Coastal communities around the world face a powerful and often devastating threat from the ocean: storm surge. This phenomenon, an abnormal rise in sea level generated by a storm, can inundate coastal and low-lying areas, causing significant damage and posing a serious risk to life and property. Understanding the forces behind storm surges is crucial for preparedness and mitigation. These are complex events driven by a combination of atmospheric and oceanic factors.
What drives a storm surge
Several factors contribute to the formation and height of a storm surge:
- Wind stress: This is generally considered the biggest factor. Strong winds from a storm push the surface water ahead of it. As this water approaches a coastline, particularly in shallow areas, it has nowhere to go but up and inland, piling up against the shore. The speed and duration of the wind, as well as the fetch (the distance over which the wind blows across the water), significantly influence the magnitude of this effect.
- Atmospheric pressure: The low atmospheric pressure at the center of a storm also plays a role, though it's a less significant contributor than wind in most cases. Lower pressure on the ocean surface allows the water level to rise slightly, as there is less force pushing down on it. This is sometimes referred to as the "inverted barometer effect."
- Shape of the coastline and bathymetry (underwater topography): These geographical features have a crucial impact on how severe a storm surge will be.
- Shallow, gently sloping continental shelves allow water to pile up more easily as it's pushed towards the coast, leading to higher surges.
- Concave coastlines (like bays) can funnel water, concentrating the surge and causing it to rise higher.
- Narrow inlets and estuaries can also amplify storm surge as the water is forced into a smaller area.
- Storm size and forward speed: Larger storms affect a greater area of the ocean with their winds, potentially generating a larger surge. The storm's forward speed can also influence the surge; a faster-moving storm might produce a higher surge at the immediate coastline, while a slower-moving storm can push water further inland, especially into bays and estuaries.
- Angle of approach: A storm hitting the coast perpendicularly is generally more likely to produce a higher storm surge than one moving parallel to the coast or at an oblique angle.
- Waves: While technically separate from storm surge, waves generated by the storm's winds ride on top of the surge and can significantly increase the total water level and the destructive power along the coast through wave run-up.
How Earth's rotation affects storm surge
The Coriolis effect, caused by the Earth's rotation, deflects moving objects (including water and air) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. In the context of storm surges, the Coriolis effect can influence the direction of water movement driven by the storm's winds. This can cause water to pile up more on one side of the storm's track relative to the coastline, potentially increasing the surge height in certain areas. For winds blowing parallel to the coast, the Coriolis effect can deflect the water flow towards or away from the shore, impacting the surge.
Understanding positive and negative surges
When we talk about storm surge, we usually mean a "positive" surge, but there's another side to the coin:
- Positive storm surge: This is the more commonly discussed type, where the sea level rises above the normal predicted tide. This is what causes coastal flooding.
- Negative storm surge: This occurs when strong offshore winds or winds blowing parallel to the coast in a specific direction (influenced by the Coriolis effect) push water away from the coastline, resulting in a lower-than-normal sea level. While not causing flooding, negative storm surges can still be hazardous, particularly for navigation, as they can lead to significantly lower water depths in harbors and coastal channels.
They are essentially opposite phenomena driven by similar storm forces but with different wind directions relative to the coast, leading to either an accumulation or depletion of water near the shore.
How tall and widespread storm surges can be
Storm surges can reach significant heights, and their inland reach can be extensive, particularly in low-lying coastal areas with shallow offshore slopes.
- Height: Storm surge heights are measured as the rise in water level above the normal astronomical tide. They can range from a few feet to well over 20 feet (about 6 meters) or more in extreme cases.
- Size/inland reach: The area affected by a storm surge can span hundreds of kilometers of coastline, and the water can push several kilometers inland, especially in flat terrain.
Examples of significant storm surges
- Hurricane Katrina (2005, United States): This hurricane famously produced a massive storm surge along the Mississippi coast, with some areas experiencing surges estimated at 25 to 28 feet (around 8-8.5 meters) above normal tide levels. The inundation extended far inland in many locations.
- Cyclone Mahina (1899, Australia): While there's some debate about the exact measurement, historical accounts suggest an extremely high storm tide (storm surge plus astronomical tide) of around 44 feet (about 13 meters) occurred in Bathurst Bay, though a significant portion was likely wave run-up on steep terrain.
- Bhola Cyclone (1970, Bangladesh): This devastating cyclone caused a storm surge estimated at over 10 meters (around 33 feet), leading to catastrophic flooding in the low-lying Ganges River Delta and immense loss of life.
- Typhoon Haiyan (2013, Philippines): Haiyan generated a massive storm surge that devastated coastal areas, with estimates reaching up to 15 feet (about 4.5 meters) in some locations, contributing significantly to the widespread destruction and death toll.
Can storms other than hurricanes cause storm surges?
While hurricanes, typhoons, and cyclones are the most common and often the most powerful generators of storm surges due to their intense winds and low pressure, other strong low-pressure systems can also cause them. Notable examples include:
- Nor'easters: These extratropical cyclones that affect the eastern coast of North America can produce significant storm surges, particularly when they stall or move slowly near the coast.
- Other intense extratropical cyclones: Strong winter storms and other deep low-pressure systems in mid-latitude regions can also generate considerable storm surges in coastal areas.
The key is the presence of strong, persistent winds pushing water towards the coast and a significant drop in atmospheric pressure, conditions that can occur in various types of intense storms, not just tropical cyclones.
