Living with flooding, part 2: balancing growth and environment in Snoqualmie River floodplain
December 23, 2009
By Dan Catchpole and Laura Geggel
The Snoqualmie River pours out of its banks last January, flooding the Valley wall to wall near Carnation. A river’s floodplain is really the river at high flow, says David Montgomery, a geomorphology professor at the University of Washington. (Photo by Alan Berner / The Seattle Times)
Flooding. It’s an all-too familiar part of life for many residents of Snoqualmie Valley. Since moving to the Valley in 2005, Snoqualmie residents Don and Nancy Ekberg have had their home flooded twice. Business-owner and resident Julie Randazzo and her husband Harold Nesland have sandbagging their pizza restaurant and bowling alley down to a science.
But flooding is part of Snoqualmie Valley’s natural rhythm, say experts. And many of them agree that fighting it is a losing battle.
Today, King County has a work-with-nature approach to flood management, which reflects both the high costs of trying to control flooding and the ecological value of flooding.
At the same time, the county and the Valley have grown very quickly in recent decades, and the county must balance development and flood management.
“The one thing about floodplains we’re sure of is that they flood,” said Dave Montgomery, river expert and geomorphology professor at the University of Washington. “Over the long run, sometimes avoidance of a hazard is the best policy.”
Along the Snoqualmie, the county has decided it is more cost effective to buy out or elevate homes to move residents out of harm’s way, rather than put in hard fixes, such as a dam or levee system.
| Relying on these solutions can cause problems down the line.
“A lot of things can go not according to plan,” Montgomery noted, pointing to the Green River’s Howard Hanson Dam as an example. |
Living with flooding | |
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After the dam and a levee system were built, the county encouraged industrial development along the Green River. But the dam has proven to be less stable than anticipated and the U.S. Army Corps of Engineers is working with local engineers to find a long-term fix. Some rivers themselves also have been altered, reducing their ability to manage high water.
In the meantime, part of King County’s industrial core is at elevated flood risk.
“How wise are we to assume that the flood controls today are always going to be there?” Montgomery asked. So, “there’s this long-term debate about what is the best land use on a flood plain. What are we willing to allow on places in a flood plain?”
Today, the county tries to find a cost effective way of minimizing risk.
“We’re at a point where we recognize we need to help people better live with flooding,” said Clint Loper, King County’s supervising engineer for the Snoqualmie River basin.
Snoqualmie Valley “has been a floodplain historically; this continues to be a floodplain,” he said, explaining the county’s view on flooding in the Valley.
The river ties the Valley together, running like a ribbon from the northern end to the south, where its three branches split off like strands of frayed rope. The river floods today much as it has since a massive sheet of ice last retreated northward thousands of years ago. It rolls down from the Cascade Mountains to the Skykomish River to form the Snohomish River and into the Pacific Ocean.
Most days, the Snoqualmie River peacefully passes along, but when the conditions are right, the river surges over its banks, flooding the fields, forests and communities in the Valley.
The Snoqualmie River’s flow – the amount of water in the channel – is unregulated. There are no dams on the forks to control its headwaters. It is a mix when it comes to flood management, with levees and revetments used in some places to protect developed areas at high-risk of flood damage even during small flows. Many of these areas are along the South Fork of the Snoqualmie River in and around North Bend.
Noticeable flooding typically occurs after a heavy rain lands on already saturated ground, said Brent Bower, a service hydrologist in the National Weather Service’s Seattle office. He is responsible for issuing flood predictions.
Those conditions occur most often in late fall through winter, but flooding can happen any time of the year.
Flooding is a natural process that can be thought of as the river at maximum flow.
“Rivers aren’t designed to carry all the flows they get. The floodplain is the river at high flow,” said David Montgomery, a river expert and geomorphology professor at the University of Washington.
The Snoqualmie River’s full force pours over the falls during the January flood. (Photo by Alan Berner / The Seattle Times)
When the Snoqualmie River floods in its natural state, it reshapes the landscape around it, forming new habitats for fish and animals, and depositing nutrient-rich soil on the valley floor.
“You can think of a river as the circulatory system of the landscape – the flow of energy and material is organized through the channel system,” Montgomery said.
The ecological importance of a river typically goes far beyond the space it occupies.
