**Exploring Localized Mixing Dynamics during Wet Weather in a Tidal Fresh Water System**

### **Ramona Stammermann and Philip Duzinski**

**Abstract:** A recently validated 3-dimensional implementation of the Environmental Fluid Dynamic Code (EFDC) for the tidal-fresh portions of the Delaware Estuary was exercised against the results of a dye release from a sewer outfall during a storm. The influence on dye distribution in the estuary resulting from variations in wind and local storm water discharges in an urban area is investigated. The modeled domain stretches 116 km from the head of tide and includes hydrologic input from 33 streams and a number of municipal and industrial discharges. Bottom roughness was parameterized from sedimentological and geophysical surveys. Model validation to-date relies upon field observations and tidal harmonics for sea level and currents derived from the NOAA-NOS 1984–1985 circulation survey and a current survey conducted by the Philadelphia Water Department (PWD). Model representation of dye distribution compared favorably for observations of concentrations in the dye plume from 10 cross-sections spanning the extent of the plume over seven tidal cycles. The dye distribution was characterized by an initial period of high local storm water and stream inflows with low wind conditions, lasting for several tidal cycles, followed by a period of reduced fresh water input and increasing wind stress. The dye experiment provided a unique opportunity to observe the performance of the model through the transition between these two very different meteorological periods, and to explore the physical conditions driving the hydrodynamics through both observations and numerical experiments. The influences of local meteorological forcing and channel morphology on lateral mixing, dispersion and longitudinal dynamics are characterized.

Reprinted from *J. Mar. Sci. Eng.* Cite as: Stammermann, R.; Duzinski, P. Exploring Localized Mixing Dynamics during Wet Weather in a Tidal Fresh Water System. *J. Mar. Sci. Eng.* **2014**, *2*, 386-399.

#### **1. Introduction**

During the City of Philadelphia's development in the 19th and 20th centuries, a combined sewer system was built, which conveys stormwater runoff and sewage together in the same pipe network [1]. Today about 60% of the City's sewered area is still served by combined sewers, especially in the older sections of the city. The remaining 40% is served by separate sewers for sewage and stormwater respectively [2]. High intensity rainfall within the City causes the combined sewers to reach their maximum capacity, and a mixture of stormwater and untreated sewage is released into rivers and tributaries.

The City of Philadelphia is regulated by the Pennsylvania Department of Environmental Protection for discharges from combined sewer overflows and storm water outfalls to the Delaware and Schuylkill Rivers. The Philadelphia Water Department (PWD) is developing a water quality model of the tidal Delaware and Schuylkill Rivers to quantify the effects of City of Philadelphia discharges on these waterbodies and meet regulatory requirements.

For this purpose a 3-dimensional hydrodynamic numerical model was developed using the Environmental Fluid Dynamics Code (EFDC) [3]. It was hydrodynamically validated against observations from the 1984 NOAA-NOS circulation survey and contemporary ongoing long term current surveys conducted by the PWD that started in August 2012. The following study shows a first attempt on assessing the model's transport capabilities by exercising it against a dye study conducted by Ocean Surveys, Inc. (OSI) for the Delaware River Basin Commission (DRBC) [4].

#### **2. Methods**

#### *2.1. Study Area*

The Delaware Estuary is located on the East Coast of the United States between Washington, D.C. and New York, NY, USA (Figure 1). The estuary spans 215 km from its mouth between Cape May, NJ, USA and Lewes, DE, USA to the head of tide at Trenton, NJ, USA. The City of Philadelphia is situated at River 147–180 km. The model domain includes the Delaware River section from 99 to 215 km and the tidal Schuylkill River. The model area begins north of the Chesapeake and Delaware Canal confluence, where a tidal gauge at Delaware City, DE, USA provides observed water levels for model forcing. The turbidity maximum zone of the Delaware Estuary reaches from 50 to 120 km. With the mean salt intrusion reaching 97 km, the model domain is generally considered to be tidal fresh water. Significant levels of salinity are only reached within the model domain during severe drought conditions when upstream river discharges are low.

Within the City of Philadelphia, there are 4800 km of sewer pipe, 455 storm water outfalls and 164 combined sewer outfalls (CSO). Most outfalls discharge directly into the Delaware and Schuylkill Rivers. Some are located along smaller non-tidal tributaries in the city area, the Cobbs, Frankford and Pennypack Creeks, which are connected as boundary conditions to the tidal model.

#### *2.2. 1997 CSO Mixing Zone Study*

The aim of the mixing zone study was to characterize a CSO discharge during a wet weather event, to identify initial dilution and mixing, and to determine the far-field impact of CSOs [4]. The study was conducted by OSI on behalf of DRBC and HydroQual, Inc. (HQI) during the period of 21–25 November 1997. Rhodamine WT dye was injected over a period of 3.5 h into a trunk sewer upstream of CSO D-39 shortly after midnight on 22 November 1997, while the CSO was actively overflowing due to a 1.1 inch storm in the region. Dye tracer concentrations were recorded during six plume mappings on the following days that coincided with either high or low slack tides (Figure 2). Contour lines of the plume during each mapping were determined by interpolation of the measured track lines.

Mappings of the observed dye data showing the interpolated dye plume projections were created by OSI. The dye concentration quickly declined with the beginning of a strong local wind event that led to a setdown of the mean sea level as can be seen in Figure 2 during 24 November 1997.

**Figure 1.** Overview over Delaware River Estuary.

**Figure 2.** Wind conditions and time table of dye injection and mapping events (shaded areas).

#### *2.3. Model Setup*

The EFDC model used for this study was developed at the Virginia Institute of Marine Science and has been applied for a wide range of environmental studies. The EFDC model solves the 3 dimensional, vertically hydrostatic, free surface, turbulent averaged equations of motion using stretched or sigma vertical coordinates and Cartesian or curvilinear, orthogonal horizontal coordinates. It solves the equations using a combination of finite volume and finite difference techniques, and allows for wetting and drying in shallow areas. Dynamically coupled transport equations for turbulent kinetic energy, turbulent length scale, salinity and temperature are also solved. Additionally, an arbitrary number of Eulerian transport-transformation equations for dissolved and suspended materials can be solved simultaneously [3,5].

The model and the dye study were used to characterize the hydrodynamics of the tidal Delaware River and the impact of stormwater and CSO discharges. A strong wind event at the end of the dye study period appeared to have considerable influence on the rate of dilution. Three model scenarios were performed to analyze the impact of wind on dye transport in the model:

