2007 and have become more prescriptive over time. The requirements include conditions related to culvert design and construction (e.g., the width and slope of the culvert) and embedment of the structure. The requirements apply to any perennial stream with a culvert, regardless of target species, which has increased the overall number of AOP projects. There are no specific requirements for bridges.

The lack of a target species makes it difficult to define an AOP project and evaluate success. GDOT considers an AOP project successful if sediment has accumulated as designed within the culvert and a headcut does not propagate through the water crossing.

In addition to the Section 404 permitting process, an Interagency Agreement (i.e., Interagency Joint Coordination Procedures, https://www.dot.ga.gov/PartnerSmart/EnvironmentalProcedures/Ecology1/References/Joint%20Coordination%20Procedures%20-%20GDOT-OES.pdf) with the USFWS, Georgia Ecological Services; Georgia Department of Natural Resources; and FHWA, Georgia Division, is in place through the Fish and Wildlife Coordination Act. This agreement allows GDOT to streamline the AOP process through a programmatic approach. GDOT is developing programmatic agreements that incorporate avoidance and minimization requirements for federally or state-protected species. As part of the development of a programmatic approach, GDOT funded a research project with the University of Georgia that aimed to incorporate connectivity criteria. The programmatic approach looks at streamlining planning and preconstruction. During construction, the programmatic approach may allow for avoiding seasonal work restrictions for certain federally listed species.

GDOT recognizes co-benefits associated with the design and construction of AOP and fish passage structures, including flood reduction and increased resilience. GDOT has found that there can be benefits to replacing culverts rather than attempting to retrofit and extend the structure’s life, especially in areas prone to intense storm events.

GDOT project costs are highly dependent on mitigation credit costs and mitigation availability related to Section 404 permit requirements. Mitigation credit costs vary from watershed to watershed by tens of thousands of dollars per credit. A mitigation forecast has found stream credits are running out in many parts of the state. For these reasons, mitigation costs need to be reduced; mitigation costs for streams have increased by at least five times since 2018. GDOT spends an estimated $30 million per year on Section 404 mitigation credits. Georgia has more than 100 active existing mitigation banks, but the state may not be able to meet the future demand for mitigation required for projects. GDOT executive management is addressing the credit shortage and allocating funds for mitigation.

In 2018, USACE released Regulatory Guidance Letter No. 18-01 (https://www.nap.usace.army.mil/Portals/39/docs/regulatory/regs/RGL-18-01-Determination-of-Compensatory-Mitigation-Credits-for-Dams-Structures-Removal.pdf?ver=2019-02-22-140711-787) that provides guidance for determining compensatory mitigation credits for the removal of obsolete dams and other structures from rivers and streams. GDOT is looking into following these guidelines with the USACE Savannah District to potentially eliminate mitigation requirements for an AOP project and to determine whether an AOP project may be able to generate mitigation credits.

4.2.1 Design Practices

GDOT designs new culverts to address both AOP and terrestrial wildlife connectivity. The GDOT Drainage Design for Highways Manual (2023a) and Perennial Stream Culvert Design Guidance (category “Design and Environmental Coordination Guidance” under the Roadway section at https://www.dot.ga.gov/GDOT/pages/DesignManualsGuides.aspx) include guidance for the design of AOP water crossings. The design community is still learning about incorporating

terrestrial wildlife passage into designs. In many cases, AOP may enable terrestrial wildlife passage when the design works as intended. AOP projects typically span the bankfull width and are intended to accumulate sediment within the structure over time. Accumulated sediment can provide dry areas that allow terrestrial species to pass through the culvert. Variability exists across different regions within the state, ranging from mountainous regions to coastal plains, that affects culvert design and sediment accumulation. Multiple culverts that were not necessarily sized for AOP seem to provide passage. In Georgia, it is not uncommon for water crossings to be designed with multiple culvert barrels. When multiple barrels are used, one of the barrels typically fills with sediment, yet it is still able to pass most flows. At crossings with multiple barrels, GDOT has started placing baffles at the entrance of one of the barrels to block off the stream until the flow is high enough to inundate the top of the baffle and flow within the barrel.

