Topic 18 Coasts, Beaches, and Estuaries

GEOL 2503Introduction to Oceanography

The Coastal Zone

• The Coastal Zone– The shoreline is constantly modified by waves– Present day shorelines

• The shore is affected by a complex interaction between sea level and local geology

– Human activity• Despite being fragile, short-lived features, humans

treat the coastal zone as a stable platform

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Basic features of the coastal zone• The shoreline is the line that marks the

contact between land and sea• The shore extends between the lowest tide

level and the highest elevation affected by storm waves

• The coast extends inland from the shore as far as ocean-related features are found

• The coastline marks the coast’s seaward edge

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The shore is subdivided:

• The foreshore area is exposed at low tide and submerged at high tide

• The backshore area is found landward of the high-tide mark

• The nearshore zone lies between the low-tide shoreline and the point where waves break at low tide

• The offshore zone is seaward of the nearshore zone

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The coastal zone

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Beaches

• A beach is an accumulation of sediment found along the landward margin of an ocean or lake – Beaches are composed of one or more berms

(the relatively flat platform composed of sand)– The beach face is the wet sloping surface that

extends from the berm to the shoreline • A beach is composed of locally abundant

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Recall from Waves:

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The longshore transport system moves sand parallel to the beach

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Beaches are often called “rivers of sand”

Sand also moves perpendicular to the shoreline

• Swash and backwash move sand toward and away from the shoreline– Net loss or gain depends on level of wave

activity• Summer typically has light waves and the berm widens• Winter typically has powerful storm waves that

erode the berm

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Erosional Shoreline Features

• Wave-cut cliffs, wave-cut platforms, and marine terraces– These features originate by the cutting action of

the surf against the base of the coast• Wave-cut platforms are flat, bench-like surfaces• An uplifted wave-cut platform is a marine terrace

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Erosional coastal features

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Wave-Cut Platform and Marine Terrace

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Erosional Features

• Sea arches and sea stacks– Rocks in headlands do not erode at the same rate

• Soft and fractured rocks erode faster than hard rocks, forming sea caves

• A sea arch forms when two sea caves meet• A sea stack forms when the arch of a sea arch falls

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Sea Arch and Sea Stack

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Depositional Coastal Features

• Spits, bars, and tombolos– A spit is an elongated ridge of sand extending

from the land into the mouth of an adjacent bay– A baymouth bar is a spit that extends across a

bay to seal it off from the ocean– A tombolo is a ridge of sand that connects an

island to the mainland or another island

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Depositional coastal features

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Sand Spit18

Deposition of a Tombolo

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Depositional shorelines

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Barrier Islands• Barrier islands are low ridges of sand that

parallel the coast 3 to 30 kilometers offshore– Found mainly along the Atlantic and Gulf

Coastal Plain– Most are 1 to 5 kilometers wide and 15 to 30

kilometers long– Probably form in several ways

• Some originate as spits • Some originate from sand piled up offshore• Some are flooded sand dunes from the last glacial period

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BARRIER ISLANDS

A cross-section profile of a typical undeveloped barrier island with the mainland to the left and ocean to the right.

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Georgia’s coast is lined with beautiful barrier islands. Only four of them are accessible by car: Tybee, St. Simons, Sea Island, and Jekyll. The rest are mostly undeveloped and preserved.

Video

• Watch the learner.org video on Waves, Beaches and Coasts to see more about shallow water waves

• An episode of the “Earth Revealed” series• http://www.learner.org/resources/series78.ht

ml

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Beach Seasonal Changes

• Winter beach (storm beach)—eroded, flat, sediment moved offshore by storm waves

• Winter berm—sediment piled on backshore

• Summer beach (fairweather beach)—steeper, sediment moved onshore by gentle waves during spring and summer

• Summer berm—formed closer to water

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Tidal Inlets

• Recall flood tides and ebb tides• Water moving in and out of channels

between barrier islands (called tidal inlets) also moves sediment in and out of the inlets

• The amount of water moving in and out of an inlet is controlled by the tidal range and the size of the water body behind the barrier islands

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Estuary vs Lagoon

• Both are semi-enclosed coastal bays• Estuaries have fresh water input, low

salinity• Lagoons have limited fresh water input,

often hypersaline• “Sound” is another term for coastal estuary

or lagoon behind a barrier island

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Little fresh water input

Major fresh water input

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Typical of North Carolina Inlets

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Typical of Georgia Inlets

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Isabel Inlet, 17 Sep 2003

Image: 23 Sep 1999

Outer Banks of North Carolina

Cape Hatteras

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Hurricane Isabel breach, Hatteras Island, NC. Photo taken Sept. 21, 2003. Courtesy Duke University Program for the Study of Developed Shorelines.

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Tidal inlets can open and close over time. Usually they are opened during storms, and closed by longshore transport during fair weather.

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Changing Sea Level• Sea level rises and falls largely controlled by land ice

volume • Glaciation is a time of increasing volume of land ice, thus

sea-level fall• Deglaciation is a time of decreasing volume of land ice,

thus sea-level rise• We currently are in a period of land ice melting and sea

level rise • Sea-level rise becomes a natural hazard when rising water

level encroaches upon human development• Difficult for coastal communities to incorporate into

planning regulations

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Sea-level rise during the past 100+ years has been about 10-15 cm total.

Global warming may increase this rate.

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Barrier island migration • Movement landward over time

• Associated with rising sea levels

• Older peat deposits found on ocean beach

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How did the Georgia coast end up looking like this?

The features in yellow are ancient barrier islands constructed during higher sea levels.

Shoreline name Elevation above today’s sea level

Estimated age

Wicomico 30 m 1.5 Ma

Penholoway 21 m 1 Ma

Talbot 13 m 600-400 ka

Pamlico 7 m 500-100 ka

Princess Anne 4 m 80-40 ka

Silver Bluff 1.5 m 40 ka

Holocene (modern) Sea level <10 ka

Ma = million years ago, ka = thousand years ago, Holocene is the name of the current geologic epoch

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Cross-section of the lower Georgia coastal plain. See how the older the barrier island complexes are, the higher elevation and further inland they are. See ages, elevations, and distance inland on previous slide. Holocene is the name of the current geologic epoch. The Holocene barrier island system is what is active today. From Henry (1968).

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