Austin’s new requirements regarding green stormwater infrastructure likely will result in the installation of more facilities such as this biofiltration pond. Photo Credit: City of Austin

The GSI requirements apply to any new public or private development or redevelopment that exceeds 8,000 square feet of impervious surface, says Liz Johnston, the environmental policy program manager for Austin’s Watershed Protection Department. Exceptions to the requirement to use GSI include sites having more than 90 percent impervious cover, sites that generate highly contaminated runoff, or a project that provides water quality treatment for currently untreated, developed off-site areas of at least 10 acres in size.

As for the volume of runoff that GSI must be capable of accepting, Austin requires that such installations adhere to the “half-inch-plus rule,” meaning they must capture the first 0.5 in. of runoff from the contributing area as well as an additional 0.1 in. of runoff for each 10 percent increase in impervious cover exceeding 20 percent of the gross site area.

Under the code’s previous requirements for water quality controls, “many sites [met] water quality treatment requirements by building a sedimentation/filtration pond,” according to a summary of the code amendments prepared by city staff. “Sedimentation/filtration devices provide some water quality benefits by filtering polluted runoff and helping control stream-channel erosion, but they do not significantly address other important ancillary goals such as supporting on-site vegetation, increasing rainwater infiltration, and reducing potable water consumption,” according to the summary.

Greater implementation of GSI in favor of conventional gray stormwater infrastructure “will provide myriad benefits, including stormwater infiltration, soil health, wildlife habitat, urban heat island mitigation, water conservation, aesthetic value, and other ecosystem services,” the summary states. “GSI also provides enhanced water quality benefits compared to sedimentation/filtration devices, including better removal of nutrients from stormwater and further reductions in erosive flows.”

Although now mandatory in Austin, GSI is hardly a new concept in the Texas capital. “Since 2013, GSI has been used by over 50 percent of all projects,” Johnston says. As a result, the regulatory requirement for GSI should not pose a problem for most projects. “Most civil engineers in town have been doing GSI for a while, so they will know how to build or design them,” she says. “But I imagine that for folks that aren't used to doing building projects in Austin, it may take a bit of a learning curve for them.”

A rain garden captures stormwater at One Texas Center, a building owned by the City of Austin. Photo Credit: City of Austin

Austin’s City Council also approved a code change requiring that stormwater within parking lots drain to parking lot islands and medians where feasible. The amendment removed an existing requirement that all parking lot landscape areas be protected by a 6 in. curb, which prevented stormwater from flowing into landscape areas.

Facilitating drainage from parking lots into their adjoining landscape areas “increases beneficial use of stormwater and reduces irrigation needs by directing stormwater into areas that are typically required to provide on-site irrigation,” according to the staff summary.

The code changes regarding GSI and parking lot drainage took effect November 7.

A biofiltration pond at a restaurant in Austin, Texas. Austin’s City Council approved a code change requiring that stormwater within parking lots drain to parking lot islands and medians where feasible. Photo Credit: City of Austin

On October 21, Boston Mayor Michelle Wu announced that the city would begin installing GI as part of projects involving curb extensions, which are also known as bulb-outs or bumpouts. Designed to improve safety at crosswalks, curb extensions reduce the distance that pedestrians must cross from one side of the street to the other, help to make pedestrians more visible to drivers, and slow down vehicles. 

“Today we are announcing Boston’s first ever green infrastructure policy and in doing so setting a new standard for safer, more sustainable roadway infrastructure throughout the city,” Wu said in an October 21 news release. “Together these elements will improve community safety, foster community engagement, and boost our climate resilience.” 

Historically, the city has used impervious materials to construct curb extensions. Under Boston’s new GI policy, five design alternatives have been developed for curb extensions:

• right-of-way (ROW) bioretention, involving such approaches as rain gardens and bioswales

• infiltration tree pits/tree trenches

• porous paving, including porous asphalt, permeable pavers, porous paver installations, and porous concrete slabs

• stone subsurface infiltration areas (with or without perforated pipe)

• one-time seeding, in which the area within the curb extension may be seeded once with a groundcover, low-grow fescue, or wildflower mix

“Integrating green infrastructure into the City's projects as a standard will help us reduce stormwater flooding, create new green space and provide more shade in all of our neighborhoods,” said Kate England, Boston’s director of green infrastructure, in the October 21 release. “We are also excited to introduce a volunteer program that will foster stewardship by creating opportunities for neighbors to come together and take care of these new features.”

“All design projects at 75 percent design or less as of October 21st will be subject to the new GI policy,” England told One Water News. Currently, the city’s Public Works and Transportation departments have “several projects in design” that call for curb extensions, she says. 

To date, Boston has approximately 50 locations with infiltration and/or GI features, England says. To help ensure the longevity of its GI installations, the city is in the process of awarding two maintenance contracts. One involves regenerative air vacuum sweeping of porous paving projects, while the other entails maintaining, replacing, and pruning vegetation in GI features in the right-of-way. Contract proposals were due November 17, and the city will award the contracts “in the coming weeks,” England says.