The City of Milwaukee has found an innovative way to store floodwaters from severe storms, which are expected to increase in frequency and intensity. Beige Alert
The City of Milwaukee is pursuing a plan to turn the basements of foreclosed homes into storm-water management facilities, topped by urban gardens.
August 26, 2014—If all goes as planned, Milwaukee, Wisconsin, will soon become the only place on the planet where a flooded basement may be a sign of relief.
The city recently released a feasibility study that examines turning the basements of some of its hundreds of foreclosed homes into storm-water detention facilities it calls “BaseTerns” and is now proceeding with a pilot project based on that study. The structures would capture storm water during heavy rains, relieving pressure on the city’s sewer system and alleviating flooding in nearby basements. They could also be topped by urban gardens as part of the city’s broader sustainability efforts.
Erick Shambarger, the deputy director of the City of Milwaukee Office of Environmental Sustainability, conceived the idea for the storm-water facilities after participating in the mayor’s Flooding Study Task Force in 2010. The group was responsible for developing storm-water solutions following major storm events in 2008 and 2010 that left many Milwaukee residents with soggy basements.
During the group’s discussions, Shambarger noticed that much of the flooding was occurring within a densely populated north-central neighborhood that had also been impacted significantly by the economic recession. The city takes ownership of properties if the owner is delinquent on their property taxes, then resells the homes it can and demolishes the ones that are beyond repair. Shambarger thought that using the basements of some of the foreclosed homes that were unsafe for occupancy and slated for demolition to provide storm-water detention would help address two of the city’s challenges at the same time.
“It just occurred to me that if we’re out there anyway demolishing some of the homes that are blighted and can’t be saved, it might make sense to try to keep the water out of occupied basements by directing it to the vacant basements,’” Shambarger says. “This is a very dense part of the city and there’s not a lot of green space, so it becomes difficult to find locations to put green infrastructure or places to manage storm water.”
The project fits within the city’s Refresh Milwaukee, a 10-year sustainability plan that includes goals for reducing the amount of storm-water runoff and clear water that enters the sewer system. Shambarger says that such reductions are necessary because the city is seeing an increasing number and growing intensity of storms as a result of climate change. Water management is also important because Milwaukee is part of the Great Lakes Basin, which holds 20 percent of the world’s fresh surface water.
“In general, the city’s sewer system works very well, but I think our combined sewer system is designed for what’s known as the 10-year storm or the 10-percent probability storm,” Shambarger says. “We’re just concerned that the frequency of the intense rain events is increasing, and now we need new solutions to manage those major rain events.”
Shambarger gained the confidence to pursue his idea further after passing it by engineers in the Milwaukee office of CH2M HILL, a design firm headquartered in Englewood, Colorado, that recently completed a Green Streets Stormwater Management Plan for the city. The city then commissioned the Milwaukee office of HNTB Corporation, an employee-owned infrastructure firm serving public and private owners and contractors across the country, with which it had an existing contract, to conduct a study to determine the feasibility of converting a basement into a new type of cistern.
HNTB examined three conversion options: filling the basement with stone and covering it with topsoil, filling the basement with rainwater-harvesting cells and covering it with topsoil, or using the existing floor of the house as a green roof and leaving the basement empty. It also considered several functionality options that included rainwater harvesting, groundwater recharge, and storm-water detention, after capturing rooftop runoff from adjacent properties and runoff from nearby streets and alleys. The options can be mixed and matched to address the needs of a specific site.
A pilot program uses the multipurpose alternative designed by
the engineering firm HNTB, which calls for a series of rain
barrels to capture runoff from nearby rooftops, the overflow going
into the basement along with runoff from the street. A pipe
attached to the basement’s floor drain will allow some water to
flow slowly into the sewer system, while the water in the rain
barrels will be used to irrigate a garden. Courtesy of the
City of Milwaukee
Using a conceptual structural analysis, HNTB determined how a basement would respond to the loads associated with the conversion. The study states that for each option, the new loads on the basement consist of the weight of the storage media, which would vary by option, as well as the weight of the stored water, the weight of the new topsoil that would cover the basement, and the live loading of people that would build and maintain the system. “Live loads for design can be chosen with the aide of ASCE 7 Minimum Design Loads for Buildings and Other Structures,” the study states. Ultimately, the firm concluded that once an existing house is razed, a typical basement will sustain those conversion loads.
Curtis Hulterstrum, P.E, M.ASCE, a senior water resource engineer for HNTB and the lead author of the study, said that he was confident that the basements could be used for some form of storm-water management and agreed that it was worthwhile to explore the idea further. “We must be creative in developing nontraditional methods of managing storm-water runoff in urban areas where space is the primary constraint,” he said in written responses to questions posed by Civil Engineering online.
Milwaukee has trademarked the name BaseTern and is moving forward with converting one basement as a pilot project. It plans to follow HNTB’s “multipurpose” alternative for the project, which calls for using a series of rain barrels to capture runoff from nearby rooftops, allowing the overflow to go into the basement along with runoff from the street. Depending on whether stone or rainwater harvesting cells are used, the basement is expected to hold between 1,800 and 6,000 cu ft of water.
The walls of the test basement will be sprayed or lined with a waterproofing material to prevent the water from seeping through, while holes will be drilled into the floor so that some of the water can infiltrate the ground. A pipe attached to the basement’s existing floor drain will allow the remainder of the water to flow slowly into the sewer system, while the water in the rain barrels will be used to irrigate the garden. A single BaseTern project is expected to cost between $34,000 and $79,000, depending on the basement conversion method used.
“During the heart of the storm, when the sewers have the most pressure on them, you’re pulling some of that pressure off and slowly allowing the water to enter the system as the storm eases,” Shambarger explains. “It ultimately goes into the sewer but at a slower rate.”
Shambarger notes that if the pilot project is a success, different designs may be used for other basements depending on whether they are located in the city’s older area where the sewers carry both sanitary and storm-water flows or areas in which the sewers are separated.
While a BaseTern may be one solution to storm-water management, Shambarger notes that it is not the only solution. “It’s one tool in the toolbox [among] many things that cities need to do to prepare for extreme weather events,” he says. “It’s no substitute for...other kinds of larger-scale flood management, but it’s a site-specific solution that can deal with a very particular problem.”