Mitigating Unprecedented Flooding Through Optimized Evaluation With Multiple Design Storms

Introduction
The City of Dearborn (City) has encountered increases in the frequency of high-intensity wet weather events, resulting in residential home flooding. On August 11, 2014, an estimated 5.6-inches of rainfall occurred resulting in expansive flooding and basement backups impacting approximately 10,000 homes. In 2021, the City experienced three additional large wet weather events causing significant flooding and basement backups, impacting more than 20,000 homes (Figure 1).

The purpose of this study was to evaluate the recurring flooding issues within the City and to develop alternatives to mitigate the basement backup effects of future flood events through identification of an overarching and prioritized sewer system plan. Several infrastructure control technologies were evaluated using optimization software for multiple design storm conditions to identify solutions that would result in the most cost-effective options to mitigate flooding. Through these analyses, the City was provided a roadmap, with order of magnitude costs, to implement projects to reduce the risk of future flooding events.

Approach/Methodology
Model Development
Development of a sewer collection system model was the first step in determining the best approach to protect residents from future flooding. Approximately forty percent of the City is served by combined sewers; thirty percent is served by a separated sewer system; and the remaining thirty percent is served by a hybrid system with the combined/sanitary sewer still having stormwater volume sources. The root cause of the flooding and CSOs in the system is multifaceted:
- Population growth and increased impervious surfaces.
- Outfalls are lower than the Rouge River elevation.
- City discharges limited sanitary and combined sewer flows to the GLWA system.

Design Criteria
Design criteria were established to develop an understanding of the level of service and performance for the different alternatives explored. The design criteria (Table 1) were established with input from the City and is inclusive of the recent extreme wet weather events.

Flood Control Technologies and Cost Development
The flood control technologies considered include the following as illustrated in Figure 2 and are technologies that have previously been implemented in various locations.
- Dewatering pump stations with wet weather treatment.
- Storage facilities.
- Gravity sewers.
- Stormwater sewer separation.

Construction, capital, operations & maintenance (life cycle costs) were developed for each flood control technology listed above.

Optimization Process
The optimization process utilized Optimizer software that evaluated 100,000+ options to identify the most cost-effective combination of technologies that meet the outlined design criteria from Table 1. Figure 3 identifies the key activities included in the three-step evaluation process.

Existing System Performance
The City's current level of service ranges from a 5-year to a 10-year storm. Figure 4 illustrates the existing system performance for the 25-year plus 30% recurrence interval flow. The figure identifies 181 million gallons of combined sewer surcharge at approximately 430 locations in the system which includes basement backup volumes. The separated stormwater system has 71 million gallons of separated stormwater surcharge at approximately 460 locations.

Alternatives Description and Evaluation
The optimization evaluation identified alternatives that include combinations of control technologies. Figure 5 illustrates the preliminary optimization solutions from the Optimizer model for the 25-year plus design criteria. The results identified five primary solution sets:
- 1A - Conveyance Only
- 2A — Dewatering Limited to 200 MGD (storage/treatment/pump stations)
- 2B — Dewatering Limited to 100 MGD (storage/treatment/pump stations)
- 1D - Stormwater Separation Only
- 1E — Full Stormwater Separation

The alternatives indicated in Figure 6 are 'preliminary' and require additional refinement for the selected alternative. The results of the evaluation indicate a significant preference for stormwater sewer separation alternatives which have a much higher services life and much lower life cycle cost and don't require chemicals, electricity for pump stations, or additional operators for facilities.

Prioritization
The Optimizer model ran a prioritization analysis of the optimized solution to help identify priority projects for implementation. Priority projects are generally determined as the largest system performance improvement for the least cost. Figure 7 illustrates the Performance Deficiency versus Cost. Careful monitoring of system performance will identify the correct end-point for implementing projects and potentially saving the City tens- to hundreds-of millions of dollars.

Conclusions and Future Work
The study's alternatives optimization effort calculated that sewer separation is the most cost-effective solution to address the City's flooding issues. Sewer separation achieves the largest system benefit at the lowest cost. It is estimated that sewer separation implementation costs are on the order of a $1.5-billion-dollar investment. However, significant success (70-80%) can be achieved with an investment of approximately 40-60% of the overall investment calculated. Figure 8 reflects this as the 'knee' of the cost/performance curve.