SHORELINE PROCESSES AND THE EVOLUTION OF COASTAL LANDFORMS

 

WATER WAVES are another agent of an EROSION, TRANSPORTATION and DEPOSITION of sediments.

Along the shores of oceans and lakes waves break against the land building it up in some places (depositing sediments) and tearing it down in others (eroding).

 

The ENERGY of the WAVES comes from the WIND.

Energy is used to:

ERODE

TRANSPORT††††† SEDIMENTS†††††

DEPOSIT

WAVE Alternating rise and fall of the water surface, produced by the flow of wind across the water surface.

Small local differences in air pressure create ondulations in the water surface.

 

Itís important to understand how waves travel and move sediments.

Waves are described or characterized by

WAVELENGHT

Distance between identical parts of the wave.

 

CREST

Highest portion of the wave with respect to the horizontal

THROUGH

Lowest portion of the wave with respect to the horizontal

PERIOD

Time it takes identical parts of successive waves to pass a fixed reference point

HEIGHT

Vertical distance between the crest and the through.

It is a key factor in determining wave energy

 

OPEN OCEAN†††† 2-5 m

HURRICANE†††††† 15 m or more

HIGHEST EVER†† 34 m (112 ft)

 

Waves travel between 30-90 km/hour

 

 

 

TSUNAMI.Results from a sudden displacement of seafloor (earthquake;fault; the collapse of a caldera)

††††††††††††††† Wave height in open sea; 1 m or so.

††††††††††††††† Wavelength in open sea; 100 to 200 km.

 

 

MOVEMENT IN WATER IN A WAVE

A particle of water moves in an orbit or circular path as the waves passes.The particle essentially returns to its original position after the wave has passed.

 

AT THE SURFACE

The diameter of the orbital path is equal to the height of the wave.

BELOW THE SURFACE

The orbit decreases in size until the motion is essentially gone at a depth equal to Ĺ of the wavelength.This is the WAVE BASE

 

 

 


 

 

WAVE REFRACTION

Most waves do not strike the shore directly, but rather meet it at an angle.When a wave approaches the coast in this manner, one end of the wave encounters shallow water first and slows down, while the rest of the waves is still in deeper water.Thus the part of the wave close to the shore slows down while the remainder continues to advance at a con tact speed.As a result the wave close to the shore bend.This process is called REFRACTION

 

 

WAVE EROSION

Occurs at sea level.Undercuts exposed bedrock.

Waves break directly o the shoreline, and thus expend the greatest part of their energy in eroding land.

 

FEATURES CAUSED BY WAVE EROSION

WAVE-CUT CLIFF

Formed by waves pounding against a rocky coastline

WAVE-CUT TERRACE or PLATFORM

Formed at the foot of a cliff.

Formed by wave erosion pushing the Wave-cut cliff steadily back.

Features resulting from differential erosion of weaker sections of the rock

SEA NOTCHS

Wave action may hollow out cavities

SEA CAVES

SEA ARCHS

When erosion, cuts through a headlands

SEA STACKS

The collapse of the roof of a sea arch leaves an isolated mass of rock, in the front of the cliff.

 

 

Since the surging water of the breaking waves must cross this terrace before reaching the cliff, it looses a certain amount of energy through turbulence and friction.So the farther the cliff retreats the less effective are the waves eroding the cliff.If the sea level remains constant, the retreat of the cliff becomes slower and slower.

 

 

EROSIONAL FEATURES


 

 

SEDIMENT TRANSPORT

Sediment is transported along the shore in a ZIGZAG pattern during the wash-backwash flow of the water.After the waves has washed up as far as it can go, the water returns to se by running down the beach face by the shortest possible route; straight downhill to the shoreline not back along the oblique route it came up (refraction).The net effect of this motion is to move the sand along the beach face.This transportation process is called LONG-SHORE DRIFT orCURRENT.

 

 

 

 

FEATURES CAUSED BY DEPOSITION OF SEDIMENTS

 

BEACH

Any strip of sediment that extends from the low-water line inland to a cliff or zone of permanent vegetation. Built of material eroded by waves from the headlands, and material brought down by rivers that carry the products of weathering and erosion from the land masses.

