geology-What percent of the U.S. population

geology-What percent of the U.S. population

Question
Coastal Processes and Problems 279

16 Coastal Processes and Problems
Topics
A. What percent of the U.S. population is expected to live within an hour’s drive of a sea coast
by the year 2025?
B. What physiographic coastal features occur along an active continental margin?
What physiographic coastal features occur along a passive continental margin?
C. In what way does the profile of an ideal surface sea wave differ from that of a sine wave?
What three variables determine the size of wind-generated waves?
D. What geometric form is described by the wave-generated motion of a floating object?
Does a floating object actually move downwind? What circumstances produce a sea breeze?
What is wave base, and how does it relate to wave length? How do marine terraces develop?
E. How are wave speed, wave length, and wave period related? In a breaking wave, how are
wave length and wave height related? Why do headlands experience erosion whereas bays
experience deposition? Why do seaports commonly occur within bays? How do longshore
currents, rip currents, and spits develop?
F. What is a stack, a tombolo, a tied island? How does each develop?
G. What are two reasons for why coastal wetlands are valued by ecologists? Why are the Isles
Dernieres—at the edge of LaFouche delta lobe—eroding so rapidly?
H. What are the five basic types of coastal installations, and what problems do they cause? How
might troublesome groins be modified so as to become beneficial groins? What problems and
solutions followed the construction of the harbor breakwater at Santa Barbara?
I. What is the where, when, and how of hurricane development? How does the Coriolis effect
influence the behavior of wind and sea currents? Why is effective wind velocity different on
the two sides (right, left) of an approaching hurricane?

A. Are we loving our coasts to death?
The 1990 Census reported that the greatest percent increases
in U.S. population during the 1980s were in coastal regions.
At present 50% of our population lives within an hour’s
drive of a coast, and that number is expected to grow to
75% by the year 2025.
• The problem: Exponential growth along our coasts is
accompanied by building in high-risk localities.
• The solution: A better understanding of coastal processes.
We attempt to protect our coastal facilities with installations
of stone and concrete—but such projects commonly either
produce unexpected results, or fail entirely in the wake of
incessant waves and currents and devastating weather.

Q16.1 Before going further, (A) can you name a
natural process that seasonally presents big problems
for coastal residents—namely the Gulf and Atlantic
coasts? Try naming a 2005 example of that process.
(B) In this context, can you name a human activity along
coasts that commonly meets with this kind of disaster?
Try naming a specific occurrence of such human
activity. Hint: Think about a particular Texas ‘island
city’ in 1900.

Q

16.2 Can you name a kind of inland waterway
management effort that also commonly meets with
disaster?

280 Coastal Processes and Problems

AMERICAN

nt

ine

ine

NORTH

nta

l ma
rgi

n

nt
A cti v e co

m

n

iv

gi

e

ar

Q16.3 (Fig. 16.1) What is the

PLATE

nt

al

North America is an example of a
continent where the physiography of
opposite coastal regions (both onshore
and offshore) differs because of
differences in proximity to global
plate boundaries (Fig. 16.1).

JUAN
DE FUCA
PLATE

co

B. Classification of coasts

Pas

s

relationship between the proximity
to a global plate boundary and the
breadth of continental shelf?

Coastal plain
Continental shelf

0

Q16.4 (Fig. 16.1) What is the
relationship between the breadth of
continental shelf and the breadth of
coastal plain?
Along our West Coast, relentless tectonic forces associated with the western
boundary of the North American plate
account for the abrupt boundary between land and sea—an example of an
active continental margin. In contrast,
our East Coast has long since departed
from the eastern boundary of the North
American plate, so land and sea have
had more time to develop an equilibrium marked by the near seamless
boundary between continental shelf and
coastal plain—an example of a passive
continental margin.
Classifying coasts is a bit difficult
because—in addition to the broader
effects of global plate boundaries—
rivers, ocean currents, and climate exert
their influences as well. But, as with all
naturalists, geoscientists are moved to
put names on things. Table 16.1 shows
a classification of coasts based on
setting and process, a classification
that is independent of proximity to plate
boundaries.

