Hurricane Forces: Wind VS Wave Forces

Thursday, September 5, 2019

Hurricane Forces - Wind VS Wave Forces

As one of many issues to be dealt with after the landfall of a major hurricane, coastal property owners and insurers of coastal properties are often faced with determining the cause of various forms of damage to their facilities. Hurricanes are somewhat unique in that they generate multiple types of forces that can affect coastal properties.

Everyone is aware of the high winds that accompany the landfall of a hurricane, and of the property damage that high winds can cause. Hurricanes also typically result in a storm surge, where a large volume of water is pushed ashore during the landfall. Other issues commonly associated with the landfall of a major hurricane include heavy rains, inland flooding, and tornadoes.

When a coastal property is subjected to both high winds and storm surge, determining what damage was caused by the winds versus what damage was caused by the surge can be difficult, as both high winds and storm surge can result in similar damage. The loss of exterior claddings, structural deformations, and other damages can result from the forces generated by both winds and storm surge. Both wind and storm surge result in the flow of a fluid, the fluid being air in high winds and/or water in storm surge, past a static body, the structure. The analysis of the flow of the fluid past the static body is similar for both conditions.

Damage photos

Doing the Math...

The calculations required to determine the force generated by winds or storm surge depend on the mass of the fluid, either air or water, the velocity of the fluid, the shape of the structure, and the area of the structure. For a typical structure, the shape and size do not change, which means that the mass of the fluid and the speed of the fluid are the most significant variables.

For a category 4 hurricane, wind speeds are a minimum of 130 miles per hour (MPH). FEMA has estimated that in a 130 MPH wind, the top portion of the storm surge coming ashore has a speed of 5.2 MPH. A comparison of these two numbers shows that the air is hitting the structure at a speed that is 25 times greater than the speed of the water that is hitting the structure. At 75 degrees, air has a mass of 0.074 pounds per cubic foot, while water has a mass of 62.4 pounds per cubic foot, or 843 times greater than the mass of air.

Therefore, while the air is striking a structure at a speed that is 25 times faster than the water, the mass of the water is 843 times greater than the mass of the air. In the formulas used to calculate the forces, the speed of the fluid is squared, somewhat reducing the difference between the mass of air and the mass of water. However, calculations have shown that forces applied to a structure by storm surge are typically much higher than the forces applied by high winds. The forces resulting from the battering of waves should also be considered when comparing wind-related forces to water-related forces.