Imagine your evening commute along a busy urban freeway in mid-summer. Your workday is done. Traffic moves at a reasonable pace. Dinner on the patio with family awaits.
Mother Nature and an aging stormwater system have other plans, however. Significant precipitation in urban settings can lead to flooding, which can bring traffic and commerce to a standstill and upend our daily routines.
For decades, the I-35W corridor south of Minneapolis, Minnesota, experienced various flooding events. Minimizing these flooding events was the core issue driving a design need.
The density dilemma
Space limitations can make urban stormwater management challenging. Unlike rural settings, which may have easier access to open areas, urban areas require engineered solutions that often must be constructed in confined spaces, adding risk, cost, and public scrutiny.
How do we add storage capacity in dense, urban areas with little or no unused space above ground? It’s a question communities grapple with every day as precipitation surge events become more frequent and intense. Runoff from years of urbanization. Changing storm patterns. Aging infrastructure. They’re all working together to test our cities’ resiliency limits.
“The system essentially serves as a massive backstop, collecting rainwater that exceeds the capacity of the existing stormwater tunnel in the medium, and then pumps it back into the main stormwater system after the storm passes.”
This was the situation at I-35W, where our client, the Minnesota Department of Transportation (MnDOT), faced a conundrum. The need for a larger stormwater-collection system was obvious. But where to put it and how to construct it to meet hydraulic needs? Any new facility would need to fit in the shoulder between the highway and a sound barrier wall parallel to a dense residential street. Because I-35W is a major artery, traffic could not be rerouted. And an ongoing construction project on a crossing highway required coordination to avoid additional congestion.
Recognizing that precipitation intensity and duration have increased since the original design, MnDOT turned to Barr to find a solution that would minimize the negative impact of surcharges from the existing stormwater pipe and keep traffic and commerce flowing today, next week, and for decades to come. Effective for today, sustainable for tomorrow.
Depth of design
Barr was hired as MnDOT’s primary engineering consultant. The Barr-led design team included TKDA and Brierley Associates, and contractor joint venture Kraemer North America and Nicholson Construction. The project successfully applied the construction manager/general contractor (CM/GC) delivery method, which stresses collaboration between all parties.
Our solution required some digging. Deep digging—figuratively and literally. After considering a number of storage-chamber configurations of various widths and depths, we ultimately decided on a final design consisting of six interconnected cells, each more than 80 feet deep and 42 feet in diameter. The system essentially serves as a massive backstop, collecting rainwater that exceeds the capacity of the existing stormwater tunnel in the medium, and then pumps it back into the main stormwater system after the storm passes.
The six 42-foot-diameter, 82- to 84-foot-deep connected tanks can collectively hold 4.5 million gallons of stormwater, the equivalent of seven Olympic-size swimming pools.
Partners down the hall
Often, stormwater-management designs are strengthened by the input of two engineering disciplines working hand in hand—hydraulics modeling and geotechnical engineering. This was the case here.
Designing and installing this storage system required answers to three questions:
1. How much storage do we need to add to contain the stormwater?
Our initial question could be answered by hydraulic modelers—problem-solving engineers who analyze and predict how water behaves in extreme weather events. Using mathematical models and computer simulations, they recreated the kind of flooding events that were stopping traffic on I-35W. Their findings identified the flow of surface water during heavy rainfall and quantified the optimal capacity and location of a right-sized storage facility.
2. Are the site’s subsurface conditions conducive to the placement of such a system?
To answer this question, we went down the hall to our geotechnical engineers and worked collaboratively with our key design partners—subsurface experts who study how soil, rock, and underground water interact with infrastructure. Their work provides key guardrails to guide design.
3. What construction approach best manages risk and achieves the design needs?
To find answers to this question, we broadened the conversation beyond the design team and consulted with:
-
MnDOT maintenance staff, who would be end users of the system
-
The contracting team, to engage a variety of considerations on construction approach, schedule, and cost implications
-
Our in-house permitting experts, to identify permitting needs early
-
The adjacent road construction project and team, to facilitate coordination between projects
“This handoff—from hydraulic modeling to geotechnical engineering to partner collaboration—was critical to developing a storage system that was not only effective but also constructible and durable.”
This handoff—from hydraulic modeling to geotechnical engineering to partner collaboration—was critical to developing a storage system that was not only effective but also constructible and durable. Without the insight of hydraulic models, a geotechnical design might underestimate water-related risks that could cause foundational or structural failures. Without geotechnical data, the hydraulic models might overlook soil-related hazards during project siting. And without early constructability input, the design option may not be practical.
Shortly after completion, Barr’s Joel Swenson, Cordelle Thomasma, Joe Welna, and Mike Haggerty toured the award-winning I-35W Stormwater Storage Facility, which Barr designed in collaboration with TKDA and Brierley Associates for MnDOT.
Meeting a moment of urgency
The first of its kind in Minnesota, this facility sits completely underground, invisible to residents and passing motorists, and can hold a combined 4.5 million gallons. That’s an impressive number in any setting—urban or rural. The fact that these six massive chambers happen to stand in between a major traffic artery and a residential area in a city of more than 3 million residents is all the more remarkable.
