It has been noted that the flood elevations for the Red River reported by the Corps of Engineers and the Federal Emergency Management Agency (FEMA) are different. FEMA currently reports a 100-year flood level for the Red River in Fargo as 38.5 feet, but plans to update this elevation to 39.5 feet this summer. (This has already been adopted in Minnesota.) The corps has used the most sophisticated modeling available and incorporated recent extreme floods, including the 2009 flood of record, to estimate the 100-year flood at 42.4 feet.
These details are bound to cause some confusion, but it is important to know that the agencies are working together and can explain the modeling and mapping differences.
1. The corps has conducted careful research and statistical analysis to develop an effective flood risk reduction solution for the Fargo-Moorhead area. The corps has been coordinating with FEMA on the analysis and is working to develop a Memorandum of Understanding which will identify how future mapping for a Fargo-Moorhead project is completed.
2. The definition of a 100-year flood is determined based on past flood events. This is the reason why 100-year flood flows can be identified as changing over time.
3. FEMA and the corps currently are using different flood flow values and stages due to differences in the period of analysis used, and updates in hydrology (amount of flow in the river) and hydraulics (water surface profiles for the river) utilized by the corps. The FEMA hydrology was originally developed in 1971 and resulted in a 100-year discharge of 29,300 cubic feet per second for the Red River at the Fargo Water Treatment Plant. The corps hydrology was updated to include streamflow through 2009 with an adjustment to account for the current wet cycle. The 100-year discharge developed by the corps is 34,700 cubic feet per second for the Red River at the Fargo Water Treatment Plant. Moving forward, the analysis will continue to be updated and the agencies will use the same results.
4. Both FEMA and the corps utilized the same hydraulic model (HEC-RAS) for the hydraulic analysis. However, there are differences in the modeling approach. The FEMA model was originally developed following the 1997 flood event and was calibrated to that event. The FEMA model is a steady-state model, meaning only the peak discharge for the flood event is utilized. The corps hydraulic model is an unsteady model, meaning the full flood hydrograph is routed through the model. The corps model was developed after the 2009 flood and has been calibrated to several historic flood events, including the 1997, 2006, 2009, and 2010 floods.
5. The corps model includes the impact that floodplain storage has on flood stages. It was necessary for the corps to use this methodology to evaluate the impact of the proposed project on flood stages. The Memorandum of Understanding is addressing how this more detailed modeling will be used for FEMA mapping once the diversion is constructed.
6. At the community of Fargo’s request, FEMA is proceeding with its 2003 data until an agreement can be reached about how to best use the updated modeling from the corps in conjunction with the Diversion Project.
Both the FEMA and corps analyses utilized the best data available at the time they were developed. However, improvements in modeling technology and a longer dataset make the corps data the most current. There is a high likelihood that future FEMA updates will more closely match the corps numbers.
- FM Area Diversion Project Receives FEMA Conditional Letter of Map Revision — September 9, 2020
- Sen. John Hoeven Reviews Inlet Construction — August 25, 2020
- CLOMR Community Notification — July 22, 2020
- U.S. Army Corps of Engineers to Conduct Geomorphic Surveys in the Fargo-Moorhead area — August 10, 2020 https://fmdiversion.gov/wp-content/uploads/2020/08/BioGeo2-225×300.png