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Perdido Key historical summary and interpretation of monitoring programs

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Perdido Key historical summary and interpretation of monitoring programs
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UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 91/009
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Work, Paul
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Gainesville
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Coastal and Oceanographic engineering Department, University of Florida
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Perdido Key (Fla)

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This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.

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UFL/COEL-91/009

PERDIDO KEY HISTORICAL SUMMARY AND INTERPRETATION OF MONITORING PROGRAMS
by
Paul Work Lynda Charles and
Robert G. Dean

June, 1991
Submitted to:
Department of the Navy Southern Division Naval Facilities Engineering Command Charleston, SC 29411-0068




UFL/COEL-91/009

PERDIDO KEY
HISTORICAL SUMMARY AND INTERPRETATION OF
MONITORING PROGRAMS
Submitted to: Department of the Navy
Southern Division
Naval Facilities Engineering Command
Charleston, SC 29411-0068
Prepared by: Paul Work Lynda Charles and
Robert G. Dean
Coastal and Oceanographic Engineering Department
University of Florida Gainesville, FL 32611

June, 1991




3 Geology
4 History of Human Impacts on Area
4.1 Tidal Inlets and Barrier Islands ......................
4.2 Pensacola Pass/Perdido Key . . . . . . . . . . . .
4.3 Historical Shoreline Changes . . . . . . . . . . . .
5 Changes Due to Beach Nourishment Project
5.1 Cross-Shore Changes . . . . . . . . . . . . .
5.2 Equilibrium Beach Profiles . . . . . . . . . . . .

TABLE OF CONTENTS

LIST OF FIGURES LIST OF TABLES I Introduction

and History

2 Environmental Conditions
2.1 Wave Climate ......
2.2 W inds ..........
2.3 Tides ...........
2.4 Major Storms ......
2.4.1 Camille .....
2.4.2 Eloise ......
2.4.3 Frederic .....
2.4.4 Elena .......
2.5 Return Periods




Evolution of Perdido Key Beach Profiles ................

5.4 Planform Changes .............
6 Summary of Perdido Key Monitoring
6.1 Macroinvertebrate Communities ......
6.2 Vegetation Monitoring ...........
6.3 Perdido Key Beach Mouse ...........
6.4 Physical Monitoring ...............
7 Plans for Future Interpretation/Prediction

REFERENCES

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

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

FIGURE PAGE
1 Location map...................................... 2
2 Geologic column of formations in western panhandle of Florida (Marsh,
1966). .. .. .. .. ... ... ... ... ... ... ... ... ... ..13
3 Geologic cross-section through western Florida's Gulf coast (Marsh,
1966). .. .. .. .. ... ... ... ... ... ... ... ... ... ..14
4 Geologic profiles through Pensacola Bay area (Marsh, 1966) .. .. ....16 5 Locations of Florida DNR survey monuments along Perdido Key. .20 6 Equilibrium throat area vs. tidal prism (O'Brien, 1969). .. .. .. ...23
7 Perdido Key shoreline history, 1858-1902 .. .. .. .. ... ... ....28
8 Perdido Key shoreline history, 1920-1978 .. .. .. .. ... ... ....29
9 Perdido Key shoreline history, 1858-1978 .. .. .. .. ... ... ....30
10 Perdido Key shoreline changes, 1858-1978. .. .. .. .. .. ... ...31
11 Pre- and post- nourishment beach profiles for range R-45, Perdido
Key, Florida .. .. .. .. .. .. .. ... ... ... .... ... ... ..33
12 Profile scaling parameter, A, vs. sediment size, D, and fall velocity, w
(Dean, 1987). .. .. .. .. .. ... ... .... ... ... ... ....35
13 Effect of sediment size on equilibrium beach profiles (Dean, 1991) .. 36
14 a) Commonly idealized form for post-nourishment beach planform; b)
Actual planform changes at Perdido Key .. .. .. .. .. .. ... ....38
15 Evolution of idealized beach nourishment project. Wave direction Ob
= 00 (normally incident waves; Hb=0.5 m). .. .. .. .. .... .....39




LIST OF TABLES

TABLE PAGE
1 Wave data for Perdido Key area ...................... 6
2 Wind data for Perdido Key area ...................... 6
3 Storms making landfall in Perdido Key/Pensacola area, 1889-1990.
H=hurricane, TS=Tropical Storm ......................... 8
4 Combined Total Storm Tide vs. Return Period for Escambia County.
From Dean and Chiu, 1986 ............................... 11
5 History of maintenance dredging at Pensacola Pass (from Hine, et al.,
1986, and Dean, 1988a; Dean 1988b) ....................... 25
6 Channel Dimension History (from Dean, 1988a; Dean, 1988b)..... 26 7 Average Grain Size Characteristics at Various Profile Locations . 37




PERDIDO KEY
HISTORICAL SUMMARY AND INTERPRETATION OF MONITORING PROGRAMS
June,1991
1 Introduction
A monitoring study of Perdido Key, Florida was initiated in 1989 to document conditions prior to construction of a major beach nourishment project. Plans call for the study to span a five-year period. The study is divided into several components, with a research team devoted to each aspect. The Coastal and Oceanographic Engineering Department of the University of Florida is responsible for documentation of physical conditions at the site; the Institute for Coastal and Estuarine Research at the University of West Florida is conducting the biological study of the island; the Gulf Coast Research Laboratory is responsible for the benthic study; and the Alabama Cooperative Fish and Wildlife Research Unit is monitoring the status of the Perdido Key beach mouse. The primary focus of this report will be on historical and expected future changes relevant to the physical study; summaries of the other components of the monitoring study are presented in Section 6.
Perdido Key is a relatively narrow, linear barrier island located in Escambia County, in the western "Panhandle" of Florida (Figure 1). It trends east-west, so that its southern shore faces the Gulf of Mexico; its northern coast abuts Big Lagoon and Old River. Perdido Pass (the entrance to Perdido Bay) forms the western boundary of Perdido Key, and the island is bordered on the east by Pensacola Pass. A major U.S. Naval facility is located in Pensacola, requiring that Pensacola Pass be maintained to a navigable depth at all times. Pensacola Pass has been dredged for over a century for navigation purposes, with the required
1




0 5 km

Figure 1.

Location map




depth increasing with time. Recent requirements to increase the depth further have provided a large quantity of sediment for beach nourishment on Perdido Key.
The eastern 10.5 km of Perdido Key is part of the Gulf Islands National Seashore, administered by the National Park Service. The beach nourishment project lies wholly within the National Seashore boundaries. Much of the dredge spoil available for beach nourishment has already been placed on the beach, initially increasing its width by approximately 150 in over a distance of 8 km. Approximately 3.4 million in 3 of the remaining suitable material will be placed in the littoral zone just offshore of Perdido Key.
The physical monitoring study involves the collection of a large amount of data that should assist in the prediction of future changes at Perdido Key. Hydrographic and topographic surveys are to be conducted annually, and sediment samples collected simultaneously throughout the project area. Wave, tide, and current data are being collected essentially continuously by an offshore wave gage and current meter. Atmospheric data are recorded by a weather station at the Perdido Key Ranger Station. These data sets will provide information necessary for calibration of numerical simulation techniques for prediction of changes at Perdido Key. Detailed analysis and prediction of changes will require a more substantial data base; only one post-nourishment survey is presently available. Based upon experience from other similar sites, however, some general expectations will be discussed.
2 Environmental Conditions and History
Very little environmental data exist for the innnediate vicinity of Perdido Key. The ongoing data collection efforts of the University of Florida's Coastal and Oceanographic Engineering Department (Work et al., 1991a) and a previous




