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Corps dams help LA, Orange counties capture $25M in December stormwater runoff

U.S. Army Corps of Engineers Los Angeles District
Published Jan. 18, 2022
Pictured is Whittier Narrows Dam, a flood-risk management and water-conservation project that serves as a central element of the Los Angeles County Drainage Area flood control system. With the help this dam and three others in the Los Angeles District’s inventory, stakeholders in Southern California captured approximately 30,000-acre feet of stormwater runoff valued at $25 million for local groundwater replenishment during heavy rainfall in December 2021.

Pictured is Whittier Narrows Dam, a flood-risk management and water-conservation project that serves as a central element of the Los Angeles County Drainage Area flood control system. With the help this dam and three others in the Los Angeles District’s inventory, stakeholders in Southern California captured approximately 30,000-acre feet of stormwater runoff valued at $25 million for local groundwater replenishment during heavy rainfall in December 2021.

LOS ANGELES –– With the help of the Los Angeles District, stakeholders in Southern California captured approximately 30,000-acre feet of stormwater runoff valued at $25 million for local groundwater replenishment during December’s heavy rainfall.

Prado and Whittier Narrows dams actively captured the runoff, with Hansen and Santa Fe dams playing a passive role in retaining widespread storm flows for groundwater replenishment instead of running into the Pacific Ocean.

Impounding and slowly releasing the water is not an automatic process, however. It’s a team effort that requires planning, expertise and close coordination between agencies, all while factoring in flood risk, changing weather forecasts and prescribed U.S. Army Corps of Engineers water-control plan requirements.

Each dam has its own operating criteria and ability to assist downstream water agencies to divert the dam’s outflow into their spreading grounds for percolation into the ground as a means of recharging the groundwater basins.

“Not every dam has the ability to recharge downstream, but those four do,” said Rene Vermeeren, chief of the Hydrology & Hydraulics Branch, in the district’s Engineering Division.

Prado and Whittier Narrows dams are more operationally intensive than Hansen and Santa Fe dams, Vermeeren added, and it was those first two that helped capture the lion’s share of runoff during the storms of Dec. 14, 24, 27 and 30.

Whittier Narrows Dam

Located in a gap in the hills forming the southern boundary of the San Gabriel Valley, Whittier Narrows Dam is a flood-risk management and water-conservation project that serves as a central element of the Los Angeles County Drainage Area flood control system. The San Gabriel and Rio Hondo rivers flow into the east and west sides, respectively, of the reservoir that impounds behind Whittier Narrows Dam.

“Whittier Narrows is complex because there’s two sides,” Vermeeren said. “There’s the San Gabriel side – which is actually the spillway for the dam – but the main component is the Rio Hondo side. That’s where a lot of our effort is focused and where coordination goes on with the Los Angeles County Department of Public Works.”

The primary purpose of the dam is flood-risk management, but both sides of the system have capacity for water conservation downstream, Vermeeren said. The LA District coordinates closely with the LA County Department of Public Works to direct the dam’s water releases into the county’s spreading grounds.

The San Gabriel River below the dam is a soft-bottom channel with hardened sides, where the county has built a series of rubber dams to impound the water for percolation. The Rio Hondo downstream is a concrete channel. About a mile downstream from Whittier Narrows, the county has gates that divert the water into their spreading grounds.

The district closely coordinates releases with the county to match their diversion capacity up to the point where the reservoir conditions require the Corps to begin making flood-control releases according to their approved water-control operating plan.

Reservoir Operations Center activation

“The Reservoir Operations Center is not activated normally because most of our dams are dry for the better part of the year, but when there’s significant rain forecasted, then we activate,” Vermeeren said. “We also work with our operations folks to man the dams for flood operations. All releases from district projects are directed by our Reservoir Regulation Section within the Hydrology and Hydraulics Branch.”

The ROC runs three eight-hour shifts for around-the-clock coverage and remains activated while there are ongoing water management activities under their direction. ROC staff has access to customized weather and river forecasts, real-time rainfall, streamflow and reservoir field gages, keeps track of all notifications made to impacted entities, and collects field reports from channel observers, among other duties.

According to Jon Sweeten, a senior engineer in the Reservoir Regulation Section, it’s not uncommon for personnel in the section to work holidays when the ROC is activated.

“Having worked in the section for about 15 years, it seems to always rain on a holiday,” said Sweeten, who takes on the role of shift leader in the ROC. “The last big event we had was 2010 on Christmas. It’s just the nature of the beast. When you sign up in Reservoir Regulation, you accept that that’s part of the deal.”

Prado Dam

Located in Riverside County on the Santa Ana River, Prado Dam is a flood-risk-reduction project protecting Orange County and the Santa Ana Coastal Plain. It also has a water conservation function in conjunction with the downstream Orange County Water District.

Though they are different projects, operations at Prado Dam during storms are similar to those at Whittier Narrows Dam.

“They also use a rubber dam to do the same kind of thing,” Vermeeren said. “They’ll inflate the rubber dam, hold water, and it also diverts water through an outlet in the levee that goes into their spreading grounds. The rate of recharge for this facility maxes out at about 600 CFS. We try to accommodate that with any kind of runoff event at Prado. If the runoff event is large, like what occurred on December 23rd, we have to go into flood-control mode and make releases larger than the water district can divert.”

“They want to capture as much water as they can, but they understand that our primary mission is flood control,” Vermeeren said. “So, they can’t capture 100 percent of all flood runoff, but they can capture some. It’s continuous coordination.”