TROY -- Thomas Zimmie has observed up close the destructive power of raging floodwaters during four decades as a dam safety expert and professor of civil engineering at Rensselaer Polytechnic Institute.

But Tropical Storm Irene had Zimmie shaking his head in disbelief and reaching for superlatives.

"This could be one of the worst ever and let's hope none of us will ever experience anything like it again in our lifetimes," said Zimmie, who has been involved in analyzing storm events that contributed to the 1977 Green Island Bridge collapse, 1987 Schoharie Creek Bridge collapse and levee failures in New Orleans after Hurricane Katrina.

By the time the numbers have been crunched, Zimmie wouldn't be surprised if the ferocity of the Aug. 28 deluge that led to historic losses and catastrophic damage across communities near streams and rivers stretching from the Adirondacks to the Catskills eclipsed a so-called 100-year flood -- meaning a flood that has a 1 percent chance of being equaled or exceeded in any single year.

"It might even turn out to be a 500-year-flood. It was that big and rare," he said.

Zimmie attributed the destruction to several factors:

A ferocious pace of rainfall. A heavy rainstorm in the Capital Region might produce 2 inches of rain in a 24-hour period, but Irene was dumping 2 inches per hour for several hours in the hardest-hit areas, with totals of 5 to 8 inches or more.

Soils already saturated by a stretch of heavy rain before Tropical Storm Irene could no longer absorb or drain the excess water, leaving soils weakened and less rigid. That caused riverbanks and hillsides to give way, triggering mudslides that swept away houses, bridges, roadways and anything in its path.

The cumulative weight of all that excess water created a lethal force. One gallon of water weighs 8.3 pounds. Multiply that by millions of gallons. Then move those thousands of tons at 106,000 cubic feet per second down the Mohawk River, for instance, more than 10 times faster than a normal flow.

Some of the watershed areas in devastated communities covered several square miles and fed into tiny streams just a few feet across. Flash floods quickly overwhelmed shallow stream beds, rendering a torrent of floodwater into a missile of mass destruction. As an analogy, think of your house's roof during a heavy rainstorm, with thousands of gallons of rainwater feeding into a four-inch drain spout that becomes an explosive geyser.

"Six inches of water is a heck of a lot and it's extremely heavy and then you factor in the topography," Zimmie said. He noted that an acre is more than 40,000 square feet, approaching 200,000 gallons of water at six inches of depth. A watershed consisting of hundreds of acres emptied into a tiny stream, produces enormous force. That helps explain why floods can be even more destructive than earthquakes.

Zimmie said the force of floodwaters that brought down the Green Island Bridge on March 15, 1977, and the Schoharie Creek Bridge over the Thruway on April 15, 1987, killing 10 motorists, was due to an effect known as scour. It's something that occurs when turbulent floodwaters scour a hole in a river bed beneath a supporting pier, causing it to fail. Scour will likely turn out to be the cause in Irene-related bridge collapses, he said.

"We know a lot about scour, but we don't know everything," said Zimmie, who studies scour using a centrifuge model at RPI. He's also a member of the National Cooperative Highway Research Program that is researching bridge scour.

The Green Island Bridge and Schoharie Creek Bridge were rebuilt stronger and more scour-proof due to a better understanding of the dynamics of scour and improved technology in design and construction.

"The good news is that our profession learned from past mistakes and we've rebuilt bridges better and stronger," he said. "I think Green Island and Schoharie are both pretty much bomb-proof now. I think it was wise to close them down and inspect them after Irene, but I don't think those bridges will have any problems."

Also, despite record flows over the top of Gilboa Dam in Schoharie County and false reports that it had been breached, the Gilboa Dam in Schoharie County did not fail. Zimmie believes it held because of emergency shoring up in 2005-2006 that included installing 80 massive steel cables across the bulk of the dam and into the bedrock below to allow it to withstand extreme water pressure. Also, crest gates were added at the top of the dam and four large siphons allowed more water to be drained quickly during Irene's torrents.

"That shoring up was a good move and the Gilboa Dam is pretty safe now," he said. "I'm sure it could take several more feet over the top and be OK. Let's hope we don't have to test that. Don't get me wrong, this was very bad. But it could have been much worse."

Reach Grondahl at 454-5623 or pgrondahl@timesunion.com.