A recent example of a culvert that was upsized was part of an emergency culvert project on SR 280 at Dobbins Air Force Base. The wider opening better matched the bankfull width and provided lower outlet velocities, which is expected to reduce potential for scour.

GDOT is working with the U.S. Geological Survey on refining bankfull width estimates. Bankfull width measurements are measured in the field by a GDOT ecologist, and regional curves are based on the drainage area. GDOT has developed the Perennial Stream Culvert Design Guidance workflow for new and replacement structures whereby the designer is to consider three main items: (1) width of the structure, (2) slope (matching existing slope within 25%), and (3) embeddedness. Retrofits have more limited options.

GDOT requires culverts to be countersunk; however, the state environmental agencies do not have any requirements that focus on the countersinking of the culvert or on the placement of channel bed material inside the structure. Therefore, GDOT does not typically specify the placement of channel bed material within the culvert as part of the design. GDOT sometimes uses concrete vanes to assist in capturing sediment. GDOT designs typically focus on setting the height of the vanes below the targeted streambed bottom elevation, allowing the vanes to trap incoming sediment and build a channel bed naturally. One criterion in design is to place a stamp within the structure to show the targeted designed depth of embedment so maintenance does not remove any sediment that has naturally accumulated within the culvert. Maintenance personnel focus on ensuring culverts pass water and do not cause roadway problems; maintenance staff are not required to fill a culvert with sediment if it is not naturally filling.

GDOT AOP designs have higher design costs compared with design costs for traditional culverts, mainly from the additional coordination required with other disciplines and agencies. The ecology group within GDOT develops the permit application and must prepare specific diagrams for AOP designs for USACE that were not traditionally prepared for culvert projects. Difficulties have arisen in finding resolutions with USACE when the project cannot meet USACE’s conditions, in some cases leading to months of additional coordination and meetings.

GDOT has different requirements for culvert extensions. GDOT has not encountered a scenario in which an agency is forcing it to replace a structure; however, GDOT is asked to justify cases when it proposes an extension as opposed to a replacement. Some culvert extension projects may correct outlet scour as part of the project.

4.2.2 Construction Practices

GDOT AOP project construction costs have likely increased because of larger culverts, additional excavation required for the larger structure, and the occasional use of baffles and vanes. Construction of AOP projects sometimes faces issues with contractors not properly installing culverts (e.g., embedding the culverts per plan), leading to the need for the contractor to redo

the work. However, construction practices have improved over time as contractors become more familiar with AOP design and construction standards. There may be a learning curve for contractors when terrestrial wildlife passage is included as part of the project.

4.2.3 Monitoring Practices

GDOT has a culvert asset management database, but interviewees did not think the database includes AOP fields for monitoring AOP projects. GDOT is not required to perform monitoring by agencies; therefore, monitoring is typically not performed. GDOT would like to have a monitoring program, but funding is an issue. It is important to GDOT to understand how the increase in dollars spent on AOP projects is improving the environment.

4.3 MaineDOT

MaineDOT’s AOP water crossing program is driven by regulatory requirements to provide passage for threatened and endangered species on potential habitat-supporting river systems as identified by the USFWS. For MaineDOT, the use of AOP water crossings became a mandated criterion for structural replacement projects on river systems that fall within the area of the state with known habitat for endangered Gulf of Maine Atlantic salmon (Salmo salar). MaineDOT does not have a programmatic requirement to replace water crossings solely for AOP; the requirement applies whenever the state prepares to replace an older crossing that has a degrading structural condition.

