PERDIDO KEY, FLORIDA Feasibility Study for Beach Restoration

Transcription

PERDIDO KEY, FLORIDA
Feasibility Study for
Beach Restoration
Ivan - NOAA
20 December 2005
Submitted to:
Neighborhood & Environmental Services Department
Escambia County, FL
Bureau of Beaches & Coastal Systems
Florida Department of Environmental Protection
Prepared by:
olsen associates, inc.
FL COA00003491
May 2006
olsen
associates, inc.
coastal engineering
Perdido Key, FL
Feasibility Study for Beach Restoration
Prepared for:
Escambia County, FL
&
Bureau of Beaches and Coastal Systems
Prepared By:
Olsen Associates, Inc.
Jacksonville, FL
May 2006
EXECUTIVE SUMMARY
This report details the findings of a comprehensive study performed to investigate the
feasibility of various shore protection alternatives for the western six miles of the Gulf of
Mexico shoreline at Perdido Key in Escambia County, FL. Olsen Associates, Inc., of
Jacksonville, FL, was contracted by the Neighborhood and Environmental Services
Department of Escambia County, FL, to conduct the investigation. Funding for the
feasibility study is provided by the Florida Department of Environmental Protection,
Bureau of Beaches and Coastal Systems (FDEP BBCS, Grant No. H5ES1), through
Escambia County, FL.
Alternatives for beach restoration for the study area must address the need to significantly
increase the level of storm protection provided by the sandy beaches of the Key to upland
infrastructure and environmental habitat, while maintaining and/or increasing the
recreational amenity value of the beach. The recent impacts of the 2004 and 2005
tropical storm seasons, most notably Hurricane Ivan in September 2004, destroyed the
primary dune system, substantially lowered the elevation of the dry beach berm, and
transported a significant quantity of sand offshore of the primary bar.
While the study area has not experienced substantial shoreline recession at the Mean
High Water Line (MHWL), the recent loss of beach volume and the lowering of the dry
beach leave upland infrastructure and habitat vulnerable to storm wave impacts and
inundation from events generating storm surges of +7ft MSL or higher (typically, the
Perdido Key, FL
i
“20-yr event” or greater). The post-Katrina FEMA berms constructed along much of the
developed area represent protection against storm events with surges lower than
approximately six to seven feet. For more severe events, the FEMA berm is expected to
be completely eroded, after which time erosion and profile deflation landward of the
seaward edge of construction becomes more likely with subsequent events.
The series of storm events in 2005 has prevented any meaningful recovery of the beaches
following Hurricane Ivan, and in some instances has exacerbated the loss of sand from
the system to offshore areas. Even under ideal future weather circumstances, which
cannot be relied upon for purposes of storm protection in the near-term, available data
suggest that only a portion of the sand transported offshore during Hurricanes Ivan,
Dennis, and Katrina would be expected to return to higher elevations along the beach
profile. Thus, for purposes of beach restoration along the Perdido Key, FL, study area, it
is not recommended that this volume of sand be relied upon to provide any level of
meaningful assistance in achieving the required level of storm protection identified
herein. The potential recovery of portions of the central and western segments of the
study area via natural littoral conditions is completely dependent upon the future storm
climate.
Recommendations
Based upon the analysis of historical shoreline changes, recent storm impacts, and
predictions of storm-induced beach profile change, it is recommended that Escambia
County, FL, and the Florida Park Service pursue the permitting and construction of a
comprehensive beach nourishment project along the westernmost six miles of the Gulf of
Mexico shoreline of Perdido Key, FL.
¾ It is recommended that the proposed project include the placement of a minimum
of 1.5 to 2.0 million cubic yards of beach compatible sand along this beach
segment, weighted toward the eastern end of the area. The minimum-volume
project is expected to prevent the loss of the existing FEMA emergency berm
during a 25- to 30-yr storm event and minimize the inundation and overwash
along the Perdido Key State Park Property (where no dune is to be constructed,
per the direction of the Florida Park Service). Additionally, such a project is
expected to minimize the deflation/erosion of the existing grade landward of the
1975 Coastal Setback Line (generally -- the seaward edge of construction) during
more severe events.
Perdido Key, FL
ii
¾ It is recommended that, as part of the proposed restoration, Escambia County
initiate the permitting of the primary nearshore borrow site identified during the
Sand Search portion of this study. While it is possible that beach-compatible sand
may become available from Pensacola Pass as part of the proposed U.S. Navy
dredging of the Federal Navigation Channel, the present uncertainty in the timing
of the two projects dictates that the County pursue an independent source of sand.
In the event sand from the Pass is utilized for the construction of the
recommended alternative, completing the permitting of the nearshore borrow site
will provide the County an additional option in the future (for emergency needs or
future renourishment, if and when necessary).
¾ It is recommended that the constructed beach restoration project be documented
with the Federal Emergency Management Agency (FEMA) for purposes of
establishing future eligibility for post-disaster financial assistance under Category
G guidelines. As demonstrated by the recent FEMA-funded reconstruction of
nearby beach restoration projects in Pensacola Beach and Gulf Shores, AL,
establishing FEMA eligibility provides significant financial assistance to rebuild
the beach in the event of a future declared disaster. For this reason, it is
recommended that the entire 6.0-mile length of the Perdido Key study area,
including the State Park, be incorporated into the beach restoration plan.
¾ It is recommended that a beach monitoring plan be established to assess the
performance of any constructed beach restoration alternative. Such a plan, which
will be required by permit, would include annual or more frequent beach profile
monitoring surveys, collection of aerial photography, and borrow site surveys at
regular intervals. Institution of such a plan would likewise serve to address the
maintenance requirements of an “engineered” beach nourishment project under
FEMA Category G Public Assistance guidelines.
¾ It is recommended that the County apply to FDEP BBCS for funding assistance
for the proposed project through the Bureau’s Beach Erosion Control Program
(BECP). Plans for a beach restoration project for Perdido Key were submitted to
BBCS as part of the 2006-07 Long Range Budget Plan. The plan should be
updated based upon this report and resubmitted for 2007-08. This study,
conducted in accordance with the BBCS 2004 Hurricane Recovery Plan for
Florida’s Beach and Dune System, is intended for submittal to FDEP to provide
the necessary information to support the budget plan and establish the project’s
Perdido Key, FL
iii
eligibility for State cost sharing in the construction process. A portion of that
eligibility determination relates to an evaluation of the critically-eroded
designation for the entirety of the study area. Data are provided herein to address
that determination.
¾ It is recommended that a dedicated source of funding be established for upcoming
beach management needs. Creation of such a fund, perhaps through tourist
development tax collections, could be used to pay for the costs of annual
monitoring of a beach restoration project, the costs of small emergency repairs to
the project, subsequent dune enhancements if desired, and the required matching
funds to pay for the repair of the beach in the event of a declared disaster.
Numerous examples of such dedicated funding sources exist, such as the beach
improvement fund established in Gulf Shores, AL.
Tasks Required for the Recommended Beach Restoration Alternative
It is strongly encouraged that the permitting and associated design tasks required for the
construction of the recommended alternative be authorized as rapidly as possible in order
to maintain the viability of various options for sand sources, especially any beach quality
sand potentially available as part of the proposed dredging of Pensacola. To that end, the
following are tasks that will be required regardless of the sand source ultimately used:
¾ Conduct a Mean High Water Line survey and formally establish an Erosion
Control Line (ECL) along the full length of the proposed project,
¾ Acquire construction easements along each beach front property where the project
is to be constructed. The easement must extend from the established ECL
northward to the landward limit of the construction project (generally, the 1975
Setback Line),
¾ Prepare permitting-level design schematics of the plan views and cross-sections
for the proposed project.
The tasks of setting an ECL and acquiring construction easements are opined to
constitute the critical path for construction of any restoration alternative. Easement
acquisition should begin as soon as possible. During that process, it may become
necessary for the County to acquire some of the easements via condemnation, thus
Perdido Key, FL
iv
adequate time must be allotted for this possibility. On a similar schedule, other tasks will
be required to permit the use of the primary nearshore borrow area:
¾ Conduct an environmental assessment of the primary nearshore borrow site,
¾ Conduct a cultural resources review/survey of the site,
¾ Prepare permitting-level design plans of the proposed excavation area.
Upon completion of these tasks and the submittal of a Joint Coastal Permit Application to
FDEP and the U.S. Army Corps of Engineers, additional tasks may be required, such as
the preparation of a Biological Assessment, design level surveys, etc.
Preliminary Opinion of Probable Cost to Construct
The cost of construction of such a project is highly dependent on a number of factors,
including, but not limited to:
¾
¾
¾
¾
The price of fuel at the time of bidding and construction,
The level of competition/availability of dredge plant,
The time of year proposed for construction,
The nature of the project design and potentially of the borrow site.
Assuming the proposed recommended alternative is constructed utilizing a
cutterhead/pipeline dredge to excavate sand from the identified primary nearshore borrow
site, it is opined that the project could be constructed for unit prices of $4.00 to $6.00 per
cubic yard, plus mobilization/demobilization fees. Thus, a first estimate of the probable
cost to construct the recommended beach restoration alternative is between $9 million
and $13 million. This first opinion of the probable construction cost is provided to
facilitate project planning, State cost-sharing, and local financing at this feasibility stage.
This cost opinion can then be applied to assess the economic justification of pursuing
construction of the recommended alternative utilizing sand excavated from the Federal
navigation channel at Pensacola Pass versus the primary nearshore site.
Perdido Key, FL
v
Perdido Key, FL
May 2006
TABLE OF CONTENTS
EXECUTIVE SUMMARY ................................................................................................i
1.0
INTRODUCTION .................................................................................................1
2.0
STUDY AREA & PHYSICAL SETTING...........................................................4
2.1
2.2
2.3
3.0
SUMMARY OF SHORELINE CHANGES AND STORM IMPACTS .........10
3.1
3.2
3.3
3.4
3.5
4.0
Basic Geologic Setting ................................................................................7
Native Beach Sediment Characteristic ........................................................7
Oceanographic Setting.................................................................................7
Long Term Shoreline & Beach Volume Changes: 1890/1895 to 1984.....10
National Seashore Disposal Activities: 1984 to 1991 ...............................16
Storm-Induced Shoreline & Beach Volume Changes: 1995 to 2005 .......17
3.3.1 Discussion......................................................................................19
3.3.2 July 2005 to Present.......................................................................29
3.3.3 Potential for Natural Recovery ......................................................29
Wave Transformation & Alongshore Transport Modeling .......................31
3.4.1 Estimates of Longshore Transport.................................................32
3.4.2 Generalized Sediment Budget .......................................................32
Observations from Storm Recession Modeling.........................................34
ALTERNATIVES FOR BEACH RESTORATION
AT PERDIDO KEY, FL......................................................................................42
4.1
4.2
4.3
4.4
4.5
No Action...................................................................................................46
Shoreline Armoring ...................................................................................47
Dune Construction .....................................................................................48
Comprehensive Beach Nourishment .........................................................49
Structural Stabilization of the Shoreline....................................................50
Continued…
Perdido Key, FL
vi
5.0
ASSESSMENT OF BEACH NOURISHMENT &
DUNE CONSTRUCTION ALTERNATIVES..................................................51
5.1
5.2
5.3
5.4
6.0
REVIEW OF POTENTIAL SAND SOURCES................................................63
6.1
6.2
6.3
6.4
6.5
7.0
Nearshore Gulf of Mexico Sand Sources ..................................................65
Pensacola Pass Federal Navigation Channel .............................................67
Pensacola Pass Ebb Shoal..........................................................................68
Gulf Intracoastal Waterway Sources .........................................................70
Upland Stockpiles (Admiral’s Island – Ft. Mcree)....................................70
RECOMMENDATIONS ....................................................................................72
7.1
7.2
7.3
7.4
8.0
Minimum Volume Requirements ..............................................................53
Beachfill Planform Alternatives ................................................................56
Previous Beach Nourishment Projects in the Region ................................57
Beachfill Project Expectations...................................................................60
Recommendations......................................................................................73
7.1.1 Recommended Alternative ............................................................73
7.1.2 Sand Source ...................................................................................73
7.1.3 FEMA Documentation of Engineered Beach ................................73
7.1.4 Establishment of a Beach Monitoring Plan ...................................74
7.1.5 Application to FDEP BBCS Beach Erosion Control Program......74
7.1.6 Creation of a Dedicated Beach Management Fund .......................74
Tasks Required for the Recommended Beach Restoration Alternative ....74
Preliminary Opinion of Probable Cost to Construct..................................75
FDEP Critical Erosion Designation...........................................................76
REFERENCES ....................................................................................................79
Appendix A Historical Beach Profile Plots - 1974-2005
Appendix B Shoreline & Beach Volume Change History
Appendix C Wave Transformation & Littoral Processes Modeling
Appendix D Storm Induced Shoreline Recession Modeling
Appendix E Aerial Photography/Shoreline Conditions
September 2005 (Post-Katrina)
Appendix F Review of Environmental Resources
Appendix G Abbreviated Glossary of Coastal Engineering Terms
Perdido Key, FL
vii
Perdido Key, FL
Prepared for:
Escambia County, FL
&
Bureau of Beaches and Coastal Systems
Prepared By:
Albert E. Browder, Ph.D., P.E.
William L. Reilly, E.I.
Erik J. Olsen, P.E.
Olsen Associates, Inc.
4438 Herschel St.
Jacksonville, FL 32210
904-387-6114
May 2006
1.0
INTRODUCTION
This report details the findings of a comprehensive study performed to investigate the
feasibility of various shore protection alternatives for the western six miles of the Gulf of
Mexico shoreline at Perdido Key in Escambia County, FL. Olsen Associates, Inc., of
Jacksonville, FL, was contracted by the Neighborhood and Environmental Services
Department of Escambia County, FL, to conduct the investigation. Funding for the
feasibility study is provided by the Florida Department of Environmental Protection,
Bureau of Beaches and Coastal Systems (FDEP BBCS, Grant No. H5ES1), through
Escambia County, FL.
The Perdido Key shoreline has been significantly impacted by storm damage from no less
than six named storms in 2004 and 2005. Most notably, Perdido Key experienced severe
storm damage from Hurricane Ivan, which made landfall on 16 September 2004 just west
of Gulf Shores, AL, approximately 18 miles west of the Florida/Alabama State Line.
Ivan produced sustained winds at landfall of 120 mph and a storm surge in excess of 14 ft
above mean sea level along most of the island (FEMA, 2005). The resulting beach
erosion destroyed the entire primary dune system and numerous individual and
Perdido Key, FL
-1-
multifamily structures along Perdido Key. The loss of the primary dune system and the
accompanying lowering of the beach profile elevation fronting upland development left
the shoreline extremely vulnerable to subsequent storm impact. In 2005, the project
study area experienced damaging sea/storm conditions from Tropical Storm Arlene,
Hurricane Cindy, Hurricane Dennis, Hurricane Katrina, and Hurricane Rita. Hurricane
Ivan in 2004 and Hurricanes Dennis and Katrina in 2005 all resulted in Emergency
Declarations for Escambia County from the Federal Emergency Management Agency
(FEMA). The intense frequency of these storms has precluded any meaningful natural
recovery and has hampered smaller-scale shore-protection efforts, such as the
construction of storm berms, etc.
The principal objective of the study is to develop a recommended shore protection/beach
restoration alternative for the westernmost 6.0-mile length of shoreline at Perdido Key,
FL, including Perdido Key State Park. The recommended plan must address the need to
significantly increase the level of storm protection provided by the sandy beaches of the
Key to upland infrastructure while maintaining and/or increasing the recreational amenity
value of the beach. The recommended alternative must acknowledge what is physically,
economically, and socially feasible for Perdido Key, FL.
Specific objectives of the study include:
¾ Analysis of historical shoreline position and beach volume changes, both longterm and short-term. Particular attention is focused upon the impacts of
Hurricane Ivan to the Perdido Key shoreline,
¾ Description of the physical processes that impact the study shoreline,
¾ Assessment of potential storm damage to upland infrastructure under existing
conditions, with emphasis on the damages caused during the recent storm seasons
(especially Hurricane Ivan),
¾ Review of potential beach restoration alternatives, with special emphasis on the
level of storm protection afforded by each and the improvement to the
recreational amenity value of the beaches of Perdido Key,
¾ Comparison of alternatives and the development of recommendations regarding
the most physically, socially, and economically feasible alternative for shore
protection at Perdido Key, FL.
In addition to addressing the objectives of the study, specific tasks associated with the
work include the collection of updated beach profile survey data for the area and the
Perdido Key, FL
-2-
acquisition of updated, geo-referenced aerial photography. A project-specific beach
profile survey was conducted in July 2005 (FDEP R-monuments R-1 to R-37, Appendix
A). That survey was performed in conjunction with a condition survey of the beaches
adjacent to Pensacola Pass and as such extended from the FL/AL State Line to the Pass
(R-1 to R-67) and onto Santa Rosa Island (R-68 to R-107). These profile data are
augmented by recent beach profile data collected by the City of Orange Beach, AL, along
the Alabama portion of the Perdido Key shoreline. Regarding aerial photography, the
original intent of the acquisition of georeferenced aerials was to post-rectify available
photography collected after Hurricane Ivan (September 2004). Following Hurricane
Katrina in late August 2005, however, the study shoreline was re-photographed,
providing the opportunity to produce a more recent set of aerial photographs for the study
(date: 3 September 2005, Appendix E).
At the time of this writing, only the developed portion of the Perdido Key, FL, shoreline
from FDEP R-monument R-26 to R-32 is formally considered by FDEP to be critically
eroded. This designation, which plays a central role in the level of State financial
participation in the construction of any beach restoration alternative, will be reviewed in
light of the severe impacts of the last two hurricane seasons. The preparation of this
feasibility study is consistent with the recommendations of the FDEP BBCS 2004
Hurricane Recovery Plan for Florida’s Beach and Dune System (FDEP/BBCS 2004).
In conjunction with the feasibility study, an offshore geotechnical investigation (i.e. sand
search) was conducted to identify suitable sources of beach-compatible sand for purposes
of cost-effective beach nourishment along the six-mile study area (Olsen Associates,
2006a). That study identified potential sand borrow sites immediately offshore of the
study shoreline. Herein, the feasibility of utilizing various sources of sand for purposes
of beach nourishment is discussed in conjunction with the recommended alternative
(Chapter 6). These sources include the offshore sites identified in the 2005 Sand Search
as well as the potential for of obtaining beach-compatible sand from the Federal
navigation channel at Pensacola Pass.
The report is organized as a brief main text supported by numerous technical appendices
detailing various aspects of the study, including historical shoreline analyses, modeling
efforts, etc. The main text focuses on those issues immediately relevant to shore
protection and beach restoration along the study shoreline. The main text presents the
recommended alternative, and outlines steps necessary for the implementation of a beach
restoration project at Perdido Key, FL.
Perdido Key, FL
-3-
2.0
STUDY AREA & PHYSICAL SETTING
Figure 2.1 depicts a location map for the present study. Perdido Key, located in
southwest Escambia County, FL, is the westernmost barrier island in Florida, extending
for approximately 15 miles from Pensacola Pass westward to Perdido Pass in Baldwin
County, AL. The westernmost 2.2 miles of the island, east of Perdido Pass, lie within the
limits of the City of Orange Beach, AL. The study area occupies the adjoining 6.0 miles
of Perdido Key, from the FL/AL State Line at FDEP survey monument R-1 eastward to
the boundary with the Perdido Key Unit of the Gulf Islands National Seashore at R-32.
