Old Bassett Creek Tunnel
Old Bassett Creek Tunnel (OBCT) is a 1.5-mile stormwater tunnel that extends under the North Loop, Near North and Harrison Neighborhoods of Minneapolis. Constructed in the early 1900s, OBCT was built to convey water from the historic Bassett Creek, encapsulating the Creek underground and reducing the impact that its flood waters had on the rapidly developing area.
Starting in 1976, the City of Minneapolis, Bassett Creek Water Management Commission and Army Corps of Engineers partnered to construct a new tunnel (the New Bassett Creek Tunnel) to convey waters from Bassett Creek into the Mississippi River. As a result, OBCT now conveys flow from Bassett Creek only under heavy rains and high (stream) flow conditions. Under typical conditions, OBCT carries water only from the 880-acres that drain directly into it via the city stormsewer system.
The history and oversight of OBCT is complex. The area draining into OBCT is overseen by two separate watershed management organizations. The Bassett Creek Water Management Commission manages the area that drains to Bassett Creek. Meanwhile, the tunnel itself, and the 880-acres that drain directly into it, are within the MWMO watershed. The MWMO has been monitoring water quality and stage at the tunnel outlet since 2008. OBCT is owned by the City of Minneapolis, which is responsible for its operation and maintenance.
Given the unique nature of the OBCT and the reduced need for using it to convey water (under most conditions), MWMO has been interested in studying its function and exploring opportunities for additional uses of the infrastructure. These uses may include stormwater treatment or storage of stormwater for reuse. MWMO is also interested in continuing our monitoring at this location.
In 2012, the MWMO and City of Minneapolis Public Works Department came together to start a study on OBCT. The purpose of this initial effort was to increase our understanding of how the tunnel is currently functioning and document its condition. An inspection of the tunnel that fall revealed more than 3,000 cubic yards of sediment and debris deposited within it. It was also noted that limited access points into the tunnel could make removing this material a challenge.
In the spring of 2017, the MWMO and staff from Barr Engineering, Co. walked the tunnel again, this time performing a more detailed analysis of its contents. Minnesota Public Radio staff joined us on the walk, documenting our efforts. In addition to collecting data on the material within the tunnel, the 2017 study also documented the structural integrity of OBCT and resulted in a plan on how to approach the effort to clean it out. Results of this work confirmed the presence of 3,900 cubic yards (approximately 6,250 tons when wet) of sediment; we documented 2,500 pounds of phosphorus, 2,000 pounds of nitrogen and hundreds of pounds of heavy metals within that sediment. In addition, the team noted significant amounts of debris. All of these materials had the potential of being mobilized and flushed into the Mississippi River. Both the city and MWMO agreed that the next step was to remove this material.
The structural inquiry performed in this project showed that OBCT is generally in “fair” condition. The City of Minneapolis can now use the results of this work to plan future tunnel maintenance and improvement activities. One example of those efforts arose in the summer of 2017, when city crews mistakenly damaged OBCT during a street improvement project. The City of Minneapolis Public Works Department used this opportunity to add an additional entry point at that location, easing access to the tunnel, for the purposes of future maintenance activities.
The first phase of clean-out of the OBCT was completed in the summer of 2018. This phase of work focused on removing sediment and debris from the upper portion of the tunnel, where the majority of material was observed. Minger Construction Company, Inc., performed the work, with oversight from city engineers. The MWMO’s capital project funds were used to pay for the effort. Over the course of eight days, in June and July, city contractors removed 935 tons of sediment and debris from 1080 linear feet of tunnel. The crews used the access hatch, installed by the city in 2017, to ease removal of the sediment.
With the first phase of work completed, the city and MWMO are discussing next steps. Future phases of work will include both additional sediment and debris removal, as well as monitoring, to observe the rate and nature of how sediment begins to accumulate back within the tunnel. Results from the 2018 efforts will be useful in planning future clean-outs, given the unique nature of the OBCT and the way that it differs from other infrastructure that the city oversees.