47.5 MB - The Whole Building Design Guide

Flood frequency analyses are performedusing historical records, and the resultsare influenced by the length of therecord. Such analyses do not accountfor recent changes to the land (uplanddevelopment or subsidence) or futurechanges (additional development, greatersubsidence, or climatic variations).neers sometimes apply computer models tosimulate different rainfall events over watersheds,to predict how much water will runoff and accumulate in channels. Other computermodels are used to characterize theflow of water down the watershed and predicthow high the floodwaters will rise.For coastal areas, both historical stormsand simulated storm surge models canbe used to predict the probability thatfloodwaters will rise to a certain level and be accompaniedby waves of certain heights. Many coastal storms will producestorm surge flooding that, depending on local topography,may extend inland significantly farther than anticipated forthe 1-percent-annual-chance flood. Statistically, such extremestorm surges occur less frequently than the 1-percent or 0.2-percent-annual-chance floods, but their consequences can becatastrophic.Planners and designers should research the relationshipbetween the flood levels for different frequency events and extremeevents, especially in hurricane-prone communities. Thedifference in flood levels may be extreme in some situations,depending on local conditions and the source of flooding. Inother areas the lower probability flood depths might not bemuch higher than the 1-percent-annual-chance flood.The National Flood Insurance Program (NFIP) is a Federalprogram that encourages communities to regulate floodhazard areas and, in return, offers property owners insuranceprotection against losses from flooding (see Sections 2.1.3.1and 2.1.3.2). The NFIP uses the 1-percent-annual-chance floodas the basis for flood hazard maps, for setting insurance rates,and for application of regulations in order to minimize futureflood damage. The 1-percent-annual-chance flood is also usedas the standard for examination of older buildings to determinethe measures to apply in order to reduce future damage.Satisfying the minimum requirements of the NFIP does notprovide adequate protection for critical facilities that need tobe functional even after low probability events. Nearly every2- MAKING CRITICAL FACILITIES SAFE FROM Flooding