Design, Construction, and Monitoring Practices for Aquatic Organism Passage (2025)
Chapter: 5 Summary of Results
CHAPTER 5
Summary of Results
The objective of this synthesis is to document current state DOT practices in the design, construction, and monitoring of AOP water crossing structures. Information for this synthesis was gathered through a literature review of current state and federal guidance documents, an online survey of DOTs for all 50 states, Puerto Rico, and the District of Columbia (a total of 42 responses [81%] were received), and case example interviews with five state DOTs. Case example interviews were selected to provide a cross section of ecoregions across the country with varied species of concern, designs, and construction and monitoring practices that provide additional insight and context into current practices.
5.1 Summary of Synthesis Findings
The findings of this synthesis study are summarized as follows:
- Most state DOT respondents 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. Thirty of the 36 survey respondents (83% of responses) identified physical condition of the structure as the primary driver for replacement, while all 36 responses identified fish or a specific fish species as the type of aquatic organism considered in design. For a few state DOT respondents, an AOP project may be one in which an AOP is a mandated primary objective.
- AOP water crossing design and construction is occurring throughout the United States, with 36 of the 42 survey respondents (86% of responses) acknowledging that they have AOP projects. Of the 36 responding state DOTs, 29 respondents (81%) evaluate AOP as a part of their typical water crossing design processes. The proliferation of AOP projects does not follow a definitive pattern, as state DOTs in all ecoregions have AOP projects.
- Evidence provided by the online survey and the interviews shows that most of the state DOTs performing AOP water crossing designs (24 of 36 responding DOTs [67%]) are using design techniques that focus on the replication of a natural channel bed within the structure. More traditional techniques, such as weirs and baffles in fish ladder–type constructs, are seemingly only used in specialized or retrofit situations.
- From the 36 respondents who acknowledged having AOP projects, 15 (42%) evaluate AOP for all water crossing projects; 13 (36%) evaluate AOP occasionally on water crossing projects for which aquatic organisms have been identified; 7 (19%) evaluate AOP occasionally, but not programmatically, on water crossing projects; and 1 (3%) rarely evaluates AOP for water crossing projects.
- From the 36 respondents, 25 (69%) have a written policy or design guidance document in place for advisement or direction on AOP design. The remaining 11 respondents (31%) do not have a policy in place.
- AOP culverts (both closed bottom and open bottom) were the most commonly identified AOP water crossing structure practice (32 of 36 respondents [89%]).
- Sizing methodologies for AOP structures vary across the United States. Of the 36 survey respondents, 22 (61%) size the AOP structure to match culvert hydraulic properties such as design velocities and depths with fish capabilities, 18 respondents (50%) size the AOP structure based on bankfull width, and nine respondents (25%) size the AOP structure based on bankfull width plus some additional width.
- Nationwide, drivers behind AOP projects and programs are diverse. These drivers for AOP range 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 under consideration. Of the 36 responding state DOTs, 14 (39%) consider resident or freshwater fish, 13 (36%) consider amphibians or reptiles, 12 (33%) consider salmonoids, and 10 (28%) consider anadromous fish in AOP design. Eight of the 36 responding state DOTs (22%) consider aquatic mammals, and the same number consider invertebrates.
- Thirty-two of the 36 responding state DOTs (89%) were not able to quantify the effect on O&M costs when incorporating AOP at a water crossing. However, respondents indicated that a reduction of maintenance costs (10 of 36 respondents [28%]) and increased resiliency (15 of 36 respondents [42%]) are potential co-benefits in AOP projects.
- The physical condition of existing water crossing structures was identified by most state DOTs (30 of 36 respondents [83%]) as being the primary prioritizing factor when considering projects for replacement with an AOP water crossing structure. The respondents identified the presence of impaired species of concern (such as rare, threatened, or endangered species) as the second most common prioritizing factor.
- Twenty-six of 36 survey respondents (72%) indicated that they consider a range of secondary benefits when prioritizing or funding AOP projects. All five case example state DOTs noted that secondary benefits are not quantified but are recognized and qualitatively identified as a potential benefit.
- Post-construction monitoring of AOP water crossing projects is not occurring extensively, with 17 of the 36 state DOT respondents (47%) stating they are performing post-construction monitoring only on an as-needed basis. Twelve of 36 respondents (33%) stated they are not performing any post-construction monitoring. In cases when post-construction monitoring is occurring, the responding state DOTs identified the primary drivers as state environmental agency requirements (16 of 23 respondents [70%]) or federal agency requirements (9 of 23 respondents [39%]).
