Effect of Climate Change on Sewer Overflows in Milwaukee

The Milwaukee Metropolitan Sewerage District and the Southeastern Wisconsin Regional Planning Commission applied an innovative watershed approach to complete a large facilities plan for the year 2020. This study of climate change is a supplement to that plan to determine whether the anticipated benefits of the improvements defined in the plan might be negated by climate change.The modeling approach used in the MMSD facilities plan was readily adaptable to the investigation of climate change impacts because the modeling approach was based on a continuous hydrologic model. The model accounts for complex hydrologic dynamics, including snow melt, which is a critical factor in the wet weather response of sewer systems in the upper Midwest.The objective of this study was not to develop the climate change scenarios, but take two of the available scenarios to evaluate the potential impact on MMSD facilities. The scenarios were used to evaluate the change in overflow frequency and volume compared to the historic climate pattern.The distribution of rainfall between large and small events was altered by climate change more than the average annual rainfall. Large wet weather events tended to become larger and more intense, while smaller events became smaller. As a result the average annual rainfall value was approximately the same.In the moderate climate change scenario the frequency of sewer overflows decreased (slightly) but the annual average volume did not change. In the extreme climate change scenario the frequency of sewer overflows was unchanged and the average annual volume increased moderately.The simulated decrease in SSOs expected to be achieved by the facility improvements in the plan was greater than the potential increase in SSOs from climate change, even in the more severe scenario. The simulated CSO volume was not changed in the moderate climate scenario, but increased in the more extreme scenario. There is a risk of increasing overflow frequency and volume with climate change, but these risks can be mitigated by facility improvements.