Toward a Sediment Transport Model of the Louisiana / Texas Shelf

Kehui (Kevin) Xu1, Courtney Harris1,Robert Hetland2 , James Kaihatu2

1 Virginia Institute of Marine Science2 Texas A&M University

(Background Image from NASA Visible Earth)

Questions1. 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 ModelRegional Ocean Modeling System (ROMS)Parallel model using a 16-cpu cluster Model Domain: ~800km × 300 km; Grid: 128 × 64 × 20Spatially uniform but temporally variable (hourly) windNo tide (tidal range <0.5m), No wave (forthcoming)

-94 -93 -92 -91 -90 -89 -88

27.5

28

28.5

29

29.5

30

30.5

longitude

latit

ude

Atchafalaya RiverMississippi River

20m

50m100m300m

(30%) (70%)

Wind Station

Atch. Bay

01/01 04/01 07/01 10/010

10

20

30

win

d sp

eed

(m/s

)

01/01 04/01 07/01 10/01 01/010

1

2

3x 10

9

Wat

er D

isch

arge

(m

3 /day

)

Mississippi

Atchafalaya

01/01 04/01 07/01 10/010

5

10

15x 10

5

Day of year 1993Sed

imen

t D

isch

arge

(to

ns/d

ay)

Mississippi

Atchafalaya

Wind, Water and Sediment Discharges in 1993

‘Storm of Century’

(USGS data from Charles Demas and Bob Meade)

0

20

40

60

80

100

-94 -93 -92 -91 -90 -89 -88

27.5

28

28.5

29

29.5

30

30.5Sediment type, mud%

longitude

latit

ude

Sediment propertiesSediment Type τcr (Pa) Ws (mm/s) Fraction

Mississippi Flocculated 0.08 1 80%

Unflocculated 0.03 0.1 20%

Atchafalaya Flocculated 0.08 1 80%

Unflocculated 0.03 0.1 20%

Sea bed Sand 0.12 10 SpatiallyVariableMud 0.10 1

20m

50m

100m300m

Sandy

Muddy

US Seabed Data from Jeffress Williams (USGS) and Chris Jenkins (INSTAAR)

ResultsSalinity,Mean Current,Wind

Miss. & Atch.sediment dispersal

Near-bottomCurrent,Resusp.from sea bed

A’

A

A A’

B’

B’

B

B

ResultsWhite line is 30 psu isohaline

A’

A

A A’

B’

B’

B

B

-94 -93 -92 -91 -90 -89 -8828

28.5

29

29.5

30la

titud

eSurface Salinity, psu

Mean Current, 0.2 m/s

0

5

10

15

20

25

30

35

-94 -93 -92 -91 -90 -89 -8828

28.5

29

29.5

30

longitude

latit

ude

Suspended sediment from two rivers, log10

kg/m2

-8

-6

-4

-2

0

Long-term Averages in 1993

Deposition of Miss. and Atch. Sediments in 199380% 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.

1 5 9 13 17 21 25 29

-10

-5

0

5

10

15

20

25

Mar

1993

win

d sp

eed

(m/s

)

-94 -93 -92 -91 -90 -89 -8828

28.5

29

29.5

30

latit

ude

Surface Salinity, psu

Mean Current, 0.2 m/s

0

5

10

15

20

25

30

35

-94 -93 -92 -91 -90 -89 -8828

28.5

29

29.5

30

longitude

latit

ude

Suspended sediment from two rivers, log10

kg/m2

bottom shear stress

0.01 Pal

-8

-6

-4

-2

0

LUMCON, 2007

LUMCON, 2007