Ivan Metocean Overview Ivan Characteristics

Ivan Metocean Overview
n
n
Focus on deep water for now
Agenda
– Ivan Wind/Wave Hindcast
– Current Hindcast
– Wave/Wind measurements
– Historical perspective
– NWS Wind Forecasting
n
Each talk followed by 55-min
questions
Ivan Characteristics
Ivan…
–
–
–
–
–
Category 33-4 Hurricane
Central pressure 939 mb
Radius=20Radius=20-30 nm
Max Wave Hmax~96 ft
Wind=92 kt (33 ft, 30 min)
API/RPAPI/RP-2A 100100-year…
– d/w Hmax=71.2 ft
– Wind=87 kt (33 ft, 30 min)
1
Hindcast Methodology
Modeling done by OWI
n Basic Steps
n
– specify storm parameters
(time history of pressure, etc.)
– Run wind model to determine
wind field every 30 minutes
– Use modeled winds to drive
wave & surge models
– Validate against site
measurements
Wind & Wave Comparison
NDBC Buoy 42040
2
Wind & Wave Comparisons
at Marlin TLP
Wave & Wind Hindcast
Summary
n
n
n
n
n
Methods & models (Gumshoe)
same as used for API RP2A.
Excellent comparisons with Ivan
measurements at buoys &
platforms
Gumshoe model works for Ivan
Hmax~96 ft; W max=92 kt (33’, 30 min)
RP2A 100100-yr:
Hmax=71 ft; W=87 kt (33’,30 min)
3
Ivan Current Hindcast
n
n
n
Review Hurricane
Currents
Hindcast Currents
from Ivan
Design Implications?
Hurricane Current
Hurricane Current:
n Generated by local wind
stress
n Strongest on right side in
DW (10’
s of km wide)
n Current peaks within 11-3
hours of max wind
n Strong inertial
component persists 33-4
days
Wind Stress
0
Current
Mixed
Layer
Momentum Transfer
-50
De pth (m )
-100
Density Stratification Limits
Momentum Transfer
-150
0
50
100
150
200
250
U (c m/s e c)
300
350
400
Hurricane Current
4
Hurricane-Loop Interaction
n
n
Varying temperature
and salinity profile
has strong influence
on hurricane current
Joint hurricanehurricane-Loop
load cases likely
important for southern
DW areas
0
-20
-40
-60
-80
-100
-120(m )
De pth
-140
-160
-180
-200
Background TS
Loop TS
0
50
100
150
U (cm/s e c)
200
250
300
Effect of Different Temperature and Salinity on
Hurricane Current profile
0.0
120
140
0
20
40
60
80 100
Time (hours)
120
140
0.0
60
80 100
Time (hours)
-4.0 -3.0 -2.0 -1.0
Vy (m/sec)
Vx (m/sec)
0.0 1.0
-1.0
40
1.0
-1.5 -1.0 -0.5
Vy (m/sec)
-2.0
-2.0
20
Vx (m/sec)
0.0
0.5
n
0
0.5
n
Current hindcast
ability not as
developed as that for
winds, waves
Little data to compare
against, no profile
data above 30 m
Bulk mixedmixed-layer
model does good job
in 5 of 6 comparisons
with ML averages
-1.0
n
2.0
Hindcasting Ability…
ML Model Compared to Measurements, Hurricane
Frederic (Sept. 1979)
5
Hindcasting Ability
n
n
n
n
Recent data shows
substantial shear in
mixed layer
M-Y 1D profile
model compares
well in DW
Bathymetry needed
around shelf/slope
Models are very
sensitive to inputs
Hindcas t Curre nts , Hurricane Lili
0
-20
-40
-60
-80
-100
De pth -120
(m )
-140
-160
ADCP
ML
M-Y 1D
HYCOM
-180
-200
0
50
100
U (cm/se c)
150
200
Profiles Near Time of Peak Current Near Genesis
Ivan Hindcasts
n
n
n
n
Commercial hindcast
available with HYCOM
Preliminary comparison
on slope with Navy
data shows reasonable
agreement
Bulk ML, MM-Y 1D profile
analyses also
performed
No DW current data for
validation
Snapshot of Ivan HYCOM Currents
6
Model Comparisons for Ivan in
DW
n
n
MixedMixed-layer depth
and average speed
from Bulk ML, MM-Y
1D profile models
similar
HYCOM mixedmixedlayer average
speed is similar,
but profile and ML
depth are
questionable
Hindcas t Curre nts , Hurricane Ivan
0
-20
-40
-60
-80
-100
