Abstract: Session F  2:10 pm (Back to Session F)
Surprising Results of a Two-Dimensional Hydraulic Model of a Combined Sewer Outfall Confluence with Tacony Creek, Philadelphia, Pennsylvania

Art Wawiernia, P.E.
AKRF, Inc.
Mount Laurel, NJ

Authors:  Art Wawiernia, P.E., AKRF, Inc.
Santosh Devotka, P.E., AKRF, Inc.
Taylor DelVecchio, E.I.T., AKRF, Inc.

The 33-square mile Tookany/Tacony-Frankford Creek watershed has been heavily impacted by rapid urbanization, including frequent combined sewer overflows. Two primary goals of the Philadelphia Water Department’s (PWD’s) Tacony Creek Reaches 4/5 stream corridor restoration project are to improve water quality and eliminate odor issues, through a strategy of filling hypoxic scour holes downstream of CSO outfalls.  A large scour hole has formed at the confluence of the T-08 CSO outfall slab and Tacony Creek, at a geomorphically unstable area with nearby eroded banks and mid-channel sediment bars. The design team theorized that the scour hole formed where, under high flow conditions, opposing currents of the Tacony Creek and the CSO create a vortex eddy. Due to concerns about whether filling only the scour hole itself could result its re-formation in mobile bed material downstream, the design team initially took a conservative approach in determining the scour hole fill extents, informed by the results of a 1D hydraulic model.  The estimated construction cost of this project component was high enough to warrant value engineering.  However, without a more detailed analysis tool, the design team could not more accurately determine where the forces responsible for scour hole formation dissipate.     

To better understand the hydraulic forces responsible for formation of the scour hole and determine if the treatment extents could be minimized without adverse geomorphic impacts, a 2D small-domain hydrodynamic model was created using SRH-2D. With it, the design team could better understand and visualize the flow interaction and eddy formation at the confluence and analyze bed mobility under the proposed treatment conditions. Not only did the model clearly confirm that the scour hole was formed by an eddy vortex, and that hydraulic forces under proposed conditions had limited potential to instigate unwanted geomorphic change elsewhere downstream, it also alerted the design team to an area of high shear stress where receding floodplain flows are funneled along the exterior outfall structure wall.  The model resulted in design changes that will save PWD approximately $1 million dollars while also protecting crucial near-stream infrastructure from hydraulic conditions that were not identified at project outset.