Replacement of the bridge crossing the Swan Lake Branch on 170th Street west of County Road P58 south of Perry began last week with the demolition and removal of the old bridge and will continue next week with installation of the new $69,800 structure.
Assistant Dallas County Engineer Bryan DeJong estimated the bridge in the 18300 block of 170th Street was built in the mid-1930s.
“The county taxpayers got about 85 years of use out if it,” DeJong said. “That’s pretty good.” He said the precast concrete box culvert that will replace the bridge should also last about a century. It will be installed by West Des Moines-based Forterra Pipe and Precast starting Monday.
Stuart-based Ganoe Excavation was removing dirt Tuesday using a shovel excavator and dump trucks. When the rear axles of one of the trucks sunk in the soft soil, it was extricated by the shovel.
The 1.28 mile stretch of 170th Street is expected to reopen around mid-October. Replacement of the parallel bridge on 180th Street, one mile to the south of the 170th Street bridge, will begin immediately following completion of the more northerly project.
The Dallas County Board of Supervisors approved five right-of-way purchase agreements in April with Dallas County landowners, paying $3,166 for 1.05 acres’ worth of permanent easements needed for the planned bridge replacements. In June the supervisors awarded contracts for construction of the box culverts to Forterra Pipe and Precast for $69,800 for the 170th Street job and $88,200 for 180th Street.
The two bridges cross the Swan Lake Branch, a tributary of the North Raccoon River. The Swan Lake Branch Watershed Plan was published in February 2018. The watershed comprises 15,775 acres south of Perry. Row crops occupy 89 percent of the watershed, with a resulting 400,000 pounds of nitrogen and 2,200 pounds of phosphorus entering the Raccoon River yearly, according to the watershed plan.
The reduction goals of the Iowa Nutrient Reduction Strategy calls for a nitrogen reduction target of 41 percent and a 29 percent reduction in phosphorus.
At present the Swan Lake Branch has very little in the way of conservation practices in place to prevent soil loss and reduce nutrients entering the waterway. Recommended conservation practices, including 6,000 acres planted in cover crops and 7,000 treated by wetlands, will cost about $2 million to implement and $335,000 annually to maintain.