Designing Culverts in Tidal Settings in New Hampshire—Unique Considerations and Applications

In a unique collaboration between the New Hampshire Department of Transportation, the New Hampshire Department of Environmental Services Coastal Program, and the Nature Conservancy, CMA Engineers, with Streamworks, PLLC, completed the planning and design of culvert replacements in tidal settings on State roadways in Rye and Stratham. The structures were chosen in a review of all coastal crossings in New Hampshire because they are tidal restrictions to a total of 48 acres of upstream salt marsh migration potential and are in poor structural condition. Two important climate change impacts will affect these and other tidal structures: increasing sea level and storm surge elevations affecting future tidal flows and increasing precipitation intensity affecting future freshwater streamflow. Comprehensive and complex hydrologic and 2D hydraulic evaluations were developed for each site that modeled both tidal and stormwater flows under both existing and future scenarios incorporating climate change. Sea level rise was considered based on relative vulnerability for each site. The alternatives in this project were evaluated considering several metrics unique to tidal settings: accommodating bi-directional flow, potential for salt marsh migration, aquatic and terrestrial organism passage, and flooding impacts. The selected and preferred alternatives included significant increases in culvert widths, from 3.5 to 15 ft at Rye and from 1.5 to 8 ft at the two adjacent culverts in Stratham. Future roadway elevations were considered for sea level rise, and structural accommodation for possible future roadway raising was accommodated at the Rye culvert. The project was awarded competitive grant funding from NOAA through the Bipartisan Infrastructure Law for final design, permitting, and construction.