Geomorphic Approach to Infrastructure Design at Road-Watercourse Intersections

floodplain area in relation to culverts and stream channel

Minnesota’s public roads intersect Minnesota’s natural perennial flowing watercourses at approximately 65,000 locations. There are numerous additional watercourse-roadway intersections when including intermittent watercourses. Road fill encroachment always will impact a watercourse's natural processes and ecological functions. Applying the Geomorphic Approach to Infrastructure Design at Road-Watercourse Intersections design approach to projects will foster natural system processes and will establish stable watercourses through time. This design approach, for the infrastructure placed upon natural watercourses, can be applied anywhere a naturally formed channel exists on the landscape.

The guidance and tools within assist designers of watercourse infrastructure place low impact, high resilient projects on Minnesota’s natural watercourses. Applied projects work with natural watercourse tendencies, towards stability, by adapting geomorphology into the site design process. Highly impacted watercourses may require multiple (adaptive) designs, through time to ultimately re-establish natural watercourse stability and function.

The Geomorphic Design Approach

Geomorphic Site Assessment

This design approach requires documentation of present day watercourse morphology and is achieved by performing a Geomorphic Assessment at Road-Watercourse Intersection. Once performed, this document is used by the designer to size and place openings through the roadway and across the watercourse. There many other uses of this assessment form, one being reference for future watercourse infrastructure designer’s to adapt future designs to a changing watercourse. Include this form with project design documentation for future reference.

Designer Note: Although not included in design approach, due-diligent site design requires hydrologic site analysis. A common result sets minimal cross sectional area requirements. Once established, apply cross sectional area, this design approach and iterative modeling to configure then optimize structure sizing and placement that best-fit’s site constraints. This is accomplished through impact assessment of velocity and shear stress analysis.  

One key to stable watercourse infrastructure design is to design independently for a channel and a floodplain. Please review Design of Channel Opening and Design of Floodplain Opening(s) below.

Design of Channel Opening

A stable channel will effectively manage its water and sediment delivered with minimal changes through time. Past design methods focus on water conveyance alone; often placing over-widened on-channel openings having no consideration to floodplain. Past design approaches commonly concentrate all flow ranges to the channel alone which can cause many detrimental impacts to the stability of the natural watercourse.  Two common channel design impacts are:

  • disrupting natural sediment transport
  • disconnection to associated floodplain

To avoid these impacts, designer should best attempt to impose stable conditions onto the watercourse.  The designer who chooses to apply this design approach will quickly understand the matrix of metrics they can use to improve site design.  To begin, the designer should pay close attention to these:

  • channel width
  • channel mean depth
  • floodplain elevation

Mean depth and floodplain elevation commonly conflict.  Designer will have to decide how to best apply the stable metrics into the site design.  As proficiency grows many other metrics can be used by the designer to establish a low-impact, high resilient infrastructure projects. 

It is highly recommended to avoid on-channel pressure flow where possible.

Channel Design Fact Sheet

Design of Floodplain Opening(s)

Throughout Minnesota, the link between channel and floodplain have often been overlooked by past watercourse infrastructure designers. Applying this design approach will allow the designer to establish floodplain connection, through the roadway, by properly sizing and placing floodplain culverts. Where possible, the design must accommodate flood flows outside the natural channel and across the entire floodplain. Floodplain culverts should be set to floodplain elevation and on floodplain slope.  Although floodplain culverts often require adjustment to best-fit site constraints and watercourse condition. For highly confined project sites, floodplain connectivity openings can be place immediately adjacent to the channel opening by imposing stable form. 

Once built, projects promote natural watercourse functions and stability. Applied designs increase infrastructure resiliency by minimizing watercourse impact and optimizing hydrologic and hydraulic response.

This design approach implements adaptive management into Minnesota’s watercourse infrastructure by documenting present day site conditions. Although highly impacted sites may require a progression of geomorphic based site designs, through time, to achieve watercourse stability and function. For many reasons, the site design specifics are best determined by the site designer; but, when this design approach is properly applied across Minnesota’s landscape, it will help ultimately achieve stable, functional watercourses throughout Minnesota.

Design Resources

Information provided within can be used to develop a geomorphic based watercourse infrastructure project. Please provide feedback and refer back to this webpage for design approach refinements and updates.

Where implemented, record documents into MPARS or return copies of the Geomorphic Assessment form with plans to:

[email protected]

All projects submitted will be monitored to improve and adapt this design approach.

Please send all field based watercourse morphology information to:

[email protected]

Other Resources
History of Minnesota’s Watercourse Infrastructure

Over the past 150 years, public invested in a variety of public watercourse infrastructure projects. These projects have commonly addressed safety and a specific goal requested by the road authority (i.e., flooding). A specific goaled design results in a project configured to perform at an ‘efficient’ flood conveyance (i.e., 100 year rainfall event) and limit flood conveyance to a single location.

Single structures attempting to address both channel and floodplain can be found Minnesota’s landscape. Although these single structure designs have served the safety requirements, they have also promoted floodplain disconnection.  These many disconnections, found across our landscape, impede a watercourses natural form and function.

The impact of watercourse infrastructure onto the natural watercourse varies across the landscape. Highly resilient watercourses may demonstrate little impact from the infrastructure placed upon it, while other watercourses have been trying to adapt to inappropriate designs since the initial roadway constructed (across the watercourse).  Please contribute your insights on Minnesota’s watercourse infrastructure history and continue to visit this webpage as we build a understanding of past watercourse infrastructure work across Minnesota.

Future of Research Project

As monitoring and watercourse data organization occur, improvement to this design approach will continue producing new design tools and design resources. Please be engaged, provide feedback and continue to visit this website for updates. All relevant questions submitted will be included in the Q&A document found within the Resources tab, so ask questions.

This design approach complies with many regulatory (FEMA and permitting) and non-regulatory (AOP) requirements.

This project is a collaboration of Minnesota water resource professionals; ask questions, provide feedback and remain involved to help improve Minnesota’s natural water resources

Questions and Feedback