A 100-year old bridge on a Level B road will be removed and replaced with a culvert. The decision was made following a discussion by the Palo Alto County Board of Supervisors Tuesday morning, Aug. 12.
Palo Alto County Engineer Joel Fantz brought the board members a draft bridge replacement agreement to look over for the bridge, which is located on a portion of 350 Street that is classified Level "B", between Sections 10 and 15 of Freedom Township. The bridge has been a topic of discussion for several months, as tenants farming land on the far side of the bridge are unable to legally transport loads over the structure, which is posted at an eight-ton limit. The bridge spans 26 feet over a channel, but in periods of high water flow, water will overtop the roadway adjacent to the bridge.
The landowners and the supervisors had agreed earlier in principal to replacing the bridge with a 10-foot culvert, but Fantz recommended to the board members that the bridge be removed and replaced with a 16-foot aluminum arch, after performing hydraulic studies on the area that indicated the 10-foot pipe would not handle the water flows.
The draft agreement prepared by Fantz would have allowed the upstream landowners to sign off on the 10-foot pipe option, releasing the county from liability for future losses due to flooding from the downsizing of the channel.
"This is a very large drainage area that falls down to a fairly flat area," Fantz explained, "The fall is about 100 foot down into this area, so it comes down fairly fast."
"Is this a drainage district?" asked board chair Ed Noonan. "No, it's not," Fantz answered.
Fantz pointed out that just a quarter-mile downstream from this spot, a 75-foot concrete slab bridge is in place to handle the water flow.
With a concern raised about water overtopping the road, Fantz noted water over the road would not be as major an issue as a culvert being washed out.
"The hydraulic modeling is sound. I've run it through the DOT and it matches exactly what is happening out there with the 26-foot bridge," Fantz said. "It says it will overtop the road every five years, which is what it does."
"What do you want to do out there?" Noonan asked.
"I've downsized it to a 16-foot aluminum arch that I think would be adequate, but not a great solution," Fantz replied.
As discussion continued, Supervisor Keith Wirtz brought up the idea of a low-water crossing option to go along with the arch, which Fantz agreed could be a possibility as well.
With an estimate of roughly $50,000 for the 16-foot aluminum arch, it was pointed out that placing three 10-foot tubes would cost nearly the same amount of money.
"You want to do things right out there, but it's for one person," observed Supervisor Craig Merrill. "I think that low water crossing has potential."
Fantz noted that in his tenure as county engineer, the existing bridge has suffered severe scouring on two occasions, requiring extensive work to short up the abutments and also repair an approach undermined by rapid water flows.
"There's a lot of water going through there then," Merrill noted. "If you think the 16-footer won't really be big enough, then I would put in the 16-footer and add a low water crossing. To me, if the 26-foot bridge is being scoured, then a 10-foot tube isn't enough."
"I guess I just want it fixed," Noonan said. "I'm not pointing any fingers, well, maybe at myself, but we've dropped the ball on this by not getting anything done."
The rest of the board members and Fantz agreed.
Fantz noted that he would order the aluminum arch and hope for delivery and installation yet this Fall.
"I'd move we put in the 16-foot arch and try a low water crossing," Supervisor Linus Solberg said, and Merrill offered a second, and asked Fantz to work with the tenant to obtain specific dimensions and measurements for their trucks to see if the existing bridge would handle the loading for a very brief time.
The motion passed on a unanimous vote.