Five months after 8.5 million gallons of raw sewage spilled from a broken mainline in Carson, an independent engineer’s report has pinpointed the cause and offered practical advice for the county agency responsible.
GHD Group’s 96-page report, released this week, investigated the causes of the December 30 incident, during which raw sewage was emptied into the Dominguez Channel, which feeds Los Angeles Harbor. Over the New Year’s holiday weekend, the leak closed seven beaches from Rancho Palos Verdes to Orange County.
Robert Ferrante, general manager of the Los Angeles County Sanitary Districts, which control this part of the sewer, wrote to its board of directors on Wednesday, May 11, that the agency had already begun implementing some of the report’s recommendations, including Inspection also includes improvements. Device.
Carson Mayor Lula Davis-Holmes did not respond to a request for comment.
The report said the rupture was primarily caused by erosion of both a 48-inch diameter, 1960s-era concrete pipe and a sewer cover at the intersection of 212th Street and South Linton Avenue.
The report said the wall of the pipe had become less than 1 inch thick. A concrete pipe of that size typically has a wall thickness of 5 inches.
Another contributing factor to the failure, the report said, was a rain event on December 30. But the weight of the two-story commercial building – adjacent to the sewer – did not play a significant role in the collapse of the pipe, according to the report.
The report said a 15-foot section of the 216th Street Relocation Trunk Sewer, “shows considerable corrosion.”
When the pipe failed, a replacement project was already underway. According to the report, this project is going to be completed by the end of this year.
According to the report, the LA County Sanitation District — an agency with 24 independent special districts — met its emergency response plan requirement of responding to a sewer spill within an hour.
Independent engineers found the bypass solution “appropriate” for the county sanitation district to troubleshoot and repair collapsed sewers using contractors.
But in its recommendations section, the report warned the agency that its heavy reliance on contractors could be problematic. The report said the agency should update the list of emergency bypass pipe contractors.
“If the collapse and (sanitary sewer overflow) occurred a day or two later, during the New Year’s holiday,” the engineers wrote, “it is unclear whether contractors will be available to respond at the earliest.”
The audit also recommended that the county agency use high resolution imaging on its closed-circuit television system, which is used to inspect sewer lines, and establish a formal sewer cover inspection program.
The county inspected the 216th Street Relocation trunk sewer in November and early December and reported there were no signs of an emergency.
It is possible that the low-resolution video images do not provide the sanitation agency with a clear enough picture of the problem, the report said. The report said that all the videos viewed by the auditors were in fact of low quality and “it is difficult to see the progress of defects and defects in videos of this quality.”
The auditors recommended that the video inspection program be improved with higher quality images and year-round tracking of defects.
The county sewer agency has already purchased high-resolution equipment for inspection, Ferrante wrote in his Wednesday letter, which accompanies the report. And officials are developing sewer cover inspection routines, he said in that letter.
The letter and the report were received by the Board on Wednesday, May 11.
The 8.6 gallons of sewage that spilled into Carson was half the size of the 17 million gallons of sludge that made its way into the ocean during the July Hyperion Water Reclamation Plant disaster. Those two spills, managed by two different sewer agencies, accounted for the most sewage released into waterways since 2007.
An independent study found that the Hyperion spill was caused by system and human error.
Meanwhile, Carson’s independent report offered a comparison of three Southern California sewer agencies.
The Carson report included a comparison of preventive maintenance and repair plans for the county agency, LA Sanitation and Environment (the Los Angeles city agency responsible for the Hyperion spill), and the Orange County Sanitation District.
Of the three agencies, the LA County district cleans sewer pipes most often. The county cleans small pipes (8-10 inches in diameter) every year, medium-sized pipes (12-27 inches) every two to four years, and large pipes (30+ inches) on a case-by-case basis.
LASAN cleans smaller pipes (less than 16 inches) on a three-year cycle, according to the report. Medium-sized pipes (16 to 30 inches) are cleaned every five or six years, while larger pipes are cleaned as needed.
Orange County’s cleaning schedule includes cleaning pipes 42 inches or smaller once every five years. Large pipes are cleaned as needed.
The report also found that the LA County Sewer Agency conducts more video inspections, although LASAN and Orange County require high-definition video.
Ferrante said in his letter that his agency has already begun implementing a number of recommendations, including:
- Purchasing new high-definition equipment to inspect sewers and covers.
- Developing a comprehensive sewer cover inspection program,
- Inspect other weak sewers.
- Spending more than $10 million to repair the failed Carson sewer line.
“We take our responsibility for protecting public health and the environment seriously,” Ferrante said, “and have worked diligently to prevent overflow from our collection system.”
Ferrante said many of the recommendations in the independent report would be implemented by the end of the year.