• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Executive summary
 Table of Contents
 List of Figures
 List of Tables
 Main
 Reference
 Appendix
 Appendix
 Appendix
 Appendix














Group Title: UFLCOEL-99002
Title: Pensacola Pass, Florida, inlet management study
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00091073/00001
 Material Information
Title: Pensacola Pass, Florida, inlet management study
Series Title: UFLCOEL-99002
Alternate Title: Pensacola Pass, FL, inlet management study
Physical Description: 1 v. (various foliations) : ill. ; 28 cm.
Language: English
Creator: Browder, Albert E
Dean, Robert G ( Robert George ), 1930-
University of Florida -- Coastal and Oceanographic Engineering Dept
Florida -- Office of Beaches and Coastal Systems
Publisher: Coastal & Oceanographic Engineering Dept., University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1999
 Subjects
Subject: Inlets -- Management -- Florida -- Pensacola Bay   ( lcsh )
Dredging -- Florida -- Pensacola Bay   ( lcsh )
Channels (Hydraulic engineering) -- Florida -- Pensacola Bay   ( lcsh )
Pensacola Pass (Fla.)   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references.
Statement of Responsibility: prepared by Albert E. Browder & Robert G. Dean.
General Note: "January, 1999."
General Note: "Prepared for Florida Department of Environmental Protection, Bureau of Beaches and Coastal Systems"--Cover.
 Record Information
Bibliographic ID: UF00091073
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 43794720

Table of Contents
    Front Cover
        Front Cover
    Title Page
        Title Page
    Executive summary
        Page i
        Page ii
        Page iii
        Page iv
    Table of Contents
        Page v
    List of Figures
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    List of Tables
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    Appendix
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Full Text

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UFL/COEL-99/002


PENSACOLA PASS, FL
INLET MANAGEMENT STUDY


by


Albert E. Browder
&
Robert G. Dean



January, 1999




Prepared for:

Florida Department of Environmental Protection
Bureau of Beaches and Coastal Systems


Coastal & Oceanographic Engineering Department
433 Weil Hall P.O. Box 116590 Gainesville, Florida 32611-6590 ---- -

UNIVERSITY OF
FLORIDA










Pensacola Pass, Florida
Inlet Management Study


Prepared By:
Albert E. Browder & Robert G. Dean
Department of Coastal & Oceanographic Engineering
University of Florida
Gainesville, FL

January, 1999





UNIVERSITY
OF
FLORIDA




EXECUTIVE SUMMARY


The University of Florida Department of Coastal and Oceanographic Engineering
(UFCOE) was contracted by the Florida Department of Environmental Protection (FDEP),
Bureau of Beaches and Coastal Systems, to conduct a study of the Pensacola Pass, FL,
entrance for purposes of developing a comprehensive plan addressing sediment management
and shoreline impacts to the adjacent islands.

Based on the present condition of the Pass and the analyses provided in this
document, the following recommendations are made regarding sediment management at
Pensacola Pass, FL:

1) It is recommended that a Pensacola Pass monitoring program be instituted,
consisting of hydrographic surveys of the bathymetry of the Pass and
adjacent shoals at least once every five years and beach profile surveys of the
adjacent shorelines in Escambia County every two years.








2) It is recommended that any beach-quality material dredged from the entrance
channel of Pensacola Pass during maintenance operations or other new work
dredging be disposed of on the adjacent shorelines. Material placed on Santa
Rosa Island should be used to construct dunes, placing the material as high
as possible on the profile to prevent its rapid return to the channel and shoal
system of the Pass. If the material is to be placed along the Perdido Key
shoreline, it should be placed on the beachface or as dunes. It is
recommended that beachface placement of material along the Perdido Key
shoreline occur in the area between 1.5 and 5.5 miles west of the Pass
(roughly between R-58 and R-37).

Based on the historical dredging record, it is estimated that once dredging
becomes necessary, requirements to maintain the Federally authorized
channel dimensions (35-ft depth by 500-ft width) may exceed 300,000 cy/yr.
Placement of that volume of material on the downdrift beaches would
represent between 4 and 6 years of equivalent net transport bypassing.

3) In the event of unacceptable shoaling in the channel between Battery 233 and
the disposal island, or in the event of further degradation of the two existing
'jetty' structures on the eastern end of Perdido Key, it is recommended that
consideration be given to the construction of sand-tight structures
immediately adjacent to the existing 'jetties.'

Given the undeveloped nature of the shorelines adjacent to Pensacola Pass, FL, at this
time more substantial engineering efforts are not recommended. It is noted that these
recommendations are made based on engineering considerations and do not consider policy
or permitting issues of the State of Florida or the National Park Service.

As part of the development of these recommendations, a comprehensive sediment
budget was developed, based on historical shoreline changes, wave refraction, and littoral
transport analyses. Some pertinent details from these analyses are summarized below:

a) The annual net littoral transport in the vicinity of Pensacola Pass, FL, is
estimated to be between 50,000 and 70,000 cy/yr, westerly directed. Further
refining of this estimate is deemed inappropriate in light of the variability in
the annual wave climate and the uncertainty in the available data.

b) Very little if any sediment is believed to naturally bypass the entrance at
Pensacola Pass, FL, due to the geometry of the dredged entrance channel.








c) Study of historic shoreline changes, even-odd analysis of those changes, and
littoral transport analysis suggests that the Pass influences the adjacent
shorelines for a distance of approximately 7.5 to 9.5 miles to either side of
the Pass. Significant evidence of Pass impacts begin to appear in the
Pensacola Beach area and increase westward toward the Pass. Along the
Perdido Key shoreline, significant Pass impacts are seen within the Gulf
Islands National Seashore (GINS) Park boundary (within 7.5 miles west of
the Pass). Just west of the boundary, the historic shoreline trend has been
stable to accretional. While Pass effects may extend westward beyond the
Park boundary, these effects may be masked by the possible addition of
sediment from an offshore source (unverifiable in this study), or a significant
decline in the breaking wave energy of the incident wave environment. The
proximity to the jetties at Perdido Pass, AL, may also play a stabilizing role.

d) Analysis of longshore transport components along Santa Rosa Island
indicates that the shoreline within the first 12 miles east of the Pass is
dominated by westerly directed transport. Wave refraction analysis of the
eastern end of the Perdido Key shoreline indicates the possibility of a net
transport reversal from predominantly westerly directed transport to easterly
directed transport (due to the refractive and sheltering effect of Caucus
Shoal). This possibility, suggested by wave forces alone, becomes highly
probable when the additional stress of flood tidal currents is considered. This
area of transport reversal is thought to lie roughly one to two miles west of
the Pass, and shifts in response to the variation in the annual wave climate.

e) The presence of a reversal in the net longshore transport direction along the
eastern Perdido Key shoreline illustrates the sink effect the Pass has on
longshore transport in both directions. Inspection of annual transport
components along Santa Rosa Island suggests that nearly all sediment
moving alongshore within 7 to 10 miles east of the Pass is transported
westward toward the Pass, while along the Perdido Key shoreline, sediment
moving alongshore within 1 to 2 miles is likely to be transported eastward
into the Pass.

f) Sediment budget analysis suggests as much as 54% of the measured erosion
losses from the 1989-1991 Perdido Key beach nourishment project was
transported eastward back into the Pass and adjacent shoals.








g) Lack of substantial shoreline change data for pre-dredge conditions (prior to
1883) limits thorough evaluation of the impacts of the dredging program on
the adjacent shorelines at Pensacola Pass, FL. Limited pre-dredge shoreline
change data suggest that the Pass was experiencing substantial migration to
the west, as evidenced by significant accretion on Santa Rosa Island. At the
same time, Perdido Key experienced a comparable degree of erosion. While
the dredging program has resulted in similar erosion rates on Perdido Key
and increased erosion along the Santa Rosa Island shoreline, the maintenance
of the channel (and the construction of the small 'jetties' on Perdido Key) has
arrested the migration of Santa Rosa Island.

h) The historical increase in the dimensions of the Pensacola Pass entrance
channel has been accompanied by a marked increase in the maintenance
dredging requirements. Since 1883, approximately 46 million cubic yards of
material have been dredged from the channel. Of that amount, 18.6 million
cubic yards have been placed on or near the adjacent shorelines. The balance
of the dredged material was placed in an offshore disposal site.

i) It is hypothesized that the large maintenance dredging volumes, combined
with the relatively low net and gross transport components, have resulted in
a draw-down of the ebb and flood shoal volumes over the 115+-yr life of the
maintained entrance channel. The ebb shoal system has evolved from a
classical bypass-bar system to a more linear ebb-shoal system with little or
no bypassing of sand across the channel.










Pensacola Pass, Florida

Inlet Management Study



TABLE OF CONTENTS


EXECUTIVE SUMMARY ............................................. i
TABLE OF CONTENTS ................................................v
LIST OF FIGURES .................................................. vi
LIST OF TABLES ................................................... xi
1.0 INTRODUCTION ................................................1
2.0 STUDY AREA .................................................. 3
3.0 DATA SOURCES ............................................. 13
4.0 DREDGING & THE PENSACOLA PASS NAVIGATION CHANNEL ... 18
5.0 SHORELINE HISTORY ........................................26
6.0 PENSACOLA PASS BATHYMETRY & CHANNEL STABILITY ...... 45
7.0 LONGSHORE SEDIMENT TRANSPORT .........................56
8.0 SEDIMENT BUDGET ......................................... 72
9.0 RECOMMENDATIONS ........................................103
10.0 REFERENCES ................................................114


APPENDIX A


APPENDIX B


APPENDIX C


APPENDIX D


COMPILATION OF AERIAL PHOTOGRAPHY OF
PENSACOLA PASS 1941-1998
PENSACOLA PASS NAVIGATION CHANNEL
CROSS-SECTION PROFILES 1989-1997
BEACH PROFILE PLOTS
ESCAMBIA COUNTY, FL 1974-1998
WAVE REFRACTION DIAGRAMS
OUTPUT FROM RCPWAVE MODEL








Pensacola Pass, Florida


Inlet Management Study


LIST OF FIGURES


Figure 2.1 Location map of Pensacola Pass, FL, located in Escambia County, FL,
in the western Panhandle portion of the state. The aerial photograph
identifies many of the relevant features discussed in the report ........ 4

Figure 2.2 Tidal elevation prediction for Pensacola Pass, FL, entrance, 1999.
Elevations plotted relative to NGVD 1929 datum.
Reference : Tidemaster 2.0, Zephyr Services, Pittsburgh, PA. ......... 8

Figure 2.3 Monthly variation in average significant wave height for UFCOE
and USACE WIS datasets, for all occurrences .................... 10

Figure 2.4 Angular distribution of incident waves (incident from) in the Pensacola
Pass area as determined from UFCOE measured data and hindcast WIS
station 44. Note the higher incidence of waves from east of shore
normal (coming from the 78 to 168 degree sector) ................. 10

Figure 2.5 Predicted storm surge elevation vs. Return period of storms for the
Pensacola Pass area. Storm surge includes the effects of wind and
wave setup, tides, and barometric pressure. Dean and Chiu, 1986 .... 11

Figure 4.1 Dredging history of Pensacola Pass, FL. The figure plots the cumulative
volume of material dredged from the entrance channel and approach to
the U.S. Navy turning basin (left axis). The figure notes six events
where sand was placed along the adjacent shorelines; the balance of the
material was deposited offshore. **The second curve presents a
simplified estimate of the increase in the dredging rate over the history
of the maintained channel (right axis). This curve was computed by
fitting the cumulative dredging data to a third order polynomial, then
taking the derivative of the resulting expression to determine a second
order expression for the effective dredging rate over time. This
effective rate may over- or underestimate the actual dredging rate for
any particular time, and is presented primarily for illustrative purposes.. 21








Figure 4.2 Schematic map of the Pensacola Pass Navigation Channel. The map
indicates the location of cross-sections used to investigate shoaling
patterns and rates. The shaded areas in the figure indicate areas of
substantial shoaling as of the July, 1997, condition survey by USACE.
Depth contours are in feet, NGVD, from 1998 condition survey. ...... 25

Figure 5.1 Historical position of the Mean High Water Line (MHWL) adjacent to
Pensacola Pass from 1856 to 1998. ............................ 27


Figure 5.2


Historical Mean High Water shoreline position rate of change adjacent
to Pensacola Pass, FL. ..................................... 28


Figure 5.3 Aerial photograph of Perdido Key, FL, indicating the pattern of beach
ridge growth along the middle of the island. Photograph dated January,
1994. Source www.terraserver.com (USGS and Microsoft Corp.). ... 30


Figure 5.4


Shoreline changes in the vicinity of Pensacola Pass, FL, relative to the
measured 1974 MHWL. Positive changes indicate seaward shoreline
advance, negative values indicate shoreline recession. ............. 31


Figure 5.5 Annual cumulative volumetric change measured eastward and westward
from Pensacola Pass, FL, for the Santa Rosa Island and Perdido Key
shorelines, respectively. The volumetric changes were computed from
the MHW shoreline changes for this time period by assuming the entire
profile moves uniformly with the MHWL over an active height of profile
of 15 ft. The grey error bands indicate the volumetric changes for a
range of profile height of 15 +/- 3 ft. ............... .......... 33


Figure 5.6


Figure 5.7




Figure 5.8



Figure 5.9


Even/odd analysis of volumetric changes between 1856 and 1978 (upper
plot) and 1974 and 1984 (lower plot). .......................... 35

Cumulative volumetric changes measured adjacent to Pensacola Pass,
FL, for the time periods 1974-1984 and 1984-1996. Changes are
measured in cy beginning at the Pass and accumulating eastward along
Santa Rosa Island and westward along Perdido Key. ............... 36

Change in position of the MHWL of the 1989-1991 Perdido Key beach
nourishment project. The plotted shoreline positions are relative to the
1989 pre-construction surveyed MHWL. ........................ 39

Cumulative volumetric changes measured along the Perdido Key beach
nourishment project immediately west of Pensacola Pass, FL. The
volume changes are computed beginning at monument R-30 and
heading eastward toward the Pass ............................ 40


-vii-








Figure 5.10 Time history of the volumetric performance of the 1989-1990 Perdido
Key beach nourishment project ................................ 41

Figure 6.1 Bathymetric contours of Pensacola Pass, FL, dated 1856. Elevations
are in feet relative to NGVD 1929. The white contour line denotes the
controlling depth over the outer bar, -21 ft NGVD.................. 45

Figure 6.2 Bathymetric contours of Pensacola Pass, FL, dated 1998. Elevations
are in feet relative to NGVD 1929. The authorized depth at the time
of the survey was 35 ft below MLLW datum. The controlling depth
was approximately -40 ft NGVD. ............................... 47

Figure 6.3 Perspective comparison of the Pensacola Pass, FL, ebb shoal system
between 1856 (pre-dredge) and 1998 (present condition). The -21 ft
NGVD contour is highlighted to indicate the controlling depth in 1856. 48

Figure 6.4 Time history of the bathymetry in the vicinity of Pensacola Pass, FL.
The vertical and horizontal scales in each plot are Northing and Easting,
respectively, ft, NAD 1927. The elevations are in ft, NGVD 1929. The
existing 1998 navigation channel is shown in all plots for reference. .. 50

Figure 6.5 Estimate of cumulative volumetric changes of the Pensacola Pass, FL,
ebb-shoal system since 1856................................. 51

Figure 6.6 Estimate of cumulative volumetric changes of the Pensacola Pass, FL,
flood-shoal system since 1856. ............................... 52

Figure 6.7 Location of channel cross-sections investigated. 1998 bathymetry
shown. ..................................................53

Figure 6.8 Evolution of channel cross-sections at Pensacola Pass, FL. See Figure
6.7 for cross-section locations ................................. 54

Figure 7.1 Center bathymetric grid used in RCPWAVE analysis. Overlapping
grids of identical size were employed to extend the grid approximately
25,000 ft to the east and west of the grid shown. Each grid consists of grid
cells 400 ft by 800 ft (cross-shore and longshore, respectively). Contours
are in feet and represent depths relative to NGVD 1929. ............. 62

Figure 7.2 Example of wave field computed by RCPWAVE analysis. This case
represents the most frequently occurring condition according to WIS
Station 44 data (1976-1995). ................................. 63


-viii-








Figure 7.3 Normalized annual longshore transport potential computed from wave
refraction analysis. Presented in the figure are the net, easterly, and
westerly transport components computed from WIS data along with the
net transport curve resulting from the application of a uniform wave
field. Positive values denote westerly transport, and vice-versa. ...... 65

Figure 7.4 Comparison of alongshore annual volumetric changes predicted by WIS
data through refraction analysis to measured historic volumetric changes
between 1856 and 1978 (Dean and Cheng, 1998). The WIS curve
is consistent with a transport coefficient of 0.05, compared to the value
published in the Shore Protection Manual of 0.39. ................. 69

Figure 8.1 Definition sketch for possible sediment flows in the vicinity of Pensacola
Pass, FL. Plot (a) illustrates the possible natural sediment flows. Plot (b)
depicts the possible mechanical sand transfers at the Pass. Not all of the
shown possible pathways apply to Pensacola Pass. Figure adapted from
Bodge (1998). ............................................ 73

Figure 8.2 Family of solutions for sediment transport pathways in the vicinity of
Pensacola Pass, FL, for 1974-1984. The figure plots values of the
shoaling rate from the east and west sides of the Pass versus the net
bypassing around the Pass for a range of net transport values, Q. All
values in cy/yr.............................................. 81

Figure 8.3 Sediment transport pathways for Pensacola Pass, FL, for 1974-1984 time
period. For this example, Case A, the net transport is 45,000 cy/yr
westerly directed and the net bypassing rate is 14,000 cy/yr. Units are
cy/yr. .....................................................84

Figure 8.4 Comparison of families of solutions for the Pensacola Pass, FL, sediment
budget for gross component ratios, r, of 0.2 and 0.5. The total shoaling
into the channel is 100,000 cy/yr, and the net change in the shoal system
is -235,000 cy/yr. ......................................... 88

Figure 8.5 Sensitivity test of Pensacola Pass, FL, sediment budget to the uncertainty
in the measured net volumetric change of the shoal system. The test
range is approximately +/- 10% of the assumed value, -235,000 cy/yr.
The limits of the test were chosen based on comparison of the resultant
downdrift impact to the measured maximum erosion downdrift. ...... 89

Figure 8.6 Typical sediment budget for Pensacola Pass, FL, for the 1974-1984 time
period. All quantities in cy/yr. The net transport is 45,000 cy/yr,
westerly directed, with zero net bypassing. ................... .... 93








Figure 8.7 Measured values and assumed ranges of volumetric changes occurring
in the vicinity of Pensacola Pass, FL, for the time period from 1991 to
1998 (post-nourishment project construction to most recent survey
condition). ............................................... 95

Figure 8.8 Family of solutions for the sediment budget of Pensacola Pass, FL,
describing the 1991-1998 time period. The figure includes data from
several estimates of the net shoal system volumetric change, AVN, and
the general trend of increasing net transport with increasing shoal system
change is shown. .......................................... 97

Figure 8.9 Sediment transport pathways in the vicinity of Pensacola Pass, FL, for
the 1991-1998 time period for a Q,,et = 70,000 cy/yr. Units are cy
except where noted otherwise.................................. 99

Figure 8.10 Typical sediment transport pathways in the vicinity of Pensacola Pass,
FL, for the 1991-1998 time period for a Qnet = 70,000 cy/yr. Units are
cubic yards (cy) except where noted ........................... 102

Figure 9.1 Location map of Pensacola Pass, FL, showing relevant features for
sediment management alternatives. ................. ........ 104

Figure 9.2 Cumulative volumetric changes measured along the Perdido Key beach
nourishment project immediately west of Pensacola Pass, FL. The
volume changes are computed beginning at R-30, summing changes
eastward toward the Pass. The figure indicates the area in the vicinity
ofR-58 where the measured erosion rates increase sharply ......... 106

Figure 9.3 Schematic of proposed structure-tightening alternative for the eastern
end of Perdido Key, FL. Structures would be constructed of rock
placed against the south side of each existing 'jetty' structure ....... 110








Pensacola Pass, Florida
Inlet Management Study

LIST OF TABLES

Table 2.1 Tidal datums for the Pensacola Pass area ......................... 7

Table 2.2 Storm History in the Pensacola Pass area (1880-1998) .............. 12

Table 3.1 Available Mean High Water shoreline position data for the Pensacola
Pass Area.................................................. .14

Table 3.2 Available Beach Profile Data for the Pensacola Pass Area. .......... 15

Table 3.3 Available Bathymetric Data for the Pensacola Pass Area. ........... 16

Table 3.4 Available Aerial Photography for the Pensacola Pass Area .......... 17

Table 4.1 Time History of the Dimensions of the Pensacola Pass Entrance
Channel. .................................................. 20

Table 4.2 Dredging History of the Pensacola Pass Entrance Channel. .......... 22

Table 7.1 Annual longshore transport rates computed from the SEDTRAN utility
using WIS Station 44 data in 16.4 ft water depth from 1976 to 1995. .. 59

Table 7.2 Input wave conditions for the RCPWAVE refraction model for
Pensacola Pass, FL. ........................................ 61

Table 8.1 Sediment budget transport components and coefficients computed from
the CEM method for the 1974 to 1984 time period. ................ 92




ACKNOWLEDGMENTS

This study was funded by the Florida Department of Environmental Protection,
Bureau of Beaches and Coastal Systems. Sidney Schofield, Viktor Adams, Roberto Liotta,
Guillermo Simon Fernandez, and Carrie Suter contributed substantially to the field data
collection portion of this study. The cooperation and assistance of these people and FDEP
personnel are gratefully acknowledged. Kevin Bodge of Olsen Associates, Inc., is gratefully
acknowledged for his assistance and for providing an advance copy of the CEM sediment
budget model for use in this study.










