Transcript Marine and Coastal Processes - Illinois Wesleyan University

Marine and Coastal Processes
• Coastal (wave) processes occur where waves break on a shore, not only on oceanic/sea coasts but also on
lakes/ponds and to some extent even rivers big and still enough to have waves.
• Marine processes include
– waves (oscillations of the water surface, without translation)
– currents (translation of water masses), as well as
– still-water deposition
– biologic activity
– tides
– long-term rises and falls of sea level
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Marine and Coastal Processes
Processes
Waves
3 Characteristics of Waves
4 Breaking waves
5 Longshore Drift
6 Waves and Rip Currents
7 Marine Deposition
8 Tides
9 Storm Surge, Tsunamis, and Coastal Flooding
10 Sea Level
11 Landforms
12 Depositional features
13 Depositional Features: Deltas
14 Erosional features
15 Emergent Coastlines
16 Submergent (Drowned) Coastlines
Characteristics of Waves
• Waves are not currents. Currents are stream-like movement water from place to place due to
differences in density, like the Gulf Stream.
• Waves are [usually] caused by wind and controlled by windspeed, duration, and fetch.
• Waves are oscillations: a water particle moves in an elliptical orbit as a wave passes and returns
to (nearly) its original position. [There is a tiny bit of translation]
• Main characteristics: height (amplitude), wavelength, wave period (or frequency).
• Water waves travel at different velocities, unlike sound and light.
– In deep water (D>1/2 λ), speed is set by wavelength, which was set by causative wind.
– In shallow water (D<1/20 λ, speed depends on water-depth.
• Wave motion dies out with depth, and how fast depends on the wavelength. (at 1/2 λ, less than
5% of surface motion)
Breaking waves
• In “deep” water, waves are smooth and oscillatory.
• In “shallow” water, waves feel bottom, form crests, and crash when the top of the wave moves faster than
the bottom, which is dragging on the seafloor. (This crashing is how whitecaps happen and what surf is. In
the surf zone and shoreward, waves are not strictly oscillatory.)
• Waves usually approach a shore obliquely and refract to become more parallel to shore as the part closer to
shore feels bottom sooner and the wave front becomes more parallel to the shoreline. This results in
concentrating energy on protrusions, and low energy in bays.
Longshore Drift
• Waves usually approach a shore obliquely and refract to become more parallel to shore as the part closer to
shore feels bottom sooner and the wave front becomes more parallel to the shoreline.
• Longshore drift: swash comes onto a beach obliquely and backwash leaves downslope (perpendicular to
the beach), resulting in a small net translation of water and sediment in the direction of the original waves.
The water is not flowing parallel to shore, but that’s the net effect of two processes.
The book is simply wrong about the existence of “longshore currents.” Such things do not actually exist.
• Longshore drift causes a continual re-working of sediment on a beach and translation parallel to the shore.
Waves and Rip Currents
• Rip currents, also known as riptides or undertows, result when water piles up on a beach. This is unstable,
and the water will make it out to sea somehow. A rip current is a fast-moving current that carries water
away from shore in a concentrated stream. It does not pull you under, but does carry you out to sea. “Rip
tides” have nothing to do with tides.
• DO NOT SWIM AGAINST THE RIPTIDE. SWIM PARALLEL TO SHORE (PERPENDICULAR TO
THE CURRENT) TO GET OUT OF THE NARROW RIPTIDE, THEN SWIM ASHORE.
Marine Deposition
• Sediments carried by rivers or eroded at the shore can be re-worked by waves and currents.
–In a high-energy near-shore environment like a beach or bar, only sand and gravel can be deposited.
–In deeper water and in bays, silt and clay can settle out.
–In even deeper water away from land, sedimentation is mostly plankton
• There are always planktonic plants and animals, many with shells of various sorts. Where terrigenous
sediment is lacking or these are very common, these dominate.
• Meteors/meteorites provide a continuous rain of dust from above (not much, but some), and where little
other sediment is deposited, these can be significant.
Tides
• The moon does not simply orbit the earth. The earth also orbits the moon, albeit in a very small orbit. Due
to the combination of gravitational and centrifugal forces (beyond the scope of this course), this results in a
twice daily rise and fall of sea-level. The earth-sun system also produces this effect, and the two are superimposed, resulting in a twice-daily rise and fall (nearly daily, it's a little longer), with variations throughout
the course of the moon’s orbit (monthly).
