Building Resilience In Boulder, Colorado For Wastewater Infrastructure

Designing for resiliency can be challenging for any project due to factors like additional cost and complexity, but can be especially challenging when the existing system is stressed. The entirety of the City of Boulder's wastewater is conveyed to the City's Water Resource Recovery Facility (WRRF) by three pipelines: a 42-inch interceptor from the west, an 18-inch inverted siphon crossing beneath Boulder Creek adjacent to the WRRF from the north, and an 18-inch aerial crossing over Boulder Creek to the WRRF from the north. The WRRF is surrounded by a FEMA-certified levee to protect it from flooding during major storm events, such as the 2013 flood. The existing interceptor was built in 1968 and follows Boulder Creek. The year following its installation, a flood caused the creek to jump it's banks and washed portions of the interceptor out, rendering a brand new wastewater facility inoperable for several weeks. This nearly happened again in the flood of 2013, when the line was exposed (Figure 1), but remained intact after crews diverted the creek back into its banks. These pipelines are known to be in poor, corroded condition, and are at risk of hydraulic overloads due to inflow and infiltration caused by extreme rainfall events. This case study reviews how the project team prioritized system resiliency when developing a long-term program to improve the City's critical wastewater pipelines, and lessons learned during the construction of these improvements. First, the team completed a condition assessment of the existing aerial crossing of Boulder Creek, then conducted a risk management assessment to identify the overall risk of maintaining the existing crossing with respect to potential system shocks such as flooding and wildfire. This led to the project team identifying an inverted siphon alignment that provides the City with the flexibility to retire the aerial crossing in the future to mitigate the risk of a catastrophic failure. Furthermore, after completion of the scour analysis of Boulder Creek to inform the siphon design, the City incorporated bank armoring improvements into the project that have the dual benefit of protecting the replacement siphon while addressing needed improvements identified in the stormwater master plan. Second, the team assessed multiple design alternatives before determining that the cost of rehabilitating the existing interceptor was justified by the significant benefits in added wastewater volume conveyance and system redundancy. This led to the design of a flow junction and diversion structure at the beginning of the new interceptor, which also will allow the City to safely complete maintenance activities to either pipeline while flow is diverted to the other pipeline. In effect, the program constructed 2.5 miles of nearly fully redundant pipeline geographically separated from the existing main, so the impacts of severe flooding can be mitigated. The City then phased the project as shown in Table 1 and Figure 2. Third, the design of the new 54-inch interceptor necessitated improvements to an earthen embankment dam. This was achieved by raising the crest of the dam and with special design features for the new pipeline to accommodate the shallow depth of cover. Furthermore, the junction of all three pipelines within the WRRF was designed to limit risk of pipeline surcharge, accommodate the shallow headworks elevation, comply with the existing FEMA-certified levee, and to accommodate the existing yard piping. A unique component of this project involves construction of a new interceptor segment adjacent to a rural community whose water supply is served by individual shallow drinking water wells. Hydrogeologic modeling showed a risk of construction dewatering impacting their water supply. A long drive (1,480 foot) microtunnel was incorporated into the design that could be installed without dewatering and was completed without any impacts to that community. Finally, working with the contractor during construction of the project, additional opportunities were identified to improve the resiliency of the pipelines, including additional embankment armoring of the new interceptor as it passed near ponds within the critical wetland area. Improvements to the constructability of the design were also identified including the dam, shallow pipeline, and siphon encasement beneath Boulder Creek. Through a thoughtful approach focused on long-term resiliency, the project team successfully prioritized, designed, and constructed infrastructure that improved the hardiness of the City's entire pipeline system entering the WRRF.