Coasts
An N. Hardy powerpoint.
What we will learn.
1.
2.
3.
4.
5.
Coastal zones
Types of coasts
Waves – how they define coast
Beach Currents
Coastal Features
1.
2.
6.
7.
8.
Erosional Coast Features
Depositional Coast Features
Coast Linearization
Emerging/Submerging Coastlines
Human Intervention
Zones (Description)
• Backshore – Further inland from the shore
• Shore – Sand and low depth areas. Between
shorelines. Consists of the littoral/intertidal
zone.
• Nearshore – Beginning of the Continental
shelf, where wave shoaling begins.
• Offshore – Begins at the continental slope,
continues out from shore.
Zones (Pictures)
Types of coasts
• Major types:
– Depositional Coast
• Deposit sediments
• Usually sandy
– Erosional Coast
• Erodes away.
• Usually rocky
Question for understanding: Which type of coast
is the Huntington Beach Pier area?
Waves – How they define coasts
• High-energy coasts
– Heavy wave activity, many storms
– Usually erosional
• Low-energy coasts
– Light wave activity, fewer storms
– Usually depostitional
Question for Understanding: Which type of
coast is Huntington Beach?
Waves – how they define coasts
• Usually seasonal changes in wave activity
• A summertime beach
• Occurs more often in summer
• Low-energy weather, builds up berm
• A wintertime beach
• Occurs more often in winter
• High-energy weather, destroys berm
Before removing this bar, guess which diagram is in summer and winter!
Did you guess right?
Waves – Cool table w/ effects
Berm/longshore bar
Light wave activity Heavy wave activity
Berm grows and
Longshore bars grow
longshore bars shrink and berm shrinks
Wave energy
Low
Time span
Long
Characteristics Summertime beach:
sandy, wide berm,
steep beach face
High
Short
Wintertime beach:
rocky, thin berm,
flattened beach face
Beach currents
• Wave movement linear to shore
– Called a longshore current
– Longshore drift/transport
• Movement of sediment by longshore current
• Create “upstreams” and “downstreams” of sediment
movement on depositional beach
– Why does longshore drift not occur further away
from the shore?
• Because waves only effect the ocean beneath them to
their wave base, which is ½ of the wavelength
Beach currents
• What is the mechanism for longshore currents?
• Water that rushes onto the beach:
– Swash is water that soaks into the beach
– Backwash flows back into the ocean
• Most water is backwash
• Backwash flows linearly down the shore
Beach Currents
• Rip Currents
– Flow seaward
– Caused when longshore currents meet and the
backwash is greater than the incoming wave mass
– REALLY COOL ANIMATION
– DANGER
• They kill people
• In order to escape, one must swim parallel to shore.
Coastal Features
• There are a lot.
• They are boring.
Coastal Features (Erosional Coasts)
• Headlands
Rocky seaward outcroppings
• Wave Cut Cliffs
– Waves cause water only up to a certain height on erosional
coasts, when the rock above the notches cut into by the waves
collapse, wave cut cliffs are formed
• Sea Caves -> Sea Arches -> Sea stacks
– Sea Caves can change into arches and stacks over time
• Marine terrace
• Blow Hole
– Outlet for sea caves
Coastal Features (Erosional Coasts)
• Picture!
Coastal Features (Erosional Coasts)
• What is the cause?
• Hydraulic Action
• Air is compressed in cracks by water moving in, may
cause cracks to widen in rocks
• Wave Pounding
• Sheer energy of the wave hits the rock
• Abrasion/Corrasion
• Waves launch seaload at the cliffs
• Most effective/rapid form of coastal erosion
– Attrition is when seaload is worn down by itself or a shoreline
Coastal Features (Erosional Coasts)
• Erosion causes (continued)
• Dissolution
– Dissolving of rocks on shore by sea water
– Can lead to caves
– Corrosion is the dissolving of rock by carbonic acid
in seawater
• Limestone cliffs are particularly vulnerable to corrosion
Coastal Features (Erosional Coasts)
• What happens to all of the eroded material?
– Eroded Material is broken down into terrigenous sediment,
then one of two things can occur:
• Transported by longshore drift to a depositional beach
• Stays at the home coast, and when enough sediment builds up a
beach is formed.
• When will it stop?
– Erosion continues until there is a beach large
enough to stop waves before they hit the rocks
Coastal Features (Depositional Coasts)
• Mostly consist of build-ups of sediment
• Spit
• Extend in the direction of longshore drift.
– Spits that continue to grow to the other side of the bay and
completely cut off the bay from the open ocean are called baymouth bars, or bay barriers
• Stupid people build structures on bay barriers, which are not stable or
permanent structures. (1m)
• Tombolo
• connect islands to either shore or another island.
Costal Features (Depositional coasts)
• Picture!
Coastal Features (Depositional Coasts)
• Barrier Islands. Very Important, and in need
of a sophomore to do a power point on them.
• Here’s the basic idea:
– They occur along depositional coastlines
– Are long offshore deposits of sand that are parallel
to shore
• Develop into their own ecosystems
– My opinion: Great learning powerpoint
Coastal Features (depositional Coasts)
• Beach Compartments
– Best comparison is describe a beach compartment as a
cycle of sediment throughout an area.
– Three main contributing factors:
1.
2.
3.
