FORCE
CALM 2-3 4 5-6 7-12

0 10 20 30 40 50 60 70 80 90 100
PERCENT

o 0i 20 30 40 50 60 TO 80 90000

January

7274

0 10 20 0 40 50 60 O 80 90 100

May

0 10 20 30 40 50 60 70 80 90 100

September

5916

S10 20 40 40 W 60 70 80 90 100

February

0 i0 20 30 40 50 60 70 80 90 100

June

9523

4

0 0 20 30 40 0 60 70 80 90 100

October

March

8543

C' 10 2o 0 1. 4. 50 6- 70 PE0 90 '00

July

7621

3

0 November

November

6481

0 10 20 30 40 50 60 70 80 90 100

April

9129

0--0

0 10 20 30 4C 50 60 70 0B 90100

August

7344

I l'I"I.. m eI"r".1.11.1. II 11 11 ,1
o i0 20 30 40 50 60 70 80 90 100

December

FIGURE 9 MONTHLY

AT DATA

NAVARRE

"SEA" ROSES

SQUARE OFF

PASS AREA

(SEE FIGURE 7)

DATA FROM REFERENCE 2

LEGEND SEA
203--OBSERVATIONS
5 % CALM
3 % CONFUSED
SLIGHT (<3 FT.)
S U MODERATE (3--5 FT.)
L 0 ROUGH (5-8 FT.)
VERY ROUGH (8-12 FT.)
HIGH (?12 FT)

c 20
30
-40

0 20 30 40 50 60 70 80 90 100(%)
SUMMARY SCALE
(ALL DIRECTIONS)

- 16 -

to the data square off Santa Rosa Island as shown in

Figure 7 and the sea indicated as originating from the

north is not of concern in considering nearshore processes.

For this data square, 80% of the sea has a characteristic

(significant) wave height less than 12 ft.

Swell

The average monthly swell roses, determined from

Reference 2 are presented in Figure 10. As for the case

of the sea roses, the predominant swell affecting the

Santa Rosa Island shoreline would propagate from the

southeast, again contributing to a net westerly littoral

drift.

Tides

The tides are of particular importance in

maintaining an inlet open under the action of littoral

drift which, unopposed, would result in the closing of an

inlet. It is valid to regard the tidal "forces" and

littoral drift "forces" in opposition, with the tidal forces

being more or less predictable and periodic and the littoral

drift forces only predictable on an average seasonal basis.

Because periods of high littoral drift could result in the

closure of an inlet, the most effective tidal characteristics

for maintaining an inlet open would be a constant tidal

range. The "forces" to maintain an inlet open would then

- 17 -

5150

0 10 20 30 40 50 60 70 80 90 00

January

4741

0 10 20 30 40 50 60 70 80 90100

March

4402

27

0 10 20 30 40 0 60 70 80 90 100

April

4694

37

0 10 20 30 40 50 60 70 80 90 100

Moy

6260

111111 ll) 11lT11 fll n00IpT

September

5329
44
0

O 10 20 30 40 50 60 70 80 90 100

June

S 7141

32

0 10 20 30 40 50 60 70 80 90 100

October

6591
53

I ..]..,ItI..11.111i. YII ,,p 1,r.TITT]f
0 10 20 30 40 50 60 70 80 90 100

July

5281

25

0 10 20 30 40 50 60 70 80 90 100

November

6860

51

0 10 20 30 40 5, 60 to 80 9 1(o

August

5206

0 10 20 30 40 50 60 0 80 90 100

December

FIGURE 10 MONTHLY SWELL ROSES

AT DATA SQUARE OFF

NAVARRE PASS AREA

(SEE FIGURE 7)

DATA FROM REFERENCE 2

LEGEND-SWELL
203-OBSERVATIONS
5 -% NO SWELL
3 % CONFUSED
S LOW (1-6 FEET)
MODERATE (6-12 FT.)
0 HIGH(>12 FT.)
c-20
()-30

-40

0 10 20 30 40 50 60 70 80 90 100(%)
SUMMARY SCALE
(ALL DIRECTIONS)

- 18 -

always be operating at an effective level to counteract

any unusually heavy littoral drift occurrence. Unfortu-

nately, the tidal range in the Santa Rosa Island area of

the Gulf of Mexico is not nearly constant, but varies

greatly from spring to neap conditions. The tidal ranges

encountered during the different field trips varied from

a low value of 0.36 ft (4 inches) to an upper range of

2.2 ft. The tide tables indicate a ratio of maximum to

minimum tidal range of approximately a factor of 18.

Figure 11 presents a plot of the predicted tides for the

month of January and a portion of February, 1971 for

Pensacola, Galveston Entrance and Miami Harbor Entrance.

The low tidal range periods are indicated when an inlet

would be highly susceptible to deposition. An example of

the measured Gulf and Sound tides obtained during the

July 1971 field trip is presented in Figure 12.

Offshore Currents

During two of the field trips, attempts were

made to install a recording current meter in a water

depth of approximately 16 ft at a location about 900 ft

offshore of the Pass Site. The first attempt was

successful and resulted in a recording of approximately 24

hours duration, however the current meter malfunctioned

during the second attempt and no data were obtained. The

data obtained during the first field trip were quite

- 19 -

0
1.0

o
-J
0
-o
I-

GALVESTON ENTRANCE

Period of Period
Low Relative of High
Susceptibility Relative
to Deposition Susceptibility
to Deposition

PENSACOLA

January, 1971

February, 1971

FIGURE II PREDICTED TIDES AT GALVESTON, PENSACOLA AND
MIAMI HARBOR ENTRANCES. NOTE DIFFERENCES
IN TIDAL RANGE VARIATIONS

- 20 -

g

-1.0-i

M

flr

Note In This Figure, The Gulf And
Sound Tides Are Not Referenced
To A Common Elevation Datum

FIGURE 12

EXAMPLE OF MEASURED TIDES IN GULF OF MEXICO,
SANTA ROSA SOUND

AND

surprising and showed a strong easterly current (1.3 ft/sec)

at the time of installation which decreased to approximately

0.5 ft/second during the 24 hour recording period. These

data are presented in Figure 13. During the second field

trip, the divers noted while installing and recovering the

current meter that there was no appreciable current. It

is believed that the current measured during the first

field trip was perhaps due to some effect of the Gulf

Stream which does form a general clockwise circulation

pattern in the Gulf of Mexico. Because the information

pertaining to this current is very limited, it is not

possible to conclude whether the effect on littoral drift

is significant, however it is noted that if the prevailing

current direction is easterly, and if the current is

significant in the surf zone area, then the effect would

be to reduce the net westerly littoral drift.

Santa Rosa Sound Currents

During one of the field trips, currents were

measured from the Navarre Bridge over a 40-hour period.

These measurements were conducted with a nonrecording

current meter and therefore were taken intermittently.

The measurements are presented in Figure 14 where it is

seen that the maximum velocities are on the order of

1 ft/sec.

