40-year Bed Changes in the Upper Atchafalaya River downstream of a Heavily Regulated Confluence

Keywords: River confluence zone; riverbed dynamics; Mississippi-Atchafalaya Rivers;
Session Type: Virtual Paper
Day: Thursday
Session Start / End Time: 4/8/2021 11:10 AM (Pacific Time (US & Canada)) - 4/8/2021 12:25 PM (Pacific Time (US & Canada))
Room: Virtual 3

Abstract

Riverbed deformation downstream of river confluences were well studied in natural rivers. However, less attention is given to bed evolution downstream of large alluvial river confluences under heavily engineered conditions, such as discharge regulation and levee confinement. Here we present four-decades of morphologic changes along the 69-km uppermost Atchafalaya River, a downstream distributary of the confluence of the Mississippi River outflow channel and the Red River. We aim to find the answers to two critical questions: (1) how do riverbed elevations downstream of a large engineered confluence adjust to controlled water discharge, and (2) whether the downstream channel can keep stable due to intensive engineering control. We utilize single-beam bathymetry data collected in 1967, 1977, 1989, 1998, and 2006 to quantify riverbed deformation of the reach after flow regulation began in 1963. Results show that between 1967 and 2006, extensive bed degradation occurred and the average bed elevation reduced by 3.7 m. A total volume of 106 × 106 m3 sediment was scoured. Bed aggravation only occurred during 1989-1998 in response to excess sediment input from the Mississippi River due to large floods. But the same amount of riverbed deposit eroded in the following eight years, demonstrating how quickly a large regulated river can diminish a perturbation owing to excess sediment input. These findings not only reveal the complexity of morphologic adjustments of a river channel in response to intensive engineering disturbances but also provide useful information for future modeling studies and management plans for the engineered river confluences.