Water & Drought

Melting snowpack and sinking land threaten Valley communities with flooding

Rushing to avoid a flood

Danny Wade of the Tranquillity Irrigation District describes how his workers have built an earthen bridge to head off a levee failing along the Fresno Slough, which is fed by the Kings River.
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Danny Wade of the Tranquillity Irrigation District describes how his workers have built an earthen bridge to head off a levee failing along the Fresno Slough, which is fed by the Kings River.

It’s a race against time this spring as water roars out of Central California’s dams and rumbles its way to the lowest-lying areas of the western San Joaquin Valley, communities where land is collapsing and water channels are growing more unstable.

State engineers are generating new maps to understand where water is stagnating in spots it once flowed freely, and to learn which communities are in the most danger of flooding. Old solutions no longer apply, water officials say, because topography is altered by what’s occurring underground.

With a hefty Sierra snowpack heading toward the months when it will melt, both Pine Flat and Millerton lakes are expected to empty and refill several times this year. Managing water flow to avoid floods is critical. High water is expected into the early summer, officials say.

Land along many local waterways has been altered by subsidence, which occurs when massive groundwater pumping causes unstable land to collapse. Those elevations in some areas have shown deterioration after the five-year drought. A recent report shows one of the largest dips is in the Tranquillity area.

“We compared information from (Tranquillity) a couple weeks ago with (high-tech detection) information taken in 2008 and we see around a 3- to 4-foot change in elevations in some places,” said Eric Tsai, a senior engineer with the state Department of Water Resources.

Put another way: A person standing on a spot of ground near Tranquillity would now be 4 feet lower because of the sinking land.

 

When land sinks, it alters the historical way water has flowed through a channel and increases the number of locations where water could overflow or compromise a levee, Tsai said.

“There is usually a gradient, but when subsidence happens (the channel) becomes more flat and it causes sedimentation,” Tsai said.

More sediment causes drag, and the leveling or pooling effect also slows water flow, he said.

When large amounts of water course down a river and hit such pools, the only place for the water to go is sideways. That causes flooding.

The most recent study by NASA’s Jet Propulsion Laboratory noted that Tranquillity’s subsidence intensified in a 16-month period ending last September, and that the elevation dropped 20 inches in some spots.

The Tranquillity area dropped between 2 and 5 inches in a previous study released in 2015 and covering about a year. The report didn’t list Tranquillity as a trouble spot. Corcoran and an area south of El Nido in Merced County were considered the most serious trouble spots.

The Tranquillity area kind of popped up and developed so quickly, now it’s equal to the maximum subsidence in Corcoran and El Nido.

Tom Farr, lead author of NASA report on subsidence

But the most recent study shows Tranquillity exceeded only by Corcoran, where land dropped 22 inches. El Nido and a 25-mile swath of the the Eastside Bypass, which stretches south into western Madera County, showed a 16-inch dropoff. Land also is sinking along the California Aqueduct near Avenal and Cantua Creek, reducing the aqueduct’s water flow capacity, state officials say.

The fast-dropping Tranquillity has become a focus for Fresno County officials who filed an emergency declaration with the state. Tranquillity Irrigation District employees are working around-the-clock to keep about 80 homes north and west of the town from flooding.

The Fresno Slough, which runs to the north of Tranquillity, draws water during wet years from the Kings River. The channel wasn’t even carrying two-thirds its normal peak capacity in February when high water threatened to flood the area.

“We had some data when it started showing up in 2014, but now it’s gotten worse,” said Tom Farr, a geologist with the Jet Propulsion Laboratory in Pasadena and lead author of the NASA subsidence report. “The Tranquillity area kind of popped up and developed so quickly, now it’s equal to the maximum subsidence in Corcoran and El Nido.”

In Tranquillity, where alkali soil turns into a quicksand-like mush when wet, the danger for leaks are even greater.

Groundwater pumping and underground geology, especially areas with clay, silts and “fine grain materials,” are making subsidence worse, Farr said.

Bridge over troubled waters

The unstable soil has meant big problems for the tiny Tranquillity Irrigation District, which historically is irrigation-oriented with little expertise in levee maintenance, said Danny Wade, the district’s general manager. “But not anymore,” he added.

