The Next 50 Years: Sediment diversions as a necessary restoration tool

This is the second post in our “The Next 50 Years” Coastal Master Plan series. Check back as we continue diving into the master plan and what it means for the people and environment of the Mississippi River Delta.

By Alisha A. Renfro, Ph.D., Staff Scientist, National Wildlife Federation

Since 1932, coastal Louisiana has lost almost 1,900 square miles of land and if bold action is not taken, another 1,700 square miles could be lost by 2060. Louisiana’s 2012 Coastal Master Plan, passed last month by the state legislature, is a 50-year strategy that aims to realistically, but aggressively, use numerous restoration tools to ensure a sustainable and more resilient coast. The plan’s analysis indicates that sediment diversions, structures that allow Mississippi and Atchafalaya River water to be moved strategically into the surrounding wetlands, are critical projects for halting land loss and building a more sustainable coastline for the future of the Mississippi River Delta.

Symbol used for sediment diversions in the 2012 Coastal Master Plan. Sediment diversions use channels and/or structures to divert sediment and fresh water from the Mississippi and Atchafalaya Rivers into adjacent basins. Source: Coastal Restoration & Protection Authority (CPRA)

Ten projects identified as sediment diversions were judged as important restoration tools in the plan. Sediment diversions mimic some of the natural processes that once built the Mississippi River deltaic region. By operating these types of control structures when river discharge is high, the large amount of sediment, that is being moved along with the water, is captured and funneled into the nearby wetlands. The introduction of sediment would not only help restore areas that are today shallow open water, but it would also help prevent future wetland loss due to rising sea levels.

Sediment diversions can build new marsh by transporting sediment-laden water from the river into the nearby basin. The heavier sand sediment is deposited quickly and over time, and a marsh platform is built. Once vegetation begins to colonize the area, sediment deposition increases as silts and clays are trapped by plants, and a new marsh will emerge. Sediment diversions can also help sustain the newly-built and existing marsh as sea levels rise, by providing a source of sediment that can be deposited on the marsh surface. Ultimately, this can increase the elevation of the marsh surface, which helps prevent prolonged flooding of the marsh, even as sea levels rise. Sediment diversion critics have voiced concerns about the introduction of fresh water into these basins and have questioned whether diversions are essential to restoring Louisiana’s coast.

This map of southeastern Louisiana shows the locations of the sediment diversion projects included in the 2012 Coastal Master Plan, represented by brown circled triangles like the one in the previous image. The brown stippled areas show the diversion impact areas. Credit: CPRA

To evaluate the necessity of the sediment diversion projects in the master plan, the authors conducted an experiment to analyze the amount of land that could be built only using other types of restoration tools. The results of this experiment indicated that by removing the sediment diversion projects from the plan, total land built would be reduced by 340 to 630 square miles over the 50 years. This indicates that sediment diversion projects are crucial projects in the plan and necessary for rebuilding the Mississippi River Delta.

A similar experiment was conducted to address questions about using a few larger sediment diversions versus multiple smaller diversions. The multiple small diversions not only affected the benefits that people get from the land, such as oysters, as much or more than larger diversions, but the smaller diversions also built significantly less land over the next 50 years.

The restoration projects selected in the 2012 Coastal Master Plan may potentially build or sustain up to 800 square miles of land over the next 50 years – land that otherwise would be lost. While sediment diversions are not the only types of restoration project identified as critical to the future of the Louisiana coastline, scientific assessment suggests they are absolutely necessary. Other types of projects, such as barrier island restoration and marsh creation, can build land quickly, but without tapping into the sustainable power, sediment and fresh water of the river, these projects cannot be maintained over the long term.

The land loss crisis in Louisiana requires aggressive action. Louisiana’s 2012 Coastal Master Plan shows us that the path forward for building a more sustainable and resilient coastline is by reconnecting the Mississippi River to the landscape it once built.