The GUIS property extends roughly 6.8 miles eastward to Pensacola Pass at R-65. The
Gulf shoreline of the Key terminates along the Pensacola Pass channel roughly 2,000 ft
north-northeast of R-65 at a small rock structure at R-67. Maps of Perdido Key can be
found on National Ocean Service (NOS) Charts 11378 and 11382.
The 6.0-mile study area is divided into three shoreline segments. From the FL/AL State
Line eastward, the first 2.4 miles of the shoreline are developed with single-familyresidences and condominiums (State Line to R-12.5). Proceeding eastward, Perdido Key
State Park occupies the next 1.6 miles of the Gulf of Mexico shoreline (R-12.5 to R21.5), and is developed with two principal parking areas with gazebos, walkovers, and
restroom facilities. Eastward thereof, the remaining 2.0 miles of the study area, from R21.5 to R-32, are developed with single-family-residences and condominiums (including
rental units). The primary East-West coastal road along Perdido Key is State Route 292
(Perdido Key Drive) from Pensacola, FL. Route 292 runs the length of Perdido Key from
approximately R-29.5 to R-1, where it transitions to HW 182 in Orange Beach, AL. East
of R-29.5, Johnson Beach Road runs eastward into the National Seashore property.
The sandy barrier island of Perdido Key varies in width from as much as 1.2 miles along
the center of the island (near R-28) to less than 400 ft in certain locations just west of
Pensacola Pass (Figure 2.1). USGS survey quadrangle sheets indicate that ground
elevations along the center of the island typically reach only +10ft above Mean Sea
Level1, while the crests of the more landward dune ridges just south of the coastal road
along the Gulf shoreline reach as high as 15 to 25 ft (see Chapter 3). For reference, the
elevation of the coastal road, Route 292, varies non-uniformly along the shoreline from
roughly +9 ft to over +15ft along the beach segment from R-1 to R-29.
1
The primary vertical datum used in this report is the North American Vertical Datum of 1988 (NAVD88).
For reference, Mean Sea Level (MSL) is approximately 0.3 ft above NAVD88, and Mean High Water
(MHW) is approximately 0.9 ft above NAVD88 (see Appendix C for further discussion).
Perdido Key, FL
-4-
490,000
MS
Perdido Key
LA
Perdido Key, FL
480,000
470,000
-5-
Northing (ft, NAD83)
Figure 2.1
460,000
*
Bear Pt.
FL
SC
1,030,000
Location map, Perdido Key, FL.
Per
dy Are
a
Perdido
Key
1,050,000
N
lands
1,060,000
Gulf Is
1,070,000
0
2.0 mi (st)
1,080,000
1.0
GRAPHIC SCALE
Pensacola
Pass
Santa
Rosa
Island
Pensacola
Bay
Pensacola
hore
l Seas
ationa
Big Lagoon
ESCAMBIA COUNTY, FL
Easting (ft, NAD83)
1,040,000
Gulf of Mexico
e y St u
dido K
PKSP
Perdido Bay
Innerarity
Pt.
Ono Island
1,020,000
Bayou St John
GA
1,010,000
Perdido
Pass
Terry
Cove
Orange Beach
TX
AL
ALABAMA
FLORIDA
500,000
Development and Public Access The 4.4-mile developed segments of the Perdido Key,
FL, Gulf of Mexico shoreline at Gulf Beach are occupied principally by large multifamily, multi-story condominium structures. There are also several single family
residences along the Gulf-front, especially along the western developed segment.
Presently on Perdido Key, there are 4,100 units that pay ad valorem taxes in Escambia
County. Those taxes generate over $16 million dollars annually for the county. Records
from Escambia County list 1,433 publicly available rental units within the
condominiums. The number of rental units available varies significantly, as buildings
damaged by Hurricanes Ivan, Dennis, and Katrina are repaired or demolished and new
buildings are constructed. Funds from the tourist development tax for Perdido Key (zip
code 32507) generate over $700,000.00 annually. Sales tax income specifically from the
Key was unavailable at the time of this writing. The developed portion of the island in
Escambia County is presently legally capped at 7,150 units and could reach 9,169 units
(based on present zoning conditions), which could increase the ad valorem and tourist
development tax income of the Key even further.
Escambia County owns two 100-ft wide public parking/beach access parcels at R-22 and
R-25. Perdido Key State Park maintains two large Gulf-front parking areas with
bathroom facilities and handicapped overwalk accesses to the beach (R-13 and R-18).
These facilities, also severely damaged in Hurricane Ivan, have reopened in Spring 2006.
The State Park also maintains two roadside dune overwalks near R-15 and R-16 (at the
Perdido Key Visitor’s Center). Just east of the developed portions of the Key, the Gulf
Islands National Seashore maintains a large public parking, picnic, and beach access area
at Rosemond Johnson Beach Park (between R-33 and R-34).
Perdido Key, FL
-6-
2.1
Basic Geologic Setting
The geologic setting of Perdido Key is similar to the coastal areas of neighboring
Alabama and Mississippi to the west rather than the majority of the Florida carbonate
platform to the east. The island sits on the north flank of the southwestward-dipping Gulf
Coast sedimentary basin. Perdido Key lies in the East Gulf Coastal Plain physiographic
province, which extends eastward to the Apalachicola River boundary with the Atlantic
Coastal Plain (a somewhat arbitrary boundary). The southern Coastal Strip, below 20 to
25 feet above sea level (generally, below the Pamlico Terrace, Healy (1975)), lies in the
Southern Pine Hills District of the province and extends eastward to Choctawhatchee Bay
in Walton County and includes Perdido Key, Santa Rosa Island, Santa Rosa Sound, and
portions of the north shoreline of the Sound (Brooks, 1982). According to the Florida
Geological Survey, the surficial sediments of the barrier islands of Escambia County,
those of immediate interest to the present study, are recent Holocene sediments composed
almost entirely of quartz (FGS, 1993). These modern beach and shoreface sands are
likely to have been derived from transgressive reworking of older delta and shallow
marine sands deposited in the Late Pleistocene Epoch.
2.2
Native Beach Sediment Characteristics
Olsen Associates (2006a) provides a detailed description of the engineering sediment
characteristics of the native beach. Surface sand samples were collected in October 2005
along six shore-normal beach profile transects and analyzed for grain size distribution,
color, and carbonate (shell) content. Figure 2.2 summarizes the grain size distribution
data for the composite distributions generated from all 29 samples collected. The median
grain size of the weighted distribution is 0.31mm and the sorting coefficient (standard
deviation) is 0.44 φ, indicative of fine to medium grained sands that are very well sorted
(all sand grains are fairly similar in size). Fine sediments in the native beach samples
averaged less than 1% by weight. The weighted composite distribution is considered to
be the “target” for purposes of assessing borrow site sand compatibility with the native
beach.
Perdido Key, FL
-7-
1
1.5
2
2.5
3
35
45
60
80
120
3.5
4
4.5
5
170
200
230
0.5
18
14
7
10
5
1/4
1/2
US STANDARD SIEVE
5/16
5/8
-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0
25
PHI:
100
0
PERCENT FINER BY WEIGHT
10
Perdido Key, FL
Native Beach Sand
Grain Size Distribution
80
20
Composite Cumulative Distribution
70
mean: 0.32mm
30
median: 0.31mm
Composite Frequency Distribution
-8ft Composite
60
40
mean: 0.28mm ; median: 0.28mm
50
50
-4ft Composite
40
60
Waterline Composite
30
70
Mid-berm Composite
20
80
Dune Composite
10
PERCENT COARSER BY WEIGHT
90
90
0
100
2
C
O
BB
LE
100
98 7 6 5 4
10
3
2
98 7 6 5 4
3
2
1
GRAIN SIZE - MILLIMETERS
AV
EL
3
PEBBLE
G
R
98 7 6 5 4
98 7 6 5 4
3
2
0.1
0.01
SAND
VERY CRS.
CRS.
MED.
FINE
V. FINE
SILT
Figure 2.2 Native beach composite grain size distributions, Perdido Key, FL, study area
(R1 to R-32, sampled October 2005). Both the frequency and cumulative distributions
are plotted, along with individual composite samples describing the cross-shore variation
of grain size (Olsen Associates, 2006).
Native Sand Color and Carbonate Content Carbonate burn tests were performed on 18
of 29 native beach samples and revealed an average carbonate content of 1.4% by weight,
with a high measurement of 2.0%. Further inspection of the individual grain size
distribution data indicate only a very small percentage of grain sizes greater than 1.0mm
in diameter (those sizes and greater are hypothesized to likely be shell fragments) (Olsen
Associates, 2006a). Sand color was documented by Olsen Associates, Inc. personnel to
be in agreement with the standards set by the Escambia County Sand and Water
Protection Ordinance for sediments present or imported to Perdido Key. That Ordinance
stipulates that sand color should meet a minimum Value of 9.25 and a Chroma of no
more than 0.5 measured on a 10YR or similar Hue scale. Value, Chroma, and Hue refer
to the Munsell Color Scale (Munsell, 1998a, b).
Perdido Key, FL
-8-
2.3
Oceanographic Setting
Appendix C describes the littoral climate of Perdido Key in terms of the tides, winds, and
waves incident to the shoreline. Tides in the area are quite small, averaging only 1.1 feet
in range over the course of the year and only slightly exceeding 2.0 feet during spring
tides. The tides are considered to be mixed but predominantly diurnal, with only one
high and low tide per day. Pendleton et al. (2004) ascribe a sea level rise rate of 2.14
mm/yr, based upon the long-term water level records at Pensacola, FL.
The characteristics of and changes to the beach profile along Perdido Key, from
elevations of +20 to -20 ft MSL, are discussed in detail elsewhere in this report. The
-20ft contour is typically reached within 1,500 to 2,000 feet of the shoreline, seaward of
which the seabed is relatively shallow and mildly sloping at approximately 1:1,000 v:h to
the edge of the DeSoto Canyon. The break in the seabed at roughly -120 to -150 ft lies
approximately 25 nautical miles offshore. As will be described elsewhere, there are
numerous shoal features, both shore-parallel and shore-oblique, that lie within that 25
nautical mile zone.
The average annual wave climate in the area is very mild, with average wave heights of
two feet or less, typically. The majority of waves incident to the shoreline arrive from the
South-Southeast or Southeast direction. A significant fraction of winter waves, however,
are directed offshore, when the passage of cold fronts generates north winds. The mild
long-term average, however, represents a significant number of calm events averaged
together with extremely high wave-height events of short-duration associated with
tropical storms and hurricanes. During such events (discussed in detail throughout this
report), offshore deepwater wave heights can exceed 50 ft. Wave heights near shore, just
seaward of the primary bar, can reach over 20 feet in height before breaking on the bar
itself. During these events, wave reformation can result in breaking waves on the beach
that reach seven to ten feet in height, depending on the surge level (see Appendix D).
NOAA’s Coastal Services Center database lists 52 tropical events (tropical depression or
stronger) that have passed within 50 miles of the FL/AL State Line since 1851, an
average of one event every three years. The same dataset lists nine events since 1995 that
have passed through the same area (one event every 1.2 years). The same database lists
32 events of Category 2 hurricane strength or greater that have passed within 125 nmi of
the State line since 1851 (4.8 per year), six of which have occurred within the last 11
years. Three such events have occurred in the last two years (Ivan, Dennis, and Katrina).
Perdido Key, FL
-9-
3.0
SUMMARY OF SHORELINE CHANGES & STORM IMPACTS
This chapter summarizes the findings of numerous analyses designed to describe the
behavior of the Perdido Key Gulf of Mexico shoreline. Historical shoreline position and
beach profile data from the FDEP BBCS database were compiled and updated with
recent beach profile survey data from numerous sources to describe the changes in
shoreline location and beach volume. This analysis is supplemented by a review of
coastal engineering activities since 1883 that have affected the study area. Further
describing the littoral environment are numerical wave refraction-diffraction modeling
results and numerical storm recession predictions. These analyses are described in detail
in Appendices B, C, and D.
3.1
Long-Term Shoreline & Beach Volume Changes: 1890/1895 to 1984
Figure 3.1 depicts the historical positions of the Perdido Key, FL, shoreline, relative to its
1890/1895 location, the oldest “complete” data set for the island. Mean High Water Line
(MHWL, +0.9ft NAVD88) position data were compiled from the FDEP database and
updated by the July 2005 beach profile survey conducted for this study. Also noted in
Figure 3.1 are the locations and dates of numerous coastal engineering activities at
Perdido Key that have influenced the shoreline position since the commencement of
dredging activities at Pensacola Pass in 1883. Observations from the long-term MHWL
shoreline change data are as follows, details may be found in Appendix B:
¾ Shoreline position changes along the first 6.5 to 7.5 miles west of Pensacola Pass are
clearly dominated by the influence of Pensacola Pass and man-induced activities
there, including the dredging program at the Pass. In the immediate vicinity of the
inlet, to R-63, the shoreline has advanced by over 700 ft since 1890/1895 as the
terminus of the island has transformed from a rounded bank2 to a defined and
essentially stabilized shoreline along R-65 to R-67. Westward thereof, the shoreline
has retreated dramatically. Recession of 350 to 500 ft was measured up through
1984, when beach disposal operations near the eastern end of the island began (see
discussion below). Figure 3.1 also indicates the approximate location of several
cross-island breaches caused by the 1906 hurricane, one near R-56, another near R35, and another in Alabama, roughly ½-mile west of the FL/AL State Line.
2
References from the 1800’s refer to Perdido Key as Foster’s Island, and the area immediately west of the
channel as Foster’s Bank. In various maps of the era, the two features appear as separate islands.
Perdido Key, FL
- 10 -
ALABAMA
FLORIDA
Old River
64
61
R0
R0
58
R0
55
R0
49
R0
46
R0
43
Escambia County
(developed)
R0
37
40
R0
R0
34
R0
31
R0
28
R0
22
Perdido Key
State Park
25
R0
R0
19
R0
13
10
Escambia County
(developed)
16
R0
R0
R0
07
04
R0
R0
01
R0
Jetty Construction
1970
Pensacola
Pass
1906
breach
Big Lagoon
52
R0
Perdido
Pass
1906
breach
67
R0
PERDIDO
KEY
Gulf Islands National Seashore
Perdido Key Unit
-800
Survey Date
1920
1934
1965
1974
1984
1996
2005
-600
-400
-200
0
200
400
1989-1990 Nourishment of 5.4 Mcy
600
800
1990-1991 Nearshore Disposal
of 3.9 Mcy
2005 Nourishment
of 0.7 Mcy (in AL)
Shoreline is seaward of 1890/1895 position
Shoreline Position Relative to 1890/1895 Location (ft)
1985 Nourishment
of 2.3 Mcy
Shoreline is landward of 1890/1895 position
1906
breach
1,000
-10,000
0
10,000
20,000
30,000
40,000
Alongshore Distance from FL/AL State Line (ft)
50,000
60,000
70,000
Figure 3.1 Historical shoreline position relative to the 1890/1895 location Perdido Key, FL.
- 11 -
¾ The data illustrate that the long-term erosional signal west of Pensacola Pass extends
through the GUIS and into Gulf Beach and the developed portion of the Key to
roughly R-26. Foster et al. (1999) developed a representative description of the longterm background shoreline change rates, focusing principally upon the 1974-1984 pre
beach disposal period (Figure 3.2). The writers calculated recession rates of up to 9
ft/yr near the Pass. Westward, near the boundary of the National Seashore with the
eastern developed portion of the island, long-term shoreline recession averages 2 ft/yr
or less (to R-26). The shoreline behavior immediately west of the Pass is attributed
principally to the channel dredging program and the refractive sink effect of the
Pensacola Pass ebb shoal. Browder and Dean (1999) developed sediment budgets of
the Pass and estimated that the long-term erosional influence of the Pass extends for
eight to nine miles west of the channel.
¾ Along the developed portions of Perdido Key at Gulf Beach, from R-26 westward to
R-21, long-term MHW changes indicate relative stability or minor shoreline advance.
Long-term shoreline change trends through Perdido Key State Park to the FL/AL
State Line increase uniformly to reach as high as 4 ft/yr of advance near R-1. Foster
et al. (1999) examined the 1974-1984 (pre-nourishment) and 1984-1996 time periods
and found variations in shoreline change rates of up to ±8 ft/yr along this segment of
the shoreline. Despite the profile-to-profile variation, the typical rate of MHW
shoreline change is relatively consistent across the various time periods shown.
¾ West of the study area, the Alabama shoreline of Perdido Key continues the general
trend of minor shoreline advance or stability for roughly another ½-mile westward, at
which point the historical influence of Perdido Pass significantly affects shoreline
change calculations. Prior to the 1890’s, the Perdido River channel followed Old
River along the north shoreline of Perdido Key, and Ono Island connected to
Alabama Point and the present-day Orange Beach shoreline. Before the early 1890’s
local interests cut a channel from the Bayou St. John area across to Perdido Pass near
what is now the western tip of Ono Island, more than ¾-mile east of the present
location of the Pass. Maps from 1911 show how the 1906 hurricane breached this
portion of Perdido Key, roughly ½-mile west of the FL/AL State Line (Mobile COE,
1967). Westerly migration of Perdido Key closed the breach before 1934, and in
1969-1970, the Pass was stabilized in its present location by the construction of two
rock jetties with a weir section in the east jetty. Douglass and Pickel (2000) describes
recent MHW shoreline changes as highly variable, with position changes of over
150ft since 1970, but exhibiting no major trend of erosion or accretion.
Perdido Key, FL
- 12 -
Perdido Key, FL
- 13 0
R037
R034
R031
Pensacola
Pass
60,000
2.3 MCy
recession
70,000
1985 Beach Disposal
(5.4 Mcy onshore, 4.0 Mcy nearshore)
1989-1991 Beach Disposal Project
10,000
20,000
30,000
40,000
50,000
1996-2005 (smoothed)
1974 to 1984 (Foster et al, 1999)
1950 to pre-Ivan (FSU BSRC)
1900 to pre-Ivan (FSU BSRC)
R040
Figure 3.2 MHW shoreline change rates for selected time periods, Perdido Key, FL. Data for the Alabama shoreline were
digitized from COE (1967) and recent aerial photography.
15
10
5
0
R001
-5
R004
-10
R007
-15
R010
-20
R022
1865/1867 to pre-Ivan (FSU BSRC)
R043
-25
R013
Historical Shoreline Change Rates
R061
-30
R016
Perdido Key Unit
R055
-35
R019
-40
R025
Escambia County
(developed)
R028
Perdido Key
State Park
Big Lagoon
R046
Escambia County
(developed)
PERDIDO
KEY
R049
-45
1906
breach
ALABAMA
FLORIDA
R052
pre-1890
1970
cut
jetty
construction
Perdido
Pass
Old River
1906
breach
R058
Rate of MHW Shoreline Change (ft/yr)
1906
breach
R064
advance
R067
¾ Browder and Dean (1999) estimated the long-term annualized cumulative erosion of
sand from the National Seashore area to be approximately 50,000 to 60,000 cy/yr. In
comparison, at least 60% or more of that value typically accretes along the remaining
eight miles of the Key. The writers estimate the long term net erosion rate along the
entire Florida shoreline of Perdido Key to be approximately 20,000 cy/yr, based on
the interpretation of volumetric changes from MHW shoreline changes.
¾ The first available set of beach profiles at Perdido Key was surveyed in 1974 for the
establishment of the 1975 Coastal Setback Line. Comparison of the 1974 to 1984
beach profile datasets indicates a net loss of almost 500,000 cy (50,000 cy/yr) of sand
along the National Seashore, extending westward into Gulf Beach to R-26. West of
R-26, volume change calculations suggest that almost the same volume of sand of
sand, accumulated along the shoreline to R-1.
