Chapter 19: Shorelines and Shoreline Processes

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Chapter 19 Shorelines and Shoreline Processes
SHORELINES and Shoreline Processes Shoreline-area
of land in contact with sea or lake, stretching
from low tide to highest elevation effected by
storm waves -shoreline processes are restricted
to a narrow zone along the shore at any
particular time -recall that shorelines move
landward/seaward through time in response to
1. sea level changes, 2. subsidence or 3.
tectonic uplift 1. Tides surface of ocean rises
and falls twice daily in response to
gravitational attraction between Moon and
Sun -flood tide higher tides which cover
portion of shoreline -ebb tide lower tides when
tide is withdrawing spring tides high tides
around equatorial region of Earth-Moon and Sun
are aligned neap tides high tides around polar
regions Sun along equator, Moon aligned with
Pole
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Tides are caused by gravitational pull of
moon. Spring and neap tides shown diurnal
Fig. 19-3, p.564
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Low tide and high tide in Turnagain Arm, Cook
Inlet Alaska. Tidal range is 10m!!
Fig. 19-2a, p.563
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Tidal power generating plant in France
p.567
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Energy from the oceans-in regions with strong
tidal surge. High tide and ebb tide.twice a day
p.566
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2. Waves oscillations of a water
surface Crest-highest point of
wave trough-lowest point of wave wavelength-
distance between successive wave crests or
troughs wave height-vertical distance from
trough to crest wave period- time for 2
successive wave crests or trough to pass any
point wave generation generated by several
different processes.... 1. most are
generated by wind blowing across surface of ocean
the longer wind blows, the higher the
waves generated. the distance the wind blows
over a body of water-known as fetch-is
the determining factor for wave height 2.
other causes, but relatively minor are- submarine
landslides, faulting, volcanic
eruptions A. Shallow water waves and
breakers -deep water swells, the water
surface oscillates and water particles move in
elliptical paths, with little lateral
net water movement -but as deep water
swells encounter shallow water they transform
from broad, shallow waves to sharp
crested waves. this begins where the waves begin
to 'feel' or intersect the sea floor-
called wave base. -as waves enter ever
shallower water they become oversteepened-the
wave crest advances faster than the wave form,
until crest plunges forward as a breaker
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2 m high waves at Jenner, CA
Fig. 19-5b, p.568
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Plunging breaker in Hawaii
Fig. 19-6a, p.569
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Spilling breaker.
Fig. 19-6b, p.569
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Waves in this lake have wavelength of 2m, so
infer that wave base is 1m deep.
Fig. 19-5a, p.568
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B. Nearshore currents and sediment transport
1. nearshore zone upper limit of shoreline to
point offshore where waves begin to break
a. breaker zone-where waves start to break to
where they do break b. surf zone- where
breaking waves rush onshore, followed by seaward
movement of water as backwash
c. width of zone depends upon wavelength of
waves 2. Wave refraction and longshore currents
-waves rarely encounter shorelines parallel
to shore, they usually encounter shorelines
at an angle -as a wave moves toward shore
the landward edge will encounter wave base first,
with the remainder of wave following after
it...this results in waves bent along
shoreline-we refer to this as wave refraction
this results in net water transport along the
shoreline- referred to as longshore current.
this direction is in the same direction as
waves longshore currents are important
because they transport and deposit large volumes
of sediment in the nearshore region Rip
currents- narrow surface currents that flow out
to sea through the breaker zone. wave heights
are generally lower over rip currents-which means
they occur over bathymetric highs (shallow
water)
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Wave refraction along coast-waves refract as
enter shallow water. Waves generate longshore
transport of sand from upper left to right.
Fig. 19-7, p.570
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Rip currents fed by currents moving parallel to
shore. Rip currents carry suspended material
seaward.
Fig. 19-8, p.571
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B. Deposition along shorelines depositional
features along shorelines beaches, spits,
baymouth bars, tombolos, barrier islands 1.
beaches-most familiar of all coastal landforms to
people -a beach can include sand dunes or sea
cliffs.... -most of sand on beaches is
transported to the seashore by streams and
redistributed along the shore by longshore
currents -man made attempts to prevent
shoreline erosion- a. groins- structure
built perpendicular from the shoreline-trap sand
on upcurrent side, beaches
preserved....but erosion occurs on downcurrent
side, which makes for irregular
beach...doesnt solve the problem!! black sand
beaches in Hawaii-sand sized grains of weathered
basalt... Seasonal changes in beaches tides
and longshore currents affect the beach extent
and topography. -normally bad storms have
negative impact on beach-removing large amount of
sand. typically these storms occur during the
winter so that a beach typically loses sand
during winter storms, but is rebuilt by summer
storms
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Seasonal changes in beach profiles. c was taken
2 years after b attributed to winter erosion
Fig. 19-9, p.574
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2. Spits, baymouth bars, and tombolos a. spit-
continuation of a beach extending into a bay or
ocean, connected to land on one end
only b. baymouth bar- is a spit that has grown
across mouth of bay, closing the bay -both
form and grow as result of longshore currents
c. tombolos- a type of spit that extends from
seashore into the ocean connecting an island
to seashore 3. Barrier Islands long, narrow
islands composed of sand and separated from
mainland by lagoon -seaward sides are smoothed
by waves, landward sides are irregular. can
contain dunes on landward side how
formed 1. spits that became detached from
shoreline, or 2. beach ridges that are bounded
by water due to subsidence
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Shoreline features a. spits and b. baymouth bar.
