The importance of extratropical and tropical cyclones on...

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Marine Geology 210 (2004) 63–78

The importance of extratropical and tropical cyclones on the

short-term evolution of barrier islands along the

northern Gulf of Mexico, USA

Gregory W. Stonea,*, Baozhu Liub, David A. Pepperc, Ping Wangd

aCoastal Studies Institute, and Department of Oceanography Coastal Sciences, Howe-Russell Geoscience Complex, Louisiana State University,

Baton Rouge, LA 70803, USAbCoastal Studies Institute, Howe-Russell Geoscience Complex, Louisiana State University, Baton Rouge, LA 70803, USA

cDepartment of Geography, University of Southern California, Los Angeles, CA 90089, USAdDepartment of Geology, University of South Florida, Tampa, FL 33620, USA

Received 21 March 2003; received in revised form 15 July 2003; accepted 3 May 2004

Abstract

Data are presented indicating the complexity and highly variable response of beaches to cold front passages along the

northern Gulf of Mexico, in addition to the impacts of tropical cyclones and winter storms. Within the past decade, an increase

in the frequency of tropical storms and hurricanes impacting the northern Gulf has dramatically altered the long-term

equilibrium of a large portion of this coast. A time series of net sediment flux for subaerial and nearshore environments has been

established for a section of this coast in Florida, and to a lesser extent, Mississippi. The data incorporate the morphological

signature of six tropical storms/hurricanes and more than 200 frontal passages.

Data indicate that (1) barrier islands can conserve mass during catastrophic hurricanes (e.g., Hurricane Opal, a strong

category 4 hurricane near landfall); (2) less severe hurricanes and tropical storms can promote rapid dune aggradation and can

contribute sediment to the entire barrier system; (3) cold fronts play a critical role in the poststorm adjustment of the barrier by

deflating the subaerial portion of the overwash terrace and eroding its marginal lobe along the bayside beach through locally

generated, high frequency, steep waves; and (4) barrier systems along the northern Gulf do not necessarily enter an immediate

poststorm recovery phase, although nested in sediment-rich nearshore environments. While high wave energy conditions

associated with cold fronts play an integral role in the evolution and maintenance of barriers along the northern Gulf, these

events are more effective in reworking sediment after the occurrence of extreme events such as hurricanes. This relationship is

even more apparent during the clustering of tropical cyclones.

It is anticipated that these findings will have important implications for the longer term evolution of barrier systems in

midlatitude, microtidal settings where the clustering of storms is apparent, and winter storms are significant in intensity and

frequency along the coast.

D 2004 Elsevier B.V. All rights reserved.

Keywords: coastal erosion; sediment transport; overwash; Hurricanes

0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.margeo.2004.05.021

* Corresponding author. Tel.: +1-225-578-6188; fax: +1-225-

578-2520.

E-mail address: [email protected] (G.W. Stone).

G.W. Stone et al. / Marine Geology 210 (2004) 63–7864

1. Introduction

Storms have played an important role in the short-

term transgressive evolution of barrier island systems

around the world, particularly during the late Holo-

cene when ‘‘stillstands’’ in sea level have greatly

reduced the rate of barrier migration across continen-

tal shelves, increasing the relative importance of

extremely energetic events such as hurricanes in

redistributing sediment (Orford et al., 1991; Roy et

al., 1994). Within century-scales and under scenarios

including rising sea level and decreasing sediment

supply to the coast, storms are likely the most signi-

ficant factor affecting shoreline migration along many

coasts, particularly where updrift sand sources are

integral to beach maintenance (Morton et al., 1995).

Among the earliest and most significant contribu-

tions to our present understanding of the impacts of

hurricanes on barrier islands was through the work of

Hayes (1964, 1967). Hayes quantified the morpho-

logical impacts of hurricanes Carla (1961) and Cindy

(1963) along the Texas coast and measured the

redistribution of sediments on the inner shelf, near-

shore, and subaerial portions of the coast. His work

demonstrated the importance of the neritic zone as a

source and sink during hurricanes.

Additional contributions on the morphological

impacts of several severe hurricanes in the Gulf of

Mexico include studies on Hurricane Audrey, 1957

(Morgan et al., 1958); Hurricane Camille, 1969

(Wright et al., 1970); Hurricane Eloise (Burdin,

1975); Hurricane Frederic, 1979 (Schramm et al.,

1980; Nummedal et al., 1980; Kahn and Roberts,

Fig. 1. Map of study sites along the

1982; Stone and Salmon, 1988); Hurricanes Juan and

Danny, 1985 (Penland et al., 1989); Hurricane Gilbert,

1988 (Penland et al., 1989; Debusschere et al., 1991);

Hurricane Andrew, 1992 (Stone et al., 1993, 1996,

1999; Stone and Finkl, 1995). Although these works

provide useful information, the majority tends to be

qualitative assessments of the impacts of hurricanes

along barrier islands (see reviews in Reimer and Jaffe,

1989; Finkl and Pilkey, 1991; Stone and Finkl, 1995).