study by Psuty (1987) are the only known studies where in-situ data were collected. Oceanographic data collected by the University of Florida include: wave height, period, and direction; tidal stage; and magnitude and direction of mean currents. All data are collected by one instrument, located in approximately 7 m of water. The gage is located offshore of the Perdido Key Ranger Station. An additional tide gage is located on the Fort Pickens pier on the Sound side of Santa Rosa Island. A weather station is located at the Perdido Key Ranger Station, collecting air temperature, wind direction, wind speed, rainfall, and wetness data. Data collection for both units commenced in January, 1990.
The study by Psuty (1987) included collection of nearshore pressure and velocity records at several locations along Perdido Key. Only five days of data were collected, however, so the information is not of significant help in an historical study.
The remaining available data for the area typically offer very coarse spatial or temporal resolution, or were obtained using very crude data collection methods (e.g. visual observations of wave heights). The review provided here is not intended to be exhaustive, but rather to provide an overview of the environmental conditions to which the study area is exposed.
2.1 Wave Climate
Weather and sea state observations obtained from ships offshore of the northwestern coast of Florida are available for the period 1859-1971, with eighty percent of the records between 1952 and 1971 (U.S. Naval Weather Service Command, 1975). The spatial resolution is so coarse, however (grid size is approximately 2.5' of latitude by 2.5' of longitude), that the data are of little value for investigation of shoreline changes along Perdido Key. The fact that ships tend to avoid severe weather also tends to bias this data source.




Additional visual observations are documented by Balsillie (1975). These observations differ in that they were obtained at a number of nearshore sites. Wave height, direction, and period were all measured visually by an observer on the beach or a pier, and longshore currents were measured by visual observation of the movement of a cloud of dye placed in the surf zone. Beach profiles were surveyed in some cases. The data collection site nearest Perdido Key was located at Fort Pickens State Park (now part of the Gulf Islands National Seashore), on the western end of Santa Rosa Island. Eight months of data (September-October, 1969, and March-August, 1970) are available, with typically 20 to 30 readings per month. While quite useful for the identification of trends, visual observations cannot be relied upon for accurate, quantitative values describing the nearshore wave climate.
Hindcast data are also available for the area. Hindcast studies typically begin with maps of the atmospheric pressure throughout the area of interest. The pressure field is used to compute the wind field, and a wave generation model is used to compute the wave climate. The usefulness of the resulting data is dependent on the accuracy of the maps and the methods by which the wind and wave climates are computed, as well as the spatial and temporal resolution of the results.
Hubertz and Brooks (1989) present the results of a hindcast study for the Gulf of Mexico, spanning 20 years (1956-1975). Data are reported at three-hour intervals, using a 55 km grid. Since one grid cell is larger than Perdido Key, this hindcast data could not be used directly for modelling of coastal processes along the island, but could be used as input to a model having finer resolution. Additionally, this data set does not include tropical storms or hurricanes.
Table 1 presents representative wave data from the sources cited above.




Table 1: Wave data for Perdido Key area.

Data Source Location Technique Period of H., m Period, sec
Record Max. Avg. Max Avg.
USNWSC (1975) Area 26 (Pensacola) Visual 1949-1971 8-10 1.1 >13
Balsillie (1975) Ft. Pickens St. Pk. Visual 9/69-8/70 0.66 (Hb) 5.78
Hubertz/Brooks (1989) Sta. 29 (Pensacola) Hindcast 1956-1975 3.7 1.0 9.6-10.5 5.2
Work et al. (1991a) Perdido Key In-situ 1/90-10/90 1.7 0.41 11.6 5.8
2.2 Winds
Wind data are available from several of the same sources discussed above.
The measured wind speeds, directions, and air temperatures at Perdido Key
were compared to values from the National Weather Service weather station at
Pensacola, with favorable agreement (Work et al., 1991a). Table 2 below summarizes wind conditions in the study area. March is generally the windiest month
of the year, with August having the least wind (U.S. Dept. of Commerce, 1981).
Wind directions are the compass heading from which the wind originates.
Table 2: Wind data for Perdido Key area.
Data Source Location Period of Record Wind Speed, m/s (mph) Wind Direction
Max. Avg. Avg.
USNWSC (1975) Area 26 (Pensacola) 1950-1971 25+ (55) 6.0 (13.5) 1290
Balsillie (1975) Ft. Pickens St. Pk. 9/69-8/70 10+ (22) ,-140*
Hubertz/Brooks (1989) Sta. 29 (Pensacola) 1956-1975 10-13 (23-29) 1340
NOS (1981) Pensacola 24 yrs 4.5 (10) (1300 hrs) ,90*
Work et al. (1991a) Perdido Key 1/90-4/91 11.2 (25.0) 3.4 (7.6) 1510

2.3 Tides
The tides in the vicinity of Perdido Key are diurnal. The mean range at the entrance to Pensacola Bay is 0.34 m (U.S. Dept. of Commerce, 1990).
2.4 Major Storms
Information regarding storms generally consists of qualitative descriptions of damages incurred or hindcast data. Reported and computed wave heights, wind speeds, storm tides, etc., are often only approximate, and spatial gradients are
6




often strong, meaning that knowledge of a parameter at one location does not imply that conditions at a nearby site are known with much certainty. In recent years, however, there have been a number of storms that have passed directly over moored, offshore data buoys, providing some rare in- -situ data of wave climate during a hurricane.
The hurricane season in the Perdido Key area generally extends from July through October. The earliest recorded hurricane was that of September, 1559, which is said to have caused severe damage in the Pensacola area (Ludlum, 1963). At least eight other major hurricanes struck the same area in the next 200 years (Dunn et al., 1967). A number of severe storms that have impacted the area since 1889 are shown in Table 3. Landfalling tropical storms and hurricanes are fairly common along the Florida panhandle, and can be expected between Pensacola and Panama City at least once every five years, on average (NOAA, 1984). This claim is also supported by the table below.
Hindcast results for the period 1956-1975 (Abel et al., 1989) yielded the following return period/significant wave height combinations (for station 29, closest to Pensacola): 5 years/5.6 m; 10 years/7.4 m; 20 years/9.7 m; 50 years/15.7 m. These values include only hurricane waves; tropical storms were not hindcast. A discussion of some of the more noteworthy storms follows. Only a brief discussion is given; future reports will attempt to quantify storm-induced changes further.
2.4.1 Camille
Hurricane Camille was the most severe to strike the U.S. Gulf Coast in recent history. It made landfall August 17, 1969, near Bay St. Louis, MS, with winds estimated at over 90 m/s (>200 mph). In Pensacola, gusts to 32 m/s (71 mph) were recorded, and storm tides reached 1.8 m, but no records of severe local damage were found (U.S. Dept. of Commerce, 1969).




Table 3: Storms making landfall in Perdido H=hurricane, TS=Tropical Storm.

Key/Pensacola area, 1889-1990.

Landfall Name Max. Wind m/s (mph) H,, m Storm Tide, m (MSL)
Sept. 23, 1889 H
July 7, 1896 H
Aug. 2, 1898 H
Aug., 1901 H 40 (90P)
Sept. 13, 1903 H
Sept. 27, 1906 H 37-45 (83-100P)
Sept. 20, 1909 29 (64P)
Aug. 11, 1911 H 36 (8OP)
Sept. 14, 1912 H 33 (74P)
July 5, 1916 H 46 (104P)
Oct. 18, 1916 H 54 (120P)
Sept. 28, 1917 H 56 (125P)
July 4, 1919 TS
Sept. 15, 1924 H
Sept. 21, 1926 H 68 (152P) 3.2
Sept. 30, 1929 31 (70P)
Aug. 31, 1932 H 40 (90P)
July 31, 1936 H
Aug. 13, 1939 H 26 (59)
Aug. 30, 1950 Baker (H) 32 (72) 1.7
Sept. 26, 1953 Florence (H) Sept. 25, 1956 Flossy (H) 39 (88)
Sept. 8, 1957 Debbie (TS) Oct. 8, 1959 Irene (TS)
Sept. 26, 1960 Florence (TS) Aug. 17, 1969 Camille (H) 32 (71P) 13.2 (offshore; Hmax=22) 1.8P
June 19, 1972 Agnes (H) Sept. 23, 1975 Eloise (H) 57 (125) 8.8 (offshore)
Sept. 12, 1979 Frederic (H) 43 (96P) 8.9 (offshore) 3.3-4.6e
Sept. 2, 1985 Elena (H) 38 (84P) 4.1 (offshore) 0.9P
Aug. 15, 1987 TS

INote: uperscript

"r denotes value for lerdido lKey/-ensacola area, not necessarily maximum for entire storm system.




