SEDIMENT TRANSPORT AND CHANNEL MORPHOLOGY OF A NATURAL AND A LEVEED ALLUVIAL RIVER

Alluvial rivers are shaped by interactions of flow and sediment transport. Their lower reaches to the world’s oceans are highly dynamic, often presenting engineering and management challenges. This thesis research aimed to investigate channel dynamics and sediment transport in a natural river and a highly engineered river in South Louisiana, in order to gain much-needed science information for helping develop sustainable practices in river engineering, sediment management, and coastal restoration and protection. Especially, the thesis research examined (1) riverbed deformation from bank to bank in the final 500-km reach of the Mississippi River, (2) bed material transport at the Mississippi-Atchafalaya River diversion, and (3) long-term and short-term flood effects on the morphological changes of the Amite-Comite River confluence. The research employed morphological, hydrodynamic, and geospatial modeling and analysis. The research found that from 1992 to 2013 the lowermost Mississippi River channel trapped 337 × 106 m3 sediment, equal to about 70% of riverine sand input from the upstream channel. The finding rejects the initial hypothesis that the highly engineered Mississippi River acts as a conduit for sediment transport. Sediment deposition mainly occurred in the immediate channel downstream of the Mississippi-Atchafalaya River diversion and the reach between RK 386 and RK 163, reflecting flow reduction and backwater influences. The bed material transport assessment revealed that in the recent decade the engineering-controlled Mississippi-Atchafalaya River diversion showed a slight disproportional transport of bed material loads. On average 24% of the Mississippi River was diverted into the Atchafalaya, but only 22% of bed material loads moved into the diversion outflow channel (i.e. 47 MT out of 215 MT). The confluence of Amite and Comite River continuously migrated about 55 m downstream between 2002 and 2017. Sediment deposition on the main channel side of the confluence mouth bar is the major driver for the confluence migration. Regression analysis shows that the increase rate of the vegetated area of the bar is highly related to the days of moderate floods. Short-term Laser scanning measurements reveal that a single flood with the intensity close to a moderate flood could double the projected surface area of the mouth bar and increased its volume by 68%. Overall, the thesis research shows the complexity of sediment transport in the lower reach of a large alluvial river, in that distinctive bed deformation can occur in different reaches because of flow deduction and backwater effects. Our study is the first try of estimating bed material load at a largely controlled bifurcation based on a simple, well-established bed material transport model. The study also highlights the importance of episodic floods on the evolution and migration of a river confluence