Wind vs. Water

Separating the Damage

Each year the Gulf Coast braces itself for hurricane season, which runs from June through November. The most active months are August and September with September typically producing the most tropical weather events. These wind and water events can wreak havoc on older dwellings and buildings that were not constructed with current building standards. Florida has led the efforts and is at the forefront in developing a building code that addresses the effects of high winds in coastal areas; this was followed immediately by the publication of the International Building Code. Each of these codes addresses the design requirements for both high wind and flood events. Following an event, Donan Engineering is repeatedly tasked with the responsibility of aiding the community in separating the damage caused by wind versus flooding. To begin this process, one must first have a clear understanding of what damage can be expected from each type of weather effect and the underlying forces of each.

Figure 1

Wind damage to enclosed buildings typically begins on the outside of the structure with the removal of roof coverings and siding. Once a breach in the building envelope (the shell of the building) occurs, the damage moves inward. Generally, damage first occurs in the form of wet building materials. However, if a breach occurs, like the breaking of a window or the collapse of a garage door, then complete destruction can occur.

Figure 2

Two basic types of flooding can occur during a tropical event: moving flood water and rising flood water. Moving water affects structures with four different components: hydrostatic pressure, hydrodynamic pressure, breaking waves, and impact damage from floating debris. There are three basic variables that must be present in order for water to move and waves to develop:

1. High winds - winds in excess of 20 mph can generate moving water and substantial waves.
2. Fetch distance - the fetch is the downwind distance over open water, with no obstructions, that a wave has to propagate. Typically, this distance must be greater than 1,500 feet.
3. Time - the time it takes for water to move and waves to develop is largely dependent on the first two variables. Generally, it takes two to three hours of consistent wind over a fetch to generate moving water and waves.

Figure 3

Removing any one of the three variables from the equation will prevent the generation of moving water, and waves will not happen. Damage that can be seen following these events ranges widely and is dependent on the height of the moving water. Low-moving water will scour foundations, inducing vertical and lateral movements in the building. Higher-moving water levels can decimate a structure.

Rising flood water can occur in bays or river areas where there is no fetch for the generation of wave action. Therefore, as the water rises, buildings are wet from the bottom up. This will affect drywall, insulation, and even wood framing members, if not mitigated immediately. Often, following a rising water event, there can be foundation movements that can be linked to the saturation of the soil.

Separating the damage between the two storm-generated forces can be difficult. This becomes problematic when the wind and wave forces are near equal. Working closely with your Donan Engineering professional can aid you in determining the sometimes subtle differences between similar damages.

Figure 4