Geomorphology 231 (2015) 182–192
Contents lists available at ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
Contrasting styles of Hurricane Irene washover sedimentation on three
east coast barrier islands: Cape Lookout, North Carolina; Assateague
Island, Virginia; and Fire Island, New York
H.F.L. Williams ⁎
Geography Department, University of North Texas, Denton, TX 76203, United States
a r t i c l e
i n f o
Article history:
Received 9 June 2014
Received in revised form 3 November 2014
Accepted 7 November 2014
Available online 16 December 2014
Keywords:
Storm surge
Overwash
Washover fan
Washover terrace
Aggradation
Foraminifera
a b s t r a c t
Storm surge and wind-driven waves generated by Hurricane Irene, which made landfall on the U.S. east coast on
August 27 2011, resulted in overwash of sandy barrier islands from North Carolina to New York State. Overwash
has significant impacts on barrier island geomorphology: it represents a sediment pathway into island interiors, a
component of island sediment budgets, and can cause considerable aggradation of backshore surfaces, important
for potentially offsetting the effects of rising sea level. This study describes the morphology, texture and microfossil content of Hurricane Irene washover deposits at three contrasting barrier island sites: Cape Lookout,
North Carolina, Assateague Island, Virginia and Fire Island, New York. At all three sites, run-up overwash
occurred, wherein waves were sufficient to overtop parts of the beach system and transport sediment inland.
However, at Fire Island, overwash was restricted by a higher elevational threshold to low spots in the beach
system coinciding with pre-existing breaches in foredunes. The result was the formation of isolated, thinner,
low-volume washover fans. At Assateague Island and Cape Lookout, lower elevational thresholds allowed
waves to overtop longer continuous sections of beach systems, resulting in the formation of laterallycontinuous, thicker, larger-volume washover terraces. Overall, the deposits lacked consistent trends in thickness
and texture (such as thinning and fining inland, reflecting a progressive reduction in overwash competence).
Thickness and texture of the deposits were both spatially variable and probably reflect infilling of low points
on the former surface and the influence of beach and foredune sediment sources. All the washover deposits
were essentially barren of foraminiferal microfossils, supporting the textural evidence that the adjacent beach
and foredunes were the predominant sediment sources.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Overwash is the flow of water and sediment over the crest of a beach
system when the run-up level of waves or the water level, often
enhanced by storm surge, exceeds the local beach or dune crest height
(Donnelly, 2008). Overwash commonly results from hurricane storm
surges and hurricane-force, wind-driven waves, although any storm
that raises water levels sufficiently to overtop the berm crest or dune
crest of a beach system can cause overwash. Depending on the relative
height of water levels and the beach system, two levels of overwash
may be defined: run-up overwash, where wave run-up is sufficient to
overtop the berm crest or dune crest; and, inundation overwash,
where the local water level, combining tide, storm surge and wave
setup, completely overtops the beach and/or dunes and the entire
beach system becomes subaqueous (Sallenger, 2000; Donnelly et al.,
2006; USGS, 2014a; Fig. 1).
⁎ Tel.: +1 940 565 3317.
E-mail address: HarryF.Williams@unt.edu.
http://dx.doi.org/10.1016/j.geomorph.2014.11.027
0169-555X/© 2014 Elsevier B.V. All rights reserved.
Overwash can significantly impact the geomorphology of barrier
islands by eroding the nearshore zone, beach, and dunes and by creating
washover deposits landward of the beach system. Under run-up
overwash conditions, the flow of water and sediment may be channeled
through low spots in the berm or dune crest and deposits isolated,
lobate-shaped washover fans in the backshore area, typically extending
tens to hundreds of meters inland (Dolan and Hayden, 1981). If run-up
overwash occurs along a longer continuous section of a beach system,
washover deposits may coalesce into a continuous sediment apron
known as a washover terrace (Morton and Sallenger, 2003). Under
inundation overwash conditions, washover sediments may extend
farther inland, forming large sheet-like deposits in the backshore area
(Williams, 2012).
