Researchers develop guidance for fish-friendly culverts in a changing climate

A wide stream running through a culvert under a roadway
Photo: Nick Bentelspacher and Kyle Weisshahn

Designing infrastructure to handle the impacts of climate change is becoming a priority for transportation agencies. Culverts are one example. Road-stream crossings are critical not only for roadway safety and longevity but also for fish and other aquatic organisms that need to move through a stream network.

“Maintaining habitat connectivity throughout a stream is important to threatened populations, because isolated populations—where fish can’t pass through culverts—can be particularly vulnerable to climate change impacts,” says Jessica Kozarek, research associate with the St. Anthony Falls Laboratory (SAFL).

Overly warm waters, or low water or drought, can make part of a stream uninhabitable for its local fish populations. In such situations, fish are more likely to survive if they can move and take refuge elsewhere, Kozarek says.

A variety of factors, however, can prevent fish from passing through a culvert. The barriers might be physical, such as steep drops that fish can’t jump over; fast water with no resting areas; or very shallow water. The barriers can also be behavioral: Lack of light, habitat, or cover can make fish unwilling to enter a culvert. These factors vary from one species of fish to another, Kozarek explains, which makes it difficult to create designs that accommodate all species.

SAFL researchers have been working toward the overarching goal of improving stream connectivity at road-stream crossings. Research has involved a range of smaller-scale projects with various partners, including several sponsored by the Minnesota Department of Transportation (MnDOT) and the Minnesota Local Road Research Board (LRRB).

An ongoing project by Kozarek and fellow SAFL research associate William Herb aims to evaluate culvert designs in relation to local stream flow and fish passage data. The researchers surveyed 50 culverts across Minnesota, recording data related to culvert design and local stream channel characteristics. They also compiled swimming criteria for fish species in the state.

The research team then used computer models to estimate stream flows for present and future climate conditions and the corresponding water velocities and depths in the culverts. These models generally found that climate change will hit differently across the state, Herb says. Summer low flows, which can create depth barriers in culverts, stand a particularly high risk of changing in the future.

A sign saying "Road may flood" with a flooded roadway in the background
Photo: Shutterstock

The study supports the current consensus for culvert design practices that recommend mimicking the structure of the natural streambed and maintaining continuity of sediment transport and debris passage, Kozarek says. Using this approach, a culvert should match or exceed the channel width, consider channel alignment and slope, and provide similar water depth and velocity as the natural channel. It should also be embedded in the streambed and include sediment to mimic the natural roughness of the channel.

“You eliminate the need to evaluate culvert velocities and the specific swimming abilities of individual fish species and life stages,” Kozarek says. “If the fish can swim through the channel itself, [they] can likely move through the culvert as well.”

These more connective culvert designs are typically larger and therefore involve a higher up-front cost, Kozarek notes. However, the research has also found a parallel between ecological connectivity and flood resiliency, since these culverts can more readily handle both high and low flows.

“Culvert designs that can adapt are more resilient to future climate scenarios,” Kozarek says.

The U researchers have developed resources for culvert designers, including the Minnesota Guide for Stream Connectivity and Aquatic Organism Passage Through Culverts, an outcome of research funded by MnDOT and the LRRB. Researchers developed virtual training based on the guide and held six sessions in 2020–2021 through the Minnesota Local Technical Assistance Program (MnLTAP). The training is now a free online course offered by MnLTAP. (MnLTAP is housed at CTS.)

Kozarek and Herb presented their findings as part of a CTS webinar on reducing climate change impacts in Minnesota.

Writer: Sophia Koch


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