While the morphological response to storms along

the northern Gulf has tended to be complex and highly

variable, the results of a detailed monitoring program

initially launched in 1995 have considerably enhanced

our understanding not only with respect to storm

effects on the study site but also poststorm adjust-

ment. Results from this effort presented here show

that hurricanes and tropical storms can be both ‘‘con-

structive’’ and ‘‘destructive’’ on barrier subenviron-

ments with respect to their sediment budget. In

addition, the significance of cold fronts in causing

beach erosion on open Gulf and bayside barrier

beaches is also important in the overall short-term

evolution of the barriers.

2. Objectives

In this paper, we present a summary of recent

findings on the morphodynamic impacts and interplay

among hurricanes, tropical storms, and cold fronts on

barrier islands along the northern Gulf of Mexico

(Fig. 1). We have selected data sets obtained from

the Florida panhandle that contain the morphological

northern Gulf of Mexico coast.

G.W. Stone et al. / Marine Geology 210 (2004) 63–78 65

signatures of six hurricanes/tropical storms and over

200 cold fronts since 1995 (Fig. 1). The most detailed

field experiments were conducted at Santa Rosa

Island (Florida) and to a lesser extent, West Ship

Island (Mississippi; Fig. 1). The longer term objective

of our research is to understand the morphological

maintenance of barrier systems in the Gulf of Mexico,

with particular emphasis on the role of midlatitude

cyclones—extratropical storms and cold fronts—on

coastal dynamics.

3. Study area

Santa Rosa Island, located along the Florida pan-

handle, extends approximately 95 km west of Destin

to Pensacola Pass (Fig. 1). The evolution and mor-

phosedimentary dynamics of the barrier are described

in Otvos (1982), Stone (1991), Stone et al. (1992),

Stone and Morgan (1993), and Stone and Stapor

(1996). This ‘‘low profile’’ barrier is of late Holocene

origin (Otvos, 1982; Stone, 1991) and supplied with

99% quartz sand from the Pleistocene ‘‘headland’’ at

Grayton Beach to the east and an internal source of

sediment along the west flank of the island (Stone,

1991; Stone et al., 1992; Stone and Stapor, 1996). The

remaining 1% of sediment comprising the area is

largely heavy minerals such as illmenite, rutile. Net

longshore transport is westward and reaches a maxi-

mum of approximately 150,000 m3/year near the

entrance to Pensacola bay (Stone et al., 1992; Stone

and Stapor, 1996). Calculated modal breaker wave

heights are 0.7 m. Tides are dominantly diurnal with

an average range of 0.43 m, although with distinct

variations between equatorial and tropic phases when

ranges of 0.15 and 0.61 m occur respectively (Stone,

1991). Prior to an increased incidence of cyclones in

1995 (Muller and Stone, 2001), the island varied from

162 to 1878 m in width with an average of approx-

imately 514 m, and foredune elevations averaging

4.12 m above Mean Low Water (MLW).

The Mississippi barriers, of which West Ship Island

is the westernmost, are characterized by a distinctive

tourmaline–kyanite ‘‘Appalachian’’ suite of heavy

minerals typical of the eastern Gulf petrologic pro-

vince (Hsu, 1960). The immediate source of barrier

island sands is considered to be eroding pre-Holocene

headlands along the Alabama–Florida coasts (i.e.,

Grayton Beach, Florida, see Stone et al., 1992), and

the reworking of sediments previously deposited on

the inner continental shelf by streams and rivers during

the Pleistocene glacial stages (Rucker and Snowden,

1989). The possibility for onshore movement of sed-

iment from the continental shelf has been reported by

Otvos (1970). Net longshore transport is westward

with computed rates reaching a maximum of around

60,000 m3/year along most of the Mississippi barriers

(Cipriani and Stone, 2001). Computed modal breaker

wave heights are approximately 50% that of Santa

Rosa Island, partially due to a decrease in deep-water

wave energy and a lower gradient more dissipative

inner shelf (Cipriani and Stone, 2001). Tidal patterns

and ranges are similar to that of northwest Florida.

