UPDATES: February 2012

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Stormwater management adaptation for future climate extremes: Minnehaha Creek Watershed case study

February 2012 (volume 7 - issue 1)

Contributed by Trisha Moore, Research Associate, St. Anthony Falls Laboratory, University of Minnesota

Funded by NOAA (National Oceanographic and Atmospheric Administration)

Project Co-Collaborators:
Latham J. Stack, Co-Principal investigator and responsible for downscaling climate change scenarios, Syntechtic International, LLC
Michael H. Simpson, Co-Principal Investigator, Antioch University, New Hampshire
Lois Eberhart, On-site leader for community education and outreach, City of Minneapolis
Jim S. Gruber, Project leader of community education, outreach, and participatory decision-making, Antioch University
John S. Gulliver, Project leader of hydrologic modeling, stormwater modeling, and development of non-pipe alternatives for stormwater management, University of Minnesota
Telly Mamayek and Leslie Yetka, On-site leaders for community education and outreach, Minnehaha Creek Watershed District
John L. Nieber, Advisor to hydrologic modeling, stormwater modeling, and development of non-pipe alternatives for stormwater management, University of Minnesota
Joel B. Smith, Advisor to climate modeling and adaptation policy, Stratus Consulting
Bruce N. Wilson, Advisor to hydrologic modeling, stormwater modeling, and development of non-pipe alternatives for stormwater management, University of Minnesota

Stormwater infrastructure – be it pipes, ponds, or rain gardens – are typically designed to convey or treat runoff flows associated with a design storm, which, in turn, is based upon the probability that a storm of a given magnitude will occur. One of the assumptions underlying this design approach is that the probability distribution upon which the design storm is based is static. Both the recent climatic record and projected trends, however, indicate increases in both the frequency and magnitude of rain events across Minnesota, casting doubt upon the assumption of a static design storm. Increases in the frequency of extreme rainfall events may also increase the frequency with which existing stormwater management systems are overwhelmed, an impact that could also precipitate a shift in flood-associated risks to public safety and property (Figure 1). There is a need, then, to evaluate future precipitation trends and the likely consequences to stormwater infrastructure.

Figure 1: Floods similar to this one in Asheville, North Carolina, 2004, may be more frequent in the future. Photo courtesy of Robert Dexter.

To address this need, SAFL researchers are teaming with the Minnehaha Creek Watershed District, the Cities of Minneapolis and Victoria, and climate researchers from Antioch University and Syntectic International to evaluate potential impacts to stormwater infrastructure in the Twin Cities metro area due to climate change. Questions that the research team will address include:

  • How is the design storm expected to change in the Twin Cities area?
  • Will the capacity of existing stormwater infrastructure be sufficient to convey this future design storm?  If not, to what degree is flooding expected to increase?  
  • Will expected changes in precipitation extremes amplify the hydrologic changes associated with land use trends? 
  • What options are available to adapt stormwater infrastructure to manage predicted climatic changes?  What are the costs of these options? 

These questions are being examined for two study sites in the Minnehaha Creek Watershed District (Figure 2). The first site, located in South Minneapolis, represents a fully developed urban watershed. The City of Victoria, selected as the second site, represents a predominantly rural watershed that is expected to transition to more urban land uses. The existing stormwater drainage network at both of these sites is being modeled using the Stormwater Management Model (SWMM) to evaluate system capacity under historic, current, and future design storm magnitudes. Future design storms are being generated through the use of coupled-climate models to project probable, future rainfall distributions and associated design storm magnitudes for the study areas. Through simulation in SWMM, relationships between the magnitude of design storm scenarios and the rate of undersized components in the existing drainage networks will be investigated.

Figure 2: Impacts of climate change on stormwater infrastructure are being studied at sites in south Minneapolis and Victoria within the Minnehaha Creek Watershed.

In the case of that flooding is predicted to increase for projected climatic scenarios, the potential to manage flooding risk at current levels through adaptation strategies will be investigated. Adaptation strategies range from replacing undersized stormwater conduits with larger pipes to adopting practices that encourage infiltration. The feasibility of adopting any of these adaptation strategies will be assessed through cost analysis.

Whereas land uses in a fully developed watershed such as that represented by the study site in south Minneapolis are not expected to change significantly, studying a developing area such as the City of Victoria affords the opportunity to superimpose climate changes on projected land use changes. To investigate the coupled effects of climate and land use change on watershed hydrology and hydraulics, the same set of design storm scenarios will also be simulated for future land use conditions in the City of Victoria. By improving our understanding of the interplay of climate and land use change, we can develop better guidelines for stormwater management in a climate-changed future.

Stakeholder engagement, apart from which advances in addressing the impacts of climate change cannot be expected, is a key component of this project. Stakeholders are being identified and involved early in order to facilitate a community-led planning process informed by modeling and cost analysis results. Through this effort, this project will provide a framework for developing local stormwater adaptation plans in both fully developed and growing communities.

We want to hear from you!!!

Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions: 

  • Do you foresee (or have you faced) other challenges related to stormwater management given current trends of more frequent, intense storms?
  • Are there any adaptive management strategies you have employed, or have begun to think about employing, to address these climate-induced challenges?