Transcript Longshore current

Coasts
Coasts
• Coastal areas join land and sea
• Coasts are temporary junctions (of land and
sea) that are subject to rearrangement by
waves, tides, sea level, biological processes
and tectonic activity
• 50% of the world’s population lives on the
coast
• The term ‘coast’ includes the shore (where
ocean and water meet) as well as marshes,
dunes, and cliffs inland of the beach
You are going to coast through this chapter
• Remember coastal margins?
• These were the submerged portions of
continental crust
• Coastal regions are constantly subject to
change, most dramatically by tides and
erosion (short-term effects) and rising or
lowering sea level (long-term effect)
• The coast is an active and dynamic place
Sea level 18,000 years ago during last ice age
Year 2100: Projected (extreme) sea level rise of 200ft
Changes in sea level: eustatic
• Several factors can influence global, or
eustatic sea level variations
– Sea level is lower during periods of glaciation
(more water held up in ice) and higher during
warm periods (when glaciers are smaller)
– High rates of seafloor spreading produce larger
rises, decreasing the volume of the ocean basin
compared to slower rates of spreading
– Thermal expansion: water occupies a greater
volume when warmed
Changes in sea level: local
• Other factors can influence regional or local
sea level changes
– Isostasy; continental plates weighed down
by ice or sediment sink deeper into the
aesthenosphere, raising local sea level!
– Wind, currents, storm surges, El Niño/La
Niña can force more (or less) water on the
shore
Isostasy and Local sea level change
Ice
Oceanic crust
Continental crust
LITHOSPHERE
AESTHENOSPHERE
Isostasy and Local sea level change
Ice
Oceanic crust
Continental crust
LITHOSPHERE
AESTHENOSPHERE
Local sea level change
• The flooding Mississippi River delta is one
example of subsidence or land sinking, caused
by an accumulation of several hundred feet of
sediments forcing the lithosphere to sink
deeper into the asthenosphere
• The continent of Antarctica is
another example; massive ice
sheets weigh down the continent
raising local sea levels
www.magazine.noaa.gov/storiesmag101.htm
Erosion
• The immense power of the waves abrades the
coasts and can cause erosion, the removal of
coastal material
• When erosive forces dominate over depositional
forces, erosion occurs
– Most rapid on high energy beaches; areas frequently
battered by large waves (ex. Maine, southern tip of
South America and Africa)
• When deposition meets or exceeds erosion, the
coast is stable or growing, respectively
– Low energy beaches (ex. NJ to Florida)
Erosion
• Erosion tends to produce a smooth coastline
(imagine what erosion does to sea cliffs over
time)
• The sediment eroded away from the coast
eventually collects as beaches and may
actually protect the shore from the incoming
energy (and erosive power) of the waves
Let’s go to the beach…
• A beach is a zone of loose particles that covers
part of a shore (dunes, vegetation and/or sea
cliffs are technically not part of the beach)
• Beaches result when waves (or rivers)
transport sediment – usually sand – to the
shore
• Beaches are constantly changing – what you
see during our field trip to Smith Point is
different from what your children will see (or
what your parents have seen)
What Smith Point used to look like
Courtesy of Prof. Pamela Lynch
What Smith Point used to look like
Courtesy of Prof. Pamela Lynch
It’s a shore thing
• The material that makes up a beach can range
from boulders, cobbles, pebbles and gravel to
very fine silt
• The black beaches of Hawai’i are made of lava!
• Some beaches consist of shells or coral
fragments
Parrotfish Poop = Paradise
• Parrotfish feed on coral engulfing the animal and
its CaCO3 skeleton; they ingest the polyp and
excrete the CaCO3, which ends up on local
beaches as soft,
fine sand between
your toes!
www.flickr.com/photos/mikesegal/1571667591/
Beaches have distinct profiles
• In general, the flatter the beach, the finer the
material from which its made
• Water from waves washing onto the beach
carries particles onshore (“swash”); water
returning to the ocean (“backwash”) carries
material back to sea
• The amount of particles transported by the
swash and backwash are not always equal,
resulting in sloped beaches
Beaches have distinct profiles
• The berm is an accumulation of sediment that
runs parallel to shore, and marks the normal
limit of sand deposition by wave action
• The peaked top of the highest berm is called
the berm crest, and is usually the highest
point on the beach; it corresponds to the
shoreward limit of wave action during the
most recent high tides
Is Oceanography fun? Shore!
Is Oceanography fun? Shore!
• The steepness of beaches changes with
storms and the seasons
– Summer and calm periods build up beaches
– Winter and storms erode and flatten
beaches
– WHY???
Light versus heavy wave activity
• During light wave activity, much of the swash
soaks into the beach, and so backwash is
reduced
– Results in a net movement of sand up the beach
• During heavy wave activity, the beach is
saturated with water from previous waves, so
very little swash soaks into the beach; more
particles are moved offshore by backwash
– Results in a net movement of sand away from the
beach
Summer versus winter beaches
• Light and heavy wave activity alternates with
the seasons
• Light wave activity dominates during summer,
and so a wide, sandy berm develops
• Heavy wave activity dominates during winter,
and transports sediment/particles offshore
• A wide berm developed over many months
during summer can be destroyed in hours by
high-energy winter storm waves
Wide,
prominent
berm
Sand
deposited
offshore
No (or
reduced)
berm
Longshore currents
• Longshore currents transport sand along the
coastline
• Recall that most wind waves approach the
shore at an angle, and then refract to break
nearly parallel to the shore
• With each breaking wave, swash moves onto
the beach at a slight angle, then gravity pulls
the backwash straight down (no angle); this
zig-zag movement of water along a shore is
called a longshore current
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Break the grip of the rip
• Rip currents is a dangerously strong current of
water flowing offshore with speeds averaging
1-8 feet per second!
• Water streams along the shore until it finds
an exit back to sea, usually a narrow
trench between sandbars
• To escape a rip current, it is extremely
important to swim parallel to shore as you
cannot outswim the rip tide current and you
will eventually exhaust yourself (and drown) in
the process. The rip current is found only
along a narrow portion of shoreline
• Topography and surging seas determine
whether a rip tide will occur; can be visualized
by rough, brown water offshore
Longshore currents
• Longshore currents transport sediments away
from the beach and towards regions
‘downwind’
• Jetties, groins, and
breakwaters serve to
intercept, or stop
the transport of
sediment away
from beaches
Longshore current
Barrier Islands
• Depositional coasts (coasts where
depositional forces dominate over erosive)
may develop narrow, exposed sand bars that
run parallel to, but are separated from, land;
known as barrier islands
• Fire Island, NY; Atlantic City, NJ; Ocean City,
MD; and Miami Beach, FL are barrier islands!
Barrier Islands
• Barrier islands protect the coast behind them,
but are very unstable themselves
What about Long Island?
Net transport of sediment is from east to west
A state park your great-grandparents
never had…
• Since the Fire Island lighthouse was
constructed 100 years ago, nearly 5 miles of
barrier island have been added to the west
forming what is now Robert Moses State Park!
www.pbase.com/image/66549546
Hurricane Paths on Long Island
The importance of dunes
• Dune plants prevent erosion by trapping
sediments in their roots
• Removal (or destruction) of
beach vegetation, and the
hardening of shorelines
(bulkheads) increases erosion
• Plant rhizomes can extend 20
feet from the plant!
Courtesy of Prof. Pamela Lynch
And don’t forget about
overpopulation!
• http://www.snopes.com/politics/quotes/gua
mtip.asp