Are we ready for El Niño?

With El Niño growing increasingly likely this winter, Orange County officials are preparing for what could be a deluge of rain by clearing drains and boosting storage for stormwater coming from the Inland Empire.

Forecasters are 90 percent sure El Niño conditions will continue through the winter, and ocean temperatures indicate this year’s El Niño could be among the strongest since 1950.

To truly make a dent in California’s drought, El Niño rain and snow would have to fall in the Sierra Nevada in Northern California near the state’s largest reservoirs and river headwaters – a prospect less likely than pouring rains in Southern California this winter, according to forecasters.

But since Orange County cities get roughly half their water from a groundwater aquifer largely recharged by local rain and recycling, a wet winter could be a boon here, replenishing the nearly depleted aquifer.

El Niño could also pose a dire threat.

In the winter of 1982-83, El Niño-related storms killed 36 people and caused $1.2 billion in damage as mudslides wiped out houses and coastal properties were flooded.

During such heavy storms, the top priority for flood control officials is to protect property and lives. Even during this drought, capturing stormwater becomes a secondary concern.

“During big rain events, our big concern is having the water reach the ocean,” said AJ Jaime, manager of Orange County Public Works Operations and Maintenance.

Only so much can be done during this calm before the storm to prepare gutters and flood channels. Most maintenance work will need to be done after the first storms of the winter, when silt and debris flood into gutters and storm drains across the county.

CLOCK IS TICKING

Still, the clock is ticking for the Orange County Water District – which manages north and central Orange County’s groundwater basin – to double the amount of floodwater it can hold behind Prado Dam near the border of Orange and Riverside counties.

Right now, the Army Corps of Engineers, which owns and manages Prado Dam, allows OCWD to store 10,000 acre-feet of water in the wetlands behind the dam during storms. The water can then be gradually released and diverted downstream to recharge basins. From there, it slowly percolates into the ground, recharging groundwater.

But if OCWD could store water just 7 feet higher behind the dam, it could double storage capacity to 20,000 acre-feet, vastly increasing the amount of water it could release into spreading basins.

OCWD and the Army Corps are just a couple of signatures away from an interim agreement to store that additional water for the winter. A study needed for a final agreement is about two years away from completion.

As a flood control measure, when water surges during a big storm, two inflatable dams that normally divert water from the Santa Ana River into recharge basins automatically deflate and the water flows straight to the ocean.

After four years of drought, there’s plenty of room in the aquifer to store that excess stormwater – and regardless of how much it rains, the basins won’t get too full.

“A lot of our recharge basins are nearly empty. They’re dry,” said Mike Markus, the general manager for OCWD. “It would take several consecutive wet winters for us to refill the basin.”

CLEAN, REPEAT

But to open the path for stormwater to percolate into the ground, OCWD must clean the silt off the bottom of its recharge basins. It’s a cleaning process that will have to be repeated later in the season.

During the winter’s first storm, the basins will likely plug up again as stormwater the consistency of chocolate milk flows in and silt settles out of the water. The next time the flow lets up, OCWD crews will pump the water out and clean the basin. Too much rain, though, and percolation rates will slow.

“If it continues to rain for several months, after time we won’t be able to take as much water,” Markus said.

Rainwater gets into the groundwater basin in other ways, Markus said. Natural aquifer recharge happens in varying degrees whereverrain falls.

But for the most part, agencies across the county are more concerned with preventing that rainwater from causing floods and are clearing drains, gutters and flood control channels.

Catch basins – where water flows from the gutter, through grates and into underground stormwater pipes – are the most likely to get clogged and need the most cleaning.

“If that gets clogged up, then usually your streets are impacted and you see flooding,” Jaime said.

Public Works owns all the large flood channels in the county, plus the unincorporated areas, such as the canyon communities. Most cities maintain their own gutters and catch basins, though OC Public Works has a contract to do that for Mission Viejo, Lake Forest and Dana Point.

Officials also are cleaning the Santa Ana River channel as a longer term maintenance project, removing sediment from where Orange and Riverside counties meet, all the way to the Pacific Ocean.

RAINS BRING DEBRIS

In coming weeks, officials also will remove sediment from five river channels, including the Santa Ana River, opening them for potential floodwater to exit to the ocean. Another project, slated to start in September 2016, will dredge more than 1 million cubic yards of sediment from the Santa Ana River between Newport Beach and Costa Mesa.

The dry times of the past four years have actually been a boon to maintenance: with less water going through storm systems, there’s less wear and tear and needed cleaning. But when the rains do come, more debris will be clogging flood channels and gutters.

“When you do get an event now, everything that’s sitting there drying up could come down and you could get your debris flows,” Jaime said.

Debris flows and are especially concerning in flood-prone areas abutting the Santa Ana Mountains, such as Silverado Canyon, Modjeska Canyon, Live Oak Canyon and other rural communities.

On hillsides scorched by wildfire, rainwater can thicken the dirt into mud, causing landslides. Other loose, dry dirt and vegetation in unburned areas also can wash down with the heavier storms, clogging drain channels.

The biggest problems arrive when several storms hit in quick succession, said Grant Sharp, Public Works’ manager of Environmental Resources. That’s what happened in December 2010.

“By the time the third or fourth storm rolled around, the ground was very saturated and when it’s more saturated, more rainfall is converted to runoff,” Sharp said. “Then you’re in a situation where almost 100percent of the rainfall you receive becomes runoff.”

SLOW AND STEADY?

Continual storms also prevent workers from clearing out clogged drainage pipes and channels, leading to more flooding. “When storms come in quick succession, it doesn’t give us as much time to go out and take care of issues,” Jaime said.

Ideally, drought-busting rain would come slow and steady, with days between showers. That would allow stormwater management officials to let water gradually soak into the ground.

“It’s during the small or medium-sized rain events they’re able to maximize recovery of water and infiltration, where they can really slow down the flow,” Sharp said.

Contact the writer: aorlowski@ocregister.com. Twitter: @aaronorlowski

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