Modeling transport and deposition of the Mekong River sediment

Z. George Xue1*Ruoying He1, J.Paul Liu1, John C Warner2

1.Dept. of Marine, Earth and Atmospheric Sciences, North Carolina State University1.Dept. of Marine, Earth and Atmospheric Sciences, North Carolina State University2.US Geological Survey, Woods Hole Science Center, Woods Hole2.US Geological Survey, Woods Hole Science Center, Woods Hole

* now at Dept. of Oceanography and Coastal Sciences, Louisiana State University* now at Dept. of Oceanography and Coastal Sciences, Louisiana State University

25% of global flux

4 billion tons/yr

Changes in Hydrology in the last 50 years*• Connection to Damming, Monsoon, and ENSO

*Xue et al., 2011. Earth Surface Processes and Landforms

“Most endangered delta due to its extreme vulnerability to sea level rise, storm surges and salinity intrusion” (IPCC 2007)

Late Holocene Sediment Budget

80% Sediment was trapped within the delta area over the past 3000 yr80% Sediment was trapped within the delta area over the past 3000 yr

Paleo Delta Front around 3000 yr BPPaleo Delta Front around 3000 yr BP

26 m

/yr

26 m

/yr

16 m/yr

16 m/yr Proximal

Distal > 200 km transporta

tion

> 200 km transporta

tion

* Xue et al., 2010. Marine Geology; Xue et al. 2014 J. Asian Earth Sci

Coastal Currents

Geyer et al.,2004;

Mekong Plume is mostly geostrophic

River PlumeRiver Plume

DownwellingDownwelling

NE WindNE Wind

16m/yr16m/yr

26m/yr26m/yr

Sediment Transport Modeling*

* Xue et al., 2012. Continental Shelf Research

Grid: 200×150 (Resolution: ~ 2 km); Vertical Layer: 20Water Depth: 5 - 153 m; Three Open Boundaries

Model Setup

IC/BC: HyCOM/NCODA (1/12IC/BC: HyCOM/NCODA (1/12oo))

Bulk Flux: NCEP (2.5Bulk Flux: NCEP (2.5oo))

Wind: QSCAT (0.5Wind: QSCAT (0.5oo))

Freshwater DischargeFreshwater Discharge

WaveWave(SWAN)(SWAN)

SedimentSediment(CSTMS)(CSTMS)

CirculationCirculation(ROMS)(ROMS)

Tide: OSU TOPEX 7.0Tide: OSU TOPEX 7.0

BC: Wave Watch III (1.25BC: Wave Watch III (1.25oo))

Suspended Sediment ConcentrationSuspended Sediment Concentration

same gridsame gridcommunicate communicate

hourlyhourly

180s time 180s time stepstep

Exp 1 : Full physicsExp 1 : Full physics(Control Run)(Control Run)

Exp 2 : Tide excludedExp 2 : Tide excluded(NTS)(NTS)

Exp 3 : Wave excludedExp 3 : Wave excluded(NWS)(NWS)

Exp 4 : Ambient CurrentExp 4 : Ambient Current excluded (NTS)excluded (NTS)

Year 2005Year 2005

The Coupled Ocean-Atmosphere-Wave-Sediment Transport The Coupled Ocean-Atmosphere-Wave-Sediment Transport Modeling system was applied to simulate the transport and Modeling system was applied to simulate the transport and

dispersal of Mekong-derived sediment in 2005. dispersal of Mekong-derived sediment in 2005.

River Inputs

160 million tons of sediment: two sediment classes,160 million tons of sediment: two sediment classes,

(30% small floc, 70% large floc)(30% small floc, 70% large floc)

sediment parameterization following Harris et al., 2008 sediment parameterization following Harris et al., 2008

Freshwater Freshwater DistributionDistribution

Sediment input (accumulative Sediment input (accumulative curve)curve)

Model ValidationValidations Validations (through satellite observations)(through satellite observations)• • Significant Wave Height: AVISO - Jason 1 (R=0.87, Significant Wave Height: AVISO - Jason 1 (R=0.87, RMSE=0.75m);RMSE=0.75m);• • Sea Level Anomaly : AVISO - Jason 1 (R=0.81, Sea Level Anomaly : AVISO - Jason 1 (R=0.81, RMSE=0.03m);RMSE=0.03m);

Jason 1Jason 1

Significant Wave Height (along-track comparison)Significant Wave Height (along-track comparison)

Sea-Level anomaly (along-track comparison)Sea-Level anomaly (along-track comparison)

Results: Seasonality

Low Flow Winter

NortheastMonsoon

High Flow Summer

Southwest Monsoon

Results: Deposition by end of simulation

Less dispersal when wave is turned off

Control Run

‘wave-off’ minus

Control Run

‘Ambient Current-off’ minus

Control run

‘tide-off’ minus

Control run

Results: Bed shear stress

The shear stress for resuspension was mainly induced by waves

Summer

Winter

Conclusion: Sediment Budget

Tide: Cross-shelf TransportWave: ResuspensionAmbient Current: Southwestward along-shelf transportAlong-shelf dispersal is less than 2% of ‘fresh’ sediment

On the delta front

Strong wave mixing and downwelling-favorable coastal current controls the southwestward along-shelf transport

Wave Orbital VelocityWave Orbital Velocity

Bottom CurrentBottom Current

% of suspended sediment% of suspended sediment

% of sediment on the seabed% of sediment on the seabed

• Coupled with WRF and look into event transport (typhoon)

• Sediment Plume Dynamics (Hyperpycnal Flow?)

• Wet-Dry and Wave-Current interaction modules for coast accretion and erosion