The Fresno Slough channel, which had a capacity of 4,500 to 4,750 cubic feet per second in 2011, the last significant wet year, was endangering a levee at 3,100 cubic feet per second in February and March. A cubic foot per second is equivalent to 7.5 gallons of water.

After more than a month of work by district staff, the channel is now carrying 4,400 cubic feet per second, and crews will attempt to add more capacity, said Wade.

Steven Haugen, the Kings River watermaster, says the million-acre-foot Pine Flat Lake will fill and empty up to four times this year, so shoring up waterways downstream is a must. The Kings River snakes its way from Pine Flat Lake toward northwestern Fresno County, and it takes about five days for water from Pine Flat to reach Tranquillity.

So far, district workers plugged troublesome leaks caused by holes in the dirt, including holes several feet underground left by rodents when the land was parched.

“This happens every five to seven years,” he said. “This year we’ve plugged 20 holes – seven in one night.”

The district is paying overtime to employees and has rented excavators, backhoes, carry-alls and dozers, ringing up a bill exceeding $75,000 for half of February and March. Employees look for leaks, which always seem to show up in the wee hours of the morning, he said.

To plug the holes, tons of dark, more absorbent dirt from a nearby grower – whose crop loss must be compensated – have been used to replace the gray alkali dirt.

“The key is 24-hour surveillance and the overtime for our guys patching up breaks,” Wade said.

The district will send the bills to the county, which issued an emergency flooding declaration for the area. Those bills will be forwarded to the state, and Wade hopes the district will be fully reimbursed.

Wade says he’s more confident today that a significant break is far less likely than he was in February.

Once we got the dam across there we went from a 95 percent to 98 percent chance of washout down to 10 percent, he said. The bridge saved the day.

Danny Wade, Tranquillity Irrigation District general manager

An earthen bridge large enough to drive a truck over was built across the most serious trouble spot. Water flow control gates were also installed to move water from one side of the bridge to the other. On the east side, Wade points to where the water is about 8 feet deeper than the west side.

“Once we got the dam across there, we went from a 95 percent to 98 percent chance of washout down to 10 percent,” he said. “The bridge saved the day.”

Because of the district’s repairs, the flow has increased from 3,100 cubic feet per second two months ago to 3,500 cubic feet per second in mid-March and to 4,400 cubic feet per second by last Wednesday.

The goal is to raise up as many nearby canal banks with the proper soil, fix Calaveras Avenue to Jefferson Avenue to make it passable in flooding and ask that more water get diverted south toward Kings County instead of north as the Kings flows into western Fresno County and Madera County.

Another proposed solution is to allow water to move from the channel to the nearby Mendota wildlife management area, but Wade said that would only be a temporary solution.

He said water is coming down the channel at such velocity that if it went into the wildlife area, it would fill within a few days.

“It’s a stand-by emergency to temporarily take pressure off to allow us to work,” Wade said of diverting water to the refuge.

If flooding occurs, Wade suspects it would move toward Mendota, where city and county officials are concerned about flooding at the city’s sewage treatment plant. But Mendota officials appear more optimistic than a few weeks ago.

“The (water) levels have been slowly increasing, “ said Cristian Gonzalez, the city’s director of planning and public works. “The city has been able to secure some low spots on the east and south side of the plant, where we were the most vulnerable.”

On the San Joaquin

About 20 miles north of Tranquillity in Madera County, subsidence is a long-term problem in pockets along the Eastside Bypass, which transports water north to the San Joaquin River from its tributaries.

Along the Eastside Bypass in western Madera County, near Red Top, water is observed moving faster than historic norms, but slowing as it travels about 10 miles north into the Merced County town of El Nido, another center of subsidence, said Reggie Hill, general manager of the Lower San Joaquin Levee District.

Rain and Sierra snow melt flows into Millerton Lake, then down the San Joaquin River to an area east of Mendota where it hits the Chowchilla Bypass, flows north, makes a western turn and becomes the Eastside Bypass.

This year, the canals are dealing with more water on a consistent basis. Flows along the channels aren’t reaching historic maximum levels because of subsidence, said Chris White, general manager of the Central California Irrigation District.

Subsidence has decreased capacity along the Eastside Bypass by 30 percent from its peak a decade ago, said White, whose district stretches from northwestern Fresno County into Stanislaus County.

