LA District Preps for Hurricane Hilary, Keeps Public Safe, Captures Water from Storm

In the week leading up to Hurricane Hilary – the first tropical storm to make landfall in Southern California in more than 80 years – the wheels were already in motion for the U.S. Army Corps of Engineers (USACE) Los Angeles District.

Tropical Storm Hilary made landfall Aug. 20, bringing heavy rain and with localized flooding to parts of Southern California and Nevada. Sporadic showers continued through Aug. 21, with the storm breaking rainfall records.

The district was prepared to ensure the safety of its workforce and communities, as well as capture additional water from the storm.


On Aug. 17, District Commander Col. Andrew Baker declared a state of emergency for the district due to the impending storm. Additionally, the district’s Emergency Management Branch activated its Crisis Management Team.


With 17 USACE-owned and operated dams across the district, including projects in Southern California, Arizona and portions of Nevada – and with a majority of them in the potential path of the storm – the district’s Water Management Team began running engineering models, based on the forecasted precipitation, to produce inundation maps.

“Because all of the district’s reservoirs were dry, the models’ simulations predicted that all reservoirs would be able to contain rainfall from Tropical Storm Hilary,” said Tim Fairbanks, chief of the LA District’s Hydrology and Hydraulics Branch, Engineering Division. “As a precaution, the team ran the models based on two times the forecasted precipitation as a worst-case scenario. Even under these conditions, the model did not result in damages.”

Col. Andrew Baker, center left, listens during an after-action review Aug. 28 about the U.S. Army Corps of Engineers Los Angeles District’s response to Tropical Storm Hilary, starting with day one leading up to the storm, throughout the storm and the two days following the storm. USACE PHOTO BY BROOKS HUBBARD

Throughout the storm, the Los Angeles District monitored its flood-risk management projects to ensure they continued to operate as designed, as well as coordinated with other agencies and counties to keep communities informed about safety in and around USACE-owned and -operated dams, levees, and basins during the storm event.

The district’s Reservoir Operations Center was activated Aug. 20 to keep internal and external partners aware of precipitation and elevation levels of the district’s dams and reservoirs in the path of the storm – reporting actual precipitation, ranging from inline to less than what was forecasted.

“All of the LA District’s flood-risk management systems performed as they were designed, with no significant issues,” Fairbanks said.


Additionally, the district’s Operations Division activated its Joint Protocol for unsheltered communities, collaborating with multiple agencies to evacuate homeless individuals from rivers and basins in and near USACE-owned dams and waterways.

“Life safety is always our priority,” said Trevor Snyder, program manager and homeless encampment liaison with the district. “The Los Angeles District activated its joint protocol for unsheltered communities, working with multiple resource providers, to ensure the most vulnerable individuals are provided shelter options during this unprecedented storm.”

More than 65 homeless individuals were provided shelter during an Aug. 19 evacuation operation at the Santa Fe Dam, which also included collaboration with the cities of Azusa and Irwindale police departments; LA County Sheriff’s Department’s Homeless Outreach Services Team; and the Los Angeles Homeless Services Authority.


The Los Angeles District also began coordinating with its Department of Defense (DOD) partner agencies prior to the storm, including the Army, Air Force, and Naval Facilities Southwest, to ensure staff members were pre-positioned at military installations to respond to potential storm operations.

Support for shoaling, dredging, and marine structures was offered to Navy and Coast Guard partners, although the district did not receive any requests. Navy Facilities Southwest requested follow-on dam safety support, if needed, to inspect dam structures at its installations.


Throughout the storm, the district was able to capture about 19,000 acre-feet of storm water – at a total value of about $19 million – for ground water recharge from its dams, in collaboration with its partners, while also providing flood protection for the public.

One acre-foot of water is equivalent to 325,851 gallons of water, one-half the size of an Olympic-size swimming pool and can serve two or more Southern California households for a year.

All projects in Los Angeles County Drainage Area, including Whittier Narrows, were operated in accordance with the district’s Water Control Manual to include water conservation during the storm.


About 75% of stormwater runoff – an estimated 7,000 acre-feet, valued at $7 million – passed through Whittier Narrows Dam and was captured downstream for groundwater recharge by the Los Angeles County Public Works.

One hundred percent of water runoff passing through Prado Dam – an estimated 12,000 acre-feet, valued at about $12 million – was captured downstream for groundwater recharge by the Orange County Water District.

“The forecasts for the region were extensively used in preparation of Hurricane Hilary’s landfall to help predict the potential impact of the storm on the dams and their downstream channels,” Fairbanks said. “This is in line with the Forecast Informed Reservoir Operation, or FIRO, concept; however, because the reservoirs were near empty, no pre-storm releases were required to help manage the event, and the dams were operated per their approved Water Control Manual.”

Some dams in the path of Hurricane Hilary, such as Sepulveda Dam, do not have recharge facilities downstream to take advantage of this opportunity.


Although Hurricane Hilary’s landfall in Southern California was a rare event, the Los Angeles District prepares for storm events year-round through tabletop and joint exercises with its partner organizations and communities.

The district’s Emergency Management (EM) Branch is funded to conduct two Flood Control and Coastal Erosion exercises per year. In fiscal year 2023, the EM Branch conducted an exercise focused on the Los Angeles River – with particular emphasis on the relationship between the Los Angeles County dams that are upstream of the USACE dams on the Los Angeles River – and an exercise with the Arizona Department of Emergency and Military Affairs, focused on supporting small towns, with Duncan, Arizona, used as the case study.

This year, the EM Branch plans to conduct an exercise with the state of Nevada, focusing on the relationship between the non-federal flood-control structures that are near the district’s Pine and Mathew dams. Emergency management specialists also participate in non-federal partnership and Silver Jackets’ exercises – most recently the Santiago Creek Dam workshop and exercise in August in Irvine, California.

The district’s Dam Safety Team conducted two tabletop exercises in fiscal year 2023 – Prado and Whittier Narrows dams. The team is scheduled to conduct seminars for Carbon Canyon, Fullerton, and Santa Fe dams in September, incorporating the San Gabriel Levee 3 and 7 into the Santa Fe Seminar.


District leaders involved in the Tropical Storm Hilary response conducted an after-action review Aug. 28 at the district headquarters building in downtown Los Angeles to identify what the team did well and what could be improved in preparation for future storm events.

“Everything went as well as it possibly could have,” said Baker, who took command of the district in July. “Everything performed exactly like it was supposed to, and there were no casualties. I was so impressed with what the district did and how we came together; the teamwork between all of the different divisions [within the district] was amazing, along with the initiative that our employees showed in this crisis.”

This article is featured in the 2023-2024 edition of America’s Engineers: The People, Programs, and Projects of the U.S. Army Corps of Engineers


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