There is some indication of a somewhat constant

rate of relative sea-level rise along the study site

during the past millennium, which is in agreement

with the ‘‘globally coherent eustatic’’ signal of 2.4

mm/year of sea-level rise (Stone and Morgan, 1993).

The late Quaternary geology of the area is reviewed in

Otvos (1982) and Stone (1991).

Historical shoreline change data (150-year record)

indicate net erosion along the east end of West Ship

Island and deposition on its western tip; there is some

indication that the barrier is experiencing rotational

instability (McBride et al., 1995). Prior to 1995, the

Gulf shoreline of Santa Rosa Island was generally

stable over a 150-year period (Stone, 1991). Erosion

rates approximating 2 m/year or more have occurred

along the north-facing back-barrier shorelines of the

northern Gulf of Mexico (Armbruster et al., 1995;

Chaney and Stone, 1996; Armbruster 1997; McBride

and Byrnes, 1997). Shorter term (2 years) monitoring

indicates erosion rates in excess of 10 m/year along the

Florida panhandle on the bayside beaches of barrier

islands (Stone, 1998). There is a growing body of

literature (Armbruster et al., 1995; Chaney and Stone,

1996; Armbruster, 1997; Stone, 1998) linking net

foreshore and backshore erosion of bayside beaches

to cold front passage over the northern Gulf of Mexico.

More specifically, the postfrontal phase is characterized

by strong winds from the north, which generate steep,

high-frequency waves that are typically erosive (Kraus

et al., 1991). Although 80% of the Atlantic and Gulf of

Mexico coastlines are located in estuaries, few process-

based studies have been conducted in these low-energy

environments, and consequently, comprehension of

Fig. 2. Typical barrier cross-section showing subenvironments used

for volumetric time series calculations.


their dynamics is lacking in the scientific literature

(Nordstrom, 1992; Nordstrom and Roman, 1996).

4. Methods

This analysis is largely based on time series topo-

graphic/bathymetric surveys conducted before and

Fig. 3. Track of Hurricane Opal from the Yucatan to landfall along the no

show the location of two National Data Buoy Center buoys from which m

after the landfall of hurricanes and tropical storms.

We selected 19 repetitive surveys across Santa Rosa

Island from February 1996 to July 2002 at 11 loca-

tions as the focus of this paper. This systematic

monitoring was established after Hurricane Opal made

landfall along the Florida panhandle in October 1995

(Stone et al., 1996). Pre-Opal profiles were surveyed

at 2 of the 11 locations, in addition to 3 nearby

locations. Beach profiles were surveyed following

standard-level and transit-survey procedures using

an electronic total station. Benchmarks established

by the Florida Department of Environmental Protec-

tion (FDEP) were used; elevation was referenced to

National Geodetic Vertical Datum (NGVD), which is

approximately 0.08 m above Mean Low Water

(MLW) at the study site.

For uniformity, shoreline location was defined as

NGVD zero. To study the different responses of the

various subenvironments, the cross-island profile was

divided into four sections as follows: Gulf beach

(foredune to � 1.5 m NGVD), dune (foredune to

bay beach dune), bay beach (dune to Mean Low

rthern Gulf of Mexico coast at Santa Rosa Island, Florida. Triangles

etocean observations were recovered during the event.


Water), and bayside platform (MLW to � 1.5 NGVD;

Fig. 2). Similar techniques were applied to West Ship

Island using a datum already established by Chaney

and Stone (1996). Apparent and significant volume

changes among the subenvironments were measured

as discussed in the following sections.

Fig. 4. Time series of central pressure (Cp), wave period (T), and

significant wave height (Hs) obtained from buoys 42036 (upper)

and 42001 (lower) during Hurricane Opal.

5. Cyclone control on morphodynamics

5.1. Overview of tropical cyclones

Since 1995, the study site has been variously

impacted by six hurricanes/tropical storms, the most

significant of which were Hurricanes Opal (1995) and

Georges [1998; Stone et al., 1996, in press; for a

detailed synoptic climatology of tropical cyclone ac-

tivity around the southeastern United States, the reader

is referred to Muller and Stone, (2001)]. On October 4,

1995, Hurricane Opal impacted more than 2000 km of

coastline along the northern Gulf of Mexico (Florida to

Louisiana). A category 3 system at landfall (Fig. 3)

along Santa Rosa Island, Hurricane Opal was the

strongest of 18 hurricanes to have impacted the study

site over a 100-year period (see details in Stone et al.,

1996; Stone, 1998). The storm was responsible for

removing foredunes along Santa Rosa Island elevated

to + 5 m (NGVD). Data from two buoys (Fig. 4)

indicated that significant wave height peaked at 8 m as

the system moved onshore with wave periods

approaching 13 s. Calculated wave heights based on

central pressure (Hsu, 1991, 1994) indicate that waves

may have approached 19.5 m while the system

remained offshore in deeper water (Stone et al., 1996).