Q16.5 Name two coasts that are
included within the same category of
‘setting’ (Table 16.1, column one), but
which occur along different kinds of
plate boundaries (Fig. 16.1).

PACIFIC PLATE

200

400 mi

CARIBBEAN PLATE
COCOS PLATE

Figure 16.1 Differences in physiography between east and west coasts of our
conterminous states reflect differences in proximity to global plate boundaries.

Table 16.1
A classification of coasts based on setting and process.
Setting

Process

U.S. example(s)

Subaerial (streams, Stream erosion
glaciers, volcanism) Stream deposition
Glacial erosion
Glacial deposition
Volcanic eruption

Estuaries (northern U.S. Atlantic Coast)
Deltas (Mississippi Delta, Louisiana)
Fjords (Alaska)
Islands (Long Island, New York)
Hawaii (amoeboid-shaped islands)

Marine (waves and Erosion
Deposition
currents)

Sea cliffs (U.S. Pacific Coast)
Beaches and barrier islands (southern
U.S. Atlantic Coast and Gulf States)
Florida Keys
Florida Everglades

Organic (corals,
mangroves)

Reefs
Coastal wetlands

We will visit a number of these coasts later

Coastal Processes and Problems 281
C. Wave anatomy
The profile of an ideal surface sea wave is suggestive of a mathematical sine wave,
but the two do differ (Fig. 16.2). Even so, physical oceanographers treat ocean
waves as sine waves in certain calculations.

Q16.6 As shown in Figure 16.2, in what most obvious way does the profile of
an ideal surface sea wave differ from that of a sine wave?
Ideal surface sea wave
Crest

Wavelength, L

Crest
Wave
height, H

Still water level
Trough

Amplitude, A
(1/2H)

Trough

Sine wave

Figure 16.2 The profile of an ideal surface sea wave differs from that of a sine wave.

Earthquakes and submarine landslides locally produce ocean waves, but the
universal wave-generating force is that of the wind. When air moves across the
surface of water, friction roughens the surface into small rounded ripples, or
capillary waves, with V-shaped troughs (Fig. 16.3). These tiny waves, which are
characterized by wavelengths less than 1.74 cm (a precise number that is governed
by the physics of wind and water), are produced by the interaction of wind and
the surface tension of water molecules. Hence the term capillary waves. As more
wind energy is added to the water surface, gravity waves develop with a form like
that in Figure 16.2. Gravity waves take their name from the fact that gravity, rather
than surface tension, is the leveling force when the wind abates.
As still more wind energy is transferred to surface waves, wave height increases
more rapidly than does wave length, resulting in a steepness that causes waves to
break as whitecaps (again, Fig. 16.3).
Three variables determine the size of wind-generated waves: (1) wind speed, (2)
wind duration, and (3) fetch (the distance over which the wind travels)å.

Q16.7 Which of the above three variables do you suppose most likely explains why the largest waves occur at sea, rather than on lakes?
Wind increasing
Capillary
waves
1.74
cm

Gravity waves

Steepness: H/L = 1/7

Breaking waves

120°

Not drawn to scale

Figure 16.3 As surface waves gain energy, they increase in height and length. When
wavelength exceeds 1.74 cm, waves assume the shape of the ideal surface sea wave shown
in Figure 16.2. When wave steepness becomes 1/7 (height divided by length), waves
become unstable and break as whitecaps.

282 Coastal Processes and Problems

D. Wave mechanics
Simple surface wave form
A floating object appears to bob up and down with passing waves. But, actually,
the object’s motion describes a circle, the diameter of which equals wave height
(Fig. 16.4). Incidentally, this circular motion of water particles is analogous to the
motion of earth particles affected by the surface wave of an earthquake. (An
earthquake also produces two additional types of waves.)

Wind and wave travel

Forward motion within the upper part
of the orbit of a wave particle is a bit
faster than backward motion within the
lower part of that orbit. This is because
of differences in resistance exerted by
the water. A sensory metaphor: One
might sense that the forward motion of
a Ferris wheel is faster than its backward motion (Fig 16.5), but that is only
perception, because of the fact that a
sense of falling is more disquieting than
a sense of rising.