Wave data were recorded at several offshore oil platforms, one close to the path followed by Camille. The gage on this platform recorded a peak significant wave height of 13.1 m, with a corresponding period of 11.5 seconds (water depth 100 in). The maximum wave height recorded was 22 m, and maximum period 18 seconds (Hamilton and Steere, 1969). Camille resulted in much coastal damage, but the most severe damage was found west of the Perdido Key area.
2.4.2 Eloise
Hurricane Eloise struck (primarily) Bay, Walton, and Okaloosa Counties in Florida on September 23, 1975. Winds up to 24 in/s (53 mph) were recorded in Pensacola during the storm, but there is no record of significant damage to the Perdido Key area.
2.4.3 Fr-ederic
Hurricane Frederic made landfall at Dauphin Island, AL, on September 12, 1979, with winds gusting to 65 in/s (145 mph). Wind gusts up to 43 in/s (96 mph) were recorded at the Pensacola Naval Air Station. Damage was most severe on Dauphin Island and in Gulf Shores, AL, but Perdido Key was also severely damaged. The history and effects of the storm are detailed in U.S. Army Corps of Engineers, Mobile District (1981).
Storm tides along Perdido Key were estimated at 3.3-4.6 m, causing erosion of the beaches and dunes, and breaches in the dune line in several places, but no permanent breakthrough occurred. Large washover fans of sediment transported across the island by this storm are still evident on the north side of Perdido Key. It was estimated that 243,000 m 3 of sand were eroded from 9.8 km of Perdido Key beaches. Three beach profiles on Perdido Key were surveyed by the Florida Department of Natural Resources subsequent to the storm.




A wave height (exclusive of runup) of 4.8 m was reported along Perdido Key, although it is not known how this value was obtained. The storm passed over a data buoy moored offshore, where a maximum wind speed of 34 in/s (76 mph) and maximum significant wave height of 8.9 m were recorded.
The road running the length of the island was seriously damaged by the storm tide. Chunks of asphalt ripped from the roadbed can be found in several places. A U.S. Air Force target vessel, the U.S.S. ex-Ozark, went aground 60 m offshore of Perdido Key after its anchor broke loose. It was later refloated and removed. Although not the strongest storm to hit the Gulf Coast in recent history, Frederic may be the most significant storm in terms of its impact on Perdido Key.
2.4.4 Elena
Hurricane Elena threatened a large section of the Gulf Coast, due to its circuitous path prior to landfall on September 2, 1985, at Biloxi, MS. Winds to 38 in/s (84 mph) were recorded at the Pensacola Naval Air Station (NRC, 1991). Unofficial weather service estimates of windspeed on Perdido Key were 40 in/s (90 mph). Storm surges in Florida were greatest in the Cedar Key and Alligator Point areas, but the most heavily damaged area in Florida was Pinellas County (Bodge and Kriebel, 1985).
2.5 Return Periods
Return periods for storm tides have been calculated for Escambia County as part of the State of Florida's Coastal Construction Control Line program, which establishes requirements for new construction along the Florida coast (Dean and Chiu, 1986). Estimates were obtained through application of a numerical model of hurricane-induced forces on the Gulf coastal waters. Results are given for two locations in Escambia County situated, respectively, east and west of the Perdido Key section of the Gulf Islands National Seashore. The water level/return period combinations are shown in Table 4.




Table 4: Combined Total Storm Tide vs. Return Period for Escambia County. From Dean and Chiu, 1986.

3 Geology
Escambia County lies in the Gulf Coastal Plain physiographic province which extends along the entire gulf coast of the United States. Coastal plain sediments, most of which were deposited during higher stands of the sea, consist of unconsolidated sands, limestones, silts and clays of Cretaceous, Tertiary, and Quaternary age. The coastal plain is approximately 200 miles wide (in the N-S direction) along the Florida panhandle. Cooke (1939) subdivides the coastal plain in this area into two topographic regions: the Western Highlands, consisting of a southwestward sloping plateau whose surface has been incised by numerous streams; and the Coastal Lowlands, consisting of relatively undissected, nearly level plains lying less than 100 feet above present sea level. The Coastal Lowlands occupy a narrow strip 10 to 12 miles wide along the coast and it is within this region that Perdido Key lies.
The coastal plain is bounded to the north by the Piedmont Plateau physiographic province where igneous and metamorphic rocks ranging in age from Precambrian to Paleozoic are exposed at the surface. These ancient rocks extend to great but unknown depths beneath the coastal plain region and are unconformably overlain by the unconsolidated coastal plain sediments which form a southward thickening wedge.

Return Period Storm Tide, m (NGVD)
(yrs) West East
500 4.7 4.4
200 3.9 3.8
100 3.5 3.4
50 3.0 3.0
20 2.2 2.2
10 1.3 1.3




The subsurface geology of Escambia County is more similar to that of the north-central Gulf Coast comprised of Alabama, Mississippi, and Louisiana to the west rather than the geology of peninsular Florida to the east. A detailed description of the geology is given by Marsh (1966) and Coe (1979). The generalized stratigraphic column for the western Florida panhandle is shown in Figure 2. Escambia County lies on the north flank of the Gulf Coast geosyncline (Barton et al., 1934; Howe, 1936) and the east flank of the Mississippi Embayment. These structures contribute to the southwestward dip which is characteristic of all the formations in the area at least as far down as the base of the Cretaceous deposits (Figure 3). Faulting has occurred to the northeast of Escambia County where a west-northwestward-trending graben, the Pollard graben, extends southward from Alabama. The major fault lines in this area are the Jay, Pollard, and Foshee faults which extend downward through the Upper Cretaceous sediments.
The most distinctive feature of coastal plain topography is the Pleistocene marine "terraces" which have been traced by previous workers along the Gulf Coast and along much of the Atlantic coast. For the state of Florida, the findings of these previous workers are summarized in map form (see Florida Geological Survey map series no. 71, 1975). These terraces represent ancient shorelines, deposited during major stillstands or slight transgressions of the sea, during the Pleistocene as sea level fluctuated with the glacial and interglacial periods which characterized this epoch. During these major stillstands or slight transgressions of the sea and similar to the depositional processes going on at present-day coastlines, formation of a barrier island chain occurred with associated lagoonal/marsh sediments being deposited on the landward side of the barrier island sequence. Inlet deposits, estuarine and channel sediments, and a seaward thinning wedge of offshore sediments were also deposited and are therefore also associated with the terrace deposits. Thus, each terrace is basically an ancient barrier island complex, preservation of which has occurred as a result of each subsequent fluctuation in sea level being less than the previous sea level rise.