A few hours before dawn on September 28, 1998,

Hurricane Georges made landfall near Biloxi along

the Mississippi Gulf Coast (Fig. 5) near category 3

strength. Waves off the Mississippi/Louisiana coast

exceeded 10 m in height with wave periods on the

order of 12–14 s (Fig. 6). The entire stretch of coast

from the modern Mississippi delta to the Florida

Panhandle, a distance in excess of 200 km, was

severely impacted by the hurricane through over-

wash and breaching of the barrier islands. Hurricane

Georges’ forward motion began decreasing as it

approached the coastline. The storm turned to the

NW and then NNW before making landfall near

Biloxi, Mississippi at approximately 1130 UTC

September 28, with maximum sustained winds esti-

mated at 47 m s� 1. Upon landfall, the system

moved over the coastline for 6 h causing significant

damage to the area. The concave-seaward configu-

ration of this reach of coast from Louisiana to

Florida was optimal for maximizing surge and wave

fields and therefore pronounced beach erosion. This

was further augmented by the fact that the system

was slow moving.

5.2. Storm impacts on morphology since 1995

Santa Rosa Island underwent extensive morpholog-

ical change during Hurricane Opal. However, at the

study site, net volume loss of sediment across the entire

profile at locations that were overwashed was remark-

ably low and constituted only 5% of the total volume

when measured to � 1.5 m (NGVD) on the gulf side of

the beach and � 1.5 m in Pensacola Bay. An example

is provided (Fig. 7), which is typical of the morphology

observed pre- and post-Hurricane Opal. Approximate-

ly 70% of the sediment eroded from the foredune–

beach–nearshore and was deposited as expansive

Fig. 5. Track of Hurricane Georges from the Atlantic Ocean to landfall along the northern Gulf of Mexico nearWest Ship Island on theMississippi

coast. Circles show the location of two National Data Buoy Center buoys from which metocean observations were recovered during the event.


overwash deposits on the interior and bayside flank of

the barrier. This is an important finding in that it

indicates a minimal loss of sediment from the near-

shore (as defined during low wave energy conditions),

beach and foredune system, offshore to the inner shelf.

Overwash fans protruded into Pensacola Bay up to 100

m at some locations. The barrier increased by an

average of approximately 40 m in width suggesting

that the system conserved mass during Opal.

The erosion–deposition couplet clearly evident

through overwash into Pensacola Bay (Fig. 7) resulted

in shoreline displacement bayward by approximately

40 m. Of most significance to this paper is that this

wedge of material was rapidly eroded after Opal. For

example, over the ensuing 2-year period, more than

20 m of bayside shoreline recession occurred prima-

rily due to waves generated in the bay during post-

frontal phases of cold fronts.

Volume change for the study site over the period

February 1996–July 2002 is presented in Fig. 8 and

Table 1. These data include the signatures of three

storms and seven cold front seasons, which contained

over 200 winter storms. The data indicate the signif-

icance of two time periods and reworking of sedi-

ments on the overwash platform. Significant deflation

occurred during the first 2 months of the post-Opal

monitoring period, when cold fronts frequently gen-

erated northerly winds with speeds in excess of 6 m

s� 1. A slight net gain of sediment, especially across

the dune field, occurred between the end of the cold

front season and the impact of Tropical Storm Jose-

phine that resulted in net loss to the system. Although

a slight gain in sediment occurred between November

1996 and July 1997, the impact of Hurricane Danny

resulted in sediment being supplied by the nearshore,

substantial accretion of the berm, and increase in dune

Fig. 6. Time series of central pressure (Cp), wind speed (Ws) and

wind gusts (Gu), wave period (T), and significant wave height (Hs)

obtained from National Data Buoy Center buoy 42040 during

Hurricane Georges.

Fig. 7. (Upper) Cross-section of Santa Rosa Island, Florida showing

pre- and post-Opal profiles. Note the erosion–deposition couplet

where essentially the same volume of sand eroded from the

nearshore–beach– fordune system was transported into the interior

and protruded as an overwash deposit into Pensacola Bay. (Lower)

Oblique aerial photograph showing extensive overwash along the

island due to Opal.


field/platform volume due to sustained, high southerly

winds and increasing density of vegetation growth in

the summer. It is also clearly evident from the data

that the downward trend in gulf beach volume indi-

cating erosion was reversed after May 2001, 6 years

after landfall of Hurricane Opal. The overwash plat-

form, however, showed an increasing volume of sand

beginning May 1997, 2 years after landfall of Opal.