While not specifically demonstrated by the available data, inspection of Figures 3.1 and
3.2 suggest that over time sand eroded from the eastern end of the island acts to feed and
stabilize the central and western portions of the island. This phenomenon is generally
supported by wave transformation and longshore transport analyses (Section 3.4), which
suggest similar patterns of increasing transport potential along the eastern end of the
island, causing erosion, and uniform or decreasing potential along the central and western
portions, promoting stability or accretion. Such a conclusion, however, assumes that
losses or gains of sand are produced strictly from longshore transport processes. While
longshore transport clearly does play a large role, other factors, such as storm-induced
overwash or possible onshore-directed contributions of sand (Stone et al, 1992), can
affect the sediment balance along the Perdido Key shoreline (discussed in Section 3.4).
It must be noted that the changes in shoreline position discussed above consider only the
Mean High Water contour3. A phenomenon characteristic of the geographic Panhandle
area is the difference in shoreline response between the lower and upper elevations of the
beach profile during storm events. During severe storms, the upper portions of the beach
profile and the frontal dunes experience substantial erosion. The eroded sand is
transported both landward and seaward. A portion of the sand transported seaward, if it
is not carried beyond the submerged primary bar in the profile, is readily available to
potentially be transported back toward the intertidal zone. Such a phenomenon
3
For some of the older datasets, the Mean High Water Line listed may actually represent a generalized
shoreline location, marking the upland limit of wave uprush (the wet/dry line) or a storm induced rack line
of debris. This possibility introduces uncertainty into the calculations of shoreline position and change rate.
Perdido Key, FL
- 14 -
frequently leads to minor accretion or reduced recession at the shoreline, as measured by
post-storm surveys. This phenomenon, which can mask the degree of shoreline recession
measured, can be clearly observed in the post-Frederic beach profile dataset and the postIvan dataset (discussed below).
Another factor that influences both the long- and short-term shoreline change data is the
occurrence of localized rhythmic variations in the shoreline that can create differences in
shoreline position of over one hundred feet along distances of as much as 1,000 ft (Figure
3.3). As indicated in Figure 3.3, the exact alongshore position of these features, some of
which tend to migrate alongshore, dramatically affects the storm-induced shoreline
response and level of protection to upland structures (see Section 3.5). These mega-cusp
features can produce substantial errors in shoreline position predictions based on longterm data. Dean (1999) investigated the variation in long-term shoreline position data of
the FDEP database for Escambia County. That research suggests that at Perdido Key a
prediction of shoreline location ten years in the future is expected to have as much as 38
feet of variation in the cross-shore direction, due in part to these rhythmic variations. A
30-yr prediction may have as much as 54 feet of variation (average of R-1 to R-32).
AL
FL
19 December 1998
Figure 3.3 Shoreline conditions at Perdido Key, FL, near the FL/AL State Line. Note
the cusp feature just east of the State Line, resulting in a large discrepancy in beach width
relative to the line of construction (photo 19 December 1998).
Perdido Key, FL
- 15 -
Also related to the variability in shoreline position measurements is the influence of
nearshore bathymetric features, specifically the impacts of shoreface-attached nearshore
ridges. Foster et al. (1999) make mention of numerous “hard features” occurring at
various points along the study area that influence the shape (but not necessarily the longterm rate of change) of the shoreline along segments of the beach as long as 2,000 to
3,000 ft. These submerged ridge features trend offshore obliquely from the shoreline
toward the southeast into waters depths of 50 ft or more, and occur in numerous locations
along the study area shoreline. One such ridge is believed to be responsible for the
creation of the large scale cusp feature shown in Figure 3.3 at the State Line. More
discussion of these features is found in Section 3.4.
3.2
National Seashore Beach Disposal Activities: 1984 to 1991
Chronic erosion along the National Seashore property resulted in the construction of two
major beach disposal projects there along (1985 and 1989-1990), using sediments
dredged from the entrance channel. Comparison of the 1984 pre-beach disposal data to
the 1996 shoreline illustrates the contribution of sand to the shoreline, totaling
approximately 7.7 million cubic yards of sand (both projects). Another 3.9 million cubic
yards of sand was placed just offshore of the National Seashore (R-46 to R-60) in a
nearshore berm in 1990-1991 (See Chapter 5). As depicted in Figures 3.1 and 3.2, the
addition of such large volumes of sand produced dramatic, but artificial, changes in
shoreline position and temporarily reversed the shoreline change rates along the National
Seashore property. Psuty and Jagger (1990) provide details of the 1985 beach disposal
project, which resulted in the placement of 2.3 million cubic yards of sand along the
shoreline immediately west of the Pass (R-65 to R-59).
Browder and Dean (1999) prepared a detailed sediment budget for Pensacola Pass based
on data from 1991 to 1998, describing the performance of the 1989-1991 Perdido Key
Beach Disposal Project. The beach disposal project resulted in the placement of
approximately 5.4 million cubic yards of sand along the eastern end of Perdido Key. It is
hypothesized that the placement of such a large volume of sand served to provide
additional sand to the shoreline west of the project area, which would positively bias any
recent MHW shoreline change estimates for the study (such as the 1996 and 1998 survey
data sets for the area). The influx of sand in such a relative short time frame, however,
may not have contributed significantly to the building of dunes alongshore. More
information on the performance of the 1989-1991 project can be found in Chapter 5.
Perdido Key, FL
- 16 -
3.3
Recent Storm-Induced Shoreline & Beach Volume Changes: 1995 to 2005
As depicted in Figures 3.1 and 3.2, MHW shoreline changes at Perdido Key over the last
eleven years differ substantially from the long-term trends due to storm impacts. Figure
3.4 plots the tracks of tropical storms and hurricanes that have significantly affected the
Perdido Key shoreline since 1995. Hurricane Ivan (2004) represents the single largest
impact to the beaches. Also notable are the impacts of Hurricanes Erin and Opal (1995),
Hurricane Georges (1998), and the storms of 2005 (Arlene, Cindy, Dennis, Katrina, Rita).
Since 1995, there have been least three named storm events formed in or passing through
the Gulf of Mexico each year (on average).
Hurricane Ivan made landfall on 16 September 2004, just west of Gulf Shores, AL,
approximately 18 to 19 miles west of the FL/AL State Line as a strong Category 3
hurricane. Reports from the National Weather Service place the radius of maximum
winds at roughly 25 to 30 miles, indicating the highest winds occurred just east of Gulf
Beach over the Perdido Key Unit of the National Seashore and the Grand Lagoon
neighborhood north of Big Lagoon (Figure 2.1). Sustained winds of almost 130 mph
were reported, and the open-coast storm surge is estimated to have been over +14 ft
NAVD88. The storm destroyed numerous single family structures and wood-frame
multi-family condominiums and completely destroyed the frontal dune along with
substantial portions of secondary dunes along the entire Perdido Key shoreline. In
numerous areas, wave overtopping and flooding across the entire island resulted in
damage to Perdido Key Drive, closing the road for several weeks following the storm.
By many statistical indicators, Hurricane Ivan represents a storm with a return period
near or exceeding 300 years (e.g. Dean and Chiu, 1986). The surge experienced during
Ivan exceeded the mapped FEMA 100-yr Base Flood Elevations for the area by several
feet in many areas and prompted a re-mapping of flood elevations. Escambia County
likewise adopted a new Ordinance (2006-4) for Coastal High Hazard Areas that now
requires that the elevation of the first habitable floor be three feet above the published
FEMA Base Flood Elevations, which range from +12ft to +16ft NAVD88 south of
Perdido Key Drive.
During the 2005 storm season, six named storms impacted the Perdido Key shoreline to
at least some degree between June and October 2005: T.S. Arlene (10 June), H. Cindy (5
July), H. Dennis (10 July), H. Katrina (29 August), H. Rita (20 September), and H.
Wilma (22 October). The intensity and frequency of these storms have restricted the
Perdido Key, FL
- 17 -
natural recovery of the beaches and dunes along the study area following Hurricane Ivan.
Hurricanes Dennis and Katrina both produced storm surges of eight to nine feet above
Mean Sea Level (estimated) at Perdido Key and resulted in Federal Emergency
Declarations for Escambia County. While the surge from these events did not result in
complete overtopping of the beach and upland flooding, the surge and accompanying
waves and runup eroded sand along the upper elevations of the beach, including the postIvan FEMA storm berms that were under construction along the developed portions of
the study shoreline.
Several datasets exist that document the impacts of the 1995-2005 storms. In addition to
the full beach profile surveys performed in 1996 and 2005, pre- and post-Hurricane Ivan
LIDAR surveys are available from the U.S. Army Corps of Engineers SHOALS program
at the Mobile District, COE. These data, high-resolution airborne scanning laser survey
data, were flown in May of 2004, then again in November/December 2004. These two
datasets generally capture the Ivan storm damages to the subaerial beaches of Perdido
Key. The May 2004 dataset was supplemented by a ground survey conducted by FDEP
in June 2004 at numerous R-monuments within the study limits.
90
95
100
80
85
NC
AL
30
TX
35
LA
SC
GA
25
Danny
FL
30
Atlantic
Ocean
Gulf of
Mexico
Katrina
20
Ivan
25
Opal
Erin
Rita
Isidore
Cindy
95
90
Dennis
Arlene
85
80
Georges
75
Figure 3.4 Tracks of hurricanes and tropical storms of particular significance to the
Perdido Key, FL, shoreline, 1995-2005.
Perdido Key, FL
- 18 -
3.3.1 Discussion -- Observations from the 1996 to 2005 timeframe are as follows (from
east to west, generally). Appendix A plots all historical beach profiles described herein.
¾ Along the National Seashore property, measured shoreline changes clearly indicate
the severe erosion that has occurred there along in the last ten years. The smoothed
shoreline change data shown in Figure 3.2 reveal recession rates exceeding -20 ft/yr
from R-55 eastward to R-64, with individual rates up to -50 ft/yr at R-62 and R-63.
Inspection of intermediate surveys and aerial photos between 1996 to 2005 indicates
that each successive storm from Hurricane Georges to Dennis has contributed
significantly to the severe erosion experienced at the eastern end of the GINS.
Further exacerbating the erosion is the interaction of this beach segment with the ebb
shoal complex at Pensacola Pass. The refractive effect of the ebb shoal creates an
area of diverging net sediment transport and a reversal in net sediment transport
direction along this segment, which receives no net inputs of sediment from
longshore transport due to the sink effect of the channel (see Section 3.4).
¾ During the intersurvey period from 1996 to 2005, the MHWL along the study area
between R-32 and R-1 retreated an average of over -1 ft/yr, compared to its long-term
trend of stability or minor shoreline advance of up to +4ft/yr (Figure 3.2 – smoothed
data). The average value includes significant variations of up to approximately ±21
ft/yr (unsmoothed data) at individual reference monuments during that time period,
and certain segments of the study area do indicate minor shoreline advance. This
variation is in keeping with the conclusions discussed in Section 3.1. These large
variations are at least partially attributed to the presence of mega-cusps alongshore.
¾ Figure 3.5 depicts pre-and post-Ivan contour changes along the study area measured
between the May 2004 and December 2004 SHOALS datasets. Consistent with
previous discussions, the average recession of the MHWL is only -16 ft, while the
recession of the +8ft NAVD88 dune toe contour averaged -91 ft. Figures 3.6 to 3.8
illustrate this phenomenon. At these locations, the post-storm beach is relatively
wide, however, the upper elevations of the beach were deflated by as much as four to
five feet in elevation. The +5ft contour advanced seaward by an average of 11ft, due
to the formation of a steep recovery berm at the post-storm wave uprush limit that
developed immediate after the storm, prior to the post-storm survey. Similar results
were reported along adjacent monitored beach restoration projects in Pensacola
Beach, FL, and Gulf Shores, AL, where surveys revealed the storm had planed the
upper beach off to typical slopes of 1:30 to 1:45 (e.g. Browder and Norton, 2005).
Perdido Key, FL
- 19 -
Perdido
Pass
R010
R007
R004
R001
R013
10,000
Perdido Key, FL
- 20 -
60,000
Average shoreline change at +8ft NAVD88: -91 ft
Average shoreline change at MHW (+0.8 NAVD88): -16 ft
70,000
Pensacola
Pass
Shoreline Position Changes
+8ft NAVD88
+5 ft NAVD88
MHW (+0.8ft NAVD88)
20,000
30,000
40,000
50,000
R022
(+) Accretion
Figure 3.5 Upper beach shoreline changes along western Perdido Key, FL, between May 2004 (pre-Ivan) and December 2004
(post-Ivan). Data provided by the U.S. Army Corps of Engineers SHOALS program.
0
R016
-200
R019
-160
R025
-120
R034
-80
R037
Average shoreline change at +5ft NAVD88: +11 ft
R040
-40
R031
Post-Storm Sand
Stockpile at R-23
R043
0
R028
Perdido Key Unit
R046
Escambia County
(developed)
R049
Perdido Key
State Park
Big Lagoon
R052
Escambia County
(developed)
PERDIDO
KEY
R055
40
ALABAMA
FLORIDA
R058
80
120
Old River
R061
Shoreline Position Change (ft)
R064
(-) Erosion
R067
Figure 3.6 Pre- and post-Ivan shoreline conditions just west of R-007 at Perdido Key,
FL. Comparison of the two photos illustrates erosion of the upper beach and the total
loss of the vegetated dune field. Note the red house in the inset photo, which was swept
off its pilings during Ivan, and the initial formation of the recovery berm (post-Ivan photo
– FDEP BBCS).
¾ The profile deflation during Ivan, as demonstrated in Figure 3.8, extended northward
well beyond the typical seaward limit of construction. This deflation during the storm
allowed the transmission of larger storm waves farther landward across the profile,
where they then impacted upland infrastructure. Referring to Figures 3.7 and 3.8, the
profile deflation at the seaward limit of construction reduced the grade elevation from
+13ft prior to the storm to +7ft during and after the storm. Assuming a +14ft surge, it
is entirely possibly that waves as high as 5.5 ft passed the typical line of construction
and impacted the structures seen in Figure 3.7, which were substantially damaged
during Ivan. Furthermore, following traditional rules of thumb for determining wave
crest elevations, it is estimated that storm waves during Ivan may have impacted
structures at elevations as high as +18 ft, as suggested by the pattern of damage seen
in Figure 3.7 (particularly the central structure).
Perdido Key, FL
- 21 -
Figure 3.7 Structural damage and shoreline conditions at FDEP monument R-27 along
Perdido Key Drive following Hurricane Ivan. The only structure still in place in this
photo after 18 months is the blue-roofed home on the north side of the road. The
shopping complex on the north side of the road was demolished subsequent to Ivan
(photo – FDEP BBCS).
R-027 Escambia County, FL
Elevation (ft, NAVD88)
20
May 2004 (LIDAR)
December 2004 (LIDAR)
15
July 2005
10
Natural Recovery Berm
5
0
-5
Seaward edge of construction (approx.)
-10
0
200
400
600
800
1000
Offshore distance from monument (ft)
Figure 3.8 Comparison of pre- and post-Ivan beach profiles at Perdido Key R-monument
R-027. Storm damage from Hurricane Ivan resulted in the complete loss of the frontal
dune (Figure 3.7). Following Hurricane Dennis in July 2005, the primary bar crest was
located roughly 200 ft seaward of its May 2004 location.
Perdido Key, FL
- 22 -
¾ Based upon the May-December 2004 SHOALS data, the beaches from R-32 to R-1
experienced a loss of over 26 cy/ft during this period as the upper beach and primary
dunes were planed off. Storm damages from Ivan resulted in the loss of almost
840,000 cy from the upper beach along the study area4. West of the study area,
surveys along the Alabama portion of Perdido Key reveal average losses of almost 23
cy/ft above 0.0 NAVD88 (April 2003 to January 2005, Olsen Associates, 2006b).
¾ Volume changes measured between the December 2004 LIDAR survey and the July
2005 profile survey indicate that 355,000 cy of sand accumulated between R-32 and
R-1 above 0.0 NAVD88. Almost 140,000 cy of this volume are attributed to the sand
recovery operations conducted by Escambia County along the developed beach
segments. In some areas, overwash fans extended over 1,000 ft from the shoreline,
across the island and into Old River and Big Lagoon. Approximately 110,300 cy
were returned from the streets/right-of-ways (ROW), and from private properties
where Right-of-Entry (ROE) was granted, (Figure 3.9). Another 29,300 cy were
obtained from maintenance dredging of the Gulf Intracoastal Waterway west of the
Theo Barrs bridge.
¾ Sand eroded from the upper beach and primary dunes was carried both landward into
overwash fans and Gulfward beyond the primary bar, where its return to the beach is
expected to be slow (on the order of years, assuming no further severe events).
Figure 3.10 depicts the large volume of sand lying beyond the primary bar in each
example profile, shown by the July 2005 post-Dennis survey (see Appendix A for all
profiles). Comparison of the February 1996 survey to the July 2005 profile survey
suggests that as much as 800,000 cy of sand lies in the area seaward of the primary
bar. Along the Alabama portion of the shoreline the net volume change Gulfward to
survey closure from April 2003 to January 2005, is near zero, although over 260,000
cy of sand were eroded from the upper beach. More discussion of the potential fate
of this material can be found at the end of this section.
¾ The City of Orange Beach, AL recently completed a beach nourishment project along
the first 1.1 miles of the shoreline immediately west of the FL/AL State Line. This
project added over 700,000 cubic yards of sand in February 2005, and was
renourished following Hurricanes Dennis and Katrina with an additional 140,000 cy
in January/February 2006 (Olsen Associates, 2006b).
4
Water clarity at the time of the December 2004 survey limited consistent data collection to depths above
-1.0ft NAVD88 (approx.).
Perdido Key, FL
- 23 -
Figure 3.9 Pre- and Post-Hurricane Ivan aerial photography, Perdido Key, FL (R-1 to R3). Note the extent of storm overwash from the beaches and the loss of dune vegetation
due to the hurricane.
Perdido Key, FL
- 24 -
20
May 2004 (LIDAR)
FEMA Emergency Berm
15
July 2005
10
5
0
-5
Seaward edge of construction (approx.)
-10
200
400
600
800
1000
20
1200
May 2004 (LIDAR)
15
July 2005
10
5
0
-5
-10
0
200
400
600
800
20
1000
May 2004 (LIDAR)
FEMA Emergency Berm
15
July 2005
10
5
0
-5
Seaward edge of construction (approx)
-10
0
200
400
600
800
1000
Figure 3.10 Example profiles at Perdido Key, FL.
Perdido Key, FL
- 25 -
¾ Another major element of damage caused by the 2004/2005 hurricanes is the loss of
dune vegetation and environmental habitat. Comparison of aerial photography from
April 2004 and September 2005 reveals the loss of over 104 acres of vegetated frontal
dune areas. On average, a vegetated zone approximately 125 ft wide from the prestorm seaward edge of vegetation landward was either eroded away or significantly
buried by overwash. As depicted by the pre- to post-storm images shown in Figure
3.11 and in previous figures, vegetated zones Gulfward of the developed areas
(generally, seaward of the line of construction), were completely stripped of native
vegetation. The reader is referred to Appendix E for post-Katrina aerial photography
of the study area.
Figure 3.11 Pre- and post-Hurricane Ivan shoreline conditions near R-23 (denoted by
the arrows), Perdido Key, FL. Note the near-planar condition of the post-storm beach
and the lack of any remaining dune vegetation. The inset photo depicts pre-storm
conditions. The initial formation of a post-storm recovery berm near the waterline is also
evident (post-Ivan photo: FDEP BBCS).