• The upshot of this is that “sea level” rises and falls twice a day, and the amount of rise/fall varies. There are
also influences due to sub-sea topography, because tides are sort-of waves.
Storm Surge, Tsunamis, and Coastal Flooding
• Storm surge
Just as you can push water by blowing on it, a storm can move a stack of water ahead of it. That’s a storm
surge. They can result in flooding near coasts and can be very devastating.
• Tsunami
–A tsunami, also known as a tidal wave (no relation to tides), is a wavetrain (series of waves) generated by
the sudden displacement of the sea-bottom due to an earthquake, or, more rarely, a submarine landslide,
an asteroid impact, or a volcanic explosion. (Pretty much anytime the water column gets shoved: it's a lot
like the waves you generate when you move in a bathtub or swimming pool.)
–Tsunami are very long wave-length waves, so they move fast (747 fast) and feel bottom at great depths.
–In deep water, most tsunamis have fairly small amplitude (like a half-meter or meter). When they feel
bottom and start to crest, a huge amount of water stacks up and can result in devastating coastal flooding.
–Sometimes, a tsunami falls then rises, sometimes it rises than falls, depending on how the fault moved
and where the site is relative to the fault.
–If there is any possibility of tsunami
• (you’re in a coastal area and have felt an earthquake,
• you're in a coastl area and you have heard about a large earthquake,
• you have been warned by authorities of a tsunami,
• the water at a beach suddenly rolls out),
get to higher ground immediately: your life depends on it. Abandon any possessions if need be. If you are
in deep water on a boat, stay in deep water.
Overall safety message:
–Below sea level: very very bad. Do not ever live below sea level, unless you’re in the US nuclear navy, or
maybe the Netherlands.
–Slightly above sea level (less than 30m=100ft elevation), pretty bad, try to avoid.
Sea Level
•It’s very hard to measure sea level in absolute terms, so geologists really
talk about relative rises and falls of sea level.
•Sea level changes can be due to
– VVV Pervasive VVV
–subsidence as underlying sediments compact (porosity decreases),
–cooling-driven subsidence (or heating-driven uplift)
–isostatic movement from changing weight of lithosphere (deposition)
– ^^^ Pervasive ^^^
–influx of ice from melting glaciers (~135 meters 10000 years ago),
–expansion or contraction of mid-ocean ridges,
–filling in of oceans with sediments
–movement of faults
–…
•Geologists spend a lot of time dealing with seas coming in (rising) and
going out (falling)
Landforms
There are two pairs of 'opposites' are at work in coastal landforms:
•Deposition vs. Erosion
In any coast, erosion, transportation, and deposition are all occurring. In
some coasts, deposition is winning and we call it a depositional coast. If
erosion is winning, we call it an erosional coast, and if the two are
exactly equal (very unlikely) we would call it a transportational coast.
•Relative sea level Rise vs. Fall
When sea levels "rises", areas that were underwater get deeper (finer
sediments get deposited) and areas that were subaerial before might
now be subjected to marine erosion. You'll often see erosional features.
When sea level "falls", it exposes submarine areas, which are often big,
flat muddy areas like the Netherlands, or broad, gently-sloping
submarine terraces to stream processes. As well as exposing previouslysubmerged areas to streams, wave erosion might start on the new coast.
Depositional features
“Depositional” coasts are dominated by deposition of sediments (from rivers) and re-working by longshore
drift. The East Coast of the U.S. from Boston southward is a good example. They are characterized by
wide, gently-sloping beaches. In a depositional coast, erosion, transportation, and deposition are all
occurring, it’s just that deposition is winning.
• Spits and baymouth bars: sand is moved along by longshore drift. Where it encounters deeper water (as
at a bay), it drops the sand, and a spit forms, which can eventually go all the way across the bay, forming a
baymouth bar.
• Sand bars and barrier islands: A sand bar is a submarine dune. A barrier island is a sand bar that makes
it above a sea level, usually due to storms and swells and high tides.
