CHAPTER 1

Introduction

This chapter describes the synthesis topic, including the need for the study and its background, study objectives, and methodology. Important definitions for this synthesis topic are included in the Glossary at the end of this document.

1.1 Background and Need

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 focused on the connection of habitat for fish and other aquatic species.

“State departments of transportation (DOTs) have traditionally designed structures to convey water under roads based on cost, hydraulic efficiency, and risk criteria. However, this design approach can result in barriers to AOP as an unintended and undesired consequence” (Lennon 2024). As these undesirable consequences are increasingly recognized, state DOTs and others seek design methodologies that consider AOP while providing safe and effective transportation infrastructure. Such methods increase the focus on habitat connection by better simulating natural stream flow conditions throughout the structure for new water crossings or for the replacement or retrofit of existing crossings.

Practices in the design, construction, and monitoring of AOP water crossing structures vary across the United States. While some state DOTs have been developing and implementing AOP programs and projects for several years, many others are in the early adoption stages and are seeking information on best practices.

1.2 Objective

The objective of this study is to document current practices of state DOTs in the design, construction, and monitoring of AOP water crossing structures. The synthesis investigates how state DOTs have implemented AOP structures and the strategies and practices that have been adopted or created. The following topics were investigated to achieve this objective:

• The extent of state DOT adoption of AOP practices
• Types of aquatic organisms typically considered in AOP design
• Current state DOT guidance documents for AOP
• Design methodologies employed for sizing and designing AOP structures
• Programmatic approaches to AOP project implementation
• Use and approaches to culvert rehabilitation and retrofits for AOP

• Impacts to project costs for AOP implementation
• The prioritization basis for replacement of current structures with AOP structures
• Construction practices for AOP structure installation
• Specifications for streambed materials
• Considerations for sizing and selection of AOP water crossing structures
• Project delivery methods
• Internal state DOT structure, staffing, and training for AOP programs
• State DOT practices in monitoring of AOP installations
• State DOT practices in asset management for AOP installations
• Secondary benefits realized by DOTs from AOP practices

1.3 Methodology

Conducting a literature review was the first step to achieving the objectives of this synthesis study. The literature review was followed by an online survey and follow-up case example interviews with select state DOTs.

The literature review focused on current state and federal publications documenting practices in AOP water crossing design, construction, and monitoring. The literature review started with the primary relevant federal guidance on AOP, with discussion of the FHWA Hydraulic Engineering Circular No. 26 (HEC-26) manual (2010), the U.S. Department of Agriculture Forest Service (USFS) stream-simulation manual (2008), and other references. The compilation of state DOT practices is included in tabular format. The references listed for the state DOTs include a mixture of references provided in response to the online survey and guidance documents found from a web-based search of state DOT drainage manuals.

The online survey was disseminated to DOTs in the 50 states, Puerto Rico, and the District of Columbia. The online survey was sent to state DOT personnel in charge of hydraulics programs (i.e., state hydraulic engineers or similar). The online survey consisted of 30 questions, with questions targeted at each of the synthesis study objectives listed, to document the state of the practice. The online survey was structured to allow state DOTs that have not implemented any AOP projects to skip the detailed AOP-focused questions. Results of the survey were compiled, with notable trends identified for this study report.

This synthesis study includes case examples developed from interviews conducted with five state DOTs. Twenty-three state DOTs volunteered to participate in the interviews in response to the last question of the online survey. The volunteer list was reduced by reviewing the individual DOT survey responses and grouping the state DOTs based on AASHTO regions, geography, and characteristics of their programs as demonstrated by the survey results. The final five DOTs were selected to represent a cross section of geographic and ecological conditions in the United States and potential AOP water crossing treatments. The case examples are valuable because they provide added context to the data collected by the online survey. The interviewers targeted discussion of details on AOP program and project drivers in each state, details on each state’s design and construction practices for AOP projects, available information in each state on the cost impacts of AOP projects, and monitoring practices undertaken in each state on AOP installations.

The final section of this study presents a summary of key findings from the literature review, online survey, and case examples. The results of the key findings were used to identify current knowledge gaps in AOP practices and suggest future research topics to aid in the evolution of these practices.