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 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.
Basic features of the coastal zone
Subdivisions of the shore
Coastal zone graphic
Beaches
Beach composition is a function of whatever the local source of sediment is.
Waves in the nearshore interact with the coast.
The longshore transport system moves sand parallel to the beach.
Sand also moves perpendicular to the shoreline
One way to classify shorelines is 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: Wave-cut cliffs, wave-cut platforms, and marine terraces.
Wave-Cut Platform and Marine Terrace
Erosional Features: sea arches and sea stacks
Sea arch and sea stack
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.
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.
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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.
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.
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.
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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.
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.