GENERALIZED GEOLOGIC COLUMN OF FORMATIONS IN THE WESTERN FLORIDA PANHANDLE 3RAPHC
SERIES SETION FORMWATION
PLEISTOCENE MARINE TERRACE DEPOSITS: Sand, IIght tan, fine to coarse
.. . .. .. x
CTRONELLE FORMATION: Sand with lenses of clay and gravel. Sand, lightPLEISTOCENE (?) yellowish-brown to reddish-brown, very fine to very coarse and
.poorly sorted. Hardpan layers in upper part. Logs and carbonace. ous zones present in places. Fossils extremely scarce except near
Sthe coast where shell beds may be the marine equivalent of the . fluvial faces of the Citronelle. .. .. .. .. .. .... .. .. .. .. .
SMIOCENE COARSE CLASTICS: Fossiliferous sand with lenses of clay and S gravel. Sand is light-gray to light-brown, very fine to very coarse and poorly sorted. Fossils abundant, mostly minute mollusks. S Contains a few zones of carbonaceous material. Lower part of coarse clastics present only in northern part of area, interfingering UPPER MIOCENE with Pensacola Clay in the central part.
PENSACOLA CLAY: Formation consists of an Upper Member and Lower Member of dark-to-light-gray, tough, sandy clay; separated by the Escambla Sand Member of gray, fine to coarse, quartz sand. Contains carbonized plant fragments, and abundant mollusks and foraminifers. Pensacola Clay Is present only in southern half of area, UPPER MIDDLE TO -- Interfingering with the Miocene coarse clastics in the central part.
LOWER UPPER MIOCENE '
LOWER MIOCENE AND CHICKASAWHAY LIMESTONE AND TAMPA FORMATION UNDIFFERENTIATED
mUPPER OLIGOCENE mpa: Limestone, light-gray to grayish-white, hard, with several beds
of clay; Chickasawhay: Dolomitic limestone, gray, vesicular. MIDDLE OLIGOCENE BUCATUNNA CLAY MEMBER OF BYRAM FORMATION: Clay, dark-gray soft, silty
to sandv. foraminiferal carbonaceous.
UPPER EOCENE OCALA GROUP: Limestone, light-gray to chalky-white foraminifers extremely
LWUPPEROCENE
abundant, esp. Lepidocvclina: corals, echinoids, mollusks, bryozoans
-- LISBON EQUIVALENT: Shaly lImestone, dark-gray to grayish cream; hard,
compact; glauconitic; with thick intervals of dense, light gray shale. MIDDLE EOCENE
E E E TALLAHATTA FORMATION: Shale and siltstone, light-gray, hard, with numerSous Interbeds of gray limestone and very fine to very coarse, pebbly sand. Foraminifers locally abundant
SHATCHETIGBEE FORMATION: Clay, gray to dark-gray, micaceous, silty, with LOWER EOCENE ---- beds of glauconitic shale, siltstone, and shaly limestone. Mollusks,
foraminifers, corals echinoids. Bashi Marl Member (about 10 feet thick) at base.
Figure 2. Geologic column of formations in western panhandle of
Florida (Marsh, 1966)




Mean Sea O~ 4 4 . m 0 m e or
400 AND cI ELLE c 0 0
800 oc) co-sse >E COL 2cL UD EE
200 sea9 C Ca a
2000" soo po' /xto I. --\OR1Note: Wells shown b
2400 S c o. 0. gUgOprojected along strike into
320 0- r0
2400
5 1000 1 20 miles
Figure 3. Geologic cross-section through western Florida's Gulf coast (Marsh, 1966)




In Escambia County remnants of these terraces are preserved as upland plateaus, flat-topped hills, low coastal plains, and benches along the rivers and bays. Three marine surfaces of Pleistocene age may be recognized in topographic profiles across the area (Figure 4). These surfaces may be associated with the Pamlico shoreline (8 m above present mean sea level) which developed during the late Pleistocene, the Penholloway shoreline (21 m) which developed during early Pleistocene, and a seaward-sloping upland surface whose elevation ranges from approximately 30 to 800 m which is probably a composite of Cooke's Hazelhurst (formerly Brandywine) terrace (Cooke, 1945) and MacNeil's "high terrace" (MacNeil, 1949).
Presently, the bulk of the northwest Florida coast is eroding with eroded material being deposited at spit termini rather than being lost offshore. The 48 kmn stretch of Santa Rosa Island between Pensacola Beach and Fort Walton Beach is the only coastal stretch prograding seaward along this area (between 1934 and 1965/69 seaward growth average rate of 0.6 m/yr).
From his study of the beach ridge plains between Pensacola, FL and Mobile Point, AL, Stapor (1973) concludes that there has been a complex history of interrupted deposition rather than constant, continuous construction along this stretch of coastline. Stapor (1973) observes that in this area: (1) net coastal erosion has replaced net seaward growth of beach ridge plains, (2) relatively young, high coastal dunes presently migrate over older beach ridge plains, and
(3) heavy mineral concentrations found along present eroding beaches are absent in beach ridge plains. From these observations, Stapor (1973) suggests there has been a shift from an economy of abundant sand (promoting beach ridge construction) to one of a shortage of sand (net coastal erosion) where exact timing of this shift for individual regions depends on the depletion of local sand supplies.
Stapor (1973) suggests that a series of longshore drift cells, rather than one well-integrated longshore drift system, characterize the northwest Florida coast,




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with some of these drift cells appearing to be interconnected, but many apparently experiencing little net exchange of sand with either adjacent cells or offshore regions. He proposes that shoreline changes from records going back to 1871 indicate that Santa Rosa Island was probably composed of several longshore drift cells, possibly experiencing net communication, but not a single, well integrated system, with Perdido Key appearing to have also been characterized by similar cellular transport.
In the north-central Gulf coast, core data (Otvos, 1979) provide supporting evidence that shoal-bar aggradation (Otvos, 1981), rather than spit segmentation (Gilbert, 1885) or mainland dune-ridge engulfment (Hoyt, 1967), has been the predominant mechanism of barrier island formation. In contrast with the transgressive Atlantic coast where barrier island evolution is typically associated with landward migration (de Beaumont, 1845), the barrier islands of the Gulf coast between Gulfport and Pensacola seem to have emerged from shoals practically "in place" and have shifted only laterally to the west in the general direction of littoral drift (Otvos, 1979).
Another barrier island type also recognized along the north central gulf coast is the secondary "composite" barrier island. This type of island is characterized by the presence of a shallow pre-Holocene (usually Pleistocene) core extending near or above present sea level which is veneered by Holocene shoreface, beach and dune deposits. During island formation the pre-Holocene core acts as a stabilizer while further seaward and longshore progradation take place. This island development pattern has been observed at eastern Dauphin Island (Otvos, 1976, 1979), central Santa Rosa Island (Otvos, 1982), Deer and Round Islands, as well as at several South Carolina, Georgia, and northern Florida islands (e.g., Hilton Head, Sapelo, Ossabaw, St. Catherines, and Wassaw Islands).




4 History of Human Impacts on Area
Since the beach nourishment project motivating this study represents a major human modification to the natural environment, it is useful to investigate the history of human impacts on Perdido Key. Due to the dynamic nature of such a site, evidence of small-scale, man-made changes to the island will be quickly erased. The focus will be on the eastern portion of Perdido Key, land which is now part of the Gulf Islands National Seashore.
Several factors combined to make Perdido Key an unlikely spot for early settlements. Its lack of freshwater, terrestrial food sources, and soil suitable for agriculture; vulnerability to storms and the accompanying flooding; and the remote location are the most obvious deterrents to the establishment of a long-term settlement. An archeological survey of Perdido Key (Prokopetz, 1974) presents a partial history of man on the island. A Fort Walton period (1400-1750 A.D.) aboriginal site was discovered, and it was suggested that several other sites from this and other periods may have existed, but sites such as this had minimal lasting impact on the physical nature of the island.
The Spanish, French, and British all claimed the Pensacola Bay area at various times from the mid-l6th century until the United States took possession from the Spanish in 1821. There is no record of any settlement or activity on Perdido Key during this time, however.
The initiation of construction of Fort McRee on the eastern end of Perdido Key in 1831 appears to have been the first significant modification to the natural condition of the island. Fort McRee was destroyed during the Civil War and abandoned afterwards. The land upon which the fort sat has been eroded; remains of the fort are thought to be in or adjacent to Pensacola Pass. A defense battery was constructed during World War I roughly 600 m from the former location of Fort McRee. Ruins of this structure are still visible.




