A blue-ribbon panel of scientists said Tuesday that the high-volume water main breaks that bedeviled Los Angeles last summer and fall were caused in part by the city’s restrictions on lawn watering, and their findings could force the city to remake its strict water conservation policy.

The city last June limited the use of lawn sprinklers to Mondays and Thursdays, and those restrictions have proved highly successful. Officials said Tuesday that in February, Los Angeles had its lowest recorded water use in 31 years.

But the water conservation policy was too much for the city’s aging network of cast-iron pipes, causing fluctuations in water pressure that strained them to the bursting point, the panel’s long-awaited report found. Its conclusions appear to put to rest other theories about the cause of the mystery, including increased seismic activity.

According to the report, on days when watering was allowed, water pressure in the pipes dropped. On days when watering wasn’t allowed, pressure increased and “accelerated the metal fatigue failures of aged and corroded cast-iron pipes,” the report said.

The result was a series of major water main breaks that flooded streets and damaged property, starting weeks after the water restrictions took effect. From July through September 2009, the city recorded 101 major breaks, compared to 42 in 2008 and 49 in 2007, the report said.

The scientists suggested the city rework its conservation plan. One alternative would be to require homes with even-numbered addresses to conserve on even-numbered days and homes with odd-numbered addresses to conserve on odd-numbered days, the team said. That, they said, would help even up pressure.

“The bottom line is, you want to create a more even usage of water pressure so you don’t have a sudden drop of water pressure at a given time of the day,” said Jean-Pierre Bardet, chairman of USC’s Department of Civil and Environmental Engineering, who headed the team. Mayor Antonio Villaraigosa and the City Council requested the panel’s probe after last summer’s spectacular blowouts.

A spokesman for the Department of Water and Power said his agency was studying the panel’s report.

A DWP spokesman said scientists at the utility “have not yet analyzed Dr. Bardet’s findings but look forward to doing so.” Joe Ramallo said the agency’s own investigation found that water rationing might have been a factor but determined that a more likely cause was corroded, aging cast-iron pipes.

Councilman Paul Koretz said that in the report’s wake, he wants the council to immediately halt the current conservation program and develop an alternative plan.

“It was such a well-intentioned program,” he said. “But I think intuitively, once somebody raised the idea, it made perfect sense; you have brittle pipes and you have dramatically increasing and decreasing water pressure.”

Officials began noticing an increase in water main breaks in late summer. Then on Sept. 5, a 5-foot-wide trunk line underneath Coldwater Canyon Avenue in Studio City exploded, sending a 10-foot gusher of water and mud into the air. Homes and businesses were flooded. The street, a major thoroughfare connecting the San Fernando Valley and the Westside, was closed for a week.

Less than 72 hours later, another main burst in Valley Village, creating a sinkhole that swallowed half a firetruck that responded to the call. Firefighters crawled out the window to reach safety.

So far, the city has been hit with 108 legal claims regarding the Coldwater Canyon pipe breaks, 41 of them from homeowners, officials said Tuesday.

Of the latter number, 25 claims have been paid, two were denied, three have pending offers and 11 are awaiting more documents, said Assistant City Atty. Joseph Brajevich. The financial terms of those settlements were not immediately available.

The Times in September reported that some experts were speculating that water rationing had played a role in the water main breaks. But at the time, other experts were skeptical, questioning why water restrictions in L.A. would cause ruptures when other cities with similar rules, including Long Beach, were not experiencing the same problems.

Bardet’s team was assigned to study last summer’s “blowouts,” the term used by the utility to describe pipe breaks that undermine the street and require the replacement of at least 100 square feet of pavement. Ninety percent of the blowouts occurred in the brittle cast-iron pipes that are among the oldest in the DWP’s network, which extends over 7,200 miles.

In addition to the water conservation findings, the panel said the city’s water pipes required work. The scientists called for a more efficient pipe-replacement plan and an improved pipe-inspection program.

Councilwoman Jan Perry said that given the findings, it is up to city officials to show why changes in the conservation rules should not occur immediately.

“We certainly don’t want to go through another summer like the one we had last year,” she said.

david.zahniser@latimes.com

jessica.garrison@latimes.com