DESIGN OF A STORAGE TUNNEL CONTROL STRATEGY TO ELIMINATE CSO AND REDUCE STORMWATER DISCHARGES TO SOUTH BOSTON BEACHES

The Massachusetts Water Resources Authority (MWRA) is completing the design of the North Dorchester Bay CSO Facilities, which will include: a 10,900 foot-long, 17-foot diameter storage tunnel constructed in soft ground; a 15-million gallons per day (mgd) dewatering pumping station at the downstream end of the tunnel; and a tunnel ventilation and odor control facility at the upstream end. The tunnel is sized to capture the CSO volume generated by the 25-year, 24- hour storm at six CSO outfalls that discharge to beaches along North Dorchester Bay in South Boston. Receiving water modeling predicts that exceedences of water quality criteria for swimming and shellfishing would still occur following completion of the project if separate stormwater discharges to North Dorchester Bay are not also controlled. While the MWRA is not responsible for managing separate stormwater under its enabling legislation or required to do so by state or federal regulations, consideration was given to the extent to which the project could be cost-effectively adapted to addressseparate stormwater impacts. MWRA determined that for storms up to the 1-year, 24-hour storm, thestorage tunnel would have sufficient capacity to capture both CSO and the separate stormwater that enters the outfalls downstream of the CSO regulators. MWRA subsequently increased the level of stormwater control to a 5-year, 24-hour storm by proposing to divert some stormwater to a less sensitive receiving water in large storms.One challenge was to develop a control strategy and accompanying surface piping/control gate structure arrangement that would allow both stormwater and CSO into the tunnel during smaller storms,and allow only CSO into the tunnel in larger storms. At the surface, separate diversion structures will be provided for stormwater and CSO in the vicinity of the tunnel drop shaft at each outfall. Hydraulically-operated gates will allow or exclude stormwater from the tunnel, and will prevent the tunnel from overfilling with CSO in a greater-than 25-year event. Weirs will be provided todivert flow towards the drop shafts, but allow unimpeded discharge to the existing outfall if the CSO and/or stormwater gates to the tunnel close. The control of the hydraulically-operated gates will be based on operator input from weather forecasts. The control system will be pre-programmed to estimate CSO and stormwater volume based on predicted rainfall. Level elements within the tunnel will continuously monitor the depth of water in the tunnel to compute the available storagecapacity remaining. During the course of a storm, the control system will compare the expected volume of rain based on updated forecasts to the available storage capacity and signal the stormwater gates to close if necessary to allow sufficient storage capacity for the expected CSO volume. The gates will also automatically close on high water level in the tunnel, to prevent hydraulic surges.This paper will present the diversion structure design concepts, the control strategy for operating the diversion structure gates, and an assessment of the predicted performance of the tunnel system using the control strategy and simulated runoff from historical rainfall data in terms of capture of CSO and stormwater that would otherwise be discharged to the South Boston beaches.