Nonanadromous Fish Passage in Highway Culverts

About the report:

Highway culverts may hinder the normal migrations of various trout species in wild trout streams, due to increased flow velocity, shallow water depths, increased turbulence, and perching. This can impede migrational movements, affecting the genetic diversity and long-term survival of some species. Often, the proper installation of culverts can reduce the adverse effects on fish while maintaining hydraulic efficiency. This study characterized the problems with existing culverts to develop guidelines for the future use of culverts in areas with high gradient streams. Installation criteria will ideally limit the use of bridges where culverts are appropriate and eliminate the use of culverts where they would create fish passage problems. This will reduce installation, maintenance, and retrofitting costs. The study concluded that culverts can be considered the primary option for crossing trout streams if the following criteria are met: the culvert can be placed on the same slope as that of the streambed the slope of the stream is less than three percent the flow velocity does not exceed 1.2 meters/second under normal flow conditions the barrel of the culvert can be properly countersunk at the outlet to prevent perching. Bridges should be used at these crossings if any of the above criteria cannot be met. Also, baffles should not be used to control streamflow velocities in new culverts, and concrete aprons should not be used at culvert outlets. If culvert bottoms could be cast to have a roughness coefficient equal to that of the streambed, this would allow greater use of culverts at stream crossings without impeding the passage of trout.

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