Longshore transportation of sand results in these
featuresa spit that closes off a bay is a
baymouth bar
Fig. 19-10, p.576
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View from space of barrier island system offshore
southern coast of Tx
Fig. 19-12, p.578
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2 models for origin of barrier islands a-b spit
forms, is breached during a storm. c-d beach
ridge forms on land, sea level rises submerging
beach ridge.
Fig. 19-13, p.578
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C. Erosion along Shorelines a shoreline that
is largely erosional has very few beaches, those
that are present are restricted to
protected areas features produced by erosion-
1. Wave-cut Platforms formed by retreat of sea
cliffs landward. that retreat rate is a
function of intensity of waves impinging on
shoreline. wave undercutting at base of sea
cliff results in upper part of cliff being
susceptible to mass wasting. erosion proceeds
long enough that beveled surface known as wave
cut platforms develops 2. sea caves- form as
waves refract around a headland, eroding from
both sides 3. sea arches- form as sea caves
merge 4. sea stacks- result from collapsed sea
arches D. Nearshore Sediment budget gains
and losses of sediment in the nearshore are the
sediment 'budget' -a positive sediment
budgetbeaches can grow -a negative
sediment budgetbeaches eventually may
recede beaches lose sediment by longshore
transport, offshore transport (storms), wind,
deposition in submarine canyons
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Wave erosion causes a sea cliff to migrate
landward-leaves a gently sloping surface
called a wave-cut platform. b. Sea cliffs and
wave-cut platform. c. Gently sloping surface
is a marine terrace. Notice the sea stacks
rising above the terrace.
Fig. 19-15, p.580
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Erosion-wave energy concentrates on headlands.
Extensive erosion of the headlands area causes
retreat throught time..straightening of
shoreline..
Fig. 19-17, p.582
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• Nearshore sediment
• Budget
• If outputs exceed inputs,
• get erosion of beach.
• b. If input exceeds output,
• get deposition along beach,
• also called accretion.
• Note the numerous forces
• that effect beach stability.

Fig. 19-18, p.583
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E. Impact of rising Sea Level on coastal areas
causes of sea level rise glaciers melting,
subsidence, local tectonic effects... -as sea
level rises we would expect landward migration of
barrier islands-significant impact on
coastal communities -people build seawalls
(Galveston) to arrest barrier island movement
pump sand onto beaches to replenish sand
losses (Atlantic City, Miami Beach) -basically
it is futile in long term to try to control
nature because in the end she will always
win....kinda like marriage.... F. Storm waves
and coastal flooding -living in the Gulf Coast
we all know that coastal flooding during
hurricanes causes more damage than high
winds -the hurricane surge is responsible for
erosion of sand on islands, which is then
transported offshore as surge retreats
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Construction of seawall in Galveston, TX to
protect city, began in 1902. Notice curved to
deflect waves upward.
Fig. 19-21, p.585
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Berm built on seaward side of beach, will be
destroyed in winter, needs to be rebuilt each
spring.
Fig. 19-22, p.587
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Erosion along sea shore-winter storms and beach
erosion caused Seashore cliff to
fail.undercutting
Fig. 19-14b, p.578
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Landward migration of barrier island- Note black
lines, these are jetties built in 1930s to
protect inlet at Ocean City. Since then,
Assateague Island has migrated 500m
landward-jetties disrupted net southward
longshore transport.
Fig. 19-19, p.584
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Beach at Miami Beach, Fla before
After US Army Corps of Engineers beach
replenishment
Fig. 19-20a, p.584
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Beach erosion effects Lower rocks, colored
grey, have been abraded because they are within
surf zone. Upper rocks are above surf zone, so
they have not been abraded.
Fig. 19-14a, p.579
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G. Types of Coasts 1. Depositional coast-
abundance of detrital sediment and deposits such
as barrier islands, wide sandy beaches,
deltas US Gulf Coast 2. Erosional coast- steep
and irregular in profile, lack well-developed
beaches characterized by sea cliffs, wave cut
platforms and sea stacks (W Coast of N
America) 3. Submergent coast- sometimes called
drowned....if sea level rises with respect to
land or land subsides, coastal regions are
flooded 4. Emergent coast- land has risen with
respect to sea level. typically in tectonically
active areas. noticeable due to exposure of old
wave cut platforms which are called marine
terraces when exposed.
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Submergent coast -extremely irregular with
estuaries such as Chesapeake Bay.
Fig. 19-23, p.588
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Emergent coast -straighter and steeper
coastline. Notice several sea stacks and sea
arch. Also marine terrace in background.
Fig. 19-24, p.589
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Fig. 19-2b, p.563
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Fig. 19-20b, p.584
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Barrier islands at Outer Banks of North
Carolina. Notice suspended material in water
column
p.586a
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p.586b
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Fig. 19-17a, p.582
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Fig. 19-17b, p.582
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Fig. 19-10a, p.576
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Fig. 19-10c, p.576
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Fig. 19-10b, p.576
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Fig. 19-12a, p.578
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Fig. 19-12b, p.578
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Fig. 19-19abc, p.584
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Fig. 19-19d, p.584
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Fig. 19-1a, p.562
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Fig. 19-CO, p.560
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Fig. 19-1b, p.562