Intercepting Maine's Largest Combined Sewer Overflow Volumes with 1-D and 3-D Dynamic Hydraulic Modeling

Since the 1990s, the City of Portland, Maine has made significant strides in mitigating combined sewer overflow (CSO) volumes. The City's next important step is the creation of the 3.5 million-gallon (MG), off-line Back Cove South Storage Facility (BCSSF), which broke ground in July 2020. This facility is designed to capture the first flush of CSO from approximately 1,435 acres within the City, reducing the annual CSO volume at outfalls 016, 017, and 018 from 150 MG to 18 MG. These outfalls account for about half of the City's and about one quarter of Maine's annual CSO volume. To ensure the best return on its substantial investment in the BCSSF, the City opted to advance the project under a design-build contract that facilitates a collaborative approach between the City, the City's Owner Project Manager (OPM), and design-build team. The purpose of this presentation is to share information on the hydraulic design approach successfully applied and the challenges, considerations, findings, and lessons learned along the way. Advanced modeling, pairing 1-D collection system and storage models with 3-D computational fluid dynamics (CFD) models of key structures, informed the hydraulic design of a combined sewer overflow (CSO) control facility to fulfill regulatory, owner, and design-build delivery requirements. This approach instilled confidence in all project stakeholders that the hydraulic design addresses CSO control requirements while preserving the operation and service level of the existing collection system. The BCSSF concept design for this storage project began as a box conduit under a major thoroughfare and active business district. This idea evolved into installing a single, large tank below a public soccer field, which mitigated traffic and business impact, but posed its own logistical problems. The field's proximity to a tidal mudflat and elevated interstate highway created significant geotechnical and structural concerns. To address the differential settlement concerns born from the one large tank concept, the idea of four interconnected tanks designed to fill sequentially was proposed by the design-build team and accepted by the project partners. Additionally, installing the tanks in two sequential phases ensures the stability of the site and adjacent features during construction. Each phase consists of excavating an area 500-feet long by 75-feet wide by 30-feet deep, installing two cast-in-place tanks, and backfilling. This approach reduces the risk of rotational slope failure related to the elevated highway and allows the construction crews more space on the field to work without disrupting traffic or the nearby public trails. In addition to protecting the integrity of adjacent areas, the project team also had to address the fundamental challenge of capturing inflow from three separate, tidally-influenced CSOs. With a diurnal tidal range of 10 feet (greater during king tides and storm surge events) and the existing pipe elevations four to five feet below mean sea level, the system hydraulics dictated that the bases of the storage tanks would need to be at least 30 feet below ground surface or around 20 feet below sea level. The design team used a dynamic stormwater management model (SWMM) to account for the tidal cycles, interconnecting outfalls, backflow prevention, storage tank connections, as well as the losses imparted by diversion and access structures. This modeling helped confirm that the flow diversion and storage is passive at all tidal stages and that interactions and flow redistribution resulting from hydraulic connection of three CSO outfalls will not negatively impact the upstream collection system. The impact of independent connections between the four tanks was incorporated into the hydraulic model. Connection elevations were based on a cumulative distribution function of recent CSO events and were designed to minimize tank cleaning frequency without sacrificing the overall project goal of storage. Additionally, the team used state-of-the-art 3-D modelling methods to confirm that the hydraulic design of CSO diversion structures meets the City's CSO control requirements. This included computational fluid dynamics (CFD) modeling of diversion structures using FLOW-3D to understand internal flow regimes during typical year peak and extreme precipitation flows. CFD modeling results were used to quantify head losses associated with diversion structures independent of tidal stage. These results were used in conjunction with the system wide SWMM model to assess impacts to upstream collection systems and set design elevations for the storage facility. Once a design water surface elevation within the tanks of -6.50 feet was established, system performance was simulated with dynamic routing under steady peak design inflows to confirm design flows would be fully intercepted, even as tanks approach full storage capacity. System performance was also evaluated under unsteady flow conditions to account for interactions between upstream collection system volume and tidal stage during precipitation events. By pairing 1-D and 3-D modeling for analysis, the team arrived at a comprehensive design, instilling confidence in the proposed approach and final design. The 1-D and 3-D model analysis indicates that the CSO diversion structures impart reasonably low head loss, even at high design flows with surcharged pipes, comparable to a typical access manhole. The most dramatic hydraulic impact was created by the proposed replacement of rectangular tide gates with inline, elastomeric backflow preventers, which impart an order of magnitude greater head loss. In addition, connecting the three CSO outfalls shifted the flow balance between the outfalls during heavy precipitation events. Left unaddressed, this would negatively impact the system upstream in a densely developed area. This required additional design modifications downstream of one diversion structure to accommodate the redistribution of flows. The passive capture of CSO flows means water surface elevation within the tanks is controlled by the tide during heavy precipitation events. The BCSSF is the latest phase of the City's Long-Term Control Plan (LTCP), which has required extensive planning, collaboration, and detailed analysis. The completion of this project will provide dramatic and immediate improvements to the water quality of Back Cove and Casco Bay, which are vital to the identity and economy of the area. The hydraulic analysis and design for this project are complete. Experience gained through the hydraulic design of this off-line CSO storage system is highly transferable and will benefit those designing similar projects that impact the hydraulics within a CSO system.