To protect public health and safety in the event of major flooding related to Howard Hanson Dam, King County employees will install and test flood walls at 12:30 p.m. Thursday at the South Treatment Plant in Renton.

The project is part of the county’s work to actively prepare for flooding and safeguard its vital regional wastewater infrastructure in the Green River Valley.

In addition to operating the South Treatment Plant in Renton – which treats about 110 million gallons of wastewater each day for east and south King County – the county’s Wastewater Treatment Division also operates the Interurban Pump Station in Tukwila and a stormwater pumping station on the Black River.

Division employees recently began clearing and grading the land around the facilities to make room for large inflatable berms called flood walls that can be quickly deployed to redirect flood water away from critical pumping and electrical equipment to protect it from damage.

Plans to maintain power at the sites are also in the works. South Treatment Plant currently has dual power feeds and onsite power generation capabilities to keep equipment running, but concerns about the possibility of losing both utility feeds has prompted the county to secure emergency generators and diesel fuel that could provide up to 18 megawatts of back up power if needed. Emergency generators will be in place to provide back up power at the pump stations.

“Even in worst-case scenario flooding, we don’t plan to evacuate South Plant,” said Christie True, division director of the Wastewater Treatment Division. “The preparations we’re making now will enable us to keep operating and processing as much flood water and wastewater as our facilities can handle.”

“We expect South Plant to treat about 340 million gallons of flood water and wastewater each day for up to 10 straight days,” said True. “That’s the equivalent of running nearly 490 Olympic-sized swimming pools through the facilities every day.”

True said that in addition to keeping the plant operating, the planning measures being undertaken now will help prevent serious damage to facilities that could leave the area without adequate sewage treatment for weeks or even months, putting human and environmental health in serious jeopardy.

The county is budgeting up to $7.5 million for preparation expenditures to protect the infrastructure, which has an estimated value of more than $1 billion. The county will pursue reimbursement for its planning investments from the federal government.

Another critical component for continued plant operation is staffing. The county is making provisions to house about 20 workers at the treatment plant for up to seven days after a flood, ferrying in relief crews and additional supplies by boat if necessary.

Flood waters likely will have the greatest impact on the local and regional conveyance lines that carry wastewater from homes and businesses to South Treatment Plant in Renton. Inflow from the flood water could overwhelm the capacity of the local and regional sewer lines, potentially forcing wastewater to back up into homes and businesses.

“The local and regional wastewater system was not designed to handle flood water,” said True. “The surging lines and associated backups would be a result of a natural disaster – the flood – and not a result of a malfunction of the sewer system.”

Local system backups could occur even outside flooded areas. Toilets and drains might not work, and home plumbing may be unusable for several days. The Wastewater Treatment Division is working closely with Public Health – Seattle & King County to ensure people are prepared to manage their own hygiene and sanitation needs in the event home plumbing systems aren’t functioning. Additional information is available at www.kingcounty.gov/safety/FloodPlan/GreenRiverValley.aspx.

Depending on the severity of impacts to the plant, the county also could issue a general curtailment advisory throughout much of its service area asking people to limit use of the sewer system. However, the county will not tell people with functional plumbing to stop flushing their toilets.

“The situation we’re facing is unprecedented,” said True. “There are a lot of unknowns about the impact of flooding on our system, but our employees will be working around the clock to prepare for major flooding and to respond if it occurs so we can protect public health and the environment.”

Additional information about flood preparedness is available on the Web at www.kingcounty.gov/floodplans.