A
ee
We

eeee!

hu
g…

B

c
g,
hu
C

C
Figure 16.5 Perception: A Ferris wheel
might produce the sensation of rotating
faster where one is descending.
D

E
Figure 16.4 This illustrates the rise and fall of a floating bottle (beginning at A and
ending at E) as the crest of a wave passes. So seasickness is caused not only by
up-and-down motion, as in car-sickness, but by forward-and-backward motion as well.

Q16.8 Does a floating object
actually move downwind? On
Figure 16.4, align a straight-edge
along imaginary points marking the
beginning (i.e., the ‘base’) of each
partial circle. What does the
orientation of the straight-edge
suggest about the motion of the
bottle—specifically, is it moving
downwind or not?

Coastal Processes and Problems 283
Wave base
Because surface waves are produced
by surface winds, the deeper the water
particle, the farther it is from its energy
source. Result: The agitation of water
diminishes as a function of depth, as
illustrated by the diminished orbits in
Figure 16.6. The depth below which
there is no excitation of water by
surface wind is called wave base. It
turns out that the distance between the
still-water level and wave base equals
one-half wavelength. To test this, do
the following:

Q16.9 On Figure 16.6, measure the
distance between the still-water level
and wave base. Double that value and
compare it with wavelength. How do
the two compare?

Wind and wave travel
Still-water level

L
2

Wave base
Figure 16.6 The depth below which there is little or no motion within the water is
called wave base. Depth to wave base, from still-water level, is 1/2 wave length (L).

Landward ho—Waves are almost invariably driven landward by prevailing
sea breezes (Fig. 16.7), which has to
do with the fact that almost everywhere
coastal land is warmer than adjacent
ocean water. So…

Q16.10 Explain the mechanism

Se

a b
ree

ze

Warmer coastal land

that generates sea breezes. Hint: It is
illustrated in Figure 16.7. (And, it is
covered on page 265 of Exercise 15,
Deserts.)
On rare occasions at night, when the air
temperature over land declines, more
gentle land breezes flow from land
to sea—happily sweeping clouds of
coastal mosquitoes with them.
Waves colliding with the sea floor—
Figure 16.8 shows a shallow sea
covering a seaward-sloping sea floor.
Wind-driven waves are advancing
toward the shoreline. Wave base,
complete with schematic oscillating
water particles, is also shown.

Cooler ocean

Figure 16.7 Sea breezes, which commonly flow at speeds of 13–19 km/hr (8–12
mi/hr) move from sea to land, reaching 15–50 km (9–30 miles) inland.

Wind and wave travel
A

B

Wave base
Seafloo

r

Q16.11 Sketch a projection of the
wave base in Figure 16.8 horizontally to a point on the sea floor. How
does the form of a wave approaching
from point B to point A change in (a)
general shape, (b) wavelength, and
(c) wave height?

Figure 16.8 This is a cross-section that includes features in Figure 16.6, plus a
sloping sea floor that rises landward to within wave base and beyond.

284 Coastal Processes and Problems
Wave forms reflect topography
Wave forms reflect sea floor topography (aka bathymetry). Shoaling conditions
(the result of currents moving across shallow sea floors) are likely to produce
breaking waves such as those that characterize shoreline surf zones (Fig. 16.9).
Shoreline surf

A

B

Figure 16.9 This is an incomplete sketch of seafloor and overlying wave forms. The shoreline surf zone—characterized by breaking waves white with foam—is shown as well.

Q16.12 From what you learned about waves colliding with the sea floor on
page 283 (Fig. 16.8), and being mindful of wave forms, trace the sea floor
from point A to point B in the reduced copy of Figure 16.9 on Answer Page
301. Hint: A breaking wave indicates that, at that spot, sea floor depth is
within wave base; and, depth to wave base can be plotted from wave length.
Capsizing of the Taki-Tooo on June 14, 2003
(Fig. 16.10) was caused in part by the 32-foot
fishing boat’s venturing out across a sand bar
topped by 16.foot-high breaking waves. Not
so incidental is the fact that none of those who
died were wearing life vests, whereas all eight
survivors were wearing vests.

