It has taken 12 years and more than $6 million worth of studies. But city engineers finally got the definitive word this week on why Los Angeles sewers overflow during rainstorms, polluting the beaches of Santa Monica Bay.
The sewer pipes are not big enough.
In dry weather, the 6,500 miles of sewers beneath the streets and lawns of Los Angeles work well enough, although they are filling up. The oldest, a brick tunnel deep under the South-Central area, was built in 1907, when officials decided to stop irrigating crops in the Inglewood area with raw Los Angeles sewage.
Sewers in Los Angeles are supposed to carry only that--the household slurry, 90% of it water, washed down showers, sinks and toilets, plus the waste from factories and industrial plants. But in any hard rain, the sewers are deluged by a wave of storm water that overwhelms the old pipes and tunnels, forcing spills into the ocean and sometimes out of manholes and onto city streets.
How all that storm water gets in the sewers has puzzled the engineers, who began investigating the mystery in 1975.
For years, the favorite culprit was thought to be the pick holes in manhole covers. In 1985, the city finally conducted hydraulic tests that proved up to 96 gallons a minute can get through the 3/4-inch holes in an ordinary manhole, and there are 100,000 manholes in Los Angeles.
But this week, the prominent consulting firm of CH2M Hill, after five years of sewer detective work with robot-controlled television cameras and smoke bombs, informed the Board of Public Works that the manholes are a nagging problem but not the major source of trouble.
As problems go, the big one is the poor condition of the aging sewer system. About 750 miles of old-fashioned, concrete sewer mains have deteriorated, some from the caustic hydrogen sulfide gas that accompanies sewage. Once the pipes have corroded, storm water is able to leak in from the soil above or seep up through the eroded bottom of the pipes.
Storm water also percolates in through thousands of defective joints linking newer ceramic sewer pipes, both under the streets and below yards where tree roots have spread tiny cracks into gaping crevasses.
"There are some major cracks in the system," said Robert Parent, who has studied the sewers for CH2M Hill since 1982.
The study, which looked at nine areas scattered around Los Angeles, also found many cases where homeowners and contractors had hooked up drains and rain spouts directly into the sewers.
These illegal connections were detected in many cases by pumping smoke into the underground sewers and watching where it reappeared. The investigators found smoke coming out of rooftop rain gutters atop houses and even coming up through lawns, which indicates a major leak in a homeowner's sewer.
All the unauthorized storm water chokes the sewers and causes overflows. Sewage spilled out of manholes and onto 70 city streets during storms in 1983, and storm water that got into the sewers in the San Fernando Valley undermined some pavement miles away in Hollywood and caused an entire street to cave in.
Even when the overflowing sewers do not pop the manhole covers, the surge of storm water threatens the treatment plants where waste is cleaned up before being discharged. In dry weather, nearly 400 million gallons a day of sewage is handled by the Hyperion treatment plant alone, but on stormy days, a billion gallons of sewage and storm water can flood through the pipes.
To protect the plant from being knocked out of service, the excess is spilled into Santa Monica Bay without conventional sewage treatment. Beaches in much of Los Angeles County were closed by health officials for nearly a week the last time that happened on Halloween.
Work Under Way
The best solution, CH2M Hill recommended this week, is for the city to spend $335 million and build new, larger sewer pipelines. The city Board of Public Works, which oversees the sewer system, was noncommittal, but some of the recommended work is already under way.
A year ago, the city began a $2.3-billion modernization of the sewers that includes a new pipeline to relieve overflowing on the North Outfall Sewer, which runs through West Los Angeles and is the source of spills into Ballona Creek. The city is under a cease-and-desist order from the state Regional Water Quality Control Board to reduce storm water spills into the creek.
CH2M Hill also recommended spending about $25 million to smoke out illegal connections to the sewers and for repairs of leaks around the city.