Southern Louisiana’s coastal marshes have collapsed over the last century and now the State is working to rebuild them. This land where current coastal wetlands sit was created around 4000 years ago by the prograding Mississippi River delta. Historically, annual floods overtopped the banks of the river and delivered sediment-laden freshwater to the surrounding area. The marshes are a productive ecosystem providing an abundance of fish for commercial fisherman and a means of subsistence for local Native American communities who live outside the levee system. Levees were constructed to prevent the Mississippi River from flooding the major cities on its banks. While this offered protection to some by preventing the river from overtopping its banks, the levee system eliminates vital freshwater and sediment inputs into the coastal marshes. Because of this, coastal Louisiana has lost 2000 square miles of land over the last century.
To address this land loss crisis, the state of Louisiana produced the Coastal Master plan, a list of projects to be constructed in the next 50 years to reduce land loss and protect people from hurricanes. The plan is expected to cost over $50 billion, with the largest investment being the $1.4 billion Mid-Barataria sediment diversion that is currently in its design phase. A sediment diversion is a controlled opening in the levee system that allows floodwater to pass through the levee into the marsh to deposit sediment, rebuilding land each time it is activated. While sediment diversions are necessary to rebuild the Louisiana Coast, the operation of these diversions can cause dangerous flooding in areas outside the immediate receiving basin, especially during the first year of operation when distributary channels have not been established. My research goals were to determine how these distributary channels will develop under different first year diversion operations and how the choice in operation will affect the flood risk for the people that live in the receiving basin.
I built a hydrodynamic and sediment transport model to analyze the impact of various first-year diversion operation scenarios on the distant receiving basin. These scenarios were run under high and normal flow conditions of the Mississippi River and compared to the baseline model run with no diversion. During a drought or low flow year, the diversion would most likely not be in operation, so this was not included. Three scenarios were modeled: 50%, 75%, and 100% of the maximum diversion capacity of 75,000 cfs. Previous research determined that the diversion should open when the Mississippi River discharge reaches 600,000 cfs and increase to the full capacity when the river reaches 1,000,000 cfs to maximize sand suspension. An example of theses operation scenarios based upon 2017 river data is shown in Figure 1.
An area of great interest to me is Grand Bayou located 17 miles south of the diversion opening. A small Native American community lives along the banks of this bayou among the deteriorating marshlands. Any increase in water level or velocity can further erode this fragile ecosystem and overtop access roads in the area. The model shows that when the diversion is at maximum capacity water levels in Grand Bayou peak at 0.45 m and 0.3 m above baseline during a high and normal river flow year, respectively. Reducing the diversion’s capacity to 50% during the first year of use would reduce the peak flood level by 33% in both river flow years. In addition, peak velocities are reduced when the diversion is operated at a lower capacity (Figure 2). These results show that a small reduction in the first year’s maximum diversion flow can mitigate the most detrimental impacts of the diversion.
My research focuses on the nexus of river dynamics, climate change, and the human made solutions to these problems. I am specifically concerned with indigenous people who are historically underrepresented in risk analysis studies and policy decisions. In all areas of my research, I focus on coproduction of knowledge through collaboration with the local community, weaving indigenous knowledge with western science to create usable results for the community. This research provided information to the people of Grand Bayou that shows the diversion operation can have a detrimental impact on their land. While sediment diversions are necessary for coastal restoration, this community will be able to use these results to advocate for strict diversion operating procedures to reduce flood risk on their land.
By Dylan Blaskey
University of Colorado Boulder