Investigating the Impact of Climate Change on Combined and Separate Sewer Overflows in Milwaukee Watersheds

Rachel M. Gregg
Posted on: 3/27/2013 - Updated on: 3/02/2020

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Rachel Gregg

Project Summary

The University of Wisconsin (UW)-Milwaukee Great Lakes WATER Institute, the Southeastern Wisconsin Regional Planning Commission (SEWRPC), and the Milwaukee Metropolitan Sewerage District (MMSD), working with several partners, modeled the potential impacts of climate change on combined sewer overflows (CSOs) and separate sewer overflows (SSOs).


Climate change is expected to cause increased air temperatures, altered precipitation, and changes in the intensity and frequency of storm events in the Midwest region. Under the “worst-case” climate change scenario considered under this study, these changes may affect the number, frequency, duration, and volume of CSOs and SSOs in Milwaukee watersheds (McLellan et al. 2011). The UW-Milwaukee Great Lakes WATER Institute, which serves on the Science Council and the Human Health and Milwaukee Working Groups of the Wisconsin Initiative on Climate Change Impacts (WICCI), and SEWRPC, which serves on both the WICCI Stormwater and Milwaukee Working Groups, worked with several partners to model the effects of climate change on CSOs and SSOs within the MMSD wastewater conveyance and storage system. The project’s aims were to estimate the effects of a range of mid-21st century climatic conditions on CSOs and SSOs, incorporate findings into the Milwaukee Working Group’s WICCI report, and develop fact sheets and outreach materials.


Between 2004 and 2007, SEWRPC and MMSD conducted infrastructure planning efforts and updated the regional water quality management plan. As part of the process, the team modeled the MMSD conveyance and storage system to evaluate the frequency of CSOs and SSOs and created a watershed model to assess pollution sources in the study area; both models relied on existing climate conditions. For this project, the team used the same models and added in downscaled climate data projections developed by the WICCI Climate Working Group (using  IPCC emissions scenario A1B) in order to examine the possible effects of altered precipitation and air temperature on flows and sewer infrastructure capacity under climate change conditions. The analyses showed that if no improvements are made to MMSD facilities beyond those currently planned and no adaptation strategies are implemented, CSOs and SSOs may increase moderately by mid-century. Continued improvements in MMSD conveyance and storage facilities could help protect water quality and human health in a changing climate.

The project team was formed as an outgrowth of the activities of the WICCI Milwaukee Working Group. In addition to the UW-Milwaukee Great Lakes WATER Institute, SEWRPC, and MMSD, other project partners included the UW-Madison Center for Climate Research and Brown and Caldwell.

Outcomes and Conclusions

This study was part of MMSD’s adaptive planning approach and is just an initial step in raising awareness regarding possible climate change effects on water resources in the region. As more reliable projections of meteorological conditions become available, additional studies are possible that may eventually inform how MMSD operates its system.

In addition, SEWRPC, along with the Great Lakes WATER Institute and scientists and engineers from UW-Milwaukee and UW-Madison, is conducting a National Oceanic and Atmospheric Administration-funded study of how climate change will affect the water quality of streams in Milwaukee watersheds, specifically in the Kinnickinnic, Menomonee, Milwaukee, and Root River watersheds, the Oak Creek watershed, and the Lake Michigan nearshore area.


Gregg, R. M. (2012). Investigating the Impact of Climate Change on Combined and Separate Sewer Overflows in Milwaukee Watersheds [Case study on a project of the Southeastern Wisconsin Regional Planning Commission]. Product of EcoAdapt's State of Adaptation Program. Retrieved from CAKE:… (Last updated October 2012)

Project Contacts

Position: Chief Environmental Engineer

Affiliated Organizations

The Great Lakes WATER Institute is a University of Wisconsin System research facility and is operated by the School of Freshwater Sciences at the University of Wisconsin-Milwaukee.

MMSD is a regional government agency that provides water reclamation and flood management services for about 1.1 million customers in 28 communities in the Greater Milwaukee Area. We serve 411 square miles that cover all, or segments of six watersheds.

Established by state law, the District is governed by 11 commissioners with taxing authority.