the mississippi river and its role in restoration...

The Mississippi River: Its Role in Restoration Efforts and Potential Effects of Climate Change

Clinton S. Willson, P.E., Ph.D.Department of Civil & Environmental Engineering

Louisiana State University

Acknowledgements• LSU Graduate Students: Erol

Karadogan, Nathan Dill, Ryan Waldron,

Joseph Tsai, Samantha Danchuk, Molly Friedmann

• Delft Technical University Hydraulic Engineering M.S. Students• LSU Undergraduate Students working at SSPM: Kevin Hanegan, Mark

Leblanc, Erin Rooney, Paul Leonard, Kyle Breaux, Brett McMann

• LA Department of Natural Resources• CREST Program• USACE – CHL• National Center for Earth‐surface Dynamics – Paola, Parker, Kim• LSU Center for Computation & Technology

River‐Dominated Delta

• Sand ‐

Sediment – Salt (3 S’s)

& Nutrients

• Not a static system– Natural detail cycle

• River changes course• Solid land to barrier islands• Barrier islands to

submerged sand bars; and

more…

– Delta is threatened by

waves, tides, and storm

surges

– Sea level rise and

subsidence

– Changes in sediment

loading

Coleman & Gagliano, 1964

Does this look like a natural Delta?

Possible Realignment of Lower Mississippi River

• The LACPR draft report does not consider this option, stating “the alternative was considered to be beyond

the scope of the current effort and could not be adequately evaluated within the scope of this effort”.

• NRC recognizes that, while controversial, there needs to be careful study of a major realignment of the lower

Mississippi River.

1st

Report from NRC on LACPR Program Review

“An evaluation of how a major realignment of the lower

Mississippi River mouth may affect sediment capture and

diversion should be conducted”

The concept has been around…

Small‐Scale Physical ModelPointe‐a‐la‐Hache

(~ RM 55)

Head‐of‐Passes (RM 0)

Model Scaling• Distorted Scale Movable

Bed Model

• E(L) = 1/12 000, E(H) = 1/500

(HIGH VERTICAL

DISTORTION)• Limits study to mostly 1D bulk movement in river

• The model is built according to:– Froude similarity law for the hydraulics; and– Schield’s

law for the inception of sediment (sand)

transport.– also utilize Re scaling to ensure turbulent flow in the river

& through diversions

• Only able to test “large‐scale”

diversions (~100‐500k cfs)

• Sediment Time Scale: 1 prototype year = 30 minutes in model time

Methodology

• Measure stage levels• Measure hydrographs• Measure dredged material• Image to obtain spatial distribution• Dye studies to obtain surface

velocities and patterns

• At conclusion of test, spatially

collect sediment and measure

amount and then sieve

1.

Run two year hydrograph in one

hour period

2.

Introduce sediment over

identical “hydrograph”

3.

Raise “sea level”

~1 ft every 30

years

SSPM Results – Large Diversion #2

Impact on Stage Level Relative Sea Level Rise

0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10

Time Interval (annual hydrogaph)

Gau

ge 2

Wat

er S

urfa

ce E

leva

tion

(ft)

Crest LD #2, Gage 2 ws el (ft) 1-2 yearsCrest LD #2, Gage 2 ws el (ft) 9-10 yearsCrest LD #2,Gage 2 ws el (ft) 19-20 yearsCrest LD #2, Gage 2 ws el (ft) 29-30 yearsCrest LD #2, Gage 2 ws el (ft) 39-40 yearsCrest LD #2 Gage 2 ws el (ft) 49-50 years

Sediment Deposited

Sediment Dredged

SSPM Results# of Years % Dredged % Deposited % Out of

Model

Base Case 100 67 28 5

Large Diversion #2 100 10 70 20

Large Diversion #2

(2)

50 6 84 10

Multiple

Diversions

100 41 55 4

Multiple

Diversions

(w/MG)

75 21 65 14

Eastern

Navigation

Channel

50 25 68 6.4

DTU Pulsed LD#2 30 41 26 32

Land loss by deltaic drowning is neither inevitable nor natural

15 cm

Seismic section: 45 km long, 1.4 km thick

These low-gradient, low-elevation delta tops are dynamic and self-maintaining

.where H is eustatic sea level,σ

spatially averaged subsidence rate,

Atop the area of the delta top (subaerial wetlands and channels),Qs total volumetric sediment supply,fr the fraction retained in the delta top, androrg the rate of storage of organic matter in the sediment column, expressed as a rate of vertical accumulation (length/time).

Delta area is set by a balance between sea-level rise + subsidence and deposition of sediment and organic matter:

Desktop Delta ModelTHE MODEL CAN REPRODUCE THE WAX LAKE DELTA’S PAST

Yellow: 38 Mt/yrWhite: 25 Mt/yr(suspended load)

VARIATION: SEA-LEVEL RISE = 4 mm/yr, SUBSIDENCE = 10 mm/yrSolid line: variant caseDotted line: base case

“Worst case”:

still 701 km2 of new land

And extra land-building due to organics is not yet included

Desktop Delta Model –

SSPM VERY preliminary comparisons

• Large Diversion w/ 2 medium‐size diversions– 50 years: 440 km2

from DDM vs. 250‐420 km2

from SSPM– 100 years: 620 km2 from DDM vs. 350‐650 km2

from SSPM

• Multiple medium‐size diversions– 100 years: 600 km2

from DDM vs. 200‐350 km2 from SSPM

• Differences most likely due to assumptions concerning independence of individual diversions

and conveyance efficiency using DDM

Is there enough sediment? Timing of the sediment?

Hydrodynamic Modeling of SSPM Area

Number of Nodes: 131,042Number of Elements: 66,468Total Mesh Area: 3530 km2

Resolution is down to: 60 m

Karadogan, 2008, in progress

USACE Adaptive Hydraulics Code- Unstructured FE- Adaptive mesh capabilities- Runs on multiple platforms including HPCs

Model Results vs Observation Data


SSPM Mesh; Water Surface Elevations, 500K, 750K, 1000K cfs


Hypothetical Diversion near Empire, LA

Bay Joe Wise

Lake Grande

Ecaille

Mississippi

River

Freepo

rt Sulp

hur Can

al

Adam’s Bay

Grand

Bayou

Bay De La Cheniere

Bastian Bay

Lake

Was

hingto

n

Diversion Channel

N

Caprien Bay

Bay Lanaux

Bayo

u Hu

erte

s

Reference Map

(Dill, 2007)

Elevations

Mesh and Boundary Conditions1000K cfs

TAIL WATER ELEVATION of 0.4 m



Mesh Adaption

Water Surface Elevations & Velocities


• Geophysical processes and geomorphic features control ecological patterns.

• Thus the structure and function of coastal ecosystems are dependent on critical processes specific to evolution of deltas.

• Links Delta Evolution to Ecological succession.

Final Thoughts

• Multiple tools are necessary – Geological data (historical and current)– Physical modeling

– High‐resolution numerical modeling

– Desktop/screening models

– Land‐building/Ecological Models

• Accurate elevation data!– Need to quantify rates and understand their

context (short‐

versus long‐term)

Final Thoughts

• Sediment– Quantity?– Occurrence/Frequency?

• Abandon Lower Mississippi River Delta?– Alternative navigation channels?– Paired with diversions in “upper”

part?

• Subsidence rates combined with eustatic

sea level rise makes the LMRD a valuable natural lab

the mississippi river and its role in restoration...

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