East Rose Street Culvert Replacement

The Duffyfield Canal conveys stormwater through dual 60-inch corrugated metal pipe culverts beneath East Rose Street before passing under the North Carolina Railroad embankment. Persistent flooding in the Duffyfield neighborhood has prompted the City of New Bern to consider culvert replacement, but the hydraulic and structural conditions at the site presented two distinct problems that needed to be understood separately.

On the hydraulic side, Neuse River stage and limited downstream conveyance beneath the railroad embankment have long influenced flood levels in the watershed. Evaluating whether culvert replacement at East Rose Street would meaningfully reduce flooding required a model capable of representing rainfall, overland flow, pump station operations, and river stage together in a single framework.

On the structural side, the existing CMP culverts show signs of deterioration that raise concerns independent of hydraulic performance. Localized sinkholes and surface subsidence above the culverts suggest potential voids, soil migration, or joint deterioration within the barrels or surrounding backfill, conditions that threaten long-term roadway stability and public safety. The existing backflow prevention and flap gate assemblies compound the problem: they require manual operation and do not fully seat, limiting their reliability during high-water events.

RiskHydro built a rain-on-grid HEC-RAS 2D model that treats the watershed as a single connected system rather than a series of isolated components. Rainfall feeds directly into the overland flow simulation, water moves through the canal and culverts, pump stations discharge at modeled operating schedules, and Neuse River stages apply as downstream boundary conditions. That integrated setup allowed the team to test culvert alternatives under realistic conditions and trace exactly where and why flood depths changed. Key tasks included:

• Building a 2D unsteady-flow model that simulates distributed rainfall, overland flow, and canal hydraulics as a connected system rather than separate analyses
• Incorporating multiple stormwater pump station discharges and operation timing as model boundary conditions to reflect real operating behavior during storm events
• Evaluating existing and proposed culvert alternatives across a range of design storms to quantify conveyance improvements and identify the configuration that performed best
• Applying precipitation frequency data from the 2025 North Carolina PMP Study to ensure the design storms used were current and regionally appropriate
• Interpreting results collaboratively with the project team to refine the preferred configuration and translate modeling findings into actionable design guidance

RiskHydro evaluated multiple culvert replacement alternatives and determined that flood levels at East Rose Street are controlled by the downstream railroad crossing, not the East Rose Street culverts themselves. While that finding narrows the hydraulic case for replacement, observed sinkholes, surface subsidence, and unreliable flap gate assemblies present independent risks to roadway stability and public safety. The findings below highlight the key hydraulic considerations that came out of the analysis.

• Culvert alternatives evaluated quantitatively, with modeled flood depths and extents across a full range of design storms for each proposed configuration
• Minimal flood reduction confirmed from culvert replacement alone or in combination with road embankment raising, providing the City with an honest assessment of what those improvements can and cannot achieve
• Railroad crossing identified as the governing constraint on flood levels throughout the watershed, redirecting the City’s planning focus toward the improvement that would make a meaningful difference
• Scalable modeling framework established that integrates rainfall, pump operations, and Neuse River stage in a single simulation, giving the City a reusable tool for evaluating future watershed improvements