By Enrique Pérez de la Rosa
WNPA Olympia Bureau
After the Oroville Dam crisis in California in February last month forced nearly 200,000 people to evacuate their California homes, Washingtonians may wonder if the dams that surround them are in danger of failing.
The good news is that dams such as Alder Dam and La Grande Dam in south Pierce County are structurally sound. The bad news is that a major earthquake could change that.
Dams in all 39 Washington counties are regulated and inspected either by the state Department of Ecology or by federal dam safety engineers.
In January 2009, a leak at the Howard A. Hanson Dam on the Green River threatened the downstream communities of Kent, Auburn, Tukwila and Renton. At that time, the U.S. Army Corps of Engineers, which is responsible for the dam's inspection and maintenance, didn't believe the structure was failing. Although danger to residents in the Green River Valley increased, there were no evacuations.
While the circumstances that led up to California's Oroville Dam crisis were unique, the likelihood of dam failures in the United States is very high, said Mark Ogden, technical specialist at the Association of State Dam Safety Officials in Lexington, Kentucky. Dams in the U.S. fail every year, though not as dramatically as Oroville, he stated.
Heavy rain that followed a long California drought raised the water in the Oroville Dam's reservoir to dangerous levels, and dam operators were forced to release water into the Feather River through damaged spillways. Repairs on the Oroville Dam have already begun, but the crisis prompted officials across the country to review the risks of dam failure and evacuation plans.
“Maybe that's the type of situation that will be more common with climate change,” Ogden said.
Dam engineers need to know how much water intake to expect every year, Ogden said. Using rainfall data collected over decades, engineers can build a dam to withstand the highest probable flood.
Changes in the climate, such as the amount of water from snowmelt to rainfall, could make designs inadequate in the future. Snowmelt runoff is slow, steady and predictable, while intake from rainfall is sudden and quick, Ogden explained.
“You have a much higher peak flow, so you need to be able to design for that, plan for that,” Ogden said.
Guy Hoyle Dodson, a dam safety engineer at the Washington Department of Ecology (DOE), said proper design is the first line of defense, and protecting the public from disasters is a priority for the state.
“Safety is our major concern,” Hoyle Dodson said. “We've been extremely successful in preventing catastrophic dam breaches.”
Of the 1,189 dams in Washington, 1,055 are regulated by the DOE, while another 15 fall under U.S. Army Corps of Engineers jurisdiction. There are 134 dams exempt from DOE control. About 10 to 15 new dams are constructed every year, according to DOE.
Most dams in the state qwew built after 1950, but the oldest dam in operation – Chelan County's Clear Lake Dam on the Chelan River – was built in 1888.
All 39 counties in the state have at least one dam. There are 56 in Pierce County, the fifth-highest number among counties statewide. King County has 126, the most.
Dams in the state receive a downstream hazard classification, a rating used to describe the potential loss of human life or property damage if a particular dam were to fail. About 37 percent of the dams under DOE jurisdiction are located above populated areas and are therefore classified as having high or significant downstream hazards.
Some of the highest-risk ratings belong to the two major dams in south Pierce County. According to a report published in 2011 by DOE on all dams in Washington, Alder Dam is 1A (more than 300 lives at risk), while La Grande Dam is 1B (from 31 to 100 lives). Any risk of seven lives or more is considered high risk by DOE.
Alder Dam, operated by Tacoma Public Utilities (TPU), was built in 1945 on the Nisqually River for $14 million. It stands 330 feet high, is 1,600 feet long, and can back up 241,000-plus surface acres of water in the form of Alder Lake, which is seven miles long. The concrete single-arch construction has hydroelectricity facilities that generate 228 million kilowatt-hours per year, serving the equivalent of 16,000 homes.
La Grande Dam is also on the Nisqually River (two miles downriver from Alder Dam) and operated by TPU. Built from concrete in 1945, it's 217 feet high, can store 3,015 surface acres of water, and generates enough electricity for about 25,000 homes each year.
Inspections are vital because repairing a dam is not a simple operation, Tacoma Power (TPU) generation manager Pat McCarty stated.
“It's not like fixing potholes in the road,” McCarty said. “When you overtop dams, good things do not happen.”
Tacoma Power has proposed recently to draw down the level of Riffe Lake at the Mossyrock Dam on the Cowlitz River in Lewis County by 30 feet each summer. The proposal came in response to updated seismic data that indicated the dam's spillway piers might be at risk of failure in a large earthquake.
In a public meeting March 3 at Mossyrock regarding the proposal, McCarty noted that a highly unlikely “intraslab” earthquake of magnitude 7.5 or greater in the vicinity of the Mossyrock Dam would create serious issues.
“But it's all in the name of public safety,” McCarty added, referring to the information at hand being available to the public.
DOE engineers perform inspections on each high-hazard dam every five years. This includes a detailed inspection of critical features like spillways, as well as an engineering analysis of the dam under extreme flood and earthquake load, Hoyle Dodson stated. During inspections, engineers look for deficiencies such as cracking in the concrete, sloping, and even animal burrows.
Low-hazard dams are inspected every 10 years by DOE engineers. Eighty inspections are planned this year, as well as some minor maintenance work, Hoyle Dodson said.
DOE also requires the owners of high hazard dams, like Tacoma Power and Seattle City Light, to perform their own inspections annually and to file an inspection form with the Department of Ecology's Dam Safety Office.
Residents who live downstream from a dam should be aware of emergency evacuation plans and emergency alert systems in their counties, Hoyle Dodson added.
“Be aware if you live below a dam,” he said. “Be prepared to leave if there is a problem.”
The Dispatch contributed to this report, which is part of coverage of state issues and the Legislature through a reporting internship sponsored by the Washington Newspaper Publishers Association Foundation.
Comments
Use the comment form below to begin a discussion about this content.
Sign in to comment