47.5 MB - The Whole Building Design Guide

2.1.1.1 Probability of Occurrence or FrequencyThe probability of occurrence, or frequency, is a statement of thelikelihood that an event of a certain magnitude will occur in agiven period of time. For many decades, floodplain managementhas been based on the flood that has a 1 percent chance of occurringin any given year, commonly called the “100-year flood.”For certain critical actions and decisions,such as planning or constructing a critical facility,the basis of risk decisions should be theflood that has a 0.2 percent probability of occurringin any given year, commonly calledthe “500-year flood.”The term “100-year flood” as an expressionof probability or frequency is often misunderstoodbecause it conveys the impressionthat a flood of that magnitude will occur onlyonce every 100 years. Actually, the 1-percentannual-chanceflood has one chance in 100of occurring in any given year. The fact thata 1-percent-annual-chance flood is experienced at a specific locationdoes not alter the probability that a comparable flood couldoccur at the same location in the next year, or even multiple timesin a single year. As the length of the period increases, so does theprobability that a flood of a specific magnitude or greater willoccur. For example, during a 30-year period (the usual lending periodfor a home mortgage), the probability that a 100-year floodwill occur is 26 percent. And during a 70-year period (the potentialuseful life of many buildings), the probability increases to 50percent. Similarly, the 500-year flood has a 0.2-percent probabilityof being equaled or exceeded in any given year, and during a 70-year period the probability of occurrence is 18 percent.Regardless of the flood selected for design purposes (the“design flood”), the designer must determine specific characteristicsassociated with that flood. Determining a flood with aspecific probability of occurrence is done in a multi-step processthat typically involves using computer models that are inthe public domain. If a sufficiently long record of flood informationexists, the design flood may be determined by applyingstatistical tools to the data. Alternatively, water resource engi-The assigned frequency of a flood (e.g.,100-year) is independent of the numberof years between actual occurrences.Hurricane Camille hit the Mississippi coastin 1969 with storm surge flooding that farexceeded previous events, and HurricaneKatrina affected much the same area.Although just 36 years apart, both stormsproduced flood levels significantly higherthan the 100-year flood.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-