Coastal environments and processes
• Sea Level - covered
earlier
• Wave energy and erosion
– Waves vs. geology
• Beaches and beach
processes
– A river of sand
• Human impact on
coastal zone
Basic conceptual distinction
• Primary coasts: Shaped
by solid earth processes
(land or geology)
– e.g. volcanic coasts
• Secondary coasts:
Shaped by ocean
– e.g. barrier islands
– Changing the supply and
movement of sand
• Estuaries: salt and
freshwater mix
– Human impacts
Coastlines - Primary processes
• Land erosion and rising sea level
Chesapeake Bay:
• Naturally, most
coastlines are shaped by
a mixture of processes
Coastlines - Primary processes
• Land erosion and rising sea level
New Zealand fjords
Low sea level: river
valley formed
Low sea level: river
valley formed
Rising sea level
drowns valley
Glaciers scour valley
and create steep
sides, U-shape
Guess when formed?
Rising sea level
drowns valley
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Coastlines - Primary processes
• Coasts built outward by land processes
Mississippi River Delta
Coastlines - Primary processes
• Coasts built outward by land processes
Cape Cod: Moraines
Sediment transported
down river
Glaciers deposit rocks
and sediment along
their fronts and edges
Deposited on broad
continental shelf: delta
Leave ridges
(moraines) that stand
above rising sea level
Only form where
coastal ocean processes
are fairly weak (waves,
tides, etc.)
Imagine similar river discharging on
W coast of N America…
Coastlines - Primary processes
• Coasts built by volcanic or earthquake activity
Coastlines - Secondary processes
Erosion = Waves vs. Geology:
Marine erosion most intense on highenergy coasts: large waves
Erosion most effective where substrate
(geology) is soft
Prince Wm Sound, AK
Earthquake uplift
created new shoreline
Gulf of California, Mexico
Isla Encantada volcanic
activity near shore
creates crater shoreline
Where tide range high, waves can
impact greater range of shoreline
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Pacific NW coast
Coastlines - Secondary features
• Tectonics and
erosion
• Sea arch, sea
stack, tombolo
• Scalloped appearance at first, as softer rock wear away
• Steep slopes as resistant rocks are eroded at base, cliffs collapse
Copyright 2005 by Andrew Alden, geology.about.com,
reproduced under educational fair use
Copyright 2005 by Andrew Alden, geology.about.com,
reproduced under educational fair use
Coastlines - Secondary features
Movement of sand and water
• Later, wave refraction concentrates energy on headlands
• Coastline smooths out as waves erode headlands; beaches grow
in quieter bays.
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Movement of sand and water
• Waves strike beach at angle
• Sand and water move up beach at angle but travel
straight back down
– Swash (upslope) and backwash (downslope)
• Net motion of sand and
water along shore longshore drift
• Dispersal of energy
from breaking waves
creates a longshore
current
• Current strength
increases with wave
angle
Rip currents
• Strong current flowing offshore
• Form where longshore currents
converge
• Visible as gaps in breakers
Longshore currents
• Coast-parallel currents near the shore
• Stronger where waves at higher angle
• Direction can vary
depending on wave
direction
• Can be established by
wave setup, where
waves of unequal
heights create “piles”
of water that disperse
as longshore currents
Where does sediment (sand)
come from?
• Sources:
– #1 is rivers
– Also: coastal erosion; windblown
• Rivers add 7 billion tons of sediment yearly to
coastal environments
• Human activity changes this!
www.miseagrant.umich.edu/
pubs/up/june04/rip2.html
• Carry plumes of
sediment
– Dams: reduce dramatically
– Colorado: 135 million tons to nearly 0
– Where rivers are undammed, sediments have likely
increased due to cultivation
www.geology.wmich.edu/ kominz/otln11.html
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Sand in motion along the shore
• Moving sand creates features: bars, spits,
beaches, islands, lagoons, and tombolos
Beach features
Typical beach profile
5
More about beaches
• Beaches change seasonally!
– Summer beaches: few storms,
broad and sandy, convex slope
– Winter beaches: frequent
storms, concave slope;
offshore bars
Beaches in
transition!