- The implementation of AOP water crossing–specific asset management systems is limited, with 29 of the 36 survey 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.
5.2 Knowledge Gaps and Suggested Future Research
The data collected under this synthesis are intended to provide information for all state DOTs to consider as they implement or advance their own AOP programs. In addition to disseminating current practices, the data collected also aided in identifying AOP practice areas for which future research efforts could help state DOTs in the ongoing management and development of AOP programs, including asset management and post-construction monitoring.
5.2.1 Secondary Benefits of AOP Water Crossing Structures
State DOTs acknowledged during both the online survey and the case example interviews that they are aware of secondary benefits (or co-benefits) of implementing AOP water crossing structures. However, they broadly stated that they are not currently quantifying secondary benefits
or using them qualitatively to justify additional expenditures beyond the minimum required. Secondary benefits identified during this synthesis included increased flood resiliency, wildlife connectivity, reduced maintenance requirements, and environmental mitigation crediting. The development of quantification methods for these secondary benefits may offer state DOTs additional tools to integrate co-benefits into the decision-making process and justify the additional capital expenditure for AOP water crossing projects and programs for both traditional project and program funding and grant-funding opportunities.
5.2.2 Cost of AOP Water Crossing Structure Implementation
In the online survey and during the case example interviews, state DOT respondents were asked about the relative cost of AOP water crossings compared with traditional hydraulic designs. The questions were targeted at design costs, construction costs, and long-term maintenance costs. Responses largely suggested that this information was not tracked and not readily available in most cases. In the online survey, 18 of the 36 respondents (50%) classified cost impacts as “unknown/information not available.” The online survey further questioned how state DOTs have seen O&M costs for AOP water crossings compare with traditional designs, with 32 of the 36 respondents (89%) indicating “unknown or have not been determined” and an additional respondent (3%) indicating O&M costs are not considered for water crossing structures.
During the case example interview process, targeted discussion focused on efforts and levels of knowledge regarding cost impacts for AOP water crossing structures. In cases when relative costs had been studied, the respondents indicated that studies had largely been limited to comparisons of construction bid tabulations. In these cases, the impact on design efforts and associated costs had not been considered, and the state DOTs expected the impact to be small (estimated at less than 10% of the total project design expenditure). The state DOTs discussed that the compilation of O&M costs was particularly difficult to define because these activities are handled at the DOT district or region level as system maintenance responsibilities. The interview subjects felt that attempting to compile the data would be a significant effort involving either (1) questioning of maintenance staff at each of the districts or regions and imprecise approximation of maintenance efforts on different assets versus other parts of the transportation system or (2) the development of a new maintenance tracking system.
5.2.3 Asset Management Benefits for AOP Programs
The online survey and the follow-up case example interviews documented that state DOTs have a low implementation rate for asset management systems that are targeted for AOP water crossings. Less than 20% of respondent DOTs identified the presence of an AOP water crossing–targeted asset management system. Fifteen of the 36 respondents to the online survey (42%) indicated that their state DOT does not have an asset management system that includes AOP structures, and 14 of the 36 respondents (39%) indicated that while the state has an asset management system, the system does not have specialized fields for tracking specialized AOP water crossing data, such as biological effectiveness monitoring and the impact of structures to aquatic species. The state DOT staff interviewed for the case examples agreed that field-collected data on AOP performance would be valuable for identifying best practices and help provide improvements in design and construction practices. Further identification of tools for the collection of field data and incorporation into an asset management database would facilitate research into the improvements discussed in the interviews.
5.2.4. AOP Structure Post-Construction Monitoring Tools
Results from the online survey show that 12 of the 36 responding DOTs (33%) do not perform post-construction monitoring and 18 of the 36 respondents (50%) only perform post-construction monitoring on an as-needed basis. Discussion of this topic during the case example
interviews documented that in the case of several DOTs, post-construction monitoring is limited because of staff resource constraints. Based on discussions during the interviews and with the synthesis panel, the synthesis study has concluded that DOT staff would benefit from training and tools to allow for rapid and effective monitoring of AOP structures. The availability of a nationwide standardized process for AOP monitoring, accompanied by training to allow a wide range of staff to support monitoring efforts, may improve monitoring participation and frequency by DOTs. Additionally, the availability of a standardized protocol would facilitate future research into AOP effectiveness by allowing monitoring data from different agencies to be combined into a single database. This future research could be used to clarify AOP design objectives and the effectiveness of various AOP design treatments, such as interior constructed channel banks and use of large wood.
