Explainer: How Hurricanes Cause Dangerous and Destructive Storm Surge

Storm surges are typically the deadliest and most destructive phenomenon accompanying a hurricane or tropical cyclone.

Here we explain how they are formed and why they are so dangerous.

WHAT IS A STORM SURGE?

When a hurricane approaches the coast, strong winds push ocean water toward land. The atmospheric pressure from the storm also helps squeeze water toward the shore. The shallower the continental shelf, the greater the threat of dangerous surf. It may take a couple of days for the waters to subside completely.

According to the U.S. National Hurricane Center, storm surge is typically the biggest threat to life and property from a hurricane.

What makes storm surge so dangerous?

Water is heavy: about 1,700 pounds (770 kg) per cubic yard. And it can move quickly in a wave, sweeping people to their deaths, shaking ships and vehicles, and pulverizing buildings and other structures. According to the hurricane center, six inches (15 cm) of fast-moving water is enough to knock over an adult. Swells become even more dangerous when they coincide with high tide.

A powerful storm surge can cause long-term damage by washing away roads, eroding beaches and contaminating land with salt water, damaging wildlife and agriculture.

WHAT WAS THE DEADLIEST STORM SURGE ON RECORD?

In 1970, a tropical cyclone formed in the Bay of Bengal and struck northeastern India and what was then eastern Pakistan. A storm surge of up to 35 feet (10.7 m) swept across barrier islands, including Bhola, the largest. The death toll was estimated at between 300,000 and 500,000 people, making it the deadliest tropical cyclone ever recorded, according to the Atlantic Oceanographic and Meteorological Laboratory.

WHAT WAS THE LARGEST STORM SURGE?

The highest storm surge recorded in historical accounts was produced by Cyclone Mahina of 1899, estimated at nearly 44 feet (13.4 m) in Bathurst Bay, Australia.

In the United States, one of the largest storm surges on record was during Hurricane Katrina that hit the Gulf Coast states in 2005. The highest storm surge mark observed was 27.8 feet (8.5 m) at Pass Christian in Mississippi, according to the National Hurricane Center. The hurricane killed about 1,500 people.

In Florida, Hurricane Michael in 2018 caused storm surges 20.6 feet (6.3 m) high and destroyed Mexico Beach in the Florida Panhandle.

HOW DOES CLIMATE CHANGE AFFECT STORM SURGE?

According to climate scientists, warmer ocean temperatures create more intense and frequent hurricanes that generate greater precipitation and a greater risk of storm surge due to rising sea levels.

HOW TO PREPARE FOR A STORM SURGE

The most important way to survive storm surge is to evacuate if you are in a flood zone. If you cannot evacuate, experts advise moving to the highest level of the building and staying indoors. They warn against attempting to drive through flooded water. REUTERS

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