Perdido Key, FL
- 26 -
¾ Figure 3.12 depicts an aerial photo history of a portion of Perdido Key State Park,
centered near the eastern primary parking area at R-18, from 1984 to 2005 (postKatrina). The 1984 aerial shows the impacts from Hurricane Frederic in 1979. In
that storm, it appears the storm surge and waves stripped vegetation back to the
landward edge of the existing series of decks and gazebos at the Park. The seaward
edge of vegetation that survived Frederic is clearly visible in April 2004, fronted by
new growth areas. Following Ivan, Dennis and, Katrina, dune erosion and overwash
resulted in the near-complete loss of vegetation landward beyond the Hurricane
Frederic limit.
¾ These denuded areas all lie within existing or proposed units of critical habitat for the
Perdido Key beach mouse. One of the primary constituent elements identified by the
U.S. Fish and Wildlife Service for the survival of the species is a habitat amongst the
primary and secondary dunes that provides food and shelter (Federal Register, 15
December 2005). Along the zone described above, the characteristics of that
constituent element have been nearly eliminated by the frequent storm impact. Many
portions of the scrub dune constituent element have been destroyed landward of the
primary and secondary dunes, as well in areas of localized washovers. Additional
discussion of environmental considerations can be found in Appendix F.
¾ Some level of natural recovery of vegetation has occurred along the study area,
typically landward of the seaward edge of construction (in the State Park, etc.).
Additionally, Escambia County plans to revegetate the constructed FEMA emergency
berm along the developed beach segments (see below), and Perdido Key State Park
plans to revegetate broad areas of the Park in an attempt to reconstitute beach mice
habitat in the primary dune area and to assist in the protection of remaining habitat
northward thereof.
Perdido Key, FL
- 27 -
Figure 3.11 Aerial photographs of parking area at R-18, Perdido Key State Park, from
1984, 2004 (pre-Ivan), and 2005 (post-Katrina). The erosional scarp from Hurricane
Frederic in 1979 is clearly visible in the upper frame, and can still be identified today.
Perdido Key, FL
- 28 -
3.3.2 July 2005 to Present -- Following the July 2005 survey, Hurricane Katrina passed
offshore of Perdido Key in late August, resulting in large storm waves and an estimated
surge of roughly nine feet along the study area. Runup from storm waves impacted the
ongoing FEMA berm construction process and resulted in additional erosion of the upper
beach. The original post-Ivan FEMA berm was designed at 163,000 cubic yards to be
placed along the 4.4-miles of developed beach-front. The impacts of Dennis and Katrina
resulted in an authorization to add an additional 100,000 cy of sand to the emergency
berms, augmented by a State contribution of another 50,000 cy. Sand for the postDennis/Katrina berm work was obtained from dredge stockpiles from the GIWW. With
the combined placement of 139,600 cy after Ivan, the impacts of Dennis and Katrina, and
the placement of an additional 116,000 cy after Dennis and Katrina, it is estimated that
perhaps as much as 256,600 cy of sand has been returned to or placed along the beaches
in the developed areas since Ivan. As of this writing the majority of the sand is believed
to still lie within the general template of the FEMA berm, constructed just south of the
seaward edge of development.
3.3.3 Potential for Natural Recovery -- Inspection of the available beach profile data
through July 2005 reveals the presence of a substantial volume of sand at the beach toe
along the seaward face of the primary bar in the beach profile (Appendix A). This sand
was transported offshore to this area principally during Hurricane Ivan and Hurricane
Dennis. Comparison of February 1996 and July 2005 beach profile data suggest that
along the entire 6-mi study area, the excess volume of sand in this region may be 800,000
to 900,000 cubic yards (over 25 cy/ft on average). The excess accumulation of sand lies
in an area seaward of the envelope of prior survey data. Comparison of pre- and postHurricane Katrina beach profile data along the adjacent Perdido Key, AL shoreline
indicate that Katrina exacerbated this problem, pushing the bar crest 50 to 100 ft farther
offshore and depositing additional sand on the seaward face of the bar (Olsen Associates,
2006b).
Of interest is the potential for portions of this material to eventually migrate ashore and
contribute to the rebuilding of the upper elevations of the beach via summer wave berm
building and aeolian transport into the dunes. It is likely that some fraction of the
material would migrate ashore under calmer wave conditions. Accomplishment of this
task would require several years of storm-free weather; the likelihood of such a weather
window is not easily predicted. Additionally, it is not expected that all the sand deposited
seaward of the bar by recent storms would return to the beaches. While storm waves and
their associated return flow were clearly able to transport sand offshore and generally
Perdido Key, FL
- 29 -
down-slope to these depths, summer wave action may not typically reach these depths
with sufficient strength to transport sand landward up the seaward slope of the bar.
Long-term volumetric and shoreline change data suggest that that the practical average
annual depth of sand motion might only reach approximately 13 ft in this area (Browder
and Dean, 1999)5. This suggests that some fraction of the sand deposited near the toe of
slope would be extremely slow to migrate and may not be moved onshore.
Olsen Associates (2001a) describes a similar scenario at Pensacola Beach, FL, following
Hurricane Georges in 1998. Pre- and post-Georges survey data indicated that over 1.8
million cubic yards of sand were displaced offshore during the storm (roughly 55 to 60
cy/ft). The primary bar crest migrated over 360 ft seaward during the storm and sand was
deposited at depths of 26 ft or deeper. During the subsequent 22-month period, survey
data indicated a loss of over one million cubic yards of sand from the submerged portions
of the profile, while the upper portions of the beach experienced only 153,000 cy of
natural accretion during the same period of time. It is hypothesized that sand in this area
can be partially mobilized by wave action in this area, where it is then available for
transport out of the surveyed project area by longshore- or offshore-directed currents.
Similar results have been documented in Panama City Beach, FL, during Hurricane Ivan
(Keehn and Armbruster, 2005). Pre- to post-Ivan surveys of the beaches revealed the
average loss of 26 cy/ft of sand in the storm. Surveys indicate significant deposition of
sand at depths deeper than the 20-ft design depth of closure for the project. Some
profiles indicated deposition from the storm to depths of 40 ft. The writes refer to the
volume of sand deposited below 20ft depth as “semi-permanent loss.”
In summary, the series of storm events in 2005 has prevented any meaningful recovery of
the beaches following Hurricane Ivan, and in some instances has exacerbated the loss of
sand from the system to offshore areas. Even under ideal future weather circumstances,
which cannot be relied upon for purposes of storm protection in the near-term, only a
portion of the sand transported offshore would be expected to return to higher elevations
along the beach profile. Thus, for purposes of beach restoration along the Perdido Key,
FL, study area, it is not recommended that this volume of sand be relied upon to provide
any level of meaningful assistance in achieving the required level of storm protection
identified herein.
5
Note that this description differs significantly from other definitions of “depth of closure” or “depth of
limiting sediment motion.” The value described herein was derived by comparing measured long-term
volumetric changes to measured shoreline changes to estimate a corresponding active height of profile
(Browder and Dean, 1999).
Perdido Key, FL
- 30 -
3.4
Wave Transformation & Alongshore Transport Modeling
Using the STWAVE numerical model (Smith et al., 1999), representative offshore wave
conditions were transformed across the nearshore bathymetry off Perdido Key to the
point of wave breaking at the shoreline (Appendix C). Using the breaking wave data,
estimates of the longshore transport potential were calculated for each representative
wave case and an average annual littoral transport curve was generated. Inspection of the
computed gradients in alongshore transport reveals areas that are expected to be
erosional, stable, or accretional (in the long-term, generally).
Results of the wave transformation analysis compare well with the long-term shoreline
change data presented previously6. The shallow waters of the ebb shoal at Pensacola
Pass tend to refract incident waves eastward, toward and into the Pass. The eastward
refractive effect is strong enough that a reversal in the direction of net longshore transport
is predicted in the vicinity of R-59 to R-61, east of which sand is directed back toward the
navigation channel (in the net). Areas of transport divergence are highly erosional, as
sand is transported away in both directions. Not coincidentally, this portion of the
shoreline is historically the most eroded segment of Perdido Key. West of the reversal
area, the resultant shadow zone north and west of the ebb shoal results in an area of
increasing westerly directed transport, as the wave climate returns to its non-shoalaffected condition. Proceeding westward, the increase in westerly net transport continues
along the National Seashore property into the eastern developed segment of Gulf Beach.
Westward thereof, the average annual net transport indicates uniform or slightly
decreasing transport conditions over the remaining five miles of shoreline to the State
Line. Such a condition, exclusive of storm impacts, suggests shoreline stability or
accretion over the long term.
Superimposed on that generally stable to accretional trend, however, are the localized
effects of shoreface-attached oblique sand ridges. Similar to the influence of the Pass ebb
shoal, these ridges can be shown to affect the local wave climate via wave refraction,
pulling wave crests toward the shallower ridge centerlines. This refraction effect results
in the creation of paired accretional and erosional zones alongshore. As the waves are
pulled toward the ridge, an area of convergent transport is created alongshore, resulting in
a stable, depositional zone. Westward thereof, the area of wave divergence left from the
6
For a shoreline with no coastal structures or other substantial cross-shore influences upon littoral
processes, this is not an unexpected result. The wave transformation analysis uses “average annual wave
conditions” to determine typical wave breaking and littoral transport conditions. Thus, the gradients in
alongshore transport should correspond well with the overall, long-term shoreline change trends.
Perdido Key, FL
- 31 -
attractive effect of the ridge produces a condition alongshore of increasing wave breaking
energy from east to west, which ultimately produces a positive gradient in alongshore
transport. The positive gradient, i.e. an increasing ability to transport sand proceeding
westward, results in the erosion of the shoreline to meet the transport demand. This
erosional trend persists westward until the localized effect of the ridge is escaped,
typically on the order of a thousand to several thousand feet west of the ridge. Such
effects can be accentuated during smaller storm events, and as such the exact location of
the accretional/erosional area shifts depending on the incident direction of the waves. In
general, however, the shoreline from the ridge crest’s point of attachment, moving
eastward, exhibits a stronger tendency for stability or accretion, while the western side of
the ridge trends more toward recession.
Such a feature exists along the western limit of the study area. The ridge near the State
Line, which attaches near R-1 or R-2, produces relative stability there along, but produces
an erosional area along the Alabama shoreline of Perdido Key that is particularly
noticeable roughly 1,000 west of the State Line. A smaller ridge feature affects the
shoreline near R-8 to R-11 (Foster et al. (1999) note the feature at R-11 near the Eden
Condominium), and several other smaller ridges are found alongshore. It must be
stressed, however, that the submerged ridge effect is realized primarily in the accretion or
erosion of sand along the lower elevations of the beach profile. During severe storm
events with significant surge levels, the refractive effect of the ridge is diminished, and in
any event the ridge has little to no influence on the surge level itself.
3.4.1 Estimates of Longshore Transport -- Numerous researchers have published
estimates of the annual net longshore transport rate in the Perdido Key/Pensacola Pass
area. These estimates range from 65,400 cy/yr to over 300,000 cy/yr, all westerly
directed (see Appendix C). Most estimates of transport tend toward smaller values of
less than 100,000 cy/yr. Browder and Dean (1999) compiled estimates of the net
longshore transport rate and performed a sediment budget analysis of the Pass which
suggested net transport rates on the order of 50,000 to 70,000 cy/yr, westerly directed.
Douglass (2001) estimated the net transport at Perdido Pass to be approximately 190,000
cy/yr.
3.4.2 Generalized Sediment Budget -- Acknowledging that the largest contributions to
shoreline and beach volume changes at Perdido Key are the impacts of major storms, an
ambient long-term sediment budget can be developed from the STWAVE/littoral
transport analysis to describe the background tendencies of the island’s Gulf of Mexico
Perdido Key, FL
- 32 -
littoral system. To relate the predicted longshore transport curve to actual volumetric
changes alongshore, an assumed “ambient” net rate of 80,000 cy/yr was applied to the
predicted longshore transport curve (Appendix C). Further, most studies of Pensacola
Pass agree that there is little if any bypassing of sand to or from Santa Rosa Island across
the navigation channel. With these assumptions, the “erosional demand” of sand from
the beach between the transport reversal area westward along the National Seashore
property and into the developed areas of Gulf Beach exceeds 80,000 cy/yr. In
comparison, Browder and Dean (1999) estimated the long-term annualized cumulative
erosion of sand from the National Seashore area to be between 50,000 and 60,000 cy/yr.
Over the long-term, it is hypothesized that less than half of the sand volume eroded from
the eastern seven miles of the island is transported westward and ultimately deposited
along the remaining eight miles of Perdido Key, as the net longshore transport decreases
to roughly 50,000 to 60,000 cy/yr near the State Line. The net transport is hypothesized
to increase sharply just east of Perdido Pass to as much as 98,000 cy/yr (Stone et al.,
1992). Douglass (2001) estimates the net longshore transport at Perdido Pass to be as
high as 190,000 cy/yr.
As discussed previously, the presence of shoreface-attached oblique sand ridges generate
localized gradients in the longshore transport curve. The predicted effect of these ridges
on the local sediment budget can be extreme, indicating losses of tens of thousands of
cubic yards per year along relatively short segments of the shoreline. In reality, the
effects of the ridges upon the shoreline is offset by the changes in shoreline orientation
that they cause. The resultant cuspate formation of the beach is a process in which the
shoreline rotates to align itself with the incoming ridge-refracted waves, thus reducing the
longshore transport potential until an equilibrium condition is established for a particular
wave condition. The equilibrium condition typically includes cuspate horn and
embayment features, which can produce substantial variations in local shoreline width
(discussed previously). Other processes, such as non-wave induced currents and small
storm events, result in the natural diffusion of sand into the eroded areas from adjacent
shorelines to reduce points of high curvature of the shoreline.
The preceding discussion of the sediment budget is based solely on gradients in the
predicted average annual longshore sand transport potential. Again, the larger influence
of individual storm impacts is not considered in the discussion of these typical, average
annual background conditions. Additionally, previous investigations (e.g. Stone et al.,
1992) have suggested the presence of onshore-directed transport across the inner
continental shelf that supplies sand to the central, Gulf Beach portion of Perdido Key.
Perdido Key, FL
- 33 -
This mechanism is difficult to directly demonstrate, but is indirectly indicated by the
increased presence of carbonate material in beach samples along Perdido Key, which is
opined to be provided only from the inner shelf, and the inability to satisfy sediment
budget calculations by any other source (Stone et al., 1992).
3.5
Observations from Storm Recession Modeling
As discussed in preceding sections, the primary factor influencing shoreline and beach
profile change along the six-mile study area of Perdido Key is the impact of recent major
storm events. The numerical model EDUNE (Kriebel, 1994) was utilized to prepare
predictions of storm-induced beach profile change upon the existing shoreline conditions
and a suite of dune restoration and beach nourishment alternatives (Appendix D). A
range of storm intensities, with storm surges from +4 to +14 ft above MSL, was applied
to each pre-storm beach profile configuration. To verify its performance, the EDUNE
model was applied to simulate the impacts of Hurricane Ivan upon the shoreline at
Perdido Key State Park. The model was found to reproduce measured dune and upper
beach recession to within 15% or less in most cases simulated.
Figure 3.12 depicts a prediction of the expected return period interval of various levels of
storm surge at Perdido Key, FL (Dean and Chiu, 1986)7. These data, used to establish the
surge level in the EDUNE model, are generated from a probabilistic model of storm
track, intensity and landfall location, based upon a ranking of storm conditions known to
have impacted the area between the late 1800’s and the mid 1980’s. Also plotted in
Figure 3.12 are measured and estimated storm surge values from several recent storm
events at Perdido Key, and the published 100-yr Stillwater surge elevation from FEMA.
Inspection of Figure 3.12 highlights the recent series of “severe” storm events
experienced along the Perdido Key shoreline. Beginning with Hurricane Georges in
1998, the Perdido Key area has experienced at least five storm events exceeding the
predicted surge level of a “20-yr storm event” in the eight tropical storm seasons of 19982005. Perhaps as many as four storms have exceeded the surge predicted for a 30-yr
event since 2002. According to the scale shown in Figure 3.12, Hurricane Ivan may be
classified as high as a 300+ yr event for Perdido Key.
7
Consistent with Dean and Chiu (1986), storm surge is defined herein as the maximum “stillwater”
surface elevation achieved during a storm event, inclusive of astronomical tides, barometric pressure
effects, and wave- and wind-induced setup. The levels reported do not include the effects of the passage of
individual waves (referred to in FEMA Flood Insurance Rate Maps (FIRMs) as the Base Flood Elevation).
Perdido Key, FL
- 34 -
Combined Total Storm Tide Elevation (ft, NAVD88)
16
Perdido Key, FL
14
Hurricane Ivan (2004)
Dean & Chiu (1986)
12
FEMA 100-yr Stillwater Flood Elevation
10
Hurricane Katrina (2005)
Hurricane Dennis (2005) & T.S. Isidore (2002)
Hurricane Georges (1998)
8
6
4
20
30
40
50 60 70 80 90
200
300
400 500
10
100
Return Period (years)
Figure 3.12 Predicted return period intervals for increasing levels of combined total
storm surge at Perdido Key, FL (adapted from Dean and Chiu, 1986 – model line 1).
The data presented in Figure 3.12 serve, however, to illustrate that the prediction of a
“100-yr storm event” or any other condition is simply a statistical indicator, used most
often to establish flood and storm insurance rates8. The conditions associated with an Nyr event are based upon the database of storm information available at the time of the
prediction. With the recent series of severe storms in this area, the conditions describing
a given return period interval may likely require revision to smaller surge levels. Herein,
terms such as “30-yr event” and “100-yr event” are used for familiarity, but each refers
more correctly to a specific storm surge elevation rather than an expected return period.
8
The term refers to a storm condition that has a 1% chance of occurrence in any given year. In terms of a
practical project time scale of ten years, the probability of occurrence of a ten-year event sometime during
that 10-yr period is 0.651 (65.1% chance). The probability of occurrence of a 30-yr event in any ten-yr
period is 0.287 (28.7% chance), and the probability that a 100-yr event will occur in any ten-yr period is
0.096 (9.6% chance). P{F-yr event in n years} = 1 - (1 - 1/F)n. This discussion does not, however, provide
guidance as to what storm condition (surge level, wave height, etc.) constitutes each n-year event.
Perdido Key, FL
- 35 -
Figure 3.13 depicts the predicted changes for a typical beach profile along the study area.
The typical profile represents a composite of profiles from R-1 to R-32. A FEMA berm
was included in the typical profile to represent the developed areas, where the postKatrina berm has been completed. Simulations of a typical no-FEMA-berm profile were
also included to provide information for the Perdido Key State Park segment (Appendix
D). Inspection of Figure 3.13 reveals how the model simulates the planing off of the
upper beach contours. In the present instance, EDUNE produces post-storm beach
profile slopes of roughly 1:30 to 1:35 (v:h) , consistent with the post-storm survey data
collected in the Pensacola Beach and Gulf Shores areas after Hurricane Ivan (e.g.
Douglass and Browder, 2005). A 30-yr storm event with a +8.2ft storm surge is expected
to completely erode the FEMA berm and result in recession of the dune toe by roughly 60
ft (typical). A 100-yr event is predicted to cause further landward retreat and profile
deflation, averaging 75 to 80 ft of dune recession. As discussed in preceding sections of
this report, the planar configuration of the post-storm beach results in large recession
values along the upper elevations of the beach and dunes, but produces little or no
recession of the Mean High Water shoreline. As seen in Figure 3.13, in most instances
EDUNE will predict that lower, submerged contours will actually advance seaward
during the storm, due to the input of sand eroded from higher elevations along the profile.
