I-94 Modernization, Modeled Drainage Study

Under the I-94 Modernization project, the Michigan Department of Transportation (MDOT) aims to make improvements to roughly 6.7 miles of depressed freeway along interstate I-94 in the City of Detroit. Freeway runoff is collected through the freeway drainage system and discharges to the local City of Detroit Sewer (DWSD), and Great Lakes Water Authority (GLWA) system through multiple gravity and pump station outlets. The DWSD/GLWA system is a combined system, meaning the system handle both wastewater and storm water. The stormwater is ultimately treated at the Water Resource Recovery Facility (WRRF) and is released into the Detroit River.

The interaction between the MDOT drainage and DWSD/GLWA system is unique, and complex as the I-94 system contributes flows to multiple DWSD/GLWA interceptors, combined sewer overflow facilities, and numerous untreated outfalls along the Detroit River. Additionally, the two systems have different levels of services as the I-94 system was designed to handle the 50Yr-24Hr design storm while the DWSD/GLWA system was designed to handle the 10Yr-1Hr design storm.

To evaluate the interaction of this dynamic system, the Regional Wastewater Collection System (RWCS) Model was used as a tool to analysis the existing conditions for the I-94 Modernization project. The RWCS model is a representation of the DWSD/GLWA sewer system and was developed in the SWMM engine. The following were the key findings.

Flooding within the I-94 project area occurs due to:

• Limitations of the I‐94 drainage system including pipe and pump station capacities
• Significant levels of surcharge in the DWSD/GLWA combined sewer system preventing I-94 drainage from discharging.
• Backflow from the DWSD/GLWA combined sewer system discharging into the I-94 system.

Proposed Improvements to I-94 including additional lanes in each direction, widening shoulders, and interchange reconfigurations, are expected to increase the peak flows and stormwater runoff generated in the service area. One of the project goals was to evaluate drainage alternatives and solutions that would address existing issues and meet MDOT standards under proposed improvement conditions.

Some of the studied alternatives included:

• Continued discharge of the I-94 drainage into the DWSD/GLWA system with controlled flow and backflow prevention.
• Complete separation of the I-94 drainage system from the DWSD/GLWA system via a storage and conveyance tunnel

This presentation explores how PCSWMM was used to evaluate the dynamic interactions between the MDOT and DWSD/GLWA systems as well as how PSCWMM was used for drainage alternative evaluation to address existing issues along with increased flows expected from freeway improvements.

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