Coastal Geol Week 3 Processes that Shape the Coast
Chaps 1 & 2 had us thinking about long-term processes, global scale changes in
terms of continental evolution and sea level change.
Now we turn to more short-term, localized processes
usually equate with specifics that give each coastal segment its own
character (e.g., the Hook in Sandy Hook…)
Break these into 3 main types of processes:
1. physical
2. chemical
3. biological
Davis considers the physical processes to be most important:
physical processes = waves + tides + currents
these physical factors cause many changes on the coast:
erosion
transport
deposition
mold features
change river courses
in this chapter we focus upon processes,
later chapters we turn to the effects - that is, the features created
I.
Waves (wave energy) at the Coast
Water wave defined: "disturbance of a fluid medium through which energy
is moved." Note that it is energy (and not particles) that are moved
forward
Analogous to E-M waves that travel through space, but no particles involved.
3 main causes of water waves :
 Wind
 Earthquake
 Gravity (moon, sun)
But it is wind that generates most of the waves we see on an ongoing basis
Wk 3 p.2

Review wave geometry
Crest
Trough
Wavelength L ( horizontal length of one cycle)
Height H ( vertical dist between crest & trough)
Steepness (= H/L)
Period - length of time it takes for one wavelength to pass a fixed point
Frequency - no. of cycles passing a fixed point per second (cycles /sec)
Steepness can be no greater than 1/7; at this point, wave topples and breaks

Wave initiation - why do waves begin?
A "disturbing force" is required - examples:
wind blowing across surface of water,
earthquake produces a tsunami
gravitational pull causes tides
atmospheric pressure diffs can produce a seiche
Mechanics a bit complex there is also a restoring force at work, mainly gravity….as wind pushes water up
into a "bunch” or hill of water, gravity tries to pull the mass of water back down to
flat position…but momentum carries the water past the flat point, and it sinks to
a trough. The basic up-and-down motion has thus been started, and in a
frictionless environment like the ocean, it keeps on going easily.
Wind blows
Water bunched up
Gravity pulls the crest down
Davis uses bungy jumper analogy; up, down, up, down til energy drained out of
cord
Wk 3 p.3

Wave size - controlled by 3 factors
Wind speed
Wind duration
Fetch distance
Once ripples have formed, more and more wind energy can be transferred
effectively
Waves get longer and bigger until a "fully developed sea" is reached.
More terminology:
Significant wave height = avg ht of highest 1/3 of waves studied in a certain time
period
II. Actual motion of water particle in a wave
A. Particle motion is circular - net effect of 4 motions, think of a box:
Fig p. 71 show the components to the circular motion
Wave travel
T1
T5
This is the sequence - 5 time slices particle starts in a trough at t1,
moves up the front side of the wave at t2,
hits the crest at t3,
starts coming down the backside at t4,
returns to the trough at t5, now slightly ahead of its orig position at t1.
Compare this to Fig on p.71 - get this travel path down mentally….
Wk 3 p.4
B. A second big mental component is that orbitals decrease with depth, and
wave base is reached at depth = 1/2 L (L = wavelength)
Fig p. 71 shows this well
III. Wind waves and swell
A. wind waves
While a gravity wave is under the influence of wind, it is known as a "wind
wave". Form of wind wave is peaked crest, broad trough:
wind
whitecap
Over time with constant wind force, periods get longer and waves get higher,
become "storm waves"
Typical wind wavelength = 50 - 100 m, storm waves twice that or more
Occasional rogue wave (120 ft high) very dangerous to ships, seems to form
quickly and then dissipate
B. Swell
Defined as waves that have traveled outside of the influence of the wind in a
local generation area, or waves that continue to travel after wind stops.
Long swell waves have low trough, low height, are not particularly steep.
Their profiles much more like a symmetrical sine wave, and long wavelength
(say 500 m):
L = 500 meters
Wk 3. P.5
Swell waves exhibit an amazing lack of friction - they can travel across an
entire ocean (!)
Power not lost because they do not "peak" in deep water - (that is, water
deeper than wave base, so no energy loss due to friction of wave dragging
along or "feeling" the bottom)
IV. Waves approaching the shore
A. breaking and shape change
Things begin to change as the wave starts to "feel the bottom" at depths of
less than 1/2 L
Example problem - wave with L = 500 m approaches shore - at what depth
does it begin to "feel the bottom"?
Note Fig p. 73 - shows change in wave shape and in elongation of orbitals
into ellipses
Waves start to back up and get "squeezed" together like an accordian as they
slow down due to friction from the bottom
Note the sequence in the figure - as waves get squeezed, their
wavelength shortens and their height increases - recall formula for steepness
(S = H/L)
So H gets bigger, L gets smaller - what happens to S?
Davis makes point that period (time that it takes for wave to travel a
wavelength) stays the same - even though wavelength and height are
changing.
Another good point - note in Fig p.73 how the elliptical orbitals become very
flat at the shore - ultimately become a back-forth motion that you see in the
swash zone.
The breaking sequence is a function of bottom of wave slowing down more at
base than at top, so not only does it steepen, it begins to topple…
Still fast
Slowing down
Wk 3 p.6
Note that the wave typically re-forms as a smaller wave which may break
again before reaching the beach and losing all energy
There are several different types of waves, corresponding to diff combinations
of bottom slope and wavelength/steepness Fig p. 75 :
Shallow slope beach:
 Swell waves of long L produce a plunging breaker (large curl, rapid loss)
 Windwaves of shorter L produce a spilling breaker (longer loss)