- 22 -

NOTE: CURRENT METER LOCATED APPROXIMATELY
900 FT. OFFSHORE OF PROPOSED INLET
SITE WATER DEPTH = 15 FT. DISTANCE.
OF METER ABOVE BOTTOM = 6 FT.

2000

TIME (hours)

2400

I I I 1 I t ~I ~ 1_1_Y 1 ~E\hL

0400

0800

1200

MAY 16

FIGURE 13

MEASURED

CURRENTS

OFFSHORE

NAVARRE

FLORIDA. MAY 15 16, 1970

2.0 -

1.0

1200

1600

MAY 15

BEACH,

I I I

I

I I I

-E-

Smoothed Curve Drawn
Through Measurements

I I I l i\li l l l l 1 1 1 l II l
1800 0000 0600 1200 1800 0000 0600
SDec. 9, 1970 Dec. 10, 1970 Dec. II, 1970

___ ___--1- 1

0

Note: See Figure 16
For Location of
Current Measurements

_ III

MEASURED AT NAVARRE BRIDGE, DECEMBER

0

10
4-
I-

9-11, 1970

FIGURE 14 CURRENTS

V. FIELD STUDIES AND RESULTS

Three separate field studies were carried out

during the study. The dates and programs carried out

during these three studies are presented in Table I.

A brief description of each of the field studies

is presented below.

Field Trip No. 1, May 13-18, 1970

During this field trip a baseline was established

which ranged approximately 1000 feet east of the centerline

of the proposed inlet to 2000 feet west of the centerline.

The baseline was located shoreward of the active beach pro-

file on the foredunes to reduce losses of the stakes. Beach

profiles and offshore soundings were conducted and the con-

toured results are presented as Plate I in the report cover

jacket. Two tide gages were installed: one was located

in the Sound in the vicinity of the south bridge section of

the Navarre Bridge; the second was installed on the Navarre

Pier near its seaward end. The tides during this period

were quite small. The predicted tides at Pensacola, Florida

ranged from 0.3 to 0.7 feet. The recorded tides are pre-

sented in Figure 15 and Figure 16 shows the locations of the

tide gages and other field measurements. From the tide

records, very little difference in Bay and Gulf tidal

- 25 -

TABLE I

Summary of Field Trip Activities and Information Obtained

Data Obtained

Dates Encompassed Gulf Sound Beach Offshore Additional Activities and
by Field Trip Tides Tides Profiles Soundings Data Obtained

May 13-18, 1970 Yes Yes Yes Yes 1. Baseline Established
(36 Hour (36 Hour 2. Offshore Currents
Duration) Duration) Measured (24 Hours)
3. Sand Samples Collected
4. Sound "Tidal Division
Line" Established

Dec. 7-12, 1970 Yes Yes Yes No 1. Sound Currents
(36 Hour (36 Hour Measured From Navarre
Duration) Duration) Bridge (40 Hour
Duration)

July 20-25, 1971 Yes Yes Yes Yes 1. Experimental "Sand Bag
(24 Hour (24 Hour Groin" for Littoral
Duration) Duration) Drift Observations
Constructed

I

I

- +0.5

E 0
0

-0.5

- +0.5

0
11
Q,

-0.5

+0.5

.'
0

w

-0.5

0900

1200

1500

1800

2100

0000

0300

0600

FIGURE 15

MEASURED TIDES IN GULF OF MEXICO AND IN SANTA ROSA SOUND
DURING FIELD TRIP OF MAY 13-18, 1970

Santo Rosa Sound Tides May 14, 1970 May 19

Gulf of Mexico Tides

I I I I I I I I I I 1 1 11

V ~J ~vl

,,

S I I I I I
0 5000
Scale (ft) ...

Navarre Bridge

Sound Tide

ound Current Measurements
Sound
soso
SOno /C Site of Orig

'--C Extent

Tide Gage

16 LOCATIONS

of Gulf Bathym

Pier

OF PRINCIPAL

MEASUREMENTS

etric Survey

CONDUCTED

FIELD TRIPS

Se.xico

Gulf

FIGURE

.--

DURING

amplitudes could be determined. An attempt was made to

determine the location of the "tidal division line" in the

Sound. This is a somewhat hypothetical separation line of

the area (to the West) which is served (alternately filled

and drained) by Pensacola Bay Entrance and the area (to

the East) which is served by the East Pass to Choctawhatchee

Bay. This line is affected greatly by winds and under

conditions of very high winds may not exist at all. The

location shown in Figure 17 was determined on May 16 by

searching until the velocity some distance to the west of

that location was toward the east and the velocity some

distance to the east of that location was directed toward

the west. Computations using the numerical model described

in Appendix I indicates that the position of the "tidal

division line" varies during a tidal period from approxi-

mately 15 miles east to 20 miles west of the Navarre Pass

Site.

Offshore currents were determined by installing a

current meter approximately 900 feet offshore of the

location of the proposed pass. The current flowed to the

east during the two day period the meter was installed,

see Figure 13. The peak current was approximately

1.3 ft/sec. Thirteen nearshore sand samples were collected

and later analyzed for grain size distribution. Table II

summarizes the results of the analysis of median diameters

- 29 -

I I I

Scale Statute Miles)oun

Santa Rosa Soun
0

"---Tidal Division Line

FIGURE 17 LOCATION OF TIDAL DIVISION LINE AT 1545 ON
MAY 16, 1970

d

TABLE II

Summary of Median Diameters of Sand Samples
Analyzed (Field Trip of May 1970)

Sample Collected Sample Location on Sample Median
at Station Beach Profile Diameter (mm)

2 ft Below Mean
Sea Level

Base of Small Scarp

Midshore Between
Waterline and Berm

2 ft Below Mean
Sea Level

Limit of Wave
Uprush

Foredune

Midway Between
Berm and Scarp

Base of Scarp

2 ft Below Mean
Sea Level

Midway Between
Berm and Scarp

2 ft Below Mean
Sea Level

Limit of Wave
Uprush

Base of Scarp

________________________ J I

0.42

0.35

0.50

0.51

0.37

0.45

0.46

0.36

0.50

0.40

0.33

0.47

0.37

- 31 -

10+00 E

10+00 E

10+00 E

0+00

0+00

0+00

10+00 W

10+00 W

10+00 W

20+00 W

20+00 W

20+00 W

20+00 W

of the samples. It is seen that the median diameter

ranges from 0.33 mm to 0.51 mm. This represents a

relatively coarse beach sand for the State of Florida.

Field Trip No. 2, December 7-12, 1970

During this field trip, the waves became quite

high on the morning of December 8, thereby precluding

the possibility of launching a boat through the surf to

conduct offshore soundings. Beach profile measurements

were conducted and are shown in Figure 18.

It is of interest to note that, in places, the

sand accumulation in the former Navarre Pass "cut" was

18" from May 1970 to December 1970 as determined by noting

the burial of the stake at Station 0+00. It is not known

whether this accumulation was primarily due to wind-blown

sand or sand transported over the berm by combinations of

high tides and waves.

