Storm surge facts for kids
A storm surge is like a giant wall of water that rushes onto the coast during a powerful storm. It's often linked to weather systems with very low air pressure, such as tropical cyclones (hurricanes or typhoons). You might also hear it called a storm flood or storm tide. It's measured by how much the water level rises above the normal tide, and it doesn't include the height of regular waves.
The main reason for a storm surge is strong winds pushing ocean water towards the shore over a long distance. Other things that make a storm surge worse include how shallow the water is, the shape of the coastline, when the storm hits compared to the normal tides, and how much the air pressure drops during the storm.
As extreme weather becomes more common and sea level rises due to climate change, storm surges are expected to pose a greater risk to people living near the coast. Communities and governments are working to protect themselves. They do this by building strong structures like flood barriers, or by using natural defenses like coastal dunes and mangroves. They also improve building rules and create plans for early warnings, education, and evacuation to keep everyone safe.
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How Do Storm Surges Form?
Several things work together to change water levels during a storm.
Wind Pushes Water
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Strong winds from a storm literally push water towards the coast. Imagine blowing across a bathtub; the water piles up on the far side. This is called "wind setup." The wind creates currents that move water in the direction the wind is blowing, causing water levels to rise on the downwind shore and drop on the upwind shore.
Air Pressure Changes
Tropical cyclones have very low atmospheric pressure at their center. This low pressure acts like a giant suction cup, allowing the ocean water level to rise slightly. For every small drop in air pressure, the sea level can rise a little. For example, a big storm with a 100 millibar pressure drop could cause the water level to rise about 1 meter (3.3 feet) just from this pressure effect.
Earth's Spin and Currents
The Coriolis effect, caused by the Earth's rotation, bends ocean currents. In the Northern Hemisphere, currents bend to the right, and in the Southern Hemisphere, they bend to the left. This bending can sometimes push currents more directly towards the shore, making the storm surge stronger. Other times, it can bend currents away, making the surge less severe.
Waves Add to the Power
Even though the main storm surge is about the overall rise in water, powerful waves also play a big role. Strong winds create large, forceful waves that travel in the same direction as the wind. When these waves crash near the shore, they carry a lot of water with them. This water can rush up a sloping beach, adding to the flood height and power, sometimes even doubling the wave's original height.
Heavy Rain's Role
Heavy rainfall from a hurricane can make flooding worse, especially in estuaries (where rivers meet the sea). Hurricanes can drop a lot of rain, sometimes 30 centimeters (12 inches) or more in just 24 hours. This rain quickly fills rivers and streams. When this freshwater flows into an estuary and meets the ocean water being pushed in by a storm surge, it can cause even higher water levels and more severe flooding.
Ocean Floor Shape Matters
The shape and depth of the ocean floor and coastline greatly affect storm surge and wave heights.
- A narrow shelf with deep water close to shore usually means a lower storm surge but taller, more powerful waves.
- A wide, shallow shelf often leads to a higher storm surge but relatively smaller waves.
For example, on Florida's southeast coast, the water gets very deep quickly. This means storm surges are not as high, but the waves are larger. However, on Florida's Gulf Coast, the ocean floor is very shallow for many kilometers offshore. These shallow areas experience much higher storm surges with smaller waves. This difference happens because in deeper water, the surge has more space to spread out. In shallow water, the surge has less room and is forced ashore by the storm's winds.
The shape of the land itself also matters. Areas that are only a few meters above sea level are at a very high risk of being flooded by a storm surge.
Bigger Storms, Bigger Surges
The size of a storm also affects how high the surge will be. A larger storm covers a wider area, pushing more water towards the coast. If a storm doubles in diameter, its area quadruples, meaning it can push a much larger volume of water, leading to a higher surge.
Different Kinds of Storm Surges
Just like tropical cyclones, other types of storms called extratropical cyclones can also cause water levels to rise along the coast. These storms can create high water levels over a large area and for longer periods.
In North America, extratropical storm surges happen on the Pacific and Alaska coasts, and along the Atlantic Coast north of 31°N. Sometimes, if there's sea ice, these surges can even cause an "ice tsunami," pushing ice far inland and causing damage. The Gulf Coast can also experience these surges, especially in winter.
For example, in November 2009, the remains of Hurricane Ida turned into a powerful nor'easter storm off the U.S. East Coast. This storm pushed water into places like Chesapeake Bay for several days. Water levels rose significantly, staying as high as 2.4 meters (8 feet) above normal in many areas.
How Do We Measure Storm Surges?
Scientists measure storm surges in a few ways. One way is to compare the actual water level at coastal stations with what the normal tide was predicted to be. The difference is the surge.
Another method involves placing special sensors called pressure transducers along the coastline before a storm hits. These sensors can be submerged and accurately measure the height of the water above them. This was first tried during Hurricane Rita in 2005.
