Official: Green River flooding would impact Issaquah, region

October 29, 2009

By Warren Kagarise

NEW — 3:27 p.m. Oct. 29, 2009

If the Green River swells from fall and winter rains, flooding could snarl traffic for Issaquah commuters, disrupt deliveries of food and fuel, and — a more remote possibility — cause local sewers to back up as floodwaters overwhelm the regional system.

Though the river winds through Auburn, Kent and Tukwila, the human and economic toll from flooding could reach Issaquah, emergency planners told City Council members Tuesday night. A Tukwila emergency planner offered a frank assessment of the potential impact of Green River floods.

Authorities expect the Green will flood because the U.S. Army Corps of Engineers will allow water to flow through the Howard Hanson Dam. The earthen abutment adjacent to the dam was weakened by severe weather last winter, and engineers worry the structure could fail if rain swelled the reservoir behind the dam.

“If that erosion is allowed to continue, the river will create a new channel and it will basically leave the engineered structure of the dam useless,” Tukwila Emergency Management Coordinator Hillman Mitchell said. “It will be an interesting sculpture up in the Cascades.”

If the Green floods, closed roads — combined with the influx of evacuees — could clog Interstate 90, Issaquah-Hobart Road and state Route 900. Debris could destroy and damage bridges spanning the Green River, and interrupt utilities; gas, power and water lines are hung beneath bridge spans.

“We could have water eight miles wide and several feet deep” in the river valley, Mitchell told the City Council. “Within this corridor, we have enormous amount of critical infrastructure.”

Authorities are also concerned about fuel disruptions because floodwaters could damage a key pump station.

The economic toll of Green River flooding could reach beyond South King County. The warehouse district in Auburn and Kent is the second-largest warehouse district on the West Coast, Mitchell said.

“Virtually all of our food and our medicine and other supplies come through those warehouses,” he said. “That is the junction for most of the supplies that are going outbound to the Pacific Rim and coming from the Pacific Rim to the United States.”

Though flooding could cause problems with sewage systems, authorities said the risk to Issaquah customers is small. Issaquah, like other Eastside and South King County cities, sends sewage to the South Treatment Plant in Renton for treatment. Mitchell said the danger to the plant is minimal, but floodwaters could overwhelm the main pipe carrying wastewater to the facility. He said authorities would likely release sewage into lakes to relieve pressure on the system and prevent backups.

Bret Heath, director of the Issaquah Public Works Operations Department, said the possibility of sewer backups is remote. Heath doubles as the city’s emergency management director.

Flooding could also interrupt telecommunications, because cities in the river valley are the site of several switching facilities.

“If those systems are knocked out, they can have far-reaching, cascading effects,” Mitchell said. “Just so you know, the power grid and the substations are controlled via telecommunications lines. So, if we lose power, we could lose telecommunications, because they need power. If we lose telecommunications, we may not be able to control our power systems and shut those off appropriately.”

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