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Rain potential analysis planned for watershed

The Altoona Water Authority plans to hire a consultant to do an in-depth analysis of the rainfall potential for the Bellwood reservoir watershed in hopes of reducing the cost of a required rehabilitation of the Bellwood Dam.

The $100,000 analysis could save hundreds of thousands – if not millions – of dollars on dam rehabilitation, because it could lead to a reduction in the size of the emergency spillway the authority will need to build, according to Mark Glenn, the authority’s consulting engineer from Gwin Dobson & Foreman.

“You don’t want to underdesign,” said Bill Kappel, chief meteorologist and president of Applied Weather Associates of Monument, Colo., with whom the authority has discussed the analysis. “(But) you don’t want to spend more than you have to.”

Like dam watersheds in most of the Northeast, the Bellwood watershed’s probable maximum precipitation is based on a world-record 1942 storm at Smethport, Pa., which reportedly dumped 30.8 inches in 4 1/2 hours.

A specific study of the Bellwood watershed could result in a different value, officials believe.

The agencies that studied the Smethport storm immediately afterward did a good job, given the available tools, Kappel said.

Clearly “a helluva lot of rain” fell, he said.

But none of the official readings for that storm produced extraordinary numbers, according to a presentation Kappel made to a gathering of Association of Dam Safety Officials in State College in 2009, as documented online.

All the extreme numbers were unofficial readings based on water captured in jars, buckets and other receptacles left outside, according to the presentation.

That casts doubt on the findings. A specific study can use 40 additional years of meteorological experience and more advanced techniques to get a more accurate picture, he said. Typically, those specific studies generate maximum precipitations that are 10 to 30 percent smaller, according to Kappel.

That doesn’t necessarily mean a maximum flood, and therefore spillway sizes are exactly proportional, but those generally follow the pattern, he indicated.

Engineer Dwight Hoare, the manager of the St. Mary’s Area Water Authority and a partner with Kappel on the State College presentation, doesn’t think the specific study of Bellwood’s watershed will convince the Department of Environ-

mental Protection to ease its spillway requirements, even if the numbers say it should.

Hoare has been urging DEP to consider re-evaluating its use of the Smethport storm for eight years, he said.

“I’ve gotten nowhere,” he stated.

Invoking “common sense,” Hoare said he simply doesn’t believe that 30 inches of rain fell in central Pennsylvania in that short a time.

Like a few other states, Pennsylvania needs a full statewide evaluation, so more reasonable results are applicable universally, he believes.

Until then, the requirements will force dam owners like the Altoona Water Authority to invest collective “billions” unnecessarily, he said. Planning estimates for the Bellwood dam rehabilitation – slated for construction in 2017 – are $11 million to $12 million, Glenn said.

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