Areas that are protected from the full force of water waves.Itís continually modified by wave and current erosion.

SPIT

A narrow ridge or embankment of sediment forming a finger-like projection from the shore into the open ocean.

Typically develops when the sediment being carried by long-shore drift is deposited where water becomes deeper, such as the mouth of a bay.

BAYMOUTH BAR or BAY BARRIER

A ridge, commonly exposed, of sand extending partially

TOMBOLO

 

TIED ISLAND

Beach of sand or sand that connects two islands or an island with mainland.

BARRIER ISLAND

 

(Atlantic City, NJ; Miami Beach, FL; and Galveston, TX)

Breached baymouth bar.

Most common North America feature of the East coast.1300 km from Long Island to Florida and the Gulf Coast (295)

First line of defense against storm waves and hurricane.

LAGOONS

 

Isolated bays

 

 

DEPOSITIONAL FEATURES

 

 

 


 

MAIN TYPES OF COASTLINES

 

SUBMERGENT

Flooded

Sea-level is relatively raised

 

EMERGENT

Exposed

Sea level is relatively lowered.

 

 

 

TWO MAIN TYPES OF COASTLINE

 

 

 

 

SUBMERGENT COASTLINES.

Previously dry land that has been recently drowned either by land subsidence or a rise in sea level. Their appearance is controlled by the erosional processes prior to the rise of the sea level.Irregular coastlines.

 

†††††††††††††††††††††††

EROSIONAL PROCESS PRIOR TO THE RISE OF SEA

TYPE†††† / Features

STREAM EROSION

ESTUARIES

They extend inland

Flooded stream valley

V-shaped passages.

Fresh-water from the river mixes with sea-water (brackish water)

Protected environment.

Reach marine life

GLACIAL EROSION

FJORDS

They extend inland.

Flooded glacial valley

Forms very steep passages (U-shaped).

Deep long narrow arm of he sea surrounded by high rocky cliffs or mountainous slopes

Very rugged shoreline.

 

 

SUBMERGENT COASTLINES

 

 

 

SUBMERGENT COAST MAP (ESTUARY)

 

SUBMERGENT COAST MAP (FJORD) MAP


 

 

DELTA.

A delta is a thick accumulation of sediments at the mouth of a river.

As the shoreline is built has built outward, the river has shifted from one position to another.

This deposition has made the area so flat that the river has commonly split into a number of channels, each of which carries a part of the river water into the ocean (The Gulf).These are called DISTRIBUTARIES (PASSES).

Part of the sediment brought down b the distributaries is accumulated in the form of offshore islands (sand bars).

Waves and tides will modify and remove some of the delta front.

MISSISSIPPI RIVER DELTA.Each year the river discharges water into the Gulf at an average rate of 15,360 m3 per second with 684,528,300 tons of sediment (at the active Balize Delta Lobe).Dariry average discharge of 1-1.5 million tons of sediments)

 

 


EMERGENT COASTLINE

Land formed under water that recently has become dry land, either by uplift of the land or drop of the sea level.

Generally straight

Characterized by the presence of extensive elevated Wave-Cut Platforms, Terraces or Platforms and Wave cut cliffs.

Erosion is the dominant agent.

†††††††††††††††††††††††††††††††††††††††††††††††††††††††††††

 


 

 

EMERGENT COAST (SIMPLIFIED) MAP

 

 

 

 

STAGES OF COAST MODIFICATION (SUBMERGENT COASTLINES)

When waves break against headlands most of their energy dissipates.Therefore the waves inside adjacent bays have lower energy and as a result sediments are deposited in the bays.As headlands erode and bays fill with sediment, an irregular coastline eventually becomes smooth

EARLY

Erosional features are dominant

Landforms: sea cliffs and stacks, wave-cut platforms, etc.

Depositional features not common.

Highly irregular coastline

MIDDLE

Depositional features begin to appear; beaches, spits.

Broad wave-cut platforms are common

LATE

Depositional features are dominant.

Landforms: extended spits, Baymouth bars, beaches, Lagoons, etc.

Reduced headlands.

Nearly linear shorelines.