Following publication by NMFS and USFWS in 2009 of the rule establishing endangered status for Atlantic salmon in Maine, MaineDOT staff consulted with USFWS on individual design features to satisfy the regulatory review on each water crossing replacement project. During these early adoption years for AOP water crossings, MaineDOT did not have a standard agreement with USFWS, nor did the reviewers have a standard requested approach for the AOP water crossing designs. This situation affected MaineDOT’s ability to deliver projects in a predictable time frame and on budget.

MaineDOT’s solution was to develop a programmatic agreement with USFWS that defined a standard process for project submissions, established the applicable area of the state for required AOP water crossing implementation, and established a standardized design procedure. The programmatic agreement is the Maine Atlantic Salmon Programmatic Agreement (MAP). The standardized design procedure, documented under the MAP, is the Maine Habitat Connectivity Design procedure. MaineDOT staff have seen a significant improvement in their project delivery process as a result of the MAP, with a streamlined agency consultation process (review periods that could take up to a year dropping to under 30 days) and an agreed-upon standardized design criteria allowing projects to remain on schedule and budget.

Outside the areas where the MAP applies, MaineDOT implements AOP water crossing design projects on a case-by-case basis. Outside the Atlantic salmon habitat, these projects are typically targeted at systems identified as supporting brook trout or alewife.

4.3.1 Design Practices

MaineDOT’s Habitat Connectivity Design is documented in Appendix B of the User’s Guide for the Maine Atlantic Salmon Programmatic Consultation (FHWA et al. 2017). The DOT’s design process typically starts with sizing the AOP water crossing structure at 1.2 times the natural channel bankfull width. MaineDOT staff will identify the bankfull width based on a field geomorphic survey of the natural river channel or by using regional curves if they are unable to

identify appropriate data in the field. Staff will then determine the height of the new structure based on hydraulic capacity and constructability; however, they view an 8-ft height (and completed 6-ft open rise) as a practical minimum to allow construction of the interior channel bed as well as post-construction access. Generally, new AOP water crossings in Maine fall under MaineDOT’s large culvert design standard that requires a 100-year storm design standard with the flooding headwater at or below the top of the structure opening; this same standard was adopted in the MAP. A 6-ft-open rise with 1.2 times bankfull width sizing almost always meets this hydraulic performance standard.

MaineDOT targets AOP water crossing designs to fit the structure into the overall channel profile with smooth transitions in the profile connecting the upstream and downstream external channel segments (vertical alignment). Target design configurations also have smooth planform shapes (horizontal alignment) with no tight bends or turns into and out of the AOP water crossing. The interior channel is designed and shaped using furnished aggregate designed to mimic natural channel processes.

Ideally, MaineDOT prefers that the constructed interior streambed material be designed to be stable for certain high-flow conditions but also match the native streambed material in the adjacent channel areas. MaineDOT does not have a standard streambed material specification but rather develops one on a project-by-project basis. MaineDOT staff stated that they like to use a bank run aggregate (rounded river stone) or gravel for the constructed channel bed because it fits the channel setting and is typically available in most of Maine. Maine’s standard for the interior constructed channel bed is for a 2-ft bed thickness. MaineDOT staff approach bed material sizing by using pebble counts or targeting stability of the D₈₄ aggregate size (size at which 84% of the rock aggregate mixture is smaller than the median) at 100-year flood conditions, depending on site conditions. MaineDOT noted that some shifting of the placed bed material is preferred, allowing for complexity of flow conditions and the channel bed. MaineDOT staff emphasized that stability of the rock aggregate used in the channel bed is important, but that they will consider smaller sizes if the design does not match the natural environment or if they expect that shifting or transport of rock gravels and cobbles from upstream will replenish the interior channel bed.

MaineDOT frequently includes grade control structures in its AOP water crossing designs, such as rock weirs or rock bands designed for stability up to the 100-year flood condition. In cases when these grade control structures are designed with drops or steps in the profile, the designers target a design whereby the drops are no larger than drops documented within the natural stream. On steeper river systems, MaineDOT has used rock bands across the channel to simulate step-pool channel bed formations. The rock bands are designed to protrude above the channel and provide grade control and a targeted drop height to a downstream pool.

