Robert Zaremba.

Overwash processes and foredune ecology, Nauset Spit, Massachusetts online

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made further migration impossible. Eventually the glacial highlands of Little
Pocket Island and Nauset Heights will be exposed to wave attack and the bar-
rier beach will be converted by erosion into a beach fronting a headland at
these locations.

2. Vegetative Response to Overwash .

The response of barrier-beach vegetation to overwash burial was studied
using U.S. Coast Survey maps and aerial photos as well as prestorm and post-
storm vegetation and elevation data. The location of dunes, salt marshes,
shrubs, and washovers was mapped along the spit system for periods from 1851
to 1978 to determine the rate of plant community development and morphologi-
cal changes along the barrier system. Vegetation and elevation data were
collected in 1977 and compared to data acquired after the February 1978
northeaster. The immediate response of major June and salt-marsh species to
overwash and the colonization of washovers were documented from these data.

IXines play an important role in the stabilization of barrier beaches.
IXines, which act as barriers to wave attack during all but the most severe
storms, provide protection for back-barrier vegetation. Salt-marsh plants
become established in low-energy, intertidal regions of the barrier. Areas
that are subject to frequent overwash or swift bay-side currents do not




support salt-uiarsh vegetation. Dunes also reduce salt-spray intensity,
which allows the establishment of salt-intolerant species to the lee of this
barrier. Finally, dunes act as sediment reservoirs for both nourishment of
beaches during storms and for washover deposits. During storms some of the
sediment removed from the i-each by wave activity is replaced by dune-face
erosion. Surges that cross the berm crest carry beach and eroded dune sedi-
ment to landward positions. This sediment is reworked by the wind
interstora periods. On Nauset Spit, prevailing winds transport much of the
washover sediment eastward. Remnant and newly established dunes trap some of
this sand, eventually recreating a continuous dune line.

In 1978 the dune line at Nauset Spit-Eastham was high and almost contin-
uous. Most of the dunes along Nauset Spit-Easthaia affected by the February
1978 overwash were leveled and the vegetative surface was eroded. Some sec-
tions of the dune at site 3 were, however, buried by up to 67 centimeters of
overwash sand without disturbing the vegetative surface. The four major dune
species found on the Nauset Spit system, Armnopkila breviligulata, Artemisia
stellev'-aruz, Lathyvus japonicus, and Solidxxgo sempevvirens, were present in
quadrats sampled before and after overwash at this location. All four species
were able to recover from deep overwash burial. Washover deposition equaled
typical high, annual evels of wind-transported sand deposition in dunes. On
low-lying, well-vegetated dunes, overwash plays an important role in dune

Following overwash, wind deflation of extensive washover flats aids in
the redevelopment of the dune line. At Nauset Spit the prevailing sand-
transpcrting winds move sand offshore; remnant dunes trap much of the material
deflated from washovers. Along the outer edges of the washover, drift mate-
rial is deposited, by spring high tides. Sand reworked from the v;a3hover also
accumulates differentially in the vicinity of emerging drift-line plants, thus
building new dunes on the fan surface through time.

Shrub communities are found in areas of North Beach protected from salt
spray. The frequency of shrubs decreases with distao.ce from the glacial head-
lands at Nauset Heights, reflecting a decrease in stability of the barrier and
a decrease in propagule availability. Shrubs have little importance in the
maintenance of the physical stability of the barrier since a cohesive sub-
strate has not been developed that could appreciably slow erosion and downward
cutting during major storms.

Many of the shrubs on northeast barrier beaches appear to recover from
overwash burial. Rosa rugosa, Myrica pensylvanic-a, and Prunus maritima, major
shrub species on Nauset, survived burial of lower plant parts. Changes in
water-table height caused by increased elevation may lead to anaerobic con-
ditions in the shrub root zones; therefore, overwash deposition may eventually
kill these plants. Major shrub communities on Nauset do not appear to grow
through washover sediment and reestablish a shrub community. isolated plants
have recovered, however, and have survived burial for many years.