The upper frame of Figure 3.13 illustrates, for existing conditions, that most portions of
the study area can survive storm surges of +4 to +6 ft with only minimal reshaping of the
beach profile. Simulations indicate that the existing FEMA berm will be eroded
completely during a storm event with a surge level approaching +7ft (just under a “20-yr”
event per Figure 3.12). Storm waves riding atop the +7ft surge are predicted to impact
the berm and eventually remove it, leading to the lowering of the beach profile at the
seaward edge of construction9 and northward thereof. At that point in the storm, upland
infrastructure becomes increasingly exposed to direct storm wave impacts and critical
habitat areas experience added inundation and overwash. Simulations predict that upon
the loss of the FEMA berm in the 20-yr event, the beach profile may experience two feet
or more of deflation at the seaward edge of construction. The predicted profile deflation
may extend for 20 to 30 ft landward, impacting upland infrastructure (pools, parking lots,
piping, foundations, etc.). For a 100-yr event, the deflation at the seaward line of
construction may exceed three feet, and the erosion may extend over 60 feet from the
construction line (Figure 3.14). Landward of the limit of deflation, overwash deposition
occurs. This phenomenon is not modeled by EDUNE (discussion – Appendix D).
9
Along the Perdido Key shoreline, the seaward limit of construction is typically the 1975 Coastal Setback
Line.
Perdido Key, FL
- 36 -
20
seaward edge of construction (typ.)
15
Typical Profile
No Project
FEMA Berm
100
10
50
30 20
10
5
MHW
0
pre-storm profile
distorted scale: 1V:10H
-5
400
500
20
600
700
Offshore Distance, Arbitrary Baseline (ft)
800
900
Typical Profile
60 cy/ft Project
15
50
100
10
30
20
10
equilibrated beach fill profile
5
MHW
0
pre-storm profile
-5
400
500
600
700
Offshore Distance, Arbitrary Baseline (ft)
800
900
Figure 3.13 EDUNE Numerical simulations of predicted beach profile response to
various storm levels. The upper frame indicates the prediction of performance under
typical existing conditions (July 2005 survey, with FEMA Berm). The lower frame
depicts a prediction of the beach profile response to the same series of storms, following
the placement and equilibration of a 60 cubic yard per front foot beach fill project.
Perdido Key, FL
- 37 -
Figure 3.14 Destruction of the Flamingo Key Condominium on Perdido Key, AL, from
storm surge impacts from Hurricane Ivan. The building stood approximately 1,000ft
west of the FL/AL State Line. Note the pools fronting the damaged buildings, collapsed
from profile deflation.
Figure 3.13 also depicts the predicted beach profile response associated with the
placement of a modest beach fill project with a typical fill density of 60 cubic yards per
lineal foot of shoreline. The simulations suggest that the modest beach fill can provide
significant storm protection benefits. The buffer represented by the additional sand
placement is predicted to protect the FEMA berm and upland infrastructure from the
impacts of the 20-yr and 30-yr storms. With the 60 cy/ft project in place, the 100-yr
storm event is predicted to erode the FEMA berm but not cause any profile deflation
landward of the construction line.
Additional discussion of the EDUNE modeling results and the selection and design of
beach fill alternatives can be found in subsequent Chapters and in Appendix D.
Perdido Key, FL
- 38 -
Alongshore Variation in Shoreline Response to Storms – Consistent with the historical
analyses of shoreline position and beach volume, the storm response modeling
demonstrates a distinct alongshore trend in the level of storm impact vulnerability
(Appendix D). The eastern developed segment of the study area is at a higher level of
risk from storm damage than the central (PKSP) and western segments of the study area,
for the same storm conditions. Assuming the post-Katrina FEMA berm is uniformly
constructed throughout the developed areas, the primary variable affecting the level of
vulnerability to upland infrastructure is the total volume of sand between the edge of
construction and the pre-storm waterline. Along the length of the study area, this
translates most directly to beach width. Appendix E provides controlled aerial
photography of the study area, collected on 3 September 2005 after Hurricane Katrina.
The aerials depict the condition of the shoreline and the locations of the 1975 Coastal
Setback Line and the 1986 Coastal Construction Control Line (CCCL).
Based upon the distance from the 1975 Coastal Setback Line to the July 2005 MHWL,
the structure fronting beach widths along the eastern segment average approximately 235
ft. Beach widths in this segment range from less than 190 ft to as much as 290 ft. Along
Perdido Key State Park (R-13 to R-21), the distance averages 270 ft and ranges from 225
to over 310 ft, due in part to local variations in the location of the Setback line. Across
the western segment of the study area10, structure fronting beach widths average 275 ft
and range from 230 ft to nearly 320 ft.
Along the widest profiles, which occur in all three segments, storm-induced profile
deflation can be limited to areas Gulfward of the line of construction. However, in nearly
every instance the FEMA berm itself cannot withstand an event with a storm surge higher
than approximately six to seven feet. While the beaches along the western portion of the
study are wider in comparison to the eastern end, the beach berm along the entire study
area is relatively low, with elevations from approximately +8ft at the toe of the
dune/FEMA berm to a seaward berm edge elevation of roughly +4ft (Appendix A). Such
a condition allows for frequent overtopping of the recreational beach berm. Following
the loss of the FEMA berm, if not rebuilt, subsequent storm impacts will result in
additional deflation of the beach profile landward of the line of construction, increasing
damage to upland infrastructure and additional overwash and inundation of
environmental habitat areas.
10
Calculations exclude the westernmost 2,500 ft (R-1, R-2, and R-3). This segment is influenced by sand
diffusion from the recently constructed nourishment project along the Alabama portion of Perdido Key.
Inclusion of the segment raises the average beach width along the western segment to over 290 ft.
Perdido Key, FL
- 39 -
In general, the east-to-west increase in beach width raises the level of storm protection
afforded by the existing beach conditions. However, local variations in the shoreline
location caused by beach cusps and other mechanisms can dramatically alter the width of
the pre-storm beach, raising or lowering the level of protection available at any given
point alongshore. In addition, variations in the cross-shore location of the Setback line
and individual structures also affect the level of exposure of any single structure (see
Appendix E). Figures 3.15 and 3.16 depict profile and plan views of storm damage near
R-31 at the Vista Del Mar Condominium. This structure experienced severe structural
damage and failure during Hurricane Ivan as a result of the loss of sand underneath the
structure. The southern wing of the condominium extended Gulfward roughly 70 ft
beyond the typical limit of construction. The deflation of the profile around this wing of
the building, estimated to be at least three feet, and the impacts of storm waves during
Ivan resulted in the settlement of the entire wing. The entire condominium building has
since been demolished.
Figure 3.15 Structural damage to the Vista del Mar Condominium on Perdido Key, FL.
The building was damaged during Hurricane Ivan in September 2004 (photo: 19 January
2005).
Perdido Key, FL
- 40 -
Figure 3.16 April 2004 pre-Ivan and September 2005 post-Katrina condition photos of
the eastern boundary of the Gulf Beach study area at the entrance to the Perdido Key Unit
of the Gulf Islands National Seashore. The Vista Del Mar condominium, which was
located seaward of the typical line of construction, was severely damaged in Hurricane
Ivan (Figure 3.15) and was eventually demolished after Hurricane Katrina.
Perdido Key, FL
- 41 -
4.0
ALTERNATIVES FOR BEACH RESTORATION AT PERDIDO KEY, FL
The previous chapter summarizes the historical and present condition of the beaches at
Perdido Key and describes the present level of vulnerability of the project area to future
storm impacts. This chapter discusses various alternatives proposed to address the need
to increase the storm protection function of the beaches and mitigate for the damages
cause during the 2004-2005 hurricane seasons, while maintaining or improving the
recreational amenity value of the beach.
As described in the previous chapter, the recent series of storms, beginning with
Hurricane Ivan in 2004, have removed large volumes of sand from the upper elevations
of the beach, eroding the natural dunes and substantially lowering the overall elevation of
the dry beach. The partial restoration of the dunes via the construction of the FEMA
berms has replaced roughly one-third of the volume lost from the upper beaches of the
study area. While the study area has not experienced substantial shoreline recession, the
recent loss of beach volume and the lowering of the dry beach leave upland infrastructure
and habitat vulnerable to storm wave impacts and inundation from events generating
storm surges of +7ft or higher (typically the 20-yr event or greater). The post-Katrina
FEMA berms constructed along much of the developed area represent protection against
storm events with surges lower than approximately six to seven feet. For more severe
events, the FEMA berm is expected to be completely eroded, after which time erosion
and profile deflation landward of the seaward edge of construction becomes more likely
with subsequent events.
Given the mild background conditions, the level of vulnerability is then weighed against
the likelihood of future storm impacts. As described in the previous chapter, existing
published estimates of the frequency of occurrence of storms with a given surge level
have underestimated these frequencies in recent years. At Perdido Key, the recreational
beach berm has been significantly overtopped at least five times during the last two
tropical storm seasons. To overtop the beach berm requires a storm surge of +4 to +5ft,
an event that has been previously estimated to have only a 10% chance of occurrence in
any given year. Similarly, the “20-yr event,” estimated to have a +7.2ft surge, has been
exceeded four times in the last two years, whereas a 20-yr event would have only a 5%
probability of occurrence in a given year. As mentioned in the preceding chapter, the
scale of return period intervals was developed with the dataset available at that time (the
mid-1980’s), which included fewer accurately described severe storm events.
Perdido Key, FL
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While it is not possible to precisely predict future storm impacts, various researchers have
opined that the present cycle of frequent severe storms may continue for several years
into the future. The research group of Dr. William Gray at Colorado State University
publishes annual predictions of the severity of the coming tropical storm season
(Klotzbach and Gray, 2006). The most recent April 2006 predictions from that group
ascribe a 47% chance of the landfall of a Category 3, 4, or 5 hurricane along the Gulf of
Mexico shoreline from the Florida Panhandle to Texas. This probability is compared to
the 100-yr average of 30% for the same area. Klotzbach and Gray predict the occurrence
of 17 named storms in the Atlantic Basin for 2006 (down from 27 in 2005). Of those 17
named storms, nine are expected to reach hurricane status, five of which are predicted to
be severe hurricanes (Category 3 or higher).
It is important to recognize that the occurrence of a hurricane in most any portion of the
Gulf of Mexico is likely to result in some measurable level of storm surge, and hence
beach erosion, along the Perdido Key shoreline. This is evidenced by the 2005 tropical
storm season, in which relatively distant events such as Hurricanes Rita and Wilma
caused storm surges and strong swell wave conditions along the study shoreline.
Hurricane Dennis made landfall at Pensacola Beach, approximately 20 miles east of the
study area. The storm surge and swell waves leading up to landfall produced a +8ft
storm surge and enough damage to trigger an Emergency Declaration from FEMA for the
Perdido Key beaches. The offshore pass of Hurricane Katrina in August 2005 resulted in
storm surge conditions of roughly nine feet along Perdido Key, and again triggered an
Emergency Declaration and the need to rebuild sand berms alongshore. These facts serve
to raise the annual probability of occurrence of an event that results in beach/dune erosion
sufficient to require corrective action or possibly an Emergency Declaration from FEMA.
Some of the less-severe events that have occurred recently result in erosion of the upper
beach and dunes, but are insufficiently severe to trigger an Emergency Declaration. This
leaves the local governmental entity with the fiscal responsibilities of repairing any
damage to the protective beach berms. An example of this problem is the impact from
Tropical Storm Arlene in June 2005. Arlene was considered to be a “minor” tropical
storm, coming ashore at Perdido Key, FL, with 60-mph winds and generating a storm
surge of roughly five feet or more along the open coast, sufficient to overtop the berm in
many areas. Along narrower areas of the shoreline, the wave runup above the storm
surge level resulted in damage to post-Ivan storm berms that were under construction.
Further east, in Pensacola Beach, the post-Ivan storm berms had just been completed, and
in some localized areas wave runup from the storm surge overtopped the berms.
Perdido Key, FL
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Objectives and Expectations – Given the conditions described herein, the objectives of
any proposed beach restoration alternative for Perdido Key, FL, are to increase the stormprotection function of the beach and maintain or improve the tourism amenity value of
the beach. More specifically, it is desirable to raise the level of storm protection afforded
by the beaches to the upland infrastructure and environmental habitat to a condition that:
1. Attempts to prevent or minimize the deflation/erosion of the existing grade
landward of the 1975 Coastal Setback Line (generally -- the seaward edge of
construction) during a storm event with a surge level exceeding +11ft NAVD88,
the “100-yr event.” Under present conditions, such an event is predicted to
completely destroy the existing FEMA berm and result in profile deflation, water
inundation and wave impacts landward of the line of construction.
2. Prevents the loss of the existing FEMA emergency berm during a storm event
with a surge level exceeding approximately +7.5 to +8.5ft NAVD88 -- the “25to 30-yr events,” and minimizes the inundation and overwash along the Perdido
Key State Park Property (where no dune is to be constructed, per the direction of
the Florida Park Service). Given the frequency of occurrence of more severe
storms in recent years and forecast for future years, this objective serves to
minimize the level of project maintenance and expense required to meet
Objective #1 at any given time. Recent events of this magnitude have not
necessarily resulted in Emergency Declarations, which provide financial
assistance from FEMA and the State of Florida to rebuild the berms.
Additionally, finding suitable sand sources for such projects has become
increasingly more difficult and more expensive as the demand for sand from
upland borrow pits has increased dramatically since Hurricane Ivan. Lastly, the
FEMA berm, once revegetated, will re-establish an important constituent element
in the critical habitat for the Perdido Key beach mouse.
It must be stressed that it is not the intent of a beach restoration project to provide
complete protection against the impacts of a 100-yr storm event. Rather, it is an
objective to provide adequate protection from conditions reaching the 25- to 30-yr storm
event level (storm surges of +7.5 to 8.5ft NAVD88) and to attempt to minimize the level
of damage caused by the surge and waves associated with lower frequency events.
Clearly the beach restoration project will not address the “100-yr” impacts from hurricane
force winds upon the upland development, nor is it practical to completely preclude all
wave overtopping and storm inundation during such events.
Perdido Key, FL
- 44 -
As an example, during extreme storm events it is desirable to minimize the exposure to
the coastal road from flowing water and waves. During Hurricane Ivan, HW 292 was
damaged in numerous places due to flowing surge waters scouring the shoulders of the
road (Douglass et al., 2004). From the County perspective, another global objective of
surviving the event is to minimize the out-of-service downtime of the rental homes and
condominiums, both of which serve the tourism industry of the County. The increase in
storm protection capability, such as through the addition of sand to the buffering beach,
minimizes damage to upland infrastructure, including the road, and habitat by decreasing
storm wave heights across the profile and in many instances burying upland structures
rather than allowing for their collapse due to profile deflation.
Engineering alternatives considered in this study are intended to satisfy the following
general constraints in meeting the stated objectives and are discussed in the context of
these constraints:
¾
Engineering:
The probability that a given alternative can be successfully
constructed and perform as intended should be high. In this
regard, the selected alternative should not negatively affect
regional or local sediment processes. This includes potential
downdrift erosive impacts. Additional engineering constraints
must consider future maintenance and/or requirements for
reconstruction of alternatives, which should be reasonable, or
ideally, minimal.
¾
Regulatory:
The selected alternative must be in accordance with local, State,
and Federal regulatory constraints.
¾
Environmental: No net loss of endangered species habitat or its quality should
result from the selected alternative. Proposed alternatives should
not significantly impact any threatened or endangered species
during construction or thereafter, and where possible should seek
to enhance, protect, or re-establish habitat. The relatively natural
aesthetic quality of the study region should be preserved. The
selected alternative should not result in reduced aesthetic quality,
relative to the conditions expected if no actions were taken.
Perdido Key, FL
- 45 -
Where it is desired to increase the storm-protection benefits of a segment of shoreline
while maintaining or increasing the recreational carrying capacity and (as opposed to
simply protecting a particular upland structure, for example), the engineering/physical
goals and constraints combine to eliminate some traditional coastal engineering
stabilization options, irrespective of economic feasibility. For purposes of discussion,
several typical coastal engineering alternatives are presented herein. An initial
assessment of each alternative is provided in order to identify those options that are likely
to meet the project objectives and constraints. Those alternatives that meet the
constraints are considered further. Where applicable, the alongshore variation in the
existing level of storm protection provided by the beaches in the study are considered.
4.1
No Action
At present, post-Katrina emergency storm berms have been constructed along the
developed segments of the study area and plans are in place to install sand fencing and
revegetate the berms with salt-tolerant vegetation (sea oats, panic grass, etc.). Along the
State Park shoreline, no berms have been constructed, although some wind-blown dune
recovery has occurred and work is underway on a large scale dune vegetation installation.
The No-Action alternative would be the continuation of this shoreline condition. No
additional measures for shore-protection, such as the placement of additional sand or the
construction of hard coastal structures, would be pursued.
Such a condition represents only a partial recovery of the upper beach and dunes to the
level that existed prior to Hurricane Ivan. Accordingly, the level of protection provided
by the buffering beach and dunes from incident storms is substantially reduced, as
discussed in Section 3.5. Storm recession modeling predicts that the FEMA berms can be
expected to survive storm events approaching a +7ft total storm surge (a “20-yr event”)11.
For more severe events, the FEMA berm is predicted to be lost entirely and profile
deflation and inundation of the upland will commence. Without reconstruction of the
berms, the vulnerability of the upland to subsequent storm impact increases dramatically.
In the event of a storm that ultimately destroys the FEMA berm, an Emergency
Declaration may or may not be issued for Escambia County. Emergency Declarations are
tied to the level of damage sustained County-wide, and if the damage is limited primarily
11
It is noted that a typical FEMA emergency berm is designed to provide immediate protection from a “5yr storm event” through the placement of a six cy/ft berm feature. In this instance, a larger volume of sand
was authorized for placement by FEMA after Hurricane Ivan due to the lack of a suitable area of sufficient
elevation upon which to construct the emergency berm.
Perdido Key, FL
- 46 -
to the sandy beaches, a Declaration may not be issued. The Declaration is necessary in
order to qualify for Federal post-disaster assistance in the repair/reconstruction of the
FEMA berms. Failing that, the County would be faced with the financial decision to
rebuild the berm (see Section 4.3 below) or accept the degraded level of storm protection
from future events.
Due to the present level of storm vulnerability along the majority of the study area, the
No Action alternative does not meet the stated objectives for beach restoration at Perdido
Key in that it does not increase the storm-protection function of the beach to the levels
specified.
4.2
Shoreline Armoring
This alternative entails the construction of rock revetments or seawalls (sheetpile, rubblemound, etc.) along sections of shorefront where buildings become imminently
endangered by erosion. This alternative is presently only permissible under State
guidelines on an individual and temporary limited basis. Locally, Escambia County
prohibits the installation of such structures seaward of the 1975 Coastal Setback Line and
strongly discourages their installation landward thereof (and again, only in a condition of
the imminent loss of the building in question).
Such an alternative is clearly not warranted in the present situation, where no structures
are presently in imminent danger from day-to-day sea conditions, nor is it desirable or
practical on a project-wide basis. Properly designed sheetpile walls for open-coast
installations can cost over $1,500 per foot of shoreline. Economics aside, this alternative
can satisfy the storm-protection aspect of the project objectives for the developed areas.
In some instances, however, perhaps along the eastern limit of the study area where the
beaches are the narrowest, construction of such armoring may actually degrade the
recreational beach via additional narrowing of the local sandy shoreline (assuming the
structure is placed on the active beach such that it is frequently in contact with the surf).
Shoreline armoring also does not typically satisfy the stated constraints in terms
environmental impacts, aesthetics, and potential alterations to sediment transport
processes.
Perdido Key, FL
- 47 -
4.3
Dune Construction
Dune restoration is the first of several “soft” engineering alternatives considered herein.
