Summary of the Executive Summary

The “Lines of Defense” include the outer continental shelf of the Gulf of Mexico, the barrier islands, bays, sounds, marsh land bridges, natural ridges, manmade ridges, flood gates, flood levees, pump stations, non-structural measures (i.e., home and building elevations, flood proofing), and evacuation routes.

Key takeaway: You must combine BOTH wetlands restoration and engineering solutions.

Since Hurricane Katrina, much has been discussed regarding the “Dutch model” of flood protection. The flood surge events in Holland are not due to hurricanes and are far lower in height than the surges in Louisiana; nevertheless, the Dutch system has achieved a high level of flood protection using major engineering structures, with results similar to what the Lines of Defense proposes for Louisiana.

However, the Dutch have not managed their coastal estuary at a sustainable level. The result has been a dramatic loss 10 of 408 (90%) of coastal wetlands and severe water quality problems which they are now retroactively attempting to remedy. In addition, the cumulative actions of the Dutch have engineered a landscape where approximately 60 percent of the population lives below sea level, similar to the New Orleans area. Thus, the Dutch will forever be actively managing their anthropogenic landscape, specifically with the challenge of future sea level rise.

Through a cooperative effort with the Netherlands and the U.S. Army Corps of Engineers, a study and report was completed that is titled “A Dutch Perspective on Coastal Louisiana Flood Risk and Landscape Stabilization” (International Research Office US Army, 2007). Although not fully appreciative of the local subtleties of our landscape, the report suggests using “multiple lines of defense” and concludes with a recommendation for southeast Louisiana titled “open estuary”. Their open coast recommendation excludes levees blocking the estuary so that the estuary may accommodate surge. The report recognizes the value and absolute necessity of a wetland buffer complementing a tight levee around New Orleans. The report also emphasizes the hydrologic aspects of the estuary and includes an eclectic approach to restoration and flood protection very similar to proposals in this report. Some striking differences are that the Dutch report does not consider the roles of pumping stations, non-structural (elevation), or evacuation routes as part of their flood risk reduction plan.

The Multiple Lines of Defense Strategy does not recommend a single continuous barrier levee for the coast.

The restoration recommendations include diversions, marsh creation, shoreline protection, barrier islands restoration, structures to reduce saltwater intrusion and oyster reef restoration.

Diversions are a key tool in the wetlands strategy.

The Multiple Lines of Defense levee alignments are intended to be fully compatible with regular moderate flooding of the wetlands with diversions and also with more extreme pulsing events which would raise water levels even higher. In all, twenty-eight diversions, three controlled crevasse-type pulsing diversions, and three land-building diversions are proposed.

“Sustaining diversions” are intended to sustain marsh land bridges identified as a Line of Defense. These diversions are generally less than 40,000 cubic feet per second (cfs) and would include structures to pass the water through the MR&T levee, such as siphons, box culverts, etc. Although the processes involved need ongoing monitoring and assessment, the working hypothesis is that nutrients will stimulate productivity that will increase resiliency and sustainability of the marsh. In addition, salinity may be managed to emulate a natural spring cycle of overflow and freshening while avoiding severe salinity spikes that may be detrimental.

“Delta-building diversions” have annual discharges over 75,000 cfs with deep-water conveyance channels through the MR&T levee to convey significant discharge from normal spring Mississippi River stages. They are intended to flow continuously over long periods to build substantial wetlands in an areas in need flood protection.

“Controlled-crevasse diversions” are intended to have periodic discharges when the river is exceptionally high so that large scale overland flow is possible without a large deep-water conveyance channel. These structures would be opened possibly every few years when there is sufficient head on the river. The controlled crevasses would likely be designed similarly to the existing Bonnet Carré or Morganza Spillways but with significant design changes to maximize restoration benefits. The use of the spillway type structure as a controlled crevasse diversion could be vital to introducing sediment into the estuary and is intended to be an important adaptive management tool that could be used more aggressively if necessary to maintain the wetland extent and functioning of the estuary.

In the spring of normal Mississippi River stage years, the diversion proposals could divert approximately 60% directly into the estuary. In the Atchafalaya River, approximately 10% of the discharge would be diverted in most years either east or west into wetlands with the remaining discharge continuing to build the Atchafalaya and Wax Lake Deltas and contribute sediment to long-shore drift.

A River Flood Restoration Action Plan is proposed to take advantage of an  exceptionally high water event on the Mississippi or Atchafalaya Rivers, such as those experienced in 1997 and 2008. In these high flood stage years, the combined proposals could divert 1,300,000 cfs of the discharge from the Mississippi River into the wetlands through diversions and spillways.