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City of Laconia Twin Culvert Replacement

City of Laconia Twin Culvert Replacement

City of Laconia Twin Culvert Replacement

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Markets: Government


  • Preparation of preliminary engineering and schematic design
  • Field investigations to define existing conditions of the roadway, culverts and manholes
  • Preparation of final design documents for construction
  • Preparation of NHDES Wetlands permit applications
  • Preparation of construction cost estimates


The twin culvert replacement project consisted of two 29-inch by 42-inch corrugated metal pipes conveying stormwater from a drainage swale under Bisson Avenue to a submerged outlet at the bottom of a bank into the Winnipesaukee River.  The corrugated metal pipes experienced severe corrosion over time, and caused a sinkhole in the roadway, limiting passage to one lane. Loureiro was performing work for the City of Laconia when the city approached us to work on this additional project. The project required Loureiro to work quickly in conjunction with department of public works staff to determine a cost-effective solution.  Loureiro proposed replacing the structure with two reinforced concrete pipes of similar hydraulic capacity. Loureiro provided civil and stormwater engineering services to execute on the proposed solution.


The proposed stormwater piping consisted of twin 30-inch reinforced concrete pipes.  The design allowed for maintaining the existing location and alignment. The proposed stormwater piping also provided an equivalent configuration, with a slight improvement to the hydraulic capacity of the existing structure, and passes flows between the 10-year and 25-year storm events.  Beginning upstream, the inverts match the existing swale elevation, and piping travels approximately 225 feet downstream, before discharging into a submerged 15 feet long riprap apron with the apron outlet invert matching existing elevation.

The proposed work maintained the profile of the existing drainage crossing and minimized upstream impacts to only those necessary for the removal of the existing metal pipes, and installation of the proposed structures.  Further downstream impacts are limited to those only necessary for the removal and installation of the structures, with the addition of a riprap apron designed to mitigate the impacts from increased flows due to the proposed crossings capacity. The project was completed within three months.


The design of the proposed work minimized potential impacts to the maximum extent practicable, and further reduced unavoidable impacts. Loureiro responded efficiently and effectively to the project and was able to get the work completed within 3 months. The city was subsequently able to open the road back up to 2 lanes, restoring normal traffic flow to the affected area.

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