Optimal Design of Sediment Diversions for Delta Restoration: lessons learned from examples

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1 Optimal Design of Sediment Diversions for Delta Restoration: lessons learned from examples Samuel Bentley 1, Angelina Freeman 2, Clinton S. Willson 1, Liviu Giosan 3, Jaye Cable 4 1 LSU 2 EDF 3 WHOI 4 UNC

2 Are diversions of water and sediment from the Mississippi River useful tools for building land in the Mississippi River Delta? Sediment diversions are man-made channels for delivering sediment-laden river water to the coastal ecosystem. Diversions have been proposed for land building in the Mississippi River s lower delta plain. However, the suitability of diversions has also 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 that Louisiana s coast is experiencing. Our analysis considered the question: given these challenges, can diversions build enough land to make a difference?

3 Given these challenges, can diversions build enough land to make a difference? Our analysis: 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. More specifically, we viewed the change of elevation in relation to what we called sink factors. These include factors such as rises in global sea level and local subsidence as well as sediment compaction. We considered several case studies as part of our analysis, including different kinds of diversions: flood control, land building, and freshwater.

4 Outline Global examples Examples in the Mississippi River Delta (review of some relevant published examples) Synthesis Conclusions

5 Global Examples Po River Delta, NE Italy: managed intensely since the Renaissance Huang He River, China: multiple channel realignments to build land in northern Yellow Sea Brazos River Delta, Texas: channel realignments have infilled coastal bays Syvitski, J. P. M., and Saito, Y., 2007, Global and Planetary Change, v. 57, no. 3-4, p

6 MRD Diversion Examples Bonnet Carré Caernarvon Wax Lake Cubits Gap West Bay

7 Spillway for flood control Bonnet Carré Spillway Located near site of >10 crevasses during 18 th and 19 th centuries Spillway flow up to 9000 m 3 /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 Nittrouer et al., 2012, Nature Geoscience

8 Spillway in operation

9 Sand versus mud distribution 2011 opening, muddy plume covers most of lake, and deposits sediment layer. Sand mostly trapped in spillway. 20 km

2011 Sand and Mud Deposition (Nittrouer et al., 2012, Fabre et al., in review) 7 Be Inventory 0.6 0.4 0.2 0.0 Average Inventory 2D Water Graph Discha

10 2011 Sand and Mud Deposition (Nittrouer et al., 2012, Fabre et al., in review) 7 Be Inventory Average Inventory 2D Water Graph Discharge 1 Cum. Sed. Disch. May Inventory Decay Days from Opening of Spillway Water Dishcarge m 3 /s 2.5e+6 2.0e+6 1.5e+6 1.0e+6 5.0e Cum. Sediment Disch. (Tons) Total Sand Deposition > 3 Mt (in spillway) Total Mud Deposition (Lake) 2.1±1.1 Mt

11 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 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

12 MRD Diversion Examples Bonnet Carré Caernarvon Wax Lake Cubits Gap West Bay

13 Caernarvon Diversion, Mississippi River Primarily designed for fresh water diversion into Breton Sound region, not operated continuously or at high flow rates (<<500 m 3 /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 Snedden, G. A., Cable, J. E., Swarzenski, C., and Swenson, E., 2007, Estuarine, Coastal and Shelf Science, v. 71, no. 1-2, p

14 Caernarvon Diversion, Mississippi River In 2011, > 4 km 2 of new land present in the receiving basin, built mostly since Lopez et al., 2011 Sediment retention rate not known, but probably high

15 MRD Diversion Examples Bonnet Carré Caernarvon Wax Lake Cubits Gap West Bay

16 Wax Lake Delta, Atchafalaya Bay Diversion built in 1944 to ease flooding in Morgan city Delta emerged subaerially 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 Roberts, H. H., 1998, Journal of Coastal Research, p Wellner et al., 2005, GCAGS Transcations

17 Wax Lake Delta, Atchafalaya Bay Growth of 1-3 km 2 /y, , depending on methodology (below from Roberts, 1998)

18 Wax Lake Delta, Atchafalaya Bay Growth rate strongly influenced by flooding events, especially 1973 flood Wellner et al., 2005

Wax Lake Delta, Atchafalaya Bay Kim et al., 2009.

19 Wax Lake Delta, Atchafalaya Bay Kim et al., 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

20 MRD Diversion Examples Bonnet Carré Caernarvon Wax Lake Cubits Gap West Bay

21 Cubits Gap Subdelta, Mississippi River Manmade cut in east bank of MR, below Venice ca Land growth and decline over ca. 150 y timescale Active growth at present, possibly due to accomodation created by subsidence. References: Coleman and Gagliano, 1964 Kolker et al., in press Roberts, 1997, JCR Wells et al., 1983 (cited in Roberts, 1997)

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

23 MRD Diversion Examples Bonnet Carré Caernarvon Wax Lake Cubits Gap 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 Closure planned

24 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 Closure planned References Andrus, 2007, LSU MS Thesis West Bay Andrus et al., 2012, State of the Coast Coleman, J. M., and Gagliano, S. M., 1964, GCAGS, v. 14, p Coleman and Prior, 1982 Kemp et al., 2011, National Audubon Society Kolker et al., 2012, ECSS

25 West Bay Region: historical subdeltas Coleman and Gagliano, 1964 Coleman and Prior, 1982

26 Island Construction ,000 yd 3 placed from Anchorage Island Dimensions: 5000 x 500 x 8 (+4 MSL) 60 acres Total BU acres built to date=511acres at $20M 4.19MCY $4.75/CY

27 Spring 2011 High Water Event From Kemp, 2011

28 Simulated Wave Height Reductions 0.18 m 0.1 m Wave reduction produced by placement of dredge-spoil islands 1 km

29 Considerations regarding diversions and subdeltas Natural subdeltas build and decline over timescales of years Sediment retention rates in natural subdeltas with open marine boundaries are 25-50% 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

30 Approaches to maximize performance: Pulsed operation Focus on times and locations that will produce maximum sediment concentration and discharge during operation Engineer basin to maximize retention of mud Reduce shear stress from waves and currents with island construction, like West Bay Reduce tidal prism to reduce tidal flow rates? Allow time for mud to consolidate between discharge pulses, to increase shear stress for erosion

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

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

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