“To the things that live in and along the river, the things that come along with the flood are pretty important,” he said.
This is especially true for salmon, which spawn in the lower Snoqualmie River.
For the people who live along the river, the effects of flooding can be devastating. Randazzo twice almost lost her business, and Ekberg suffered thousands of dollars of damage to his home, not to mention the stress that came along with watching water rush into his new home.
They aren’t the first humans to have endured flooding along the Snoqualmie.
Humans have been drawn to the benefits of the Snoqualmie River for thousands of years. Snoqualmie Tribe’s ancestors were hunters and gatherers in the fertile lower Valley. White settlers came for the Valley’s agricultural and timber lands.
Flooding was of little consequence when the population was sparse, said Gardiner Vinnedge, a North Bend historian.
At first, “there just weren’t that many people here to notice it and they didn’t have that much at stake,” Vinnedge said.
Logging led to more flooding and dead trees jammed the Snoqualmie River. As farmers moved in, they began to drain swampy areas and straighten waterways, such as Gardiner Creek, named for Gardiner Vinnedge’s great-grandparents. The creek meanders down from Rattlesnake Mountain and through Forster Woods, but straightens when it hits Meadowbrook Farm. When the settlers straightened it, its water began to move faster.
“You begin messing with the natural patterns and you begin to get changes,” Vinnedge said. “You’d begin to have washouts and silt and more severe flooding, but it would have been very local.”
People began building permanent, more valuable structures they didn’t want flooded. They started bringing in fill, like gravel, and built on top of it. Railroad beds, roads and culverts changed the landscape.
After being hit by bad floods in 1959, levees were built in and around North Bend. The city saw a decline in flooding after that, which prompted further development, including the Factory Stores, the Nintendo warehouse and the Shamrock Park neighborhood.
“(We have) massive new investments, and now they have to be protected,” Vinnedge said.
In King County, more than $7 billion worth of development was built over time in the floodplain, according to a 2007 county estimate.
“We’re in a region that’s growing,” and local, county and state policy is designed to allow development to happen, Loper said.
King County has been changing its development policies to increasingly restrict development in floodplains. To qualify for the federal flood insurance program, municipalities must adhere to FEMA’s strict guidelines for development in floodways – the federally-designated core of a floodplain.
Loper couldn’t say whether or not the county would build the levees in the same way today as they were originally built, but it is in the process of a $5 million levee restoration and improvement project on the South Fork.
While dredging hasn’t been done in upper Snoqualmie Valley for years, the county widened the river’s channel just above the falls in 2004-05, which had a similar affect to dredging. The project was designed to reduce flood elevations upstream in the city of Snoqualmie by 1.5 feet during a 100-year flow.
Flood management often comes down to a cost analysis, according to experts.
The question becomes “how much does society want to subsidize people on a flood plain?” Montgomery said.
Growth has come, in large part, by building on previously undeveloped lands. Between the late 1970s and 2002 in Washington, about 1.2 million acres of forestland, almost all of it privately owned, were converted to other land uses, including development, according to the state Department of Natural Resources. That is a little more than the size of the Olympic and Mt. Rainier National Parks combined.
Each year between the late 1980s and 2004, over 30,000 acres of forestland around Puget Sound were lost, much of it being used for rural-residential and urban development, according to Luke Rogers at the UW College of Forest Resources. That is an area roughly half the size of Seattle.
Development typically means more runoff when it rains. The faster and more runoff there is, the worse flooding will likely be.
“The biggest change is if it gets developed into towns, buildings, streets, which have 100 percent runoff,” said Bower.
However, the amount of development in Snoqualmie Valley likely falls far short of what would be needed to cause a noticeable rise in flood levels, he said.
The Valley’s flood storage capacity could have even possibly increased as former logging areas converted back to forest, he said.
New development in a river’s floodplain must meet strict guidelines before work can begin, but building in uplands can affect flooding, as well.
In King County, all development outside a floodplain must still minimize its effect on changes in runoff characteristics. For large developments, solutions include structures such as storm-water holding ponds.
No engineering solution will be as good as the natural setting, Loper said.
Dan Catchpole: 392-6434, ext. 246, or editor@snovalleystar.com.
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