Determining water loss: Storm surge, tide surge or rainwater flood
How do the characteristics of different water loss events compare?
The standard ISO property, homeowners and dwelling coverage forms each contain a water exclusion that extends to exclude loss caused by storm surge. It seems that recent hurricane paths have come further inland and resulted in an inordinate amount of flood loss. How can one determine if a loss in an inland area is from flooded rainwater or a storm surge?
Of all the damage caused by hurricanes, storm surge presents the greatest threat to life and property along the coast. Hurricanes produce strong, powerful winds, but it’s actually the water that causes most of the destruction, accounting for nearly 90% of fatalities, with about half of those due to storm surge. The size and strength of a storm surge can break down barriers such as dunes, levees and barricades and sweep through an area, swiping away everything in its path, flooding communities, and leaving death and devastation in its wake.
Storm surge is an undefined term in insurance policies, so what exactly is a storm surge?
According to the National Oceanic and Atmospheric Administration (NOAA), the definition of a storm surge is as follows: Storm surge is the abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide. The surge is caused primarily by a storm’s winds pushing water onshore. The amplitude of the storm surge at any given location depends on the orientation of the coastline with the storm track; the intensity, size, and speed of the storm; and the local bathymetry.
Put more simply, as a hurricane closes the gap between sea and land, its winds push a huge volume of ocean water ashore, which is a storm surge.
The storm surge begins over the open ocean as strong hurricane winds push the ocean waters around, causing water to pile up under the storm. As winds swirl around the storm, the seawater is pushed into a mound at the storm’s center. The height and extent of this pile of water depend on the size and strength of the hurricane. The faster the wind speed, the more water is able to pile up. At the center of a hurricane, air pressure is low. Low air pressure causes a slight bulge in the ocean, which adds to the pile of water that causes the storm surge, allowing for about 5% of the mound.
The tide time is an important factor in the impact a storm surge will have at landfall. At high tide, the water is already elevated, so if landfall occurs during high tide, it will cause the surge to reach higher levels and push the water further inland. This is what happened when Hurricane Isaias hit Myrtle Beach, S.C., in August, when the storm surge was only four feet, but the water level was more than ten feet above normal.
Storm surge vs. storm tide
However, according to the NOAA, a storm surge should not be confused with a storm tide. A storm tide is defined as the water level rise due to the combination of storm surge and the astronomical tide. Even though the rise in water level can cause extreme flooding in coastal areas when a storm surge coincides with normal high tides and can reach up to 20 feet or more. The storm surge is created by water being pushed by the force of winds moving cyclonically around the storm.
Then there is also a growing concern over rising sea levels and its influence on storm surge. As water warms, it expands, so as global temperatures rise, it raises the sea level. Melting of ice sheets and glaciers adds to this. Together, they elevate the background ocean height, meaning that with the higher elevation, the hurricane storm surge can bring the water further inland.
Another contributing factor is the depth or shallowness of the seafloor; for example, if the seafloor gradually slopes away from the coastline, it’s more likely to have a higher storm surge than if there is a steep drop-off of land into the sea. Contrast this with Louisiana and Miami — the storm surge will be greater in Louisiana than in Miami because the seafloor in Louisiana is wide with a gradual shallow slope, while in Miami, the seafloor drops off very quickly.
The shape of the coastline also affects the shape of the surge. For example, when a storm surge enters a bay or river, the shape of the land can act as a funnel, sending the water surge higher. The wind speed significantly affects this, as slow-moving storms tend to cause more storm surge in coastal bays and estuaries, such as that seen with Hurricane Sally in Florida and Louisiana.
The angle of the storm’s trajectory also affects the surge; a hurricane that heads straight towards a coastline is more likely to cause a higher storm surge than one that hits the coast at an oblique angle or travels parallel to the coast.
At what point does the water stop being from the hurricane surge and become rainwater? Storm surge is not related to the amount of rain that falls within a storm. Based on the earlier information about storm surge, we can determine that normal rainwater, regardless of how heavy it is, would not meet the definition of a storm surge. Once the storm moves inland, the wind is no longer pushing the water cyclonically across the ocean, and it is no longer a storm surge. Instead, it is now a rainstorm, which could produce flooding.
Karen L. Sorrell, CPCU, (ksorrell@alm.com) is an editor with FC&S Expert Coverage Interpretation, the authority on insurance coverage interpretation and analysis for the P&C industry. It’s the resource agents, brokers, risk managers, underwriters, and adjusters rely on to research commercial and personal lines coverage issues. Visit www.NationalUnderwriter.com for more information.
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