By Gregg Thielman, Houston-Moore Group
One of several studies researching potential design improvements and changes to the Fargo-Moorhead (FM) Area Diversion Project concerns increasing the flow of the Red River through Fargo-Moorhead. The study follows the April 3rd signing of the Record of Decision (ROD) on the Project’s Final Feasibility Report and Environmental Impact Study (FEIS).
In July, 2012, Houston-Moore Group, under contract with the FM Diversion Authority, completed the study to evaluate options for increasing the flow and resultant residual Red River of the North flood stage through the Diversion Project’s flood damage reduction area. This study is commonly being referred to as the “flow through town” study.
The options serve as an alternative environmental mitigation project to the proposed fish passages on the Red and Wild Rice River control structures for the project. As a secondary benefit, if proposed mitigation measures to allow more flow through Fargo-Moorhead are completed early in the construction phase, the measures will help mitigate flood risk during flood events before the diversion channel is completed by lessening the extent of required temporary flood fighting measures.
In addition to evaluating benefits, the study looked at costs to construct the various flood protection measures, as well as an initial estimate of capital and operation/maintenance costs that would result from allowing higher residual flood stages through the protected area.
Currently, the Federally Recommended Plan (FRP) proposes a resultant 100-yr residual flood stage (RS) through the protected area of approximately RS 31 feet. Additional residual flood stage options were evaluated in this study between RS 30 feet and RS 37 feet using the U.S. Army Corps of Engineers’ unsteady HEC-RAS hydraulic model.
The hydraulic analysis showed higher flows through Fargo/Moorhead would reduce the frequency and duration of the staging of water. However, none of the alternatives for higher flows through town would significantly reduce upstream staging or impacts, and would have minimal impacts on the peak upstream staging elevation.
Also, it should be noted that the analysis concluded increasing flows through town would not cause downstream impacts.
The table above presents the resultant 100-year peak discharge and corresponding existing condition frequency of this peak discharge for the flood stages that were evaluated as part of the study.
A review of the “flow through town” study by agency representatives, the local Technical Team, and the U.S. Army Corps of Engineers, concluded that passing additional flow through the protected area provides significant benefits to the project from environmental, economic, social and agricultural aspects. For instance, increasing flows to 35 feet would reduce the predicted frequency of use of the Diversion from once every 3-4 years to once every 10 or more years. Put another way, a 35-foot stage during a 100-year flood event would result in no operation of the diversion channel for floods smaller than a 10-year flood event. This also means that based on historic records, with a 35-foot stage in place, the diversion channel would not have operated during any past summer flood events.
To be able to manage the 35-foot stage through the protected area, mitigation measures would be needed in several areas, most notably a levee and floodwall along 2nd St. North in Fargo near City Hall and the Fargo Civic Center. In addition to providing long-term benefits after the Diversion is operational, these mitigation measures will provide flood protection benefits to the communities in the interim until the diversion is constructed.
The full study results on increasing river flows through Fargo-Moorhead, and results of the other post-feasibility studies, will be presented to the Diversion Authority on Thursday, September 13.
- NDSU study published examining the agricultural impacts from the FM Area Diversion Project — May 12, 2020
- Diversion Authority and MCCJPA ask City of Moorhead to Assist in Obtaining Five Property Easements — April 28, 2020
- USACE FY 2020 Work Plan Provides $100 Million for Project — February 10, 2020
- Agreement Reached to End Lawsuits and Compensate for Impacts from the Fargo-Moorhead Area Diversion Project — October 26, 2020