Porous Asphalt at Yauger Park LID Stormwater Facility

The goal of the Yauger Park Phase I project was to improve stormwater flow control, and give some treatment to the water.  Porous Asphalt was chosen to demonstrate how infiltrating the incoming stormwater can reduce the area needed for treatment ponds, and therefore allow for more viable building space.

The compliance path for this project was prescriptive.  The City of Olympia already has standards of how to lay porous asphalt.  The 1993 AASHTO Empirical Equation for Flexible Pavement Design and experience with porous asphalt projects within the city, was used to determine how the asphalt at Yauger Park was designed and laid down.

The other reason for using porous asphalt is for demonstration and educational purposes.  The porous asphalt demonstrates how to combine two uses into one.  Creating a more dynamic use of land.  The site, once the signs are up, will educate the public on how porous asphalt enables direct stormwater infiltration.

Prior to construction of the Low-Impact Development project, Yauger Park was designed to provide stormwater facilities to accommodate approximately 66-acre feet of detention of winter runoff from a 570 acre basin.  It included a stormwater facility, with some minimal water quality treatment features in the form of swales, pre-settling ponds, and a small “wetpond” near flow control structures.  While the facility did attenuate flows and reduce downstream flooding, the facility has overtopped in recent years, confirming that it was undersized for the contributing flows.  During the drier season, Yauger Park provides recreation facilities including trails, a parking lot, a disc golf course, and several ball fields.

The Yauger Park LID project had a few purposes when first conceived.  The primary purpose was to increase the stormwater detention capacity of the park.  Another aspect was to have some treatment of the stormwater coming in before being discharged downstream.  The other was as a demonstration project.

The idea is that each aspect of the stormwater facility can treat the incoming stormwater on its own, or work together.  The porous asphalt was conceived as a way to infiltrate and filter the stormwater.  The rain and stormwater coming infiltrates through the asphalt into the sublayers.  In the sublayers contaminants are filtered out before being discharged.  The porous asphalt has the added benefit to the public to demonstrate how this technology works and holds up over time.

From the start there were some challenges to the porous asphalt.  The fact that the parking lot was to frequently be underwater in winter months meant finding a mix that would hold up frequent flooding.  Having a thickness that was easy for repaving was also a consideration.  The contractor and city decided on a mix that was previously used at other city projects.  However, during the construction process, a different mix was used.  The new mix has a stiffer binder and higher oil content (“DESIGN SUMMARY REPORT-FINAL:YAUGER PARK REGIONAL STORMWATER FACILITY EXPANSION-PHASE I” DCG, December 2011), this makes the mix more durable.

When visiting the site, the porous asphalt has what looks like divots on the surface.  In fact it is the top course being rubbed off by car tires.  As people turn their tires while the car is in a stationary position, the very top layer is rubbed off.  While this cosmetically looks bad, it has little effect on the filtration properties of the porous asphalt.

Overall, the porous asphalt works in concert with the rest of the facility, and all on its own.  The asphalt has been functioning very well, and regular maintenance is key to porous asphalt life and performance.

Updates

Probable Construction Cost Update – An Engineer’s Opinion of Probable Cost was developed for the project using the City of Olympia construction cost database, and estimated at $2,814,000 without State Sales Tax. The estimated construction cost for Additive Bid Schedule #1 for the bicycle shelter construction is $49,000 without State Sales Tax.