What is storm surge? Rising water and flooding the most deadly parts of a hurricane

What is storm surge, and how serious is it? You’ve heard WRAL meteorologists discuss storm surge and the damage high water levels can cause, especially during major hurricanes. As it turns out, storm surge can be the most deadly part of a hurricane.

What is storm surge?

The National Oceanic and Atmospheric Administration (NOAA) defines storm surge as the rise in seawater level caused solely by a storm. When combined with normal tides, storm surge can increase the ocean’s water level by 30 feet or more.

According to NOAA, storm surge is the water pushed toward shore by hurricane-force winds while storm tide is the total observed seawater level during a storm, a combination of storm surge and normal high tide.

Storm surge is deadly

When people think of the life-threatening dangers from hurricanes, they probably think of strong winds, but most hurricane deaths are actually caused by flooding and storm surge.

According to the National Hurricane Center, nearly half of all U.S. deaths from tropical systems are due to storm surge — far fewer than the 10% of deaths in a hurricane caused by winds.

Storm surge can cause devastating flooding — fast — and is why it’s so important for people in coastal communities to evacuate when they’re told to do so. Alerts are issued when flood waters reach 3 feet above ground level, which is considered life-threatening. Warnings go out to the public 48 hours to 36 hours before flooding is expected.

It’s extremely dangerous to ignore evacuation orders during a hurricane. In this 3D model, animation shows what just 3 feet, 6 feet and 9 feet of water can do to a home.

Just a few inches of water in your home can knock out power, and as the water rises, furniture and other objects can begin to float. Up to 3 feet of water inside a home will force a resident to look for higher ground, such as a second floor or attic.

Once water inside a home rises to 6 feet, the flooding is life-threatening, especially due to sharp and electrically-charged objects that could be floating around in the water.

Water levels up to 9 feet will reach the ceilings in most homes and cause further danger.

How high can storm surge get?

Stronger storms will produce a larger storm surge.

In 2005, Hurricane Katrina, a Category 3 storm at landfall, produced a 28-foot storm surge in Louisiana, according to NOAA. In 2011, Hurricane Irene, a Category 1 storm at landfall, produced an 8-foot to 11-foot storm surge in North Carolina.

How much damage can storm surge do?

The National Weather Service estimates the damage storm surge can do when combined with hurricane-force winds.

Category 1 storm, 4 to 5 feet above normal tide: Damage to shrubbery, trees, poorly constructed signs, and unanchored mobile homes. Low lying coastal roads inundated.

Category 2 storm, 9 to 12 feet above normal tide: Damage to roofing materials of buildings; some wind and door damage. Major damage to exposed mobile homes. Coastal roads and low lying escape routes made impassable.

Category 3 storm, 9 to 12 feet above normal tide: Large trees blown down, some damage to roofing materials of buildings, windows and doors. Some structural damage to small buildings. mobile homes destroyed. Serious flooding at the coast, large structures damaged by waves and debris.

Category 4 storm, 13 to 18 feet above normal tide: Shrubs and trees blown down, extensive damage to roofing materials, windows, and doors. Complete failure of roofs of many small residences and destruction of mobile homes. Major damage to lower floors of structures near shore.

Category 5 storm, storm surge greater than 18 feet above normal tide: Shrubs and trees blown down; considerable damage to roofs of buildings. Very severe and extensive damage to windows and doors. Complete destruction of mobile homes. Massive evacuation of residential areas on low ground within 5 to 10 miles of shore possibly required.

Not just oceans cause flooding

Rising ocean levels and storm surge aren’t the only contributors to serious flooding — rising river levels can also deliver dangerous impacts during hurricanes.

In September 2018, Hurricane Florence dumped 8 trillion gallons of water across North Carolina just two years after Hurricane Matthew brought devastating rainfall to the same area. Both hurricanes brought record flooding to N.C.’s rivers.

During Matthew, the Lumber River at Lumberton crested at 28 feet. Two years later, during Florence, it crested at 29 feet, the largest in history, even covering parts of Interstate 95.

The Neuse River in Goldsboro also crested to record levels during Matthew, at 29.74 feet. Two years later in Florence, it reached the third-highest crest on record, 27.6 feet.

The Cape Fear River in Fayetteville topped out at 58.94 feet during Matthew, the sixth-highest crest on record. In both instances, rivers remained above flood stage for weeks after the storms passed.

Take action during severe weather

Download the WRAL News app to get severe alerts and warnings when there is hurricane or flood risk in your area. Evacuate if you are ordered to do so, bringing pets and emergency supplies.