Oyster River (New Hampshire) Culvert Analysis Project

In this project the Piscataqua Region Estuaries Partnership (PREP) performed a vulnerability assessment of the hydrology and drainage system of the Oyster River watershed in southeastern New Hampshire and its capacity to adapt to climate change impacts and increased development. The project used a geographic information system (GIS) based watershed model to identify the road/stream crossing culverts that are vulnerable to failure due to extreme storms with increased precipitation, in combination with development of the watershed. The project found that while approximately one quarter of culverts in the study area are undersized, upgrading culvert capacity is an adaptation activity with a positive benefit/cost ratio.

The project modeled runoff and peak flow rates based on current land use and precipitation, as well as under two different future development scenarios and two greenhouse gas emission scenarios. One development scenario assumed continued development consistent with current practices, while the other assumed a shift to low impact development (LID) techniques (i.e., techniques that minimize runoff through onsite storage and reduction of impermeable coverage). The coupled-climate model was downscaled for two global greenhouse gas emission scenarios derived from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios.    

The project reverse-engineered the culverts to determine the required capacity necessary to accommodate peak flows from increased precipitation and development. The analysis also included the design of simple culverts with sufficient capacity for the tested scenarios, as well as the replacement and marginal costs that would be incurred replacing at-risk culverts. Finally, the individual culverts were given a ranking based on their vulnerability to increased flows and the potential threat to the community of a culvert failure. This analysis provides community decision-makers with guidance both for prioritizing culvert upgrades and for planning LID ordinances.

The project found that under the most likely projected precipitation scenarios for New England, 23 to 30 percent of the watershed’s culverts are undersized. Of these undersized culverts, upgrades can be prioritized based on the risk associated with culvert failure in order to maximize efficiency and affordability. The project also found that the use of LID methods can significantly mitigate the impacts of climate change and population growth. The analysis indicates that adaptation to climate change through upgrading culvert capacity has a positive benefit/cost ratio, as the incremental cost for upgrading to culverts with sufficient capacity for the most pessimistic scenario is a minimal percentage of the total life-cycle expenditure.

The Oyster River Culvert Analysis Project presents a simple model that generates specific estimates of infrastructure vulnerabilities; the study’s results can be used by local decision-makers within the Oyster River watershed, and its methodology can be applied to conduct similar vulnerability assessments in other watersheds.

EPA’s Climate Ready Estuaries program awarded a $50,000 grant to fund this project.  PREP convened a technical team to conduct the pilot project, which involved both field research and hydrological modeling. PREP staff, researchers from Antioch New England University, and staff from the Town of Durham, the New Hampshire Fish and Game Department, and the Strafford Regional Planning Commission performed fieldwork to inventory and map all of the watershed’s culverts and collect data on culvert capacity and the surrounding ground cover, slope, soil, permeability, and land use.

 

This Adaptation Clearinghouse entry was prepared with support  from the Federal Highway Administration. This entry was last updated on October 30, 2015.

 

Publication Date: March 2010

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