Assessing Culverts in Minnesota: Fish Passage and Storm Vulnerability

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 modeling 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 modeling 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.