Two tide gages were installed at the same locations

described for the previous field trip. The predicted tides

at Pensacola Bay during this trip ranged from 1.2 feet to

2.1 feet. The tide records for the period December 8-11

are shown in Figure 19. It was found again that there

was little difference in tidal amplitude between the Sound

and Gulf and also that the tidal lag was between 3 and 4

hours.

- 32 -

-Station 20 + 00 W

100

Distance From Baseline (ft)

N200

Station 18+00 W

O<)ro 100 Base200
Distance From Baseline (ft)

-Station 16+00 W

0 _

100

200

Distance From Baseline (ft)

Station 14+00 W

O) 100 2 __00
Distance From Baseline (ft)

FIGURE 18 BEACH PROFILES, DECEMBER
S33 -

9, 1970

C-l

w-J
8--s/

4-
YV)

(0
5s

'iu<

C-1

0
* S

_-J

W<

(

+ 101-.- I I I -

v

p IF I

LAJ

0

1.J<
Ui

Distance From Baseline (ft)

0

Station 8+00 W
OS~ O
O ,. ,,,

I tuu J-
Distance From Baseline (ft)

Station 6+00 W

,\ I

Distance From Baseline (ft)

18 (CONTINUED) BEACH PROFILES,
DECEMBER 9, 1970
34 -

-Station 12+00 W

0 100- 200
Distance From Baseline (ft)

Station 10+00 W

0o. .,,,, .

-

I<

Z.0
Io

+10

_-

SU<
0!

FIGURE

2W

IW

+k

c.

1u u

100_

S-Station -4+00 W

-|,^ I

200

- 2

0

Lu<

FIGURE 18 (CONTINUED)

BEACH PF
DECEMBER

- 35 -

PROFILES,
9, 1970

Distance From Baseline (ft)

--Station : 2+00 W

o 100 200
Distance From Baseline (ft)

---- Station 0+00

S100 200
o----------------- ----t-^s^- ----- -----
Distance From Baseline (ft)

Station 2+00 E

200Distance From Baseline (ft)
Distance From Baseline (ft)

I-
_jl
U,

50
WE
U<

4t-
0

>

_.-

-I,
CX

C

+IC

+IC10

IV

0

Station 10+00 E

0_200
Distance From Baseline (ft)

BEACH

DECEMBER

PROFILES,

9, 1970

- 36 -

-Station 4+00 E

0 ___ ___) 2(o0
Distance From Baseline (ft)

>u)

0s

o

0

Wl<
* .<

0-J

Ld'

+1

_-

W)
w-

FIGURE

18 (CONTINUED)

i
~tn

.. +1.0

E+ 0.5
52 0
0
W o

0. -0.5
w--

-1.0

+1.0
o

-.0

+ 1.0
6Q
.+0.5

E
.0O

, 0
a

0 -0.5

'~
.+0.5

**s-

0900 120 1500 1800 2100

4-
-Note: December 10, 1970
Gulf and Sound Tides
Not Referenced to Same Datum

Sound

I I I

S 0300 s0600

december II, 1970

FIGURE 19

MEASURED TIDES IN GULF OF MEXICO AND IN SANTA ROSA
SOUND DURING FIELD TRIP OF DECEMBER 7-12, 1970

- 37 -

- Gulf

II I I I I I

~7=-~

-

L

D-

-l.v .

A current meter was also installed offshore,

however the high wave activity caused a mechanical

failure and no data were obtained. It was noted during

installation and retrieval of the current meter, that

net currents were relatively small.

Sound currents were measured over a 40-hour

period. The results of these measurements have been

presented as Figure 14. The maximum velocities were in

excess of 1 ft/sec. The significance of the Sound currents

and their relationship to the tides will be discussed

later when the calibration of the numerical model is

presented.

Field Trip No. 3, July 19-23, 1971

In addition to the type of information collected

on previous field trips, a temporary sand bag groin was

installed to act as a partial littoral drift barrier. The

hydrographic information collected during this field trip

included beach and offshore soundings and Gulf and Sound

tide records.

The Gulf tidal range measured was approximately

2 ft and the tidal lag between Gulf and Sound was in the

range 3h 20m to 4h 20m. See Figure 20 for the tide

records which were measured during a 48-hour period.

The contoured results of the beach profiles and

offshore soundings are presented as Plate II in the cover

- 38 -

SI___

July 21, 1971
Sound
I Gullf
I111 I11 I I I I11 I

July 22 197I1

I II I I

0900 1200 1 0 18 2100 03 0
Time (Hours)

f^ A^ ^

-I.ut- -v--

+1.0

July 22, 1971 July 23, 1971
SSound
-Gulf

Note: Gulf and Sound Tides
Not Referenced to Same
Datum
-I.0

FIGURE 20 MEASURED TIDES IN GULF OF MEXICO AND IN SANTA
ROSA SOUND DURING FIELD TRIP OF JULY 19-23 1971

- 39 -

E

2
w

/'N

TI II

jacket of this report. In order to carry out these

surveys, it was necessary to reestablish much of the

baseline which had been destroyed by four-wheel vehicles

and other extraneous activities.

An experimental sand bag-groin was constructed

at the site of the planned Navarre Pass. The purpose of

this groin was to perform as a partial littoral drift

barrier and through observations of the impoundment, to

provide a qualitative indication of the direction and

persistence of the nearshore littoral drift. Two

photographs of the groin under construction are shown in

Figure 21. Unfortunately, due to settlement, the groin

was only effective for a period of 2 months and the

groin was not maintained because the required sand bags

were not available for a period of 4 to 6 months. The

performance of the groin will be discussed in "Section

VI Littoral Drift" and photographs (kindly taken by

R. W. Slye) will be presented.

- 40 -

July 1971 Groin Under Construction

I_ .. .-- -

July 1971 Groin Nearing Completion
Extreme Low Tide

FIGURE 21. EXPERIMENTAL GROIN UNDER CONSTRUCTION

VI. LITTORAL DRIFT

Introduction

In considering the establishment of a new inlet,

the magnitude and direction of sand transported in the

nearshore region (littoral drift) by waves and possibly

currents are most important factors and also the most

difficult to establish accurately.

Jetties act to block sand from entering inlets,

thereby rendering them more suitable for navigation. In

performing this function, jetties interrupt the natural

flow of sand (littoral drift) along the shore with the

resulting accumulation of sand on the updrift side of the

jetties. Since the waves maintain their sand-transporting

capacity downdrift of the jetties, serious erosion can

occur with long-term degradation of the downdrift beaches.

In addition to interrupting the natural flow of

sand along a beach, the interaction of inlet currents and

sediment causes bars to be built offshore and in the inner

bay. The material comprising these bars is derived from

the natural sand system and therefore represents a loss

to that system. Of course, once these bars have been

established to near-capacity, then subsequent annual losses

are reduced.

