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Recessed culverts are often installed in Minnesota to facilitate aquatic organism passage (AOP) by providing a natural streambed through the culvert. The least expensive option when installing a recessed culvert is to allow the culvert to fill in with sediment naturally over time; however, previous field studies suggest that in many cases, sediment fails to deposit within the culvert. The objective of this research was to understand the function of a culvert set below the streambed elevation under various sediment transport conditions. Laboratory experiments were designed to assess the performance of recessed culverts across a range of geomorphic characteristics representative of Minnesota streams. These experiments explored the functionality of a culvert that is prefilled with sediment representative of the stream as a part of the installation process against one that is empty after installation and assessed the potential for headcutting and downstream degradation. The experiments evaluated the need for artificial roughness installations within recessed culverts in high gradient streams. Three sets of experiments were conducted examining: 1) the effect of sediment grain size, slope, and flow hydrograph on sediment transport through a single recessed box culvert, 2) the effect of bed roughness structures on sediment stability in a single recessed box culvert in high-gradient streams, and 3) the effect of culvert offset and skew on sedimentation in multi-barrel culverts.

Culverts can act as barriers to fish passage for a number of reasons including insufficient water depth or excess velocity. In addition, concern is being raised over behavioral barriers where culvert conditions elicit an avoidance response that deters or slows fish movement. Long culverts can block sunlight creating a potential behavioral barrier as fish approach a long, dark culvert. Scant information exists on low light as a potential barrier to fish passage, particularly with warm water species, such as the federally endangered Topeka Shiner. As some older culverts are being replaced with longer total lengths to improve safety by extending the culvert through re-engineered road embankments, information is needed to 1) determine when and if light mitigation strategies are necessary, and 2) to design appropriate light mitigation strategies if necessary. Based on literature review, field monitoring, and laboratory experiments, the effect of light on fish passage for Topeka Shiner and other small prairie stream fish was indiscernible. Therefore, no light mitigation for large box culverts (up to 150 feet in length) can be recommended for similar fish communities. Culverts that are very long or have very small openings may benefit from additional light.

This guide assists Minnesota culvert designers in identifying; selecting; and implementing appropriate designs for maintaining aquatic organism passage (AOP) and stream connectivity at road-stream intersections. It was synthesized from existing literature and culvert design documents; a survey of Minnesota practitioners; research; and input from local; regional; and national experts. Culvert designs that create excessive velocity; physical barriers; or shallow depth can disrupt AOP and may be detrimental to the continuity of water flow; sediment; and debris transport vital to stream health. Conversely; the recommended culvert designs in this guide account for stream parameters such as slope and substrate that vary across Minnesota landscapes. A set of best practices captures critical design elements based on stream characteristics; which can be summarized as follows: 1.) Design the culvert to be similar to the stream channel (reference reach) by matching its slope; alignment; bankfull width; and flow depth to maximize AOP; 2.) Provide a continuous sediment bed with roughness similar to the channel; while maintaining continuity of sediment transport and debris passage; and; 3.) Design for public safety; longevity; and resilience. Culvert design that improves AOP and accounts for sediment transport is expected to reduce long-term maintenance costs and increase culvert life span.

Culverts at road-stream crossings can create barriers to movement within a stream network that can have dramatic consequences for fish populations by fragmenting habitat. Culverts can become barriers when flow conditions exceed fish swimming ability, e.g., for drop at the outlet and insufficient depth or excess flow velocity. In this project, we use a simple modelling framework to assess 50 culverts throughout Minnesota to: a) determine what fraction of these culverts currently present a fish passage barrier for both high flows (velocity barrier) and low flows (depth barrier) and b) to summarize design parameters that most affect passibility (e.g., culvert width). The estimated high and low flows are fed into the HY-8 culvert hydraulics model, and the resulting velocity and depths are compared to published fish swimming capabilities. We also assess future (2061-2080) high-and low-flow fish passage conditions for five culvert sites using global climate model outputs, Hydrologic Simulation Program Fortran (HSPF) runoff models, and the fish passibility modelling framework. Both low-and high-flow conditions in streams are very responsive to future climate, with either positive or negative future changes, depending on which global climate model is used. This study concludes that maintaining a low-flow channel or embedded culvert barrel will make culverts more passable during changes in low flows and ensuring culvert widths equal to or greater than the bankfull channel width in combination with embedded sediment will help mitigate increases in high fish-passage flows and high peak flows.

Culverts can act as barriers to fish and aquatic organism passage by a number of mechanisms including insufficient water depth or excess velocity; perched culvert outlets; excess turbulence or behavioral barriers. In Minnesota; maintaining unimpeded fish passage is a concern due to our high-quality fisheries; however; much of the fish passage research in the United States has been conducted on the coasts with anadromous species. Many different methods have been used in Minnesota and nationally to facilitate passage including retrofitting existing culverts. In larger culverts; methods such as baffles or weirs have been installed to create resting areas for fish in culverts with high velocities; but these can be costly; difficult to install in tight culverts; can create maintenance issues; and may only be applicable to larger fish species. Recent research from New Zealand demonstrated the effectiveness of mussel spat rope in facilitating fish passage in steep; perched; or high-velocity culverts. The installation of these ropes in Minnesota box culverts was investigated as a low-cost; low-maintenance method to facilitate fish passage; specifically focusing on 1) the hydrodynamic performance of the ropes; 2) the use of these ropes by Minnesota fish species; and 3) evaluation of field installations in typical box culverts. This project used a combination of physical laboratory measurements on rope hydrodynamics; fish laboratory experiments on use of ropes; and two field demonstration sites. Ropes created low-velocity areas and cover that were utilized by small fish to navigate shallow flows over smooth boundaries.