Proposal would give some relief to flood-prone Willmar intersection
WILLMAR — The elevation of stormwater in the 10th Street and Kandiyohi Avenue Southwest intersection could be reduced by about half a foot if a deep, dry pond is constructed in the northwest corner of the flood-prone intersection, an engineer’s analysis shows.
The analysis indicates a reduction of 5 inches is attainable by constructing a 7- to 8-foot-deep pond, which would connect to the existing storm sewer by a 30-inch-diameter pipe. When stormwater fills the sewer, the water would back up into the pond. As the pressure comes off the storm sewer, the water in the pond will go down.
The cost of digging the pond and installing the pipe is approximately $75,000 to $80,000, said Jared Voge, project manager at Bolton and Menk of Willmar.
Voge is serving as interim city engineer until the city fills the vacant engineer’s position.
The most cost-effective method for reducing flood elevations in the city, however, is to continue to implement the city’s stormwater management ordinance, Voge said. The ordinance requires that all properties be developed with no increases in stormwater leaving the properties.
Voge said preventing additional water from entering the city’s overloaded storm sewer network will help in preventing flood elevations from increasing.
Voge presented the recommendations to the City Council’s Public Works/Safety Committee earlier this month after being directed by the council to investigate potential improvements associated with the city’s stormwater system.
The committee received the recommendation as information and directed Voge and city staff to meet with Kandi-yohi County officials and determine if the proposed Grass Lake restoration project and a proposal to intercept flows from Peach Creek to Lake Wakanda would affect flooding problems in Willmar.
In his analysis, Voge said the “tailwater effect’’ is the primary limiting factor in Willmar’s stormwater management system.
The tailwater effect is the result of lack of elevation change — about six-tenths of a foot — along County Ditch 23A from the city’s stormwater system to Lake Wakanda, located southeast of the city.
“The amount of fall between Willmar and Lake Wakanda is approximately half a foot. It’s really flat,’’ said Voge.
The tailwater comes from Peach Creek and one other watershed located east of Willmar. Both flow into Lake Wakanda.
“There is a significant drainage area that contributes to the Lake Wakanda watershed,’’ said Voge.
“As that water increases during storm events, then essentially what we have is water from Lake Wakanda backing up into the city storm water system and ultimately creating difficulties with drainage throughout the city,’’ he said.
The tailwater effect is evident during periods of heavy rainfall at the intersection of 10th and Kandiyohi, as well as other low locations around Willmar.
Voge said the 5-inch reduction in flood elevation at 10th and Kandiyohi will result in less frequent periodic flooding in the vicinity of the intersection if a pond is built where a vacant lot is now located.
However, he notes that construction of the pond will not eliminate flooding at the intersection.
As part of the analysis, Voge studied the possible benefit of creating a larger holding pond by acquiring several properties in the vicinity of the intersection.
But it was determined this would not eliminate intersection flooding and the benefit would not be much more than with just the corner pond.
To eliminate the tailwater effect, Voge said the city would need approximately 26 acres of land at a cost of about $20 million for stormwater storage, pumps, easements and additional analysis.
“The 26 acres would be a very costly project. That is the ultimate solution to address the tailwater concerns,’’ he said.
Again, the most cost-effective way to reduce or limit flood elevations in the city, Voge said, is by enforcing the city’s stormwater ordinances.
Also, he recommends the city identify a funding source for projects because the cost of stormwater treatment and maintenance in Minnesota will continue to increase.
“It is equally important that prior to investigating potential revenue streams that a clear goal be identified moving forward,’’ he says.
“The costs associated with incremental improvements are significantly less than the costs associated with a solution to reduce the Lake Wakanda tailwater effect within the city of Willmar’s storm sewer system.’’