- 42 -

L

In considering the establishment of a new inlet,

the sand must be recognized as a valuable resource and

the sand transfer as a natural process. Interruption of

the sand transfer or the net loss of a significant amount

of sand from the active system will definitely lead to a

significant adverse effect On the downdrift beaches. In

recognizing the significance of these processes and the

necessity of maintaining the stability of the Santa Rosa

Island beaches, the cutting and stabilizing of a new inlet

should be planned to minimize any net loss of sand to the

system and also to provide for the mechanical transfer of

the sand interrupted by the presence of the jetties. A

number of attempts have been made to design jettied inlets

such that sand is prevented from interfering with:

navigation, yet the currents and waves still provide for

the natural transfer of sand. A survey of jettied inlets

will demonstrate that this approach has not proven

successful and that the only effective concept is to

provide for the artificial bypassing of sand.

Littoral Drift Estimates

Littoral drift estimates can be based on field

measurements or on calculation procedures using wave data.

Each of these approaches has advantages and disadvantages.

Good accuracy in field measurements of littoral

drift requires a near-complete trap (e.g. a long jetty)

- 43 -

and reasonably long records in which the trap impoundment

history is documented and/or records are kept of the amount

of material removed or added to maintain stability of the

downdrift shoreline. Littoral drift calculations are

based on wave measurements and/or observations; to date

(1972) the calculation procedures have not been developed

and verified to the degree that a high degree of confidence

is warranted. The most often applied calculation procedure

(3) does not account for many presumably important

parameters, including

(1) sand size

(2) sand specific gravity

(3) beach slope

(4) beach roughness

In attempting to develop the best estimate of

littoral drift, all sources of information should be

reviewed with relative confidence based on the particular

circumstances attending each measurement or calculation.

Field Measurements.--The available field

measurements in the Navarre Pass area are generally based

on the westward rate of growth of the western ends of

barrier islands and on the accretion behind the eastern

jetty at Perdido Pass.

Based on the rate of growth of the western end

of Santa Rosa Island, and the dredged quantities in the

- 44 -

Bay Entrance and on the shoals, the U. S. Army Corps of

Engineers (4) has concluded that the average annual

westward and eastward drift are 130,000 and 65,000 cubic

yards, respectively, resulting in an annual net westward

drift of 65,000 cubic yards.

In 1938, F. F. Escoffier (5) analyzed the westward

growth rates of the eastern shore of East Pass (entrance to

Choctawhatchee Bay), the results available at that time

indicated an annual deposition rate of 26,300 cubic yards,

although Escoffier noted that this quantity is undoubtedly

smaller than the net littoral drift due to some bypassing of

material past the inlet.

The Corps of Engineers (4) estimates the westerly

and easterly drift components at Perdido Pass to be

130,000 and 65,000 cubic yards per year resulting in a

net westerly drift of 65,000 cubic yards per year. During

the period May 1969 to March 1970, the Corps measured the

deposition inside the Perdido Pass weir jetty to be

146,000 cubic yards. Hurricane Camille occurred within

this period and may account for the higher than anticipated

impounded quantities. For the period 1934-1953 (before the

weir jetty system was constructed), J. W. Johnson (6)

analyzed accretion at the eastern bank of Perdido Pass

and concluded the annual deposition to be 200,000 cubic

yards; presumably this would correspond approximately to the

net westerly littoral drift.

- 45 -

Calculations of Littoral Drift.--D. S. Gorsline (7)

conducted a one-year study of Gulf beaches extending from

Keaton Beach, Florida to Gulf Shores, Alabama. His study

included monthly surveys of fifteen beaches in the study

area and wave observations. Gorsline carried out

calculations which indicated the gross drift rates at

Pensacola to be approximately 200,000 cubic yards per year

with a net westerly drift of 78,500 cubic yards/year.

It should be stressed that Gorsline's calculations at

each location were based on only one observation per

month over a period of one year. There is a good

likelihood, therefore that his results are not representative

of average annual conditions.

T. L. Walton (8) has carried out computations

of littoral drift along all of the sandy beach segments of

the State of Florida. The calculations are based on

long-term wave observations collected by military and

commercial ships. The wave characteristics are transformed

to shore using standard procedures and drift is calculated

based on the usual relationship (3). In comparing his

predictions with other estimates for the Florida East

Coast, Walton found generally good agreement for portions

of the northern Florida east coast, however his calculated

values were much higher than estimates based on impounded

quantities along the lower Florida east coast. This

difference was attributed, at least in part, to the

- 46 -

proximity of the Gulf Stream and its effect in causing an

increase in height of waves propagating from the north-

east. This would qualitatively explain the differences

noted. In the Navarre Pass area, Walton's calculated

annual westward and eastward drifts are approximately

400,000 and 100,000 cubic yards, respectively, resulting

in a net westward littoral drift of 300,000 cubic yards

per year.

The Coastal Engineering Research Center (CERC)

collects shore-based observations in a program entitled

"Littoral Environmental Observations" (LEO). The LEO data

are generally taken daily and include visual estimates of

breaking wave height and breaking wave direction in terms

of a coastal sector method, see Figure 22. These data

provide an alternate basis of estimating littoral drift,

using the usual calculation procedure and assuming that

the wave conditions reported are representative for the

entire 24-hour period. Data were provided by C. J. Galvin

and A. De Wall of CERC for four locations: Navarre Beach,

Grayton Beach, Beasley Park and Crystal Pier. The period

over which data were available ranged from 8 months at

Navarre Beach to 24 months at Beasley Park and Crystal

Pier. See Figure 23 for a map of the four observation

locations. Table III summarizes the littoral drift values

calculated from the LEO data.

- 47 -

If No Waves,
Fill in Zero

Observer
Observer

LAND
WAVE DIRECTION CODE FOR WAVES AT BREAKING

FIGURE 22 COASTAL SECTOR METHOD USED
ENGINEERING RESEARCH CENTER
REPORTING WAVE DIRECTION

BY
IN

COASTAL

- 48 -

OCEAN

5

Shoreline

I

Pensacola Bay

thatchee Bay

GULF OF MEXICO

Note: Arrows and Numbers Indicate
Calculated Directions and
Net Annual Littoral Drift
(in Cubic Yards/Year) Using
Leo Data

FIGURE 23

LOCATIONS OF CERC LEO DATA USED IN LITTORAL
DRIFT CALCULATIONS. DRIFT DIRECTIONS AND NET
ANNUAL RATES ALSO SHOWN

TABLE III

Summary of Calculated Littoral Drift Using LEO Data

Results Averaged Annual
Duration of Data Calculated Littoral Drift Net Drift
Location Available Interval (Cubic Yards) (Cubic Yards/Year)

Navarre Beach 8 months Jan. 1, 1970 to
Sept. 1, 1970 158,000(W)* 237,000(W)

Grayton Beach 24 months Dec. 1, 1970 to
Dec. 1, 1971 53,345(W) 52,100(W)

Dec. 1, 1971 to
Dec. 1, 1972 50,776(W)

Beasley Park 24 months Jan. 1, 1971 to
Jan. 1, 1972 30,064(W) 45,200(W)

Jan. 1, 1971 to
Nov. 1, 1971 50,321(W)

Crystal Pier 12 months July 1, 1971 to
July 1, 1972 253,331 253,331(W)

*(W) denotes drift from East to West.