-120(m )
De pth
-140
-160
ML
M-Y 1D
HYCOM
-180
-200
0
50
100
150
U (cm/se c)
200
250
300
Profiles Near Time of Peak Current in DW
Model Comparisons Continued
n
n
Bulk ML, MM-Y 1D
profile model predict
similar currents
across storm track
M-Y 1D predicts
higher mean speeds
on 100 m
HYCOM result
includes Ulysses
eddy currents, so
not shown
Comparing ML and M-Y Models for Ivan at Fixed Latitude
250
M-Y
ML
200
Mean U, 100 m (cm/sec)
n
150
100
50
0
-90.5
-90.0
-89.5
-89.0
-88.5
-88.0
-87.5
-87.0
-86.5
-86.0
-85.5
Longitude
7
Summary of Currents
n
n
n
n
Ivan model efforts hampered by
lack of data for validation
Bulk ML, MM-Y 1D profile models
with limited prior validation yield
similar results for Ivan in DW
HYCOM results in DW are
questionable – need to
investigate
M-Y 11-D profile model should be
used to derive criteria for shallow
draft platforms (Bulk ML model
suitable for spars)
Industry Site Measurements
at Marlin and Medusa
n
Marlin TLP
– Wind at top of crane
– Wave radars on SE
(noisy) & SW sides
– High sampling rates
n
Marlin
Medusa Spar
– Wave radars on SE
(noisy) & NW sides
– High sampling rates
Medusa
8
Wind Spectrum at Marlin
4102.2
2.0 m/skts
R e feat
re 170
nce Wind,
5 1.8kts
m Ele
ft, 81.5
atvatio
33 nft
17 :3 0 - 1 8:30
34.8
83.5m/s
ktsReference
at 173 ft,
Wind,
67.5
52.7
kts
m at
Elevation
33 ft
15:30 - 16:30
3500
1500
Energy Density, m2/Hz-s 2
NPD
API, lo freq
API, mid fre q
API, hi freq
Iva n at Ma rlin
2000
1000
500
0
0.001
0.01
0.1
NP D
3000
AP I, lo fre q
2500
AP I, mid fre q
2000
AP I, hi fre q
Iva n a t Ma rlin
1500
1000
500
0
0.001
1
0.01
0.1
1
Fre q ue n c y , Hz
Fre que ncy, Hz
1-sec gust factor = 1.36
1-min gust factor = 1.15
1-sec gust factor = 1.34
1-min gust factor = 1.14
•Gust factors agree reasonably well with NPD
•Earlier spectrum agrees with NPD model but later
spectrum is deficient in very low frequency energy
Wave Time Series at Marlin
Ivan Waves at Marlin
100.00
Hmax=86.3 ft
90.00
80.00
height (feet)
2/Hz-s 2
Energy
EnergyDensity,
Density,mm2/Hz-s2
2500
Hs,max=50.6 ft
70.00
60.00
Hs
50.00
Hmax
40.00
Cmax
30.00
20.00
10.00
0.00
0
5
10
15
20
25
time (hours) on 15 Sep
9
Wave Time Series at Marlin
16:30 - 17:30 on 15 Sep
Ivan Waves at Marlin, 16:30 - 17: 30 on 15 Sep
wave elevation (ft)
60
40
20
0
-20
-40
-60
0
600
1200
1800
2400
3000
3600
time (sec)
60
60
40
40
20
20
0
0
-20
-20
-40
-40
-60
700
seconds
H = 73.5’
800
-60
1100
seconds
1200
H = 86.3’
Wave Height Distribution
at Marlin
10
Wave Height Distribution
at Medusa
Platform Damage at
Petronius and Pompano
Petronius
Pompano
11
Petronius Platform Damage
Damage at 54.1’–57.4’above storm
water level (after accounting for 2.6’
storm surge, tide, and setdown)
Hs=51.1’
Tp=15.4s
Forristall distribution integrated over entire storm
100%
percentile
80%
60%
hindcast
1.03 * hindcast
40%
damage height
above SWL
20%
0%
40
50
60
70
Corresponds to 60th –80th
(40th –70th) percentile
estimate of maximum
crest from Ivan hindcast
max crest (fee t above SWL)
Pompano Platform Damage
Damage at 54.1’–61.3’above storm
water level (after accounting for 1.6’
storm surge and tide)
Forristall distribution integrated over entire storm
100%
percentile
80%
60%
hindcast
40%
1.03 * hindcast
20%
Damage height
above SWL
0%
40
50
60
70
max crest (m above SWL)
Hs=45.4’
Tp=16.8s
60o
Hs=29.2’
Tp=10.2s
Hs=34.9’
Tp=16.8s
Corresponds to 95th –99.9th (90th –
99th) percentile estimate of maximum
crest from Ivan hindcast
Unlucky? Crossing wave trains?
