EP003-0006
Four decades of channel dynamics of a large regulated alluvial river: Results from the uppermost Atchafalaya River

Monday, 7 December 2020
Poster
Bo Wang1,2, Yi-Jun Xu3, Ming Tang4, Wei Xu5, Frank T-C Tsai6 and Laurence C Smith1, (1)Brown University, Providence, RI, United States, (2)Louisiana State University, Baton Rouge, LA, United States, (3)Louisiana State University, School of Renewable Natural Resources, Baton Rouge, LA, United States, (4)East China Normal University, State Key Laboratory of Estuarine and Coastal Research, Shanghai, China, (5)East China Normal University, Shanghai, China, (6)LSU, Department of Civil and Environmental Engineering, Baton Rouge, LA, United States
Abstract:
Many rivers in the world are engineered by societies to meet their needs, such as navigation, flood control, and other purposes. These engineering efforts often oppose the natural tendency of river channel development, especially in alluvial rivers, to sediment transport, flooding, and migration. Previous studies have established the knowledge that under natural conditions, alluvial river confluence zones experience channel scour followed with middle channel bar development. Less care is given to the bed evolution downstream of large alluvial river confluences under man-controlled conditions, such as discharge regulation and levee confinement. Here we conducted a study focused on four-decadal morphologic changes of the 69-km uppermost Atchafalaya River, which is downstream of the confluence of the Mississippi River outflow channel and the Red River. The flow from the Mississippi River into the Atchafalaya is strictly regulated and sediment transport in this large river is highly complex. The primary purpose of this study is to test two hypothesis: (1) riverbed adjustments downstream of a large confluence take a long time after increased water discharge, and (2) confluence scour in engineering-control large alluvial rivers can be more intensive and last longer than natural small rivers, preventing mid-channel bar building downstream. We utilized single-beam bathymetry survey data collected by the U.S. Army Corps of Engineers in 1967, 1977, 1989, 1998, and 2006 to quantify channel deformation of the reach after the flow regulation began in 1963. Changes in average riverbed elevation and sediment volume were estimated. Results show that extensive bed degradation occurred between 1967 and 2006 and the average bed elevation reduced by 3.7 m. Based on the channel bed assessment, a total volume of 106 × 106 m3 sediment was scoured from the uppermost 69 kilometers of the Atchafalaya over the 40 years. The decadal volume changes in bed material were, respectively, -82 × 106 m3, -26 × 106 m3, +68 × 106 m3, and -66 × 106 m3 during 1967-1977, 1977-1989, 1989-1998, and 1998-2006. The bed aggradation occurred during 1989-1998, which may have been caused by the upstream building of two diversion channels in 1986 and 1991, as part of the Old River Control Complex. The locations of the deposition and erosion during 1989-1998 and 1998-2006 were highly symmetric, indicating that the deposited bed materials during 1989-1998 had been very likely removed by the flow during 1998-2006. These findings from the field measurements have not only revealed the complexity of morphologic adjustments of a river channel in response to intensive engineering disturbances but also provide useful information for further modeling studies on bed deformation downstream of a large river confluence.
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