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What is a Storm Surge and How is it Formed?
A storm surge is a destructive weather phenomenon that forms near sea coasts by hurricanes or cyclones. In this article, we shall learn what is a storm surge and how does it form. We’ll also look at what safety tips one can take to minimize the damage caused by these surges.
The largest storm surge on record occurred during the 1970 Bhola Cyclone, destroying thousands of homes and killing over 500,000 people near the Bay of Bengal.
Whenever a hurricane moves near coastal areas, storm surges are the biggest and most common threat to life and property. This phenomenon is commonly found in low-pressure systems, and the severity of the storm surge wave depends on the tides, shallowness of the water in the area, and the angle at which the water is to the hurricane.
A storm surge is not to be confused with a tsunami, because although both events can lead to the formation of huge, destructive waves of water, a tsunami can only be formed by an earthquake or other seismic activity. On the other hand, a storm surge is formed primarily due to the high velocity winds of a hurricane, and to a lesser extent, low-pressure conditions. It should also not be confused with a storm tide. When a storm surge and high tide combine their forces, it forms an even stronger surge, known as a storm tide. Let us now look at some more interesting information about storm surges.
Causes of Storm Surges
There are many factors that contribute towards the formation and propagation of a storm surge. Let us look at each of them closely:
- Strength and Size of the Storm: During a hurricane, the water level rises to form storm surges, where the strength and speed of the winds are the highest. Usually, the largest surges occur in the direction of where the wind is blowing. Due to the rotation of the earth, the surge occurs towards the right side of the hurricane in the northern hemisphere, and towards the left side in the southern hemisphere. A larger storm will also cause a larger surge.
- Atmospheric Pressure: The force exerted by the atmospheric pressure is a smaller factor in the formation of a storm surge. The atmospheric pressure is the highest at the edges of the storm, and gradually reduces as it nears the center. Due to the low pressure, the water bulges outwards, starting off a high surge.
- Bottom Conditions Near Shore: Another minor factor determining the strength of a surge is whether the coastal slope is steep or shallow, and rough or smooth. A shallow and smooth ocean floor near the coast can dramatically enhance the speed and power of the storm surge, while a steep climb with rough obstructions can slow and sometimes even stop a storm surge. A wider shore will have a higher surge than a narrower shore.
- Distance from Storm Center to Shore: For a storm surge to achieve maximum potency, the distance between the eye of the storm and the shore should neither be too close nor too far. If the distance is less, the surge cannot gather enough velocity to gain power. However, if the storm is too far, the surge will lose its gathered energy by the time it reaches the shore.
- Tides: The gravitational force of the sun and moon cause low and high tides. If the storm surge occurs during a low tide, the intensity will be significantly reduced. However, a storm surge during high tide will cause a storm tide capable of heavy destruction.
- Sea Waves: When waves break onto the beach, they may collect into pools, eventually making it easier for the surge to overcome the friction of the beach, and move even further inland.
- Freshwater: Usually, before a storm reaches land, most coastal areas receive heavy rainfall, causing water levels to rise. This is especially true in areas that have a river delta, causing bigger and stronger storm surges.
- Shape and Angle of Coast to the Storm: A shore with a convex shape will have a lower surge as compared to a concave shore. Also, if the storm is moving parallel to the shore, it will cause lower and weaker storm surges as compared to a storm moving perpendicular to the coast.
Storm Surge Formation Process
When a hurricane is in deep ocean waters, the circulating wind pushes the ocean surface to create a vertically circulating column of water, where the surge is barely visible. However, as the storm moves closer to the shore, the water which is being pushed downwards by the wind cannot move any lower, so the water forces itself from the sides towards land, causing a storm surge wave. Although low pressure also contributes to the surge, its influence is very small, i.e., around 5%. An average storm surge can reach heights of around 15 feet, while a storm tide will reach around 17 feet. However, larger and smaller ones have also been observed and recorded. On the other hand, the width of these waves depends upon the shoreline length, with larger surges being recorded at longer beaches.
Storm Surge Scale
There are 6 categories of storm surges, depending on the height of the surge and the wind speed of the storm. Let us look at them in some detail:
- Tropical Storm: With a poor wind speed of 40 – 75 mph, the chances of a noticeable storm surge is low, with the wave coming in at heights of 3 – 5 feet. Usually, any storm surge of this size will cause no damage.
- Category 1: Minimal: The wind speed of these storms is slightly higher, and can range between 75 – 95 mph. The surges range around 5 – 7 feet in height, which can cause minor damage to poorly constructed buildings or objects that are not anchored.
- Category 2: Moderate: With increasing size and wind speed of around 95 – 110 mph, the surges come in heights of 7 – 12 feet. They can cause considerable damage to mobile homes, vehicles, and small watercraft.
- Category 3: Extensive: Strong winds of 110 to 130 mph cause surges of 12 – 15 feet. These storms can cause structural damage to small buildings, while poorly built buildings and mobile homes can be completely destroyed. Flooding can reach deep inland, and floating debris can slightly damage large buildings.
