Diversion Basics – Notes for the Expert Panel, 10 AM, Tuesday, 28 October 2014, Baton Rouge by R. Eugene Turner
An excellent overview of the science behind river diversions and why they do not build land. The starting point for learning about diversions.
Navigation Issues: Big River Coalition: Comments on Mid-Barataria Sediment Diversion Permit Application (2017) – Big River Coalition
This report documents how a natural diversion – a break in the river levee – lead to significant land loss, not land gain.
“Quantifying the effects of active natural and constructed crevasses is critical to the planning and success of future ecosystem restoration activities. This document provides a historical overview of landscape changes within the vicinity of the natural crevasses near Fort St. Philip, Louisiana. A significant event influencing landscape change within the Fort St. Philip study area was the breaching of the eastern levee of the Mississippi River. Initially, the river water that was diverted through these crevasse channels physically removed significant marsh areas within the study area. These initial direct impacts were succeeded by several decades of larger regional loss patterns driven by subsidence and other episodic events (e.g, hurricanes and floods), and recent localized land gains. These increases in land area are potentially the long-term results of the Fort St. Philip crevasses, and the short-term impacts of delta management activities. However, for the majority of the 1956-2008 period of analysis, the crevassing of the eastern bank of the Mississippi River levee was a loss accelerant in the Fort St. Philip area.”
The first meeting of the Expert Panel on Diversion Planning and Implementation focused on uncertainty and the ways in which scientific and engineering uncertainty could be understood and reduced to the maximum extent possible. Discussions centered around six themes: (1) the importance of data, (2) the absence of analogs, (3) uncertainty in ecological outcomes, (4) uncertainty in social and economic outcomes, (5) design and operational uncertainties, and (6) framing expectations in light of uncertainties. Panel recommendations covered methods by which biophysical data should be collected and disseminated, social data that should be incorporated and linked to biophysical data, models that should be developed and used as an ensemble, coordination and communication that should be undertaken, and types of additional simulations and experiments that should be conducted.
The Louisiana Coastal Restoration Authority then said it would be going forward, seemingly in the face of this report:
Teal, J.M., R. Best, J. Caffrey, C.S. Hopkinson, K.L. McKee, J.T. Morris, S. Newman and B. Orem. 2012. Mississippi River Freshwater Diversions in Southern Louisiana: Effects on Wetland Vegetation, Soils, and Elevation. Edited by A.J. Lewitus, M. Croom, T. Davison, D.M. Kidwell, B.A. Kleiss, J.W. Pahl, and C.M. Swarzenski. Final Report to the State of Louisiana and the U.S. Army Corps of Engineers through the Louisiana Coastal Area Science & Technology Program; coordinated by the National Oceanic and Atmospheric Administration. 49 pages.
Professor Roberts makes a convincing case that river sediment diversions are the only meaningful way to build coastal line, but that the amount of coast that can be built is very limited. At best, a small part of the coast can be preserved through the use of one or two large diversions, positioned inland, rather than at the edge of the coast. Multiple small diversions will build no meaningful land. This raises several difficult questions:
1) Where will the diversions be built?
2) Major diversions will slow river flow during floods, causing sediment to drop out in the channel which would otherwise be carried out into the gulf. This will create shoals in the river and interfere with navigation. This will require additional dredging, which may still be unable to keep the river open to an adequate depth.
This is a report prepared for the Coastal Protection and Restoration Authority of Louisiana in 2011. It is designed to support the coastal restoration plan and should be read very critically.
Sediment Diversions for Delta Restoration (slides that go with the report)
This is a modeling document prepared for the Corps.