Chapter 15. Shorelines

 

The waters of the oceans are constantly in motion. This movement is much better recognisable along the shore, which is the dynamic interface between land and ocean called coastal zone.

Here we can observe waves, tides, and the ever shaping shoreline

Waves, currents and tides

Waves

The energy of waves are coming from wind. When wind blows over the water, some of the energy is transferred to the water surface and its undulation is forming the waves that move through the water.

Height of wave

The vertical distance between the crest  and the trough.

In the ocean normal waves are 1-15 feet high, but storms such as hurricans can induce waves with 50 feet height. The highest wind wave ever measured was 112 feet.

Crest

high point of a wave

trough

low point of a wave

Wavelength

The horizontal distance between two wave crests.

In most of the ocean the lengtsh of wind waves are 130-1300 feet (40-400m) and their speed  is 15-55 miles/hour in deep water.

Wave period

It is the time passing between two crests passing through at a stationary point.

Factors, influencing wave height, length, and period

1. wind speed

2.the length of the time the wind has blown

3.The fetch, or distance that the wind has travelled across the open water.

The stronger the wind the steepness and the height of the waves increase till a critical point, where breakers or white caps form. Whitecaps are the places to loose energy, and waves can not grow higher.

When wind stops or change direction the waves that were created will continue to travel independently from local winds. Gradually they will change into swell.

Swell

Lower, longer waves, carrying storm energy to a distant shore.

Movement of the wave

It is important to know that the wave form is moving, not the water itself. Each water particle moves in a circular path as the wave passes through, and they return to their original position. The diameter of the circular orbit of the particle on the surface is equal with the wave height. To larger depth this diameter becomes smaller, and dissapear at the depth of 1/2 wavelengthWhen the particle is in the crest it moves with the wave direction, when it is in trough it moves opposite to the wave direction. This is called wave oscillation.

As long as the wave moving in deep water, it is not affected by water depth.

When it approaches to the shore it starts to be affected by the bottom.

Wave base

When the botton is equal to the 1/2 wavelength, the wave starts to feel the bottom. When it happens the circular water movement becomes oval, the wave slow down and its length decreases. Along with this the height of the wave increases and becomes steeper till it reaches the critical point and breaks. (surfing) The wave oscillation becames wave translation when the water itself starts to move and advances up to the shore.

Surf

The zone of turbulent water created by breaking waves is called surf.

swash

is the sheet of turbulent water collapsing from braking waves and advancing upon the beach.

Backwash

The water moving backward to the ocean.

Wave refraction and coastal currents

Usually waves strike a coast at an angle, when part of the wave entering to the shallow water sooner than the rest. The other part of the wave is still in deeper water, so moving fast and pivots around the slow shallow water segment (turning marching band). This wave refraction or banding causes the last arriving portion of the wave to be almost parallel to the coast before braking. (From 50-60 to less than 5)

As each of them brakes and strikes the coast its surf pushes the swash ahead of it up the beach at a small angle. Backwash then returns to the sea perpendicular to the shoreline.

Wave impact along an irregular coast is different because of the refraction. The headland would be more attacted than the bay. So arosion of headlend, and deposition in the bay will eventually straighten the shoreline.

Longshore current

The combined swash and backwash of the waves creates a turbulent current, that moves sediment or swimmers along the shore nearly parallel to the shore line.

After:http://www.gvsu.edu/videticp/longshore.htm

Speed of this current is 0.25-1m/sec. It can be higher than this, and be powerful enough to carry sediment in suspension into long distances (some sand on the outer banks of North carolina is coming from the Rocky coast of Maine, 1500 km North).

The width of longshore current is equal to the width of surf zone.

Rip current

This is known by swimmers as undertow, flow straight out to sea, channelled water and sediment movement perpendicular to shoreline. This occur when water in the surf zone accumulates moves back seaward. They usually interfere with incoming waves. It is easy to see them, because of the discoloration. (rip currents have lots of sediment they would be darker). Another sign, that at rip currents incoming waves would break earlier (Swimmers caught in these should swim parallel with the shoreline to get out) DO NOT TRY TO STRUGGLING AGAINST THE CURRENT!!!!! For people surfing it is good to travel with seaward.

 

After: http://marine-science.sc.edu/classes/msci505/penn/sld008.htm

TIDES

Tides are twice a daly rise and fall of the surface of oceans and large lakes, which moves the shorelines alternately landward and seaward. For the water to rise in one place they must be pulled away from another part of the earth.

 

Causes:

1. The gravitational pull of the moon and the sun and

2. The force produced by the rotation of earth-moon system.

 

The tides are response to gravitational and centrifugal forces. The gravitational attraction of the Moon periodically reinforced or opposed by the sun, pulls a buge of water toward it, while the centrifugal force of rotation of the earth-Moon system forces an opposing mass of water to be flung outward on the opposite side of the earth. The earth rotates through these two bulges every 24 hours; thus there are two high and two low tides each day(Atlantic) (SEMIDIURNAL).