In recent decades Perdido Key has emerged as a beach resort area. The intracoastal waterway through Big Lagoon was authorized in 1933 and dredged in the early 1940's to a depth of 3.6 mn (12 ft) and a width of 38 m (125 ft). Condominiums and other housing units have been constructed along with service industry facilities (restaurants, gas stations, etc.). Because of its incorporation into the Gulf Islands National Seashore in 1971, the eastern half of Perdido Key has not been developed in this manner, but has been affected by man. Aerial photos from 1987 show two short rock groins in the vicinity of Florida Department of Natural Resources (DNR) survey monument R67. (See Figure 5 for locations of survey monuments). These groins are approximately 100 mn long and extend 15 mn into Pensacola Pass. These groins are presently still visible, but deteriorating. No other hard structures for erosion control are visible in the 1973 photos or have been built within the National Seashore boundaries since that time.
Several construction projects have been completed for use by visitors to the National Seashore. A visitor center and parking lot were constructed near monument R34. A paved road leads from this area north to a boat launching ramp on Big Lagoon. The access road to the park and leading east as far as monument R44 was paved prior to National Park Service acquisition of the land. Major storms (such as Hurricane Frederic in 1979) have damaged the road. Blocks of asphalt ripped up from the roadbed during storms can be found immediately north of the road in several places. A lighted Coast Guard navigation tower is located near monument R67 on the eastern end of the island.
Off-road vehicles formerly resulted in significant impacts to Perdido Key (Shabica and Cousens, 1983). Aerial photos from 1973 reveal multiple vehicle tracks along the length of the National Seashore, with typically one track along each coast and another down the center of the island. The destruction of vegetation that occurred resulted in a National Park Service decision to ban recreational




/ 0 1

Meteorological

2 3 4 5km
I I -===

Approximate Westerly Park Boundary

NOTE:
R-40 Is Florida Department of Natural Resources Monumented "Range 40"

Figure 5. Locations of Florida DNR survey monuments along Perdido Key




off-road vehicle use on Perdido Key in 1979. Following recovery of the vegetation, off-road vehicle use was again permitted in August, 1981. Since the mid-1980's, only National Park Service and research vehicles have been allowed on the beaches of the Perdido Key section of the Gulf Islands National Seashore.
Beach nourishment represents the most obvious human impact on Perdido, Key. In July, 1985, approximately 1.86 million m' of sand was placed on the south shore of Perdido Key for beach nourishment, between monuments R59 and R65 (Hine et al., 1986). The 1989-90 beach nourishment project motiviating this study is much larger in scope, nourishing the region from monument R41 to R64 and involving up to 8 million m' of material by the time it is completed. The description of the effects of a change of this magnitude will follow the discussion of inlet/barrier island dynamics presented in the next section.
4.1 Tidal Inlets and Barrier Islands
Dredging of a tidal inlet for navigation improvement often disrupts the relationship between the inlet and adjacent barrier islands. Because of the long history of dredging at Pensacola Pass, it is plausible to suggest that this may have significantly affected Perdido Key. A general discussion of the interaction between a tidal inlet and the adjacent lands is therefore appropriate.
Tidal current velocities through an inlet are a function of the tidal range, the surface area of the bay behind the inlet that is subject to tides, and the cross-sectional area of the inlet throat. The tidal range and the area of the bay are generally fixed by nature; man typically increases the inlet throat area by increasing the depth for navigation purposes. Since the volumetric fiowrate through the inlet remains unchanged and the cross-sectional area is increased, the velocity must decrease unless the width is decreased correspondingly.




O'Brien (1931; 1969) found that for a natural tidal inlet, there is generally an equilibrium cross-sectional area that is a function of the volume of flow, or tidal prism, that must enter and leave the inlet during each tidal cycle (Figure 6). If the throat area is increased, the velocity through the inlet is reduced, allowing deposition of sediment in the channel. Conversely, if the throat area is decreased (typically by a storm forcing sediment into the inlet), the increased velocities will tend to scour material out until the inlet returns to its equilibrium condition. O'Brien found that the peak spring tide velocity associated with equilibrium is approximately one meter per second. Unfortunately the equilibrium configuration rarely provides adequate depth for safe navigation of modern vessels.
In its natural state, flood waywardd) and ebb (seaward) tidal flows will tend to flush sediments in and out of a tidal inlet to maintain its equilibrium geometry. The sediment carried out of the inlet will be transported until the flow velocity decreases below some threshold required for sediment motion. Since sediment is transported in this manner during both flood and ebb tides, large deposits of sand are typically found both seaward and wayward of the inlet throat. The depth of water over these shoals is generally less than that in the inlet, making them a serious impediment to navigation.
The offshore wave climate generally limits the size of the ebb tidal shoal. Larger waves tend to force material from the shoal back onshore, until a dynamic equilibrium is reached between the onshore stress exerted by the waves and the offshore stress exerted by the ebb tidal flow.
The local wave climate is also important to the inlet/barrier island "sediment budget". At most sites, the dominant waves tend to come from some direction other than shore-normal, driving alongshore current which carries sediment along the beach. Neglecting changes in the cross-shore direction, it can be said that as long as there is no spatial variation in this longshore sediment trans-




A MINIMUM FLOW AREA (ft2)

Figure 6.

Equilibrium throat area vs. prism (O'Brien, 1969)

z
C',
z C',
a
-J
0
IL
P-

7
10




port rate, there will be no change in the planform of the beach. Maintenance of the inlet/barrier island equilibrium condition therefore requires that sediment be passed from the updrift barrier island across the ebb tidal shoal to the downdrift barrier island. Stabilization of an inlet with jetties and/or dredging through the ebb tidal shoal effectively interrupts this longshore transport of sediment, typically resulting in accretion updrift and erosion downdrift of the inlet.
4.2 Pensacola Pass/Perdido Key
The above discussion is relevant to the case of Pensacola Pass, a natural tidal entrance which has been dredged for over a century. The longshore sediment transport rate in the area has been estimated at 200,000 m3/yr, from east to west (Dean, 1988a). Thus in its natural condition, sediment should be passed from Santa Rosa Island onto the ebb tidal shoal and over to Perdido Key west of the inlet, the ebb shoal acting as a "bridge" to carry the sediment across the inlet. Dredging of Pensacola Pass has effectively cut through the ebb tidal shoal, leaving Middle Ground and East Bank shoals east of the pass and Caucus Shoal to the west. Because the deepened channel effectively interrupts the longshore transport of sediment across the inlet, erosion of Perdido Key should be expected. This has been found to be the case.
Erosion downdrift of a deepened tidal inlet is often mitigated by sand bypassing, where sediment accreting updrift of the inlet and filling the channel is dredged and placed on the downdrift beaches. At Pensacola Pass, however, most dredged material has been disposed of offshore, removing material from the sediment budget. Table 5 provides the available data regarding dredging quantities and disposal areas. As shown in the table, historical records indicate that approximately 28 million m3 of material has been dredged from Pensacola Pass (prior to initiation of the ongoing project); all but 7.2 million m3 of this material has been




Table 5: History of maintenance dredging 1986, and Dean, 1988a; Dean 1988b).

at Pensacola Pass (from Hine, et al.