Wave enhancement by platform?
12
Wave / Platform Interaction
80’wave at Ekofisk
Model tests
What is the height of the undisturbed wave crest (green water)
that might be inferred from local platform damage?
Measurement Summary
n
n
n
n
Wind spectra fit standards
No evidence of “freak”
(rogue) waves
Distributions of measured
wave crests fit design
standards
Damage provides no
compelling evidence for
criteria change
13
Ivan Characteristics
Ivan…
–
–
–
–
–
Pressure 939 mb (93%)
Radius=25 nm (25%)
Forward Spd=10
Spd=10 kt (50%)
Wind=92 kt (33 ft, 30 min)
Max Wave Hmax~96 ft
API/RPAPI/RP-2A 100100-year…
– d/w Hmax=71.2 ft
– Wind=87 kt (33 ft, 30 min)
Key Questions
n
n
n
What return interval was Ivan?
Was Ivan statistically
“unexpected”?
Should criteria be increased &
if so in what part of Gulf?
Satellite image of Ivan
14
Ivan’
s Return Interval?
Site-to-Site Variability
n
n
Model hindcast
(GUMSHOE) gives large
sitesite-toto-site variability
Causes of variability
1. Water depth & fetch
2. Insufficient sample of
severe storms
3. Regional differences
100100- yr
96°
90°
85°
80°
100100- yr & max Hs along the 600 ft isobath
based on Gumshoe site hindcast
Insufficient Sample of
Severe Storms
14 Jan 2005
60
50
Weibull fit: k = 1.20
30
Hs (ft)
Hs (ft)
40
40
20
10
0
-60
20
-40
-20
0
20
Dist from center (miles)
40
60
Parametric model crosscross-section of the Hs in
Hurricane Camille.
10
20
40
60
Return Period
100
200
Extremal fit for site near maximum of
Camille. W/o Camille, Hs100 is 4 ft lower.
Given small size of storms & infrequent occurrence, we need several
several
hundred years of data to sufficiently reduce this “noise”
15
Regional Variations
n
Recent work suggests real
regional variations
– Severity
– Frequency
n
Severity Contours
Possible physical causes
– Persistent Loop water
– Gulf geometry
– Atmospheric influences
Storm frequency contours
Removing the “Noise”
n
n
n
10-4 JIP is addressing issue
Solution 1: combine (pool)
sites that are similar but
not identical sites
Solution 2: develop a
deductive model
Uncertainty of nn-yr Hs
Model
Gumbel
Site
Gumbel
Pooled
100-yr
±2.5’
10k-yr
±5.0’
±1.6’
±2.0’
Pooling reduces uncertainty
16
Choosing the Optimal
Pooling Size
n
n
n
n
Apply “crosscross-validation”
(Chouinard,1992, OTC)
Optimal dist. ~ 100 miles
1010-4 JIP has found
similar results
Results that follow use
pooling at 55-7 sites
Will also use this 100100-mi
scale in another key way
Cross-Validated Square Error for Rate Estimation
(for omni-directional > 15mb rate)
0.2112
0.211
0.2108
CVSE
n
0.2106
0.2104
0.2102
0
50
100
150
200
250
300
Sigma(distance) [km]
What Return Interval
Was Ivan?
