The Pipeline Less Traveled By: Resolving Design Challenges along a Trail through Critical Wetland Areas

The City of Boulder, Colorado, is installing a new 2.5-mi sanitary sewer transmission main to convey the majority of the city's wastewater flows to the Boulder Water Resource Recovery Facility. The final mile of the 42-in. and 54-in. diameter interceptor runs through Sawhill Ponds, owned by the State of Colorado, and Walden Ponds, owned by Boulder County, before connecting into the headworks of the wastewater treatment facility. These critical habitat areas and wetlands are beloved and include a trail network that is well used by the surrounding public. To design the pipeline through this unique area, the project team coordinated closely with the city, county, and state to identify critical habitat and to carefully coordinate the alignment to navigate existing wetlands, trees, and structures. Addressing these challenges also required a redesign of a dam spillway separating two of the ponds that the new 54-in. interceptor crosses between, as well as designing for a crossing of a FEMA-certified levee into the treatment facility. Furthermore, due to the shallow grade (3.8′ to top of the 54-in. pipe), the project team had to work closely with manhole and pipeline suppliers to design custom shallow manhole structures that are traffic rated, allow for safe operation, and align with the desires for the use of the future trail system. This case study reviews how the project team weighed multiple approaches to mitigate the unique design challenges for this project. By communicating closely with stakeholders, suppliers, and permitting agencies, the project team successfully selected an alignment and prepared a design that not only resolved the unique site's challenges, but also improved the Sawhill Ponds and Walden Ponds wetland areas. The presentation includes an assessment of the overall constructability through this area as well as the unique design details that were prepared.