What if the Oroville Dam had collapsed?

If California’s Oroville Dam were to collapse, an estimated $21.8 billion of building value would be at risk in an area where many of the communities are uninsured, Risk Management Solutions Inc. said Friday.

The Newark, California-based catastrophe risk modeler produced two “what-if” scenarios that simulated what might happen if the Northern California dam either collapses suddenly or experiences a controlled breach.

The worst-case scenario is modeled assuming the height of the dam wall becomes “zero” instantaneously, such as an explosion due to sabotage. For the controlled breach scenario, RMS said the dam wall is gradually lowered at two meters per hour to simulate what could have happened had the erosion of the dam continued.

Last month, pressure from a combination of heavy rainfall and melting snow that had filled Lake Oroville punctured a 300-foot hole in the dam’s main concrete spillway. RMS said that at least 188,000 residents were ordered to evacuate on February 12, while trucks and helicopters dumped over 1,000 tons of material on the weakened dam.

RMS said the scenarios highlight the risks of aging infrastructure that may not be able to withstand extreme weather. RMS said the National Inventory of Dams indicates that of the 90,580 situated across the U.S., with over 30% exhibiting significant to high hazard potential due to structural deterioration.

“In our modeling,” RMS said in a statement, “the amount of discharged water would be expected to overwhelm local flood control measures over 100 miles downstream, as was evidenced by the widespread flooding in the simulations. We estimate that under either scenario, about $21.8 billion of building value would be at risk, which, for the communities near Oroville, would be a huge problem because much of it is uninsured.”

Most property owners rely on the National Flood Insurance Program, RMS said, which is most commonly sold to homes in the 100-year flood plain along the Feather River. RMS said its modeling shows the area that would be inundated by breach is much greater than the 100-year-old flood plain coverage area.

“Residents living outside the 100-year flood zone are not required to purchase flood insurance, and therefore most do not,” RMS said.

RMS said that while dam failure is rare, it is not a negligible risk and cited a situation during the 1971 San Fernando earthquake where the lower Van Norman Dam was a near-breach and forced the evacuation of 80,000 people.