Tuesday's storm sent high water coursing down the Rubio Wash in the San Gabriel area, causing destruction that undermined a retaining wall, put a house at risk of collapse, and sent some 250,000 gallons of sewage toward the Los Angeles River and San Pedro Bay.
Stormwater that flooded the channel eroded a temporary concrete retaining wall that was erected as part of a rail bridge construction project next to the home, in the 5300 block of Pondosa Avenue (map), officials said.
The stormwater took out a temporary sewer bypass and permanent sewer line at the site, spewing sewage into the Rubio Wash, which leads to the Rio Hondo, a tributary of the L.A. River.
The destruction put a nearby home's foundation in danger, Los Angeles County Sheriff's Department Sgt. Ed Mackenzie said in a news release.
The home was red-tagged due to the damage, which was connected to work on a nearby rail bridge over the wash, according to Kerjon Lee, a spokesman for the county Department of Public Works.
The work is part of the that Alameda Corridor-East project, a series of rail crossing improvements designed to increase safety and ease the flow of train traffic through the San Gabriel Valley.
The home's residents were evacuated and no one was reported injured, Mackenzie said. Fire officials and building inspectors were on scene.
The retaining wall that failed was installed in preparation for planned lowering of the wash, according to Paul Hubler, a spokesman for the Alameda Corridor-East Construction Authority. When the wall collapsed, a dirt embankment next to the home was scoured out by stormwater, Hubler said.
As part of the rail bridge work, a sewer needed to be relocated, according to Steve Highter, a spokesman for the Sanitation Districts of Los Angeles County. The stormwater overwhelmed a temporary relocation of the sewer, and left a 24-inch sewer line pouring into the wash, he said.
The flow began around 7 a.m. and wasn't shut off till 12:30 p.m., though crews were able to divert some of the flow from the was in the meantime, Highter said.
The resulting 250,000-gallon sewage spill prompted the closure of beaches in Long Beach, some 33 miles away, the Long Beach city health officer announced.
Meanwhile, it's not clear if the San Gabriel-area home will be salvageable, Hubler said.
"It's too early to tell, but what we definitely know is that it was not safe to remain in the home," Hubler said.
The family that lived at the home relocated to a hotel in the area, he said.
Aerial video showed the home hanging over the side of the scoured-out embankment, like a miniature cliff. A hillside that would have supported a walkway along the side of the house was no longer there.
A red tag was placed on one of the windows at the home.
KTLA's Steve Bien contributed to this report.