Upper Mississippi River and Illinois River Flow Frequency Study

Location

Upper Mississippi River Basin

Description

The current Upper Mississippi River and Illinois River flow frequency study, completed in 2004, was created using a period of record ending in 1998. Since then, frequent and record setting major floods have occurred. The flood of 2019 was not only historic with regards to flood heights, but also in its duration. The Mississippi River was above flood stage at Cape Girardeau for 145 days. In addition, many communities between Dubuque, Iowa and St. Louis, Missouri have experienced the majority of their top 10 flood crests in the past 20 years. Significant flooding on the UMR occurred in 2001, 2008, 2011, 2013, 2014, 2018, and 2019. The Illinois River has experienced the same unprecedented flooding. At Peoria, IL (on the Illinois Waterway), 5 of the 10 highest flood crests have occurred since 1998, including the record crest in 2013.

This effort will update the flood flow frequency profiles on the Upper Mississippi River from Birds Point (Mile 0.0) to Anoka, MN (864.8), and the Illinois River from Grafton (Mile 0.0) to Dresden Island Lock and Dam (Mile 271.5). Previous methodologies will be reviewed and modified as needed to confirm they meet the current state of the practice of flood frequency calculation. The study will utilize input data gathered during the previous flow frequency effort, augmented with new observed data to complete a 126-year period of record (1898 to Present).

Completion of the proposed study, utilizing the updated period of record and new federal guidelines for determining flood flow frequencies, Bulletin 17C, will improve confidence and reliability of flow frequencies on the UMR and IWW to support flood risk management planning and communication and the USACE Dam and Levee Safety Programs. This will provide an opportunity to incorporate available climate change data into the analysis of a large river system using the latest USACE Guidance. Updated frequency profiles and associated documentation will be published at the completion of this effort.

Background
The benefits of an updated flow frequency study on the Upper Mississippi and Illinois Rivers will be realized immediately and ensuring water resource decisions are made using current and accurate data will provide a considerable value to the nation. The Corps continually engages with watershed stakeholders through its various programs and authorities. These stakeholders routinely ask what the effects these recent common and historic floods are on the previously calculated frequencies and have requested this data be included when making water resource decisions.

From FY2018 to present, the Corps provided technical assistance to the Upper Mississippi River Basin Association (UMRBA) on a Planning Assistance to States effort to identify Upper Mississippi River actions that have consensus and could have an immediate benefit. One of the high leverage actions identified in the UMRBA’s draft report included updating the flow frequencies. This information was viewed as fundamental to future planning and decision making. In addition, the Corps has been advancing the development of HEC-RAS hydraulic models on the Upper Mississippi and Illinois Rivers. The updated models on the Upper Mississippi River are complete and can be immediately utilized for updating the flow frequencies, which will save time and money.

Status

The project was partially funded from FYs 2021 thru 2023 for development of a detailed scope of work, inventorying existing data and performing most of an ECB 2018-14 climate assessment.

With additional funding, major tasks in this effort include:

• HEC-HMS models will be used to compute local inflow for the period of record.
• Existing HEC-RAS models will be used to route the period of record flows and compute profiles for frequency analysis.
• Existing CWMS models from regulated basins within the study area will be utilized as needed to route inflow and provide regulated vs unregulated flow information.
• Development of an interagency team which will include USACE (MVS, MVP, and MVR), FEMA, USGS, NWS, NRCS, and state agencies from Missouri, Illinois, and Iowa.
• Internal review and coordination will be conducted within the HH&C CoPs from MVD and HQ, and with HEC, the USACE Climate Change team, and the USACE hydrology committee, as required. Additional outside expert assistance may also be required.

In FY21, $180,000 in funding was provided through an on-going Lower Mississippi River Basin Flow Frequency Study to develop an advanced scope of work for the UMR Flow Frequency Study. The UMR is a major contributor to the flows on the Lower Mississippi River, and any data developed from the UMR flow frequency study must be consistent and compatible with this on-going study. In FY22, an additional $200,000 in combined funding through the Lower Mississippi River Basin Flow Frequency Study and FPMS was provided to continue coordination and complete Phase I of the study. In FY23, partial funding was received for the study once again via Mississippi River and Tributaries project, with a total allocation of $150,000.  In FY 2024, the project received $1,000,000 in earmark appropriations which will enable the team to complete the remaining tasks associated with Phase I of the study. 

Additional Information

Authority

FY23 and beyond – Water Resources and Development Act (WRDA) 2022

Summarized Project Costs

Major Work Current Year

FY 2024: The team will complete Phase I of the study. Major activities included completion of data inventorying, near completion of the ECB 2018-14 climate assessment, and development of a draft Hydrologic Engineering Management Plan.

Major Work Item Next Fiscal Year

FY 2025: If funded, the District will complete Phase II of III of the Flow Frequency Study.  The study is identified as a $1,000,000 earmark in the DRAFT FY25 Appropriations Bill.