North Salem Street Dam

The planned pond removal needs to meet applicable NCDEQ Dam Safety requirements. The hazard classification review informs the flows the permanent breach channel must convey. The project also involves a complex downstream crossing, with a culvert that has multiple bends and connects to stormwater structures. The owner needed the analysis to account for those conditions while coordinating dam removal with future site planning.

RiskHydro built a 2D HEC-RAS model to evaluate downstream impacts from a sunny-day breach and support the hazard classification review. Just below the dam, flow passes through a culvert with multiple bends that ties into stormwater structures. We represented that crossing with HEC-RAS pipe network features so the model reflects how water actually moves through it.

We also developed HEC-HMS hydrologic modeling for the contributing watershed using precipitation data from the North Carolina Probable Maximum Precipitation Study. The modeling evaluated frequency-based storm events and probable maximum precipitation to provide inflow hydrographs for review of the planned breach channel.

We based the hydrologic model on existing watershed conditions. RiskHydro reviewed the preliminary site drainage plan, which routes most of the existing drainage area to stormwater controls downstream of the dam, and documented assumptions that could warrant revisiting if site plans change.

Key tasks included:

• Evaluating potential downstream impacts with a sunny day breach analysis
• Developing a 2D HEC-RAS hydraulic model of the breach and downstream flow path
• Representing a complex culvert crossing and stormwater structures using HEC-RAS pipe network features
• Supporting the dam’s hazard classification review
• Developing HEC-HMS hydrologic modeling for the contributing watershed
• Preparing inflow hydrographs for frequency-based storms and probable maximum precipitation
• Reviewing preliminary site drainage and documenting assumptions for future coordination

Merit Group has a clear picture of the dam safety requirements for the pond removal, and the project team has the hydrology and hydraulics needed to evaluate the breach channel. Watershed assumptions are documented so the analysis can be checked quickly if site plans change.

• 2D breach model of the downstream flow path. The HEC-RAS model captures how breach flow moves through the multi-bend culvert and connected stormwater structures.
• Hydrologic modeling for the breach channel. HEC-HMS hydrographs provide flows for evaluating the channel under frequency-based storms and the probable maximum precipitation.
• Site drainage assumptions documented. The analysis records how the preliminary site plan affects runoff to the channel and when changes would call for updated hydrology.