After a storm surge has passed, teams of surveyors map "high-water marks" (HWMs) on land. These marks show the highest point the floodwaters reached. They take photos and write descriptions. By carefully analyzing these marks, they can figure out how much of the flooding was due to the storm surge itself, separate from regular waves or rain. These measurements are very important for understanding storms and planning for future events.
Predicting Surges with SLOSH
The U.S. National Hurricane Center uses a computer model called SLOSH to predict storm surges. SLOSH stands for Sea, Lake and Overland Surges from Hurricanes. This model is quite accurate, usually within 20 percent.
To make a prediction, SLOSH uses information about the storm, such as its central pressure, size, speed, path, and strongest winds. It also considers local features like the shape of the land, the direction of bays and rivers, the depth of the ocean floor, and the normal tides. All this information is put into a special grid called a SLOSH basin. There are many overlapping SLOSH basins covering the southern and eastern coasts of the U.S.
Sometimes, for one storm, they might use more than one SLOSH basin. For instance, when Hurricane Katrina hit in 2005, the model used basins for both Lake Pontchartrain/New Orleans and the Mississippi Sound. The model's final output shows the highest water level that occurred at each location, called the Maximum Envelope of Water (MEOW).
To account for any uncertainty in a storm's path or strength, scientists often run the model many times with slightly different inputs. This creates a map of "Maximum of Maximums" (MOMs), showing the worst-case flooding scenarios. These studies help communities plan for evacuations and build safer structures.
What are the Impacts of Storm Surges?
Storm surges can cause a lot of damage and affect many lives during cyclones. They can destroy buildings, roads, and other structures, and even weaken the foundations of homes.
Unexpected flooding in coastal areas can catch people off guard, leading to a tragic loss of life. The 1970 Bhola cyclone, for example, caused a devastating loss of life in the Bay of Bengal region.
Beyond direct damage, storm surges can also change the land. They can harm soil fertility, push saltwater into freshwater sources (called saltwater intrusion), damage wildlife habitats, and spread chemicals or other harmful substances from storage areas.
Protecting Ourselves from Surges
Even though weather forecasts warn us about hurricanes and severe storms, some areas have special storm surge warnings because the risk of coastal flooding is so high. Countries like the Netherlands, Spain, the United States, and the United Kingdom have these systems. Educating people in coastal communities and creating local evacuation plans are also key ways to keep everyone safe.
After a terrible flood in the North Sea in 1953, people started building large dams and storm-surge barriers. These barriers are usually open, allowing ships to pass freely, but they can close quickly when a storm surge threatens the land. Famous examples include the Oosterscheldekering and Maeslantkering in the Netherlands, the Thames Barrier protecting London, and the Saint Petersburg Dam in Russia.
Another modern idea, used in the Netherlands, is building floating homes in wetland areas. These homes are held in place by strong poles. These wetlands can then absorb floodwaters and surges without damaging the floating structures, while also protecting regular buildings on higher ground, as long as dikes prevent the main surge from coming in.
Other ways to adapt include building homes on stilts or elevated foundations to avoid flooding. Restoring natural protections like mangrove forests and dunes also helps, as they can absorb some of the surge's energy.
For areas on the mainland, a storm surge is usually a bigger threat when the storm comes from the ocean towards the land, rather than approaching from inland.
The "Reverse" Storm Surge
Sometimes, before a storm surge hits, the water can actually be pulled away from the shore. This happened on the western Florida coast in 2017, just before Hurricane Irma made landfall, revealing land that is usually underwater. This interesting event is known as a reverse storm surge or a negative storm surge.
Famous Storm Surges in History
One of the most impactful storm surges on record was caused by the 1970 Bhola cyclone, which led to a tragic loss of life, affecting hundreds of thousands of people in the low-lying Bay of Bengal region. This area is very vulnerable to tropical cyclone surges.
In the 21st century, Cyclone Nargis in May 2008 affected over 138,000 people in Myanmar. Another devastating surge was caused by Typhoon Haiyan (Yolanda) in 2013, which affected over 6,000 people in the central Philippines and caused an estimated $14 billion (USD) in economic losses.
The 1900 Galveston hurricane, a powerful Category 4 storm that hit Galveston, Texas, caused a devastating surge. This event resulted in a significant loss of life, affecting thousands of people, making it one of the deadliest natural disasters in U.S. history.
One of the highest storm tides ever recorded was during the 1899 Cyclone Mahina in Bathurst Bay, Australia. It was estimated at almost 13.4 meters (44 feet), though much of this was likely due to waves running up the steep coast. In the United States, Hurricane Katrina in August 2005 produced a maximum storm surge of over 8.5 meters (28 feet) in southern Mississippi. Another record surge in the same area was from Hurricane Camille in 1969, with a storm tide of 7.5 meters (24.6 feet). More recently, Hurricane Sandy caused a storm surge of 4.3 meters (14 feet) in New York City in October 2012.
See also
In Spanish: Marejada ciclónica para niños
- Coastal flooding
- Ishiguro Storm Surge Computer
- Meteotsunami
- Rogue wave
- Tsunami-proof building
