Post-Construction Assessment of the North Dorchester Bay Tunnel System

The Massachusetts Water Resources Authority (MWRA) is currently conducting a combined sewer overflow (CSO) Post Construction Monitoring and Performance Assessment, a multi-year study to measure the performance of its $912 million long-term CSO control plan (LTCP). One of MWRA's completed activities is the North Dorchester Bay (NDB) Tunnel performance assessment. The purpose of this presentation will be to provide a brief summary of the MWRA LTCP, the NDB Tunnel system, and the work performed to demonstrate that the NDB Tunnel system has attained the levels of CSO and separate stormwater control called for in the LTCP. While numerous LTCPs nationwide have been completed, and recommended CSO control projects implemented, there is less industry experience with the conduct of successful post-construction monitoring programs. Accordingly, the NDB Tunnel performance assessment approach was developed based on original and creative thinking about how to best demonstrate that the operational tunnel system was meeting LTCP requirements. This presentation will be of benefit to others preparing to conduct LTCP post-construction assessments. For the NDB Tunnel system, performance was assessed using data collected by tunnel system instrumentation and logged in the MWRA SCADA system. Data were available following start-up of tunnel system operation in May 2011. December 2018 was the data cut-off date, providing over 7 ½ years of data for review. As a first step, MWRA rainfall summary records were reviewed to identify larger storm events, those with approximately a 1-year recurrence interval or larger. A total of 19 storm events meeting this size criterion were identified. Specific data to be obtained from the SCADA system were identified by reviewing tunnel system record drawings, the Operation and Maintenance (O&M) Manual, and a NDB tunnel control strategy document developed and used by MWRA for tunnel system operation. The MathWorks computational software package MATLAB was used to process SCADA system data and generate plots for system assessment. A total of 17 plots were prepared and inspected for each of the 19 storm events. Each plot was inspected to compare measured (SCADA) data to physical system characteristics such as CSO liquid level versus overflow weir height at a tunnel overflow point. Plots were also inspected to review operational decisions, such as operation of the pump station used to empty the tunnel after storm events and maintain capacity for storage during subsequent storms. Over 300 total plots were inspected to assess overall tunnel system performance and operation. Nearly all plots prepared and inspected exhibited consistent and appropriate tunnel system performance for each location. A few of the plots did not exhibit data fully consistent with expected performance. A listing of the storm event dates and locations for this subset of plots was prepared and reasons for observed inconsistencies were assessed. Most of these minor inconsistencies were explained by normal sensor or SCADA system outages. For example, in locations where redundant level elements are installed, differences in level measured by the two sensors account for some of the minor inconsistencies. Several plots showed fluctuations in levels indicative of tide gates not seating properly. Findings indicative of minor inconsistencies were relayed to MWRA so inspections could be scheduled to investigate and take corrective action as appropriate. As noted above, the NDB Tunnel system performance assessment is complete and has demonstrated that this marquee project, constructed to improve water quality along the South Boston Beaches, has met its design goals based on documented performance during storm events.