SUMMARY

Design, Construction, and Monitoring Practices for Aquatic Organism Passage

Aquatic organism passage (AOP) water crossing design is an evolving field at the nexus of the built environment and the natural world. When successful, AOP water crossings provide sustainable transportation infrastructure and environmental benefits via connected habitats for fish and other aquatic organisms. Practices in the design, construction, and monitoring of AOP water crossing structures vary across the United States. While some transportation agencies have been developing and implementing AOP programs and projects for several years, many others are in early adoption stages and are seeking information on practices for continued program growth.

The objective of this study is to document current practices of state departments of transportation (DOTs) in the design, construction, and monitoring of AOP water crossing structures. The study aims to identify the extent of state DOT adoption of AOP practices, design methodologies employed for sizing and designing the AOP structures, impacts to project costs for AOP implementation, construction practices for AOP structure installation, and practices in monitoring AOP installations.

The synthesis objective was achieved using a three-phase approach that consisted of a literature review, an online survey, and case example interviews. The literature review focused on current state and federal publications documenting practices in AOP water crossing design, construction, and monitoring. An online survey that consisted of 30 questions targeted each of the synthesis study objectives. The online survey was sent to the lead hydraulics staff for DOTs in each of the 50 states, Puerto Rico, and the District of Columbia. Responses were received from 42 of the 52 survey recipients (81% response rate), with 36 of the 42 respondents (86%) acknowledging that they have AOP water crossing projects. Last, case example interviews were conducted with five state DOTs to provide further context to the information collected from the literature review and the online survey. The five state DOTs were selected to represent a cross section of geographic and ecological conditions in the United States and thus potential AOP water crossing treatments.

The following key findings, based on information provided by the 36 state DOTs (of 42 responding agencies) that indicated having AOP water crossing projects, were identified from this synthesis study:

• Thirty of the 36 responding state DOTs (83%) define an AOP project as a new or retrofit project for which consideration of aquatic organisms, primarily fish, is a secondary objective of the project, with improving the transportation system being the primary objective. For 13 of 36 responding state DOTs (36%), the severity of the AOP barrier, habitat improvement, or specific species of concern are a factor in the prioritization of the removal, rehabilitation, or replacement of AOP barrier road crossing structures.

• AOP water crossing design and construction is occurring throughout the United States. Of the 36 responding states with AOP projects, 29 states (81%) evaluate AOP as a part of their typical water crossing design processes.
• Evidence provided by the online survey and the interviews shows that 27 of the 36 responding state DOTs (75%) performing AOP water crossing designs are using design techniques that focus on replicating a natural channel bed geometry or materials within the structure.
• Sizing methodologies for AOP structures vary across the United States. Of the 36 respondents, 22 (61%) size the AOP structure to match culvert hydraulic properties such as design velocities and depths with fish capabilities, 18 (50%) size the AOP structure based on bankfull width, and 9 (25%) size the AOP structure based on bankfull width plus some additional width.
• Nationwide, AOP projects and programs are driven by multiple reasons, ranging from legal mandates to ad hoc projects and programs driven by structural integrity or environmental stewardship considerations.
• AOP design and project requirements vary across the United States depending on the quantity and type of fish species taken into consideration.
• Thirty-two of 36 respondents (89%) did not know the effect on operations and maintenance costs when incorporating AOP at a water crossing.
• Thirty of 36 responding state DOTs (83%) identified the physical condition of existing water crossing structures as being the primary prioritizing factor when considering projects for replacement with an AOP water crossing structure.
• Twenty-six of 36 survey respondents (72%) indicated that they consider a range of secondary benefits when prioritizing or funding AOP projects. Information gathered during the five case example interviews indicated that the five interviewed state DOTs do not quantify secondary benefit considerations, but rather qualitatively identify them as a potential benefit.
• Post-construction monitoring of AOP water crossing projects is not occurring extensively: 12 of 36 state DOT respondents (33%) do not currently perform post-construction monitoring of AOP structures, while 17 of 36 respondents (50%) perform post-construction monitoring as needed.
• The implementation of AOP water crossing–specific asset management systems is limited, with 29 of the 36 state DOT respondents (81%) stating either that they have no asset management systems that include the water crossings or that their systems do not include AOP fields.

In addition to identifying the current state of the practice, this synthesis study identified areas of knowledge gaps for which additional research would provide valuable tools for state DOTs as they continue to develop their AOP programs:

• The development of quantification tools for the secondary benefits of AOP water crossing structures and methodologies to integrate the quantified secondary benefits into the capital improvement decision-making process.
• Updated research into the costs of implementing AOP water crossing structures, with documentation of design, construction, and long-term maintenance costs.
• Documentation on the benefits of asset management systems for AOP programs and the development of tools for the collection and incorporation of field data into asset management databases.