Nonstationary streamflow effects on backwater flood management of the Atchafalaya Basin, USA

A non-stationary increasing streamflow trend has been observed on the Mississippi River and other major river basins around the world. The current study analyzed the non-stationary streamflow effects (NSFEs) on flood management in backwater areas adjacent to the Atchafalaya Basin floodway in Louisiana, USA. A continuous simulation hydrology model coupled with a quasi-two-dimensional hydrodynamic model of the basin floodway and surrounding regions was used to develop over 180 simulation scenarios by superimposing local flood events (early summer 2014 and late summer 2016) against 90-years' worth of daily Atchafalaya River streamflow hydrographs. The NSFE on the Atchafalaya River induced substantial reductions in the performance of major flood regulating structures with seasonal effects based on the annual flood cycle. Capacity reductions at the structures were demonstrated to trigger a cascade of effects in ostensibly protected backwater areas including amplification of erosion potential near the levee and within tidal passes during early summer floods. Increases in mean and peak flood levels on the order of 15-20 cm during local storm events were shown to extend as far as 20 km away from the floodway protection levee during both early and late summer local flooding scenarios. Low-lying areas closest to the levee were adversely affected during both the high (early summer) and low flow (late summer) periods of the annual discharge cycle. The approach and findings of this study are relevant for risk management in river basins around the world affected by NSFEs.