Pensacola Pass, Florida
Inlet Management Study


Prepared By:
Albert E. Browder & Robert G. Dean
Department of Coastal & Oceanographic Engineering
University of Florida

January, 1999






UNIVERSITY
OF
FLORIDA



1.0 INTRODUCTION


The University of Florida Department of Coastal and Oceanographic Engineering
(UFCOE) was contracted by the Florida Department of Environmental Protection (FDEP),
Bureau of Beaches and Coastal Systems, to conduct a study of the Pensacola Pass, FL,
entrance for purposes of developing a comprehensive plan for the Pass.

By Florida Statutes 161.142(4) and 403.021(9)(b), Pensacola Pass is exempt from the
inlet management plan requirements of Statute 161.142(1,2) in recognition of the need to
maintain certain deep-draft navigation entrances in the State of Florida. The statute
exemption aside, the purpose of this study is to address the intent of F.S. 161.161(1)(b-1)
which states:

"(b) Evaluate each improved coastal beach inlet and determine whether the inlet is a
significant cause of beach erosion. With respect to each inlet determined to be a
significant cause of beach erosion, the plan must include:
1. The extent to which such inlet causes beach erosion and recommendations to
mitigate the erosive impact of the inlet, including, but not limited to, recommendations
regarding inlet sediment bypassing; modifications to channel dredging, jetty design, and
disposal of spoil material; establishment of feeder beaches; and beach restoration and
beach renourishment;..."








and the intent of F.S. 161.142 "Declaration ofpublic policy relating to improved navigation
inlets":

"(1) All construction and maintenance dredging of beach-quality sand should be placed
on the downdrift beaches; or, if placed elsewhere, an equivalent quality and quantity of
sand from an alternate location should be placed on the downdrift beaches.
(2) On an average annual basis, a quantity of sand should be placed on the downdrift
beaches equal to the natural net annual longshore sediment transport."



1.1 Report Organization

This management study is organized as follows. The following chapter (Chapter 2)
describes the location and limits of the study area and describes the physical setting. The
third chapter details the sources of data used in preparation of the study. Chapter 4 describes
the dredging history and recent condition of the navigation channel itself. Chapters 5
through 8 describe the physical analyses performed and the results obtained in developing
the sediment budget and the study recommendations. Chapter 9 presents and evaluates
several options for coastal engineering measures at the Pass. Chapter 10 provides the
references listed in the study. The executive summary included at the beginning of the report
outlines the study recommendations and provides a brief description of the pertinent
technical details.








2.0 STUDY AREA


2.1 Location and Study Limits

Pensacola Pass is located in the westernmost portion of the State of Florida in
Escambia County (Figure 2.1). The Pass connects the Gulf of Mexico to Pensacola Bay and
Big Lagoon. The Pass is bordered to the east by Santa Rosa Island, a sandy barrier island
stretching roughly 48 miles to East Pass in Okaloosa County, FL. Perdido Key borders the
Pass to the west. Perdido Key is also a sandy barrier island, approximately 15 miles in
length, reaching westward to Perdido Pass in Baldwin County, AL1. The pass is bounded
to the north by the City of Pensacola, FL, and the Pensacola Naval Air Station (PNAS).

The Pass is located at north latitude 30019.5', west longitude 87018.5' (roughly N
492,690 and E 1,114,020 in the NAD27 Florida North Grid State Plane Coordinate system).
The GulfIntracoastal Waterway (GIWW) passes through the project area, with the Pass lying
at mile marker St. M 180 (St M 0.0 is located at Harvey Lock, LA, (St. M measures statute
miles)). Map descriptions of Pensacola Pass may be found on National Ocean Service
(N.O.S.) nautical charts 11378, 11382, and 11384, among others.

The limits of the present study extend westward to the Florida/Alabama state line,
a distance of over 12 miles, and to the east over 27 miles to the Okaloosa County line. The
practical limits of the study are dictated more by the measured effects of the Pass and the
limits of available data (roughly 8 to 12 miles to either side of the Pass). Much of the Gulf
of Mexico shoreline included in the study lies within the Gulf Islands National Seashore,
administered by the National Park Service. The study area extends into the Pass northward
and eastward to approximately the U.S. Navy turning basin.



2.2 Coastal Structures

Several old coastal structures lie within the study area. While the Gulf of Mexico
shoreline of Santa Rosa Island has no structures in the study area, the northern bay shoreline
has several small shore perpendicular structures and the Ft. Pickens Pier, each of which
affect the local shoreline position somewhat along the northern shoreline. These structures
do not appear to affect the east/west position of Santa Rosa Island nor do they significantly
prevent the migration of sand into the navigation channel.

1
Approximately 2.8 miles of Perdido Key lie in Baldwin County, AL. The
remaining 12+ miles of Perdido Key are in Escambia County, FL.












Pensacola Pass, FL


Escambia County, FL
U.S.A.


Escambia
County


FLORIDA


Pensacola
Bay


Gulf of Mexico


Figure 2.1 Location map of Pensacola Pass, FL, located in Escambia County, FL, in
the western Panhandle portion of the state. The aerial photograph
identifies many of the relevant features discussed in the report.


Beach


Perdido Key


Santa Rosa Island








On the Perdido Key shoreline, two shore-normal structures exist that serve to dictate
the position of the Perdido Key shoreline inside the pass (see Figure 2.1). These structures,
referred to locally as 'the jetties,' were built in the World War I era, and consist of a
combination of rusted steel sheetpile, marble stone, and large-aggregate concrete block. The
structures, approximately 300 ft in length, are somewhat dilapidated and allow a significant
amount of sand to flow through and over them. These structures, particularly the
southernmost structure, appear to act somewhat as a training groin, dictating the western
edge of the navigation channel while also establishing the position of the shoreline in that
area. The role of these structures in the study recommendations will be discussed in
subsequent chapters.

Old military forts and batteries exist on all sides of the Pass. Ft. Pickens lies on the
Santa Rosa Island side of the shoreline, along with Batteries Worth, Cooper, Langdon, and
234. Batteries Slemmer, Center, and 233 lie on the Perdido Key side of the Pass, several
hundred feet west of where Ft. McRee was located (no traces of Ft. McRee are seen on
Perdido Key, although the name Ft. McRee continues to appear on many maps). Fort
Barrancas and its batteries reside on the mainland north of the Pass, near the Pensacola
Lighthouse. These forts are either operated as museums by the National Park Service or are
abandoned; their history is briefly described in Chapter 4. None of these military structures
play a direct role in the hydrodynamic or littoral processes of the Pass, but they represent the
only significant development within the first several miles to either side of the Pass, and their
locations are noted for reference.



2.3 Geologic Setting

Pensacola Pass, Santa Rosa Island, and Perdido Key lie in the Gulf Coastal Plain
Section of the Coastal Plain Province (Brooks, 1982). The geology of the area has been
described in several investigations. Work et al., 1991, prepared a review of the geology of
Escambia County and the Perdido Key area as part of the Perdido Key monitoring efforts
for the U.S. Navy. Work et al. describe the sediments along the shorelines adjacent to
Pensacola Pass as unconsolidated sands, limestones, silts, and clays of various ages. They
describe the most distinctive feature of coastal plain topography, that being the Pleistocene
marine terrace deposits identified by Marsh, 1966. Examples of these terrace deposits, which
were formed during higher stands of sea level, include the Pamlico Terrace (roughly 26 ft
above present day sea level) and the Penholloway Terrace (roughly 70 ft above sea level).
Marsh, 1966, describes these terraces as composed of fine to coarse sand, light tan in color.








Brooks, 1982, provides a description of the physiographic region of Escambia
County, accompanied by complete geologic maps. Brooks described the Pleistocene areas
also identified by Marsh, 1966. Fronting the Pleistocene marine terrace deposits, which are
found on the mainland of Pensacola, are the barrier islands themselves, Santa Rosa Island
and Perdido Key. Brooks describes these areas as Holocene deposits, less than 4,500 yrs in
age (i.e. more recent than the Pleistocene marine terrace deposits to the north) consisting of
"...undifferentiated sand, shell, clay, marl, peat...."

Otvos (1979) suggests that the barrier islands of the area formed more-or-less in place
from detrital sediments, termed 'shoal-bar aggradation' by Otvos, rather than other
mechanisms such as spit segmentation from the mainland or mainland dune-ridge
engulfment by rising seas. Otvos (1979) also notes that these islands appear to have shifted
only laterally to the west, as opposed to retreating barrier islands.

Coastal Sediments Of more immediate interest to the coastal engineering analysis
of the Pass is the composition and size distribution of the beach sediments in the active zone.
The beaches along Perdido Key and Santa Rosa Island are well-known for their sugar-white
appearance and texture, prompting Perdido Key to be consistently named in the top 20
recreational beaches in the United States (Leatherman, 1998).

Gorsline (1966) describes the beach sands along the Florida Panhandle as texturally
homogenous quartz sands. For the beach stations Gorsline studied at Fort Pickens and Gulf
Beach, he reports an average grain size in the swash zone of 0.32 mm. Balsillie (1975)
reports the results of the Littoral Environment Observation (LEO) program, in which swash
zone samples were collected monthly for 12 months during 1969-1970. The results of the
collection for the Ft. Pickens station showed an average grain size of 0.43 mm.

Work et al. (1991) reported the results of sand sampling prior to the 1989-1991
Perdido Key beach nourishment project. The measured average grain size varies from 0.36
to 0.38 mm along the dune and beach face to 0.30 to 0.32 mm in the submerged nearshore
region (out to 8 m water depth). Otay and Dean (1994) indicate that during the dredging
process for the Perdido Key beach nourishment project, pockets of fine muds of Pleistocene
origin were dredged and placed in isolated areas along the beach. These pockets of fine
sand/mud, containing a very high fraction of fines, appear to have winnowed out of the
project over the first year or two of the project (based on surficial sediment sampling).








2.4 Oceanographic Environment


Tides Table 2.1 lists the tidal datums for the Pensacola Pass entrance (Balsillie,
1987). The astronomical tides in the area are characterized as mixed, predominantly diurnal,
with a mean range of 1.2 ft. Figure 2.2 illustrates the predicted time history of tidal elevation
for 1999 for Pensacola Bay, FL. The plot indicates the biweekly variation of tide due to the
phases of the moon, as well as the seasonal variation.


Table 2.1 Tidal datums for the Pensacola Pass area


Datum Elevation (ft, NGVD)
Mean Higher High Water (MHHW) 1.18

Mean High Water (MHW) 1.12
Mean Tide Level (MTL) 0.49
Mean Low Water (MLW) -0.13
Mean Lower Low Water (MLLW) -0.20
NGVD National Geodetic Vertical Datum, 1929


Wave Climate Data describing the wave climate in the vicinity of Pensacola Pass
were obtained from two primary sources. Otay and Dean (1994) present wave climate data
collected by UFCOE as part of the 1989-1991 Perdido Key beach nourishment monitoring
project. This data set represents wave height, period, and direction information for the
period between 1990 and 1994, in an average water depth of 19.7 ft.

The second source of wave information is hindcast data provided by the U.S. Army
Corps of Engineers (USACE) Wave Information Study (WIS) dataset for the Gulf of Mexico
(Tracy et al. 1996). This dataset represents wave climate data for the twenty-year period
from 1976 to 1995, and includes the effects of tropical storms and hurricanes. The hindcast
predictions are listed for a water depth of 16.4 ft.















2










0



-1 I I I


0 50 100 150 200 250 300 350
Julian date, 1999 (days)

Figure 2.2 Tidal elevation prediction for Pensacola Pass, FL, entrance, 1999. Elevations plotted relative to NGVD 1929
datum. Reference : Tidemaster 2.0, Zephyr Services, Pittsburgh, PA.








Figure 2.3 presents the variation in monthly average significant wave height obtained
from both sources for all wave occurrences (onshore and offshore). The average annual
significant wave height is roughly 2 ft or less. The mean period determined from the
USACE WIS data is 4.3 seconds, while the mean period measured from the UFCOE data is
slightly longer at 5.0 seconds. The angular distribution of onshore-directed waves for both
datasets is plotted in Figure 2.4. For a shore-normal bearing of 168 degrees with respect to
true north, the UFCOE data show a fairly uniform angular distribution with a 60% / 40%
split between waves incidentfrom the east and west, respectively (with respect to shore
normal). The USACE data are somewhat more skewed toward waves incident from the east,
indicating a 70% / 30% split between easterly and westerly incident waves.

Winds Work and Kaihatu (1997) present four years of wind speed and direction
data collected as part of the UFCOE monitoring project for the 1989-1991 Perdido Key
beach nourishment project. These data were collected at the National Park Service Ranger
Station on Perdido Key. Work and Kaihatu indicate that the mean measured wind speed
during that period was 7.8 mph, with the windiest months being March and April. The
USACE (1993) reports that the average annual wind speed measured at the Pensacola Naval
Air Station is 9 mph. Wind directions as reported by both Work and Kaihatu (1997) and
USACE (1993) suggest that winds originate more often from the north (offshore breezes)
and from the east, although neither dataset indicates a strong tendency. It is noted that these
values do not reflect the extremely high winds that occur during episodic events, such as
hurricanes and other tropical storms.

Currents No tidal current study was conducted as part of this report, but published
tidal current information are available (e.g. Degnon, 1996). These tidal current tables,
produced by the National Ocean Service, predict the maximum flood and ebb currents
expected in the channel. For Pensacola Pass, FL, the average maximum flood and ebb
currents are 1.6 knots and 1.8 knots, respectively. Maximum listed current speeds in the
tables are 2.8 knots (flood) and 3.1 knots (ebb). Degnon, 1996, states that current speeds
of up to 8 knots have been reported in the Caucus channel area.

Work and Kaihatu (1997) list the current measurement results from the Perdido Key
beach nourishment project from two wave/current meters installed offshore of the project.
The first gage, located in 19.7 ft of water offshore of the Johnson Beach Park ranger station,
recorded an average current speed of 0.16 knots with a predominant heading of 135 degrees
(southeast) at an elevation of 5.6 ft above the seabed. The second gage, located on Caucus
Shoal in 16.4 ft of water, recorded an average current speed 4.6 ft above the seabed of 0.23
knots directed into the Pass at a heading of 30 degrees with respect to true north.











\ _0
-I -








-- UFCOE 19.7 ft water depth 1/92 to 5/94
---- USACE WIS 16.4 ft water depth 1/76 to 12/95


Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec


Month
Figure 2.3 Monthly variation in average significant wave height for UFCOE and
USACE WIS datasets, for all occurrences.


300


50 100 150 200 250
Direction of Wave Approach (degrees, 0 = True North)


Figure 2.4 Angular distribution of incident waves (incident from) in the Pensacola
Pass area as determined from UFCOE measured data and hindcast WIS
station 44. Note the higher incidence of waves from east of shore
normal (coming from the 78 to 168 degree sector).


C


L3) 4-S



0)>
(U








Storms Some of the major changes to the natural shoreline witnessed in this area
are the result of episodic events, i.e., hurricanes and other such storms. In the 1990's, the
Pensacola Pass area has been impacted by at least 5 named hurricanes, most recently
Hurricane Georges in September, 1998.


Dean and Chiu (1986) modeled the storm surge elevation resulting from storms of
various return periods. Figure 2.5 plots the results of this analysis. The figure predicts that
for a 100-yr storm event the storm surge elevation, inclusive of tides, wind and wave setup,
and barometric pressure, will be approximately +11 ft NGVD. This would easily inundate
the barrier islands on both sides of the Pass, leaving only the higher dune crests exposed.


Table 2.2 presents a list of major storms/hurricanes that have impacted the study area
over the last 120 years. These data have been compiled from numerous sources, including
the U.S. Army Corps of Engineers, UFCOE reports, and the database of the Hurricane
Prediction Center of the National Oceanic and Atmospheric Administration. Where possible
estimates of the storm surge in the Pensacola Pass area are provided (many of the estimates
are eye-witness accounts, and are therefore provided in a qualitative sense only).


C1
C)

Ez
cn

-o >
0)
aC)C
Ev
o0
.o'a


10 20 50 100 200
return period (yrs)


500


Figure 2.5 Predicted storm surge elevation vs. Return period of storms for the
Pensacola Pass area. Storm surge includes the effects of wind and
wave setup, tides, and barometric pressure. Dean and Chiu, 1986.


-11-










Table 2.2 Storm History in the Pensacola Pass area (1880-1998)


Date Name Storm Surge/Comment**
August, 1880 hurricane
September, 1882 hurricane
September, 1889 hurricane
October, 1893 hurricane
July, 1896 hurricane
August, 1898 hurricane
August, 1901 hurricane
September, 1903 hurricane
September, 1906 hurricane 10 12 ft MSL
September, 1909
August, 1911 hurricane category 1 hurricane
September, 1912 hurricane
July, 1916 hurricane
October, 1916 hurricane
September, 1917 hurricane 4.5 7.5 ft MSL
July, 1919 tropical storm
September, 1924 hurricane
September, 1926 hurricane 10.5 ft MSL
September, 1929
August, 1932 hurricane
July, 1936 hurricane
August, 1939 hurricane
September, 1947 hurricane
August, 1950 Hurr. Baker 5.6 ft MSL
September, 1953 Hurr. Florence 3 ft. MSL (category 1 hurricane)
September, 1956 Hurr. Flossy category 1 hurricane
September, 1957 TS Debbie
October, 1959 TS Irene
September, 1960 TS Florence
August, 1969 Hurr. Camille 5.9 ft MSL
June, 1972 Hurr. Agnes
September, 1975 Hurr. Eloise
September, 1979 Hurr. Frederic 10.8 15.1 ft
September, 1985 Hurr. Elena 3.0 ft MSL
October, 1985 TS Juan 4 6 ft MSL
August, 1987 tropical storm
August, 1992 Hurr. Andrew offshore pass of Pensacola
August, 1995 Hurr. Erin 4.9 ft MSL
October, 1995 Hurr. Opal 7.9 ft MSL
July, 1997 Hurr. Danny
September, 1998 Hurr. Earl offshore pass of Pensacola
September. 1998 Hurr. Georges 7.7 ft MSL
** storm surge estimates for the Pensacola Pass area, not necessarily the peak storm surge for individual storm.
The primary sources for these data are Bares, 1998, and Work et al. 1991.


-12-








3.0 DATA SOURCES

This chapter lists the many sources of data used in preparing the inlet management
study. Many of the sources are unpublished references; where possible, an attempt has been
made to provide the simplest means of access to these data. It is noted that this list does not
encompass all the available data in the area. Efforts were made to collect reasonably
complete sets of data for a particular year in order to substantially contribute to the
development of the management plan. An up-to-date survey of the Pensacola Pass
bathymetry and the adjacent beach profiles was conducted as a part of this study in
July/August, 1998.