Constructed interior channel banks are also targeted for design stability up to the 100-year flood condition, when the AOP water crossing structure is large enough for the inclusion of banks. MaineDOT staff stated that they see a lot of value in incorporating constructed interior banks and frequently target them for their inclusion in a project. However, on smaller structures (widths less than plus or minus 8 ft) where interior banks are not feasible, MaineDOT staff typically specify having a V-shaped channel graded into the bed with rock clusters placed along the sides of the structure or the edge of the channel. MaineDOT staff noted that for both the interior constructed channel banks and the grade control structures, they prefer the use of angular quarry rocks over the bank run aggregates they rely on for the channel bed.

MaineDOT does not use culvert AOP retrofit practices in areas of the state covered by the MAP for salmon but does consider and use the practices on a case-by-case basis in other areas of the state. Culvert retrofit practices implemented by MaineDOT include interior weirs, cast-in-place

baffles, culvert slip-lining with welded-in weir plates, concrete outlet structures, and downstream grade control or rock ramps to “bathtub” the culvert (i.e., low slope or no slope with water backed through). MaineDOT staff feel that they have seen good success with retrofit projects. They have performed passive integrated transponder tagging studies on a few weir baffle structures placed for brook trout passage. MaineDOT staff have learned that the success of culvert retrofit projects relies on getting the downstream grade control to connect with the culvert outlet or on inclusion of an engineered fishway.

4.3.2 Construction Practices

MaineDOT staff have refined their approach to AOP water crossing construction as they have gained applied experience. At the start of each AOP construction project, the staff will require a preconstruction meeting with the contractor to review the plans, specifications, and any special details of the waterway construction. MaineDOT staff feel that early and frequent interaction with the contractor is important, with field support provided by a combination of construction support, regional staff, and environmental staff. MaineDOT seeks to have this early interaction before streambed construction starts to catch any deficiencies in the AOP construction practices and advise on correct practices. As the projects continue through construction, MaineDOT’s interactions decrease to more periodic inspections because of staffing constraints. MaineDOT staff also noted that the inspection and approval process for materials is an important part of each project that is stressed to field staff because undersized rock or similar materials could undermine the stability of the channel design.

MaineDOT staff agree that the relative cost of an AOP water crossing structure over a traditional hydraulic design culvert is significant and measurable. The staff noted that construction of the interior channel bed and related features could account for around a 10% increase in the total cost of a project but that the larger sizes of an AOP water crossing have a much greater impact on cost.

4.3.3 Monitoring Practices

MaineDOT’s monitoring of AOP water crossings follows the plan defined in the MAP. The agreement includes a prescriptive process for visual inspections with profile data collection 1 year and 3 years after construction, and a final visual inspection 5 years after construction. The monitoring process is targeted at checking the geomorphic stability of the constructed AOP channel bed.

MaineDOT has a statewide database for large water crossing structures that tracks typical NBIS features. The statewide database does not have any specific features to note structures that are AOP water crossing structures, but pictures of each water crossing are included in the database and could be used to identify AOP crossings. The MaineDOT Hydrology and Stormwater Division maintains an ad hoc database of its AOP water crossing projects using a spreadsheet and Google Maps. Since 2001, MaineDOT has intentionally designed and constructed more than 400 crossings for AOP using a variety of approaches and achieving a range of success.

4.4 MnDOT

MnDOT started its focused implementation of AOP water crossing structures around 2009. At that time, MnDOT convened a working group that recognized the need and identified enhanced AOP water crossing designs as an emerging focus area for replacement structure design. Findings of this early work are documented in MnDOT Research Report 2009-20, Cost Analysis

of Alternative Culvert Installation Practices in Minnesota (Hansen, Nieber, and Lenhart 2009). Minnesota’s current primary guidance document is the Minnesota Guide for Stream Connectivity and Aquatic Organism Passage Through Culverts (Hernick et al. 2019).