A number of important geomorphological processes are associated
with washover deposition. Erosive scarping of the beach face and
dunes of a barrier island usually causes some offshore sediment transport, which, depending on the degree of post-storm beach recovery,
may represent a loss to nearshore sediment budgets (Hawkes and
Horton, 2012). However, some of the eroded sediment is commonly
transported landward of the beach system by overwash. This inland
H.F.L. Williams / Geomorphology 231 (2015) 182–192
183
Nguyen et al., 2006). This paper reports on the characteristics of
washover deposits of Hurricane Irene (U.S. landfall August 27, 2011). Although a relatively weak storm, attaining Category 1 status on the SaffirSimpson Hurricane Wind Scale at landfall in the U.S., Hurricane Irene
paralleled a long segment of the U.S. east coast, resulting in washover
deposition on sandy barrier islands from North Carolina to New York
State. The objectives of the study were to document geomorphic impacts of washover deposition in backshore areas, in terms of washover
thickness, morphology, volume and mass; and to provide a set of modern analogs to aid in identification of prehistoric washover deposits, by
documenting textural characteristics and microfossil contents of representative washover deposits from a range of contrasting barrier island
settings along the east coast of the U.S.
2. Hurricane Irene: synoptic history and meteorological summary
Fig. 1. a. Barrier island beach system showing storm-elevated water level Rlow (combining
tide, surge and wave set-up), wave run-up level Rhigh, dune base Dlow and dune crest Dhigh.
The dashed line represents the swash excursion about wave set-up (solid line). Interior
topography and vegetation vary. b. Run-up overwash (Rhigh N Dhigh) and washover deposit.
Dashed line represents beach and dune erosion and former interior surface buried by
washover deposit. c. Inundation overwash (Rlow N Dhigh) and washover deposit. Modified
from Sallenger (2000) and USGS (2014a).
transfer of sediment may represent a significant pathway for the input
of sediment into backshore environments and thus play an important
role in coastal sediment budgets (Stone et al., 2004; Gares and White,
2005; Turner et al., 2006; Williams, 2012). If overwash reaches the
bayside of the island, the gradual landward migration of the island
occurs—a process known as “rollover” (Orford and Carter, 1982;
Donnelly et al., 2006). Washover deposition can cause considerable
aggradation of backshore surfaces—a process important for maintaining
coastal environments against detrimental effects of rising sea level and
preserving the function of marshes as a barrier to storm waves (Wang
et al., 2006; Day et al., 2007; Williams and Flanagan, 2009; Williams,
2011; Woodruff et al., 2013). Washover deposits also play a central
role in paleotempestological research, because they often form
anomalous sand beds recording storm overwash into muddier, more
organic-rich backshore settings, such as coastal ponds, lakes and
marshes (Liu, and Fearn, 1993, 2000; Donnelly et al., 2004; Williams,
2013).
Empirical field data on washover sedimentation, required, for
example, for improved modeling of geomorphic effects of storm
overwash on barrier islands, is limited (Leatherman, 1976; Morton
and Sallenger, 2003; Gares and White, 2005; Donnelly et al., 2006;
Hurricane Irene originated from a tropical wave over the west coast
of Africa on August 15, 2011. After crossing the Atlantic, the storm
moved west-northwest across the northeastern Caribbean Sea and
intensified into a hurricane as it crossed the island of Puerto Rico on
August 22. The storm continued to strengthen and became a Category
3 hurricane on the Saffir-Simpson Hurricane Wind Scale, with a peak
intensity of 194 km/h, as it moved over the southeastern Bahamas
on August 24. The hurricane weakened and turned to the northnorthwest as it crossed the Bahamas, becoming a Category 2 hurricane
on August 25. Passing well offshore of Florida and Georgia, Hurricane
Irene made landfall as a Category 1 hurricane near Cape Lookout,
North Carolina, on August 27. The hurricane then moved offshore, still
following a north-northwest track, and made a second landfall at
Brigantine Island, New Jersey on August 28, after which it weakened
to a tropical storm and continued to move inland passing over Coney
Island, Brooklyn and Manhattan in New York City (Avila and Cangialosi,
2011; Fig. 2). Intense rainfall from the storm resulted in record flooding
and sediment mobilization in the U.S. Northeast (Ralston et al., 2013;
Magilligan et al., 2015; Yellen et al., 2014).
