CRANSTON — An explosion and a rupture of the main dam at the Scituate Reservoir.

An estimated 70-foot wall of water, at its peak, moving down the north branch of the Pawtuxet River.

Large swaths of five cities and towns, T.F. Green Airport and parts of Route 95 flooded. People stranded on the tops of houses and cars. An unknown number of casualties.

A scenario worthy of a disaster movie — but not as farfetched as it first appears, officials say — was the basis for a practice drill Thursday at the headquarters of the Rhode Island Emergency Management Agency.

Pretending that a lone wolf terrorist had blown a gaping hole in the dam, about 100 emergency management personnel gathered to test the state’s public and internal lines of communication, a new 800 MHz radio network, and an emergency action plan mandated by a three-year-old amendment to a state dam safety law.

Many more people participated without traveling from federal and state agencies and the municipalities most affected: Scituate, West Warwick, Coventry, Warwick and Cranston.

Among their tasks: Cope with a loss of potable water in the drinking-water system that serves 60 percent of Rhode Island. Among their responses: ask the military to send two water-desalinization plants on barges, have the Poland Spring water company immediately ship hundreds of thousands of liters of bottled water, and activate a little-known mutual aid compact among water systems called RI WARN.

Officials said the scenario could occur if there was a catastrophic breach of the Gainer Memorial Dam and a massive release of water into the Pawtuxet River floodplain.

Edward Johnson, deputy director of state EMA and a colonel in the Air Force Reserve, said the drill exposed a few glitches in the state’s preparation, such as a need to have emergency management personnel become more conversant with Web EOC — as in emergency operations center — which is an e-mail communications system.

A representative of the Coast Guard said in a closed-door post-exercise evaluation, according to EMA spokesman Steve Kass, that the EMA had demonstrated unbelievable improvement in the years since he had participated in a drill there.

J. David Smith, state EMA executive director, said, “It’s a validation of all the hard work and training that the people have done.”

The state EMA flexes its muscles from time to time on major exercises, such as a response to a severe hurricane, in which they worked closely with their Connecticut counterparts, and another imaginative disaster in which the populace was bombed with anthrax from a helicopter piloted by people posing as movie makers.

But the agency’s emergency operations center often is activated for small-scale real-life events, too, such as searches for missing people.

The script for the catastrophe at the Gainer Memorial Dam said that there had been a prolonged rainstorm and that the reservoir was filled to the brim and the river swollen. At capacity, the reservoir, owned by the City of Providence, contains 54 billion gallons and is held back by a 3,200-foot-long earthen dam crossed by Route 12, locally known as Tunk Hill Road.

The sabotage unleashes a cascade that, over the course of five-plus hours, overspreads many square miles and inundates the airport to a depth of perhaps 10 feet, until it dissipates into Greenwich Bay.

Clustered at telephones and computer keyboards, representatives from agencies such as the state Department of Health, Division of Public Utilities, the Red Cross and National Weather Service talk out problems — reported electrocutions from downed power lines, a tour bus stranded on 95 in rising water, and the like — with one another as well as local emergency management agencies.

At 9:50 a.m., incident commanders put their heads together. They debate whether an all-out evacuation would be necessary.

State police Lt. Darren Delaney says his agency is assisting.

“We cannot take over from Scituate. This is a local incident so far. … They’re very sensitive in this state [about] us coming in.”

At some point, Johnson asks the governor to declare a state of emergency and for the governor to ask the president to declare central Rhode Island a disaster area.

While the drill lacks the tension of a real incident, there is a note of urgency as the officials continue to discuss an evacuation. The question is: Where will they be sent, and by what routes?

At a mock news conference, a tight-lipped Delaney tells reporters that there had been an explosion of unknown origin and that it was under investigation. Kass later discloses that it had been an act of terrorism.

gsmith@projo.com