Mitigating land loss in coastal Louisiana by controlled diversion of Mississippi River sand

Abstract

After the 1927 flood of record on the Mississippi River, the Bonnet Carré Spillway in Louisiana was constructed as a flood control operation. When it is opened, the spillway diverts floodwaters from the Mississippi River to Lake Pontchartrain¹, to reduce the water discharge flowing past New Orleans. During the 2011 Mississippi River flood, which had the highest peak discharge since 1927, the Bonnet Carré Spillway was opened for 42 days, from 9 May to 20 June. During this period, the average spillway discharge of 6,010 m³ s⁻¹ amounted to 10–20% of the total river flood discharge. Here we present measurements of the areal extent and thickness of new sediments in the floodway, following the 2011 Mississippi flood. Only the upper 10–15% of the river water column was skimmed into the floodway. Yet, we conservatively estimate that 31–46% of the total sand load carried by the Mississippi River during the period of spillway opening was diverted. We find that local river conditions led to increased concentrations of suspended sand in the upper water column and thus led to diversion of sand from the river into the spillway. We conclude that an appropriate design of engineered river diversions in Louisiana can help mitigate coastal wetland loss.