Beaver Dam Bridge
Date Completed: April 2011
Cost: $10,000,000
$9,500,000 funded by FHWA grant
Located on County Highway 91 in northwest Arizona just north of Littlefield Arizona, the Beaver Dam Wash Bridge, originally built in 1951, is located approximately 0.9 miles north of the Interstate 15 Traffic Interchange. In January 2005, a major flood along the Beaver Dam Wash was generated by a warm rain on a record snowpack in the 575-square-mile watershed reaching into Nevada and Utah. High flows in the wash eroded the south approach road and bridge abutments.
The County applied and received a grant from the Federal Highway Administration (FHWA) first for the temporary repair, and later for the eventual replacement of the CH 91 Bridge. The total project cost for temporary repair, engineering evaluations, engineering design and bridge replacement construction was approximately ten million dollars, and the County's financial contribution to that was approximately half a million dollars. The County hired consultants to evaluate and justify the need for the bridge replacement and managed the engineering design process for the new bridge. The County also developed an agreement with ADOT to manage the construction.
An evaluation of the bridge required a determination of the 100-year flood discharge at the project site. After numerous calculations, the model estimated a 100-year peak flow of 21,000 cubic feet per second (cfs) for the watershed. Studies document that the January storms caused two major changes in the hydraulic/structural integrity of the existing bridge.
The Beaver Dam Wash Bridge Alternative Analysis Report November 2005 concluded that a 408 foot long bridge with longer spans and wire-tied riprap mattress lined guide banks should replace the existing bridge. It also recommended relocating the bridge and moving the south abutment 120 feet further south to better match the new channel alignment. The new bridge would also provide a safer pedestrian crossing, less maintenance and longer life span. The new bridge was designed to withstand a 500- year storm and was built on piles extending 60 feet into the ground to be safe from erosion and scour at the piers.
