East Coast MARE Ocean Lecture May 16, 2012 - Surf's Up! All About Waves at the Coast
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Transcript of East Coast MARE Ocean Lecture May 16, 2012 - Surf's Up! All About Waves at the Coast
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Surfs Up!
All About Waves at
the CoastProf. Tom Herrington
Ocean Engineering Program Director
Stevens Institute of Technology
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Tonight’s Lecture
• What happens when waves reach the
coast
• Surf zone currents
• Wave & beach interactions
• Shoreline evolution
• Shoreline of the future
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Anatomy of a Wave
L
T
H
T
2
Wave Number: L
k2
Wave Phase Speed T
LC
h
(Always Constant for an individual wave)
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Wave Growth
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Wave Development Limit
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Fully developed seas
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Interesting Mathematical Wave Properties
• Wave are dispersive
(longer waves move
faster) period and length
are not independent!
• In deep water (h>L/2)
tanh(kh) ~ 1.0
• In shallow water (h<L/20)
tanh(kh) ~ kh
• Between deep and
shallow water must use
full equation
)tanh(2 khgk
L
h
Lg
T
2tanh
222
2
2gTLo 2
gT
T
LC
TghL ghC
L
h
Lg
T
2tanh
222
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-In Deep-Water -
Waves have Ideal
Shape and thus
Propagate Energy
but not Mass
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Deep and Shallow-Water Wave Regions
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Wave Motion and Particle MotionProgressive Waves
Waves which interact with the sea floor are known as shallow-water
waves. The orbits of the water molecules become elliptical.
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Wave Refraction
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Bending of Shallow-
Water Wave Fronts Due
to Change in Bottom
Depth. The Leading
Edge of a Wave Front
Enters Shallower Water
and Slows While the
Remaining Front
Continues at Higher
Speed. The Net Result is
a Rotation of Wave
Fronts To Become
Parallel with Bottom
Depth Contours.
Wave Refraction:
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Wave Focusing and Spreading
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Examples
Headland Focusing
Embayment wave spreading
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Wave train breaking
Really amazing rule of thumb
Breaking wave height can be estimated by: hHb 78.0
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Waves Break
by
Plunging and
Spilling
• depends on the
slope of the bottom
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Surf Similarity Parameter
• Ratio of slope steepness and local wave
steepness
ξ> 2.0 – surging/collapsing breaker
2.0 <ξ< 0.4 – plunging
ξ< 0.4 – spilling
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Plunging wavesPlunging Breaker at
Avon-by-the-Sea, NJ
Spilling Breaker at
Ocean City, NJ
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Surf Zone Physical Processes
• Currents in the surf zone are generated by the variation in Wave Height across surf zone and the angle at which the waves approach the coast.
• Generates both cross-shore and alongshore currents
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Cape May, NJoblique wave approach
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Flow in the Surf Zone is Very Complex!
From Svendsen and Lorenz (1989)
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Longuet-Higgens (1970)
Alongshore Current Forcing
bbbL gHSV cossin4.41
b
bH
g
S
: beach slope
: gravity
: breaking wave height
: wave angle at breaking
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Longshore Currents
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Cross-shore wave generated motion
Svendsen, et al. (1987)
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Cross-shore Current Forcing
dx
dgh
dx
dSR xx
Masselink and Black (1995)
Wave setup balances
the gradient in the
cross-shore directed
radiation stress
2
16
3gdHS xx
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What is a Rip Current?
• Narrow seaward
moving current of up
to 3 knots
• Related to mass
transfer of water
trough the breaker
zone
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Flow through gaps in the sandbar
system
Hass, et al. (2000)
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Rip Current Formation between Groins
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When do Rip Currents Occur?
…Extremely Large Wave Events…
April 18, 2003
Ocean City, NJ
44009: Hm0=13 ft, Tp =10 sec
Surfer/Lifeguard drowned at this beach
…Tropical Cyclone Swells…
Hurricane Fabian
Sept. 4, 2003
Surf City, LBI
44009: Hm0=5.75 ft, Tp=14 sec
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…Low energy Wave Events…
July 5, 2003
44009: Hm0=3 ft, Tp=6 sec
Over 100 rescues along NJ coast
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Analysis of Observed Rip Current Events
In New Jersey
• Revealed 2 conditions:
1. Extreme waves (> 8 ft)
with periods > 8 sec
2. Long-period swell of any
height
• Rate of wave energy
propagation to coast appears
to be important
nECP
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Rip Current Index
• In order to weight large swell
higher than wind waves and
smaller swell a Rip Current
Index (RI) that is the ratio of
swell energy flux to wind
wave energy flux multiplied by
the ratio of the wave height to
water depth appears
reasonable.
• Developed from wave buoy
data located 20 n.m. off
NJand Stevens Coastal
Monitoring Network
windwave
swell
P
P
h
HRI **1.0
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July 4th FabianIsabel
Juan
Evaluation of Rip Current Index Against known Events in 2003
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What about Notched Groins?
What are these things?
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Current and Sediment Measurements Near Notched
Groin
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-15
-15
-15
-10
-10
-5
-5
0
5
10
10
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Measured Alongshore Current through Notched
Groin
Rankin, et al. (2003)
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Waves as Agents of Coastal Change
• Shoreline Changes occur on many temporal scales:
– Seconds (Every wave)
– Daily (Tides)
– Seasonally (Changing wave climate)
– Decadal (Extreme storms)
– Centuries (Sea level changes)
– Millennia (Global climate changes)
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Seasonal Coastal Changes
• The cross-shore extent of the beach undergoes erosion and accretion on a seasonal basis
– In the summer and fall, small waves transport sand up onto the beach
– In the winter and spring, large storm waves erode sand
– Transition provides natural protection for the beach.
