overview : increased lsu investments in coastal science and engineering: expanded coastal studies...
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Overview : Increased LSU investments in coastal science and engineering: expanded Coastal Studies Institute Sediment diversions for coastal restoration: lessons learned from examples Environmental controls on deltaic land building Examples in the Mississippi River Delta Synthesis - PowerPoint PPT PresentationTRANSCRIPT
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.
Governor's Office of Coastal Affairs June 2013
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.
Governor's Office of Coastal Affairs June 2013
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.
Governor's Office of Coastal Affairs June 2013
MRD Diversion ExamplesBonnet 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
Governor's Office of Coastal Affairs June 2013
Governor's Office of Coastal Affairs June 2013
Spillway in Operation
Governor's Office of Coastal Affairs June 2013
0 10 205 Kilometers
LegendDelta_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
Governor's Office of Coastal Affairs June 2013
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 Spillway0 10 20 30 40 50 60 70
7 Be
Inve
ntor
y0.0
0.2
0.4
0.6
Wat
er D
ishc
arge
m3 /
s
0
2000
4000
6000
8000
10000
Cum
. Sed
imen
t Dis
ch. (
Tons
)
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
Governor's Office of Coastal Affairs June 2013
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 concentration
2011 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
Governor's Office of Coastal Affairs June 2013
MRD Diversion Examples
Bonnet Carré
Caernarvon
Wax Lake
Cubits Gap
West Bay
Governor's Office of Coastal Affairs June 2013
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
Governor's Office of Coastal Affairs June 2013
Wax Lake Delta, Atchafalaya BayGrowth of 1-3 km2/y, 1973-2012, depending on methodology (below from Roberts, 1998)
Governor's Office of Coastal Affairs June 2013
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
Governor's Office of Coastal Affairs June 2013
Wax Lake Delta, Atchafalaya BayKim 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
Governor's Office of Coastal Affairs June 2013
MRD Diversion ExamplesBonnet Carré
Caernarvon
Wax Lake
Cubits Gap
West Bay
Governor's Office of Coastal Affairs June 2013
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
Governor's Office of Coastal Affairs June 2013
West Bay Region: historical subdeltas
Coleman and Gagliano, 1964 Coleman and Prior, 1982
Governor's Office of Coastal Affairs June 2013
Spring 2011 High Water Event
From Kemp, 2011
Governor's Office of Coastal Affairs June 2013
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
Governor's Office of Coastal Affairs June 2013
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
•
Governor's Office of Coastal Affairs June 2013
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
Governor's Office of Coastal Affairs June 2013
Acknowledgements:Many authors and researchers conducted work used in our report. Thanks to all of you.
Questions?
Supplemental discussion materials on following pages
Governor's Office of Coastal Affairs June 2013
MRD Diversion ExamplesBonnet Carré
Caernarvon
Wax Lake
Cubits Gap
West Bay
Governor's Office of Coastal Affairs June 2013
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.
Governor's Office of Coastal Affairs June 2013
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
Governor's Office of Coastal Affairs June 2013
MRD Diversion ExamplesBonnet Carré
Caernarvon
Wax Lake
Cubits Gap
West Bay
Governor's Office of Coastal Affairs June 2013
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)
Governor's Office of Coastal Affairs June 2013
Cubits Gap Subdelta Growth (Wells et al., 1983)