Minnesota does not have an externally mandated requirement to replace water crossing structures that may be barriers to fish passage. Rather, MnDOT’s AOP programs are focused on a partnership with the Minnesota Department of Natural Resources (MnDNR) to facilitate waterway construction permitting, recognizing that environmental stewardship is an important aspect of MnDOT’s efforts. Following this understanding, MnDOT will, on a case-by-case basis, incorporate AOP design features into water crossing structures that are scheduled for replacement during the course of their traditional CIP. In most cases, the AOP water crossing structure will be used to replace a culvert in poor condition or as part of a large corridor improvement project. In ideal cases, the replacement water crossings will be identified and AOP design needs defined 5 years in advance and will follow the state’s typical 5-year capital improvement plan.

MnDOT develops an early notification memo that identifies project impacts such as the planned water crossing replacement project. The early notification memo is presented to MnDOT’s Office of Environmental Stewardship for initial review and identification of MnDNR coordination needs. In cases when MnDOT has identified that the project will affect public waters regulated by MnDNR, the early notification memo is presented to MnDNR through MnDOT’s in-house MnDNR liaison. MnDOT staff have learned that use of the in-house MnDNR liaison allows for efficient and expedited coordination with MnDNR on environmental issues, including early identification of AOP water crossing needs.

4.4.1 Design Practices

MnDOT has identified the following seven best practices for AOP water crossing design and stream connectivity:

1. Design the culvert slope to match the stream channel slope.
2. Place the culvert to best match stream alignment.
3. Design the culvert opening to bankfull channel width or slightly greater.
4. Provide culvert flow depth comparable to channel flow depth for AOP (not overly wide and too shallow).
5. Provide a continuous sediment bed with roughness similar to the channel.
6. Maintain continuity of sediment transport and debris passage, similar to adjoining reaches.
7. Design for safety of the general public, longevity, and resilience.

The design of AOP water crossings in Minnesota starts with Chapter 3, Hydrology, and Chapter 5, Culverts, of the MnDOT Drainage Manual (2024). These chapters set the base design standards for the waterway crossing, including design storm, overtopping, and headwater requirements. These base design standards are then combined with as many of the seven best practices as possible to develop the AOP structure recommendation.

During the scoping phase, the design team performs an initial sizing evaluation for the AOP structure based on a field inspection of the pre-project water crossing structure, investigation of historic structure inspection reports, and other available historical information (e.g., maintenance history). The field investigation documents the existing structure’s performance based on evidence of scour, sedimentation within the structure, stability of any riprap placements, and other notable evidence of channel instability. Based on the documented evidence from the scoping phase, the design team develops an initial recommendation for AOP structure sizing. Hydrologic and hydraulic studies following recommendations from Chapters 3 and 5 of the Drainage

Manual and the seven best practices are incorporated during conceptual design to further refine the AOP structure sizing recommendation.

MnDOT staff have experienced some challenges with regulatory agencies during the waterway construction permitting process related to water crossing structure size increases and related impacts on flooding performance. These challenges have included requirements for FEMA Conditional Letter of Map Revision studies and applications in cases when upstream headwater levels are shown to decrease. Other agency concerns include increased flood volumes pushing downstream, which have been viewed as a water quality impact because of the loss of upstream storage from undersized culverts. In other cases, MnDOT has addressed complications from downstream property owner complaints regarding increased AOP water crossing structure sizes causing increased flooding. As a result of these lessons learned, MnDOT, in many cases, has targeted AOP water crossing structure sizes that achieve “no-rise” floodplain conditions to avoid permitting complications, project delays, and added project costs.