3.1 Shoreline Position Data

Table 3.1 presents the available data used in the study that describe the position of
the Mean High Water Line. Historic data for this area come primarily from old nautical
charts or the actual "boat sheets" from the U.S. Coast and Geodetic Survey and the National
Ocean Service. Many of these charts were digitized for shoreline position by Florida
Department of Environmental Protection (FDEP) personnel for inclusion in the extensive
FDEP shoreline position and beach profile database. This database can be accessed via the
Internet through the FDEP website, www2.dep.state.fl.us/water/beaches/



3.2 Beach Profile Data

Table 3.2 lists the available beach profile data used in the study. The first set consists
of the profiles taken by FDEP in establishing the first Coastal Construction Control Line in
Escambia County in 1974. The remainder of the profiles were collected either by FDEP or
by the University of Florida Department of Coastal and Oceanographic Engineering (UF
COE) for the Perdido Key Beach Nourishment monitoring program or for this study.









Table 3.1 Available Mean High Water shoreline position data
for the Pensacola Pass Area.


Date Data Limits Source

1856-1858 R-47 to R-103 FDEP database1

1895 R-27 to R-99 FDEP database

1902 R-57 to R-80 FDEP database

1920 R-1 to R-166 FDEP database

1934 R-1 to R-214 FDEP database

1967 R-9 to R-168 FDEP database

1974 R-1 to R-214 FDEP database (CCCL)

1978 R-1 to R-214 FDEP database

1984 R-l to R-214 FDEP database (CCCL)

1985 R-1 to R-214 FDEP database

1988 R-1 to R-2142 FDEP database

1989-1997 R-25 to R-67 UF COE (8+ datasets)

1993 R-1 to R-2142 FDEP database

1995 R-68 to R-214 FDEP database (post-Opal)

1996 R-1 to R-214 FDEP database

1998 R-20 to R-214 UF, FDEP

The FDEP shoreline position database is a compilation of many different sources of data, such as U.S. Coast
and Geodetic Survey, National Ocean Service, etc. The database provides a consistent dataset of shoreline
position, therefore this reference is chosen. The reader is referred to the FDEP database for specific sources
of individual survey sets.

2Shoreline position reported for every third profile at FDEP R-monuments.


-14-









Table 3.2 Available Beach Profile Data for the Pensacola Pass Area.


Date Data Limits Source

1974 R-1 to R-214 FDEP database (CCCL)

1984 R-1 to R-214 FDEP database (CCCL)

1985 R-1 to R-214 FDEP database

1988 R-l to R-2141 FDEP database

1989-1997 R-25 to R-67 UFCOE (8+ datasets)

1993 R-1 to R-2141 FDEP database

1995 R-25 to R-214 UF/FDEP (Pre/post-Opal)

1996 R-l to R-214 FDEP database

1998 R-20 to R-214 UFCOE, FDEP

Shoreline position reported for every third profile at FDEP R-monuments.




3.3 Bathymetric Data

Table 3.3 lists the available bathymetric data used in the study. These data include
the T- and H-sheets (boat sheets) produced from the National Ocean Service and its fore-
runner the U.S. Coast and Geodetic Survey. Datasets prior to 1930 were digitized by UF
personnel for this study. The later datasets were compiled from data provided on CD-ROM
from the National Ocean Service (www.nos.noaa.gov). The 1998 bathymetric survey of
Pensacola Pass was conducted by UFCOE personnel as a part of this study.

It is noted that the National Ocean Service (NOS) produces nautical chart updates
every few years. These updates, however, do not represent all new surveys of the area, and
frequently only include updated obstruction information from boaters in the area. In this
study, only the comprehensive surveys of the area are included, with supporting, anecdotal
information from nautical charts used where appropriate.


-15-









Table 3.3 Available Bathymetric Data
for the Pensacola Pass Area.


3.4 Aerial Photography

Aerial photography of the Pensacola Pass area was obtained from several sources.
The University of Florida Libraries System maintains a database of aerial photos from all
Florida Counties. These photos are primarily historic aerials from the Florida Department
of Transportation (FDOT). Recent aerial photographs were obtained directly from FDOT.
Additionally, aerials were obtained from the Microsoft/U.S. Geologic Survey joint website
at www.terraserver.com. Color oblique photography was collected by UF personnel in
December, 1998, as a part of this study.

A compilation of the historic aerial photography is contained in Appendix A.


-16-


Date Data Limits Source

1856 floodshoal, ebbshoal, USCGS boat sheet H-585
throat

1881 floodshoal, ebbshoal, USCGS boat sheet H-1497
throat

1920 floodshoal, ebbshoal, USCGS boat sheet H-4103
throat

1934 floodshoal, ebbshoal, USCGS boat sheet H-5823
throat & CD-ROM

1981 floodshoal, ebbshoal, NOS CD-ROM
throat

1998 floodshoal, ebbshoal, UFCOE
throat









Table 3.4 Available Aerial Photography
for the Pensacola Pass Area.


-17-


Date Areal Extent Source

1940 flood, ebb**, throat UF Libraries

1954 flood, ebb, throat UF Libraries

1958 flood, ebb, throat UF Libraries

1974 flood, ebb, throat UF Libraries

1981 flood, ebb, throat FDOT

1983 flood, ebb, throat FDOT

1986 flood, ebb, throat FDOT

1989 flood, ebb, throat FDOT

1992 flood, ebb, throat FDOT

1993 flood, ebb, throat FDOT

1994 flood, ebb, throat USGS (Internet)

1997 flood, ebb, throat FDOT

1998 flood, ebb, throat (oblique) UFCOE
* flood and ebb refer to floodshoal and ebbshoal area, respectively.








4.0 DREDGING & THE PENSACOLA PASS NAVIGATION CHANNEL


4.1 The Federal Role in the Pensacola Pass Navigation Channel


This section briefly outlines the involvement of the United States Federal
Government in the development of Pensacola Pass, beginning in the early 1820's. The U.S.
Army Corps of Engineers (USACE) and the U.S. Navy (and Army) have played dominant
roles in the development of the navigation channel and adjacent shorelines.


Pensacola and Pensacola Bay were ceded to the United States from Spain in 1819,
following the capture of Ft. San Carlos de Barrancas by Andrew Jackson in 1818. Formal
control of Pensacola by the United States occurred in 1821 (Coleman and Coleman, 1995).
A territorial government was established in March, 1822 (Coleman, 1988). The U.S.
Congress authorized the establishment of a naval facility at Pensacola in 1825, and
proceeded to consider plans for the fortification of Pensacola Harbor'. Surveys of the area,
including Santa Rosa Island, the Barrancas area north of the mouth, and Foster's Bank2 had
been conducted as early as 1822.


Construction of Ft. Pickens on Santa Rosa Island began in 1829 and was completed
in 1834. Ft. McRee, on Foster's Bank/Perdido Key was constructed between 1834 and 1839.
The fort north of the mouth, Ft. Barrancas, was built between 1839 and 18443. During that
time, the Navy Yard was established (Florida was admitted to the Union in 1845). During
the construction of the Navy Yard and the forts, no work on the navigation channel or harbor
channels was conducted. Maps dating from 1821 through 1856 indicate a controlling depth
over the bar at the mouth of the Pass of approximately 22 to 23 feet MLW, at most.




1
For example, refer to House Document No. 88, 19th Congress, 2nd Session,
1827, Contemplated Fortifications in Pensacola Harbor.
2
Perdido Key was at one time in the 1800's named Foster's Island, or
Foster's Bank. Foster's Bank appears as a spit or shoal just to the
east of Foster's Island in a 1830 chart by the Board of Engineers, but
apparently rejoined with the island shortly thereafter.
3
As an aside, in 1861 Fort Barrancas became the location of the first
shots fired during the U.S. Civil War (fired on January 8th, months
before the fighting began at Fort Sumter in South Carolina)(Coleman and
Coleman, 1995).


-18-








The first work on the navigation entrance to Pensacola Bay was authorized by the
River and Harbor Act of June 18, 1878, which authorized the construction of an entrance
channel through the outer bar to a depth of 24 feet below MLW datum. Subsequent acts in
1894, 1899, 1902, 1935, 1937, and 1945 brought the dimensions of the entrance channel to
32-ft depth below MLW with a corresponding width of 500 ft. The existing project,
authorized by the River and Harbor Act of 19624, establishes the size of the Federally
authorized project to be 35-ft deep (MLW) with a corresponding width of 500 ft.

While the dimensions of the Federally authorized project are presently set at 35 ft by
500 ft, the U.S. Navy has contributed to the maintenance of the channel since 1958. At that
time, the U.S. Navy requested the entrance channel be maintained to 37 ft depth below MLW
with a channel width of 800 ft. These dimensions were maintained via dredging until 1989-
1991, at which time the U.S. Navy requested an increase in the channel depth to 44 ft to
accommodate the homeporting of an aircraft carrier at Pensacola. The dredging work
completed in 1991 to provide this homeporting created an entrance channel with a depth of
48 ft (44 ft plus 2 ft advance maintenance dredging and 2 ft allowable overdredge) and a
width of 800 ft. No dredging of the entrance channel has been conducted since 1991. The
planned aircraft carrier homeporting was not carried out.



4.2 Dredging Records

Table 4.1 chronicles the dimensions of the Pensacola Pass entrance channel since
1881. The table was compiled from actual surveys and government documents indicating
the authorized project dimensions.

The increase in the depth and width maintained in the entrance channel has increased
the dredging requirements within the channel. The increase in dredging requirements,
however, is much greater than the relative increase in depth, as is shown in Figure 4.1, which
presents the cumulative volume of material dredged from the entrance channel and approach
to the Navy turning basin since dredging commenced in 1883. The figure also presents an
'effective' dredging rate computed by fitting the cumulative volume change data to a third-
order polynomial then taking the derivative to determine a smoothed dredging rate over the
life of the maintained channel.


4
The relevant portion of the River and Harbor Act of 1962 can be found in
House Document No. 526, "Pensacola Harbor, Florida," 87th Congress, 2nd
Session, August 27th, 1962.








Combined with Table 4.2, which details as much as possible the dredging history at
Pensacola Pass, it is seen that over the 115-yr history of the Pass since dredging commenced,
roughly 45.8 million cubic yards (mcy) were removed mechanically from the channel. Of
that amount, 18.6 mcy have been disposed of on or near the adjacent shorelines. Prior to the
most recent dredging/renourishment efforts in 1989-1991, however, only 25% of the dredged
volume had been placed alongshore, with the remainder placed in an offshore disposal site
(Table 4.2).

Further inspection of Figure 4.1 indicates the increase in overall annual dredging rate
as well as the events in which 'new work' was undertaken to deepen the channel to meet the
authorized project dimensions. Prior to the most recent dredging events, the annual dredging
rates (say, between 1960 and 1985) were over 300,000 cy/yr. As will be discussed in detail
in a later chapter, this amount represents more material than is supplied to the Pass from the
net littoral drift and the adjacent eroding shorelines. This implies that the dredging
operations are serving to 'draw down' the shoals. That is, the shoals are decreasing in
volume as a result of the maintenance of the deep navigation channel.




Table 4.1 Time History of the Dimensions of the Pensacola
Pass Entrance Channel.


Date Depth (ft, MLW) Width (ft) Authorized/Actual

1881 24 80 authorized
1885 22.5 80 actual
1890 24 120 actual
1894 30 300 authorized
1902 30 500 authorized
1928 30 500 actual
1945 32 500 actual
1959 37 800 actual
1962 35 500 presently authorized
1989-1991 44 800 Navy requested
1991 48 800 actual
1997 40 600 actual*
** per July, 1997, condition survey by USACE.


































Figure 4.1 Dredging history of Pensacola Pass, FL. The figure plots the cumulative volume of material dredged from the entrance
channel and approach to the U.S. Navy turning basin (left axis). The figure notes six events where sand was placed
along the adjacent shorelines; the balance of the material was deposited offshore. **The second curve presents a
simplified estimate of the increase in the dredging rate over the history of the maintained channel (right axis). This
curve was computed by fitting the cumulative dredging data to a third order polynomial, then taking the derivative of
the resulting expression to determine a second order expression for the effective dredging rate over time. This effective
rate may over- or underestimate the actual dredging rate for any particular time, and is presented primarily for
illustrative purposes.











Table 4.2 Dredging History of the Pensacola
Pass Entrance Channel.


Year Disposal site volume (cy) cumulative volume (cy)

1883.5 offshore 8,400 8,400
1885.4 offshore 37,000 45,400
1885.6 offshore 16,300 61,700
1886 offshore 55,500 117,200
1891.5 offshore 14,100 131,300
1893.5 offshore 27,900 159,200
1894.6 offshore 92,600 251,800
1896 offshore 392,400 644,200
1897.5 offshore 300,800 945,000
1897.6 offshore 7,500 952,400
1898.4 offshore 255,100 1,207,500
1899.4 offshore 295,600 1,503,100
1900 offshore 480,000 1,983,100
1901 offshore 324,400 2,307,500
1902 offshore 184,400 2,491,900
1905 offshore 200,100 2,692,000
1906 offshore 592,500 3,284,500
1907 offshore 286,400 3,571,000
1908 offshore 855,400 4,426,400
1909 offshore 307,400 4,733,700
1910.6 offshore 381,900 5,115,700
1911 offshore 956,100 6,071,800
1914 offshore 385,800 6,457,600
1915 offshore 19,000 6,476,600
1917 offshore 244,600 6,721,200
1921 offshore 137,300 6,858,500
1922 offshore 247,200 7,105,700
1927 offshore 787,400 7,893,100
1930 offshore 924,700 8,817,800
1932.4 offshore 674,900 9,492,700
1933.7 offshore 769,100 10,261,800
1934.2 offshore 584,700 10,846,400
1934.8 offshore 691,900 11,538,300
1935.7 offshore 371,500 11,909,800
1937.7 offshore 465,600 12,375,400
1938.7 offshore 512,700 12,888,200
1939 offshore 112,900 13,001,000
1940.6 offshore 303,400 13,341,100
1940.8 offshore 659,200 13,995,100
1946 offshore 215,800 14,125,900
1947.4 offshore 846,200 15,041,400
1947.8 offshore 121,400 15,172,200
1948 offshore 472,200 15,564,600
1950.5 offshore 396.300 15.957.000


-22-








1951.6 offshore 230,200 16,218,600
1953.9 offshore 546,700 16,741,800
1955.9 offshore 264,200 17003,400
1958.9 offshore 272,100 17,264,900
1959.7 Santa Rosa Pt. 971,800 18,311,300
1959.7 Santa Rosa Pt. 3,948,700 22,235,200
1959.8 Santa Rosa Pt. 2,011,600 24,197,100
1964.9 offshore 2,482,500 26,682,200
1967 offshore 1,225,500 27,990,100
1968 offshore 941,700 28,905,700
1969 offshore 218,400 29,167,300
1970 offshore 239,400 29,428,900
1971.1 offshore 171,300 29,559,700
1971.8 offshore 1,563,000 31,129,200
1975.2 offshore 1,098,700 32,175,600
1981.2 offshore 654,000 32,829,600
1983.9 offshore 113,800 32,960,400
1984 offshore 915,600 33,875,900
1985.5 Perdido Key 2,432,800 36,361,000
1987 offshore 196,200 36,491,800
1990 Perdido Key 5,362,600 41,854,400
1991 Perdido Key (offshore berm) 3.923.900 45.778.300
sources Hine et al. (1986), Dean (1988), Work et al. (1991)


The draw-down of the shoals due to dredging increases with increasing channel
depth, as seen in Figure 4.1 and Table 4.1. As shown in Figure 4.1, the effective dredging
rate has increased from roughly 150,000 cy/yr in the 1880's to roughly 800,000 cy/yr as of
the most recent dredging efforts. When the channel is dredged, the adjacent side can slump,
depositing material from the shoals into the navigation channel to be dredged out at the next
event. Since much of the dredged material prior to 1990 was disposed of offshore, this
represents a loss of material from the littoral system. Additional factors contributing to this
draw-down may be additional shoaling of the channel caused by a reduction in magnitude
of the tidal currents through the enlarged entrance channel. Other effects of the deepening
of the channel are discussed in subsequent chapters.



4.3 Present Condition


As shown in Table 4.1 the present controlling dimensions of the channel appear to
be roughly 40 ft by 600 ft. These dimensions are based on channel condition surveys
conducted on a more-or-less annual basis by the U.S. Army Corps of Engineers, the most
recent of which occurred in July, 1997. Inspection of the time history of the channel cross-
sections since 1990 reveals the primary shoaling patterns in the channel.


-23-








Figure 4.2 depicts a schematic of the entrance channel indicating the locations where
cross-sections were digitized for investigation. The suggested shoaling patterns are shown
by the shaded areas in the figure. Plots of the cross-sections are contained in Appendix B.

The shoaling patterns provide a substantial amount of information relating to the sand
flows around the Pass. The shoaling/accretion witnessed along the western tip of Santa Rosa
Island was obviously expected as the island has a tendency to migrate westward. Along
these cross-sections (60+20N and 69+20N), the initial dredging cut into the submerged bank
off Santa Rosa Island some 400 to 800 ft. This area has since recovered, almost entirely,
since the dredging in 1989-1991.

The second area of shoaling lies just off the line of the Perdido Key shoreline (sta
10+50N and 20+50N). It is believed that this shoaling is a result of sand "spilling" off the
eastern end of Perdido Key during flood tide and during easterly directed wave events. This
area is also shoaled by sand stripped from the tip of Santa Rosa Island on the ebb tide. Sand
is carried from the tip out onto Middle Ground shoal, where some of the material then falls
into the channel in this area.

The last area experiencing significant shoaling lies farther offshore along the channel
between stations 29+50S and 89+50S. Not surprisingly, this area is where the original
bypass bar for the channel was located.

Volume change calculations of the channel itself from the time of the post-dredge
survey (1990-1991) to the most recent condition survey by the USACE (1997), indicate that
the channel has shoaled by as much as 1.9 mcy. This represents an annualized shoaling rate
of 313,700 cy/yr. To date, the shoaling of the channel since 1991 has not necessitated
maintenance dredging since the aircraft carrier homeporting was not carried out. The U.S
Army Corps of Engineers continues to monitor the condition of the Pass, and will continue
to maintain the entrance channel at the Federally authorized dimensions (35 ft deep by 500
ft wide).

Gulf Intracoastal Waterway The Gulf Intracoastal Waterway runs east-west
through Pensacola Pass and is maintained by the U.S. Army Corps of Engineers. The
presently authorized project calls for a 12-ft deep, 125-ft wide channel from Apalachee Bay
to the Texas/Mexico border, as presently authorized by the River and Harbor Act of 1966,
House Document 481, 89th Congress, 2nd Session. The Ft. McRee land cut, created in the
mid-1950's north of the disposal island and south of Sherman's Cove, is periodically dredged
to maintain authorized dimensions. No records of the GIWW dredging were obtained.


-24-















I i I I-


500,000-


498,000


496,000-


494,000-


492,000-


490,00 0


488,000-


486,000-


484,000-


482,000-1
N

480,000 --
1,108,000


S.Federal
navigation
Channel


1,112,000


Pensacola
Bay


89.50S


1,116,000


1,120,000


Easting (ft, NAD 1927)


scale: 1" = 4,000ft

Schematic map of the Pensacola Pass Navigation Channel. The map
indicates the location of cross-sections used to investigate shoaling
patterns and rates. The shaded areas in the figure indicate areas of
substantial shoaling as of the July, 1997, condition survey by USACE.
Depth contours are in feet, NGVD, from 1998 condition survey.


Z. /- 69+20N
S/ 69+20N 80+24N
/8"80N 4\
53+54. V



S40+50

30+50N
S20t50N

'10+50DN
0+.0
7-84S

019-50S

29+50S
839+50S
Wt:' 6 49+50S


'69 +50S
'.4 .


Gulf
of
Mexico


Figure 4.2


sS








5.0 SHORELINE HISTORY

This chapter details the changes in shoreline position and sand volume measured
from the available data beginning in 1856 (Chapter 3). The objective of this chapter is to
describe the overall shoreline behavior, to address the volumetric gains and losses that have
occurred on either shoreline, and to determine the longshore extent of influence of the Pass
as measured by the shoreline data.

Unlike other tidal entrances in the State of Florida that have a major milestone in
their histories (i.e., the construction ofjetties, the opening of the entrance via man or storm,
etc), Pensacola Pass has existed more or less in its present condition for over 300 years (and
probably much longer). The most significant change is the pattern of dredging of the
entrance channel, which began in 1883. The lack of a major event in the history of the Pass
presents some difficulty in choosing a suitable baseline, since for this Pass any changes from
the baseline may be small, making the noise in the system difficult to separate from the
actual trend. The approach chosen herein is to evaluate as many of the datasets as possible
to investigate consistent patterns of shoreline behavior.



