overview: increased lsu investments in coastal science and engineering: expanded coastal studies...

Overview:• Increased LSU investments in coastal science and

engineering: expanded Coastal Studies Institute

• Sediment diversions for coastal restoration: lessons learned from examples1.Environmental controls on deltaic land building2.Examples in the Mississippi River Delta3.Synthesis4.Conclusions

Focus Areas for Investment

Coastal is at the center

Expanded Coastal Studies Institute• Focus of LSU investment• Crossing six colleges and schools• >20 faculty• ~100 faculty, staff, technicians, graduate

students• Top coastal scientists and engineers in

water and sediment dynamics, and wetland growth

• New salary, graduate assistantships, facilities

Sediment Diversions for Delta Restoration: lessons learned from examples

Samuel Bentley1, Angelina Freeman2, Clinton S. Willson1, Mitch Andrus1,3

1LSU Coastal Studies Institute2Environmental Defense Fund

3Royal Engineering

Governor's Office of Coastal Affairs June 2013

Are diversions of water and sediment from the Mississippi River useful tools for building land in the Mississippi River Delta?

The suitability of diversions for delta restoration has been the subject of vigorous scientific and policy debate, particularly given projected sea level rise and subsidence rates as well as the catastrophic rate of land loss Louisiana’s coast is experiencing.

Our analysis considered the question: given these challenges, can diversions build enough land to make a difference?

We considered the land building capacity of a diversion to be the ability of deposited sediment to increase the elevation of a land or seabed surface.


Controls on land building by river-sediment diversions

Supply components:Sediment delivery from river: influenced by river stage, location of conveyance channel. This can be optimized.

Sediment retention in receiving basin: controlled by sediment depositional properties (grain size, cohesion) and resuspension by wind and waves in the receiving basin. This varies from <20% to ~100% and should be a target for engineering optimization

Plant production and contribution to soil volume (peat, root mass). This can probably be optimized.


Controls on land building by river-sediment diversions

“Sink” components:Subsidence: local lowering of the land surface by subsurface movement. Choose locations with lowest subsidence rates (upstream, mostly)

Compaction: local reduction of sediment volume by dewatering. Sand compacts less after deposition, but the river carries much more mud.

Global sea level rise. This is beyond the control of our state legislature.


MRD Diversion Examples

Bonnet Carré

Caernarvon

Wax Lake

Cubits Gap

West Bay

Bonnet Carré Spillway

• Spillway for flood control• Located near site of >10 crevasses during 18th and 19th

centuries• Spillway flow up to 9000 m3/s, about 20% of MR flow• Sand captured mostly in spillway, mud dispersed to lake• Millions of tons of sediment discharged per event

References:Davis, D. W., 2000, in Colten, C. E., ed., University of Pittsburgh Press.Fabre et al., Marine Geology, in reviewLane, R. R., Day, J. W., and Day, J. N., 2006, Wetlands, v. 4, p. 1130-1142.Nittrouer et al., 2012, Nature Geoscience



Spillway in Operation


0 10 205 Kilometers

Legend

Delta_Inv

-0.720040 - 0.000000

0.000001 - 0.200000

0.200001 - 0.400000

0.400001 - 0.800000

0.800001 - 1.600000

Relative 2011 sedimentation in lake from Spillway operation


2011 Sand and Mud Deposition (Nittrouer et al., 2012, Fabre et al., in review)

Total Mud Deposition (Lake) 2.1±1.1 Mt

Total Sand Deposition > 3 Mt(in spillway)

2D Graph 1

Days from Opening of Spillway

0 10 20 30 40 50 60 707 B

e In

vent

ory

0.0

0.2

0.4

0.6

Wat

er D

ishc

arge

m3 /

s

0

2000

4000

6000

8000

10000

Cum

. S

edim

ent

Dis

ch.

(Ton

s)

0.0

5.0e+5

1.0e+6

1.5e+6

2.0e+6

2.5e+6

Average Inventory Water DischargeCum. Sed. Disch.May Inventory Decay


Total Sediment Discharge and Comparisons

• Sand supply to spillway appears to be delivered at concentrations above mean river sand concentration

• Mud supply is in equilibrium with mean river concentration2011 new mud deposition of 1.1-3.3 Mt for the entire lake.

