Stormwater Team Leader Matt Eblen (left) and Senior Consultant Jerry Johnson (right) observe the scour that has occurred downstream of the primary spillway at Tarsney Lake.
Tarsney Lake provides a scenic area for fishing and recreation in eastern Jackson County; however, the bridge over the lake’s primary spillway, which is composed of seven metal arch pipes, has deteriorated and fallen into disrepair threatening the integrity of the road and the dam’s embankment.
To ensure this critical piece of infrastructure continues to function for nearby residents who rely on the bridge, Jackson County hired surveying and engineering firm McClure Engineering Company (MEC) to analyze the dam’s hydraulics and design structural solutions for the rehabilitation or replacement of the bridge and spillway.
“It’s not what you would typically think of as a bridge, because it’s composed of seven, 42-inch pipes that are side by side,” said Senior Project Manager Gary Strack, P.E. “However, because the waterway opening is more than 20 feet, it qualifies as a bridge according to the Federal Highway Administration rules.”
The bridge and spillway structures are more than 50 years old, and provide the only access to and from approximately 50 homes on the east side of the lake. One of the project’s challenges is a 15-20 foot drop downstream of the bridge. This significant drop increases the water’s velocity coming out of the pipes and has led to severe erosion along the spillway adjacent to the dam’s embankment.
“If the erosion of the embankment continues, the embankment and the road could fail from the scour,” said Stormwater Team Leader Matt Eblen, P.E., who will perform the hydraulic analysis of the dam. “Additionally, if the pipes continue to rust and fail, the water could seep under the pipes and also undermine the embankment.”
In addition to the rehabilitation or replacement of the bridge, the team will assess how to mitigate the erosion of the embankment, which could include building the channel to a shallower depth and lining it, or installing a drop structure to reduce the velocity of the water flowing through the bridge. MEC will also evaluate the structural integrity of the dam’s emergency spillway on the west end of the dam.
The Tarsney Lake bridge project is being funded through the Missouri Local Public Agency Off-System Bridge Replacement and Rehabilitation (BRO) program through the Missouri Department of Transportation (MoDOT). The BRO program currently funds up to $18.9 million per year to repair or replace deficient bridges on roads throughout Missouri.
“We have been conducting BRO projects since 1995,” said Strack, who presented on the BRO program at the 2017 County Commissioners Association of Missouri training conference. “This project allows us to leverage our collective knowledge and experience with BRO projects, along with Matt’s expertise working with dams and spillways.”
The BRO program provides 80 percent federal dollars with a 20 percent local funds match for bridge repairs and replacements. Each time MoDOT inspects a bridge, it is given a rating based on its structural and functional conditions. Bridges that receive a sufficiency rating of 50-75 percent qualify for rehabilitation funding, while bridges that receive a rating of less than 50 percent qualify for replacement.
“No matter what set of solutions we design and ultimately recommend for Jackson County, the site is anticipated to require phased construction in order to maintain both residential and emergency access for the homes, which are located as far as a half mile from the bridge and spillways,” said Strack.
Construction on the project is anticipated to begin in late 2017.
