So my collection system model predictions and my metering depth data don't agree, now what? The tackling of hydraulic depth discrepancies in the Evergreen-Farmington Sewage Disposal System

The Evergreen-Farmington Sewage Disposal System (EFSDS) is currently operating under an Administrative Consent Order (ACO) by the Michigan Department of Environmental Quality (MDEQ). The EFSDS system is monitored through an extensive network of nearly 80 sewage flow meters that each record depth, velocity, and flow rate readings. As the Office of the Oakland CountyWater Resources Commissioner (WRC) prepared to make a Long-Term Corrective Action Plan (LTCAP) for the EFSDS, WRC asked its consultants, Applied Science, Inc. (ASI) and Orchard, Hiltz and McCliment (OHM) to run an EPA Storm Water Management Model Version 5 (SWMM5) model of the EFSDS system created in 2007.Rainfall data from two large wet weather events that occurred (in September 2008 and March 2009) were used in the SWMM5 model.The modeled hydrographs for these two large events agreed well with the recorded flow rates throughout the EFSDS. However, there were several locations where modeled depths were significantly lower than measured. Elevated sewage depths in the EFSDS often drive sanitary sewer overflow (SSO), so special attention was paid to model predictions of depth for the events. The SWMM5 results indicated that sewage levels should have remained within the pipe or slight surcharging should have been experienced. Many meter records agreed with this result. However, in some locations, the metered depths showed significant surcharging that had not been predicted by the model. Some of this actual surcharging was related to the occurrence of SSO during these two large events. These model-versus-meter disagreements were termed “hydraulic discrepancies.” In all, thirteen hydraulic discrepancies were identified and seven of these were designated as high priority, meaning that they directly related to SSOs in the EFSDS.The WRC was left with a difficult question: How can we use the model for long-term planning when we can see that the model results do not agree with observed data?It was quickly realized that to answer this question the WRC would have to embark upon a comprehensive effort. A unique approach was taken that merged field investigation efforts, technical data review, and model upgrades through pairing the sewer maintenance field staff and the modeling staff members for field investigations. This promoted immediate and open communication between the modeling and field investigation efforts and built WRC's institutional knowledge of these system issues.Through the combined modeling and investigation effort, the EFSDS model continues to be updated to truly reflect reality and be used for long-term planning.