Project Number: BC-7
A proposal for the development of a feasibility study by the BCWMC Engineer was approved at the July 2019 BCWMC meeting. The study got underway in the fall 2019 and was completed in spring 2020. At their meeting in May 2020, the BCWMC approved implementation of the project to dredge all 3 lagoons to a depth of 6 feet (Alternative 2-all).
The total cost of this project is estimated at $3,259,000 including feasibility study, design, construction, landfill disposal of contaminated sediment, and administration. Project costs are likely to be paid from a variety of sources including grant funds, ad valorem property taxes collected over multiple years by Hennepin County on property within the Bassett Creek Watershed, and up to $1,200,000 in BCWMC Closed Project Account funds.
The proposed project to dredge accumulated sediment in the Main Stem of Bassett Creek in Wirth Park is in the Bassett Creek Watershed Management Commission’s (BCWMC) current CIP (Table 5-3, as amended in 2018), listed as project BC-7. The proposed project will remove sediment that has collected in three of the seven “lagoons” created along the Main Stem of Bassett Creek in Wirth Park, between Golden Valley Road and Trunk Highway 55. These lagoons were originally created in the mid-1930s by Civilian Conservation Corps crews. A 2015 study found that the lagoons remained relatively unchanged until the early 1990s, when large amounts of sediment began to appear in the lagoons. The sedimentation appeared to slow or stop by the mid-1990s. The sedimentation in Lagoon “E” (immediately to the east of the chalet and parking area) is of concern to City of Minneapolis and Minneapolis Park and Recreation Board (MPRB) staff because the sedimentation may be contributing to flooding of the parking lot and a short section of Theodore Wirth Parkway. The sedimentation has resulted in the creation of new “islands” in the creek/lagoons that reduce the flow capacity and floodplain storage of the creek.
In addition to improving flow capacity and floodplain storage, the project will improve habitat for fish and macroinvertebrates and will improve downstream water quality by trapping sediment in the lagoons, thus minimizing sediment and nutrients passing downstream within Bassett Creek and on to the Mississippi River. The project will result in a reduction of approximately 600 pounds of total phosphorus and 156,000 pounds of total suspended solids per year that currently flows downstream in the creek and into the Mississippi River.
Almost 40,000 cubic yards of accumulated sediment will be removed through this project. Due to high levels of toxic substances in the sediment, all dredged material is likely to require landfill disposal. Disposal of contaminated sediment is very expensive, resulting in the high cost of the project. Despite the high cost, however, the cost per pound of total phosphorus removal (one measure often examined when considering project implementation) is extremely low at $270/pound of total phosphorus removed.