Wednesday, June 14, 2006

Flood volume highest ever

Big Wood flows far exceeded any previously recorded volume

Tom Downey walks Sunday with his dog, Fish, in the recently flooded Della View neighborhood in Hailey. Local emergency officials are now saying that the 2006 flooding event on the Big Wood River was the largest ever in terms of the river?s overall flow. Photo by David N. Seelig

Based on river data obtained over the past century, this year's flood event in the Wood River Valley can be considered the largest ever recorded in terms of the overall volume of water that flowed down the Big Wood River.

Measured at the Bullion Street Bridge in Hailey at its peak Sunday, May 21, the Big Wood River reached a height of 7.92 feet and was flowing at a whopping 7,800 cubic feet per second. Official flood stage on the Big Wood River is 6 feet as measured at the Bullion Street Bridge.

Compared to this year's flow, the next most significant measurement was taken on May 30, 1983. That measurement placed the Big Wood River's flow at 7.93 feet, one-hundredth of an inch above this year's height, but with an overall flow significantly less at 6,150 cubic feet per second.

The reason for the discrepancy, Blaine County disaster services coordinator Chuck Turner explains, likely has to do with the higher rate in which the Big Wood was flowing this May as compared to flows in 1983. "It was moving faster," Turner said. "That's probably why."

Comparing higher river crests measured in the decades before the 1983 flooding event is even more difficult, he said.

As for those historical river crests, Turner said the biggest difference in the records is in how and where the river was measured. The current Bullion Street Bridge with its associated river gauge was built prior to the 1983 flooding, but well after notable flooding events in 1921, 1958, 1965 and 1967, he said.

"It's apples and oranges," Turner said.

During the flooding in 1921, the river height on the Big Wood was measured at an all-time high of 10.66 feet. After seeing this year's flows, Turner questions the accuracy of those figures. "That's almost hard to believe," he said.

For the 1921 figures to have been properly measured and to be compared against more recent figures, the 1921-era gauge would have had to been placed at the exact same height and location as the current gauge, Turner said.

Barring any unforeseen circumstances that may arise in the coming days and weeks, the Wood River Valley has likely seen the last of the year's flooding, he said.

The Big Wood should continue to rise and fall between 4.5 and 5 feet for the rest of the week, after which it will begin a steady decline, Turner said. "The experts say we're past the really high water."

In Hailey, the city's public works manager, Ray Hyde, also believes the area has seen the last major flooding for the year.

Last week's rise in levels on the Big Wood River—its second peak—have already begun to subside, Hyde said. For the second time in a month, the Big Wood rose above the official flood stage of 6 feet on June 8 and hit a peak height of 6.24 on June 9.

The Big Wood River was flowing at 5.39 feet at 7 a.m. Tuesday.

While certain roads in Hailey have experienced some damage due to the two flooding events, the extent of the damage won't be fully known until river levels drop some more, Hyde said.

At Lawrence Heagle City Park in Hailey, the city still has a certain amount of post-flood cleanup to contend with, he said.

Still, things are looking positive, Hyde said. "Overall we're getting there. There's no longer water over the roads. We no longer have the (emergency) signs out."

To the south of the Wood River Valley, areas below Magic Reservoir on the Big Wood River experienced two flooding events that lasted nearly a full month combined, Gooding County Sheriff Shaun Gough said. During those flooding events, flows coming into the reservoir exceeded the overall outflows and caused water to pour over the spillway at the Magic Reservoir dam.

The first flood episode took place in April, Gough said, and was due to snowmelt from the Camas Creek drainage, northwest of the reservoir. The second took place in May and was due to snowmelt from the Big Wood River drainage, he said.

While certain areas of Gooding County experienced some flooding due to the high flows, the area experienced very little property damage, Gough said Tuesday. "We didn't lose any houses. Only a few outbuildings got flooded."

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