5.1 Pre-Dredging & Long Term Mean High Water Shoreline Changes

Figure 5.1 presents the position of the Perdido Key and Santa Rosa Island Mean High
Water Lines (MHWL) along the first 10,000 ft to either side of the Pass where FDEP R-
monuments presently exist. The figure indicates the general trend of shoreline behavior in
the area since 1856. Santa Rosa Island has progressively advanced westward, up until the
significant dredging efforts of the last roughly 50 years, which have served to hold the
average position of the shoreline on Santa Rosa Island approximately fixed in this area. The
shoreline along Perdido Key has advanced along the first 3,000 ft (interior to the Pass). This
deposition appears to coincide with the ongoing deepening of the channel, which has created
a much more linear ebb shoal feature from Caucus Shoal than what existed in 1856. The
shoreline at the eastern end of Perdido Key has changed from a gently curving shoreline
heading toward the Pass to a very angular shoreline, taking a sharp turn northward into the
Pass in the lee of Caucus Shoal.

Figure 5.1 also illustrates the effect the southernmost jetty has on the Perdido Key
shoreline. While the shoreline history indicates fluctuations along most of its length, in the
immediate vicinity of the southern jetty, the shoreline has remained fairly stable since the
1930's, which appears to be the last time any modifications were made to this structure.


-26-










494,000





491,000


488,000 I I 1 I I I II

1,105,000 1,108,000 1,111,000 1,114,000 1,117,000


494,000


491,000





488,000


1,105,000


1,120,000 1,123,000 1,126,000


1,108,000 1,111,000 1,114,000 1,117,000 1,120,000 1,123,000
Easting (ft, NAD 1927)


1,126,000


Figure 5.1 Historical position of the Mean High Water Line (MHWL) adjacent to Pensacola Pass from 1856 to 1998.








The lower plot of Figure 5.1 also describes the pre-dredging shoreline behavior.
While these data are greatly limited in alongshore extent and time, they do provide insight
into the 'natural' behavior of the island/pass system. These data were digitized from the U.S.
Coast and Geodetic Survey boat sheets, and their accuracy should be considered accordingly.
However, the 1881 survey sheet also contains the 1857 shoreline position, so that a direct
comparison may be made with greater confidence. During the 24-yr period between 1857
and 1881, the Santa Rosa Island shoreline advanced an average of over 180 ft westward,
illustrating the island's natural tendency to migrate westward. Coleman and Coleman (1995)
mention that when the site for Ft. Pickens was selected in 1829, the western edge of the fort
was situated only about 200 yards from the water. They report that the fort itself is now well
over 4,000 ft from the western waterline. As expected, the Perdido Key shoreline retreated
significantly, averaging 250 ft of recession along the measured shoreline, during which time
it appears that the island was breached somewhere north of the present day jetties.

The available shoreline change data were analyzed to determine the historic rate of
change of the MHW position since 1856. Where data from 1856 were unavailable, the
longest time span available was analyzed. Figure 5.2 presents the results of the analysis
from this study and from another separate study conducted by Dean & Cheng (1998). Both
results are obtained from the same FDEP datasets, only the chosen years of analysis vary.


-80,000


-40,000 0 40,000 80,000
Distance east of Pensacola Pass (ft)


120,000


Figure 5.2 Historical Mean High Water shoreline position rate of change adjacent to
Pensacola Pass, FL.








From Figure 5.2 the following observations regarding the long-term shoreline
behavior are made:

1) The Santa Rosa Island shoreline along the first 10,000 to 15,000 ft appears
to be directly influenced by the Pass, experiencing large fluctuations in
shoreline position throughout the time period investigated. Shoreline
changes ranging from 8 ft/yr of advance to 5 ft/yr of recession are measured.
This erratic behavior of the shoreline is consistent with the findings of Dean
and Cheng (1998), who documented shoreline changes around the state of
Florida and indicated that the largest and most erratic shoreline changes occur
immediately adjacent to tidal entrances (as indicated by the standard
deviation of measured shoreline changes in an area).

In this same stretch, the trend of increasing seaward advance (or decreasing
shoreline recession) does suggest that the western tip of the island has had a
tendency to be accretional. It is noted that this dataset does not extend
westward to the terminus of the island, which obviously has a trend toward
accretion. The large erosional signal seen at the last datapoint (R-68)
indicates the fluctuations in shoreline position this area experiences.
Inspection of aerial photography (Appendix A) indicates that the shoreline in
the vicinity of R-68 and R-69 occasionally experiences recession reaching
landward to and beyond the vegetation line.

2) Moving eastward along Santa Rosa Island, at approximately 20,000 ft east of
the Pass (just east of Pensacola Beach), the shoreline has experienced an
overall recession of the shoreline on the order of 2.5 to 3.0 ft/yr. This
recession extends eastward, diminishing to zero over a distance of 40,000 to
60,000 ft (near the eastern end of Pensacola Beach). Eastward of the 60,000
to 80,000 ft mark extending to 120,000 ft (22.7 miles), the shoreline appears
to be net stable to somewhat accretional.

3) The first 10,000 feet of shoreline along Perdido Key indicate similar
fluctuations in shoreline position over time as seen on Santa Rosa Island.
The first 3,000 ft of shoreline (inside the Pass) appear to be stable to
accretional, due in large part to the presence of the southern terminal jetty.
The next 7,000 ft of the Gulf of Mexico shoreline along the easternmost
portion of Perdido Key has experienced recession of as much as 4 ft or more
per year over this time period.








4) The trend of shoreline recession that begins at the eastern end of Perdido Key
extends westward, diminishing to zero at a distance of approximately 40,000
ft (7.6 miles) west of the Pass, near the western boundary of the National
Park on Perdido Key. West of the park, the shoreline history indicates
smoothly increasing seaward advance of the shoreline, reaching roughly 4
ft/yr at the Florida/ Alabama State Line. The State Line lies less than 3 miles
from Perdido Pass, which is a tidal entrance stabilized by rock jetties. At this
point, the influence of Perdido Pass most certainly comes into play, and
likely has a stabilizing influence upon the shoreline. Another possible reason
for the stability of the shoreline in this region may be a supply of sediment
from offshore. Stone et al. (1992) have suggested this mechanism based
upon inspection of the accretional beach ridges seen in the upland portion of
Perdido Key between R-34 and R-22. These beach ridges can be seen in
Figure 5.3, and extend in an approximately east to east-southeast direction.
The implications of onshore directed transport on the shoreline behavior and
sediment budget will be discussed in Chapter 8 Sediment Budget.

5) The 1856-1998 dataset indicate the effect the 1985 and 1989-1991
nourishment projects have had in offsetting the shoreline recession in that
area (R-40 to R-64 on Perdido Key). The effects of the 1989-1991
nourishment project will be discussed in detail elsewhere in this chapter.


Figure 5.3 Aerial photograph of Perdido Key, FL, indicating the pattern of beach
ridge growth along the middle of the island. Photograph dated January,
1994. Source www.terraserver.com (USGS and Microsoft Corp.).


-30-








It is important to recognize that the values and trends of shoreline advance or retreat
presented above represent data collected at discrete points in time during different times of
the year. In this discussion, it is assumed that seasonal variations over such a long time
period would be small relative to the overall change, and it is noted that the effects of
individual storms are not expressly included here.



5.2 Recent Shoreline Behavior


Figure 5.4 presents the change in MHWL position over the last 24 years. The data
in the figure are plotted relative to the 1974 MHWL. The recent shoreline behavior closely
mimics the longterm behavior, with the obvious exception of the large nourishment project
on Perdido Key in 1989-1991. The MHWL changes immediately adjacent to the Pass
indicate large fluctuations in shoreline position along the Santa Rosa Island shoreline. Some
of these changes can be linked to the dredging operation in 1989-1991, which cut the western
tip of Santa Rosa Island back nearly 600 ft in places. The subsequent readjustment of the
shoreline may have resulted in the large shoreline recession values measured just to the east
of the tip of the island (the FDEP R-monuments do not fully extend westward).


15
-- 1984

S 10 _-- 1996
S 1998
S 5
a)





I. i y
-10 8 .- M0
C')




v o S
a) n 3
I Perdido Key W Santa Rosa Island


I I I


-80,000


I I I


I I I


I I I


-40,000 0 40,000 80,000
Distance East of Pensacola Pass (ft)


I I I


120,000


Figure 5.4 Shoreline changes in the vicinity of Pensacola Pass, FL, relative to the
measured 1974 MHWL. Positive changes indicate seaward shoreline
advance, negative values indicate shoreline recession.


-31-








Similar to Figure 5.2, shoreline recession appears to extend roughly 40,000 ft to
either side of the pass, at which point the shoreline begins to recover. Moving eastward
through Pensacola Beach (R-108 to R-140), the shoreline change as measured by these
datasets is fairly small, indicating a fairly stable shoreline, although these data do not
expressly include the effects of individual storms.

Along the western shoreline, the most obvious occurrence is the beach nourishment
project on Perdido Key. The 1998 survey indicates that the shoreline within the project
limits is over 200 ft seaward of the 1974 shoreline position. A detailed description of the
performance of the Perdido Key beach nourishment project is included in this chapter.
Westward of the nourishment project, the shoreline behavior is very similar to the long term
trend, indicating that the shoreline is increasingly depositional toward the State Line. Again,
the increasing influence of the east jetty at Perdido Pass in Alabama may be a large
contributor to the stability of the shoreline in this area (as may the possible addition of sand
from offshore).



5.3 Pre-Dredging & Long Term Volumetric Changes

Beach profile data collected along the Santa Rosa Island and Perdido Key shorelines
beginning in 1974 allows for quantification of the volumetric changes that have occurred
there along. Using the recent measured beach profile changes and shoreline position
changes, estimates may be made of the volumetric changes occurring prior to 1974.

Using data from the 1974-1984 and 1984-1996 data sets, an estimate was made of
the ratio between volumetric change and shoreline position change. The resulting ratio may
be thought of as the active height of the beach profile. This technique assumes that the entire
profile moves the same distance, landward or seaward, as the measured shoreline position.
Having determined the active height of the profile, estimates of the historic volumetric
changes since 1856 can be made using the data shown in Figure 5.2. The average active
height of the beach profile was determined to be approximately 15 ft. This assumes that the
height of the beach berm is roughly 5 ft and the depth of closure is 10 ft'. Inspection of
individual beach profiles along both sides of the Pass suggests that this is a reasonable value.




1
The reader is encouraged to inspect the plots of beach profiles for both shorelines in Appendix C to
evaluate this choice of the active height of profile.









Using the technique described above, the volume change associated with the pre-
dredging time period, 1857 to 1881, was calculated. During the 24-yr period, the western
end of Santa Rosa Island accumulated roughly 1.2 mcy of sand, advancing Gulfward and
westward an average of 182 ft along a total measured reach of 11,900 ft. The Perdido Key
shoreline, along the present-day monitored reach of 10,200 ft, eroded some 1.4 mcy of sand,
retreating an average of 247 ft along the measured reach.


Figure 5.5 depicts the cumulative average annual volumetric change determined from
the above method for the 1856-1984 (pre-nourishment) time period. For clarity only one
curve from Figure 5.2 is plotted. The curve in Figure 5.5 represents the accumulated losses
or gains of sand traveling away from the Pass to the East and West along Santa Rosa Island
and Perdido Key, respectively. Areas of higher erosion or accretion are indicated by the
steeper slope of the curve. The grey error band in the figure indicates the range in the
answers if the active height of profile was off by 3 feet in either direction.


1856-1984
accretion


0
erosion Santa Rosa land

ey
Perdiao Key M




2 c3

5 0 |
------ coci ----- --------- --------- S n
o: g ^o:


I I I


I I


I I I


I I I


-40,000 0 40,000
Distance east of Pensacola Pass (ft)


80,000


120,000


Figure 5.5 Annual cumulative volumetric change measured eastward and westward
from Pensacola Pass, FL, for the Santa Rosa Island and Perdido key
shorelines, respectively. The volumetric changes were computed from the
MHW shoreline changes for this time period by assuming the entire
profile moves uniformly with the MHWL over an active height of profile
of 15 ft. The grey error bands indicate the volumetric changes for a range
of profile height of 15 +/- 3 ft.


-33-


20,000



0 0
(0
Co
I
E -20,000

cu

7 -40,000
E


| -60,000
<


-80,000








The results plotted in Figure 5.5 indicate that the first 40,000 ft of shoreline along
Santa Rosa Island experiences an average of roughly 30,000 cy/yr of erosion, or 0.75 cy/ft/yr
(for this time period). This stretch of beach reaches to the eastern boundary of the Gulf
Islands National Seashore (R-107). Along the next 60,000 ft of shoreline, through Pensacola
Beach and eastward toward Navarre, the shoreline eroded at a rate of approximately 20,000
cy/yr, or 0.33 cy/ft/yr. Eastward toward the Santa Rosa-Okaloosa County Line, the shoreline
shows signs of stability to mild accretion.

Along the Perdido Key shoreline, the long-term trend suggests that the first 40,000
ft of shoreline experiences an average of 50,000 cy/yr of erosion (1.3 cy/ft/yr). Similar to
the previously discussed shoreline changes, westward of the National Park the shoreline has
historically been stable to accretional, accumulating an average of 30,000 cy/yr (Icy/ft/yr).



Even/Odd Analysis of Long-Term Volumetric Changes Figure 5.6 presents an
even/odd analysis of the long term volumetric changes plotted in Figure 5.5. This analysis
separates the volumetric changes into a symmetric (even) and anti-symmetric (odd)
component about the Pass (Berek and Dean, 1982). The symmetric component describes the
extent to which the Pass affects both shorelines equally; alternatively, the even component
describes any existing background erosion prevalent along the entire study area. The anti-
symmetric component may reveal any impounding of sand on one side of the Pass, generally
updrift, and a corresponding erosion of sand on the downdrift side.

Figure 5.6 illustrates the even and odd components of volumetric change measured
between 1856 and 1978 and between 1974 and 1984. The 1856-1978 data are shoreline
positions converted to volumetric changes using an active height of 15 ft, as discussed
previously. Inspection of the even components reveals a pattern similar to the historic
changes presented above. In Figure 5.6, however, the analysis suggests that the longshore
extent of the erosion may reach another 10,000 ft to either side, extending up to 50,000 ft
from the Pass in both directions. The odd component of the longer period (upper plot)
suggests some impoundment within the first 10,000 to 20,000 ft east of the Pass on Santa
Rosa Island, with a corresponding erosional signal along Perdido Key.

One possible result of the even and odd volumetric changes along the Perdido Key
shoreline is that the two components may distinguish between the erosional effect of the Pass
and the depositional effect of the Perdido Pass jetty and/or the possible addition of sand to
the Perdido Key shoreline in the Gulf Beach area from offshore. That is to say, if these
possible depositional mechanisms were not present, the resulting rate of erosion would be


-34-






















-60,000 -40,000 -20,000 0 20,000 40,000 60,000
Perdido Key Santa Rosa Island

(1974-1984



I r




_I
I I i l I l l i l I I I I l I I I I I
-- Th 1~ fl6l fl4A A~ Nlll


-60,000 -40,000 -20,000 0 20,000 40,000
Distance East of Pensacola Pass (ft)


60,000


Figure 5.6 Even/odd analysis of volumetric changes between 1856 and 1978 (upper
plot) and 1974 and 1984 (lower plot).



higher, and the erosional longshore extent of Pensacola Pass might be longer. This is seen
in the plot in the even components, which reach erosion rates of 1.5 to 2.0 cy/ft/yr, compared
with the average erosion rate of 1.3 cy/ft/yr. The odd component along Perdido Key reaches
accretion rates of 1 cy/ft/yr. This method may allow a more quantitative estimate of how
much more erosion would be present, a possibility that will be explored in the development
of the sediment budget (Chapter 8).









5.4 Recent Volumetric Changes


Figure 5.7 depicts the volumetric changes measured since 1974. These volumetric
changes were computed directly from the surveyed beach profiles using the average end-area
method between profile lines. Again the changes are plotted in a cumulative fashion,
beginning at the Pass and accumulating to the east and west along Santa Rosa Island and
Perdido Key, respectively. The first set of changes, between 1974 and 1984, mirror the long-
term averages shown in Figure 5.5. Along Santa Rosa Island, the shoreline eroded roughly
540,000 cy over the ten-year period, averaging 54,000 cy/yr of erosion along the first 40,000
ft east of the Pass. Similarly, along the Perdido Key shoreline, 460,000 cy of sand eroded
from the beach; averaging 46,000 cy/yr of erosion along the first 40,000 ft of shoreline west
of the Pass. Beyond 40,000 ft to either side of the Pass, the 1974-1984 datasets reflect the
same stable to accretive trends seen in the longterm volumetric and shoreline changes and
the shoreline changes.


3,000,000

2,500,000

2,000,000

1,500,000

1,000,000

500,000

0

-500,000

-1,000,000

-1,500,000

-2,000,000


-80,000


-40,000
Distance


0 40,000
East of Pensacola Pass (ft)


80,000


Figure 5.7 Cumulative volumetric changes measured adjacent to Pensacola Pass, FL,
for the time periods 1974-1984 and 1984-1996. Changes are measured in
cy beginning at the Pass and accumulating eastward along Santa Rosa
Island and westward along Perdido Key.


C
(n

E cu
0)
cu U,
a). 0c a)
Scu cu ci
U.c) W
CU CU)0
C: a-
M W
6 0)a
5 a)


0

E>F0 CU
Uo)~

cuor

oaE
= M C
6 M,








The most recent time period shown in Figure 5.7, from 1984 to 1996, presents a
different picture of shoreline behavior. First and foremost, the 1989-1991 beach nourishment
project represents a substantial addition to the shoreline along the first 25,000 ft of Perdido
Key (5.4 million cubic yards (mcy) added to the beach, another 3.9 mcy of sand to the
nearshore area). Earlier during this time period, roughly 2.4 mcy were placed along the
Perdido Key shoreline between R-65 and R-59, 2,000 to 8,000 ft west of the Pass. As of the
1996 survey, the nourishment efforts resulted in the addition of approximately 2.2 mcy of
sand to the shoreline on Perdido Key, extending westward 40,000 ft to the point where the
shoreline has historically been stable to accretional.

Along the Santa Rosa Island shoreline, the measured erosion immediately east of the
Pass is noticeably higher than the historical data report. During the 1984-1996 timeframe,
the shoreline eroded over 1.0 mcy, an annual average of 83,000 cy/yr along the first 40,000
ft. This represents an increase of 2.7 times over the historical average. Additionally, the
increased erosion signal continues to extend eastward another 30,000 ft, through Pensacola
Beach to its eastern limit. This increased erosional signal may be a result of the deepening
of the navigation channel for the Homeport project in 1989-1991.



5.5 Performance of the 1989-1991 Perdido Key Beach Nourishment Project

The large beach nourishment project constructed on the eastern end of Perdido Key
in 1989-1991 has been extensively monitored by the University of Florida Coastal and
Oceanographic Engineering Department, beginning in 1989 with a pre-construction survey.
The project was most recently surveyed in July, 1998, as a part of the condition survey of
Pensacola Pass for the Inlet Management Study. This section will describe the performance
of the project since construction, with emphasis on its impact and importance to the Pass
itself and its sediment budget. The reader is referred to Browder and Dean (1997) and Otay
and Dean (1994) for detailed descriptions of the monitoring program and performance
results.

Project Description As a result of the U.S. Navy plan to bring the aircraft carrier
U.S.S. Kitty Hawk to Pensacola for homeporting, the Pensacola Pass entrance channel was
deepened to a navigable depth of 44 ft MLW. Including overdraft and advance dredging, the
channel was deepened to nearly 48 ft MLW. This deepening (some of which was
maintenance material, most of which was 'new work' dredging) made available some 9.3
mcy of material for disposal, some of which undoubtedly was sand 'recycled' from the
previous dredging/nourishment project in 1985. 5.4 mcy of sand was pumped directly onto


-37-








the shoreline of Perdido Key between FDEP monuments R-64 and R-40, between 2,000 and
25,000 ft west of the Pass. This portion of construction was completed in 1989-1990. An
additional 3.9 mcy of sand was dredged from the channel and placed in a nearshore berm
in approximately 19 to 20 feet water depth between monuments R-48 and R-60 (roughly
13,000 ft alongshore).