Because Hurricane Irene essentially paralleled the coast from North
Carolina to New York State, it generated strong winds and significant
storm surges along approximately 1400 km of coastline. The highest
storm surge (difference between observed water level and predicted
tide level) reported by a tide gage was about 2.2 m NAVD88 at Oregon
Inlet, North Carolina, although post-storm surveys suggest that surge
heights of up to about 3.4 m occurred in parts of Pamlico Sound
(McCallum et al., 2012). Storm surge heights recorded by tide gages
farther north along the Atlantic coast from the mouth of Chesapeake
Bay to southeastern Massachusetts, were generally between 0.6 and
1.4 m (Avila and Cangialosi, 2011). Maximum sustained wind speeds
were 139 km/h at landfall near Cape Lookout, North Carolina, declining
to 119 km/h about 18 h later as the storm made landfall in New Jersey.
Hurricane Irene had a large wind field and strong winds were experienced well to the east of its track, including the southern coast of Long
Island, New York, and states in the southern part of New England
(Hwind Scientific, 2011).
3. Study areas
Potential sites for studying washover sedimentation were reported
by National Seashore Park staff, based on ground surveys, or identified
by comparing pre- and post-Hurricane Irene aerial photographs that
closely bracketed landfall. In North Carolina and Virginia, washover
fans were identified on Rapid Response Imagery aerial photographs
obtained by the U.S. National Geodetic Survey (2011) within 1–3 days
following Hurricane Irene's landfall. Aerial photographs obtained by
the USDA Farm Service less than two months before landfall at both
sites confirmed that these washover fans were recent deposits attributable to Hurricane Irene overwash. Rapid Response Imagery did not
extend to New York State, but Fire Island National Seashore Park staff
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Fig. 2. Track of Hurricane Irene and location of study areas: A. Fire Island, New York; B. Assateague Island, Virginia; C. Cape Lookout, North Carolina.
reported widespread washover along the south shore of the island
immediately following passage of the hurricane (M. Bilecki, Fire Island
National Seashore, personal communication, October 21, 2011). The
presence of Hurricane Irene washover deposits was later confirmed by
comparing aerial photographs obtained by the USDA Farm Service
three months before and by NASA—six months after, Hurricane
Irene's landfall. Three barrier island sites were selected for study to
cover a range of geographic locations and contrasting topographic
conditions—Fire Island, New York, Assateague Island, Virginia and
Cape Lookout, North Carolina (Figs. 2, 3).
At the Fire Island study area, large well-vegetated dunes, up to about
5 m high, back much of the beach, although there are common breaches
through the dunes, some several hundred meters across, where the
dunes have essentially been planed down to the level of the beach
berm crest. The elevation of the beach berm crest, including breaches
through the foredunes, is generally in the range of 3–4 m NAVD88
(Fig. 4; USGS, 2014b). The nearest tide gage station to the study area,
Montauk New York, about 88 km to the east, recorded a storm tide
(maximum water elevation during passage of storm) of 1.244 m
NAVD88 (Fanelli and Fanelli, 2011). The nearest wave buoy to the
study area, Breezy Point NY, about 76 km to the southwest, recorded a
maximum wave height of 7.95 m during passage of the storm (NOAA,
2014).
Topography at the Assateague Island and Cape Lookout sites is much
more subdued. Both sites have small vegetated dunes, with frequent
breaches, creating about 1 m of relief along the dune crest. Farther
inland, the backshore slopes gently down to marshes densely vegetated
by grasses and stands of shrubs and woodlands. Beach system crest
elevations are generally in the range of 2–3 m NAVD88 at the Cape
Lookout site and ~ 2 m NAVD 88 at the Assateague Island site (Fig. 4;
USGS, 2014b). The nearest tide gage stations to the Cape Lookout
study area, Wilmington NC (~ 144 km south), Oregon Inlet NC
(~138 km north) and Duck NC (~169 km north), recorded storm tides
of 0.865 m, 2.1 m and 0.925 m NAVD88, respectively. The nearest tide
gage station to the Assateague Island study area, Ocean City, Maryland,
about 50 km to the north, recorded a storm tide of 0.929 m NAVD88
(Fanelli and Fanelli, 2011). Maximum wave heights recorded during
passage of the storm were 8.64 m at Onslow Bay NC, about 80 km
southwest of the Cape Lookout study area and 6.36 m at Delaware
Bay, about 72 km northeast of the Assateague Island study area
(NOAA, 2014).