Rivers supply sediment to coast
Sediment moving due to longshore transport
Sediment drained down into the ocean through submarine
canyons
– Beach Starvation
•
Human structures, such as a dam, limit rivers, slowing
depositing of sediment
Coastal Features (Depositional Coasts)
• Picture!
Coast Linearization
• Coast Linearization tells of the age of a
coastline and how it has developed due to
erosional and depositional factors.
– Not boring like coastal features, actually pretty fun
to learn about.
– Is a convergence of all of the stuff you just learned
about
Coast Linearization!
• The best example of coast linearization is the
Mediterranean Sea.
• For Sophomores: The Mediterranean Sea dried up a long time ago, but then was
refilled from the Atlantic.
– When the Mediterranean first filled up with water, it had a lot of highenergy waves from the waterfall hitting it
– Basicly, a new coastline.
– We will follow what would ideally happen to this new coastline in the
next few slides.
Coast Linearization
• When the waves first hit the rocks of the
Mediterranean, an erosional coast was formed.
– It was erosional in most areas
– Irregularities in the rocks caused headlands and
coves to form.
• At first, the coast becomes non-linear
Coast Linearization
• Important: Wave Refraction
– Caused by uneven wave shoaling
– Wave Refraction is the diversion of waves from coves, and
the convergence of waves onto headlands.
• Effectively Creates a High-energy coast on the headlands
• Effectively creates a low-energy coast in coves
• Concept recall: what do low energy coasts do that highenergy coasts do not do?
– ANSWER: they build
berm,you
or click!
a beach
Guessabefore
Did you guess right?
Coast Linearization
Coast Linearization
• What’s going on now?
– A beach is formed in coves that protect the rock
their from erosion
– Headlands have no protection and have waves
being directed towards them because of wave
refraction.
– Headland decline is much much faster than the
decline of coves at this point
• COOL ANIMATION: CLICK HERE
Coast Linearization
• Headlands retreat
– Leave behind sea stacks and archs
– The coast is straightened
• Sediment builds up, and the coast becomes
depositional
Coast Linearization
• Overall concept:
• Most of the time,
– A younger coast is erosional
– An older coast is depositional
Emerging/Submerging Coastlines
• Two things can move coastlines.
• Surprise! Sea level changes.
• Land moves up and down as well.
– Coasts that are rising above sea level are called
emerging shorelines
– Coasts that are sinking below sea level are called
submerging shorelines
Emerging Coastlines
• Evidence is shown in:
– Marine terraces
– Stranded beach deposits
– Evidence of sea life found
above sea level
– Sea caves above water
level
Submerging Coastlines
• Drowned beaches
• Submerged dune topography
• Drowned river valleys
Emerging/Submerging Coastlines
• Mechanisms on land:
• Tectonic movements
– Passive margins
• Moving away from a spreading center
• Usually submergent
– Active margins
• Tectonically active
• Usually emergent
Emerging/Submerging Coastlines
• Isostatic Adjustment
– Earth’s crust, especially continental crust, is
floating.
– Crust sinks when
• Crust sinks under accumulation of heavy loads of ice,
vast piles of sediment, outpourings of lava
– Crust rises when
• Heavy loads are removed
– A very slow process
Emerging/Submerging Coastlines
• Eustatic changes in sea level
– More ice = sea level lower
– Less ice = sea level higher
– Other Causes?
• Formation/destruction of inland lakes
• Changes in sea floor spreading rates
– Faster spreading = raise sea level
– Slower spreading = lower sea level
– The slowest
• Thermal expansion of water
– For every 1°C, sea level changes about 2 meters
Emerging/Submerging coastlines
• Link to ice ages
– According to isostatic adjustment, continents sink
under ice
– However water level also raises when water melts
from ice packs
– Water also expands under heat
– Which wins?
• Global warming raises sea level because thermal
expansion and melting ice win.
Emerging/Submerging Coastlines
• Important word: Pleistocene
– Estimated Sea level change to be 190 meters
• 120 below
• 70 above
– Most Coastal Areas have evidence of both
emergence and submergence
– Most changes due to ice sheets forming/melting
and taking/releasing water
• Thermal = 10m/5C
Emerging/Submerging Coastlines
Emerging/Submerging Coastlines
• Global warming
– Increase in average ocean temperature of .6°C
over the past 140 years
– Eustatic sea level
change of 4-10 in. up.
– Global warming
raises sea level.
Human Intervention
• Due to Eustatic sea level rise, 70% of sandy
beaches are eroding
• 60% of humans live near the coast
• Rising sea level threatens human structures
– we try to stop it
Human Intervention
• Hard Stabilization
– Structures built by humans to stop beach erosion.
• Doesn’t work very well.
– Groins and Jetties
•
•
•
•
Built perpendicular to shore
Groins create groin field
Protect harbor entrances
Both composed of large blocky material called rip-rap
Human Intervention
Human Intervention
– Breakwater
• Built to create harbors free of waves
• create many problems, such as erosion downshore and
unwanted tombolos.
• Dredging solves some of these issues
• Picture is very explanatory:
Human Intervention
Human Intervention
– Seawalls
• Constructed parallel to shore
• Designed to stop erosion
• Very expensive
Human Intervention
• Alternatives to hard stabilization
– Construction Restrictions
• Stop building of structures close to coastlines
– Beach Replenishment/Beach Nourishment
• Sediment is added to a beach to stop erosion
– Relocation
• Moving of structures
Human Intervention