Flyover Showing Geomorphology and Coastal Processes Along the Atlantic Shoreline

19 Pages·2.098 MB·English

by  S. Jeffress Williams(auth.)

FRONTISPIECE: LANDSAT satellite photograph of
the middle Atlantic coastal plain showing a
variety of late Quaternary ·coastal landforms.
Ayover Showing Geomorphology
and Coastal Processes
Along the Atlantic Shoreline
Cape Henlopen, Delaware to Cape Charles, Virginia
July 17-18,1989
Field Trip Guidebook T219
s.
Leader: Jeffress Williams
American Geophysical Union, Washington, D.C.
Leader:
S. Jeffress Williams
U.S. Geological Survey
914 National Center
Reston, VA 22092
Published 1989 by American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C 20009
ISBN: 0-87590-553-6
Printed in the United States of America
COVER: The 100-ki1ometer chain of 18 pristine barrier islands and
lagoons along the Virginia coast is among the best examples in the
world that remain.
CONTENTS
PAGE
Introduction 1
Purpose of fly-over field trip 1
Delmarva Coastal Zone.............................................. 3
Delaware Coast................................................ 3
Maryland Coast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3
Virginia Coast 4
Geomorphology and Coastal Geology 6
Process of Coastal Evolution 6
Stratigraphy of Holocene Transgressive Deposits 8
Delaware Coast )
Maryland Coast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8
Virginia Coast..................................... . . . . . .. 8
Holocene Sea-Level History 10
Modern Sea-Level Rise 11
Future Coastal Evolution 11
Acknowledgments 12
References 12
Figures
1. Location Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2
2. Aerial photo of the Delaware Coast 3
3. Aerial photo of Ocean City, Maryland 4
4. Aerial photo. of Ocean City Inlet, Maryland 4
5. Aerial photo of Gargathy Inlet, Virginia 5
6. Virginia coast change map 5
7. Quaternary geologic map 7
8. Delaware coast change map 8
9. Delaware coast dip sections 9
10. Maryland coast change map 9
11. Shoreline change map of Fishing Point, Virginia 10
12. Southern Delmarva Peninsula map 10
13. Virginia coast dip sections 11
14. Holocene sea-level curve for the Delaware coast 11
Table 1. Recent coastal erosion and sea-level rise rates 12
vii
IGC FIELD TRIP T219:
GEOMORPHOLOGY AND COASTAL PROCESSES ALONG THE ATLANTIC SHORELINE
J
CAPE HENLOPEN DElAWARE TO CAPE CHARLES VIRGINIA
J J
S. Jeffress Williams
U.S. Geological Survey, Reston, Virginia
INTRODUCTION sediment in-filling and down-cutting
associated with multiple sea-level
The more than 4,000 km of barrier fluctuations over the past 2 million
islands along the U.S. Atlantic coast and years. These evolutionary processes are
Gulf of Mexico comprise one of the continuing at the present time with
largest, most diverse and most studied additional significant human impacts
coastal barrier chains in the world. during the past several decades. Coastal
Within this chain, the coastal compartment erosion and pollution of Chesapeake Bay
from Delaware Bay to Chesapeake Bay offers and the rivers entering the Bay are now
superb examples of a variety of coastal finally being recognized as serious
and nearshore environments and shoreline problems, and efforts to mitigate these
features. are becoming a high priority with Federal,
The flight path of the field trip will State and local governments.
extend from Washington, D.C. at the Fall
Line, east across the Atlantic Coastal Purpose of fly-over field trip
Plain to Cape Henlopen, Delaware, at the
entrance to the Delaware River estuary,
The one-day overflight will enable trip
and then south along the Maryland and participants to observe, photograph and
Virginia coast to Cape Charles, Virginia
study a wide variety of landforms and
(Fig. 1). The subaerial part of the U.S. geologic features within the Middle
Atlantic Coastal Plain is about 185 km
Atlantic Coastal Plain Province. Emphasis
wide, while the width of the submerged will be on late Quaternary coastal
continental shelf portion is approximately
features and the processes responsible for
100 km. The portion of Coastal Plain that
their formation and evolution. Beginning
we will fly over, as well as that along
in Washington, D.C., we will fly east to
the rest of the Atlantic margin, has Cape Henlopen, Delaware, and then south,
relatively low physiographic relief and along the coast at a low altitude, to Cape
consists of a seaward-dipping and Charles at the entrance to the Chesapeake
thickening wedge of only slightly deformed
Bay estuary (Fig. 1). Examples of late
siliclastic sedimentary units of Pleistocene and Holocene landforms to be
Cretaceous, Tertiary and Quaternary age.