• DO NOT BUY A HOUSE ON BARRIER ISLAND UNLESS YOU WOULD ALSO BUY ONE ON ICE.
Depositional Features: Deltas
Deltas are acted on by waves and tides as well as the river. Typically, coarse sediment gets deposited
nearshore, and is re-worked by waves. Fine sediment gets deposited further offshore.
Deltas are a major source of oil.
• If sediments are not re-worked at all, you get a river-dominated (crow's foot) delta like the Mississippi.
• If the sediments are re-worked by longshore drift, you get a wave-dominated delta like the Nile’s.
• If the sediments are re-worked by the on-shore/off-shore currents of the tides, you get a tide-dominated
delta like the Brahmaputra’s
Erosional features
Waves usually contain sand and are moving rapidly, so they can be very abrasive. Coastlines are zones of
very intense erosion due to waves: much of the energy from marine storms travels away from the storm and
affects coasts.
• Wave cut cliffs Waves cut at the shore, and the cliff collapses above the resulting notch, leaving a cliff.
• Wave cut platforms Wave erosion largely stops below sea level (depth of wave base, actually), so the
retreat of a cliff leaves behind a flat area slightly below sea level.
• Sea caves, arches, and stacks: Sea caves result where an easily eroded piece of rock is removed. A sea
arch happens when a sea cave or two goes all the way through. If the arch collapses, the seaward part is
still there and is a stack. Stacks also happen when some chunk of rock doesn't get eroded as rapidly as its
neighbors. (Sea stacks are erosional remnants; with a cave cutting through, it's a sea arch.)
• DO NOT BUY A HOUSE OVERLOOKING THE SEA, UNLESS YOU WOULD ALSO BUY ONE BUILT
ON TOP OF A SNOWDRIFT.
This diagram is confusing
Emergent Coastlines
A coastline is emergent if it has recently emerged from the sea, due to a relative drop in sea level.
• A good artificial example is Holland, where you have huge, flat, muddy plains that used to be below
sea level.
• Another set of good examples is California, where submarine terraces are exposed by tectonic uplift.
• When sea level drops, the gently-sloping beach faces and submarine terraces are exposed to stream
processes. This usually progresses very quickly.
• A lot of preserved organic matter decays. Recently exposed sediments have lots of organic matter that
starts decaying and smells bad
• Areas that were previously under enough water to avoid erosion can now be exposed at the surface
and be subject to wave erosion and you get sea cliffs (good photos). Sometimes, you get broad flat
plains which do not make good photos.
• Baselevel of streams leading into the coast also drops, and stream erosion increases.
• You can also end up with submarine features well above sea level, like marine platforms exposed
above sea level.
Submergent (Drowned) Coastlines
A coastline is submergent if it has recently submerged into the sea (Sea
level "rose")
Sea level rose over 100 meters after the last ice age, which only ended
10,000 years ago, Many coastlines have not adjusted to this change.
• A "rise" in sea level floods land that's below the new sea level, giving
rise to a coastline that looks like a normal, usually stream-dominated
landscape with all the area below some contour shaded in blue.
–Sometimes, hills become isolated islands, like off Maine.
–Some such coastlines have numerous estuaries (drowned river mouth
with brackish water).
• Streams keep bringing in sediments, which form deltas where the
stream enters the estuary, and the delta eventually fills in the valley.
• Wave action gradually smoothes out the inlets and promontories by
eroding headlands and depositing sediments in inlets.
• Drowned coastlines usually have numerous inlets, that make good
harbors.
Coastal Processes
•Waves are a powerful force for erosion and deposition.
–In deep water, waves are oscillatory and particles move in small
orbits around their rest-position, but in shallow water, they feel
bottom and move water onshore, and gravity moves it back offshore.
–Wave refraction causes waves to focus into headlands and away from
bays, resulting in rapid erosion of headlands and deposition in bays,
and hence a straighter shoreline.
–Longshore drift is the movement of sediment in a 'virtual' current
parallel to shoreline.
•All coasts have erosion and deposition and transportation occurring
simultaneously.
–Erosional coasts can have sea cliffs and offshore marine platforms
–Depositional coasts are generally very flat, and sometimes have slow
subsidence
•In addition to waves, there is deposition due to biologic activity, and, in
quiet water, deposition of clays.