n
n
~2500 yr Hs at site
where peak occurred
Exceeded Hs100 over
~150 mile swath
Ivan peak vs Gumshoe N-yr Hs along 28.25°N
17
How Does Wind Compare?
n
n
~700~700-yr Wind Spd
(33 ft, 30 min)
Exceeded 100100-yr
over ~60 mile swath
Ivan peak vs Gumshoe N-yr Wind along 28.25°N
How Current Compare?
Ivan P e ak Curre nt vs. N-Ye ar Value s at Fixed Latitude
n
n
n
~700~700-yr 5050-m
Exceeded 100100-yr
over ~ 20 mi swath
Storm that causes
100100-yr Hs does not
usually cause 100100yr wind or current
400
Ivan
R=100
R=1000
350
300
250
200
150
100
U nifo50
rm C u rre n t o n U p pe r 5 0 m (c m /s e c )
0
-90
-89.5
-89
-88.5
-88 -87.5
Longitude
-87
-86.5
-86
-85.5
Ivan peak vs N-yr Layered Model along 28.25°N
18
Was Ivan Statistically
Unexpected?
n
n
n
Intuition: expect one, 100100-yr
storm in 100 yrs in entire Gulf
Fact: expect Hs100 exceeded
somewhere in Gulf every 4 yrs
Because… …
– Must treat Gulf as statistically
independent regions
– Assume “regions”in Gulf are
100 mi apart
– Expect a 25002500-yr Hs in 100 yrs
– That sounds like Ivan!
25 sites, ~ 100 mi apart
Ivan not a major surprise based on prepre-Ivan distribution
Metocean Summary
1.
2.
3.
4.
5.
6.
7.
8.
Ivan generated peak Hmax~ 96 ft
Highest in 100 yrs but not by much
Ivan generated ~2500~2500-yr Hs using the
prepre-Ivan extremal distribution
Ivan peak wind & current ~ 700700-yr event
Could argue Ivan is an “outlier”
But new designs in Eastern Gulf should
include Ivan
Under peak of Ivan, a d/w facility could
have seen wave loads ~30% higher
then present design but still << then
100% factor of safety
Further work … .
a. Look at metocean in shallow sites
b. Review API metocean guidelines
c. Obtain more upper waterwater-column
currents
19
HURRICANE FORECASTING
Frances
Charley
Jeanne
Ivan
2005
2005 Season
Season
Forecast
Forecast
Dr
DrGray
GrayForecast…
Forecast…
NOAA
NOAAFORECAST
FORECAST
15
15Named
NamedStorms
Storms
12-15 Named
12
12-15
NamedStorms
Storms
88 Hurricanes
Hurricanes
66-9
-9 Hurricanes
Hurricanes
44 Intense
Intensehurricanes
hurricanes
33-5
-5 Major
MajorHurricanes
Hurricanes
Factors supporting an active hurricane season
•Warmer than normal sea surface
temperature
•“La Nada”
•MultiMulti-decadal Atlantic signal
20
2005 HURRICANE
SEASON
2005 SEASON
7 Tropical Storms
n 2 Major Hurricanes (Dennis & Emily)
n
21
22
MAJOR HURRICANES
5 YEAR AVERAGE
23
Errors cut in half in 15 years
24
Hurricane Charley Minimum Central Pressure
9 - 14 August 2004
1020
1010
Pressure (mb)
1000
990
980
970
BEST TRACK
960
Sat (TAFB)
Sat (SAB)
950
Sat (AFWA)
Obj T-Num
940
AC (sfc)
Surface
930
8/9
8/10
Charley deepened from
964 mb to 941 mb
in 4 h 35 min near
landfall –NIGHTMARE!
