Sediment Diversions for Delta Restoration: Lessons Learned from Examples

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 • 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 consideredthe 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

Bonnet Carré Caernarvon Wax Lake Cubits Gap West Bay MRD Diversion Examples 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, inColten, 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 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) 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 • MudDischarge is the same order as for West Bay • ~10% of Wax Lake Delta annual discharge Governor's Office of Coastal Affairs June 2013

Bonnet Carré Caernarvon Wax Lake Cubits Gap West Bay MRDDiversionExamples 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

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 Kolker et al., 2012, ECSS Governor's Office of Coastal Affairs June 2013

West Bay Region: historical subdeltas Governor's Office of Coastal Affairs June 2013 Coleman and Gagliano, 1964 Coleman and Prior, 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 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? Governor's Office of Coastal Affairs June 2013

Supplemental discussion materials on following pages

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

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

Sediment Diversions for Delta Restoration: Lessons Learned from Examples