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Why the water main in Westwood broke has not yet been definitively determined, though the obvious culprit is wear and tear. The pipe was more than 90 years old.

According to the L.A. Department of Water and Power, 20 million gallons of potable water were lost. The leak took more than 24 hours to be contained. 

DWP said the shut-off had to be done gradually. In a press release on July 29, the agency said three nearby valves had to be closed to shut off the water leak. Immediately shutting down water flowing through the broken pipe could have damaged other lines, creating even more problems.

Mark Gold, a professor at UCLA who has taught classes on water and sewage systems in Southern California for a decade, said that response was "prudent".

One of the reasons the break resulted in such an enormous water loss is the size of tube. The water main measured 30 inches across. That makes it one of the larger tubes buried underground in L.A. At full capacity, a 30-inch pipe can move 75,000 gallons of water per minute.

That pressure was provided without the use of pumps. According to Mayor Eric Garecetti's office, that particular water main is fed by a reservoir at a much higher elevation, in the canyons north of Los Angeles.

Controlling the break would have been much easier had it been a smaller pipe, such as one of the city's 6-inch-wide lines, according to Gold. He said less water would have escaped if a six-inch pipe burst, and the nearest valve could have been shut off with less disturbance to the water pressure of nearby pipes.

Shutting down the geyser in Westwood immediately could have caused smaller pipes that led to the broken main to themselves break under the strain of increased pressure if the value closest to the leak was suddenly shut down. Instead, DWP said the contributing lines had to be closed gradually, making sure that the pressure in every connected pipe never exceeded the maximum it could handle.

And pipes downstream from the break are also at risk of breaking when a valve above them is shut off suddenly, Gold said, because of a sudden drop in pressure.

"It’s more likely to be the upstream valves," Gold said. "But any extreme change in pressure could cause problems."

L.A. City Councilman Paul Koretz introduced a motion Wednesday asking the city to find funding sources that would speed up the Department of Water and Power's replacement of aging water pipes. Even though the DWP expects to spend $2 billion on infrastructure reliability over the next decade, it would take the utility more than 300 years to replace all of the city's water pipes at the current rate.

This story has been updated.

You can reach this article's author Graham Clark by email or Twitter.