Monitoring Monitoring of the nourishment project has consisted of beach profile
surveys conducted annually and biennially since the 1989 pre-construction survey. Surveys
consist of 25 to 33 beach profiles surveyed from FDEP monuments out to as much as 16 ft
water depth. Additional boat surveys extend the profiles to 30 to 40 ft depth. Boat surveys
have also been conducted of the nearshore berm area since 1992 (5 surveys in all).

Waves, currents, and winds were also monitored during the first four to five years of
the project. These data are summarized in Chapter 2.

Results Figure 5.8 depicts the history of the MHWL position since completion of
the beach nourishment project in 1990. The figure indicates that the initially constructed
project advanced the shoreline between R-40 and R-64 seaward an average of over 440 ft,
resulting in an increase in the beach planform area of 250 acres. In the first year following
construction, the shoreline retreated landward an average of 120 ft. This retreat resulted from
a combination of natural beach profile equilibration and erosion from within the project
limits.

The surveyed 1993 shoreline indicates that after 3 years, the shoreline had retreated
an average of 200 ft, roughly 45% of its initially constructed width. As of the most recent
condition survey, July, 1998, the shoreline remains an average of 150 ft seaward of its pre-
project position within the project limits. It is important to note that between the 1993 and
1998 surveys, the Pensacola Pass area was severely impacted by Hurricanes Erin and Opal
in 1995, as well as Hurricane Danny in 19972. Further inspection of Figure 5.8 illustrates
the effect the Pass has had on the project performance. The western and central portions of
the project have faired noticeably better than the eastern end of the project, immediately
adjacent to the Pass. In fact, the shoreline near monument R-62 had retreated landward of
its pre-project position by 1997.



2
The reader is referred to the storm history in Chapter 2, and also to Dean & Lin (1995) and Browder &
Dean (1997) for more detailed monitoring reports of Perdido Key addressing the impacts of Hurricanes
Erin and Opal.








L original construction limits
-r- -- - S S S S U U 5 5 ( -'(Llo ( N ~ C 0 0~~


S' '' '. 0 (0 (0 (0of of (o (O W
0 0 000000000 0 0 0 0 0 0 00000
K z ^ ..1111 i.,


E-0
2-

(n0



co0


I I I


__ __ _ 4


0-



200



400-


I I II I I II

-30,000 -20,000
Distance East of Pensacola Pass (ft)


00/L

^/


I I I I


10,000


Figure 5.8 Change in position of the MHWL of the 1989-1991 Perdido Key beach
nourishment project. The plotted shoreline positions are relative to the
1989 pre-construction surveyed MHWL.


Figure 5.8 also shows that very little spreading of sand westward from the beach
nourishment project occurred until the 1993-1998 time frame, during which the two major
hurricanes occurred. Obviously, there has been a significant amount of sand 'drawn' off the
eastern end of the channel into the Pass.

Figure 5.9 plots the cumulative volume changes along the monitored area since 1990,
measured from R-30 heading eastward toward the Pass. Figure 5.9 illustrates that over the
7.8 years of monitoring, the nourishment project has lost approximately 2.5 million cubic
yards (mcy) of sand from the project limits, with only a small fraction appearing west of the
project limits. Inspection of aerial photography suggests that some of the placed material has
migrated around the eastern tip of Perdido Key, where it is shoaling the shallow channels of
the Intracoastal Waterway and the old channel to Big Lagoon. Additionally, condition
surveys of the navigation channel suggest that perhaps as much as 2 mcy of sand have
shoaled the channel since 1991 (see Chapter 4). It is reasonable to assume that a significant
fraction of this shoaling originated from the nourishment project, meaning the placed sand


-200


1990
1991
1993
1998


seaward


Si ; 1


600


-40,000


I


'


j/-








original construction limits



:0.0

S2 -0.5 version


S-1.0 -



r 0$ 1993-1997 (3.1 years)
E E -2.0 E D 1997-1998 (1.5 years)
0 A-- 1990-1998 (7.8 years)

-2.5 1 ------- --

-40,000 -30,000 -20,000 -10,000 0
Distance East of Pensacola Pass (ft)

Figure 5.9 Cumulative volumetric changes measured along the Perdido Key beach
nourishment project immediately west of Pensacola Pass, FL. The volume
changes are computed beginning at monument R-30 and heading eastward
toward the Pass.



simply returned to the Pass. The likelihood of this occurrence is further supported by the
shape of the curves plotted in Figure 5.9. The areas where the slope of the curve is the
greatest indicate the areas of highest volumetric change, in this case, erosion. Figure 5.9
indicates that the area of highest volumetric change is along the eastern end of the project
limits from R-58 eastward, followed by the end of the project in the vicinity of R-45 to R-44.
The steep slope of the volumetric change curve at the eastern end is a definitive indicator that
the Pass plays a strong role in the performance of the nourishment project. This mechanism
will be discussed further in later chapters.

Additional data from Figure 5.9 illustrate that as much as half of the erosion
measured within the project limits occurred in the latter half of the project life. Again this
is attributed to the impacts of Hurricane Erin and Hurricane Opal. The surge levels
associated with these storms, combined with the tidal currents in the area, are believed to
have resulted in a significant fraction of the erosion measured during this time period.
Another mechanism to be considered in determining the fate of any sand eroded from the








beach during these hurricanes is overwash. Inspection of aerial photography flown
immediately after Hurricane Opal indicates substantial overwash deposits. These deposits
may account for a substantial fraction of the eroded volume (see Section 5.6). Following the
1993-1997 time frame, however, very little volumetric loss is witnessed. This may be a
result of beach recovery in which sand that was carried into the ebb shoal system has slowly
returned to the beach.


Figure 5.10 presents the percent of sand volume remaining within the surveyed
project limits over the life of the project. As of the 1998 condition survey, the project has
retained roughly 56% of the placed volume within the project limits. The figure indicates
that, as discussed, the western half of the project has fared better than the eastern half, having
retained 62% of its placed volume versus 50% for the eastern half, again pointing up the
effect the Pass has on the adjacent shoreline. Figure 5.10 clearly indicates the impact the two
hurricanes have had on the Perdido Key beach nourishment project. The figure suggests that
Hurricane Opal was particularly damaging. The figure also shows, however, that the overall
performance of the project, measured volumetrically, has been fairly consistent, decreasing
in volume at a generally decreasing rate over time.


100


E
c
0)
C =




> 0


0 1 2 3 4 5 6
Time in years after project completion


7 8


Figure 5.10


Time history of the volumetric performance of the 1989-1990 Perdido Key
beach nourishment project.








The nearshore berm, completed in 1991, has shown very little change since
construction. Bathymetric surveys of the berm conducted in 1997 indicated the possibility
of some erosion on the seaward side of the berm, but the losses were minimal. Very little
landward migration has been measured. While the horizontal centroid of the berm has
migrated forward as much as 200 ft, this is due primarily to the loss of volume from the
seaward edge of the berm. The landward edge of the berm has migrated landward as much
as 165 ft, but some of this migration may be due simply to spreading. It is believed that the
berm does provide some degree of storm protection. It is hypothesized that during Hurricane
Opal, storm waves measured to be as large as 25 ft offshore (significant height), were
breaking on the offshore side of the berm (Browder & Dean, 1997).

Comparison of project performance to historical trends Inspection of Figures
5.8 and 5.9 reveals erosion rates much higher than those measured historically. From Figure
5.9, the measured erosion rate over the life of the project, expressed on an annual basis, is
greater than 300,000 cy/yr, or 12 cy/ft/yr. This is approximately 9.5 times greater than the
historical trend suggests. If the effects of erosion that occurred during the first year of the
project are neglected, during the time the project was equilibrating in planform and profile,
the measured rate is roughly 11 cy/ft/yr. Over certain one-year intervals during the project
life, the measured losses indicated an erosion rate of only 6.6 cy/ft/yr, but that value is 5
times higher than the 1.3 cy/ft/yr historical rate.

These substantially increased erosion rates measured during the project life are a
result of the substantial change to the system represented by the nourishment project. Other
possible contributors to the increased erosion are the direct impact of Hurricane Opal and the
possibility of an increased sink effect of the newly deepened navigation channel. This last
effect is supported by the concurrent increased erosion witnessed along the Santa Rosa
Island shoreline (see Figure 5.7). The material placed on Perdido Key was of a similar grain
size distribution as the native sediment, with only small pockets of finer material appearing
over the project limits. These possibilities and their implications play a large role in the
suggested recommendations for the management of Pensacola Pass.



5.6 Consideration of Hurricane Impacts on Shoreline Performance

As shown in the previous section, hurricanes have played a significant role in the
shoreline changes adjacent to Pensacola Pass, most notably in the 1990's. During the present
decade, at least six major named hurricanes have impacted the Pensacola Pass area (see
Chapter 2, Table 2.2). For each storm, varying degrees of storm-specific data exist. First








and foremost, UF personnel have collected extensive data relating to Hurricane Opal,
including post-hurricane recovery data, aerial photography, and meteorological data. For the
three most recent storms, Danny, Earl, and Georges, limited data are available. As they
relate to this study, the impacts of hurricanes are divided into three areas: (1) erosion from
the surveyed beach face and immediate nearshore area, (2) overwash deposits of sand in the
dunes landward of(l), and (3) increased shoaling in the Pass itself. The most germane issues
are those that impact the sediment budget of the Pass. For Pensacola Pass, only limited data
exist that describe channel shoaling (#3). Issues 1 and 2 are briefly addressed below.

Beach Erosion In most all instances, the lack of a recent pre-hurricane shoreline
survey makes exact analysis of hurricane impacts difficult. In reference to Hurricane Opal,
sparse data from earlier in 1995 exist along the Santa Rosa Island shoreline, and a post-
Hurricane Erin survey exists for the National Park shoreline on Perdido Key. Prior to these
surveys, FDEP conducted a shoreline condition survey in 1993 that measured the beach
profile to wading depth at every third R-monument in the county.

Browder and Dean (1997) measured the erosion of the Perdido Key Beach
nourishment project between 1993 and 1997 (see Figure 5.9). They estimate that roughly
1.3 mcy of sand eroded from the nourishment project limits during that time as a result of
the impacts of Hurricanes Erin and Opal in 1995 and due to average annual erosion
mechanisms. Analysis of FDEP profile data between 1993 and 1996 suggest that roughly
1 mcy of sand eroded from the Perdido Key shoreline while an additional 2 mcy eroded from
the Santa Rosa Island shoreline between Pensacola Pass and the Okaloosa County line. For
comparison, Leadon (1996) reported that nearly 8 mcy of sand eroded from the Florida
Panhandle shoreline as a result of the impacts of Hurricane Opal.

FDEP personnel and personnel from the United States Geological Survey (USGS)
reported that the passage of Hurricane Georges in September, 1998, resulted in shoreline
recession of up to 130 ft along Santa Rosa Island, and recession of up to 70 ft along Perdido
Key. The storm surge during Georges was reported to be 7.7 ft at Pensacola Beach.

Overwash Considerations The elevated surge levels of hurricanes often lead to the
deposition of significant quantities of sand in the upland areas just landward of the beachface
itself. A fraction of this deposited sediment originates from coastal dunes which are eroded
during the storm. Additional sediment may originate from elsewhere alongshore as the surge
level varies along the coast. It is noted that while in developed areas these overwash deposits
are frequently considered to be 'hurricane damage,' overwash is a natural mechanism by
which the barrier island system migrates or restores itself. Thus after a hurricane, the total


-43-








erosion may be limited by the inclusion of overwash volume, since this volume is displaced
and not necessarily lost from the system.

Limited overwash measurements were conducted as part of the monitoring of
Hurricane Opal and the recovery of the Panhandle shoreline following that storm.
Measurements of the depth of overwash deposits in the Navarre Beach area near the
Okaloosa County line indicated average depths of deposition of 16 inches. The areal extent
of overwash fans in the county was measured by Dean and Suter, 1998 to estimate the
volume of sand deposited as overwash during the storm. Along Santa Rosa Island, overwash
fans covered 53.8 million ft2. On Perdido Key, 6.9 million ft2 of fan area was measured.

Using the measured areas, estimates of the volume associated with the fans were
made. These values carry with it a great deal of uncertainty, based on the limited depth
measurements made. The first estimate is obtained by applying a consistent 16 inch depth
of overwash deposit at all areas. This results in a total deposited volume of 3 mcy, which
is actually greater than the measured erosion throughout the county. This result is possible,
considering the substantial amount of sediment placed in suspension by the hurricane waves
and the decreasing surge level across the county from east to west, however, this estimate
is likely too high.

The second estimate assumes that the depth of overwash deposit decreases linearly
across the county from east to west from the Okaloosa County line to R-34, where the last
overwash fan is seen on aerial photographs. It was assumed that any overwash fan seen on
the aerials was at least 6 inches in thickness. This assumption and the linear interpolation
result in a total overwash deposition of 2.3 mcy. The vast majority of the deposited volume
lies in the eastern portion of the county, near Navarre Beach. In this area, deposits of over
24 inches in thickness were found. In total, the estimated volume ofoverwash deposited on
Santa Rosa Island is 2.1 mcy. Perdido Key received roughly 200,000 cy of sand as overwash
deposited by Hurricane Opal.

The uncertainty associated with estimating overwash deposit thicknesses from limited
data is clearly recognized. The measured and estimated erosion/deposition volumes along
Santa Rosa Island suggest that overall the net change to the system was slightly positive
(meaning more sand was deposited in the upland than was eroded from the beach). While
this result seems counterintuitive, and may well be somewhat incorrect, it is clear that a
significant amount of sediment was deposited in the upland along Santa Rosa Island (ref.
Leadon, 1996). No data regarding overwash are available for other such storms.


-44-








6.0 PENSACOLA PASS BATHYMETRY & CHANNEL STABILITY


This chapter details the evolution and present condition of the Pensacola Pass shoal
system and adjacent submerged areas. Bathymetric data dating back to 1856 were compiled
to provide a time history of the condition of the ebb and flood shoals as well as the
navigation channel. The present condition of the navigation channel itself was discussed in
Chapter 4.


Bathymetric History Figure 6.1 depicts the bathymetric contours digitized from
the 1856 United States Coast and Geodetic Survey boat sheet #585 (see Chapter 3). The
contours are plotted relative to the National Geodetic Vertical Datum, 1929, and have been




500,000





495.000


Santa Rosa
Perdido Iddl Island
in slnede
490 000-


SEast
o0 \ Bank
Z 40Caucus V
485 000 4.r Shoal





480,000-
1,110,000 1,115,000 1,120,000 1,125,000
Easting (ft, NAD 1927) scale: = 5000 ft

Figure 6.1 Bathymetric contours of Pensacola Pass, FL, dated 1856. Elevations are
in feet relative to NGVD 1929. The white contour line denotes the
controlling depth over the outer bar, -21 ft NGVD.


-45-








corrected for sea level rise assuming a constant rate of rise of 2.5 mm/yr. Figure 6.1 clearly
shows the developed bypass bar extending from East Bank off Santa Rosa Island across to
Caucus Shoal off Perdido Key. The figure indicates that in 1856, the controlling depth over
the outer bar was approximately 21 ft relative to NGVD datum, as denoted by the white
contour line in the figure.

Figure 6.1 also shows that the shoreline along Perdido Key (then Foster's Island)
extends continuously all the way up to the present-day aircraft carrier turning basin. At the
time of the survey there was no communication between Pensacola Bay and Big Lagoon.
Coleman (1988) reports that in September, 1846, a storm closed the channel between the two
water bodies just north of Ft. McRee. Another channel was apparently opened mechanically
just south of the fort shortly thereafter, resulting in erosion that encroached upon the fort
foundation. This action prompted the construction of temporary 'jetties' (actually groins in
this application) along the Perdido Key shoreline inside the Pass, presumably in the vicinity
of the existing south jetty. Another channel was excavated north of Ft. McRee; Coleman
(1988) reports that this channel was allowed to close naturally over the next few years,
leading up to the 1856 survey shown in Figure 6.1. Later surveys of the area show a channel
between Big Lagoon and Pensacola Bay in various locations until the 1950's, at which time
the Ft. McRee Land Cut of the Gulf Intracoastal Waterway was constructed just south of
Sherman's Cove on the U.S. Navy base.

Figure 6.1 illustrates that in 1856, Middle Ground and East Bank were separated by
a deep embayment, and that Middle Ground was a much smaller feature than it is now. The
natural entrance channel shown in Figure 6.1 runs between Caucus Shoal and East Bank and
around Middle Ground before turning eastward behind Santa Rosa Island, following the
island shoreline closely, much as it does today.

Figure 6.2 depicts the condition of Pensacola Pass as surveyed for this study in
August, 1998. Comparison of the two bathymetries over the 142-yr time span show some
stark differences. The construction of the entrance channel cut through the southeastern tip
of Caucus shoal, creating a linear channel from the throat of Pensacola Pass to deep water.

Qualitatively, the effect of the construction and continued deepening/maintenance of
the entrance channel has been to linearize the ebb shoal system of the Pass. Comparison of
Figures 6.1 and 6.2 illustrates the growth of Middle Ground Shoal into a linear shoal feature
lining the east side of the channel. Similarly, Caucus Shoal has evolved from a bulbous
component of the bypass bar into a highly linear ebb shoal feature along the western margin
of the channel. These changes have effectively severed the bypass bar system.























CN
03)
- 490,000-

Z


C
' 485,000-
0
Z


475,000


1,105,000 1,110,000 1,115,000 1,120,000 1,125,000
Easting (ft, NAD 1927)
scale: 1"= 5,000 ft

Figure 6.2 Bathymetric contours of Pensacola Pass, FL, dated 1998. Elevations are
in feet relative to NGVD 1929. The authorized depth at the time of the
survey was 35 ft below MLLW datum. The controlling depth was
approximately -40 ft NGVD.



Additionally, the deep embayment witnessed between Middle Ground and East Bank
in the 1856 bathymetry has evolved into a smaller, shallower marginal tidal channel running
along the Santa Rosa Island shoreline in the lee of Middle Ground. This represents a
significant degree of impoundment of littoral material, caused by the interruption of the sand
flows across the bypass bar of the Pass.


I


I


I


I








Figures 6.1 and 6.2 also show the westward migration of Santa Rosa Island and a
narrowing of the throat width at the Pass. This narrowing appears to be accompanied by an
increase in depth of the channel at the throat, much deeper than the maintained channel
depth. The changes in the throat cross-section will be further discussed later in this chapter.

Figure 6.3 emphasizes the drastic changes in the configuration of the ebbshoal system
at Pensacola Pass since the initiation of dredging. Figure 6.3 presents a perspective view of
the entrance channel for both the 1856 and 1998 bathymetries. The controlling depth
contour in 1856, -21 ft, is shown in both figures to highlight the impact of the deep dredge
cut and the complete interruption of the bypass bar system.


Figure 6.3


Perspective comparison of the Pensacola Pass, FL, ebb shoal system
between 1856 (pre-dredge) and 1998 (present condition). The -21 ft
NGVD contour is highlighted to indicate the controlling depth in 1856.


Santa Rosa Island


1856














1998








Figure 6.4 presents a time history of the bathymetric contours of Pensacola Pass. The
plots in this figure illustrate the concepts presented above, showing the effects of the
construction of the Pass and the changes in the ebb shoal system. Also visible in Figure 6.4
is the effect of the construction of the Ft. McRee Land Cut of the GIWW. The cut was
constructed in the mid 1950's. The construction appears to have caused a recession of the
interior Pass shoreline, associated with the creation of a spoil island separating the old
channel from the GIWW. The recession witnessed in this area may also be a result of the
dredging operations carried out in the navigation channel (see Appendix A).

Hine et al. (1986) estimated the volume of sand contained in the ebb and flood shoals
of Pensacola Pass, FL. Their estimate of the volume of sand contained in the ebb shoal
system, exclusive of the navigation channel itself, was 18.03 mcy. Hine et al (1986) also
estimated the volumetric loss of the ebb shoal system between 1877 and 1984 to be -1.25
mcy (taken from the published N.O.S. nautical chart #11384). This estimate of the volume
lost lies well within the range of uncertainty in any volume change calculation in this area.
The area of the Pensacola Pass ebb shoal system, including Caucus Shoal, Middle Ground,
and East Bank is on the order of 5,300 acres (2.29x108 ft2). To provide an idea of the
sensitivity of the measurement, a 0.5 ft difference over that entire area results in a volumetric
change of 4.24 mcy, over three times the volume change reported by Hine et al. (1986) over
a 107-year period. When the uncertainty in each survey is considered, coupled with the
possible inclusion of uncertainties in both horizontal and vertical datums (sea level rise,
shifts to NAD 1927, etc) the difficulty in accurately determining the volume change of the
shoal system of Pensacola Pass becomes readily apparent. It is with this caveat that the
following volume change computations are provided.

