Transcript Coastlines - HRSBSTAFF Home Page

Coastlines:
oceanic borders
G.Burgess
2009.
Dynamic environment
• Zone where, ocean
waters meet continental
margins and the
atmosphere
• Marked by beaches,
rocky shores, cliffs, etc.
• Constantly changing
shape and form
Erosion
• Erosion is destructive waves wearing away the coast.
There are four main processes which cause coastal
erosion. These are corrasion/abrasion, hydraulic action,
attrition and corrosion/solution.
• Corrasion/abrasion
– When waves pick up beach material (e.g. pebbles)
and hurl them at the base of a cliff. (physical action)
• hydraulic action
– When waves hit the base of a cliff air is compressed
into cracks. When the wave retreats the air rushes
out of the gap. Often this causes cliff material to
break away.
• Attrition
– when waves cause rocks and pebbles to bump into
each other and break up.
• Corrosion/solution
– when certain types of cliff erode as a result of weak
acids in the sea. (chemical action)
Transportation
• Transportation is the movement of material in the sea
and along the coast by waves.
• Longshore Drift
– movement of material along the shore by wave
action.
• happens when waves moves towards the coast at an
angle.
• The swash (waves moving up the beach) carries
material up and along the beach. The backwash carries
material back down the beach at right angles. This is
the result of gravity. This process slowly moves material
along the beach.
• longshore drift provides a link between erosion and
deposition. Material in one place is eroded, transported
then deposited elsewhere.
Transportation
• material moves in four different ways;
– Traction
– large material is rolled along the sea floor.
– Saltation
– beach material is bounced along the sea
floor.
– Suspension
– beach material is suspended and carried by
the waves. .
– Solution
• material is dissolved and carried by the water.
Deposition
• Deposition
– when eroded material is dropped by constructive
waves.
– happens when the wave loses energy and is not
able to carry the suspended materials
– Helps to sort materials by size
• Heavy materials (large rocks) drop first and
lighter materials (sand/silt) drop last.
– Deposition creates a range of landforms.
Beaches
•
•
The beach is the area between the lowest spring tide level and the
point reached by the storm waves in the highest tides.
Every beach is different but they are usually made up of material
deposited on a wave-cut platform.
Sand Dunes
• created by strong winds (not by coastal erosion or deposition)
• sand is blown up a beach forming small hills.
• as depression and hills are formed, more sand gets trapped until
there is enough that plants may take root.
• Dunes are held together by long-rooted grasses such as marram
grass. The root systems hold the grains of sand so that they cannot
be blown away
• Root systems enable the collection of moisture that enables the
growth of other plants and development of new food webs
Spit
• Longshore drift moves material along a coastline.
• Where there is an obstruction or the power of the waves
is reduced the material is deposited.
• Where rivers or estuaries meet the sea deposition often
occurs.
• over the years materials build up forming a long ridge of
material, called a spit.
Salt Marshes
• A salt marsh is a coastal marsh that forms on mud flats.
• They usually form in very sheltered inlets and estuaries,
or behind spits (places where fine sediment
accumulates).
• form as vegetation builds up on mud flats .
Sand Bar
• Waves transport sand making a trough
• This forms a division that separates how materials are
deposited
• Material in backwash is deposited on sand bar and the
rest is deposited on the beach
Bays, inlets
• Usually formed by erosion and
transportation of materials.
• Wave action will transport the soft rock or sediment to
other locations
• Factors of transport; storm surges, glaciation (melt and
movement of ice during ice age)
• Narrow bay = fjiord
• Large bay = gulf
• Small rounded inlet = cove
• Headland (Cape Breton Island): raised area protruding
out into the ocean.
Human Influences
1.
Breakwater
• An artificial structure of a durable material that
interrupts the progress of waves to shore.
• used to dissipate some of the oceans wave energy
• can be made of a build up of rocks, cement,
timbers, boulders, etc.
• usually found at the mouth of a harbour to protect
the area on the other side of the breakwater
What do Breakwaters do?
•
•
•
interrupt the wave energy heading to the beach,
weakening the longshore current and allowing sand
to accumulate
eventually causes the sand of the beach to approach
the breakwater; filling in the area the breakwater is
designed to protect the coastline properties.
Also, it may cause erosion further down the coast
because of the diverted energy
Human Influences (continued)
2.
•
•
•
Groin
A structure extending from the beach into the water.
help to trap sand from the current, accumulating sand
on the updrift side.
Usually make erosion worse on the downdrift side
where the beach is sand deprived
Human Influences (continued)
3. Sea Wall
• A wall constructed of a durable substance that protects
the beach or shoreline from eroding.
• the protection is usually temporary because the sea
wall is only as good as the substrate it is made of.
• they can increase beach erosion because they are
deflecting wave energy.
• Also, high waves can crash over the wall and erode
both sides