By ‘soft’ it is meant that the solution is predominantly non-structural and is typically
more in keeping with the existing conditions of the Perdido Key shoreline, which has no
coastal structures along the open coast between Pensacola Pass and Perdido Pass. In the
present context, dune restoration would entail the construction of a second dune feature
seaward of the existing FEMA berm. Such a scenario would seek to maintain the
integrity of the FEMA berm over the course of minor storm impacts by adding volume
and elevation to the upper beach immediately seaward of the berm, leaving the berm in
place in the event of a severe storm.
Such a scenario would involve the placement of approximately 10 cubic yards of sand per
foot of shoreline along the 4.4-mile developed portion of the study area. This would
require approximately 240,000 cubic yards of sand. Such a volume is typically most
efficiently delivered to the beach by truck from upland borrow pits, located off-island.
Delivering 240,000 cy of sand requires roughly 16,000 individual truck trips over the
local roads and bridges. Recent experience with the delivery of sand to the beach in such
fashion indicates that the in-place cost of upland truck-haul sand can exceed $20.00/cy.
Storm recession modeling of this alternative does indicate an enhanced level of storm
protection provided by the additional dune feature. Construction of the dune feature and
its expected dune enhancement may reduce the width of the recreational beach by 60 to
70 feet, which may be problematic along the narrower eastern segments of the study area
(assuming it is intended to prevent foot traffic and recreational activity on the new dune
feature). Dune restoration is permittable by all regulatory agencies, and provides low
level enhancement to environmental resources (particularly sea turtles, Appendix F).
From an aesthetic and sediment processes standpoint, dune restoration is deemed
beneficial.
Given the present condition of the shoreline, dune construction alone may only be a
viable option for the extreme western end of the study area, within ½- to 1-mile from the
State Line. For most of the proposed project length, storm recession modeling suggests
that while the dune feature increases the level of storm protection somewhat, it will be
damaged by storm events with surges of less than +7ft, indicating that a frequent
maintenance interval may be required. Along the eastern portion, dune construction by
itself is no longer considered to be a viable shore protection alternative. Along the
Perdido Key, FL
- 48 -
Perdido Key State Park shoreline segment, Park Service personnel have requested that no
dune feature be constructed. The addition of 10 cubic yards of sand per foot of shoreline,
placed lower along the sandy beach, is expected to have only a minimal level of
improvement in storm protection characteristics over existing conditions. Given the
relatively high cost of truck haul sand, such a scenario is deemed infeasible for the Park
shoreline.
Given these caveats, the construction of a dune restoration or dune enhancement feature,
combined with other shore-protection measures only, is considered for additional review.
4.4
Comprehensive Beach Nourishment
This alternative entails the engineered placement of a large quantity of beach compatible
material along the shoreline for purposes of elevating the dry beach and advancing the
shoreline Gulfward (i.e., widening the beach), and replenishing the volume of sand lost
over some period of time. Over the project’s effective life, this alternative satisfies the
goals of increasing the storm-protection function of the beach as well as significantly
improving the recreational carrying capacity of the beach. Beach nourishment is a widely
recognized means of beach restoration and is generally acceptable to all regulatory
agencies.
This alternative meets all of the previously identified constraints.
Comprehensive beach nourishment can include a dune restoration/enhancement feature to
further improve storm-protection benefits.
Accordingly, the beach nourishment
alternative, in several forms, is considered feasible and is reviewed further in subsequent
sections of this report12.
In comparison with the dune construction alternative described in the previous section,
comprehensive beach nourishment is typically constructed via dredging with direct beach
fill placement. As a result, the minimum economically feasible placement volumes are
much greater than those associated with truck haul projects. Beach nourishment projects
constructed via small hopper dredges typically require a minimum fill volume density of
30 to 35 cubic yards of sand per foot of shoreline, compared to the 10 cy/ft truck haul
presented above. Cutterhead-suction/pipeline dredges operate at much higher production
levels and typically require minimum fill densities of 60 cubic yards per foot or more.
12
Mobile District COE (1993) prepared a reconnaissance report for a beach erosion control and storm
damage reduction project, and found, prior to Hurricane Opal, that a beach/dune restoration project was
feasible and justifiable in the Federal interest. Financial considerations on the part of the County precluded
the further development of the project at that time, which would have been the formulation of a formal
feasibility study (at that time, the study was estimated cost $1.25M and take 36 months to complete).
Perdido Key, FL
- 49 -
These constraints play an important role in the selection of any beach restoration
alternative that includes the placement of significant volumes of sand.
4.5
Structural Stabilization of the Shoreline
This alternative entails the construction of various coastal structures, such as groins or
breakwaters. Such structures are intended to modify littoral transport processes to retain
sand in their immediate vicinity, either by blocking the alongshore transport of sand
(groins) or by reducing the wave energy reaching the shoreline, which typically results in
the deposition of sand normally transported by the larger wave climate (breakwaters).
Both types of structures can be built in a wide array of configurations, including both
submerged and emergent designs. The storm protection benefit created by the structures
is principally the result of the accreted sand trapped by the structures, not by the
structures themselves. Typically, and ideally, the length of shoreline protected by the
accreted sand is much longer than the shoreline directed protected by the structures.
Coastal structures that are intended to create and hold additional beach area along the
open coast are generally appropriate only in highly erosional areas typified by large
gradients in alongshore transport. These conditions frequently occur near the ends of
littoral cells (near inlets, rocky headlands, etc.). Such an example would be the eastern
end of Perdido Key, adjacent to Pensacola Pass. Along the present study area, however,
background shoreline changes are generally mild, thus precluding the need for hard
structures.
Additionally, any structural field intended to restore, elevate, or increase the area of the
recreational beach would require the concurrent placement of a volume of sand
equivalent to or exceeding the expected impoundment volume of the structural field.
Such a requirement is dictated by prudent design practice and is expected as part of any
State permit issued by the FDEP. In effect, the structures would be constructed in
conjunction with, and at great additional expense to, the construction of some level of
beach nourishment project that would initially fill the field. Along the study area, it is
highly unlikely that regulatory agencies would permit the construction of hard, permanent
structures in lieu of or as an adjunct to beach nourishment. Accordingly, given the overall
coastal processes for the project area, structures as a means of beach stabilization are not
considered appropriate at this time.
Perdido Key, FL
- 50 -
5.0
ASSESSMENT OF BEACH NOURISHMENT &
DUNE RESTORATION ALTERNATIVES
Review of the available options for beach restoration, the background shoreline change
trends, and the predicted levels of storm vulnerability for the study area indicate that the
stated objectives for beach restoration can be achieved via some level of beach
nourishment and dune reconstruction. Storm-recession modeling indicates that the
reconstruction of the upper elevations of the beach through the placement of beach
compatible sand can significantly improve the storm protection function of the buffering
beach. Sand placement can provide protection for storm events exceeding the 30-yr
event and can minimize the level of damage incurred by storms as severe as the 100-yr
event – with the appropriate caveats and expectations (see Chapter 4). Structural
alternatives, such as seawalls, revetments, groins, breakwaters, or other hard structures,
are generally unwarranted, due to the favorable background shoreline change conditions,
the relative expense of the structures, and their unwanted presence along this otherwise
contiguous sandy coastline.
Simulations of various sand placement alternatives were conducted using the EDUNE
model for various segments of the study area to determine the volume of sand required to
achieve relatively uniform storm protection levels alongshore (Appendix D). The range
of alternatives considered consists of:
¾
¾
¾
¾
Dune construction, 10 cy/ft
Dune and beach fill construction, 30 cy/ft
In each case, except the stand-alone dune construction, model simulations were
performed on equilibrated beach fill construction templates. Figure 5.1 depicts a typical
60 cy/ft beach fill configuration simulated via EDUNE. The equilibrated profile
prediction indicates a relatively minor retreat of the construction template to the
equilibrated condition, due principally to the broad shallow submerged terrace feature
typical of many profiles along the study area (see Appendix A).
Perdido Key, FL
- 51 -
Perdido Key, FL
- 52 -
-10
-5
0
5
10
15
20
25
450
500
100
FEMA
Berm
+12ft
varies
550
600
existing typical profile
30
40 ft
1
beach
fill
800
75 ft
+5ft
predicted initial
equilibrated beach fill profile
40
650
700
750
Offshore Distance (arbitrary baseline, ft)
+9ft
160 ft
325 ft (typ.)
15
850
1
900
950
MHW
beach fill
construction
template
Typical Construction Template
60 cy/ft Project
Figure 5.1
Typical beach fill construction template for Perdido Key, FL. Template assumes the placement of 60 cubic yards of
beach fill per foot project shoreline. A prediction of the initial equilibrated beach profile is included. Using the equilibrated profile as
pre-storm input to the storm recession model EDUNE, the predicted post-storm configuration for a 30-yr and 100-yr event are plotted.
The beach fill template is designed to generally replicate or slightly exceed the pre-Ivan
beach profile condition by reconstructing a wider dune field, gently sloping the dry
recreational beach berm down from the dune toe to a seaward berm elevation of
approximately +5ft NAVD88, and further sloping the seaward face of the construction
template to its intersection with the existing seabed. The sloping beach berm and
seaward face are intended to create a more natural and “turtle-friendly” beach profile
from the initial construction template. It is expected that the dune feature constructed as
part of the beach restoration project would tie into the FEMA berm at the seaward edge
of any vegetation line existing at the time of construction (potentially creating a natural
swale between the two features, much like what existed there previously). Further, with
the installation of salt-tolerant vegetation over the new dune feature, the recovery of the
vegetated areas that existed prior to Hurricane Ivan can be greatly accelerated (see
Chapter 3 for photo examples). Adjustments in template beach width and volume are
typically instituted in the area between the seaward toe of the dune and the beginning of
the seaward slope of the dry beach berm.
5.1
Minimum Volume Requirements
The range of dune/beach fill templates simulated in the EDUNE model were assessed to
determine the approximate range of beach fill volume densities required to meet the
stated project objectives along the length of the study area. This approach generates the
“minimum volume project.” Figure 5.2 depicts a plan view of the alongshore distribution
of fill volume density associated with the minimum project. The distribution shown in
Figure 5.2 has been smoothed somewhat to remove some of the noise associated with
profile-to-profile variations in the EDUNE results and the position of the setback line
itself13. Based on these assumptions, the minimum fill volume project requires the
placement of approximately 1.5 million cubic yards of sand. As demonstrated throughout
this report, the volumetric requirements generally decrease in an east-to-west direction
across the study area. Correspondingly, the eastern limits of the study area, generally
through Gulf Beach, require more than twice the volume of sand necessary along the
western segment of the study area.
13
In addition, a 15% tolerance is ascribed to the fill volume densities to attempt to account for profile-toprofile and between-profile variations in shoreline conditions.
Perdido Key, FL
- 53 -
R024
R022
R023
R021
R020
R019
R017
R014
R015
R013
R011
R012
R009
R010
R007
R008
R006
Perdido Key, FL
30,000
R032
R005
R004
R003
R002
R001
- 54 -
Beachfill Volume Density (cy/ft)
35,000
GUIS
Perdido Key Unit
Figure 5.2
Suggested beachfill planform for minimum volume beach nourishment project at Perdido Key, FL. Dashed lines
represent a ±15% variation in alongshore placement density to account for profile-to-profile and between profile variations.
10,000
15,000
20,000
25,000
R016
5,000
R018
0
R026
R025
100
R027
80
R029
60
R031
Beach Fill: 1,500,000 cubic yards
(minimum fill volume)
R030
40
R028
Escambia County
(developed)
R033
Perdido Key
State Park
Big Lagoon
R034
Escambia County
(developed)
PKSP
Old River
R035
20
0
ALABAMA
FLORIDA
PERDIDO
KEY
R036
Near the boundary with the Perdido Key Unit of the GUIS, the placement of a minimum
of approximately 70 cy/ft of sand is required to meet the project objectives. Placement of
that volume is anticipated to result in a modest 70 to 80 ft seaward advance of the
shoreline after equilibration. To provide for the placement of this volume up to the
boundary of the developed portion (at the former Vista Del Mar location), it will be
necessary to taper the beach fill down to existing conditions along the adjacent National
Seashore property. The taper precludes the creation of adverse end effects on Park
property and to minimize volumetric end losses of the otherwise blunt terminus of the
fill. Preliminary discussions with GUIS personnel indicate that such a taper onto the Park
property is acceptable along the first 2,500 ft of the Park shoreline, sufficient to reach the
eastern edge of the Johnson Beach Park area (to R-34).
Proceeding westward to the public access at R-22 at the intersection of River Road and
Perdido Key Dr., minimum volume requirements along Gulf Beach approach 60 cy/ft.
Along the Perdido Key State Park segment, volume requirements gradually decrease
from 60 cy/ft to approximately 35 cy/ft as the beaches get wider, consistent with the longterm trends discussed in Chapter 3. Along this 1.6-mile segment of the study area, no
dune feature shall be constructed. The State Park is presently installing an array of salttolerant native species of plants across broad portions of the beach in the Ivan/Katrina
overwash zone and in several cross-shore blowout areas throughout the length of the
Park. While the lack of the higher-elevation dune feature raises the vulnerability level of
the upland Park areas to storm surge inundation and overwash, it is noted that the
objectives of the Park service differ somewhat from the project objectives along the
developed portions of the study area. Park personnel seek to balance the directive to
maintain and preserve the Park in as natural a state as is presently possible while seeking
to protect the upland habitat (and the minimal level of infrastructure in the Park) from
future damage.
Along the western 2.4 miles of the study area, minimum fill volume requirements
necessary to meet the project objectives decrease from approximately 35 cy/ft at the Park
boundary to the FL/AL State Line. This fill volume is expected to raise the upper
elevations of the dry recreational beach by two to four feet (typical), but will only result
in a seaward advance of the shoreline of 40 to 50 feet, at most, upon equilibration. Along
the westernmost 2,500ft, recent diffusion of sand from the Orange Beach, AL, beach fill
project in 2005 has created a very wide beach fill condition, wherealong minimum fill
requirements may only be 10 cy/ft (approx.).
Perdido Key, FL
- 55 -
5.2
Beachfill Planform Alternatives
The minimum fill volume project described in the previous section addresses the basic
requirements to meet the project objectives. Adjustments and increases to the minimum
plan should be considered to account for constructability and advance nourishment
considerations. These issues are tied principally to the source of sand ultimately selected
to construct the project. As of this writing, the County has several options for the sand
source (Chapter 6). These options involve the use of different construction techniques,
each of which has different minimum fill volume requirements (and unit costs).
The planform depicted in Figure 5.2 generally represents the minimum volume project
that could be economically constructed via hopper dredge. A fill volume density of
approximately 30 cy/ft is considered to be the minimum cost-effective placement volume
for a hopper dredge. Below that value, the losses and inefficiencies of the hydraulic
placement of the fill begin to exceed a manageable level and the per-unit cost of sand
placement rises. Similarly, for a typical ocean-going hydraulic cutterhead/pipeline
dredge, minimum cost-effective fill volume densities are approximately 60 cy/ft, due to
the higher production levels of the dredges.
Increasing the typical project volume fill density to approximately 60 cy/ft along the
entire project length raises the total project placement volume from 1.5 to 2.0 million
cubic yards of sand. Such an option would significantly raise the level of protection
afforded the western four miles of the study area, including Perdido Key State Park. The
added sand would act as advance nourishment to this segment, further protecting the
newly constructed frontal dune, the FEMA berm, and the recent dune enhancements from
higher frequency storm events. Assuming the additional volume is manifest as an
increase in beach width following the template of Figure 5.1, an addition of 10 cy/ft
results in a seaward advance of approximately 10 ft14.
14
This result is in near-perfect agreement with the traditional rule-of-thumb calculation that one cubic yard
of fill results in 1 foot of seaward advance. In the present instance, this agreement results from the choice
of the design berm elevation of +9ft and the typical depths of equilibrated fill placement of 16 to 20 ft deep
(9ft plus 18ft (average) = 27ft).
Perdido Key, FL
- 56 -
5.3
Previous Beach Nourishment Projects in the Region
Several full-scale beach nourishment or beach disposal projects have been constructed in
the Escambia County, FL / Baldwin County, AL region in the last 15 years. These
projects serve as full-scale prototypes to demonstrate the expected behavior of any beach
nourishment project along the Perdido Key, FL, study area. These projects have
experienced both “typical” weather conditions and the severe impacts of numerous
hurricanes during their project lives.
5.3.1 Perdido Key, FL, 1989-1991 Beach Disposal Project -- This project was
constructed as part of the deepening of the entrance channel at Pensacola Pass in
Escambia County, FL. Over 11 million cubic yards of material were dredged from the
channel to deepen it from -38 ft to -48 ft MLLW. Of that volume, 5.4 million cubic yards
(Mcy) were directly placed along the eastern 4.5 miles of Perdido Key, immediately west
of Pensacola Pass (R-40 to R-64). An additional 3.9 Mcy of sand were placed in a
nearshore berm along the center 2.5 miles of the project in roughly 20 ft water depth
(Browder and Dean, 2000). The fill volume density of the Perdido Key project, roughly
225 cy/ft, well exceeds that of the proposed project and stands as one of the largest
projects constructed in Florida (in terms of section fill density). During its life, the
disposal project has been impacted by Hurricanes Opal, Georges, Ivan, Dennis, and
Katrina, as well as numerous other tropical events
Over the nearly 15-yr monitoring period since construction, approximately 4.4 million
cubic yards of sand have eroded from the project limits. As of July 2005, approximately
17% of the volume of sand placed in 1989-1991 remains within the original project
limits. As described in Browder and Dean (2000), however, the eastern half of the
project has experienced much higher losses compared to the western half, due to the
strong transport reversal and sink effect of the channel (Chapter 3.0). As of the July 2005
survey, the eastern half of the project retained none of the sand volume placed in 19891991, and had lost an additional 125,000 cubic yards. Conversely, the western half of the
disposal project retained over 42% of the volume placed there along15. While the
beaches and the upland area along the eastern 4.5 miles of Perdido Key have sustained
substantial damage from the series of severe storms, the beach disposal project has no
doubt succeeded in maintaining the basic geomorphic integrity of the island compared to
other areas in the region, such as the western end of Santa Rosa Island and the western
end of Dauphin Island, AL.
15
The volumetric values discussed herein do not include overwash sand volumes north of the project limits.
Perdido Key, FL
- 57 -
5.3.2 Gulf Shores, AL, 2001 Beach Restoration Project -- In early 2001, approximately
1.63 million cy of sand were placed along the eastern 3.1-mi commercial segment of the
Gulf Shores, AL, Gulf of Mexico shoreline. The $4.3M project, funded entirely by the
City of Gulf Shores, utilized a hydraulic cutterhead/pipeline dredge to place the sand at
an average volume density of almost 100 cy/ft. The project borrow site was located more
than a mile offshore of the eastern project limit in 28 to 30 ft water depths. Project
construction resulted in an average advance of the shoreline of over 160 ft. After three
years of monitoring (May 2004, pre-Ivan), 70% of the volume of sand placed above MSL
was retained above that elevation and 100% of the placed volume remained within the
surveyed alongshore limits of the project. Prior to Ivan, the project shoreline was, on
average, 105 ft wider than pre-project conditions (Olsen Assoc., 2004).
The impact of Hurricane Ivan in September 2004 resulted in the loss of approximately 30
cy/ft of sand from the project limits above -12ft NAVD88. It is estimated that
approximately 60% of the lost sand was transported seaward beyond the crest of the
primary bar. MHW shoreline recession averaged 53 ft, while upper elevations of the
beach receded by as much as 165 ft (Douglass and Browder, 2005). Subsequent recovery
efforts included the retrieval of a substantial percentage of the overwash sand from the
streets and upland properties and the eventual renourishment of the project as part of a
more recent, larger beach restoration project in the area (described below). The
renourishment project was funded principally by FEMA Category G public assistance
funds, made available due to the locally funded construction of the original 2001 project.
