Muti-Agency Collaboration in Mitigating SSO Risk in Buried Stream Crossing

The Foundry Branch, traversing Glover Park in northwest Washington, DC, has historically been a picturesque creek, but recent climate change-induced record rainfall has transformed it into a volatile waterway, exacerbating the risk of Sanitary Sewer Overflows (SSOs) at a crucial crossing. This crossing, where an encased sanitary sewer on shallow piers discharges into a 27-in. interceptor, has experienced significant SSO events, prompting DC Water to initiate a Capital Improvement Project (CIP) aimed at a comprehensive overhaul. The project encompasses not only the replacement of the existing sewer crossing but also enhancements to the receiving sanitary trunk sewer, marking a significant undertaking within a wetland area with complex permitting processes involving the National Park Service (NPS), the District Department of Energy & Environment (DOEE), and the US Army Corps of Engineers (USACE). Estimated to span 4−5 years from design inception to construction completion, this initiative underscores the challenges posed by environmental regulations and climate change alike. In response to two SSO incidents within an 8-month span in 2022 and 2023, DC Water conducted a geo-structural evaluation of the creek crossing and engaged Spiniello Companies for a site assessment, uncovering significant upstream infiltration. Through collaborative discussions with NPS, DOEE, and USACE, DC Water devised an emergency work plan, earning the requisite approvals for immediate intervention. This plan included the removal of obstructive vegetation, the creation of an access route within the dense woodland, and the demolition of the existing crossing to make way for an innovative solution. The implemented design features a 12-in. PVC sewer line, supported by helical anchors and set at a 2% slope, encased to ensure durability. This new crossing, adorned with a stepping stone surface per NPS request, serves dual functions as both a sewer conduit and a pedestrian pathway. Further, the design integrates flowable fill encasement for the sewer segment and a concrete headwall on piles at the creek crossing, significantly bolstering the embankment erosion resistance. In presenting this resilient infrastructure solution, DC Water aims to showcase how engineering innovation can effectively mitigate the impacts of climate change on urban water management systems. The project not only addresses immediate SSO concerns but also sets a precedent for future-proofing critical infrastructure in environmentally sensitive areas. By employing an oversized sewer crossing on erosion-resistant piles and installing an upstream monolithic manhole with a watertight frame and cover, DC Water demonstrates a commitment to safeguarding the community and the environment against the challenges of the coming decades.