Mark Pestrella is the director of L.A. County Public Works, which oversees 27 spreading grounds and 14 dams that both hold most of our local water supply as well as prevent massive flooding in the cities below.
Pestrella said he isn’t losing sleep over a megaflood. His biggest concern? The increasing severity of smaller, but intense storms — like many we experienced this winter. At times, some of those storms could be considered 100-year and 200-year events (or more severe in some areas) due to the intensity of the rainfall at certain times.
Such storms are becoming more dangerous as increasingly severe fires bring ashy sediment into the reservoirs, Pestrella explained. The county’s dams in the San Gabriel mountains are taking in a lot more muddy sediment when rain falls over large burn scars.
“Every one of the 14 major reservoirs are getting injected with sediment at an accelerated pace,” Pestrella said.
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You've probably heard terms like "100-year flood," "500-year flood" and "megaflood." These are terms that have been developed based on historical rainfall and hydrological data to provide estimates on flooding of differing severities.
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These terms also help designate flood insurance maps used by the Federal Emergency Management Agency, or FEMA. They also help engineers know how to build to withstand certain extreme scenarios. L.A.'s flood control system is built to withstand 100-year flood flows, but aging infrastructure challenges that.
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“It rains at different amounts in different places and we see these events that come and go, and they may not exactly match what we've designed for, but the hope is that we've done a reasonably good job planning for this type of event so that the consequences are not too catastrophic,” said Brett Sanders, a UC Irvine engineering professor whose research focuses on flood resilience.
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- A 100-year flood doesn't occur every 100 years. Rather, it has a 1% chance of occuring any given year.
- A 500-year flood has a 0.2% chance of occurring any given year
- A megaflood is most commonly considered to be a 1000-year flood, or a flood that has a 0.001% chance of occurring any given year.
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But all of those chances are going up as the climate crisis changes the global water cycle. A recent UCLA study found chances of a megaflood have doubled due to the changing climate.
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Read more about what's causing bigger floods here.
In this winter’s storms, as a result of that built up sediment, several dams briefly overflowed, though not enough to cause dangerous flooding in neighborhoods below them, Pestrella said.
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The county estimates it has some 15 million cubic yards of sediment to move to make room for more water in the dams. That could involve deepening debris basins, an effort that is likely to cost roughly $500 million. Pestrella said the siloed nature of state and federal funding makes it difficult to expand funding sources beyond local taxes, where the department gets most of its budget.
“To give you an idea what that does to our budget, we collect about $300 million a year; 85% to 90% of that money right now is spent on maintenance,” Pestrella said.
Pestrella said in the water infrastructure world, water conservation and flood control are still seen as separate responsibilities for separate agencies.
“The federal government has not valued water conservation in the West for some time,” Pestrella said. “That’s manifested itself in a lack of authority for their public works department — the Army Corps of Engineers — to actually help us with this.”
That’s left local agencies, such as L.A. County Public Works, almost solely responsible for maintaining existing infrastructure. When that maintenance spills into the broader tasks of water supply and flood control, more needed funding can be hard to get.
“The state of California has basically put the maintenance requirements on the local agencies, the counties,” Pestrella said. “Types of activities like reservoir restoration are considered maintenance. So for instance, after an emergency declaration, there's no money for us. Even though that sediment's coming from a fire that occurred or is being accelerated, there's no line item in there for us to get the dollars for that.”
In plain terms: “Maintenance … it's just not sexy,” Pestrella said. “People want to see things built.”
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Maybe you remember the concept of the water cycle from elementary school: When water from lakes, rivers and the ocean evaporates, it condenses to form clouds, then falls back to the earth in the form of rain or snow.
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But as our climate heats up, water on the landscape, such as mountain snow and rain-fed streams and lakes, evaporates more quickly and dries out soils faster. That leaves less water for humans and the animals and plants that rely on that surface water, and also leads to a “thirstier” atmosphere, which then dumps that evaporated water in the form of increasingly intense storms and atmospheric rivers.
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That’s why the climate crisis is making California’s drought-to-deluge cycle even more extreme.
But as the climate crisis fuels more intense storms and longer droughts, building bigger dams and levies isn’t the way forward, even with the challenge of increasing mudflows in the system, Pestrella said.
“I think we've learned that these are highly expensive systems to maintain and they have high impact on the environment. ” Pestrella said. “They aren't necessarily the most effective way to protect people and to conserve water. But early land use decisions have somewhat locked us into the system that we have in Los Angeles.”
Pestrella said that’s why the county is embarking on an education campaign this summer in the most flood-prone areas of the county, such as Sun Valley, South L.A. and communities along the region’s major rivers, to educate communities about their flood risk, learn about flood insurance and otherwise prepare for future disasters.
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Fewer than 2% of Angelenos at risk of major flood damage have flood insurance, according to the Federal Emergency Management Agency, or FEMA. It’s a challenge because the people most at risk also have some of the lowest incomes in the county, a reason communities have fought expanded Federal Emergency Management Agency flood maps that would require insurance.
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Do you live in a flood zone? You can check out this map to find out.
