Elmer’s Two-Mile Creek Underpass Boulder, Colorado

This federally funded, 83-ft. long, 20-ft. wide underpass crosses Iris Avenue and Elmer’s Two-Mile Creek in west Boulder. As a combined-use facility for path users and creek flow, the structure is sized to accommodate the 100-year flood within the structure, and the 5-year flood is contained behind the floodwall while providing ADA accessibility.

A Structure Designed for Flood Resilience and Accessibility

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. Geometry is very tight on the north side of the project, requiring close coordination with a homeowner’s association for location of retaining walls and trail access points while providing ADA accessibility at several locations. Due to high groundwater conditions and tailwater elevations, a lift station was required to handle both stormwater and the structure underdrain system. In addition, a low-water crossing is designed with collapsible railings to fold down during a major flood event. Otak provided complete engineering services in bringing this structure to completion while allowing traffic on Iris Avenue to be maintained throughout construction.