Southwest Renton’s commercial and industrial area is most at risk if the worst happens – a massive flood of the Green River caused by a weakened Howard Hanson Dam.

But the worst-case scenario only stands a 1 to 5 percent chance of occurring, based on modeling done by City of Renton.

It’s unlikely that any of Renton’s residential areas on the Valley floor would feel a flood’s impact – unless the worst happens.

However, city officials are continuing to plan – and perhaps spend up to $800,000 – to prepare for what Mother Nature and the dam throws at Renton over the next several years.

The city is asking that businesses and residents plan, too.

Businesses need to figure out how to continue their operations, elsewhere if necessary, and protect their equipment and material.

Residents need to follow the long-preached emergency guidelines – enough food, water, medication, etc., for three days, if not more.

The Renton City Council received a 90-minute briefing Monday night on the work and planning under way to protect the city in the event of a major flood.

The presentation was made by Gregg Zimmerman, the city’s public works director, and Ron Straka, the city’s utility engineering supervisor.

Jay Covington, the city’s chief administrative office, told the council and those in the audience that Renton is “very well protected,” short of a severe flood. And, with all the current preparations, the city is in a much better situation to handle floods in the future, he said.

Other cities in the Green River Valley, including Tukwila, Kent and Auburn, are doing the same, because last January’s storms damaged the right abutment of the dam, but not the dam itself.

The dam’s operators, the U.S. Army Corps of Engineers, is working around the clock to fix the leaky abutment. But the corps has warned the cities downriver that it could cause flooding on the Green because it can’t store as much water as normal in the reservoir due to the damaged abutment.

But, because of the dam repairs and efforts to raise height of the river’s levees, the risk of catastrophic flooding is now one-in-four chance, rather than what the corps previously estimated at a one-in-three chance, according to the corps.

Straka took the City Council through a detailed explanation of dam operations. The river’s flow is measured in cubic feet per second, or cfs. The river’s level for the Green River Valley is measured at a gage in Auburn.

Today, the corps maintains a maximum flow of about 12,000 cfs on the Green to avoid most flooding. That level is only reached during major storms; however, the city has heard the corps may keep the river that high during fair weather just to keep more storage capacity behind the dam.

Straka presented three maps to the council that outlined what would happen at progressively higher flows of the Green River.

• At a flow of 13,900 cfs, no flooding would occur in Renton. There’s a 10 to 25 percent chance of such a flow occurring.
• A flow of 17,600 cfs is considered a 100-year flood, which could result in water depths of 1 to 3 feet in the area around Southwest 27th Street. There’s a 5 to 10 percent change that flow will occur.
• A flow of 25,000 cfs would create a “serious problem,” resulting in water depths of 6 to 10 feet on the Valley floor. That would threaten such sensitive facilities as the Olympic Pipeline, the Black River Pump Station and Burlington Northern Santa Fe Railroad. There’s a 1 to 5 percent chance such a flow would occur.

Potentially, flood waters could reach south Renton neighborhoods in a catastrophic event and some sewer backups could occur on the Valley but are much less likely on the hillsides above.

Officials stress that the flow – and the potential damage – are basically a snapshot in time of a flood. Different impacts would occur at different flood levels.

Another variable is that the river’s flow is affected, sometimes significantly, by the amount of water from streams and runoff below the dam. And if a major storm has sent the Green River over its banks, that same storm could open up another disaster front for the city – the Cedar River.

Another concern, even with a relatively small flood, is the loss of power. Puget Sound Energy officials have said the utility will shut off power if there is standing water, according to Straka and Zimmerman.

PSE has told the city it will attempt to keep any outages within the flooded areas, but it can’t make promises, Zimmerman said.

In any scenario, businesses and residents will have time to decide whether it’s time to seek higher ground. Flood waters would reach Renton about 12 hours after being released from the dam.

The Valley cities are working one a single standard when to order an evacuation, in order to avoid confusion, said Zimmerman. Police would oversee the evacuation, basically securing the perimeter around the flooded areas.

Costs just to prepare for the flood and protect key facilities could run between $400,000 and $800,000. The city would not receive federal reimbursement because a disaster has not been declared.

The city is in line to receive 500,000 sandbags and 10,000 supersized sandbags from the Corps of Engineers. The city is talking with property owners about leasing space to fill those sandbags and to store debris until it can be hauled away.

The city also is considering purchasing a $26,000 sandbag-filling machine. It also has to buy its own sand. The sandbags would be available to the general public.