4. Methods
Fieldwork on the selected washover deposits was conducted in
November 2011 (Fire Island), about two months after landfall, and in
January 2012 (Assateague Island and Cape Lookout), about four months
after landfall (Fig. 3). It is possible that the deposits were reworked to
some extent by washover generated by winter storms prior to fieldwork. However, no obvious evidence of reworking (e.g. channels eroded
into fan surfaces, lag deposits formed by the removal of finer sediments)
was observed during fieldwork. In addition, a review of wave data from
nearby buoys shows that maximum wave heights generated by winter
storms were considerably smaller than those resulting from Hurricane
Irene. At Fire Island, nearby waves reached a maximum of 3.29 m on
October 30, 2011 (buoy station 44065); in contrast, waves during the
hurricane reached 7.95 m and were accompanied by a storm surge of
~ 1.24 m. Nearby Assateague Island, a maximum wave height of
4.86 m was recorded on October 29, 2011 (buoy station 44009);
waves during the hurricane reached 6.36 m and were accompanied by
a storm surge of ~0.929 m. At Cape Lookout, waves reached a maximum
of 4.58 m on November 5, 2011 (buoy station 41036); during the hurricane, waves were up to 8.64 m high and were accompanied by a storm
surge of ~1.29 m (NOAA, 2014).
The thickness of washover deposits was determined by digging
spade pits down to the contact of the deposit with the underlying buried
H.F.L. Williams / Geomorphology 231 (2015) 182–192
185
Fig. 3. Pre- and post-Hurricane Irene aerial photographs of the three study areas showing washover deposits formed August 27–28, 2011; a–b. Fire Island, New York, September 19, 2010
and March 6, 2012; c–d. Assateague Island, Virginia, June 20, 2011 and August 29, 2011; e–f. Cape Lookout, North Carolina, July 10, 2011 and August 28, 2011. Washover deposits used in
this study are outlined in photos b, d and f. (Imagery: a, c, e; Google, USDA; b; Google, NASA; d, f; National Geodetic Survey).
surface. Pit locations were recorded by handheld GPS with a reported
accuracy of 10 m (Garmin, 2007). An attempt was made to position
pits along transects oriented perpendicular to the coastlines of the
study areas to allow sampling of washover deposits from near the
shoreline to progressively farther inland. This approach worked fairly
well on the sheet-like washover deposits at Assateague Island and
Cape Lookout, but the presence of multiple branching lobes of washover
fans on Fire Island caused pit placement to be more scattered. A
partially buried line of fence posts at Assateague Island presented a
serendipitous opportunity to include an additional pit transect along
the fence line.
Identification of the basal contact of washover sediments was
relatively easy in distal parts of the deposits, where there were marked
lithological contrasts between the sandy, low-organic-content
washover sediments and the typically muddy, organic-rich, buried
surfaces. In more proximal areas of washover deposition, identification
of the base of deposits was more problematic because the sandy
backshore surface was overlain by sandy washover sediments with little
lithological contrast. However, buried dune grasses (Ammophilia
breviligulata), still rooted and in upright growth position, were fairly
common at all three study areas, and could be used to estimate
the depth of washover sedimentation (Schwartz, 1975; Wang and
Horwitz, 2006; Hawkes and Horton, 2012). The inland edge of washover
deposition formed an easily-identified abrupt boundary with the
densely-vegetated backshore and was commonly marked by relatively
steeply dipping avalanche faces. The proximal edge of washover
deposits was less distinct, but its position could be estimated as lying
between the most seaward documented washover deposits and the
eroded beach (Figs. 3, 4, 5).
Samples of washover sediments were collected for determination of
textural characteristics, microfossil content and bulk density. At the Fire
Island and Assateague Island study areas, two surface samples were
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H.F.L. Williams / Geomorphology 231 (2015) 182–192
Fig. 4. Topographic conditions at study areas. a. Fire Island, New York; 3–5 m high vegetated dunes backing the beach berm crest. Spade is about 1 m long. b. View inland from the dune
crest at the Fire Island site, showing several small washover lobes, part of Fan A in Fig. 3b. Bellport Bay (lagoon) is visible in the distance. c. Cape Lookout, North Carolina; the backshore,
covered by a sheet-like washover deposit, slopes gently down from a line of low dunes at the back of the beach. View south. Crouching figure on right provides scale. d. View to southwest
across distal part of the washover deposit at Cape Lookout, showing encroachment of sediment onto densely-vegetated marshland and abrupt landward edge of washover deposit.
collected from the beach (“low beach” and “upper beach”), the presumed source of at least some washover sediment. Five more sample
points spanned proximal to distal locations across the washover
deposits. At these sampling points, samples (~ 3-cm-thick intervals)
were collected from the top, middle and base of the deposit to allow
investigation of lateral and vertical trends in textural characteristics
and microfossil contents. Samples of the buried surface underlying
sampling points at the Assateague Island study area were also collected
to allow comparison of microfossil content between the former surface
and the overlying washover sediments.
