GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 18. Coasts, Beaches, and Estuaries
POWERPOINT SLIDE SHOW NOTES
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Topic 18. Coasts, Beaches, and Estuaries
The coast, or coastal zone, is one of the most dynamic environments on Earth. It is
where land, air, and water meet, and a zone of intense human activity. There is no set
distance inland or offshore to define the coastal zone. It extends anywhere there is
some kind of interaction between the land and sea. The star indicates the approximate
location of Jekyll Island in Glynn County, Georgia. The Georgia coast is very flat and has
the highest tidal range in the southeastern United States, so the effects of the sea are
felt far inland. The two yellow dots on the inset map are separated by 15 miles straightline distance. A study at Skidaway Institute of Oceanography documents saline water
and tidal currents a minimum of 15 miles inland
Beaches are part of the coastal zone right at the water’s edge. The term “beach” does
not imply a sediment size or composition. A beach is simply an accumulation of
sediment found along the landward margin of an ocean or lake. A beach is under
constant motion by the forces of wind, waves, and tides. The composition of beaches is
controlled by whatever local material is available, be it river sand, volcanic rock, shells, or
ground up pieces of coral. These pictures show two strikingly different beach sediment
compositions and sizes. On the left are light-colored shells and shell fragments, many of
which are pebble- and cobble-size. On the right are dark volcanic sediments of fine-tomedium sand size. What controls the sediment that will be found on a particular beach?
Whatever the local supply of sediment happens to be.
In Georgia, our beaches are mostly quartz and other minerals derived from igneous and
metamorphic rocks. The source ultimately was erosion of the Appalachian Mountains.
Some sediment came from reworking of older deposits as modern sea level has risen.
And some shell material also has been moved onshore by waves and tides.
There are two main types of coasts, determined by whether they are erosional or
depositional. Erosional shorelines develop when the features of the coast are remnants
of eroding rock. Depositional shorelines develop when coastal sediments are moved and
deposited by ocean waves and currents.
Erosional coastal features originate by the cutting action of the surf against the base
coastal cliffs. Remnants of the cliffs can remain as sea stacks and arches. Planation
forms a wave-cut bench; if it is tectonically uplifted it becomes a marine terrace.
Sediment from the eroded rock can form small sandy beaches.
The processes involved in coastal cliff retreat
Wave-Cut Platform and Marine Terrace
Sea arch and sea stack
Depositional coastal features are the result of sediment being transported and deposited
by marine processes including waves, tides, and wind.
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A spit is an elongated ridge of sand attached to land on one end and extending into an
adjacent bay. A baymouth bar is a spit that extends entirely across a bay to seal it off
from the ocean. Sometimes a baymouth bar will be cut dredged open in order to gain
access to a harbor. And they sometimes can be cut through during storms. A tombolo
can be formed when an offshore island blocks wave energy allowing sand to be
deposited to connect land to the island. A barrier island is similar to a spit except it is
not connected to land. It is unconsolidated sediment. The entire island slowly moves
inland over thousands of years as sea level rises.
Waves in the nearshore interact with the coast.
The longshore transport system moves sand parallel to the beach.
The beach can be broken into several components or environments depending on
several factors. And there are several different ways to subdivide the beach and coastal
area. Note on this diagram there are 3 levels of divisions.
Row 1 is the big picture. The most inland term is coast. Remember how we talked about
the term “coastal zone” and erosional or depositional coast? Well, this is not quite the
same thing. This is just a term to sort of mean inland from the active beach. Remember,
the true coastal zone can extend inland and offshore for many miles. Next is shore,
which is the part of the beach from the coastline to the low-tide shoreline. The
nearshore extends from the low-tide shoreline to a water depth of L/2. Remember that?
It’s the water depth at which waves start to interact with the seafloor. It’s not labeled on
this diagram. It is also known as “wave base.” Beyond that is true offshore.
In Row 2 you can see that the shore is broken into backshore, which runs from the
coastline to the high-tide shoreline; and foreshore, which runs between the high-tide
and low-tide shorelines. The beach face is the part of the foreshore that is submerged,
and where the beach starts to slope regularly seaward.