MnDOT’s approach to AOP water crossing design uses a standard special provision for the constructed channel bed material that includes the use of large angular riprap. The riprap component of the interior constructed channel bed is designed for stability for the selected design event, commonly the 50-year return period storm event flood conditions. The interior bed material may be shaped to form a bankfull-sized trapezoidal channel if the targeted water crossing structure is sufficiently wide enough to allow for incorporation of the full channel width. MnDOT’s typical AOP water crossing structure relies on the large riprap component of the constructed channel bed material to provide long-term stability to the interior channel, without the use of large boulder sills, weirs, or other bed retention–type structures. In select cases, where site conditions do not allow for the design of a 50-year stable bed, MnDOT will incorporate large boulder structures into the channel to support bed material retention under flood conditions.

MnDOT has worked with MnDNR, in limited cases, to perform retrofit projects to existing culverts that are determined to be fish barriers. Retrofitting is not a preferred practice; however, it provides a viable course of action to provide partial uplift for aquatic passage conditions in a structurally functional culvert that is not planned for replacement to satisfy regulatory needs. MnDOT’s experience with retrofits is varied and targeted at the specific aquatic needs of a crossing as defined by MnDNR. Examples of retrofits performed in the state include cast-in-place concrete baffles, downstream rock ramps to raise tailwater conditions, and mussel spat rope installations (Kozarek and Hernik 2018).

4.4.2 Construction Practices

MnDOT and MnDNR have each researched the added capital costs for construction of AOP culvert over standard culvert replacements. MnDNR’s research concluded that added AOP design requirements increased the construction costs of individual projects by 20% to 30%. MnDOT’s research was performed by compiling historical construction bid tabulations for comparable AOP versus non-AOP culvert replacements and concluded that added AOP requirements increased construction costs by 33%.

MnDOT staff have gained experience from working in the field with contractors during the construction of AOP water crossing structures related to the sourcing and specification of the constructed channel bed material. Construction observations provided by MnDOT staff include problems with improperly or excessively mining native channel bed aggregates from adjacent channel reaches, vague specifications, or specifications that require material that is not commercially available. Staff noted that these problems drove project issues, including contractor complaints and construction change orders. MnDOT has adopted a random riprap class special

provision for use in the constructed channel bed in response to these gained experiences. Material requirements for the MnDOT’s random riprap class special provision are 67% random riprap and 33% virgin course filter aggregate (D₅₀ of 3/8 in.; D₁₀₀ of 1 in.).

The special provision notes that the random riprap class is to be based on the AOP guidance manual and hydraulic recommendation letter. Minnesota DOT staff believe that use of the random riprap special provision is a good compromise that meets MnDNR requirements while satisfying MnDOT construction oversight staff and minimizing contractor issues.

MnDOT coordinates with MnDNR throughout the construction process, with MnDNR staff providing in-field guidance while the contractor is installing the constructed channel bed. MnDOT has learned that this engagement helps the contractors properly place and shape the channel bed materials and satisfies MnDNR. This engagement also allows MnDNR staff to work with the contractor to field engineer details in the channel bed that provide flow diversity and minimize flow concentration along the structure sides.

4.4.3 Monitoring Practices

MnDOT does not currently have a monitoring program that collects long-term data specific to its AOP water crossing structures. Monitoring practices are currently limited to the requirements of the NBIS program for structures of 20 ft in length, with only standard NBIS data collection occurring.

4.5 WSDOT

The focus of AOP for WSDOT is primarily on fish passage, specifically passage for salmonids. Aquatic organisms other than salmonids are of concern, but by meeting Washington State water crossing requirements, the regulatory agencies assume that passage is provided for other aquatic organisms. Washington State prioritizes water crossing sites for fish passage installations based on the length of habitat gain above the crossing, without considering other barriers. The potential gain in fish habitat is the determining factor for prioritizing barrier removals.

The WSDOT fish passage program has been in place since the 1990s but previously had limited funds. The program is now driven by a federal injunction and is expected to continue indefinitely. The focus of the federal injunction is currently on Western Washington, an area that includes everything north of the Columbia River drainage and west of the Cascades. The rest of the state continues to remove fish barriers; however, funding for projects outside Western Washington typically comes from grants, partnerships, and other sources. The federal injunction requires WSDOT to provide 90% of the potential habitat above identified barriers within the case area by 2030. There are more than 2,000 crossings on fish-bearing streams in the state, and around 400 crossings are targeted for replacement by 2030.

