High hazard dams

After the 1983 floods, officials took several steps to prevent further problems at Mountain Dell. The biggest was building the modern Little Dell Dam just upstream in 1993. After it was completed, Mountain Dell was never allowed to fill to more than two-thirds its capacity.

Also, Hooton said numerous improvements were completed at Mountain Dell, including raising its height and performing studies to ensure it should be safe during an earthquake. "The improvements have really taken the worry out of running that dam," Hooton said.

Surprise inside

Marble said repair work has often brought unhappy surprises once crews dug into dams for repairs.

For example, at Rocky Ford Dam, he said, officials worried that extra seepage they had observed could eventually lead to erosion and create sinkholes that could break it apart. Once they started digging to put in some drains, "We found a very large sinkhole already developing. . . . We worried it could happen and found it already had."

Marble said similar findings came in repairs at many dams. For example, at Porcupine digging showed that seepage and saturation was much more extensive than feared — and had saturated almost the entire dam. "It wasn't something that you could just look at the dam (from the outside) and see from a safety inspection," he said.

To help find hidden and unpleasant surprises among the many dams awaiting upgrades, Marble said, virtually all have added special equipment in recent years to help monitor interior saturation, seepage and dam movement. He said that helps give early warning if conditions are deteriorating and quick action is needed.

That is a vast improvement over conditions found back in 1988.

"In 1988, I would bet there were no more than 10 high hazard dams that were fully instrumented for performance evaluation," Marble said. "Now virtually all of the high hazard dams are instru- mented."

Also of note, records show that all high hazard dams overseen by the state are being inspected every year now, which was not always the case back in 1988. However, Marble said, plans call for inspecting some rarely filled flood control dams every other year to allow more attention on higher risk dams that constantly store water.

Also, all but eight of the high hazard dams in Utah have completed "emergency action plans" — about how to handle emergencies and failures and whom to contact downstream. Few had them back in 1988, but they were later required by law. The only dams lacking them now have only recently been elevated to "high hazard" status, which triggers requirements for such plans.

Federal efforts

The federal government has also made big strides in recent years to upgrade the large dams it owns or inspects.