People in Hunters may not need an ark for the next “hundred-year storm” after all.
Public officials are planning to put a giant drain through an abandoned 74-year-old earthen dam that has been declared the most dangerous dam in the state. A 6-foot-diameter pipe will let water pass through the dam before it can spill over the top and cause the structure to collapse from erosion.
State Ecology Department experts say failure of the dam 2 1/2 miles east of Hunters could send a 6- to 7-foot-tall wave of water crashing through the town of 200 people. Hunters is at the mouth of a narrow canyon that opens onto Lake Roosevelt.
The 65-foot-high dam on Hunters Creek was built in 1921 by the Hunters Fruit Co. to irrigate apple orchards on the Columbia River benchland on the other side of town. A three-mile-long wooden flume carried water to the orchards.
But the flume is gone and a 100-foot-long wooden spillway is badly dilapidated. Ecology Department engineers say the spillway would collapse if the reservoir overflowed, and the subsequent erosion would wash out the dam.
Many people doubt a storm would fill the reservoir. The last time that happened was in January 1974, when the small existing drain got clogged. A debris trap has been installed since then and Stevens County officials keep a closer eye on the dam.
Even if the drain got plugged again, state Transportation Department engineers calculated that only 20 feet of water would collect in the reservoir during the so-called “hundred-year storm” - the worst storm likely to occur over the course of a century.
Ecology Department dam safety engineer Bruce Barker said the Transportation Department based its study on a 24-hour summer thunderstorm while the Ecology Department used a 72-hour winter rainstorm. He said the Ecology model more closely matches the 1974 storm, in which rainfall was compounded by a chinook wind that melted the snow pack.
The reservoir has been mostly empty since it was abandoned in 1957. The dam’s only purpose has been to hold up the Springdale-Hunters county road that passes across the top.
Because of that, the department ordered Stevens County to take corrective action last year even though the dam is owned by the Dale Ranch. The Dale family was ordered to fix the dam by Dec. 15 last year if the county failed.
Unimpressed with the state agency’s assessment of the risks, county commissioners appealed to the state Pollution Control Hearings Board. A hearing that had been scheduled this month was tabled after Ecology officials offered a $150,000 grant from the state Flood Control Assistance Account.
“We’ve always argued that it’s not really a problem, but if they will give us some money, we’ll fix it,” County Engineer Jerry Bryant said. “But, until now, our grants have all been turned down.”
The county had sought $516,000 to bore a bigger drain through the bottom of the dam.
Now, though, officials plan to place a new drain 22 feet above the old one. The project is estimated to cost only $166,000 because the pipe can be placed by digging a trench from the top of the dam.
The county has agreed to put up $16,000, and the Ecology Department has agreed to relinquish jurisdiction when the work is completed.
Bryant said officials plan to seek bids in May and have the four- to six-week project completed this summer. He said the road probably will be closed for four weeks, causing a 24-mile detour over the Addy-Cedonia Road.
The only alternatives, he said, are poorly maintained dirt roads such as Locke Road, which would shorten the detour to 12 miles.