Date Source Disposal Area Quantity, rmT

1883 4/30-6/30 1885 5/25-6/30 1885 7/1-8/9 1886
1891 7/1-7/28 1893 6/14-6/30 1894 7/1-8/15 1896
1897 4/20-7/19 1897 7/1-7/12 1898 2/8-7/27 1899 2/8-6/30 1900
1901
1902
1905
1906
1907
1908
1909
1910 7/1-10/7 1911
1914
1915
1917
1921
1922
1927
1930
1932 4/24-6/30 1933 7/1-10/5
1934 1/23-4/16 1934 9/16-10/31 1935 8/22-9/22 1937 8/22-9/18 1938 8/14-9/17 1939
1940 7/1-7/22 1940 9/9-10/10 1946
1947 4/30-6/30 1947 10/14-10/31 1948
1950 6/1-6/30 1951 7/1-7/15 1953 11/2-11/22 1955 11/9-11/19 1958 11/13-11/19 1959 7/7-8/9 1959 7/12-9/4 1959 9/30-11/30 1964 9/28-12/12 E 1967
1968

Entrance Chi.
Entrance Oh.
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Entrance Oh.
Entrance Oh.
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Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
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Entrance Oh.
Entrance Oh.
Entrance Oh.
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Entrance Oh.
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Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
Entrance Oh.
,ntrance and Harbor
Entrance Oh.
Entrance Oh.
Entrance Oh.
ntrance and Turning
Entrance Oh.
Entrance Oh.

Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Open Water Santa Rosa Pt. Santa Rosa Pt. Santa Rosa Pt. Open Water Open Water Open Water

Continued

6400 28,300
12,500 42,400 10,800 21,300 70,800 300,000 230,000 5700 195,000 226,000 367,000
248,000 141,000 153,000
453,000 219,000
654,000 235,000 292,000 731,000 295,000
14,500 187,000 105,000 189,000 602,000 707,000 516,000 588,000
447,000 529,000
284,000 356,000 392,000 86,300 232,000
504,000 165,000
647,000 92,800 361,000 303,000 176,000
418,000 202,000 208,000
743,000 3,019,000 1,538,000 1,898,000 937,000 720,000




Table 5: Continued

Table 6: Channel Dimension History (from Dean, 1988a; Dean, 1988b).

Date Source Disposal Area Quantity, rn
1969 1/4-1/10 Entrance Oh. Open Water 167,000
1970 Entrance and Turning Open Water 183,000
1971 9/1-10/19 Entrance Oh. Open Water 1,195,000
1971 1/15-1/22 Entrance Oh. Open Water 131,000
1975 1/31-2/14 Entrance Oh. Open Water 840,000
1981 2/8-2/19 Entrance Oh. Open Water 500,000
1983 12/13-12/23 Entrance Oh. Open Water 87,000
1984 Entrance Oh. Open Water 700,000
1985 6/20-7/31 Entrance Oh. Perdido Key 1,860,000
1987 Entrance Oh. Open Water 150,000
1989-91 Entrance Oh. Perdido Key Ongoing

Year Depth (in) Width (in) Authorized or Actual 1881 7.3 24 Authorized
1885 6.9 24 Actual
1890 7.3 37 Actual
1896 9.1 91 Authorized
1902 9.1 152 Authorized
1935 9.8 152 Authorized
1959 11.3 244 Actual
1988 13.4 244 Authorized
1989 14.6 244 Authorized




removed from the local sediment budget by offshore disposal. Table 6 presents channel dimensions.
In general, the more the depth of a channel is increased beyond its "natural" depth, the greater the dredging rate required to maintain its depth. Part of this is due to the reduced tidal velocities as described earlier, but the increased slopes of the channel walls also lead to slumping of material into the channel. This could lead to erosion of both islands adjacent to the inlet.
4.3 Historical Shoreline Changes
Historical shoreline change data are often useful for calibration of numerical models of shoreline change that can in turn be used to predict future changes. Digitized shoreline position data were obtained from the Florida Department of Natural Resources (DNR). Figures 7 through 10 show shoreline changes at Perdido Key, spanning over a century. The data obtained from the Florida DNR provide shoreline positions in a artesian state plane coordinate system. A coordinate transformation was used to generate the referenced figures. The new coordinate system was defined so that its origin is near the center of Pensacola Pass, and the direction of the y-axis approximates the average shore-normal azimuth of 167.8', measured from North. The x-axis therefore corresponds to distance from Pensacola Pass. Shoreline changes are simply defined as the change in the y-coordinate between the two surveys.
Inspection of the figures reveals that the most significant erosion on Perdido Key has been within the Gulf Islands National Seashore, corresponding to O< x <10,500 rn in the figures. West of this region (x >10,500 m), a net accretion is evident. The eroded and accreted volumes approximately balance; one possible interpretation is that material eroded from the eastern end of Perdido Key has been transported westward and deposited west of the National Seashore.




PERDIDO KEY SHORELINE HISTORY

800.0
700.0
600.0
- 500.0 C:
400.0 rr3
ED)
CC
C 300.0

LL1
0 C 0.0
1 200.0
CD

0.0
-100.0
-200.0

4000.0 6000.0 8000.0 10000.0 12000.0 14000.0 16000.0
DIST. FROM PENSACOLA PRSS, M
Figure 7. Perdido Key shoreline history, 1858-1902




PERDIDO KEY SHORELINE HISTORY

DIST. FROM PENSACOLR PASS, M
Figure 8. Perdido Key shoreline history, 1920-1978

800.0 700.0 S600.0

U
to LJ
H
CC
1
CD)
U

500.0
400.0 300.0
200.0 100.0
0.0
-100.0
-200.0




PERDIDO KEY SHORELINE HISTORY
800.0
1858
--- ------------------ 1890/95
700.0 -1934t
- - - - -1978
600.0 /
--------------------------,7
LL.Ju .\ ,"
500.0
t! "\"'" ..~.............."..... .... "" "kr.
300.0.
S-. / -,. \
cc i "":..'-4.
200.0 -.
100.0
:1
Sii '"'
0.0
-100.0
-200.0 I
0.0 2000.0 4000.0 6000.0 8000.0 10000.0 12000.0 14000.0 16000.0 18000.0 20000.0 22000.0
DIST. FROM PENSRCOLA PASS, M

Figure 9. Perdido Key shoreline history, 1858-1978




PERDIDO KEY SHORELINE CHNGES

250.0
200.0
150.0
1 100.0
CD
CE 50.0
U
Z 0.0
-J
n-" -50.0
(F)
-100.0
-150.0 -

____________1858-90/95
---------------1890/95-1934
- -- -- ---19314-1978
- -- -- -- 1890/95-1978

I/ \\

.1 '~/- /

/

'I t
l'/
i'

2 0
2000.0 4000.0

1
6000.0
DIST.

