USING 1D2D HYDRAULIC MODELING TO ASSESS THE EFFECTIVENESS OF PROPOSED FLOW-CONNECTIVITY ENHANCEMENT PROJECTS IN THE ATCHAFALAYA RIVER BASIN, LOUISIANA

The Atchafalaya River (AR) in Louisiana, the nation’s largest river swamp, is the principal distributary of the Mississippi River (MR). After the AR Basin (ARB) was designated a federal floodway following the destructive 1927 MR flood, it was extensively modified to accommodate a portion of the MR flow to mitigate flooding in s. Louisiana. It currently receives a mandated 30% of the combined flows of the MR and Red River through the Old River Control Structure. Anthropogenic modifications and associated increased flows resulted in major geomorphic change along the AR, and altered connectivity between the AR and its floodplain.

Still, the floodplain wetlands provide many ecosystem services, including flood protection, biogeochemical cycling, wildlife habitat, and cultural resources. Decades of research and personal observation have convinced a diversity of stakeholder groups that restoring flow connectivity is essential to maintaining the ARB’s ecological integrity. However, spatially explicit data regarding hydrologic and geomorphic conditions remain a major research need to inform adaptive management practices.

To address this data gap, we collaborated with agencies and used existing geospatial datasets to develop a 1D2D hydraulic model using SOBEK 2.12 to realistically model a range of physical parameters (residence times, inundation extent, water depths, velocity) over a large spatial scale. While many restoration projects focus on short reaches and are based on opportunistic funding availability, a major goal here was to design a project that could assess already-proposed flow-connectivity enhancement projects in the 174 km² Flat Lake Water Management Unit (FLWMU), a hydrologic subunit of the AR that was previously identified as a key area for restoring connectivity. Calibration runs indicate good agreement with observed flows; the three mainstem gages closest to FLWMU have MAEs (m) of 0.10, 0.27, and 0.12 and RMSEs (m) of 0.11, 0.27, and 0.15 respectively. An additional verification step examining inundation goodness of fit (F≈65) is comparable to accuracies reported in other studies. Ongoing research involves stepped implementation of the 171 recommended enhancement projects using fixed-discharge scenarios to quantify connectivity improvements; preliminary results will be discussed.