Estimating the Future Frequency of Bonnet Carré Spillway Openings

The Bonnet Carre Spillway (BCS) is a flood control project built after the Great Mississippi River Flood of 1927. Between its completion in 1931 and 2020, the BCS has been opened a total of 15 times. However, the rate of openings has increased, from once every 9 years from 1931-2010, to once every 2 years from 2010-2020. Furthermore, some of these openings are historically anomalous in their season timing, such as having a new earliest opening (2016), and their intra-annual frequency, such as the first double opening in one year (2019) and the longest consecutive opening (2019).

This study utilizes ERA5 reanalysis total precipitable water (TPW) between 1991-2020 to train a self-organizing map (SOM) to characterize large-scale TPW patterns associated with lower Mississippi River (MR) flooding and BCS operation. Eighteen historical TPW patterns are resolved by the SOM (i.e., nodes), and node preferences during BCS operation years are identified. Only 0.83% of days with low TPW centered over the MR basin occurred during the during the record-setting 2019 BCS opening instead of the expected frequency of 3.3%. Meanwhile, days with moderately high TPW stretching across the entire MR basin corresponded to 5.85% during 2019. Thus, this particular BCs opening was associated with a reduction in the SOMs drier patterns and an increase in its moistest nodes. TPW projections from a global climate model (GCM) from 2020 to 2100 are then inserted into the SOM to diagnose future changes in BCS-associated moisture patterns.