An aerial photography analysis indicated that shrub communities became
established on new washover substrate in as little as 26 years. On Nauset,
shrub species generally invade bare, stable washover pediment that is
protected from high levels of salt spray. Ammophila breviligulata may have
grown briefly in the area, but undoubtedly does not alter the sandy surface


Salr marshes are important in contributing organic material to the bay and
in providing stability to the barrier by the formt»tioa of peat. Over many
years, salt marshes accumulate fine-grained sediment and organic material,
building cohesive peat. Peat prevents excessive scouring during overwash,
which could lead to inlet formation. Peat outcrops along the ocean beach also
slow shoreline erosion a i, as at the southern end of Nauset Spit-Eastham,
salt-marsh peat has apparently slowed the lateral, northward migration of
Nauset Inlet.

Salt-marsh vegetation is generally not eroded by overwash surges. After
crossing the dune ?oae, surges slow due to flow divergence and essentially
stop where ponded bay waters are encountered.

Salt-marsh plants on Nauset Spit-Eastham were buried by as much as 110
centimeters of sand. Only Spartina patens and Spartina altemiflora were able
to recover, in restricted areas, from more than 10 centimeters of overwash
deposition; Spartina patens recovered from up to 33 centimeters of burial.
Plants at prestorm high elevations recovered better than plants at lower
elevations. Data obtained from shallow wells indicated that the ground water
table is raised beneath washover deposits. Anaerobic conditions resulting
from waterlogging may limit the recovery of Spartina patens at low elevations
and in areas of deep burial.

Spartina altemiflora recovered from as much as 22 centimeters of washover
sand in 1978. Unlike Spartina patens, burial recovery was best at lowest
elevations. Higher elevations are less frequently flooded and soil salinity
may increase beyond the tolerance levels of Spartvna altemiflora.

Many of tb° Spartina patens and Spartina altemiflora plants that had
recovered from burial in 1978 were either dead or dying by 1980. The larvae
of a weevil had invaded the cortex of many Spartina altemiflora plants that
were yellow and dying; many Spartina patens plants that had recovered in 1978
were dead in 1980 and had rotted at a point 5 to 10 centimeters before the
surface. Sediment deposition may have raised the elevation-water table height
to a point that plant vigor was lost and waterlogging produced anaerobic con-
ditions, weakening and ultimately killing the plants.

The aerial photography analysis indicated that salt marshes can form in
as little as 10 years on newly placed washovers, but nev; salt marshes do not
form on barren, intertidal washovers until overwash pressures are reduced.
Salt marshes are established either by rhizome outgrowth from recovering or
adjacent marsh plants, by plant fragments eroded from creek margins, or by
seed. Following overwash, both Spartinae are able to expand rapidly by
rhizome extension. Plants at the margins of washovers will colonize inter-
tidal washovers at a rate of approximately 1 meter per year. Blocks of salt-
marsh peat (mainly Spartina altemiflora) with living plants are frequently
eroded along creek margins and carried by tides to intertidal positions where
roots develop, anchoring the peat block. Many of these blocks are ice-rafted
to the upper range of Spartina altemiflora. Fragments of both Spartina
altemiflora and Spartina patens are also occasionally found among drift
material deposited following overwash at the outer margin of washovers
by spring tides. Spartina altemiflorVL seedlings in drift lines did not
survive the dry summers during the course of this study. Spartina patens
var. monogyna survived in drift lines and expanded energetically during the
following years. Seeds of both Spartina altemiflora and Spartina patens



germinate at the upper edge of the intertidal zone. During the 3 years of
this study, Spavtina seedlings were not found on recent washovers but were
present in other upper-intertidal areas protected from overwash.

