Results from Ann Arbor’s Sanitary Sewer Wet Weather Evaluation Project

THE PROBLEMIn the early 2000’s, the City of Ann Arbor decided to implement a footing drain disconnection (FDD) program as the selected alternative to address their sanitary sewer wet weather issues. They have performed approximately 2,700 FDDs from their sanitary sewer system between 2001 and 2012. The FDDs were focused on five priority districts around the City that experienced a high frequency of basement backups. In 2012, the City experienced several large storms that led to flooding issues around the City and some basements were flooded due to a variety of factors such as overwhelmed sump pumps, power outages, and surface flooding. As a result, the City Council elected to partially suspend the FDD program until technical evaluations could be completed and a Citizens Advisory Committee (CAC) made recommendations on the appropriate path forward.GOALS AND OBJECTIVESThe goals of the technical evaluation project were to address the following questions:• How effective has the FDD program been?• What is the risk of future basement backups from the sanitary sewer system?• What long-term improvements should the City implement to improve the sanitary sewer system?• How to effectively engage the public on the long-term plan?TECHNICAL EVALUATIONSEvaluating flow removals from an FDD program in a sanitary sewer system can be challenging due to varying wetness conditions. For this reason, the evaluation approach included a robust process comprised of three scientific methods:1. Scatter plots with regression analysis to assess statistical significance2. Meter correlations with control districts3. A continuous hydrologic model using the H2Ometrics Antecedent Moisture ModelWhen three independent methods are all in agreement, it provides a strong confidence in the results and a good basis for assessing the effectiveness of the FDD program. These three methods provided a valid, scientific method of evaluating the FDD effectiveness that accounts for varying wetness conditions on the flows.The overall project included flow metering, FDD evaluation, hydrologic modeling, hydraulic modeling, capacity assessment, alternatives evaluation, and an extensive public engagement program. Throughout the project, information was shared with the public through multiple channels including public meetings, the City's web site, social media, blog postings, video documentaries, and a Citizens Advisory Committee. The Citizens Advisory Committee was empowered to make recommendations to City Council for the long-term plan. By engaging the public in this manner the City was able to ensure that both engineering optimization and community values are reflected in the final plan.RESULTSThis presentation will review the results of the study including the techniques used to evaluate the effectiveness of the FDD program, the results of the evaluation, a comparison of the flows removed from the FDD program to commonly used standards, and provide an overview of the public engagement process and results.A summary of the results from the City’s five (5) priority FDD districts is outlined in the tables below. The paper will review the derivation of these results, the statistical methods used to quantify statistical significance, and provide commentary on the explanation for the variances across the districts based on system characteristics and hydrologic flow components. The paper will review how this information was used to quantify the risk of future basement backup and support the hydraulic modeling and alternative evaluations to address wet weather flows. The extensive public engagement program conducted as part of the study will be outlined.