HOWARD HANSON DAM, Wash. - When Central Washington University first approached the U.S. Army Corps of Engineers about hooking up a Global Positioning System antenna to the area surrounding the Howard Hanson Dam, the idea was to add another node to their network that monitors stress in the continent's tectonic plate in an effort to anticipate earthquakes.
The Army Corps then asked CWU if they would wire up the whole dam to keep a very close eye on any changes in the dam.
"All dams move," says Ron Burkhard, the Corps operations manager for Howard Hanson and Mud Mountain dams. Those are the two earthen flood control dams located in the Cascade mountains that provide flood protection to people living in the Green and White River valleys below.
He says even big concrete dams in Eastern Washington can move several inches as they expand and contract between hot summer days and frigid winter nights. And for many of those same reasons, along with soil conditions and water pressure, Howard Hanson also moves.
Burkhard says the GPS array is all about making the dam safer.
It's not movement, but the type of movement that engineers are looking for, particularly after an earthquake. GPS would give engineers that information instantly. A particular kind of movement could signal the opening of cracks in the dam or settling in the dam. The GPS can monitor those movements in terms of millimeters.
But it's not just earthquakes. CWU and the Army Corps says the GPS system will alert them to any shift in the dam for any reason.
Rex Flake, the field engineer with CWU who is leading the installation of the system, says it's the densest GPS array he's seen. There are 13 antennas, most of them mounted to the dam. Several are attached to the natural abutment that ties into the dam.
It was in that abutment where the Corps found excessive leaking more than a year ago. That forced the Corps to limit how much water the dam can hold during the worst winter storms, opening valley communities like Kent, Auburn, Renton and Tukwila up to the potential of flooding for the first time since about 1960. Over the winter, contractors pumped a million gallons of slurry-like grout into the underground voids in the abutment to dramatically slow down leaking.
Since the so-called "grout curtain" was installed the Corps says the odds of a major flood have dropped from about 1-in-4 to about 1-in-25. In a typical year, the odds are 1-in-140.
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