City study finds permeable pavement manages stormwater but needs costly upkeep on arterials

Sharp Avenue permeable-pavement trial showed benefits for stormwater control and pollutant reduction but revealed durability and maintenance trade-offs for arterial streets.

City environmental manager Trey George told the Spokane Climate Resilience and Sustainability Board on July that the 2018 Sharp Avenue installation of porous asphalt and pervious concrete — a trial funded in part by the Washington State Department of Ecology — was installed to test durability, minimize roadway flooding and evaluate water-quality treatment.

The study matters because permeable pavement is often proposed as a way to reduce runoff in built-up urban areas where conventional swales or large green spaces are not feasible.

City staff described the project design, monitoring and maintenance history, findings on pavement condition and pollutant removal, and the equipment and cost required to restore infiltration after clogging.

"Porous pavement will inherently plug with sediment and requires specialized maintenance to function long term," George said. He identified a specialized cleaning machine used to restore infiltration as the key maintenance tool and estimated its purchase cost at about $650,000.

Board members were shown performance data from repeated monitoring between 2019 and 2024. The pavement-condition index (PCI) for ordinary HMA asphalt remained near its initial 100 rating, while the porous asphalt and pervious concrete sections declined to PCI values around the high 70s in the study period, indicating faster wear. George said joints in concrete sections concentrate wheel loads and are common failure points.

Monitoring also included targeted sampling of inflow and effluent from the pavement using underdrains and manhole sampling, separate from the median swales installed as redundancy. The effectiveness study, done to meet the city's MS4 permit requirements, reported modest reductions in several pollutants including diesel-range organics (DRO), oil-range organics (ORO) and total phosphorus; some metals (lead, cadmium, zinc) showed reductions as well. The team cautioned that background variability upstream could create apparent negative or positive changes for some analytes.

The city cleaned the pavement after five years of routine street maintenance had allowed clogging to reduce infiltration nearly to zero; the specialized cleaner achieved roughly 1,000 square feet per hour and cost about $0.60 per square foot for the 49,000-square-foot trial area after multiple passes. George said more frequent cleaning would reduce the number of passes required to reach target infiltration rates.

Staff emphasized appropriate uses for permeable pavement: "This technology is challenged on arterials because of higher traffic forces and joint stress," George said. He recommended prioritized applications in low-traffic areas such as bicycle lanes, parking lanes and some parking lots, where durability is more favorable and maintenance burdens are lower.

Board members discussed costs, the potential for engineered soils or biochar under pavements to increase treatment, stormwater treatment trade-offs with swales, reuse of removed pavement materials and heat-island effects. City staff said pervious asphalt costs were roughly 2.3 times conventional asphalt and pervious concrete about 6.1 times conventional pavement in the city's cost estimate, and that those multipliers should be compared to the downstream stormwater infrastructure and operational costs that permeable pavement could reduce.

The city will finalize the effectiveness study report to Ecology and circulate the report and presentation slides to the board when available.

The presentation included design drawings, monitoring data and photographs documenting cracking, raveling and spalling at joints and stressed areas. Board members requested copies of the final report and follow-up information about lifecycle costs and potential funding for required maintenance equipment.