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Porous Asphalt at Yauger Park LID Stormwater Facility

Case Study by Samuel Rowswell, Kraig Chalem
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Porous Asphalt at Yauger Park LID Stormwater Facility
Site and Stormwater
Low-Impact Development (LID)
Porous Pavement
City of Olympia
12816310300
Fran Eide, City Engineer, City of Olympia Public Works
Tom Hill, Building Official, City of Olympia Community Planning and Development
City of Olympia
Infrastructure
250900
Tanja Wilcox, J. A. Brennan Associates, PLLC
Eric Davido, Davido Consulting Group, Inc.
BCE Engineers

Abstract

The Yauger Park Low Impact Development (LID) Project provides for enhanced water quality treatment, additional storage volume and improvements to the recreational facilities at the City of Olympia’s Yauger Park regional stormwater facility.  In this case study, the permeable pavement (porous Asphalt) at the Yauger Park Low Impact Development (LID) Project is examined.  Federal funding under the “American Recovery Act” was used to build a demonstration project employing environmentally sensitive Best Management Practices (BMPs).  There are several environmental innovations being used at this one site, and permeable pavement is just one.

Permitting Process

The Yauger Park Low Impact Development (LID) project is on property owned by the City of Olympia and for all intents and purposes it is considered to be private property and subject to the same regulatory standards as any other private development.  A porous pavement parking lot was installed to demonstrate how innovative methods can be employed by property owners and developers to help meet the City’s requirements to comply with all applicable Stormwater Regulations, and Building Standards, and Engineering Standards.  In the process, the facility helped the City meet performance standards for its own National Pollutant Discharge Elimination System (NPDES) permit and evolving LID Best Management Practices (BMP’s). According to City staff no variances or deviations from the standards were sought or granted, although approval of the hybrid system was innovative.

Due to the significant modification of the designated site plan, and addition of parking, appurtenant structures, and functions, the project was subject to: Land Use Approval (LUA), State Environmental Policy Act (SEPA), Stormwater Regulations, Building Standards, and Stormwater Engineering Standards.

Code RequirementCompliance Path
Drainage Design and Erosion Control Manual For Olympia:
Chapter 5.3.4 Permeable/Porous Pavements
Prescriptive path following a close outline of code, approved by City Stormwater Engineer using the city stormwater manual code for permeable/porous pavements
Washington State Department of Ecology, Stormwater Management Manual for Western Washington, Volumes 1-5, 2005 No alternative path was needed.
Limitations on seasonal grading in Percival Creek Basin per City of Olympia Stormwater Manual Usage of Clear Creek Solutions Modeling formulas to determine seasonal grading limitations. Summary of results in David Consulting Group Report
Washington State Department of Ecology, Construction Stormwater General Permit Prescriptive state regulations followed in creation of porous asphalt and dealing with stormwater during construction.

Project Description

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.

Design / Build Process

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.

Lessons Learned

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.

Project Contacts
Owner: Andy Haub, Project Manager City of Olympia Public Works Owner: Eric Christensen, ead Design Engineer City of Olympia Public Works Approving Offcial: Tom Hill, Building Official City of Olympia Community Planning and Development
Approving Offcial: Fran Eide, City Engineer City of Olympia Public Works Designer: Erik Davido,, Design Team Project Manager, Civil Design Lead Davido Consulting Group, Inc. Designer: Tanja Wilcox, Landscape Architect Design J. A. Brennan Associates, PLLC
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