"The flooding that would occur below the dam would be by not because of the cause of the Whittier Narrows Dam but because of the flooding itself. So the entire L.A. basin would be flooded from the rains," Col. Aaron Barta from the U.S. Corps of Engineers said.
Some homes are next to the Whittier Narrows Dam, which was built 60 years ago. There is concern it could fail in a rare and very large storm.
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"The other way it could fail is that water could come underneath the dam and carry the soil out of the dam, and the same could happen and it would make the dam collapse," said lead engineer of the project Doug Chitwood.
The plan includes a major upgrade.
"We're going to come back in and put in a roller-compacted concrete on the face of the dam and it will have a stair step look to it, and it'll come up along the top and up to the crest," Chitwood said.
The Army Corps of Engineers said it's about $600 million, and it's the agency's highest priority.
The plan is to begin construction in 2021 and have the dam completed with the new refurbishments in 2025.