The flood fight on the Green River is on in earnest.

On Monday, City of Kent crews continued filling 20,000 giant 1.7-ton sandbags that later this week will begin appearing on top of 12 miles of the Green River levees.

The sandbags will give the levee 3 feet more of freeboard in the event of a major storm and flood and may reduce levee damage and flooding in the Green River Valley.

The Kent crews started last Friday night and plan to have the big bags in place by Nov. 1. The river cities of Auburn, Renton and Tukwila, as well as King County, are planning similar protective measures.

Nov. 1 is the start of the traditional flood season on Western Washington rivers.

Meanwhile, Col. Anthony Wright, local commander of the U.S. Army Corps of Engineers, told a King County committee Monday that the Corps plans to have interim repairs to Howard Hanson Dam done by Nov. 1.

He estimated the Corps’ work at the dam, combined with local levee-raising efforts, will reduce the risk of a catastrophic flood this fall and winter anywhere by from 25 percent to 30 percent.

Current repairs to the weakened dam are only temporary, he said. Permanent repairs might take five years, meaning the flood danger to the Green River Valley is far from over.

Following last January’s major flooding, increased seepage found flowing through the right abutment at the dam meant it could no longer hold back as much water as it has for the past 47 years.

As a result, the Corps might have to release more water during a flood event, which could inundate portions of the Green River Valley, creating a lake 35 miles long, a mile wide and 3 to 12 feet deep.

The stakes are high. There are thousands of residents and hundreds of businesses in the potential flood zone. Major infrastructure such as highways, water and sewer lines as well as services dot the valley floor. The Green River Valley also is one of the West Coast’s major distribution centers.

The temporary repair involves pouring a grout curtain 160 feet deep along a portion of the abutment where depressions occurred last January. The permanent repair now being designed is a concrete wall similar to the dam itself along the length of the abutment.

On Monday, the Corps also delivered some 750,000 sandbags to the area, including about 750 super bags like those being readied in Kent.

The City of Kent is spending $1.5 million to sandbag 12 miles of the city side of the Green River.

The sun was shining Monday on one of two city crews that are each working 10-hour shifts seven days a week to fill the bags.

They were staged off Military Road at about South 231st Way and South Riverview Boulevard on the west hill above the Green River. The only water present was in jugs to keep crew members hydrated.

Every minute or so, they filled one of the giant white sandbags with 3,365 pounds of dirt. Forklifts fed a sandbag filling machine and then stacked the bags along the hillside.

Tim LaPorte, deputy public works director for Kent, said they will begin transporting the big bags to the levee by the end of this week or at the beginning of next week. The city was sorting bids Monday from companies to do the work, he said.

The bags will be placed down the middle of the Green River Trail, which runs on top of the river levee.

Michelle Witham, Kent city spokeswoman, said the city will have to close the popular north-south trail while the sandbags are in place.

She said the city will use another 20,000 small sandbags to secure a black vinyl covering placed over the big bags. The black wrap will protect the bags from rain and ultraviolet rays that can break down the vinyl, she said.

LaPorte said the levees in Kent are not designed to allow water to flow over them. If a catastrophic flood did overtop the levees, the water could damage the levees or cause them to fail.

The giant sandbags hopefully will prevent overtopping, he said.

Some of the giant sandbags were being hauled Monday and placed around a sewage pump along the west side of the Green River.

Mike Archbold: 253-597-8692

mike.archbold@thenewstribune.com