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How much storm water is LA catching?

89.3 KPCC

By Sharon McNary

January 23, 2017


Los Angeles County storm water capture systems have shunted enough water from rain-swollen rivers into percolation ponds this rain season to serve the annual water needs of about a half-million people, an official said Monday.

More than 22 billion gallons of storm water has been collected since mid-October  along the San Gabriel and Los Angeles rivers, said Steven Frasher, spokesman for the Los Angeles County Public Works Department.

However, most of the water that falls on the region is still lost to the Pacific, partly because the kinds of investments made over the years in spreading grounds along the San Gabriel River have been lagging along the Los Angeles River, said Mark Gold of the UCLA Institute of the Environment and Sustainability

“You see a storm year like this and you see all the water that ends up going through the LA River and Ballona Creek and Dominguez Channel, and you say, “Wow. That could have been our water supply for the next year,” Gold said.

“I think this storm here has really demonstrated where the shortcomings are in our local water system,” Gold said. “We’ve barely scratched the surface on what we can do in the eastern San Fernando Valley in trying to capture more of that precious rainfall from the sky and have it actually infiltrate into the ground and get into our groundwater supply.”

Why do we lose so much rain water?

The flood control system was initially built to speed water to the ocean to avoid damage to communities along the rivers during heavy rain storms. So it took decades for the region to adapt to the idea of capturing that water for later use.

Every few decades from the first settlements to early 1900s, big rainstorms would cause flooding and destruction along the rivers that run through Los Angeles and surrounding counties. And while local governments raised some money to channelize parts of the river and build dams,  the 1938 flood became was a turning point.

In late February and early March 1938, record-setting rain caused a disastrous flood on the Los Angeles River. Homes were swept away, bridges torn out. That’s back when the L.A. River was a natural river.

After that storm, Congress authorized federal money to build a new system designed to flush stormwater out to the Pacific as fast as possible.  The Los Angeles and the San Gabriel rivers were  mostly lined with concrete.  Orange County’s Santa Ana River and some of its larger creeks also were lined for much of their reach. That system was mostly built out by 1960.

In recent decades, as the region has struggled through repeated droughts, the sight of all that water being lost to the Pacific Ocean has motivated water agencies to install more projects to capture storm runoff. A network of spreading grounds has been built up along the region’s rivers. In some places, rubber dams are used to redirect the flow of water.

However, some of these groundwater aquifer recharge projects go back to the late 1930s, like one along the Rio Hondo tributary of the Los Angeles River. They’ve become more common in the past two decades.

Can those spreading grounds absorb all this rain?

Nope. There’s just too much rain coming down at once for the spreading grounds to soak up. That’s where the dams come in.

High up in the San Gabriel  Mountains is a series of dams that capture and control rainwater falling on the slopes. The Morris, San Gabriel and Cogswell dams are visible alongside Highway 39 above Azusa. Farther down in the San Gabriel Valley are the Santa Fe and Whittier Narrows dams.

And the water in those dams is parceled out to the spreading grounds a bit at a time over many months.

How much rain is being saved this way?

L.A. County Flood Control estimates that from the latest storm, they were able store 433 million gallons of stormwater. That’s about 656 Olympic-size pools of water, about enough to serve the water needs of 10,600 people a year.

Since the rainy season started in mid-October, the spreading grounds have saved enough water to serve more than a half-million residents, Frasher said.

All that rain takes months or years to percolate down through layers of soil and rock to filter into the groundwater where it can be pumped out.

How else are we hanging onto all this rain?

There are some pretty remarkable water saving projects in the works.

One in Sun Valley is a series of human-made caverns built underneath a park’s baseball field. The water from flood-prone Sun Valley flows to the park and drains into these catacombs to be filtered into the groundwater. More projects like these are being built across the region.

What still needs to be done?

Where local history is full of mega-projects like large dams and river-fed groundwater recharge fields covering hundreds of acres, others see the potential water supply that could come from micro-projects like residential rooftop water capture systems put on millions of homes and businesses.

“Right now there are a number of these missed opportunities when we get these wonderful rainstorms,” said Cindy Montanez, CEO of Treepeople. The water advocacy nonprofit is collaborating in a project with the large local water utilities including Los Angeles DWP to make such projects achievable.

“The Trump administration has said they want to spend more on water infrastructure. We hope that means that our front yards and distributed storm water will be seen as an opportunity for storm water capture, and not just build big dams that are more difficult to site in California,” Montanez said.

She also hopes the region can get new federal funding to clean pollution from underground water aquifers in the San Fernando Valley, making them more suitable for storing large amounts of stormwater.

Gold, the UCLA sustainability expert, said he’d like to see the state put money toward aquifer cleanup and other stormwater storage projects from the $7.5 billion that voters approved for water projects in Prop 1. Like Montanez, Gold also sees potential for more groundwater recharge with “green streets” designs that use porous ground coverings to soak water into the earth.

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