5.3.3 Pensacola Beach, FL, 2003 Beach Restoration Project -- The 8.1-mile Gulf of
Mexico shoreline was nourished in 2002-2003 via the placement of 4.25 million cubic
yards of sand. The borrow site for the project was located 3.5 to 4.0 miles directly
offshore of the site in 65 ft water depths. The $15.3M project, funded by the Santa Rosa
Island Authority, Escambia County, FL, and the FDEP BBCS, utilized a hopper dredge to
place the sand at an average volume density of almost 100 cy/ft. Project construction
resulted in an average advance of the shoreline of 190 ft. Over the first year of the
project (May 2004), monitoring surveys describe the equilibration of the project,
indicating an average shoreline retreat of 52 ft and the erosion of 21% of the sand volume
placed above MSL. However, 100% of the placed sand was found by survey within the
full project limits (Browder, 2004).
The impact of Hurricane Ivan in September 2004 resulted in the displacement of
approximately 70 cy/ft of sand from the project limits above -14ft NAVD88. It is
Perdido Key, FL
- 58 -
estimated that over 1.5 million cubic yards of sand were transported seaward beyond the
crest of the primary bar. MHW shoreline recession averaged 52 ft, while upper
elevations of the beach receded by as much as 200 ft (Browder and Norton, 2005). Part
of the increased volume change and beach recession is attributed to the lack of complete
equilibration of the beach fill project at the time of storm impact. The Santa Rosa Island
Authority reported that 440 structures island-wide were destroyed during Ivan. All 440
structures were built prior to the establishment of the Flood Insurance Rate Maps
(FIRMs) in 1974. Only five post-FIRM structures were substantially damaged, all of
which were constructed prior to the 1987-1996 upgrades of local building codes.
Following Ivan, almost one million cubic yards of sand were retrieved from upland areas,
sifted to remove debris, and returned to the sandy beaches within the project limits. The
remaining losses were replaced via a renourishment of the project in 2005-06. Similar to
the Gulf Shores project, the locally-constructed Pensacola Beach project qualified for
FEMA post-disaster public assistance under Category G guidelines. The renourishment
project was thus funded principally by FEMA public assistance dollars. During the
course of construction of that project, additional beach damage was incurred due to
Hurricanes Dennis and Katrina. Repair of the losses from those FEMA-declared
disasters was likewise principally funded via FEMA under Category G guidelines.
5.3.4 Orange Beach/Gulf State Park/Gulf Shores 2005 Beach Restoration Project -This large scale, three-party regional project was constructed in 2005-06 along 15.3 miles
of beaches in Baldwin County, AL. The original beach restoration project, planned
before Hurricane Ivan and built with local funds, included only 11 miles of the beach and
4.5 million cubic yards of sand, excavated from three separate borrow areas along the
project length. Following Ivan’s landfall, the decision was made to include the FEMAreimbursable beach segment along the 2001 Gulf Shores project and to expand segments
in Orange Beach, AL. The original $21.5M project was constructed principally by
cutterhead/pipeline dredge and resulted in the placement of over 6.0 million cubic yards
of sand in several different segments in each community. The easternmost segment,
described in Chapter 3.0, lies along the 1.3-mile segment of Perdido Key immediately
west of the FL/AL State Line. During construction, the project was impacted by Tropical
Storm Arlene, and Hurricanes Cindy, Dennis, Katrina, Rita, and Wilma. Damage
incurred from Hurricanes Dennis and Katrina resulted in the re-pumping of numerous
segments of the project, now eligible for FEMA post-disaster assistance under Category
G guidelines. Ultimately, the gross volume of sand pumped reached nearly 8.0 million
cubic yards of sand.
Perdido Key, FL
- 59 -
5.4
Beachfill Project Expectations – Perdido Key, FL
The coastal engineering analyses provided in this report, particularly those results
describing the long-term shoreline changes and the predictions of storm recession,
indicate that the construction of a beach nourishment project along the study area can be
expected to perform quite well and to provide a significant increase in the level of storm
protection afforded to upland infrastructure and environmental habitat. Long-term
shoreline changes suggest that the study area experiences very little erosional stress
outside of those times when major storm events occur (Chapter 3.0). Therefore,
predictions of the performance of any beach fill configuration are principally dependent
on the storm climate it experiences over its lifetime. Predictions of the expected storm
performance of various configurations of the beach fill are discussed elsewhere in this
report.
Outside of storm events, other processes affecting beach fill longevity include the longterm background erosion and the alongshore diffusion of sand from the beach fill.
Diffusion, or alongshore spreading of sand from the project to the adjacent beaches,
begins at the ends of the fill, where the shoreline perturbation of the project must
transition back into the adjacent shoreline. The alteration of shoreline orientation caused
by the perturbation produces an increase in longshore transport, leading to erosion of the
fill at the ends. Figure 5.3 depicts a schematic of the diffusion process, indicating the
sand that will accumulate along the beaches to either side of the initially constructed fill.
This process can be modeled analytically or numerically to estimate the long-term fate of
the beach fill material, in the absence of significant storm events16. Appendix B contains
a description of the development of such a model for this project and the assumptions and
variables used in the predictions. The primary piece of information sought from this
analysis is the overall volumetric percentage of sand remaining within the original
construction limits over time. It is not the goal of the model to specifically address the
irregular behavior of the fill at every location along the project length, but the prediction
does describe the time rate of loss of sand from the project limits to within 10 to 15%
using reasonable values for input.
16
In the present discussion, a significant storm event is one that results in the more-or-less permanent
removal of a significant quantity of sand from the active beach system. This can be through overwash of
sand or the transport of sand into deep water beyond the assumed depth of closure of the model
Perdido Key, FL
- 60 -
pre-project shoreline
cross-shore distance
land
beach fill
initial
construction
berm
beach fill
diffusion
over time
Gulf
beach fill diffusion
w/ background erosion
alongshore distance
Figure 5.3
Schematic planview of beach fill diffusion, with and without simple
background erosion.
Figure 5.4 plots the predicted time history of the percentage of beachfill sand remaining
within the hypothetical project limits for varying lengths of fill along the Perdido Key
shoreline. For simplicity, a uniform beach fill volume density of 60 cy/ft was used for all
predictions. Different beach fill lengths are depicted, up to the full six-mile study area
length, to demonstrate the strong influence of project length on fill longevity. It can be
shown that the success of a project varies directly with the square of the project length.
By increasing the length of the fill, the losses are minimized (as a percentage of the
overall fill volume; the absolute losses at the ends changes very little). Such a model can
also easily accommodate a background erosion rate (assuming the background erosion
rate is attributed to something other than wave-generated alongshore diffusion). As
shown in Figure 5.4, two different rates were modeled, a no-background erosion case and
a minor -1 cy/ft/yr case.
Results of the diffusion modeling suggest that in the absence of storm events that remove
substantial quantities of sand from the alongshore littoral system, a 6-mile beach fill
project along the Perdido Key shoreline can be expected to retain approximately 80% or
more of the originally placed volume after five years. In comparison a shorter 3-mile fill
would be expected to retain only 60% of its original volume after five years, while a 1-
Perdido Key, FL
- 61 -
mile project would only retain 25%. Given that the background rates along the study area
suggest stability or minor accretion in some areas, these predictions may be pessimistic
for some beach segments.
Assuming a full six-mile project is constructed, the modeling suggests that the vast
majority of the beach fill will be in place for many years, ready to withstand the first
major storm impact to the project during that time. The specific response of the beach
then depends on the timing of the impact of a major event (or events) during the life of
the project. In the event a major storm impacts the project over the course of its useful
life, the post-storm project will continue to provide storm protection without
maintenance, albeit in a diminished capacity. For this reason, the future renourishment of
a constructed beach restoration project would more likely be triggered by the impact of a
specific severe storm event, rather than the year-to-year degradation of the fill. For
purposes of funding source planning, a renourishment interval of roughly eight to ten
years should be considered.
100
90
Percent of Construction Volume
Remaining Within Project Limits (%)
L = 6 miles (no bkg
80
L=6
70
L=3
L=
60
d.)
miles
(-1 cy
/ft/yr)
miles
(no
bkgd.)
3m
iles
(-1
cy/f
t/yr)
50
40
L=1m
ile (no
30
L=
20
1m
ile (1
bkgd.)
cy/ft
/yr)
10
0
0
1
2
3
4
5
6
Project Life (years)
7
8
9
10
Figure 5.4 Projections of beach fill gross volumetric performance as a function of
project segment length, L, and prevailing background erosion rate (in parentheses),
assuming the lack of impacts from major storm events.
Perdido Key, FL
- 62 -
6.0
REVIEW OF POTENTIAL SAND SOURCES
The recommended alternative for beach restoration at Perdido Key, FL, calls for the
placement of a minimum of 1.5 to 2.0 million cubic yards of beach compatible sand
along the 6.0-mile beach segment. This chapter discusses the potential sources of sand
to meet this requirement and the delivery systems and construction methods generally
associated with each. Figure 6.1 indicates the location of various potential sand sources
discussed in this Chapter. The various sources of sand are considered in the context of
the immediate volume of sand needed (1.5 to 2.0 million cubic yards), as well as:
¾
¾
¾
¾
¾
sediment quality of a prospective source,
the total volume of sand available from each source,
future maintenance requirements for Perdido Key (emergency berms, etc.),
proximity of the source to project area,
logistics of acquiring and delivering sand to project area.
To provide a sense of scale for the placement of 2.0 million cubic yards of sand, a typical
dump truck carries 15 cubic yards per load. At that rate, construction of the project via
upland truck haul sand sources would require over 133,000 trips by dump truck.
Assuming for simplicity a minimum effective sand placement rate of 15,000 cubic yards
per day, 1,000 dump truck loads per day would be required over a four to five month
period. Such a project, assuming a sufficient upland source were available, is clearly not
feasible from a traffic, logistics, and road/bridge-impacts standpoint. Additionally, truck
haul sand is estimated to cost four to five times as much as sand delivered from other
potential sources, due to the high demand for fill sand in the region after Hurricanes Ivan,
Dennis, and Katrina. For these reasons, building even the minimum-volume restoration
project via truck-haul from upland sand sources is eliminated from consideration herein.
In the future, small maintenance requirements for the project could be constructed via
truck-haul, depending on the actual scope of the work (such as for the construction of
post-storm berms, etc.).
Perdido Key, FL
- 63 -
490,000
Perdido Key
MS
Perdido Key, FL
480,000
470,000
- 64 -
Figure 6.1
460,000
*
Bear Pt.
FL
SC
1,030,000
1,050,000
N
lands
1,060,000
Gulf Is
1,070,000
0
1,080,000
1.0
2.0 mi (st)
Pensacola
Pass
GRAPHIC SCALE
Pensacola Pass
Federal Channel
hore
l Seas
ationa
Santa
Rosa
Island
Pensacola
Bay
Pensacola
Big Lagoon
Admiral's Island
ESCAMBIA COUNTY, FL
Easting (ft, NAD83)
1,040,000
Gulf of Mexico
Offshore Borrow Site
(Olsen Assoc. 2006)
a
dy Are
Perdido
Key
GIWW
Prospective sand sources in the vicinity of Perdido Key, FL.
Per
e y St u
di do K
PKSP
Perdido Bay
Innerarity
Pt.
Ono Island
1,020,000
Bayou St John
GA
1,010,000
Perdido
Pass
Terry
Cove
Orange Beach
TX
LA
AL
ALABAMA
FLORIDA
500,000
6.1
Nearshore Gulf of Mexico Sand Sources
Olsen Associates (2006a) conducted a nearshore sand search in conjunction with this
study to assess the availability of beach-quality sand in nearby State Waters. The
objective of the search was to identify a sufficient quantity of beach-compatible sand in
the vicinity of the project area to cost-effectively construct a large-scale beach
nourishment project. In May 2005, 57 20-ft sand Vibracores were collected by Alpine
Ocean Seismic Survey (AOSS), Inc., of Norwood, NJ, under the on-board direction of
Olsen Associates, Inc. Figure 6.2 plots the locations of each Vibracore.
Based upon the native beach sand characteristics, a primary borrow site has been
preliminarily identified for potential use in a beach nourishment project to restore the
project shoreline. This roughly 300-acre area, located over 1.1 miles offshore between R6 and R-12, is estimated to contain as much as 6.0 million cubic yards of beachcompatible material. A secondary site, which may contain up to 2.0 million cubic yards
of sand, has also been identified (Figure 6.2). Vibracore logs, photographs, and grainsize-distribution data are provided for all Vibracores in the companion geotechnical
report to this study (Olsen Associates, 2006a).
The location of the primary potential borrow site relative to the project area is such that
the project could be constructed via hydraulic cutterhead/pipeline dredge. Maximum
pipeline lengths would reach approximately 30,000 ft at the extreme eastern limit of the
project, potentially requiring a booster pump, but a manageable length nonetheless. This
method of construction would provide one of the most efficient, and hence, cost-effective
means of placing large volumes of sand on the shoreline. Daily production for these
types of dredges can average over 40,000 cy/day, and can reach as high as 70,000 cy/day
on short pipeline lengths under ideal conditions.
Olsen Associates (2006a) identifies several tasks required to ultimately utilize this site,
including cultural and environmental resources investigations, a final borrow site design
and wave field/shoreline impact analysis, and the acquisition of a Submerged Lands
Easement from the State of Florida. All of the tasks listed are part of the “typical”
permitting process for a beach nourishment project in Florida, and no particularly unusual
permitting problems are anticipated with the use of the preliminary site identified. Of the
potential sources of sand identified in Figure 6.1, the primary nearshore source depicted
in Figure 6.2 is opined to be the most cost-effective means of delivering large volumes of
sand to the Perdido Key shoreline for purposes of beach restoration.
Perdido Key, FL
- 65 -
480,000
Perdido Key, FL
- 66 -
470,000
BC-34 BC-35
BC-36
BC-2
BC-1
BC-32 BC-31 BC-33
R-1
BC-3
460,000
PK-18
PK-13
PK-09
PK-15
PK-57 PK-52
SECONDARY
SITE
PK-12
R-31
PK-54PK-14
PK-17
PK-55
PK-10
PK-08
PK-56 PK-53
PK-16
R-28
1,040,000
0
0.5
PEN-91-1
R-37
1,050,000
1.0 miles (st)
R-34
Big Lagoon
GRAPHIC SCALE
PRELIMINARY OFFSHORE
BORROW SITE
PK-11
R-22
Easting (ft, NAD83)
1,030,000
R-19
R-25
Gulf of Mexico
R-16
Old River
Perdido
Key
Figure 6.2 Location of Vibracores collected off Perdido Key, FL (May 2005). From Olsen Associates (2006)
1,020,000
Other Researchers (dates vary)
2002 Baldwin County, AL (OAI)
2005 Perdido Key, FL (OAI)
PK-24
PK-07
PK-30
R-13
PK-28
PK-51
PK-34
PK-35
PK-23
PK-01
PK-05
PK-03
PK-50
PK-32
PK-47
PK-38
PK-33 PK-19
PK-22 PK-48
PK-06
PK-25 PK-36
PK-20 PK-26 PK-39
PK-42
PK-02
PK-04
PK-27PK-21
PK-37
PK-44
PK-41
PK-40
PK-49
PK-43
PK-46 PK-45
PK-29
PK-31
R-7
R-10
Ono Island
Vibracore Data Sources
PEN-92-7
1,010,000
BC-7
BC-154
BC-154j
BC-42
BC-149
BC-4
A90
A93
R-4
Bear
Point
Bayou St John
Caswell
ALABAMA
FLORIDA
490,000
6.2
Pensacola Pass Federal Navigation Channel
Concurrent with this study, the U.S. Navy is developing plans to dredge the Federal
navigation channel back to its 1990-1991 configuration, which consists of an ocean
channel width of 800-ft and a depth of -48ft MLLW17 (including over-dredge and
advance-maintenance dredging volumes). Permit applications submitted to the State of
Florida indicate that, as of this writing, as much as 8.0 million cubic yards of sand may be
available for placement along the adjacent beaches or in nearshore disposal areas. Figure
6.3 depicts the July 2005 condition of the channel and identifies the location of 84
Vibracores collected by the Mobile District, USACE, as part of the project design.
A project of this magnitude mirrors the 1989-1991 Homeporting project at Pensacola
Pass, which included the removal of over 11 million cubic yards of material to achieve
the Navy’s design width and depth. A brief description of that project and its
performance over time can be found in Chapter 5 and in Browder and Dean (2000). As
of the time of this writing, a final disposal plan is still under development. Parties
involved in the decision-making process including the U.S. Navy, the Mobile and
Jacksonville USACE Districts, the National Park Service, the State of Florida, and
Escambia County. Once a specific disposal plan is formulated, the bulk of the permitting
process at the Federal level can commence.
It is opined that sand dredged from Pensacola Pass is potentially a very suitable source of
beach nourishment sand for the project area at Perdido Key. Construction of any beach
fill project along the Perdido Key study area using this sand source would necessitate the
use of a hopper dredge, which is expected to extend the duration of the work and increase
the unit cost of sand. Issues regarding the timing of the channel dredging and the costsharing and permitting responsibilities remain to be resolved. A more detailed
investigation of the geotechnical data from the Pass Vibracores is underway as part of a
companion task to this study. Depending on the final cost-sharing arrangement, the costeffectiveness for the County and State Park to build the recommended alternative using
this sand source may prove to be the most feasible option.
An additional element of the U.S. Navy proposal consists of the maintenance dredging of
the channel every two years (approx.). The maintenance is expected to generate
approximately 550,000 cy. Participation by the County in the maintenance program may
17
These dimensions exceed the Federally-authorized channel dimensions of 500-ft width and -38 ft MLW
depth. The additional width and depth are maintained at the request of the U.S. Navy.
Perdido Key, FL
- 67 -
provide a suitable source of beach-quality sand for future needs, such as addressing hotspot erosion areas or for the construction of dune restoration projects or post-storm
emergency sand berms.
While permitting issues associated with the dredging of the Pensacola Pass channel
would not be the specific responsibility of Escambia County, the timing associated with
the permitting may ultimately effect the County’s decision regarding beach restoration
activities. These issues principally relate to the protection of endangered species,
specifically sea turtles in the channel and nesting on the beaches in the disposal areas and
gulf sturgeon, including their critical habitat in the channel and along the adjacent
shorelines. None of these issues are believed to be especially problematic for the
completion of dredging, but they may result in constructability issues, such as
construction-time windows and increased monitoring requirements. In particular, any use
of a hopper dredge in the channel would require the implementation of the standard
turtle/sturgeon protection requirements (draghead deflectors, relocation trawling for sea
turtles and gulf sturgeon). At this time it is opined that sea turtle nesting densities in the
project area are low enough that permission can be obtained to place sand on the beach
during turtle nesting season, with the proper monitoring and relocations plans in place.
Appendix F contains additional information on the protection of environmental resources.
6.3
Pensacola Pass Ebb Shoal
Figure 6.3 also depicts the location of 28 Vibracores collected in 2000 and 2001 as part
of the Pensacola Beach, FL, Sand Search. Olsen Associates (2001) describes a potential
borrow site occupying a 319-acre area along the eastern boundary of the entrance channel
and including a portion of the entrance channel itself. The specific area evaluated is
estimated to contain approximately 6.2 million cubic yards of sand. Once properly
permitted, this site could provide a very suitable source of sand for large-scale beach
nourishment for Perdido Key. As with the channel itself, the distance from the site to the
project ranges between 7.8 and 13.6 miles, requiring that a hopper dredge be used for
construction. It is expected that permitting and environmental protection issues would be
similar for both the channel dredging and the excavation of sand from this site.
