Retrofitting an Existing Stormwater System to Meet Numerical Effluent Limits - Stormwater Management at Seattle-Tacoma International Airport

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ABSTRACT
Owned and operated by the Port of Seattle, Seattle-Tacoma International Airport (STIA) is the 28th busiest airport in the world. A Section 401 Water Quality Certification was issued to the Port to allow its expansion projects to proceed. This certification required existing portions of the airport be retrofitted to meet current stormwater standards. In addition, the Port’s renewed NPDES permit contained new effluent limits for stormwater discharges that will be effective starting in 2008. Port management initiated a new capital program focused exclusively on improving the existing stormwater systems and designing new systems to ensure compliance with these conditions.

A key programmatic consideration was cost – the airline industry has been suffering through its worst recession in history. Since the Port passes a significant portion of the cost of building and operating new projects onto the airlines in the form of landing fees and passenger emplanement charges, scrutiny on any spending was intense and cost justification was paramount.

Another key driver was schedule. A timeline of only 5 years was available from the creation of the program to the successful implementation of all the facilities. Because of the high development density of the existing airport, any stormwater improvements would be difficult to implement. Potentially innovative technologies or combinations thereof would be necessary for implementing the suite of Best Management Practices (BMPs) necessary to meet effluent limits. Finally, all stormwater improvements needed to be consistent with the airport’s wildlife hazard management plan. An FAA Advisory Circular specifically discourages open water within 10,000 foot of an active runway. Working cooperatively with the Airport Biologist, techniques were developed to mitigate any wildlife attraction posed by the new stormwater infrastructure.

A total of 13 new flow control facilities were needed to be built to detain runoff and release it at controlled rates. Updated hydrologic modeling was performed using site specific calibrated runoff estimates to determine design criteria for the new flow control facilities. Significant reductions in the sizes of the facilities could be achieved through using more refined information than what was used in a preliminary analysis and through optimization of the stage-storage and orifice sizing for selected facilities. In addition, the Port’s participation in local Basin Planning in its southernmost drainage basin allowed a basin-specific flow control standard to be applied that resulted in further reductions of stormwater detention volumes. Altogether, the aggregated changes allowed a reduction in detention volume required from a planning number of 344 acrefeet to a regulatory-approved volume of 122 acre-feet, or 65 percent. This work allowed detention ponds to be built instead of more expensive underground concrete vaults in all but 2 of the sub-basins. Vaults are approximately three times more expensive than ponds for a given volume. Wildlife mitigation measures include nets and linings which reduce attractiveness to birds. Overall, the cost of the stormwater flow control facilities was reduced to less than one-third of the original budget.

The Port had 13 regulated stormwater outfalls at the start of the program. In 4 of the outfalls, it was predicted that future effluent limits could be met without additional treatment BMPs. However, the Washington State Department of Ecology (Ecology) required the Port to conduct a site specific study to determine all known, available, and reasonable methods of treatment (AKART) that should apply at STIA. It was determined that commonly used “Basic Treatment” BMPs should be applied wherever practicable. Modeling predicted that an attainment probability of at least 97 percent would be achievable at each regulated outfall after installing BMPs.

It was found that runoff from runways and taxiways is relatively clean after it passes through grassy areas, termed vegetated filter strips, which are located adjacent to the pavement. The newly created third runway was designed to incorporate properly sized vegetated filter strips wherever practicable. In existing airfield areas, the Port optimized the ability of vegetated filter strips to provide water quality treatment wherever practicable. Conveyance systems were modified where short-circuiting of runway and way runoff directly to catch basins occurred, forcing the runoff through grassy areas. In addition, re-vegetation, regrading, and soil improvements were implemented in areas where the vegetated filter strips in existing infield areas were found to be deficient.

Landside activities at STIA include runoff from roads, buildings, and parking areas. There are 6 different landside subbasins, each with different challenges. In the northeast (SDN1) subbasin, BMPs include coating metal roofs, a wet pond incorporated into a detention pond and bioswales along roadway shoulders. In three of the other subbasins, runoff from pollution generating surfaces was eliminated either by changing activities or eliminating runoff up to the water quality design storm. In other subbasins, where land availability allowed, bioswales, Ecology Embankments, and vegetated filter strips were installed, coupled with numerous source controls.

In one case, a new $4,000,000 54-inch diameter conveyance pipe was micro-tunneled nearly 2,000 feet to the end-of-pipe treatment facility, consisting of a detention pond followed by media filtration. The detention facility has been designed to provide flow equalization to optimize treatment. At least 91 percent of the annual runoff volume will be treated. In conclusion, an airport is essentially a small city with runoff characteristics not unlike urban runoff. At STIA, the successful retrofit of the airport to meet numeric effluent limits portends of what municipalities might be required to do in the future.

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