Monticello council members recently approved spending $10,000 to fix a problematic stormwater pond near the Interstate 94-County Highway 18 interchange.
City Administrator Jeff O’Neill described the problem in a background memo Oct. 14. The pond repair expenditure was approved as a consent agenda item.
According to O’Neill, when the I-94 and County State Aid Highway 18 interchange was constructed, a storm water pond system was designed to accommodate new improvements associated with the project. The pond located on the north side of the westbound I-94 off-ramp, referred to as Pond No. 5, accommodates direct drainage from the Union Crossings development as well as segments of County 18 and the I-94 off ramp. When the pond fills up, the water overflows to the adjacent pond to the east over a berm. O’Neill reported there isn’t a pipe connection between the ponds due to an existing storm sewer pipe that is located under the berm. The pond design was intended for both pond cells on either side of the berm to fill up and equalize. However, the berm has experienced erosion over the years when the pond overflows. A cable concrete mat was installed in 2009 to alleviate the erosion; however, it has since been undermined and eroded due to the sandy soils and is in need of repair. O’Neill reported city staff and WSB & Associates, Inc., the city’s consulting engineering firm, have monitored the situation over the past four years and have spoken to several contractors to obtain feedback on how to correct the problem. O’Neill said filling the eroded area with clay-like material and placing poly fabric was recommended, along installation of a 6-inch concrete spillway with wire mesh over the eroded area with a 6-inch curb around the perimeter of the spillway. The cable concrete would be capped with a 4-inch concrete mat, with the mesh being tied to the concrete.
An erosion control blanket would also be installed and seed placed around the spillway. “These repairs should assist in spreading out the volume of flow overflowing from Pond No. 5 within the concrete spillway, thereby reducing erosion,” O’Neill stated in his background memo, adding city staff received two quotes from concrete contractors for the project. Monticello Public Works staff will help in filling in the eroded area with clay-like soils and install the blanket.
“WSB has indicated that since the original maintenance and repair of the erosion did not last as long as they anticipated, WSB will contribute at no charge their past and current design fees as well as contribute one-third of the funds needed for the repair,” O’Neill wrote. However, he advised council members that due to a conflict in storm sewer grades in relation to the pond grades in this location, erosion is anticipated to continue to some extent in the future and future periodic maintenance should be anticipated. City staff will also pursue Minnesota Department of Transportation (MnDOT) funding to cover their share of the project per a cost participation policy which indicates that their participation is based on their contributing drainage to the pond. According to O’Neill, the city estimates this amount could be as high as 27 percent of the cost. Remaining funding would come from the city’s stormwater trunk fund, which includes funds for pond repair work. According to O’Neill, the current balance of this fund is slightly more than $1 million. “This expense would be a good candidate for funding via a stormwater utility fund, but such a fund isn’t currently in place,” O’Neill noted, adding the one-to-two-day project should be completed before month’s end.
Contact Tim Hennagir at firstname.lastname@example.org