Even after contractors complete interim repairs to the federally owned and operated Howard Hanson Dam, there is still a “1 in 4 chance” that water may need to be released, causing the Green River to overtop its banks and flood the valley.

That’s what Col Anthony Wright, district commander of the U.S. Army Corps of Engineers, told a committee of the King County Council Monday in Seattle.

Wright told the Council’s Committee of the Whole that he expects contractors to wrap up interim repairs to the seeping right abutment of the dam and test them by Nov. 1. Combined with work underway to raise the heights of the Green River levees, Wright estimated the risk of catastrophic flooding will be reduced from one-in-three to one-in-four.

While his goal is to minimize the risk, Wright stressed that those in the potential flood zone need to make the necessary preparations.

“Water will go were it wants to,” said Wright. “We can do some things to adjust that, but people looking at maps thinking they are dry should not use that as a reason not to get flood insurance or take appropriate precautions.”

After the Army Corps discovered excessive amounts of water seeping through the earthen abutment, the product of 20,000-year-old landslide, abutting the dam after last January’s record rainfall, the Corps hired a contractor to inject grout into the earthen material in an effort to prevent a washout that could threaten the integrity of the dam. The primary holes have been drilled, and the drilling of secondary holes is almost complete. When the so-called “grout curtain” or seepage barrier finished, engineers will run tests.

Wright stressed that even with positive test results, he cannot risk filling the reservoir behind the dam to capacity this winter because the grout curtain will reduce but not completely stop the seepage.

The Corps is working with the King County Flood Control District to ensure that levees along the Green River are upgraded to federal standards, mostly by removing vegetation from around the tops of the levees so that federal inspectors can check for damage. Those upgrades will make the levees eligible for federal rehabilitation and restoration funding if they sustain damage in a flood.

“If the levee is damaged while it is in an ineligible status, I cannot support the repair with federal funds,” said Wright. “For post-flood repairs, it has to be in the program in order for me to federally cost-share it. Our inspectors are ready to go out when the upgrades are complete.”

Wright praised the cooperation of King County, the Flood District and the local cities for their preparations with the public, calling it a “textbook example of all echelons of government working very closely together.” He said 750,000 sandbags were being delivered for distribution today, with 750 of those sandbags being “supersacks” or doorway-sized plastic and fiber bags filled with rock and gravel.

Col. Wright said the sandbags will help prevent the overtopping of levees, but he cautioned communities against getting into what he called “levee wars,” the building of levee protections higher than those erected in other areas.

“This creates the risk of damaging the levees under their own weight” and causing them to collapse, Wright said. “Building the system up to capacity is an appropriate balance to prevent overtopping while still not placing too high of an over-burden.”

Wright acknowledged that the grout curtain is a temporary repair, and that the best-case scenario for finishing a permanent repair — such as a concrete cut-off wall which could be placed along the earthen abutment — could be three years off. He said that length of time includes designing the repair, acquiring the federal funding and actual construction time.

Wright is the federal official responsible for making the final decision on whether to release water from the dam, or risk the catastrophic failure of the dam due to excessive pressure on the weakened structure.

“Col. Wright made it clear that we need to be prepared for an increased risk of flooding for the next five years,” said Councilmember and committee chairman Bob Ferguson. “The Council is united in its willingness to work with the federal government and other jurisdictions to make sure everything is being done to be prepared.”

“I want to thank Col. Wright and the Army Corps of Engineers for their incredible work,” said Councilmember Julia Patterson, chair of the King County Flood Control District. “When the Council first learned about the problem, we wanted to make sure the federal government, including the Army Corps, was doing everything it could to fix the problem. We are now assured beyond a doubt that this is happening.”

Further briefings flood threat will be heard in the Committee of the Whole. The County is also hosting a series of public meetings on the issue.