Why San Francisco Is at Risk of 20ft Waves—and Why They May Get Even Bigger

Enormous waves of up to 19 feet were forecast for early this week in San Francisco.

"There is a northwest swell that is coming in ahead of [Tuesday's] cold front," Dalton Behringer, a forecaster with the National Weather Service, told SFGate.

In the end the waves on Monday and Tuesday did not reach 19 feet, according to NOAA buoy data, but they did get to around 10 feet, roughly as tall as a UHaul truck.

"A number of factors can cause large waves like those forecast for San Francisco this week," Shane Keating, an ocean fluid dynamicist at UNSW Sydney, in Australia, told Newsweek.

"Storms and atmospheric fronts create large swells that can travel tens of thousands of kilometers across the ocean. When these waves reach the shore, they slow down, causing energy to pile up and the wave to suddenly grow in height. Large waves can also be created by winds, currents, or focusing by the seabed shape," he said.

Some areas of the world are more prone to larger waves than others because of their local climate and the structure of the coastline. These areas are at a higher risk of being damaged by the power of these waves, especially during storms.

"Large waves, especially when combined with strong winds, can raise the normal water levels and erode beaches and dunes. This can be a serious issue, particularly when houses/infrastructure are built on dunes or low areas. Large waves move a lot of sand and in some cases they can also bring sand from deeper waters to the shore," Javier Leon, a senior lecturer in physical geography at the University of the Sunshine Coast in Australia, told Newsweek.

The largest wave ever recorded was the result of a landslide in July 1958 in Alaska's Lituya Bay, which triggered a 1,719 feet high tsunami. The official largest open-water wave ever was recorded in 2013 in the North Atlantic Ocean and was 62.3 feet high.

The waves seen in San Francisco this week can be dangerous, with the National Weather Service issuing a beach hazard statement on Monday.

"The most dangerous large waves are known as 'rogue waves' (in the open ocean) or 'sneaker waves' (inshore)," Keating said. "These waves are caused by instabilities and interactions among waves as they travel in a group. Although rogue/sneaker waves are not the largest waves in the ocean, they are dangerous because they are unusually large compared to the sea state. This makes them hard to predict and they can appear without warning."

According to the National Weather Service, large waves can cause injuries by slamming down onto beachgoers, or knocking them off their feet and sweeping them into the water.

Waves are expected to increase in size and therefore become more dangerous to life and infrastructure in the future because of the effects of climate change.

"Climate change is increasing sea surface temperatures so storms are getting larger. Overall, global wave energy is increasing due to climate change," Leon said. "Larger waves, combined with higher sea levels due to sea-level rise, are increasing the risk of coastal hazards such as flooding and erosion, especially around densely populated coastal communities."

The waves off the coast of San Francisco are not expected to exceed 8 feet for the rest of the week, although there are several small craft advisory warnings along the coastline because sea conditions are still dangerous.