Identifying river inflow to a combined sewer system through comprehensive river level and flow meter analysis

The Great Lakes Water Authority (GLWA) has expanded and calibrated the Storm Water Management Model (SWMM) of the City of Detroit West Side combined sewer system.  This system includes approximately 22,000 acres of combined sewer area that can discharge to the Main Rouge River through 17 untreated outfalls.  The updated SWMM model will be used for planning long term combined sewer overflow (CSO) control alternatives to mitigate the impact of CSO on the Main Rouge River.

A significant amount of data was collected to support the development and calibration of the West Side Model.  Data collection included point rainfall, radar rainfall, sewer flow, USGS river flow, river level, and sewer level throughout the West Side System.  In addition, sewer level data was also collected along the eastern boundary of the system.  Due to the configuration of the system, direct measurements of CSO were not possible.  Instead, clusters of flow meters were installed within the vicinity of the outfalls.  These clustered flow meters were used to calculate overflow through meter math.  River level boundary conditions have a significant impact on the frequency and volume of CSO.  To fully understand the system, river level monitors were also installed at each outfall.  The concentration of flow meters on the interceptor and level sensors in the river provided a unique opportunity to perform a flow meter balance analysis. 

Flow meter balance analysis is a matter of comparing the sum of upstream meter flow to downstream meter flow in areas without significant incremental tributary area.  In instances when the sum of the upstream flow meters does not balance with the downstream flow meters, the engineer must decide if:

1. One or more of the flow meters is collecting inaccurate data
2. Overflow to the river is occurring (CSO)
3. Illicit inflow is occurring to the combined sewer system (river inflow)

Flow meter balance calculations were completed around every outfall where applicable based on meter groupings. Flow meter balance coupled with the river level data afforded the opportunity to perform comprehensive QA/QC of the flow data and identify periods when high river levels were causing direct river inflow to the combined sewer system. This paper will explain the process used to collect and analyze the system flow and river level data, described how decisions were made to identify quality data, presents findings of the analysis, and corrective action that was performed to improve the system based on these findings.

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