Samples of known volume were collected from the side of pits using
a thin-walled metal cylinder. These samples were dried and weighed to
provide bulk density in grams per cm3. At the Assateague Island study
area, bulk density samples were collected from the top, middle and
base of the washover deposit at each sampling point; at the Fire Island
study area, bulk density samples were collected from the middle of
the washover deposit at each sampling point. A more limited set of
samples was collected from the Cape Lookout study area because
sampling was curtailed by heavy rain. At this site, samples were
collected from one sample point on the distal part of the washover
deposit. Samples for textural and microfossil analysis were collected at
~ 10 cm intervals from the top to the base of the deposit (0.68 m
depth). Samples for bulk density determination were collected from
the top, middle and base of the deposit at this location (sampling
point 1, Fig. 6d).
The washover boundaries in Fig. 6 were digitized and a GIS was
used to calculate the area of washover within the digitized washover
boundaries. At the Fire Island and Cape Lookout sites, stands of dense
vegetation apparently acted as barriers to washover deposition, forming
“islands” of negligible sedimentation; these patches of vegetation were
excluded from washover areas. The volume of each deposit was found
by multiplying the area by the average thickness of washover
sediments, based on thicknesses recorded at the spade pits. Deposit
volumes were multiplied by the average bulk density for each site to
find the mass of each washover deposit.
Particle size analysis was conducted on sediment samples using
sieves at 0.25 phi unit intervals from 0 phi (1 mm) to 4 phi (63 μm).
A few samples contained coarse shell fragments that were retained
in the 1 mm sieve – these were removed prior to weighing of
the sieve fractions. The sieve data was analyzed using the GRADISTAT
program (Blott and Pye, 2001), which provided particle size parameters using graphical procedures proposed by Folk and Ward (1957).
Microfossil analysis was conducted on 5 cm3 samples of washover
sediment and sediment underlying the washover deposits (from
the buried former surface). Samples were passed through 500 μm
and 63 μm sieves and the fraction remaining in the 63 μm sieve
was examined under a dissecting microscope to identify and count
foraminifers. Foraminifers were identified to the genus or species
level.
5. Results and discussion
5.1. Morphology of washover deposits
There are clear contrasts in the morphology of washover deposits
between Fire Island and the sites at Assateague Island and Cape Lookout.
Given the relative elevations of beach systems at the study areas and
local observed storm tide levels, it is unlikely that inundation overwash
H.F.L. Williams / Geomorphology 231 (2015) 182–192
187
Fig. 5. a. Base of washover deposit at Assateague Island marked by sharp contact between sandy washover sediment and buried marsh surface. b. Buried dune grasses at the Fire Island
study area; estimated base of washover deposit (dashed line) is between below-ground rhizome (arrowed) and above ground plant stems (trowel is about 0.25 m long). c. Abrupt distal
edge of washover deposit at the Assateague Island study area, with steeply dipping avalanche faces (spade is about 1 m long).
occurred at any of the three sites. Instead, run-up overwash likely
occurred. This is supported by nearby wave data, which suggests local
wave heights exceeded the beach berm or foredune crest elevation at
all three sites (NOAA, 2014). The washover deposits at Fire Island are
the result of run-up overwash channeled through low spots in the
beach system created by pre-existing foredune breaches and following
branching pathways through the hummocky topography of the
backshore to create isolated, multi-lobed washover fans. Due to lower
elevations at the Assateague Island and Cape Lookout sites, run-up
overwash occurred along longer continuous sections of the beach
system, forming washover terraces along many km of backshore.
These deposits form a continuous blanket of sediment with lobes or
narrow “fingers” of sedimentation extending inland, where overwash
was channeled through low spots or around dense vegetation stands
in the backshore.
There is no consistent pattern to the thickness of washover at Fire
Island or Assateague Island, but at Cape Lookout, washover sedimentation generally becomes thicker in the more distal part of the deposit,
suggesting a wedge-shaped profile that thickens inland (Fig. 6). This
suggests that more sediment was transported over proximal parts of
the deposit than deposited or, if deposited, it was later remobilized as
the storm intensity increased (Leatherman et al., 1977). The transported
sediment accumulated in the lower-lying, more densely-vegetated
marsh in distal areas of the deposit. At Fire Island and Assateague Island,
washover thicknesses are highly variable, with no consistent spatial
trend, suggesting a strong influence of the pre-existing topography.