I
8000.0
FROM

I I
-4

I I
10000.0 12000.0
PENSACOLA

I I I 1
14000.0 16000.0 18000.0 20000.0 22000.0
PASS, M

Figure 10. Perdido Key shoreline history, 1858-1978

-200.0




Data collected as part of the ongoing monitoring of the nourished beach will help clarify the sediment transport processes affecting Perdido Key. Future work will hopefully add to the available historical database and allow a more detailed analysis.
5 Changes Due to Beach Nourishment Project
A beach nourishment project can change the character of a beach in a number of ways. The focus here will be on physical changes, primarily the shape and size of the beach. Because of the complex nature of most shoreline change problems, sediment transport is generally divided into components: cross-shore (perpendicular to the beach) and longshore (shore-parallel) transport. Both modes are important subsequent to a beach nourishment project. Cross-shore changes will be discussed first.
5.1 Cross-Shore Changes
Figure 11 provides a representative example of conditions immediately before and after the beach nourishment project. The horizontal axis denotes distance from the survey monument, and the vertical indicates elevation. Common traits of the pre-nourishment profiles sampled, including this example, are a sequence of dunes reaching 3-5 m in elevation, seaward of which a relatively narrow, unvegetated beach is present. The subaqueous portion of the profile is characterized by a planar slope in the swash zone, a distinct breakpoint bar, and a very mild slope beyond the -5 m contour.
The pre-nourishment condition can be considered representative of an "equilibrium" beach profile. With this in mind, the profile immediately after completion of the beach nourishment is clearly oversteepened and thus out of equilibrium. The dry beach width has increased roughly 150 m, the nearshore slope is




I .......I... ........ ... .... ..... .J . ... -- -- -- -- -- -- -- . L .... ---.. : ..... . . ... .' ..:.. .. .... L. . -...-...- ........ ..... .. .
......... : ...... ... .... .............. ........ ... .......... .... . .. ... ................ ........ ........ ... .... .. .. ....... ...
--4 -- ... .. ... --4-- ........ .--i.... .......... .......... .... -4.... .........i .. .. -- ...... ... .... ......... -:... ...
. .... .... ... .... ..* ,. .
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T....... ........ ..,.. ... .. ........ ..,.,., ... .. ... .,.... . .... ....,... .,. .,.. .....,....... ., ... -,-.- -.- --.-.-... .,.. .... .,7......... ... ... ... ...... ... .. ... ...... ... ... ..... .......
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o ... .. .. ... .. .............. .....
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.. .+ .. ...... .+... ....... ---- . ... .... .... ... ..... ,.. ........ .. ...... .... ....: :
::: ::: :::::::::::: ::: :: :: ::: ::::: :: :: ::: :: :::::::::: ::.::........:: ::: :..:: ........... ..............-- -.- .... ....... ."!" ... ... .... "I ........ ............. ... ... ...... .. .. -. ...... : :::: ::::; :
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. .. ........ ........ -- --- --- -- ......... --- --- -- ---* ---. .--- ,-:- -:-, .... -... ........
- - - .. . . . . r- - - - - - - - - - - - - - - - - - ... . . . . . . . - --.. - --. -.. . - - - - - - - - - ---.. . . . .. ... --.. .. .... .
,' : : : : . .. .. ------.. . . . .,. . . . . ..... ... .....L 1.. I..... I..,........I ... ... ..... .. .... . . .. .
... -t ... ... ... - .. ....... - ....... ----- -------- --- ..--- --- .... . .. . . ----- .. .. .. - . .. .. . .. - .. .
.. . .. .J- --L. ..S .. .... . .

-400

-300

-200

-100

GULFWAHRO DISTANCE FROM MONUMENT (METERS)

500

SEPTEMBER. 1990
- MEASURED PROFILE
-- -CONSTRUCTED PROFILE

OCTOBER, 1989
- MEASURED PROFILE
-- -CONSTRUCTED PROFILE

RANGE:R-45 1/2
SURVEY BEAR(HAG.N) :1600
PROJECT:PERDIDO KEY

Figure 11. Pre- and post-nourishment beach profiles for range R-45, Perdido Key,
Florida




much steeper, and no bar exists. With time, it is anticipated that the nourished beach will also evolve toward an equilibrium condition.
5.2 Equilibrium Beach Profiles
Bruun (1954) first proposed the simple relationship describing equilibrium beach profiles, based on data from the Danish North Sea coast and Mission Bay, California:
h(y) = A Y2/3 (1)
where h is the water depth at some distance y offshore of the still water line, and A is a scaling parameter dependent primarily on sediment characteristics. Later studies by Dean (1977) and Moore (1982) helped establish the relationship between sediment size and the scaling parameter, A, shown in Figure 12.
The equilibrium beach profile theory predicts an unrealistic infinite slope at the shoreline, and monotonically increasing depths offshore, thus ruling out offshore bars, but has been found to be of use in many situations. Dean (1991) discusses many applications of the theory.
As indicated in Figure 12, larger sediment sizes correspond to larger values of the A parameter, and therefore steeper beach slopes. If two beaches can each be characterized by a single sediment size, one having coarse sediment and the other fine, the resulting profiles are as shown in Figure 13.
5.3 Evolution of Perdido Key Beach Profiles
The previous discussion has emphasized the importance of sediment size on beach profiles. Table 7 illustrates the sediment sizes at the various sampling depths at Perdido Key before and after nourishment of the beach. Note that these values have been obtained by averaging values from each beach profile, 34




SEDIMENT FALL VELOCITY, w (cm/s)

10.0

100.0

0.01 1.0
F C
0.10
0.01
0.01

SEDIMENT SIZE, D (mm)

Figure 12.

Profile scaling parameter, A, vs. sediment size, D, and fall velocity, w (Dean, 1987).

0.1 1.0 10.0 100.0




DISTANCE OFFSHORE (M) 100
z 0 =.6 mm
I
LU
0

Effect of sediment size on equilibrium beach profiles (Dean, 1991)

Figure 13.




so there is more variation than indicated here, but little variation is evident either spatially or temporally. Since the nourishment operation did not markedly change the sediment sizes, the nourished beach should be expected to evolve to an equilibrium profile that differs little from the pre-nourishment condition. Equilibrium beach profile theory holds that if there is no change in sea level or sediment size, the effect of a beach nourishment project is to simply displace the profile seaward with no change in form. The time required for equilibration of a nourished beach profile is not well-known; one benefit of the ongoing monitoring study will be the acquisition of data defining this time scale.
Table '7: Average Grain Size Characteristics at Various Profile Locations Gulf Side
Location Pre-Nourishment Post- Nourishment
Average D5o (mm) Average Dso (mm)
Dune 0.36 0.35
Berm 0.37 0.38
Beachface 0.38 0.39
-1 m 0.36 0.28
-2 m 0.30 0.34
-5 m 0.32 0.32
-8 in/End of Line 0.32 0.32
5.4 Planform Changes
A beach nourishment project generally increases the beach width by nearly a constant amount over a fixed length of beach. With an initially straight beach, the post-nourishment planform can then be idealized as shown in Figure 14a.
The simple planform change idealized in Figure 14a lends itself well to analytical solutions describing the shoreline evolution in time. With the additional assumption of a spatially and temporally uniform wave climate (often a very restrictive assumption), the shoreline evolution is as shown in Figure 15. Numerical




Idealized Beachfil
T
1 . . . . .. . . . .

01 2 3g45 D 6 7 8, 910
Longshore Distance, km

Perdido Key Beachfill

1 1 1
E
C.
0
a)
C

30 35 40 45 50 55 60 65
Range Number

Figure 14. a) Commonly idealized form for post-nourishment
beach planform; b) Actual planform changes at
Perdido Key
38

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




Initial Condition
................
T =1 year T= 3 years T =5 years T= 10 years

Longshore Distance, km

Figure 15.