• Beach at UCSB in
1975 and 1983
• El Niño storms
• Beaches can change in a
single storm
– Offshore bars migrate
shoreward, accrete onto beach
= growth
• Beaches are always
changing, in motion
When river waters reach the sea
Hypothetical sand
budget of a beach
• Current velocity decreases
• Sand settles out; ocean waves and currents
move it around
• Smaller particles (mud, clay) remain in
suspension
• Sand on beaches
• Mud and clay out to sea
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When river waters reach the sea:
delta formation
Types of deltas
• Where rate of sediment supply
exceeds the redistribution by
currents and waves
• Sediment accumulates to form a
delta
• Here, waves and currents have
smoothed the delta’s edges
Barrier islands: fragile!
Nile delta
Barrier islands: Ocean City MD
Texas shoreline before Hurricane Carla
Texas shoreline after Hurricane Carla
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Barrier island morphology
Coastlines - Biological formation
• Coasts can be formed or modified by biological action
• Role of vegetation
• Which way is the island migrating?
Coral reefs (Great Barrier
Reef, Australia)
Mangrove swamps
(Florida)
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Coastlines - Human impacts
• Dams, other construction can cut off sediment
supply to coasts: beach starvation
• Beach nourishment: adding sand to replace lost
sand
Upham Beach, FL
• Before, after, and 1 year
after: beach nourishment
costly and temporary
– Expensive!
– Taxpayer funded!
• Added sand is often
finer than beach
sand, so it is more
easily lost.
• Offshore habitat
disruption
http://coastal.er.usgs.gov/wfla/video/vidphotos.html
Effect of groins/jetties
Human impacts on beaches
structures to “protect” beach
Groins or jetties: Piles of rock perpendicular
to beach
Seawalls: Walls protecting cliff
Breakwaters: Offshore piles of rock parallel
to shore
Sediment accretion upstream, but erosion downstream
Longshore drift direction
Accretion
Erosion
None of these actually preserve or protect the
beach in any meaningful way!
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Effect of seawall
Effect of breakwater
• Sediment on either side eroded; eventually behind
• Sediment accumulates directly behind breakwater
• Beach erodes on downstream side
Ocean City (Fenwick Isl)
Fenwick/Assateague, MD
• 1933: Hurricane opened inlet
• 1934-5: ACE stabilized inlet with
jetties
• Sand accumulates N, erosion to S
• Islands migrating landward:
Assateague Isl
• Where is erosion occurring? Accretion?
• Which way is longshore current moving?
– Fenwick: Ocean City stranded as island
moves landward? (maintained by costly
beach nourishment)
– Assateague: Erosion of about 12m (40ft)
per yr; rapid landward migration of N tip
(likely to merge with mainland in the
next few decades)
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Estuaries
• A body of water partly surrounded by land,
where fresh water from a river mixes with
ocean water
– E.g. Chesapeake, Hudson, SF Bay
• Extremely productive
– High nutrients, well-ventilated
– Reduced competition and predation (stressful)
– Nurseries for young fish and shellfish
• Heavily developed/exploited
– Well over half of US estuaries are lost
Estuary formation
Estuaries: Human impacts
• Harbor construction
and shipping
• Sewage outfalls
• Pollution
• Draining and filling
of wetlands for
construction, river
channelization
• Diversion of fresh
water
http://www.davenewbould.co.uk/postersgallery/pages/Sunlight%20on%20the%20Mawddach%20Estuary.htm
http://www.ens-newswire. com/ens/dec2004/2004-12-02-10.asp
Estuary formation, cont’d
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Types of estuaries: Salt-wedge
•
•
•
•
Tidal currents weak, river flow strong
Strongly stratified: fresh surface, salty deep
Strong surface flow, weaker deeper
Seds from river: sand near river; muds and clays
in lower estuary
Types of estuaries: Partially mixed
• Stronger tidal currents, river flow moderaate
• Weakly stratified: surface and deep = few ppm
different
• Stronger landward deep flow than in salt-wedge
• Moves sed into estuary!
Types of estuaries: Well-mixed
•
•
•
•
Tidal currents dominate, river flow weak
Not stratified
Flow follows tide always
Seds from offshore
Types of estuaries: Reverse
•
•
•
•
River is saltier than ocean
Slight stratification
Flow is landward at surface, seaward deep
Example: Colorado River delta
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Chesapeake Bay
• Partially-mixed type
• Drowned river valley
• Salinity ranges from fresh
to near seawater
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