**4. Conclusions**

To meet its regulatory obligations for combined sewer overflow control, the City of Philadelphia Water Department is developing a hydrodynamic and water quality model to determine if changes made to the stormwater infrastructure will result in improvements in receiving stream water quality. Validation of an EFDC hydrodynamic model was successfully completed as demonstrated through model skill metrics.

Exercising the model against the results of a dye study demonstrated that this tidal fresh water riverine system model, using a detailed and bathymetrically accurate grid, and forced at the downstream boundary with observed water levels, successfully represents the dynamics of the advection and dispersion of dye transport. The selected model domain proved to be appropriate with the observed water level forcing at Delaware City driving the model to adequately represent the tidal and subtidal (meteorologically-induced) effects, including those that originated down-bay, from the Chesapeake and Delaware Canal, and remotely from the continental shelf. Numerical experiments conducted with and without the application of wind showed that the model responded as expected to meteorological forcing through a local, down-bay stress and yielded results consistent with findings of other Delaware Estuary researchers. That is, large-scale wind stress forcing on the lower Delaware Bay caused a setdown at the model lower boundary, resulting in a barotropic response observed in both the dye study and the model. Local wind forcing internal to the model domain was shown to exert little influence on the hydrodynamics driving the dye advection and dispersion.

Bathymetric interactions play an important role in lateral mixing. With an estimated mixing length on the order of the local tidal excursion, full lateral mixing could theoretically be possible over the course of the dye study. Comparison of the dye plume to the bathymetry suggested that bathymetric steering forces the major part of the dye into the deep navigational channel preventing it from fully distributing laterally.

These validation and dye study exercises demonstrate that our model is a reliable tool for a broad range of applications for the City of Philadelphia. Future use of the model will include exploring more advanced mixing dynamics through a new dye study that includes a more complete plume coverage. Other applications include scenarios for early response to pollutant spills and strategic planning related to climate change impacts on infrastructure vulnerability and salt intrusion on the City's drinking water source.

### **Acknowledgments**

We want to acknowledge the assistance of James Smullen, CDM Smith, a consultant to the Office of Watersheds of the PWD, as well as support from our PWD colleagues.

#### **Authors Contributions**

The authors contributed equally to research and manuscript writing.
