Flow Frequency Study of The Upper Mississippi River

The Saint Louis, Rock Island and Saint Paul Districts of the U.S. Army Corps of Engineers, Mississippi Valley Division are working jointly on a study of Flow Frequency on the Upper Mississippi River, UMR-FFS. Refined flood frequency information from this effort is essential for resilience of the many water management programs and projects along the Upper Mississippi and Illinois rivers; such as evaluation of levee height increases and developing flood risk management strategies. A previous study was conducted in 2004 and incorporated a period of record through 1998. Since that time, there have been several significant flood events which include the 2001, 2008, 2011, 2013, 2014, 2018, and 2019 floods on the Mississippi River and 2013 on the Illinois. The 2019 flood on the Mississippi produced a new record for the duration of days above flood stage (145 days at Cape Girardeau). This will be used in conjunction with existing data gathered during the previous flow frequency study.

In addition to a more extensive period of record, this study will leverage newer hydrologic guidance for assessing flow frequency, such as methodology in USACE ECB 2018-14 and USGS Bulletin 17C. The ability to provide estimates of uncertainty around flow frequency has increased in importance for supporting initiatives of the Corps and partner agencies. Also being utilized are a newly completed suite of HEC-RAS hydraulic models for the Upper Mississippi and Illinois River main stems. The last phase of these UMR hydraulic models was the Illinois River portion finished in 2021. These models cover approximately 864 miles of the Mississippi River and approximately 272 miles of the Illinois River and are similar in coverage to the extent anticipated for this flow frequency study.

The Scope of Work for the UMR-FFS is partitioned into 3 phases with Phase 1 concluding in 2022. Phase 1 consisted of compiling necessary data records, an inventory of hydraulic structures, qualitative assessment of climate change considerations and definition of the overall engineering plan for executing the remainder of the study. Phase 2 will include determination of the period of record, the hydrologic development of unregulated and regulated daily flow time series on the Upper Mississippi and Illinois Rivers, as well as hydraulic routing with HEC-RAS. The hydrologic routing for tributary flows will be accomplished with a combination of HEC-ResSim and HEC-HMS models. Phase 3 will consist of annual peak frequency analysis, development of hydraulic profiles, generation of final study products and report, as well as coordinated review with State and Federal agencies. Deliverables from this project will include a web-based, visual database for the Mississippi River Flow-Frequency Assessment to host the data products generated in Phase I of the study effort, the regulated and unregulated datasets produced in Phase II, and the final prepared frequency curves generated in Phase III.