Experimental Groin at Navarre

An experimental sand bag groin was constructed

at the site of the original Pass. The purpose of the

groin was to obtain information regarding the variability

and (hopefully) magnitudes of littoral drift. The groin

was constructed on July 22, 1971 and extended 100 ft

seaward of the mean high water line. The groin was about

3 ft high by 8 ft wide; two photographs showing the groin

under construction have been presented as Figure 21.

The groin was reasonably effective in trapping

the nearshore portion of drift for a period of approximately

2 months, after which the portion of the groin traversing

the beach face was undermined and settled significantly

(about 4-6 ft). At that time, it was planned to rebuild

the groin by adding more sand bags. Unfortunately the

sand bags were not available* for a period of 4 to 6

months and the experiment was discontinued.

A brief photographic history of the groin is

presented in Figure 24.** During the period August 5 to

August 8, 1971, impoundment occurred on the west side of

the groin. On the morning of August 9, impoundment was

evident on the east side of the groin which remained the

*The factory had experienced a fire.

**Captain R. W. Slye of the Santa Rosa County Beach
Administration kindly offered to photograph the groin.

- 51 -

August 5, 1971 Note Slight Build-up
0800 From West

August 8, 1971 Continued Accretion
0800 on West Side of Groin

FIGURE 24. PHOTOGRAPHIC HISTORY OF NAVARRE EXPERIMENTAL GROIN
(Photographs Taken by Captain R. W. Slye)

0r

I

August 9, 1971
0900

- Accretion is Now Apparent on
East Side of Groin. Some
Transport of Sand Over Groin
is Evident

August 20, 1971 -

Some Evidence of Lessened
Easterly Drift Compared to
Photograph of August 9, 1971

FIGURE 24. PHOTOGRAPHIC HISTORY OF NAVARRE EXPERIMENTAL GROIN (continued)
(Photographs Taken by R. W. Slye)

Ln
CW

August 28, 1971
0800

- Accretion on Western
Side of Groin Compare
With Photographs of
August 9 and 20, 1971

September 2, 1971
0830

- Evidence of Drift Reversal
Compared to August 28, 1971
Photograph. Also Note
Lowering of Middle Portion
of Groin Due to Undermining

FIGURE 24. PHOTOGRAPHIC HISTORY OF NAVARRE EXPERIMENTAL GROIN (continued)
(Photographs Taken by R. W. Slye)

cn
41h

CTI
(en

7 ,,.
1-" "A

September 8, 1971
1200

- Same General Accretion
Situation as Shown on
September 2 Photograph

September 13,
0900

1971 Groin Has Been Flanked
With Scarping to East
(Also See Following
Photograph of Same Date)

FIGURE 24. PHOTOGRAPHIC HISTORY OF NAVARRE EXPERIMENTAL GROIN (continued)
(Photographs Taken by R. W. Slye)

September 13, 1971
0900

- Showing Effect of High
Tides and Easterly Drift
and Escarpment

September 22, 1971
1130

- Drift Accumulation on
East Side of Groin
Indicates Reversal
From September 13
Photograph

FIGURE 24. PHOTOGRAPHIC HISTORY OF NAVARRE EXPERIMENTAL GROIN (continued)
(Photographs Taken By R. W. Slye)

October 1, 1971 -
0830

Groin Profile Has Been
Lowered Significantly
Due to Undermining.
Groin is Now Generally
Ineffective for Drift
Impoundment

October 7, 1971 Drift Passes Over
0800 Groin in Beach Face
Region

FIGURE 24.

PHOTOGRAPHIC HISTORY OF EXPERIMENTAL GROIN (continued)
(Photographs Taken by R. W. Slye)

U,

o0
00

<-

1

1. ~ce
r
r
r
Ir
.' ,
.~ -
r --hlrli
'rr.
''~~- ~C- 11
r~r
I
rr_
i
4 -;i
1 L ~ ; ~a~z4i~c; ~4*~-~-~-s
c~ 5~' rv;k? ~p~~-
C I
--gr r I" r"r
1
i" ~C. i
'I *i C, r
.,I
I f
e --L --
I. h~ ;~v";;
e
I
r
Ci) i
-t, ~g~p~,i.~- i~ i -r

October 14, 1971

0900

V j

-l.

- Beach Accretion Has November 30, 1971

Nearly Completely 0800
Buried Shoreward

One-Third of Groin

b

- Groin Ineffective as

Littoral Drift Impediment.
Photograph Taken at Low

Tide

FIGURE 24. PHOTOGRAPHIC HISTORY OF NAVARRE EXPERIMENTAL GROIN (continued)
(Photographs Taken by R. W. Slye)

4 010

December 16, 1971 Final Photograph of
0800 Experimental Groin

FIGURE 24.

PHOTOGRAPHIC HISTORY OF NAVARRE EXPERIMENTAL GROIN (continued)
(Photographs Taken By R. W. Slye)

r

dominant side through August 20. By August 28, the

drift had reversed again and the impoundment was on the

west side of the groin. On September 2, 1971, impoundment

had occurred on the east side and the first groin subsidence

is evident. By September 13, 1971, the groin had been

flanked by high waves and tides and the impoundment was

on the west side of the groin. By September 22, the drift

evidence was from the east. The photographs on October 1,

1971, and thereafter show that the upper one-third of the

groin had subsided to such an extent that it would no

longer be effective in impounding littoral drift.

Although it is clear that the groin installation

was not effective to obtain quantitative evidence

regarding the littoral drift, it is of interest that

during the two month period over which it was effective,

the impoundment indicated nearshore drift reversal at

least six times. Furthermore, because the nominal

interval between photographs is one week but was as great

as eleven days, it is likely that more reversals than

noted had taken place. It is noted that the months of

August and September are not expected to be the months of

heaviest nor most persistent drift. The sea and swell

charts for this period, however, do indicate that for

average August and September months, net drifts to the

west are to be expected.

- 60 -

Although it is not possible to draw strong

conclusions from the experimental groin due to the short

period over which it was effective, it does appear that

drift rates based on the ship data would yield drift

rates to the west that would be unrealistically high.

Summary and Recommendations

Several littoral drift estimates in the Navarre

Pass area have been presented. These estimates all

indicate a net westward drift with net magnitudes ranging

from 65,000 to 300,000 cubic yards per year. This range

represents a factor of 4.6 which is not too surprising

considering the present state of knowledge of littoral

drift quantities.