Q16.13 What two simple but
valuable lessons should be learned
from the Taki-Tooo tragedy? Hint:
One has to do with pathways at sea,
the other with on-board precautions.

Portland
OREGON
Detail
Pacific
Ocean

Figure 16.10 Rivers emptying into
Tillamook Bay produce currents that keep
the harbor channel swept free of mud and
silt. But the mouth of the bay tends to
become barred by sand drifting southward
along the coast.

Tillamook
Bay
Boat
capsized
killing
eleven

Wi l s o n

Riv

er

Tr as k R iv e r
0

5 mi

0
5 km
AP graphic

D, JUNE 17, 2003

THE REGISTER-GUAR

y
The Taki-Tooo traged
ar

J e t t ie

s

Garibaldi

Ch

a

nn
el

e treacherous Tillamook
GARIBALDI, Ore.—Th capsizing of the Takisandbar—one factor in the people—hasn’t been
Tooo and the loss of 11 Corps of Engineers
my
dredged by the U.S. Ar s of po un din g su rf,
19 76 . De ca de
sin ce
d the rock jetties built to
meanwhile, have erode ells and whitewater at
sw
shelter boats from the
re than anything else,
the harbor’s entrance. Mo ves as a haunting
ser
the Taki-Tooo tragedyand unpredictability of
reminder of the power rability of all who sail
the sea—and the vulne
upon it.

Sand b

raises
Charter boat disaster
ns
safety concer

Taki-Tooo disaster: http://www.registerguard.com/news/2003/06/17/ed.edit.charterboat.phn.0617.html

101

Coastal Processes and Problems 285
Marine terraces
Given the turbulence of the surf zone, sediment suspended in the water provides
grist for the proverbial mill—grinding away at rocks and installations at the
water’s edge, plus flattening the seafloor down to the depth of wave base
(Fig. 16.11). The result: A marine terrace (aka wave-cut platform, aka wave-cut
bench).
Wind and wave travel

Wave base

Wave-cut terrace

A

Monterey
Peninsula

Figure 16.11 Where the elevation of the sea is stable—relative
to that of the land—for a sufficient period of time, erosive action
down to wave base can sculpt a marine terrace.

Marine terraces, hidden from view during their development, can be exposed
through a rising of the land, a lowering
of the sea, or some combination of the
two. Because of its position along an
active plate margin marked by vertical movements of continental crust,
our west coast is replete with exposed
marine terraces. A notable example is at
Pebble Beach, California.

ea

Inset A
Cal
ifo

B

Figure 16.12, on page 302 of the
Answer Page draw a profile (i.e., a
side-view) from NW to SE along the
line-of-sight indicated by the arrow
in Figure 16.12B. Show the relative
elevations of each of the two marine
terraces and that of the Pacific Ocean.

rn

ia

15

Pebble Beach Golf Links

2

16
3

1

14

4

Line-of-sight in C
6

17

N

1,000 ft

C
Higher marine terrace

Lower marine terrace

7

11

9
10

8

18

0

12
13

5

web site pebblebeach.com.

Q16.14 From the information in

of
B

Carmel Bay

The area of Pebble Beach Golf Links,
described by some as “the greatest
meeting of land, sea, and sky in the
world,” exhibits innumerable exposed
marine terraces. Figure 16.12C shows
a view from hole 5 toward hole 6.
Figure 16.12 AB. Famed Pebble
Beach Golf Links. C. Looking southeastward from hole 5 toward hole 6
(as indicated by the arrow in B). After

Ar

286 Coastal Processes and Problems
E. Wave refraction

Figure 16.13 Wave-generated orbits of water particles become distorted and flattened where
they meet the sea floor.

-40 ft

decrease in wavelength and increase
in wave height have to do with conservation of energy? Hint: A water
particle within a wave crest has both
kinetic energy (energy of motion) and
potential energy (energy of elevation).

Wave base

-30 ft

Q16.15 What does the simultaneous

Orbits drag on bottom

Wind and wave travel

-20 ft

Where ‘stacks’ of orbits of downwarddiminishing wave motion encounter the
seafloor (at wave base), frictional drag
distorts the stacks, (a) flattening orbits,
(b) decreasing wavelength, and
(c) increasing wave height (Fig. 16.13).

