In 2016, the runoff from an intense rainstorm overwhelmed an existing 84” CMP culvert on Lone Bear Road (CR 112) in northern Natrona County. The crossing was washed out rendering the road impassable. Natrona County Road and Bridge contracted with WLC to conduct a feasibility study on the crossing to provide several alternatives and associated cost estimates to repair the crossing. WLC conducted a drainage study of the contributing drainage basin to estimate peak runoff rates for various return periods. WLC presented the alternatives, including a prefabricated bridge, open bottom arch, concrete box culverts, low water crossing, and multiple culverts, to the Natrona County Commissioners. The Commissioners selected the dual 10’ diameter culvert alternative with the option of installing two additional 10’ diameter culverts if the bid prices were favorable. The dual culvert alternative would convey approximately 1,300 cfs. This conveyance is more than the 10-year peak runoff and less than the 25-year peak runoff. Intense evaluation of the culvert/channel geometry and associated erosion effects indicated substantial channel armoring was necessary. The channel was widened and 2.5’ of riprap lined sides of the channel upstream of the crossing. The upstream face of the roadway embankment was paved with concrete and a substantial headwall was installed. The final configuration of the crossing was 4-10’ diameter CMP culverts. This configuration conveys the estimated 50-year peak runoff or approximately 3,500 cfs before overtopping the roadway.