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Row 3 gives some of the detail of the beach. The beginning (or end, depending on your
perspective) of the beach is dunes or some other sharp break in slope or change in
composition. We saw rocky cliffs on the erosional beach. Thus, the coastline is the
demarcation of the inland environment. A berm is a break in slope on a beach. Walk
along a berm and you will notice the beach sloping down to the water on one side, and
down toward the dunes on the other. Berms are built by sand being pushed onshore by
waves and building up the beach. The high-tide shoreline marks how high the water
level rises during the high tide. The shoreline is where the water level is at any given
time. It changes continuously as the tidal level changes. The low-tide shoreline is where
the shoreline is at low tide. The difference between the low-tide and high-tide
shorelines can be significant in high tidal range, gently sloping coasts.
The exposed portion of the beach above the low-tide shoreline is often referred to as the
wet beach. The part of the beach above the high-tide shoreline is often called the dry
beach or the recreational beach.
The beach and dunes are often referred to as a single beach-dune system. Wind moves
sand from the beach to the dunes. Dunes store sand until a storm tears them apart and
rearranges the system.
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Sand also moves perpendicular to the shoreline
Depositional coastal features: spits, bars, and tombolos
Depositional coastal features
Longshore transport of sediment will form a spit in front of an ocean embayment. A spit
is connected to land on one end.
A tombolo forms in the quiet area behind an offshore island when sediment collects and
connects the island to land.
Bay barriers, also called bay mouth bars, are often cut through for harbor access.
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Barrier islands
Cross section of a typical barrier island.
Georgia barrier islands
Watch the video “Waves, Beaches and Coasts” on learner.org, part of the Earth Revealed
series.
Beaches change significantly from summer to winter, mostly as a result in offshoreonshore sediment movement. Older terminology is to classify beaches as winter versus
summer, but better terms are storm versus fairweather. Storms are more common in
the winter, but can occur any time. Storm waves carve into the beach and move
sediment offshore to form sand bars. Fairweather waves move that sand back onshore
and rebuild the beach.
Storm (winter) – fairweather (summer) beach profiles. A berm is a feature that can be
erosional or depositional. It is a dividing line between beach sloping down toward the
water on one side, and down toward the land on the other.
Tidal inlets are gaps between barrier islands through which flood and ebb tidal currents
move. The ebb and flood tidal currents carry water and sand in and out of inlets.
A tidal inlet usually has a single main channel but can also have many smaller associated
channels and sand deposits. Inlets are very dynamic and can have major influence on
the barrier islands.
A quick word about the bodies of water behind barrier islands. The technical terms are
estuary and lagoon, but we often call them all lagoons, and sometimes we use the term
sound.
Estuaries are water-mixing zones known as important breeding ground for many land,
air, and sea animals. They are highly productive ecosystems, and unfortunately, have
been seriously degraded by human development.
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A lagoon has little fresh water input and may become very salty because of evaporation.
An estuary has major fresh water input and may have low salinity (called brackish water)
and have almost all fresh water during floods.
The flood tide brings sediment into the estuary (labeled lagoon, but really an estuary)
behind the barrier islands and creates a sand deposit called a flood-tidal delta. Likewise,
the ebb tide brings sediment out of the lagoon into the ocean and creates a sand deposit
called an ebb-tidal delta. Tidal delta shape is controlled by the balance between the tidal
flow and wave energy.
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In much of North Carolina, particularly the Outer Banks, ocean wave energy is dominant
over tidal currents. This is called a wave-dominated coast. Here flood-tidal deltas are
deposited in the calm waters of broad lagoons and can grow quite large with irregular
shapes and many channels. However, the tidal range is low, so ebb-tidal delta sediments
are reworked into small rounded sand bodies.
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In Georgia, the setting is the opposite of North Carolina. Here, the wave energy is low
and the tidal range high. This is called a tide-dominated coast. The estuaries are not as
wide as in North Carolina, but the tidal range is so high that many more inlets are need
to accommodate tidal water moving through the inlets. The result is that Georgia barrier
islands are shorter and stubbier than North Carolina’s barrier islands. Salt marsh thrives
in Georgia because of the tidal range but also because there is a lot more sand to be
carried into the estuaries and forming a platform on which the salt marsh grasses can
grow. Broad salt marshes are one of the most beautiful features of the Georgia coastal
zone. They are even visible in many stretches along I-95.