Evolution of idealized beach nourishment project. Wave direction eb = 00 (normally incident waves; Hb = 0.5 m)




(computer-based) solutions of the governing equations for this beach nourishment problem are often employed to allow simulation of more realistic geometries and wave climates.
The planform changes described above result totally from longshore gradients in the longshore sediment transport rate. The local wave climate and shoreline orientation are the key variables in determining the magnitude of this transport. An accurate prediction of the expected changes at Perdido Key would require application of a refraction model to determine the wave climate throughout the project domain. A realistic forecast of the offshore wave climate would also be required. Future work will discuss further the anticipated planform changes at Perdido Key.
6 Summary of Perdido Key Monitoring
This section provides an overall summary of all of the monitoring components that are underway on Perdido Key. The purpose is to attempt to integrate results into a comprehensive framework to characterize the effects of the beach nourishment project and to identify any unexpected impacts of the nourishment.
The main information available to date is that available in the progress reports for the first year by the various investigators. These progress reports describe experimental techniques and design, and pre-nourishment conditions, but to date do not encompass nourishment conditions.
A brief review is provided below of each of the study components.
6.1 Macroinvertebrate Communities
This study is being conducted by Heard et al. (Rakocinski, 1990) at the Gulf Coast Research Laboratory in Ocean Springs, Mississippi. Experimental design includes an extensive field sampling program, laboratory analysis and characterization of the macroinvertebrate samples.




Four transects oriented approximately perpendicular to shore have been established. Along each of these transects, nine sampling stations were located at the following distances in meters from the shoreline: 0, 25, 50, 75, 100, 150, 300, 500 and 800. These 36 stations were complemented by eleven swash zone samples ranging from DNR Monuments 34 to 67. In addition, four lagoon stations were established. Sampling methods included use of box-core, yabby pump, berm trawl and kicknet equipment. Water quality data included temperature, dissolved oxygen, salinity and pH. Sediment samples were collected at the various stations.
The samples have been analyzed and the results and additional data stored in databases. Analysis of biota includes presentation, as a function of distance offshore, of the following variables: species richness, total density, and diversity. Also, for the four transects, the following sediment characteristics were plotted as a function of offshore distances: median grain diameter, percent silt/clay and the standard deviation of the sediment size.
The "level of disturbance" was identified as the most significant factor in the distribution and other characteristics of the macroinvertebrates. This "level of disturbance" is believed to be due to turbulence, primarily wave-induced. It is hypothesized that the nearshore communities which, by their presence, are wellsuited to an energetic region will recover quickly following beach nourishment. However, the offshore communities will be slower to adapt.
Various advanced statistical tests are reviewed as to their appropriateness in identifying a causal relationship between beach nourishment and changes in macroinvertebrate density or other characteristics.
41




6.2 Vegetation Monitoring
This component of the study is being conducted by Gibson et al. at the University of West Florida. The general objectives of the study include characterization of the types, densities and distributions of vegetation prior to and following nourishment and, identifying differences and quantifying rates of colonization.
The first annual report (Gibson and Looney, 1990) presents monitoring results of Autumn, 1989 and Spring and Summer, 1990, prior to and during nourishment, respectively. Permanent plots along thirteen cross-island transects were established and monitored. The plots are located at specified distances along each transect and extend from the mean high water (MHW) line on the Gulf side to the waterline on the Lagoon side. Thus the number of plots per transect depends on the island width, varying from 10 to 39. Monitoring includes the vegetative cover within each plot and the buildup of sand within each plot. Specific attention was focused on Uniola Paneculata (sea oats) to determine the number of seedlings and total number per quadrate. Sampling in four macroplots was continued. These 15 m x 25 m plots include: pioneer beach, swale marsh and woods and have been monitored regularly since 1983. Plans are to conduct monitoring during the Fall, Spring and Summer seasons. The data collected during the Autumn, 1989 and Spring, 1990 monitoring have been analyzed and developed into a data base. Special statistical analysis techniques have been applied to classify the "statistical and ecological relevance" of the various vegetation types.
Analysis results include the identification of '79 new plant species in the Gulf Islands National Seashore of Perdido Key. Statistical procedures identified nine basic vegetation types in Autumn, 1989 and again in Spring, 1990. Species abundance and distribution of vegetation types was established.




It is anticipated that the vegetation data base formed before and during nourishment coupled with the statistical methods employed will provide a good basis for evaluating post-nourishment data and thus identifying effects.
6.3 Perdido Key Beach Mouse
The study of the Perdido Key Beach Mouse is being carried out under the direction of Dr. Nicholas Holler, Unit Leader of the Alabama Cooperative Fish and Wildlife Research Unit at Auburn University. The overall purpose of this study is to identify any impact on the mouse population and patterns of concentration. Methodology used is primarily live trapping and observation of tracks and droppings. Also vegetation data is being collected as a possible correlative parameter with mice density.
In conjunction with this project, four trapping efforts have been documented: October, 1989, and January, April and July, 1990; however, earlier trapping data are available for July and December, 1988 and June, 1989. The trapping transect encompassed from DNR Monument 65 near the eastern end of Perdido Key to approximately 7 km west. Live traps were located at nominal spacings of 10 m except in those internal areas lacking vegetation. The results available to date, expressed in terms of total individuals trapped, show fluctuations but no negative effects which seem to be related to the dredging which commenced in Fall, 1989. The numbers trapped ranged from 19 in December, 1988 to 90 in April, 1990. To attempt to remove any seasonal fluctuations, the numbers trapped in July, 1988, June, 1989 and July, 1990 are 55, 67 and 73 which shows an increasing trend. These are preliminary data and interpretation of cause and effect is probably premature.
Plans are to continue trapping at 50 selected stations. In addition to trapping, plans are to collect fecal pellets from 30 animals per trapping sequence. Sea oat




and beach grass density will be noted at each of the 50 stations in Fall, 1990, Spring, 1991 and seed production will be documented in the Fall season. Each season, 30 Santa Rosa beach mice will be taken and a stomach contents analysis conducted.
This program promises to provide valuable data on the impact /non-impact and adaptability of the Perdido Key Beach Mouse. Of considerable interest will be the rate at which the mouse population migrates gulfward as the vegetation propagates onto the now relatively barren "new beach.
6.4 Physical Monitoring
The physical monitoring is being conducted by the University of Florida and includes upland, lagoon and offshore profiling, sand sampling, wave and tide measurements, meteorological measurements and ground photographic documentation. The data are to be interpreted to provide a basis for predicting the physical performance of the project. A historical substudy is underway to characterize the geological history of the area and quantify as well as possible anthropogenic effects. The rate of sand deposition by the sand fencing will also be documented.
Field studies have been carried out in August-November, 1989 and AugustNovember, 1990. An in-situ recording wave gage and weather station were installed in January, 1990 and a shore-connected wave gage was installed in early 1991.
Analysis of the survey data has identified a total of 4.3 million m' (5.6 million yd') of sand added as contrasted to the pay quantity of 5.4 million ino. The difference is interpreted, at least in part, as due to significant erosion of material placed in the vicinity of the western end of the project. The stability of this area is poor due to the proximity of the deepened channel and this sand was placed first, thereby having a greater time to evolve before the post-nourishment




survey. Comparison of the pre- and post-nourishment sediments demonstrates the material to be of generally good quality. However, the 5 m contour samples contained greater fines (silts and clays) from the post-nourishment sampling compared to the pre-nourishment. This is believed to be fine material that has been washed out from the nourishment and concentrated in the nearshore area. The weather data have been compared to results from the Pensacola Naval Air Station and generally good results were found. Plans include the correlation of wave data with profile and planform evolution results to compare expected and actual project performance.
7 Plans for ]Future Interpretation/Prediction
The data collected as part of the beach nourishment physical monitoring study will allow investigation of a number of physical processes. Both cross-shore and longshore sediment transport rates will be known, allowing calibration, and hopefully improvement, of numerical models for prediction of beach changes over time. Prediction of project lifetimes and the degree and rate of sediment compaction and overwash are also of interest during the feasibility study stage of a beach nourishment project. Any sorting of the newly-placed sediment that occurs will influence the resulting shape of the beach profile, thus the cross-shore distribution of grain sizes is also an important relation. The effects of the deepened channel on both Perdido Key and on the dredging rate required for channel maintenance will be important when determining the feasibility of other, similar projects including channel maintenance.
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