Considering the estimates available, it is

believed that the net annual littoral drift is something

less than 200,000 cubic yards to the west. It will be

recommended that the inlet maintenance be planned to

provide transfer of 200,000 cubic yards per year with the

understanding that the actual amount required is expected

to be less than this value. This represents a responsible

approach to the problem of maintaining the littoral drift

and it is realistic to reduce the amount of sand transfer

below that planned, however, the financial and equipment

problems attendant with increasing the sand transfer above

that originally planned argue against arranging for a

lesser amount.

- 61 -

VII. SUMMARY OF NUMERICAL MODEL CALCULATIONS

Introduction

In order to represent the behavior of Navarre

Pass and its interaction with adjacent entrances a

computer method which simulated the flows into and through

the Santa Rosa Sound system was developed and applied.

This method is called a "numerical model" as opposed to a

hydraulic model and has the advantage of rationally

incorporating the interaction of Navarre Pass with the

tides and flows in Santa Rosa Sound and also with the

flows through the entrances to Choctawhatchee and Pensacola

Bays.

The basis for and evaluation of the numerical

model are described fully in Appendix I "Numerical Model

of the Bay System Affecting Navarre Pass." After evalua-

tion for the present situation in which no flows occur

through Navarre Pass, the model was modified and used to

evaluate the effect of Navarre Pass on flows through

neighboring inlets and also to calculate the expected

velocities through Navarre Pass. Appendix II "Stability

of Navarre Pass" presents an evaluation of the tendency of

Navarre Pass to close by comparing the sedimentary stability

- 62 -

with Rollover Pass, Texas which is an artificial inlet

which grew rapidly after opening. The results of applying

the numerical model are described in detail in Appendixes

I and II and are presented briefly in the following sec-

tions.

Results Obtained Using the Numerical Model

Effect of Navarre Pass on Entrances to Pensacola and
Choctawhatchee Bays

The percentage changes in flows through Pensacola

and Choctawhatchee Bays due to the influence of Navarre

Pass were evaluated for various Gulf tidal ranges. These

results are tabulated in Table I-4 (Appendix I). It was

found that, as expected, the presence of Navarre Pass

would decrease the total inflows and outflows through the

entrances to Pensacola and Choctawhatchee Bay Entrances.

The largest percentage effect was on Pensacola Bay

Entrance due to the Sound being of greater width between

Navarre Pass and Pensacola Bay than between Navarre Pass

and Choctawhatchee Bay.

For a tidal range of 1.5 ft (approximate average),

the percentage reductions in the maximum flows in and out

of Pensacola and Choctawhatchee Bays are 2.0% and 0.1%,

respectively. For a more complete summary refer to

Table I-4 in Appendix I. The reduction in tidal flows

- 63 -

into Pensacola Bay will result in an associated

reduction of 1.8% in equilibrium cross-sectional flow

area into this Bay. For Choctawhatchee Bay the equilibrium

flow area will be reduced by 0.1%.

Maximum Velocities Through Navarre Pass

The peak velocities averaged over the Pass cross-

section are calculated to vary from 1.27 ft/sec to 3.54

ft/sec for Gulf tidal ranges varying from 0.5 to 2.0 ft,

respectively. This range of velocities is well within

acceptable limits for small craft navigational safety. A

more complete summary of maximum velocities is presented

in Table I-5 of Appendix I.

Relative Stability of Navarre Pass

Computations were carried out in Appendix II to

compare the tendency of Navarre and Rollover (Texas)

Passes to remain open. These two passes have respective

histories of closure and growth following their initial

openings. The calculations showed that the geometric and

tidal conditions at Navarre Pass are much less conducive

to remaining open without jetties than at Rollover Pass.

These calculations simply reinforce the known requirement

for jetties at Navarre Pass.

- 64 -

VIII. RECOMMENDED DESIGN OF NAVARRE PASS INLET

Functional Design

In developing a functional design of Navarre Pass,

the primary factors considered were: (1) minimum adverse

effects on adjacent beach stability through effective sand

by-passing and placement of initial sand dredged, (2) nav-

igational safety for craft using the Pass, (3) improvement

of water quality within the Sound adjacent to the Pass, and

(4) a minimum of required costs associated with the periodic

maintenance of the Pass.

Some of the factors noted above conflict, for

example the effective by-passing of sand will be fairly

expensive. In the recommendations pertaining to the layout

and planning for the Pass, the highest priority will be

given to beach stability and navigational safety.

Recommended Layout of Navarre Pass

Prior to discussing the recommended layout of

Navarre Pass, it is emphasized that it is not intended to

present a final detailed design, but rather a workable

conceptual design which is in accordance with the objec-

tives presented in the preceding section. The Santa Rosa

County Beach Administration and their engineers will make a

- 65 -

detailed engineering design and will make modifications to

the recommended layout which facilitate or reduce the cost

of construction. These modifications, however, should not

significantly impair the performance of the design.

The recommended layout of Navarre Pass is presented

as Plate III in the jacket in the back cover of this

report. The main components of the Pass include: (1) a

weir jetty and deposition basin for the trapping and

retention of sand until by-passed to the downdrift (west)

side of the Pass, (2) a training wall on the Pass sides to

provide lateral stability of the cut, (3) a navigational

channel, 12 feet deep by 150 feet wide extending through

the Island to the 12 feet contour on the Gulf side and to

the Intercoastal Waterway on the Sound side, and two

jetties extending into the Gulf, and (4) either mechanical

or vegetative control of wind drift of sand. Each of these

features is discussed separately below.

1. Weir Jetty and Deposition Basin-A weir jetty

and deposition basin (sand trap) are recommended with the

weir section 400 ft long, oriented parallel to shore and

with the weir crest elevation at the approximate present

mean sea level contour. The design and construction of

the weir are to be such that minor required increases in

weir elevation can be accomplished by the addition of

stone. In considerations of weir stability, the design

should account for the expected variations in sand

- 66 -

elevations on both sides of the weir. The weir design

recommended is similar to that at Hillsboro Inlet, see

Figure 25. The weir section at Hillsboro Inlet is 200

ft long and the elevation of the weir crest is at MSL.

The eastern end of the weir should be tied into the

natural dune system in order to prevent flanking and a

short adjustable groin should be located at the eastern

end of the weir in order to provide a control on the

stability of the updrift (eastern) beaches.

The expected performance of the weir/deposition

basin is as follows. After initial or maintenance

dredging of the basin, the predominately westward littoral

drift will deposit in the basin at the eastern end of the

basin. If the tides and waves are low during this period,

a spit will grow toward the west and will be located on

the Gulfward side of the weir. During periods of high

tides and/or high waves, the sand forming this spit will

be carried further into the deposition basin and the weir

will be re-exposed. For the dimensions of the basin

shown, the volumetric storage below MSL are 48,000 and

64,000 cubic yards based on a 1:3 side slope and maximum

basin depths of 12 and 18 ft below MSL respectively.

