Elmer’s Two-Mile Creek Underpass: Boulder, Colorado

Otak provided complete engineering services for this federally funded 83-ft. long, 20-ft. wide underpass crossing Iris Avenue in west Boulder. The underpass structure is a combined-use facility for path users and creek flow. Geometry is very tight on the north side of the project, which required close coordination with a homeowner’s association for location of retaining walls and trail access points.

The Elmer’s Two-Mile Creek underpass provides a safe and inviting pedestrian connection beneath Iris Avenue. Natural stone and boulder retaining walls, plantings and colored concrete were incorporated in the design to provide an inviting and aesthetically pleasing facility. The project was designed to provide ADA accessibility to several locations within the project limits.

The structure was sized to accommodate the 100-year flood within the structure, and the 5-year flood is contained behind the floodwall. A low-water crossing was designed with collapsible railings to fold down during a major flood event. Due to high groundwater conditions and tailwater elevations, a lift station was required to handle both stormwater and the structure underdrain system. An extensive landscape plan was included in the project to soften the retaining walls and build support from the adjacent property owners. Traffic on Iris Avenue was maintained during construction.