Using the available bathymetric data, estimates of the volumetric changes in the ebb
and flood shoals were made. Attempts were made to correct the older surveys for sea level
rise, and all surveys were brought to a common horizontal datum, the North American
Datum of 1927, and vertical datum, the National Geodetic Vertical Datum of 1929. For each
available intersurvey period, the volume change was computed for both the ebb shoal and
flood shoal systems; the ebb shoal system was further separated into two portions, Caucus
Shoal west of the channel and Middle Ground and East Bank east of the channel.


-49-








500,000-



495,000-



490,000-



485,000-



480,000-


475,000
1,105,000 1,110,000 1,115,000 1,120,000 1,125,000


500 000-



495 000-



490 000-



485 000-



480 000-


475,000 1 I I 1 I
1,105,000 1,110,000 1,115,000 1,120,000 1,125,000

I500 I
500,000-1qR


495 000-



490 000-



485 000-



480 000-


I I I I


1881


scale: 1" = 10,000 ft
all plots
475,000- I I I I
1,105,000 1,110,000 1,115,000 1,120,000 1,125,000
I I I I


- 500



- 495



- 490



- 485


- 480 000-


475,000 1
1,105,000 1,110,000 1,115,000 1,120,000 1,125,000


1,105,000 1,110,000 1,115,000 1,120,000 1,125,000


1,105,000 1,110,000 1,115,000 1,120,000 1,125,000


Time history of the bathymetry in the vicinity of Pensacola Pass, FL. The
vertical and horizontal scales in each plot are Northing and Easting,
respectively, ft, NAD 1927. The elevations are in ft, NGVD 1929. The
existing 1998 navigation channel is shown in all plots for reference.


Figure 6.4








Figure 6.5 plots the cumulative changes in ebb shoal volume since 1856.
Uncertainties in the volume change calculations notwithstanding, Figure 6.5 suggests that
overall, the volume of the ebb shoal system of Pensacola Pass has been decreasing over its
monitored lifetime. This includes the period between 1856 and 1881, prior to the initiation
of dredging in 1883. In general the trend of change for Caucus shoal has been erosional,
while the trend for East Bank and Middle Ground after 1881 is accretional. The net effect
has been an overall loss of material, since Caucus Shoal is substantially larger than the other
two features. The 1935 dataset appear somewhat anomalous, showing accretion of both
portions of the ebb shoal. This discrepancy could not be resolved, the data are plotted herein
for completeness, and to provide further evidence of the uncertainty in the calculations.
Given the uncertainty in the values, a detailed discussion of volume change rates is
inappropriate. This fact will be taken into account in any sediment budget calculations.


0 30 60 90 120
years from 1856


150


Figure 6.5 Estimate of cumulative volumetric changes of the Pensacola Pass, FL,
ebb-shoal system since 1856.


-51-








Figure 6.6 illustrates the estimated cumulative volumetric changes in the flood shoal
system. The trend in volumetric change here is much more definitive, the flood shoal system
appears to have consistently loss material since 1881, with no noticeable increase in the
erosion rate over time. Two curves are presented in the figure to account for the different
areas of the flood shoal used in the computations. Inspection of Figure 6.4 indicates that
only a limited area of the flood shoal was surveyed in some years. Thus the two curves
represent the volume change of both the smaller area and a larger area which includes the
smaller area and reaches to the U.S. Navy turning basin. The two curves do not differ greatly
in magnitude, indicating that most of the change associated with the flood shoal is contained
in the area immediately north of the throat of the Pass.


5
-- flood shoal small area
E 6 -+- flood shoal large area






0 30 60 90 120 150


years from 1856

Figure 6.6 Estimate of cumulative volumetric changes of the Pensacola Pass, FL,
flood-shoal system since 1856.



Interpretation of Volumetric Changes The high degree of uncertainty in the shoal
volume change computation presents somewhat of a problem in providing information to
determine an appropriate sediment budget. However, the approach to be followed in this
report is to evaluate a range of possible values for all volume changes and transport rates.
Thus, it is desirable to identify a reasonable range of values for these shoal volume changes
to apply to the sediment budget determination.

Inspection of the bathymetric changes and computed estimates of volume changes
suggests that both the ebb shoal and flood shoal systems are eroding. This conclusion, while
very simplistic in itself, is useful to the sediment budget preparation. As stated above,
Caucus Shoal appears to be eroding, while Middle Ground and East Bank appear to be stable








to mildly accretional. This is bore out by visual inspection of the bathymetry, irrespective
of the absolute depths in the surveys. The embayment between Middle Ground and East
Bank appears to be infilling as Middle Ground extends out along the channel as a linear ebb
shoal feature, while Caucus Shoal is eroding as the sand in the shoal appears to be drawn
closer to the channel into a more linear ebb shoal feature. These observed changes,
combined with the known dredging history, will be used in the sediment budget preparation
(Chapter 8).


Channel Stability No hydraulic field studies or numerical model studies were
conducted as part of this work. The following discussion provides some general information
on the characteristics of the entrance channel at Pensacola Pass, based on historical
bathymetry, tide measurements, and published analytical techniques.


Figure 6.7 indicates two cross-sections of the channel overlaid on the 1998 surveyed
bathymetry. The time evolution of these two cross-sections is shown in Figure 6.8. The
figures indicate that the channel has narrowed considerably over the 142-yr period presented.
The migration of Santa Rosa Island westward, combined with the stabilization of the interior
shoreline of Perdido Key, appears to have resulted in the deepening of the channel.


Figure 6.7


Location of channel cross-sections investigated. 1998 bathymetry shown.


-53-


I--
) 494,000- Santa Rosa
SBI Island A
z
S 492 000-

'c Perdido
-t0
Z 490.000- --
Middle
'Gr und
"1 Cau u
488,000
1,110,000 1,112,000 1,114,000 1,116,000 1,118,000 1,120,000
Easting (ft, NAD 1927)
scale: 1" = 3,000 ft










0

-10

i -20
z
-30

I -40

w -50

-60


0

-10

> -20
z
S-30
0
U -40

w -50

-60


1,000


2,000 3,000 4,000 5,000 6,000 7,000
2,000 3,000 4,000 5,000 6,000 7,000


0 1,000 2,000 3,000 4,000 5,000 6,000
Distance from common baseline (ft)


Figure 6.8 Evolution of channel cross-sections at Pensacola Pass, FL. See Figure 6.7 for cross-section locations.


1856 throat
cross-section -


If I- -- 1856
S- ---- 1881
-, .- 1935
SECTION A-A' 1998


I I I


7,000








This deepening, from approximately 41 ft to roughly 65 ft, is accompanied by only
a slight reduction in the overall cross-sectional area of the channel. The channel throat cross-
sectional area at A-A' in 1856 is measured to be 125,600 ft2. In 1998, the throat location has
migrated along with Santa Rosa Island to section B-B', and its area is 115,500 ft2, only an
8% reduction in area. The width of the channel at the throat, however, has reduced from
roughly 5,900 ft in 1856 to 3,700 ft in 1998. The combination of the pressure to move the
channel from the migrating tip of Santa Rosa Island and the stabilizing presence of the
structures on Perdido Key has resulted in the scouring of the channel to a greater depth. The
pressure of the channel on the Perdido Key shoreline is apparent in Figure 6.7 as the contours
bend around the southern jetty.

Several investigators have studied the relationship between the cross-sectional area
of a tidal entrance and its associated tidal prism. Most notably, O'Brien (1969) and Jarrett
(1976) related the two values in the following format:

AC = cP" 6.1


where Ac is the inlet cross-sectional area, c and n are constants (c has units of length-' and
n is dimensionless), and P is the tidal prism, expressed in consistent units. Using this
relationship, O'Brien (1969) found for unstabilized inlets that c = 2x10-5 ft-1 and n = 1.0,
while Jarrett (1976) reported values ofc = 1.04x105 ft1' and n = 1.03.

The difficulty in applying Equation 6.1 to Pensacola Pass lies in determining the tidal
prism, P. No tidal study was conducted for this report, and no published information on the
value of the tidal prism was found in the literature. As an estimate, the area of the bay was
computed and multiplied by the tidal range. The area of the bay was found to be
approximately 4x109 ft2 (92,000 acres). For simplicity a tide range of 1.0 ft was assumed.
Using these values, a cross-sectional area of 90,000 ft2 was computed (the two approaches
yielded answers that differed by less than 1.3%). This is only slightly smaller than the
measured throat area of 115,500 ft2 presented above. One consideration must be noted,
however, that the tidal prism used is most likely an overestimate of the true tidal prism, since
the area used encompasses all of Pensacola and Escambia Bays.

The simple analysis presented suggests that the channel throat area is larger than
needed for the tidal flows in the area. This suggests the fairly obvious result that the
measured 1998 channel is expected to continue to shoal (i.e. the tidal currents are not
sufficient to maintain the throat area at its present dimensions).


-55-








7.0 LONGSHORE SEDIMENT TRANSPORT


Chapter 7 presents a review of published estimates of the littoral transport rate in the
vicinity of Pensacola Pass, FL, and several analyses performed as part of this study to
determine the patterns and magnitudes of littoral drift in the area. These analyses include an
inspection of the littoral drift information inferred from shoreline and bathymetric changes
and a wave refraction analysis performed on the existing bathymetry.

Two important aspects of the present discussion are the emphasis on gross
components of transport and their role in the sediment budget of the Pass, and the importance
of applying a range of possible values to all transport components. As has been discussed
in previous sections, many calculations of volumetric change carry a substantial degree of
uncertainty. This uncertainty requires that a more flexible approach be taken to analyzing
sediment transport rates in the area. These topics are frequently not addressed in the
published literature, but will be shown to play an important role in the sediment transport
pathways around the Pass.



7.1 Published Estimates of the Net Longshore Sediment Transport Rate

Several authors have presented estimates of the net longshore sediment transport rate
in the vicinity of Pensacola Pass, FL. Johnson (1956) presented a compilation of measured
rates of net longshore transport for various points in the United States. Johnson references
a 1954 U.S. Army Corps of Engineers Report from the Mobile, AL, district in which the net
transport at Perdido Pass, AL, was estimated to be 200,000 cy/yr, westerly directed. This
rate was determined using the deposition rate of sand behind shoreline structures, and may
be closer to the value of the gross transport rate in the area (Balsillie, 1975, reference to
unpublished CERC report). Gorsline (1966) compiled one year of monthly observations
from single beach profile stations at fifteen locations across the Florida Panhandle and Big
Bend. Using the wave data and profile changes measured, he estimated the net transport at
Pensacola Beach and Gulf Beach to be westerly directed at 78,000 cy/yr and 98,000 cy/yr,
respectively, with a gross transport estimate of approximately 196,000 cy/yr.

Balsillie (1975) reports results from the U.S. Army Coastal Engineering Research
Center (CERC) Littoral Environment Observation program in which coastal observations of
wave height, period, and direction were compiled between 1969 and 1970, and applied using
the CERC formula for longshore transport (USACE, 1984). For the Fort Pickens State Park








area east of the Pass, Balsillie reports a net westerly directed transport rate between 239,000
cy/yr and 334,000 cy/yr, depending on the value of the proportionality constant used in the
equation. The gross transport rate was estimated to be between 309,000 and 432,000 cy/yr.
Walton (1976) compiled shipboard wave and wind observations to estimate gross and net
transport components for various points around the state of Florida. For the Pensacola Pass
area, Walton reported a net transport of 280,000 cy/yr westerly directed, with a gross
transport rate of 530,000 cy/yr.

Stone et al. (1992) provide a thorough synopsis of previously published transport
estimates and introduce an independent estimate of transport along the western Florida
Panhandle shoreline. Based on two separate wave data sets, Stone et al. computed the
breaking wave field along the Florida Panhandle, having traced the individual waves to shore
over the measured bathymetry in the area. The longshore transport is then computed in a
manner similar to the CERC formula. Using this method, Stone et al. (1992) compute a net
westerly directed transport for the Fort Pickens/Pensacola Pass area of between 163,000
cy/yr and 196,000 cy/yr. On the western side of the Pass at Gulf Beach, their method
projects a westerly transport of between 33,000 cy/yr and 65,000 cy/yr. Stone et al. also
report a reversal in the direction of net transport along the eastern Perdido Key shoreline, an
indication of the sink effect of the Pass.

Stone et al. (1992) also present a hypothesis for a non-integrated, cellular longshore
drift system in the Panhandle area. The system is broken into cells by various littoral
barriers, one of which is Pensacola Pass. Perdido Key is broken up into two cells, the eastern
cell experiencing easterly net transport due to the Pass, and the western cell is thought to
receive sediment from the inner shelf area just offshore. The division point between the cells
is presented as somewhere to the east of the developed Gulf Beach area of Perdido Key.

Data collected as a part of this study from the Mobile, AL, district of the U.S. Army
Corps of Engineers indicates that for Perdido Pass, AL, the annual dredging rate is on the
order of 100,000 to 150,000 cy/yr. This could potentially provide an upper limit estimate
of the gross transport rate in the Perdido Pass area.

Synopsis The published literature regarding the net and gross transport rates in the
vicinity of Pensacola Pass exhibit a substantial range in values. For example, estimates of
the net transport in the area range from 33,000 cy/yr to 334,000 cy/yr. The range in rates is
partially explained by the method of determination of each estimate. Rates that consider
only the incident wave climate may not consider the availability of an adequate sediment


-57-








supply to the area (or an additional supply of sediment from a non-longshore wave driven
source). Conversely, rates that are derived from measured changes in beach and shoal
volume may include gross components of transport in the net rate, and/or may not consider
the uncertainty in historic survey data. The published literature do agree that the general
direction of net transport is westerly. As with the measured bathymetric changes, this
conclusion in and of itself is useful in applying sediment budget techniques, and the
published upper bound on the net transport value also provides a useful limit.



7.2 Littoral Drift Description from WIS Data

Estimates of the net and gross rates of longshore transport can be determined from
the Wave Information Study (WIS, Tracy et al. 1996) time series of wave and wind data for
the Gulf of Mexico (see also Chapter 2). Using the Shoreline Modeling System (Gravens
et al., 1991), the time series of wave data is used to compute the average breaking wave
conditions and thus the resultant longshore transport along either an idealized shoreline,
using the SMS SEDTRAN utility, or along a detailed shoreline, using the Generalized Model
for Simulating Shoreline Change (GENESIS, Hanson, 1987). Herein, the GENESIS model
was not applied. SEDTRAN was used to provide an estimate of the ambient longshore
transport components based on offshore data and the CERC formula (USACE, 1984).

As presented in Chapter 2, the nearest WIS station to Pensacola Pass is Station 44 for
the 1976-1995 data set, which includes the effects of tropical storms and hurricanes. This
data set was employed with the SEDTRAN utility to evaluate the average annual transport
components and the yearly variability in the dataset. Table 7.1 presents the results of 20
individual 1-year runs of the SEDTRAN utility. The results of the analysis indicate a large
range of annual net transport values over the 20 year period, from as low as 50,000 cy/yr in
1989 to as high as 460,000 cy/yr in 1983. The average annual net transport predicted by
SEDTRAN is 244,000 cy/yr, westerly directed. This estimate assumes that the shoreline is
straight and oriented with a shore-normal bearing of 168 degrees.

Wang et al., 1998, present data from streamer traps collected around the Southeast
United States. These data suggest that for low energy coastlines like those in the Florida
Panhandle, the sediment transport coefficient used in the CERC formula (USACE, 1984)
should be reduced, perhaps by as much as a factor of 10. This suggests the net transport
could be as low as 25,000 cy/yr. These streamer trap data, however, may not represent the
total transport, and hence the estimates of Wang et al. may be too low.









Table 7.1 Annual longshore transport rates computed from the SEDTRAN utility using
WIS Station 44 data in 16.4 ft water depth from 1976 to 1995.


Year Qes Q Qwr Qet Qgrss (QeJQwet)
1976 -33,000 180,000 150,000 210,000 0.18
1977 -56,000 350,000 300,000 410,000 0.16
1978 -44,000 160,000 120,000 200,000 0.28
1979 -54,000 380,000 330,000 430,000 0.14
1980 -44,000 270,000 220,000 310,000 0.16
1981 -56,000 210,000 160,000 270,000 0.27
1982 -22,000 370,000 350,000 390,000 0.06
1983 -46,000 510,000 460,000 560,000 0.09
1984 -31,000 250,000 220,000 280,000 0.12
1985 -42,000 450,000 410,000 490,000 0.09
1986 -33,000 200,000 170,000 240,000 0.17
1987 -40,000 310,000 270,000 350,000 0.13
1988 -43,000 360,000 310,000 400,000 0.12
1989 -66,000 120,000 50,000 180,000 0.55
1990 -23,000 240,000 220,000 260,000 0.10
1991 -48,000 280,000 240,000 330,000 0.17
1992 -51,000 210,000 160,000 260,000 0.24
1993 -38,000 270,000 230,000 310,000 0.14
1994 -72,000 220,000 140,000 290,000 0.33
1995 -70,000 400,000 330,000 470,000 0.18

averages -46,000 290,000 244,000 336,000 0.18
notes: 1) positive transport is defined as being to the right (westerly) when facing offshore
2) SEDTRAN reports values to one decimal place in scientific notation.


Also determined from the SEDTRAN analysis was the ratio of westerly directed to
easterly directed transport. As shown in Table 7.1, the majority of the transport is westerly
directed, totaling 80% of the gross transport rate. This analysis is for a generic, straight
shoreline and does not consider the irregular bathymetry of the ebb shoal system of the Pass.
The ratio of transport components will be used in the sediment budget model in Chapter 8.


-59-








7.3 Littoral Drift Description from Wave Refraction Analysis


The grid-based Regional Coastal Processes Wave Propagation Model (RCPWAVE,
Ebersole et al., 1986) was used in conjunction with the 1976-1995 WIS dataset described
previously to study the wave refraction patterns in the vicinity of the Pass and to estimate the
average annual breaking wave conditions alongshore. From the average annual wave
conditions, an estimate of the average annual transport components and their variation
alongshore was developed.

Table 7.2 presents the range of wave conditions studied. In total, 9 of 11 conditions
were run to establish the average annual conditions'. These 11 cases were determined by
sorting the WIS Station 44 dataset into bins according to wave period and angle of incidence.
Since the WIS station in this area is hindcast for 16.4 ft water depth, each condition was
transferred to deeper water, 60 ft, via linear wave theory to input into the offshore row of the
refraction model. As discussed in Chapter 2, the dataset shows a dominance of waves
incident from the east, with the most frequently occurring case being 1.8 ft waves incident
from the east, roughly 14 degrees east of shore-normal in deep water.

Figure 7.1 illustrates the center bathymetric grid used in the refraction analysis. Each
cross represents a corer of a grid cell where the water depth is reported. Grid cells were
established at 800 ft lengths alongshore and 400 ft length in the cross-shore direction. This
resulted in a bathymetric grid of 75 cells by 100 cells, the limit of the RCPWAVE model.
Three overlapping grids were employed to cover the shoreline roughly 60,000 ft to either
side of the Pass. Each condition in Table 7.2 was input to the model, and the incident wave
field was computed. Figure 7.2 presents an example of the wave field for case #5, the most
frequently occurring case. For clarity only 15% of the data points are shown. The computed
height and angle are represented by the orientation and relative size of the arrows. Plots of
all cases investigated are contained in Appendix D.









1
The extreme wave angle cases were not run on the refraction grid; the
results from these cases are unreliable. The percent occurrences for
these cases were included in the next adjacent angle bin.












Table 7.2 Input wave conditions for the RCPWAVE refraction model
for Pensacola Pass, FL.


Case # H, Incident Angle (degrees Wave Percent
(60 ft depth, ft) wrt shore normal) Period (s) Occurrence

1 4.2 87.4 3.0 9.0

2 1.7 57.8 3.0 11.8

3 1.7 33.8 3.0 19.5

4 6.5 48.0 6.0 0.8

5 1.8 13.5 4.0 24.3

6 5.1 13.0 6.0 4.1

7 1.7 -10.2 3.0 12.4

8 4.7 -10.1 6.0 2.5

9 1.4 -33.8 3.0 8.0

10 1.4 -56.8 3.0 4.3

11 2.5 -84.2 3.0 2.6


Figure 7.2 illustrates the refractive effect of the ebb shoal of Pensacola Pass and the
effect of the increasing shoreline curvature of Santa Rosa Island approaching the Pass. The
shallow depths of the shoal tend to bend incident waves toward the mouth of the Pass,
focusing energy there. Along the Perdido Key shoreline, the refractive effect of Caucus
Shoal is such that the breaking wave angles reverse from easterly incident (positive) to
westerly incident (negative) in the lee of the shoal. The increasing curvature of the western
tip of Santa Rosa Island increases the breaking wave angle approaching the Pass. Both of
these effects have a strong effect on the sediment transport pathways around the Pass.


