Vulnerability Assessment

Risks posed by flooding are increasing as extreme precipitation events become more intense and more frequent. SNHPC conducted a region-wide assessment of culverts and small bridges within the region to evaluate the risks associated with extreme precipitation events. Extreme precipitation causes inland flooding which is the most destructive type of historic weather pattern in the region. Such rainfall events continue to threaten the functioning of region’s transportation network. To avoid and minimize disruptions to road networks and reduce the need for costly repairs under emergency conditions, vulnerabilities to the network need to be identified and a plan for adaptation and/or mitigation put in place. A working group was formed to address this need and developed a system for prioritizing culverts and small bridges based on sensitivity to and magnitude of risks posed to these assets.

Overtopping of Culvert

This assessment and prioritization effort provide local communities and regional agencies with an additional decision-making tool to help them select adaptation and mitigation strategies for highest-risk locations. The assessment also creates a framework to share information and consider impacts to the road network. Communities within the SNHPC region can explore their Culverts and Small Bridges via an online GIS map which inventories data for many of these stream crossings.

Click here to visit the GIS Stream Crossing Map

The vulnerability assessment activities encouraged increased coordination and communication among relevant agencies. As a result, the report provides a means to increase coordination around emergency events and considers impacts of extreme precipitation on other system types which rely on the functionality of the road network.

The assessment results support the following recommendations for action to mitigate and adapt to known vulnerabilities within the SNHPC region:

•   Improve the accessibility of stream crossing inventory data to municipal staff.

•   Emphasize the need for coordination between NH DOT and Municipal Public Works.

•   Refine the criteria for prioritizing risk to culverts and small bridges on an annual basis.

•   Plan for mitigation in advance of the most severe projected changes in weather patterns.

You can read the full Vulnerability Assessment Report here: (coming soon)

Vulnerability Assessment Graphic

Vulnerability Assessment History in the Region

2016 SNHPC/Milone & MacBroom Prioritization Report

In 2016, SNHPC sought to extend the availability of data on stream crossings for the Piscataquog River Watershed by developing a Prioritization Model. SNHPC hired Milone & MacBroom, Inc. to build upon the model developed by Trout Unlimited in 2014 and provide a methodology for prioritizing stream crossings based on four criteria: geomorphic compatibility [GC] to measure how the crossing structure inhibits the flow of the stream, structural condition [STR] to determine is repair or replacement of the crossing structure is needed, aquatic organism passage [AOP] to identify if fish and other wildlife can navigate through the crossing structure, and criticality [C] to assign a value of the crossing to the functionality of the road network. The cumulative output of the four criteria was a measure of overall risk. This risk information was packaged for consumption by municipalities to allow them to plan for a more resilient local road network and enhance capital improvement planning and budgeting. Much of the information developed in this prioritization model was use at the inception of the SADES stream crossing program to evaluate and prioritize replacement of high-risk stream crossings state-wide.

Read the full Piscataquog River Watershed Culvert Prioritization Model Report

2013 SNHPC/Trout Unlimited Assessment

In 2013, SNHPC initiated the “Piscataquog River Watershed Stream Crossing Vulnerability Assessment Project”. This assessment featured the work of Colin Lawson from Trout Unlimited and Tom Ballestero from the University of New Hampshire; this team developed a GIS model to measure hydraulic capacity of stream crossings in the watershed area to determine the maximum flow of water for a given stream crossing during a storm event.

The results of the modeling included a hydraulic capacity rating for 400 stream crossings in the Piscataquog River Watershed into one of three categories corresponding to the ability of the crossing to pass peak flow through a stream crossing. This model was run for the watershed under precipitation frequencies ranging from 2-year up to 100-year storm events in order to better understand the hydraulic vulnerability by categorizing vulnerability of crossings under each storm-year scenario.

Read the full Piscataquog River Watershed Stream Crossing Vulnerability Assessment