3. Barrier Migration Model.

Overwash and the vegetative response tc overwash have played important
roles in tha landward migration of Nauset Spit. Washovers along Nauset can bi
divided into two types based on their si^e and influence in barrier migration:
washover fans and flats. Washover fans form as breaches through the dune
line and result in small amounts of sand deposition to the lee of the dune
field (Fig. 132). Along the Nauset Spit system, these deposits are frequently
placed on salt-marsh vegetation; recovery from burial occurs only at the edges
of the washover where deposits are less than 34 centimeters for Spavtina
patens and 23 centimeters for Spavtina alt evni flora, tost marsh plants do not
recover from burial, leaving the washover fan barren and subject to wind
deflation. At Nauset Spit, the prevailing west winds carry washover sand to
the beach and to the back dunes adjacent to the washover throat (Fig. 132, c).

The percentage of sand returned to the beach and lost from the system
depends on the general form of the washover, prevailing wind directions, and
the width of the washover throat. At the washover studied in detail along
Nauset Spit-Eastham, 52 percent of the sediment deflated from the fan was
added to the landward margin of the barrier dunes and 22 percent was returned
to the ocean beach. Although 62 percent of the original deposit was redis-
tributed by winds and tides, the entire washover remained above the general
elevation range of salt-marsh species. The eventual plant communities that
form on small washover fans are dependent on the elevation of the deflated
surface as it is stabilized by vegetation.

Because the Nauset barrier system is oriented north to south and prevail-
ing sand-transporting winds are from tha west, there is little opportunity for
dune-building on small washover fans since there is only a limited amount of
sand in upwind positions. Low dunes may form on a washover fan, hut washover
sediment is reduced as dunes coalesce acrobP the washover thro<.-t, and new
dunes on washovers rapidly reach a maximum height and begin to deflate. After
several years (5 to 7), a small washover fan appears as a crescent-shaped rise
on the salt marsh adjacent to the dune line. The dune line may have migrated
slightly landward as sand deflated from the washover and accumulated in back-
bar-ier dunes (Fig. 132, d). Tine washover itself may he colonized by supra-
tidal vegetation, which has built very low dunes. Otherwise, these dunt3 may
have deflated and Anrnophila bveviligulata-domiv.^zed vegetation may be out-
coicpeted by species adapted to periodic saltwater flooding, such as Spavtina
patens var. rnonogyna and Agvopyvon punge.m. The net result of snall-scale
overwash is that the dune line is displaced slightly landward.

In the second case, large-scale washovers play a very important role in
barrier migration (Fig. 133). Prior to overwash, the barrier beach may con-
sist of a continuous dune line backed by salt marsh. During a major storm the
barrier dune is eroded to a point where low elevation dunes and blowouts are
overtopped by overwash surges. These surges erode an increasingly wide chan-
nel through the dune line by lateral cutting until broad sections of the dunes
are entirely flattened (Fig. 133, c). During overwash, large volumes of sand
may be carried from the beach and dune to the back barrier. Some of this
sediment may be transported into the bay, resulting in landward extension of
the barrier unit.


- -Ji!


y (



Figure 132. Model of role of overwash and vegetative response tor
small washover features.



Figure 133. Model of role of overwash and vegetative response to overwaeh
In the landward migration of a northeastern barrier beach.


Following overwash, organic debris is left on Che washover surface In
large clumps. Organized drift lines are also deposited along the outer mar-
gins of washover flats by spring high tides. The surface of the salt marsh
is generally increased to elevations above the natural range of salt-marsh
species. Fragments of dune plants present in drift lines regenerate and
seeds germinate leading to the establishment of dune vegetation. As with the
smaller washovers along Nauset Spit, the westerly winds deflate these wash-
overs (Fig. 133, d). Most of the deflated sediment is returned to the ocean
beach because extensive sections of the dune line have been leveled. Rhizome
extension from surrounding dunes plays a smaller role in the stabilization and
revegetation of large washovers than it does on smaller fans due to the large
ratio of fan area to vegetative perimeter.

