Sediment Diversions for Delta Restoration

Report
Overview:
• Increased LSU investments in coastal science and
engineering: expanded Coastal Studies Institute
• Sediment diversions for coastal restoration: lessons
learned from examples
1.Environmental controls on deltaic land building
2.Examples in the Mississippi River Delta
3.Synthesis
4.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 Institute
2Environmental Defense Fund
3Royal Engineering
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 Examples
Bonnet Carré
Caernarvon
Wax Lake
Cubits Gap
West Bay
Governor's Office of Coastal Affairs June
2013
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 review
Lane, 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
Spillway in Operation
Governor's Office of Coastal Affairs June
2013
Relative 2011 sedimentation in lake from Spillway
operation
Legend
Delta_Inv
-0.720040 - 0.000000
0.000001 - 0.200000
0.200001 - 0.400000
0.400001 - 0.800000
0.800001 - 1.600000
0
5 10
20 Kilometers
Governor's Office of Coastal Affairs June
2013
2011 Sand and Mud Deposition
(Nittrouer et al., 2012, Fabre et al., in review)
Average Inventory
Water
Discharge
2D
Graph
1
7Be Inventory
0.6
8000
6000
0.4
4000
0.2
2000
0.0
Water Dishcarge m3/s
10000
2.5e+6
2.0e+6
1.5e+6
1.0e+6
5.0e+5
0.0
0
0
10
20
30
40
50
60
70
Days from Opening of Spillway
Total Mud Deposition (Lake)
Total Sand Deposition > 3 Mt
Governor's Office of Coastal Affairs June
2.1±1.1 Mt
(in spillway)
2013
Cum. Sediment Disch. (Tons)
Cum. 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 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 construction
References:
Allen et al., 2012, Estuaries and Coasts
Kim, 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 Bay
Growth 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 Bay
Growth 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 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
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
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
References
Andrus, 2007, LSU MS Thesis
Andrus et al., 2012, State of the Coast
Coleman, J. M., and Gagliano, S. M., 1964, GCAGS, v. 14, p. 141-154.
Coleman and Prior, 1982
Kemp et al., 2011, National Audubon Society
Governor's Office of Coastal Affairs June
Kolker et al., 2012, ECSS
2013
West Bay Region: historical
subdeltas
Coleman and Gagliano,
Governor's Office of Coastal Affairs June
1964
Coleman
and Prior,
2013
1982
Spring 2011 High Water Event
From Kemp, 2011
Governor's Office of Coastal Affairs June
2013
Estimated Wave Height Reductions from island construction
0.18 m
0.1 m
Wave reduction
produced by
placement of
dredge-spoil
islands
Increase sediment
retention
1 km
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 mayGovernor's
be built
to benefit several regions, and not
Office of Coastal Affairs June
2013
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
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?
Governor's Office of Coastal Affairs June
2013
Supplemental
discussion
materials on
following pages
MRD Diversion Examples
Bonnet 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 References
Lopez et al., Basics of the Basin Proceedings, 2011
Lane, 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 Examples
Bonnet 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, 1964
Kolker et al., in press
Roberts, 1997, JCR
Wells et al., 1983 (cited in Roberts, 1997)
Governor's Office of Coastal Affairs June
2013
Cubits Gap Subdelta Growth
(Wells et al., 1983)
Governor's Office of Coastal Affairs June
2013

similar documents