Town of Erwin Stormwater Master Plan

Erwin’s stormwater system includes more than six square miles of urbanized watershed, over 970 pipes, and more than 1,275 stormwater structures. Like many municipal drainage systems, it has been constructed and modified over many decades as the Town has grown. Recurring localized flooding, aging infrastructure, and limited capital funding created a need for a system-wide understanding of where the stormwater network is most vulnerable and which improvements would provide the greatest benefit.

For Town staff and elected officials, the challenge was not simply identifying where flooding had been reported. The Town needed a technical basis to understand how the full drainage system performs during design storms, how failures in one part of the system influence flooding elsewhere, and where future stormwater investments should be prioritized.

A static map or isolated pipe analysis would not provide that level of understanding. The Town needed an advanced hydrologic and hydraulic modeling platform capable of evaluating rainfall, pipe flow, hydraulic surcharge, and overland flooding together.

RiskHydro built a fully integrated 1D–2D rain-on-grid PCSWMM model to simulate rainfall, storm drain conveyance, and surface flooding within one connected framework. The model uses the SWMM5 computational engine through PCSWMM and directly links the underground stormwater network to a two-dimensional surface mesh, allowing floodwaters to leave the pipe system, spread across the ground surface, and re-enter the drainage network where hydraulic conditions allow.

This approach reflects how urban flooding actually occurs. Rather than treating pipe capacity and surface inundation as separate problems, the model evaluates the full interaction between rainfall, drainage infrastructure, terrain, and flow paths. Key tasks included:

• Developing a municipal-scale PCSWMM model representing over 970 pipes, 1,275 structures, and a two-dimensional rain-on-grid surface mesh across more than six square miles.
• Simulating a design storm suite, including the 2-, 10-, 25-, and 100-year, 24-hour events, to characterize existing flood risk and identify system-wide hydraulic deficiencies.
• Evaluating model results to identify five priority areas where drainage improvements could meaningfully reduce flooding, improve conveyance, or address known infrastructure limitations.
• Developing conceptual stormwater improvements for the five priority areas, including pipe and culvert upgrades, rerouting concepts, roadway crossing improvements, channel improvements, and other capital planning opportunities where appropriate.
• Preparing maps, flood depth results, model outputs, and 3D visualizations through CHI StormCity to help communicate technical results to municipal staff, project partners, and decision-makers.

The Town of Erwin now has a system-wide technical foundation for stormwater planning and capital improvement prioritization. The PCSWMM model provides a consistent way to evaluate existing flood risk, compare drainage limitations across the Town, and identify where future investments are most likely to improve performance.

The work supports municipal decision-making by moving stormwater planning from a reactive process based primarily on complaints and isolated observations to a model-supported process grounded in hydraulic performance data.

Key project outcomes include:

• Municipal-scale PCSWMM modeling platform
  • A town-wide integrated 1D–2D model covering more than six square miles of drainage area, over 950 pipes, and more than 1,250 structures.
• System-wide flood risk characterization
  • Flood depths, inundation extents, pipe surcharge, and overland flow paths were evaluated across the design storm suite to identify the most vulnerable locations in the system.
• Five priority areas identified
  • RiskHydro used model results to identify priority areas where conceptual improvements could support meaningful flood reduction, infrastructure upgrades, and capital planning.
• Conceptual improvements developed
  • Drainage improvement concepts were developed for five priority areas, giving the Town a practical framework for advancing future design, funding, and implementation.
• Decision-ready visual communication
  • Maps, model outputs, and 3D visualizations were prepared to help communicate complex hydraulic results in a format that is accessible to technical and non-technical audiences.
• Scalable modeling framework
  • The model can be updated as new survey, infrastructure data, development plans, or future stormwater improvements become available, giving the Town a long-term tool for stormwater management.