Influences of Channel Dredging on Avulsion Potential at the Atchafalaya River

In 1950, the U.S. Army Corps of Engineers (USACE) reported a rapid increase of water discharge from the Mississippi River to its distributary channel; the Atchafalaya River. If not prevented by man-made structures, the complete capture of the Mississippi River by the Atchafalaya River was predicted. The USACE report cites multiple causes for the observed increase in discharge partitioning, yet fails to assess the largescale channel dredging operations conducted throughout the Atchafalaya River Basin during the 1930's and 1940's as a potential cause for the increased discharge. To assess the role man-made interventions, specifically channel dredging, played in the increase of discharge partitioning down the Atchafalaya River, this study incorporates a one-dimensional backwater flow model based on conservation of fluid mass and momentum equations and utilizes the geological and engineering data of the Atchafalaya, Mississippi and Old River Systems compiled by the USACE from 1880--1950. Two models were developed from 75 channel cross-sections measured during hydrographic surveys of the Atchafalaya Basin and river systems in 1916--17 and 1950, representing the pre-dredging and post-dredging conditions of the Atchafalaya River. A third model was adapted from the 1916--17 pre-dredging model and incorporates the dredging of a 4 meter deep channel from Morgan City, Louisiana to the headwaters of the Atchafalaya River at Simmesport, Louisiana. Based on this one-dimensional modeling approach, comparison of the 1916--17 Pre-Dredging (16%) and Proposed Dredging (26%) models of discharge partition percentages flowing into the Atchafalaya River from the Mississippi River indicates that dredging is potentially associated with an increase of Atchafalaya River flow partition discharge of +10% under 1916--17 historically measured discharge conditions of 18,000 (m3/s) total discharge above the bifurcation (TDAB). By comparison, the historically measured discharge partition percentages recorded by the USACE for 1916--17 (11%) at 18,000 (m3/s) TDAB and 1950 (22%) at 25,000 (m3/s) TDAB indicate a similar increase in discharge partitioning of +11% change between 1916--17 pre-dredging and 1950 post-dredging conditions. However, due to the limitations of the one-dimensional model to simulate flow through additional downstream bifurcations, further multi-dimensional analysis is needed before definite causation can be warranted.