Sheetflow Erosion Control Erosion Control for the CESCL

September 19, 2013

Lessons in Solving Big Weather-Related Problems

Grading and Excavation Contractor
September-October 2003

Lessons in Solving Big Weather-Related Problems

When it comes to controlling erosion and sediment in bad weather, construction of a third runway at Seattle-Tacoma International Airport, near Seattle, WA, qualifies as a genuine challenge. Providing a foundation for the 8,500-ft.-long, 150-ft.-wide runway in the hilly terrain will require an estimated 17 million yd.3 of fill. Since construction began in 1997, about 5 million yd.3 have been placed. It will take a large fleet of dump trucks, running 20 hours a day, three and a half years to bring in the rest. Then there’s all that wet weather the area is famous for, especially in late fall and winter. Two years ago, for example, the project was drenched with about 5 in. of wind-driven rain in one 36-hour period.

As the erosion control/stormwater engineer for the Port of Seattle, which owns the project, David Jenkins is one of several people in charge of protecting the fill slopes and surrounding streams and wetlands from the impact of erosion during construction. “At the heart of our approach – from project engineers to contractors to inspectors – is our desire to sleep at night,” he says. “We don’t want any violations of water-quality regulations.”

In addition to doing as much earthwork as possible during the dry months and installing sediment control measures along the perimeter of the project, the ESC program features a complex system of berms, pipes, and ditches that directs runoff from disturbed areas to two holding/treatment pond complexes.

Some of the lessons Jenkins has learned include the following:

How to Build Stable Embankments

“The construction permit conditions and language in the stormwater manual talk about designing and constructing fill slopes to minimize erosion, but no one really explained how to do that,” he notes. “This project has given me a good education.”

At first the embankments were built using the same material throughout and included several benches to break up water flows down the slope face. Now they are being built without benches using finer sandy, gravelly fill material for the inner portion of the cross-section and coarser particles of the same material on the outer 20-ft.-thick band. Several feet of rocky, granular materials provide a drainage layer at the base of the embankment.

“We’ve maintained slope surface integrity with this current method, plus it’s easier to build the embankments this way and we can put more fill material in a smaller footprint,” he relates. “Also, all embankments are constructed with positive drainage so that no water flows over the slope face except what falls from the sky.”

How to Clean Tires

The project also includes what Jenkins terms “the biggest tire wash facility I’ve ever seen” to prevent sediment tracking onto public roads as trucks leave the site. The automatic high-pressure spray system, which recycles the water, includes a 15,000-gal. water holding tank and a 100-hp pump. As the truck rolls onto a grate, it breaks an electric eye, triggering jets of water that shoot up into the undercarriage to wash off any dirt or mud. In severe conditions, the truck is also treated with a second clean-water rinse system.

How to Treat Stormwater

Runoff from unstable areas, including haul roads, is routed to one of two treatment sites. Each features a conventional sedimentation pond (sized to treat bigger storms and smaller particles than local guidelines require), where heavier sediment particles settle out before the water is pumped to one of three lined treatment cells. There, the stormwater is treated in batches with a chemical coagulant (Calgon 2953 CatFloc) to remove fine soil particles. Once the treated water meets pH, turbidity, and other water-quality requirements – typically after one to two hours of contact, circulation, and settling – it can be released into the nearby creek.

“I wasn’t a big fan of chemical treatment of construction stormwater in the past,” Jenkins says. “But I’m sold on it now. Not a drop of water from the site is discharged into creeks without being tested and treated if necessary. Over the past five seasons we’ve treated [more than] 500 million gallons of runoff without violating any stormwater quality standards. That helps everyone sleep better at night.”

Grading and Excavation Contractor
September-October 2003

No Comments »

No comments yet.

RSS feed for comments on this post. TrackBack URL

Leave a comment

You must be logged in to post a comment.

Powered by WordPress