Taking the LID approach

By: Tom Gorrivan, EI, CPSWQ

Rapid growth in Olympia, Wash., has lead to an increase in storm-water pollution. With nine streams, four lakes, four large wetlands and 6 miles of shoreline within the city’s 24-sq-mile limits, it is not surprising that protecting water quality is of utmost importance. Managing runoff in accordance with the Western Washington Phase II Municipal Stormwater Permit, issued by the Washington State Department of Ecology (WDOE) in January 2007, is the city’s responsibility.

Runoff from two blocks of Decatur Street and much of the surrounding neighborhood was piped north to the Schneider Creek Watershed and ultimately to Puget Sound without treatment. With no land available to build large storm-water ponds—the conventional option for storm-water treatment—the city turned to an innovative low-impact-development (LID) approach. The project was funded as a demonstration project through a WDOE grant to test and monitor new LID methods.

LID design seeks to minimize runoff by treating and infiltrating it onsite. “We installed Tensar Biaxial (BX) geogrids on the entire 600-ft project,” said Craig Anderson, from the city’s Public Works Water Resources Department. “Then we incorporated three different LID storm-water treatment systems every 200 ft.”

The geogrids permit heavy construction on sites with weak soils, eliminating all or most other foundation or soil stabilization work. Structural geogrids provide a reliable, cost-effective method for increasing the stability of weak soils.

Two of the systems use regular asphalt pavement combined with a treatment system where treated flow is discharged into a perforated pipe. It is then released into the subsoil beneath the road surface. The first treatment system uses Stormwater Management’s StormFilter system, and the second uses a rain garden. The third system is porous asphalt pavement.

The StormFilter system removes TSS, soluble metals, oil, grease and total nutrients from storm water. Siphon-actuated filter cartridges, which trap and adsorb pollutants, feature a surface cleaning system that prevents blinding and extends maintenance intervals.

“We chose the filter systems because we had experience with most of them,” Anderson said. “Personally, I like the CatchBasin StormFilter because during the construction phase, it was the most straightforward method and easiest to construct.”

Following construction, 100% of storm-water runoff from these two blocks will be treated onsite. Each system will be measured for the amount of water soaking, pollutants removed, runoff receiving water quality treatment, durability and maintenance.