Adapting to Change in the Lowermost Mississippi River: Implications for Navigation, Flood Control and Restoration of the Delta Ecosystem

Abstract

The Lowermost Mississippi River (LMMR), from the Gulf of Mexico to 520 km above Head of Passes, remains critical for flood conveyance and transport of agricultural and industrial bulk products from the central United States. The US Army Corps of Engineers (USACE) has managed it with little change for 80 years using the levees and spillways constructed under the Mississippi River and Tributaries project (MR{&}T). At the same time, public demand for reconnection of the Mississippi to the deteriorating delta ecosystem has grown. Significant sediment diversion projects have been authorized downstream of New Orleans to restart deltaic wetland building to conserve fish and wildlife resources and reduce hurricane flood risk to delta communities. Recent research and observations from the back-to-back record 2011 high-, and 2012 low-discharge events indicate that LMMR hydraulics have changed significantly, and that sea level rise, subsidence and a reduction in sand transport through the Plaquemines-Balize birdsfoot delta (PBD) now favor formation of new, unregulated outlets upstream. During the peak of the 2011 flood, only 27 {%} of the 65,000 m3-s−1 discharge entering the LMMR reached the Gulf via Head of Passes, compared to 36 {%} passing through the two outlets of the shorter Atchafalaya distributary. About 20 {%} of the water lost from the LMMR occurred through unregulated flow overbank and through small, but growing, distributaries between New Orleans and Head of Passes. Adding delta restoration to existing USACE missions will require adjusting the MR{&}T but has potential to lower flood flow lines and reduce navigation dredging costs sufficiently to allow LMMR ports to accommodate larger, Post-Panamax ships.