Crews Removing KK River's Concrete Bed
[excerpted from JSOnline.com]
Excavating equipment started breaking apart and removing the 8-inch-thick concrete bed of the Kinnickinnic River on Tuesday, downstream of the S. 6th St. bridge as part of a flood management project.
Concrete slabs that extend 500 feet east of the bridge are to be replaced with large stones by December, said Patrick Elliott, a watercourse project manager with the Milwaukee Metropolitan Sewerage District.
MMSD's commission hired Edgerton Contractors Inc. of Oak Creek to take out the slabs and restore the riverbank within the 1,000-foot-long stretch between the S. 6th St. bridge and I-94.
Edgerton will crush the concrete for reuse as gravel in road projects, according to Elliott.
Work is being done to boost the volume of floodwater that can pass through this section of the river now that the city has built a new bridge at S. 6th St., Elliott said.
The bridge's wider span allows larger flood flows to push beneath it and lessens the risk of upstream flooding.
Before Edgerton Contractors could start the job, however, the company encountered soil contaminated with toxic chemicals known as polycyclic aromatic hydrocarbons, or PAHs, downstream of the concrete bed.
PAHs form during incomplete combustion of coal or gasoline, and historic industrial activity along the river is the likely source of the chemicals.
Removing 4,100 tons of contaminated black muck and disposing of it at a landfill added $320,000 to the cost of the original $2.13 million contract.
MMSD received a $1.56 million federal Great Lakes Restoration Initiative grant to help pay project costs.
Muck was dug out of a 38-foot-wide section of river channel to a depth of 3 to 5 feet.
A new channel with a more natural curve was dug and large stones were placed there.
The contractor subsequently eliminated steep slopes and stabilized stream banks downstream of the concrete bed, Elliott said.
After the concrete is removed, retaining walls will be built on both riverbanks to form terraces that will prevent erosion of the slopes and maintain the channel's flood-carrying capacity.
MMSD also has begun buying and removing homes and duplexes in the river's floodplain upstream of S. 6th St. to S. 16th St., as part of a separate $49.9 million flood control project.
Up to 83 structures are targeted for removal in the next several years to provide space for widening the stream channel and eliminating flooding in that section of the river.