Buckets of rain, but few problems found on Plateau
April 30, 2009 · Updated 1:03 PM
By Shawn Skager
Although the recent spat of consecutive days of rain in Western Washington fell short of the biblical 40 days and 40 nights or the state record of 34 days, the amount of water that has fallen from the sky has been prodigious.
Since the rains began Dec. 19 more than 13 inches were recorded at SeaTac International Airport and two single day records - 1.33 inches on Jan. 5 and .94 inches on Jan. 10 - have been set, according to the National Weather Service.
The result of all the rain has been predictable with sodden hillsides sliding and rivers surging over their banks and across roadways.
Also predictable has been the performance of two of the area's dams, Mud Mountain on the White River and Howard Hanson on the Green River, both operated by the Seattle district office of the Army Corps of Engineers.
Although on first glance the uninformed may be alarmed by the amount of water flowing out and being held back by the dams, Ken Brettmann, hydrological engineer for the Corps of Engineers, says the dams are operating as they are supposed to.
“I think we're in good shape right now,” Brettmann said. “There actually not that full. We've had an incredibly wet run. We've been holding back water, since Tuesday.
“As much outflow as you see, there is more inflow coming in,” he continued. “Both projects are performing as they were designed. We're actually storing water to help mitigate downstream flooding.”
According to Brettman, both the Howard Hanson and Mud Mountain dams were constructed to control flooding further downstream, rather than on the Plateau.
“The Mud Mountain Dam was built (in 1948) for protection for the lower Puyallup Valley, between Puyallup and Tacoma where the population density is,” he said. “The primary concern is the lower Puyallup. That's why the project was built. Secondly, is the White River.”
The White River, which originates on the shoulders of Mount Rainier, joins the Puyallup River in Sumner.
The Howard Hanson Dam, which was completed in 1962, controls flooding along the course of the Green River, which winds through the Kent Valley before becoming the Duwamish River and emptying into Elliot Bay.
According to Brettmann, one of the questions he frequently fields is why the outflow isn't restricted or cut off to control flooding on the upper White and Green rivers.
“Flood control is a balancing act, looking downstream and mitigating flooding,” Brettmann said. “The last thing you want to do is fill your project and not have any room for more inflow. If we're storing too much, we run the danger of not having any flood control.”
So far Brettmann said the efforts have been successful. The Puyallup River recently crested at 20,000 cubic feet per second (cfs) of water, well below the 50,000 cfs considered flood stage.
The Hanson has also fulfilled it's purpose, Brettmann said, despite flooding along the Green River in Kent.
“The only thing one can control is the inflow into the project,” he said. “There is also a sizable drainage area downstream that we can't control. We've had very high runoff in local creeks in the lowlands, which we normally don't have.”
“It's a tough thing to forecast,” he said. “The amount of rains can really matter.”
Brettmann said although forecasters may predict a front moving through Thurston county, a slight variation to the north can make a huge difference.
“They might be off by a 100 miles which will put more rain in your basin,” he said. “Or the forecasters might predict two inches and instead four inches fall, that can make a big difference.”
Shawn Skager can be reached at firstname.lastname@example.org