The washover is probably thicker where low-lying areas were infilled
and thinner where the surface was higher; a similar pattern was
observed by Hawkes and Horton (2012) in a study of Hurricane Ike
washover in Texas.
Inland extent of washover is comparable between the four sites,
ranging from 109 to 173 m at Fire Island Fan A and Fan B, respectively,
to 138 to 182 m at Assateague Island and Cape Lookout respectively.
However, average washover thickness is greater at Assateague Island
(0.76 m) and Cape Lookout (0.66 m) than at Fire Island Fan A
(0.27 m) and Fan B (0.38 m) and consequently, the volume of washover
deposition per meter of coastline is greater at Assateague Island and
Cape Lookout (97 m3/m and 119 m3/m, respectively) than at Fire Island
Fan A and Fan B (33 m3/m and 49 m3/m, respectively) (Table 1). These
results are consistent with washover volumes reported in previous
studies, ranging from 20 to 225 m3/m (Morton and Sallenger, 2003).
The contrast in washover volumes deposited at Fire Island and at
Assateague Island and Cape Lookout reflects the different styles of
washover sedimentation that occurred at these sites; washover fans
typically contain a lower percentage of eroded beach and foredune
sediment than washover terraces, which, because of the lower
elevational threshold for overwash occurrence, are characterized by
deeper, more frequent landward directed wave-induced surges of
overwash capable of transporting greater amounts of sediment
(Morton and Sallenger, 2003).
5.2. Texture of washover deposits
All washover samples subjected to textural analysis were classified
as 100% sand. Eight out of fifteen samples from Fire Island Fan A were
categorized as “well-sorted medium sand”, as were both samples from
the surface of the adjacent beach; of the seven remaining samples, five
were classified as “moderately well-sorted medium sand”, one was
“moderately sorted coarse sand” and one was “moderately well-sorted
coarse sand”. Ten out of fifteen washover samples from Assateague
Island were categorized as “moderately well-sorted medium sand”,
the same as both samples from the adjacent beach; two samples were
classified as “well-sorted medium sand”, one was “well-sorted fine
sand”, one was “moderately-sorted medium sand” and one was
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H.F.L. Williams / Geomorphology 231 (2015) 182–192
Fig. 6. Washover deposit sampling points and depth of washover sediment. See Fig. 3 for locations. a–b. Fire Island, New York; c. Assateague Island, Virginia; d. Cape Lookout, North Carolina
(Imagery: a, b; Google, NASA; c, d; National Geodetic Survey). Washover deposits at Fire Island and Cape Lookout surround “islands” of dense vegetation which received negligible
sedimentation.
“moderately well-sorted coarse sand.” At Cape Lookout, five out of
seven samples were classified as “moderately well-sorted medium
sand” and two were “well-sorted fine sand” (Folk and Ward, 1957;
Table 2). Although texture varies within the washover deposits, many
of the samples from Fire Island and Assateague Island have similar
texture to samples of the adjacent beaches. This supports the likelihood
Table 1
Physical characteristics of washover deposits.
Washover deposit
Inland extenta(m)
Area (m2)
Average thickness (m)
Volume (m3)
Mass (tonnes)
Volume per meter of coastline (m3/m)
Fire Island Fan A
Fire Island Fan B
Assateague Island
Cape Lookout
109
173
138
182
5,842
18,207
41,390
21,455
0.27
0.36
0.76
0.66
1,577
6,555
31,456
14,160
2,098
8,718
45,926
20,674
33
49
97
119
a
Proximal to distal edge of deposit.
H.F.L. Williams / Geomorphology 231 (2015) 182–192
189
Table 2
Textural characteristics of washover deposits.
