The Big Dig—the massive, multi-billion-dollar project that rerouted Boston’s Central Artery—stirred up plenty of controversy during its two decades of construction. Some of the staunchest opposition came from environmental groups concerned about elevated traffic flows negatively impacting air quality in the region. To quell the groups’ concerns, the state agreed to extend the commuter rail line south of the city. Establishing commuter rail service on the new, 18-mile-long Greenbush rail line through the towns of Braintree, Weymouth, Hingham, Cohasset, and Scituate, Mass., allowed the state to divert 5,000 daily automobile trips to and from the city.
|Seven new stations with associated parking lots serve the new 18-mile-long Greenbush rail line south of Boston.|
The seven new stations were developed to support at least 20 percent more parking spaces than the projected average parking demand of 3,100 total spaces. However, paving of these areas can significantly impact stormwater runoff. Green space serves as a natural filter to remove pollutants from stormwater. As these locales are turned into impervious areas such as buildings and parking lots, associated surface pollutants are picked up by stormwater and carried into nearby storm drains, and then directly into lakes and streams. The addition of impervious surfaces also leads to a rapid increase in the percentage of stormwater runoff volume, and associated flooding.
It is also important to remove sediment from stormwater before it enters nearby water bodies. Large concentrations of sediment contained in stormwater harm aquatic life. Suspended sediment results in turbidity, which reduces the light penetration necessary for sustaining plant and animal habitats. Sediments also attract and bind other harmful pollutants such as hydrocarbons, heavy metals, and nutrients. Moreover, increased sedimentation clogs storm drains, resulting in an increased flooding risk and more frequent and costly system maintenance.
To prevent these problems, the Massachusetts Bay Transportation Authority (MBTA) worked closely with state and local regulatory authorities to ensure stormwater drainage systems were designed to avoid potential contamination of water supplies, rivers, or wetlands. In accordance with Massachusetts Department of Environmental Protection (DEP) regulations, stormwater runoff from impervious areas must be treated by systems that achieve at least 80-percent removal of suspended solids at peak storm flows.
Moreover, several Greenbush stations faced more stringent standards to protect adjacent Areas of Critical Environmental Concern (ACECs). ACECs are identified and nominated at the community level and reviewed and designated by the Massachusetts Secretary of Environmental Affairs "to receive special recognition because of the quality, uniqueness, and significance of their natural and cultural resources."
STV Inc., a Boston-based engineering firm sharing responsibility for design of the railway, parking lots, and storm drainage systems, chose to install 27 Downstream Defender units from Hydro International for treating stormwater. These units use vortex separation to remove sediment, oil, and floatable materials from stormwater runoff, typically in half the footprint of conventional stormwater treatment structures. The units were evaluated by the Massachusetts DEP during a multi-year review process to determine whether the technology was appropriate and was sized correctly for the project. In 2004, the DEP gave a green light to the project after its evaluation process.
The flow-modifying internal components of the Downstream Defender offer an alternative to simple swirl-type devices and conventional oil/grit separators with less turbulence and headloss. The device consists of a concrete cylindrical vessel with two ports at ground level to provide access for inspection and clean-out of pollutants. It is designed so that the static water level is determined by the elevation of the outlet pipe invert. The inlet pipe enters the Downstream Defender below the static water level to minimize turbulence.
|The Downstream Defender stormwater treatment device has two ports at ground level to provide access for inspection and clean-out of pollutants.|
The Downstream Defender cleans the water before it discharges from the site or enters an infiltration system for groundwater recharge. Pollutants are separated from the stormwater by vortex and gravitational forces as the water flows through the device. Stormwater is introduced tangentially into the side of the vessel, initially spiraling around and down the perimeter, then inward and upward around a central shaft component in a continuous spiraling motion. As oil and floatables enter the vessel, they rise to the water surface and are trapped by a dip plate in the outer annular space. Low-energy vortex motion directs sediment inward along a benching skirt and into a protected sediment storage zone. Storing pollutants outside the treatment flow path improves efficiency and prevents pollutant "washout," a phenomenon that plagues many stormwater treatment devices whereby high storm events wash previously captured pollutants out of the treatment device.
"The design of the Downstream Defender is appealing," said Paul Tyrell, a former STV design manager who oversaw portions of the engineering project. "Other alternatives require a crane for installation, increasing the cost and complication of the project. With the [smaller-sized] Downstream Defender units, you just need a typical backhoe, which is already on the job site. Plus, it fits right into precast manholes, [decreasing] the time and cost involved with installation."
|Treated stormwater enters an infiltration pond for groundwater recharge.|
The engineering firm also finds the units easier to maintain than traditional stormwater treatment methods. "With many solutions, there are multiple covers, and it’s very easy for maintenance crews to make a mistake," Tyrell said. "The 4-foot Downstream Defender just has a center cover. You just remove the cover and use a [vacuum] truck to clean out the device."
The Downstream Defender installations were completed in the spring of 2006, with the Greenbush line extension coming on line the following year. Communities along the new route now have a cost-effective, sustainable means to treat stormwater runoff, with minimal maintenance needs.
Dave Scott is stormwater products manager for Hydro International. He can be contacted at email@example.com.