After the storm: A few facts about King County’s February widespread flooding

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Five months ago, long-range weather experts told us this winter’s western Washington weather would be influenced by “La Nada” – the “anything is possible” middle pattern that is neither La Nina nor El Nino.

 

The rainstorm that drenched western Washington beginning Feb. 5 and led to widespread flooding across much of the region was one for the record books. Thanks to that storm, the official rain gauge at Sea-Tac Airport has received half a foot more rain since Jan. 1 than our already gaudy average for this time of year.

So it was not too surprising that on Wednesday, Feb. 5, King County’s Flood Warning Center opened for the fifth time since Jan. 1 – that’s five flood events over the first six weeks of the year.

By the time the Flood Warning Center closed early in the evening on Tuesday, Feb. 11, flood center workers had taken roughly 1,000 calls during this current activation – one of the highest call volumes over the past several years.

And the County’s award-winning Flood Warning App was accessed more than 24,500 times at the peak of the flooding Feb. 6-7 – a record for any two-day period since it was introduced in late 2012.

Unlike the previous four flood events, this flood caused significant impacts to King County residents, as well as county roads, trails, and other infrastructure.

Also unusual about this flood was where it hit the hardest.

Typically, the Snoqualmie and Tolt rivers lead the way in flood impacts, such as closed roads, inundated fields, and neighborhoods becoming isolated by floodwaters.

This time, Issaquah Creek and the Cedar River saw the highest flows, with homes, roads, bridges and more sustaining significant flood damage.

Issaquah Creek soared to a Phase 4 flood alert level, with the Hobart gauge recording its highest flood elevation since 2006. The high flows resulted in emergency evacuations for more than 200 people living along the creek in Issaquah, a multi-day closure of Issaquah-Hobart Road while crews made emergency repairs, and several more major problems.

Lake Sammamish – which receives Issaquah Creek at the southern end of the lake and sends flows down the Sammamish River at the lake’s northern end – recorded its highest elevation in 23 years, leading to concerns from lakeside residents with flooding on their property.

The Sammamish River’s low gradient watercourse means lakeside residents have to deal with higher-than-normal lake levels long after stream flows feeding into the lake have receded.

The Cedar River didn’t fare any better, with the Landsburg gauge recording its highest flow since 2009. Flooding led to the closure of a four-mile section of State Route 169, while a neighborhood that lost access to and from their homes relied on an emergency route using a portion of the county’s Cedar River Trail for access.

Although constrained by levees, the Cedar River was flowing too high and too fast to be contained along its entire length. A portion of the Riverbend lower levee was breached by swift flows, sending a portion of the river’s flow into Cavanaugh Pond – a King County Natural Area.

Engineers are concerned that as flows from the pond area move downstream to rejoin the river’s mainstem, additional erosion could cut into the riverbank immediately adjacent the Cedar River Trail – threatening not only the trail itself, but a fiber optic line located underneath the trail.

The Green River also recorded major flood flows, with recordings at the Auburn gauge reaching their highest levels since 1996. Road closures were common throughout the Green River Valley upstream of Auburn, as floodwaters fanned out across the valley at levels that hadn’t been seen in a decade or longer.

Flood patrol crews were in the field for six straight days beginning in the evening of Feb. 12, with two-person teams working 12-hour shifts to serve as the Flood Warning Center’s eyes and ears. They observed flood conditions, checked on levees and other flood structures, and reported findings back to River and Floodplain Management colleagues.

Those observations helped King County identify damage to many river facilities, with more problems becoming visible as floodwaters recede. These detailed inspections will continue for several days as the water levels take place once water levels drop.

So even though flood flows are largely a memory a week after the peak of this flood event, the impacts from high, swift waters will be something King County will be working to address for weeks or months to come.

About the King County Flood Control District
The King County Flood Control District is a special purpose government created to provide funding and policy oversight for flood protection projects and programs in King County.  The Flood Control District’s Board is composed of the members of the King County Council. The Water and Land Resources Division of the King County Department of Natural Resources and Parks develops and implements the approved flood protection projects and programs. Information is available at kingcountyfloodcontrol.org/.

About the King County Water and Land Resources Division

The Water and Land Resources Division works to protect the health and integrity of King County’s natural resources. Employees work to reduce flood risks, monitor water quality and restore wildlife habitat; manage, and reduce the harmful impacts from stormwater, noxious weeds and hazardous waste; create sustainable forestry and agriculture; and protect open space to support all of these efforts.

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