Dancing with the Models. SWMM and Physical Models used to Optimize Design of the Cemetery Brook Drain Tunnel

Overview: Through a delicate dance of information exchange, both a computational and physical model supported the design of the Cemetery Brook Drain (CBD) Tunnel Project (Project) for the City of Manchester, NH, Department of Public Works, Environmental Protection Division. The Project is part of the City's combined sewer overflow (CSO) abatement program and will redirect Cemetery Brook, the largest of five brooks, from the combined sewer system through the construction of a two mile long, 12-ft internal diameter drainage tunnel with seven vortex drop shaft structures and an energy dissipation outfall. The roughly $200M project will provide the City with the stormwater infrastructure to implement brook removal and sewer separation in the Cemetery Brook CSO basin. The SWMM model for the Cemetery Brook basin was developed in 2021 using the US EPA Stormwater Management Model (SWMM). The SWMM has been used since the late 1990s to support planning and design of sewer separation projects in Manchester. Clemson Engineering Hydraulics, Inc. (CEH) built a physical hydraulic model of the proposed CBD tunnel for CDM Smith. The physical modeling included portions of the main tunnel, vortex drop shaft structures as well as the outfall energy dissipation structure. The physical model was used to evaluate the hydraulic conditions within the tunnel, drop shaft structures, and the outfall and develop recommended modifications as needed to remediate any adverse hydraulic phenomena. This presentation focuses on the input/output exchange between the SWMM model and the physical model for the CDB tunnel project. The SWMM model was used to feed operation conditions of the tunnel, drop shaft structures and the outfall into the physical model in the laboratory setting. The results of the physical modeling tests were then added into SWMM to fine-tune collection system performance. The purpose of this presentation is to educate the audience about unique information exchange between the two models on a unique and challenging design project. Approach The City's existing collection system in the Cemetery Brook basin contains four functional sewer areas: separated, combined, separated upstream/recombined downstream, and stream discharge from Stevens Pond. Sanitary flow from all functional areas plus runoff from upstream surface waters is routed to the combined Cemetery Brook Conduit, where it either continues to the City's wastewater treatment plant or discharges to the Merrimack River as CSO during wet weather. In the future, all functional sewer areas in the basin will be separated, with brook flow, stormwater and surface water disconnected from the combined sewer, directed to the new CBD Tunnel, and discharging to the Merrimack River. The objectives of the SWMM model were twofold: -Hydrology: simulate the future post-separation runoff response in the basin and determine how much surface water and stormwater flow enters the tunnel from each drop shaft during the 100-yr, 50-yr, 25-yr, 10-yr and 5-yr design storm events. -Hydraulics: size lateral pipes leading to the drop shaft structures and confirm the alignment and the size of the tunnel that were established during the conceptual engineering phase. As a result, important input parameters were sent to CEH to proceed with further testing using physical models: a) the tunnel peak flows and water depths for all seven drop shaft structures for selected design storms, and b) the proposed tunnel size and drop shaft pipe sizes. Based on information received from CDM Smith, CEH built a physical model with a 1:9.3 scale. The prototype model included a portion of the inlet pipe, de-aeration chamber, and the tunnel, and 200 feet of tunnel upstream and 300 feet (prototype) downstream of the drop shaft connections (Figure 1). Given that the full length of conveyance tunnel was not simulated, CEH utilized hydraulic grade line information obtained from the SWMM model to simulate the water levels upstream of the drop shafts. The objectives of the physical model tests concerning improvements of the SWMM model included: -Confirm tunnel size and drop shaft pipe sizes for the selected design storms. -Confirm drop shaft inlet losses. In SWMM, the connection of the collection system piping to the drop shaft vortex inlet structures was assumed as free pipe discharge. The hydraulic rating curves developed during lab testing were used to back check the model. Results and Benefits A unique exchange of model results was completed at the end of the modeling phase. The peak flows and depths, drop shaft pipe sizes, and the tunnel size developed in SWMM were successfully used in the laboratory setting. Physical testing confirmed that drop shaft pipes modeled in SWMM were adequately sized to pass up to 25-year design flows through each of the drop shafts. The connections between the collection system piping to the drop shaft vortex inlet structures were confirmed and improved; the SWMM model was updated with rating curves that were developed in the lab from water depths observed in the approach channel to vortex inlet at each drop shaft. Figure 2 shows an example of the peak hydraulic grade line (HGL) at the drop shaft and tunnel connection before and after the SWMM model was updated with the results from the physical lab tests. The figure on the left shows the connection as free pipe discharge and figure on the right shows the peak HGL updates with rating curves. Physical testing also showed one interesting phenomenon - at high flows, as soon as water enters the tunnel from the drop shaft structure at a 90-degree angle, a hydraulic jump forms, travelling rapidly upstream. Figure 3 show hydraulic jump observed during testing. To avoid formation of the hydraulic jump, the physical model suggested a modified design approach, in which the adit is either installed into the conveyance tunnel at a 20-degree angle or installed from above at 45-degrees (in the downstream direction) and installing a baffle plate near the crown of the conveyance tunnel. These lab observations may be incorporated into the model but require further engineering evaluations. Current Status Numerical and physical modeling of the CBD design was completed in early 2023 and additional refinements were completed in fall 2023. The Cemetery Brook tunnel is currently in the final design phase with a completion date in mid-2024. Bidding and construction will follow, and a roughly 3-year construction duration is planned with completion in fall 2027. Conclusion Physical models have been rare due to budget constraints and advances in computational techniques. This project proved the benefit of using physical modelling on complex conveyance projects and the benefits of mutual exchange of information between physical and computational models. The result is an improved system design through confirmation from two distinct modeling methodologies thereby increasing reliability and confidence in the performance of the completed infrastructure.