The overwash platform may have reached an equilib-

rium volume in the summer of 2001, because the last

three surveys indicate little variability.

During the first 2 months of monitoring, the bay-

side beach eroded 0.6 m3 m� 1 (volume per linear

meter of transect), while during the next 5 months, 4/

96–9/96, approximately 0.4 m3 m� 1 of sediment

accumulated. Tropical Storm Josephine caused ero-

sion of approximately 0.4 m3 m� 1, and this rate

increased to 0.5 m3 m� 1 during the cold front season

of late 1996 and early 1997, through July 1997.

During Hurricane Danny, prolonged southerly winds

resulted in wind-driven sediment transport across the

island and deposition of 0.2 m3 m� 1 of sediment

along the bayside beach. However, net loss of sedi-

ment was experienced along the bayside beach during

the entire period of monitoring, and as discussed later

in this paper, this is a direct function of cold front

forcing during the winter. The bayside subtidal zone

did not demonstrate any discernible morphological

pattern of change throughout the monitoring period,

although it experienced erosion of approximately 1.1

m3 m� 1 during the initial 18-month period.

The impacts of Hurricane Georges were detectable

from the point of landfall along the Mississippi coast,

east to the Florida panhandle. The study site along

Santa Rosa Island experienced significant overwash,

particularly at narrow portions. An example of the

response and poststorm adjustment is provided in Fig.

Fig. 8. Volume change for each of the barrier subenvironments from 2/96–8/02 at the Santa Rosa Island site, Florida.


9. The most pronounced retreat was measured at 0.5

to 1.0 m NGVD, corresponding to the berm and

backshore, and ranged between 20 and 60 m. A

considerable volume of sediment eroded from the

berm was transported across the island to form a

series of overwash terraces. Data indicate a predom-

inant depositional trend along the gulf side berm and

backshore over the 3-month period following hurri-

cane landfall.

6. Discussion

6.1. Influence of storms on morphodynamics

Field monitoring indicated the importance of over-

wash events in supplying sediment to the bayside

beaches along Santa Rosa Island, and the importance

of cold fronts in rapidly eroding the overwash depos-

its and causing a significant amount of deflation on

Table 1

Sediment volumes of the entire study area and each subenviron-

ments: (calculated by BMAP to � 1.5 m NGVD)

Survey Entire area

(106 m3)

Gulf beach

(106 m3)

Dune

(106 m3)

Bay beach

(106 m3)

Bay beach

and bayside

platform

(106 m3)

Feb-96 5.903 1.173 3.974 0.295 0.749

Apr-96 5.886 1.195 3.945 0.291 0.737

May-96 5.871 1.185 3.947 0.291 0.731

Jun-96 5.830 1.141 3.952 0.292 0.729

Jul-96 5.841 1.152 3.950 0.292 0.731

Aug-96 5.859 1.165 3.952 0.293 0.733

Sep-96 5.828 1.127 3.952 0.293 0.740

Tropical storm Josephine

Oct-96 5.673 0.982 3.940 0.291 0.742

Nov-96 5.666 0.980 3.938 0.290 0.740

Dec-96 5.670 0.968 3.945 0.290 0.742

Jan-97 5.741 1.048 3.941 0.290 0.743

Feb-97 5.746 1.048 3.947 0.290 0.742

Mar-97 5.780 1.088 3.945 0.289 0.738

Hurricane Danny

Jul-97 5.721 1.013 3.963 0.286 0.736

Sep-97 5.644 0.940 3.961 0.287 0.734

Jun-98 5.739 1.058 4.011 0.282 0.731

Tropical depression Georges

Oct-98 5.646 0.861 4.090 0.279 0.682

Jan-99 5.638 0.831 4.095 0.280 0.692

Mar-99 5.614 0.837 4.086 0.288 0.701

Jul-99 5.660 0.953 4.083 0.287 0.686

Nov-99 5.577 0.846 4.084 0.285 0.675

Mar-00 5.585 0.829 4.105 0.283 0.680

Mar-01 5.685 0.796 4.185 0.276 0.659

Feb-02 5.826 0.962 4.149 0.267 0.742

Jul-02 5.959 1.115 4.163 0.271 0.718

Tropical storm Isidore

Oct-02 5.905 0.955 4.250 0.273 0.721

Fig. 9. Profile comparisons pre- and post-Hurricane Georges at West

Ship Island, Mississippi (upper and middle) and Santa Rosa Island,

Florida (lower).


the subaerial portion of the barrier. These data have

added significantly to our understanding of the longer

term morphological maintenance of barrier islands in

the Gulf and the importance of extratropical and

winter storms. Few studies have been conducted on

the poststorm adjustment phase of barrier islands.

