Toward a Sediment Transport Model of the
Louisiana / Texas Shelf
Kehui (Kevin) Xu1, Courtney Harris1,
Robert Hetland2 , James Kaihatu2
1 Virginia Institute of
Marine Science
2 Texas A&M University
(Background Image from NASA Visible Earth)
Questions
1. Does the sediment from Mississippi and Atchafalaya
Rivers mix with each other or not?
2. How is sediment transported and resuspended by
wind-driven currents?
3. How does flocculation impact sediment dispersal on
the shelf?
4. How do waves resuspend sediment?
5. Linking this hydrodynamic-sediment model to
MCH2 and a biogeochemical model.
The Model
Regional Ocean Modeling System (ROMS)
Parallel model using a 16-cpu cluster
Model Domain: ~800km × 300 km; Grid: 128 × 64 × 20
Spatially uniform but temporally variable (hourly) wind
No tide (tidal range <0.5m), No wave (forthcoming)
30.5
(70%)
(30%)
30
Atchafalaya River
Mississippi River
Atch. Bay
latitude
29.5
29
20m
28.5
50m
100m
28
300m
Wind Station
27.5
-94
-93
-92
-91
longitude
-90
-89
-88
Wind, Water and Sediment Discharges in 1993
wind speed (m/s)
30
‘Storm of Century’
20
10
Sediment Discharge (tons/day)
Water Discharge (m 3/day)
0
01/01
04/01
07/01
10/01
9
3
x 10
Mississippi
Atchafalaya
2
1
0
01/01
04/01
07/01
10/01
01/01
5
15
x 10
Mississippi
Atchafalaya
10
5
0
01/01
04/01
07/01
Day of year 1993
10/01
(USGS data from Charles Demas and Bob Meade)
Sediment properties
Sediment
Type
τcr (Pa)
Ws (mm/s)
Fraction
Mississippi
Flocculated
0.08
1
80%
Unflocculated
0.03
0.1
20%
Flocculated
0.08
1
80%
Unflocculated
0.03
0.1
20%
Sand
0.12
10
Mud
0.10
1
Spatially
Variable
Atchafalaya
Sea bed
Sediment type, mud%
30.5
100
Muddy
30
80
latitude
29.5
60
20m
29
40
50m
28.5
100m
300m
28
27.5
-94
-93
20
-92
-91
longitude
-90
-89
-88
US Seabed Data from Jeffress Williams (USGS) and Chris Jenkins (INSTAAR)
0
Sandy
Results
Salinity,
Mean
Current,
Wind
Miss. &
Atch.
sediment
dispersal
Near-bottom
Current,
Resusp.
from sea bed
Results
White line is 30 psu isohaline
A
A
B
B’
B
B’
A’
A’
A
A’
A
A’
B
B’
B
B’
Long-term Averages in 1993
Surface Salinity, psu
35
30
30
25
latitude
29.5
20
29
15
10
28.5
5
Mean Current, 0.2 m/s
28
-94
-93
-92
-91
-90
-89
-88
0
Suspended sediment from two rivers, log10 kg/m2
0
30
-2
latitude
29.5
-4
29
-6
28.5
-8
28
-94
-93
-92
-91
longitude
-90
-89
-88
Deposition of Miss. and Atch. Sediments in 1993
80% flocculated sed.
20% unflocculated sed.
50% flocculated sed.
50% unflocculated sed.
Conclusions
• Sediment from the Mississippi and Atchafalaya Rivers
both contribute to sedimentation on the inner shelf
southeast of the Atchafalaya Bay.
• Mississippi sediment is more widely dispersed than
that from the Atchafalaya.
• Strong winds during storms can suspend seabed
sediment extensively and may greatly rework
sediment accumulation.
Future works
• Estimate sediment resuspension by waves.
• Continue to compare ROMS model estimates with LaTex
observation data.
• Generate the sediment concentration output files, and
link turbidity to light attenuation in the biogeochemical
model.
• Run the model for 2006 and 2007, and compare with
field data collected in cruises for the MCH2 (Mechanisms
Controlling Hypoxia, 2) project.
Acknowledgement
•
•
•
•
•
•
Steven DiMarco (TAMU)
Bob Meade, Charles Demas and Jeffress Williams (USGS)
Chris Jenkins (INSAAR)
Aaron Bever, J. Paul Rinehimer, and Mary Ann Bynum (VIMS)
Chris Bording and Tom Crockett (William & Mary)
The sediment model is being developed as part of the NOPP
(National Oceanographic Partnership Program) CSTM (Community
Sediment Transport Model).
• The model was run on the cluster SciClone at the College of William
& Mary.
25
20
wind speed (m/s)
15
10
5
0
-5
-10
1
Mar
1993
5
9
13
17
21
25
29
Surface Salinity, psu
35
30
30
25
latitude
29.5
20
29
15
10
28.5
5
Mean Current, 0.2 m/s
28
-94
-93
-92
-91
-90
-89
-88
0
Suspended sediment from two rivers, log10 kg/m2
0
30
-2
latitude
29.5
-4
29
-6
28.5
0.01 Pal
-8
bottom shear stress
28
-94
-93
-92
-91
longitude
-90
-89
-88
LUMCON, 2007
LUMCON, 2007