This wetland system cleans storm water flowing to the Phalen Chain of Lakes.
The Kohlman Basin Project was designed to reduce phosphorus levels in storm water from Willow Creek and Kohlman Creek before discharging into Kohlman Lake. This improvement was identified in the 1987 Phalen Chain of Lakes Watershed Study as a needed improvement to protect the water quality of the chain. The project area includes the KSTP property south of Beam Avenue at Highway 61 and a wetland area north of the old Countryview golf course, which is now Carmax and Costco in Maplewood.
The project involved diversion of the flows from the creeks into a sedimentation basin and then through the wetland area south of Beam Avenue. A new pipeline was installed through the old golf course and driving range to bring the Willow Creek flows to the sedimentation basin located near the old Ramsey County compost site at the east end of the wetland area. Water then is directed into the wetland area and over a series of permeable weirs installed to spread the stormwater flows through the wetland vegetation.
The pipeline, sheet piles for the weirs and initial grading for the sedimentation basin were completed in 1996 and 1997. The final grading and permeable weir installation was completed during the winter of 1998-1999. The berm north of the old golf course was completed during the winters of 1999-2000 and 2000-2001. Those pipes have since been replaced by an open channel as part of the development of the golf course property, and a roadway was placed on the north side in the location of the berm.
In 2007 a water quality improvement project in Kohlman Basin involved the construction of permeable limestone barriers on the downstream side of the existing weirs to remove soluble phosphorus from runoff. The permeable limestone barrier is a engineered combination of large and small size limestone rock. As flows pass through the barrier, the soluble phosphorus binds with the calcium in the limestone and settles in the wetland. Over time the limestone will dissolve and need to be replaced.
The larger limestone rocks/boulders that add structural support to the barrier also provide substrate for periphyton (attached algae) growth and uptake of phosphorus. The limestone substrate can provide an eventual sink for the phosphorus taken up by the algae.