currents and sediment transport in the mississippi canyon and effects of hurricane georges

Currents and Sediment Transport in the Mississippi

Canyon and Effects of Hurricane Georges

Mary Jo Richardson

Wilford Gardner

Cheryl Burden

Vernon Asper

Thanks for inviting us to HCMR

• Mary Jo Richardson (Texas A&M University)• Wilford Gardner (Texas A&M University - Fulbright Fellow)

Currents, temperature and particulate matter concentration

• May - July 1998

• Aug - Nov 1998

• Current meters, thermographs, light-scattering sensor, sediment traps, cores, ROV video camera

Canyons

• Conduits of sediment to deep water

• Oscillatory currents - internal waves

• Storm events - hurricanes

• Winter storm cooled - Mediterranean

Mississippi Canyon station locations and bathymetry. Contours

are in 100 m intervals.

A. The upper portion of the map shows station locations from the 1998 cruise,

including core, LAPS/ROV and mooring locations.

B. The lower portion of the map shows station locations from the 1994 cruise. Transect lines and LAPS stations are numbered. Inset shows

the study area in relation to the Louisiana coastline and Mississippi River Delta.

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Configuration of instrument mooring.

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Back-scattered light (measured from the LSS) vs. particulate

matterconcentration (PM) for the May

1998 cruise. All data are included; no division was made

between surface and deeper water.

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Temperature in Canyon axis at

10

11

12

13

14

15

16

17

18

19

20

5/18 5/28 6/7 6/17 6/27 7/7 7/17

Temperature (°C)

333.5 mab

Figure 4 A

A

mab

Temperature in Canyon axis at

10

11

12

13

14

15

16

17

18

19

20

8/7 8/17 8/27 9/6 9/16 9/26 10/6 10/16 10/26 11/5 11/15

Temperature (°C)

33 mab3.5 mab

HurricaneGeorges

Figure 4 B

B

Temperatures at 33 and 3.5 mab for

A) deployment 1 and

B) B) deployment 2.

Vertical displacement of 50 m 2C change - saw tooth pattern

• Up/down-canyon flow, bed shear stress, particle concentration, and temperature for days 172-181 at the canyon axis mooring site. Current velocity and temperature scales are reversed to match up/down-canyon flow. Deployment 1.

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•Up/down-canyon flow, cross-canyon flow, bed shear stress, and temperature for days 270-279 at the canyon axis mooring site. Current velocity and temperature scales are reversed to match up/down-canyon flow. Deployment 2.

Progressive vector plot of currents at 3.5 mab during the first mooring deployment. Net transport is mostly cross-canyon (South) at 50 mab and

down-canyon at 3.5 mab (East) with a cross-canyon drift. X's denote midnight.

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Progressive vector plot of currents at 3.5 mab during the second mooring deployment. Net transport is down canyon (East). X's denote midnight. The passage of Hurricane Georges is denoted in year days. Note unequal axes that also differ from the unequal axes of Figure 8.

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Conclusions

• Currents oscillated with diurnal periodicity• Currents exceeded critical bed shear stress

30% (normal) 50% (hurricane)• Sediment trap collection increased 70-130 X

during hurricane• Trap fluxes better related to input from nearby

rivers and resuspended sediment more than current strength, compared with other canyons

Special thanks to Aris (for inviting us) and Victoria (for sharing her office)