One Size Storm Does Not Fit All: Development of a Wet Weather SSO Elimination Plan for the City of Baltimore using Continuous Model Simulation

The City of Baltimore is required under a consent decree to mitigate sanitary sewer overflows (SSOs) from its wastewater collection system. The consent decree specified the use of a design storm approach to establish the level of service of the sewer system. The City proposed that the recommended level of SSO control be based on a more rigorous continuous model simulation approach utilizing historical precipitation data instead of the traditional design storm approach. A continuous simulation approach provides a greater level of confidence in the level of control of SSOs, since the return frequency of peak flows in the sewer system corresponds to the return frequency of SSOs, which do not necessarily coincide with the return interval of the storm event. The continuous simulation methodology will allow the City to provide appropriate collection system improvements and ensure that the desired level of control of wet weather SSOs is achieved. The methodology and development of a wet weather SSO mitigation plan for the City based on continuous model simulation are discussed.