- Category 4: Extreme: One of the most common types, with wind speeds of 130 – 155 mph, the storm surges can get to heights of 15 – 20 feet. These surges can cause heavy flooding well inland, and can erode beaches severely. Large and small buildings can be considerably damaged, while any un-anchored object is likely to be destroyed.
- Category 5: Catastrophic: When hurricanes blow winds that go faster than 155 mph, they create huge surges that can reach 20 – 25 feet, and in rare cases, even higher. Larger buildings can get heavily damaged, while the smaller ones can be completely destroyed. Deep and strong flooding inland can damage lower floors of most buildings. Usually, large-scale evacuation is needed in the region.
There is usually some loss of life, when a Category 2 or stronger surge hits. Combined with the force of the hurricane itself, big storms can claim thousands of lives. Sometimes, when the direction of the wind is towards the ocean, a negative storm surge will occur. These surges cause lesser damage, but can damage ships that are docked at ports. It is important to note that the above scale will differ in different coastal areas due to terrain factors in the region.
Storm and Sea Level Rise
Now that we have seen why it is dangerous, let us look at another concerning factor that is making storm surges much more dangerous – rise in sea levels due to global warming.
Although the temperature rise of 3.5 degree Fahrenheit does not seem much, it is gradually increasing the level of the oceans around the world. Researchers studying storm surges concluded that the frequency of the biggest destructive surges will reduce from once in a century to once in a decade.
It is predicted that sea levels are set to rise by 3 feet by 2100. And while this change can be devastating, there is enough time to plan for it. The Gulf of Mexico, the Pacific coast, and other low-lying coasts are candidates for increasing storm surges, and it is important for the industrialists and governments to consider this while creating infrastructure at the coast, and work on environmental protection.
Hurricane Sandy Sea Surges
Although hurricanes like Katrina and Ike caused devastating damage, and were the cause for destructive storm surges, super storm Sandy of 2012 is deeply ingrained in the memory of the US. The immense size and intensity of the storm was damaging for a large part of the eastern coast. Storm surges by Hurricane Sandy reached areas like New York City, the Jersey shore, Staten Island, Long Island, New England, etc., with such intensity and mass that inland flooding was extensive, and such a scene had not been seen since the 1820s. Poor planning, despite an idea of potential damage, caused severe losses, especially in parts like the NYC subway system. Here’s more information on Hurricane Sandy.
Safety Tips and Measures Before, During, and After a Storm Surge
The NOAA (National Oceanic and Atmospheric Administration) has put in place many tide stations, pressure sensors, and high water marks across the United States coastal areas. They can use these machinations to get reliable measurements of a storm surge by getting real-time data, measuring still water, and other weather forecasting techniques. The organization has also recently deployed a computer model called SLOSH (Sea, Lake, and Overland Surges from Hurricanes) to predict storm surges and the extent of inland flooding, that can form a very effective warning system.
If you live in an area that is prone to storm surges, there are a few things you can do to keep yourself and your close ones safe.
Before a Storm Surge
- Make regular checks of your home for any areas vulnerable to floods. If you find any, make prompt repairs.
- Check with your local authorities to learn how to use sandbags to protect your house from floods.
- Store all important items such as valuables and documents in the upper floors of the home.
- Prepare and train your family with an emergency kit and plan. Plan a temporary second residence in case you have to evacuate.
- Make sure you and your family know first aid and CPR.
- Learn the maps of the local area well.
- Board or shutter your windows.
- Turn off the electricity and gas supply during such a disaster.
- Keep a battery-operated radio with you at all times to receive emergency broadcasts.
- If you hear a storm forecast, get all your emergency supplies ready and keep them nearby.
- Close all the basement entrances.
- Keep your vehicle fully fueled and ready for evacuation.
During a Storm Surge
- Stay inside your home, away from any windows.
- Be alert and constantly look out for warnings or instructions from officials.
- Pay careful attention to evacuation plans before driving away in your vehicle.
- Do not drive in flood water, as just a 2-feet deep storm surge is capable of washing your vehicle off the road.
- Inform your neighbors if you plan to leave the area before any evacuation warnings have been given.
- Fatal storm surges can take place even in weaker categories of storms, so don’t take such events lightly.
- Always obey evacuation orders. Staying back after such instructions are given is extremely life-threatening.
After All-Clear Signal
- Be careful of damaged roads, broken or dangling electric wires/cables, and report any of these to the authorities.
- When going back into your home, check for gas leaks.
- Find and discard any spoiled food or water.
Over and above the given instructions, some measures need to be taken by the people of the locality to reduce the effects of a storm surge in the area.
- Creating designation for shelters.
- Planning evacuation routes.
- Specifying which areas need to be evacuated.
- Working on the efficiency of the emergency services, like the fire and police department, with the authorities.
- Working on protecting any surrounding wetlands, such as estuaries, mangroves, and marshes, which work in blunting the force of a storm surge.
The above information and safety tips should help you prepare yourself and your locality for a storm surge eventuality, and help save many lives.