Last summer’s heavy rainfall is still costing the city of Wichita.
The city is spending about $400,000 to repair a section of the retaining wall along the east bank of the Arkansas River just south of the Lincoln Street Bridge, officials said this week.
But the officials noted that even with the repairs, the city is spending less by originally allowing use of a less-expensive wall design over a more costly one that might have prevented the problem.
The wall was a small part of a $13.7 million, nearly two-year project that also included rebuilding the bridge and adding a new dam downstream. The work was completed in early 2013.
But last summer’s heavy rain rearranged some of that work.
After a record 5.83 inches of rain fell in the first five days of August 2013 – on top of heavy rain through late July – erosion at the base of part of the wall caused it to collapse and the sidewalk on top of it to wash out, city engineer Gary Janzen said.
“It was such a unique project and hard to predict,” he added. “It was some of the highest flows the river had seen in 30 years.”
The contractor is replacing about 80 to 100 feet of the 600-foot-long wall, including the section with a lookout that curves out into the river and is partially submerged in the water.
The river’s level was lowered when work began about a month ago. Repairs are expected to be completed in a few weeks.
Part of the problem was caused by the type of wall that was used, said Alan King, director of public works and utilities.
When the project – funded in part by $6 million in federal and grant money – was put out for bids, it included two options for the wall: A precast wall that was constructed off-site and put in place and a wall that was constructed on-site.
The contractor with the winning bid, Dondlinger & Sons Construction Co., chose the precast wall. That saved at least $700,000 and three to four months in construction time.
The city appreciated the less expensive choice because the city was “having budget problems,” King said.
Even after having to redo part of the wall and improve the foundation around it, the city will be $300,000 ahead of where it would have been if the more expensive option had been selected, King noted.
The problem centered on the design of the precast wall, which is 20 to 25 feet high.
Sheets of steel were placed deep underground and three to five feet in front of the wall to provide overall stability of the east bank – not just the wall, Janzen said.
“Basically it was to stop us from losing everything from the bank into the river,” he added.
The precast design works “99 out of 100 times,” King said.
But the gap between the wall and the steel sheets “presented a problem we didn’t expect,” Janzen said. “The heavy flow from the big rains caused erosion.”
A wall built on site would have had wide, concrete footings that would have extended toward the steel sheets and partially eliminated the gap, though there’s no guarantee that would have prevented the erosion, Janzen said.
The precast wall had a different design that didn’t include concrete footings.
To correct the problem, repairs include pouring a concrete pad in the gap to stop erosion.
“This won’t happen again,” Janzen said. “The wall will work fine.”
Money for the repairs comes out of the project’s original budget, he said.
Water will be released from the dam to fill the river after the job is completed. The smaller walls on the west bank weren’t affected.