Smithfield staff recommend upsizing Second Street culvert; Fourth Street fixes would require property acquisition

Engineers hired by the Town of Smithfield recommended the town move ahead with design and permitting to enlarge the culvert under Second Street to reduce frequent flooding, while noting that meaningful mitigation at Fourth Street would require property acquisition or easements and significantly higher cost.

The study presented to the Town Council by Perry Ross of Withers and Ravenel analyzed Spring Branch, a FEMA-studied stream with about a 700-acre drainage area that runs through downtown and discharges to the Neuse River. Ross said the firm modeled 2-year, 10-year, 25-year and 100-year storm events and focused on reducing inundation for the 2- and 10-year storms because the larger events are dominated by backwater from the Neuse.

The study found Second Street acts as a downstream bottleneck. Existing conditions include a 65-inch corrugated metal pipe under Second Street. Ross summarized three reasonable options: adding a 54-inch overflow pipe adjacent to the existing pipe, replacing the culvert with a concrete box culvert, or replacing it with a structural arch-type metal culvert. "They all offer significant drops in flooding immediately upstream of Second Street," Ross said, pointing to model runs that showed 2- and 10-year backwater reductions in the range of about 5.2 to 5.9 feet depending on the alternative. He also said the larger replacements produced modest reductions even in the 100-year scenario because of downstream tailwater in some cases.

By contrast, Fourth Street sits in a tight, built-up section of downtown where the channel is effectively constrained by building walls and an older stone wall. Ross said the canal and adjacent structures limit the benefit of enlarging the culvert because the channel itself becomes the constraint. "Everything we looked at pretty much had no flood mitigation benefit" while maintaining the existing properties, he said. As a result the team modeled bypass options using narrow corridors to the north or south; those provided limited benefit only for the 2-year events and required easements or acquisition of small parcels. The study's most effective remedies for Fourth Street require acquiring adjacent properties to lay back the channel and realign the stream corridor.

Ross presented cost and benefit tradeoffs and recommended a phased approach: prepare full engineering plans and permitting and implement the Second Street improvements first because they lie within the public right-of-way and show the largest near-term benefit; in parallel, explore easement or acquisition feasibility for Fourth Street so the town can return with a preferred alternative if acquisition is possible. The presentation noted a current project allocation of $1,400,000 and cautioned that that amount is likely sufficient for the Second Street options but not for property acquisition and Fourth Street reconstruction.

Town staff described next steps: produce a signed engineering report, advance construction drawings and permitting this summer, submit those documents to the grant agency for review, and then, if approved, bid and construct the Phase 1 Second Street work with a target of early next construction season. The study identified permitting and coordination with the grant agency (described in the presentation as NCDEQ funding and the grant program mentioned in the slides) as necessary steps before bidding.

Council members asked detailed operational questions during the discussion, including how the models treat backwater from the Neuse and whether devices such as flap gates or backcheck valves could be incorporated. Ross said flap gates to limit backflow from the Neuse could be added at the downstream crossing and modeled; he noted the strategy is commonly used to allow the channel to drain during the local flood peak and then be closed to limit river backwater as the Neuse rises days later. Town Manager Scott confirmed staff would study adding a flap-gate scenario to the modeling and examine historical records to estimate how frequently a backflow-prevention device would have reduced damage.

Ross and staff also explained technical details presented in the study: the Second Street site showed a 65-inch existing corrugated metal culvert; an adjacent 54-inch pipe was modeled as an overflow option; structural plate (arched) metal culvert and concrete box culvert replacements were modeled and had similar preliminary cost estimates and similar hydraulic performance for the 2- and 10-year design storms. The Fourth Street analysis highlighted constraints including an active sewer and water line running through or adjacent to the culvert and very limited lateral space (about 30 to 40 feet) between existing structures, which limited the size of any bypass culvert to roughly a 48-inch pipe unless property was acquired.

The council did not take a formal vote on the recommendations at the meeting. Ross said the firm would incorporate council comments into the final study report and proceed with designs for Second Street if the town directs staff to move forward.

Ending note: staff signaled they will return with a finished report and recommended construction documents for the Second Street culvert and continue to explore acquisition or easement options for Fourth Street before asking Council to fund or approve additional work that would go beyond the current allocation.