Section 3.1: Mississippi River Sediment Diversion
The most ambitious of the engineering projects designed to stop, and potentially reverse, ongoing processes of coastal erosion involves the diversion of sediment from the Mississippi River to locations in the Mississippi River Delta Gulf Coast that are experiencing erosion.
The basic concept behind sediment diversion relates to some of the geological principles discussed earlier in this primer. If sediment is not being deposited along the coastline, then it is often already in the process of eroding. We have engineered the Mississippi River in such a way that sediment is propelled out into the Gulf of Mexico through the main river channels of the Birdfoot Delta, preventing sediment flow through the distributaries and flanking alluvial wetland of the Mississippi to the delta’s coastline. This means that sediment no longer reaches sections of the Louisiana coast that it did even in the relatively recent past—a key geological root of coastal erosion.
Sediment diversion would effectively reopen certain distributaries flowing from the main channel of the Mississippi River, which would carry sediment to portions of the coast that no longer receive it. One such effort is the Mid Barataria Sediment Diversion Project. This project would open a sediment diversion canal from the main channel of the Mississippi River at Myrtle Grove in Plaquemines Parish, which would drain into Barataria Bay. In addition to providing a source of sediment to Barataria Bay, which would help stabilize and perhaps rebuild land surfaces, this diversion would also increase the flow of freshwater, fostering the regeneration of coastal wetland habitats.
In addition to the Mid-Barataria Sediment Diversion Project, there are at least five other potential diversion projects that may be built in the future: the Lower Barataria, Mid-Breton, Lower Breton, Atchafalaya River, and Maurepas diversions. Together, all of the planned diversion projects will cost around $5 billion and will dramatically reshape wetland landscapes from Vermillion Bay to the Chandeleur Sound. Both the Mid- Barataria and Mid-Breton diversions are in the final stages of planning in 2019 and are due to be constructed over the coming several years.
These sediment diversion projects are controversial in terms of their impacts on the on the saltwater wetlands around Plaquemines, St. Bernard, Lafourche, and Jefferson Parishes. Above all, many residents of these parishes who rely on fishing for saltwater species such as oysters and shrimp are concerned that the major influx of freshwater from the Mississippi River will degrade the salt-water marsh ecosystems and otherwise negatively impact fish and shellfish species—as has happened this year with the protracted opening of the Bonnet Carre spillway.
More generally, important questions remain about some of the socioeconomic and political calculations having to do with the diversion projects. As these are very large-scale and long-term engineering approaches to coastal restoration, they are intended to produce results at the scale of decades to millennia; in other words, the time scales at which the geology of the Mississippi has always worked in the past. In contrast, the diversion projects are likely to have serious immediate impacts on local fishing communities, which will no doubt have negative economic and social consequences for certain members of those communities.
The inherent complexity of human interactions with coastal landscapes almost always leads to extremely difficult ethical decisions having to do coastal restoration strategies. The controversy surrounding the sediment diversion projects is a major manifestation of this problem but it will also appear again in the following sections having to do with other coastal restoration strategies.