The rash of water main breaks that flooded streets, homes and businesses and snarled traffic throughout Los Angeles over the last few months may have been triggered by minor increases in pressure because of an unusually full reservoir and fluctuations after a trunk line ruptured in Studio City in September.
In a report prepared for the City Council and made public Tuesday, Department of Water and Power officials wrote that they still don't know conclusively what caused the rash of breaks. They also are still waiting for input from scientists at USC and the Jet Propulsion Laboratory who are analyzing seismic activity and other issues to see if they are factors.
But after analyzing dozens of incidents, officials determined that many of the breaks occurred in corroded, deteriorated cast-iron pipe. This type of pipe causes greater damage to streets when it ruptures, leading to more "major blowouts," the report states.
Corroded pipe also is more susceptible to breaking when subjected to minor increases in pressure. A cluster of leaks in July and August, for example, coincided with an increase in reservoir elevation -- and a corresponding increase in static pressure of about 4 pounds per square inch -- at the Lower Franklin Reservoir.
The report said the pressure was still "within the normal operating range," but that it could have been enough to stress already aging pipes. Officials also speculated that a rupture in the city's nearly 100-year-old trunk line Sept. 5 -- an incident that sent a geyser shooting 10 to 15 feet in the air and then flooded homes and businesses in Studio City -- may have accelerated the number of breaks because of "operational changes required to maintain service and supply."
After that incident, officials wrote, 26 gate valves were moved to isolate the line and make repairs. Numerous other changes also were made throughout the system to "maintain pressure and flow" throughout the system.
The report cast doubt on the theory that new watering restrictions that allow sprinklers to run only on Mondays and Thursdays were a factor in the breaks.
But engineers cannot be sure -- especially because they have noticed some pressure changes on Mondays and Thursdays. As a result, DWP officials last month installed "data loggers" to monitor the issue going forward.
DWP spokesman Joe Ramallo also noted that the problem appears to have subsided. So far this month, there have been only three major breaks. That compares with 34 major blowouts in the first 18 days of September. But Ramallo added that officials are braced for more breaks in December and January as colder weather stresses pipes.