Sitea
Depth
Mean grain size (μm)b
Sortingb
Descriptionb
Fire Island Fan A
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
Upper Beach
Low Beach
Top
Middle
Base
Top
Middle
Base
Top
Middle
Base
Top
Middle
Base
Top
Middle
Base
Surface
Surface
463
374
399
452
526
413
425
441
517
436
540
405
412
487
363
359
387
1.375
1.473
1.511
1.409
1.516
1.440
1.359
1.368
1.776
1.364
1.459
1.602
1.370
1.377
1.351
1.381
1.413
Well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Well sorted medium sand
Well sorted medium sand
Moderately sorted coarse sand
Well sorted medium sand
Moderately well sorted coarse sand
Moderately well sorted medium sand
Well sorted medium sand
Well sorted medium sand
Well sorted medium sand
Well sorted medium sand
Well sorted medium sand
Assateague Island
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
Upper Beach
Low Beach
Top
Middle
Base
Top
Middle
Base
Top
Middle
Base
Top
Middle
Base
Top
Middle
Base
Surface
Surface
362
434
386
398
460
410
312
406
542
419
289
278
383
256
243
399
337
1.497
1.584
1.463
1.535
1.482
1.515
1.371
1.821
1.579
1.591
1.542
1.474
1.522
1.325
1.354
1.609
1.462
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Well sorted medium sand
Moderately sorted medium sand
Moderately well sorted coarse sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Well sorted medium sand
Well sorted fine sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Cape Lookout
1
1
1
1
1
1
1
0–2 cm
14–16 cm
24–26 cm
34–36 cm
44–46 cm
54–56 cm
65–67 cm
290
287
342
323
323
246
206
1.593
1.539
1.595
1.488
1.489
1.405
1.360
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Moderately well sorted medium sand
Well sorted fine sand
Well sorted fine sand
a
b
See Fig. 6 for locations.
Derived from Folk and Ward (1957) graphical methods.
that the eroded beaches were the sources for much of the washover
sediments. Other probable sediment sources include the foredunes,
which were eroded at all three sites, particularly by widening of
pre-existing breaches, the intertidal and shallow sub-tidal zones, and,
possibly, eolian inputs generated by strong winds before, during and
after passage of the storm.
Lateral and vertical trends in texture were investigated by plots
of mean particle size (Figs. 7, 8). At the single sampling point at Cape
Lookout, washover texture initially coarsens with depth (~ 290 μm
to 323 μm) and then fines with depth to the base of the deposit
(~ 323 to 206 μm; Fig. 7). It is tempting to conclude that this reflects
strengthening and then waning of the storm (Leatherman et al.,
1977). However, many of the vertical profiles at Fire Island and
Assateague Island do not conform to this pattern; profiles range from
fining with depth, coarsening with depth, fining at mid-depth to coarsening at mid-depth (Fig. 8). Lateral trends in texture of the top, middle
and base of washover at Fire Island and Assateague Island also show
considerable variability, fluctuating between fining inland and coarsening inland. At Fire Island, the average particle size of washover (average
of top, middle and base at each sampling point) undergoes little overall
change between proximal and distal parts of the deposit. At Assateague
Island, the average particle size undergoes an overall fining trend with
distance inland (Fig. 8).
Previous studies of textural trends in washover deposits have
reported fining and/or thinning of washover with distance inland
(Kochel and Wampfler, 1989; Switzer and Jones, 2008; Williams,
2009, 2010, 2012; Hawkes and Horton, 2012). Kochel and Wampfler
(1989), for example, reporting on washover fans on Assateague Island,
suggested inland fining trends they observed may reflect the loss of
competence coincident with rapidly expanding flow across mid-fan
and back-fan areas. The deposit at Fire Island (Fan A) does not display
an overall inland fining trend and at Assateague Island, although
there is an overall fining trend inland in the average particle size
of the deposit, there is also much variability in texture within the
deposit, which is inconsistent with a progressive reduction in flow
competency along proximal–distal trends (Fig. 6). Instead, it may be
that sediment textures in this study are more heavily influenced by
sediment sources (the beach, foredunes and possibly some contribution
190
H.F.L. Williams / Geomorphology 231 (2015) 182–192
Fig. 7. Vertical trends in mean particle size, sample point 1, Cape Lookout.
from the inter-/sub-tidal zones) and how these sources changed
over the duration of the storm, rather than changing flow dynamics as
the overwash moved inland (Leatherman et al., 1977; Hawkes and
Horton, 2012).
5.3. Microfossil content of washover deposits
Very few of the samples analyzed for microfossil content contained
any foraminifera. All 17 samples from Fire Island (2 beach, 15 washover)
and all 7 samples of washover from Cape Lookout were completely
barren of microfossils. At Assateague Island, only the surface samples
of washover at locations 6 and 7 contained foraminifera (Fig. 6). The
sample from location 7 contained 1 specimen of Elphidium spp. and
the sample from location 6 contained 3 specimens of Elphidium spp.