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How Do We Know This?
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Seasonal Beach Changes
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We even know where motion stops…
Depth of ClosureHallermeier (1981) and Birkemeier (1985)
2
2
9.5775.1e
eec
gT
HHh
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…and which way the sand is moving!
3
00070.0
Tw
H
L
Hcritical
s
Profile Transitions between eroded
(“bar”) profile and accreted (”berm”)
profile on a seasonal basis
Kraus and Larson (1988)
3
00070.0
Tw
H
L
Hcritical
s
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Episodic Change:
Driven by 3 Components1. Extreme astronomical tides
2. Storm surge generated by intense storms or prolonged onshore
winds can generate significant departures from predicted water elevations and wave attack high up on beach berm.
3. Large waves generate mass transport of water toward coast,
increasing flood levels and extent of wave attack.
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Prolonged storm surge at Atlantic
City during March 1962 Nor’easter
Storm Evolution March 5 – 8, 1962Water Level recorded at Atlantic City Steel Pier
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Large-scale Damage
Ocean City, NJ. Note the transport of water across the island by waves
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How can we predict potential storm damage?
Not Easily!
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Results
Not so conclusive
• Longer storm
duration more
important than
wave height and
surge unless…
• If duration is
about equal
wave heights
dominate
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Wave Climate Long Term Coastal Change
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How do we know this?
• Wave Hindcasts based
on historic weather data
• Offshore Buoy Data
• Neashore Wave Data
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Long Term Wave ClimateWave Height Distribution by Month (1980 - 1999)
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
1 2 3 4 5 6 7 8 9 10 11 12
Month of Year
Wave H
eig
ht (ft)
127
130
135
138
140
145
Pecent Occurrence of Wave Direction November (1980 - 1999)
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 45 90 135 180 225 270 315 360
Direction (deg N)
Occurr
ence (
%)
127
130
135
138
140
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We can use this to our advantage!
WAVE DIRECTION & PERCENT OCCURRENCE
Long Branch, NJ
0
5
10
15
20
25
30
35
40
0 45 90 135 180
Direction (deg N)
Occurr
ence (
%)
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Long Branch Feeder Beach
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Shoreline
Evolution
• Elevation Change
• Offshore contours are becoming parallel to shoreline
• Deflation of feeder feature
• 100,000 cu.yds. Per year transport to north
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Long Term Change
(Sea-level Rise)
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Recent Sea Level Changes
• 18,000 years ago, at the
height of the last
glaciation, sea level
was 130 m lower than
today.
• Sea level continues to
rise by about 1 foot per
century in New York
City.
• A rise in sea level of up
to a meter is predicted
for the coming century.
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Global Temp and Sea level changes for past 18,000 yearsbased on radiometric age dating of corals
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Modern Record Sea level rise about 30 cm/centuryspeculation: greenhouse gas>>global warming >>melting ice
3.8 mm/yr in New Jersey
NJ Tide Gauge Records
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Rising 5-10mm
Rising 1-3mm
Rising 1-3mm
Rising 3-5mm
Falling1-3mm
Falling 5+mm
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Glacial Rebound
Tectonic Factors Affecting
Sea Level
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North American Glacial
Rebound
Rebound occurs much more
slowly than ice melting.
Even though the ice has
been gone for 10,000
years, North America is still
rebounding at 1 to 2 mm/yr.
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Projected sea-level rise for the 21st century. The projected range of global-averaged sea-level rise from
the IPCC (2001) assessment report for the period 1990–2100 is shown by the lines and shading (the
dark shading is the model average envelope for the range of greenhouse gas scenarios considered, the
light shading is the envelope for all models and for the range of scenarios, and the outer lines include
an allowance for an additional land-ice uncertainty). The AR4 IPCC projections (90% confidence limits)
made in 2007 are shown by the bars plotted at 2095, the magenta bar is the range of model projections
and the red bar is the extended range to allow for the potential but poorly quantified additional
contribution from a dynamic response of the Greenland and Antarctic Ice Sheets to global warming.
What does the future hold?
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Considerable Uncertainty
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What does this mean for the Coast?
StormMeas. Elev.1Surge2012221003
Sept. 1944 8.96 ft 4.17 ft 9.81 ft 11.43 ft
March 1962 8.80 ft 3.43 ft 9.42 ft 11.05 ft
Dec. 1992 9.14 ft 4.28 ft 9.39 ft 11.01 ft
Oct. 1991 8.93 ft 4.48 ft 9.19 ft 10.81 ft
1. Relative to MLLW at Atlantic City
2. Adjusted for historic sea level rise of 3.8 mm/yr
3. Adjusted for IPPC Max Sea Level Rise Projection of 1.9 ft by 2100
Adjusted Flood Levels at Atlantic City
INCREASED FLOOD LEVELS
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What does this mean for the Coast?
1990
2100
Change in MHHW Line
80 – 100 ft
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Food for Thought ?
Changes in the speed that ice travels in more than 200 outlet glaciers indicates
that Greenland's contribution to rising sea level in the 21st century could be
significantly less than the upper limits some scientists thought possible.
The finding comes from a paper funded by the National Science Foundation
(NSF) and NASA and published in today's journal Science.
May 4, 2012
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Useful Data Sources:
Waves: http://www.ndbc.noaa.gov/
Tides: http://tidesandcurrents.noaa.gov/
Stevens: http://hudson.dl.stevens-tech.edu/maritimeforecast/
Stevens Storm Surge: http://hudson.dl.stevens-tech.edu/SSWS/
NJ Beach Data: http://www.gannet.stockton.edu/crc/index.asp
Thank you