Though difficult and expensive to repair, the brick-paved streets that still exist in some Pittsburgh, Pennsylvania neighborhoods are a treasure worth saving. Dellrose Street, an aging, 900 ft. long, brick road, was in need of repair, but the city of Pittsburgh wanted to limit traditional stormwater infrastructure, such as pipes and catch basins.
To save the aesthetics of the neighborhood, they hired ms consultants, inc. to design a permeable paver solution for controlling stormwater runoff volumes and peak runoff rates that would traditionally be routed off-site via storm sewers. Jason Borne, a stormwater engineer for ms consultants who worked on the project says, “What we try to do is understand the in situ infiltration potential of the subsoils to determine the most efficient natural processes for attenuating flows; either through infiltrating excess water volume back into the soil or through slow-release off-site.” He used the Saturo Infiltrometer to get an idea of how urban fill material would infiltrate water.
Green Infrastructure Aids Natural Infiltration
As Borne and his team investigated what they could do to slow down the runoff, they decided permeable pavers would be a viable solution. He says, “There’s not much you can do once you put in a hardened surface like a pavement. Traditional pavement surfaces accelerate the runoff which requires catch basins and large diameter pipes to carry the runoff off-site. We were interested in investigating what some of the urban subsoils, or urban fill would allow us to do from an infiltration perspective. As we started looking at some of these subsoils, we decided a permeable paver system would be ideal for this particular street.”
Infiltrometers Determine Natural Infiltration Potential
Once the water flowed into the aggregate, the team began to figure out ways to slow it down and promote infiltration. Borne says, “Basically we came up with a tiered subsurface flow barrier system. We had about 60 concrete flow barriers across the subgrade within the aggregate base of the road. We needed so many because the longitudinal slope of the road was fairly significant. Behind each of these barriers we stored a portion of the stormwater that would typically run off the site. The ideal is to remove the stored water through infiltration–to get it down to the subgrade and away, so we used infiltrometers to help us establish where we could maximize infiltration and where we might need to rely on other management methods.”
A Need for Faster Test Times Inspires a Comparison
Borne says that USDA soil surveys are too generalized for green infrastructure applications in urban areas and only give crude approximations of the soil hydraulic conductivity. Understanding the best way to promote natural infiltration requires a very specific infiltration rate or hydraulic conductivity for the location of interest. He says, “The goal is to excavate down to the desired elevation before construction and find out, through some kind of device what the infiltration potential of the subsoil is. Typically we use a double ring infiltrometer, but it’s a very manual device. We’re constantly refilling water, and it requires us to be on-site and attentive to what’s happening. We can’t really multitask, especially in areas of decently infiltrating soils where the device might run out of water in 30 minutes or less. So, in the interest of saving water and time, we used the automated Saturo infiltrometer and the manual double ring infiltrometer concurrently for comparison purposes.”