• If this is correct, suggests close to 100% retention of spillway discharge

• Compare to 25-50% retention for diversions entering open embayments

• Mud Discharge is the same order as for West Bay• ~10% of Wax Lake Delta annual discharge



Bonnet Carré

Caernarvon

Wax Lake

Cubits Gap

West Bay


Wax Lake Delta, Atchafalaya Bay

• Diversion built in 1944 to ease flooding in Morgan city• Delta emerged sub-aerially in 1973, after major flood• Unmanaged delta, most mature example of bay-head

delta that might result from diversion constructionReferences:Allen et al., 2012, Estuaries and CoastsKim, W., Mohrig, D., Twilley, R., Paola, C., and Parker, G., 2009, EOS Trans AGU, v. 90, no. 42, p. 373-374.Roberts, H. H., 1998, Journal of Coastal Research, p. 882-899.Wellner et al., 2005, GCAGS Transcations


Wax Lake Delta, Atchafalaya BayGrowth of 1-3 km2/y, 1973-2012, depending on methodology (below from Roberts, 1998)


Wax Lake Delta, Atchafalaya BayGrowth rate strongly influenced by flooding events, especially 1973 flood. Also, mud sustains regional wetlands

1974 2002

Wellner et al., 2005


Wax Lake Delta, Atchafalaya Bay

Kim et al., 2009. Delta growth maintained at relative sea level rise of 7 mm/y, and model results suggest growth possible at higher relative rate of sea level rise



Bonnet Carré

Caernarvon

Wax Lake

Cubits Gap

West Bay


West Bay• Manmade uncontrolled diversion• Created in 2004, in location of

historic subdelta documented by Coleman others

• Land emergent in 2011, following 2010 island construction and 2011 flood

ReferencesAndrus, 2007, LSU MS ThesisAndrus et al., 2012, State of the Coast

Coleman, J. M., and Gagliano, S. M., 1964, GCAGS, v. 14, p. 141-154.Coleman and Prior, 1982Kemp et al., 2011, National Audubon SocietyKolker et al., 2012, ECSS


West Bay Region: historical subdeltas

Coleman and Gagliano, 1964 Coleman and Prior, 1982


Spring 2011 High Water Event

From Kemp, 2011


Estimated Wave Height Reductions from island construction

1 km

0.18 m

0.1 m

Wave reduction produced by placement of dredge-spoil islands Increase sediment retention


Considerations regarding diversions and subdeltas• Natural subdeltas build and decline over timescales of 75-150 years

• Sediment retention rates in natural subdeltas with open marine boundaries are 25-50%. Mud mostly lost to coastal ocean. We need to keep the mud, too.

• Increasing sediment retention rates (as for Bonnet Carre, near 100%, and possibly Caernarvon, and wave reduction in West Bay) may shorten timescales of growth

• The Mississippi has limited capacity for simultaneous operation of large diversions (each at 5-25% of total water discharge)

• Multiple diversions may be built to benefit several regions, and not all have to operate simultaneously

•


Approaches to maximize performance:Pulsed operation

Locate receiving basins where subsidence is lowest.

Focus on times and locations that will produce maximum sediment concentration and discharge during operation

Design receiving basin to maximize retention of mud• Reduce shear stress from waves and currents with island

construction, like West Bay

Allow time for mud to consolidate between discharge pulses, to increase shear stress for erosion


Acknowledgements:Many authors and researchers conducted work used in our report. Thanks to all of you.

Questions?

Supplemental discussion materials on following pages



Bonnet Carré

Caernarvon

Wax Lake

Cubits Gap

West Bay


Caernarvon Diversion, Mississippi River• Primarily designed for fresh water diversion into Breton

Sound region, not operated continuously or at high flow rates (<<500 m3/s)

• Opened to allow flow following DWH Spill, and in 2011• Inland basin, surrounded by wetlands

Selected ReferencesLopez et al., Basics of the Basin Proceedings, 2011Lane, R. R., Day, J. W., and Thibodeaux, B., 1999, Estuaries and Coasts, v. 2, p. 327-336.Snedden, G. A., Cable, J. E., Swarzenski, C., and Swenson, E., 2007, Estuarine, Coastal and Shelf Science, v. 71, no. 1-2, p. 181-193.


Caernarvon Diversion, Mississippi River• In 2011, > 4 km2 of new land present in the receiving basin, built

mostly since 2006. Lopez et al., 2011• Sediment retention rate not known, but probably high



Bonnet Carré

Caernarvon

Wax Lake

Cubits Gap

West Bay


Cubits Gap Subdelta, Mississippi River• Manmade cut in east bank of MR, below Venice ca.

1862• Land growth and decline over ca. 150 y timescale• Active growth at present, possibly due to

accommodation created by subsidence.

References:Coleman and Gagliano, 1964Kolker et al., in pressRoberts, 1997, JCRWells et al., 1983 (cited in Roberts, 1997)


Cubits Gap Subdelta Growth (Wells et al., 1983)

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