Water rules may drain builders - But new manual will 'help environment'
Thurston County, WA - Stormwater ponds for many projects in Thurston County will need to be built bigger to comply with new state and local standards for drainage and erosion control.
On the other hand, the new rules will increase the cost of construction, especially on poorly draining soils, and are likely to make some property unavailable for development, said Steve Hatton, a professional engineer with Hatton Godat Pantier in Tumwater.
Book on stormwater
At issue is the rewrite of the stormwater manual for Thurston County, Olympia, Lacey and Tumwater to comply with state Department of Ecology stormwater requirements developed for Western Washington in August 2001.
The local manual that guides development of stormwater controls on public and private property was last revised in 1994 and may be ready for local government adoption late this year.
In the past 10 years, much has been learned about the effects of stormwater and how better to control it, noted Mark Blosser, an Olympia project engineer.
For years, engineers calculated the size of stormwater detention ponds based on a "100-year storm" model that added up worst-case rainfall over 24 hours.
But in Western Washington, stormwater ponds are more likely to overflow -- sending untreated water to nearby streams -- by one or two inches of rain each day over four or five days.
In response to a new hydrology computer model, the new manual calls for bigger detention ponds on poorly draining soils.
City engineers have estimated a pond size increase of 10 percent to 100 percent on poor soil. Hatton said it could be a 30 percent to 200 percent increase in pond size.
That means, in some cases, the stormwater pond could eat up 20 percent to 40 percent of the buildable land in a project.
"Fewer lots means the cost per lot will go up," Hatton said. "And some land will be completely taken out of use."
"If the ponds aren't big enough, we'll just keep blowing out the creeks with stormwater," said Eric Hielema, Lacey water resources engineer.
Retention ponds, which slowly release the stormwater to the groundwater and work best in well-drained soils, are the more common type of stormwater pond used in South Sound -- especially in Lacey and Tumwater.
The local manual will try to be flexible with retention ponds, Blosser said, but they may need to be built larger -- but not as deep -- in areas prone to flooding.
A lot of property in Thurston County wouldn't meet the new Ecology manual requirements for retention ponds. The new requirements increase the distance that must be maintained between the groundwater and the bottom of the pond, Hatton said.
The threshold for when stormwater control rules kick in is dropping enough to affect single- family homes with 2,000 square feet or more of impervious surface, including roofs, patios and driveways.
At a minimum, the new regulations will probably require an engineered erosion control plan for most new homes, Hatton said.
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