Fargo-Moorhead Metropolitan Area Flood Risk Management and Unsteady HEC-RAS Modeling

Background

As a result of the record-setting 2009 spring flood event, an accelerated schedule was adopted by the US Army Corps of Engineers (USACE) and the cities of Fargo and Moorhead for an ongoing feasibility study to develop solutions to address flooding in the Fargo-Moorhead (F-M) metro area.

Two alternatives were determined to be most feasible—one in Minnesota and one in North Dakota—that would divert the Red River around the surrounding communities of Fargo and Moorhead. Houston Engineering, Inc. (HEI) developed the original hydraulic model to evaluate potential downstream impacts, which is also being used for future design as the project moves forward.

The Project

HEI worked closely with the USACE to provide the F-M metro area with a hydraulic model that accurately depicts flooding scenarios for existing conditions and with project conditions. HEI conducted the complex analysis using the USACE Hydrologic Engineering Center River Analysis System (HEC-RAS) in an unsteady environment.

HEI’s extensive hydrology and hydraulic experience helped determine the best approach to accurately model the Red River and its tributaries within the flat glacial Lake Agassiz bed. HEI utilized the latest LiDAR technology to create an unprecedented model consisting of more than 3,500 cross sections and 900 storage areas along 650 miles of river (380 on the Red River). This included 20 tributaries, 642 lateral structures, and 1,477 storage area connections.

The model was calibrated to the 2009 spring flood, and the calibration was verified with flood events from 1997, 2006, and 2010. For calibration purposes, HEI developed an innovative process to accurately account for hydrologic contributions from ungaged drainage areas and automate the model input processes. Understanding and distributing the local hydrology is a vital component to matching the stream gage data during calibration.

The unsteady analysis incorporates stage, discharge, and volume within the entire flood hydrograph. It also accounts for timing of the flood wave as it passes through the F-M project area. The final alternative set forth in the feasibility study included a diversion that would pass water around the cities and utilize a staging area that would detain water and attenuate the peak and timing of the flood hydrograph. This analysis could not be accomplished using a steady state HEC-RAS model.

The innovative technology used in this project garnered the praise from the American Council of Engineering Companies of North Dakota. HEI was awarded the 2012 Engineering Excellence Award for Category A: Studies, Research, and Consulting.