Possibly the hardest-hit area, near the Madera County community of Red Top, is where the Eastside Bypass meets Ash Slough.

“The (water flow) numbers we had in 2006 would have flooded this whole area,” White said.

Land sank by about 15 inches per year from 2008 to 2012.

The (water flow) numbers we had in 2006 would have flooded this whole area.

Chris White, Central California Irrigation District general manager

On the San Joaquin River, west of Red Top, land has fallen about 6 inches per year. The river, he said, is supposed to have 3 feet of space to the top of the channel, but water is cresting near the top, indicating subsidence.

About 4 million acre-feet of water is expected to run into and out of Millerton Lake, an equivalent of eight Millertons, said Michael Jackson, South Central California Area office manager for the federal Bureau of Reclamation in Fresno.

“That’s a number that is reachable,” he said.

If temperatures warm slowly without sudden heat and the amount of rain slows in the spring, the lake will fill up more slowly. The concern for flooding will escalate with additional rain, warmer storms at high elevations and sudden heat waves that speed snow melt.

For now, the lake is about 40 percent of its 520,000 acre-feet capacity after dumping 9,000 cubic feet per second for about three weeks from late February to mid-March, which is well over the normal maximum release of 8,000. At that time, the lake was more than 87 percent full.

And, though this year is unusual, dealing with snow melt potentially into July is not unusual for a heavy rain year. He recalled a year when releases went into October.

“If we make it into July with no damage, then everything is probably manageable,” Jackson said, “but right now, we don’t know.”

This year may be a turning point for subsidence since property owners are spending millions digging “sinking basins” to use flood water for recharge. The idea is to have water available at shallower depths to reduce deep groundwater pumping and subsidence in the dry years that are certain to return, White said.

“If you don’t pump the deep aquifer, you don’t get subsidence,” White said.

California Aqueduct sinking

The California Aqueduct is sinking most significantly in an area north of Avenal Cutoff in Fresno County, affecting flows headed toward Southern California.

Another site along the aqueduct, near Cantua Creek in Fresno County, also is a longtime problem. It dropped 8 inches between 2013 and 2016.

NASA’s Farr said land near the “Avenal hot spot” along the California Aqueduct has dropped 25 inches between 2013 and 2016.

“It just keeps going,” Farr said. “Avenal has developed the whole time, impinging on the aqueduct. I think it will cost a lot  a high-ticket item.”

Historical repair costs from subsidence are difficult to evaluate because for many years it was deemed a maintenance issue, said Jeanine Jones, interstate resources manager for the state Department of Water Resources.

The cost to fix the Avenal Cutoff portion of the aqueduct could exceed $10 million, she said. To repair the aqueduct from subsidence damage and return it to its original flow capacity of 50 years ago is likely to take hundreds of millions, she said.

David Rennie, a principal engineer with the state Department of Water Resources, said the aqueduct carries 20 percent – about 1,650 cubic feet per second – less through Cantua Creek to the Kettleman City area, including Avenal, than when it opened.

The shortfall hasn’t affected scheduled water deliveries yet, and it’s not clear when it will, he said.

“Each area is a little bit different, but we’ve looked at how flows have changed over that entire stretch and that number is on par with what we’ve observed,” he said.

Westlands Water District, which has contractors pumping groundwater in the area, is evaluating NASA’s subsidence information, said Gayle Holman, the district’s spokeswoman.

“It’s a good opportunity for us to use the Sustainable Groundwater Management Act and the preparation we have for our sustainability plan to look at the relationship between groundwater pumping and subsidence.”

NASA’s study examines topography from satellite images, she said, but “there’s a lot going on underneath the soil. We are not seeing the direct impacts that maybe this report is portraying.”

About one-third of the district is fallowed, roughly 200,000 acres, a record amount, Holman said.

For the coming growing season, she anticipates a sharp drop in groundwater pumping. The district also is recharging where possible. Last year, about 550,000 acre-feet of groundwater was pumped districtwide. Farmers are on water meters, she said, and with a 65 percent water allocation for 2017, Holman anticipates groundwater use to drop to about 200,000 acre-feet.

“This (year), hopefully, will prove to be a turning point for the last five years,” she said, “but we still have a lot of work to do.”

Marc Benjamin: 559-441-6166, @beebenjamin

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