Coastal storm surges caused by hurricanes and tropical storms are perhaps the most damaging part of these natural disasters. Hurricane Katrina led to an estimated $75 billion in damage in the New Orleans area alone, with at least 1500 deaths directly or indirectly resulting from the storm surge.
Storm surges are likely to become more common — and more dangerous — as hurricanes and tropical storms intensify. Here’s what you need to know about coastal storm surges and how to better predict and protect yourself from storm surge flooding.
What is a storm surge?
A storm surge is a temporary rise in sea level beneath a storm. Storm surges are caused primarily by a storm’s winds pushing water onshore, but there are a number of complex factors that impact the strength of a storm surge. Storm surges are impacted by changes in the storm’s intensity, forward speed, size, the angle at which the storm approaches the coast, central pressure, and characteristics of the coast (e.g. geographical features like estuaries or high cliffs).
How does a storm surge form? As winds swirl in a hurricane or tropical storm, seawater is pushed into a “mound” at the storm’s center. The faster the wind speed, the more water piles up — accordingly, Category 4 and 5 storms produce higher water mounds than Category 1 or 2 storms.
Low air pressure at the center of the hurricane also contributes to the pile-up of seawater. About 5% of the mound that becomes the storm surge is caused by low air pressure. As the storm approaches the coast, this mound makes landfall and turns into storm surge flooding.
The shape of the coastline and the seafloor also impact the strength of the storm surge as it makes landfall. Coastal areas with a shallow seafloor are more likely to be affected by a large storm surge compared to areas where the seafloor is deep. Likewise, coasts that bow inward are more likely to see larger storm surges: bays are vulnerable because the pile of water is funneled into a small area.
A storm surge is often (mistakenly) confused with a storm tide. A storm tide occurs when a high tide and storm surge coincide. Storm tides can cause water to rise 20 feet or more above sea level, dangerously high for low-lying areas and coastal communities.
Why are storm surges so dangerous?
Storm surges can devastate communities — even those tens of miles inland, causing flooding of 30 feet or more from the coast. Hurricane Ike, a Category 2 hurricane in 2008, caused storm surges 15 - 20 feet above normal tide levels, leading to property damage estimated at nearly $25 billion.
“When a storm surge reaches land, the wall of waves can rush miles inland, battering anything in its path. Under the weight of that water—approximately 1,700 pounds per cubic yard—beaches erode and buildings can crumble. Storm surges can also flood inland rivers and lakes, contributing to billions of dollars of flood damage,” wrote National Geographic.
Perhaps most concerning, however, is the danger to human lives that storm surges present. One study found that storm surges accounted for nearly 50% of all Atlantic tropical hurricane deaths from 1963 to 2012, mainly due to drowning.
For these reasons, it’s important to take storm surges seriously. Here’s what you can do to protect your home, family, and community.
How to prepare for a coastal storm surge
FEMA estimates that 90% of natural disasters in the US involve a flood. When Hurricane Sandy hit New York and New Jersey, flooding, high winds, and a 13+ foot storm surge caused $70 billion in damages. As storms become more intense, it’s important for communities to prepare accordingly.
Improve storm surge forecasting
Coastal flood warnings are informed by computer models, such as the NWS “SLOSH” model — Sea Lake and Overland Surges from Hurricanes. This model estimates storm surge heights by accounting for atmospheric pressure, size, forward speed, and tracking data. This creates a model for the wind field driving the storm surge using three estimates.
- Deterministic approach: this approach uses physics equations to create a single simulation based on a "perfect" forecast. This approach is heavily dependent on accurate meteorological inputs, which, as we’ve covered, can change quickly and have huge impacts on a storm surge.
- Probabilistic approach: also known as “P-Surge”, this approach uses statistics from past forecasts to generate scenarios based on distributions of cross-track, along track, intensity, and size errors.
- Composite approach: this approach predicts storm surge by “running SLOSH several thousand times with hypothetical hurricanes under different storm conditions. The products generated from this approach are the Maximum Envelopes of Water (MEOWs) and the Maximum of MEOWs (MOMs) which are regarded by NHC as the best approach for determining storm surge vulnerability for an area since it takes into account forecast uncertainty.”
The deterministic approach is perhaps the least accurate. “At the time emergency managers must make an evacuation decision, the forecast track and intensity of a tropical cyclone are subject to large errors, thus a single simulation of the SLOSH model does not always provide an accurate depiction of the true storm surge vulnerability,” wrote the National Hurricane Center.
Emergency management services tend to use MEOWs and MOMs in developing evacuation zones. Unfortunately, flood maps are notoriously outdated, impacting how emergency services may use these tools to accurately evacuate or focus their rescue efforts.
[Read more: Flood Maps Are Outdated - Here’s How to Fix Them]
Moored buoy systems such as Sofar Ocean’s Smart Mooring are deployed along coasts to develop stronger early warning systems, provide coastal communities with enough time to evacuate, shelter in place, or receive aid after a storm has made landfall. The data used for storm surge models can help families prepare well in advance.
Preparing for a storm surge
The Canadian government recommends taking the following steps when a storm surge is forecasted to impact your area.
- Check supplies, including medications, radio, flashlight, and batteries.
- Keep your emergency kit close at hand in case you have to evacuate.
- Close any basement windows
- Fill your car’s gas tank.
Local officials will also let you know in advance if you need to surround your house with sandbags to prevent water from entering. You should also learn how to turn off gas and electricity before the storm hits.
If you find yourself hit with a storm surge, stay inside and away from the windows, ideally on the side of the house that’s downwind from the hurricane. Monitor the storm’s progress and listen to the radio or TV broadcast: try to reach rescue officials who will be coordinating evacuation plans. Do not drive through floodwaters and try to stay out of the water in order to avoid hypothermia or drowning.
To learn more about mitigating the risks of hurricanes and tropical storms, visit the Sofar Ocean blog.