Curiously, once a wave has been
generated, its period is unchanging.
(Wave period is the time required for
two successive crests, or troughs, to
pass a fixed point.) Therefore, the speed
of a wave (= speed of water particles
within a wave) diminishes as wave
length diminishes.

crest

W

av

e

ba

se

Wa v e

Figure 16.14 A contour map of the sea floor showing a solitary wave crest being refracted
as it moves from water in which depth exceeds wave base into water in which depth is less
than that of wave base.

Q16.16 What is the period of waves

+80

moving at 10 m/s, if their wavelength
is 64 m?

Ba y
+60
-20

Figure 16.14 shows the crest of a single
wave on a contour map of the sea floor
where the wave moves from water
depths greater than wave base into
water depths less than wave base. The
dragging of orbiting water particles as
they encounter the seafloor slows the
advance of the wave, causing the wave
crest to refract or bend.

of the wave crests is in position to
refract with the slightest additional
motion? Hint: You must solve for
both wave base and water depth.

dla

nd

-60

+20

ea

H

H

ea

dlan

d

-80
Wave crest X

-100

Figure 16.15 shows wave crests X, Y,
and Z approaching shore. Crests are
shown as straight lines, but topography
on the seafloor should cause refraction.

Q16.17 In Figure 16.15, which one

+40

-40

Wave crest Y

-120
Wave crest Z

N

0

100 ft

Contour interval: 20 ft

Figure 16.15 This is an artificial representation of straight wave crests approaching an
irregular shoreline from south to north. Contours show land elevations and water depths.

Coastal Processes and Problems 287
Erosion of headlands,
deposition within bays

Ba y

dlan

e cr es

t
ea

d

dlan

Figure 16.16 Lines of force (arrows) associated with advancing waves are perpendicular to
wave crests. So as a wave refracts (bends), lines of force either converge or diverge. These
arrows are vectors of sorts, i.e., an arrow points in the direction of applied force, and the
weight of an arrow indicates the amount of relative force.

Mazatlán

M

E

X

I C

N

O
0
0

50 mi
50 km

Inset map
Puerto Vallarta

Guadalajara

Q16.19 What is there about the
physical geography of the Mexican
coastline shown in Figure 16.17
that sets Puerto Vallarta apart from
neighboring coastal communities?

d

Wave crest

constructed within bays. Why is that?
(Seaports also commonly develop
within estuaries—i.e., drowned river
mouths—to facilitate inland
commerce.)

Famed sand beaches of the world,
e.g., Waikiki (Hawaii), Lloret de Mar
(Spain), Puerto Vallarta (Mexico)—the
list goes on—are all within bays (Fig.
16.17).

av

H

Q16.18 Seaports are typically

ea

H

• Lines of force are perpendicular to
wave crests.
• When a wave crest refracts (bends),
lines of force are redirected so as to
continue to be perpendicular to the
wave crest.
• Lines of force converge (thereby
concentrating force) on headlands.
• Lines of force diverge (thereby
dissipating force) within bays.

w

Headlands (peninsulas) are sites of
erosion (producing sea cliffs), whereas
bays are sites of deposition (producing
sand beaches). This is because wave
energy (i.e., the grist mill mentioned
earlier) is concentrated on headlands for
reasons illustrated in Figure 16.16 and
tabulated as follows:

Pa

ci

Punta
de Mita

Manzanillo

fi
c

O

Playa de
Puerto Vallarta

ce

an
Cabo
Corrientes

Figure 16.17 Puerto Vallarta is growing in its
popularity as a tourist attraction in Mexico,
largely because of its sand beach.

288 Coastal Processes and Problems
Longshore currents

e
cr
es
t

are critical concepts in the business of coastal management. Question: Where—earlier in this exercise—did it
appear that a longshore current and transport specifically led to fatalities?

av

hore curre nt

Q16.20 Longshore currents and longshore transport

W

Longs

Waves striking a coast at an angle—the usual condition—produce a longshore current (Fig. 16.18). Longshore
currents can, in turn, effect longshore transport of coastal
sediments, prompting the aging 16 mm film title:
“Beach, A River of Sand.”

