Floods Caused by 100-Year Rainfall


Laguna received 3.44 inches of rain within six hours on Dec. 22, exceeding what’s expected in a 100-year storm, a flood control expert said this past Monday.
The deluge flooded Laguna Canyon homes and businesses as well as downtown shops, causing an estimated $10 million in damages, and would have overwhelmed flood control improvements proposed a decade earlier even if they had been built, said Kevin Onuma, manager of the Orange County Flood Control District.
A 100-year rain unleashes 3.36 inches of water within six hours, which the county has set as its highest measurement of rain in its flood protection plan. Even though that amount of rainfall occurs every 100 years on average, there’s a one percent chance in any year to receive that much rain. “It’s rare and infrequent,” Onuma said, “yet achievable.”
In 2001, the county and the federal Army Corps of Engineers proposed installing a large-capacity underground channel from Beach Street to the ocean to prevent flooding of Laguna’s downtown business district. The “ultimate” plan that would handle a 100-year rain, said Onuma, was to install two large-capacity channels, but due to other flood-control priorities there was only enough county and federal funds for one of the required conduits.
After a year of review, Laguna’s City Council rejected the proposal. If the county had pursued “the ultimate project,” said Vic Opincar, a civil engineer who then served on a committee to study the sewer system, “it would have made sense.”
Regardless, the proposed channel could have better handled the 2,200 cubic feet of water per second that stormed down the canyon on Dec. 22, according to Herb Nakasone, the now-retired director of the county’s public works department who engineered the proposed storm-drain improvement plan 10 years ago. The existing downtown channel’s intake is 800 cubic feet of water per second, nearly three times less.
The single proposed pipe, however, would not have contained runoff from a 100-year rain, stated Onuma. “It would have provided approximately a 10-year capacity, which is 2.2 inches for a six-hour period,” he explained. “Given the amount of rain that fell, it probably would not have totally prevented flooding; it might have lessened it because more water would have been in the underground storm drain.”
Had the county installed two 2,200 cfs channels, one under Forest Avenue and the other under Broadway, rain from a 100-year storm would have gone unnoticed on the streets of Laguna. Even though one channel would have handled a 10-year event, two would have handled the ultimate storm.
Rain doesn’t fall in equal parts, explained Onuma. “It isn’t linear,” he said. “Runoff from a 10-year storm is not one-tenth of a 100-year storm. It’s more like 60 percent.”
Onuma added that the underground water capacity of the proposed pipe at Beach Street would have matched the water volume from the canyon and likely would have lessened the bottleneck at that channel conversion, which caused spill-over flooding downtown.
“With less water on the streets, the potential for damage would have been less,” he surmised.
But to Mayor Toni Iseman, who was the first council member to oppose the project in 2002, the solution was more complicated than just one larger storm drain. “There were so many ways this particular design wasn’t working,” she said.
The major problem wasn’t mismatched channel sizes at Beach Street, where water spouted 15 feet in the early hours of Dec. 22, pointed out Opincar, the civil engineer. “The water pops up 15 feet there because it can’t force its way through two little pipes under Coast Highway,” he said, “and it backs up.”
“Those of us who work in this field recognize that the deficiency in the size of conduits in Laguna under Coast Highway is so severe that it doesn’t take much of a storm to cause flooding downstream,” confirmed Nakasone.
In addition to widening the pipes under Coast Highway, Opincar conceded that improving the channel transition at Beach Street is mandatory. “The committee felt that could have been done for less than $1 million 10 years ago, but that was 10 years ago,” he said.
The downtown fix, however, didn’t solve the bigger upstream flooding problems and was a poor use of city time and money, according to the committee. “Why would you tear up Broadway for a year, spend $5 million or more [the committee’s estimate] relocating utilities when the city should have looked harder at improving the capacity of what we had,” he said, “which, of course, went totally ignored by the city staff for the last 10 years?”
Now, new developments upstream, such as housing tracts, the 73 toll road, and a wider Laguna Canyon Road, exacerbate downtown flooding. Even with additional water-retention basins installed to divert water volume, “the flows that come through Laguna Canyon along the new highway improvement will always create a problem for the downtown area unless something is done with the conduit under the highway,” Nakasone stated.
Iseman said the county’s inattention to first resolving upstream problems was another reason she didn’t support the project. She pointed to recent storm damage in Laguna Canyon at Anneliese’s School and in the Sun Valley Drive neighborhood. “This pipe would not have addressed those things,” she said.
Geologist Katie Maes’s canyon home took on three inches of water and her garage walls were blown out by the force of last month’s flooding. She pointed to lack of upstream drainage as the culprit. Maes, who was hired to work on the Bluebird Canyon mudslide in 2005, claims more pavement in the canyon as a result of the toll road and a wider, tilted and resurfaced Laguna Canyon Road contributed to flooding. “With all that new hardscape,” she said, “it’s more likely that we’re going to have more events like this.”
Maes said each time the canyon road is resurfaced, it adds a few more inches to its height.  “The road is now above our house,” she said. Lack of proper drainage as well as the raised road and new construction inland with inadequate filtration increases flood potential, she added. The flooding, she said, isn’t just coming from runoff in the canyon; it’s from Laguna Woods and Aliso Viejo as well. Maes suggests raising the elevation of the homes in the flood plain, but pointed out that most insurers require that rebuilding replicate what was damaged.
Besides traffic detours through neighborhoods and late-night construction noise, another concern about the rejected project was toxic leaks from underground fuel tanks at the corner gas stations, a problem discovered in the design-approval process.

“The federal law says that whoever disturbs them is responsible for cleaning them up, not the person who caused the problem in the first place,” said Iseman, who was later joined in her opposition to the project by then-councilmembers Wayne Baglin, Steve Dicterow and Paul Freeman.  Councilwoman Cheryl Kinsman supporte the higher-capacity pipe plan.
The county and the Army Corps would have contributed $9.7 million to the one-pipe project 10 years ago, according to city reports. The city expected to spend about $1.8 million moving utilities out of the way, though some experts predicted the costs would be  higher. Flood damage during the record-setting storm last month was estimated at $3.2 million in downtown alone with canyon damage adding another $7 million.
The county’s original recommendation to split storm runoff from the canyon into two underground channels beneath Forest Avenue and Broadway, creating two ocean outlets, would meet the standard for handling the 100-year level of rainfall, said Onuma. That remains the county’s solution.

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