observed and discussed include: Cape
From Cape Henlopen, Delaware, south Henlopen spit rapidly prograding north
approximately 200 km to Cape Charles
into Delaware Bay, the eroding headland
Virginia at the entrance of Chesapeake and sand-starved bay-mouth barrier coast
Bay, the Delmarva (Delaware-Maryland of Delaware, and the highly developed
Virginia) coastal compartment consists of barrier spit containing Ocean City,
a variety of marine and estuarine Maryland, with its closely spaced high
environments and landforms. These have rise buildings and a recently completed
been dominated by late Quaternary sea beach nourishment project. Downdrift from
level rise of at least 100 m and also the the jettied Ocean City Inlet, northern
resultant marine transgression that is Assateague Island will offer a classic
continuing to the present time. example of a sediment-starved coast due to
The return flight to Washington, D.C., man-made causes. Farther south Fishing
will afford an excellent view of the lower Point spit is an example of natural causes
parts of Chesapeake Bay, the largest and of littoral-drift disruption. Along the
most biologically diverse and productive Virginia coast, 18 short and narrow,
estuary in the U.S. (Fig. 1). The modern pristine, barrier islands and associated
Bay originated about 5 ka when sea level lagoonal wetlands are undergoing rapid
rose high enough to flood the deeply natural changes.
scoured ancestral fluvial valleys of the Our return flight will offer excellent
more than 12 rivers that converged at the viewing of lower parts of the Chesapeake
axis of the Bay. Recent seismic Bay as well as the Potomac River and other
reflection surveys in Chesapeake Bay rivers that enter the Bay.
reveal a complex geologic history of
T219:
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CITY
VIRGINIA
I
-1-
ATLANTIC OCEAN
N
1
25 0 25
MILES
25 0 25
KILOMETERS
36°+1-----------+---....:..;J...:...:~~-----~-.-.....~..------_+-----------r
FIGURE 1 Location map of the Middle Atlantic (Delmarva) coastal region.
Trip extends from Washington, D.C. east across the Coastal Plain to Cape
Henlopen, Delaware and south 200 km to Cape Charles, Virginia at the
entrance to Chesapeake Bay. The return flight path follows the western
shore of the Bay back to Washington, D.C.
T219: 2
Preserve. The cylindrical concrete towers
DEl.MARVA COASTAL ZONE
that may be obvious from the air represent
some of the few remaining military obser
Delaware Coast
vation towers that were built along the
U.S. Atlantic coast during WW II. Built
The coast from Cape Henlopen, Delaware,
on the more stable headland coast to the
south to Cape Charles, Virginia, is as
south, Rehoboth Beach and Dewey Beach
varied in character as a resort area and
situated on the narrow barrier are popular
nature preserve as it is in its Quaternary
resort towns that have grown considerably
geologic history and the dynamic waves,
in the past 30 years. South of Dewey, the
currents and winds which shape the
bay-mouth barrier comprising the 12-km
present-day shoreline. Our first view of
long Delaware Seashore State Park is
the Delaware coast will be on the southern
backed by Rehoboth and Indian River bays.
shore of the Delaware Bay estuary. Lewes,
These drowned ancestral river valleys have
a town of Dutch heritage, has a 357-year
been recently mapped from seismic profiles
history as a fishing port that continues
and found to extend continuously from the
to the present. Many of the town's
mainland, under the barrier and seaward
historical buildings have been refurbished
across the inner shelf.
and the Early American atmosphere is
Next, Indian River Inlet, important for
preserved in spite of some recent low-rise
tidal exchange with the bays and as a
development.
