City of Irvine
2020 Local Hazard Mitigation Plan
• In March 2003, 3 to 7 inches of rain fell on Southern California, causing region-wide flooding. Water reached depths of up to three feet on some roadways, causing over 1,000 vehicle collisions. • In January 2011, California received a Presidential Declaration for the Severe Winter Storms, Flooding, and Debris and Mud Flows that occurred over a nearly three-week period. During this incident, the State of California incurred well over $75 million in damages, of which over $36 million occurred within Orange County. Much of the damage impacted private and public property, as well as critical infrastructure. • In 2014 heavy rains affecting most of Southern California caused flooding on a section of Bastanchury Road that was nearly a foot deep. Nearby weather stations reported that more than an inch of rain had fallen in a span of three hours. • In September 2015, flooding of roadways caused severe traffic congestion across Southern California, including Orange County. In the City of Los Angeles, 7,300 people lost power for most of the day, and there were more than 500 traffic collisions across the entire region as a result of the road conditions. • 2017 Winter Storms 66 included three storms over six days inundating southern California. Heavy rains, combined with already saturated soil, produced flash flooding across much of Orange County. Streets flooded with 1 to 3 feet of water in Huntington Beach, Santa Ana, and Newport Beach. The storms resulted in a Presidential Disaster Declaration for 16 counties throughout the state.
Risk of Future Events
There is no indication that the severe rainfall that leads to flooding will abate in the future, either in Irvine or the greater region of Southern California. While Irvine may experience prolonged periods of dry or wet years, flood events will likely continue to impact the city. For areas within the 100-year and 500-year flood hazard zones, the likelihood of flooding to occur on an annual basis is 1% and 0.2%, respectively.
Climate Change Considerations
Climate change is expected to alter the frequency of intense precipitation events throughout California, including Irvine. Intense rainfall is expected to occur more frequently (perhaps twice as often by the end of the 21st century) and potentially drop more rain (up to 40 percent more). These projected changes likely mean that Irvine will experience more frequent and more intense flooding, potentially leading to erosion, dam failure, tree mortality, and other potential hazards.
Geologic Hazards
For the purposes of this plan, geologic hazards include landslides and subsidence.
Description
Landslide Landslides occur when earth on slopes become destabilized, typically after heavy rains, when the precipitation saturates the soil and makes it less stable, or when significant erosion from rainfall destabilizes the ground. Slopes that have recently burned face a greater risk from rain-induced landslides,
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