The series of major water main breaks that occurred around Los Angeles last year was caused by the city's water conservation program, which put too much pressure on the city's aging cast iron pipes, according to a city report released Tuesday.
The report is a step forward in solving a mystery that bedeviled city officials and engineers and enraged some residents who had to endure the flooding and road damage.
A team of scientists charged with looking at the pipe breaks concluded that the city should rework its conservation plan, which limited the use of sprinklers to Mondays and Thursdays.
One alternative would be to require homes with even-number addresses to conserve on even-numbered days and requiring homes with odd-numbered addresses to conserve on odd-numbered days, the team said.
"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, a USC engineering professor who headed the team.
The investigation team, which appeared before the council's Energy and Environment Committee, found a connection between the city's water-rationing program and the increase in pipe breaks last summer, particularly with cast iron pipes.
At various locations in the L.A. Department of Water and Power distribution system, water pressure fell significantly on Mondays and Thursdays after the beginning of the water-rationing program on June 1, 2009, the report said.
"Those water pressure drops on these days were caused by an increased water flow during the watering of lawns," the report said. "As a result, the cyclic levels of water pressure increased and accelerated the metal fatigue failures of aged and corroded cast-iron pipes."
Mayor Antonio Villaraigosa, the council and the DWP agreed last year to restrict the use of sprinklers to 15 minutes a day on Mondays and Thursdays. No watering is allowed between 9 a.m. and 4 p.m. Hand-watering -- using a hose with a nozzle -- is allowed on a daily basis, although not between 9 a.m. and 4 p.m.
The city had 101 breaks during 2009, compared to 42 in 2008 and 49 in 2007, according to the report. Last fall, The Times reported that some experts believed the city's recent decision to allow sprinklers to run only on Mondays and Thursdays may have played a role in the breaks.
They say that if more water flows through the system on those two days when people water their lawns and then pressure suddenly changes on other days, it could put added stress on already aging pipes.