Estimate of decadal‐scale riverbed deformation and bed‐load sediment transport during flood events in the lowermost Mississippi River.

Riverbed changes are important to applications in engineering in the environment, such as navigation, salt‐water intrusion and habitat quality. This study employed decadal hydrographic data over the past 50 years (1963–2013), as well as daily water stages and discharges to outline the long‐term channel morphological changes and bed‐load sediments deposited by floods in the lowermost Mississippi River (LMR). To generate accurate bathymetric digital elevation models (DEMs), we interpolated the hydrographic survey in a channel‐centred coordinate system by employing ordinary kriging with anisotropy. Riverbed deformations were estimated by subtracting the elevations between DEMs at two different times to generate a series of DoDs (DEM of difference). The results from DoDs exhibit temporal riverbed change patterns and also reveal net degradation after the completion of the Old River Control Structure in 1963, except for the period 1992–2004. Spatially, over 32% of the total area experienced continuous erosion, with only 0.4% having continuous aggradation. Historical erosion over the past 50 years could be removing the alluvial veneer in sections of the river, and exposing more non‐alluvial sediments, particularly between river kilometre RK 230 and RK 40. Our analysis also shows that riverbed deformation patterns are mainly controlled by flood events, backwater effects, as well as the drawdown effect near the Bonnet Carré spillway. In this study, we also examine whether the LMR traps bed‐load sediment under flood conditions. The probability distribution estimated by Monte Carlo simulation suggests only low amounts of bed‐load sediments are deposited on the channel bed during floods. Spatially, the short‐term bed‐load erosion/deposition pattern roughly matches the decadal‐scale riverbed deformation estimated from DoDs. Moreover, this study finds that the trends between the thalweg changes and riverbed deformation are similar, suggesting that both of these patterns are controlled by the same factors. [ABSTRACT FROM AUTHOR]