Moreover, those that have been conducted (cf., Sexton

and Moslow, 1981; Thieler and Young, 1991; Sexton

and Hayes, 1991; Dingler and Reiss, 1995) are limited

to annual topographic surveys, which significantly

reduce the resolution of morphological recovery of

the barrier system. Profiles are typically conducted

during the same month in successive poststorm years,

restricting conclusions to the net amount of sediment

deposited on the barrier during the poststorm phase

(Dingler and Reiss, 1995; Sexton, 1995).

Given that >90% of sand comprising largely the

subaerial barrier mass at the study site could be

accounted for after two severe hurricanes, the impli-

cations are highly significant for (a) the source of

material during poststorm recovery, and (b) the pre-

vailing concepts that sediment eroded from in partic-

ular the upper shoreface during storms is transported


offshore to the continental shelf and lost from the

barrier system. These issues have not been adequately

addressed in the barrier island literature. In addition,

these preliminary data have potentially significant

bearing on the application of existing models to the

upper shoreface–foreshore (see Swift 1976; Swift et

al., 1985).

Barrier islands along the northern Gulf of Mexico

are oriented generally east–west from southern Loui-

siana to the Florida panhandle (Fig. 1). North-facing

beaches on the bay side of these barrier islands are

particularly susceptible to wave attack during north-

erly winds, which generally accompany the passage of

cold fronts. Significant erosion has been observed in

areas where the adjacent fetch in the sound or bay is

relatively long (Stone and Morgan, 1993; Chaney and

Stone, 1996; Armbruster et al., 1995; Armbruster,

1997).

Back-barrier beaches along the northern Gulf are

characterized by a specific nearshore morphology. A

nearly vertical erosional scarp is commonly observed

at the high water mark. Between high and low water

levels, the shoreface generally has a steep slope,

f 10j. A flat platform with a slope of typically less

than 0.5j, which is referred to as the low tide terrace

(Nordstrom, 1992; Nordstrom et al., 1996), extends

up to several hundred meters into the bay/sound along

many beaches. Along the northern Gulf however, this

feature usually remains subtidal.

During winter seasons, three distinct end-member-

type cold fronts impact the northern Gulf of Mexico

(Lewis and Hsu, 1992; Dingler et al., 1993; Pepper,

2000; Pepper and Stone, 2002; 2004): (1) the midlat-

itude cyclone, (2) the Arctic surge, and (3) the Gulf

cyclone. The extratropical weather systems produce

local variations in wind direction, intensity, and du-

ration, and thereby significantly affect wave and

sediment transport processes along the back-barrier

beach. Fronts cross the northern Gulf approximately

30 times each year and with the exception of tropical

storms and hurricanes are the only known natural

mechanism to generate relatively high waves in these

bay/sound environments. A common regional weather

phenomenon accompanying the passage of cold fronts

is the strong postfrontal northerly wind. Due to the

sheltering effect of the island, southerly winds are not

capable of generating high waves along the back-

barrier beach. Influences of the Gulf swell are mini-

mal except in the vicinity of the tips of the islands and

inlets. Strong northerly winds, therefore, constitute the

primary driving force for modifying the bayside

barrier beach system.

The general morphological characteristics (Fig. 2)

of these back-barrier beaches are fundamentally dif-

ferent from the features developed by storm overwash

processes (Schwartz, 1981; Otvos, 1982), which are

critical to the origin and supply of sediment to these

systems (Stone, 1998). These differences indicate the

existence of a general mechanism/s governing the

dynamics and morphological maintenance of the

back-barrier beach.

6.2. Postfrontal forcing

It is reasonable to assume that due to the shelter-

ing effect of barrier islands along the northern Gulf

(Stone and McBride, 1998), nearshore sediment

transport along the back-barrier beach is not signif-

icant during southerly winds. Where the northerly

wind has a long fetch and is capable of generating

relatively high waves, water-level setup occurs. Our

work to date indicates that the strong northerly wind

is the primary energy supply to the back-barrier

beach system in terms of significant sediment trans-

port and morphological change. We acknowledge the

longer term role of relative sea-level rise in these

areas but take the position that the critical forcing

mechanism/s for mobilizing and transporting sedi-

ment are a primary function of wave–current and

wind forcing. The area is microtidal, and thus tidal

currents have an insignificant role in sediment resus-

pension in shallow water.

