addressing climate change adaptation in regional transportation plans

Addressing Climate Change Adaptation in Regional Transportation PlansA Guide for California MPOs and RTPAswill consider only potential, not actual, risk the estimate of likelihood can beperformed solely for the climate stressor.Determining stressor likelihood varies widely in degree of difficulty, although, aswith other uncertainties treated in this module, the preference is for a quick,sketch level methodology rather than a rigorous but time consuming approach.There are three primary perspectives for considering stressor likelihood, averageannual frequency of occurrence (frequency), average annual exceedanceprobability (probability), and average recurrence interval, which express thesame phenomenon using different terminology.Average annual frequency of occurrence. Events that are described by thenumber of days (or other time periods) meeting or exceeding thresholdvalues, such as days ≤95°F temperatures, ≤1” rainfall, or the upper onepercentrainfall event 21 , can be considered in terms of their average annualfrequencies. These events are often associated with maintenance andoperational impacts or asset deterioration, rather than major damage,although with each event there may be a remote, and potentially increasing,likelihood of more significant impacts (for example a high-heat inducedconcrete blow-up that causes a motorcycle fatality). Historical averageannual frequencies can be derived from weather station records collected bythe National Weather Service, with detailed information available on-linefrom the National Climatic Data Center.Average annual exceedance probability (AEP). When events are describedby their annual likelihood of occurrence, they are referred to in terms of theirexceedance probabilities. These stressors typically include flood events,runoff volumes, and significant rainfall events. The FEMA floodplains, forexample, represent estimated flood coverage areas for (commonly) 1-percentand 0.2-percent chance flooding events (although common convention, theterms 100-year and 500-year to describe floodplains are misleading, insteadreferring to average recurrence interval).Average recurrence interval (ARI). The NOAA Atlas 14 provides estimatesof rainfall intensity and depth ranges (associated with 90-percent confidenceintervals) for a matrix of event durations (in minutes, hours, and days) andaverage recurrence intervals from 1 to 1,000 years (periods betweenexceedance events are random). For the “24-hour” rainfall event, forexample, the user can view the range (upper and lower bounds of theconfidence interval) of absolute rainfall in inches expected to recur every100 years, on average (e.g., 5.4 inches, with a range of 4.56 to 6.52 inches, forSacramento). The average recurrence interval can be adjusted through21 In this case, the one-percent precipitation event denotes the values that fall into the topone percent of all precipitation events; not to be confused with the one-percent chancerainfall event.11-10 Cambridge Systematics, Inc.