Depending on the quantities of net westerly littoral

drift, the basin would require maintenance dredging and

by-passing to the west side of the inlet on a frequency

ranging from 1 to 3 times a year if carried out on a

- 67 -

25 WEIR JE
HILLSBORO

TTY

SYSTEM

INLET,

FLORIDA

- 68 -

FIGURE

demand basis. Because the heaviest littoral drift is

expected during the winter, the required dredging may be

more frequent during this season. Due to the present

uncertainties in net littoral drift magnitudes, it will be

difficult to realistically address the problem of mainte-

nance dredging and by-passing to the west if this is

planned to be done on a contract basis. An alternate

concept providing flexibility would be a relatively small

custom dredge built for and operated under the direction

of an agency established for the overseeing of the Pass

operation and maintenance. This would also allow any small

amount of maintenance dredging required in the channel or

at the tips of the jetty to be carried out during relatively

calm wave conditions which would be difficult to schedule

in advance on a contract basis. The Hillsboro Inlet

District has successfully operated their small custom

dredge for by-passing and minor maintenance dredging in the

channel and marina for over eight years.

The possibility of the northern portion of the

deposition basin providing a recreational facility could

be considered. The wave energy at this point will be

reduced and the beach slope could be controlled. A

marina occupying a portion of the basin is another

possibility, but would be reduced in value due to land

access problems, especially if a bridge spanning the

Pass is not constructed.

- 69 -

2. Training Wall on Sides of Cut--The banks of

the cut should be stabilized with sheet piling or rubble

protection to prevent sloughing and erosion of the sides.

The consequences of not providing a means for bank

stabilization will be a widening and shoaling cross-

section, and increased maintenance and possibly a migrating

interior channel which would also result in the need for

increased maintenance. There are advantages of reduced

reflection of boat waves if a rubble mound bank protection

is chosen. The cut through St. George Island (Figure 26)

is an example of erosion if no training walls are

provided.

3. Navigational Channel and Jetties-It is

understood that the Navarre Pass Committee desires a

12 ft deep channel to match the depth of the Intercoastal

Waterway through Santa Rosa Sound. The desired width of

the 12 ft depth portion is 150 ft with a somewhat greater

width of the remaining portion of the cut. Some of these

features are flexible and can be varied within limits of

safe navigational consideration. The channel shown in

Plate III is 12 ft deep for a width of 150 ft and is a

reduced depth, say 6 ft, over the remainder of the 400 ft

width. The reduced depth portion of the channel will

serve as a fishing area for small boats, or as a safe area

for boats experiencing engine trouble, etc. Also, a wider

inlet, immediately past the tips of the jetties, is

- 70 -

FIGURE 26

ST. GEORGE ISLAND CUT. NOTE
EROSION WHERE BANK PROTECTION

IS NOT

PROVIDED

- 71 -

I

favorable psychologically to operators of entering craft.

The Sound Side of the channel could be provided with

short (30 ft) rubble mound structures to keep drift out

of the channel. An alternate and reasonable approach

would simply be to accept some minor dredging of the Pass

in this area.

The jetties should extend to approximately the

12 ft Gulf contour and a design entrance width of

approximately 170 ft is recommended. The jetties should

be provided with a core so that the possibility for sand

being carried through the jetties is minimal. The eastern

jetty is extended further seaward than the west jetty

because the predominant wave action is from the east and

entering craft can first "duck behind" the protection of

the east jetty and can then contend with the presence of

jetties on both sides in comparatively protected waters.

4. Stabilization of Sand Against Wind Drift-

Inspection of the dune system in the Navarre Beach area

indicates that wind drift is an effective agent for sand

transport. In the interest of reducing the maintenance

dredging in the Pass and in preventing erosion of the land

features, it is important that the areas near the Pass be

provided with mechanical (sand fences) or natural

(vegetative) control against wind erosion. This is

particularly important along the boundary of the Pass

- 72 -

where presently-existing vegetation will necessarily be

removed by the excavation activity.

Disposition of Initially Dredged Material

The initial dredging of the Pass and deposition

basin will result in approximately 400,000 cubic yards of

beach quality sand. It is recommended that at least 90%

of this material be placed on the western side of the

inlet to be used as "feeder sand" for the down drift beaches

while the near Pass bathymetry is adjusting to the presence

of the Pass system and while the deposition basin is

filling. The material should be placed so as to cause a

seaward extension and an increase in elevation of the

existing down drift shores. If the material is distributed

over 2000 ft of beach on the down drift side of the Pass,

approximately 200,000 cubic yards of sand will be required

to advance the shoreline seaward a distance of 100 ft.

The remaining 160,000 (or so) cubic yards could be used to

raise the elevation of this section of the beach. This

remaining portion placed on the newly-established beach

would thereby : raise its elevation and not damage the

existing vegetation. This amount of material would result

in a new dune approximately 16 ft high by 150 ft wide and

2000 ft long.

- 73 -

Alternate Designs

Only one design for Navarre Pass has been

presented. This design is considered to be the best

choice from a functional standpoint, however it is

recognized that the present design will be somewhat more

expensive to construct than others. One alternate design

which could be less expensive would be one similar to that

at Perdido Pass, Alabama. This design incorporates a weir

section as part of the eastern jetty and the interior

region adjacent to the eastern jetty serves as the deposi-

tion basin. Apparent drawbacks to this design would appear

to include the possibility of undesirable wave conditions

inside the jetties during periods of high tides and waves,

and the possible difficulty of sand encroaching on and

causing shifting of the navigational cut.

- 74 -

IX. SUMMARY AND CONCLUSION

Summary

The results of this study have indicated that:

1. Navarre Pass would only reduce slightly the
equilibrium cross-sectional flow areas into
Pensacola (1.8%) and Choctawhatchee (0.1%)
Bays.

2. The velocities through Navarre Pass would be
well within the limits considered safe for small
craft navigation.

3. Planning for the artificial transfer of sand
should be based on an annual rate of 200,000
cubic yards to the west. Initial disposition of
sand dredged should be as a feeder beach on the
west side of the Pass.

4. The Pass would cause a localized moderation of
salinities and increased flushing in the waters
adjacent to the Pass.

5. The tide and geometric characteristics are such
that Navarre Pass will always tend to close;
jetties are therefore essential to the stability
of the Pass.

Conclusion

Based on the results of this study, it is concluded

that if proper financial provision is made for the

construction and maintenance of the inlet, there should

be no significant adverse hydrographic effects to the

stability of the Santa Rosa Island Beaches, nor to the

adjacent waters.

- 75 -

X. REFERENCES

1. Bruno, R., Personal Communication.

2. Oceanographic Atlas of the North Atlantic Ocean,
U. S.Navy Oceanographic Office, Publication No. 700,
Section IV, Sea and Swell, 1963.

3. Shore Protection, Planning and Design, Technical
Report No. 4, Coastal Engineering Research Center,
U. S. Army Corps of Engineers, Third Edition, June
1966.

4. National Shoreline Study, Regional Inventory Report,
South Atlantic-Gulf Region, Puerto Rico and the
Virgin Islands, U. S. Army Corps of Engineers,
South Atlantic Division, Atlanta, Georgia, August 1971.

