Environment & Science

Whittier Narrows Dam risk spurs flood planning in downstream cities

The Army Corps of Engineers says Whittier Narrows Dam is among the least safe in its system of dams. If the spillway gates, shown in February 2016, open prematurely in a big storm, that could contribute to downstream flooding. Erosion or overtopping of the earthen dam also posed failure and flood risks.
The Army Corps of Engineers says Whittier Narrows Dam is among the least safe in its system of dams. If the spillway gates, shown in February 2016, open prematurely in a big storm, that could contribute to downstream flooding. Erosion or overtopping of the earthen dam also posed failure and flood risks.
Sharon McNary/KPCC

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When the U.S. Army Corps of Engineers dubbed its Whittier Narrows dam unsafe last year, the announcement came with a jarring surprise: None of the cities downstream from the dam had a flood evacuation or emergency plan.
    
One year later, officials who run the city closest to the dam are still working their way through the complicated issues that go into planning an evacuation when there's a risk of flooding.
    
“If we had to evacuate what would that look like? What would be the route to travel,” said Pico Rivera City Manager Rene Bobadilla. “How much flooding will occur if that there was ever a major breach? And where would the shelters be?”
    
City officials have known since 2008 that the dam had risks. That’s when it was assigned the Corps’ second-worst rating.  That risk heightened in May 2016 when further study put the Whittier Narrows Dam in the worst category. The dam was deemed “unsafe” and “critically near failure.” 

The Whittier Narrows Dam went up in 1957 to keep the San Gabriel and Rio Hondo rivers from flooding homes in a stretch of LA County from Pico Rivera to Long Beach. Normally the dam is dry and what little water comes down the rivers flows through at a measured pace.
    
But the risk of dam failure and flooding is heightened if the automatic spillway gates open prematurely or if the earthen dam foundation erodes further, the Army Corps study concluded. Storm runoff from extreme rain also posed a significant risk.

Bobadilla says Pico Rivera is on a low-lying basin with little high ground for evacuation centers. Bridges over the rivers and the 605 Freeway are the main routes out of the city. The city is six months into its emergency planning, with funding from a $200,000 grant. The city will roll the plan out to the public once it’s done, he said.

Until then, he said he's confident the dam won’t fail.

The Corps has begun several interim risk reduction measures recommended by the study, said spokesman Jay Field. When the water level behind the dam rises, the Corps will step up inspections of the toe of the dam, that is, where the slope of the earthen dam meets the ground.

The Corps will deliver sand and gravel to places where it could be used to control floodwater. It is also providing flood maps to disaster managers in the downstream cities. They also have annual meetings to coordinate communication and evacuation warnings, Field said in an email.

Long Beach already experienced significant flooding when its storm drain system was overwhelmed in last winter rains, said Public Works Director Craig Beck. But even as the city improves its storm drains, those empty into flood channels and the L.A. and San Gabriel Rivers.

“If either of these drainage systems were compromised by upstream capacity issues, Long Beach could potentially experience even greater flooding than we experienced this year,” Beck said.

Officials of several other cities along the San Gabriel River and the Los Angeles County Flood Control District were unable to provide information about their respective flood planning in the year since the Corps raised the issue of the lack of flood evacuation plans.