navigation route, was artificially opened
Cape Henlopen sp~t (Fig. 2) and the
in 1929 and is maintained by two large
surrounding 10 km of dunes, beaches,
rock jetties. The eastward offset in
ponds and swales are part of the wel1
beach width adjacent to the jetties that
maintained Delaware State Park and Nature
we will see is a classic jetty-effect that
we will see demonstrated even more
dramatically at Ocean City inlet. Such
offset at Indian River inlet is proof of
net longshore transport northward. The
Seashore Park continues south as a natural
beach to the town of Bethany Beach, site
of the recently built Sea Colony high-rise
development. Like Rehoboth Beach, Bethany
too is built on a headland portion of the
coast. Historical erosion trends and
geomorphic evidence suggest that this
stretch of coast is a zone of divergence
for longshore transport. Net littoral
sand drift travels north from Bethany to
Cape Henlopen spit, while net sand
transport is south along the coast from
Bethany Beach to Cape Charles.
South of the Seashore Park at the
Delaware-Maryland boundary is the town of
Fenwick Island consisting of low-rise
development stretching from Little
Assawoman Bay across the barrier to the
ocean. Fenwick is a popular family-style
resort and less crowded in-season than
Rehoboth and Bethany to the north, or
Ocean City to the south.
Maryland Coast
One of the most popular resorts in the
Middle Atlantic region, Ocean City,
located at the southern end of Fenwick
Island, has been an important beach resort
for more than a century. At first people
from Washington, D.C. and Baltimore
FIGURE 2 Southerly view of Cape Henlopen reached the coast by train and then. later
spit and the Delaware headland coast. by automobile. A boardwalk was
Tidal currents and refracted waves carry constructed in the 1920's along with
sand around the tip of the spit as several large hotels. Development was
evidenced by the submarine sand ribbons. slow but steady until a dramatic boom in
Photo ca 1980. construction took place from the early
T219: 3
1950's until the late 1970's, after which Figure 4, the result is a westerly offset
vacant land and financing for construction in the once straight barrier island of
of high-rise condominiums and apartments more than 500 m.
became scarce. In the late 1970's
th~
Federal and State laws to reduce
bulkheading and dredging and filling of
wetlands came into effect. Still, today
Ocean City beaches are overcrowded in the
summer and the massive concrete and glass
buildings at the water's edge are so tall
that shadows are cast on the beaches by
midafternoon (Fig. 3).
FIGURE 4 Oblique southwest view of Ocean
City Inlet showing the 500 m offset
between Fenwick Island (north) and
Assateague Island (south) due to jetty
effects on the net southerly littoral
drift over the past 50 yrs. Photo ca
1980.
FIGURE 3 Oblique view of the highly
The most damaging storm to hit Ocean
developed barrier beach at Ocean City,
City within historic times and the most
Maryland (ca 1980). Protective dunes that
recent major storm, was the "Ash
once existed have been removed, and the
Wednesday" or "Five-High" northeast storm
buildings constructed too close to the
that hit the coast in early March 1962.
water allow little room for storm
It caused major erosion and flooding along
processes and seasonal and longterm
much of the Atlantic coast. For two days
fluctuations of the shoreline. Many of
over five high-tide cycles all of Fenwick
these buildings experienced flooding and
Island except the highest dune areas were
sustained damage immediately following
repeatedly overwashed by storm waves
their construction. Beach replenishment
superimposed on the 2-m surge. Property
along Fenwick Island in the summer of 1988
damage to Ocean City alone was estimated
will likely provide some temporary
at $7.5 million (Dolan and others, 1980).
protection to the buildings as well as a
With the extent of dense development
widened recreational beach.
today, damage from a similar such storm
would likely be hundreds of millions of
dollars.
