West Park Stormwater Completion Project

Background
In 2010, the City undertook a project to make various improvements to West Park, which included:
• Seating built into the hill in front of the band shell
• New access stairs from the Huron entrance
• New pathways from Seventh Street
• Relocation of basketball court
• Planning of a community garden
• Regrading open field play area for better drainage
• Boardwalk through proposed wetland/pond area

In addition to these improvements, the project also enhanced stormwater management, which included features such as:
• Eight swirl concentrators to remove sediments (and thus some pollutants) from Allen Creek;
• A series of bioswales and wetlands to help remove pollutants from surface water runoff;
• Disconnection of stormwater drainage from North Seventh Street from the underground pipe, instead discharging the stormwater runoff to the surface through the bioswales and wetlands, thus receiving additional treatment.

Outstanding Issues
During the construction of the project, unanticipated flooding was experienced on the south branch of Allen Creek during some of the large storms experienced in the summer of 2010. In addition, in November of 2010, after the stormwater work had been completed, one of the swirl concentrators on the north branch collapsed, creating a sinkhole near the northwest corner of the park. Subsequent investigation revealed that the remaining swirl concentrators on the north branch were in various stages of failure as well.

In response to the south branch flooding, the weir that served to divert flow into the swirl concentrator units was removed due to concerns over its performance. The weir on the north branch was removed in November of 2010 after the collapse of the swirl concentrator unit. Currently, the weirs are still removed, leaving the below-ground sewer system to function essentially as it did prior to the 2010 construction.

In 2011, the City engaged the engineering consulting services of Orchard Hiltz & McCliment (OHM) to analyze the failure of the swirl concentrator units; and to evaluate the original diversion structure design and propose any changes.

Based on OHM’s report, it appears the failure of the swirl concentrator units was caused primarily by a manufacturing defect. OHM also provided some recommendations for changes to the design of the diversion structures. OHM’s contract was subsequently extended to provide engineering plans and specifications for replacing the diversion structures.

There are also some areas where the turf grass and prairie grass have not been satisfactorily established. Ongoing restoration activities will also be occurring in these areas to assure that acceptable restoration of the park open spaces has occurred.

Current Work & Schedule
As of July 9 the repair work on the swirl concentrator units has been completed. The old covers have been removed, the new covers are now in place, and the structures have been re-buried.  

As of Oct. 29, the majority of the work to rebuild the diversion structures has been completed and both paths into the park have been reopened. The contractor still has some work to be completed, but it should not have a major impact on access to the park. One of the structures at the northwest corner of the park will remain fenced off until the portion of the weir that was damaged during installation can be repaired and reinstalled. In the spring, the contractor will return to the site and touch up the restoration work and reseed as necessary.

 Thank you very much for your patience during this project.

Questions/Contact Information
For further information or questions, contact the project manager for the project, Nick Hutchinson at 734-794-6000 ext. 43633, or by e-mail at nhutchinson@a2gov.org.  If you would like to be added to the e-mail update list for this project, send an e-mail to the address above and put “West Park e-mail list” in the subject line.

Last Updated:  10/11/12