Cedar Creek Dam Safety & Flood Risk Evaluation

The dam sits within Gander Development’s proposed Cedar Creek community and predates current dam safety regulation. It wasn’t listed in the NCDEQ Dam Safety Program inventory, and Gander Development needed to know whether it 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 a defensible hazard classification and culvert sizing that accounted for the dam’s failure potential together, not as two disconnected analyses.

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 the same models fed directly into culvert sizing for 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
• Sized culverts for the proposed roadway crossings using the same hydrologic and downstream flood data developed for the dam safety evaluation
• Visited the site with the dam owner and a contractor to review embankment conditions and discuss rehabilitation and constructability
• Recommended targeted dam rehabilitation measures based on field observations

NCDEQ Dam Safety approved the recommended exempt classification 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 the pond and greenway preserved as an amenity rather than a long-term compliance liability.

• Exempt status approved on first submittal, with no additional review cycles required from NCDEQ Dam Safety
• Incremental risk isolated from baseline flooding, showing the dam failure itself 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 without the long-term cost of state dam safety compliance
• 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
• Culvert sizing built on the same flood data, avoiding a duplicate analysis effort for the community’s proposed roadway crossings
• Rehabilitation path identified in the field, backed by a site visit with the dam owner and contractor that produced targeted, field-verified repair recommendations