Shannon O'Sullivan grew up a few blocks from Devil's Gate Dam and has often hiked in its sediment-choked reservoir. She says a county project to remove 1.7 million cubic yards of sediment from the reservoir will destroy too many trees that have taken root there. She visited the site Nov. 27, 2018 (Sharon McNary/KPCC/LAist)
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In Pasadena, the Hahamongna Watershed Park behind Devil's Gate Dam is a hidden gem of a hiking area, filled with trees and plants.
But it's being stripped of its trees, and soon hundreds of trucks will be removing tons of dirt every day.
It's part of a long-delayed project to reduce the flood threat from the Devil's Gate Dam to hundreds of homes and the 110 freeway to the south along the Arroyo Seco Channel.
Locals who have been enjoying the beauty of the park are deeply unhappy.
"These trees are 20 and 30 feet tall. Starting now, the trees, the plants, the birds, the burrowing owls, the lizards, and the frogs, they will just be mowed under," said Shannon O'Sullivan. She grew up close to the park and dam and hikes there daily.
Shannon O'Sullivan stands in a dry creekbed that runs through the Hahamongna Watershed Park Nov. 27, 2018. This portion of the nature area is to be preserved. (Sharon McNary/KPCC/LAist)
The dam's reservoir, east of the 210 Freeway, has become so full of sediment that county engineers warned a series of intense storms could clog the valves that control the flow of water from the reservoir to the Arroyo Seco Channel.
"It's very possible that this winter we could have a storm come in that could generate enough sediment to actually bury our outlet works," said Daniel Lafferty, assistant deputy director overseeing water projects for the Los Angeles County Flood Control District. "It is actually an imminent threat to the folks who are living downstream."
Lafferty says that if the dam works get buried in sediment, he's no longer running a dam -- he's got an uncontrolled waterfall.
Why was the Devil's Gate Dam built?
Devil's Gate Dam is an important element of L.A. County's network of 14 flood control dams and reservoirs. It was the county's first flood control dam and is named for a rock below the dam that appears to have a devilish face in profile.
Devil's Gate Dam as it appeared in 2015 following a rainstorm. (Sharon McNary/KPCC/LAist)
The Arroyo Seco had always been a natural channel carrying water, sand, even steelhead trout from the mountains to the Los Angeles River north of Downtown and on to the Pacific Ocean.
The dam across the Arroyo was completed in 1920, six years after a flood killed 43 people downstream. When first constructed, the reservoir could hold 7 million cubic yards of water or sediment.
Since then, every big storm in a watershed that drains 31 square miles of the San Gabriel Mountains into the reservoir has carried water and sediment, and in stronger storms, even rocks and tree trunks can flow down to the dam.
The dam has valves below the top that are used to control the flow of water into the Arroyo Seco Channel. The channel was lined with concrete by 1940, work that enabled construction of the Pasadena 110 Freeway alongside the channel.
How long has the risk been building up?
Over the decades, there have been periodic, less drastic cleanouts than the one underway now. But rains following the 2009 Station Fire washed 1.3 million cubic yards of sediment into the already half-full reservoir.
A 2011 hydrology report commissioned by L.A. County Public Works concluded that a series of intense rainstorms could put enough new sediment into the reservoir that it would clog the valves that control the flow of water in the dam. At that point, any new runoff would continue to fill the dam, and eventually overtop it, flowing without control into the Arroyo Seco Channel.
If enough water runs uncontrolled into the Arroyo, it could slop over the sides at the S-bends where the channel changes direction. More than 500 homes along the Arroyo could be flooded, and so could the segment of the 110 freeway (recently named the Arroyo Seco Parkway), the report said.
Adding to the risk to downstream homeowners, they are not covered under federal flood insurance because they are not in a federally designated flood zone. That is because the risk from the potential overflow of the Devil's Gate Dam arose recently, in the past decade, and has not been included in federal flood maps.
Why didn't this work get done years ago?
While most stakeholders agree that sediment needs to be removed, neighbors of the dam and environmentalists with the Arroyo Seco Foundation and the Pasadena chapter of the Audubon Society challenged the project's large scale.
They sought limits on the number of truck trips per day, vehicle emissions, the volume of dirt to be removed, and the destruction of plants, animals and the homes the animals made atop the sediment buildup in the dam's reservoir.
The L.A. County Board of Supervisors approved removing 2.4 million cubic yards of sediment in 2014, and revisited the issue last year following a court challenge. They voted to reduce the volume to 1.7 million cubic yards. The hole will be shallower than planned but the footprint of the excavation remained about the same, Lafferty said.
Work to clear the reservoir of trees and vegetation began the day after Thanksgiving.
Tree clearing has begun, part of a big dig to remove 1.7mm cubic yards of sediment behind Devil's Gate Dam. @LACoPublicWorks says new sediment borne on storms this winter could clog the dam's valves and flood homes and the 110 Fwy along the Arroyo Seco. pic.twitter.com/XowArgzPxC
The reservoir digging will run from April to November over the next four years. The county is permitted to run as many as 425 dirt-hauling trucks in and out every workday.
Shannon O'Sullivan takes a photo of an earth-moving machine that is about to strip trees from the Devil's Gate Dam reservoir area Nov. 27, 2018. (Sharon McNary/KPCC/LAist)
At the same time, a 70-acre natural area of the Hahamongna Watershed Park that surrounds the reservoir will be cleared of invasive and non-native plants, and native plants will be added as seeds and seedlings.
While there have been challenges to sediment removal plans at other dams in L.A. County in the past, the proposal for Devil's Gate Dam has a greater impact on the public because it's in the middle of a highly urbanized area, Lafferty said.
This story is part of Elemental: Covering Sustainability, a multimedia collaboration between Cronkite News, Arizona PBS, KJZZ, KPCC, Rocky Mountain PBS and PBS SoCal.
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