Perdido Key, FL
- 68 -
500,000
490,000
Perdido Key, FL
480,000
- 69 -
R-49
R-58
R-61
1990-91 DISPOSAL BERM
R-55
1,070,000
Other Researchers (dates vary)
2001 Pensacola Beach, FL (OAI)
2004 Pensacola Pass (COE)
PN-53-04
PN-54-04
PN-55A-04
PN-55-04
P-48
PN-1-04
P-46
P-41
P-45
P-42
1,080,000
1,090,000
PN-59-04
0
R-82
0.5
1,100,000
1.0 miles (st)
R-85
Pensacola
Bay
PN-70-04
GRAPHIC SCALE
R-79
Santa Rosa
Island
R-76
P-40
P-44
P-43
P-39
P-50
P-38
P-35
P-34
P-37
PN-3-04PN-2-04
PN-4-04
P-51 P-47
PN-5-04
P-36
P-33
PN-6-04
P-49
PN-7-04
PN-8-04
P-56
R-73
Gulf of Mexico
P-52
P-57
PN-9-04
P-53
PN-10-04
P-55
P-54
PN-11-04
PN-12-04
P-58
PN-14-04
PN-13-04
PEN-93-5
P-59
PN-69-04
PN-68-04
PN-62-04
PN-67-04
PN-63-04PN-64-04
PN-66-04
PN-66A-04
PN-65-04
Ft. Pickens
R-70
MIDDLE
GROUND
PN-61-04
PN-37-04
PN-34-04
PN-35-04
PN-32-04
PN-33-04
PN-30-04
PN-31-04
PN-28-04
PN-77-04
PN-29-04
PN-72-04
PN-26-04
PN-27-04
PN-24-04
PN-25-04
PN-78-04
PN-22-04
PN-71-04
PN-23-04
PN-20-04
PN-79-04
PN-21-04
P-62
PN-18-04
PN-19-04
PN-16-04
PN-17-04
P-60
P-61
PN-15-04
PN-40-04
PN-38-04
PN-36-04
PN-80-04
PN-39-04
PN-41-04
PN-42-04
PN-45-04
PN-44A-04
PN-44-04
PN-81-04
PN-43-04
PN-47A-04
PN-47-04
PN-46-04
PN-73-04
PN-82-04
PN-51-04
PN-50-04
PN-84-04
PN-74-04
PN-49-04
PN-48-04
PN-83-04
PN-52-04
CAUCUS
SHOAL
R-64
R-67
PN-56A-04
PN-56-04
PN-58A-04
PN-57A-04
PN-57-04
PN-58-04
PN-60-04
Pensacola
Pass
Easting (ft, NAD83)
PENSACOLA PASS
NAVY CHANNEL
R-52
Vibracore Data Sources
1,060,000
R-43
R-46
Big Lagoon
Perdido
Key
NAS Pensacola
PN-76-04
PN-75-04
R-88
Figure 6.3 Location of Vibracores collected within the Federal Navigation Channel at Pensacola Pass, FL. Vibracores collected by
Mobile District USACE and by Olsen Associates (2002). Channel/ebb shoal survey - July 2005.
470,000
6.4
Gulf Intracoastal Waterway Sources
The Gulf Intracoastal Waterway (GIWW) borders Perdido Key to the north, separating
the island from the mainland (Figure 6.1). The channel is periodically dredged by the
Mobile District of the U.S. Army Corps of Engineers, and the material produced is
typically of beach-compatible quality (although that fact has not been verified and the
sediment quality most likely varies). Dredged material is stockpiled alongside the
channel in most instances (see Section 6.5). Dredge records obtained from the Mobile
District from 1995 to 2005 indicate that the annualized volume of sand dredged from the
channel varies dramatically, and is principally a function of storm events that shoal the
channel. Records indicate that the segment of the channel from the Theo Barrs Bridge
westward to the north shore of Ono Island, AL, has been dredged six times since 1995.
This effort has produced over 330,000 cubic yards of material in total. The channel was
most recently dredged in 2005, when 153,000 cubic yards of sand was dredged. In
contrast, dredging work in 1995 yielded only 8,900 cubic yards and dredging in 1998 and
2002 produced only 23,100 cubic yards (Mobile District, personal communication).
While material dredged from the GIWW typically does provide beach-quality sand, the
quantities of sand available from this source are far too small to address the immediate
full beach restoration needs. However, dredging of the GIWW may continue to meet
post-storm sand needs for emergency uses. This source was used in 2004 and 2005 to
rebuild berms after Ivan, Dennis, and Katrina. Approximately 145,000 cy of sand was
trucked from a disposal area along the GIWW just west of the Theo Barrs bridge (at a
cost of $16.05/cy, not including COE dredging costs).
6.5
Upland Stockpiles (Admiral’s Island – Ft. McRee)
One such stockpile of sand from the GIWW exists just west of Pensacola Pass at
Admiral’s Island, north of the eastern terminus of Perdido Key and immediately south of
the Ft. McRee Land Cut of the GIWW from Big Lagoon to Pensacola Pass and Pensacola
Bay. This site, created by the GIWW land cut in the 1950’s, has been proposed in the
past as a source of beach-compatible sand. Sediment quality testing performed for the
U.S. Army Corps of Engineers recently suggests that the majority of material in the
island site would meet the Escambia County Sand Protection Ordinance. A Preliminary
Restoration Plan for Fort McRee (The “Ft. McRee Disposal Area” and “Admiral’s
Island” refer to the same feature) was prepared in 2002 by the Mobile District USACE to
investigate the feasibility of excavating up to 1.4 million cubic yards of beach-quality
Perdido Key, FL
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sand, transporting it in dump scows, and placing it in a feeder berm at the east end of
Pensacola Beach18.
The island occupies an area of approximately 45 acres and stands roughly 25 feet above
Mean Sea Level. The total volume of material available is dependent on the desired postconstruction configuration of the disposal area, which typically might include
containment berms and a prescribed minimum elevation for constructability. As an
extreme example, assuming the entire stockpile island was available for excavation down
to ambient lagoon-bed level (assumed to be -2ft Mean Sea Level), the volume of sand
available may be almost two million cubic yards (45 acres at 27 feet thick).
The sand available in the Admiral’s Island disposal pile represents a significant volume
of sand relative to the volume required by the recommended project alternative. The
logistics of excavation and transport to the project area, however, present substantial
challenges and, hence, increased degree of difficulty and cost. The cost of retrieval may
ultimately be competitive with upland truck haul sand sources. While it is not expected
that this disposal island sand be used for any initial restoration of the Key, it is proposed
that the feasibility of retrieving this sand be investigated further.
18
The PRP was formulated prior to the 2002-2003 Pensacola Beach, FL, Beach Restoration Project, which
placed 4.25 million cubic yards along Pensacola Beach.
Perdido Key, FL
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7.0
RECOMMENDATIONS
Alternatives for beach restoration for the westernmost six miles of shoreline at Perdido
Key, FL, including Perdido Key State Park, have been evaluated in order to address the
need to significantly increase the level of storm protection provided by the sandy beaches
to upland infrastructure and environmental habitat while maintaining and/or increasing
the recreational amenity value of the beach. The recent impacts of the 2004 and 2005
tropical storm seasons, most notably Hurricane Ivan in September 2004, destroyed the
primary dune system, substantially lowered the elevation of the dry beach berm along the
entire length of the study area, and transported a significant quantity of sand offshore of
the primary bar, where its mechanical recovery is infeasible and natural recovery is
deemed unlikely on any useful project time scale.
While the study area has not experienced substantial shoreline recession, the recent loss
of beach volume and the lowering of the dry beach leave upland infrastructure and habitat
vulnerable to storm wave impacts and inundation from events generating storm surges of
+7ft MSL or higher (typically, the “20-yr event” or greater). The post-Katrina FEMA
berms constructed along much of the developed area represent protection against storm
events with surges lower than approximately six to seven feet. For more severe events,
the FEMA berm is expected to be completely eroded, after which time erosion and
profile deflation landward of the seaward edge of construction becomes more.
The series of storm events in 2005 has prevented any meaningful recovery of the beaches
following Hurricane Ivan. In some instances it has exacerbated the cumulative loss of
sand from the beach/dune system to offshore areas. Even under ideal future weather
circumstances, which cannot be relied upon for purposes of storm protection in the nearterm, available data suggest that only a portion of the sand transported offshore during
Ivan, Dennis, and Katrina would be expected to return to higher elevations along the
beach profile (see section 3.3.3). Thus, for purposes of beach restoration along the
Perdido Key, FL, study area, it is not recommended that this volume of sand be relied
upon to provide any level of meaningful assistance in achieving the required level of
storm protection identified herein. The potential recovery of portions of the central and
western segments of the study area via natural littoral conditions is completely dependent
upon the future storm climate.
Perdido Key, FL
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7.1
Recommendations
7.1.1 Recommended Alternative -- Based upon the analysis of historical shoreline
changes, recent storm impacts, and predictions of storm-induced beach profile change, it
is recommended that Escambia County, FL, and the Florida Park Service initiate the
permitting and construction of a comprehensive beach nourishment project along the
westernmost six miles of the Gulf of Mexico shoreline of Perdido Key, FL. Such a
project should include the placement of a minimum of 1.5 to 2.0 million cubic yards of
beach compatible sand along this beach segment, weighted toward the eastern end of the
area (reference Chapter 5). The minimum-volume project is expected to prevents the loss
of the existing FEMA emergency berm during a storm event with a surge level exceeding
approximately +7.5 to +8.5ft NAVD88 -- the “25- to 30-yr events,” and minimizes the
inundation and overwash along the Perdido Key State Park Property (where no dune is to
be constructed, per the direction of the Florida Park Service). Additionally, such a
project is expected to minimize the deflation/erosion of the existing grade landward of
the 1975 Coastal Setback Line (generally -- the seaward edge of construction) during a
storm event with a surge level exceeding +11ft NAVD88, the “100-yr event.”
7.1.2 Sand Source -- It is recommended that Escambia County, FL, initiate the
permitting of the primary nearshore borrow site identified during the Sand Search. While
it is possible that beach-compatible sand may become available from Pensacola Pass as
part of the Navy dredging, the present uncertainty in the timing of the two projects
dictates that the County pursue an independent source of sand. In the event sand from the
Pass is utilized for the construction of the recommended alternative, completing the
permitting of the nearshore borrow site will provide the County an additional option in
the future (for emergency needs or future renourishment, if and when necessary).
7.1.3 FEMA Documentation of Engineered Beach -- It is recommended that the
constructed beach restoration project be documented with the Federal Emergency
Management Agency (FEMA) for purposes of establishing future eligibility for postdisaster financial assistance under Category G guidelines. As demonstrated by the recent
FEMA-funded reconstruction of nearby beach restoration projects in Pensacola Beach
and Gulf Shores, AL, establishing FEMA eligibility provides significant financial
assistance to rebuild the beach in the event of a future declared disaster. For this reason,
it is again recommended that the entire 6.0-mile length of the Perdido Key study be
included in the beach restoration plan.
Perdido Key, FL
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7.1.4 Establishment of a Beach Monitoring Plan -- It is recommended that a beach
monitoring plan be established to assess the performance of any constructed beach
restoration alternative. Such a plan, which will be required by permit, would include
annual or more frequent beach profile monitoring surveys, collection of aerial
photography, and borrow site surveys at regular intervals. Institution of such a plan
would likewise serve to address the documentation requirements of an engineered beach
nourishment project under FEMA Category G Public Assistance guidelines.
7.1.5 Application to FDEP BBCS Beach Erosion Control Program -- It is recommended
that the County apply to FDEP BBCS for funding assistance for the proposed project
through the Bureau’s Beach Erosion Control Program (BECP). Plans for a beach
restoration project for Perdido Key were submitted to BBCS as part of the 2006-07 Long
Range Budget Plan. The plan should be updated based upon this report and resubmitted
for 2007-08. This study, conducted in accordance with the BBCS 2004 Hurricane
Recovery Plan for Florida’s Beach and Dune System, is intended for submittal to FDEP
to provide the necessary information to support the budget plan and establish the
project’s eligibility for State cost sharing in the construction process.
7.1.6 Creation of a Dedicated Beach Management Fund -- It is recommended that a
dedicated source of funding be established for upcoming beach management needs.
Creation of such a fund, perhaps through tourist development tax collections, could be
used to pay for the costs of annual monitoring of a beach restoration project, the costs of
small emergency repairs to the project, subsequent dune enhancements if desired, and the
required matching funds to pay for the repair of the beach in the event of a declared
disaster. Numerous examples of such dedicated funding sources exist, such as the beach
improvement fund established in Gulf Shores, AL.
7.2
Tasks Required for the Recommended Beach Restoration Alternative
It is strongly encouraged that the permitting and associated design tasks required for the
construction of the recommended alternative be pursued as rapidly as possible in order to
maintain the viability of various options for sand sources, especially any beach quality
sand made available as part of the proposed dredging of Pensacola Pass (see following
section). To that end, the following are tasks that will be required regardless of the sand
source ultimately used:
Perdido Key, FL
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¾ Conduct a Mean High Water Line survey and establish an Erosion Control Line
(ECL) along the full length of the proposed project,
¾ Acquire construction easements along each beach front property where the project
is to be constructed. The easement must extend from the established ECL
northward to the landward limit of the construction project (generally, the 1975
Setback Line,
¾ Prepare second phase engineering/permitting-level design schematics of the plan
views and cross-sections for the proposed project.
The tasks of setting an ECL and acquiring construction easements are opined to
constitute the critical path for construction of any restoration alternative. Easement
acquisition should begin as soon as possible. During that process, it may become
necessary for the County to acquire some of the easements via condemnation, thus
adequate time must be allotted for this possibility.
On a similar schedule, other tasks will be required to pursue the use of the primary
nearshore borrow area:
¾ Conduct an environmental assessment of the primary nearshore borrow site,
¾ Conduct a cultural resources review/survey of the site,
¾ Prepare permitting-level design plans of the proposed excavation area.
Upon completion of these tasks and the submittal of a Joint Coastal Permit Application to
FDEP and the U.S. Army Corps of Engineers, additional tasks may be required, such as
the preparation of a Biological Assessment, design level surveys, etc.
7.3
Preliminary Opinion of Probable Cost to Construct
An initial opinion of the probable cost to construct the recommended alternative is based
upon the construction of the beach nourishment project utilizing a cutterhead/pipeline
dredge to excavate sand from the primary nearshore borrow site identified by Olsen
Associates (2006a) and described in Chapter 6.0. Basic components of the recommended
alternative are:
¾ Project Length:
¾ Recommended volume:
Perdido Key, FL
6.0 miles
2.0 million cubic yards (60 cy/ft project)
- 75 -
¾
¾
¾
¾
Minimum volume:
Berm Elevation:
Dune Feature:
Borrow Site:
1.5 million cubic yards
+9 ft, sloping to +5 ft NAVD88
5 to 7 cy/ft, crest elevation of +12 ft NAVD88*
1.1 miles offshore of R-10, Perdido Key
* Dune to be built in developed areas only, assumed to be constructed seaward of FEMA berm.
A project of this scope, constructed by cutterhead/pipeline dredge, is expected to require
approximately three months to complete, conservatively allowing for weather-related
downtime. The cost of construction of such a project is highly dependent on a number of
factors, including, but not limited to:
¾
¾
¾
¾
The price of fuel at the time of bidding and construction,
The level of competition/availability of dredge plant,
The time of year proposed for construction,
The nature of the project design and potentially of the borrow site.
Based upon recent previous experience with beach nourishment projects of a similar
magnitude, the present cost of fuel, analytical predictions, and informal discussions with
qualified ocean dredging contractors regarding the proposed project, it is opined that the
project could be constructed for unit prices of $4.00 to $6.00 per cubic yard, plus
mobilization/demobilization fees. Thus, a first estimate of the probable cost to construct
the recommended beach restoration alternative is between $9 million and $13 million.
This first opinion of the probable construction cost is provided to facilitate project
planning and financing at this feasibility stage.
The cost opinion provided above can then be applied to assess the economic justification
of pursuing construction of the recommended alternative utilizing sand excavated from
the Federal navigation channel at Pensacola Pass versus the primary nearshore site.
Present discussions of the Pass project have yet to address the potential cost of delivering
sand to the proposed project limits via hopper dredge from the channel, a sail distance of
between 6.0 and 13.0 miles. At this time, a dredged material disposal plan is being
formulated. Once that task is completed, an initial cost opinion for the channel dredging
project can be developed. At that stage, there will, in all probability, be the need to create
a cost-sharing plan between the various parties. The choice of sand source to construct
the recommended alternative can then be assessed by the County.
Perdido Key, FL
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7.4
FDEP Critical Erosion Designation
At the time of this writing, only the developed portion of the Perdido Key, FL, shoreline
from FDEP R-monument R-26 to R-32 is considered to be critically eroded (based
strictly upon the interpretation of long-term MHWL changes. Pending certain access
criteria, critically eroding shorelines are eligible for state cost sharing for beach
restoration. The FDEP “Guidelines – Florida Beach Erosion Control Program” provides
the following definition for a “Critical Erosion Area”, (FDEP BBCS, 2006):
“Critical Erosion Area” is a segment of shoreline where
natural processes or human activities have caused or
contributed to erosion and recession of the coastal system
to such a degree that upland development, recreational
interests, wildlife habitat or important cultural resources
are threatened or lost.
Critical erosion areas may also
include peripheral segments and gaps between identified
critical erosion areas which, although they may be stable
or slightly erosional now, their inclusion is necessary for
continuity of management of the coastal system or for the
design integrity of adjacent beach management projects.”
Based upon the analyses presented herein, it is the opinion of Olsen Associates, Inc. that
the western six miles of Perdido Key, FL, from FDEP R-Monuments R-1 to R-32, meet
the definition provided above, and thus should be considered fully eligible for State costsharing in the construction of the recommended beach restoration alternative. This
opinion is based on the following factors:
¾ The substantial storm-induced erosion of the subaerial beach since the 2004
hurricane season and the present level of exposure of remaining upland
infrastructure and habitat to future storm damage (Chapter 3),
¾ The need to maintain and improve the integrity of the evacuation route, S.R. 292,
immediately adjacent to the beach strand along this segment for both Perdido
Key, FL residents and residents of Orange Beach, AL, the neighboring
community to the west,
¾ The need to maintain and enhance the design integrity of the proposed beach
restoration project, and to jointly contribute to the regional stability of the Perdido
Key littoral cell, which is shared by the States of Florida and Alabama.
¾ The need to restore and protect the upland habitat of the endangered Perdido Key
beach mouse and the nesting sea turtle habitat along Perdido Key, and lastly
¾ The need to ensure continuity of management of the Perdido Key coastal system.
Perdido Key, FL
- 77 -
In regard to the extension of the Critical Erosion Designation, it is likewise fiscally
important to include the entire beach segment in a comprehensive engineered beach
restoration plan in order to be eligible for post-disaster assistance from FEMA in the
event of a declared disaster. This assistance will be critical to rebuilding the protective
beach at a time when the County and State may likely face significant post-storm
expenditures in areas extending well beyond the beaches.
Perdido Key, FL
- 78 -
8.0
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Key, FL, Gulf Island National Seashore,” Center for Coastal and Environmental
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PERDIDO KEY, FLORIDA Feasibility Study for Beach Restoration

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