While building an embankment for a new runway at the Seattle-Tacoma International Airport in Washington, the Port of Seattle is also developing some new erosion control measures to help protect the water quality of nearby creeks and potentially boost compliance with state water-quality standards beyond what is required.

"We want to go the extra mile in keeping sediment on-site and protecting water quality during construction," maintains David Jenkins, erosion control and stormwater engineer for the port. "We are shooting for a system that will be even better than the prescribed state guidelines and that will fit right in with the way we are building the embankment."

Crew placing straw in a test plot

Construction of test plots on the third runway embankment

Jenkins is managing a yearlong test of erosion control methods on the surface of the embankment. The test involves 12 treatment plots where various applications of a soil-binding agent, polyacrylamide, and varying quantities of straw mulch have been applied to both smooth and textured soil to see which combination works best, has the greatest longevity, and is most cost-effective. Port Construction Services, a branch of the Port of Seattle, constructed the test plots. Northwest Erosion Control of Seattle installed the material. The effect on sediment and runoff will be measured over the seasons.

Results of the test will determine what combination of materials provide optimum rainfall protection as fill material is brought to the new runway site during the major construction phase of the embankment. About 17 million yd.³ of fill is needed to bring the new runway site up to the same elevation as the two existing runways. More than 3 million yd.³ has already been placed, and another 2 million yd.³ will be placed by March. The major embankment building contract is scheduled for 2002-2004.

The new information from the test is expected to enhance the existing embankment erosion control measures, which include the use of binding agents, hydroseeding, and terracing to stabilize the surface, and the detention of runoff in grass-lined ditches and ponds before chemical treatment and release.

The test results will also increase the body of knowledge in the field of erosion control for construction projects, expanding on what is known about the effectiveness of various erosion control methods, says Jenkins.

He chose to test methods that have been less thoroughly examined for construction sites. "It seems everybody in the world is looking at blankets and hydroseeding, including the Texas Transportation Institute, and I didn't want to reinvent the wheel. I wanted to look at some very specific things, one of which is what actual benefit you get by tracking a slope rather than leaving a smooth slope. My underlying philosophy was to do something more operational than product-oriented.

"I also wanted to look in particular at polyacrylamide. The agricultural industry has done a lot of research on it, and I've seen bits and pieces of information on it for construction. I wanted to see, on a larger scale, what it would do." Jenkins plans to finish drying and weighing sediment samples over the winter and to present the test results sometime in the spring.

"We're committed to continuous improvement in our environmental safeguards during construction of the third runway," states Michael Cheyne, program manager for the runway. "We welcome the opportunity to use our embankment as a learning tool to benefit the construction erosion control industry." The third runway is expected to help Sea-Tac Airport sharply reduce flight delays and give it the ability to operate more efficiently in all types of weather. When low clouds are present–about 44% of the time–Sea-Tac is permitted to use only one of its two existing runways for arrivals because the runways are close together. Directing all arriving air traffic onto a single runway causes delay. The third runway will allow planes to land on two runways in all but the worst weather.

The embankment is being designed with various drainage features for handling runoff on a permanent basis. The surface will be planted with grass and other plants for stability. The port is working with regulators to finalize an extensive stormwater management plan that spells out how runoff will be controlled, stored, and treated to protect areas creeks from flooding, contamination, and sediment.