Cedar Creek Dam

The dam sits within Gander Development’s proposed Cedar Creek community. It predates current dam safety regulation and did not appear in the NCDEQ Dam Safety Program inventory, so its jurisdictional status was an open question rather than a matter of record. Gander Development needed to know whether the dam would trigger state regulation before finalizing the community’s layout and road network.

Gander Development wanted to keep the pond as a community amenity, with a greenway around it that would let residents connect with nature. Before committing to that plan, they needed to understand the actual risk the dam posed and what long-term regulatory compliance would cost if the dam were classified as jurisdictional.

Two roadway crossings are also proposed downstream of the dam. Gander Development needed to understand how breach flooding would interact with those crossings before committing to the road network, so that the layout reflected the dam’s actual failure consequences rather than an assumption about them.

RiskHydro approached the dam as a flood risk problem, not just a desktop classification exercise. We built hydrologic models in HEC-HMS to generate reservoir inflow for the governing spillway design flood, then used two-dimensional HEC-RAS modeling to route both breach and non-breach flows downstream and compare the results side by side. Running that comparison across sunny day and hydrologic loading conditions gave Gander Development a defensible basis for hazard classification and a clear picture of how breach flooding would interact with the community’s proposed road crossings. Key tasks included:

• Simulated sunny day breach scenarios at normal pool and maximum pool
• Simulated a hydrologic loading breach scenario, with failure occurring at peak reservoir elevation during the spillway design flood (SDF)
• Routed each scenario downstream in a 2D HEC-RAS model to capture inundation extent, water surface elevation, and velocity at downstream road crossings
• Ran a corresponding non-breach SDF scenario and compared it to the breach results to isolate the incremental depth, velocity, and inundation caused by the dam failure itself, separate from the storm event alone
• Evaluated how those incremental changes in depth and velocity affected hazard to passenger vehicles at the crossing
• Modeled existing and proposed development conditions to see how the community’s grading and road layout would interact with breach inundation
• Visited the site with the dam owner to observe and document existing embankment and spillway conditions

NCDEQ Dam Safety approved the recommended classification and exempt determination on the initial JDHC submittal, with no additional review cycles required. Gander Development moved forward with the Cedar Creek community knowing the dam’s actual flood risk, not an assumed one, and with both existing and proposed conditions evaluated rather than assumed.

• Intermediate hazard classification supported, and exemption with it, approved by NCDEQ Dam Safety on the first submittal with no additional review cycles required
• Incremental consequences isolated from baseline flooding, showing that under existing conditions, dam failure did not meaningfully increase depth, velocity, or vehicle hazard at the downstream crossing beyond what the storm event alone would produce
• Pond and greenway amenity preserved, letting Gander Development move forward with the community’s design intent while the dam remains outside state dam safety jurisdiction under current conditions
• Flood risk profile in hand before final design, giving the development team a known basis for grading and road layouts instead of discovering constraints later