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, my research shows the operation of this diversion can cause dangerous flooding in areas outside the immediate receiving basin, during the first year of operation when distributary channels have not been established.

In specific, Grand Bayou is 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.