8/11
8/12
8/13
8/14
8/15
8/16
Date (Month/Day)
25
FORECAST
IMPROVEMENT
Better Observations
n Improved Computer Models
n
How Do We Track A Hurricane?
n
Satellite Imagery
GOES East and Goes West
Visual, IR, WV
Every 1515-30 minutes (rapid update for research)
Used to determine location, motion, and intensity
n
Aircraft Reconnaissance
USAF CC-130 - Primary Mission Operations
NOAA PP-3 - Primary Mission Research
NOAA GG-IV – High Altitude Operations
More accurate than satellite
n
Doppler Radar
250 nm range for reflectivity tracking
125 nm range for Doppler velocity estimates
Location, wind, motion, rainfall estimates and tornado detection
26
High Resolution Vis
RECONNAISSANCE FLIGHT PATH
Aircraft “ALPHA”
Pattern
27
NOAA G-IV AIRCRAFT
28
TRACKING AND FORECASTING
HURRICANES IN 2004
Frances
Charley
Jeanne
Ivan
2004 Track Guidance (1st Tier)
29
2004 Track Guidance (1st Tier)
Intensity Forecasts
30
GFS TRACK FORECASTS FOR IVAN FROM 9/7/04 12Z –9/11/04 12Z HAD A
SIGNIFICANT RIGHT BIAS.
31
GFS TRACK FORECASTS FOR IVAN FROM 9/13/04 12Z –9/15/04 12Z WERE
EXCELLENT IN SPECIFYING IVAN’S LANDFALL LOCATION ON GULF COAST.
IVAN
CHARACTERISTICS
Typical Cape Verde Storm
n Southern Most Major Hurricane
n Reached Category 5 Three Different
Times
n Was a Category 5 for over 30
consecutive hours.
n Weakened and made landfall as Cat 3
n
32
IVAN TRACK
FORECAST ERRORS
# HOURS
12
24
36
48
72
96
120
NHC
24
47
79
108
161
222
289
FSSE
21
38
58
81
126
171
199
10 YR AV
44
78
112
146
217
248
319
33
WIND PROBABILITY
PRODUCT
Experimental Product in 2005
n Available on NHC Homepage
n Graphical and Text
n Could become Operational in 2006
n
34
35
n Gene
Hafele
n Warning and Coordination
Meteorologist
n gene.hafele@noaa.gov
n 281-337-5074 x 223
36
HURRICANE-INDUCED
SEAFLOOR FAILURES
IN THE
MISSISSIPPI DELTA
by
James R. Hooper, FugroMcClelland Marine Geosciences
&
Joseph N. Suhayda, Consulting
Oceanographer
HURRICANE-INDUCED
SEAFLOOR FAILURES
IN THE MISSISSIPPI DELTA
Presented to the
2005 Offshore Hurricane Readiness and Recovery Conference
American Petroleum Institute
Houston, Texas
July 26-27, 2005
James R. Hooper, P.E.
Senior Consultant
Fugro-McClelland Marine Geosciences, Inc.
1
Mississippi River
New Orleans
Mississippi River
Delta
THE DELTA FRONT
SEAFLOOR HAS A BAD
REPUTATION.
CONSIDER SHELL’
S NEW
SP-70B PLATFORM
SHORTLY AFTER
INSTALLATION IN 1969.
2
SP-7OB
Water
Depth
~300 ft
Seafloor
Seafloor
16 Piles
Driven to
~400 ft
SP-70
PLATFORM
1969
SP-7OB
Seafloor
Hurricane Camille Waves
Seafloor
Failure Zone
~100 ft
Seafloor sliding
SP-70
SEAFLOOR
FAILURE
3
SP-7OB
Hurricane Camille Waves
Seafloor
Seafloor
Soil Force Against
16 Piles & 24 Well Conductors
SP-70
SOIL FORCES
SP-7OB
Seafloor
Hurricane Camille Waves
Piles Fail at
Base of Jacket
Seafloor
Soil Motion
SP-70
SOIL FORCES
4
SP-70
PLATFORM
FAILURE
1969
MAJOR HURRICANE
HISTORY
•9 major hurricanes (Category 3 or
greater) passed near the delta
1900 - 2004.
