MetroWest Water Supply Tunnel
From Wikipedia, the free encyclopedia
 MetroWest Water Supply Tunnel |
|
| Official name | MWWST |
|---|---|
| Begins | Northborough |
| Ends | Weston |
| Maintained by | MWRA |
| Length | 17.6 mi (28.3 km) |
| Diameter | 14 ft (4.3m) |
| Capacity | 1.893e06 m³ per day (500 MGD) |
| Construction began | 1996 |
| Opening date | 2003 |
| Geographical Data | |
| Coordinates | |
The MetroWest Water Supply Tunnel (MWWST) is an advanced underground aqueduct that supplies potable water to Boston residents. It is part of the Massachusetts Water Resources Authority (MWRA) water supply system
Contents |
[edit] Information
This aqueduct starts at the John J. Carroll Water Treatment Plant near Northborough, Massachusetts and ends at a MWRA terminal in Weston, Massachusetts. It is about 17.6 miles long (28.3 km) and is constructed far below ground level, mostly in bedrock. It comprises several vertical risers called shafts, lined with steel, used to make connections throughout the system. It is built underneath portions of Marlborough, Southborough, Framingham, Wayland, and Weston Massachusetts, with a wye intersection 235 feet (71.6 m) below the Massachusetts Turnpike toll booths at State route 128.
[edit] History
In 1989, the Massachusetts Water Resources Authority (MWRA) issued a planning and design contract for a second transmission main to provide redundancy for the Hultman Aqueduct. As originally conceived, the project consisted of a tunnel combined with reconstruction of an existing, unused gravity aqueduct, the Sudbury Aqueduct. During feasibility studies, it was recognized that costs and environmental and community impact issues related to reconstruction of the Sudbury Aqueduct through an urban/suburban area compared unfavorably with a full length tunnel in rock, deep under existing structures and facilities. The alignment of the tunnel generally coincides with the existing Hultman Aqueduct and is constructed in permanent underground easements below several hundred private properties. The full length, unreinforced concrete lined, pressure tunnel design concept was selected and the facility was named the MetroWest Water Supply Tunnel.
[edit] Redundancy
It was determined that the tunnel would be redundant to the existing Hultman Aqueduct by connecting Shaft C on the existing Cosgrove Tunnel in the west with Shaft 5 on the existing City Tunnel, and with the Weston Aqueduct Terminal Chamber Area in the east end. These criteria resulted in terminal connections at Shafts D, 5A and W, respectively, and definition of the general alignment of the tunnel facility. Additional major connections to existing MWRA transmission facilities resulted in Shaft E, Shaft L, Shaft NW and Shaft NE. Connections to serve community clients resulted in the five small diameter riser shafts to supply water to customers along the tunnel alignment.
[edit] Technical data
The water pressure hydraulic grade level (HGL) corresponding to the maximum water surface elevation of the Wachusett Reservoir, feeding the proposed facility by gravity from an overflow elevation of 120.4 m (395 ft Boston City Base Datum, BCB), was selected by the MWRA for design criteria. HGL is used, instead of elevation, data because it is significantly more precise than elevation data. Because 2.31 feet of elevation translates into 1 psi of pressure (for water), calculating pressure to 1 psi precision requires elevation data that's accurate to roughly 2 feet. Normal elevation data that is accurate to the nearest 10 feet will result in pressure that is accurate to roughly 4 psi.
[edit] Pressure
A 120.4 m (395 ft) HGL is used for design of the tunnel from Shaft D to Shaft NW at the Norumbega Reservoir, so water could continue to be stored for distribution to the Boston area. The remaining downstream portion of the MWWST facility, Shaft NE to Shaft 5A andShaft W, is designed for a HGL of 88.4 m (290 ft BCB). The design transient pressure was determined by hydraulic analysis of the overall future transmission system, including accounting for characteristics of control mechanisms, e.g., shutting times for major valves or catastrophic type changes in flow rate. The resulting design transient pressure for the MWWST facility is 15.2 m (50 ft) of head increase, or approximately 137.9 KPa (20 psi). This transient pressure is used for design of impermeable lining system components, i.e., localized steel linings in the shafts and tunnels.
[edit] Flow Rate
The design flow rate used in hydraulic analysis for the MWWST facility was 1.893e06 m³ per day (500 MGD). In addition to meeting the fundamental hydraulic requirements of operation, such as flow capacities and operating hydraulic characteristics, additional important operations and maintenance provisions require identification and incorporation in the design, including provisions for future reentry into the tunnels and associated unwatering and safe future access for men and some essential equipment. For the MWWST tunnel conduits, all projected flow velocities are less than 3 m (10 ft) per second and thus acceptable for all envisioned types of lining systems.
