Next California worry: Flood risk from dam
Californians have endured the dual hardships of wildfires and mass power outages meant to prevent them, not always effectively.
Now comes word that desert communities in the Golden State could be at risk of flooding because of concerns that the Mojave River Dam could breach in an extreme weather event.
A decision by the U.S. Army Corps of Engineers about a danger most Californians didn’t even know existed serves as a reminder that everyone is at the mercy of Mother Nature, and preventive measures can go a long way toward sparing life and property.
What did the US Army Corps do?
It switched the Dam Safety Action Classification of the Mojave River Dam from low to high urgency of action, meaning steps must be taken to safeguard communities close to the river – such as Hesperia, Apple Valley, Victorville and Barstow – from flood hazards.
A risk assessment determined that, in an extreme weather event, water could flow over the nearly 50-year-old dam and it could breach, endangering 16,000 residents downstream and property valued at $1.5 billion. Floodwaters could reach as far as 140 miles away.
What measures must be taken?
The most immediate is raising awareness about preparation. Residents are encouraged to assemble an emergency kit, sign up for phone alerts and formulate evacuation plans.
“We’re going to be enacting a lot of emergency-preparedness activities,” said Kristen Bedolla, dam safety program manager for the Corps’ Los Angeles District. “As we turn the corner toward winter, we’re definitely going to be more proactive in working with our downstream partners.”
The agency will conduct further studies to assess whether the dam – which goes through regular maintenance and upgrades – needs further hardening. Interim measures also are being considered before the rainy season arrives in the coming weeks.
How big is the risk?
It’s minimal, but it does exist. The dam is about 200 feet high, and its highest watermark ever, 72 feet in 2005, did not reach the halfway point. The emergency spillway has never been used.
Lillian Doherty, chief of the operations division for the L.A. District, said the chances of water flowing over the dam are 0.01% percent in any given year.
Why bother, then? Perhaps the names Katrina and Harvey will answer that question. Those hurricanes brought unthinkable damage to states along the Gulf of Mexico. In February 2017, both spillways for the Oroville Dam in Northern California were damaged by floodwaters, and 190,000 people had to flee their homes.
Is climate change a factor?
Of course. Although the Mojave River Dam, built in 1971, has been performing as designed, old weather models no longer accurately predict what conditions to expect. Evidence of climate change has been a motivator in the decision to further assess the dam’s ability to withstand extreme floods.
“That is our mission, to reduce the risk for the communities we serve,” Doherty said. “With climate change, we are seeing more severe events on a more regular basis.”
Floods in the desert, really?
The Mojave Desert, primarily located in southeastern California and southern Nevada, is regarded as the driest desert in North America, with an average yearly precipitation of less than 5 inches. The western part is not quite as parched, averaging 6.7 inches, mostly stemming from winter storms rolling east from the Pacific. When that happens, the water can accumulate quickly in areas not used to it.
“We have a very reactive system,” Doherty said. “Just a little bit of rain can cause huge flash floods in any area, including the desert.”