Dunes begin to develop in the location of drift material. In contrast
to the case of small washovers, these dunes continue to build as overwash
continues to add sediment to the back of the washover in upwind positions
relative to the drift lines. The lack of constraining foredunes allows over-
wash to take place for several years (5 to 10), augmenting this sand supply.
Drift-line dunes are usually not eroded during overwash since they are located
in landward positions. During the final stages of dune recovery, washover
passages through the foredunes periodically coalesce during windy, interstorm

Eventually the dune line becomes continuous and the back barrier deflates
to intertidal elevations at which moist sand will not saltate (Fig. 133).
The net result of large-scale overwash is that after many years (10 to 20),
all barrier features are displaced landward. New dunes, resulting from sand
accumulation around vegetation initiated in drift lines, coalesce with vege-
tation expanding by rhizome extension from remnant dunes. New salt marsh
forns in the lee of these dunes, and the barrier beach as a whole is displaced
landward with the establishment of the same general physiographic features and
vegetative composition.

4. Engineering Implications .

All sections of the Nauset Spit system are subject to draraatic changes
either by Inlet activity or overwash. The four units designated during this
study are eroding at progressively faster rates with distance from the major
source of sediment along Outer Cape Cod, the glacial cliffs. Increased ero-
sion rates lead to more rapid landward migration and more unstable conditions.
The outer shoreline appears to be readjusting toward a slightly more southwest
to northeast orientation. Mannsade structures along all sections of the spit
system will be subject to destruction during storms. The most stable unit,
Nauset Spit-Eastham, appears to be undergoing a longer migration cycle than
other sections of the spit system.

Artificial creation and maintenance of dunes and salt marshes can be used
to extend various periods of the migration cycle but will not alter the basic
biogeological process. With the initiation of a new inlet through North
Beach, the town of Chatham will be subject to wave assault and severe erosion.
Dune stabilization will not prevent the eventual formation of an inlet through
this section cf North Beach, which is eroding at a rate of 5.8 meters per year
and is only 110 meters wide in soee areas. Salt marshes cannot serve aa an
inlet deterrent because of the length of titre required to establish a thick
peat layer.


* .; / • - ' >

Nauset Spit-Orleans will also narrow through time until it will overwash,
an inlet will form, and the spit north of Nauset Heights will be lost— perhaps
fusing to Nauset Spit-Eastham as the inlet channel shifts. Oane stabilization
along Nauset Spit-Orleans will only increase the rate of barrier narrowing as
occasional overwash widening will be prevented.

Overwash will continue at Old North Beach and Nauset Spit-Eastham. Exten-
sive dune stabilization can reduce overwash activity for a period of time
resulting in calra back-barrier conditions necessary for the establishment of
salt-marsh vegetation. Artificially established dunes will continue to narrow
in the absence of washover sediment in upwind positions, and these foredunes
will eventually be destroyed with incessant shoreline erosion. Salt-marsh
peat behind the barriers will continue to restrict inlet formation.

Without artificial dune and salt-marsh establishment, new dunes and salt
marshes will form along those parts of the barrier within the correct eleva-
t tonal ranges. However, it may be many years before high dunes are once again
present on Nauset Spit-Eastham. A continuous dune line may not reform and
appear at; it did prior to 1978. New dunes, if artificially established,
snould be constructed well landward of the berm crest to allow for future
shoreline erosion. Natural dunes will form toward the back of washovers in
drift lines and expand eastward.

Salt marshes can also be established effectived on washover flats. Care
should be taken to plant Spartina patens and Spavtim. alterrdflova at eleva-
tions within their natural range along the spit system. Plantings should also
be conducted only in areas that are not subject to continued overwash or high-
energy conditions (.swift currents near inlet channels or long fetch directions
on the bay side).

Rapid shoreline erosion, high rates of littoral drift, and harbor condi-
tions render rigid structures ineffective in a practical sense. By working in
association with natural processes, segments of the migrational cycle can be
expanded but not restricted.



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