-61-



















-~~'"'~it"~~' i --lg ~*~~iU~


.-.i .: ..







+ -+ITCLI



+ +-


Pensacola
Pass
I I


... . .. ....
S:::: 1:: : :






f|j1. || |


lr+ +
+ +:ti


+ +

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


I I I I


* -I-- I


scale: 1" = 10,000 ft


Figure 7.1 Center bathymetric grid used in RCPWAVE analysis. Overlapping grids of identical size were employed to extend the
grid approximately 25,000 ft to the east and west of the grid shown. Each grid consists of grid cells 400 ft by 800 ft
(cross-shore and longshore, respectively). Contours are in feet and represent depths relative to NGVD 1929.


--7-. 7. -. c7-














Case 05
Ht = 1.79 ft
T = 4.0 s
Angle = +13.5 degrees


Pensacola
Pass


Wave height scale:

1.0ft

S4.0ft


scale: 1" = 10,000 ft


Figure 7.2 Example of wave field computed by RCPWAVE analysis. This case represents the most frequently occurring
condition according to WIS Station 44 data (1976-1995).











Breaking wave conditions were computed from each of the refraction plots and used
to determine the potential average annual transport components alongshore, assuming the
percent occurrences of each case as shown in Table 7.2. The transport components were
determined from the CERC formula (USACE, 1984):

Q = 7,500 (0.0884)pg3/2H5/2sin(2ab) 7.1


The value of Q in Equation 7.1 has units of cy/yr, where p is the density of seawater, g is
gravitational acceleration, Hsb is the significant breaking wave height, and ab is the breaking
wave angle with respect to the local shoreline. This equation assumes a sediment transport
coefficient, K, of 0.39.

The use of Equation 7.1 has been evaluated by a number of investigators (e.g. Bodge
and Kraus, 1991, Wang et al., 1998). These investigators generally consider the results
provided by Equation (1) to be unrealistically high, particularly for low to moderate energy
coasts. Therefore, in this analysis the approach will be to investigate the differences in the
magnitude of transport alongshore and the alongshore gradients in transport. Thus the values
computed from Equation 7.1 are non-dimensionalized by the peak value of net transport
computed along the shoreline.

Figure 7.3 presents the annualized longshore transport potential as determined from
the refraction analysis. Curves are presented to illustrate the alongshore variation in the
westerly-directed, easterly-directed, and net transport potential based on the WIS data set.
In this manner, areas of high erosion or deposition are indicated by large changes in transport
potential, shown in the plot as the areas along the individual curves where the slope is
greatest (either negative or positive).

An additional curve is plotted in Figure 7.3 to indicate the net transport gradients that
would result from a uniform wave field. As shown in Chapter 2, the WIS data set is skewed
somewhat to easterly incident waves (westerly transport). The uniform wave field curve is
shown to illustrate the effect of changing the average annual wave climate to a more westerly
orientation, which is certainly a possible occurrence. Three equally weighted wave
conditions were applied to the refraction grid (2.5 ft waves of 6 second period at angles of
+30, 0 ,and -30 degrees respectively).



















1.5

S- westerly transport (positive Q)


C-
4 1.0


CC 0.5



0.0

) -0.5
S- (easterly transport (negative Q) -)
-1.0

-60,000 -40,000 -20,000 0 20,000 40,000 60,000
Distance East of Pensacola Pass (ft)
Qnet WVS
-- -- Qwest WIS
-----Qeast- WIS
Qnet Uniform Wave field


Figure 7.3 Normalized annual longshore transport potential computed from wave refraction analysis. Presented
in the figure are the net, easterly, and westerly transport components computed from WIS data along
with the net transport curve resulting from the application of a uniform wave field. Positive values
denote westerly transport, and vice-versa.








It is important to recognize that this analysis and its interpretation are based solely
on the incident wave climate, and that the results DO NOT include the effects of currents,
either tidal or otherwise. In addition, the transport potential assumes that a sufficient amount
of sand exists to be transported. Given the lack of hard structures or natural rock exposure
in the area, this is a reasonable assumption. Given these caveats, the results shown in Figure
7.3 are interpreted as follows:

1) Along Santa Rosa Island (SRI), from Pensacola Beach into the Gulf Islands
National Seashore near R-100, the transport is uniformly increasing and is
dominated by westerly directed transport. The easterly component of gross
transport diminishes to a negligible amount approaching Pensacola Pass.
This is related to the shoreline curvature of SRI, which promotes westerly
transport for most wave events. This suggests that this reach would be
uniformly erosional.

2) Between R-100 and R-85, the refraction model suggests that the transport
potential is fairly uniform and again dominated by westerly transport,
indicating that this area would typically be less erosional or more stable than
the adjacent areas.

3) From R-85 westward toward the Pass to R-70, a stretch of approximately 2
miles, the westerly transport increases noticeably, suggesting increasing
erosion rates along this reach. It is interesting to note that the ebb-shoal
system of the Pass ties into the Santa Rosa Island shoreline in the vicinity of
R-85 (ref. Chapter 6).

4) The remainder of the SRI shoreline inside the Pass appears to be depositional
based on wave refraction analysis. This is to be expected given the degree
of sheltering provided by Middle Ground and East Bank. Again, however,
it must be stressed that this analysis does not consider the effects of tidal
currents, which obviously impact this shoreline.

5) Figure 7.3 suggests that in the case of a uniform wave field, the net transport
gradients behave similarly to the WIS data set, but are more pronounced.
The uniform wave field analysis suggests that the SRI shoreline up to R-70
would experience greater erosion rates than predicted by the WIS data set
(and presumably any model using that data, such as GENESIS).








6) Overall, along the Santa Rosa Island shoreline, the vast majority of sand
entering the system here will be transported westward toward the Pass. This
is consistent with the large accretion witnessed at the very tip of SRI, and the
shoaling records of the navigation channel provided by USACE.

7) Inside Pensacola Pass along the Perdido Key shoreline, refraction results
were not computed. The shoreline position is fixed by the southernmost
jetty, and the shoreline overall is thought to be depositional, based on profile
surveys. This depositional behavior is limited by the presence of the south
jetty and the navigation channel, both of which act to hold this shoreline in
a more or less fixed position.

8) The first 30,000 ft of shoreline west of the Pass will be discussed in terms of
a comparison between the WIS data and the uniform wave field case.
Immediately in the lee of Caucus Shoal, both curves indicate a decrease in
transport potential, reflecting the sheltering effect of the shoal. From
approximately R-61 westward to the GINS park boundary at R-32, the WIS
data set predicts a gradually increasing westerly directed transport, suggesting
that this area would be uniformly erosional, and that the eroded material
would be directed westward.

The uniform wave field case, however, contains a significantly higher
contribution of easterly directed transport events and predicts that a reversal
in the net transport direction occurs along the Perdido Key shoreline
somewhere within the Park limits, roughly R-40 in Figure 7.3. This reversal
has been discussed in previous published literature (Stone et al., 1992) and
is a common occurrence at many tidal inlets. In this case, the reversal occurs
approximately 28,000 feet from the Pass, and the easterly directed net
transport increases noticeably to the east toward the Pass.

Both of these cases represent an "average" annual condition, and it is
reasonable to expect in any given year that the wave climate would fall
anywhere between these two cases or even outside the range of these two
conditions. Inspection of historic volumetric changes and individual
refraction plots strongly suggests that there is a reversal point somewhere
along this shoreline, probably farther east than shown in Figure 7.3.
Referring back to the monitoring results from the Perdido Key nourishment
project, the reversal point may occur closer to R-56 or R-58. There is no








reason to discuss this reversal point in terms of a fixed point alongshore, as
it surely shifts alongshore from season to season and year to year. However,
it is appropriate to recognize that there most likely is a reversal point
somewhere within the Park limits on the eastern end of Perdido Key. This
reversal point (or area) is subject to increased erosion rates, since sand is
transported from this area in both directions.

9) From the Park boundary at R-32 westward toward the FL/AL state line, the
net and westerly directed transport components diminish, suggesting an
accretional or stable shoreline. A slight increase in the easterly directed,
seasonal component is noticed. The uniform wave field case behaves
similarly, with a slightly higher seasonal behavior.

This behavior along the westerly segment of Perdido Key provides an
alternative explanation for the stability of the shoreline in this area. Stone et
al., 1992, hypothesized that this area is supplied with sediment from the inner
shelf region via cross-shore transport, and support their argument with the
presence of beach ridges in the upland area of Perdido Key (Figure 5.3). The
refraction analysis suggests that this area would be depositional due to a
decreasing longshore transport potential.

Figure 7.4 presents a comparison of the WIS net annual transport curve shown in
Figure 7.3 to the measured long-term volumetric changes shown in Figure 5.2 (converted
from MHW changes). The resulting curve from the refraction analysis shows quite a bit of
variation, but in general tracks well with the measured changes, particularly along the
Perdido Key shoreline, where both data sets indicate the change from erosion to stability and
accretion near the GINS Park boundary.

To compare the two data sets, the WIS transport curve was adjusted via the sediment
transport coefficient K, to provide reasonable agreement with the measured changes (K is
embedded in Equation 7.1). To achieve this agreement, it was found that a much lower
transport coefficient, K = 0.05, was necessary, as compared to the published value ofK =
0.39 from the Shore Protection Manual (USACE, 1984). This choice of K results in net
transport values at the boundaries of the refraction grid (nearly twelve miles from the Pass)
of roughly 55,000 to 70,000 cy/yr.


-68-


























IT -



Qnet WIS
1856-1978 measured change


-60,000


-40,000


-20,000 0 20,000
Distance East of Pensacola Pass (ft)


40,000


60,000


Figure 7.4 Comparison of alongshore annual volumetric changes predicted by WIS data through refraction
analysis to measured historic volumetric changes between 1856 and 1978 (Dean and Cheng, 1998).
The WIS curve is consistent with a transport coefficient of 0.05, compared to the value published in
the Shore Protection Manual of 0.39.








Close to the Pass, Figure 7.4 may provide some insight into the effect of the tidal
currents into and out of the Pass. It is hypothesized that tidal currents flowing into the Pass
in marginal flood channels serve to increase the erosion potential alongshore close to the
Pass. Along the Perdido Key shoreline this appears to be true, and the erosive effects of the
currents may play their strongest role within roughly the first mile west of the Pass. Along
the Santa Rosa Island shoreline, however, this hypothesis may not hold true. The measured
long-term changes suggest accretion, whereas the refraction analysis suggests erosion along
much of the first 2.5 miles east of the Pass. It is unclear why this might be, although the
marginal flood channels on the east side may be less channelized than on the west side.



7.4 Interpretations of Littoral Drift Estimates

The literature review and analytical results presented in this chapter present a wide
range of estimates for the components of littoral transport in the vicinity of Pensacola Pass,
FL. This range represents both the variability in the annual wave climate and the different
techniques used in estimating the transport values, along with the uncertainty in each
technique.

While it is important to recognize that there is a range of values that describe the
longshore sediment transport values in the area, it is desirable to obtain a limited range that
describes the average annual conditions in order to develop reasonably accurate sediment
budgets upon which to base engineering decisions about the Pass.

The values of net transport discussed in this chapter range from 25,000 cy/yr to over
300,000 cy/yr. Inspection of shoaling rates at adjacent tidal entrances suggests that the actual
value may be closer to the lower limit rather than the upper. At Perdido Pass, the annual
dredging rate at this relatively shallow but regularly dredged entrance is between 100,000
and 150,000 cy/yr. The dredging rate may be thought of as an indicator of the shoaling rate
of the entrance from both sides of the entrance, i.e., it may be an indicator of the gross
transport rate. At Pensacola Pass, the shoaling rate (or the trapping rate) would be expected
to be higher than at Perdido Pass, but that effect would be due to the hydraulics of the
entrance, and would not accurately describe the ambient transport rates in the area.

Recent literature also suggests that the transport rate may in fact be much lower than
previously thought, with an upper bound of perhaps 200,000 cy/yr along the Santa Rosa
Island shoreline. The refraction analysis presented herein indicates lower values of net
longshore transport, in the range of 55,000 to 70,000 cy/yr


-70-








In short, based only on the data presented herein and the published literature, it is
difficult to determine a single reasonable value of sediment transport. The following chapter
will develop sediment budgets for various time periods for Pensacola Pass. The approach
used will indicate the reasonable range of net transport values, based on the measured
volumetric changes around the inlet. The values from the sediment budget analysis will then
be compared to the estimates presented in this chapter for comparison.








8.0 SEDIMENT BUDGET

This chapter presents a detailed analysis of the sediment transport pathways in the
vicinity of Pensacola Pass, FL, and a sediment budget for the Pass that provides a means of
evaluating the engineering alternatives discussed in the following chapter. The sediment
budget is developed using the approach outlined in the Coastal Engineering Manual (CEM),
presently being prepared by the U.S. Army Corps of Engineers. The sediment budget
method was written for the USACE by Bodge, 1998.

An important feature of the sediment budget presented in this chapter is the concept
of a family of solutions for the budget. This implies that for each component of the budget
(i.e. net transport, gross components, etc.) there is a range of possible values, and that the
combination of all the possible values comprises a family of solutions. As stated in previous
chapters, this approach is considered to be particularly appropriate in light of the uncertainty
associated with many of the measured volumetric changes in the vicinity of the Pass.



8.1 CEM Sediment Budget Method

Bodge (1998) presents a method of accounting for the sediment transport pathways
and resulting shoreline changes in the vicinity of a tidal inlet, based on measured volumetric
changes in the area, limited wave climate information, and limited certainty of the
relationship between wave characteristics and sediment transport. The method describes
each transport component as a fraction of the two gross components of littoral drift, the left-
and right-directed gross components. In applying this analysis to Pensacola Pass, a notation
consistent with the CEM was adopted in which the westerly directed transport is defined as
positive, right-directed transport (as defined by an observer facing offshore), and vice-versa.

Figure 8.1 presents a schematic of the possible sediment transport pathways used in
the method; the pathways have been adapted from Bodge's simplified inlet to those
applicable to Pensacola Pass. The upper plot presents the possible natural sediment
pathways, while the lower plot defines the possible mechanical sediment transfers. The
variables shown in Figure 8.1 are defined as follows:

L = left (east) directed transport outside the limit of influence of the Pass.

R = right (west) directed transport outside the limit of influence of the
Pass.









Gulf Of Mexico


Pensacola Bay S. ( a


Gulf of Mexico


ngnt
shoreline


left I
shoreline


( Pensacola Bay


* ; ,. (b )


I

Figure 8.1 Definition sketch for possible sediment flows in the vicinity of Pensacola
Pass, FL. Plot (a) illustrates the possible natural sediment flows. Plot (b)
depicts the possible mechanical sand transfers at the Pass. Not all of the
shown possible pathways apply to Pensacola Pass. Figure adapted from
Bodge (1998).


-73-


00000








pl, P2 = fraction of the left and right directed transport, respectively, that
naturally bypasses the Pass.
mi, m2 = fraction of the right and left directed transport, respectively, that is
eroded from the left and right shorelines, respectively, into the Pass.
This may be considered to be the amount that the Pass increases the
littoral drift rate directed toward the Pass.
ji, j2 = fraction of the right and left directed transport, respectively, that is
impounded along the left and right shorelines, respectively.
S, = quantity of sediment added to the inlet shoal system from upland
sources, such as riverine input.
D, DR = mechanical transfer of sediment from the inlet shoal system to the left
and right shorelines, respectively (defined as positive quantities).
DB = mechanical bypassing of sediment (defined as positive from the left
to the right shoreline).
Do = mechanical transfer of sediment from the inlet shoal system out of the
littoral system (e.g. offshore disposal of dredged material).

Another important quantity used in the CEM method is the ratio of updrift to
downdrift transport at the boundaries of the study area. This ratio, r, is the ratio of the gross
components of transport sufficiently far away from the effects of the Pass. In this fashion,
the left and right directed components of transport can be expressed as functions of the net
transport, Q, and r (assuming that right directed transport is positive and downdrift):


R -Q L- rQ 8.1
1-r 1-r


Other quantities that are either defined at the onset or determined from the analysis
include:

A VG = gross volume of sediment shoaling the Pass.
A VN = net volume of sediment shoaling the Pass (i.e. the net change in the
floodshoal/ebbshoal/throat system).
AVL = net volume change along the left shoreline.
A VR = net volume change along the right shoreline.

These quantities can be defined as functions of the net transport Q, the ratio r, and the
fractions defined previously.








The philosophy of the CEM method is to investigate a range of possible values for
Q for all possible combinations of p,j, and m, given some measured volumetric changes,
such as A VL, A VN, and Do, for example. Doing so allows for determination of all the
pathways shown in Figure 8.1(a) under a range of conditions, and creates a family of possible
solutions.

Briefly, the method is applied by balancing combinations of measured volumetric
changes with the chosen, m, andp parameters to produce either the downdrift volumetric
response or the net change in the shoal system of the Pass. Using the model to determine the
downdrift volumetric response is one means of estimating the extent of downdrift impact of
the Pass, but it also assumes that the gross and net changes in the shoal system are fairly well
established. Using the method in the other direction, assuming the downdrift volumetric
change and determining the shoal system volumetric change, assumes then that the length
of downdrift shoreline impacted by the Pass is known in advance; which is usually not the
case.

It is noted that the overall balance of volumetric changes can be determined without
knowledge of the transport coefficientsj, m, andp:

AVL = -(AVR + Do + (AVN S,)) 8.2



AVR = -(AVL + Do + (AVN S,)) 8.3


As Bodge states:
"In this way, the global volumetric impact of the inlet to the downdrift shoreline can be
computed without reference to, or assumption of, measured downdrift shoreline changes,
ambient longshore transport rates, or detailed mechanics of the inlet's transport pathways.
While data regarding these latter phenomena are ultimately useful to the development of an
inlet sand management plan and a detailed sediment budget, Eqs. (8.2 and 8.3) show that
such data are not fundamentally required in order to assess the inlet's volumetric impact to the
adjacent shorelines."

While this is true, Equations 8.2 and 8.3 cannot provide an estimate of the net and
gross littoral drift components, nor do they address the individual transport components at
the inlet or the amount of bypassing around the inlet.








To produce the family of solutions, a range of discrete values for the net annual
transport, Q, is chosen. Using the ratio r, the bypassing coefficients p,2 are determined from
the following equations, as presented by Bodge (1998) for a system where the right shoreline
(Perdido Key) is downdrift:

p, = 1 + r(m2,-j) - (AV, + AVN + D + DR + Do -S.) 8.4




1 1-r
P2 = 1 + -[mI ji + -- (AV+DB-D,) 8.5
r Q


where P1,2 are evaluated for a wide range of ml,2 andj,,. With all values either chosen or
calculated, the amount of sediment entering the shoal system from the left and right can then
be determined. Referring to Figure 8.1 these values are:

S, = (m, + 1 jl p) 8.6
1-r



SR = (2 + 1 2 -P2) r 8.7
1-r


The net amount of sediment bypassing the Pass is found from:

P (P rp2) 8.8
1-r


Equations 8.4 through 8.8 assume that the right shoreline is the downdrift shoreline
and hence the downdrift volumetric change is determined from Equation 8.3. With
reasonable limits applied to the range of Q, m, andj, the range of possible solutions that can
exist is calculated. This range is frequently large and requires further investigation.

The family of possible solutions can be narrowed into a more useful range of possible
answers using certain reasonable assumptions. Some of these assumptions can be as simple
as requiring that Q be positive (i.e. westerly directed in this case). Other assumptions may
be more detailed, such as placing limits on the net bypassing rate or the amount of sediment
expected to shoal the inlet from the left or right shoreline. These assumptions should be








based on measured volumetric changes where possible and always with sound engineering
judgement. Following the assumptions, a much smaller, 'practical' family of solutions is
obtained upon which engineering decisions may be based.



