Ballentine Farms Pond 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 watershed sits in a fast-growing part of Wake County, and the Town of Fuquay-Varina’s 2040 land use plan shows significant development planned within the drainage area. Designing for existing conditions alone could leave the channel undersized within a few years of construction. The analysis needed to account for both where the watershed is today and where it is headed.

RiskHydro built a HEC-HMS hydrologic model for the contributing watershed and ran it for both existing and future land use conditions across all required design storm events. Precipitation inputs came from the 2025 North Carolina PMP Study, which provides state-specific PMP values and updated rainfall distributions consistent with current NCDEQ Dam Safety Program guidance. The watershed is primarily low-density residential and forested land, with significant development anticipated under the Town of Fuquay-Varina’s 2040 Community Vision Land Use Plan. Key tasks included:

• Delineating the contributing watershed using a LiDAR-derived DEM and verifying hydraulic connectivity to the dam outlet
• Translating the Town of Fuquay-Varina 2040 Land Use Plan into future land cover conditions to capture anticipated increases in impervious cover within the watershed
• Applying precipitation depths from the 2025 NC PMP Study for the 10-year, 25-year, 50-year, 100-year, and 1/3 PMP design storms, with temporal distributions selected in accordance with NCDEQ guidance
• Running the HEC-HMS model for both existing and future 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

The analysis produced peak discharges for all design storm events under both existing and future conditions. Future conditions produced higher peak flows across the board, as expected given the level of development anticipated in the watershed. 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 design flow established for future conditions as the basis for permanent breach channel sizing
• 100-year peak flow established for future conditions as the basis for channel armoring and stabilization requirements
• Both existing and future conditions modeled, ensuring the breach channel is designed for the watershed as it will be, not just as it is today
• Precipitation inputs sourced from the 2025 NC PMP Study, consistent with current NCDEQ Dam Safety Program hydrologic design guidance
• Complete technical memorandum delivered to the design team, documenting methodology, data sources, and results for NCDEQ submittal support