•Camille passed over the delta with
a central pressure of ~909 mb.
•Ivan passed east of the delta with a
central pressure of ~935 mb.
5
Mobile, Alabama
New Orleans
IVAN, 2004
Mississippi River
Delta
CAMILLE, 1969
HURRICANE CAMILLE
•South Pass 70 landslides caused the
seafloor to move downslope by more
than 3000 ft in some areas.
•One new 24-well platform toppled and
two others were badly damaged by
seafloor landslide failures.
•Pipelines were also damaged and
destroyed by landslides.
6
HURRICANE IVAN 2004
•Delta-front landslides during Ivan were
similar in size & character to those
experienced during Camille.
•One platform in ~480 ft water depth
toppled by landslides (MC-20).
•Pipelines were also damaged and
destroyed by landslides.
Path
o
f Iva
n’s C
e
nter
Western Limit of
Hurricane Force
Winds
From Fugro Chance, Inc.
Region of delta
front discussed
during this talk.
STUDY REGION PLATFORMS
AND PIPELINES
7
WHY IS THE DELTA FRONT
PRONE TO SEAFLOOR
FAILURES SO DESTRUCTIVE
TO PLATFORMS AND
PIPELINES ?
Pass
A’Loutre
Head of Passes
THE
BIRDSFOOT
DELTA
we
ut
h
ss
Pa
So
h
ut
So
st
Pa
ss
NE Pass
SE Pass
8
~1300-1400
1840
2005
18
m
i
1684
1684
1684
1840
1840
2005
2005
GROWTH OF THE DELTA
Consider this
Profile line
across the
seafloor
GROWTH OF THE DELTA FRONT
9
From Coleman et al, 1980
So
ut
hw
es
tP
as
s
So
ut
h
Pa
ss
Pr
of
ile
lin
e
1874 DELTA
FRONT BATHYMETRY
1979 BATHYMETRY
MAP
From Coleman et al, 1980
So
ut
hw
es
tP
as
s
So
ut
h
Pa
ss
Mudflow
lobes
1979 BATHYMETRY
MAP
1979 DELTA
FRONT BATHYMETRY
10
Depth Below Sea Level, Feet
Mudlobe Advance = 2.7 mi
0
1979
Mudlobe in 1874
Mudlobe in 1979
200
1874
400
Pre-Delta
(600-700 YBP)
600
800
0
2
4
6
8
10
12
Distance from South Pass, Miles
SEAWARD ADVANCE OF THE DELTA
MUDFLOWS MOVE THE
DELTA FRONT SEAWARD
11
DEPOSITION IN THE DELTA
•Deposition rates 1-2 ft/yr in front of the
major passes.
•Sediments primarily low permeability
clay with silt.
•Results in weak strength profiles more
than 300-ft thick, and numerous
landslides that create a unique seafloor.
From Coleman et al, 1980
Region of
Mudflow Gullies
Region of
Mudflow Lobes
SEAFLOOR GEOMORPHOLOGY
12
Mudflow Gullies
or Channels
SEAFLOOR
GEOMORPHOLOGY
6 –10 miles
Mudflow
Lobes
From Coleman et al, 1980
Slump
Failure
at Head
of Gully
Mudflows
in the
Gully
Mudflow
Lobe at
End of
Gully
From Coleman et al, 1980
ELONGATE LANDSLIDES
MUDFLOW ORIGIN, SEDIMENT
TRANSPORT, AND DEPOSITION
13
MUDFLOW GULLIES
•Mobile sediment typically 40 to 80
feet thick.
•Gully lengths up to 6 miles.
•Major mudflow activity occurs
several times a year in some
gullies, and every few years in
others.
Newest
Mudflows
Discharging
from Gullies
Older Mudflows
Newer Mudflows
SEDIMENT STRENGTH
PROFILES
STACKED MUDFLOW ORIGIN
14
MUDFLOW LOBES
•Individual flow thickness ranges
from a few feet to ~50 feet.
•Stacked mudflows are more than
100-ft thick in some areas.
•Mudflow lobes tend to remain
stable until triggered by large storm
waves.
