Minnesota has over 92,000 miles of streams and rivers and approximately 142,000 miles of roads. The necessity for culvert structures occurs at tens of thousands of locations. For decades, their design has included regulatory requirements that they provide for game fish movement. However, no single design method has been successfully applied throughout the state. This has led to uncertainty in a designs approval during the permit review-approval process and unpredictability of achieving design goals after installation.
Over the past ten years there have been a series of studies conducted to evaluate various culvert design parameters and their applicability to the state. This has led to the resulting 'Minnesota Guide for Stream Connectivity and Aquatic Organism Passage (AOP) through Culverts'. This guide was developed by University of Minnesota researchers at the St. Anthony Falls Laboratory and the Department of Bioproducts and Biosystems Engineering, with input from the Minnesota Department of Transportation (MnDOT), Minnesota Department of Natural Resources (MnDNR), US Forest Service (USFS), and input from local, regional, and national experts. The guide also draws on the fields of fluvial geomorphology, biology, and other related fields.
The guide assists Minnesota culvert designers in identifying, selecting, and implementing culvert designs appropriate for invertebrates, reptiles, amphibians and non-game fish species as well as game fish. In addition to meeting traditional hydraulic and roadway capacity goals, these culvert structures are intended to facilitate the movement and lifecycle needs of fish and other aquatic organisms at road-stream intersections. This includes addressing the closely related need to maintain healthy stream connections for movement of water, sediment, and debris through stream networks.
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 seven 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 increased culvert life span.