Rising temperatures are creating major headaches for engineers trying to figure out the best ways to address environmental impacts ranging from increasing severity of storms to persistent droughts.

“We live on a knife’s edge on the West Coast,” David Curtis, a longtime engineer and vice president of West Consultants, based in Folsom, California, told the Portland chapter of the Society of American Military Engineers (SAME) during its monthly luncheon earlier this month. “Atmospheric rivers have a profound effect. (California) gets five storms (annually) on average. If we get two (fewer), that is 20 percent less water; now you are in a drought.”

Those same atmospheric rivers provide much of the rain and snowpack in the Pacific Northwest. A warming atmosphere has a profound effect on the atmospheric rivers’ severity, according to Curtis. When temperatures increase, more water vapor ends up on the air. Once the atmospheric rivers reach West Coast mountains, an uplift can create enormous storms with devastating impacts. The changes also mean more precipitation falls as rain instead of snow.

Last year, a high-powered atmospheric river nearly triggered a catastrophe at Oroville Dam, north of Sacramento. The 770-foot dam is the tallest in the U.S. and helps supply water to nearly 20 million people.

“The main (spillway) is over 3,000 feet long from the top of the dam to the bottom,” Curtis said. “The emergency spillway is 1,700 feet wide. It is a massive structure.”

But a series of winter storms provided more runoff than the reservoir could hold. The problem was compounded when the main spillway used to release water and pressure from the reservoir developed a 35-foot-wide hole, forcing the dam operators to stop using it.

The dam’s auxiliary and emergency spillways were not designed to handle the amount of water that needed to be released. The resulting erosion in the auxiliary spillway threatened to damage the emergency spillway, which had never been used.

Had that erosion worsened, a catastrophic wall of water would have inundated cities downstream. That threat prompted evacuations by more than 180,000 people. A major catastrophe was averted when dam operators decided to continue using the damaged main spillway, along with the auxiliary and emergency spillways.

Nevertheless, between the evacuations and damage to the dam itself, the cost of the storms’ impact was more than $1.2 billion. This came despite highly accurate forecasts as to what to expect.

“They forecasted future information within four or five percent,” Curtis said. “That difference pushed it over the spillway and started the erosions.”

Such events are expected to become more common as the climate continues to warm, according to Curtis.

“The natural variability scale of the West Coast is really strong,” he said. “It is being compounded by the stuff we are doing to the atmosphere.”

The Oroville Dam case shows the importance of taking these issues into account when planning new projects or improving existing infrastructure, Curtis said. Despite the best forecasts and planning, people along the West Coast are going to have to learn to live with higher risks due to a warming atmosphere, he said.

“At the end of the day, it’s not an engineering question,” he said. “It is an economic risk question.”

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