Hurricanes often are responsible for creating tidal inlets. They push water into the broad
sounds, and as they pass the reversing winds push water seaward against the back side
of the narrow, low-elevation barrier islands. Occasionally, the water erodes through an
island creating a new inlet. Hurricane Isabel hit the Outer Banks of North Carolina in
2003 opening a new inlet.
A new inlet is often called a breach when it first forms. Tidal flow is responsible for
maintaining and/or enlarging the new inlet. The tidal range is low in North Carolina so it
would have been difficult for this inlet to survive very long before it was choked with
tidal sediment. However, the inlet was filled artificially in order to rebuild the road.
Sometimes inlets migrate away from their original position. The old inlet closes because
of lack of tidal current activity, and what was once an active flood tidal delta becomes
inactive, or relict. Relict flood tidal deltas can eventually be incorporated into the barrier
island sand body.
Changing sea level
Land ice was at a maximum about 18,000 years ago, so sea level was at a low point.
Since then, land ice has been melting and sea level has been rising. Projections are for a
continued rise in sea level and an acceleration of the rate of rise.
As sea level changes, the shape of the coast changes. Sea level has risen and fallen
countless times during geologic history.
The sea level change at any location is a combination of the global change in sea level
plus the change in land level caused by tectonics or subsidence because of natural
compaction or ground water withdrawal. While the general trend in most places is a rise
in sea level, the year-to-year change can vary.
The green depicts extent of land during the last glacial maximum when sea level was
nearly 400 feet lower than today.
The projected shoreline (green) if the Greenland ice sheet melts, causing a 7-meter rise
in sea level.
Rising sea level is one of the factors needed in order for barrier islands to form. Storms
during times of rising sea level erode the fronts (ocean sides) of barrier islands and throw
the sediment up on top of the islands or completely over them into the estuary.
Continuation of this process drives entire islands landward and up the coastal plain in a
process called barrier island migration. Peat deposits form from accumulated vegetation
in the estuary. Peat deposits found on the ocean beach indicate that the barrier island
has migrated landward over the peat deposits and exposing them on the front side of
the island.
Georgia’s coast records a history of many rises and falls in sea level. Each rise occurs
during a single deglaciation period and its associated rise in sea level. Each rise in sea
level drove a line of barrier islands landward. Falling sea level leaves the ancient barrier
islands stranded in place. Subsequent rises created new lines of barrier islands which
were left stranded by falls during glaciation.
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The highest and furthest inland ridges must be the oldest. Ages, elevations, and distance
inland decrease in a seaward direction. A rising sea level will erode all previous barrier
islands up to its highest elevation and furthest inland location. Thus, the six ridges of
ancient barrier islands in Georgia are only the minimum number of rises and falls. There
may have been several more that were eroded by the next sea level rise. The only thing
we can say with certainty is that since the time of deposition of any single ancient barrier
island complex, the sea level has never since been that high nor higher.
A cross section of the six ancient barrier island ridges in Georgia. The Holocene island is
the one being deposited today during the current rising sea level, and it has collided with
the Silver Bluff shoreline in many places. If sea level continues to rise, the Holocene
shoreline may eventually erode away the Silver Bluff shoreline.
Many of Georgia’s islands are what are referred to as composite islands. That is, they
consist of a core of Sliver Bluff island with modern sediments of the Holocene sea level
rise added along the front and/or sides. The approximately 50,000 year old Silver Bluff
(Pleistocene age) barrier islands are remnants of the previous shoreline location. By
about 18,000 years ago, glaciation and falling sea level had moved the shoreline some 70
miles eastward. Since then, deglaciation has caused sea level to rise and a line of
Holocene barrier islands to be created and migrated landward. Around 5,000 years ago,
the Holocene islands were beginning to collide with the Silver Bluff islands. This drawing
shows the evolution of Jekyll Island. Today, continued sea level rise is driving erosion
along the center stretch of Jekyll island, eroding into the Silver Bluff deposits.