Some places on the earth especially where the water body has restricted access to the open ocean such as Gulf of Mexico, Caribbean sea, there is just one high and low tide. This type of tide is called DIURNAL . There are places when the time between two high is not the same, called MIXED TIDAL PATTERN. (Pacific)

 

 

 

 

 

 

Tidal interval

Time between two high tide

 

 

Tidal range

The difference between sea level at high and low tide. It is varies from place to place in response to the factors such as coastline, the depth of water, accessto the ocean, and the topography of the ocean floor. (average on Pacific coast 1.5-3m) Florida bay less than 0.7m. The highest tidal range occurs in Bay of Fundy average 15m.

 

Spring tide

Sun has a tidal effects also because of its gravitational force, but since it is much further away it is less important than the moon. However when the moon and the sun lines up (full moon, new moon ), it will result in an abnormally high and low tide increasing the tidal range (every two weeks).

 

Neap tide

When the moon is halfway on its orbit around the earth it is 90 away from the sun. This will result in an abnormally low high tide called neap tide.

 

 

 

 

 

 

 

Horizontal tidal currents

FLOOD TIDE

The tidal movement we just described is the vertical rise and fall of the ocean water. There are also horizontal currents, especially in bays or sounds. The flood tide is the incaming current accompanies rising tide

 

Ebb tide

This accompanies falling tide

 

Slack water

The time between flood and ebb tide where there is no current

 

 

Speed of tidal current

High 4.6 miles/hour at Golden Gate

Could be as fast as 11miles/hour

 

 

Processes that shape the coasts

Coastal erosion

Probably most important source are waves where the erosion is caused by swash and backwash. It is important because the braking waves have huge energy. the pressure of the Atlantic waves for example 10,000 kg/m2. (1000 tons weight steel could be moved suring storms.(abrasion)

Wave erosion is much bigger along shores of unconsolidated sediments. It is also depend on the orientation of the coast line relative to the prevailing wind direction.

 

 

Landforms produced by coastal erosion

 

 

 

1.Sea caves

 

 

2. Sea arch

 

 

3. sea stack

 

Coastal transport and deposition

When there is an access sediemnt in the water it will start to deposit. Most of the timre it will form BEACH.

It is defined as the dynamic, relatively narrow segment of the coast that is washed by waves, tide. It may contain sand, silt, or gravel sometimes even cobble depending on the energy level of the coast. From the sand on the beach dunes often forms by wind blowing and transporting the sediment.

Boundaries of beach

low-tide line to the sand dune fiels, or where the permanent vegetation starts.

Parts of the beach

Foreshore

low-hightide line

Backshore

High tide line - to sea cliff or vegetation line

Beach face

the steepest part of the beach

 

 

 

 

 

Longshore drift

It is the movement of sediment parallel the shore by longshore current. It happens on the beach and in the surf zone.

This sediment after a while will be deposited and form different forms such as

 

Spit

Fingerlike ridge of sediment that extend into open water

 

After: http://www.gvsu.edu/videticp/beach_drift.htm

baymouth bar

A ridge of sediment that cuts a bay off from the ocean

   

Tombolo 

rare, it is a bar of sediment connecting former island to the mainland

 

After: http://www.geo.wvu.edu/~kite/Geol221_2001Lect23Coastal/sld059.htm

Human-induced coastal deposition

Some human-made structure will disturb the balance between the amount of sediemnt delivered to the shore and the amount removed by longshore currents. This cause beaches to grow in some places and shrink in other places.

 

BREAKWATERS

 

 

 

 

 

 

GROINS

 

 

 

JETTIES

 

 

 

BEACH NOURISHMENT

Is replacing sand to eroding beaches

Can causeproblems suchs as in Miami

Sand for replenishment was brought form Everglades

-sand silt and clay to replace the coarse graded sediment of Miami beach. The clay silt particle washed out from the sediment and the water became turbid and caused big damage to the reef.  

http://marine-science.sc.edu/classes/msci505/penn/sld021.htm

 

Types of coasts

Primary coast

Formed by nonmarine processes:

glacial erosion: fjords Northeast of Atlantic(Maine)

Stream deposition: Gulf coast of Luisiana

Carbonate reef (Florida, Bahamas)

 

 

Secondary coast

Formed mostly by ongoing marine processes

 

 

Barrier islands

 

 

 

 

Changing sea level

Sea level is changing. Its cause is mostly external, but some people think that is enhanced by human influence!!!

Global warming!!!!!

So what!!!!!

Discuss!!!!!

Ice ages!!!