Installing culverts is a necessary part of most transportation projects, allowing water to pass under roads and other transportation infrastructure. However, when culverts create obstacles for fish and other aquatic life, the consequences can be serious.

“Culverts can create barriers in a stream network that fragment aquatic organism populations,” says Jessica Kozarek, a research associate at the University of Minnesota’s St. Anthony Falls Laboratory (SAFL). “This leaves fish vulnerable to dying off by chance events and can lead to a longer-term loss of genetic diversity.”

To allow fish to pass through culverts, many are installed slightly below streambed level. The expectation is that sediment from the stream will be carried into the culvert, creating a consistent streambed. But does this strategy actually work? A new study from SAFL researchers reveals that it may not always achieve the desired results—and outlines new strategies for making culverts more fish-friendly.

A model stream in the St. Anthony Falls Lab. Photo: SAFL

A model stream in the St. Anthony Falls Lab. Photo: SAFL

To determine whether setting a culvert below the streambed was enough to enable aquatic organisms to pass through a culvert, researchers created several model streams in their laboratory that represented Minnesota streams with three different gradients, or slopes. Using these models, they tested the effectiveness of two installation methods: one in which the culvert was placed below the streambed but not filled with sediment and another in which the culvert was pre-filled with sediment.

“We found that pre-filling the culvert with sediment that replicates the streambed as part of the installation process helped prevent upstream erosion and the development of vertical drops that can become barriers to aquatic movement,” Kozarek says. “In addition, pre-filling the culvert helped ensure that the sediment remained inside the culvert when flows were high and when water moved quickly during rainstorms.”

For streams with steep slopes, researchers also found that adding structures such as steps, boulders, and riffles was critical to the stability of the sediment within the culvert. An experiment that used these structures inside the culvert showed that they helped stabilize the streambed upstream of the culvert.

Based on these experiments, the researchers developed design recommendations for embedded culverts where maintaining a natural streambed to preserve fish and other aquatic organism passage is a design goal. These recommendations include making the culvert as wide as the width of the stream when water levels are highest, analyzing each installation site to predict sediment movement into the culvert, pre-filling the culvert with sediment that matches the sediment in the existing streambed, and installing structures within the culvert for streams with steeper slopes to maintain sediment stability and provide resting places for fish traveling upstream.

“A lot of what was done in the past was not based on research; it was mostly just opinion. This study provides us with real data from testing in a lab setting to help us better design culverts,” says Petronella DeWall, MnDOT bridge waterway engineer. “Moving forward, MnDOT will be using these recommendations when designing culverts in environmentally sensitive areas.”

Reprinted with permission from the May 2015 issue of CTS Catalyst, a publication of the University of Minnesota Center for Transportation Studies.