Hydrologic Issues for Dams - Association of State Dam Safety Officials

are limited to record lengths of less than about 30 years; basic rainfall and temperature data areavailable for some stations for up to 150 years, but in most cases are limited to less than 100 years.Historical DataHistorical data can provide a means for extending the length of record for many types ofdata, in particular for observations of the most extreme events. These data are most commonly usedto extend streamflow records of peak discharge prior to organized stream gaging. Historicalobservations can provide information for other types of data such as weather patterns and thefrequency of extreme storm events, or changes in land use or vegetation that may be significant torunoff modeling calculations. However, as with any type of historical data, the accuracy and validityof the observations must be carefully assessed and compared to the other types of data used in theanalysis.Paleoflood DataPaleoflood hydrology is the study of past or ancient flood events which occurred prior to thetime of human observation or direct measurement by modern hydrological procedures (Baker, 1987).Unlike historical data, paleoflood data do not involve direct human observation of the flood events.Instead, the paleoflood investigator studies geomorphic and stratigraphic records (various indicators)of past floods, as well as the evidence of past floods and streamflow derived from historical,archeological, dendrochronologic, or other sources. The advantage of paleoflood data is that it isoften possible to develop records that are 10 to 100 times longer than conventional or historicalrecords from other data sources in the western United States. In addition, the paleoflood record is along-term measure of the tendency of a river to produce large floods. In many cases, paleofloodstudies can provide a long-term perspective, which can put exceptional annual peak dischargeestimates in context and assist in reconciliation of conflicting historical records.Paleoflood data generally include records of the largest floods, or commonly the limits on thestages of the largest floods over long time periods. This information can be converted to peakdischarges using a hydraulic flow model. Generally, paleoflood data consist of two independentcomponents. One component is a peak discharge estimate; the second is a time period or age overwhich the peak discharge estimate applies. Paleoflood studies can provide estimates of peakdischarge for specific floods in the past, or they can provide exceedance and non-exceedance boundsfor extended time periods. Each of these differing types of paleoflood data must be appropriatelytreated in flood frequency analyses.Extrapolation Limits for Different Data TypesThe primary basis for a limit on credible extrapolation of extreme flood estimates derivesfrom the characteristics of the data and the record length used in the analysis. The data used in theanalysis provide the only basis for verification of the analysis or modeling results, and as such,extensions beyond the data cannot be verified. Different risk assessments require flood estimates fordifferent ranges of annual exceedance probability (AEP), and therefore analysis procedures and datasourcesshould be selected to meet project requirements. The greatest gains to be made in providingPaper 18 – Swain, Bowles, Ostenaa 128