Vaughan Park Dam

NCDEQ Dam Safety Program requirements set specific design standards for permanent breach channels on High Hazard dams. The channel has to pass the 1/3 PMP storm event without re-impounding the former reservoir, and it has to be lined or stabilized up to the water surface elevation associated with the 100-year peak flow. Meeting those thresholds requires defensible peak inflow values developed in accordance with current NCDEQ hydrologic guidance.

The development context added a wrinkle. The planned Vaughan Park project would not just increase impervious cover within the existing watershed, but also reroute drainage, changing the contributing area boundary itself. The hydrologic analysis had to reflect both scenarios accurately, because using the wrong watershed configuration would result in a breach channel that was either undersized or oversized.

RiskHydro built a HEC-HMS hydrologic model for the contributing watershed and ran it for both existing and proposed post-development conditions across all required design storm events. Precipitation inputs came from the 2025 North Carolina PMP Study, consistent with current NCDEQ Dam Safety Program guidance. The proposed conditions analysis accounted for changes to both the drainage area boundary and land cover resulting from the planned development, and hydrographs were compared across scenarios to quantify the effect on peak inflows and timing. Key tasks included:

• Delineating existing and proposed watershed boundaries to capture drainage rerouting associated with the planned Vaughan Park development
• Developing existing and post-development land cover conditions and assigning TR-55 curve numbers based on NRCS hydrologic soil group data
• Applying precipitation depths and temporal distributions from the 2025 NC PMP Study for the 10-year, 25-year, 50-year, 100-year, and 1/3 PMP design storms using NCDEQ’s Gridded PMP Tool and Basin AEP Tool within ArcGIS Pro
• Running the HEC-HMS model for both existing and proposed conditions to develop peak inflow hydrographs for all design storm events, including the governing 1/3 PMP and 100-year events that drive breach channel sizing and armoring requirements
• Comparing hydrograph results across conditions to quantify the influence of drainage rerouting and land cover change on peak inflows and timing

The analysis produced peak inflow discharges for all design storm events under both existing and proposed conditions. Results showed that inflows are greater under existing conditions due to the larger contributing drainage area, and that post-development inflows decrease as drainage is redirected away from the dam, despite the increase in impervious cover associated with the mixed-use development. The governing design values give the breach channel designer a technically defensible, NCDEQ-compliant basis for sizing the channel and specifying the required armoring extent.

• 1/3 PMP and 100-year design flows established for both existing and proposed conditions as the basis for breach channel sizing and armoring requirements
• Drainage rerouting quantified, with post-development inflows lower than existing conditions due to a reduced contributing area, despite increased impervious cover
• Both watershed configurations modeled, giving the design team a complete picture of how the development affects hydrologic loading on the breach channel
• Precipitation inputs sourced from the 2025 NC PMP Study, consistent with current NCDEQ Dam Safety Program hydrologic design guidance
• Regulator-ready technical documentation delivered to the design team, supporting breach channel design and NCDEQ Dam Safety Program review