Samples of the buried surface underlying the washover at Assateague
Island contained larger numbers of agglutinated marsh species. The
buried surface at location 7 contained 33 specimens of Jadammina
macrescens, 3 specimens of Haplophragmoides wilberti and 15 specimens
of Trochammina inflata. Low numbers of Jadammina macrescens were
found underlying the washover at locations 4, 5 and 6 (8, 5 and 3
specimens, per 5 cm3, respectively). The buried surface at location 3,
in the most proximal part of the washover deposit, was barren of
foraminifera.
The rarity of foraminifera in washover deposits at the three study
areas supports the textural evidence that the sources of washover
sands were predominately the adjacent eroded beaches and foredunes.
Foraminifera rarely live in these harsh environments. When they do
occur it is usually because of post-mortem transport and tests are
often subject to destruction by abrasion and dissolution (Murray,
1973; Haslett et al., 2000). Elphidium spp. are very common in tidal
flat and shallow sub-tidal zones on the coast of Virginia (Culver et al.,
1996). The presence of these species in the washover supports the
possibility that some inter-/sub-tidal sediment was eroded and
transported inland by the storm. Washover sediments barren of foraminifera have been reported in previous studies (Horton et al., 2009), but
in some cases foraminifera are abundant in washover (Collins et al.,
1999; Hippensteel and Martin, 1999; Hawkes et al., 2007; Williams,
2009, 2010). Clearly these variations in the abundance of foraminifera
in washover deposits reflect the source area(s) of sediments; a relatively
large inter-/sub-tidal component will probably result in relatively
abundant foraminiferal contents. Washover sediments derived
primarily from the beach and foredunes will probably be largely barren
of foraminifera.
6. Conclusions
This study demonstrates that differing levels of overwash result in
contrasts in the morphology and volume of resulting washover
deposits. At Fire Island, run-up overwash was channeled through low
spots created by pre-existing breaches in foredunes and deposited relatively thin, low-volume washover fans. Run-up overwash also occurred
at Assateague Island and Cape Lookout, but because of lower elevational
thresholds at these sites, storm waves overtopped longer continuous
sections of the beach system and deposited thicker, larger-volume
washover terraces. The larger washover terraces presumably reflect
deeper, more frequent landward directed surges of overwash capable
of transporting greater amounts of sediment from eroded beaches and
foredunes.
Previous studies of washover have documented thinning and fining
of deposits with distance inland, probably reflecting progressively
decreasing flow competence. The washover deposits in this study do
not conform to this pattern, but are instead characterized by a high
degree of spatial variability in thickness and texture. A possible explanation is that sediment textures of washover examined in this study
reflect the character of sediment sources and how these sources
changed over the duration of the storm, rather than changing flow
dynamics as the overwash moved inland. Variability in thickness of
washover probably reflects the influence of pre-existing topography,
wherein washover is thicker where low areas were filled in and thinner
where washover covers higher points on the former surface. Microfossils are very rare in the washover deposits and this most likely also
reflects beach and foredune sediment sources, with only a minor contribution from inter-/sub-tidal zones. Geomorphologically, the washover
deposits have significant impacts on these barrier islands; they represent a sediment pathway and component of local sediment budgets,
transferring up to 119 m3/m of eroded beach and foredune sediments
into island interiors and accounting for up to an average of 0.76 m of
aggradation of backshore surfaces.
Acknowledgments
This material is based upon work supported by the National Science
Foundation under Grant No 1157527. P. Strong assisted with field and
laboratory work. M. Bilecki, B. Commins, R. Beavers and S. Strickland
of the National Park Service assisted in obtaining site access. C. Hapke,
USGS, suggested sites for the study. The paper was improved by the
comments of two anonymous reviewers.
H.F.L. Williams / Geomorphology 231 (2015) 182–192
191
Fig. 8. Textural trends in washover deposits: a. Vertical textural trends, Fire Island Fan A (dashed lines represent uncertainty in textural characteristics between sampling points). b. Lateral
textural trends, Fire Island Fan A. c. Vertical textural trends, Assateague Island (dashed lines represent uncertainty in textural characteristics between sampling points). d. Lateral textural
trends, Assateague Island.
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