Town of Erwin Stormwater Master Plan

Erwin’s stormwater system spans more than six square miles of urbanized watershed, with over 950 pipes and 1,250 structures that have been built up incrementally over decades. Recurring localized flooding is a known problem, the infrastructure is aging, and development pressure continues to grow. For Town staff trying to decide where to spend limited capital improvement dollars, the core difficulty was the absence of a system-wide picture: without reliable data on where the system fails and how badly, there was no defensible way to prioritize projects or make the case for funding.

A static snapshot of existing conditions was not going to be enough. The Town needed a flexible modeling platform capable of evaluating multiple improvement scenarios so that staff and elected officials could compare alternatives, understand tradeoffs, and build a long-term capital plan grounded in real hydraulic performance data.

RiskHydro built a fully integrated 1D-2D rain-on-grid PCSWMM model that simulates rainfall, pipe conveyance, and surface flooding together in a single continuous framework. Rather than treating underground infrastructure and surface flooding as separate analyses, the model connects them directly: water that overtops the pipe system flows onto the surrounding terrain, and the model tracks where it goes from there. That approach reflects how flooding actually works at the street and parcel level. Key tasks included:

• Building an integrated 1D-2D model incorporating over 950 pipes, 1,250 structures, and a rain-on-grid surface mesh that routes overland flow across real terrain as water exits the pipe system
• Running a full design storm matrix across the 2-, 10-, 25-, and 100-year, 24-hour events under existing conditions to characterize current flood risk and pinpoint infrastructure deficiencies across the system
• Developing and analyzing five improvement scenarios to evaluate the hydraulic performance and relative benefit of alternative capital investments, giving the Town a direct comparison of what each option actually does
• Delivering results through StormCity (CHI) and NEER’s web-based mapping platform, making flood depths, extents, and infrastructure performance accessible to both technical staff and elected officials

The Town of Erwin now has a consistent, system-wide picture of how its stormwater infrastructure performs under a range of storm conditions. That foundation changes what is possible in planning conversations: instead of relying on complaint histories or engineering judgment alone, Town staff and elected officials can point to quantified flood depths, mapped extents, and scenario-by-scenario comparisons when making capital investment decisions. The work delivered six outcomes that directly support that goal.

• Unified model platform delivered covering the Town’s full municipal stormwater system, producing a town-wide flood risk picture that did not previously exist
• Flood depths, extents, and infrastructure surcharge quantified across the full design storm suite, identifying the highest-risk locations and most vulnerable assets across the system
• Five improvement scenarios analyzed with scenario-by-scenario comparisons, giving Town staff and elected officials a clear, defensible basis for capital prioritization
• Interactive flood maps and 3D visualizations created to communicate underground infrastructure performance and surface flood risk to both technical and non-technical audiences
• Scalable, updatable modeling framework established that can incorporate future development, climate projections, or new infrastructure improvements as the Town’s needs evolve
• Technical foundation provided to advance Erwin’s long-term stormwater resilience through evidence-based capital improvement planning