The MetroWest Water Supply Tunnel (MWWST) is an advanced underground aqueduct that supplies potable water to residents of much of

Greater Boston Greater Boston is the metropolitan region of New England encompassing the municipality of Boston (the capital of the U.S. state of Massachusetts and the most populous city in New England) and its surrounding areas. The region forms the northern ar ...

. It is part of the

Massachusetts Water Resources Authority The Massachusetts Water Resources Authority (MWRA) is a public authority in the Commonwealth of Massachusetts that provides wholesale drinking water and sewage services to certain municipalities and industrial users in the state, primarily in th ...

(MWRA) water supply system, having entered operation in November 2003. This aqueduct starts at the

John J. Carroll Water Treatment Plant The John J. Carroll Water Treatment Plant (CWTP) is a water treatment plant operated since 2005 by the Massachusetts Water Resources Authority (MWRA) to treat water bound for Greater Boston. The plant is located at the town lines of Marlborough, N ...

Marlborough, Massachusetts Marlborough is a city in Middlesex County, Massachusetts, United States. The population was 41,793 at the 2020 census. Marlborough became a prosperous industrial town in the 19th century and made the transition to high technology industry in the ...

, and ends at an MWRA terminal in

Weston, Massachusetts Weston is a town in Middlesex County, Massachusetts, Middlesex County, Massachusetts, about 15 miles west of Boston. At the time of the 2020 United States Census, the population of Weston was 11,851. Weston was incorporated in 1713, and protectio ...

. It is about long (28.3 km) and is constructed far below ground level, mostly in bedrock. It includes several vertical risers called shafts, lined with steel, used to make connections throughout the system. It is built underneath portions of

Marlborough Marlborough may refer to: Places United Kingdom * Marlborough, Wiltshire, England ** Marlborough College, public school * Marlborough School, Woodstock in Oxfordshire, England * The Marlborough Science Academy in Hertfordshire, England Austral ...

, Southborough,

Framingham Framingham () is a city in the Commonwealth of Massachusetts in the United States. Incorporated in 1700, it is located in Middlesex County and the MetroWest subregion of the Greater Boston metropolitan area. The city proper covers with a popul ...

, Wayland, and

Weston Weston may refer to: Places Australia * Weston, Australian Capital Territory, a suburb of Canberra * Weston, New South Wales * Weston Creek, a residential district of Canberra * Weston Park, Canberra, a park Canada * Weston, Nova Scotia * W ...

, Massachusetts, with a wye intersection 235 feet (71.6 m) below the

Massachusetts Turnpike The Massachusetts Turnpike (colloquially "Mass Pike" or "the Pike") is a toll highway in the US state of Massachusetts that is maintained by the Massachusetts Department of Transportation (MassDOT). The turnpike begins at the New York state li ...

former toll booths at State Route 128.

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 The Hultman Aqueduct forms part of the water supply system of eastern Massachusetts, managed by the Massachusetts Water Resources Authority (MWRA). The aqueduct extends from Southborough to Weston, connecting the Cosgrove Tunnel to the distributi ...

. As originally conceived, the project consisted of a tunnel combined with reconstruction of the

Sudbury Aqueduct The Sudbury Aqueduct is an aqueduct in Massachusetts. It runs for from Farm Pond at Waverly Street in Framingham to Chestnut Hill Reservoir in Boston’s Chestnut Hill neighborhood. A later built extension main runs from the Farm Pond gateho ...

, a 19th-century construction that had been taken out of service in 1978. 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. After fewer than seven years of service, on the morning of May 1, 2010, the tunnel's connection to the City Tunnel Extension failed, dumping its full capacity of 8 million gallons per hour into the

Charles River The Charles River ( Massachusett: ''Quinobequin)'' (sometimes called the River Charles or simply the Charles) is an river in eastern Massachusetts. It flows northeast from Hopkinton to Boston along a highly meandering route, that doubles b ...

and forcing 2 million water customers in Boston and surrounding communities to boil their water before drinking it. Emergency supplies from such locations as the

Chestnut Hill Reservoir Chestnut Hill Reservoir is a reservoir created in 1870 on existing marshes and meadowland to supplement the city of Boston's water needs, located in its namesake neighborhood of Chestnut Hill. A 1.56 mile jogging loop abuts the reservoir. Chestnu ...

were exposed to air and therefore were susceptible to contamination by animal waste.

Redundancy

It was determined that the tunnel would be redundant to the existing Hultman Aqueduct by connecting Shaft C on the existing

Cosgrove Tunnel The Cosgrove Aqueduct, also called the Cosgrove Tunnel, forms part of the water supply system for the communities of the Greater Boston area in eastern Massachusetts that are served by the Massachusetts Water Resources Authority. It is a man-made t ...

in the west with Shaft 5 on the existing City Tunnel, and with the

Weston Aqueduct The Weston Aqueduct is an aqueduct operated by the Massachusetts Water Resources Authority (MWRA). Now part of the MWRA backup systems, it was designed to deliver water from the Sudbury Reservoir in Framingham to the Weston Reservoir in Weston. ...

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.

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 of elevation translates into 1 psi of pressure (for water), calculating pressure to 1 psi precision requires elevation data that's accurate to roughly . Normal elevation data that is accurate to the nearest will result in pressure that is accurate to roughly 4 psi.

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 an 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.

Flow rate

The design flow rate used in hydraulic analysis for the MWWST facility was 1.893 million m³ (500 million USgal) per day. 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 workers 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.

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).

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.

Notes

References

WMWST Tunnel information