Project Profile: Detention Chambers Help Alleviate Flooding
Winchester, MA, is a picturesque suburb located 8 miles north of Boston. The town’s McCall Middle School is widely known for its Manchester Field, the main athletic field used by both students and residents. When the town noticed that flooding occurred in the area of Manchester Field and the adjoining neighborhoods, it engaged engineering firm AECOM Environment to help solve the problem.
The AECOM engineers looked into the existing stormwater conditions in the area of the school and nearby Main Street, Mystic Avenue, Cutting Street, and Manchester Road. They found that four stormwater systems were pieced together to collect runoff from the neighboring area. The school and athletic fields’ drainage system tied in with the Main Street system at a manhole on the northeast corner of the football field. The combined system continued flowing west through a siphon that crossed the Massachusetts Water Resources Authority (MWRA) sewer main. Here, the parking and skate park section tied in and the system continued southeast parallel to Manchester Road. At the basketball courts, the system collecting runoff from Mystic Avenue, Cutting Street, and Manchester Road tied in. Ultimately, a 27-inch outfall discharged runoff from all these systems into the Aberjona River.
After analyzing the conditions, the engineers concluded that the system was greatly undersized and not adequate to collect and discharge small-scale storm events, even when there was no resistance downstream from the Aberjona River flooding. AECOM recommended upgrading the system for storm events less than and including the 25-year, 24-hour storm. Because the school was located in the Federal Emergency Management Agency’s 100-year floodplain of the Aberjona River, the site might still experience flooding. Therefore, the upgrade aimed to reduce or eliminate flooding mostly during high-frequency storm events.
The $1.2-million project consisted of two phases and included the construction of an innovative, integrated stormwater management system. In its plan, AECOM incorporated other proposed upgrades to Manchester Field planned by the town, such as the addition of a new track and field. The plan included stormwater connections from the new fields and the realignment of the main stormwater line draining the area up gradient of Main Street.
While the new design proposed to maintain the existing stormwater system alignment, it incorporated underground detention chambers provided by Cultec Inc. According to Jacob San Antonio, P.E., water resource engineer at AECOM who worked on the project, underground detention was the only suitable best management practice for this 4-acre site.
“The site did not have enough space for an aboveground detention solution, so we went underground,” says San Antonio. “We designed Cultec systems as detention galleys to attenuate peak discharge rates to prevent the existing system’s surcharges and increase storage when the Aberjona River level rises above the low ground areas.”
Although Cultec chambers have open bottoms allowing for infiltration, for this design, the engineers did not assume that infiltration would occur due to the minimal separation between the installed chambers and groundwater. AECOM estimated the seasonal high groundwater level at an elevation of 9.2 feet. The bottoms of the chambers were located at an elevation of 10.5 feet during the first phase and 12 feet during the second phase.
The integrated stormwater management system included several other elements:
- The existing Manchester Road drainage system had two separate stormwater pipes conveying water to the main trunk line under Manchester Field. They formed a loop that caused stormwater to back up and flood Manchester Road; therefore, the engineers abandoned that drain line.
- AECOM increased the size of drainage pipes in the major trunk lines in Manchester Road to reduce the hydraulic grade lines throughout the system and increased slope for positive drainage.
- A check valve was installed on the Manchester Road drainage system tie-in to Manchester Field trunk line. This allowed stormwater from the Manchester Field trunk line to continue to drain as the river levels rose above the low ground areas on Manchester Road and prevented river water from backing into these areas.
- The engineers had to relocate the Manchester Field trunk line to avoid interference with the track proposed as part of Manchester Field’s other improvement efforts. They also had to slope drain lines for positive drainage and added a check valve to the outfall.
The first stage of the project commenced in Manchester Road and Cutting Street with the installation of new storm drain lines. The work then progressed into the basketball courts and track and field areas where a Cultec detention system was installed. It was designed to collect stormwater runoff from the abutting neighborhoods along Manchester Road, Mystic Avenue, Maxwell Road, Lloyd Street, and Cutting Street.
This phase presented the engineers with a significant challenge. The siphon under a 48-inch-diameter MWRA sewer had to be replaced to connect the drainage from the neighborhood streets into the Cultec system. The new drainage system also conveyed more water from the neighborhoods through the siphon, so the existing 12-inch line was upgraded to a 30-inch line.
The new siphon was installed approximately 10 feet into groundwater using pipe-jacking techniques, which required a significant amount of dewatering. The process included developing sumps at the bottom of the excavations pits, well point dewatering around the pipe-jacking work area, and an additional 12-inch-wide, 30-foot-deep well between the excavation pits.
“We had to manage an enormous amount of groundwater on site,” confirms Anthony Amaral Jr., assistant project manager with contractor Green Acres Landscape & Construction Co. Inc. “During the excavation, we had to keep numerous large pumps with filtration tanks running 24/7.”
Green Acres Landscape & Construction installed 1,356 Recharger 150 HD chambers in three beds fed by two header lines. The system occupied 36,000 square feet and provided 496,584 cubic feet of storage.
“Cultec was instrumental in helping us launch this project smoothly,” says Amaral, who has worked with Cultec in the past. “A representative helped us every step of the way, clarifying details and specifications and answering questions about the system’s installation. He was also on site during the installation to assist in any way he could.”
The Recharger 150 HD is 33 inches wide and 18.5 inches tall and offers 4.89 cubic feet of storage per linear foot. This lower profile chamber works well for installations with depth restrictions.
“We selected this chamber for Phase I due to the limited separation between the low spot in Manchester Road and expected high groundwater,” explains San Antonio. “Additionally, the footings for the basketball hoops extended four feet into the ground right above the chambers, limiting the available space for installation of the underground units.”
As the second-phase location had a greater separation to the groundwater, the engineers selected the Recharger V8, Cultec’s largest-capacity chamber. It is 32 inches high and 60 inches wide, with a bare chamber capacity of 8.68 cubic feet per linear foot. Its parameters allowed fewer units to be used to achieve the desired storage capacity, which resulted in less labor and a smaller installation area.
The second-phase installation of a Cultec system took place under the teachers’ parking lot behind McCall Middle School. Two beds fed by one header line were installed, employing 556 Recharger V8 units. The system took up 22,000 square feet and offered 527,076 cubic feet of storage. The beds were designed to handle the runoff from the neighborhoods along Main Street to the east of the school.
The entire construction period for both phases took about 10 months, including a two-month suspension of work due to winter weather.
The Manchester Field stormwater management improvements were classified as a redevelopment project under the Massachusetts Department of Environmental Protection (MADEP) stormwater management policy. Designed to upgrade the existing stormwater system and improve water quality in the Aberjona River, the project resulted in no net increase of impervious area and needed to meet the MADEP’s standards only to the maximum practicable extent.
As a result of the upgrade, the stormwater system at Manchester Field reduced surcharging of catch basins and manholes and the ensuing flooding of the area. Cultec’s detention chambers were instrumental in providing the necessary runoff storage and allowing the aboveground land to be used for the school’s athletic facilities.