[edit] Aqueduct diameters
The selected inside diameter of the MWWST tunnel conduit is 4.3 m (14 ft) from Shaft D to the Norumbega Reservoir and 3.7 m (12 ft) for the remainder to Shaft 5A and Shaft W. The shaft conduits vary from 3.7 m (12 ft) to 2.4 m (8 ft), depending on the location, metering, hydraulic and future access requirements. There are five small-diameter community riser shafts of 0.51 m (30 in).
[edit] Maintenance
Incorporated into the design are considerations for filling and unwatering (e.g., air release/vacuum valves, filling/unwatering procedures and requirements for pumping), as well as the need and means for metering of flows during operation. Accommodations to inspect and maintain the tunnel is access through the shaft conduits by proper sizing of manholes and access hatches, invert gradients (the tunnel must generally slope toward the access shafts) and locating the metering devices at accessible locations.
[edit] MWRA water system overview
The MWRA and the Massachusetts Department of Conservation and Recreation (MDCR) own and operate the collection, treatment, distribution, and storage facilities that supply drinking water to some forty municipalities in the metropolitan Boston area. This water system design was based upon the purchase and subsequent protection of an entire watershed. This design assures that the water remains as pristine as possible. However, modern regulations require that all supplies of drinking water be chemically treated regardless of the source[1]. Additions to the MWRA water system throughout its history have resulted in redundancies that allow major sections of the water system to be shut down for repair or maintenance.
[edit] Water flow
Water flows from the MWRA's main storage facility, the Quabbin Reservoir in central Massachusetts, through the Quabbin Aqueduct to the Wachusett Reservoir in and around Boylston and Clinton [2]. Tributary rivers and streams comprising the Wachusett watershed, a 108 square mile (280 square kilometer) drainage basin, also feed the Wachusett Reservoir. At the eastern end of the Wachusett Reservoir, water enters the Cosgrove Tunnel at the Cosgrove Intake. The Cosgrove Tunnel feeds both the MetroWest Water Supply Tunnel (MWWST) and the Hultman Aqueduct. The MWWST starts from the Carroll Water Treatment Plant in Marlborough. The Hultman branches off at Framingham in two directions. The smaller branch, the Weston Aqueduct, empties into the Weston Reservoir in Weston. The main branch continues to the Norumbega Reservoir, also located in Weston[2].
[edit] Redundancy
Water can be treated with chlorine as it leaves the Wachusett Reservoir in an emergency, and again as it leaves the Norumbega Reservoir. This is to provide for a backup to the new water treatment facility, the John J. Carroll Water Treatment Plant, which started operation on July 27, 2005[3]. This plant is of modular design and provides ozonation for primary disinfection, chloramination for residual disinfection, fluoridation, and pH control.
[edit] John J. Carroll water treatment plant
Located at the town lines of Marlborough, Northborough, and Southborough, Massachusetts, this facility replaces the one used previously only for pH control[4]. It comprises four ozone generators with diffusers and five concrete contact chambers with a volume of 11.3 million gallons (42.7 million liters). The plant has a capacity of 275 million gallons (1.04 billion liters) per day, on an average day or 405 million gallons (1.53 billion liters) per day, at peak level. It cost US$340 million[3].
[edit] Electrical generation
The system includes three hydropower stations, with a total capacity of 8 MW[5]. Water released to the Swift River flows through the turbines at Winsor Station below the Winsor Dam. Water transferred from Quabbin to Wachusett can pass either through the turbines at Oakdale or through bypass pipes when flow requirements exceed turbine ratings. Water released from Wachusett into the Cosgrove Tunnel passes through the Cosgrove turbines[2][3].
MWRA power generation
|
The Quabbin Aqueduct connects the two reservoirs, and relies upon gravity to accommodate the three separate operational needs. First, diversion of water from the Ware River into the Quabbin Reservoir uses this aqueduct. Second, water transfer from the Quabbin Reservoir to the Wachusett Reservoir, through a hydropower station or a bypass pipe, uses it as well. The bypass valves are non-regulating valves, and when opened, only the head in the Quabbin Reservoir and the physical characteristics of the aqueduct govern the flow. Because the turbines are flow limited, the bypass mechanism permits transfer rates nearly twice as high as are possible through the turbines. Operationally, the single aqueduct fulfills three purposes, but only one operational mode is possible at a given time [6].
[edit] MWRA references
- ^ Safe Drinking Water Act. Retrieved on 2006-12-01.
- ^ a b c MWRA water system. Retrieved on 2006-12-01.
- ^ a b c Water system history. Retrieved on 2006-12-01.
- ^ John J. Carroll water treatment plant. Retrieved on 2006-12-01.
- ^ Electrical power generating plants. Retrieved on 2006-12-07.
- ^ Water system configuration. Retrieved on 2006-12-01.
