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.
G.W. Stone et al. / Marine Geology 210 (2004) 63–7866
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.
G.W. Stone et al. / Marine Geology 210 (2004) 63–78 67
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.
G.W. Stone et al. / Marine Geology 210 (2004) 63–7868
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.
G.W. Stone et al. / Marine Geology 210 (2004) 63–78 69
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.
G.W. Stone et al. / Marine Geology 210 (2004) 63–7870
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).
G.W. Stone et al. / Marine Geology 210 (2004) 63–78 71
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
G.W. Stone et al. / Marine Geology 210 (2004) 63–7872
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
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.
G.W. Stone et al. / Marine Geology 210 (2004) 63–78 73
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.
G.W. Stone et al. / Marine Geology 210 (2004) 63–7874
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).
G.W. Stone et al. / Marine Geology 210 (2004) 63–78 75
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
G.W. Stone et al. / Marine Geology 210 (2004) 63–7876
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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