Likelihood & Risk
Probability and risk concepts are used to quantify a homeowner or community's hurricane hazard. The likelihood of being exposed to a hazard of some given level or higher is known as the "exceedance probability." The exceedance probability is defined as a percent or fraction. For example, a 1% (or 0.01) annual flood exceedance probability means there is a 1 in 100 chance in any given year that the flood elevation will be at that level or higher.
Another popular probability term is "return period." Return period is defined as the inverse of the exceedance probability, expressed in years. In our example, a 1% flood exceedance probability has a return period of 100 years (1/0.01). The return period represents the average number of years between events of that magnitude or higher. The return period does not represent the actual number of years between such events. More specifically, it is plausible that an area can experience the 100-yr event two years in a row, or even twice in one year.
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Example of storm surge versus annual exceedance probability (left) and return period (right).
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Acknowledgements & Credits: This material is based upon work supported by the Texas Department of Public Safety's Division of Emergency Management. Background photo courtesy of Mark Moran (Creative Commons license CC BY 2.0)
© 2019 Jennifer L. Irish & Steven M. Quiring. All rights reserved.
© 2019 Jennifer L. Irish & Steven M. Quiring. All rights reserved.