Figure 16.18 Waves striking a north–south coast from the
northeast produce a current moving southward, dragging
coastal sediments with it. (Notice the refraction of the wave
crests as they enter shallow water.) On the beach, wave
crests push water and sand up the beach in the direction of
wave motion, while, within wave troughs, gravity drives
water and sand directly downslope. This zigzag motion
results in the longshore transport of beach sands.

Offshore
sand bar
North
Wave-driven water and sand
(up across slope)
Gravity-driven water and sand
(directly down slope)

Rip currents

Hall, Jr. and Anthony Ervin tied (yes, tied!) in the 50 m
freestyle event. Their time: 21.98 seconds. Question:
could Gary and Anthony make it to shore swimming
against a rip current moving at the rate of 8 km/hr?

TS

one

Surf z

Ri

0

p

cu

rre

nt

Q16.21 In the Sydney 2000 Olympics, Americans Gary

REN

curre

Rip currents move slowly by automobile standards, e.g., 8
km/hr (5 mi/hr), but that’s fast enough to get swimmers into
big trouble.

CUR
RIP

shore

Figure 16.19 This is a sketch of an actual lowaltitude photograph taken along the coast near
La Jolla, California. Rip currents and their
direction of flow are indicated with arrows. In
this case, rip currents run directly opposite of
the motion of onshore waves.

GER

Long

In cases where the sea floor is relatively smooth, backwash
from the surf zone returns to the sea as thin sheet-flow along
the sea floor. Standing in waste-deep water, one can sense
that seaward flow at ankle level. It is commonly called
undertow. But where the sea floor is scalloped into channels and ridges at some angle to the surf zone, backwash can
become concentrated within the channels, much like surface
water finding its way into gullies. This channeled water,
which is difficult to detect from the shore, can be swift and
strong—giving rise to the name rip currents (Fig. 16.19).
Of course, undertow and rip currents grade into each other.

DAN

nt
25 m

Coastal Processes and Problems 289
Land forms shaped by currents—Cape Cod, Massachusetts
The complexity of currents along our
Atlantic coast presents special
challenges to coastal developers and
managers. The converging, diverging,
and swirling of currents shape myriad
land forms in this region, including
curious sand spits (Fig. 16.20).

Q16.22 On the map in Figure 16.20,

feature. In fact, a number of spits bear
the name ‘hook,’ e.g., Sandy Hook,
New Jersey—a picturesque part of
Gateway National Recreation Area.

find peninsulas or islands that appear
to be spits shaped by currents. Place
an arrow along each spit indicating the direction of the current that
shaped it, and record its map coordinates (letter-number) on page 302
of the Answer Page. Hint: Do not be
dismayed by currents that appear to
converge or diverge over short distances. Such irregularities are typical
of the sea.

A disclaimer: Larger islands in Figure
16.20 are neither spits nor sand bars,
but gravel bodies left by retreating
glaciers. But most of the delicately
narrow peninsulas, along with the
hook-shaped features, consist of sand
strewn about by present-day currents.

A sand spit is basically an extension of
a beach or bar into deeper water of a
bay mouth. Currents entering the bay
commonly shape a spit into a hook-like

Figure 16.20 Sand spits of various shapes
and sizes along our Atlantic coast are
graphic testimonies to the complexity of
currents that shaped them.

A

B

C

D

E

F

G

H

I

J

K

L

M

1

N

O

Ca

Q

R
1

p

2

e

2

P

C

3

od

3
4

4

Plymouth
Cape Cod Bay

5

5

6

6

7

7

8

8

9

MAINE
NEW
HAMPSH
IRE

9
South Yarmouth

10

10

11

MASS .

11
New Bedford

12

East Falmouth

12

13

13

14

14

15

15
ineyard
tha’s V
Mar

16

16

Chappaquiddick
Island

17

17

18

18

Nantucket Island

19

0

20

0

N

A

B

C

D

E…