5. Escoffier, F. F., "Study of East Pass Channel,
Choctawhatchee Bay, Florida," United States Engineers
Office, Mobile District, Mobile, Alabama, 1938.

6. Johnson, J. W., "Nearshore Sediment Movement,"
Bulletin, American Association of Petroleum Geologists,
Vol. 40, 1956, pp. 2211-2232.

7. Gorsline, D. S., "Dynamic Characteristics of West
Florida Gulf Beaches," Vol. 4, Marine Geology, 1966,
pp. 187-206.

8. Walton, T. L., "Littoral Drift Computations Along
the Coast of Florida by Use of Ship Wave Observations,"
M.S. Thesis, Coastal and Oceanographic Engineering
Department, University of Florida, 1972.

9. O'Brien, M. P., "Estuary Tidal Prisms Related to
Entrance Areas," Civil Engineering, Vol. 1, No. 8,
pp. 738-739, 1931.

10. Escoffier, F. F., "The Stability of Tidal Inlets,"
Shore and Beach, Vol. 8, No. 4, pp. 114-115, 1940.

- 76 -

11. Keulegan, G. H., "Tidal Flows in Entrances. Water
Level Fluctuations of Basins in Communication with
Seas," Third Progress Report, National Bureau of
Standards Report, No. 1146, 1951.

12. O'Brien, M. P., "Equilibrium Flow Areas of Inlets on
Sandy Coasts," Journal, Waterways and Harbors Division,
ASCE, Vol. 95, No. WW1, pp. 43-52, Feb. 1969.

13. Rouse, H., "Elementary Mechanics of Fluids," John Wiley
and Sons, Inc., 1956.

- 77 -

APPENDIX I

NUMERICAL MODEL OF THE BAY SYSTEM AFFECTING
NAVARRE PASS

Introduction

The purpose of the numerical model is to provide a

means of realistically representing the hydraulics of the

system and any changes that would occur due to the opening

of Navarre Pass. Because of the extreme length (approxi-

mately 50 miles) of Santa Rosa Sound, the construction of

a hydraulic (physical) model was ruled out during the

conduct of the project.

In the following sections of the Appendix, the

governing differential equations will be presented and

cast into finite difference form for numerical solution;

this provides the basis for simulating the tides and

currents that would occur at any locality in the system

represented. Two representations of the numerical model

will then be discussed: (1) In the calibration phase,

data collected during the study will be used to assess the

validity of and/or modify the numerical model, and (2) With

the validity established, the Pass will be introduced into

the numerical model and the hydraulics of the inlet and/or

- 78 -

the effect of the Inlet on the tide in Santa Rosa Sound

and on the hydraulics of the Pensacola Bay Entrance and

East Pass will be investigated. Figure I-1 presents the

geographic area represented in the numerical model.

Derivation of the Numerical Model

Governing Differential Equations

The differential equations governing the flow in

bay systems are the depth-integrated equations of motion

and continuity.

Equation of Motion.--The vertically integrated

differential equation of motion can be written for the

x-direction in a semi-linearized form as

= g D + (I-1)
at ax p ( b

in which

q = discharge per unit width in the x-direction

t = time

g = gravitational constant

D = total depth = h + n

h = depth referred to mean sea level

n = tide displacement above mean sea level due
to astronomical, wind and barometric tides

x = horizontal distance coordinate aligned with
bay axis

- 79 -

j

G U L F

O F

System
Model

SLimits Encompassing Bay
i Represented in Numerical

say
" y \' '

a Bay .

!a.Son

M E X -LC 0

Z-1 BAY SYSTEM REPRESENTED IN NUMERICAL

MODEL

FIGURE

* ..

f""Ce

p = mass density of water

T = wind stress in x-direction on air-water
n interface

T = frictional stress on bottom of water column
b

The quantities T and Tb can be expressed as

T = CfP U2 cos B (1-2)

S f qjlq (1-3)
b 8D2

in which

C = wind stress coefficient

0.0013, U < 23.6 ft/sec (1-4)

0.0013 + 0.00295 1.0 2 U > 23.6 ft/sec

Pa = mass density of air

U = wind speed at 30 ft reference elevation

S = angle of wind vector relative to the bay axis

f = Darcy-Weisbach friction factor (Reference 13, page 201)

Equation of Continuity.-The equation of continuity

in one dimension is expressed as

i + q R (1-5)
at ax w

in which the righthand side represents the effect of runoff,

- 81 -

R = runoff in cubic ft/sec per foot of bay
length

w = width of segment considered

It is noted that in the present application of the

model, the wind stress, T and direct precipitation and

runoff will be taken as zero, however they have been

included here for completeness.

Finite Difference Equations

In order to employ Equations (I-1) and (1-5) for

realistic geometries and Gulf tides, it is necessary to

cast these equations into finite difference form. The

time- and space-staggered procedure is used in which the

equation of motion is applied between midpoints of adjacent

segments (i.e., across a segment boundary) at full time

steps, At, and the equation of continuity is applied for

each segment at half time step increments.

Finite Difference Form of the Equation of

Motion.--Equation (I-1) can be expressed in finite

difference form for the total flow, Qn, onto the nth

segment, as:

Q +At T wD g n- n 1
Q n [P n n n-1 (1-6)
Q = (I-6)
n +w At f IQ n
1 + (
8(Dw)2

- 82 -

in which the over-barred quantities represent averages

based on the nth and (n-l)th segments. The prime indicates

the value at time t + At whereas unprimed quantities are

known from calculation at time t, and w is the width of

the bay segment, see Figure I-2 for the variable

representations and Figure I-3 for the numerical model

representation of the area of concern.

Finite Difference Equation of Continuity.-Equation

(1-5) can be written in finite difference form as

qR At
At 1 ( At
n n Ax w n n+1l w

where the primes indicate the unknown quantities as before

and the terms on the right hand side are known from

calculations at previous times.

Boundary Conditions

The boundary conditions for this problem are the

flows through the inlets and may be expressed, for example,

for Destin Pass as:

Ac /2g no qGI sign(nl0 G) (
Ken + Kex + ft/4R

in which

- 83 -

Variables Represented
at Segment Midpoints:

Variables Represented
ot Segment Junctions
Q

V7/ / // //

On i *lln, hn
I k p

n51' Segment

*ln+li hn+1

- + x

FIGURE I-2

ILLUSTRATION
REPRESENTATION

OF BAY SEGMENT

- 84 -

n-I

I -.

i -o,+,

Pensacolo Bay
Segment (

---_ ---- ------ --- H-- --l- --t -

--Pensacola Bay Entrance -Navarre Pass Destin Pass-
( Site

GULF OF MEXICO

FIGURE I-3 SCHEMATIZATION OF PENSACOLA BAY / CHOCTAWHATCHEE BAY / SANTA ROSA SOUND /
GULF OF MEXICO SYSTEM

Ii

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