Mudflows
in Gullies
New Mudflow
Lobes & Lobe
Failures
Platform &
Pipeline Failures
From Fugro Chance, Inc.
MAP OF SEAFLOOR INSTABILITY FEATURES
OFFSHORE SOUTH PASS AREA
STUDY REGION
15
From Coleman, et al, 1980
6
Mudlobes
200
4
400
2
600
0
10,000
20,000
30,000
0
40,000
Seafloor Gradient, Percent
Depth Below Sea Level, Feet
0
Distance from Arbitrary Origin, Feet
SEAFLOOR PROFILE IN
SOUTH PASS AREA
SW
25 ft
NE
410 ft
Single flow units
~70 ft Stacked Mudflows
Pre-Delta Parallel Layers
STACKED MUDFLOW EXAMPLE
16
Depth Below Sea Level, Feet
Mudflow
Gully
Transport
Zone
0
Slow
accumulation
zone
200
Overflow Zone
400
600
0
10,000
20,000
30,000
40,000
Distance from Arbitrary Origin, Feet
THE FATE OF MUDFLOW
SEDIMENTS
WAVE-TRIGGERED
SEAFLOOR FAILURES
17
Of course, the wave is moving
Pressure
increase
seafloor
Pressure
Decrease
So the seafloor pressure
is also moving
WAVE-INDUCED
PRESSURE ON SEAFLOOR
Wave
Crest
Wave
Trough
Upward
Motion
Downward
Motion
seafloor
Lateral
Motion
SEAFLOOR MOTION
18
CYCLIC WAVE-BOTTOM
PRESSURES
•Cyclic pressure & sediment motion
beneath the waves gradually
weakens the sediment.
•Weak sediment in the gullies slowly
moves downslope during a storm.
•Cyclic pressures may cause slope
failure of some mudflow lobes.
SEDIMENT STRENGTH
19
Depth Below Sea Level, Feet
0
0
Strength, KSF
2
4
6
20% of
typical
strength
Typical
strength
in
Gulf of
Mexico
200
400
DELTA FRONT
STRENGTH
PROFILES
600
0
0
0.2
0.4
0.6
0.8
Soil Strength, KSF
Depth Below Seafloor, Feet
20
B1
40
B4
60
80
B3
B2
STRENGTH
VARIATION IN
A SINGLE
MUDFLOW
LOBE
100
120
20
THIS STRENGTH
VARIABILITY IS
TYPICAL OF MUDFLOW
LOBES
Soil Strength, KSF
Depth Below Seafloor, Feet
0
0
0.2 0.4 0.6 0.8 1.0
Upper
Bound
50
100
STRENGTH
PROFILES FOR
SLOPE
STABILITY
150
Lower
Bound
200
21
Depth Below Sea Level, Feet
STABILITY ANALYSES
0
200
Mudflow Lobe 300 ft WD
Gully Region
400
Mudflow Lobe
400 ft WD
600
0
10,000
20,000
30,000
40,000
Distance from Arbitrary Origin, Feet
TYPICAL SEAFLOOR PROFILE
22
RESULTS OF STABILITY
ANALYSES
•Moderate-size waves can fail the
sediments in mudflow gullies.
•Mudflow Lobes with Upper-Bound
strength profiles appear to be
stable during intense hurricanes.
•Mudflow Lobes with Lower-Bound
strength profiles appear to fail
during intense hurricanes.
SUMMARY
1. Sediments accumulate in shallow
water and, over time, move
downslope in gullies as mudflows.
2. These gully mudflows are
triggered by waves typical of small
to large hurricanes.
23
SUMMARY (cont.)
3. Intense hurricanes (e.g., Ivan and
Camille) create large waves causing
large pressures on the seafloor.
4. During Ivan/Camille-size storms,
existing mudflow lobes may fail, and
mudflows from gullies may overrun
previously deposited mudflow lobes.
SUMMARY (cont.)
5. Large-scale seafloor failures are
the primary geologic process for
seaward growth of the delta.
6. Past rates of seaward growth of
the delta front will likely be
maintained, and seafloor failures
will continue to occur.
24