Engineering for Changing Rainfall in Southeast Michigan

Over the last ten years, several significant flooding events have occurred in the southeast Michigan. On August 12, 2014, 6 inches of rain fell over a 4-hour period and caused damage to 75,000 homes and businesses with an estimated $1.8 billion dollar loss (NOAA). In the last 4 years (June 22, 2017 and June 26, 2021) southeast Michigan has experienced two 1000-year events. The Southeast Michigan Council of Governments, SEMCOG, is leading an initiative to review and plan for changing rainfall patterns in the region. Model projections of future precipitation vary greatly, but the trends of increasing frequency and intensity of extreme precipitation events is anticipated to continue. Climate models project the Great Lakes region to experience a greater increase in total precipitation than most other regions. The amount of precipitation falling in the most intense 1 percent of precipitation events has increased significantly in the Midwest (42%) from 1958 to 2016. These numbers are projected to increase by another 40% or more by late century (GLISA). SEMCOG formed a working group comprised of local public works, state and county transportation departments, county drain commissions, local universities, and consultants to collaborate on design alternatives for resilient infrastructure into the future. The group evaluated use of current rainfall data, collected examples of national and international resilient infrastructure best practices, polled participants on comfort level of approaches and prepared a list of suggested alternatives. Ideas and approaches reviewed include: Using recent observed historical events as design storm for future projects. Changing the target level of service for the design criteria. Adding level of service design criteria to accommodate large cloudburst events. Incorporating the confident intervals in the existing rainfall statistics for future planning. Using a flat rate increase on the rainfall data to represent future predictions. Basing future rainfall projections on average annual projected temperature changes. Using detailed projected IDF statistics for mid- and end-of-century rainfall. Adopting a risk management approach for selecting design criteria based on potential hazards. Challenges and barriers to reaching consensus on best practices exist including: Interconnectedness of transportation stormwater systems with regional combined sewer systems. Varying design standards across different stormwater sectors. Funding priorities focused on GI and challenges addressing flooding through state programs. Varying CIP planning schedules that lack institutional frameworks to collaborate across infrastructure sectors. Uncertainty in economic impacts for infrastructure projects using the listed approaches. Lack of public understanding about declining levels of service in water infrastructure. The working group agrees on the need for consensus on best practices in the region. It's critical that the regulators support local efforts to address changing rainfall. Presently, transportation and water infrastructure sectors have varying design requirements. Current efforts include a guidance to the member communities on how to address the changing rainfall for infrastructure design purposes, economic impact analyses for selecting different rainfall conditions, and increasing the public's understanding on what the rainfall changes mean for their communities.