The engineering industry agrees. In September, the stormwater storage facility was honored as a national Public Works Project of the Year by the American Public Works Association. In February, Barr received three awards from the American Council of Engineering Companies of Minnesota as part of its Engineering Excellence Awards competition. And, last month, Barr was recognized nationally by the American Council of Engineering Companies with one of only 16 Engineering Excellence Honor Awards.
“While communities can’t control where stormwater collects to dangerous levels, they do have a say in how to capture it.”
We’re honored and humbled to be recognized by our peers in a competitive setting. More than that, we’re inspired by seeing all the innovative ways our industry is responding to stormwater challenges. As severe weather events grow more frequent, the need for larger stormwater-collection systems will become more urgent. And as cities densify, tight construction spaces will become more common or may even worsen. Fortunately, clients like MnDOT and industry leaders are rising to the challenge with a spirit of innovation and technological advancement. New modeling software, near-real-time monitoring instrumentation, and innovative stormwater management practices are part of an ever-expanding toolbox.
While communities can’t control where stormwater collects to dangerous levels, they do have a say in how to capture it. Finding custom solutions for precise site challenges is what Barr does every day. Contact us to start a conversation about how to adapt your stormwater infrastructure.
About the authors
Mike Haggerty, vice president, senior geotechnical engineer, has more than 15 years of geotechnical engineering experience on projects involving tunnels, mines, dams, and wind turbines. His work frequently involves the inspection and rehabilitation of existing tunnel infrastructure and the design of underground space. Mike has served as onsite field engineer and geologist for numerous projects; directs drill crews for hollow-stem-auger, air-rotary, sonic, dual-casing, rock-coring, and cone-penetration-test investigations; and has installed a variety of geotechnical instrumentation.
Joe Welna, senior civil engineer, has more than a decade of civil and geotechnical engineering experience on projects involving dams, tailings basins, and stormwater pipes and tunnels. His work includes geotechnical investigations, design and field engineering, construction observation, and foundation improvements, including grouting and seepage cutoffs using permeation and compaction grouting and micropile proof testing. Joe primarily serves power, mining, and municipal clients in Minnesota, Michigan, and Wyoming. He has conducted numerous stormwater tunnel inspections for the City of Minneapolis and watershed districts in the Minneapolis and St. Paul metropolitan area.
Related projects
Flood-control-system upgrades and certification
When the City of Oslo, Minnesota, faced a FEMA deadline to recertify its levee system or risk losing flood insurance eligibility, the city hired Barr to evaluate the levee system, design remedial measures, and manage construction to meet FEMA and USACE standards. Barr’s team identified significant deficiencies, including unstable riverbanks and inadequate levee structures, and implemented extensive improvements, including new levee construction, pump station upgrades, and floodwall installation. We conducted detailed geotechnical investigations and modeling to guide the design. Ultimately, Barr delivered all required documentation and secured FEMA accreditation for the rehabilitated flood-risk management system.
Multi-phase levee alterations for the City of Des Moines, Iowa
The Des Moines Levee Alterations Project (DMLAP) is a multi-phase initiative aimed at upgrading the levee systems along the Des Moines and Raccoon Rivers to meet FEMA levee accreditation standards under 44 CFR 65.10. Barr provided detailed design and construction project management, including raising levee and floodwall crest elevations, modifying gatewell and closure structures, and implementing seepage mitigation and embankment protection measures. A key component involved assessing geotechnical stability and constructing a seepage relief trench system, eliminating the need to relocate existing utilities. The project required coordination with the US Army Corps of Engineers, including securing Section 404 and 408 permits, which govern wetland impacts and alterations. Upon completion, the project will enhance flood risk management and resilience for the City of Des Moines while complying with FEMA requirements.
Rushford levee system remediation
When the City of Rushford, Minnesota, experienced a catastrophic flood that exceeded the levee design flow due to record-breaking rainfall, Barr collaborated with the city and the U.S. Army Corps of Engineers to implement corrective measures to the levee and flood conveyance system that would comply with FEMA and USACE standards. The primary corrective measures included over 3,600 lineal feet of seepage collectors, 450 relief wells, reinforced channel armoring, and a new 800-foot-long levee. Barr also evaluated the freeboard protection based on anticipated future sedimentation and designed necessary roadway and utility modifications.
Image gallery (below)
1. Over the past several decades, severe storm events led to significant flooding and explosive stormwater geysers that posed public safety risks and disrupted traffic and commerce near 42nd Street on I-35W—a major artery in Minneapolis with an increasing volume of traffic that is expected to reach more than 250,000 vehicles per day within the next decade.
2. The design and construction approaches were developed to address several critical challenges, including complex hydraulic conditions, deep excavation with a high groundwater table, and adjacent highway reconstruction—all in a restricted space wedged between the interstate and a residential area.
3–4. The completed facility features six 82- to 84-foot-deep connected chambers that together have the capacity to hold 4.5 million gallons of stormwater—the equivalent of seven Olympic-size swimming pools.