Assessing Stream Crossings for Vulnerability
Increasingly, road-stream crossings are threatened by climate change impacts like more frequent and intense storms and flooding.
In the context of transportation, vulnerability is defined as the degree to which a system is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extremes. Vulnerability is a function of the character, magnitude, and rate of climate variation to which a system is exposed, its sensitivity, and its adaptive capacity.
Assessing road-stream crossing structures for vulnerability to flooding and possible structure failure can diversify funding sources available for improving crossings as well as potential partners for the work. The results of a vulnerability assessment can be overlaid with fish passage assessments to identify sites where replaced culverts will meet multiple co-benefits (i.e., ecological, social, and economic).
Because comprehensive vulnerability data are often not available, several different approaches have been developed; these can be adapted for different places based on data availability, budget, and timeline.
Approaches for Assessing Vulnerability
Outreach approach: Direct outreach to municipal and state transportation agencies can be an effective way to compile vulnerability data, especially where little data currently exists. Local transportation staff can often pinpoint individual vulnerable road-stream crossings on a map and provide past flooding history.
Washington State Department of Transportation (WSDOT): As part of a FHWA climate change resilience pilot project, WSDOT used scenario planning in a series of statewide workshops to create a qualitative assessment of climate vulnerability on Washington’s transportation assets. During regional workshops, agency staff from several departments and geographies provided expert opinion on the vulnerability of multiple assets (such as bridges, culverts, airports, ferry terminals, and rail lines).
New York State Department of Transportation (NYSDOT): NYSDOT created a statewide GIS data layer of flood-vulnerable locations based on information from their staff. NYSDOT developed a structured, stakeholder-based approach to qualitatively assess facility risk. With support from a FHWA climate change resilience pilot project, The Nature Conservancy (TNC) compiled similar information at the municipal level. Direct outreach and in-person interviews with county and town transportation authorities resulted in mapped vulnerable road-stream crossings in NY’s Lake Champlain Basin.
The Vermont Culvert Geomorphic Compatibility Screening Tool: The Vermont Agency of Natural Resources commissioned development of a screening tool for evaluating geomorphic stability at road-stream crossing structures. The sum of five variables – percent bankfull width, sediment and debris continuity, slope (structure slope versus channel slope), approach angle, and bank erosion – collectively helps to identify road-stream crossing structures at risk for failure due to incompatibility with natural stream processes. Used in concert with aquatic organism passage assessments, the geomorphic compatibility tool helps determine how culverts influence both the physical and biological components of a stream channel.
Hydraulic capacity modeling approach: Cornell University, in partnership with the Northeast Regional Climate Center and Hudson River Estuary Program, developed an approach to identify undersized culverts vulnerable to flooding under current and future precipitation conditions. Using a combination of culvert inventory field data and culvert capacity and peak discharge models for several climate scenarios, culvert suitability (defined as culverts that can convey peak storm runoff associated with >= 5 year storm) can be calculated.
Planning tools: The Federal Highway Administration and partners have developed several planning tools for assessing climate impacts on transportation infrastructure.Enhanced StreamStats: A new, enhanced web-based tool developed by the USGS Troy, NY office, estimates the magnitude of future peak flows for streams and rivers in New York State and the Champlain Basin in Vermont. The combination of climate models, greenhouse gas scenarios, and time periods, can provide up to 30 sets of peak flow magnitude estimates for each stream watershed delineated. Transportation engineers can use this tool to determine design discharges for culvert designs.