By Gregg Thielman and Lee Beauvais, Houston-Moore Group

A critical tool for the study and design of the Fargo Moorhead Metro Area Diversion Project has been LiDAR data that is available for the Red River Valley. LiDAR is an acronym that stands for Light Detection and Ranging. LiDAR is a highly accurate aerial mapping technique that links the use of laser and Global Positioning System (GPS) technology to scan the surface of the earth. This creates a detailed 3D surface that can be used to generate Digital Elevation Models (DEMs) and contours that can be used for various design, modeling, and mapping activities.

LiDAR-based topographic data for the F-M area first became available in 1999 when LiDAR was acquired as part of a detailed Flood Insurance Study (FIS) for the Red and Wild Rice Rivers being conducted by FEMA Region VIII. Subsequent updates and expansion of the collection area occurred in 2002, 2005, 2006, 2008, and 2011. In 2008, the International Water Institute also coordinated a basin-wide LiDAR collect for the entire United States portion of the Red River Basin. Funding for the basin-wide collect was provided by a combination of local, state, and federal agencies. The availability of the LiDAR data during the feasibility study was a critical factor in being able to complete the study in such a short time-frame.

Throughout the feasibility study, LiDAR was used to create the detailed hydrologic and hydraulic models for design and impact evaluation of the various project alternatives. These models include the detailed unsteady HEC-RAS hydraulic model that was created to quantify project impacts outside the protected area. HEC-RAS is hydraulic modeling software developed by the U.S. Army Corps of Engineers Hydrologic Engineering Center (USACE – HEC) used to develop water surface profiles in rivers and streams. LiDAR provided the topographic data needed to generate the cross sections and storage cells included in the model, as well as the ground surface used for mapping project impacts. Ultimately, the unsteady HEC-RAS model was used for the hydraulic design of the Diversion Project during feasibility, including the staging area and Storage Area 1 features used to mitigate downstream river stage impacts. The unsteady HEC-RAS model is also being used for current design activities.

LiDAR was also used to develop the detailed HEC-HMS hydrologic models that have been created for the Red River Basin upstream from the FM Area. HEC-HMS is software developed by the USACE – HEC to simulate the precipitation-runoff processes of watersheds. These models can be used to evaluate potential retention sites in the sub-watersheds upstream of the project area. Retention includes structures that can temporarily and/or permanently store water within the Basin. The models can also help quantify the benefits of the retention sites on the sub-watersheds and on the Red River mainstem.

Other uses of LiDAR for the Diversion Project include: design activities, both during feasibility and current design activities; floodplain mapping for alternatives; evaluating wetland impacts; evaluating cultural impacts; and estimating right-of-way requirements.

Overall, the availability of LiDAR has enabled the project team and the USACE to develop some of the most sophisticated hydrologic and hydraulic models of any river system in the United States. The models of the Red River and its tributaries are highly detailed and accurate, which enabled the project team and USACE to conduct the detailed design and analysis needed for the Diversion Project.

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