Characterizing the Impact of Natural Hazards

  In July 2009 Ricardo Alvarez submitted an abstract for a presentation at the 34th Annual Hazards Workshop, where he was an invited panelist. This abstractc may be viewed at the following link: Characterization of Impact. In his presentation Ricardo submitted the arguments outlined below.

The practice of hazard mitigation provides human societies with the best tool to reduce the potential for damage from the impact of natural hazards to human activity, the built-environment, and the natural environment. Mitigation measures are most effective when designed and implemented to counteract specific causes of damage, which means it is critical to identify and understand the damage components of each specfic hazard.

Along these lines Ricardo Alvarez proposes it is critically important to understand the causality of damage, meaning the cause and effect of the impact of a hazard; what happens when a building is hit by a hazard? In order to answer this question we must first understand that natural hazards have damage components, which are those components of the hazard capable of causing direct damage.

Take for example a hurricane: what are the damage components of a hurricane? In general terms the main damage components of a hurricane are wind and water. Being more specific, damage components of a hurricane include (a) wind-velocity pressure, (b) hydrodynamic pressure from storm surge, (c) hydrostatic pressure from flooding or water-logged soils, (d) wave impact, also from storm surge, (e) wind-driven rain, (f) extreme precipitation [rain, hail], (g) lightning, (h) flying debris and (i) floating debris. We could continue this discussion on hurricane damage components byclassifying wind-velocity pressure into positive pressure, when the wind pushes on the surfasce of an object or building, and negative pressure, when the wind ‘pulls’ or creates suction on a surface. Or we could describe the various effects caused by the wind interacting with a building.

It is clear that by following the process outlined above  we can gain a much better understanding of how damage is caused when specific damage components of a hurricane impact or interact with a building, and also acquire a clear understanding of cause and effect.  However since the purpose of this brief article is not to go into a detailed study of causality of damage, let us focus on the following: each damage component of a hurricane will apply loads (forces) to a building upon impact. Such loads can be quantified, measured, in terms we can all understand and use; for example positive wind-velocity pressure applies a load pushing on a building surface with a force typically measured in pounds per square foot  (psf).

Knowing this, and if we also know the characteristics of a given hurricane as it approaches a coastal community, it becomes possible to describe the magnitude and performance of the various damage components, describe the expected effects of these damage components as they interact with a specific building, estimate the magnitude of the loads that will be applied to the building.On that basis and knowing more about the design criteria used for the building, the types of materials used in iuts contruction, and other factors sich as shape, height etc. it is possible to estimate the type and amount of damage that could be caused by the impact of that hurricane on that building. We call this process the characterization of impact. Characterization of impact is an effective tool for the practice of hazard mitigation and risk assessment.