Steep slope beach:
Swell waves on steep beach produce collapsing breaker (looks like
spilling)
Wind waves on steep beach produce surging breaker
Beach slope and other factors also influence other changes produced in waves:

Reflection (bounce back the energy)

Diffraction (spread the energy)

Refraction (bend the energy)
B. Reflection
If surface is hard, energy retained in bounce-back is close to 100% (Fig p. 76)
Steeper the beach, bluff, or structure, the higher the proportion of reflected
energy from that feature.
Typical beach reflects about 20 %.
Angle of incidence = angle of reflection (just basic physics)
i
r
Wk 3 p.7
C. Diffraction
"spreading of energy" along the wavefront (Fig p. 77)
mechanism is "lower waves propagated sideways"
D. Refraction
A bending of the wavefront if it approaches at an angle (which it almost
always does)…this causes the orthogonals (lines of force  to the wavecrest) to
either bunch together or diverge (fig p.79)
Notice the longterm consequences here - lots of energy focused on headlands,
trying to wear them down, while energy is spread out in bays, encouraging
deposition - headlands wear down, bays fill up
Tendency is for coast to try and straighten itself out over the long term, come into
equilibrium with the erosive and depositional properties at work
Other waves at work include:
V. Tsunami (seismic sea wave, aka "tidal wave") fig p.81
 "most destructive of all waves"
 triggered by major underwater events:
- earthquake
- volcanic eruption
- major landsliding
tsunami is Japanese for "harbor wave", because it is this setting where tsunami
is most dangerous
- energy that sets them in motion is very large,
- they travel incredibly fast (800 kph = 480 mph)
- wavelength can be huge, on order of 150 km
in deep water, wave height may be slight, only 1/2 meter, so not noticed in the
open ocean…
on continental shelf, wave will start to feel bottom, slow and steepen, perhaps as
far out as 50 km…
Wk 3 p. 8
when wave finally hits a coast, it can be 40 m (120 feet) high (as high as a rogue
wave at sea), with HUGE amt of energy
- tsunami in Awa, Japan killed 100,000
- many in Lisbon, Portugal eqk of 1755
recurring phenomenon is that sometimes trough arrives first, which pulls much of
the ocean out of the coastal zone, looking like a super-low tide…people may
have tendency to go out into the bay to see the strange phenomenon, then are in
really bad spot when the crest of the tsunami hits
other bad tsunamis include:
- 1883 Krakatoa eruption in Indonesia set off several tsunami, 40 m
high, swept over islands, including moving a ship 3 km inland
- 1946 Aleutian Islands eqk triggered tsunamis throughout the Pacific;
30 m waves in Alaska, a 15 m wave in Hawaii only 5 hours after the
quake
- 1964 Alaskan eqk, huge tsunami in Alaska
tsunami warning system now in place throughout Pacific - provides a few hours
of warning for potential victims
VI. Standing waves and Seiches
Standing wave doesn't move forward, but rather "sloshes" back and forth
Fig p. 82 good picture - central node acts as pivot, ends are the ones that go up
and down
These waves have low height, long wavelength
Atmospheric pressure diffs are often the catalyst, sometimes a tsunami will set
up the sloshing
Example: Lake Erie can have 6-foot differential from W end to E end
High baro pressure
Low baro pressure
node
Wk 3 p.9
Some interesting seiche accounts by Davis…
Great Lakes and some Swiss lakes are famous for seiches - Reversal of currents
- People swept off docks
- Sunbathers "swept off beaches of Lake Michigan"
VII. Longshore and rip currents
A. longshore currents - shore-parallel, set up by waves approaching at relatively
low angle to the coast:
breakers
wave crests
shoreperpendicular
total force
force
shore - parallel force
beach
Ls current typically sets up between shoreline and the breaker zone, acts like a
"shallow river channel"
- Typical speeds are 10-20 cm/sec, but can be as fast as 1m/sec (over 2
mph)
B. Ls currents work with wave energy to carry large amt of sand (aka "littoral
drift" because this is the littoral zone)
How does the process work? Sand is brought into suspension by wave
energy, longsh current drives the sediment-laden water
Longsh current can come from opposite directions, depending upon wind
direction at given instant in time…
But if a generally prevailing wind direction exists, net direction of transport will be