8.2 Application of CEM Method to Pensacola Pass, FL

To apply the CEM sediment budget method to Pensacola Pass, it is desirable to
establish the known quantities in the area. In reality, however, none of the quantities are
truly 'known,' rather, reasonable ranges may be applied to each variable. As is the case at
nearly any site of coastal engineering interest, some quantities are more sound than others.

To begin, it is desirable to choose a time period for analysis during which mechanical
operations were either constant or minimal, and during which volumetric changes are as well
documented as possible. This allows for the determination of what might be considered
baseline or background conditions. In other words, use of a stable baseline time period
allows for estimation of appropriate values of Q,j, m, and p to use for other time periods.

Inspection of the time history of Pensacola Pass and the available data over the past
142+ years indicates that the period between 1974 and 1984 provides the best time frame to
meet these requirements. During this time, the dredging rate appears to be fairly constant
at 335,000 cy/yr (Figure 4.1, all of which was disposed of offshore), the shoreline changes
are well established via complete beach profile surveys, and no beach nourishment or sand
bypassing projects were performed. Two potential drawbacks to this choice of time period
are the lack of definitive bathymetric survey data and the impacts of Hurricane Frederic in
1979. Given the sparseness of bathymetric survey data, this limitation must be accepted.
The hurricane's impact on the beach is accounted for, but any overwash volume is not.

Chapter 7 presents information regarding the net littoral drift in the vicinity of
Pensacola Pass, FL. While it is difficult to determine a small, practical range of possible net
transport values, the available literature and the analysis in Chapter 7 suggest that a
reasonable, large range of Q to investigate might be 0 to 250,000 cy/yr, all westerly
directed. Additionally, the results of Chapter 7 suggest that the ratio of left- to right-directed
transport is approximately 0.2. This value may be low, as indicated by the uniform wave
field case presented in Chapter 7. Hence values ofr = 0.2 and r = 0.5 will be investigated.

Chapter 5 presented a detailed volume change analysis of the 1974-1984 time period.
The analysis shows that Santa Rosa Island experienced a net erosion of 54,000 cy/yr during


-77-








this time. Inspection of the sediment transport potential in Chapter 7 (Figure 7.3) indicates
that it is likely that nearly all the eroded sediment along this reach was transported to the
west toward the Pass. Hence this value is applied in the method with a reasonable level of
confidence. On the Perdido Key shoreline, during this time period roughly 46,000 cy/yr of
erosion was measured. The amount of erosion that is attributable to the Pass is not so clear
here. The potential for sediment supply from an offshore source may mask a higher erosion
rate caused by the Pass, or, not all the erosion measured should be attributed to the Pass.
Given this dilemma, the downdrift erosion rate will be investigated in tandem with the shoal
system volumetric change.

Chapter 6 presents the available bathymetric change data in the vicinity of the Pass.
These data carry the largest degree of uncertainty, and must be treated accordingly.
Inspection of Figures 6.5 and 6.6 suggests that for the 1974 to 1984 time period, the
measured change in the shoal system could be as much as 340,000 cy/yr of erosion. This
change reflects the change over the entire shoal area, and is exclusive of the maintained
navigation channel itself (which, due to the regular dredging schedule, would only show a
change of between -33,500 cy/yr and +33,500 cy/yr for this time period).

Equation 8.3 can be used to investigate the relationship between the measured
downdrift volume change and the net change in the shoal system, since it is these two
quantities that carry the bulk of the uncertainty. If it is assumed that the amount of measured
erosion, 46,000 cy/yr, is the impact due only to the Pass, the resultant shoal volume change
is 235,000 cy/yr of erosion. Herein, it will be assumed initially that the net change in shoal
volume is -235,000 cy/yr, coupled with the downdrift erosion of -46,000 cy/yr.

A reasonable argument against using the above combination of values might be to
investigate the possibility that the impact to the Perdido Key shoreline is the measured
erosion, -46,000 cy/yr, plus or minus 50% of that value, where the plus infers that not all the
measured erosion is attributable to the Pass (in this case 23,000 cy/yr of erosion would be
the extreme limit), and the minus suggests that the actual erosion rate due to the Pass is
masked by an offshore supply of sediment to the shoreline (i.e. the Pass causes 69,000 cy/yr
of erosion along Perdido Key, masked by a 23,000 cy/yr accretion from offshore, also an
extreme limit). This would result in a combination of [net shoal volume change, Perdido
Key volume change] of between [-258,000 cy/yr, -23,000 cy/yr] and [-212,000 cy/yr, -69,000
cy/yr], respectively.

It is the opinion of the authors that the true values lie either near the
measured/calculated quantities [-235,000 cy/yr, -46,000 cy/yr] or that the erosion rate along


-78-








Perdido Key attributable to the Pass is actually higher than the measured erosion rate. The
addition of sand from an offshore source is a possible mechanism for reducing the erosion
(ref. Stone et al., 1992). This is supported by the shoreline change analysis and the results
of even/odd analysis (Chapter 5). Lacking other data to resolve this problem, the
combination of -235,000 cy/yr and -46,000 cy/yr will be used for this period and the
sensitivity of the method to these values investigated. It is noted that choosing one of these
two values based on the other, or some combination thereof, adds a constraint on the method,
which ideally is employed with a known value of shoal volume change in order that the
downdrift impact might be found (and subsequently the length of alongshore impact).

Care must be taken at this stage to distinguish between the gross and net volumetric
change in the shoal system, and to distinguish between maintenance dredging and 'new
work' dredging performed in order to maintain the deep navigation channel. The gross
quantity of material that shoals the Pass is the sum of all material transported into the
channel from the littoral drift and the adjacent shorelines (generally a positive quantity)'.
The net quantity that shoals the Pass is the gross quantity minus any maintenance dredging
quantity; this quantity may be positive or negative, depending on the dredging rate.

The annual volume of dredged material has two component sources. During this time
period, the channel depth was not increased, therefore there was no new-work dredging
performed. The sources of the dredged material then include material brought into the Pass
from the littoral drift gross components and eroded material from each shoreline, as well as
material from the ebb shoal that has been drawn off the shoal or slumped directly into the
channel. The gross volumetric change in the shoal system can then be found as the sum of
the dredging rate and the selected/assumed net volumetric change in the shoal system. This
value is +100,000 cy/yr into the Pass, for this time period.

The gross change of 100,000 cy/yr may seem to be an obvious result from Equation
8.3, however, this amount that enters the Pass comes from four different sources, as shown
in Figure 8.1. The sources include sediment eroded directly from the two adjacent shorelines
and sediment impounded from the two gross littoral drift components. For example, it will
be shown that of the 46,000 cy/yr of sediment eroded from along Perdido Key, less than half
is transported back into the Pass (for this time period). The remainder of the loss, while still
attributable to the Pass, results from erosion needed to meet the requirement of the gross
transport components at the far downdrift boundary of the study area, where it is assumed

1
Riverine input would also be included in the gross shoaling rate. For
Pensacola Pass, FL, however, this value is assumed to be negligible.


-79-








that the gross transport components return to the same values applied at the updrift boundary.
Inspection of the normalized longshore transport curve in Chapter 7 indicates that the net
transport returns to the updrift value roughly 40,000 ft west of the Pass.

At this point, appropriate ranges of m andj are chosen. Bodge (1998) presents
several methods of determining these ranges. One method is to examine the increase in
transport potential as described by wave refraction analysis (Chapter 7, Figure 7.3). This
analysis provides an adequate value for the Santa Rosa Island Side, where the transport is
seen to increase by nearly a factor of 2 approaching the Pass (suggesting m, = 1.0 by Bodge's
definition). On the Perdido Key side of the Pass, the increase is not so easily distinguished
due to the possibility of a reversal in transport alongshore. For the range of m2, the measured
cumulative volumetric changes along Perdido Key are investigated. The change in slope of
the cumulative change curve indicates the increase in transport toward the Pass. While the
initial range of ml,2 to investigate will be 0.1 to 6.0 for both, it is anticipated that the true
answer lies between m = 0.5 and m = 3.0, based on the available data and analyses for this
time period. It is noted that the wave refraction analysis does not consider tidal currents,
hence the value of m, chosen from Figure 7.3 may be somewhat low.

The selection ofj is determined by the amount of impoundment along each shoreline.
For an unstabilized entrance, this value is somewhat difficult to determine as compared to
a jettied inlet. As an example, for a jettied inlet, it might be possible to intercept a large
fraction of the gross transport directed toward the inlet, thus resulting in aj value close to
1.0. While the ebb shoal system of Pensacola Pass is quite substantial and might be thought
of as a low weir jetty (ref. Work and Dean, 1995), it certainly does not have the trapping
capability of a traditional rock structure. Since the method is fairly simple to apply, the full
range of impoundment coefficients will be investigated (0 < j,2 < 1.0). It is anticipated,
however, that the limit ofjl,2 in the case of Pensacola Pass will be less than 0.5 or 0.6.

The set of values and variables for the 1974-1984 time period at Pensacola Pass is
shown in Figure 8.2. These values were applied to the CEM method to generate a family of
solutions (FS), plotted in the figure. Figure 8.2 presents lines of constant net transport
plotted as a function of the net bypassing computed from Equation 8.8 and the shoaling rate
from each side of the channel, computed from Equations 8.6 and 8.7.


-80-











100,000


90,000






w
S80,000
E
2
4-







easterly we
net bypassing ne
60,000 i 1 1

-20,000 0



FIXED OR KNOWN QUANTITIES


Do = 335,000 cy/yr
VI = -54,000 cy/yr
r=0.2


0 t 10,000














000
U)















20,000 40,000 60,000
Jet Bypassing (cy/yr)


ASSUMED VALUES/ASSUMPTIONS:

Vnet = -235,000 cy/yr
Vr = -46,000 cy/yr
ml, m2 = .2 to 6
Net bypassing is westerly directed
and less than half of Qnet
and less than half of Qnet


Hatched Area
Additional Assumptions

At least 10% of the total shoaling
originates from the Perdido Key side

ml, m2 < 3.0
j1,j2 < 0.5

Figure 8.2 Family of solutions for sediment transport pathways in the vicinity of
Pensacola Pass, FL, for 1974-1984. The figure plots values of the
shoaling rate from the east and west sides of the Pass versus the net
bypassing around the Pass for a range of net transport values, Q. All
values in cy/yr.








Figure 8.2 provides a substantial amount of information on one figure. The figure
illustrates a series of assumptions applied to the large FS in order to obtain a more practical
refined limit of the possible solutions. To begin, the FS indicates that for the input
parameters listed, the range and magnitude of possible net transport values is fairly small,
ranging from 20,000 cy/yr to just over 85,000 cy/yr. In the figure, it is assumed that the net
transport is westerly directed, and that the net bypassing (if any is present) is westerly
directed also. An additional assumption is that the net bypassing rate is no more than 50%
of the net transport rate at the study area boundary. This last assumption is regarded as fairly
conservative, it is reasonable to believe that there is little or no bypassing of any kind across
the deep navigation channel.

Figure 8.2 indicates that for all the possible solutions to the sediment budget, the vast
majority of the shoaling material in the Pass originates from the Santa Rosa Island side (over
70%). This is the result of the westerly-dominated drift in the area, reflected by the transport
ratio, r = 0.2. This fact is further supported by the behavior of the ebb shoal system on either
side of the navigation channel (Figure 6.4). The shoals on the eastern side of the channel,
Middle Ground and East Bank, appear to be stable to accretional over their measured
history, while Caucus Shoal on the west side is strongly erosional.

It is unlikely, however, that all the shoaling originates from the Santa Rosa Island
side of the channel. Therefore, it is assumed that at least 10% of the annual gross shoaling
rate originates from the Perdido Key side of the channel (combined shoreline erosion and
easterly directed gross transport impoundment). This assumption further reduces the FS in
Figure 8.2, placing an upper bound on the FS. Additionally, it is assumed that in reality the
presence of the Pass only results in m values up to 3.0.

The resultant FS after applying a limited number of reasonable assumptions is shown
in Figure 8.2 as the small cross-hatched area. This area encompasses a range of net transport
values from 25,000 to 70,000 cy/yr. These values are somewhat smaller than the
conventional wisdom for transport in this area, but they are not unreasonable by any means.

The objective at this point is to investigate the actual values ofj, m, and p that
comprise the small FS in Figure 8.2 to find a reasonable combination of coefficients. The
computed values of these coefficients depend strongly on SL and SR, since the sum of these
two values must equal 100,000 cy/yr, as dictated by the measured quantities. A useful
feature of this method is that after the above assumptions have been made, the range of
coefficients that can satisfy those assumptions is substantially reduced (as one might expect).
This limits the amount of judgement that must go into the selection of individual answers.








To further explain the CEM method and investigate the implications of various
solutions within the small FS, four example cases, shown in Figure 8.2, will be discussed:

Case A) In this example a solution in the middle of the small FS is chosen. The
resultant values are:

Q = 45,000 cy/yr
P = 14,000 cy/yr
SL = 85,000 cy/yr
SR= 15,000 cy/yr
mI = 1.3, m2 = 0.9,j, = 0.5,j2 = 0.35, p, = 0.29, p2 = 0.20

In general, it was desirable to choose solutions that resulted in the lowest
bypassing coefficients p,,2. This is of course a subjective decision based on
engineering judgement. Given the wide, deep, navigation channel that
extends seaward of the historic bypass bar location, this decision seems
justified. This resultant case is shown graphically in Figure 8.3, with each
transport component properly labeled. This figure will be described to some
detail, since it generally applies to all cases. All values have been rounded
to the nearest 1,000 cy/yr.

At the updrift and downdrift boundaries, the chosen value of Q, 45,000 cy/yr,
results in gross transport components of 56,000 cy/yr westerly and 11,000
cy/yr easterly, based on r = 0.2. As stated, the downdrift boundary is allowed
to 'float' to a distance where the ambient transport is unaffected by the Pass
and returns to the value updrift of the Pass. The input volume changes, Do,
AVL, AVN, and the computed AVR, are shown.

For this set of coefficients, the net bypassing was computed to be 14,000
cy/yr, which is a fairly small absolute value but is 31% of the net transport
value. Figure 8.3 illustrates that the net volumetric change measured along
the Santa Rosa island shoreline, -54,000 cy/yr, is comprised of three
elements. Roughly 73,000 cy/yr erodes from the shoreline on westerly
directed events and enters the shoal system. Approximately 28,000 cy/yr are
deposited along the Santa Rosa Island shoreline from the westerly transport.
Another 9,000 cy/yr erodes from the shoreline during easterly directed wave
events, resulting in a net change of -54,000 cy/yr.










1974-1984


Do = 335,000


11,000



56,000


CASE A
Qnet = 45,000 cy/yr


11,000



56,000
56,000


40,000


12,000'' 1000


- ... 73,000 Ii
-


Net Change Perdido Key
AVR = -46,000 cy/yr


Net Change Santa Rosa Island
AVL = -54,000 cy/yr


SNet change
in shoals, AV, =
-235,000 cy/yr
-_ *...-: -1-.


Sediment transport pathways for Pensacola Pass, FL, for 1974-1984 time period. For this example, Case A, the net
transport is 45,000 cy/yr westerly directed and the net bypassing rate is 14,000 cy/yr. Units are cy/yr.


Figure 8.3


r=02








Along the Perdido Key shoreline, the net change of -46,000 cy/yr is also the
result of three components. 4,000 cy/yr are impounded from the easterly
directed transport. Roughly 10,000 cy/yr are estimated to erode from the
Perdido Key shoreline and enter the shoal system. The balance of the net
change results from erosion due to westerly directed wave events and
amounts to 40,000 cy/yr of erosion. This large fraction of the measured net
change on Perdido Key (87%, 74% of the gross change) highlights the
impoundment effect of the Pass and the impact to the downdrift shoreline.
For this case example, 85,000 cy/yr of sediment is deposited in the channel
from the updrift (east) side. As a result, the downdrift shoreline must erode
in order to re-establish the ambient gross transport rates at the boundaries.

Figure 8.3 also illustrates an important issue in the discussion of sediment
budgets in the vicinity of tidal entrances. In Figure 8.3, the net volumetric
impact of the Pass on the Perdido Key shoreline is higher in magnitude than
just the ambient littoral drift (albeit only slightly in this case). This means
that simply reinstating the ambient transport by bypassing the net littoral drift
may not be sufficient. At the same time, it is also seen that of the net
transport shown in this case, 31% of that amount is bypassed around the
entrance in the net.

The point to the above example is that the sediment budget effect of a tidal
entrance is not simply the interruption of some or all of the net littoral drift
incident upon on the pass. A tidal entrance acts as a sink of sediment to
transport in both directions, and therefore the volumetric impacts of the Pass
may be greater or less than the net transport value.

Case B) In this example, a solution with no net bypassing is investigated. The
resultant values are:

Q = 30,000 cy/yr
P=0
SL = 84,000 cy/yr
SR= 16,000 cy/yr
m, = 1.5, m2 = 1.5,j = 0.25,2 = 0.30,p, = 0.01,p2 = 0.05








In this example, the net transport was selected at 30,000 cy/yr with no net
bypassing. With a lower net transport, the ml,2 values must increase to meet
the fixed requirement that 100,000 cy/yr shoal the Pass. With zero net and
gross bypassing, the Perdido Key shoreline must provide all of the westerly
transport requirement at the downdrift boundary by eroding at a calculated
rate of 38,000 cy/yr under westerly directed transport events.

Overall, the numerical values of this example do not vary significantly from
the Case A example. This is primarily attributed to the overall low net
transport values. In order to meet the requirement that 100,000 cy/yr shoal
the Pass, most of the material must originate from the adjacent shorelines.

Case C) In this example, a solution with the highest net transport rate in the small FS
is chosen. The resultant values are:

Q = 70,000 cy/yr
P = 34,000 cy/yr
SL = 90,000 cy/yr
SR= 10,000 cy/yr
mI = 0.9, m2 = 0.5,j, = 0.4,j2 = 0.5,p, = 0.47,p2 = 0.41

For the assumption given that at least 10% of the total shoaling volume must
originate from the west side of the Pass, Case C represents the highest
resulting net transport value, Q = 70,000 cy/yr. As seen in Figure 8.2, this
also implies that the net bypassing is near its highest value, 49% of the net or
34,000 cy/yr westerly directed.

Since the higher net transport means more material is supplied to the Pass
from outside the control volume/study area, the localized increase in transport
will decrease for both shorelines, as seen in the lower m,i2 values listed above.
These m,i2 values represent the highest values of transport increase that can
generate this set of shoaling/net transport values. Alternatively, the choice
of the lowest m values that meet the constraints results in higher direct
trapping of littoral drift sediment by the Pass itself. These lower transport
increase coefficients also result in higher impoundment coefficients j,2 along
each shoreline.








Ultimately, since the combinations of the three coefficients all have to result
in the dictated shoaling rates and shoreline changes, the choice of the
coefficients is primarily one of matching the physical conditions. Herein, the
available data suggest that the ebb shoal platform acts to sharply increase the
local transport into the Pass, hence the higher values ofm are chosen. On the
western side of the Pass, the Perdido Key shoreline itself contributes less
material to shoal the inlet, but, while the net bypassing is substantially higher
in this case, the Perdido Key shoreline still must erode some 46,000 cy/yr to
satisfy the gross westerly transport component at the downdrift boundary. In
this case, bypassing an amount equal to the net littoral drift, 70,000 cy/yr,
would result in placing more sediment west of the Pass than is eroded
annually from the downdrift shoreline.



Case D) In this example, a solution with a low net transport rate and the highest
fraction of shoaling from the Perdido Key shoreline is chosen. The resultant
values are:

Q = 35,000 cy/yr
P= 17,000 cy/yr
SL = 72,000 cy/yr
SR = 28,000 cy/yr
mI = 1.5, m2 = 2.7,j, = 0.45,j, = 0.4, p = 0.41, p2 = 0.08

Once again, the lower value of the net transport results in increased erosion
from either shoreline into the Pass in order to meet the shoaling requirement.
In particular, the m2 value is substantially higher, 2.7, than the previous cases.
Additionally, the selection of the highest input from the Perdido Key side
results in a bypassing rate that is nearly 50% of the net rate, 17,000 cy/yr,
westerly directed.



As discussed, the chosen value of r = 0.2 was based on the WIS wave data, and
appears to be slightly skewed to the east. To test the sensitivity of the CEM to the ratio of
gross components, the above analysis was tested with r = 0.5. Figure 8.4 presents a
comparison of the family of solutions resulting from both cases. Figure 8.4 illustrates that
an increase in r results in lower possible values of net transport for the given parameters
because of the increase in magnitude of the gross components. The previous limited FS


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