Establishing time-varying tidal and non-tidal water surface boundary conditions can improve typical year estimations for green infrastructure based Long-Term Control Plan wet-weather models

The Philadelphia Water Department (PWD) maintains hydrologic and hydraulic models of the City’s combined sewer collection system for planning, management, and compliance purposes. These wastewater collection system models have been developed by the PWD using the United States Environmental Protection Agency Storm Water Management Model (USEPA SWMM) to represent the collection systems that convey combined and sanitary flow to the water pollution control plants. PWD relies on these models to evaluate the effectiveness of existing and proposed combined sewer overflow (CSO) control measures. These models are the foundation of Green City, Clean Waters, Philadelphia’s Wet-Weather Long-Term Control Plan to reduce stormwater entering the combined sewer system through the implementation of green stormwater infrastructure (GSI), and thus reduce wet weather overflows (PWD 2009).The receiving waters in the Philadelphia area include the tidally influenced Delaware and Schuylkill rivers and both non-tidal and tidal extents of the main tributaries Tacony/Frankford Creek and Cobbs Creek. The tidal extent of the tributaries is approximated to end at man-made dams built across the creeks. Historically, an annual average water surface elevation has been used as the model boundary condition for the outfalls at the receiving waters. However, water level data analyses have been used to develop continuous timeseries to represent surface water level for a typical year, and have been used in recent model validation exercises. Observations suggest that timing and variation of tide and water level can have an impact on overall collection system hydraulics. A time-varying outfall boundary condition is important because of the influence of surface water elevation on the head of the collection system and resulting impacts to system capacity and CSO discharge during wet-weather. Accurate models are valuable because Philadelphia is required to meet a Water Quality Based Effluent Limit that includes CSO volume and pollutant load reductions and the implementation of green stormwater infrastructure. This is aimed at compliance with water quality requirements of the Clean Water Act and National CSO Policy by the end of the 25-year implementation period of the plan.The Green City, Clean Waters program aims to reduce wet-weather overflows in the Philadelphia combined sewer collection system using the collection system model evaluated on a typical year basis as an analysis tool. The typical year modeling approach was developed to evaluate the effectiveness of current and proposed CSO control measures under average annual conditions. The typical year represents average annual hydrologic conditions through the development of rainfall input timeseries to represent average, or typical, annual input to drive the models. Typical year rainfall was developed based on calendar year 2005 rainfall with modifications to select events. Likewise, recent efforts to improve model accuracy during the implementation phase of the program have led to the development of a typical year tidal timeseries to represent average annual dynamic tidal boundary conditions. Analyses of the water surface levels at the receiving water model boundaries were used to develop the boundary conditions for both tidal and non-tidal reaches.This paper will present how the typical year boundary conditions were developed for the collection system models, including rainfall and tidal and non-tidal outfall boundaries of the receiving waters. Methods to develop a representative 12-month rainfall record, or typical year, will be explored, as well as the development of a corresponding typical year tidal timeseries to improve the boundary conditions of the models. Frequency analysis of observed water levels and analysis of empirical cumulative distribution function of long-term data were used to develop the tidal timeseries. Transforming the tidal observations over a region of five classified tidal zones is also explored. The paper will also discuss how existing open channel models can be utilized to develop boundary conditions for non-tidal reaches, and the value of utilizing multiple sources of water surface level information to improve overall model prediction.