Flood control sometimes a dam difficult decision

November is historically the wettest month of the year in the Seattle area and that means flooding could be just days or weeks away. We can’t control the weather but we do control one factor that could be the difference between a bad flood and a disaster.

Multiple dams are located strategically along rivers that drain to the Puget Sound basin. They’re used to store drinking water, generate electricity and for flood control. The challenge is deciding when to hold back water and when to open the flood gates. It’s a dam regulator’s paradox.

“The strategy is to store the peak of the flood, not the entire flood,” said meteorologist Larry Schick. “You shave off that peak, store it behind the dam during the flood and then you release it in a safe and prudent way after the flood.”

There’s a risk to holding back too much water in an effort to protect people and property downstream from a catastrophic flood.

“It takes instead of one week, for instance, to let all that water go out, it will take two weeks and that leaves the whole system a little more vulnerable, a little more (at risk of) a subsequent storm that would have less capacity behind the dam,” Schick explained.

An example of the paradox, in November 2006, on the Skagit River, there was a forecast that rain and runoff would double the capacity of Upper Baker Dam and officials were prepared to release water, which would have created a severe flood downstream if that forecast had panned out. Luckily, said Schick, the storm nudged south, easing the stress on the Skagit system and dam regulators were able to minimize flood impact.

The winter weather outlook for the Pacific Northwest calls for an El Nino, which typically means less of a flood risk. But National Weather Service hydrologist Brent Bower says the El Nino winter of ’06, including November, was among our worst in recent memory.

“We had some record flooding in that year and in fact, November is our wettest month.”

This week, the Executive Board of the King County Flood Control District approved a proposed 2015 budget of $55.6 million, which includes money for several flood reduction projects. They include gravel removal in the Cedar River, levee improvements along the Green River and programs to elevate homes and build livestock pads in the Snoqualmie River basin.

While we’re talking disaster preparation and prevention, based on rainfall totals an official government threshold is about to be met in Seattle, meaning the start of landslide season.

“Despite the fact that we have yet to experience widespread flooding or soil saturation this season, October is already much wetter than normal, so it shouldn’t be much of a surprise that landslide season is here,” said Seattle Public Utilities meteorologist James Rufo-Hill in a news release.

Neighborhood rain gauges have recorded almost four inches of rain over the past two weeks, which the utility says is certain to push Seattle over the landslide threshold, which has been developed by the city and the U.S. Geological Survey.