ENGINEERING WATER AND SEDIMENT DIVERSIONS ALONG THE LOWERMOST MISSISSIPPI RIVER FOR COASTAL WETLAND MITIGATION EFFORTS IN LOUISIANA: NEW INSIGHTS PROVIDED BY THE 2011 MISSISSIPPI RIVER FLOOD AND THE OPENING OF THE BONNET CARRé SPILLWAY

After the 1927 flood of record on the Mississippi River, the United States Army Corps of Engineers constructed the Bonnet Carré Spillway (BCS) in Louisiana to prevent failure risk of the engineered Mississippi River levees that protect surrounding communities and infrastructure, including the city of New Orleans. The BCS diverts floodwaters from the Mississippi River to Lake Pontchartrain, thereby reducing the water discharge flowing past New Orleans. In the 81 years since construction, BCS has been opened ten times, diverting water for between 13 and 75 days. The 2011 Mississippi River flood, which had the highest peak discharge since 1927, necessitated opening the BCS for 42-days (May 9-June 20), and during this period, average spillway discharge (6010 m³ s^-1) amounted to 10-20% of the total river discharge. Sediment-laden water entered the adjacent floodway and was guided for 10 km into Lake Pontchartrain. The 2011 flood event thus presented a unique opportunity to examine the operation of the BCS as a possible analogy for evaluating how water and sediment diversions along the Mississippi River could be effectively engineered to mitigate coastal land loss in Louisiana. Field studies were conducted both during and after the BCS opening, in order to assess the quantity and character of sand routed from the river and into the spillway. Sand, rather than mud, is crucial for land and delta growth because it settles proximally to the fluvial source and provides a stable substrate for vegetation growth, which then aids mud deposition. Our findings show how local river conditions impacted the diversion of sand from the river into the spillway, by influencing channel bed sediment composition, and promoting elevated suspended sand concentrations within the water column. We conservatively estimate that 31-46% of the total sand load carried by the Mississippi River during the period of spillway opening was diverted into the floodway, by skimming only the upper 10-15% of the water column. Therefore, the results of this study have important implications for the design of engineered river diversions in Louisiana to mitigate coastal wetland loss, by demonstrating that sand can be diverted from the main channel if the appropriate morphological controls are present.