DOWL provided a historic channel migration study, evaluation of flood hydrographs, calibration of a HEC-RAS hydraulic model to the observed flood events, evaluation of a number of alternative replacement structures, analysis of potential scour for the two bridges operating in tandem, and coordination with the various state and federal resource agencies for approval of the recommended replacement bridge. The bridge crosses the Yellowstone River, and the area experienced back-to-back near 100-year floods. A Governor’s Upper Yellowstone River Task Force was organized to ensure that future projects affecting the river are designed to preserve the integrity, beauty, and values of the Yellowstone River Floodplain. The United States Corps of Engineers also performed a floodplain delineation study of this section of the river. The existing US Highway 89 bridge was a five-span, 502-foot long structure located immediately upstream, and within the backwater influence of a 377-foot-long railroad bridge which added to the hydraulic modeling challenges.
Montana Department of Transportation
Drainage and River Systems