Ocean City inlet, marking the southern
end of the city, was opened during the
Virginia Coast
"great hurricane of 1933" by storm-surge
overwash from the bay side rather than the
To the south of Ocean City inlet is
more usual process of inlet breeching from
Assateague Island, a 50-km-Iong natural
ocean waves. To maintain the inlet as a
barrier that has prograded south into
navigation channel, twin stone jetties
Virginia past Chincoteague Island and
were constructed by the U.S. Army Corps of
terminates at Fishing Point, a recurved
Engineers shortly thereafter. As will be
spit (Figure 1). Assateague Island
evident, the jetties have functioned to
displays all the geomorphic elements of a
stabilize the inlet, but they have also
pristine barrier-beach system: swash
greatly affected the littoral processes
bars, storm berm, dunes, washover sand
adjacent to the inlet. Net southerly
flats, back barrier lagoons and tidal
longshore drift has accreted the beach at
marshes, and old tidal inlets that have
the north jetty and some sand has been
been filled by littoral drift. Relict
diverted onto the ebb-tide shoal off the
inlets, however, are still obvious from
inlet throat, while erosion on Assateague
the curved beach ridges present. At
Island immediately south of the inlet has
present there are no active tidal inlets
greatly accelerated over the past 50 years
along Assateague Island, but McBride
(Dolan and others, 1980). As shown in
T219: 4
(1986) shows that at least 10 ephemeral islands vary considerably from Wallops
inlets have been present within historic Island on the north to Fishermans Island
time. at the Chesapeake Bay entrance. Historic
Chincoteague Island is a Holocene age maps and geologic evidence show that the
barrier that has been shielded from the four northern islands are migrating
ocean since Assateague migrated south. landward parallel to the mainland, while
Located on the island, the small fishing from Parramore south, the islands have
village of Chincoteague has access to the tended to rotate clockwise and pivot about
ocean by means of Chincoteague Inlet. In their central axes. Fishermans Island,
spite of encroachment by Fishing Point however, has undergone rapid expansion
spit, the inlet has naturally maintained over the past century, a condition most
its position for at least the past 139 unusual among the almost 200 Atlantic and
years. Unique in the way of animal life Gulf of Mexico coastal barriers (Fig. 6).
to Assateague and Chincoteague islands,
herds of small ponies roam free over the
islands and marsh. Their origins, some
feel, go back to the early settlers and
the ponies as well as the Chincoteague
region were made famous in the book "Misty
of Chincoteague" by Marguerite Henry.
The 90-km-long chain of segmented
barrier islands are separated by tidal
inlets (Fig. 5) and backed by pristine
lagoons and marshes. From Assateague
Island south to Cape Charles, they are the
longest stretch of undeveloped barrier
islands along the U.s. Atlantic coast. Of
the 18 barriers, all or part of 13 are
owned and protected by The Nature
--1980
Conservancy, a public national ···············1852 I\
conservation organization. The remaining N
--Bridge
five islands are under the jurisdiction of
the State of Virginia or Federal agencies.
AtlanticOcean
~...---a.~----I..}iNauticalMiles
2Kilometers
FIGURE 6 Historic shoreline changes of
Smith Island and Fishermans Island between
1852 and 1980 clearly demonstr~te the con
trasting ways transgressive barrier
islands respond to coastal processes.
Smith Island has eroded about 700 m on the
seaward side whereas Fishermans Island has
expanded due to accretion from littoral
drift moving south along the Virginia
coast.
Except for the odd fishing shack and a
recent development of summer homes on
FIGURE 5 Southerly view of Gargathy Inlet,
private land on northern Cedar Island, the
northern end of Metomkin Island, Virginia.
Virginia barrier islands are uninhabited
Note the well developed ebb-tide delta and
and in a natural and pristine condition.
numerous storm washover fans on the low
However, this has not always been so. As
relief barrier. Photo ca 1980.
early as 1608 Captain John Smith, the
settler of Jamestown, landed on Smith
Island. Shortly thereafter a salt
All of the Virginia barriers are sand through-evaporation operation was
starved due to the efficient trapping of established. In the 17th and 18th
most of the lit-toral sand at the Fishing centuries, people lived temporarily on the
Point spit. As a result, their average islands and hunted waterfowl, but the
erosion rates of almost 6 m/yr are among islands were most widely used for open
the highest of any coastal barriers along live-stock grazing. During the early
the Atlantic Coast. However, the historic 1700's, pirates r~portedly visited the
erosion and migration patterns of the islands periodically to hide or take on
T219: 5