| Abstract
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Hydraulic modeling was performed in support of the Yellowstone River Corridor Cumulative Effects Assessment (CEA). The CEA is a joint effort of the Yellowstone River Conservation Districts Council and the U.S. Army Corps of Engineers, Omaha District (Omaha District). For complete information on the processes and data used for the models, please see the Final Hydraulics Report included with each downloadable file.The hydraulic analysis characterizes the impacts of human development, including man-made structures, encroachments, storage, diversions, and depletions on the Yellowstone River hydraulic profiles and flood boundaries by comparing current and "prehistoric" conditions. Human impacts have been evaluated by modeling two geometry files and two flow files. The geometry files include "Developed" conditions (i.e., the file represents current conditions with existing bridges, embankments, diversion dams, etc.) and "Undeveloped" conditions (i.e., human made structures will be manually removed from the geometry file.) The flow files include "Regulated" conditions (i.e., hydrologic discharges representing current conditions resulting from human-controlled reservoirs, irrigation canals, etc.) and "Unregulated" conditions (i.e., hydrologic discharges representing natural conditions). Four scenarios have been evaluated for a range of flood events using the hydraulic model by modeling combinations of the "Developed" and "Undeveloped" geometry and "Regulated" and "Unregulated" flow files.The hydraulic analysis utilized the US Army Corps of Engineers river modeling software HEC-RAS and geospatial software ArcGIS and the Geo-RAS extension for model geometry development and floodplain mapping. Flow information was obtained from an intensive hydrology study of the basin performed jointly by Montana U.S. Geological Survey (USGS) and Omaha District. Hydraulic models and inundations were developed for the Yellowstone River reaches for Sweet Grass, Stillwater, Yellowstone, Treasure, Rosebud, Custer, Prairie, Dawson, Richland and McKenzie Counties in Montana and North Dakota.For each combination of scenarios and events evaluated in each county, the data available for download include a inundation depth grid with 2.5-meter cells, a inundated area shapefile developed from the outline of the inundation depth grid, and HEC-RAS model result spreadsheets showing modeled stream flow, velocity, depth, and other information for stations at approximately every 1000 feet along the river.Data sources used in the models include aerial photography and LiDAR-derived digital elevation models and partial bathymetry collected in 2004 and 2007 by Aerial Surveys International, LLC, Eisenbraun and Associates, Merrick and Company, and Chris Ransome and Associates, available through the Yellowstone corridor clearinghouse at the Montana State Library. Hydrology was obtained for the Regulated and Unregulated flows for the lower reach from the USGS (Streamflow Statistics for Unregulated and Regulated Conditions for Selected Locations on the Yellowstone, Tongue, and Powder Rivers, Montana, 1928-2002 (2013). https://pubs.usgs.gov/sir/2013/5173/) and for the upper reach from US Army Corps of Engineers, Omaha District (Yellowstone River Corridor Study, Upper Yellowstone River Hydrology, 2011). USGS Montana, Yellowstone River Corridor Bridge Survey (2005-2009), unpublished.
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