"That's a huge question mark," said Brad Iarossi, an engineer who speaks about dam safety for the American Society of Civil Engineers.
A properly working dam should have been capable of handling the 13-inch downpour storms dropped upstream, they said.
The dam has three floodgates, which were designed to be raised or lowered to regulate water levels in the lake and on the Maquoketa River, which forms the lake. They were supposed to be open after heavy rains to allow water to flow out of the lake and into the river.
But one of the gates was inoperable, and it was partly closed when lake waters rose late last week. The lake water kept rising until it poured over the top of the dam, then chewed through an earthen portion of it. Most of the 440-acre lake drained out through the new hole.
Ron Corso, a Virginia engineer who is spokesman for the Association of State Dam Safety Officials, said Internet photos of the dam taken after the breach appear to show a significant floodgate problem. He said he hadn't seen details of water flows at the time, but logic dictates that a partially closed floodgate could have contributed to the catastrophe.
"What doesn't go through the gates goes over the dam," he said.
The homeowners association that owns the 88-year-old dam said this week that a contractor had been working to fix the problem, but that even if the gates had been fully functional, the floodwater still would have gone over the top of the dam. The association president said the two other gates were open farther than they were designed to open, and the dam was letting out more than 100 percent of the water it was designed to release.
Kevin Baskins, a spokesman for the Iowa Department of Natural Resources, said Wednesday the department will investigate.
"While acknowledging that functionality of the gates is a legitimate question at this point that needs to be answered, we need to be able to review all of the data involved with this incident and be able to provide a definitive answer on what happened based on professional engineering analysis rather than speculation," he wrote in an e-mail.
A DNR inspector last summer gave the association until Dec. 31, 2009, to fix the gate problem, which was caused by 2008 flooding.
The association's president said this week that the work was delayed by issues involving financing and availability of a contractor. The group's executive director said the association did not have insurance on the dam.
Delaware County Engineer Anthony Bardgett also said Wednesday that he believes the broken floodgate could have contributed to the dam's failure. He said the gate appeared to have been closed more than halfway. He said he expects state officials to sort out what happened.
Iarossi said that if the dam was releasing as much water as it was designed to, as the owners say, officials should investigate whether the dam's apparatus was sufficient for the size of the lake behind it.
He said other old dams around the country have been improved with the addition of gates or emergency spillways.
The dam failed after torrential storms upstream dropped as much as 13 inches of rain in 48 hours.
Iarossi and Corso both said the dam should have been able to cope with that much water.
"It was a huge rain, but in the world of dams, that's nowhere near what we design them to handle," Corso said.
Even if fully operational floodgates had failed to keep up with the water flows, they might have prevented a breach. That's because small amounts of water over a dam aren't necessarily fatal, he said.
"We have lots of examples of earthen dams that were overtopped and survived," Corso said.
Iarossi said replacing a dam like the one at Lake Delhi could easily cost tens of millions of dollars. He said the project could have trouble gaining much federal money, because many publicly owned dams are competing for assistance with badly needed repairs.
When asked to guess how long a replacement dam might take to design, gain government permits and complete, Iarossi paused.
"I'd rather try to pick today's lottery numbers," he joked. But he said such a project generally takes at least several years.
Corso said the fact that the dam had a public road going over it could improve the chances of receiving rebuilding money from the Federal Emergency Management Agency.