downwind
Example - Sandy Hook prevailing longsh drift direction is from ____ to ____.
What general direction do waves come from to set up this drift direction??
Wk 3 p.10
Trapping of sand carried by longsh current is commonly done on purpose (by
groins) or inadvertently by large jetties installed by US gov't to preserve shipping
channels)
jetties
longsh current
waves still
eroding
waves
Orig shoreline
New shoreline
"Massive sandbars, spits, and other accumulations attest to the carrying power of
longshore currents."
C.Rip currents - movement of water offshore, in a direction  to land
Cause: wave energy piles up water along shore, actually elevates the
local sea level nearshore to higher than that offshore.
This sets up hydraulic gradient (water flows from high head to low head), and
water tries to escape back to offshore area. Easiest way for this to occur is
through channels in nearshore sand bars (fig p.85)
Swimmers beware - swim parallel to shore to get out of current, but Don’t try to
Fight It - don't swim into it, swim across it to get out.
Note how rip current is a key element to a nearshore circulation cell that consists
of incoming wave energy, lonsh current, and rip current.
Wk 3 p.11
VIII. Tides
A. Complicated to visualize
Flood tide - rising water culminates in high tide
Ebb tide - falling water cuminates in low tide
Why do tides happen? Gravitational attraction of sun, moon, and Earth to each
other.
Gravity distorts the "liquid envelope" of the Earth (the ocean) quite
easily…visualize the Earth as spinning on it's axis inside what is essentially an
elliptical water jacket.
Example: look down on Earth from N pole, imagine a palm tree at Equator
spinning around once in a 24-hr period.
N
moon
Notice how twice the palm tree is drowned (high tide), twice it's fine (low tide)
Good Fig p.88
B. Tidal cycles
Tidal cycle is based upon lunar day, which is almost 25 hrs/day, not 24. This is
why high & low tide times shift every day, don't stay the same.
Also note Fig p.89 - spring and neap tides - these controlled by whether or not
moon and sun are lined up with Earth (spring), or if they are 90o to Earth (neap)
Wk 3 p.12
C. Travelling tides
Complications occur to the simple picture due to Coriolis effect, continental
blockage of tide energy, etc.
Irregularities combine to cause unique tidal character for for "each location on
Earth"
D. Tidal Range
"avg tidal range over open ocean is 1/2 m" (about 1.5 ft)
tidal crests move pretty quickly, 10-20 kph (6-12 mph)
funnel-shaped inlets cause tide to steepen and increase in height
Davis notes that theoretical prediction is pretty difficult, most common method is
to rely upon tidal gauge data collectied historically: first gauge developed by
Lord Kelvin in 1882. Pen attached to float draws line on slowly rotating drum to
Produce records like Fig p.93
As you'd expect from Fig p. 89 and fig p.94, biggest range form high to low tides
are the spring tides, smallest range are the neap tides
IX. Storm Surge
Very dangerous, large, temporary elevations of sea level due to low pressure
storm systems
Low pressure sort of "Sucks up" the sea surface like a huge vacuum cleaner…
Low
Ocean surface
As low moves inland, so does the bulged-upward sea surface, which floods lowlying coastal areas with tragic results
Wk 3 p.13
Some examples of devastating surges
Hugo, South Carolina, 1989 - 11 m (33 feet!) storm surge on top of a 3 m (9 ft)
spring tide
Devastating combo of high storm surge and high wind-driven waves by
hurricane-force winds - Galveston, TX, 1900 - 5m (15 ft) storm surge with 7 m
(21 ft) waves created a "tsunami-sized wave" that overran Galveston barrier
island and killed 5,000 Americans.
X. Tidal Currents
Currents are assoc with each cycle of flood and ebb - water moving onshore,
water moving offshore (Fig p. 99)
Current speed dictated by "tidal prism" - product of vertical tidal range and area
discharging
Rule of thumb - large range produces high currents (need to move water quickly),
But large bays produce them too, even with lower tidal range.
Final point - asymmetry to tidal current time-velocity curve - ebb tide takes longer
to empty a bay than the flood tide does to fill it…Davis suggests that this is due to
the fact that there is more volume to empty at ebb due to addition of fresh water
from river mouth coming offshore.