Flood Control and Water Quality: Balancing Gray and Green Infrastructure in Detroit

The Detroit Water and Sewerage Department (DWSD) has completed the design of a $25 million project to reduce flows to the City's combined sewer system, thereby having a major impact on a nearby combined sewer overflow (CSO) that currently discharging to the adjacent Rouge River during most wet weather events. The project includes a near-complete sewer separation within a 214-acre residential neighborhood on the far west side of the City of Detroit. The project is known as the Far West Detroit Stormwater Improvements Project ('Far West'). Key Learning Objectives: 1. Highlight the benefits of using adjacent park lands to install a large scale naturalized extended detention feature that provides water quantity and quality benefits beyond sewer separation. 2. Summarize a methodology to demonstrate how the design meets water quality (TSS reduction) objectives. 3. Demonstrate how a regional sewer model can be used to evaluate project impacts. 4. Discuss the establishment of a pre- and post-construction flow metering program to quantify the impacts of the project. A large park (Rouge Park) is adjacent to the residential neighborhood which provided a unique opportunity to include Green Stormwater Infrastructure (GSI) design components (see Figures 1 and 2). The design consisted of a linear extended detention conveyance channel that incorporates native plantings, floodplain benches, and plunge pools (Figures 1 & 2). This will allow for treatment and flow attenuation of stormwater runoff prior to discharge to the Rouge River, an impaired waterway draining one of the nations' oldest and most industrialized watersheds. Water quantity was addressed in the design through a conveyance channel at the proposed separate storm sewer outfalls from the Far West neighborhood. This conveyance channel provides ample storage volume for both frequent and infrequent hydrologic events, with a focus on extended detention for smaller storms, up to the 2-year recurrence interval. The channel was also designed to be a natural feature that would enhance Rouge Park, and the design includes the establishment of a pedestrian pathway to allow area residents to walk along the edge of the channel. Natural, native vegetation along the channel banks will discourage people from climbing into the channel and will provide additional water quality benefits through erosion protection and nutrient uptake. To quantify the potential TSS removal of the open channel, the calculation procedure for sizing sediment containment systems described in the State of Florida Erosion and Sedimentation Control Designer and Reviewer Manual (2013) was used. This procedure uses a design soil particle size and calculates the apparent removal effectiveness of design size (and larger) particles suspended in runoff waters using the settling velocity from Stokes' Law, design flow rates and sediment containment geometry. The design soil particle size for this assessment is 75 microns based on a geotechnical report that provided a grain size distribution for three boring locations where the samples were collected within the top 12 inches of the soil surface. Finally, two storm events, the Water Quality Event (1-inch) with a one-hour duration and the Channel Protection Event (2-yr 24-hr), were modeled in SWMM and HEC-RAS to provide the hydrology and hydraulics of the channel for sediment transport calculations (Figure 3). For the Water Quality Event, the calculated TSS effectiveness of both channels for the 75-micron size is 100% and the net effectiveness of all suspended particles is 82%. For the Channel Protection Event, the apparent effectiveness of the north channel for the 75-micron size is 100% and for the south channel it is 97%. The net effectiveness of all suspended particles is 82% for the north channel and 80% for the south channel. This approach for calculating TSS removal met with regulatory approval which mandates a 80% TSS removal rate. Water quality impacts to the Rouge River extend beyond the GSI components of this project. The partial separation of a 214-acre neighborhood will have a profound impact on the frequency and volume of CSOs at the Rouge River. The Great Lakes Water Authority (GLWA) developed a regional SWMM model of the collection system tributary to their Water Resource Recovery Facility, and this SWMM model was used to evaluate the impacts of the Far West project. The modeling effort revealed that this project, in concert with related hydraulic improvements downstream of the CSO, will decrease the CSO frequency to once in ten years. The 10-year recurrence interval peak flow in the combined sewer, upstream of the CSO, will be reduced by approximately 200 cfs. In addition to system modeling, pre- and post-construction flow metering is being conducted within the project area. A flow meter was installed on an existing 72-inch combined sewer at the downstream end of the Far West project area and a rain gage was also installed on area neighborhood school roof. On June 25-26, 2021, metro Detroit was hit with a large rainfall event causing regionwide flooding. The Far West rain gage measured 5.0 inches of rainfall in a 24-hour period; fortunately, the flow data from this event looked very reliable and serves as a useful design storm benchmark against which post-construction conditions can be compared. Preliminary modeling of this event demonstrates that the proposed system will function without surface flooding and with a considerable reduction in CSO volume. This project balances sewer separation and CSO flow/volume reduction with additional treatment of surface water. This enhances the water quality in the Rouge River in two ways: 1) enhanced water quality through CSO volume reduction, and 2) extended detention of separated stormwater, reducing TSS loading and peak flows to the Rouge River. Furthermore, DWSD is able to meet the commitments in their CSO NPDES Permit through the implementation of GSI. The project has been bid and the construction contract award is expected in late 2021, with construction commencing in early 2022 and ending in 2026.