Wednesday, December 12, 2007

Pervious Parking Lot Stands Up to Torrential Rains

By Christopher Dunagan (Contact)Originally published 12:25 p.m., December 5, 2007Updated 12:25 p.m., December 5, 2007

During the peak of Monday's storm, with water pouring from the sky, no water could be seen running off one parking lot on Auto Center Way.

The parking lot, made of various types of pervious pavement, was installed last summer at the office of the Home Builders Association of Kitsap County.

Rainwater rapidly penetrated through the pavement and into an underlying bed of gravel, said Art Castle, executive director of the Home Builders Association. When the gravel bed nears capacity, an "underdrain" directs the water into the city's storm sewers.

A bioretention cell, or "rain garden," captures all the runoff from the roof of the office. While the rain garden did fill up with water during the storm, it did not overflow, Castle said. Rain gardens are generally designed to contain typical rainfall over a two-day period. "This was a worst-case scenario," he added.

These kinds of projects are called low-impact development, or LID. They're not designed to prevent flooding, Castle said, but they can slow down runoff by storing the water for a period of time.

"Flow is delayed, and that takes off the peaks," Castle said. "Even if there is outfall, the water has been delayed and it gets water-quality treatment along the way."

The same techniques can be used for private homes, where local rules provide major allowances in stormwater design when using pervious paving. Essentially, the first 5,000 square feet of pervious pavement placed over native soil are exempt from stormwater calculations. That can save months of delay and extensive engineering costs, Castle said.

For a series of video clips shot by Castle during the storm, go to

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