The frontal event has distinct meteorological and

sea state signatures associated with its pre- and

postfrontal phases. In Fig. 10, we present a time

series of wind speed (A), wind direction (B), and

significant wave height (C) for a frontal passage in

December 2002. The data were obtained from a

WAVCIS (www.wavcis.lsu.edu) metocean station lo-

cated at the 6-m isobath in Mississippi Sound, north

of West Ship Island. Winds blow from the south

prior to arrival of the front at the site and reach a

maximum speed of approximately 7 m/s. Southerly

wind speeds decrease as wind direction veers clock-

wise to the north over a matter of a few hours (B).

Wind speeds rapidly increase across the Sound and

http:\\www.wavcis,lsu.edu

Fig. 10. Time series of wind speed (A), wind direction (B), and

significant wave height (C) during a frontal event obtained from a

WAVCIS metocean array in Mississippi Sound.


peak at approximately 13.5 m/s. During the prefron-

tal phase, wave heights are low in the Sound, < 0.2

m (C), due to the sheltering effect of West Ship

Island. Wave response to wind forcing is rapid, and

heights of 0.6 m are attained. Spectral evolution of

the wave field is presented in Fig. 11 and shows

clearly energy concentrated in the higher frequency

bands due to wind veering to the north. Northerly

winds persist for several days and maintain high

wave energy levels along the north-facing beach

resulting in erosion. As wind speed decreases wave

energy levels show a concomitant decrease and

winds veer clockwise to the east during this phase,

resulting in extremely low wave energy conditions in

the Sound.

The data support the contention that winter cold

fronts play the key role in north-facing beach erosion

over short-term time scales. The prevalence of fronts

over the Florida site explains the continued loss of

sediment from the bay beach as presented in Fig. 8.

What is less clear, however, is the actual pathway of

sand transport during these events. Our data indicate

that sediment is being removed from the foreshore

during postfrontal events. The transport and ultimate

fate of the material eroded is not yet understood. In

addition, the relative significance of longshore and

offshore transport is not clear. The presence of oblique

transverse bars observed at numerous locations along

the northern Gulf (Zapel, 1984) and other estuarine

beaches (cf., at Fire Island, New York, Nordstrom et

al., 1996) indicates a combination of longshore and

cross-shore transport. The strike of the oblique trans-

verse bars cannot be explained by one mechanism

alone, and we hypothesize a genetic relationship with

the sum of longshore and cross-shore transport vec-

tors. Longshore sediment transport is driven by

oblique incident waves. Our preliminary data indicate

a sequence of events during the postfrontal northerly

winds. The existence of the commonly observed

erosional scarp extending along the high water mark

and the flat extensive platform indicates to us a

significant contribution of sediment transported from

the upper shoreface offshore. The mechanisms driving

this apparent offshore sediment transport are poorly

understood.

A field experiment carried out along the Florida

site during frontal passages revealed the possibility of

near-bottom bayward flow. A bottom boundary layer

tripod was deployed at the 1-m isobath and captured

two fronts over a 9-day period. The time series of

significant wave height and cross-shore velocity are

presented in Fig. 12 for the duration of both events.

Two intervals are apparent when offshore currents

occur as the significant wave height increases with

wind speed and northward veering. Between fronts

when wave energy is low, onshore currents occur, but

Fig. 11. Spectral evolution of waves from the WAVCIS array in Mississippi Sound located on the 6-m isobath, north of West Ship Island

(upper); corresponding evolution of wind speed showing the onslaught of northerly winds during a postfrontal phase (lower). Note the

correlation between the sudden increase in wind speed and energy in the high-frequency (0.35 Hz) band.


as wave energy increases with the second postfrontal

period, offshore currents are apparent. While similar

offshore currents due to event-related water-level

setup have been observed along high-energy open

coasts (e.g., Wright et al., 1991), it is not yet clear if

this is a reasonable mechanism by which sediment is

Fig. 12. Time series of significant wave height and cross-shore

current direction obtained from Pensacola Bay on the north shore of

Santa Rosa Island, Florida. Two cold fronts were captured in the

time series, and there is evidence of bayward currents during these

events.

reworked from low-energy beaches and transported

offshore to the nearshore.

6.3. Implications for short-term barrier evolution

Changes in volume for the respective barrier sub-

environments are presented in Fig. 13. A fourth-order

polynomial curve has been added to each to identify

apparent trends in the data. Since landfall of Hurricane

Opal in October 1995, the study site along Santa Rosa

Island has shown a net decrease in volume until the

summer of 2000 when an upward trend in the curve

indicates an increasing sediment volume (Fig. 13A).