WSDOT recognizes various co-benefits associated with the design and construction of AOP and fish passage structures, including flood reduction and increased resilience. WSDOT has seen reduced maintenance activities with fish passage water crossings compared with traditional culverts. With larger fish passage water crossing structures, the risks of scour and flood damage have decreased. Other co-benefits depend on the project but may include wildlife, wetland, and floodplain connectivity.

4.5.1 Design Practices

An ideal WSDOT fish passage project involves collaboration and agreement with co-managers, including Tribal Nations and the Washington Department of Fish and Wildlife (WDFW).

Designs vary from project to project and require significant coordination and agreement with co-managers. A WSDOT fish passage water crossing project design goal is to maintain fish passage throughout the life of the infrastructure. The WSDOT Hydraulics Manual (2024b) provides guidelines and practices for designing fish passage projects.

For sizing AOP and fish passage structures, WSDOT starts with measuring bankfull width in either the existing reach or a reference reach. An agreed-upon bankfull width is determined collaboratively with co-managers early in the design process. The starting point for determining a minimum structure span uses the agreed-upon bankfull width in a formula developed by WDFW, which is 1.2 times the bankfull width plus 2 ft. A more thorough discussion on the required assessments for determining final structure size is provided in the WSDOT Hydraulics Manual (2024b).

To design a channel streambed, WSDOT starts with a pebble count in a representative reach and aims to have the design D₅₀ be within 20% of the D₅₀ assessed from the pebble count. The WSDOT Hydraulics Manual (2024b) includes a flow chart to determine whether the streambed should be stable or mobile, taking into account the characteristics of each stream. If a stable bed is required, equations such as the Bathurst equation are used to determine the sediment size for stability.

WSDOT incorporates woody material into its fish passage water crossing designs. The WSDOT Hydraulics Manual (2024b) categorizes wood into three types: slash (less than 2 in. in diameter), small wood (4 in. or less in diameter), and large wood (greater than 4 in. in diameter and larger than 6 ft in length). Large woody material is often used outside the structure to provide complexity in the system, whereas small wood, slash, and boulders are typically used inside the structure. Co-managers would like to use large wood within the structure. However, WSDOT has concerns with the potential effects this may have on structures. WSDOT water crossing structures are larger than bankfull width; therefore, there are typically no plugging issues. Stability calculations are performed on wood and on the potential for downstream risks associated with wood becoming mobile.

In addition to woody material, WSDOT incorporates other channel complexity features into its fish passage designs. These complexity features are necessary to prevent some channels from becoming a plane-bed and to maintain fish passage over the design life of the infrastructure. A channel becoming a plane-bed is typically considered a barrier for fish passage. In addition to channel complexity features, interior channel banks are often constructed to facilitate fish passage and reduce the likelihood of the channel becoming a plane-bed. WDFW requirements for sizing streambed material often cause channel banks to wash out and a plane-bed to form. Therefore, streambeds require the use of complexity features such as meander bars made of large rocks or boulders to maintain the banks.

WSDOT has seen the cost of fish passage projects at least double over the last 3 years, primarily because of demand. There are many crossings that need to be designed and few experts with AOP and fish passage experience. Additionally, general materials and supplier costs have increased. WSDOT has developed training for design of fish passage projects and requires anyone designing a project to be certified through the training (https://wsdot.wa.gov/engineeringstandards/project-management-training/training/hydraulics-hydrology-training). WSDOT starts with WDFW for fish passage guidance but expands beyond that, according to the WSDOT Hydraulics Manual (2024b).

WSDOT cannot extend the life of an existing water crossing structure identified as a barrier if it is within the injunction case area. However, if a water crossing is constructed using current design guidance and is identified as a barrier, a retrofit or repair can be completed.