A Really Complex Stormwater Model: Salters Creek 2D SWMM

Engineers from the City of Hampton and GKY & Associates, Inc. recently completed a study of the 2,500-acre Salters Creek Watershed, which discharges to the Hampton Roads Harbor. The area has a relatively flat, low-lying topography with drainage infrastructure that is more than 70 years old. The City undertook this effort to reduce future flooding impacts on parcels and structures—with an eye toward climate change impacts.

The terrain, storm drain system, design rainfall, and tailwater elevations drive the hydraulics. The watershed configuration and buildout level do not lend themselves to adding flood storage in new impoundments or natural floodplains. To relieve flooding, the drainage network has to be improved to provide increased flow conveyance to the outfalls. There are no feasible, suitable locations to add flood storage.

The EPA SWMM model can be configured as a combined 1D/2D finite element mesh system (for above-ground flows) with traditional 1D pipe hydraulics (for underground pipe systems). This approach—using a terrain-based mesh to model surface hydraulics—produces more reliable results than conventional 1D modeling. However, it is computationally intensive. Fortunately, powerful, multi-core processors are now relatively inexpensive and have opened the door to much more complex and useful modeling than was possible even three or four years ago.

The Salters Creek Watershed Study took over two years to complete and overcame many challenges in collecting field data for the drainage system inventory. The project engineers decided to capture considerable detail in the modeling, producing SWMM models that are between 2 and 5 GB in packaged form and can take more than 37 hours each to run on the fastest desktop computers available in 2022. There are 1,798 subcatchments (averaging 1.39 acres each), over 3,200 1D junctions, and 3,400 1D conduit links. The PCSWMM complexity index for the models is over 4,600,000 (indicating an enormous number of input parameters such as pipe lengths, invert elevations, subcatchment slopes, Manning coefficients, soil parameters, node connections, etc.). These models are the most detailed GKY has produced to date.

This presentation will highlight how the models were built and used and demonstrate why this complex approach was appropriate and beneficial. Particular emphasis will be given to overcoming practical challenges—from both modeling and project delivery perspectives.