SHERMAN OAKS - A municipal water main bursts at 2 a.m. Tuesday and water gushes across Mulholland Drive near Sherman Oaks, soaking two backyards and caking several intersections with mud.

A short time later, two separate water lines rupture in South Los Angeles, causing a sinkhole, flooding and street closures.

But for the Department of Water and Power, it's back to business as usual in Los Angeles.

"The (number of water line breaks) have returned back to normal," said DWP spokesman Joe Ramallo, noting a big dip in main line breaks in October compared with September.

"What we've seen is a lot of increased media coverage on what in the past had been minor breaks that no one noticed," Ramallo said.

So far in October, there have been 18 water main breaks, including the latest on Mulholland Drive. That puts October on pace to drop sharply from September, when 44 significant breaks were reported, Ramallo said.

With some 7,200 miles of pipe running below Los Angeles streets, there are typically up to five breaks or leaks each day. About 225 breaks each year are considered major, averaging about 19 each month, Ramallo said.

As with most water line ruptures, it's unknown what caused the 12-inch, cast-iron pipe to burst in the 13600 block of Mulholland Drive and disrupt the well-heeled streets of Benedict Hills Estates tucked below in Benedict Canyon.

But for residents and local motorists, the aftermath of the

"This street is usually empty, but this morning it was packed," said carpenter Oscar Pastrana, 30. "People lost their steam and were honking their horns."

Pastrana left his Winnetka home at 6:20 a.m. for a job on Mulholland Drive and wound up sitting in traffic for more than an hour.

The rupture Tuesday forced the closure of Mulholland Drive between Benedict Canyon Drive and Coldwater Canyon Avenue until about 1 p.m. while DWP crews worked on repairs, making for a grueling morning for commuters.

Water flowed into two backyards, including one with a tennis court, in the 9700 block of Donington Place, causing minor damage, Ramallo said.

Debbie Allen, a Benedict Hills Estate resident, jogged down Deep Canyon Drive and shook her head at the sight of mud piled a few inches thick in intersections at Suffolk Drive and Donington Place. She wondered about the recent spate of broken water mains.

"It doesn't make much sense. It's very concerning," Allen said. "It's like at your home - when one thing goes wrong, everything goes wrong."

DWP officials and outside experts from Cornell University, the Jet Propulsion Laboratory and the University of Southern California are studying the causes for the peculiar surge of water main breaks in September. The panel should produce its report in two weeks, Ramallo said.

Some are questioning if recent seismic activity is a potential culprit.

Others have theorized the city's water conservation measure is to blame, causing a surge in water use on Mondays and Thursdays, the only days residents are allowed to irrigate their lawns.

Ramallo said soil conditions and the way the pipes were originally laid have a lot to do with the success or failure of the equipment.

Pipes installed today are sealed in plastic and have multiple polywraps to make sure nothing touches them, a change from the past when pipes were laid in the ground without the knowledge that nearby rocks and debris could cause problems over time, Ramallo said.

A massive rupture Sept. 8 in the 12200 block of Hartsook Street in North Hollywood, which formed a sink hole that consumed half a firetruck, had eight feet of corroded metal, Ramallo said.

"It had to do with the soil and the way the pipe was laid," Ramallo said of the pipe, noting it was placed in the ground 40 years ago. "The way they lay the pipes now is entirely different from the way they used to."

The city says its conservation efforts have saved 10 billion gallons of water from June through August over the same period last year. Water lost during the spike in pipe ruptures accounts for less than half of one percent of what customers have conserved, Ramallo said.

He said it was impossible to quantify how much water has been lost on a break-by-break basis.

Still, Tuesday's break near Sherman Oaks was enough to accumulate at least 6 inches of mud outside the home of Benedict Hills Estates resident Cary Medill. The 63-year-old man on Tuesday peered out his front door at the messy sight and wondered when DWP bulldozers would come by to scrape away the mud.

"It stops right in front of my house. How lucky can you get?" Medill said with a chuckle. "Still, it could be worse."

Medill and several other long-term residents of the canyon community who awoke early Tuesday to the buzz of helicopters and shining search lights had thought a motorist drove off the cliff of Mulholland Drive, an unfortunate but common occurrence off the isolated, windy thoroughfare.

News of a water main break instead of a car crash brought a sigh of relief to several residents, including Randy Levin-Cohen.

"Our initial fear was that someone drove over the cliff. We're glad no one was hurt," Levin-Cohen said. "A water main is repairable. The mud can be washed away."