The signature of both cold fronts and hurricanes/

tropical storms is clearly evident in the data. Over an

approximate 1000-day period between landfall of Hur-

ricanes Opal and Georges, the study site experienced a

net sediment loss of 0.23 million m3 of sediment.

Approximately 50% (0.11 million m3) of this material

was recovered during a 5-month period after Georges.

During the study period, the interaction and cumulative

impacts of tropical cyclones (during summer and fall)

and postfrontal circulation (in winter and spring) ap-

pear to play an integral role in the longer term mor-

phodynamic evolution of the coast along the study site.

Fig. 13. Time series of volumetric change from 2/96 to 8/02 along the study site at Santa Rosa Island, Florida for the entire section of the study

site (A), Gulf beach (B), dune system (C), bay beach (D), and bay platform (E).


Over the shorter term (approximately 7 years), field

monitoring indicates the importance of overwash

events in supplying sediment to the bayside beaches

along Santa Rosa Island, and the importance of cold

fronts in rapidly eroding these deposits through aeolian

deflation and wave erosion of their marginal lobes.

There is also evidence indicating that catastrophic

events similar to hurricane Opal disrupts the system to

the extent that poststorm recovery does not occur for

several years. This was evident after Opal when the

barrier experienced net erosion over a period in excess

of 200 days prior to being impacted by a tropical


storm. Thus, although the barrier conserved mass after

a catastrophic hurricane through overwash processes,

cold fronts during the ensuing winter played an im-

portant role in deflation of the barrier surface and

erosion of the marginal lobe of the overwash deposits.

Recovery of the Gulf beach/nearshore did not begin to

occur until the summer of 2001, some 6 years after

landfall of Opal (Fig. 13B). As of the summer of 2002,

the beach/nearshore environment remained lower than

that post-Opal when the initial surveys commenced.

The absence of an immediate poststorm recovery

trend in the Gulf beach/nearshore was not the case in

the dune environment; a net accumulation of sand in

the dune system was apparent by the summer of 1996,

nearly 1 year after landfall of Opal (Fig. 13C).

Accumulation continued throughout the nearly 7-year

record.

The bay beach environment eroded throughout the

entire period due to frontal impacts (Fig. 13D). The

nearshore platform in the bay showed a general

decrease in volume following Opal until midyear

2001 when rapid accumulation of sediment began

and continued to the end of the time series in mid-

2002 (Fig. 13E).

The data presented suggest that weaker hurricanes,

such as Danny and Georges, can rework considerable

amounts of sediment to the berm and relict overwash

terrace. Bay beach accretion through overwash or

aeolian deposition is short lived however, and frontal

passage during the ensuing winter typically results in

net loss to the system. This sequence of events implies

that the barrier crosses a threshold initiated by a

catastrophic storm, and over the shorter term, barrier

degradation results due to net sediment loss from the

system in its entirety.

7. Conclusions

Five important conclusions have been reached

based on the data presented here: (1) low-lying barrier

islands, which are vulnerable to overwash, can con-

serve mass during catastrophic hurricanes (e.g., Hur-

ricane Opal); (2) less severe hurricanes can promote

rapid dune/berm aggradation and contribute sediment

to the entire barrier system; (3) cold fronts play a

critical role in the poststorm adjustment of the barrier

by deflating the subaerial portion of the overwash

terrace and eroding its marginal lobe along the bayside

beach; (4) considering seasonal time scales, poststorm

recovery along the Gulf-facing beach/nearshore is not

immediate and did not occur at the study site until

approximately 6 years after the event; and (5) bay-

facing beaches showed a continued decline in volume

throughout the entire study period, a phenomenon we

attribute to locally generated high-frequency waves

due to the passage of cold fronts along the northern

Gulf of Mexico. There remains a need for field experi-

ments to determine the source(s) and mechanisms

responsible for sedimentation in these environments

over various time and spatial scales, and the develop-

ment of models that elucidate the morphosedimentary

dynamics of other midlatitude, microtidal barriers.

Acknowledgements

This work was funded by the National Science

Foundation under awards AGS 9625709 and

EAR9903984. We appreciate the cooperation of the

National Park Service, Gulf Islands National Sea-

shore, in allowing access to the field site and helping

facilitate field work. We also acknowledge the Coastal

Studies Institute Field Research Group for helping

conduct field work. Mary Lee Eggard and Clifford

Duplechin assisted with cartography.

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