12 Mar 2025

Old River Control Low Sill Structure: Monitoring and Performance

Abstract: The Old River Low Sill Structure (ORLSS) is located approximately 25 mi south of Vidalia, LA, and is part of the Old River Control Complex. The structure is founded on steel piles and is a 566-foot-long concrete structure constructed in 1959. The structure resembles a dam with 11, 44-foot-wide gates to prevent migration of the Mississippi River’s current course to a hydraulically shorter course down the Atchafalaya River. A re-analysis was undertaken by the US Army Engineer Research and Development Center to investigate the current level of stability and the potential for adjusting the operating limitations placed on the structure because of damage during the 1973 flood. The investigation was performed by reviewing the design and post-design stability analyses, analyzing performance data, and performing an updated stability analysis. The stability analysis was performed with monitoring data that spanned several decades. Using historic monitoring data in the stability analysis, with a wide range of loading conditions, it was shown that uplift conditions were not as severe as previously considered. Performance data coupled with the stability analysis show that the structural performance supports the current operating conditions.